Sensors_**2009**, \(9\), 6626-6651; doi:10.3390/s90906626  
_Article_  
**Energy Balanced Strategies for Maximizing the Lifetime of Sparsely Deployed Underwater Acoustic Sensor Networks**  
**[PERSON] \({}^{1,\star}\), [PERSON] \({}^{1}\), [PERSON] \({}^{2}\), [PERSON] \({}^{1}\) and [PERSON] \({}^{1}\)**  
\({}^{1}\) Department of Computer Science, Ocean University of China, Qingdao, China;  
E-Mails: [EMAIL_ADDRESS] (Z.-W.G. ); [EMAIL_ADDRESS] (H.F.); [EMAIL_ADDRESS] (Y.F.) \({}^{2}\) Department of Computer Science & Engineering, Hong Kong University of Science and Technology, Hong Kong, China; E-Mail: [EMAIL_ADDRESS] (K.-S.W.) \({}^{\star}\) Author to whom correspondence should be addressed; E-Mails: [PERSON]L_ADDRESS];  
[EMAIL_ADDRESS]; Tel. : +86-532-66781127; Fax: +86-532-66781127. _Received: 9 June 2009; in revised form: 9 August 2009 / Accepted: 10 August 2009 / Published: 24 August 2009_  
**Abstract:** Underwater acoustic sensor networks (UWA-SNs) are envisioned to perform monitoring tasks over the large portion of the world covered by oceans. Due to economics and the large area of the ocean, UWA-SNs are mainly sparsely deployed networks nowadays. The limited battery resources is a big challenge for the deployment of such long-term sensor networks. Unbalanced battery energy consumption will lead to early energy depletion of nodes, which partitions the whole networks and impairs the integrity of the monitoring datasets or even results in the collapse of the entire networks. On the contrary, balanced energy dissipation of nodes can prolong the lifetime of such networks. In this paper, we focus on the energy balance dissipation problem of two types of sparsely deployed UWA-SNs: underwater moored monitoring systems and sparsely deployed two-dimensional UWA-SNs. We first analyze the reasons of unbalanced energy consumption in such networks, then we propose two energy balanced strategies to maximize the lifetime of networks both in shallow and deep water. Finally, we evaluate our methods by simulations and the results show that the two strategies can achieve balanced energy consumption per node while at the same time prolong the networks lifetime. **Keywords:** acoustic communications; energy efficiency; underwater acoustic sensor networks; data propagation; energy balancing Introduction  
Underwater Acoustic Sensor Networks (UWA-SNs) have recently been drawing much attention because of their potential applications ranging from oceanographic data collection, environment monitoring, structure monitoring, tactical surveillance to disaster prevention [1; 2]. However, UWA-SNs are very different from existing terrestrial sensor networks due to the properties of the underwater environments. Firstly, UWA-SNs use acoustic signals to communicate, thus the propagation delay is large due to the slow acoustic signal propagation speed (\(1.5\times 10^{3}m/s\)). Secondly, the underwater acoustic communication channel has limited bandwidth capacity because of the significant frequency and distance dependent attenuation. Currently, the limit on available underwater bandwidth is roughly 40 km\(\times\)kbps [3; 4]. Thirdly, due to economics and the potentially large areas of interest in the ocean, UWA-SNs are mainly sparse networks nowadays [2; 3]. For such networks, instead of randomly deploying the sensor nodes, it is common to deploy the nodes manually with help of ships [5]. To deploy such a long-term UWA-SNs, one of the main challenges is the limited energy resources of the sensors because they are battery-powered and it is even harder to recharge node batteries in underwater environments. With such sparsely deployed networks, the energy balanced dissipation of individual nodes become very important, and it is desirable to distribute energy consumption of each node evenly, thus nodes may die together and we can change their batteries or replace them by new nodes simultaneously for the whole area.