OPTIMUM DIVERSITY FOR THE FH/MFSK WAVE-FORM IN A SLOWLY FADING CHANNEL

The detonation of high altitude nuclear devices results in a severly disturbed ionosphere causing significant performance degradations for satellite communications links [1-6]. These disturbances range from deep fades in signal amplitude and phase to widely varying time dispersion characteristics. Once the prompt effects (i.e., less than several seconds) have subsided, it is typical to model the transmission medium as a slowly fading channel with Rayleigh signal statistics [6-8]. The actual performance of a given waveform design in a slowly fading channel is thus of great importance in the design of robust satellite communications links. Various techniques have been used in waveform design to improve performance including spread spectrum, forward error correction (FEC) coding, and interleaving. The purpose of this paper is to investigate the impact of applying chip diversity through a frequency hopped spread spectrum scheme together with FEC coding. Specifically, an expression is derived for the optimum diversity of a frequency hopped M-ary frequency shift keyed (FH/MFSK) class waveform. This expression is then used to generate bit error rate (BER) data versus input signal-to-noise ratio (SNR) for a FH/MFSK waveform with optimum diversity combined with several FEC coding techniques.