Spread Spectrum Communications Historical and Technical Overview

A s we all know, the RF spectrum is a finite and exceedingly valuable resource. The practical limits of the existing spectrum, together with the exponential need for new communications, has created an increasing demand for what little free or under-utilized space remains. In general, the policy for traditional commercial services has been to place limits on RF power and bandwidth to minimize interference and maximize the number of channels that can be assigned to a given band. For example, although the FCC continues to license a few "clear channel" AM broadcast stations, most frequencies are assigned to multiple stations. The nature of the service, local and regional geography, as well as signal propagation qualities at the operating frequency define the minimum physical spacing between stations sharing the same frequency. The inter-station distance ranges from a few hundreds of meters for low-power cellular radio nodes to hundreds or thousands of kilometers for high-powered commercial broadcast stations. Although congestion may not be obvious to the casual listener on the AM or FM broadcast bands, the current explosion in cellular telephone systems and wireless computer networks is a driving force behind the quest for more communications channels. l These and other new technologies that rely on relatively low-power, high-density environments operating at UHF and above suggest that new ways of managing signal bandwidth are critical to maximizing spectrum utilization. In general, the type of modulation and information filtering used determine the RF signal bandwidth requirements. For example, where a properly filtered voice modulated AM signal requires a bandwidth of about twice that of the modulating frequency, SSB and other techniques can be used to reduce the RF bandwidth requirements significantly. On the amateur bands, the use of SSB instead of AM has resulted in tremendous spectrum savings, as well as increased efficiency of communications. However, despite the savings of spectrum by SSB. the bands remain crowded. The introduction of the newer spread spectrum techniques can further increase efficiency of spectrum use. Paradoxically, techniques such as spread spectrum are designed to increase spectrum utilization by generating an extremely wide signal relative to the information to be transmitted. In conventional RF communications, the amplitude , frequency, or phase of an RF carrier is varied in accordance with the information to be transmitted. With the exception of those that use FM with a high modulation index, the band-width requirements of these systems are simply a …