[Paper Review] Rate-Splitting Multiple Access for Overloaded Cellular Internet of Things

Yijie Mao, Enrico Piovano+1 more|arXiv (Cornell University)|Aug 3, 2020

Advanced Wireless Communication Technologies29 references4 citations

TL;DR

This paper proposes Power Partitioning–Rate Splitting Multiple Access (PP–RSMA) for overloaded cellular IoT networks with heterogeneous Channel State Information at the Transmitter (CSIT). By jointly serving high-end users (partial CSIT) and IoT devices (statistical CSIT) in the same time slot via power-splitting precoding, PP–RSMA achieves optimal Degrees of Freedom (DoF) at high SNR and provides explicit sum rate gains over Time Partitioning–RSMA and baseline schemes, while being robust to CSIT inaccuracy and flexible for QoS constraints.

ABSTRACT

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.

Motivation & Objective

• Address the challenge of serving a massive number of IoT devices with heterogeneous CSIT quality in overloaded cellular networks.
• Overcome limitations of conventional SDMA and NOMA in overloaded, imperfect-CSIT scenarios.
• Design a multiple access scheme that maintains high spectral efficiency and robustness under partial or statistical CSIT.
• Enable joint transmission of high-end and low-end IoT users in the same time-frequency resource with QoS flexibility.
• Achieve optimal DoF and sum rate performance under practical CSIT constraints.

Proposed method

• Propose two strategies: Time Partitioning–RSMA (orthogonal time slots) and Power Partitioning–RSMA (non-orthogonal joint transmission).
• Use rate-splitting with common and private streams, where common streams are precoded to align interference for users with partial CSIT.
• Implement power partitioning across users: private streams for high-SNR users, common streams for statistical CSIT users.
• Apply DoF analysis at high SNR to prove optimality of PP–RSMA in overloaded MISO BCs with heterogeneous CSIT.
• Derive sum rate expressions and compare PP–RSMA with TP–RSMA and baseline schemes (e.g., SDMA, NOMA) at finite SNR.
• Use induction and facet analysis to prove achievability of the DoF region, validated via converse from [29].

Experimental results

Research questions

• RQ1 Can RSMA achieve optimal Degrees of Freedom in an overloaded MISO BC with two groups of users having different CSIT qualities?
• RQ2 How does PP–RSMA compare to TP–RSMA and conventional multiple access schemes (SDMA, NOMA) in terms of sum rate and robustness?
• RQ3 To what extent does PP–RSMA maintain performance under imperfect or statistical CSIT?
• RQ4 Can PP–RSMA support diverse Quality of Service (QoS) rate constraints for heterogeneous users?
• RQ5 What is the achievable DoF region for an overloaded MISO BC with partial and statistical CSIT?

Key findings

• PP–RSMA achieves the optimal Degrees of Freedom (DoF) in an overloaded MISO BC with heterogeneous CSIT qualities at high SNR.
• At finite SNR, PP–RSMA provides explicit sum rate gains over TP–RSMA and all baseline schemes, including SDMA and NOMA.
• The DoF region of PP–RSMA matches the outer bound derived from [29], proving optimality.
• PP–RSMA is robust to CSIT inaccuracy due to its interference management via common streams and power partitioning.
• The scheme is flexible and can meet individual QoS rate constraints for all users, including those with statistical CSIT.
• The proposed method achieves the DoF region through power partitioning and time-sharing over facets of the polyhedral region, validated via induction and converse proof.