Rate-Splitting With Hybrid Messages: DoF Analysis of the Two-User MIMO Broadcast Channel With Imperfect CSIT

Most of the existing research on degrees-of-freedom (DoF) with imperfect channel state information at the transmitter (CSIT) assume the messages are private, which may not reflect reality as the two receivers can request the same content. To overcome this limitation, we therefore consider the hybrid unicast and multicast messages. In particular, we characterize the optimal DoF region for the two-user multiple-input multiple-output (MIMO) broadcast channel (BC) with imperfect CSIT and hybrid messages. For the converse, we establish a three-step procedure to exploit the utmost possible relaxation. For the achievability, since the DoF region is with specific three-dimensional structure regarding antenna configurations and CSIT qualities, we verify the existence or non-existence of corner point candidates via the feature of antenna configurations and CSIT qualities categorization, and provide a hybrid message-aware rate-splitting scheme. Besides, we show that to achieve the strictly positive corner points, it is unnecessary to split the unicast messages into private and common parts. This implies adding a multicast message may mitigate the rate-splitting complexity.

I. Introduction

The emergence of the upcoming sixth generation mobile communications (6G) will provide extremely reliable, ultra-fast, and ubiquitous wireless connectivity with significantly elevated performance, as opposed to those in existing communication standards and systems [1], [2], [3]. It is expected that 6G can achieve 50 times higher peak data rate, 10 times reduced latency, and 100 times higher reliability than that of existing mobile communications systems. Typical 6G services are upgraded version of enhanced mobile broadband (eMBB), ultra reliable low latency communications (URLLC), and massive machine type communications (mMTC). One of the main challenges of 6G is the technique of multiple-access, which should support massive receivers with high data rate and are resilient to errors of channel state information at the transmitter (CSIT). Conventional multiple-access techniques, e.g., orthogonal multiple-access (OMA) and non-orthogonal multiple-access (NOMA), cannot address this challenge due to the following reasons: 1) OMA is very resource-consuming for supporting massive receivers 2) Both OMA and NOMA cannot be adaptive and robust to errors of CSIT, leading to degraded performance. In this regard, the rate-splitting multiple-access (RSMA) stands out as a viable solution, which not only can support massive receivers but also is adaptive and robust against errors of CSIT [4], [5], [6]. Specifically, it was found in [4] that NOMA is in fact a special case of RSMA. The foundations of RSMA stem from information-theoretic research, and the concept of RSMA then further moves to engineering practice with advanced wireless communication applications. Next, we will review the related work to the problem.