One of the most relevant challenges in future 6G wireless networks is how to support a massive spatial multiplexing of a large number of user terminals. Recently, extremely large antenna arrays (ELAAs), also referred to as extra-large MIMO (XL-MIMO), have emerged as an potential enabler of this type of spatially multiplexed transmission. These massive configurations substantially increase the number of available spatial degrees of freedom (transmission modes) while also enabling to spatially focus the transmitted energy into a very small region, thanks to the properties of near-field propagation and the large number of transmitters. This work explores whether multiplexing of multiple orthogonal polarizations can enhance the system performance in the near-field. We concentrate on a simple scenario consisting of a Uniform Linear Array (ULA) and a single antenna element user equipment (UE). We demonstrate that the number of spatial degrees of freedom can be as large as 3 in the near-field of a Line of Sight (LoS) channel when both transmitter and receiver employ three orthogonal linear polarizations. In the far-field, however, the maximum number of spatial degrees of freedom tends to be only 2, due to the fact that the equivalent MIMO channel becomes rank deficient. We provide an analytical approximation to the achievable rate, which allows us to derive approximations to the optimal antenna spacing and array size that maximize the achievable rate

翻译：未来6G无线网络面临的关键挑战之一是如何支持大量用户终端的大规模空间复用。近年来，超大规模天线阵列（ELAA）——亦称超大规模MIMO（XL-MIMO）——已成为实现此类空间复用传输的潜在关键方案。凭借近场传播特性与大规模发射器配置，这些巨型阵列不仅显著增加了可用空间自由度（传输模式），还能将发射能量聚焦于极小区域。本文探究在近场条件下，多正交极化复用是否能提升系统性能。我们聚焦于由均匀线性阵列（ULA）与单天线用户设备（UE）组成的简化场景，证明当发射端与接收端均采用三种正交线性极化时，视距（LoS）信道近场区域的空间自由度可达3。然而在远场区域，由于等效MIMO信道秩亏，最大空间自由度仅趋近于2。我们提出可达速率的解析近似表达式，并据此推导出最优天线间距与阵列尺寸的近似解，以实现系统可达速率最大化。