This paper tackles the problem of designing proper uplink multiple access (MA) schemes for coexistence between enhanced mobile broadband+ (eMBB+) users and massive machine-type communications+ (mMTC+) devices in a terminal-centric cell-free massive MIMO system. Specifically, the use of a time-frequency spreading technique for the mMTC+ devices has been proposed. Coupled with the assumption of imperfect channel knowledge, closed-form bounds of the achievable (ergodic) rate for the two types of data services are derived. Using suitable power control mechanisms, we show it is possible to efficiently multiplex eMBB+ and mMTC+ traffic in the same time-frequency resource grid. Numerical experiments reveal interesting trade-offs in the selection of the spreading gain and the number of serving access points within the system. Results also demonstrate that the performance of the mMTC+ devices is slightly affected by the presence of the eMBB+ users. Overall, our approach can endow good quality of service to both 6G cornerstones at once.

翻译：本文针对终端中心型无小区大规模MIMO系统中增强移动宽带+（eMBB+）用户与大规模机器类通信+（mMTC+）设备之间的共存问题，研究了合适的上行多址接入（MA）方案设计。具体而言，提出了针对mMTC+设备采用时频扩频技术。结合信道估计不完善的假设，推导了两种数据服务可实现（遍历）速率的闭式界。利用合适的功率控制机制，我们证明了在相同的时间-频率资源网格上有效复用eMBB+与mMTC+业务流是可行的。数值实验揭示了在系统内扩频增益与服务接入点数量选择中的有趣权衡。结果还表明，mMTC+设备的性能受eMBB+用户存在的影响较小。总体而言，我们的方法能够同时为6G的两大基石提供良好的服务质量。