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How much is it worth to let us do it
From: David Farber <farber () linc cis upenn edu>
Date: Fri, 29 Oct 93 05:41:24 -0400
This rule turns the conventional wisdom of microwaves upside down. For example, it is true that microwaves don't travel far in the atmosphere. You don't want to use them to transmit 50,000 watts of Rush Limbaugh over 10 midwestern states, but to accommodate 200 million two-way communicators will require small cells; you don't want the waves to travel far. It is true that microwaves will not penetrate most buildings and other obstacles, but with lots of small cells, you don't want the waves to penetrate walls to adjacent offices. Microwaves require high-power systems to transmit, but only if you want to send them long distances. Wattage at the receiver drops off in proportion to the fourth power of the distance from the transmitter. Reducing cell sizes as you move up the spectrum lowers power needs far more than higher frequencies increase them. Just as important, mobile systems must be small and light. The higher the frequency, the smaller the antenna and the lighter the system can be. All this high-frequency gear once was prohibitively expensive. Any functions over two gigahertz require gallium arsenide chips, which are complex and costly. Yet the cost of gallium arsenide devices is dropping every day as their market expands. Meanwhile, laboratory teams are now tweaking microwaves out of silicon. In the world of electronics - where prices drop by a third with every doubling of accumulated sales - any ubiquitous product will soon be cheap. The law of the telecosm dictates that the higher the frequency, the shorter the wavelength, the wider the bandwidth, the smaller the antenna, the slimmer the cell and ultimately, the cheaper and better the communication. The working of this law will render obsolete the entire idea of scarce spectrum and launch an era of advances in telecommunications comparable to the recent gains in computing. Transforming the computer and phone industries, the converging spirits of Maxwell, Shannon and Shockley even pose a serious challenge to the current revolutionaries in cellular telephony. The New PC Revolution: PCN Many observers herald the huge coming impact of wireless on the computer industry, and they are right. But this impact will be dwarfed by the impact of computers on wireless. In personal communications networks (PCN), the cellular industry today is about to experience its own personal computer revolution. Just as the personal computer led to systems thousands of times more efficient in MIPS per dollar than the mainframes and minicomputers that preceded it, PCNs will bring an exponential plunge of costs. These networks will be based on microcells often measured in hundreds of meters rather than in tens of miles and will interlink smart digital appliances, draining power in milliwatts rather than dumb phones using watts. When the convulsion ends later this decade, this new digital cellular phone will stand as the world's most pervasive PC. As mobile as a watch and as personal as a wallet, these PICOs will recognize speech, navigate streets, take notes, keep schedules, collect mail, manage money, open the door and start the car, among other computer functions we cannot imagine today. Like the computer establishment before it, current cellular providers often seem unprepared for this next computer revolution. They still live in a world of long and strong - high- powered systems at relatively low frequencies and with short- lived batteries - rather than in a PCN world of low-power systems at microwave frequencies and with batteries that last for days. Ready or not, though, the revolution will happen anyway, and it will transform the landscape over the next five years. We can guess the pattern by considering the precedents. In computers, the revolution took 10 years. It began in 1977 when large centralized systems with attached dumb terminals commanded nearly 100 percent of the world's computer power and ended in 1987 with such large systems commanding less than one percent of the world's computer power. The pace of progress in digital electronics has accelerated sharply since the early 1980s. Remember yesterday, when digital signal processing (DSP) - the use of specialized computers to convert, compress, shape and shuffle digital signals in real time - constituted an exorbitant million-dollar obstacle to all-digital communications? Many current attitudes toward wireless stem from that time, which was some five years ago. Today, digital signal processors are the fastest-moving technology in all computing. Made on single chips or multichip modules, DSPs are increasing their cost- effectiveness tenfold every two years. As radio pioneer Donald Steinbrecher says, "That changes wireless from a radio business to a computer business." Thus, we can expect the cellular telephone establishment to reach a crisis more quickly than the mainframe establishment did. The existing cellular infrastructure will persist for vehicular use. As the intelligence in networks migrates to microcells, the networks themselves must become dumb. A complex network, loaded up with millions of lines of software code, cannot keep up with the efflorescent diversity and creativity among ever more intelligent digital devices on its periphery. This rule is true for the broadband wire links of fiber optics, as intelligent switching systems give way to passive all-optical networks. It is also true of cellular systems. Nick Kauser, McCaw Cellular Communications' executive vice- president and chief of technology, faced this problem early in 1991 when the company decided to create a North American Cellular Network for transparent roaming throughout the regions of Cellular One. "The manufacturers always want to sell switches that do more and more. But complex switches take so long to program that you end up doing less and less," says