Orthogonal beamforming for rural broadband wireless access with limited feedback

We consider an overlay mode of an orthogonal frequency division multiple access (OFDMA) based cellular system using orthogonal beamforming to provide broadband wireless access for rural users. In this overlay mode, since the signals transmitted to rural users should not interfere with mobile users of higher priority in the OFDMA-based cellular system, the beamforming vectors to rural users have to be orthogonal to the channel vectors of mobile users, which becomes a beamforming constraint for rural users. However, with limited feedback of channel state information (CSI) at a base station (BS), the orthogonal beamforming constraints may be not achieved. In such a practical case, conventional feedback schemes in which the channel direction information is uniformly quantized for a fixed codebook can be used. Unfortunately, it may not be efficient due to the orthogonality constraints. In this paper, we propose a new multiple beam selection (MBS) approach with limited feedback for rural users by taking into account the orthogonality constraints. Simulation results show that the performance improvement of the proposed approach over the conventional ones is generally about 10% when a BS is equipped with an array of 6 elements.