Utilizing Improper Gaussian Signaling for Downlink Rate-Splitting Multiple Access with Imperfect Successive Interference Cancellation

TL;DR

This paper demonstrates that improper Gaussian signaling (IGS) effectively mitigates residual interference in downlink rate-splitting multiple access (RSMA) with imperfect successive interference cancellation, outperforming proper Gaussian signaling (PGS).

Contribution

It introduces IGS into RSMA to counter residual interference, deriving optimal solutions and developing a SAC algorithm for non-convex sum rate maximization.

Findings

IGS maximizes the private rate at maximum impropriety.

Closed-form solutions for common rate maximization are derived.

Numerical results show IGS outperforms PGS, especially with higher SIC imperfection.

Abstract

To mitigate the residual interference from imperfect successive interference cancellation (SIC) in Rate-Splitting Multiple Access (RSMA), this paper incorporates improper Gaussian signaling (IGS) into the downlink RSMA framework. Unlike existing RSMA--IGS works that embed impropriety within IQ-imbalanced frameworks, we show that IGS alone effectively counters SIC-induced residual interference. For a basic SISO setup with IGS on the common stream and PGS on private streams, we establish three key results: the optimal impropriety degree for private rate maximization attains its maximum; closed-form optimal solutions with rigorous monotonicity conditions are derived for common rate maximization; and a soft actor-critic (SAC) algorithm is developed for the non-convex sum rate problem. Numerical results show that IGS consistently outperforms PGS, with the gain widening as SIC imperfection increases.