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.)

"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.

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)

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

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.

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

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)."

Click here for a large, readable image.

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

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."]

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.

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

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.

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