Fairness in a Class Barring Power Control Random Access Protocol for Crowded XL-MIMO Systems

This article focuses on conceiving a random access (RA) protocol able to provide connections for a vast number of devices subject to intensive collisions in an extralarge massive multiple-input–multiple-output (XL-MIMO) system equipped with a uniform rectangular array (URA). The recently proposed strongest user collision resolution protocol for XL-MIMO (SUCRe-XL) presents a bias since devices closer to the base station (BS) are favored, while those farther ones are rejected, leading to poor connectivity and excessive access delays to most users not very close to the BS. The novel RA protocol for XL-MIMO scenarios proposed in this article is based on access class barring with power control (ACBPC). Exploring the visibility regions (VRs) intrinsic to XL-MIMO scenarios as a new degree of freedom, the proposed grant-based ACBPC-XL protocol can improve the probability of resolving pilot collisions. The protocol is able to manage a higher number of users in crowded applications, diminishing the access delay while improving connectivity fairness along the entire cell area since the access performance does not depend on the distance between users and BS. Numerical results indicate that in average $\approx $ 76% of users have improved connectivity when employing the ACBPC-XL instead of the SUCRe-XL protocol.

I. Introduction

Fundamental services within the fifth-generation of wireless networks (5G), as massive machine type communication (mMTC) and crowded mobile broadband (cMBB) use cases, demand highly efficient procedures to deal with massive user equipment (UE) access attempts [1]. In such scenarios, the number of available pilot sequences is often lower than the number of UEs. Therefore, implementing collision resolution protocols becomes essential to allow coherent communication.