SPREAD SPECTRUM SYSTEMS RADIO COMMUNICATION BASIC INFORMATION

As radio communication systems proliferate and traffic increases, interference problems become more severe. Interference is nothing new, of course, but it has been managed reasonably successfully in the past by careful regulatory control of transmitter locations, frequencies, and power levels.

There are some exceptions to this, however. Two examples are CB radio and cordless telephones. In fact, wherever government regulation of frequency use is informal or nonexistent, interference is likely to become a serious problem.

Widespread use of such systems as cordless phones, wireless local-area networks, and wireless modems by millions of people obviously precludes the tight regulation associated with services such as broadcasting, making bothersome interference almost inevitable. One approach to the problem, used by cellular radio systems, is to employ a complex system of frequency reuse, with computers choosing the best channel at any given time.

 However, this system too implies strong central control, if not by government then by one or more service providers, each having exclusive rights to certain radio channels. That is, in fact, the current situation with respect to cellular telephony, but it can cause problems where several widely different services use the same frequency range.

The 49-MHz band, for instance, is currently used by cordless phones, baby monitors, remote controlled models, and various other users in an almost completely unregulated way. Similarly, the 2.4-GHz band is shared by wireless LANs, wireless modems, cordless phones—and even microwave ovens!

Another problem with channelized communication, even when tightly controlled, is that the number of channels is strictly limited. If all available channels are in use in a given cell of a cellular phone system, the next attempt to complete a call will be blocked, that is, the call will not go through. Service does not degrade gracefully as traffic increases; rather, it continues as normal until the traffic density reaches the limits of the system and then ceases altogether for new calls.

There is a way to reduce interference that does not require strong central control. That technique, known as spread-spectrum communication, has been used for some time in military applications where interference often consists of deliberate jamming of signals. This interference, of course, is not under the control of the communicator, nor is it subject to government regulation.

Military communication systems need to avoid unauthorized eavesdropping on confidential transmissions, a problem alleviated by the use of spread spectrum techniques. Privacy is also a concern for personal communication systems, but many current analog systems, such as cordless and cellular telephone systems, have nonexistent or very poor protection of privacy.

For these reasons, and because the availability of large-scale integrated circuits has reduced the costs involved, there has recently been a great deal of interest in the use of spread-spectrum technology in personal communication systems for both voice and data.

The basic idea in spread-spectrum systems is, as the name implies, to spread the signal over a much wider portion of the spectrum than usual. A simple audio signal that would normally occupy only a few kilohertz of spectrum can be expanded to cover many megahertz.

Thus only a small portion of the signal is likely to be masked by any interfering signal. Of course, the average power density, expressed in watts per hertz of bandwidth, is also reduced, and this often results in a signal-to-noise ratio of less than one (that is, the signal power in any given frequency range is less than the noise power in the same bandwidth).

It may seem at first glance that this would make the signal almost impossible to detect, which is true unless special techniques are used to “de-spread” the signal while at the same time spreading the energy from interfering signals. In fact, the low average power density of spread-spectrum signals is responsible for their relative immunity from both interference and eavesdropping.