Rate-Splitting Multiple Access for Multi-Antenna Joint Radar and Communications

Dual-Functional Radar-Communication (DFRC) is a promising paradigm to achieve Integrated Sensing and Communication (ISAC) in beyond 5G. In parallel, Rate-Splitting Multiple Access (RSMA), relying on multi-antenna Rate-Splitting (RS) by splitting messages into common and private streams at the transmitter and Successive Interference Cancellation (SIC) at the receivers, has emerged as a new strategy for multi-user multi-antenna communications systems. In this paper, we show that RSMA can be used as a unified and powerful multiple access and interference management strategy for multi-antenna DFRC system, where the base station has a dual communication and radar capability to simultaneously communicate with downlink users and probe detection signals to azimuth angles of interests. To that end, we design the message splits as well as the precoders of the communication (private and common) streams and radar sequence to jointly maximize the Weighted Sum Rate (WSR) and minimize the radar beampattern approximation Mean Square Error (MSE) subject to the per-antenna power constraint. We make key and refreshing observations. First, the proposed RSMA-assisted DFRC framework embraces existing DFRC frameworks based on Space-Division Multiple Access (SDMA) as subschemes. Second, RSMA exploits synergies in DFRC by making a better use of the common stream and the SIC receivers for the triple function of managing interference among communication users, managing interference between communication and radar, and enhancing radar sensing. Third, by contrasting with existing SDMA-based DFRC schemes where transmitting an additional radar sequence enhances the WSR-MSE tradeoff, RSMA-assisted DFRC frameworks with and without radar sequence achieve the same tradeoff performance and outperform all schemes using SDMA or orthogonal resources. Therefore, by enabling RSMA in DFRC, the system performance is enhanced and the system architecture is simplified since there is no need to use an additional radar sequence. We conclude that RSMA is a powerful multiple access and interference management strategy for DFRC.

I. Introduction

In 5G and beyond, many promising applications, such as autonomous vehicles, Wi-Fi sensing, and extended reality, have encouraged the Integration of Sensing and Communication (ISAC). As a typical and reliable means of sensing, radar has been widely used in both civil and military scenarios. Due to the scarcity of the spectrum resources, the 4th and 5th generation wireless communication systems are competing with radar applications in the S-band (2-4GHz), C-band (4-8GHz) and possibly millimeter-wave (mmWave) band, which may result in severe spectrum congestion and hamper the higher data rate demand in future wireless communications [2]. As a long-term solution, communication and radar spectrum sharing has attracted much interest and significant potential for ISAC. Depending on the levels of radar-communication integration, the study of joint radar-communication can be generally classified into three categories, namely, co-existence, co-operation and co-design, as discussed in [3], [4]. Following these distinctions, a variety of spectrum sharing techniques are compared in [5] with a short discussion of their possible applications.