## 1 Introduction  
Underwater acoustic sensor networks (UASNs) are attracting huge interest due to their wide variety of applications, such as environmental monitoring, resource investigation, detection of phenomena related to natural disasters, _etc._ Naturally, all these mission-critical applications highly require reliable data transmission techniques. To design a feasible transmission scheme, it is important to take into account the harsh and limited conditions of the underwater environment. Radio signals suffer from severe path losses in underwater scenarios, thus acoustic signals are typically employed. However, underwater acoustic links also suffer from path losses, time varying multi-path fading, motion-induced Doppler spread and aquatic noise. As a result, the underwater acoustic link often offers a high bit error rate (BER). To establish reliable communications under such poor channel conditions, an efficient retransmission scheme is needed. In general, Automatic Repeat reQuest (ARQ) schemes have been used to achieve high reliability in data transmission. Especially, in underwater acoustic sensor networks, the S&W ARQ protocol has been employed as the only way of retransmission due to the half duplexing mode of underwater acoustic modems [1,2], but high BER of the acoustic channel along with long propagation delays are making it more challenging to achieve high throughput efficiency with the conventional S&W ARQ. In recent years, several modifications in S&W ARQ have been used to improve its efficiency. Cooperation-based ARQ is one of these alternatives which utilizes the communication channels of neighbor nodes to achieve a form of cooperative diversity that can provide significant performance improvement. Particularly for UASNs, CARQ has been studied to achieve throughput enhancements [3], among other benefits [4, 5, 6, 7]. For terrestrial wireless sensor networks (WSNs) too, cooperation-based ARQ strategies have been studied and proved very efficient [8,9]. As evident from the past literature, ARQ techniques have always benefitted from the utilization of FEC codes for improving their performance; obviously, the cooperation-based ARQ schemes are also eligible to employ them. For terrestrial wireless networks, the expected enhancement in performance when a cooperation-based ARQ is combined with the error correction technique of IR-HARQ, to form a cooperative hybrid ARQ, has also been verified [10, 11, 12]. Naturally, this concept of cooperative hybrid ARQ can be applied to UASNs as well, so as to extract the best performance, at least from the link layer, under the constraints of the harsh underwater environment. However, the performance results of cooperative hybrid ARQs obtained for terrestrial sensor networks cannot be directly applied to underwater sensor networks, because of the significant distinctions in BER and propagation delay (sound speed = 1,500 m/s, speed of radio signals = 3 \(\times 10^{8}\) m/s). Especially for underwater environments, the one-by-one transmission of the redundancies of IR-HARQ might accumulate enough long delays to nullify any performance gains achieved by error-correction coding. Thus, in order to fully extract the performance gains of C-HARQ, the protocol needs to be designed and studied exclusively for underwater scenarios. To the best of our knowledge, a cooperative hybrid ARQ (C-HARQ) protocol for UASNs has not yet been designed and investigated; hence this paper presents a C-HARQ protocol which employ RCPC codes to implement IR-HARQ along with cooperation features. Moreover, sensor networks have large numbers of nodes located in close proximity with each other, which makes it important to consider the effect of multiple neighbor nodes on throughput and energyefficiency when designing any cooperative protocol for UASNs. However, the previous analysis considers only single relay assisted cooperation [10; 11; 12].