Mobile Phones - The Basics

IX. Code Division Multiple Access -- IS-95
Code Division Multiple Access has many variants as well. InterDigital, for example, produces a broadband CDMA system called B-CDMA that is different from Qualcomm's narrowband CDMA system. A CDMA system assigns a specific digital code to each user or mobile phone 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.

A. 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. IS-136 is an AT&T offering, the newest of the TDMA services, which still retains an AMPS operating mode. Both IS-54 and 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.

(Oh, IS-54 , for your information, recently went away by that name, absorbed by the latest revision of interim standard 136. IS-54 is now IS-136. No, I don't think they mean to confuse us with their language, it just seems that way. And, since I am digressing slightly here, 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. )

PCS1900, the closest thing we have to GSM in North America, operates at higher frequencies than conventional cellular. It can use TDMA or CDMA. PCS1900 is not compatible with other services, but I have seen a Sprint phone which has two bands and two modes. It uses their PCS service where available but has a mode which lets the phone choose AMPS service if PCS1900 isn't available. That's not a feature of PCS but rather a hardware fix, two phones in one. 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.

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. Phil Karn, KA9Q, one of the principal engineers has written that 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 mobile 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 phone 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. Because CDMA is so important to cellular radio, especially for its future, I want to discuss it at length. I've taken many comments on CDMA from the Cellular Development Group's website. They are the principal industry group pushing CDMA forward.

A 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.

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." Don't panic about that last phrase. Instead, let's get comfortable with CDMA terms by seeing see how this transmission techniques work.

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.

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