Quaternary Transmission Over Satellite Channels with Cascaded Nonlinear Elements and Adjacent Channel Interference

The performance of coherent QPSK, offset QPSK, and MSK modulation systems is investigated via computer simulation for TDMA transmission over satellite channels with cascaded nonlinear elements, up- and downlink fades, intersymbol interference, adjacent channel interference, and thermal noise. Three satellite transponder concepts are considered: TWTA transponders nominally operated at their saturation power under clear sky conditions, TWTA transponders nominally operated at input overdrive, and hard-limiting transponders. The performance of these modulation techniques in terms of bit error rate versus energy per information bit over thermal noise power density E_{b}/N_{0} has been evaluated for the three system concepts as a function of various system parameters. For transponder BT products in the range of 1.5-2.0, modem filter characteristics and waveform shaping have been carefully and nearly "optimally" selected to combat adjacent channel interference, which is the major source of impairments, especially under uplink fades. It has been found that, for certain system environments, MSK and OQPSK outperform QPSK. As a byproduct of this investigation, various means to minimize the effects of adjacent channel are proposed.