Mutual Interference of QPSK and Spread Spectrum Signals

A comparison of 8-ary FSK with differentially encoded binary phase shift keying [DEBPSK] spread spectrum systems was made for application to a proposed satellite communications system. The spread spectrum signals are to operate on the same satellite transponder as a bank of QPSK signals. The two candidate modulation techniques are found to be equally deteriorating to QPSK when they operate at the same total carrier power, provided that the FSK power is distributed over a sufficient number of tones. On the other hand, the QPSK signals constitute a somewhat greater interference for 8-ary FSK than for DEBPSK. The result is a slight advantage for DEBPSK in this application.