  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. We introduce Rate-Splitting Multiple Access
(RSMA), a new multiple access based on multi-antenna Rate-Splitting (RS) 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 optimum Degrees-of-Freedom (DoF) in an overloaded MISO BC
with heterogeneous CSIT qualities and then show at finite SNR that, by marrying
the benefits of PP and RSMA, PP-RSMA achieves explicit sum rate gain over
TP-RSMA and all baseline schemes. 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 draw the conclusion that
PP-RSMA is a powerful framework for cellular IoT with a large number of
devices.
