# Energy-Efficient Channel Coding Strategy for Underwater Acoustic Networks
## 2 Related Work  
The use of optimized FEC channel coding strategies for energy saving has been extensively investigated for terrestrial wireless networks, as, for instance, in [8; 9; 11]. For instance, in [11], the authors investigate the trade-off between transmission and processing energy consumption in wireless sensor networks. The study identifies and selects the appropriate complexity of the error control code to be used in each sensor node, in order to maximize the network lifetime. A detailed study developed in [8] established the optimal coding rate selection as a function of the transmit distance between communicating nodes for Additive White Gaussian Noise (AWGN) and Rayleigh fading channels in terrestrial setups. The energy efficiency achievable by ARQ schemes when the code rate of the error-correcting code is optimized is investigated in [9]. In the case of UAN, the optimization of channel coding strategies has also been investigated due to their potential energy savings. In [12], we investigated the use of binary frequency-shift-keying (FSK) modulation and convolutional error correcting codes in UAN. The operation frequency, SNR and code rate are optimized for a target frame error rate (FER). The results show that the proper choice of code rate has a great impact on the overall energy consumption and lifetime of the underwater communication devices. In [13], it was analyzed how much energy is required to successfully transfer an information bit over a multi-hop underwater acoustic network while considering the optimum number of hops, retransmissions, code rate, and SNR. In [14], an optimization process is used to obtain the optimal code rate, SNR and modulation order for energy efficient underwater communications with ARQ. The use of fountain codes in UAN was experimentally investigated in [15]. The transmitter sends coded packets with a predetermined redundancy over a simplex link with no feedback. The strategy uses Orthogonal Frequency Division Multiplexing (OFDM) and the authors investigate the relation between FER and the number of required transmissions. In [16], the use of random linear fountain codes associated with an adaptive power control strategy for energy savings in UANs is considered, in order to reduce the energy consumption. A recent study presented in [5] investigates a reliable transmit strategy for UAC which employs fountain codes and ARQ schemes. The authors optimize the number of transmitted coded packets in order to satisfy a pre-defined reliability at the receiver, assuming a stop-and-wait ARQ technique. An initial study of the energy consumption of UAN employing fountain codes is carried out in [17], whose main idea is to transmit an undetermined amount of coded bits so as to ensure a given target FER at the receiver without the need for retransmissions. Note that [17] employs a fixed number of coded bits, while the optimization in terms of the code rate may thus lead to a very small number of information bits, which may be a problem in longer distances, since the code rate becomes very small. In [18], a scheme in which the number of required coded packets is determined as to achieve a specified reliability criterion is proposed. The transmitter adjusts its power and the number of coded packets using the channel state information that is obtained via a feedback channel from the receiver. A new protocol for underwater acoustic communication using fountain codes was proposed in [19]. However, the protocol employs a hybrid strategy using automatic repeat request (ARQ), which requires a feedback communication channel. In [20] the authors addressed the problem of stopping sets occurring during fountain decoding in the UAN communication. In particular, they focused on improving the fountain encoding and decoding algorithms. A hybrid protocol, FOCAR, was proposed in [21]. It integrates fountain codes with hop-by-hop retransmission-upon-failure, requiring a feedback channel and employing selective repeat ARQ.