Downlink beamforming for mobile radio systems with frequency division duplex

The capacity of cellular mobile radio systems can be increased with smart antennas. The following two concepts can be utilized. First, smart antennas enable the concept of space division multiple access (SDMA) so that transmission from a base station to several mobiles at the same time and carrier frequency is possible. Second, cochannel interference in cellular systems can be reduced so that the frequency reuse is improved. The purpose of downlink beamforming is to transmit as much power as possible to the desired mobile and as little power as possible to any undesired mobile. In the case of systems with frequency division duplex (FDD) the fast fading processes for uplink and downlink are different. But, if the frequency separation is not too large, the uplink and downlink waves exhibit the same directional dependence. Therefore, downlink beamforming has to be carried out in an average sense based on the uplink channel measurements. The paper discuss the optimization of downlink beamforming for systems with FDD. It answers the question whether beamforming should be optimized in such a way that the transmitted power is mainly sent into the direction of the path with minimum average attenuation or whether it is better to distribute the power to all desired paths. The advantage from concentrating the power to the main path is that the interference power to other mobiles may be reduced due to a higher degree of freedom for beamforming. In the case of power distribution on all paths, the advantage from higher path diversity may be obtained