Wayne Perry, McCaw vice-chairman. Each time Kauser tried to change software code in one of McCaw's Ericsson switches, it might have taken six months. Each time he wanted to add customer names above a 64,000 limit, Ericsson tried to persuade him to buy a new switch. The Ericsson switches, commented one McCaw engineer, offer a huge engine but a tiny gas tank. The problem is not peculiar to Ericsson, however; it is basic to the very idea of complex switch- based services on any supplier's equipment. When McCaw voiced frustration, one of the regional Bell operating companies offered to take over the entire problem at a cost of some $200 million. Instead, Kauser created a Signaling System 7 (SS-7) network plus an intelligent database on four Tandem fault-tolerant computers, for some $15 million. Kauser maintains that the current services offered by North American Cellular could not be duplicated for 10 times that amount, if at all, in a switch-based system. Creating a dumb network and off- loading the intelligence on computer servers saved McCaw hundreds of millions of dollars. The law of the microcosm is a centrifuge, inexorably pushing intelligence to the edges of networks. Telecom equipment suppliers can no more trap it in the central switch than IBM could monopolize it in mainframes. Kauser should recognize that this rule applies to McCaw no less than to Ericsson. His large standardized systems with 30- mile cells and relatively dumb, high-powered phones resemble big proprietary mainframe networks. In the computer industry, these standardized architectures gave way to a mad proliferation of diverse personal computer nets restricted to small areas and interlinked by hubs and routers. The same pattern will develop in cellular. Could "Charles" Upend McCaw? Together with GTE and the regional Bell operating company cellular divisions, McCaw is now in the position of DEC in 1977. With its new ally, AT&T, McCaw is brilliantly attacking the mainframe establishment of the wire-line phone companies. But the mainframe establishment of wires is not McCaw's real competition. Not stopping at central switches, the law of the microcosm is about to subvert the foundations of conventional cellular technology as well. Unless McCaw and the other cellular providers come to terms with the new PC networks that go by the name of PCNs, they will soon suffer the fate of the minicomputer firms of the last decade. McCaw could well be upended by its founder's original vision of his company - a PICO he called "Charles." Just as in the computer industry in the late 1970s, the fight for the future is already under way. Complicating the conflict is the influence of European and Japanese forces protecting the past in the name of progress. Under pressure from EEC industrial politicians working with the guidance of engineers from Ericsson, the Europeans have adopted a new digital cellular system called Groupe Speciale Mobile (GSM) after the commission that conceived it. GSM is a very conservative digital system that multiplies the number of users in each cellular channel by a factor of three. GSM uses an access method called time-division multiple access (TDMA). Suggestive of the time-sharing methods used by minicomputers and mainframes to accommodate large numbers of users on centralized computers, TDMA stems from the time-division multiplexing employed by phone companies around the world to put more than one phone call on each digital line. Thus, both the telephone and the computer establishments are comfortable with time division. Under pressure from European firms eager to sell equipment in America, the U.S. Telephone Industry Association two years ago adopted a TDMA standard similar to the European GSM. Rather than creating a wholly new system exploiting the distributed powers of the computer revolution, the TIA favored a TDMA overlay on the existing analog infrastructure. Under the influence of Ericsson, McCaw and some of the RBOCs took the TDMA bait. Thus, it was in the name of competitiveness and technological progress, and of keeping up with the Europeans and Japanese, that the U.S. moved to embrace an obsolescent cellular system. It made no difference that the Europeans and Japanese were technologically well in our wake. Just as in the earlier case of analog HDTV, however, the entrepreneurial creativity of the U.S. digital electronics industry is launching an array of compelling alternatives just in time. Infusing cellular telephony with the full powers of wide and weak - combining Shannon's vision with computer advances - are two groups of engineers from MIT who spun out to launch new companies. Qualcomm Inc. of San Diego, is led by former professor Irwin Jacobs and telecom pioneer Andrew Viterbi. A Shannon disciple whose eponymous algorithm is widely used in digital wire-line telephony, Viterbi now is leading an effort to transform digital wireless telephony. The other firm, Steinbrecher Corp. of Woburn, Mass., is led by an inventor from the MIT Radio Astronomy Lab named Donald Steinbrecher. Like Bernie Bossard and Vahak Hovnanian, the leaders of Qualcomm and Steinbrecher received the ultimate accolade for an innovator: They were all told their breakthroughs were impossible. Indeed, the leaders at Qualcomm were still contending that Steinbrecher's system would not work just weeks ago when PacTel pushed the two firms together. Now they provide the foundations for a radical new regime in distributed wireless computer telephony. Signals in Pseudonoise Ten years ago at Linkabit, the current leaders of Qualcomm conceived and patented the TDMA technology adopted as the U.S. standard by the Telephone Industry Association. Like analog HDTV, it was a powerful advance for its time. But even then, Viterbi and Jacobs were experimenting with a Shannonesque technology. A classic example of the efficacy of wide and weak, CDMA exploits the resemblance between noise and information. The system began in the military as an effort to avoid jamming or air- tapping of combat messages. Qualcomm brings CDMA to the challenge of communications on the battlefronts of big-city cellular. Rather than compressing each call into between three and 10 tiny TDMA time slots in a 30-kilohertz cellular channel, Qualcomm's CDMA spreads a signal across a comparatively huge 1. 