Weighted cross-layer cooperative beamforming for wireless networks

In wireless networks, implementing cooperative beamforming (CB) can enable long-range communications in an energy efficient manner. By appropriately weighting and forwarding message signals, the cooperating nodes form one or more beams to cooperatively transmit one or more message signals to the desired destinations. In this paper, a cross-layer CB framework recently proposed by the authors is revisited and optimal weight design is considered. For single-beam beamforming, closed-form optimal weights that maximize the received signal-to-noise ratio (SNR) or signal-to-interference-plus-noise ratio (SINR) under a transmit power constraint are derived. It is shown that these weights also achieve maximal spectral efficiency. For multibeam beamforming, determining the weights that maximize spectral efficiency is in general a difficult problem, and two suboptimal weight designs (co-phasing weights and nulling weights) are proposed. Co-phasing weights allow desired signals to combine coherently at destinations and require local channel state information (CSI); nulling weights completely cancel interference at destinations and require global CSI.