# Energy-Efficient Channel Coding Strategy for Underwater Acoustic Networks
## 2 Related Work
They optimize the data block size on every intermediate node by using a centralized optimization procedure that requires packet error rate information from every hop. The study does not consider the use of specific FEC codes and their interaction with fountain codes. In [22], the authors proposed a broadcasting algorithm for UANs, where a source node disseminates information to a number of nodes randomly placed within a given geographical area. A mathematical model to characterize the performance of a hybrid ARQ based fountain code scheme was derived. No practical broadcasting protocol was proposed and the use of FEC codes is not addressed. In [23], a hybrid ARQ scheme based on fountain codes for the transmission of multicast messages in underwater channels is proposed. The scenario considers the use of fountain codes with a stop-and-wait ARQ strategy in order to enhance the performance of broadcast communications. The use and selection of FEC code parameters are not investigated. In [24], a fountain-based reliable unicast transmission scheme by exploiting the estimating process of channel state information (CSI) is proposed. The scheme employs a specific class of fountain codes known as Raptor codes. In order to improve the goodput, the authors apply stochastic optimization to adapt the parity ratio. Our study differs from [24] in the sense that we consider a two-layered coding scheme and formulate the optimization process focusing on energy consumption. Additionally, we employ a relatively simple class of FEC codes (BCH codes), with respect to the implementation, which is a desirable characteristic for the encoding and decoding processes in sensor nodes with limited energy resources. In [25], the authors investigate a layered coding approach that uses error-correction coding within each packet and erasure-correction coding across the packets. They addressed a wireless link scenario and determined the optimum trade-off in splitting redundancy between error-correction and erasure-correction codes. The results determine the optimum trade-off in splitting redundancy between error-correction coding and erasure-correction codes, which depends on the fading statistics and the average SNR of the wireless channel. The study assumes a theoretical premise that the system employs capacity-achieving channel coding. Their optimization aims at achieving a given error performance with a maximum rate. Differently, besides considering an underwater channel model and other related parameters, our approach considers practical block codes, fast fading and we model the optimization problem from an energy consumption perspective. Thus, in our work, the coding parameters are optimized focusing on maximizing the network energy resources, not the network rate, which may be more desirable in the case of sensor networks. Against the above background, in this work, we propose a mixed FEC/fountain coding scheme for UAN using a two-layer approach, in which we are able to adjust the rate of the FEC code and the number of coded packets in the fountain code as to achieve a given target reliability while minimizing the energy consumption. Moreover, the proposed method operates without a feedback channel, being therefore suitable for multicast networks. Our study finds applicability in protocol development, network topology planning and network performance optimization.