Private Lines
About Private Line

Private Line covers what has occurred, is occurring, and will ocurr in telecommunications. Since communication technology constantly changes, you can expect new content posted regularly.

Consider this site an authoritative resource. Its moderators have successful careers in the telecommunications industry. Utilize the content and send comments. As a site about communicating, conversation is encouraged.

Writers

Thomas Farely

Tom has produced privateline.com since 1995. He is now a freelance technology writer who contributes regularly to the site.

His knowledge of telecommunications has served, most notably, the American Heritage Invention and Technology Magazine and The History Channel.
His interview on Alexander Graham Bell will air on the History Channel the end of 2006.

Ken Schmidt

Ken is a licensed attorney who has worked in the tower industry for seven years. He has managed the development of broadcast towers nationwide and developed and built cell towers.

He has been quoted in newspapers and magazines on issues regarding cell towers and has spoke at industry and non-industry conferences on cell tower related issues.

He is recognized as an expert on cell tower leases and due diligence processes for tower acquisitions.

January 01, 2006

Posted by Tom Farley & Mark van der Hoek at 08:55 PM

Cell and Sector Terminology

With cellular radio we use a simple hexagon to represent a complex object: the geographical area covered by cellular radio antennas. These areas are called cells. Using this shape let us picture the cellular idea, because on a map it only approximates the covered area. Why a hexagon and not a circle to represent cells?

When showing a cellular system we want to depict an area totally covered by radio, without any gaps. Any cellular system will have gaps in coverage, but the hexagonal shape lets us more neatly visualize, in theory, how the system is laid out. Notice how the circles below would leave gaps in our layout. Still, why hexagons and not triangles or rhomboids? Read the text below and we'll come to that discussion in just a bit.

Notice the illustration below. The middle circles represent cell sites. This is where the base station radio equipment and their antennas are located. A cell site gives radio coverage to a cell. Do you understand the difference between these two terms? The cell site is a location or a point, the cell is a wide geographical area. Okay?

Most cells have been split into sectors or individual areas to make them more efficient and to let them to carry more calls. Antennas transmit inward to each cell. That's very important to remember. They cover a portion or a sector of each cell, not the whole thing. Antennas from other cell sites cover the other portions. The covered area, if you look closely, resembles a sort of rhomboid, as you'll see in the diagram after this one. The cell site equipment provides each sector with its own set of channels. In this example, just below , the cell site transmits and receives on three different sets of channels, one for each part or sector of the three cells it covers.

Is this discussion clear or still muddy? Skip ahead if you understand cells and sectors or come back if you get hung up on the terms at some later point. For most of us, let's go through this again, this time from another point of view. Mark provides the diagram and makes some key points here:

"Most people see the cell as the blue hexagon, being defined by the tower in the center, with the antennae pointing in the directions indicated by the arrows. In reality, the cell is the red hexagon, with the towers at the corners, as you depict it above and I illustrate it below. The confusion comes from not realizing that a cell is a geographic area, not a point. We use the terms 'cell' (the coverage area) and 'cell site' (the base station location) interchangeably, but they are not the same thing.

Click here if you want an illustrated overview of cell site layout

WFI's Mark goes on to talk about cells and sectors and the kind of antennas needed: "These days most cells are divided into sectors. Typically three but you might see just two or rarely six. Six sectored sites have been touted as a Great Thing by manufacturers such as Hughes and Motorola who want to sell you more equipment. In practice six sectors sites have been more trouble than they're worth. So, typically, you have three antenna per sector or 'face'. You'll have one antenna for the voice transmit channel, one antenna for the set up or control channel, and two antennas to receive. Or you may duplex one of the transmits onto a receive. By sectorising you gain better control of interference issues. That is, you're transmitting in one direction instead of broadcasting all around, like with an omnidirectional antenna, so you can tighten up your frequency re-use"

"This is a large point of confusion with, I think, most RF or radio frequency engineers, so you'll see it written about incorrectly. While at AirTouch, I had the good fortune to work for a few months with a consultant who was retired from Bell Labs. He was one of the engineers who worked on cellular in the 60s and 70s. We had a few discussions on this at AirTouch, and many of the engineers still didn't get it. And, of course, I had access to Dr. Lee frequently during my years there. It doesn't get much more authoritative than the guys who developed the stuff!"

Jim Harless, a regular contributor, recently checked in regarding six sector cells. He agrees with Mark about the early days, that six sector cells in AMPS did not work out. He notes that "At Metawave (link now dead) I've been actively involved in converting some busy CDMA cells to 6-sector using our smart antenna platform. Although our technology is vendor specific, you can't use it with all equipment, it actually works quite well, regardless of the added number of pilots and increase in soft handoffs. In short, six sector simply allows carriers to populate the cell with more channel elements. Also, they are looking for improved cell performance, which we have been able to provide. By the way, I think the reason early CDMA papers had inflated capacity numbers were because they had six sector cells in mind."

Mark says "I don't recall any discussion of anything like that. But Qualcomm knew next to nothing about a commercial mobile radio environment. They had been strictly military contractors. So they had a lot to learn, and I think they made some bad assumptions early on. I think they just underestimated the noise levels that would exist in the real world. I do know for sure that the 'other carrier jammer' problem caught them completely by surprise. That's what we encountered when mobiles would drive next to a competitors site and get knocked off the air. They had to re-design the phone.

Now, what about those hexagon shaped cell sites?
Mark van der Hoek says the answer has to do with frequency planning and vehicle traffic. "After much experimenting and calculating, the Bell team came up with the solution that the honeybee has known about all along -- the hex system. Using 3 sectored sites, major roads could be served by one dominant sector, and a frequency re-use pattern of 7 could be applied that would allow the most efficient re-use of the available channels."

A cell cluster. Note how neatly seven hexagon shaped cells fit together. Try that with a triangle. Clusters of four and twelve are also possible but frequency re-use patterns based on seven are most common.

Mark continues, "Cellular pioneers knew most sites would be in cities using a road system based on a grid. Site arrangement must allow efficient frequency planning. If sites with the same channels are located too closely together, there will be interference. So what configuration of antennas will best serve those city streeets?"
"If we use 4 sectors, with a box shape for cells, we either have all of the antennas pointing along most of the streets, or we have them offset from the streets. Having the borders of the sites or sectors pointing along the streets will cause too many handoffs between cells and sectors -- the signal will vary continously and the mobile will 'ping-pong' from one sector to another. This puts too much load on the system and increases the probablity of dropped calls. The streets need to be served by ONE dominant sector."

Do you understand that? Imagine the dots below are a road. If you have two sectors facing the same way, even if they are some distance apart, you'll have the problems Mark just discussed. You need them to be offset.

............................................................................
<-------Cell Site A ---------> <------Cell Site B------->
.............................................................................


"For a more complete discussion of the mathematics behind the hex grid, with an excellent treatment of frequency planning, I refer you to any number of Dr. Bill Lee's books."

Posted by Tom Farley & Mark van der Hoek at 09:09 PM

Basic Theory and Operation

Cell phone theory is simple. Executing that theory is extremely complicated. Each cell site has a base station with a computerized 800 or 1900 megahertz transceiver and an antenna. This radio equipment provides coverage for an area that's usually two to ten miles in radius. Even smaller cell sites cover tunnels, subways and specific roadways. The area size depends on, among other things, topography, population, and traffic.

When you turn on your phone the mobile switch determines what cell will carry the call and assigns a vacant radio channel within that cell to take the conversation. It selects the cell to serve you by measuring signal strength, matching your mobile to the cell that has picked up the strongest signal. Managing handoffs or handovers, that is, moving from cell to cell, is handled in a similar manner. The base station serving your call sends a hand-off request to the mobile switch after your signal drops below a handover threshold. The cell site makes several scans to confirm this and then switches your call to the next cell. You may drive fifty miles, use 8 different cells and never once realize that your call has been transferred. At least, that is the goal. Let's look at some details of this amazing technology, starting with cellular's place in the radio spectrum and how it began.

The FCC allocates frequency space in the United States for commercial and amateur radio services. Some of these assignments may be coordinated with the International Telecommunications Union but many are not. Much debate and discussion over many years placed cellular frequencies in the 800 megahertz band. By comparison, PCS or Personal Communication Services technology, still cellular radio, operates in the 1900 MHz band. The FCC also issues the necessary operating licenses to the different cellular providers.

Although the Bell System had trialed cellular in early 1978 in Chicago, and worldwide deployment of AMPS began shortly thereafter, American commercial cellular development began in earnest only after AT&T's breakup in 1984. The United States government decided to license two carriers in each geographical area. One license went automatically to the local telephone companies, in telecom parlance, the local exchange carriers or LECs. The other went to an individual, a company or a group of investors who met a long list of requirements and who properly petitioned the FCC. And, perhaps most importantly, who won the cellular lottery. Since there were so many qualified applicants, operating licenses were ultimately granted by the luck of a draw, not by a spectrum auction as they are today.

The local telephone companies were called the wireline carriers. The others were the non-wireline carriers. Each company in each area took half the spectrum available. What's called the "A Band" and the "B Band." The nonwireline carriers usually got the A Band and the wireline carriers got the B band. There's no real advantage to having either one. It's important to remember, though, that depending on the technology used, one carrier might provide more connections than a competitor does with the same amount of spectrum. [See A Band, B Band

Mobiles transmit on certain frequencies, cellular base stations transmit on others. A and B refer to the carrier each frequency assignment has. A channel is made up of two frequencies, one to transmit on and one to receive.]

Learn more about cellular switches

-------------------------------

Notes:

[A Band, B Band] Actually, the strange arrangement of the expanded channel assignments put more stringent filtering requirements on the A band carrier, but it's on the level of annoying rather than crippling. Minor point.

Posted by Tom Farley & Mark van der Hoek at 09:17 PM

Cellular frequency and channel discussion

American cell phone frequencies start at 824 MHz and end at 894 MHz. The band isn't continuous, though, it runs from 824 to 849MHz, and then from 869 to 894. Airphone, Nextel, SMR, and public safety services use the bandwidth between the two cellular blocks. Cellular takes up 50 megahertz total. Quite a chunk. By comparison, the AM broadcast band takes up only 1.17 megahertz of space. That band, however, provides only 107 frequencies to broadcast on. Cellular may provide thousands of frequencies to carry conversations and data. This large number of frequencies and the large channel size required account for the large amount of spectrum used.

Thanks to Will Galloway for corrections

The original analog American system, AT&T's Advanced Mobile Phone Service or AMPS, now succeeded by its digital IS-136 service, uses 832 channels that are 30 kHz wide. Years ago Motorola and Hughes each tried making more spectrum efficient systems, cutting down on channel size or bandwidth, but these never caught on. Motorola's analog system, NAMPS, standing for Narrowband Advanced Mobile Service provided 2412 channels, using channels 10 kHz wide instead of 30kHz. [See NAMPS] While voice quality was poor and technical problems abounded, NAMPS died because digital and its inherent capacity gain came along, otherwise, as Mark puts it, "We'd have all gone to NAMPS eventually, poor voice quality or not."[NAMPS2]

I mentioned that a typical cell channel is 30 kilohertz wide compared to the ten kHz allowed an AM radio station. How is it possible, you might ask, that a one to three watt cellular phone call can take up a path that is three times wider than a 50,000 watt broadcast station? Well, power does not necessarily relate to bandwidth. A high powered signal might take up lots of room or a high powered signal might be narrowly focused. A wider channel helps with audio quality. An FM stereo station, for example, uses a 150 kHz channel to provide the best quality sound. A 30 kHz channel for cellular gives you great sound almost automatically, nearly on par with the normal telephone network.

