Robust Transmission in Downlink Multiuser MISO Systems: A Rate-Splitting Approach

Abstract

Abstract-We consider a downlink multiuser MISO system with bounded errors in the Channel State Information at the Transmitter (CSIT). We first look at the robust design problem of achieving max-min fairness amongst users (in the worstcase sense). Contrary to the conventional approach adopted in literature, we propose a rather unorthodox design based on a Rate-Splitting (RS) strategy. Each user's message is split into two parts, a common part and a private part. All common parts are packed into one super common message encoded using a public codebook, while private parts are independently encoded. The resulting symbol streams are linearly precoded and simultaneously transmitted, and each receiver retrieves its intended message by decoding both the common stream and its corresponding private stream. For CSIT uncertainty regions that scale with SNR (e.g. by scaling the number of feedback bits), we prove that a RS-based design achieves higher max-min (symmetric) Degrees of Freedom (DoF) compared to conventional designs (NoRS). For the special case of non-scaling CSIT (e.g. fixed number of feedback bits), and contrary to NoRS, RS can achieve a non-saturating max-min rate. We propose a robust algorithm based on the cutting-set method coupled with the Weighted Minimum Mean Square Error (WMMSE) approach, and we demonstrate its performance gains over state-of-the art designs. Finally, we extend the RS strategy to address the Quality of Service (QoS) constrained power minimization problem, and we demonstrate significant gains over NoRS-based designs.Index Terms-MISO-BC, degrees of freedom, linear precoding, max-min fairness, quality-of-service, robust optimization.

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

“…Though originally introduced for the two-user Single-Input Single-Output Interference Channel (IC) in [2], RS has become an underpinning communication-theoretic strategy to tackle modern interference-related problems and has recently been successfully investigated in several Multiple-Input Single-Output (MISO) Broadcast Channel (BC) settings, namely, unicast-only transmission with perfect Channel State Information at the Transmitter (CSIT) [3], [4] and imperfect CSIT [5]- [13], (multigroup) multicast-only transmission [14], as well as superimposed unicast and multicast transmission [15]. Results highlight that RS provides significant benefits in terms of spectral efficiency [3], [6], [7], [9], [13]- [15], energy efficiency [4], robustness [8], and CSI feedback overhead reduction [6], [12] over conventional strategies used in LTE-A/5G that rely on fully treating interference as noise (e.g. conventional multiuser linear precoding and Space Division Multiple Access -SDMA) or fully decoding interference (e.g.…”

“…This is the first paper to show analytically how RS unifies, outperforms, and specializes to SDMA, OMA, NOMA, and multicasting as a function of the disparity of the user channel strengths and the angle between the user channel directions. To that end, the paper differs from, and nicely complements, past works that analytically studied the rate performance of RS with imperfect CSIT [6], [9], [12] or looked at RS from an optimization perspective [3], [7], [8].…”

Linear Precoding and Stream Combining for Rate Splitting in Multiuser MIMO Systems

“…The maximum value of the numerator is achieved when w (12) where β j is the angle between w k and r k,j . The sum rate performance can be found by using (12) in (7) and in (8).…”

“…Substituting (26) and (27) in (10) we obtain the SINR, which can be used in (7) and (8) to obtain the sum rate.…”

Cooperative Rate Splitting for MISO Broadcast Channel With User Relaying, and Performance Benefits Over Cooperative NOMA

“…In contrast to NOMA that relies on some users to fully decode the messages of other users (i.e. fully decode interference), Rate-Splitting (RS) also exploits SIC but exploits a more flexible framework of non-orthogonal transmission, which enables to partially decode interference and partially treat remaining interference as noise, and hence provide significant benefits in terms of spectral efficiency [13], [15]- [20], energy efficiency [21], robustness [22], and CSI feedback overhead reduction [15], [23]. In addition, on the standpoint of encoding structure, the combination of a common stream decoded by all users and private streams dedicated to each user also provides a fundamental compatibility with other advanced techniques [24].…”