Performance of a carrier-recovery loop in a satellite modem

Abstract

The paper describes a carrier recovery loop for the coherent demodulation of a digital satellite modem for mobile systems. The loop is data aided, employing decision feedback. The modem transmits a band-limited differentially encoded quaternary phase-shift-keyed (DEQPSK) signal over a satellite link. Several different earth stations are assumed to have simultaneous access to a given transponder, in an arrangement of frequency-division multiple access (FDMA). Thus the DEQPSK signals that occupy the immediately adjacent frequency bands can introduce adjacent-channel interference (ACI) into the transmitted DEQPSK signal. The high-power amplifier (HPA) at the earth station transmitter introduces AM-AM and AM-PM conversion effects into the DEQPSK signal, but the satellite transponder is linear. A baseband predistorter operates on the baseband signals at the input to the modulator and assumes a prior knowledge of the HPA characteristics. A carrier-recovery loop is used at the receiver to generate the in-phase and quadrature reference carriers employed by the coherent demodulator. The performance of the system is studied by computer simulation, over a range of different conditions and with different arrangements of the modem, and the results are used to determine the preferred modem design.