In this paper, we investigate the uplink transmission for user-centric cell-free massive multiple-input multiple-output (MIMO) systems. The largest-large-scale-fading-based access point (AP) selection method is adopted to achieve a user-centric operation. Under this user-centric framework, we propose a novel inter-cluster interference-based (IC-IB) pilot assignment scheme to alleviate pilot contamination. Considering the local characteristics of channel estimates and statistics, we propose a location-aided distributed uplink combining scheme to balance the relationship among the spectral efficiency (SE), user equipment (UE) fairness and complexity, in which local partial minimum mean-squared error (LP-MMSE) combining is adopted for some APs, while maximum-ratio (MR) combining is adopted for the remaining APs. A corresponding AP selection scheme based on a novel proposed metric representing inter-user interference is proposed. We also propose a new fairness coefficient taking SE performance into account to indicate the UE fairness. Moreover, the performance of the proposed scheme is investigated under fractional power control and max–min fairness (MMF) power control. Simulation results demonstrate that the channel estimation accuracy of our proposed IC-IB pilot assignment scheme outperforms that of the conventional pilot assignment schemes. It is also shown that compared with the benchmark LP-MMSE combining, the proposed location-aided combining trades 13.45\(\%\) average SE loss for 26.61\(\%\) UE fairness improvement and 28.58\(\%\) complexity reduction when \(\gamma = 0.6\). And by adjusting the threshold \(\gamma\), a good trade-off between the average SE, UE fairness and computational complexity can be provided by the proposed scheme. Furthermore, the proposed scheme with fractional power control can better demonstrate the advantages of trade-off performance than MMF power control and full power transmission.

在本文中，我们研究了以用户为中心的无蜂窝大规模多输入多输出 (MIMO) 系统的上行链路传输。采用最大-大规模基于衰落的接入点(AP)选择方法，实现以用户为中心的操作。在这个以用户为中心的框架下，我们提出了一种新颖的基于集群间干扰 (IC-IB) 的导频分配方案，以减轻导频污染。考虑到信道估计和统计的局部特性，我们提出了一种位置辅助的分布式上行链路组合方案，以平衡频谱效率（SE）、用户设备（UE）的公平性和复杂性之间的关系，其中局部最小均方误差部分AP采用LP-MMSE合并，其余AP采用MR合并。提出了一种基于表示用户间干扰的新提出的度量的相应AP选择方案。我们还提出了一个新的公平系数，将 SE 性能考虑在内，以指示 UE 的公平性。此外，在分数功率控制和最大最小公平（MMF）功率控制下研究了所提出方案的性能。仿真结果表明，我们提出的 IC-IB 导频分配方案的信道估计精度优于传统的导频分配方案。还表明，与基准 LP-MMSE 组合相比，建议的位置辅助组合交易 13.45 在分数功率控制和最大最小公平（MMF）功率控制下研究了所提出方案的性能。仿真结果表明，我们提出的 IC-IB 导频分配方案的信道估计精度优于传统的导频分配方案。还表明，与基准 LP-MMSE 组合相比，建议的位置辅助组合交易 13.45 在分数功率控制和最大最小公平（MMF）功率控制下研究了所提出方案的性能。仿真结果表明，我们提出的 IC-IB 导频分配方案的信道估计精度优于传统的导频分配方案。还表明，与基准 LP-MMSE 组合相比，建议的位置辅助组合交易 13.45当\(\gamma = 0.6\)时，\ (\%\)平均 SE 损失为 26.61 \(\%\) UE 公平性提高和 28.58 \(\%\)复杂性降低。并且通过调整阈值\(\gamma\)，所提出的方案可以提供平均 SE、UE 公平性和计算复杂度之间的良好折衷。此外，与MMF功率控制和全功率传输相比，所提出的具有分数功率控制的方案可以更好地展示权衡性能的优势。