Constructive Interference Based Constant Envelope Precoding

We present a new multiple-input-multiple-output (MIMO) transmission scheme for generic phase-shift-keying (PSK) modulations in the multi-user (MU) downlink channel, where Constant Envelope Precoding (CEP) is combined with concepts of interference exploitation. In the proposed approach, multi-user-interference (MUI) is treated as a resource for increasing the signal-to-interference-and-noise-ratio (SINR) at the receiver side, in contrast with conventional precoding schemes from the literature which aim to minimize MUI. Two different CEP schemes are presented: a first technique, based on the application of the cross-entropy solver, and a two-step approach, based on an initial relaxation of the power constraints and a subsequent enforcement of per-antenna power constraints. The benefits of the proposed algorithms are evaluated in terms of computational costs and achievable symbol error rate (SER) in a perfect channel state information (CSI) scenario for different modulation orders. The analytical and numerical results show that interference-exploitation concepts are able to further extend the benefits of classical CEP