Cellular runs in two blocks from, getting specific now, 824.04 MHz to 893. 97 MHz. In particular, cell phones or mobiles use the frequencies from 824.04 MHz to 848.97 and the base stations operate on 869.04 MHz to 893.97 MHz. These two frequencies in turn make up a channel. 45 MHz separates each transmit and receive frequency within a cell or sector, a part of a cell. That separation keeps them from interfering with each other. Getting confusing? Let's look at the frequencies of a single cell for a single carrier. For this example, let's assume that this is one of 21 cells in an AMPS system:

Cell#1 of 21 in Band A (The nonwireline carrier)

Channel 1 (333) Tx 879.990 Rx 834.990

Channel 2 (312) Tx 879.360 Rx 834.360

Channel 3 (291) Tx 878.730 Rx 833.730

Channel 4 (270) Tx 878.100 Rx 833.100

Channel 5 (249) Tx 877.470 Rx 832.470

Channel 6 (228) Tx 876.840 Rx 831.840

Channel 7 (207) Tx 876.210 Rx 831.210

Channel 8 (186) Tx 875.580 Rx 830.580 etc., etc.,

The number of channels within a cell or within an individual sector of a cell varies greatly, depending on many factors. As Mark van der Hoek writes, "A sector may have as few as 4 or as many as 80 channels. Sometimes more! For a special event like the opening of a new race track, I've put 100 channels in a temporary site. That's called a Cell On Wheels, or COW. Literally a cell site in a truck."

Cellular network planners assign these frequency pairs or channels carefully and in advance. It is exacting work. Adding new channels later to increase capacity is even more difficult. [See Adding channels] Channel layout is confusing since the ordering is non-intuitive and because there are so many numbers involved. Speaking of numbers, check out the sidebar. Channels 800 to 832 are not labeled as such. Cell channels go up to 799 in AMPS and then stop. Believe it or not, the numbering begins again at 991 and then goes up to 1023. That gives us 832. Why the confusion and the odd numbering? The Bell System originally planned for 1000 channels but was given only 666 by the FCC. When cellular proved popular the FCC was again approached for more channels but granted only an extra 166. By this time the frequency spectrum and channel numbers that should have gone to cellular had been assigned to other radio services. So the numbering picks up at 991 instead of 800. Arggh!

You might wonder why frequencies are offset at all. It's so you can talk and listen at the same time, just like on a regular telephone. Cellular is not like CB radio. Citizen's band uses the same frequency to transmit and receive. What's called "push to talk" since you must depress a microphone key or switch each time you want to talk. Cellular, though, provides full duplex communication. It's more expensive and complicated to do it this way. That's since the mobile unit and the base station both need circuitry to transmit on one frequency while receiving on another. But it's the only way that permits a normal, back and forth, talk when you want to, conversation. Take a look at the animated .gif below to visualize full duplex communication. See how two frequencies, a voice channel, lets you talk and listen at the same time?


Full duplex communication example. The two frequencies are paired and constitute a voice channel. Paths indicate direction of flow.

Derived from Marshal Brain's How Stuff Works site (external link)

------------------------------

Notes:

[Adding channels] "The channels for a particular cell are assigned by a Radio Frequency Engineer, and are fixed. The mobile switch assigns which of those channels to use for a given call, but has no ability to assign other channels. In a Motorola (and, I think, Ericsson) system, changing those assigned channels requires manual re-tuning of the hardware in the cell site. This takes several hours. Lucent equipment allows for remote re-tuning via commands input at the switch, but the assignment of those channels is still made by the RF engineer, taking into account re-use and interference issues. Re-tuning a site in a congested downtown area is not trivial! An engineer may work for weeks on a frequency plan just to add channels to one sector. It is not unusual to have to re-tune a half dozen sites just to add 3 channels to one." Mark van der Hoek. Personal correspondence.

[NAMPS] Macario, Raymond. Cellular Radio: Principles and Design, McGraw Hill, Inc., New York 1997 90. A good but flawed book that's now in its second edition. Explains several cellular systems such as GSM, JTACS, etc. as well as AMPS and TDMA transmission. Details all the formats of all the digital messages. Index is poor and has many mistakes.

[NAMPS2] "Only a few cities ever went with NAMPS, and it didn't replace AMPS, it was used in conjunction with AMPS. We looked at it for the Los Angeles market (where I spent 7 years with PacTel/AirTouch) but it just didn't measure up. The quality just wasn't good, and the capacity gains were not the 3 to 1 as claimed by Motorola. The reason is that you cannot re-use NAMPS channels as closely as AMPS channels. Their signal to noise ratio requirements are higher due to the reduced bandwidth. (We engineered to an 18dB C/I ratio for AMPS, whereas we found that NAMPS required 22 dB.) [See The Decibel for more on carrier interference ratios, ed.] Also, market penetration of NAMPS capable phones was an issue. If only 30% of your customers can use it, does it really provide capacity gains? The Las Vegas B carrier loved NAMPS, though. At least, that's what Moto told us. . . though even under the best of conditions NAMPS doesn't satisfy the average customer, according to industry surveys. There's no free lunch, and you can't get 30 kHz sound from 10 kHz. But the point is moot - - NAMPS is dead." Mark van der Hoek. Personal correspondence. (back to text)

[Adding channels] "The channels for a particular cell are assigned by a Radio Frequency Engineer, and are fixed. The mobile switch assigns which of those channels to use for a given call, but has no ability to assign other channels. In a Motorola (and, I think, Ericsson) system, changing those assigned channels requires manual re-tuning of the hardware in the cell site. This takes several hours. Lucent equipment allows for remote re-tuning via commands input at the switch, but the assignment of those channels is still made by the RF engineer, taking into account re-use and interference issues. Re-tuning a site in a congested downtown area is not trivial! An engineer may work for weeks on a frequency plan just to add channels to one sector. It is not unusual to have to re-tune a half dozen sites just to add 3 channels to one." Mark van der Hoek. Personal correspondence.

Posted by Tom Farley & Mark van der Hoek at 09:29 PM

Channel Names and Functions

Okay, so what do we have? The first point is that cell phones and base stations transmit or communicate with each other on dedicated paired frequencies called channels. Base stations use one frequency of that channel and mobiles use the other. Got it? The second point is that a certain amount of bandwidth called an offset separates these frequencies. Now let's look at what these frequencies do, as we discuss how channels work and how they are used to pass information back and forth.

Certain channels carry only cellular system data. We call these control channels. This control channel is usually the first channel in each cell. It's responsible for call setup, in fact, many radio engineers prefer calling it the setup channel since that's what it does. Voice channels, by comparison, are those paired frequencies which handle a call's traffic, be it voice or data, as well as signaling information about the call itself.

A cell or sector's first channel is always the control or setup channel for each cell. You have 21 control channels if you have 21 cells. A call gets going, in other words, on the control channel first and then drops out of the picture once the call gets assigned a voice channel. The voice channel then handles the conversation as well as further signaling between the mobile and the base station. Don't place too much importance, by-the-way, to the setup channel. Although first in each cell's lineup, most radio engineers place priority on the voice channels in a system. The control channel lurks in the background. [See Control channel] Now let's add some terms.

When discussing cell phone operation we call a base station's transmitting frequency the forward path. The cell phone's transmitting frequency, by comparison, is called the reverse path. Do not become confused. Both radio frequencies make up a channel as we've discussed before but we now treat them individually to discuss what direction information or traffic flows. Knowing what direction is important for later, when we discuss how calls are originated and how they are handled.

Once the MTSO or mobile telephone switch assigns a voice channel the two frequencies making up the voice channel handle signaling during the actual conversation. You might note then that a call two channels: voice and data. Got it? Knowing this makes many things easier. A mobile's electronic serial number is only transmitted on the reverse control channel. A person tracking ESNs need only monitor one of 21 frequencies. They don't have to look through the entire band.

So, we have two channels for every call with four frequencies involved. Clear? And a forward and reverse path for each frequency. Let's name them here. Again, a frequency is the medium upon which information travels. A path is the direction the information flows. Here you go:

--> Forward control path: Base station to mobile

<-- Reverse control path: Mobile to base station

------------------------------

--> Forward voice path: Base station to mobile

<-- Reverse voice path: Mobile to base station

One last point at the risk of losing everybody. You'll hear about dedicated control channels, paging channels, and access channels. These are not different channels but different uses of the control channel. Let's clear up this terminology confusion by looking at call processing. We'll look at the way AMPS sets up calls. Both analog and digital cellular (IS-136) use this method, CDMA cellular (IS-95) and GSM being the exceptions. We'll also touch on a number of new terms along the way.

Still confused about the terms channels, frequency, and path?, and how they relate to each other? I understand. Click here for more: See channels, frequencies, and paths.


The control channel and the voice channel, paired frequencies upon which information flows. Paths indicate flow direction.

-----------------------------------

Notes:

[Control channel] "Is the control channel important? Actually, I can't think of a case where it would not be. But we don't think of it that way in the business. We have a set-up channel and we have voice channels. They are so different (both in function and in how they are managed) that we never think of the set-up channel as the first of the cell's channels -- it's in a class by itself. If you ask an engineer in an AMPS system what channels he has on a cell, he'll automatically give you the voice channels. Set up channel is a separate question. Just a matter of mindset. You might add channels, re-tune partially or completely, and never give a thought to the set-up channel. If asked how many channels are on a given cell, you'd never think to include the set-up channel in the count." Mark van der Hoek. Personal correspondence.

Channels, frequencies, and paths: Cellular radio employs an arcane and difficult terminology; many terms apply to all of wireless, many do not. When discussing cellular radio, which comprises analog cellular, digital cellular, and PCS, frequency is a single unit whereas channel means a pair of frequencies, one to transmit on and one to receive. (See the diagram above.) The terms are not interchangeable although many writers use them that way. Frequencies are measured or numbered by their order in the radio spectrum, in Hertz, but channels are numbered by their place in a particular radio plan. Thus, in cell #1 of 21 in a cellular carrier's system, the frequencies may be 879.990 Hz for transmitting and 834.990 Hz for receiving. These then make up Channel 1 in that cell, number 333 overall. Again, in cellular, a channel is a pair of frequencies. The frequencies are described in Hz, the channels by numbers in a plan. Now, what about path?

Path, channel, and frequency, depending on how they are used in wireless working, all constitute a communication link. In cellular, however, path does not, or should not, describe a transmission link, but rather the direction in which information flows.The forward path denotes information flowing from the base station to the mobile. The reverse path describes information flowing from the mobile to the base station. With frequency and channel we talk about the physical medium which carries a signal, with path we discuss the direction a signal is going on that medium. Is this clear?

Posted by Tom Farley & Mark van der Hoek at 09:46 PM

AMPS Call Processing

AMPS call processing diagram -- Keep track of the steps!

Let's look at how cellular uses data channels and voice channels. Keep in mind the big picture while we discuss this. A call gets set up on a control channel and another channel actually carries the conversation. The whole process begins with registration. It's what happens when you first turn on a phone but before you punch in a number and hit the send button. It only takes a few hundred milliseconds. Registration lets the local system know that a phone is active, in a particular area, and that the mobile can now take incoming calls. What cell folks call pages. If the mobile is roaming outside its home area its home system gets notfied. Registration begins when you turn on your phone.

Posted by Tom Farley & Mark van der Hoek at 09:49 PM

Registration -- Hello, World!

A mobile phone runs a self diagnostic when it's powered up. Once completed it acts like a scanning radio. Searching through its list of forward control channels, it picks one with the strongest signal, the nearest cell or sector usually providing that. Just to be sure, the mobile re-scans and camps on the strongest one. Not making a call but still on? The mobile re-scans every seven seconds or when signal strength drops before a pre-determined level. Next, as Will Galloway writes, "After an AMPS phone selects the strongest channel, it tries to decode the data stream and in particular the System ID, to see if it's at home or roaming. If there are too many errors, it will switch to the next strongest channel. It also watches the busy/idle bit in the data stream to find a free slot to transmit its information." After selecting a channel the phone then identifies itself on the reverse control path. The mobile sends its phone number, its electronic serial number, and its home system ID. Among other things. The cell site relays this information to the mobile telecommunications switching office. The MTSO, in turn, communicates with different databases, switching centers and software programs.

