A Practical Optimization Approach for QPSK/TDMA Satellite Channel Filtering

This paper reports the results of computer and laboratory hardware simulation studies concerning optimum pulseshaping filter characteristics and configurations to be employed with conventional QPSK signaling in a nonlinear channel. An understanding of the factors and issues contributing to transmission impairments was obtained through an initial hardware simulation which was confirmed and extended with computer simulation. Based on this understanding, the critical characteristics of the channel pulseshaping transmit and receive filters were determined, promising filter candidates were implemented, and a complete system laboratory hardware simulation study was configured to evaluate the candidate filters using an INTELSAT IV satellite. This study emphasizes the performance tradeoff between the nonlinear channel-induced bit-error-rate impairment and the adjacent channel interference levels caused by HPA spectrum regeneration of filtered QPSK as a function of filter configuration. The results show that a more gentle rolloff Nyquist or wider bandwidth non-Nyquist transmit filter produces similar significantly improved performance compared to a conventional sharp rolloff Nyquist transmit filter in nonlinear satellite channels. For the wider non-Nyquist filter case, employing a Nyquist filter at the demodulation also benefits performance.