High-throughput multicarrier satellite transmission using linearized traveling wave tube amplifiers

Traveling wave tube amplifiers (TWTAs), used in satellite transponders, are nonlinear devices. Often, multiple, frequency multiplexed, digitally modulated carriers are transmitted through a satellite transponder. Additionally, square-root raised cosine transmit and received filtering is frequently used to reduce bandwidth requirements, and a result is a large peak power to average power ratio for each carrier at the TWTA input. In this situation, when the TWTA is operated close to its saturation point, there is large bit error rate performance degradation due to the intermodulation products produced by the nonlinearity. Operation of the TWTA with large input and output backoffs provides more linear operation at the expense of TWTA output power. A linearizer, which may be implemented as an instantaneous nonlinearity inserted in front of the input to the TWTA, may be designed such that the cascade combination of the linearizer and the TWTA has a considerably more linear characteristic than the TWTA alone. The optimal TWTA operating point, which minimizes the performance losses both due to bit error rate degradation and TWTA output backoff, is obtained with less TWTA output backoff using the linearizer. In this paper, the performances for both the TWTA and the linearized TWTA are obtained by computer simulation, and the performance advantage of the linearized TWTA is determined for extremely bandwidth efficient, frequency multiplexed, square-root raised cosine pulse shaped QPSK modulated carriers. A modest power efficiency advantage of the linearizer is shown for this extremely bandwidth efficient, multicarrier case.