The local system registers the phone if everything checks out. Mr. Mobile can now take incoming calls since the system is aware that it is in use. The mobile then monitors paging channels while it idles. It starts this scanning with the initial paging channel or IPCH. That's usually channel 333 for the non-wireline carrier and 334 for the wireline carrier. The mobile is programed with this information and 21 channels to scan when your carrier programs your phone's directory number, the MIN, or mobile identification number. Again, the paging channel or path is another word for the forward control channel. It carries data and is transmitted by the cell site. A mobile first responds to a page on the reverse control channel of the cell it is in. The MTSO then assigns yet another channel for the conversation. But I am getting ahead of myself. Let's finish registration.

Registration is an ongoing process. Moving from one service area to another causes registration to begin again. Just waiting ten or fifteen minutes does the same thing. It's an automatic activity of the system. It updates the status of the waiting phone to let the system know what's going on. The cell site can initiate registration on its own by sending a signal to the mobile. That forces the unit to transmit and identify itself. Registration also takes place just before you call. Again, the whole process takes only a few hundred milliseconds.

AMPS, the older, analog voice system, not the digital IS-136, uses frequency shift keying to send data. Just like a modem. Data's sent in binary. 0's and 1's. 0's go on one frequency and 1's go on another. They alternate back and forth in rapid succession. Don't be confused by the mention of additional frequencies. Frequency shift keying uses the existing carrier wave. The data rides 8kHz above and below, say, 879.990 MHz. Read up on the earliest kinds of modems and FSK and you'll understand the way AMPS sends digital information.

Data gets sent at 10 kbps or 10,000 bits per second from the cell site. That's fairly slow but fast enough to do the job. Since cellular uses radio waves to communicate signals are subject to the vagaries of the radio band. Things such as billboards, trucks, and underpasses, what Lee calls local scatters, can deflect a cellular call. So the system repeats each part of each digital message five times. That slows things considerably. Add in the time for encoding and decoding the digital stream and the actual transfer rate can fall to as low as 1200 bps.

Remember, too, that an analog wave carries this digital information, just like most modems. It's not completely accurate, therefore, to call AMPS an analog system. AMPS is actually a hybrid system, combining both digital and analog signals. IS-136, what AT&T now uses for its cellular network, and IS-95, what Sprint uses for its, are by contrast completely digital systems.

-------------------

Notes

Bits, frames, slots, and channels: How They Relate To Cellular

Here's a little bit on digital; perhaps enough to understand the accompanying Cellular Telephone Basics article. This writing is from my digital wireless series:

Frames, slots, and channels organize digital information. They're key to understanding cellular and PCS systems. And discussing them gets really complicated. So let's back up, review, and then look at the earliest method for organizing digital information: Morse code.

You may have seen in the rough draft of digital principles how information gets converted from sound waves to binary numbers or bits. It's done by pulse code modulation or some other scheme. This binary information or code is then sent by electricity or light wave, with electricity or light turned on and off to represent the code. 10101111, for example, is the binary number for 175. Turning on and off the signal source in the above sequence represents the code.

Early digital wireless used a similar method with the telegraph. Instead of a binary code, though, they used Morse code. How did they do that? Landline telegraphs used a key to make or break an electrical circuit, a battery to produce power, a single line joining one telegraph station to another and an electromagnetic receiver or sounder that upon being turned on and off, produced a clicking noise.

A telegraph key tap broke the circuit momentarily, transmitting a short pulse to a distant sounder, interpreted by an operator as a dot. A more lengthy break produced a dash.. To illustrate and compare, sending the number 175 in American Morse Code requires 11 pulses, three more than in binary code. Here's the drill: dot, dash, dash, dot; dash, dash, dot, dot; dash, dash, dash. Now that's complicated! But how do we get to wireless?

Let's say you build a telegraph or buy one. You power it with, say, two six volt lantern batteries. Now run a line away from the unit -- any length of insulated wire will do. Strip a foot or two of insulation off. Put the exposed wire into the air. Tap the key. Congratulations. You've just sent a digital signal. (An inch or two.) The line acts as an antenna, radiating electrical energy. And instead of using a wire to connect to a distant receiver, you've used electromagnetic waves, silently passing energy and the information it carries across the atmosphere.

Transmitting binary or digital information today is, of course, much more complicated and faster than sending Morse code. And you need a radio transmitter, not just a piece of wire, to get your signal up into the very high radio spectrum, not the low baseband frequency a signal sets up naturally when placed on a wire. But transmission still involves sending code, represented by turning energy on and off, and radio waves to send it. And as American Morse code was a logical, cohesive plan to send signals, much more complicated and useful arrangements have been devised.

We know that 1s and 0s make up binary messages. An almost unending stream of them, millions of them really, parade back and forth between mobiles and base stations. Keeping that information flowing without interruption or error means keeping that data organized. Engineers build elaborate data structures to do that, digital formats to house those 1s and 0s. As I've said before, these digital formats are key to understanding cellular radio, including PCS systems. And understanding digital formats means understanding bits, frames, slots, and channels. Bits get put into frames. Frames hold slots which in turn hold channels. All these elements act together. To be disgustingly repetitive and obvious, here's the list again:

Frames

Slots

Channels

Bits

We have a railroad made not of steel but of bits. The data stream is managed and built out of bits. Frames and slots and channels are all made out of bits, just assembled in different ways. Frames are like railroad cars, they carry and hold the slots which contains the channels which carry and manage the bits. Huh? Read further, and bear with the raillroad analogy.

A frame is an all inclusive data package. A sequence of bits makes up a frame. Bit stands for binary digit, 0s and 1s that represent electrical impulses. (Go back to the previous discussion if this seems unclear.) A frame can be long or short, depending on the complexity of its task and the amount of information it carries. In cellular working the frame length is precisely set, in the case of digital cellular, where we have time division multiplexing, every frame is 40 milliseconds long. That's like railroad boxcars of all the same length. Many people confuse frames with packets because they do similiar things and have a similiar structure. Without defining packets, let just say that frames can carry packets, but packets cannot carry frames. Got it? For now?

A frame carries conversation or data in slots as well as information about the frame itself. More specifically, a frame contains three things. The first is control information, such as a frame's length, its destination, and its origin. The second is the information the frame carries, namely time slots. Think of those slots as freight. These slots, in turn, carry a sliced up part of a multiplexed conversation. The third part of a frame is an error checking routine, known as "error detection and correction bits." These help keep the data stream's integrity, making sure that all the frames or digital boxcars keep in order.

The slots themselves hold individual call information within the frame, that is, the multiplexed pieces of each conversation as well as signaling and control data. Slots hold the bits that make up the call. frequency for a predetermined amount of time in an assigned time slot. Certain bits within the slots perform error correction, making sure sure that what you send is what is received. Same way with data sent in frames on telephone land lines. When you request $20.00 from your automatic teller machine, the built in error checking insures that $2000.00 is not sent instead. The TDMA based IS-136 uses two slots out of a possible six. Now let's refer to specific time slots. Slots so designated are called channels, ones that do certain jobs.

Channels handle the call processing, the actual mechanics of a call. Don't confuse these data channels with radio channels. A pair of radio frequencies makes up a channel in digital IS-136, and AMPS. One frequency to transmit and one to receive. In digital working, however, we call a channel a dedicated time slot within a data or bit stream. A channel sends particular messages. Things like pages, for when a mobile is called, or origination requests, when a mobile is first turned on and asks for service.

1. Frames

Behold the frame!, a self contained package of data. Remember, a sequence of bits makes up a frame. Frames organize data streams for efficiency, for ease of multiplexing, and to make sure bits don't get lost. In the diagram above we look at basis of time division multiplexing. As we've discussed, TDMA or time division multiple access, places several calls on a single frequency. It does so by separating the conversations in time. Its purpose is to expand a system's carrying capacity while still using the same numbers of frequencies. In the exaggerated example above, imagine that a single part of three digitized and compressed conversations are put into each frame as time goes on.

2. Slots

IS-54B, IS-136 frame with time slots

Welcome to slots. But not the kind you find in Las Vegas. Slots hold individual call information within the frame, remember? In this case we have one frame of information containing six slots. Two slots make up one voice circuit in TDMA. Like slots 1 and 4, 2 and 5, or 3 and 6. The data rate is 48.6 Kbits/s, less than a 56K modem, with each slot transmitting 324 bits in 6.67 ms. How is this rate determined? By the number of samples taken, when speech is first converted to digital. Remember Pulse Amplitude Modulation? If not, go back. Let's look at what's contained in just one slot of half a frame in digital cellular.

IS-54B, now IS-136 time slot structure and the Channels Within

Okay, here are the actual bits, arranged in their containers the slots. All numbers above refer to the amount of bits. Note that data fields and channels change depending on the direction or the path that occurs at the time, that is, a link to the mobile from the base station, or a call from the mobile to the base station. Here are the abbreviations:

G: Guard time. Keeps one time slot or data burst separate from the others. R: Ramp time. Lets the transmitter go from a quiet state to full power. DATA: The data bits of the actual conversation. DVCC: Digital verification color code. Data field that keeps the mobile on frequency. RSVD: Reserved. SACCH: Slow associated control channel. Where system control information goes. SYNC: Time synchronization signal. Full explanations on the next page in the PCS series.

Still confused? Read this page over. And don't think you have to get it all straight right now. It will be less confusing as you read more, of my writing as well as others. Look up all of these terms in a good telecom dictionary and see what those writers state. Taken together, your reading will help make understanding cellular easier. E-mail me if you still have problems with this text. Perhaps I can re-write parts to make them less confusing.

Posted by Tom Farley & Mark van der Hoek at 09:57 PM

Pages: Getting a Call

Okay, your phone's now registered with your local system. Let's say you get a call. It's the F.B.I., asking you to turn yourself in. You laugh and hang up. As you speed to Mexico you marvel at the technology involved. What happened? Your phone recognized its mobile number on the paging channel. Remember, that's always the forward control channel or path except in a CDMA system. The mobile responded by sending its identifying information again to the MTSO, along with a message confirming that it received the page. The system responded by sending a voice channel assignment to the cell you were in. The cell site's transceiver got this information and began setting things up. It first informed the mobile about the new channel, say, channel 10 in cell number 8. It then generated a supervisory audio tone or SAT on the forward voice frequency. What's that?

Posted by Tom Farley & Mark van der Hoek at 09:58 PM

The SAT, Dial Tone, and Blank and Burst

[Remember that we are discussing the original or default call set up routine in AMPS. IS-136, and IS-95 use a different, all digital method, although they switch back to this basic version we are now describing in non-digital territory. GSM also uses a different, incompatible technique to set up calls.]

An SAT is a high pitched, inaudible tone that helps the system distinguish between callers on the same channel but in different cells. The mobile tunes to its assigned channel and it looks for the right supervisory audio tone. Upon hearing it, the mobile throws the tone back to the cell site on its reverse voice channel. What engineers call transpond, the automatic relaying of a signal. We now have a loop going between the cell site and the phone. No SAT or the wrong SAT means no good.

AMPS generates the supervisory audio tone at three different non-radio frequencies. SAT 0 is at 5970 Hz, SAT 1 is at6000 Hz, and SAT 2 is at 6030 Hz. Using different frequencies makes sure that the mobile is using the right channel assignment. It's not enough to get a tone on the right forward and reverse path -- the mobile must connect to the right channel and the right SAT. Two steps. This tone is transmitted continuously during a call. You don't hear it since it's filtered during transmission. The mobile, in fact, drops a call after five seconds if it loses or has the wrong the SAT. [Much more on the SAT and co-channel interference] The all digital GSM and PCS systems, by comparison, drops the call like AMPS but then automatically tries to re-connect on another channel that may not be suffering the same interference.

