[Paper Review] Analyzing Grant-Free Access for URLLC Service

Yan Liu, Yansha Deng+3 more|arXiv (Cornell University)|Feb 18, 2020

IoT Networks and Protocols15 references4 citations

TL;DR

This paper proposes a spatio-temporal analytical framework to evaluate grant-free (GF) uplink access for URLLC services using three HARQ-based schemes—Reactive, K-repetition, and Proactive—under contention-based resource access. It introduces the latent access failure probability as a key metric and shows that Proactive GF achieves the lowest failure probability under short latency constraints, while K-repetition performs better under longer delays, depending on the value of K.

ABSTRACT

5G New Radio (NR) is expected to support new ultra-reliable low-latency communication (URLLC) service targeting at supporting the small packets transmissions with very stringent latency and reliability requirements. Current Long Term Evolution (LTE) system has been designed based on grantbased (GB) (i.e., dynamic grant) random access, which can hardly support the URLLC requirements. Grant-free (GF) (i.e., configured grant) access is proposed as a feasible and promising technology to meet such requirements, especially for uplink transmissions, which effectively saves the time of requesting/waiting for a grant. While some basic GF access features have been proposed and standardized in NR Release-15, there is still much space to improve. Being proposed as 3GPP study items, three GF access schemes with Hybrid Automatic Repeat reQuest (HARQ) retransmissions including Reactive, K-repetition, and Proactive, are analyzed in this paper. Specifically, we present a spatiotemporal analytical framework for the contention-based GF access analysis. Based on this framework, we define the latent access failure probability to characterize URLLC reliability and latency performances. We propose a tractable approach to derive and analyze the latent access failure probability of the typical UE under three GF HARQ schemes. Our results show that under shorter latency constraints, the Proactive scheme provides the lowest latent access failure probability, whereas, under longer latency constraints, the K-repetition scheme achieves the lowest latent access failure probability, which depends on K. If K is overestimated, the Proactive scheme provides lower latent access failure probability than the K-repetition scheme.

Motivation & Objective

• To address the limitations of grant-based access in meeting URLLC's stringent 1ms latency and 1−10−5 reliability requirements.
• To analyze contention-based grant-free uplink access with Hybrid ARQ (HARQ) retransmissions for URLLC services in 5G NR.
• To develop a tractable spatio-temporal analytical framework to evaluate reliability and latency performance under different GF HARQ schemes.
• To define and compute the latent access failure probability as a unified metric for URLLC reliability and latency evaluation.
• To compare the performance of three GF HARQ schemes—Reactive, K-repetition, and Proactive—under varying latency and load conditions.

Proposed method

• Proposes a spatio-temporal analytical framework combining stochastic geometry and point process models to analyze interference and outage in contention-based GF access.
• Models the downlink and uplink interference using Poisson cluster processes and derives Laplace transforms of aggregate intra- and inter-cell interference.
• Introduces the latent access failure probability as the complement of transmission success probability, accounting for both interference and feedback timing.
• Derives closed-form expressions for transmission success probability under K-repetition and Proactive HARQ schemes using binomial inclusion-exclusion and generalized hypergeometric functions.
• Uses the 3GPP-defined K-repetition scheme and a novel Proactive scheme where retransmissions occur regardless of feedback, with feedback only available after the 4th repetition.
• Applies the generalized hypergeometric function $_2F_1$ to model the interference distribution and derive the Laplace transform of inter-cell interference.

Experimental results

Research questions

• RQ1 How does the latent access failure probability vary across Reactive, K-repetition, and Proactive grant-free HARQ schemes under different latency constraints?
• RQ2 What is the impact of the number of repetitions K in the K-repetition scheme on the overall reliability and latency performance?
• RQ3 How does feedback availability after the 4th repetition affect the performance of the Proactive HARQ scheme compared to the Reactive scheme?
• RQ4 Under what conditions does the Proactive scheme outperform the K-repetition scheme in terms of access failure probability?
• RQ5 How does the proposed analytical framework accurately capture the trade-off between reliability and latency in contention-based grant-free uplink access?

Key findings

• Under short latency constraints (e.g., 1ms), the Proactive HARQ scheme achieves the lowest latent access failure probability due to its proactive retransmission strategy.
• Under longer latency constraints, the K-repetition scheme achieves lower failure probability than Proactive, with performance dependent on the value of K.
• If K is overestimated, the Proactive scheme outperforms the K-repetition scheme in terms of failure probability, indicating a performance trade-off based on K selection.
• The transmission success probability for the K-repetition scheme is derived in closed form using binomial coefficients and the generalized hypergeometric function $_2F_1$.
• The Proactive scheme’s success probability depends on feedback availability from the 4th repetition onward, leading to a dynamic change in the number of interfering users from the 5th repetition.
• The analytical framework successfully captures the interplay between interference, feedback timing, and retransmission strategies, enabling accurate performance evaluation of GF access for URLLC.