An HF modem with increased immunity against multipath

摘要

A direct-sequence spread-spectrum technique with low spreading factor is investigated for a 1200-baud modem with increased immunity against multipath propagation. Spread-spectrum techniques are used in conjunction with two DQPSK (differential quadrature phase shift keying) systems overlapping almost totally in the frequency domain. They system has increased immunity against multipath and reduced receiver complexity with not much more bandwidth than a system using a single PSK channel. The price being paid for this advantage is the loss of a few dBs on the noise performance of the system.