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Guest Column

CDMA: Past, Present, And Future

Andrew J. Viterbi
Viterbi Group, LLC

With all the alphabet soup in the titles of the multitude of wireless conferences these days, we lose sight of what the initials stand for. Here UPC stands for Universal Personal Communication.

But perhaps more importantly, it can also stand for universal service to all the worldís population, more than half of which has never had access to telephony.

Universal is a term which has two meanings: first it can stand for "Anywhere, Anytime" availability and access, which only mobile communications can offer.

For these, mobility is hardly meaningful; the issue is how possibly can they be connected to the developed world within the first decade of the new century? And the answer can only be through wireless fixed local loops, another theme of this conference.

Now let me turn to the word Personal, usurping its true meaning in the conferenceís title to describe rather some personal experiences and express some personal views. Iíve been extremely fortunate to have participated in the four-decade evolution of digital wireless and spread spectrum from its origins in military and space applications through its current commercial diffusion. The abridged titles of my three books gives a brief summary of my technical activities: "Coherent Communications"í "Digital Communications and Coding"; and the most recent "CDMA and Spread Spectrum." And this last one relies heavily on all the previous subject matter.

Itís also the topic which continues to fully occupy my interests and activities. But, more importantly, CDMA is the technology which in my opinion will best fulfill the promise of Universal Personal Communication. And let me tell you why. The primary issue in wireless multiple access communication is how to cope with interference. This can be approached by separating users in frequency, time and space, all of which limit the number of users supportable by each base station; and even this does not completely avoid the self-interference through multipath, resulting in fading, or the interference from remote base stations which reuse the same frequency band; the smaller the reuse factor, the greater the user capacity, but the more severe the interference.

Or one can tackle the problem head-on, accept that every user or every base station will interfere with every other user or base station and share the spectrum among all user transmitters in one direction and among all base station transmitters in the other. To achieve this, one must create benign interferers, and employ signal processing techniques to extract individual user signals from their benignly interfering neighbors. The first task is fulfilled by spreading the spectrum such that every user appears as wideband noise to every other user. The second task is fulfilled by a collection of tightly related and mutually supportive techniques. Foremost among these is power control. For the challenge to true spread spectrum (meaning that generated by direct sequence spreading rather than frequency hopping) has always been the "near-far" problem. In the earlier military scenarios, directional antennas were the only remedy. But for civilian systems, tight and accurate power control of each individual userís transmitter, so that it transmits just enough power to ensure reliable reception, solves the near-far problem; beyond this it serves the dual purposes of minimizing interference to other users and maximizing the battery life of portable terminals. Other important processing measures to enhance spread spectrum signal reception include "Rake " receivers to turn detrimental multipath into cooperative signal energy and "Soft Handoff" among base stations or among different antenna sectors of the same base station. These two features can be implemented by the same receiver technique since two or more base stations transmitting the same signal appear to the "Rake " receiver as time-delayed versions of the same signal, which is equivalent to the case of multipath. Another means for reducing interference, by better than half, is through use of a variable rate voice coder, which reduces its transmission rate and hence transmitted power, during periods of voice inactivity or reduced activity. Finally, error-correcting coding permits the receiver to operate at a lower signal-to-interference ratio, and antenna switching or adaptive techniques provide spatial interference reduction.

All these tightly connected and integrated signal processing techniques are either made possible or at least facilitated by spread spectrum signaling. They provide a capacity, in terms of users per MHz per base station, which is several times that of other multiple access techniques, along with extended coverage for each base station or enhanced portable battery life, and better voice clarity. This message has only recently been widely accepted and understood. QUALCOMM began this development in 1989, held numerous demonstrations, large-scale field tests, forums and debates over the next four years, in a climate that was generally less than friendly. A number of political and public relations roadblocks were erected by the competition to delay or derail the process. Only a few service providers and fewer manufacturers agreed with us initially. But by 1993, with their help we had gained enough support to establish the TIA standard, IS-95. Thereafter scores of manufacturers, for both handsets and infrastructure joined the CDMA consortium, now known as "cdmaOne". Hong Kong was the first region to be offered commercial CDMA and South Korea became the first country where CDMA digital users exceeded the number of analog users. The vast majority of North American operators have selected CDMA for digital telephony, with continent-wide coverage and serving the majority of the population. Even though service initiation has begun in earnest in the U.S. only this year, there are already well over one million CDMA subscribers with service in over 100 cities of North America, two million subscribers in South Korea and five million worldwide. There is now little risk in predicting that CDMA will ultimately dominate the North American market and, even with the considerable head start of the European digital standard GSM, that CDMA will gain wide acceptance in Asia and South America as well.

