Spread spectrum goes commercial

The use of spread-spectrum techniques to achieve more efficient utilization of available frequency spectra is examined. The two main spread-spectrum techniques, direct sequence and frequency hopping, are explained. In frequency hopping, the transmitter repeatedly changes (hops) the carrier frequency from one frequency to another. Direct-sequence transmission spreads the spectrum not by periodically changing the frequency but by modulating the original (information) baseband signal with a very wide-baseband digital signal. The wideband modulating signal's amplitude changes continually between two states, high and low, arbitrarily called +1 and -1, respectively, with the sequence of highs and lows being pseudorandom. Applying spread spectrum to code-division multiaccess (CDMA) communication, in which each user is assigned an identification code (a distinct sequence of frequencies for frequency hopping or +1 s and -1 s for direct-sequence modulation) is discussed. The use of CDMA for cellular radio, where it promises a capacity of over 1000 users per cell, by the authors' calculations, is considered. The potential of spread spectrum for relieving spectrum congestion is addressed.<
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