In this paper, we investigate a cell-free massive MIMO system with both access points (APs) and user equipments (UEs) equipped with multiple antennas over jointly-correlated Rayleigh fading channels. We study four uplink implementations, from fully centralized processing to fully distributed processing, and derive their achievable spectral efficiency (SE) expressions with minimum mean-squared error successive interference cancellation (MMSE-SIC) detectors and arbitrary combining schemes. Furthermore, the global and local MMSE combining schemes are derived based on full and local channel state information (CSI) obtained under pilot contamination, which can maximize the achievable SE for the fully centralized and distributed implementation, respectively. We study a two-layer decoding implementation with an arbitrary combining scheme in the first layer and optimal large-scale fading decoding (LSFD) in the second layer. Besides, we compute novel closed-form SE expressions for the two-layer decoding implementation with maximum ratio (MR) combining. In the numerical results, we compare the SE performance for different implementation levels, combining schemes, and channel models. It is important to note that increasing the number of antennas per UE may degrade the SE performance.

中文翻译：

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联合相关瑞利衰落信道上多天线用户无小区大规模 MIMO 的上行链路性能


在本文中，我们研究了一种无小区大规模 MIMO 系统，该系统的接入点 (AP) 和用户设备 (UE) 在联合相关的瑞利衰落信道上配备了多个天线。我们研究了从完全集中式处理到完全分布式处理的四种上行链路实现，并利用最小均方误差连续干扰消除 (MMSE-SIC) 检测器和任意组合方案推导了它们可实现的频谱效率 (SE) 表达式。此外，全局和局部MMSE组合方案是基于在导频污染下获得的完整和局部信道状态信息（CSI）而导出的，这可以分别最大化完全集中式和分布式实现的可实现的SE。我们研究了双层解码实现，第一层采用任意组合方案，第二层采用最佳大规模衰落解码（LSFD）。此外，我们还为具有最大比率（MR）组合的两层解码实现计算了新颖的封闭式 SE 表达式。在数值结果中，我们比较了不同实施级别、组合方案和渠道模型的 SE 性能。值得注意的是，增加每个 UE 的天线数量可能会降低 SE 性能。