Space-Diversity Effects on Line-of-Sight Propagation Paths,

This paper discusses the effects of space-diversity reception on the performance of analog and digital radio-relay systems. It reviews the existing results valid for analog systems and describes some single-frequency measurements of the dependence of the improvement factor on the vertical antenna separation. It was found that the prediction formula proposed by Hosoya gives a good approximation to the measured dependence. The effect of the frequency-selectivity of the fading was taken into account for a performance estimation of digital radio links. Fading events measured in a 43-MHz bandwidth at 9.5 GHz with 3 antennas spaced 60 lambda apart were described with an effective two-path model. Assuming maximum-power combining of the main and diversity branches the performance of 140-Mbits M-QAM systems M 16,64,256 were evaluated for a roll-off factor of alpha 0.5. results were obtained for idealized systems no timing- and carrier phase jitter, perfect Nyquist filtering without channel distortions optionally equipped with an adaptive transversal equalizer with 5 taps adjusted by the zero-forcing algorithm. A substantial improvement was obtained if both countermeasures were applied as compared to the case where either diversity or equalization was used. Furthermore, the improvement factor was reduced if the modulation scheme was changed from 16 QAM to 64 and 256 QAM keeping the transmitted power constant. This can be explained by the reduced flat fade margin and increased sensitivity of the higher-level modulation schemes to frequency selective fading.