Downlink beamforming for mobile communications
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This dissertation introduces modeling and prediction of the downlink (transmission from base station to mobile user) spatial signature (the vector channel). The primary application of this technique is to time-division duplex wireless communications systems. In these systems, downlink beamforming performance based on spatial signatures suffers when the mobile user is truly mobile and not in line-of-sight with the base station. In these cases, the uplink spatial signature no longer resembles the downlink spatial signature. Chapter 2 shows that the spatial signature behavior with mobile movement can be modeled as an autoregressive process. The coefficients derived from observations of the uplink spatial signatures can be used to predict the downlink spatial signatures that follow. The predictions increase the downlink received power or signal-tonoise ratio, depending on the downlink beamforming method used, by as much as viii 10 dB in comparison to beamforming without prediction. These predictions can be used instead of channel coding to double the data throughput and allow reliable communication to two co-channel users traveling as fast as 105 km/h. This dissertation also includes a chapter that considers issues necessary for the practical implementation of downlink beamforming with prediction in frequencydivision duplex systems.