Frantic to save her party, the hostess tries several solutions. First she disperses her guests into different rooms, one pair to a room. With no noise at all in the room except for the pair's own conversation, each couple converses unimpeded, luxuriating in silence. This solution is frequency division multiple access, the technique used by analog mobile systems, which dedicates a pair of narrow-frequency channels solely to one pair of users in a cell. Since the frequencies cannot be reused except by users in cells at least two diameters away, the system also invokes space division multiplexing.
Alas, while some guests now have a surfeit of silence, many still have none. This is a Manhattan cocktail party, and there are not enough empty rooms (channels) in the apartment
A simple spatial division won't do. Taking another stab at the problem, the hostess now places three pairs of conversationalists in each room. All three pairs may converse, but they must take turns. Every 20 seconds, say, one pair gets to talk; the other two must be silent. Adapting to the problem, the guests find that, when it is their turn, they tend to talk faster than usual. And while waiting, they search for economical turns of phrase, to encode more meaning in less transmission time. The result is time division multiple access (TDMA), made possible by digitizing the voice signals so we can send them in rapid bursts, combined with some speech compression codes to save bits. It also triples the number of conversations the hostess can accommodate.
Unfortunately, this is a paradigm party of legend, a party of the decade. Guests continue to throng in at a rate of millions per week. Fortunately, the solution dawns on the hostess: Her party is ever so cosmopolitan, with Japanese, Koreans, Indians, and polyglot Chinese and Europeans. So she asks everyone to return to the main room and speak as softly as they can manage, but each pair in a different language. Mirabile dictu, everyone can hear and understand the signal meant for him or her. It's a little better if a few guests leave; a little worse if a few more show up, but satisfactory in any event. In the real world of wireless this is, of course, code division multiple access (CDMA).
Adding silence, taking away noise, the solutions have progressed on a continuum from the purely physical to the predominantly logical. In the first, the analog solution, our only resources were physical—separating the speakers by space. In the TDMA solution, we employed two physical dimensions and built our isolation chamber out of both space and time. TDMA plied logic as well, compressing communications to fit better into those slots. But only the final solution, CDMA, is predominantly logical—assigning each speaker a different language, or code. Coding the messages into different languages provides virtual silence, since listeners easily filter out the sounds of languages they do not understand, readily identifying them as "noise" rather than signal.
Adding silence in this way is only part of the story. Also crucial is silence removal.
Consider again our hostess. Her first solution—each conversational pair got its own room—wasted silence. With each speaker occupying an entire channel, much of what flows through—some 65%, in fact—is silence. In typical phone conversations, each speaker talks about 35% of the time, with both parties silent about 30% of the time. And because the analog signals in the communication "look" just like the sound waves they imitate, they cannot be compressed or accelerated. TDMA, despite employing digital compression, still wastes 65% of its bandwidth on empty but rigidly scheduled time slots.
While TDMA wastes silence, CDMA spends logical millions of instructions per second (mips) to both save and manufacture silence. Under the guidance of Qualcomm cofounder Andrew Viterbi, who created crucial coding algorithms used in nearly all digital communications systems, Qualcomm developed the variable rate vocoder. In TDMA or CDMA, a vocoder condenses the 64,000 kilobits-per-second (Kbps) digitized version of human speech down to between 8 and 13 Kbps. But during moments of silence, CDMA's variable rate vocoder will output at as little as one-eighth of the full rate. The momentarily silent user opens up space in the channel for other conversations. The variable vocoder alone accounts for a 250% increase in the capacity of a CDMA cell, but it would be pointless in a TDMA system, which shares time but cannot share silence.
Thanks to the law of large numbers in CDMA, the salvaged silence is spread out across the shared 1.25 megahertz channel in a variant of statistical multiplexing, the basic economizing principle behind any shared channel, such as an Ethernet or the Internet. As with an Ethernet, there is only a "soft" limit on the number of users: Adding one more will increase the interference in the channel only marginally. Our hostess need not panic if a late guest shows up at the door.