Nevertheless, the negative propaganda and public relations campaigns continue. This in spite of Forbes magazine recognizing its past misunderstanding of the technology ("Wrong Call" October 6), and even the Wall Street Journal now desisting from general and personal attacks and admitting instead on September 20 that "South Koreaís bet on cellular phone technology may pay off." The battleground seems to have now shifted to the trade press with an article in Telephony magazine ("Blind Faith" September 8) rehashing for the nth time previously discredited negative claims about CDMA capabilities, complexity and cost. In fact, these claims are belied by the dozens of cellular and PCS operators and their millions of customers which use the technology daily. But in addition to the commercial success of CDMA, and the ever expanding universe of manufacturers and service providers joining the "cdmaOne" organization, we now have the grudging admission of the two dominant European (GSM and TDMA) wireless manufacturers, and the dominant Japanese telecommunications operator, that CDMA is a valid technology. In fact, they have gone as far as placing their considerable power and prestige behind a 3rd Generation proposal to the International Telecommunications Union, which uses a 4.1MHz spreading sequence and which they have labeled Wideband CDMA (W-CDMA). So at this point, we should simply declare "Victory" and retire. But not so fast, for this too is a clever PR ploy. It lets our erstwhile detractors and future competitors label IS-95 as the Narrowband CDMA standard. Now "wide" is a relative term. No one can argue that 4.1MHz is wider than the 1.23 MHz of the IS-95 spreading sequence by more than a factor of 3, but then 1.23 MHz is more than 4.5 times the 271K symbol rate of GSM and more than 25 times the 48.6K symbol rate of the North American TDMA standard IS-136. But, so what? Is wider necessarily better? Perhaps and perhaps not. What matters is what one gains or loses by wider-spreading.

To begin with, we need to distinguish between voice telephony and high speed data. Itís easy to say that data at 2 Mbps is better served with a 4 MHz modulation bandwidth than by a 1 MHz bandwidth, but, for digital telephony, very high quality vocoders require only a maximum data rate of 8 Kbps to 13Kbps, and with improved algorithms, over time this may yet be reduced to 4Kbps or less. Spreading to 1.23 MHz provides a healthy processing advantage over interference as well as multipath resolution of better than 1msec. Wider spreading permits resolution of more closely spaced multipath components. But is this necessarily good? Aside from the complexity required to acquire and track the additional components, which may be minimized by advancing technology, thereís the more serious issue of channel measurement inaccuracy, which is inversely proportional to the energy per component and hence increases as the number of multipath components increases. Thus wider bandwidths and lower data rates (requiring less transmitted energy) are counter-productive to measurement accuracy and for too wide spreading they can thus cause overall degradation.

But enough of past and present standards battles, politically and economically motivated. What about the future? Especially, what about data? Here, contrary to common wisdom, Iíll make another controversial distinction: "All bits are not created equal." Voice and data can coexist, but to the detriment of both. Let me hold off on data and on explaining the difference. Letís first explore improvements in voice telephony. Only a knave or a fool would argue that what we have today is the end-all of CDMA perfection. I have argued that IS-95 has incorporated many critical and excellent features, which in fact, are being imitated by the newly proposed standards just mentioned. Some of these proposals involve just parameter changes, designed to counter the competition, without noticing that it would also impact operators which have implemented CDMA, forcing them to scrap their current systems and start over, hardly a palatable alternative given their large capital investments. Other proposed changes may also be ill conceived but for less obvious reasons; time prevents me from elaborating.

Rather let me comment on improvements to IS-95 which are easily implemented and are backward compatible to existing infrastructure. The first is to abandon the reverse link Walsh-function modulation, which lends itself to non-coherent reception at the base station, replacing it by a simpler biphase modulation, aided by an auxiliary continuous pilot on the quadrature channel, which lends itself to coherent demodulation at the base station. This, in fact, makes the reverse channel quite similar to the forward channel modulation. Why then did we not use this fairly commonplace approach for the reverse link in the first place? Conservatism is the reason. We feared that in a rapidly varying multipath channel, phase could not be tracked accurately unless an inordinate percentage of the power was assigned to the unmodulated pilot; hence we opted for noncoherent reception. This proved to be an overly cautious decision. Second, if tight power control is a necessary feature, tighter power control is even better. This has been found to be particularly true for the forward link from base station to user, where power control was correctly deemed to be less critical and hence made very infrequent, but this turned out to limit performance. Also, means for reducing latency of the power control loops in both directions, will considerably improve performance.

Another improvement certain to enhance capacity and coverage involves adaptive antenna array techniques. A combination of spatial and signal processing, this will primarily benefit the reverse link and will be rendered more feasible by the coherent demodulation just mentioned. Iím convinced that spatial signal processing will be far more effective at interference reduction, as well as simpler, than purely temporal signal processing techniques such as interference cancellation. And finally, as noted previously, vocoder algorithms will continue to improve, yielding lower peak and average bit rates.

