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Cellular Basics Series

I Introduction

II Cellular History

lII Cell and SectorTerminology

IV Basic Theory and Operation

V Cellular frequency and channel discussion

VI. Channel Names and Functions

VII. AMPS Call Processing

A. Registration

B. Pages: Getting a Call

C. The SAT, Dial Tone, and Blank and Burst

D. Origination -- Making a call

E. Precall Validation

VIII. AMPS and Digital Systems compared

IX. Code Division Multiple Access -- IS-95

A. Before We Begin -- A Cellular Radio Review

B.Back to the CDMA Discussion

C. A Summary of CDMA -- Another transmission technique

D. A different way to share a channel

E. Synchronization

F. What Every Radio System Must Consider

G. CDMA Benefits

H. Call Processing -- A Few Details

X. Appendix

A. AMPS Call Processing Diagram

B. Land Mobile or IMTS

C. Early Bell System Overview of Amps

D. Link to Professor R.C. Levine's .pdf file introducing cellular. (100 pages, 374K)


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WiWCellular Telephone Basics

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(Page 2) Cellular Telephone Basics cont. . .

lII Cell and SectorTerminology

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?
 
hex and circle gif
 
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.
 
hex and circle group

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.
 
 
sectors
 

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."

sectors

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."
Seven hex cluster

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."

IV 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

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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. (back to text)

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