Stochastic Augmented Projected Gradient Methods for the Large-Scale Precoding Matrix Indicator Selection Problem

In this paper, we consider the large-scale precoding matrix indicator (PMI) selection problem at the receiver in wireless communications. The selection is based on the channel capacity of the PMI matrix in a pre-designed codebook. The quality of the PMI matrix is essential in achieving higher spectral efficiency. We first derive two novel formulations including a partial permutation-matrix model and an indicator-vector model for the original problem. The discrete constraints in the formulations make the problem NP-hard. Then we propose a stochastic projected gradient method augmented by block coordinate descent under various strategies. We show that the algorithms terminate in finite steps and produce sufficient descent at each iteration when the step size is chosen properly. Extensive experiments demonstrate that our proposed algorithms are able to find better PMI matrices more efficiently compared to the existing methods.