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