Rate-Splitting Multiple Access for Integrated Sensing and Communications: A First Experimental Study

A canonical use case of Integrated Sensing and Communications (ISAC) in multiple-input multiple-output (MIMO) systems involves a multi-antenna transmitter communicating with $K$ users and sensing targets in its vicinity. For this setup, precoder and multiple access designs are of utmost importance, as the limited transmit power budget must be efficiently directed towards the desired directions (users and targets) to maximize both communications and sensing performance. This problem has been widely investigated analytically under various design choices, in particular (a) whether or not a dedicated sensing signal is needed, and (b) for different MIMO multiple access techniques, such as Space Division Multiple Access (SDMA) and Rate-Splitting Multiple Access (RSMA). However, a conclusive answer on which design choice achieves the best ISAC performance, backed by experimental results, remains elusive. We address this vacuum by experimentally evaluating and comparing RSMA and SDMA for communicating with two users $(K = 2)$ and sensing (ranging) one target. Over three scenarios that are representative of \emph{vehicular} ISAC, covering different levels of inter-user interference and separation/integration between sensing and communications, we show that RSMA without a dedicated sensing signal achieves better ISAC performance – i.e., higher sum throughput (up to $50%$ peak throughput gain) for similar radar SNR (between $20$ to $24{\rm dB}$) – than SDMA with a dedicated sensing signal. This first-ever experimental study of RSMA ISAC demonstrates the feasibility and the superiority of RSMA for future multi-functional wireless systems.

💡 Research Summary

The paper presents the first experimental comparison between Rate‑Splitting Multiple Access (RSMA) and Space‑Division Multiple Access (SDMA) in a multiple‑input multiple‑output (MIMO) Integrated Sensing and Communications (ISAC) scenario. The testbed consists of a multi‑antenna transmitter serving two single‑antenna users while simultaneously ranging a single target placed at broadside, emulating a vehicular environment. Both communication and radar functions share the same 100 MHz OFDM waveform derived from IEEE 802.11ac‑VHT, and the transmitter power is constrained to a fixed budget.

RSMA is implemented by splitting each user’s message into a common part and a private part, generating three data streams (one common, two private). Linear precoders are applied to each stream: a weighted maximum‑ratio transmission (MRT) precoder for the common stream that balances gain toward both users and the target, and either MRT or zero‑forcing (ZF) precoders for the private streams, also weighted toward the target direction. Four key parameters control the power and direction allocation:

• t_comm – fraction of total power devoted to communication,
• t_p – fraction of the communication power allocated to private streams,
• α_c – weight between the user‑combined direction and the target direction for the common precoder,
• α_p – analogous weight for each private precoder.

A dedicated sensing signal can be added (SDMA case) with its own precoder that points solely at the target, consuming the remaining power (1 – t_comm). The authors evaluate four design combinations: RSMA with and without the dedicated sensing signal, and SDMA with and without it.

Three vehicular‑representative scenarios are defined, differing in inter‑user interference levels (low, medium, high) and in how tightly sensing and communication are integrated. For each scenario, a simulation‑assisted parameter sweep identifies a set of well‑chosen (t_comm, t_p, α_c, α_p) values that approximate the Pareto frontier of the achievable performance region, defined by the pair (sum throughput, post‑processing radar SNR).

Experimental results show that the RSMA performance region strictly contains the SDMA region. In particular, RSMA without a dedicated sensing signal achieves the same radar SNR (20–24 dB) as SDMA with a dedicated sensing signal, while delivering up to 50 % higher sum throughput. The advantage is most pronounced in high‑interference scenarios where the common stream of RSMA efficiently mitigates inter‑user interference and simultaneously provides sufficient illumination of the target. Conversely, SDMA requires a dedicated sensing waveform to reach comparable radar SNR, which reduces the power available for data transmission and limits throughput.

The study confirms several theoretical predictions: (i) RSMA’s common stream can serve as a “virtual user” for the radar target, eliminating the need for a separate sensing waveform; (ii) the four‑parameter precoder design, though heuristic, is sufficient to expose the fundamental trade‑off between communication and sensing; (iii) adjusting α_c and α_p offers a practical knob to prioritize either function in real‑time operation.

Overall, the work provides concrete experimental evidence that RSMA is a superior multiple‑access strategy for ISAC systems, delivering higher spectral efficiency and lower hardware complexity than traditional SDMA. These findings have direct implications for the design of future 6G standards and vehicular communication‑radar convergence, where flexible, power‑efficient, and low‑latency operation is essential.