This paper investigates the impact of non-ideal user equipment (UE) hardware on a cell-free (CF) massive MIMO (mMIMO) network with centralized operation under spatially correlated channels. The minimum mean-squared error (MMSE) estimator can be derived with the help of the generic non-ideal UE hardware model. It is demonstrated that even if the effective signal-to-noise ratio approaches infinity, pilot contamination and imperfect hardware can cause a non-zero estimation error floor. After that, a lower bound is determined for the ergodic uplink capacity of the centralized CF mMIMO network under non-ideal UE hardware. Moreover, the optimal receive combining vector is obtained to maximize the uplink spectral efficiency (SE). The maximum ratio (MR) and regularized zero-forcing (RZF) combining schemes are offered as alternatives in light of the computational complexity of the MMSE receiver. Comparing the RZF to the MMSE scheme under different levels of hardware impairments, our findings indicate that the RZF receiver suffers a negligible loss in total SE. For MR combining, a novel closed-form uplink achievable SE expression is obtained based on the MMSE estimator and the use-and-then-forget bounding technique. This expression gives vital insights into the achievable uplink performance with UE hardware impairments. Besides, for various hardware impairment factors, the impact of pilot sequence length on average sum SE is disclosed for different receive combining schemes. To increase the overall SE of the max-min fairness scheme, a heuristic fractional power control scheme with UE hardware impairments is developed, which can essentially avoid sacrificing the SE of other UEs while maximizing the SE of the unluckiest UE in the whole network. Finally, our theoretical performance analysis and power control algorithm are validated by simulation results, and fundamental design guidelines are provided for selecting hardware satisfying the practical UE requirements.

本文研究了非理想用户设备 (UE) 硬件对空间相关信道下集中操作的无小区 (CF) 大规模 MIMO (mMIMO) 网络的影响。可以借助通用非理想 UE 硬件模型导出最小均方误差 (MMSE) 估计器。事实证明，即使有效信噪比接近无穷大，导频污染和不完善的硬件也会导致非零估计误差基底。之后，确定非理想UE硬件下集中式CF mMIMO网络的遍历上行容量的下界。此外，获得最佳接收组合向量以最大化上行链路频谱效率（SE）。考虑到 MMSE 接收器的计算复杂性，提供最大比 (MR) 和正则化迫零 (RZF) 组合方案作为替代方案。将 RZF 与 MMSE 方案在不同级别的硬件损伤下进行比较，我们的研究结果表明，RZF 接收器的总 SE 损失可以忽略不计。对于 MR 组合，基于 MMSE 估计器和使用后忘记边界技术获得了一种新颖的封闭形式上行链路可实现 SE 表达式。该表达式为了解 UE 硬件损伤时可实现的上行链路性能提供了重要的见解。此外，对于各种硬件损伤因素，针对不同的接收组合方案公开了导频序列长度对平均总和SE的影响。为了提高最大-最小公平方案的整体SE，开发了一种具有UE硬件损伤的启发式分数功率控制方案，该方案可以本质上避免牺牲其他UE的SE，同时最大化整个网络中最不幸的UE的SE。最后，我们的理论性能分析和功率控制算法通过仿真结果得到验证，并为选择满足实际UE要求的硬件提供了基本的设计指南。