Orthogonal beamforming for overlay mode of OFDMA-based rural broadband wireless access

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 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 to mobile users, which becomes a beamforming constraint for rural users. It is shown that the signal to interference-plus-noise ratio (SINR) can be reasonably high for rural users in overlay mode despite the beamforming constraint and the proposed overlay mode can offer broadband access to rural users.