Rethinking Grant-Free Protocol in mMTC

TL;DR

This paper proposes a two-stage protocol for mMTC that dynamically estimates active devices to improve identity detection stability, addressing limitations of current grant-free methods with fixed preamble lengths.

Contribution

It introduces a novel two-stage protocol with an estimation phase for active devices and a dynamic preamble allocation scheme to enhance detection performance.

Findings

The proposed protocol outperforms existing methods in identity detection accuracy.

Dynamic preamble allocation improves stability across varying active device counts.

Algorithms effectively estimate active devices, reducing detection complexity.

Abstract

This paper revisits the identity detection problem under the current grant-free protocol in massive machine-type communications (mMTC) by asking the following question: for stable identity detection performance, is it enough to permit active devices to transmit preambles without any handshaking with the base station (BS)? Specifically, in the current grant-free protocol, the BS blindly allocates a fixed length of preamble to devices for identity detection as it lacks the prior information on the number of active devices $K$. However, in practice, $K$ varies dynamically over time, resulting in degraded identity detection performance especially when $K$ is large. Consequently, the current grant-free protocol fails to ensure stable identity detection performance. To address this issue, we propose a two-stage communication protocol which consists of estimation of $K$ in Phase I and detection of identities of active devices in Phase II. The preamble length for identity detection in Phase II is dynamically allocated based on the estimated $K$ in Phase I through a table lookup manner such that the identity detection performance could always be better than a predefined threshold. In addition, we design an algorithm for estimating $K$ in Phase I, and exploit the estimated $K$ to reduce the computational complexity of the identity detector in Phase II. Numerical results demonstrate the effectiveness of the proposed two-stage communication protocol and algorithms.