Abstract: A phase lock loop for a digital input signal has a phase detector, a loop filter, a digital voltage controlled oscillator (VCO), an initial phase difference calculator, a center frequency difference calculator and an input buffer memory. In an initial training mode prepared in the PLL operation, an optimum initial phase and an optimum center frequency of the VCO to complete a lock-in state is searched for the input signal stored in the buffer memmory. By estimating the initial phase difference and the center frequency difference between the input signal and the VCO output with repetative kick-offs in calculators, optimum values mentioned above are obtained.

Abstract: In a satellite communication system including a plural small earth stations (3-8), a relay earth station (2) and a satellite (1), information transmission is undertaken by primary modulation of carrier wave, and transmission of operation status of each small earth station (3-8) is carried out by secondary modulation of said carrier wave. Primary modulation may be BPSK or multiphase PSK (MPSK), and secondary modulation may be PSK, ASK or FSK. Said operation status may be receive signal quality in each small earth station, and the relay earth station (2) may adjust transmit power to said small earth station such that the receive signal quality at small earth stations satisfies desired criteria irrespective of rain attenuation.

Abstract: A phase synchronization circuit for processing a received signal, the phase of which is detected by a first reference carrier from a fixed oscillator and a second reference carrier orthogonal to the first reference carrier. Two baseband signals are thus obtained and sampled with a certain period. Converters convert these two signals into digital values. Phase and amplitude values at each sample point are obtained from these digital values orthogonal to each other. The frequency and phase differences between the received signal and each of the first and second reference carriers are estimated by an optimization method by utilization of pluralities of phase and amplitude data obtained with a fixed period of time. The voltage-controlled oscillator and the phase shifter are controlled with the estimated values for generating a recovered carrier of a phase synchronized with the received signal.

Abstract: A satellite communication among a plurality of small earth stations (very small aperture terminal;VSAT) through a satellite is performed by using a relay earth station which has higher power than a small earth station. A signal goes from an originating small earth station to a destination small earth station, through a satellite, a relay earth station, and a satellite by using two hops propagation. Communication is carried out in digital form. The relay earth station regenerates digital signals from small earth stations. In the process of the regeneration of the digital signals, the error correction technique is applied to reduce the error rate, and a frequency compensation technique is also employed. Thus, a small simple earth station can provide high quality of communication with high C/N. A reference clock signal for operating a small earth station is derived from a reception signal from a satellite.

Abstract: A phase slip detector and systems employing said detector comprises a frequency detector for detecting an instantaneous value of received carrier frequency and an integrator for integrating the output of said detector, and determines an occurrence of phase slip by detecting the output level of the integrator larger than predetermind threshold level. Application of the phase slip detector to a carrier recovery system makes it possible to obtain a correct recovered carrier without the effect of phase slip, and application of the detector to a demodulated data compensation system makes it possible to obtain received or demodulated data compressed in generation of burst error and free from the effect of the phase slip.