Abstract :
An analytical study to predict multipath fading outages in terrestrial digital radio systems is described. The method, which is quite general, makes specific use of previously reported statistics on multipath fading. It is applied, in this study, to 6- and 11-GHz systems carrying 90 Mbits/s per RF channel. For purposes of comparing alternate approaches, three different modulations are considered, namely, (i) conventional QPSK at 45 Mbits/s, using two polarizations per channel; (ii) Offset-QPSK at 45 Mbits/s using two polarizations per channel; and (iii) 8-level PSK at 90 Mbits/s, using one polarization per channel. In all cases, a fourthorder Butterworth transmit filter is assumed which enforces FCC spectral emission requirements. The channel is treated as a two-path propagation medium, the receiver is assumed to be coherent, and various practical assumptions are invoked regarding the recovered carrier phase, timing phase, receive filter response, bit error rate objectives and fade margin. Results are given in terms of expected yearly fraction of multipath outage per repeater hop vs. hop length. We find that the choice of operating band has only a minor effect on the results; that OffsetQPSK systems suffer somewhat more multipath outage than conventional QPSK systems; that 8-level systems suffer more multipath outage than 4-level systems (although this difference may be offset by cross-polarization effects in the latter); and that all the approaches considered require repeater spacings of 15 mi or less in order to meet short-haul outage objectives over average terrain.
