The extra-large multiple-input multiple-output (XL-MIMO) architecture has been recognized as a technology for supporting the massive MTC (mMTC), providing very high-data rates in high-user density scenarios. However, the large dimension of the array increases the Rayleigh distance (dRayl), in addition to obstacles and scatters causing spatial non-stationarities and distinct visibility regions (VRs) across the XL array extension. We investigate the random access (RA) problem in crowded XL-MIMO scenarios; the proposed grant-based random access (GB-RA) protocol combining the advantage of non-orthogonal multiple access (NOMA) and strongest user collision resolutions in extra-large arrays (SUCRe-XL) named NOMA-XL can allow access of two or three colliding users in the same XL sub-array (SA) selecting the same pilot sequence. The received signal processing in a SA basis changes the dRayl, enabling the far-field planar wavefront propagation condition, while improving the system performance. The proposed NOMA-XL GB-RA protocol can reduce the number of attempts to access the mMTC network while improving the average sum rate, as the number of SA increases.

翻译：针对高用户密度场景下大规模物联网通信（mMTC）的需求，超大规模多输入多输出（XL-MIMO）体系结构被认为是一种支持高数据速率的技术。然而，由于数组的大尺寸，加之障碍物和散射物的影响，导致XL-MIMO存在空间非统计性和不同的可视范围（VR）。本文研究了拥挤的XL-MIMO场景下的随机接入（RA）问题，并提出了基于授权的随机接入（GB-RA）协议，该协议结合了非正交多址（NOMA）与特定信道对冲解决方案。该协议命名为NOMA-XL，可以允许两个或三个共冲突用户选择相同的导频序列在同一个XL子阵列（SA）内接入网络。在SA基础上的接收信号处理改变了Rayleigh距离（dRayl），从而实现了远场平面波前传播条件，提高了系统性能。所提出的NOMA-XL GB-RA协议可以在SA的数量增加时减少接入mMTC网络的尝试次数，同时提高平均总速率。