Distributed massive MIMO networks are envisioned to realize cooperative multi-point transmission in next-generation wireless systems. For efficient cooperative hybrid beamforming, the cluster of access points (APs) needs to obtain precise estimates of the uplink channel to perform reliable downlink precoding. However, due to the radio frequency (RF) impairments between the transceivers at the two en-points of the wireless channel, full channel reciprocity does not hold which results in performance degradation in the cooperative hybrid beamforming (CHBF) unless a suitable reciprocity calibration mechanism is in place. We propose a two-step approach to calibrate any two hybrid nodes in the distributed MIMO system. We then present and utilize the novel concept of reciprocal tandem to propose a low-complexity approach for jointly calibrating the cluster of APs and estimating the downlink channel. Finally, we validate our calibration technique's effectiveness through numerical simulation.

翻译：分布式大规模MIMO网络被预期在未来无线系统中实现协作多点传输。为实现高效的协作混合波束成形，接入点（AP）集群需要获取精确的上行信道估计，以执行可靠的下行预编码。然而，由于无线信道两端收发器之间的射频（RF）损伤，全信道互易性无法保持，这会导致协作混合波束成形（CHBF）性能下降，除非采用合适的互易性校准机制。我们提出了一种两步法来校准分布式MIMO系统中的任意两个混合节点。进而，我们引入并利用互易串联这一新概念，提出一种低复杂度的联合校准AP集群与估计下行信道的方法。最后，通过数值仿真验证了我们校准技术的有效性。