Rate-Splitting for Max-Min Fair Multigroup Multicast Beamforming in Overloaded Systems

Abstract

Abstract-In this paper, we consider the problem of achieving max-min fairness amongst multiple co-channel multicast groups through transmit beamforming. We explicitly focus on overloaded scenarios in which the number of transmitting antennas is insufficient to neutralize all inter-group interference. Such scenarios are becoming increasingly relevant in the light of growing lowlatency content delivery demands, and also commonly appear in multibeam satellite systems. We derive performance limits of classical beamforming strategies using DoF analysis unveiling their limitations; for example, rates saturate in overloaded scenarios due to inter-group interference. To tackle interference, we propose a strategy based on degraded beamforming and successive interference cancellation. While the degraded strategy resolves the rate-saturation issue, this comes at a price of sacrificing all spatial multiplexing gains. This motivates the development of a unifying strategy that combines the benefits of the two previous strategies. We propose a beamforming strategy based on rate-splitting (RS) which divides the messages intended to each group into a degraded part and a designated part, and transmits a superposition of both degraded and designated beamformed streams. The superiority of the proposed strategy is demonstrated through DoF analysis. Finally, we solve the RS beamforming design problem and demonstrate significant performance gains through simulations.

Precoder Design For Multi-Group Multicasting With a Common Message

“…While NoRS schemes saturate in the interference limited region (high SNR), the MMF rate for RS schemes keeps increasing. Results confirm that observations of [8] on the usefulness of RS in an overloaded multi-group multicast also carries on to the case where a common message is additionally transmitted. Fig.…”

“…In this letter, we build upon the benefits demonstrated in [8]- [10] and look at RS in multigroup multicasting with a common message. Compared to [8], this letter considers the presence of a common message. Compared to [9], this letter considers an RS strategy.…”

Rate-Splitting Unifying SDMA, OMA, NOMA, and Multicasting in MISO Broadcast Channel: A Simple Two-User Rate Analysis

“…which coincides with (14) when P → ∞, and leads to a high SNR non-zero (whenever 0 < t ⋆ < 1) sum-rate gap between RS and SDMA (t = 1) given by…”

“…From the above system model, both SDMA and RS can achieve such a DoF by precoding s 1 and s 2 using zero-forcing (ZF). On the other hand, OMA, NOMA, and multicasting can achieve at most a DoF of 1 (irrespectively of how the precoders and power allocation are optimized), which leads to a rate loss at high Signal-to-Noise Ratio (SNR) in general multi-antenna settings, as already highlighted in [3], [14].…”

“…The value of t that maximizes R s is the solution of ∂Rs ∂t = 0, which is written as t = − a 2b − c 2d . Since t ≤ 1, the optimal value t ⋆ is given in closed form by (14) at the top of the next page. For t ⋆ < 1, RS yields a non-zero sum-rate enhancement over SDMA.…”