(Page Nine) Cellular Telephone Basics continued . . .

 

IX 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)

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.

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 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.)

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.

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.

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)

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. ---> next page, please -->

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

Pages in This Article (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) Next page -->