Performance characteristics of QPSK signal passing nonlinear channel in digital ship earth station system

This paper presents the out‐of‐band emission characteristics and the BER performance of the QPSK signal passing a class‐C power amplifier in a ship earth station transmitter and compares them with those of the QPSK signal passing a solid‐state linear power amplifier operating near saturation. The study is made mainly by computer simulation. The results show that the BER performance degradation for the class‐C power amplifier is approximately 0.9 dB at BER of 10‐6 with appropriate filtering when adjacent‐channel interference is absent. This BER performance is almost the same as that for the linear power amplifier. However, the class‐C power amplifier generates a higher level of out‐of‐band emission over a wide frequency range than the linear power amplifier, which in an SCPC system degrades the BER performance of adjacent channels. It is quantitatively shown that larger carrier spacing should be adopted for class‐C power amplifiers than for linear power amplifiers to cope with this degradation. Finally, the feasibility of using a class‐C power amplifier for the ship earth station transmitter is discussed for the digital system employing 36‐kbps QPSK which has been developed for possible INMARSAT application. It is concluded that system operation with 50‐kHz carrier spacing is feasible, while operation with 25‐kHz carrier spacing is infeasible when class‐C transmitters are used. On the other hand, system operation with 25‐kHz carrier spacings is feasible when linear transmitters are used.