Hybrid Automatic Repeat Request Methods Research Articles

Highly-Efficient Low-Latency HARQ Built on NOMA for URLLC: Radio Resource Allocation and Transmission Rate Control Aspects

Hybrid automatic repeat request (HARQ) is an essential technology that efficiently reduces the transmission error rate. However, for ultra-reliable low latency communications (URLLC) in the 5th generation mobile communication systems and beyond, the increase in latency due to retransmission must be minimized in HARQ. In this paper, we propose a highly-efficient low-latency HARQ method built on non-orthogonal multiple access (NOMA) for URLLC while minimizing the performance loss for coexisting services (use cases) such as enhanced mobile broadband (eMBB). The proposed method can be seen as an extension of the conventional link-level non-orthogonal HARQ to the system-level protocol. This mitigates the problems of the conventional link-level non-orthogonal HARQ, which are decoding error under poor channel conditions and an increase in transmission delay due to restrictions in retransmission timing. In the proposed method, delay-sensitive URLLC packets are preferentially multiplexed with best-effort eMBB packets in the same channel using superposition coding to reduce the transmission latency of the URLLC packet while alleviating the throughput loss in eMBB. This is achieved using a weighted channel-aware resource allocator (scheduler). The inter-packet interference multiplexed in the same channel is removed using a successive interference canceller (SIC) at the receiver. Furthermore, the transmission rates for the initial transmission and retransmission are controlled in an appropriate manner for each service in order to deal with decoding errors caused by error in transmission rate control originating from a time varying channel. We show that the proposed method significantly improves the overall performance of a system that simultaneously provides eMBB and URLLC services.

Markov‐based analysis for cooperative HARQ‐aided NOMA transmission scheme in 5G and beyond

AbstractIn the fifth generation (5G) wireless communication system, the non‐orthogonal multiple access (NOMA) scheme has recently been proposed as a possible candidate for improving spectral efficiency. We look at power‐domain NOMA in this study, in which, multiple simultaneous transmissions are allowed at the same channel with different transmission powers. To achieve reliable transmissions in NOMA, the hybrid automatic repeat request (HARQ) scheme can be integrated into NOMA. The performance of a downlink NOMA system using the Type I HARQ method is investigated in this research. This type of HARQ does not entail expensive device which is considered one of the requirements for a massive 5G‐based Internet of Things network. Unfortunately, in the literature, it has been shown that deriving closed‐form expressions for the outage probabilities of HARQ‐aided NOMA systems is intractable due to the incorporation of multiple fractional random variables. Consequently, in this article, we follow a simple yet efficient methodology to study our considered HARQ‐aided NOMA system. The HARQ‐assisted NOMA system on the downlink is modeled as an absorbing discrete‐time Markov chain (DTMC). Thereafter, under perfect and imperfect channel estimation, we use this model to examine system performance in terms of outage probability and estimated number of retransmissions. Interestingly, we demonstrate that our DTMC‐based analysis is straightforward, repeatable, and accurate. Moreover, in this article, we propose a cooperative HARQ (CHARQ) scheme for the downlink NOMA system. According to CHARQ, the users cooperate with the base station in the retransmission process that will result in improving the performance of NOMA. Monte Carlo simulations have been conducted to not only verify the accuracy of our DTMC‐based analysis, but also manifest the superiority of CHARQ over HARQ.

Channel Constrained Multiple Selective Retransmissions for OFDM System: BER and Throughput Analysis

Hybrid automatic repeat request (HARQ) retransmission is an effective method to combat time-variant channel fading. Conventional HARQ methods initiate retransmission of failed packet without exploiting channel knowledge. When a packet fails, retransmission of data corresponding to high gain sub-carriers of an orthogonal frequency division multiplexing (OFDM) modulation is not necessary and deteriorates the throughput of the communication system. In this paper, we propose accumulated channel norm constrained selective retransmission (CNSR) at physical layer (PHY) in conjunction with HARQ method, which enhances throughput of the transceiver without compromising latency. When a packet fails, medium access control (MAC) layer requests retransmission of the failed packet. The proposed CNSR method initiates retransmission of data at PHY layer transmitted over low signal-to-noise ratio sub-carriers using outdated channel information. The transceiver pair continues retransmission of the failed packet until target channel norm of the sub-carriers is achieved or maximum number of retransmissions are completed. We maximize throughput by optimizing threshold on channel norm for OFDM modulation. We also present an upper bound on bit error rate and throughput analysis of the proposed method. The proposed method achieves higher throughput as compared with the existing Chase combining with selective repeat and conventional Chase combining schemes with and without forward error correction codes.

Fast-Decodable MIMO HARQ Systems

This paper presents a comprehensive study on the problem of decoding complexity in a Multi-Input Multi-Output (MIMO) Hybrid Automatic Repeat reQuest (HARQ) system based on Space-Time Block Codes (STBCs). We show that there exist two classes of fast-decodable MIMO HARQ systems: independent and dependent STBC structures. For the independent class, two types of protocols namely, fixed and adaptive threshold-based are presented and their effectiveness in both computational complexity reduction and spectral efficiency preservation are discussed. For the dependent class, a fast Sphere Decoder (SD) algorithm with a low computational complexity is proposed for decoding process of HARQ rounds. Two new concepts are introduced to leverage the fast-decodable notion in MIMO HARQ systems. Simulation results show that the proposed fast-decodable MIMO HARQ protocols in both classes provide a significant reduction in the decoding complexity as compared with the original MIMO HARQ method.

Selective HARQ Transceiver Design for OFDM System

We present a novel selective Hybrid Automatic Repeat reQuest (HARQ) based transceiver design for orthogonal frequency division multiplexing (OFDM) system. The proposed method is bandwidth efficient and has lower complexity as compared to conventional HARQ method adopted by communication standards such as long-term evolution (LTE). Our transceiver design introduces an additional retransmission layer at OFDM modulation level, which is independent of conventional HARQ methods. Instead of calculating computationally expensive soft information and applying forward error correction (FEC) on the soft information, receiver requests retransmission of information symbols corresponding to the subcarriers that have signal-to-noise ratio (SNR) below a set threshold at modulation level. We also provide criteria for selective retransmission and throughput analysis with new selective retransmission approach. We demonstrate that with limited feedback at modulation layer level, the proposed method enhances throughput of the system in all SNR regimes. The proposed selective HARQ method provides great flexibility for an application to optimize throughput based on its bit error rate (BER) requirement.