Delay Tradeoff and Adaptive Finite Blocklength Framework for URLLC

TL;DR

The paper addresses ultra-reliable low-latency communications (URLLC) where end-to-end delays span sub-millisecond to seconds and packet sizes are often short, necessitating a finite blocklength (FBL) analysis. It introduces an adaptive blocklength framework that jointly optimizes time-frequency resources (blocklength per frame via variable TTI and bandwidth) to balance transmission, queuing, and retransmission delays under reliability constraints, informed by the tradeoffs introduced by FBL. A case study demonstrates a dynamic queuing and resource-allocation approach, employing DRL methods (DDPG and Multi-DQN) to solve multi-user scheduling problems and shows that adaptive strategies can substantially reduce over-the-air delay compared to fixed blocklength designs, especially for grant-free access. The work provides a principled, practical path to URLLC in 6G by integrating FBL theory, access protocol considerations, adaptive framing, and learning-based optimization, enabling flexible QoS across diverse IoE tasks.

Abstract

With various time-sensitive tasks to be served, ultra-reliable and low-latency communications (URLLC) has become one of the most important scenarios for the fifth generation (5G) wireless communications. The end-to-end delay from the sub-millisecond-level to the second-level is first put forward for a wide range of delay-sensitive tasks in the future sixth generation (6G) communication networks, which imposes a strict requirement on satisfying various real-time services and applications with different stringent quality of service (QoS) demands. Thus, we need to find out new delay reduction framework to satisfy the more stringent delay requirements. In this article, a state-of-the-art overview of end-to-end delay composition and delay analysis combined with access protocols are elaborated. We reveal the tradeoff relationship among transmission delay, queuing delay, and retransmission times with the change of blocklength in the finite blocklength (FBL) regime. Based on the tradeoff and combining the grant-free (GF) random access (RA) scheme, we propose the adaptive blocklength framework and investigate several effective algorithms for efficiently reducing the over-the-air delay. Numerical results show that our proposed framework and schemes can significantly reduce the over-the-air delay for URLLC.

Figures (4)

• Figure 1: The end-to-end delay components.
• Figure 2: Tradeoff relationship, adaptive blocklength framework, and variable TTI-based dynamic queuing process.
• Figure 3: The convergence comparison of DDPG, Multi-DQN, and Single-DQN.
• Figure 4: Delay performance comparison.