The Effect of Selective Fading on Digital Radio

In the course of development of the DRS-8 Digital Radio System a series of digital propagation trials were conducted over a 51 km path at 8 GHz to determine the effect of multipath propagation on a 40 MHz bandwidth digital radio system. It was found that the effects of frequency selective fading result in unacceptably high system unavailability unless adaptive equalization and space diversity are employed. Specifically the results are: 1) Multipath induced outage is much higher than would be predicted from the measured flat fade margin of the equipment. For a nondiversity system, the probability of outage for a single hop in the worst fading month is approximately
1 \times 10^{-3}
. This is 700 times the objective for a long haul system and corresponds to an effective fade margin of only 27 dB. 2) The primary cause of outage is inband distortion caused by the frequency selectivity of the multipath fading process. 3) Phase adaptive space diversity combining is very effective in reducing the amount of fading. In addition it affords some increase in the effective fade margin of the radio, i.e., it reduces the severity of inband distortion for a given fade depth. Outage for the System with space diversity combining was
2.6 \times 10^{-4}
which is about 18 times the long haul objective. 4) A simple adaptive linear amplitude equalizer in conjunction with the phase adaptive space diversity combining provides an additional improvement of approximately a factor of 20, reducing the multipath outage to levels compatible with long haul availability objectives. 5) This same equalizer when applied to a non-diversity channel provides an improvement of about 2. Thus the preconditioning effect of phase adaptive space diversity combining is necessary to reduce distortions sufficiently that they can be dealt with by a relatively simple adaptive equalizer. These results apply to wideband digital radio which operates over long hops using a two dimensional modulation system such as QAM or multiphase PSK.