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The authors 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.
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