ITS name hardly trips off the tongue. But thanks to a boffin from Bell Labs in New Jersey, plus the rise of the microprocessor and some engineers in San Diego who wouldn't take no for an answer, code-division multiple access (CDMA) technology is now conquering the wireless world. Its story is a textbook example of a broader principle of technological innovations, that while a simple way of doing something is sometimes the best, a more complicated approach—provided it can be made sufficiently cheap and reliable—can be better still.

CDMA is a form of “spread-spectrum” technology which, as its name suggests, involves spreading a radio signal out over a range of frequencies—in contrast to the usual approach of transmitting on a single, well-defined frequency. The origins of CDMA go back to 1948, when Claude Shannon, a researcher at Bell Labs, published “A Mathematical Theory of Computation”, which provides the mathematical underpinning for modern computing and communications technologies. Among other things, Shannon popularised the term “bit” (short for “binary digit”), the smallest possible unit of information. He also investigated the theoretical limits of data transmission over a noisy channel.

Combining a message with an apparently random sequence of ones and zeroes, Shannon observed, would produce a spread-spectrum signal that resembled random noise. A synchronised receiver equipped with the same pseudo-random code could, however, “de-spread” the signal and reconstruct the original message. But to any other receiver, the signal would be indistinguishable from background noise. As a result, several such transmissions could co-exist in the same swathe of spectrum—though eventually a limit would be reached, when the total level of background noise caused by the multiple transmissions would be too high for reliable communication.

When Shannon outlined this idea, it was entirely theoretical. The electronic hardware required to build such a system did not exist at the time. Even as electronic components improved and fell in price, his observation did not seem to have any practical application, since there were other, simpler, ways to divide up radio spectrum between multiple users. But by the 1980s, when analogue mobile phones started to become popular, the limited spectrum available for wireless telephony began to become increasingly congested. At the same time, the cost of computing power was plunging, following the trajectory of Moore's law (which states, roughly, that the cost of a given amount of computing power halves every 18 months). Though few were aware of it, Shannon's scheme was becoming more practical by the day.

Eureka on the I-5

In the autumn of 1985, as he drove from Los Angeles to San Diego, Irwin Jacobs, a telecommunications engineer, was thinking about mobile phones. At the time, his newly founded company, Qualcomm, was evaluating the design of a satellite-based mobile-telephone system for Hughes, an aerospace firm. As with analogue cellular phones of the time, the idea was that each phone would talk to the base-station (in this case, a satellite) on its own radio frequency. But, Dr Jacobs realised, this was very inefficient. Usually during a phone call, only one person is speaking at a time, so there is no need to transmit anything back in the other direction. Giving up an unused radio channel when it was not needed would result in a significant increase in capacity. But dynamically assigning and reallocating channels, particularly with the second-long delays associated with satellite-based systems, would be impractical.

Then Dr Jacobs remembered Shannon's suggestion: several spread-spectrum signals that resemble background noise can share the same spectrum, up to a certain limit. Dr Jacobs wondered if handsets could be designed so that they stopped transmitting when the user was listening rather than talking. The result would be to lower the level of background noise—and hence the level of interference—for other users sharing the same spectrum. Because the capacity of a spread-spectrum system is limited by the interference caused by other users, lowering the amount of interference (by ceasing transmission whenever possible) increases the number of users that can be supported by the system. “That was the lightbulb,” recalls Dr Jacobs.

Another Qualcomm engineer, Klein Gilhousen, realised that the spread-spectrum approach would have another benefit. In cellular networks, adjacent cells cannot use the same radio frequencies to communicate with handsets—otherwise interference will result in the overlap areas. The standard way around this is to use a mosaic pattern, so that only some of the available frequencies are used in a given cell, and none of those frequencies is used in any neighbouring cell. But this reduces the capacity of the system. With a spread-spectrum approach, on the other hand, handsets and base-stations do not transmit on particular frequencies, but across a broad frequency range; and each user's transmission looks like noise to other users. This means all the spectrum can be reused in adjacent cells, providing a further boost in capacity.

