Intermediate relay station of a digital microwave communication system

Abstract

A microwave digital communication system PSK-modulates a microwave carrier by using a main signal and, further, frequency-modulates it by using a service channel (SC) signal for monitoring and controlling the system. An intermediate relay station employing a space diversity reception system converts a received microwave into an intermediate frequency (IF) to extract the SC signal, and converts the IF signal into a microwave to transmit, thus performing heterodyne relay. The intermediate relay station generates a phase error signal corresponding to the frequency deviation of the IF received signal, by using the IF received signal and an IF reference signal having a fixed frequency equal to the carrier frequency of the IF received signal, and extracts the phase error signal as an SC signal. The SC signal is inputted as a control signal to a VCO for generating a receiving-side local oscillation signal so that the frequency of that follows the variation of the frequency deviation of the received microwave. Thereby, the IF received signal is phase-locked by the IF reference signal, thus suppressing the frequency modulation component of the IF received signal caused by the SC signal. Because the intermediate relay station causes no increase in the frequency modulation factor while adding an SC signal, this relay station facilitates easily regeneration of the carrier performed by the receiving terminal station.