# Energy-Efficient Channel Coding Strategy for Underwater Acoustic Networks
## 1 Introduction  
Underwater acoustic networks (UANs) are used in a wide range of applications [1], including biological ecosystem monitoring, oceanography data collection, fishing exploration, pollution control, sub-aquatic surveillance [2; 3; 4] and underwater autonomous vehicles [5]. Basically, a UAN consists of sensor nodes disposed at different depths in the underwater environment, which are interconnected by gateways using acoustic links and surface stations to receive and process the collected data. The underwater acoustic channel is affected by attenuation, noise and multipath propagation [6], which leads to long propagation delays, transmit errors and reduced channel bandwidth [7].  
In order to tackle these problems, the UAN may employ channel coding techniques similar to those used in wireless sensor networks (WSNs); these techniques can improve the system performance and reduce the energy consumption. Energy efficiency in UAN may be even more important than in WSN because the impairments imposed by the underwater environment can be extremely challenging. Typically, in UAN communication, the transmit energy consumption is 100 times greater than the reception one [3]. Additionally, it may be difficult and costly to replace the batteries of the sensor nodes. Error control strategies, such as forward error correction (FEC) codes and fountain codes can beused in order to improve the transmit efficiency and reduce energy consumption at the transmitter and receiver. The use of fountain codes is motivated by the long propagation delays introduced by classical automatic repeat request (ARQ) schemes in the underwater acoustic channel, which may considerably reduce the throughput when multiple retransmissions are required. Moreover, ARQ schemes cannot be applied in scenarios when a feedback channel is not available. Additionally, the fountain code approach may be useful in multicast scenarios where receivers experience different channel conditions that are unknown to the transmitter.  
Motivated by the proposed energy-efficient optimization of FEC coding schemes for WSNs presented in [8; 9] and the benefits of fountain codes [10] for erasure channels, we investigate the energy efficiency by jointly using FEC and fountain coding schemes in underwater acoustic links. We aim at defining the best FEC encoding parameters for the mixed FEC/Fountain strategy in order to reduce the energy consumption in the communication link and therefore extend the lifetime of the UAN.  
This paper proposes a mixed FEC/Fountain transmission strategy for the underwater acoustic channel. An energy-efficient communication link is obtained through the use of optimized FEC code parameters. We model and evaluate the energy consumption of different channel coding schemes for a \(K\)-distributed multipath fading channel. We determine the optimal coding parameters to be selected as a function of the link distance or operating signal-to-noise ratio (SNR) at the receiver. The results show that the energy consumption can be significantly reduced by the proposed optimization.  
The remainder of this paper is organized as follows. Section 2 discusses related work. The underwater acoustic channel and the system model are described in Section 3. The proposed mixed FEC/Fountain transmit strategy is presented in Section 4. The results are shown in Section 5. Conclusions are summarized in Section 6.