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：在本文中,我们研究了无细胞(CF)超大型多投入-多流量(XL-MIMO)系统(XL-MIMO)的上链性能,这是未来无线通信的一个很有希望的技术。更具体地说,我们考虑了多基台和多用户设备(UES)的实用情景。为此,我们为任何组合方案得出了精确可实现的光谱效率表达式。值得指出的是,我们用最大比率(MR)组合得出了CF XL-MIMO的封闭式SE表达式。数字结果表明,CF XL-MIMO的SE性能与小细胞XL-MIMO相比可以大大改进。有趣的是,一个较小的天线间距导致平天线之间的相关性水平更高。最后,我们证明,增加U天线的数量可能会降低SE与MR合并的性能。