Pre-distortion techniques and bandwidth efficient modulation for military satellite communications

In bandwidth limited satellite channels, higher order modulations can be used to increase data throughput (as measured by bit/s/Hz). This paper compares two modulations with 4 bit/s/Hz efficiency, 16-QAM and 16-amplitude phase shift keying (16-APSK). It is shown that 16-APSK offers better performance in nonlinear satellite channels with adjacent channel interference (ACI). The nonlinearity of the channel is due to the high power amplifier (HPA) at the transmitter. The nonlinearity can be reduced by using larger output back-offs (OBO) at the transmit HPA. However, it is desirable to keep the OBO as low as possible in order to lower the costs associated with the HPA. Pre-distorting the transmitted signal suitably can lower the distortion due to the HPA, without a large OBO. When the HPA characteristics are completely known, the transmitter can pre-distort the signal based on the instantaneous transmitted power. This is called open-loop predistortion. Typically, the amplifier characteristics are usually not completely known. The existing methods in the technical literature estimate the pre-distortion required by assuming that the transmitter has access to the HPA output. However, in military satellite communications such a feedback path is also typically not present, e.g., onboard ships. This paper develops a closed-loop pre-distortion method when the transmitter cannot access the output of the HPA and the link is full-duplex. Here, the receiver estimates the pre-distortion required and communicates it to the transmitter. The performance of the closed-loop predistortion technique is characterized by simulations.