In the near future, it is envisioned that cellular networks will have to cope with extensive internet of things (IoT) devices. Therefore, a required feature of cellular IoT will be the capability to serve simultaneously a large number of devices with heterogeneous demands and qualities of channel state information at the transmitter (CSIT). In this paper, we focus on an overloaded multiple-input single-output (MISO) broadcast channel (BC) with two groups of CSIT qualities, namely, one group of users (representative of high-end devices) for which the transmitter has partial knowledge of the CSI, the other group of users (representative of IoT devices) for which the transmitter only has knowledge of the statistical CSI (i.e., the distribution information of the user channels). We introduce rate-splitting multiple access (RSMA), a new multiple access based on multi-antenna rate-splitting (RS) technique for cellular IoT. Two strategies are proposed, namely, time partitioning–RSMA (TP–RSMA) and power partitioning–RSMA (PP–RSMA). The former independently serves the two groups of users over orthogonal time slots while the latter jointly serves the two groups of users within the same time slot in a non-orthogonal manner. We first show at high signal-to-noise ratio (SNR) that PP–RSMA achieves the optimal degrees-of-freedom (DoF) in an overloaded MISO BC with heterogeneous CSIT qualities. We then show at finite SNR that PP–RSMA achieves explicit sum rate gain over TP–RSMA and all baseline schemes by marrying the benefits of PP and RSMA. Furthermore, PP–RSMA is robust to CSIT inaccuracy and flexible to cope with quality of service (QoS) rate constraints of all users. The DoF and rate analysis helps us in drawing the conclusion that PP–RSMA is a powerful framework for cellular IoT with a large number of devices.

中文翻译：

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重载蜂窝物联网的速率拆分多路访问

在不久的将来，预计蜂窝网络将不得不应对广泛的物联网 (IoT) 设备。因此，蜂窝物联网的一个必需特性将是能够同时为大量具有异构需求和发射机（CSIT）信道状态信息质量的设备提供服务。在本文中，我们关注具有两组 CSIT 质量的过载多输入单输出 (MISO) 广播信道 (BC)，即一组用户（代表高端设备），其中发射机具有部分CSI 的知识，另一组用户（代表物联网设备），发射机只知道统计 CSI（即用户信道的分布信息）。我们引入了速率分割多路访问 (RSMA)，一种用于蜂窝物联网的基于多天线速率分离 (RS) 技术的新多址接入。提出了两种策略，即时间分区-RSMA（TP-RSMA）和功率分区-RSMA（PP-RSMA）。前者在正交时隙上独立地为两组用户服务，而后者在同一时隙内以非正交的方式联合为两组用户服务。我们首先在高信噪比 (SNR) 下表明 PP-RSMA 在具有异构 CSIT 质量的过载 MISO BC 中实现了最佳自由度 (DoF)。然后，我们在有限 SNR 下证明，通过结合 PP 和 RSMA 的优点，PP-RSMA 实现了优于 TP-RSMA 和所有基线方案的显式总速率增益。此外，PP-RSMA 对 CSIT 的不准确性具有鲁棒性，并且可以灵活地应对所有用户的服务质量 (QoS) 速率限制。