无蜂窝大规模MIMO的接入点间同步与空口校准技术

摘要：无线通信系统中多个天线单元相干协作发送（CJT）技术的性能严重依赖于发送端已知信道状态信息（CSI）。为了避免CSI反馈，通常假设系统工作在时分双工（TDD）模式。利用TDD空口上下行信道的互易性，根据上行探测信道获得下行CSI。但是，实际系统中由于节点间存在同步误差及通道的非理想因素，上下行整体信道并不互易。研究了面向无蜂窝大规模MIMO的网络侧时频同步以及空口校准技术。基于现网商用远端天线单元以及同步技术，给出了大规模组网的同步和校准方案。结合试验系统，探讨了CJT对同步和校准的性能的要求以及无蜂窝大规模MIMO实现方式和节点天线配置，给出了校准相位的预测性能。

关键字：无蜂窝大规模MIMO；相干协作发送；空口校准；节点间同步

Abstract: The performance of coherent joint transmission (CJT) techniques with multiple access points in wireless communication systems is highly dependent on known channel state information (CSI) at the transmitter. To avoid CSI feedback, it is generally assumed that systems operate in time division duplex (TDD) mode, and then the downlink CSI is obtained according to the uplink sounding signals by using the reciprocity of the uplink and downlink channels. However, due to the synchronization error between access points (AP) and the non-ideal hardware factors, the uplink and downlink channels are not reciprocal. In this paper, the time-frequency synchronization of the wireless network and over-the-air calibration technologies for cell-free massive MIMO are studied, including phase tracking and over-the-air (OTA) calibration. Based on the commercial RRU and synchronization technology, synchronization and OTA calibration schemes for large-scale network are discussed. Finally, based on the experimental system, the performance requirements of CJT for synchronization and calibration are discussed, as well as the implementation of cell-free massive MIMO and the antenna configuration of the AP, and the prediction of the phase of the calibration coefficient is studied.

Keywords: cell-free massive MIMO; coherent joint transmission; OTA calibration; inter-AP synchronization