Scheduling Versus Contention for Massive Random Access in Massive MIMO Systems

TL;DR

This paper compares scheduled and contention-based massive access schemes in massive MIMO systems, showing that scheduling with minimal feedback significantly improves successful transmissions and sum rate over traditional contention methods.

Contribution

It introduces a coordinated access scheme using activity detection and a single feedback message, demonstrating its advantages over existing contention-based approaches.

Findings

Scheduled access increases successful transmissions per slot.

Reduced interference leads to higher sum rate.

Minimal feedback cost enhances overall system performance.

Abstract

Massive machine-type communications protocols have typically been designed under the assumption that coordination between users requires significant communication overhead and is thus impractical. Recent progress in efficient activity detection and collision-free scheduling, however, indicates that the cost of coordination can be much less than the naive scheme for scheduling. This work considers a scenario in which a massive number of devices with sporadic traffic seek to access a massive multiple-input multiple-output (MIMO) base-station (BS) and explores an approach in which device activity detection is followed by a single common feedback broadcast message, which is used both to schedule the active users to different transmission slots and to assign orthogonal pilots to the users for channel estimation. The proposed coordinated communication scheme is compared to two prevalent contention-based schemes: coded pilot access, which is based on the principle of coded slotted ALOHA, and an approximate message passing scheme for joint user activity detection and channel estimation. Numerical results indicate that scheduled massive access provides significant gains in the number of successful transmissions per slot and in sum rate, due to the reduced interference, at only a small cost of feedback.