Sensors_**2012**, _12_, 1827-1845; doi:10.3390/s120201827  
_Sensors_  
**ISSN 1424-8220**  
www.mdpi.com/journal/sensors  
_Article_  
**AURP: An AUV-Aided Underwater Routing Protocol for**  
**Underwater Acoustic Sensor Networks**  
**[PERSON] \({}^{\star}\), [PERSON], [PERSON] and [PERSON] \({}^{\star}\)**  
School of Electrical Engineering, University of Ulsan, Ulsan 680-749, Korea;  
E-Mails: [EMAIL_ADDRESS] (A.K.A. ); [EMAIL_ADDRESS] (H.O.) \({}^{\star}\) Authors to whom correspondence should be addressed; E-Mails: [EMAIL_ADDRESS] (S.Y. );  
[EMAIL_ADDRESS] (S.K.). _Received: 19 December 2011; in revised form: 20 January 2012 / Accepted: 7 February 2012 / Published: 9 February 2012_  
**Abstract:** Deploying a multi-hop underwater acoustic sensor network (UASN) in a large area brings about new challenges in reliable data transmissions and survivability of network due to the limited underwater communication range/bandwidth and the limited energy of underwater sensor nodes. In order to address those challenges and achieve the objectives of maximization of data delivery ratio and minimization of energy consumption of underwater sensor nodes, this paper proposes a new underwater routing scheme, namely _AURP_ (AUV-aided underwater routing protocol), which uses not only heterogeneous acoustic communication channels but also controlled mobility of multiple autonomous underwater vehicles (AUVs). In AURP, the total data transmissions are minimized by using AUVs as relay nodes, which collect sensed data from gateway nodes and then forward to the sink. Moreover, controlled mobility of AUVs makes it possible to apply a short-range high data rate underwater channel for transmissions of a large amount of data. To the best to our knowledge, this work is the first attempt to employ multiple AUVs as relay nodes in a multi-hop UASN to improve the network performance in terms of data delivery ratio and energy consumption. Simulations, which are incorporated with a realistic underwater acoustic communication channel model, are carried out to evaluate the performance of the proposed scheme, and the results indicate that a high delivery ratio and low energy consumption can be achieved. **Keywords:** underwater sensor networks; _ad hoc_ routing; AUVsIntroduction  
Underwater acoustic sensor networks (UASN) can provide a cost-effective underwater monitoring solution by tackling problems of traditional ocean monitoring approaches which rely on off-line monitoring [1] or underwater infrastructures [2], where costly underwater optical cables are used for communications between sensors and base stations. For example, UASN can be deployed without significant infrastructures for ocean environmental monitoring [3], seismic monitoring [4], underwater tactical surveillance [5], ecology and habitat monitoring. However, designing an UASN brings about several new challenges due to the power limitations of underwater sensor (U-sensor) nodes and the unfavorable characteristics of the underwater acoustic channel such as limited bandwidth, multi-path effect, fading, large propagation delay, and high bit error rate [4] (As an example of limited underwater communications, only about 48 Kbps data rate can be achieved for 2 kilometer distance over 23 KHz bandwidth [6], while terrestrial microwave communications can achieve over 100 Mbps for a distance of several kilometers [7]). More specifically, these limitations of the underwater communication make it hard to design a high performance network [8], _i.e._, a low available data rate, high variation of propagation delay depending on the distance, and a high bit error rate result in low network performance in terms of throughput and delivery ratio [9]. When multi-hop transmissions are required for data delivery in a large scale underwater network, it is obvious that the performance degradation becomes more severe due to the accumulated effects of the communication limitation over multiple forwarding processes.