Achieving high throughput and reliability in underwater acoustic networks for transmitting distributed and large volume of data is a challenging task due to the bandwidth-limited and unpredictable nature of the acoustic channel. In a multi-node network, such as in the Internet of Underwater Things (IoUT), communication link efficiency varies dynamically: if the channel is not in good condition, e.g., when in deep fade, channel coding techniques may fail to deliver the information even with multiple retransmissions. Hence, an efficient and agile collaborative strategy is required to allocate appropriate resources to the communication links based on their status. The proposed solution adjusts the physical and link-layer parameters collaboratively for a Code Division Multiple Access (CDMA)-based underwater network. An adaptive Hybrid Automatic Repeat Request (HARQ) solution is employed to guarantee reliable communications against errors in poor links. Results were validated using data collected from the LOON testbed-hosted at the NATO STO Centre for Maritime Research and Experimentation (CMRE) in La Spezia, Italy-and from the REP18-Atlantic sea trial conducted in Sept'18 in Portuguese water.
Using numerical simulations, authors in @cite_14 used the random linear packet coding to control the packet loss in a hierarchical definition of packets in the stop &wait ARQ protocol for the channels with a long propagation delay. @cite_28 , the authors applied fountain codes to HARQ in underwater networks to reduce retransmissions and achieve optimal broadcasting policies. An adaptive coding approach based on the IR-HARQ was proposed in @cite_15 to improve the packet error rate in a time-slotted underwater acoustic network. @cite_27 , we proposed a scheme based on HARQ that exploits the diversity gain offered by independent links of an underwater acoustic Multiple Input Multiple Output (MIMO) channel. A large number of papers can be found in the literature that investigate the efficiency of point-to-point HARQ, especially in the terrestrial environment.
@cite_28 @cite_27 @cite_15 @cite_14
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This paper explores hybrid ARQ policies based on fountain codes for the transmission of multicast messages in underwater channels. These rateless codes are considered because of two nice properties, namely, they are computationally lightweight and do not require to know the channel erasure probabilities at the receivers prior to transmission. In this paper, these codes are used together with a Stop and Wait ARQ to enhance the performance of broadcast communications. First, we present a dynamic programming model for the characterization of optimal broadcasting policies. The derived broadcasting rules are then compared against plain ARQ schemes via Monte-Carlo simulation. Our results show that digital fountain codes are a promising technique for the transmission over underwater channels as their performance, in terms of delay, reliability and energy efficiency, clearly dominates that of plain ARQ solutions. This paper is a preliminary study on the topic and encourages us towards the design of practical HARQ protocols for the underwater medium. Achieving robust and reliable data communications in the harsh underwater environment is still a challenging issue due to the fast-changing and unpredictable nature of the acoustic channel. Notwithstanding the use of acknowledgement messages at the link layer, data transmission protocols still require more efficient error control strategies so to achieve a higher link reliability. A novel solution based on hybrid automatic repeat request (HARQ) is proposed that exploits the diversity gain offered by independent links in an underwater acoustic Multiple Input Multiple Output (MIMO) system. Irrespective of the channel regime, the proposed scheme aims at reducing the probability of retransmission and at increasing the link reliability via packet-level codeword selection and waterfilling coding. Encoding and decoding algorithms at the transmitter and receiver, respectively, are designed, multiple evaluation metrics are defined, and computer-based simulation results are presented to quantify the performance improvement of the proposed methods. Underwater acoustic communication networks (UWANs) have recently attracted much attention in the research community. Two properties that set UWANs apart from most radio-frequency wireless communication networks are the long propagation delay and the possible sparsity of the network topology. This in turn offers opportunities to optimize throughput through time and spatial reuse. In this paper, we propose a new adaptive coding method to realize the former. We consider time-slotted scheduling protocols, which are a popular solution for contention-free and interference-free access in small-scale UWANs, and exploit the surplus guard time that occurs for individual links for improving transmission reliability. In particular, using link distances as side information, transmitters utilize the available portion of the time slot to adapt their code rate and increase reliability. Since increased reliability trades off with energy consumption per transmission, we optimize the code rate for best tradeoff, considering both single and multiple packet transmission using the incremental redundancy hybrid automatic repeat request (IR–HARQ) protocol. For practical implementation of this adaptive coding scheme, we consider punctured and rateless codes. Simulation results demonstrate the gains achieved by our coding scheme over fixed-rate error-correction codes in terms of both throughput and consumption of transmitted energy per successfully delivered packet. We also report results from a sea trial conducted at the Haifa harbor, which corroborate the simulations.