25-megahertz swath of the cellular spectrum. This allows many users to share the same spectrum space at one time. Each phone is programmed with a specific pseudonoise code, which is used to stretch a low-powered signal over a wide frequency band. The base station uses the same code in inverted form to "despread" and reconstitute the original signal. All other codes remain spread out, indistinguishable from background noise. Jacobs compares TDMA and CDMA to different strategies of communication at a cocktail party. In the TDMA analogy, each person would restrict his or her talk to a specific time slot while everyone else remains silent. This system would work well as long as the party was managed by a dictator who controlled all conversations by complex rules and a rigid clock. In CDMA, on the other hand, everyone can talk at once but in different languages. Each person listens for messages in his or her own language or code and ignores all other sounds as background noise. Although this system allows each person to speak freely, it requires constant control of the volume of the speakers. A speaker who begins yelling can drown out surrounding messages and drastically reduce the total number of conversations that can be sustained. For years, this problem of the stentorian guest crippled CDMA as a method of increasing the capacity of cellular systems. Spread spectrum had many military uses because its unlocalized signal and cryptic codes made it very difficult to jam or overhear. In a cellular environment, however, where cars continually move in and out from behind trucks, buildings and other obstacles, causing huge variations in power, CDMA systems would be regularly swamped by stentorian guests. Similarly, nearby cars would tend to dominate faraway vehicles. This was termed the near-far problem. When you compound this challenge with a static of multipath signals causing hundreds of 10,000-to- 1 gyrations in power for every foot traveled by the mobile unit - so-called Rayleigh interference pits and spikes - you can comprehend the general incredulity toward CDMA among cellular cognoscenti. Indeed, as recently as 1991, leading experts at Bell Labs, Stanford University and Bellcore confidently told me the problem was a show-stopper; it could not be overcome. Radio experts, however, underestimate the power of the microcosm. Using digital signal processing, error correction and other microcosmic tools, wattage spikes and pits 100 times a second can be regulated by electronic circuitry that adjusts the power at a rate of more than 800 times a second. To achieve this result, Qualcomm uses two layers of controls. First is a relatively crude top layer that employs the automatic gain control device on handsets to constantly adjust the power sent by the handset to the level of power received by it from the base station. This rough adjustment does not come near to solving the problem, but it brings a solution into reach by using more complex and refined techniques. In the second power-control step, the base station measures the handset's signal-to-noise and bit-error ratios once every 1.25 milliseconds (800 times a second). Depending on whether these ratios are above or below a constantly recomputed threshold, the base station sends a positive or negative pulse, either raising or lowering the power some 25 percent. Dynamic Cells Passing elaborate field tests with flying colors, this power- control mechanism has the further effect of dynamically changing the size of cells. In a congested cell, the power of all phones rises to overcome mutual interference. On the margin, these high- powered transmissions overflow into neighboring cells where they may be picked up by adjacent base station equipment. In a quiet cell, power is so low that the cell effectively shrinks, transmitting no interference at all to neighboring cells and improving their performance. This kind of dynamic adjustment of cell sizes is impossible in a TDMA system, where adjacent cells use completely different frequencies and fringe handsets may begin to chirp like Elmer Fudd. Once the stentorian voice could be instantly abated, power control changed from a crippling weakness of CDMA into a commanding asset. Power usage is a major obstacle to the PCN future. All market tests show that either heavy or short-lived batteries greatly reduce the attractiveness of the system. Because the Qualcomm feedback system keeps power always at the lowest feasible level, batteries in CDMA phones actually are lasting far longer than in TDMA phones. CDMA phones transmit at an average of two milliwatts, compared with 600 milliwatts and higher for most other cellular systems. A further advantage of wide and weak comes in handling multipath signals, which bounce off obstacles and arrive at different times at the receiver. Multipath just adds to the accuracy of CDMA. The Qualcomm system combines the three strongest signals into one. Called a rake receiver and co- invented by Paul Green, currently at IBM and author of Fiber Optic Networks (Prentice Hall, 1992), this combining function works even on signals from different cells and thus facilitates hand-offs. In TDMA, signals arriving at the wrong time are pure interference in someone else's time slot; in CDMA, they strengthen the message. Finally, CDMA allows simple and soft hand-offs. Because all the phones are using the same spectrum space, moving from one cell to another is easy. CDMA avoids all the frequency juggling of TDMA systems as they shuffle calls among cells and time slots. As the era of PCN microcells approaches, this advantage will
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