Hybrid Automatic Repeat Request Feedback Research Articles

Swift HARQ Based on Machine Learning for Latency Minimization in URLLC

Ultra-reliable low-latency communication (URLLC) has been introduced in the 5th Generation (5G) radios for mission-critical applications that demand strict reliability and latency traffic to guarantee the rapid delivery of short packets (up to 1 ms) with a success probability rate of 99.999%. The challenging reliability and latency requirements of URLLC have significant impact on the air-interface design, especially on the Hybrid Automatic Repeat reQuest (HARQ) mechanism. This study focuses on satisfying link latency requirements by reducing the delay that arises in the presence of the HARQ operation. To this end, we propose a Swift HARQ protocol empowered by machine learning techniques to estimate the decodability of a packet early enough within its maximum number of allowable retransmission attempts. This can allow the transmitter to react faster by dropping the non-decodable packets, or activating the repetition mode where parts of the HARQ feedback can be omitted. As shown through system-level simulations, the proposed model achieves a delay reduction of more than 50% compared to the traditional HARQ, and increases the system throughput by up to 40% when multiple HARQ retransmissions are required.

A HARQ Combining Method for FDD-Based Flexible Duplex System

We propose a hybrid automatic repeat request (HARQ) message combining method for frequency division duplex (FDD)-based flexible duplex systems. Utilizing a link condition between the base station (BS) and the user equipment (UE) while combining HARQ messages can reduce the throughput performance degradation caused by HARQ message bundling, which is necessary when the number of HARQ messages exceeds that of resources to report them. We analyze the average throughput performance according to the number of HARQ messages associated with multiple HARQ feedback bits and propose an algorithm to obtain an optimal solution. Moreover, we show the superiority of the proposed algorithm by evaluating the average throughput performance and the computational complexity.

Opportunistic spectrum access for cooperative unlicensed femtocells in two-tier heterogeneous networks utilizing HARQ feedback

The co-channel deployment of femtocells can lead to severe cross-tier interference to macrocell users which is resolved through the use of cognitive radio technology in which the femtocells, acting as unlicensed users, opportunistically access the spectrum of the primary macrocell in a cognitive way. This research proposes a cooperative scheme to improve the spectrum access opportunity for unlicensed femtocell in a two-tier heterogeneous network by exploiting the hybrid-automatic repeat request (HARQ) feedback information of macrocell transmission assuming the macrocell spectrum is considerably underutilized. This HARQ based scheme is very useful to get rid of the ‘hidden node problem’ and ‘exposed node problem’ encountered in conventional ‘listen-before-talk’ strategy used by the cognitive users. The outage performance of macrocell user is also improved due to cooperation from the femtocell. Performance analysis of the scheme is presented in Nakagami fading environment for a single femtocell scenario and compared with the conventional non-cooperative spectrum access via numerical examples. Appreciable improvements in terms of macrocell outage, macrocell and femtocell throughput over the conventional non-cooperative spectrum access have been found by considering a fixed macrocell traffic arrival rate. Moreover the dependence of femtocell location on the performance of the proposed scheme is further explored.

Finite Block-Length Analysis of the Incremental Redundancy HARQ

This letter studies the power-limited throughput of a communication system utilizing incremental redundancy (INR) hybrid automatic repeat request (HARQ). We use some recent results on the achievable rates of finite-length codes to analyze the system performance. With codewords of finite length, we derive closed-form expressions for the outage probabilities of INR HARQ and study the throughput in the cases with variable-length coding. Moreover, we evaluate the effect of feedback delay on the throughput and derive sufficient conditions for the usefulness of the HARQ protocols, in terms of power-limited throughput. The results show that, for a large range of HARQ feedback delays, the throughput is increased by finite-length coding INR HARQ, if the sub-codeword lengths are properly adapted.

Multi-user diversity with two-step channel state information feedback

This study proposes the performance of a scheduling scheme using two-step quantised channel state information (CSI) feedback. Considering fixed and random access networks, the results are obtained under both short- and long-term power constraints and for both single- and multi-layer transmission techniques. Further, the authors evaluate the asymptotic performance of the proposed scheme and show that, while the users scheduling is based on quantised CSI feedback in the first round, the second round of partial CSI feedback can be provided by either channel quantisation or hybrid automatic repeat request feedback, leading to the same forward channel average rate. In comparison with the case of no CSI feedback, the results indicate substantial average rate increment with limited number of feedback bits. Also, the average rate increases via successive partial CSI feedback when compared with one-step CSI feedback schemes.