Qualcomm and Hughes went on to build a simulator to test these ideas, but the satellite-telephony plan was abandoned. So, in 1988, Dr Jacobs decided to apply it to terrestrial cellular telephony instead. By then, the mobile-phone industry was in the midst of a heated argument over the best way to replace the analogue system with a second-generation (2G) digital system. It was clear that a digital system would be more efficient and would support more users, but there were two competing approaches.

Taking turns to speak

One, called “frequency-division multiple access” (FDMA), was similar to the old analogue system—in the sense that each handset within a given cell would operate on its own radio frequency. But improvements due to digital encoding would mean that three digital channels could be squeezed into each analogue channel, tripling the number of available frequencies, and hence the number of users that could be supported. The alternative, called “time-division multiple access” (TDMA), would squeeze more users on to each frequency by having their handsets take turns to transmit and receive—a principle known as “time-slicing”. In America, the industry decided to adopt the TDMA approach, and a similar system, called GSM, was adopted as a Europe-wide 2G standard.

Roberto Padovani, Qualcomm's chief technology officer, explains the difference between these approaches by analogy with a dinner-party conversation. FDMA is akin to a party at which everybody talks simultaneously, but each pair of speakers converses at a different musical pitch, from booming bass to piping treble. A rota system in which party-goers took turns to speak would be like TDMA. And everybody talking at once, only in different languages (so that other conversations are rendered incomprehensible), would be equivalent to the spread-spectrum approach, by now dubbed CDMA.

Wavering sceptics

Dr Jacobs started talking to American operators in 1989, but they were sceptical. For a start, compared with TDMA or FDMA, CDMA was extremely complicated, which would make handsets much more expensive. And the industry had just emerged from a bruising battle between FDMA and TDMA.

Getting CDMA to work meant solving a number of problems. First, there was the difficulty of synchronising the pseudo-random codes used to spread the signal out at the transmitter, and de-spread it at the receiver. These codes change a million times a second. And the system will only work if the transmitter and receiver are perfectly synchronised.

The Qualcomm team started by synchronising the base-stations using the atomic clocks of the global-positioning system (GPS) satellite network. But that left the problem of synchronising handsets with base-stations. This is done, essentially, by trial and error. The radio signal (which resembles noise) is plucked from the air by the receiver, and the appropriate pseudo-random code is then applied to de-spread a chunk of the signal. Most of the time, the result is still noise. The code is then re-applied with a small time shift, and is repeatedly shifted forwards and backwards in time until an intelligible signal emerges.

What seems impossibly complex today may well seem simple tomorrow—thanks to the relentless advance of Moore's law.

This technique can also be extended to provide better coverage inside buildings and built-up areas, where reflections of the radio signal can cause interference. The time-shifting trick used to de-spread and extract the signal can also be used to identify delayed reflections of the signal, which can then be combined with the original using something called a “rake receiver”. Again, this involves lots of clever software in the handset.

A second hurdle was the so-called “near-far” problem. The basic principle—that all transmissions should look like background noise to all but the intended receiver—works only if all transmissions are of equal intensity. This means that a handset near a CDMA base-station has to transmit at a lower power than a handset far away, so that both signals are of equal strength at the base-station receiver. Otherwise, the nearby handset will drown out the more distant handset's signal.

To prevent this, CDMA handsets adjust their power output 800 times per second in response to signals from nearby base-stations. If a handset's signal becomes too weak, the base-station asks it to increase the transmission power; if it is too strong, the handset reduces the power. The result is that the handset always transmits at the lowest possible power level—which has the additional benefit of extending battery life. Once again, the near-far problem is solvable, but at the cost of making the handset a good deal more complex.

