We consider a downlink multiuser MISO system with bounded errors in theChannel State Information at the Transmitter (CSIT). We first look at therobust design problem of achieving max-min fairness amongst users (in theworst-case 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 apublic codebook, while private parts are independently encoded. The resultingsymbol streams are linearly precoded and simultaneously transmitted, and eachreceiver retrieves its intended message by decoding both the common stream andits corresponding private stream. For CSIT uncertainty regions that scale withSNR (e.g. by scaling the number of feedback bits), we prove that a RS-baseddesign achieves higher max-min (symmetric) Degrees of Freedom (DoF) compared toconventional designs (NoRS). For the special case of non-scaling CSIT (e.g.fixed number of feedback bits), and contrary to NoRS, RS can achieve anon-saturating max-min rate. We propose a robust algorithm based on thecutting-set method coupled with the Weighted Minimum Mean Square Error (WMMSE)approach, and we demonstrate its performance gains over state-of-the artdesigns. Finally, we extend the RS strategy to address the Quality of Service(QoS) constrained power minimization problem, and we demonstrate significantgains over NoRS-based designs.
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