We study distributed algorithms for joint adaptation of precoding and combining filters in frequency division duplex (FDD) multiple-input multiple-output (MIMO) cellular systems. Our approach extends bi-directional training (BiT), designed for time division duplex (TDD) systems, to FDD systems where uplink/downlink reciprocity may not apply. An analysis of the performance loss due to different uplink-downlink frequencies in a point-to-point scenario, shows that a direct application of BiT gives mismatched precoders with substantial performance degradation. We first propose an algorithm assuming each base transceiver station (BTS) knows the uplink and downlink channels of mobiles within the cell. We then consider the scenario where neither the BTSs nor the mobiles have a priori channel state information. Our proposed approaches assume angular reciprocity characterized by angles of arrival/departure that vary predictably with frequency. Hence spatial beams corresponding to angles of arrival can be turned around to the corresponding angles of departure in the paired band. We present three methods, differing in how the angular decomposition is applied and used, namely, to reconstruct the combiner directly, or to reconstruct the received signal. Simulation results indicate that when the multipath is sufficiently sparse, most of achievable gain with channel reciprocity and TDD can be recovered.

中文翻译：

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频分双工的双向训练方法


我们研究频分双工 (FDD) 多输入多输出 (MIMO) 蜂窝系统中预编码和组合滤波器联合自适应的分布式算法。我们的方法将专为时分双工 (TDD) 系统设计的双向训练 (BiT) 扩展到上行链路/下行链路互易性可能不适用的 FDD 系统。对点对点场景中不同上下行频率造成的性能损失的分析表明，直接应用 BiT 会导致预编码器不匹配，从而导致性能大幅下降。我们首先提出一种算法，假设每个基站收发信台（BTS）都知道小区内移动设备的上行链路和下行链路信道。然后我们考虑 BTS 和移动设备都没有先验信道状态信息的情况。我们提出的方法假设角度互易，其特征在于到达/离开的角度随频率可预测地变化。因此，对应于到达角的空间波束可以转向成对频带中对应的离开角。我们提出了三种方法，不同之处在于如何应用和使用角度分解，即直接重建组合器或重建接收信号。仿真结果表明，当多径足够稀疏时，可以恢复大部分具有信道互易性和TDD的可实现增益。