Precoding techniques are key to dealing with multiuser interference in the downlink of cell-free (CF) multiple-input multiple-output systems. However, these techniques rely on accurate estimates of the channel state information at the transmitter (CSIT), which is not possible to obtain in practical systems. As a result, precoders cannot handle interference as expected and the residual interference substantially degrades the performance of the system. To address this problem, CF systems require precoders that are robust to CSIT imperfections. In this paper, we propose novel robust precoding techniques to mitigate the effects of residual multiuser interference. To this end, we include a loading term that minimizes the effects of the imperfect CSIT in the optimization objective. We further derive robust precoders that employ clusters of users and access points to reduce the computational cost and the signaling load. Numerical experiments show that the proposed robust minimum mean-square error (MMSE) precoding techniques outperform the conventional MMSE precoder for various accuracy levels of CSIT estimates.

翻译：预编码技术是处理无小区多输入多输出系统下行链路中多用户干扰的关键。然而，这些技术依赖于对发射机信道状态信息的精确估计，这在实际系统中无法实现。因此，预编码器无法按预期处理干扰，残余干扰会显著降低系统性能。为解决此问题，无小区系统需要能够抵抗CSIT不完美的鲁棒预编码器。本文提出新型鲁棒预编码技术以减轻残余多用户干扰的影响。为此，我们在优化目标中加入一个加载项，以最小化不完美CSIT的影响。我们进一步推导出采用用户与接入点集群的鲁棒预编码器，以降低计算成本与信令开销。数值实验表明，在不同CSIT估计精度水平下，所提出的鲁棒最小均方误差预编码技术均优于传统MMSE预编码器。