Excellent .pdf file from Paul Bedell on co-channel interference, carrier to interference ratio, adjacent channel interference and so on, along with good background information everyone can use to understand cellular radio. (280K, 14 pages in .pdf)

The file above is from his book Cellular/PCs Management. More information and reviews are here (external link to Amazon.com)

The cell site unmutes the forward voice channel if the SAT gets returned, causing the mobile to take the mute off the reverse voice channel. Your phone then produces a ring for you to hear. This is unlike a landline telephone in which ringing gets produced at a central office or switch. To digress briefly, dial tone is not present on AMPS phones, although E.F. Johnson phones produced land line type dial tone within the unit. [See dial tone.]

Can't keep track of these steps? Check out the call processing diagram

Enough about the SAT. I mentioned another tone that's generated by the mobile phone itself. It's called the signaling tone or ST. Don't confuse it with the SAT. You need the supervisory audio tone first. The ST comes in after that; it's necessary to complete the call. The mobile produces the ST, compared to the SAT which the cell site originates. It's a 10 kHz audio tone. The mobile starts transmitting this signal back to the cell on the forward voice path once it gets an alerting message. Your phone stops transmitting it once you pick up the handset or otherwise go off hook to answer the ring. Cell folks might call this confirmation of alert. The system knows that you've picked up the phone when the ST stops.

Thanks to Dwayne Rosenburgh N3BJM for corrections on the SAT and ST

AMPS uses signaling tones of different lengths to indicate three other things. Cleardown or termination means hanging up, going on hook, or terminating a call. The phone sends a signaling tone of 1.8 seconds when that happens. 400 ms. of ST means a hookflash. Hookflash requests additional services during a conversation in some areas. Confirmation of handover request is another arcane cell term. The ST gets sent for 50 ms. before your call is handed from one cell to another. Along with the SAT. That assures a smooth handoff from one cell to another. The MTSO assigns a new channel, checks for the right SAT and listens for a signaling tone when a handover occurs. Complicated but effective and all happening in less than a second. [See SIT]

Okay, we're now on the line with someone. Maybe you! How does the mobile communicate with the base station, now that a conversation is in progress? Yes, there is a control frequency but the mobile can only transmit on one frequency at a time. So what happens? The secret is a straightforward process known as blank and burst. As Mark van der Hoek puts it,

"Once a call is up on a voice channel, all signaling is done on the voice channel via a scheme known as "Blank and Burst". When the site needs to send an order to the mobile, such as hand off, power up, or power down, it mutes the SAT on the voice channel. This is filtered at the mobile so that the customer never hears it. When the SAT is muted, the phone mutes the audio path, thus the "blank", and the site sends a "burst" of data. The process takes a fraction of a second and is scarcely noticeable to the customer. Again, it's more noticeable on a Motorola system than on Ericsson or Lucent. You can sometimes hear the 'bzzt' of the data burst."

Blank and burst is similiar to the way many telco payphones signal. Let's say you're making a long distance call. The operator or the automated coin toll service computer asks you for $1.35 for the first three minutes. And maybe another dollar during the conversation. The payphone will mute or blank out the voice channel when you deposit the coins. That's so it can burst the tones of the different denominations to the operator or ACTS. These days you won't often hear those tones. And all done through blank and burst. Now let's get back to cellular.

--------------------

Notes:

[Dial tone] During the start of your call a "No Service" lamp or display instead tells you if coverage isn't available If coverage is available you punch in your numbers and get a response back from the system. Imagine dialing your landline phone without taking the receiver of the hook. If you could dial like that, where would be the for dial tone?

[Much more on the SAT and co-channel interference] The supervisory audio tone distinguishes between co-channel interferrors, an intimidatingly named but important to know problem in cellular radio. Co-channel interferrors are cellular customers using the same channel set in different cells who unknowingly interfere with each other. We know all about frequency reuse and that radio engineers carefully assign channels in each cell to minimize interference. But what happens when they do? Let's see how AMPS uses the SAT in practice and how it handles the interference problem.

Mark van der Hoek describes two people, a businessman using his cell phone in the city, and a hiker on top of a mountain overlooking the city. The businessman's call is going well. But now the hiker decides to use his phone to tell his friends he has climbed the summit. (Or as we American climbers say, "bagged the peak.")

From the climber's position he can see all of the city and consequently the entire area under cellular coverage. Since radio waves travel in nearly a straight line at high frequencies, it's possible his call could be taken by nearly any cell. Like the one the businessman is now using. This is not what radio engineers plan on, since the nearest cell site usually handles a call, in fact, Mark points out they don't want people using cell phones on an airplane! "Knock it off, turkey! Can't you see you're confusing the poor cell sites?"

If the hiker's mobile is told by the cell site first setting up his call to go channel 656, SAT 0, but his radio tunes now to a different cell with channel 656, SAT 1, instead, a fade timer in the mobile shuts down its transmitter after five seconds. In that way an existing call in the cell is not disrupted.

If the mobile gets the right channel and SAT but in a different cell than intended, FM capture occurs, where the stronger call on the frequency will displace, at least temporarily, the weaker call. Both callers now hear each other's conversation. A multiple SAT condition is the same as no SAT, so the fade timer starts on both calls. If the correct SAT does not resume before the fade timer expires, both calls are terminated

Mark puts it simply, "Remember, the only thing a mobile can do with SAT is detect it and transpond it. Either it gets what it was told to expect, and transponds it, or it doesn't get what it was told to expect, in which case it starts the fade timer. If the fade timer expires, the mobile's transmitter is shut down and the call is over."

[SIT] "A large supplier and a carrier I worked for went round and round on this. If their system did not detect hand-off confirmation, it tore down the call. Even if it got to the next site successfully. Their reasoning was that, if the mobile was in such a poor radio frequency environment that 50 ms of ST could not be detected, the call is in bad shape and should be torn down. We disagreed. We said, "Let the customer decide. If it's a lousy call, they'll hang up. If it's a good call, we want it to stay up!" Just because a mobile on channel 423 is in trouble doesn't mean that it will be when it hands off to channel 742 in another cell! In fact, a hand-off may happen just in time to save a call that is going south. Why?"

"Well, just because there is interference on channel 423 doesn't mean that there is on 742! Or what if the hand-off dragged? That is, for whatever reason the call did not hand off at approximately half way between the cells. (Lot's of reasons that could happen.) So the path to the serving site is stretched thiiiiin, almost to the point of dropping the call. But the hand-off, almost by definition in this case, will be to a site that is very close. That ought to be a good thing, you'd think. Well, the system supplier predicted Gloom, Doom, and Massive Dropped Calls if we changed it. We insisted, and things worked much better. Hand-off failures and dropped calls did not increase, and perceived service was much better. For this and a number of other reasons I have long suspected that their system did not do a good job of detecting ST . . ."

Posted by Tom Farley & Mark van der Hoek at 10:03 PM

Origination: Making a call

Making a mobile call uses many steps that help receive a call. The same basic process. Punch out the number that you want to call. Press the send button. Your mobile transmits that telephone number, along with a request for service signal, and all the information used to register a call to the cell site. The mobile transmits this information on the strongest reverse control channel. The MTSO checks out this info and assigns a voice channel. It communicates that assignment to the mobile on the forward control channel. The cell site opens a voice channel and transmits a SAT on it. The mobile detects the SAT and locks on, transmitting it back to the cell site. The MTSO detects this confirmation and sends the mobile a message in return. This could be several things. It might be a busy signal, ringback or whatever tone was delivered to the switch. Making a call, however, involves far more problems and resources than an incoming call does.

Making a call and getting a call from your cellular phone should be equally easy. It isn't, but not for technical reasons, that is setting up and carrying a call. Rather, originating a call from a mobile presents fraud issues for the user and the carrier. Especially when you are out of your local area. Incoming calls don't present a risk to the carrier. Someone on the other end is paying for them. The carrier, however, is responsible for the cost of fraudulent calls originating in its system. Most systems shut down roaming or do an operator intercept rather than allow a questionable call. I've had close friends asked for their credit card numbers by operators to place a call. [See cloning comments]

Can you imagine giving a credit card number or a calling card number over the air? You're now making calls at a payphone, just like the good old days. Cellular One has shut down roaming "privileges" altogether in New York City, Washington and Miami at different times. But you can go through their operator and pay three times the cost of a normal call if you like. So what's going on? Why the problem with some outgoing calls? We first have to look at some more terms and procedures. We need to see what happens with call processing at the switch and network level. This is the exciting world of precall validation.

-------------------

Notes:

[Clone comments] "You could make more clear that this is due to validation and fraud issues, not to the mechanics of setting up the call, since this is pretty much the same for originations and terminations."

"By the way, at AirTouch we took a big bite out of fraudulent calls when we stopped automatically giving every customer international dialing capability. We gave it to any legitimate customer who asked for it, but the default was no international dialing. So the cloners would rarely get a MIN/ESN combo that would allow them to make calls to Colombia to make those 'arrangements'. Yes, the drug traffic was a huge part of the cloning problem. We had some folks who worked a lot with law enforcement, particularly the DEA. Another large part of it was the creeps who would sell calls to South America on the street corners of L.A. Illegal immigrants would line up to make calls home on this cloned phone."

"Actually, even though it's an inconvenience, being cloned can be fun if you are an engineer working for the carrier. You can do all kinds of fun things with the cloner. Like seeing where they are making their calls and informing the police. Like hotlining the phone so that ALL calls go straight to customer service. It would have been fun to hotline them to INS, but INS wouldn't have liked that."

Posted by Tom Farley & Mark van der Hoek at 10:09 PM

Precall Validation: Process and Terms

We know that pressing send or turning on the phone conveys information about the phone to the cell site and then to the MTSO. A call gets checked with all this information. There are many parts to each digital message. A five digit code called the home system identification number (SID or sometimes SIDH) identifies the cellular carrier your phone is registered with. For example, Cellular One's code in Sacramento, California, is 00129. Go to Stockton forty miles south and Cellular One uses 00224. A system can easily identify roamers with this information. The "Roaming" lamp flashes or the LED pulses if you are out of your local area. Or the "No Service" lamp comes on if the mobile can't pick up a decent signal. This number is keypad programmable, of course, since people change carriers and move to different areas. You can find yours by calling up a local cellular dealer. Or by putting your phone in the programming mode. [See Programming].

This number doesn't go off in a numerical form, of course, but as a binary string of zero's and ones. These digital signals are repeated several times to make sure they get received. The mobile identification number or MIN is your telephone's number. MINs are keypad programmable. You or a dealer can assign it any number desired. That makes it different than its electronic serial number which we'll discuss next. A MIN is ten digits long. A MIN is not your directory number since it is not long enough to include a country code. It's also limited when it comes to future uses since it isn't long enough to carry an extension number. [See MIN]

The electronic serial number or ESN is a unique number assigned to each phone. One per phone! Every cell phone starts out with just one ESN. This number gets electronically burned into the phone's ROM, or read only memory chip. A phone's MIN may change but the serial number remains the same. The ESN is a long binary number. Its 32 bit size provides billions of possible serial numbers. The ESN gets transmitted whenever the phone is turned on, handed over to another cell or at regular intervals decided by the system. Every ten to fifteen minutes is typical. Capturing an ESN lies at the heart of cloning. You'll often hear about stolen codes. "Someone stole Major Giuliani's and Commissioner Bratton's codes." The ESN is what is actually being intercepted. A code is something that stands for something else. In this case, the ESN. A hexadecimal number represents the ESN for programming and test purposes. Such a number might look like this: 82 57 2C 01.

The station class mark or SCM tells the cell site and the switch what power level the mobile operates at. The cell site can turn down the power in your phone, lowering it to a level that will do the job while not interfering with the rest of the system. In years past the station class mark also told the switch not to assign older phones to a so called expanded channel, since those phones were not built with the new frequencies the FCC allowed.

