Experimental Study of Advanced MU-MIMO Scheme with Antenna Calibration for the Evolving LTE TDD System

As the 3GPP long term evolution (LTE) system actively evolves, advanced multi-user multiple-input multiple-output (MU-MIMO) schemes have attracted increasing interest in the engineering field. We have proposed a QR decomposition (QRD) based MU-MIMO scheme for the evolving LTE time division duplex (TDD) system to achieve high spectrum efficiency, assuming ideal channel reciprocity. In order to investigate the promising performance of TDD MU-MIMO in practice, we implement the proposed QRD-based TDD MU-MIMO scheme together with an actual over-the-air (OTA) antenna calibration scheme into our FPGA-based hardware testbed for the evolving LTE TDD system to examine its performance under the constraint of practical channel reciprocity. Measurement results show that channel reciprocity indeed could be achieved with the OTA antenna calibration and the QRD-based MU-MIMO scheme can effectively make use of this channel reciprocity to achieve high spectrum efficiency data transmission for both downlink and uplink. Moreover, the influence of antenna calibration on the TDD MU-MIMO performance is further investigated and several considerations related to antenna calibration are addressed.