Modulation-Specific Multiuser Transmit Precoding and User Selection for One-Dimensional Signaling

Massive deployment of low data rate Internet of things and eHealth devices that require high reliability motivates the development of practical precoding and user selection techniques. In this paper, we show that throughput and communications reliability can be improved by incorporating knowledge of modulation type in the design of the multiuser transmit precoder. The transmission of low data rate one-dimensionally modulated signals in a broadcast channel is considered. The transmit precoding matrix is determined by minimizing the weighted sum of error probabilities of users. Although the proposed minimum probability of error (MPE) precoding problem is nonconvex and highly nonlinear, it is solved by the alternating minimization of two convex subproblems. A reduced-complexity version of convex MPE precoding is then introduced, which exponentially reduces the complexity of the problem. Numerical results show that the proposed precoding techniques significantly improve system performance in broadcast channels. A user selection algorithm, compatible with MPE precoding, is also proposed that selects users if their error probabilities can approach zero. Based on line packing principles in Grassmannian manifolds, it is shown that the number of selected users could potentially be more than the number of transmit antennas, which translates to supporting more simultaneous users in the shared channel compared to existing user selection methods.