We consider the downlink of a cell-free massive multiple-input multiple-output (MIMO) system with single-antenna access points (APs) and single-antenna users. An iterative robust minimum mean-square error (RMMSE) precoder based on generalized loading is developed to mitigate interference in the presence of imperfect channel state information (CSI). An achievable rate analysis is carried out and optimal and uniform power allocation schemes are developed based on the signal-to-interference-plus-noise ratio. An analysis of the computational costs of the proposed RMMSE and existing schemes is also presented. Numerical results show the improvement provided by the proposed RMMSE precoder against linear minimum mean-square error, zero-forcing and conjugate beamforming precoders in the presence of imperfect CSI.

中文翻译：

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无小区大规模 MIMO 系统的鲁棒 MMSE 预编码和功率分配


我们考虑具有单天线接入点（AP）和单天线用户的无小区大规模多输入多输出（MIMO）系统的下行链路。开发了一种基于广义加载的迭代鲁棒最小均方误差 (RMMSE) 预编码器，以减轻存在不完美信道状态信息 (CSI) 时的干扰。进行可实现的速率分析，并根据信号与干扰加噪声之比开发最佳且统一的功率分配方案。还对所提出的 RMMSE 和现有方案的计算成本进行了分析。数值结果表明，在存在不完美 CSI 的情况下，所提出的 RMMSE 预编码器相对于线性最小均方误差、迫零和共轭波束形成预编码器提供了改进。