Performance of a carrier-recovery loop in a satellite modem

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. Adjacent-channel interference can be introduced 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 and the results are used to determine the preferred modem design.<>