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17958 Hughes, David  
 Scientific American  April 1998
 Reproduced with permission. Copyright April 1998  Scientific American Inc. All rights reserved

Dicing information into digital bundles and transmitting them at low power over different frequencies can enable millions of people to send and receive simultaneously. 
 by David R. Hughes and Dewayne Hendricks

Conventional wisdom holds that radio airspace is a valuable -- and limited -- resource that has to be rationed, like water in a desert. That mind-set comes from traditional transmitters and receivers, whose operation must be restricted to narrow, dedicated slices of the electro magnetic spectrum to minimize interference. Thus, governments have parceled out and licensed radio channels like real estate. In the U.S., the Federal Communications Commission (FCC) has sometimes used a cash-bidding process to allocate precious frequency bands for a variety of purposes, including commercial television and radio broadcasts; military, marine and police transmissions; taxi dispatchers; CB communications; and ham radio operators and cell phone consumers. 

Recent advances in digital communications, though, have opened the door to an entirely new model. Transmitters can now deploy so-called spread spectrum techniques to share channels without running afoul of one another. Information can be diced into tiny electronic bundles of 1s and Os and then transmitted over radio waves, with each packet sent over different channels, or frequencies, at low power. New studies have shown that, theoretically, millions of radio transmitters within the same metropolitan area can successfully operate in the same frequency- band while transferring hundreds of megabits of data per second. 

Such shared use of the spectrum challenges customary practices. In the past, when allocating narrow frequency bands for exclusive commercial purposes, the government has granted licenses to firms, such as those offering cell phone and personal communications services (PCS); Those licensees charge consumers for services, just as conventional telephone companies bill their customers. In the new economic model, the middleman becomes unnecessary: consumers can communicate with one another directly and at no charge, even when they are kilometers apart and when myriad other people are using the same radio channels. This fundamental change has led to revisions in the regulatory policies of the U.S. and foreign governments, which have now designated certain frequency bands for free, unlicensed use of spread-spectrum radios by the public.

Enough Spectrum for Everyone

What is the basis of this technology revolution? Traditional radios work by broadcasting information over a single, narrow channel with high power. By operating in as skinny a sliver of the electromagnetic spectrum as possible, a transmitter thus makes room for other devices to operate in neighboring frequencies without interference. But radio engineers now know that the opposite way of transmitting data --  by smearing, or spreading, information across a wider chunk of the spectrum at low power -- is more efficient. 

The concept is counterintuitive: instead of carving a pie into a finite number of small slices, the technology allows a larger sharing of the whole pie by permitting an all but unlimited number of individuals to take barely noticeable bites.  Although spread-spectrum radios use more bandwidth than necessary, by doing so the devices avoid interference because the transmissions are at such minimal power, with only spurts of data at any one frequency. In fact, the emitted signals are so weak that they might be almost imperceptible above background noise. This feature results in an added benefit of spread spectrum: other radios have great difficulty eavesdropping on the transmission. In practice, only the intended receiver may even know that a transmission is taking place. 

The covert nature of spread spectrum was initially its main attraction. During World War II, the U.S. military became interested in an intriguing device that actress Hedy Lamarr had co-patented, The concept was simple enough -- instead of broadcasting information over a single channel, where the enemy might stumble upon the transmission, the device switched channels continually, broadcasting a little bit of information here and a little bit there, in accordance with a secret code known only to the transmitter and the intended receiver. This repeated hopping of frequencies would make it extremely difficult for the enemy to pluck the entire transmission from the surrounding noise. But Lamarr's device was deemed impractical because it relied on an unwieldy mechanical contraption to perform the frequency hopping.

Subsequent advances in electronic circuitry, however, have made spread spectrum feasible. Semiconductor chips, crammed with thousands of transistors, can broadcast digitized packets of data in a seemingly random pattern over many channels. The receiver, designed to hear the signals in accordance with the precise and proprietary sequence of the sending radio, is able to pull the fragmented information in the right order from the different frequencies. In addition, when the receiver encounters missing or corrupted packets, it can signal the transmitting radio to resend those packets.  Also, a technique called forward error correction can be used to improve the chances that the data are received correctly the first time. 

