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John R. Juroshek

Abstract: The possibility of overlaying a spread-spectrum system into frequency bands containing conventional FM land-mobile systems is examined. Overlaying here is interpreted as meaning the unrestricted operation of spread-spectrum and FM mobiles throughout the same service area and on the same frequency. The report assumes conventional spread-spectrum and FM systems where a single base serves a large urban area. The small cell narrow coverage concept is not discussed. A theoretical compatibility study is described that concludes that significant interference would result to existing FM systems. This conclusion assumes spread-spectrum transmitter powers~comparable to existing FM systems. The report considers land-mobile operating frequencies of 150 and 900 MHz. The study also examines the reverse problem of interference from FM to spread spectrum. Curves are prepared showing separation requirement for various channel multipath conditions. A computer simulation program is also described· that simulates the operation of a spread-spectrum system in a multiple FM interferer environment. The conclusions are that an overlayed spread spectrum system also would receive significant interference. The report also describes a frequency hopping, spread-spectrum system that is programmed to miss those FM channels in use at a given locality. The advantages obtained with this technique are briefly discussed.

Keywords: multipath; interference; land mobile radio; spread spectrum; frequency modulation; propagation

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