Cell-free massive MIMO is one of the core technologies for future wireless networks. It is expected to bring enormous benefits, including ultra-high reliability, data throughput, energy efficiency, and uniform coverage. As a radically distributed system, the performance of cell-free massive MIMO critically relies on efficient distributed processing algorithms. In this paper, we propose a distributed expectation propagation (EP) detector for cell-free massive MIMO, which consists of two modules: a nonlinear module at the central processing unit (CPU) and a linear module at each access point (AP). The turbo principle in iterative channel decoding is utilized to compute and pass the extrinsic information between the two modules. An analytical framework is provided to characterize the asymptotic performance of the proposed EP detector with a large number of antennas. Furthermore, a distributed iterative channel estimation and data detection (ICD) algorithm is developed to handle the practical setting with imperfect channel state information (CSI). Simulation results will show that the proposed method outperforms existing detectors for cell-free massive MIMO systems in terms of the bit-error rate and demonstrate that the developed theoretical analysis accurately predicts system performance. Finally, it is shown that with imperfect CSI, the proposed ICD algorithm improves the system performance significantly and enables non-orthogonal pilots to reduce the pilot overhead.

翻译：无蜂窝大规模MIMO是未来无线网络的核心技术之一，有望带来超高可靠性、数据吞吐量、能效和均匀覆盖等巨大优势。作为一种根本上的分布式系统，无蜂窝大规模MIMO的性能关键依赖于高效的分布式处理算法。本文针对无蜂窝大规模MIMO提出了一种分布式期望传播（EP）检测器，该检测器包含两个模块：位于中央处理单元（CPU）的非线性模块和位于各接入点（AP）的线性模块。利用迭代信道解码中的Turbo原理，计算并在两个模块之间传递外信息。本文提供了一个分析框架，以表征所提出的EP检测器在大规模天线配置下的渐近性能。此外，还开发了一种分布式迭代信道估计与数据检测（ICD）算法，以处理信道状态信息（CSI）不完善的实际场景。仿真结果表明，在误码率方面，所提方法优于现有无蜂窝大规模MIMO系统检测器，并验证了所提出的理论分析能准确预测系统性能。最后，研究表明，在CSI不完善情况下，所提出的ICD算法显著提升了系统性能，并允许使用非正交导频减少导频开销。