Cooperative beamforming can provide significant performance improvement for relaying systems with the help of the channel state information (CSI). In time-division duplexing (TDD) mode, the estimated CSI will deteriorate due to the reciprocity mismatch. In this work, we examine the impact and the calibration of the reciprocity mismatch in relaying systems. To evaluate the impact of the reciprocity mismatch for all devices, the closed-form expression of the achievable rate is first derived. Then, we analyze the performance loss caused by the reciprocity mismatch at sources, relays, and destinations respectively to show that the mismatch at relays dominates the impact. To compensate the performance loss, a two-stage calibration scheme is proposed for relays. Specifically, relays perform the intra-calibration based on circuits independently. Further, the inter-calibration based on the discrete Fourier transform (DFT) codebook is operated to improve the calibration performance by cooperation transmission, which has never been considered in previous work. Finally, we derive the achievable rate after relays perform the proposed reciprocity calibration scheme and investigate the impact of estimation errors on the system performance. Simulation results are presented to verify the analytical results and to show the performance of the proposed calibration approach.

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IEEE Transactions on Communications
Department of Systems and Computer Engineering

Nie, R. (Rongjiang), Chen, L. (Li), Zhao, N. (Nan), Chen, Y. (Yunfei), Yu, F.R, & Wei, G. (Guo). (2020). Relaying Systems with Reciprocity Mismatch: Impact Analysis and Calibration. IEEE Transactions on Communications, 68(7), 4035–4049. doi:10.1109/TCOMM.2020.2982632