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Automatic repeat request (ARQ) schemes, and in particular hybrid-ARQ (HARQ) schemes, which jointly adopt forward error correction (FEC) and ARQ, are essential to provide reliable data transmission in wireless communications systems. However, the feedback from the receiver to the transmitter and the retransmission process used in ARQ incurs significant cost in terms of power efficiency, throughput, computational power and delay. Unfortunately, such drawbacks can limit their applications to several current and emerging technologies. More specifically, the increasing number of wireless users has create spectrum scarcity, relying on small-size batteries create power constraints, deployment of real-time applications boost the demand for ultralow delay networks, and the ultra-small low-cost internet of things (IoT) devices has limited signal processing and computation capabilities. Consequently, extensive research efforts have been dedicated to overcome the limitations inherent in HARQ. This survey paper provides an extensive literature review of the stateof-the-art HARQ techniques and discusses their integration in various wireless technologies. Moreover, it provides insights on advantages and disadvantages of particular ARQ types and discusses open problems and future directions.

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The bit error rate (BER) performance of turbo product codes (TPC) has been considered extensively in the literature. However, other performance metrics such as throughput can be more informative in particular systems. In this letter, the throughput performance of hybrid automatic repeat request (HARQ) is considered using TPC with iterative hard and soft decision decoding. Monte Carlo simulation and semi-analytical solutions are developed to evaluate the throughput of HARQ-TPC system for a wide selection of codes. The obtained results reveal that the coding gain advantage of the soft over hard decoding is reduced significantly when throughput is adopted as the performance metric, and it actually vanishes completely for some codes. When adaptive coding is used, the soft decoding advantage is limited to about 1.4 dB.Index Terms-Hybrid automatic repeat request, turbo product codes, subpacket systems, error detection, iterative decoding. I. INTRODUCTIONT URBO product codes (TPC), alternatively referred to as block turbo codes, are powerful forward error correction (FEC) codes that can provide high coding gain [1]. TPC are constructed by serially concatenating two linear block codes separated by an interleaver [2]. TPC support a wide range of codeword sizes and code rates, and they are included in some recent communication standards such as the IEEE-802.16 for fixed and mobile broadband wireless access systems [3] and the IEEE-1901 for broadband power line networks [4].The ultimate coding gain of TPC is achieved by performing a number of soft-input soft-output (SISO) iterative decoding processes that are applied to each row and column in the codeword matrix, which requires considerable computational power [5]. Consequently, reducing the computational complexity of TPC has received significant attention in the literature as reported in [5], [6] and the references listed therein. The computational complexity constraint of TPC becomes even more severe for systems that employ automatic repeat request (ARQ) protocol because particular packets have to be retransmitted, and hence decoded several times. The techniques that employ both FEC and ARQ are usually referred to as hybrid ARQ (HARQ) [7].In the literature, Al-Dweik et al. [5], [8], [9] proposed new techniques to reduce the SISO decoders complexity Manuscript