Performance of a Digital Beamforming Concept in a GSM1800 Downlink: A Simulation Study

A smart antenna concept was studied in the context of a GSM1800 downlink. In practice, it is not possible to estimate the actual downlink channel when using frequency division duplexing (FDD). Therefore, the current approach is based on uplink direction-of-arrival (DoA) estimation. The downlink beam is then directed to the DoA obtained from the uplink. The purpose was to study the dependence of downlink performance on the angular and delay distribution of the desired signal. The downlink beamforming was studied in this work using simulations with 2D channel models for different test environments. The probability of error in DoA estimation of the desired mobile station was evaluated in conjunction with DoA averaging. In addition, downlink BER performance was examined in a noise-limited situation and compared to the results of the corresponding uplink and a conventional 2-branch GSM receiver. With a 1*8-element antenna configuration, downlink performance degradation compared to the corresponding uplink case was approximately 1.3 dB, 2.4 dB, and 0 dB in typical Urban, Suburban, and Rural environments. The losses were greater in environments with very large angular spread. It was also found that increasing accuracy in uplink DoA estimation does not necessarily correspond to improved downlink performance.