# Coordinated Beam Selection in Millimeter Wave Multi-User MIMO Using Out-of-Band Information

## TL;DR

This paper introduces a novel out-of-band information-based framework for coordinated beam selection in multi-user millimeter wave MIMO systems, leveraging lower band spatial data and device-to-device communication to reduce interference and improve spectral efficiency.

## Contribution

It proposes a decentralized, out-of-band aided beam selection scheme that uses inter-user coordination via device-to-device communication to suppress interference in multi-user mmWave systems.

## Key findings

- Significant spectral efficiency gains achieved.
- Effective suppression of co-beam interference.
- Decentralized coordination enhances multi-user performance.

## Abstract

Using out-of-band (OOB) side-information has recently been shown to accelerate beam selection in single-user millimeter wave (mmWave) massive MIMO communications. In this paper, we propose a novel OOB-aided beam selection framework for a mmWave uplink multi-user system. In particular, we exploit spatial information extracted from lower (sub-6 GHz) bands in order to assist with an inter-user coordination scheme at mmWave bands. To enforce coordination, we propose an exchange protocol exploiting device-to-device communications, where low-rate beam-related information is exchanged between the mobile terminals. The decentralized coordination mechanism allows the suppression of the so-called co-beam interference which would otherwise lead to irreducible interference at the base station side, thereby triggering substantial spectral efficiency gains.