The switch process this information along with other data. It first checks for a valid ESN/MIN combination. You don't get access unless your phone number matches up with a correct, valid serial number and MIN. You have to have both unless, perhaps, if you call 911. The local carrier checks its own database first. Each carrier maintains its own records but the database may be almost anywhere. These local databases are updated, supposedly, around the clock by two much larger data bases maintained by Electronic Data Systems and GTE. EDS maintains records for most of the former Bell companies and their new cellular spin offs. GTE maintains records for GTE cellular companies as well as for other companies. Your call will not proceed returned unless everything checks out. These database companies try to supply a current list of bad ESNs as well as information to the network on the tens of thousands cellular users coming on line every day.

A local caller will probably get access if validation is successful. Roamers may not have the same luck if they're in another state or fairly distant from their home system. Even seven miles from San Francisco, depending on the area you are in. (I know this personally.) A roamer's record must be checked from afar. Many carriers still can't agree on the way to exchange their information or how to pay for it. A lot comes down to cost. A distant system may still be dependent on older switches or slower databases that can't provide a quick response. The so called North American Cellular Network attempts to link each participating carrier together with the same intelligent network/system 7 facilities.

Still, that leaves many rural areas out of the loop. A call may be dropped or intercepted rather than allowed access. In addition, the various carriers are always arguing over fees to query each others databases. Fraud is enough of a problem in some areas that many systems will not take a chance in passing a call through. It's really a numbers game. How much is the system actually loosing, compared to how much prevention would cost? Preventive measures may cost millions of dollars to put in place at each MTSO. Still, as the years go along, cooperation among carriers is getting better and the number of easily cloned analog phones in use are declining. Roaming is now easier than a few years ago.

AMPS carries on. As a backup for digital cellular, including some dual mode PCS phones, and as a primary system in some rural areas. See "Continues" below:

---------------------------

Notes:

[Programming]Thorn, ibid, 2 see also "Cellular Lite: A Less Filling Blend of Technology & Industry News" Nuts and Volts Magazine (March 1993)

[MIN] Crowe, David "Why MINs Are Phone Numbers and Why They Shouldn't Be" Cellular Networking Perspectives (December, 1994) http:/www.cnp-wireless.com

[Continues] AMPS isn't dead yet, despite the digital cellular methods this article explores. Besides acting as a backup or default operating system for digital cellular, including some dual mode PCS phones, analog based Advanced Mobile Phone Service continues as a primary operating system, bringing much needed basic wireless communications to many rural parts of the world.

I got an e-mail in late 2000 (11/12/2000) from a reader who lives in Marathon, Ontario, Canada, on the tip of the North Shore of Lake Superior. As he refers to the Lake, "The world's greatest inland sea!" He reports, "We just got cell service here in Marathon. It is a simple analogue system. There is absolutely no competition for wireless service. Two dealers in town sell the phones. In the absence of competition there are no offers of free phones; the cheapest mobiles sell for (and old analogue ones to boot!) $399.00 Canadian . . ." And you thought you paid too much for cellular.

More recently I got an e-mail from a reader living in Wheatland, Wyoming. He, too, has only analog cellular (AMPS) to use.

Posted by Tom Farley & Mark van der Hoek at 10:13 PM

AMPS and Digital Systems compared

The most commonly used digital cellular system in America is IS-136, colloquially known as D-AMPS or digital AMPS. (Concentrate on the industry name, not the marketing terms like D-AMPS.) It was formerly known as IS-54, and is an evolutionary step up from that technology. This system is all digital, unlike the analog AMPS. IS-136 uses a multiplexing technique called TDMA or time division multiple access. The TDMA based IS-136 uses puts three calls into the same 30kz channel space that AMPS uses to carry one call. It does this by digitally slicing and dicing parts of each conversation into a single data stream, like filling up one boxcar after another with freight. We'll see how that works in a bit.

TDMA is a transmission technique or access technology, while IS-136 or GSM are operating systems. In the same way AMPS is also an operating system, using a different access technology, FDMA, or frequency division multiple access. See the difference? Let's clear this up.

To access means to use, make available, or take control. In a communication system like the analog based Advanced Mobile Phone Service, we access that system by using frequency division multiple access or FDMA. Frequency division means calls are placed or divided by frequency, that is, one call goes on one frequency, say, 100 MHz, and another call goes on another, say, 200 MHz. Multiple access means the cell site can handle many calls at once. You can also put digital signals on many frequencies, of course, and that would still be FDMA. But AMPS traffic is analog.

(Access technology, although a current wireless phrase, is, to me, an open and formless term. Transmission, the process of transmitting, of conveying intelligence from one point to another, is a long settled, traditional way to express how signals are sent along. I'll use the terms here interchangeably.)

Time division multiple access or TDMA handles multiple and simultaneous calls by dividing them in time, not by frequency. This is purely digital transmission. Voice traffic is digitized and portions of many calls are put into a single bit stream, one sample at a time. We'll see with IS-136 that three calls are placed on a single radio channel, one after another. Note how TDMA is the access technology and IS-136 is the operating system?

Another access method is code division multiple access or CDMA. The cellular system that uses it, IS-95, tags each and every part of multiple conversations with a specific digital code. That code lets the operating system reassemble the jumbled calls at the base station. Again, CDMA is the transmission method and IS-95 is the operating system.

All IS-136 phones handle analog traffic as well as digital, a great feature since you can travel to rural areas that don't have digital service and still make a call. The beauty of phones with an AMPS backup mode is they default to analog. As long as your carrier maintains analog channels you can get through. And this applies as well as the previouly mentioned IS-95, a cellular system using CDMA or code division multiple access. Your phone still operates in analog if it can't get a CDMA channel. But I am getting ahead of myself. Back to time division multiple access.

TDMA's chief benefit to carriers or cellular operators comes from increasing call capacity -- a channel can carry three conversations instead of just one. But, you say, so could NAMPS, the now dead analog system we looked at briefly. What's the big deal? NAMPS had the same fading problems as AMPS, lacked the error correction that digital systems provided and wasn't sophisticated enough to handle encryption or advanced services. Things such as calling number identification, extension phone service and messaging. In addition, you can't monitor a TDMA conversation as easily as an analog call. So, there are other reasons than call capacity to move to a different technology. Many people ascribe benefits to TDMA because it is a digital system. Yes and no.

Advanced features depend on digital but conserving bandwidth does not. How's that? Three conversations get handled on a single frequency. Call capacity increases. But is that a virtue of digital? No, it is a virtue of multiplexing. A digital signal does not automatically mean less bandwidth, in fact, it means more. [See more bandwidth] Multiplexing means transmitting multiple conversations on the same frequency at once. In this case, small parts of three conversations get sent almost simultaneously. This was not the same with the old analog NAMPS, which split the frequency band into three discrete sub- frequencies of 10khz apiece. TDMA uses the whole frequency to transmit while NAMPS did not.

This is a good place to pause now that we are talking about digital. AMPS is a hybrid system, combing digital signaling on the setup channels and on the voice channel when it uses blank and burst. Voice traffic, though, is analog. As well as tones to keep it on frequency and help it find a vacant channel. That's AMPS. But IS-136 is all digital. That's because it uses digital on its set-up channels, the same radio frequencies that AMPS uses, and all digital signaling on the voice channel. TDMA, GSM, and CDMA cellular (IS-95) are all digital. Let's look at some TDMA basics. But before we do, let me mention one thing.


Wonderful information on IS-136 here. It's from a chapter in IS-136 TDMA Technology, Economics, and Services, by Harte, Smith, and Jacobs (1.2mb, 62 pages in .pdf)

Book description and ordering information (external link to Amazon.com)

I wrote in passing about how increasing call capacity was the chief benefit of TDMA to cellular operators. But it is not necessarily of benefit to the caller, since most new digital routines play havoc with voice quality. An uncompressed, non-multiplexed, bandwidth hogging analog signal simply sounds better than its present day compressed, digital counterpart. As the August, 2000 Consumers Digest put it:

"Digital cellular service does have a couple of drawbacks, the most important of which is audio quality. Analog cellular phones sound worlds better. Many folks have commented on what we call the 'Flipper Effect." It refers to the sound of your voice taking on an 'underwater-like' quality with many digital phones. In poor signal areas or when cell sites are struggling with high call volume, digital phones will often lose full-duplex capability (the ability of both parties to talk simultaneously), and your voice may break up and sound garbled."

Getting back to our narrative, and to review, we see that going digital doesn't mean anything special. A multiplexed digital signal is what is key. Each frequency gets divided into six repeating time slots or frames. Two slots in each frame get assigned for each call. An empty slot serves as a guard space. This may sound esoteric but it is not. Time division multiplexing is a proven technology. It's the basis for T1, still the backbone of digital transmission in this country. Using this method, a T1 line can carry 24 separate phone lines into your house or business with just an extra twisted pair. Demultiplexing those conversations is no more difficult than adding the right circuit board to a personal computer. TDMA is a little different than TDM but it does have a long history in satellite working.

More on digital: http://www.TelecomWriting.com/PCS/Multiplexing.htm

What is important to understand is that the system synchronizes each mobile with a master clock when a phone initiates or receives a call. It assigns a specific time slot for that call to use during the conversation. Think of a circus carousel and three groups of kids waiting for a ride. The horses represent a time slot. Let's say there are eight horses on the carousel. Each group of kids gets told to jump on a different colored horse when it comes around. One group rides a red horse, one rides a white one and the other one rides a black horse. They ride the carousel until they get off at a designated point. Now, if our kids were orderly, you'd see three lines of children descending on the carousel with one line of kids moving away. In the case of TDMA, one revolution of the ride might represent one frame. This precisely synchronized system keeps everyone's call in order. This synchronization continues throughout the call. Timing information is in every frame. Any digital scheme, though, is no circus. The actual complexity of these systems is daunting. You should you read further if you are interested.

Take a look into frames

There are variations of TDMA. The only one that I am aware of in America is E-TDMA. It is or was operated in Mobile, Alabama by Bell South. Hughes Network Systems developed this E-TDMA or Enhanced TDMA. It runs on their equipment. Hughes developed much of their expertise in this area with satellites. E-TDMA seems to be a dynamic system. Slots get assigned a frame position as needed. Let's say that you are listening to your wife or a girlfriend. She's doing all the talking because you've forgotten her birthday. Again. Your transmit path is open but it's not doing much. As I understand it, "digital speech interpolation" or DSI stuffs the frame that your call would normally use with other bits from other calls. In other words, it fills in the quiet spaces in your call with other information. DSI kicks in when your signal level drops to a pre-determined level. Call capacity gets increased over normal TDMA. This trick had been limited before to very high density telephone trunks passing traffic between toll offices. Their system also uses half rate vocoders, advanced speech compression equipment that can double the amount of calls carried.

Before we turn to another multiplexing scheme, CDMA, let's consider how a digital cellular phone determines how to choose a digital channel and not an analog one. Perhaps I should have covered that before this section, but you may know enough terminology to understand what Mark van der Hoek has to say:

"The AMPS system control channel has a bit in its data stream which is called the 'Extended Protocol Bit.' This was designed in by Bell Labs to facilitate unknown future enhancements. It is used by both CDMA and TDMA 800 MHz systems."

"When a dual mode phone (TDMA or CDMA and AMPS) first powers up, it goes through a self check, then starts scanning the 21 control or setup channels, the same as an AMPS only phone. Like you've described before. When it locks on, it looks for what's called an Extended Protocol Bit within that data stream If it is low, it stays in AMPS. If that bit is high, the phone goes looking for digital service, according to an established routine. That routine is obviously different for CDMA and TDMA.

'TDMA phones then tune to one of the RF channels that has been set up by the carrier as a TDMA channel.Within that TDMA channel data stream is found blocks of control information interspersed in a carefully defined sequence with voice data. Some of these blocks are designated as the access or control channel for TDMA. This logical or data channel, a term brought in from the computer side, constitutes the access channel."

I know this is hard to follow. Although I don't have a graphic of the digital control channel in IS-54, you can get an idea of a data stream by going here.