Electronic technologies have enabled another method of spectrum spreading: direct sequence, in which the transmitted information is mixed with a coded signal that, to an outside listener, sounds like noise. In this alternative to frequency hopping, each bit of data is sent at several different frequencies simultaneously, with both the transmitter and receiver synchronized, of course, to the same coded sequence. 

More recently, further advances in chip technology have produced digital signal processors that can crunch data at high speed, use little power, and are relatively inexpensive. The improved hardware allows more sophisticated spread-spectrum techniques, including hybrid ones that leverage the best features of frequency hopping and direct sequence, as well as other ways to code data. The new methods are particularly resistant to jamming, noise and multipath -- a frequency-dependent effect in which a signal reflects off buildings, the earth and different atmospheric layers, introducing delays in the transmission that can confuse the receiver. 

In 1985 the FCC finally permitted the unlicensed use of spread-spectrum radios by the public, albeit with several restrictions. The radios must operate under FCC regulations in what are called the unlicensed industrial, scientific, medical (ISM) bands.  More important, the radios are forbidden to operate at greater than one watt, and the transmissions must be spread a minimum amount across the assigned spectrum. 

These restraints notwithstanding, the 1985 (and later) FCC rules have already spawned the development, manufacture and marketing of a wide range of "no license required" products. Because mass manufacturing has yet to occur, spread spectrum products for data transmission from the 60 or so current vendors carry premium price tags that have limited the technology mainly to large organizations, such as businesses, schools and libraries. Today a radio that can handle near-Ethernet traffic (10 megabits per second, suitable for high-speed computer communications} up to a distance of about 40 kilometers (25 miles) costs $11,000. Devices with lower capability -- operation at T1 speeds (1.5 megabits per second) to a range of 25 kilometers or so -- cost $1,500. For very short ranges, such as for communications within a building, wireless local-area network (LAN) cards for personal computers are priced as low as $250. 

There is every reason to believe these prices will drop as manufacturing volumes increase to meet the growing market demand for higher bandwidth and secure wireless connections from PCs to the Internet. In the future, people may, for example, routinely rely on wireless transmission to reach a central system that would then connect to a traditional network of ground-based lines.  We predict that reliable, secure data radios operating at T1 or higher speeds to a range of more than 30 kilometers will soon cost less than $500 each. 

Where is this technology headed? Transmitters and receivers are becoming fully digital. That trend, combined with the rapid development of cell-based wireless systems, will open up a wide range of services based on spread spectrum. The services will rely on intelligent networks containing "smart" transceivers and switches as basic components. Such devices would, for instance, know which of the varied spread-spectrum techniques to use in a given situation to ensure that the information will be transmitted without error. These new networks will increasingly involve a diverse mixture of links and switches, some ground-based, that are owned by different entities. 

The Internet today represents the best example of the self-regulating mechanism that will be necessary in the new radio environment. The creation of a similar, decentralized structure for the optimal sharing of the radio spectrum will require a substantial effort by a combination of telecommunications experts and entrepreneurs working with the various regulatory bodies around the world. We believe the deployment and growth of such a system is achievable through increasingly "smart" electronics, and we envision a self-governing set of protocols that are built into these intelligent devices. As advanced radios are deployed; society must tackle the crucial issue of incorporating both positive and negative incentives within the network infrastructure itself to make the best use of a shared common resource -- the radio spectrum. 

The Authors

DAVID R. HUGHES and DEWAYNE HENDRICKS recently helped to install spread-spectrum radios in Ulaanbaatar to connect eight scientific and educational institutions in that Mongolian city. Hughes is a principal investigator for the National Science Foundation Wireless Field Tests. Hendricks is CEO of Warp Speed Imagineering in Fremont, Calif.

Further Reading

SPREAD SPECTRUM COMMUNICATIONS HANDBOOK. Second revised edition. M. K. Simon, J. K. Omura, R. A. Scholtz and B. K. Levitt. McGraw-Hill, 1994.


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