# Joint User Selection, Power Allocation, and Precoding Design with Imperfect CSIT for Multi-Cell MU-MIMO Downlink Systems

## TL;DR

This paper introduces a novel optimization framework and an efficient algorithm, GPIP, for joint user selection, power allocation, and precoding in multi-cell MU-MIMO systems with imperfect CSIT, improving spectral efficiency.

## Contribution

It proposes a new non-convex optimization approach and the GPIP algorithm for joint design in multi-cell MU-MIMO systems with imperfect CSIT, extending to cooperative base stations.

## Key findings

- GPIP converges to a local maximum of the spectral efficiency.
- The proposed method outperforms conventional user selection and linear precoding.
- System-level simulations demonstrate significant performance gains.

## Abstract

In this paper, a new optimization framework is presented for the joint design of user selection, power allocation, and precoding in multi-cell multi-user multiple-input multiple-output (MU-MIMO) systems when imperfect channel state information at transmitter (CSIT) is available. By representing the joint optimization variables in a higher-dimensional space, the weighted sum-spectral efficiency maximization is formulated as the maximization of the product of Rayleigh quotients. Although this is still a non-convex problem, a computationally efficient algorithm, referred to as generalized power iteration precoding (GPIP), is proposed. The algorithm converges to a stationary point (local maximum) of the objective function and therefore it guarantees the first-order optimality of the solution. By adjusting the weights in the weighted sum-spectral efficiency, the GPIP yields a joint solution for user selection, power allocation, and downlink precoding. The GPIP is also extended to a multi-cell scenario, where cooperative base stations perform joint user selection and design their precoding vectors by sharing global yet imperfect CSIT within the cooperative BSs. System-level simulations show the gains of the proposed approach with respect to conventional user selection and linear downlink precoding.