Subcarrier averaging based blind beamforming and equalization for multiuser OFDM systems with CFO

Recently, Chi and Chen et al. proposed a subcarrier averaging based blind beamforming framework for a multiuser orthogonal frequency division multiplexing (OFDM) system, which can work well in block fading channels and achieve full multipath diversity gain by maximal ratio combining. However, when the channel is not static in an OFDM block or the frequency synchronization is not accurate, the basic assumption of “block fading” for the above blind beamforming framework will not be valid. In this paper, the decorrelation character of multiuser OFDM system with carrier frequency offsets (CFOs) based on subcarrier averaging is firstly proved. Thereby, the subcarrier averaging based blind beamforming method can be applied to extract each path signal. In order to reduce the effects of CFOs, a blind equalizer in frequency domain is further proposed to equalize inter-carrier interference (ICI). Simulations verify the effectiveness of the proposed blind receiver structure with CFOs, even in the fast fading channels with doppler frequency shifts¹.