Beamforming and Transmission Power Optimization

Beamforming-array signal processing is a powerful means of increasing capacity and coverage of the mobile communication networks. It supports the next generation broadband cellular system to efficiently carry populated traffic over a limited spectrum due to its co-channel interference mitigation capability. This paper presents performance comparison of three beamforming methods namely null steering, minimum variance distortionless response (MVDR) and minimum mean square error (MMSE) beamforming. It is observed that MMSE beamforming is most suitable for practical non-line-of-sight multipath environment. The work is further extended to downlink beamforming problem for multicell network which is formulated as minimization of transmitted power by all base stations (BSs) subject to SINR constraints at each mobile. The conventional per cell basis beamforming and coordinated multicell beamforming are implemented to achieve downlink transmit power optimization using downlink beamforming. Both algorithms use uplink (UL)-downlink (DL) duality concept to solve complicated DL beamforming by using dual uplink problem. Per cell basis beamforming treats intercell interferences as a part of background noise while coordinated multicell beamforming considers intercell interferences for weight vector calculation. Thus, per cell basis optimization does not lead to a joint optimal solution which is possible by coordinated multicell beamforming. The important feature of coordinated beamforming algorithm is that, it leads to distributed implementation in time division duplex (TDD) system. The distributed implementation is less complex and requires only beamforming level coordination unlike centralized implementation which requires system level coordination and thus it is more complex. Thus, in this paper per cell basis beamforming and coordinated multicell beamforming has been implemented and compared for mobile wimax multicell network. The simulation results proved that coordinated beamforming outperforms per cell basis beamforming for different values of SINR and user locations.