"Remember, the term 'channel' may refer to a pair of radio frequencies or to a particular segment of data. When data is involved it constitutes the 'logical channel'.' In TDMA, the sequence differentiates a number of logical channels. This different use of the same term channel, at once for radio frequencies and at the same time for blocks of data information, accounts for many reader's confusion. By comparison, in CDMA everything is on the same RF channel. No setting up on one radio frequency channel and then moving off to another. Within the one radio frequency channel we have traffic (voice) channels, access channels, and sync channels, differentiated by Walsh code."

------------------

Notes:

[More bandwidth] "The most noticeable disadvantage that is directly associated with digital systems is the additional bandwidth necessary to carry the digital signal as opposed to its analog counterpart. A standard T1 transmission link carrying a DS-1 signal transmits 24 voice channels of about 4kHz each. The digital transmission rate on the link is 1.544 Mbps, and the bandwidth re-quired is about 772 kHz. Since only 96 kHz would be required to carry 24 analog channels (4khz x 24 channels), about eight times as much bandwidth is required to carry the digitally (722kHz / 96 = 8.04). The extra bandwidth is effectively traded for the lower signal to noise ratio." Fike, John L. and George Friend, UnderstandingTelephone Electronics SAMS, Carmel 1983

[TDMA] There's a wealth of general information on TDMA available. But some of the best is by Harte, et. al:

Wonderful information on IS-136 and TDMA here. It's from a chapter in IS-136 TDMA Technology, Economics, and Services, by Harte, Smith, and Jacobs (1.2mb, 62 pages in .pdf)

Book description and ordering information (external link to Amazon.com)

Posted by Tom Farley & Mark van der Hoek at 10:21 PM

Code Division Multiple Access: IS-95

Code Division Multiple Access has many variants as well. InterDigital (external link), for example, produces a broadband CDMA system called B-CDMA that is different from Qualcomm's (external link) narrowband CDMA system. In the coming years wideband may dominate. But narrowband CDMA right now is dominant in the United States, used with the operating system IS-95. I should repeat here what I wrote at the start of this article. I know some of this is advanced and sounds like gibberish, but bear with me or skip ahead two paragraphs:

Systems built on time division multiplexing will gradually be replaced with other access technologies. CDMA is the future of digital cellular radio. Time division systems are now being regarded as legacy technologies, older methods that must be accommodated in the future, but ones which are not the future itself. (Time division duplexing, as used in cordless telephone schemes: DECT and Personal Handy Phone systems might have a place but this still isn't clear.) Right now all digital cellular radio systems are second generation, prioritizing on voice traffic, circuit switching, and slow data transfer speeds. 3G, while still delivering voice, will emphasize data, packet switching, and high speed access.

Over the years, in stages hard to follow, often with 2G and 3G techniques co-existing, TDMA based GSM and AT&T's IS-136 cellular service will be replaced with a wideband CDMA system, the much hoped for Universal Mobile Telephone System (external link). Strangely, IS-136 will first be replaced by GSM before going to UMTS. Technologies like EDGE and GPRS(Nokia white paper) will extend the life of these present TDMA systems but eventually new infrastructure and new spectrum will allow CDMA/UMTS development. The present CDMA system, IS-95, which Qualcomm supports and the Sprint PCS network uses, is narrowband CDMA. In the Ericsson/Qualcomm view of the future, IS-95 will also go to wideband CDMA.


Excellent writing on this transition period from 2G to 3G and beyond is in this printable .pdf file
, a chapter from The Essential Guide to Wireless Communications Applications by Andy Dornan. Many good charts. (454K, 21 pages in .pdf)

Ordering information for the above title is here (external link to Amazon.com)

Whew! Where we were we? Back to code division multiple access. A CDMA system assigns a specific digital code to each user or mobile on the system. It then encodes each bit of information transmitted from each user. These codes are so specific that dozens of users can transmit simultaneously on the same frequency without interference to each other, indeed, there is no need for adjacent cell sites to use different frequencies as in AMPS and TDMA. Every cell site can transmit on every frequency available to the wireline or non-wireline carrier.

CDMA is less prone to interference than AMPS or TDMA. That's because the specificity of the coded signals helps a CDMA system treat other radio signals and interference as irrelevant noise. Some of the details of CDMA are also interesting. Before we get to them, let's stop here and review, because it is hard to think of the big picture, the overall subject of cellular radio, when we get involved in details.

Posted by Tom Farley & Mark van der Hoek at 10:30 PM

Before We Begin: A Cellular Radio Review

We've discussed, at least in passing, five different cellular radio systems. We looked in particular at AMPS, the mostly analog, original cellular radio scheme. That's because three digital schemes default to AMPS, so it's important to understand this basic operating system.We also looked at IS-54, the first digital service, which followed AMPS and is now folded into IS-136. This AT&T offering, the newest of the TDMA services, still retains an AMPS operating mode. IS-54 and now IS-136 co-exist with AMPS service, that is, a carrier can mix and match these digital and analog services on whatever channel sets they choose. IS-95 is a different kind of service, a CDMA, spread spectrum offering that while not an evolution of the TDMA schemes, still defaults to advanced mobile phone service where a IS-95 signal cannot be detected.

Confused by all these names and abbreviations? Consider how many different operating systems computers use: Unix, Linux, Windows, NT, DOS, the Macintosh OS, and so on. They do the same things in different ways but they are all computers. Cellular radio is like that, different ways to communicate but all having in common a distributed network of cell sites, the principle of frequency-reuse, handoffs, and so on.

If an American carrier uses these words or phrases, then you have one of these technologies:

If your phone has a "SIM or smart card" or memory chip it is using GSM

If your phone uses CDMA the technology is IS-95

If the carrier doesn't mention either word above, or if it says it uses TDMA, then you are using IS-136

And iDEN is, well, iDEN, a proprietary operating system built by Motorola (external link) that, among others, NEXTEL uses.

PCS1900, although not a real trade name, usually refers to an IS-95 system operating at 1900MHz. Usually. If you see a reference to PCS1900 as a GSM service then it is a TDMA based system, not a CDMA technology. PCS1900 in CDMA is not compatible with other services, but it has a mode which lets the phone choose AMPS service if PCS1900 isn't available. Want more confusion? Many carriers that offer IS-136 and GSM, like Cingular, refer to IS-136 as simply TDMA. This is deceptive since GSM is also TDMA. Whatever. And since we are reviewing, let's make sure we understand what transmission technologies are involved.

Different transmission techniques enable the different cellular radio systems. These technologies are the infrastructure of radio. In frequency division multiple access, we separate radio channels or calls by frequency, like the way broadcast radio stations are separated by frequency. One call per channel. In time division multiple access we separate calls by time, one after another. Since calls are separated by time TDMA can put several calls on one channel. In code division multiple access we separate calls by code, putting all the calls this time on a single channel. Unique codes assigned to every bit of every conversation keeps them separate. Now, back to CDMA, specifically IS-95. (Make sure to download the .pdf files to the left.)

Posted by Tom Farley & Mark van der Hoek at 10:32 PM

Back to the CDMA Discussion

Qualcomm's CDMA system uses some very advanced speech compression techniques, utilizing a variable rate vocoder, a speech synthesiser and voice processor in one. Vocoders are in every digital handset or phone; they digitize your voice and compress it. Phil Karn, KA9Q, one of the principal engineers behind Qualcomm, wrote about an early vocoder like this:

"It [o]perates at data rates of 1200, 2400, 4800 and 9600 bps. When a user talks, the 9600 bps data rate is generally used. When the user stops talking, the vocoder generally idles at 1200 bps so you still hear background noise; the phone doesn't just 'go dead'. The vocoder works with 20 millisecond frames, so each frame can be 3, 6, 12 or 24 bytes long, including overhead. The rate can be changed arbitrarily from frame to frame under control of the vocoder."

This is really sophisticated technology, eerily called VAD, for voice activity detection. Changing data rates allows more calls per cell, since each conversation occupies bandwidth only when needed, letting others in during the idle times. Some say VAD is the 'trick' in CDMA that allows greater capacity, and not anything in spread spectrum itself. These data rate changes help with battery life, too, since the mobile can power down in those moments when not transmitting as much information.

Several years ago CDMA was in its infancy. Some wondered if it would work. I was not among the doubters. In May, 1995 I wrote in my magazine private line that I felt the future was with this technology. I still think so and Mark van der Hoek agrees. Click here if you want to read his comments or continue on this page if you want to learn more about this technology.

Posted by Tom Farley & Mark van der Hoek at 10:33 PM

Summary of CDMA: Another transmission technique

Code division multiple access is quite a different way to send information, it's a spread spectrum technique. Instead of concentrating a message in the smallest spectrum possible, say in a radio frequency 10 kHz wide, CDMA spreads that signal out, making it wider. A frequency might be 1.25 or even 5 MHz wide, 10 times or more the width a conventional call might use. Now, why would anyone want to do that?, to go from a seemingly efficient method to a method that seems deliberately inefficient?

The military did much early development on CDMA. They did so because a signal using this transmission technique is diffused or scattered -- difficult to block, listen in on, or even identify. The signal appears more like background noise than a normal, concentrated signal which you can easily target. For the consumer CDMA appeals since a conversation can't be picked up with a scanner like an analog AMPS call. Think of CDMA in another way. Imagine a dinner party with 10 people, 8 of them speaking English and two speaking Spanish. The two Spanish speakers can hear each other talking with out a problem, since their language or 'code' is so specific. All the other conversations, at least to their ears, are disregarded as background noise.

CDMA is a transmission technique, a technology, a way to pass information between the base station and the mobile. Although called 'multiple access', it is really another multiplexing method, a way to put many calls at once on a single channel. As stated before, analog cellular or AMPS uses frequency division multiplexing, in which callers are separated by frequency, TDMA separates callers by time, and CDMA separates calls by code. CDMA traffic includes telephone calls, be they voice or data, as well as signaling and supervisory information. CDMA is a part of an overall operating system that provides cellular radio service. The most widespread CDMA based cellular radio system is called IS-95.

Download this! In these pages from Bluetooth Demystified (McGraw Hill), Nathan Muller presents good information on CDMA, spread spectrum, spreading codes, direct sequence, and frequency hopping. (6 pages, 509K in .pdf)

Bluetooth Demystified ordering information (external link to Amazon)

Posted by Tom Farley & Mark van der Hoek at 10:36 PM

A different way to share a channel

Unlike FDMA and TDMA, all callers share the same channel with all other callers. Doesn't that sound odd? Even stranger, all of them use the same sized signal. Imagine dozens of AM radio stations all broadcasting on the same frequency at the same time with the same 10Khz sized signal. Sounds crazy, doesn't it? But CDMA does something like that, only using very low powered mobiles to reduce interference, and of course, some special coding. "With CDMA, unique digital codes, rather than separate RF frequencies or channels, are used to differentiate subscribers. The codes are shared by both the mobile station (cellular phone) and the base station, and are called "pseudo-Random Code Sequences." [CDG] Don't panic about that last phrase. Instead, let's get comfortable with CDMA terms by seeing see how this transmission technique works.

As the Cellular Development group puts it, "A CDMA call starts with a standard rate of 9600 bits per second (9.6 kilobits per second). This is then spread to a transmitted rate of about 1.23 Megabits per second. Spreading means that digital codes are applied to the data bits associated with users in a cell. These data bits are transmitted along with the signals of all the other users in that cell. When the signal is received, the codes are removed from the desired signal, separating the users and returning the call to a rate of 9600 bps."

Get it? We start with a single call digitized at 9600 bits per second, a rate like a really old modem. (Let's not talk about modem baud rates here, let's just keep to raw bits.) CDMA then spreads or applies this 9600 bit stream by using a code transmitted at 1.23 Megabits. Every caller in the cell occupies the same 1.23 Megabit bandwidth and each call is the same size. A guard band brings the total bandwidth up to 1.25 Megabits. Once at the receiver the equipment identifies the call, separates its pieces from the spreading code and other calls, and returns the signal back to its original 9600 bit rate. For perspective, a CDMA channel occupies 10% of a carrier's allocated spectrum.

