Interference Exploitation Precoding Made Practical: Closed-Form Solutions with Optimal Performance

TL;DR

This paper introduces practical closed-form precoding schemes for constructive interference in MU-MISO systems, achieving optimal performance with efficient solutions that outperform traditional methods.

Contribution

It derives closed-form solutions for CI precoding with optimal performance, including a novel iterative scheme that is more efficient than conventional quadratic programming methods.

Findings

The proposed schemes achieve optimal CI exploitation in MU-MISO downlink.

The iterative closed-form scheme outperforms traditional QP algorithms in efficiency.

Numerical results confirm the optimality and practicality of the proposed methods.

Abstract

In this paper, we propose closed-form precoding schemes with optimal performance for constructive interference (CI) exploitation in the multiuser multiple-input single-output (MU-MISO) downlink. We first consider an optimization where we maximize the distance between the constructive region and the detection thresholds. The cases of both strict and non-strict phase rotation are considered and can further be formulated as convex optimization problems. For optimization with strict phase rotation, we mathematically derive the optimal beamforming structure with Lagrangian and Karush-Kuhn-Tucker (KKT) conditions. By formulating its dual problem, the optimization problem is further shown to be equivalent to a quadratic programming (QP) over a simplex, which can be solved more efficiently. We then extend our analysis to the case of non-strict phase rotation, where it is mathematically shown that a K-dimensional optimization for non-strict phase rotation is equivalent to a 2K-dimensional optimization for strict phase rotation in terms of the problem formulation. The connection with the conventional zero-forcing (ZF) precoding is also discussed. Based on the above analysis, we further propose an iterative closed-form scheme to obtain the optimal beamforming matrix, where within each iteration a closed-form solution can be obtained. Numerical results validate our analysis and the optimality of the proposed iterative scheme, and further show that the proposed closed-form scheme is more efficient than the conventional QP algorithms with interior-point methods, which motivates the use of CI beamforming in practical wireless systems.