In this paper, we investigate the uplink performance of cell-free (CF) extremely large-scale multiple-input-multipleoutput (XL-MIMO) systems, which is a promising technique for future wireless communications. More specifically, we consider the practical scenario with multiple base stations (BSs) and multiple user equipments (UEs). To this end, we derive exact achievable spectral efficiency (SE) expressions for any combining scheme. It is worth noting that we derive the closed-form SE expressions for the CF XL-MIMO with maximum ratio (MR) combining. Numerical results show that the SE performance of the CF XL-MIMO can be hugely improved compared with the small-cell XL-MIMO. It is interesting that a smaller antenna spacing leads to a higher correlation level among patch antennas. Finally, we prove that increasing the number of UE antennas may decrease the SE performance with MR combining.

翻译：本文研究了无蜂窝（CF）超大规模多输入多输出（XL-MIMO）系统的上行性能，该系统是未来无线通信领域的一项有前景的技术。具体而言，我们考虑了多基站（BS）和多用户设备（UE）的实际场景。为此，我们推导了任意合并方案下精确的可达频谱效率（SE）表达式。值得指出的是，我们还推导了采用最大比（MR）合并时CF XL-MIMO的闭式SE表达式。数值结果表明，与小型蜂窝XL-MIMO相比，CF XL-MIMO的SE性能可得到显著提升。有趣的是，较小的天线间距会导致贴片天线间更高的相关性水平。最后，我们证明：在MR合并下，增加UE天线数量可能会降低SE性能。