The current cellular systems are conventional cell-centric based architectures, and thus the inter-cell interference greatly affects the quality of service (QoS) of users located at the border of those service areas. For that reason, future wireless systems will rely on a cell-less deployment by using an alternative promising network infrastructure known as cell-free (CF) technology. Here, in contrast to the current networks, there are no concepts of cells or cell boundaries since it is a disruptive paradigm where a large number of geographically distributed access points (APs) simultaneously and jointly serve a much smaller number of user terminals (UTs) to fulfill the exponentially increasing number of users. Therefore, this paper aims to contribute on this field and considers the design of hybrid analog/digital multi-user equalizer for broadband millimeter wave (mmWave) CF massive multiple-input multiple-output (mMIMO) systems, using a distributed beamforming approach (processing is distributed by APs and central unit (CU)) to reduce the fronthaul requirements. First, only a low complexity analog precoder is applied to the transmitter, while a joint design of APs hybrid equalizer and CU combiner are considered by minimizing the average mean square error (MSE) between the transmitted and CU estimated signals. To achieve this joint design, a cyclic minimization method is used to iterate between APs hybrid equalizer and CU combiner, where the information is exchanged between APs and CU based on a set of error variances to improve detection performance. The simulation results show that the proposed precoder and the hybrid linear equalizer can achieve performance results very close to those of the fully digital system. Moreover, just two iterations of the proposed algorithm are already enough to match the full centralized approach performance.

中文翻译：

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用于协作毫米波无蜂窝大规模 MIMO 的分布式混合均衡


当前的蜂窝系统是传统的以小区为中心的架构，因此小区间干扰极大地影响位于这些服务区域边界的用户的服务质量(QoS)。因此，未来的无线系统将依赖于无蜂窝部署，使用另一种有前景的网络基础设施，即无蜂窝 (CF) 技术。与当前网络相比，这里不存在小区或小区边界的概念，因为这是一种颠覆性范例，其中大量地理上分布的接入点（AP）同时联合地为数量少得多的用户终端（UT）提供服务以满足呈指数级增长的用户数量。因此，本文旨在为这一领域做出贡献，并考虑使用分布式波束成形方法（处理由 AP 和中央单元 (CU) 分配，以减少前传要求。首先，仅将低复杂度模拟预编码器应用于发射机，同时通过最小化发射信号和CU估计信号之间的平均均方误差(MSE)来考虑AP混合均衡器和CU组合器的联合设计。为了实现这种联合设计，使用循环最小化方法在AP混合均衡器和CU组合器之间进行迭代，其中基于一组误差方差在AP和CU之间交换信息以提高检测性能。仿真结果表明，所提出的预编码器和混合线性均衡器可以获得非常接近全数字系统的性能结果。 此外，所提出的算法只需两次迭代就足以匹配完全集中式方法的性能。