In June 1989, Dr Jacobs presented his ideas to a meeting of the Cellular Telephony Industry Association, an American industry body, to see if anybody could find a hole in his scheme. Despite widespread scepticism, nobody did. So the Qualcomm team decided to build a demonstration system, to be unveiled in November 1989.

Two base-stations were set up in San Diego, and a mobile unit, equivalent to a handset, was installed in the back of a van. After a few speeches, the idea was to test the system by driving around in the van and making phone calls. But just as he finished his speech, Dr Jacobs noticed a technician frantically signalling him to continue talking. Something had gone wrong. He started talking again, and over an hour later he was given another signal that all was well. (It turned out that the GPS receiver in one of the base-stations was faulty, making it necessary to wait for it to re-synchronise.) The demonstration then proceeded smoothly.

Even so, there was still much scepticism. Would a system that worked in San Diego work in the concrete canyons of Manhattan? Would it really be possible to pack all the technology required to support CDMA into a small handset? The answer was yes in both cases. By November 1991, Qualcomm was demonstrating its technology with base-stations and handsets ready for large-scale production, and had begun the process of licensing its technology to equipment manufacturers. By then, several operators were interested in the system, though it took until 1993 for CDMA to be approved as an industry standard. The first CDMA network was switched on in Hong Kong in 1995, closely followed by two more networks in South Korea, and two in America. European regulations, however, require operators to use GSM technology, to ensure compatibility and roaming between countries.

But CDMA's technical superiority meant that it was the logical technology on which to base third-generation (3G) wireless telephony. The result was another bruising industry battle. Rather than adopt Qualcomm's CDMA technology, European and Japanese equipment makers devised their own version of CDMA, called W-CDMA, which is more compatible with their existing GSM network infrastructure. Qualcomm, meanwhile, devised a 3G version of CDMA, called CDMA2000. There are now CDMA2000 networks operating in the United States, South Korea and Japan.

W-CDMA has taken longer to get out of the starting blocks for a number of reasons. It is a new, untested standard, and getting equipment from different vendors to work together has proved harder than expected. But W-CDMA networks are gradually being launched across Europe. To complicate the picture further, China has developed its own 3G standard, called TD-S CDMA. As its name suggests, it, too, is a version of CDMA.

Room for improvement

In one form or another, then, CDMA is set to become the dominant technology for mobile telephony around the world, as 2G networks are upgraded to 3G. But there is still room for improvement. Dr Jacobs believes that versions of CDMA, optimised for high-speed data transmission, will ultimately combine the benefits of high-speed but short-range WiFi connections, widely used these days on laptop computers, with the universal coverage of a cellular network. Already, several such networks, based on a CDMA technology called EV-DO, are being built in South Korea and America. While WiFi makes sense within homes, offices and universities, says Dr Jacobs, he is not convinced that current efforts to build commercial WiFi hotspots will succeed.

Suggesting that CDMA will triumph over WiFi is daring enough. But what of that other great wireless hope, “ultra-wideband” (UWB) transmission, which is the focus of much interest from venture capitalists and start-ups alike? Dr Jacobs believes UWB is unsuitable for use in wide-area networks and will mainly be used for short-range data transfer—say, between video cameras and personal computers. When it comes to telephony, he believes CDMA will be the standard for years to come. “For voice, I don't see much competition,” he says.

Perhaps the greatest lesson from the story of CDMA is that what seems impossibly complex today may well seem simple tomorrow—thanks to the relentless advance of Moore's law. Without it, Dr Jacobs admits, CDMA handsets would have been too large, and the base-stations would have been too expensive. “Our argument was that Moore's law will take care of cost, size and power for us,” he says. As Qualcomm has packed more features into the CDMA chips it sells to handset-makers, the latest CDMA2000 handsets have come to support colour screens, picture messaging, video clips and GPS positioning, The next step will be to build CDMA chips that can handle multiple flavours of CDMA, plus GSM, to allow truly global roaming with a single handset. It seems a daunting task now. But thanks to Moore's law, it will seem only half as daunting in 18 months.