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：超大规模多输入多输出（XL-MIMO）架构已被公认为支持大规模机器类通信（mMTC）的一项技术，可在高用户密度场景下提供极高的数据速率。然而，阵列的大尺寸增大了瑞利距离（dRayl），同时障碍物和散射体也会导致空间非平稳性，并在XL阵列扩展范围内产生不同的可见区域（VR）。我们研究了拥挤XL-MIMO场景下的随机接入（RA）问题；所提出的基于授权的随机接入（GB-RA）协议结合了非正交多址接入（NOMA）的优势以及超大规模阵列中最强用户冲突解决算法（SUCRe-XL），命名为NOMA-XL，该协议允许两个或三个冲突用户在相同XL子阵列（SA）中选择相同导频序列时接入网络。基于SA的接收信号处理改变了dRayl，实现了远场平面波传播条件，同时提升了系统性能。随着SA数量的增加，所提出的NOMA-XL GB-RA协议能够减少接入mMTC网络的尝试次数，同时提高平均和速率。