Coordinated beamforming using semidefinite programming

In this paper, we study a coordinated beamforming (CBF) scheme for a multi-cell scenario where precoding/beamforming vectors for all coordinating base stations (BSs) are jointly designed in a manner that each BS transmits to its local users. We minimise the total transmit power across coordinating BSs while keeping the signal-to-interference-plus-noise ratio (SINR) at each user above the required level. We formulate the optimisation problem for CBF in a standard semidefinite programming form using instantaneous channel state information. Then, taking into consideration the uncertainty in the estimation of channel covariance matrix, we formulate the CBF problem based on the second order statistical properties of channel so that the robustness of the designed system parameters is guaranteed within a tolerable channel estimation error. We show that, although this robustness comes at the expense of increased overall power, a significant reduction in signaling overhead can be attained with a minor increase in total transmit power at moderately low SINRs.