Frequency-selective scintillation effects and decision feedback equalization in high data-rate satellite links

Ionospheric irregularities produce signal amplitude and phase scintillations in transionospheric satellite channels. When the electron-density fluctuation is large, the channel coherence bandwidth is small, causing the scintillations to decorrelate at different frequencies within the bandwidth of a wide-band signal. Frequency-selective scintillation introduces delay spread and intersymbol interference, which can severely limit the usable capacity of the channel. This paper, largely tutorial in nature, relates adaptive equalization techniques, which have been developed previously for other dispersive channels, to the problem of wide-band satellite links operating through a highly disturbed transionospheric channel. Pertinent features of the ionospheric channel and characteristics of signal scintillations are summarized. Effects of frequency-selective scintillation on high data-rate wide-bandwidth digital communication links are described in some detail. Intersymbol interference caused by the loss of channel coherence over the signal bandwidth is shown, via simulations, to severely degrade the performance of a conventional coherent binary PSK receiver. An adaptive decision feedback equalization technique is then shown to be a powerful method of mitigating frequency-selective disturbances in high data-rate satellite links.