Abstract: An improved reliability microwave communications system combining discrete state modulation with diversity transmission and reception. Signals are transmitted and received in both horizontal and vertical antenna polarization using QPSK modulation techniques in the preferred embodiment. At the receiving terminal the received signal with the best quality as determined by predetermined receiver parameters is selected as the output data stream. Switching between received signals is accomplished using a hitless switch such that no data is lost when switching between received data streams. Redundant switching and transmitting circuitry may also be provided to further increase the reliability of the system.

Abstract: A digital data communication system employing quaternary phase-shift keying for transmission by a microwave radio link. The digital modulation of the quadrature channel is delayed from the in-phase channel by one-half the time duration of a bit so that the transmitted phase does not experience a quantum shift in excess of 90.degree.. This permits the use of an injection locked oscillator as an amplifier of the microwave transmission.