-----------------------

Notes:

Probably the best reference is the paper "On the System Design Aspects of Code Division Multiple Access (CDMA) Applied to Digital Cellular and Personal Communications Networks" by Allen Salmasi and Klein S. Gilhousen [WT6G], from the Proceedings of the 41st IEEE Vehicular Technology Conference, St Louis MO May 19-22 1991.

There are also several papers on Qualcomm's CDMA system in the May 1991 IEEE Transactions on Vehicular Technology, including one on the capacity of CDMA.

Musings from a Wireless Wizard

Q. So, Mark van der Hoek, what would it take to have cell phones stop dropping calls?

A. What is required is a network with a cell site on every corner, in every tunnel, in every subterranean parking structure, every office building, perfectly optimized. Oh, and you have to perfectly control all customers so that they never attempt to use more resources than the system has available. What people don't realize is that this kind of perfection is not even realized on wireline networks. Wireline networks suffer from dropped and blocked calls, and always have. They have it it a lot less than a wireless network, but they do have it. And a wireless network has variables that would give a wireline network engineer nightmares. Chaos theory applies here. Weather, traffic, ball games letting out, earthquakes. Hey, in our Seattle network, for the hour after the recent earthquake, the call volume went from an average of 50,000 calls to over 600,000. Oh, that reminds me! You can't guarantee "no drops" until you can guarantee that the land line network will never block a call! So now you have to perfectly control all of that, too! You see, it's not just about the air interface. It's not just about the hardware. . .

Thanks again to Mark van der Hoek

Posted by Tom Farley & Mark van der Hoek at 10:44 PM

Synchronization

To make this transmission method work it is not enough just to have a fancy coding scheme. To keep track of all this information flying back and forth we need to synchronize it with a master clock. As the CDG puts it, "In the final stages of the encoding of the radio link from the base station to the mobile, CDMA adds a special "pseudo-random code" to the signal that repeats itself after a finite amount of time. Base stations in the system distinguish themselves from each other by transmitting different portions of the code at a given time. In other words, the base stations transmit time offset versions of the same pseudo-random code."

Arrgh. Another phrase with the word 'code in it, one more term to keep track of! Don't despair. Even if "pseudo-random code" is fiercesomely titled, it's chore is simple to state: keep base station traffic to its own cell site by issuing a code. Synchronize that code with a master clock to correlate the code. Like putting a time stamp on each piece of information. CDMA uses The Global Positioning System or GPS, a network of navigation satellites that, along with supplying geographical coordinates, continuously transmits an incredibly accurate time signal.

Posted by Tom Farley & Mark van der Hoek at 10:45 PM

What Every Radio System Must Consider

Radio systems, like life, demand tradeoffs or compromises. The CDG says, "CDMA cell coverage is dependent upon the way the system is designed. In fact, three primary system characteristics-Coverage, Quality, and Capacity-must be balanced off of each other to arrive at the desired level of system performance." Wider coverage, normally a good thing, means using higher powered mobiles which means more radio interference. Increasing capacity means putting more calls into the same amount of spectrum which means calls may be blocked and voice quality will decrease. That's because you must compress those calls to fit the spectrum allowed. So many things must be balanced. As the saying goes, radio systems aren't just sold, they are engineered.

Posted by Tom Farley & Mark van der Hoek at 10:47 PM

CDMA Benefits

The CDG states that CDMA systems have seven advantages over other cellular radio transmission techniques. (GSM and IS-136 operators will contest this list.) CDG says benefits are:

1.Capacity increases of 8 to 10 times that of an AMPS analog system and 4 to 5 times that of a GSM system
2.Improved call quality, with better and more consistent sound as compared to AMPS systems
3.Simplified system planning through the use of the same frequency in every sector of every cell
4.Enhanced privacy
5.Improved coverage characteristics, allowing for the possibility of fewer cell sites
6.Increased talk time for portables
7.Bandwidth on demand

Good, readable information on CDMA is here:
http://www.cellular.co.za/celltech.htm

Posted by Tom Farley & Mark van der Hoek at 10:48 PM

Call Processing: A Few Details

IS-95, as I've mentioned before, is another cellular radio technique. It uses CDMA but is backward compatible with the analog based AMPS. IS-95 handles calls differently than TDMA schemes, although registration is the same. IS-95 queries the same network resources and databases to authenticate a caller. One thing that does differ IS-95, besides the different transmission scheme, are handoffs. It's tough transferring a call between cells in any cellular radio system. Keeping a conversation going while a cellular user travels at seventy miles per hour from one cell to the next finds many calls dropped. CDMA features soft handoffs, where two or more cell sites may be handling the call at the same time. A final handoff gets done only when the system makes sure it's safe to do so. Check out the file just below for a better summary:

Paul Bedell writes an excellent summary of CDMA, including information on soft handoffs, in this .pdf file. It's just six pages, about 273K.

It's from his book Cellular/PCs Management. More information and reviews are here (external link to Amazon.com)

I hope the above comments were helpful and that you visit the CDG site soon. Let's finish this article with some comments by Mark van der Hoek. He says that the most signifigant feature of CDMA is how it delivers its features without a great deal of extra overhead. He notes how CDMA cell sites can expand or contract, breathing if you will, depending on how many callers come into the cell. This flexibility comes built into a CDMA system. Here are some more comments from him:

"CDMA is already dominant, and 3G will be CDMA, and everyone knows it. The matter was really settled, though some still won't admit it, when Ericsson, the Big Kahoona of GSM, Great Champion of The Sacred Technology, capitulated to Qualcomm by buying Qualcomm's infrastructure division. The rest is working out the details of the surrender. TDMA just can't deliver the capacity. In fact, I understand that the GSM standard documents spell out TDMA as an interim technology until CDMA could be perfected for commercial use."

"A further note on CDMA bandwidth. IS-95 CDMA (Qualcomm) uses a bandwidth of 1.25 MHz. Anyone know why? I have fun with this one, because few people, even in the industry, know the answer. PhDs often don't know the answer! That's because it is not a technical issue. The key to the matter can be found in the autograph in one of my reference books, "Mobile Communications Design Fundamentals" by William C. Y. Lee. The inscription reads, 'I am very glad to work with you in this stage of designing CDMA system, with my best wishes. Bill Lee, AirTouch Comm Los Angeles, CA March 22, 1995'."

"Dr. Lee is a major figure in the cellular industry, but few know of the contribution he made to CDMA. Dr. Lee was one of the engineers at Bell Labs in the '60s who developed cellular. He later came to work for PacTel Cellular (later AirTouch) as Chief Science Officer. Qualcomm approached him in 1992 or 1993 about using CDMA technology for cellular. TDMA was getting off the ground at that time, and Qualcomm had to move fast to have any hope of prevailing in the marketplace. They proposed to Dr. Lee that PacTel fund them (I think the number was $100,000) to do a "Proof of Concept", which is basically a theoretical paper showing the practicality of an idea. Dr. Lee considered Qualcomm's proposal, and said, "No." Qualcomm was shocked. Then Dr. Lee told them we'll fund you 10 times that amount and you build us a working prototype."

"It is not too much to say that we have CDMA where it is today in part because of Dr. Lee. Qualcomm built their prototype system piggybacked on PacTel's San Diego network. During the development phase it was realized that deployment of CDMA meant turning off channels in the analog system. (What we call "spectrum clearing".) "How much can we turn off?" was the question. Dr. Lee considered it, and came back with the answer, "10%". Well, that worked out to 1.25 MHz, and that's where it landed. (All of this according to Dr. Lee, who is a brilliant and genuinely nice person.) By comparison, though, 3rd generation systems will have a wider bandwidth, than the 1.25 MHZ bandwidth used for CDMA in IS-95 . The biggest discussion about 3G is now what kind of CDMA will be used. Bandwidth is the sticking point. Will it be 3.75 MHz or 5 MHz? You can see discussions on it at the CDG site (external link)."

Posted by Tom Farley & Mark van der Hoek at 10:52 PM

Appendix

Four pages of content to supplement this article.

Posted by Tom Farley & Mark van der Hoek at 10:53 PM

AMPS Call Processing

This is AMPS call processing for analog and digital services, CDMA or IS-95 excluded. There are two parts to this diagram, click on the links below to see the readable images. I've split the diagram in this way to make it quicker to download. If you want to see the whole graphic at once then click here.


Click here for a large, readable image.


Click here for the large image of this thumbnail.
Click here for the entire diagram.

Posted by Tom Farley & Mark van der Hoek at 10:56 PM

Land Mobile or IMTS

Learn the present by looking at the past. Here's some great reading on the transition from mobile telephone service to cellular. It outlines the IMTS system that influenced tone signaling in AMPS, and gives some clear diagrams outlining AMPS' structure. This is from the long out of print A History of Engineering and Science in the Bell System: Communications Sciences (1925 -- 1980), prepared by members of the technical staff, AT&T Bell Laboratories, c. 1984, p.518 et. seq.:

More on IMTS! (1) Service cost and per-minute charges table / (2) Product literature photos / (3) Briefcase Model Phone / (4) More info on the briefcase model / (5) MTS and IMTS history / (6) Bell System (7) Outline of IMTS / (8) Land Mobile Page 1 (375K) / (9) Land Mobile Page Two (375K) / (10) The Canyon GCS Briefcase Telephone

A History of Engineering and Science in the Bell System: Communications Sciences (1925 -- 1980)

Channel Availability

Mobile telephone service began in the late 1940s. By the seventies, it included a total of thirty-three 2-way channels below 500 megahertz MHz), as shown in Table 11-2. The 35-MHz band, which is not well suited to mobile service (because of propagation anomalies), is not heavily used. The other bands are fully utilized in the larger cities. In spite of this, the combination of few available channels per city and large demand has led to excessive blocking. The FCC's recent allocation of 666 channels at 850 MHz for use by cellular systems (described below) should change this situation. This allocation is split equally between wire-line and radio common carriers (each is allocated 333 channels). In many areas, the wire-line carrier will be the local operating company.

Use of conventional systems on the new channels would increase the traffic-handling capacity by a factor of about 10. The cellular approach, however, will increase the capacity by a factor of 100 or more. How this increase is achieved is discussed later in this section. The potential for very efficient use of so valuable and limited a resource as the frequency spectrum was a persuasive factor in the FCC's decision.

Transmission Considerations

Radio propagation over smooth earth can be described by an inverse power law; that is, the received signal varies as an inverse power of the distance. Unlike fixed radio systems (for example, broadcast television or the microwave systems described in Chapter 9), however, transmission to or from a moving user is subject to large, unpredictable, sometimes rapid fluctuations of both amplitude and phase caused by:

Shadowing: This impairment is caused by hills, buildings, dense forests, etc. It is reciprocal, affecting land-to-mobile and mobile-to-land transmission alike, and changes only slowly over tens of feet.

Multipath interference: Because the transmitted signal may travel over multiple paths of differing loss and length, the received signal in mobile communications varies rapidly in both amplitude and phase as the multiple signals reinforce or cancel one another.

Noise: Other vehicles, electric power transmission, industrial processing, etc., create broadband noise that impairs the channel, especially at 150 MHz and below.

Because of these effects, radio channels can be used reliably to communicate at distances of only about 20 miles, and the same channel (frequency) cannot be reused for another talking path less than 75 miles away except by careful planning and design.

In a typical land-based radio system at 15 or 450 MHz, one channel comprises a single frequency-modulation (FM) transmitter with 50- to 2;0-watt output power, plus one or more receivers with 0.3- to 0.5 microvolt sensitivity. This equipment is coupled be receiver selection and voice-processing circuitry into a control terminal that connects one or more of these channels to the telephone network (see Figure 11-34). The control terminal is housed in a local switching office. The radio equipment is housed near the mast and antenna, which are often on very tall buildings or a nearby hilltop.

Click here for a larger image

Conventional System Operation

Originally, all mobile telephone systems operated manually, much as most private radio systems do today. A few of these early systems are still in use but because they are obsolete, they will not be discussed here.


