Multipath Fading Effects on Digital Radio

An experimental study of multipath fading effects on an 8-PSK 6 GHz digital radio system was conducted on a 26.4 mi test link near Atlanta, Georgia. Results were obtained for a non-diversity arrangement and a space-diversity arrangement employing a cophasing combiner operating at 70 MHz and fed by two antennas separated by 30 ft. Multipath fading was found to have a significant impact on bit error rate performance, i.e., the digital radio system was fragile by comparison to conventional analog FM radio. Modest in-band linear amplitude dispersion, 0.2 dB/MHz for example, was sufficient to cause aBERgeq10^{-3}. Such dispersion occurred at (average power) fade depths as shallow as 20 dB. However, average fade depth was a poor indicator of BER performance. The measured hop missed short haul outage objectives by an order of magnitude without space-diversity, and was close to acceptable when diversity was activated.