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

Abstract

We consider a downlink multiuser MISO system with bounded errors in the\nChannel State Information at the Transmitter (CSIT). We first look at the\nrobust design problem of achieving max-min fairness amongst users (in the\nworst-case sense). Contrary to the conventional approach adopted in literature,\nwe propose a rather unorthodox design based on a Rate-Splitting (RS) strategy.\nEach user's message is split into two parts, a common part and a private part.\nAll common parts are packed into one super common message encoded using a\npublic codebook, while private parts are independently encoded. The resulting\nsymbol streams are linearly precoded and simultaneously transmitted, and each\nreceiver retrieves its intended message by decoding both the common stream and\nits corresponding private stream. For CSIT uncertainty regions that scale with\nSNR (e.g. by scaling the number of feedback bits), we prove that a RS-based\ndesign achieves higher max-min (symmetric) Degrees of Freedom (DoF) compared to\nconventional designs (NoRS). For the special case of non-scaling CSIT (e.g.\nfixed number of feedback bits), and contrary to NoRS, RS can achieve a\nnon-saturating max-min rate. We propose a robust algorithm based on the\ncutting-set method coupled with the Weighted Minimum Mean Square Error (WMMSE)\napproach, and we demonstrate its performance gains over state-of-the art\ndesigns. Finally, we extend the RS strategy to address the Quality of Service\n(QoS) constrained power minimization problem, and we demonstrate significant\ngains over NoRS-based designs.\n