Field Trial of Downlink CoMP with OTA Reciprocity Calibration for TD-LTE-Advanced System

Coordinated Multi Point (CoMP) technology has been considered as a promising means to mitigate inter-cell interference and has great potential to significantly improve the system capacity as well as the spectral efficiency for cell-edge users. TDD system like TD-LTE-Advanced is especially suitable for downlink CoMP operation because accurate channel state information (CSI) can be obtained from uplink measurement with greatly reduced system overhead by assuming channel reciprocity. However the reciprocity error due to Tx/Rx RF mismatch will severely degrade CoMP performance in practical TDD system and how to calibrate the remote RF units across different base stations during CoMP transmission has been a challenge in real system implementation. This paper validates the effectiveness of Over-the-air (OTA) reciprocity calibration method for TDD CoMP by a real-time experimental system. Extensive field measurements demonstrate that by using OTA calibration, the channel reciprocity can be well leveraged by TDD CoMP transmission and significant spectral efficiency gain can be obtained from coherent joint transmission based downlink CoMP over the traditional single-cell transmission