Power-efficient transmission design for MISO broadcast systems with QoS constraints

This paper considers a novel transmitter optimization problem for MISO broadcast systems, where a multi-antenna base station (BS) serves a set of single-antenna mobile stations (MSs) with Quality of Service (QoS) targets formulated in terms of Symbol Error Probability (SEP): data inputs are drawn from a discrete alphabet, precoding is performed by the BS to generate appropriate signals transmitted over the channel, symbol-by-symbol detection is performed at each MS, and a minimum SEP target is imposed at each MS as the QoS target. Starting with the system adopting standard constellations and with focus on 16-QAM, the SEP constraints are formulated and characterized by a set of convex relaxations on the received signals. The resulting problem is convex, and can be efficiently solved by the primal-dual interior point method. Numerical results demonstrate that the proposed precoder provides significant transmission power reduction compared with the linear zero-forcing (ZF) precoder under the same SEP targets. Turning to the system with Tomlinson-Harashima type constellations, a similar convex relaxation approach is applied and the corresponding convex optimization problem is presented. The proposed precoding scheme is shown to outperform both the previous scheme (based on regular constellations with convex relaxation) and the standard Tomlinson-Harashima precoding using zero forcing.