# Multiplexing More Streams in the MU-MISO Downlink by Interference Exploitation Precoding

## TL;DR

This paper develops an interference exploitation precoding method for MU-MISO downlink systems where the number of streams exceeds transmit antennas, deriving optimal structures and demonstrating significant performance gains.

## Contribution

It introduces a novel optimal precoding structure for scenarios with more streams than antennas, using pseudo inverse and dual problem formulation.

## Key findings

- Optimal precoding structure derived using pseudo inverse.
- Closed-form expression for the precoding matrix in terms of dual variables.
- Numerical results show substantial performance improvements.

## Abstract

In this paper, we study the interference exploitation precoding for the scenario where the number of streams simultaneously transmitted by the base station (BS) is larger than that of transmit antennas at the BS, and derive the optimal precoding structure by employing the pseudo inverse. We show that the optimal pre-scaling vector is equal to a linear combination of the right singular vectors that correspond to zero singular values of the coefficient matrix. By formulating the dual problem, the optimal precoding matrix can be expressed as a function of the dual variables in a closed form, and an equivalent quadratic programming (QP) formulation is further derived for computational complexity reduction. Numerical results validate our analysis and demonstrate significant performance improvements for interference exploitation precoding for the considered scenario.