Spectrum Efficiency for Multiple Independent Spread-spectrum Land Mobile Radio Systems

The use of spread-spectrum modulation in a common type of land-mobile radio service is studied. The radio service contains many networks operated independently, similar to the business land-mobile radio service. Network independence implies (among other things) that there is no central control of a transmitter's radiated power. A congested urban environment is assumed. If a band is allocated exclusively to spread-spectrum modulation for such a service, base stations and mobiles must operate on different channels, and all base station antennas must be located within a relatively small area. In this case, the worst interference is in the mobile-to-base channel. An explicit formulation of the "far-near" problem and a powerful computer calculation both show that in such a service spread-spectrum modulation is less spectrum efficient than conventional FM modulation. In fact, the spectrum efficiency of the spread-spectrum systems is inversely proportional to the cube root of the processing gain. However, overlaying a spreadspectrum system on conventional bands shows some promise.

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