Reciprocity-based time-division duplex (TDD) Massive MIMO (multiple-input multiple-output) systems utilize channel estimates obtained in the uplink to perform precoding in the downlink. However, this method has been criticized of breaking down, in the sense that the channel estimates are not good enough to spatially separate multiple user terminals, at low uplink reference signal signal-to-noise ratios, due to insufficient channel estimation quality. Instead, codebook-based downlink precoding has been advocated for as an alternative solution in order to bypass this problem. We analyze this problem by considering a "grid-of-beams world" with a finite number of possible downlink channel realizations. Assuming that the terminal accurately can detect the downlink channel, we show that in the case where reciprocity holds, carefully designing a mapping between the downlink channel and the uplink reference signals will perform better than both the conventional TDD Massive MIMO and frequency-division duplex (FDD) Massive MIMO approach. We derive elegant metrics for designing this mapping, and further, we propose algorithms that find good sequence mappings.
翻译:对等基于时间的基于时间的多输出(TDD) MIMO(多重投入多输出)系统利用在上行获得的频道估计进行下行编码前的预编码,但这一方法被批评为破碎,因为由于频道估计质量不足,频道估计不足以在空间上分解多个用户终端,使用低上端链接信号信号到噪音比率,因此频道估计质量较低。相反,提倡以基于代码的下行连接预编码作为替代解决办法,以绕过这一问题。我们通过考虑“光束电网世界”来分析这一问题,并有一定数量的可能的下行连接频道实现。假设终端能够准确地探测下行连接通道,我们表明,在对等性维持的情况下,仔细设计下行连接频道和上行连接信号之间的绘图将比传统的TDD Massive MIMO和频谱分解混合MIMIMU(FD) Massive MIMIMO方法都好。我们为设计这一绘图提供了优等量的计量标准,并进一步提出找到良好序列制图法。