We consider the problem of user activity detection for grant-free massive connectivity, where a base station, equipped with a large number of antennas, serves a large number of machine type devices. Two massive multiple-input multiple-output (mMIMO) systems are studied, namely, a collocated mMIMO with physically collocated antennas and a cell-free mMIMO with geographically separated antennas. By utilizing the sporadic transmission nature and the asymptotical orthogonality of channel vectors associated with different users, we propose a correlation-based sparse signal recovery scheme to detect active users. Simulation results are provided to evaluate the performance of the proposed method. Our results show that the cell-free mMIMO achieves much better user activity detection performance than the collocated mMIMO, and thus is more suitable for grant-free massive connectivity. An empirical analysis is performed to explain the superiority of the cell-free system over the collocated system.

中文翻译：

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大规模MIMO系统中的无授予大规模连接：与无蜂窝并置

我们考虑了无授权大规模连接的用户活动检测问题，其中配备大量天线的基站为大量机器类型的设备提供服务。研究了两个大规模的多输入多输出（mMIMO）系统，即具有物理并置天线的并置mMIMO和具有地理上分开的天线的无小区mMIMO。通过利用零星的传输性质和与不同用户相关联的信道向量的渐近正交性，我们提出了一种基于相关的稀疏信号恢复方案来检测活跃用户。仿真结果提供了评估该方法性能的方法。我们的结果表明，与并置的mMIMO相比，无蜂窝mMIMO可以实现更好的用户活动检测性能，因此更适合于无赠款的大规模连接。进行了一项经验分析，以解释无单元系统相对于并置系统的优越性。