Cell-free (CF) multiple-input multiple-output (MIMO) systems generally employ linear precoding techniques to mitigate the effects of multiuser interference. However, the power loss, efficiency, and precoding accuracy of linear precoders are usually improved by replacing them with nonlinear precoders that employ perturbation and modulo operation. In this work, we propose nonlinear user-centric precoders for CF MIMO, wherein different clusters of access points (APs) serve different users in CF multiple-antenna networks. Each cluster of APs is selected based on large-scale fading coefficients. The clustering procedure results in a sparse nonlinear precoder. We further devise a reduced-dimension nonlinear precoder, where clusters of users are created to reduce the complexity of the nonlinear precoder, the amount of required signaling, and the number of users. Numerical experiments show that the proposed nonlinear techniques for CF systems lead to an enhanced performance when compared to their linear counterparts.

翻译：无蜂窝（CF）多输入多输出（MIMO）系统通常采用线性预编码技术来抑制多用户干扰。然而，通过引入采用扰动与模运算的非线性预编码器替代线性方案，可有效改善线性预编码器的功率损耗、效率及预编码精度。本文针对CF MIMO系统提出非线性的用户中心化预编码器，其中不同接入点（AP）簇服务于CF多天线网络中的不同用户。每个AP簇基于大尺度衰落系数进行选择，该聚类过程可生成稀疏非线性预编码器。我们进一步设计了降维非线性预编码器，通过创建用户簇来降低非线性预编码器的复杂度、减少所需信令开销及用户数量。数值实验表明，与线性方案相比，本文提出的CF系统非线性技术能够实现更优的性能表现。