And last what youíve all being waiting for Ė what about high speed data? Everyone wants to connect to the Internet and why not by wireless means, avoiding the nuisance of unfriendly line connectors for our laptops in each new location. Here common wisdom dictates higher bit rates. ISDN is insufficient; 384 Kbps is better; so why not 2 Mbps? Faster is better. But is speed really the main issue? For wireline connections, we buy a 56 Kbps modem and find that our line will only support 24 Kbps. So what is the consequence? Latency: the delay time we have to sit at our terminal waiting for the downloading from the Net to be completed. But if weíre used to varying rates, and hence latencies, from our wireline modems, why shouldnít we expect this in wireless. If the base station is in a closet down the hall in our office, we should expect a 2 Mbps downloading rate, but if itís five miles away from our vehicle which is being driven (by someone else) on an expressway, might we not be quite satisfied with ISDN speeds or lower? This is what differentiates data from voice. We can tolerate and even expect latencies of many seconds in receiving data, but voice conversations are intolerant of delays any greater than 100 milliseconds. So for voice telephony, we must allocate an inordinate amount of our common resources, usually power, to the weakest users, thereby limiting the total throughput for the collection of users served by one base station. With variable latency, we have much more flexibility. To begin with, latency need not be inversely proportional to data rate, because in a packet-switched network, we can assign more packets to lower rate users. We may thus impose a "fairness criterion" that the most disadvantaged userís latency be no greater than N times that of the highest rate user. For voice, we must make N = 1. For data, studies with real traffic and measured channel quality statistics show that we can more than double overall throughput in the forward direction by letting N = 8.

The flexibility of packet transmission allows other improvements. One is the use of longer interleaving depths, intolerable by voice because of the inherent delay. In this context, I would be forgetting my heritage, if I did not mention "turbo codes", a mixture of simple short convolutional codes, long interleavers and better soft decision decoding, which permit data rates to approach within 60% to 80% of the Shannon coding limit (an amazing feat), thus increasing current throughputs by more than 60%. Last, we should note that data transmission is often asymmetric. We may wish to download megabytes from the Web, but need only send kilobits worth of requests. For the forward link all these data packet network features and improvements taken together may lead to a forward link throughput enhancement of at least a factor of 4. This is in sharp contrast with circuit-switched voice and serves to demonstrate why "all bits are not created equal."

All this argues for the desirability of a separation of voice and data, which assumes, of course, that customer data requirements approach those for voice. For the near-term, data can coexist with circuit-switched voice, employing the IS-99 and IS-657 data standards. The quite attainable goal then should be for a smooth transition to packet-switched higher throughput data service protocols with an air interface which is compatible with current voice and data services.

Let me conclude then by summarizing my various messages. Spread spectrum CDMA is here to stay. The religious wars are over. The heretics have been confounded and a few may even have been converted. Some conversions have been under duress and some may not yet have fully shed their heresies, but itís only a matter of time. Various improvements can and should be implemented, but Third Generation Systems better be backward compatible if theyíre to find a willing market among operators. Data transmission differs fundamentally from voice through relaxed latency requirements. With Internet-type packet protocols, we can exploit these differences to increase throughputs several-fold.

Data and voice users can coexist on the same carrier but to the detriment of the capacity of both. As long as voice requirements dominate wireless demand, this is a tolerable condition. As connectivity to the Internet or any other massive data resource becomes the dominant application, great improvements in spectrum efficiency will mandate the separation of voice and data on separate frequency carriers with different network protocols. The technologies for this transition are already available. Market forces and operator choices will determine how rapidly they are deployed.

About the Author

Dr. Andrew Viterbi is a co-founder and retired Vice Chairman and Chief Technical Officer of QUALCOMM Incorporated. He spent equal portions of his career in industry, having previously co-founded Linkabit Corporation, and in academia as Professor in the Schools of Engineering and Applied Science, first at UCLA and then at UCSD, at which he is now Professor Emeritus. He is currently president of the Viterbi Group, a technical advisory and investment company.

His principal research contribution, the Viterbi Algorithm, is used in most digital cellular phones and digital satellite receivers, as well as in such diverse fields as magnetic recording, voice recognition and DNA sequence analysis. More recently, he concentrated his efforts on establishing CDMA as the multiple access technology of choice for cellular telephony and wireless data communication.

Dr. Viterbi has received numerous honors both in the U.S. and internationally. Among these are four honorary doctorates, from the Universities of Waterloo (Canada), Rome (Italy), Technion (Israel) and Notre Dame, as well as memberships in the National Academy of Engineering, the National Academy of Sciences and the American Academy of Arts and Sciences. He has received the Marconi International Fellowship Award, the IEEE Alexander Graham Bell and Claude Shannon Awards , the NEC C&C Award, the Eduard Rhein Foundation Award and the Christopher Columbus Medal. He has received an honorary title from the President of Italy and he has served on the U.S. Presidentís Information Technology Advisory Committee.

Viterbi serves on boards of numerous non-profit institutions, including the University of Southern California, UC Presidentís Council for the National Laboratories, MIT Visiting Committee for Electrical Engineering and Computer Science, Mathematical Sciences Research Institute, Burnham Institute and Scripps Cancer Center.



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