Performance degradation of analog FM system due to spread spectrum overlay

The investigation of spectrum overlay of a spread spectrum system on the existing narrowband FM broadcasting system is presented. The overlaid spread spectrum (SS) system is assumed to utilize direct sequence (DS) spreading, using maximal length pseudorandom sequences. We studied the performance degradation of the analog FM system due to the interference produced by the SS signal, through laboratory subjective and objective measurements, for various types of spreading scenarios and for different carrier frequency differences (Δf). The RF protection ratios for the FM receivers are derived, along with SINAD measurements. Finally, these experimental results have been compared with the theoretical study of the FM receiver´s audio frequency SNR output due to SS interference.