Even more auspicious for CDMA in the coming era of the wireless Web: The hostess need not despair even if scores of guests decide to deliver speeches with PowerPoint slides or transmit lengthy PostScript files. Because the CDMA system spreads all the data across all the available spectrum all the time, it can accommodate the bursty bitstreams characteristic of the Internet.
Thus CDMA exploits processing to add silence where it is needed and takes it away where it would be wasted. But CDMA's greatest feat and the essence of the system is that it uses processing to turn noise itself into silence.
In any communication channel, all the transmission power ends up either as noise or signal, and some of it ends up as both, because even a well-shaped signal is noise to a user trying to receive a different signal. In TDMA, there are only two possible fixes. The sender can raise the power of the transmission, thus making it more likely that the bits will be discernible through the noise. But any additional power will show up as additional noise on adjacent channels, confronting them with the same choice. (The guests at the cocktail party have started to shout.) Or the sender can add more bits—e.g., in the form of more elaborate error correction—but this will decrease the information rate of the channel. (The guests at the party have started to repeat themselves.)
CDMA soaks up all this overflow of noise and bits with DSP millions of operations per second (mops). The CDMA transmitter first multiplies the information bits by a pseudorandom noise code and then spreads the resulting apparently randomized signal across a slice of spectrum more than 100 times the bandwidth of the original signal. Like a platoon spreads out to avoid death from a single grenade, the spread signal cannot be wiped out by noise in any narrow portion of the channel.
When the signal is spread by a factor of more than 100, its energy is necessarily spread as well, and so is the energy of the other users of the same spectrum. Collectively, the signals of dozens of users have acquired the essential characteristics of, if not quite white, then "gray" Gaussian noise. The result is scores of decibels of "processing gain": the apparently magical power of hearing a soft sound above a much louder one of similar pitch. Abandoning the attempt to power past competing users, CDMA lowers the energy of all signals to the minimum needed to reach the receiver. Because the receiver has the matching noise code—which is inverted to delete the noise through destructive interference—the message can be extracted from the background drone. What would have been a cacophony of competing voices now appears as a low murmur, like the celestial hum of a Gregorian chant against which the soloist stands in vivid relief.
STRAIGHT TO THE OUTSOURCE
Of course, the CDMA profile of risk and reward offered by Qualcomm and others will not suit everyone. Attempting to reduce exposure to political spikes and marketing noise, many of you will spread your wireless investments across such technologies as LMDS (local multipoint distribution system), MMDS (microwave multipoint distribution system), Metricom's (nasdaq: MCOM) frequency-hopping "Ricochet," wireless optics, Bluetooth, and the world's thousands of cellular and personal communications service (PCS) vendors.
On the other hand, readers can choose the one company whose capabilities play across the entire range of these technologies. That company is Wireless Facilities Inc. ( (nasdaq: WFII)) of San Diego, California. With more than 1,000 wireless professionals with previous work experience from Bell Labs to Bell South, WFI provides outsourced services covering the gamut of wireless functions.
Among Wireless Facilities' customers were all the first 12 American deployers of PCS, including both Sprint PCS and AT&T partners TeleCorp PCS (nasdaq: TLCP) and Triton PCS (nasdaq: TPCS). With overseas ventures in Mexico, Poland, India, Nigeria, Singapore, Australia, and the United Kingdom, to name but a few, WFI services the globe.
With a growing backlog of projects, WFI is situated at the heart of the wireless new world. As the company increasingly concentrates on advanced data projects, it will necessarily have to become chiefly a CDMA specialist. Anticipating this paradigmatic destiny—and appreciating its current promise—we think WFI is an up-and-comer.
George Gilder is a contributing editor at Forbes ASAP. He coauthors the monthly Gilder Technology Report with Richard Vigilante. For a newsletter subscription, call 1-800-292-4380.