The capabilities of multi-antenna technology have recently been significantly enhanced by the proliferation of extra large array architectures. The high dimensionality of these systems implies that communications take place in the nearfield regime, which poses some questions as to their effective perfomrance even under simple line of sight configurations. In order to study these limitations, a uniform linear array (ULA) is considered here, the elements of which are three infinitesimal dipoles transmitting different signals in the three spatial dimensions. The receiver consists of a single element with three orthogonal infinitesimal dipoles and full channel state information is assumed to be available at both ends. A capacity analysis is presented when the number of elements of the ULA increases without bound while the interelement distance converges to zero, so that the total aperture length is kept asymptotically fixed. In particular, the total number of available spatial eigenmodes is shown to depend crucially on the receiver position in space, and closed form expressions are provided for the different achievability regions. From the analysis it can be concluded that the use of three orthogonal polarizations at the transmitter guarantees the almost universal availability of two spatial streams, whereas the use of only two polarizations results in a more extensive region where maximum multiplexing gain is available.

翻译：近年来，超大规模阵列架构的普及显著增强了多天线技术的性能。这些系统的高维度特性意味着通信发生在近场区域，即使在简单的视距配置下，其实际性能仍存在疑问。为研究这些限制，本文考虑一种均匀线性阵列（ULA），其阵元为三个在空间三维方向上发射不同信号的无限小偶极子。接收端由包含三个正交无限小偶极子的单阵元构成，并假设收发两端均具备完整的信道状态信息。当ULA阵元数量无限增加而阵元间距趋近于零时（保持总孔径长度渐近恒定），本文提出了容量分析框架。特别地，研究证明可用空间本征模总数关键取决于接收端在空间中的位置，并针对不同可达性区域给出了闭式表达式。分析表明：发射端采用三个正交极化可保证两个空间流几乎普遍可用，而仅使用两个极化则能在更广泛的区域内实现最大复用增益。