Performance Comparison between Distributed Beamforming and Clustered Beamforming

The performance of transmit beamforming (BF) systems in terms of the spatial correlation coefficient and channel gain variance (CGV) are analyzed over Rayleigh fading channels. We derive bit error rate (BER) of distributed BF (DBF) systems as a function of CGV of distributed channel links. As for clustered BF (CBF), we express its BER as the explicit function of the correlation coefficient. Based on the BER formulas, the required transmit signal-to-noise ratios (SNRs) for target BERs are derived in DBF and CBF as another performance measure. Using these expressions, the performance of DBF is compared with the spatially correlated CBF systems on CGV and correlation coefficient plane. Analytical results enable us to verify the fact that if one of the distributed antennas has significantly larger CGV than the others, it is better to employ CBF rather than DBF and vice versa. As the correlation among the CBF antennas increases, the area where DBF performs better increases. Via extensive performance evaluations for various cases, we quantitatively analyze the sensitivity of the performance gap between CBF and DBF according to the CGV ratio of DBF and the spatial correlation of CBF. The overall performance trends of CBF and DBF are almost the same irrespective performance measures, i.e., BER or required transmit SNRs. It is remarkable that the BER gap is almost invariant to SNRs and the required SNR gap is almost invariant to the target BER. We perform Monte Carlo simulations to verify the analytical results.