Combining calibration schemes on a real-time multiuser MIMO-OFDM system with implicit feedback

Multiple-input multiple-output (MIMO) systems with implicit feedback utilize uplink channel state information (CSI) for downlink transmit beamforming, relying on over-the-air channel reciprocity. We develop real-time hardware for multiuser MIMO transmission with implicit feedback based on orthogonal frequency division multiplexing (OFDM), with field programmable gate arrays (FPGAs). This paper presents indoor experimental results by the real-time hardware, employing several calibration schemes for uplink CSI. In particular, we investigate performance of our proposed scheme, weighted-combining calibration (WCC), in which multiple calibration coefficients are calculated and then combined with weights based on channel gain, to improve the calibration accuracy. It is shown that WCC improves real-time throughput considerably, compared with equal gain-combining calibration (EGCC), a scheme which combines coefficients with equal weighting.