Forschiingsiristit,iit (lor Doiil,sdicw Biindcspost D-6 100 D iimis t ad t , W. Gor iriariy AbstractThe pa.per describes propa.ga,tion experiments pcrformed on a test link using different diversity configiirations. The results for the angle-diversity ant.cnna showed that the dispersive na,ture of the fading is ma.inly ca.used by ground reflectlions or ray bending a.t, ground-based layers. Outage calciilat.ions were performed for 16 QAM and 256 QAhl digital radio systems with different pa.rametcrs using the observed propagation cha.nne1 chara.cterist,ics. The rcsults showed tha.t propa.gation impairments can strongly be reduced by a.ngle-diversity reception a.s compared with single reception or horizonta.1 spacc diversity. Vertical spa.ce diversitay with a 1a.rge antenna separation, however, outperformed angle diversitmy, a t least, for the bea,m oricnta.tions chosen in this expcriincnt,. 24.2.1.
The paper describes propagation experiments and the performance of angle-diversity reception on two different test links. One of the test links exhibited a good path clearance with a pronounced height difference between transmitter and receiver terminals, whereas the second link ran parallel to the earth's surface with smaller path clearance given by the radius of the first Fresnel zone. The results for both test links showed that the dispersive nature of the fading is caused mainly by ground reflections or ray bending at ground-based layers. Outage calculations were performed for 16 QAM and 64 QAM digital radio systems with different parameters using the measured propagation channel responses. The results show that propagation impairments can be reduced strongly by angle-diversity reception as compared with single reception for both links. With additional adaptive equalization, outage can be avoided on both links if the flat-fade margin is large enough. I IntroductionThe transmission performance of high-capacity digital radio systems is degraded by frequency-selective fading occurring at short time intervals under certain meteorological conditions. In order to minimize or eliminate any transmission degradation angle diversity has recently gained much attention as a countermeasure against propagation effects [1]. An advantage of this technique is that only one antenna is needed and no further installation space is required as compared with conventional space diversity. Under multipath propagation conditions the effect of this technique is based on the slightly different angles-of-arrival of the indirect delayed waves and the direct wave.Owing to the different beam orientations, the wave components add up in a different way resulting consequently in different frequency-selective fading at the two diversity branches.There are two possibilities to determine the efficiency of angle-diversity reception: firstly, the transmission quality of a digital radio system with different diversity configurations is monitored simultaneously and the observed bit-error ratios for these configurations are compared. Secondly, the channel-transfer functions are measured simultaneously for the diversity 379
The height dependence of the field strength was studied on a line-bf-sight link, 43 km in length, at 9.3 GHz. At the receiving site, 14 antennas sere mounted one above the other at 2 m spacing. Height profiles with deep fades or marked enhancements of the signal caused by multipath propagation were investigated. The parameters of a propagation model with two or three partial waves were evaluated from the measured data. These parameters w'ere the amplitudes, the differences of the angles of incidence, and the phase differences. Only a small part of the events is covered by the two-path model. The three-path model proves to be much more adapted to this purpose.
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