HARQ-Aided RSMA for Integrated Satellite–Terrestrial Networks

This paper presents a non-orthogonal retransmission framework for integrated satellite-terrestrial networks (ISTNs). This framework integrates hybrid automatic repeat request (HARQ) with incremental redundancy (HARQ-IR) and rate splitting multiple access (RSMA). HARQ-IR and RSMA are utilized for downlink retransmission and multi-user interference management, respectively, to address the requirements for extensive and highly reliable concurrent connections. Employing the inclusion-exclusion principle, we establish precise upper and lower bounds for the exact outage probability (OP), which function as approximations. We also examine the asymptotic OP, which yields significant insights and informs a partial HARQ-IR-RSMA scheme. We propose a joint common-private power allocation (JCPPA) algorithm based on alternating optimization (AO) to enhance the energy efficiency (EE) of the partial retransmission scheme while adhering to power and outage probability (OP) constraints. The non-convex problem is addressed effectively via asymptotic OP, which allows for the decomposition into common and private power optimization subproblems utilizing the Dinkelbach method. The common power optimization subproblem is convex and can be solved using the CVX toolbox. The private power optimization subproblem is addressed through variable substitution and the application of the Lagrangian dual algorithm. The numerical results indicate the accuracy and superiority of the proposed special scheme regarding outage performance and energy efficiency when compared to benchmarks.