Multipath compensation of a DQPSK communication link

Based on the signal space concept of orthogonality it is shown herein that an optimum signal for communication through a strong multipath environment is a signal orthogonal to time delayed versions of itself. A sub-optimum but realizable modulation technique called FSK-DQPSK is analyzed. Essentially the technique consists of stepping the DQPSK carrier frequency through a known sequence of closely spaced frequencies, so chosen as to be orthogonal to one another. The performance characteristics of FSK-DQPSK are evaluated. Specifically, the FSK-DQPSK technique will cause an increase in the transmitted spectrum's width, an increase in the receiver lock time, and a significant improvement in the receiver's performance in strong random multipath. The FSK-DQPSK modulation technique is demonstrated to be a sort of in-channel frequency diversity which can be implemented by relatively minor modifications to the basic DQPSK transmitter and receiver.