One of the key ideas for reducing downlink channel acquisition overhead for FDD massive MIMO systems is to exploit a combination of two assumptions: (i) the dimension of channel models in propagation domain may be much smaller than the next-generation base-station array sizes (e.g., 64 or more antennas), and (ii) uplink and downlink channels may share the same low-dimensional propagation domain. Our channel measurements demonstrate that the two assumptions may not always hold, thereby impacting the predicted performance of methods that rely on the above assumptions. In this paper, we analyze the error in modeling the downlink channel using uplink measurements, caused by the mismatch from the above two assumptions. We investigate how modeling error varies with base-station array size and provide both numerical and experimental results. We observe that modeling error increases with the number of base-station antennas, and channels with larger angular spreads have larger modeling error. Utilizing our modeling error analysis, we then investigate the resulting beamforming performance rate loss. Accordingly, we observe that the rate loss increases with the number of base-station antennas, and channels with larger angular spreads suffer from higher rate loss.

减少 FDD 大规模 MIMO 系统下行链路信道获取开销的关键思想之一是利用两个假设的组合：(i) 传播域中信道模型的维度可能远小于下一代基站阵列大小（例如，64 个或更多天线），以及 (ii) 上行链路和下行链路信道可以共享相同的低维传播域。我们的信道测量表明，这两个假设可能并不总是成立，从而影响依赖于上述假设的方法的预测性能。在本文中，我们分析了使用上行链路测量对下行链路信道建模的误差，该误差是由上述两个假设的不匹配引起的。我们研究了建模误差如何随基站阵列大小而变化，并提供了数值和实验结果。我们观察到建模误差随着基站天线数量的增加而增加，具有较大角展的信道具有较大的建模误差。利用我们的建模误差分析，我们随后研究了由此产生的波束成形性能速率损失。因此，我们观察到速率损失随着基站天线数量的增加而增加，并且具有较大角展的信道遭受较高的速率损失。