More recent systems (the MJ system at 150 KHz and the MK system at 450 KHz) [Improved Mobile Telephone Service or IMTS, ed.] provide automatic dial operation. Control equipment at the central office continually chooses an idle channel (if there is one) among the locally equipped complement of channels and marks it with an "idle" tone. All idle mobiles scan these channels and lock onto the one marked with the idle tone. All incoming and outgoing calls are then routed over this channel. Signaling in both directions uses low-speed audio tone pulses for user identification and for dialing. Compatibility with manual mobile units is maintained in many areas served be the automatic systems by providing mobile-service operators. Conversely, MJ and MK mobile units can operate in manual areas using manual procedures.

One desirable feature of a mobile telephone system is the ability to roam; that is, subscribers must be able to call and be called in cities other than their home areas. The numbering plan must be compatible with the North American numbering plan. Further, for land-originated calls, a routing plan must allow calls to be forwarded to the current location. In the MJ system, operators do this. Because of the availability of the MJ system to subscribers requiring the roam feature, the MK system need not be arranged for roaming.. .

[Editor's note. IMTS authority Banner

Free Telecom Magazines through TradPub.com. Click here to go there

wb6nvh/Motadata.htm">Geoff Fors (external link) makes these important points: "There are some errors in AT&T's history of mobile telephone data. The UHF MK system mobiles did not have manual capability and could not roam. The MK head, the handheld device you actually made phone calls with, was a stripped-out version of Motorola's "FACTS" control head. What was stripped out was the Roam and the Manual features, and the operator-selected-channel option. MK phones were not popular and are very rare today."]

Posted by Tom Farley & Mark van der Hoek at 11:07 PM

Early Bell System Overview of Amps

Cellular Concept. Although the MJ and MK automatic systems offer some major improvements in call handling, the basic problems, few channels and the inefficient use of available channels still limit the traffic capacity of these conventionally designed systems. Advanced Mobile Phone Service overcomes these problems be using a novel cellular approach. It operates on frequencies in the 825- to 845 MHz and 870-to 890-MHz bands recently made available by the FCC. The large number of channels available in the new bands has made the cellular approach practical.

A cellular plan differs from a conventional one in that the planned reuse of channels makes interference, in addition to signal coverage, a primary concern of the designer. Quality calculations must take the statistical properties of interference into account, and the control plan must be robust enough to perform reliably in the face of interference. By placing base stations in a more or less regular grid (spacing them uniformly), the area to be served is partitioned into many roughly hexagonal cells, which are packed together to cover the region completely. Cell size is based on the traffic density expected in the area and can range from 1 to 10 miles in radius.

Up to fifty channels are assigned to each cell to achieve their regular reuse and to control interference between adjacent cells. This is illustrated in Figure 11-35, where cell A' can use the same channels as cell A. Because of the inverse power law of propagation, the spatial separation between cells A and A' can be made large enough to ensure statistically that a signal-to-interference ratio greater than or equal to 17 dB is maintained over 90 percent of the area. Maintenance of this ratio ensures that a majority of users will rate the service quality good or better.

Cellular systems also differ from conventional systems in two significant ways:

High transmitted power and very tall antennas are not required.

Wide FM deviation is permissible without causing significant levels of interference from adjacent channels.


Click here for a larger image


The latter is responsible for the high voice quality and high signaling reliability of the Advanced Mobile Phone Service.

In any given area, both the size of the cells and the distance between cells using the same group of channels determine the efficiency with which frequencies can be reused. When a system is newly installed in an area (when large cells are serving only a few customers), frequency reuse is unnecessary. Later, as the service grows, a dense system will have many small cells and many customers), a given channel in a large city could be serving customers in twenty or more nonadjacent cells simultaneously. The cellular plan permits staged growth. To progress from the early to the more mature configuration over a period of years, new cell sites can be added halfway between existing cell sites in stages. Such a combination of newer, smaller cells and original, larger cells is shown in Figure 11-36.


Click here for the larger image

One cellular system is the Western Electric AUTOPLEX-100. In this system, a mobile or portable unit in a given cell transmits to and receives from a cell site, or base station, on a channel assigned to that cell. In a mature system, these cell sites are located at alternate corners of each of the hexagonal cells as shown in Figure 11-36. Directional antennas at each cell site point toward the centers of the cells, and each site is connected by standard land transmission facilities to a 1AESS switching system and system controller equipped for Advanced Mobile Phone Service operation (called a mobile telecommunications switching office, or MTSO). Start-up and small-city systems use a somewhat more conventional configuration with a single cell site at the center of each cell.

The efficient use of frequencies that results from the cellular approach permits Advanced Mobile Phone Service customers to enjoy a level of service almost unknown with present mobile telephone service. Grades of service of P(0.02) are anticipated,compared to today's all-too-common P(0.5) or worse. At the same time, the number of customers in a large city can be increased from a maximum of about one thousand for a conventional system to several hundred thousand. Also, because of the stored-program control capability of MTSOs equipped with the lAESS system, Custom Calling Services and man other features can be offered, some unique to mobile service. Other, smaller, switches provided by Western Electric or other vendors are also available to serve smaller cities and towns.

System Operation: Unlike the MJ and MK systems, Advanced Mobile hone Service dedicates a special subset of the 333 allocated channels solely to signaling and control. Each mobile or portable unit is equipped with a frequency synthesizer (to generate any one of the 333 channels) and a high speed modem (10 kbps). When idle, a mobile unit chooses the "best control channel to listen to (by measuring signal strength) and reads the high-speed messages coming over this channel. The messages include the identities of called mobiles, local general control information, channel assignments for active mobiles and "filler" words to maintain synchronism. These data are made highly redundant to combat multi-path interference. A user is alerted to an incoming call when the mobile unit recognizes its identity code in the data message. From the user's standpoint, calls are initiated and received as they would be from any business or residence telephone.

As a mobile unit engaged in a call moves away from a cell site and its signal weakens, the MTSO will automatically instruct it to tune to a different frequency, one assigned to the newly entered cell. This is called handoff. The MTSO determines when handoff should occur by analyzing measurements of radio signal strength made by the present controlling cell site and by its neighbors. The returning instructions for handoff sent during a call must use the voice channel. The data regarding the new channel are sent rapidly (in about 50 milliseconds), and the entire retuning process takes only about 300 milliseconds. In addition to channel assignment, other MTSO functions include maintaining a list of busy (that is, off-hook) mobile units and paging mobile units for which incoming calls are intended.

Regulatory Picture. The FCC intends cellular service to be regulated by competition, with two competing system providers in each large city: a wire-line carrier and a radio common carrier. To prevent any possible cross-subsidization or favoritism, the Bell operating companies must offer their cellular service through separate subsidiaries. These subsidiaries will be chiefly providers of service and, in fact, are currently barred from leasing or selling mobile or portable equipment. Such equipment will be sold by nonaffiliated enterprises or by American Bell Inc.

Posted by Tom Farley & Mark van der Hoek at 11:11 PM

Link to Professor R.C. Levine's article

Editor's note: Professor Richard Levine has shared his time and experience with privateline.com readers for many years. His cellular introduction in .pdf remains the best long form piece to that subject on the web. Look below. Consider his resume, please, if you need a consultant or expert witness.

Richard C. Levine, ScD, PE (TX)
Manager
Beta Scientific Laboratory
PO Box 836224
Richardson, TX 75083-6224

Telephone: 972-233-4552

Web site: http://www.betalab.org/ (external link)
E-Mail: r.levine@betalab.org


Introducing cellular radio by Levine (374K in .pdf)


SPECIALIZATIONS

* Expert Witness
* Evaluation of patents
* Evaluation of technical products
* Technology training

Independent telecommunications consultant since 1990. Broad and deep knowledge of digital switching and transmission technology in the public switched telephone network, radio, signal processing, antennas, etc.

Adjunct Professor teaching graduate electrical engineering department courses in Digital Telephony and Digital Switching at Southern Methodist University, Dallas, TX. Co-author (with Lawrence Harte) of Cellular and PCS: The Big Picture (McGraw-Hill Book Co.,1997) and GSM Superphones (McGraw-Hill).

Major participant in design of Nortel DMS-MTX 800 MHz Digital Cellular system (IS-54/IS-136 technology), and participated in the standards development of IS-54 and North American Authentication and Encryption Standard used in IS-136 and IS-95. Involved in staff training and system debugging for original 900 MHz GSM digital cellular systems installations in Germany and France. Familiar with other full range cellular/PCS/SMR/ESMR and short-range or low-tier PCS systems such as: Nextel (Motorola iDEN), Geotek/PowerSpectrum, FHMA/TDMA, and DECT/DCT/PWT.

Developed and delivered industrial training courses to quickly familiarize technical staff with GSM/PCS-1900 and IS-136. These courses have been given to staff of numerous US and foreign manufacturers of both mobile handset, mobile data, and base equipment. Hold numerous patents and also an experienced evaluator of patent and technological intellectual property items for valuation or litigation cases. Designed and evaluated/debugged systems in Brazil, Britain, Canada, France, Germany, Finland, Israel, Mexico, and USA.

http://www.ewh.ieee.org/r5/dallas/cn/consultants/levine_r.htm

July 08, 2006

Posted by Ken Schmidt & Mark van der Hoek at 03:37 PM

Cell Tower Lease Expert

If you need specific help with questions regarding cell towers, one of our editors can help.

Ken Schmidt, a cell tower expert, can answer questions regarding cell towers, cell tower leases, lease negotiations, and lease buyouts. He is knowledgeable on most areas regarding cell towers including cell tower valuations and lease valuations.

March 06, 2007

Posted by Tom Farley & Mark van der Hoek at 12:26 PM

Q&A: Cell Tower Capacity

Dear Mark van der Hoek:


Q. Do you know how much capacity cell towers have? I'm on our local school board for a small rural district of about 2,000 students. There was discussion last night about in case of an emergency the students should not be able to use their cell phones because it would overload the cell towers and interfere with emergency personnel.


A. I can't give you an absolute answer because there are numerous variables. Perhaps the biggest is, how many cellular companies (carriers) provide service to your location? Obviously, the more the merrier as far as capacity. Assuming they have a fairly equal market share, of course.


However, the rural nature of your location and your (relatively) small population make it safe to make a few assumptions. It's not likely that any cellular carrier is going to serve your town with more than one, or at the MOST, two cell sites. Then, assuming you have, let's say, 5 wireless providers, that gives us a MAXIMUM of 10 sites to serve your town. Of course, that will be 5 sites that are likely to be dominant at the school, with 5 sites that could possibly take some overload. Realistically, it's probably 5 sites period, and those sites are probably going to be a mix of single and three sectored sites. Let's be generous and assume that 3 of the 5 carriers have three sectored sites, and all three are configured such that 2 of their 3 sectors are able to serve the school. That gives us (2*1) + (3*2) = 8 sectors to provide service at your school. Given that a single sector can carry anywhere from 7 (GSM) to 20-something (CDMA) calls at one time, that gives a capacity at your school of somewhere between (7*8 = 56) and (25*8 = 200) calls at one time.


While this is very much a "back of the napkin" exercise, oversimplified and with a lot of room for error, I do think your concern is well founded. I've probably been overly generous with the number of carriers and sites, and of course, if you have fewer carriers and fewer sites, the picture is even worse.


The sad thing is that even back in the analog days, we had the technology to deal with this. The engineers at Bell Labs who developed the technology foresaw this kind of thing, and built in a mechanism to prioritize traffic. Each phone was to be assigned an "Access Overload Class", and phones owned by bona fide emergency agencies would have a special ACCOC assigned. In an emergency, the cellular operator would simply deny channels to everyone BUT the emergency personnel. However, the FCC in a mistaken egalitarian zeal, decreed that such discrimination was unfair, and could not be implemented. So, a good idea died at the hands of a bureaucracy. The technology is STILL there, but cannot be used.


Mark van der Hoek

Front Royal, Virginia

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