An efficient method for enhancing TDD over the air reciprocity calibration

Channel reciprocity is regarded as one of the most important advantages of TDD systems. However, the RF front-ends contribute to the channel experienced, and their TX/RX mismatches might cause reciprocity error, and thus degrade system throughput. Therefore, channel reciprocity calibration is a crucial issue. Among the existing calibration methods, Over-the-air (OTA) calibration is a promising approach because no extra hardware cost is needed. Since the OTA calibration precision highly depends on the link quality between Base Station (BS) and the calibration User Equipment (UE), how to select the most proper UE for calibration is an important issue. In this paper, efficient calibration UE selection schemes are proposed to enhance OTA calibration for both single-cell and Coordinated Multi-Point Transmission (CoMP) scenarios. System level simulation results demonstrate that the proposed scheme can well improve the system throughput. For single-cell scenario, with the proposed scheme, the system average throughput almost achieves the ideal reciprocal case. For CoMP scenario, although system throughput is more sensitive to reciprocity error, the proposed scheme can still improve the average throughput around 13%.