# UCMAC: A Cooperative MAC Protocol for Underwater Wireless Sensor Networks
## 1 Introduction  
In recent years, researchers have actively studied numerous applications of underwater wireless sensor networks (UWSNs), including tactical surveillance, disaster prevention, and oceanographic observation [1]. In particular, studies have proposed an array of medium access control (MAC) protocols to enable communicating nodes to access the shared underwater channel. Compared to terrestrial radio signals, underwater acoustic signals suffer from high attenuation, long propagation delays, severe multipath fading, and high bit error rates (BER) in the channel. Achieving reliable delivery of data packets under such poor channel conditions necessitates the use of the automatic repeat request (ARQ) and/or the forward error correction (FEC) techniques at the link layer. Traditionally, ARQ methods are divided into three categories: stop-and-wait (S&W), go-back-n (GBN), and selective repeat (SR) [2]. In the S&W method, a transmitter waits for an acknowledgement (ACK) from a receiver after transmitting a data packet and then retransmits the data packet if it fails to receive the ACK. Many existing MAC protocols for the UWSNs [3; 4; 5] employ the S&W method due to its simplicity and suitability for half-duplex channels. However, long propagation delays in the acoustic signals make the S&W method inefficient because the sender's lengthy waiting time wastes resources. In the GBN and SR methods, on the other hand, senders can transmit data packets continuously while waiting for receivers' ACKs. This can increase network throughput at the expense of receiver processing complexity, but it requires the full-duplex system for concurrent delivery of ACK packets, which has yet to prove workable in the bandwidth-limited underwater channel. Another newapproach is the so-called _cooperative ARQ_, which is based on the cooperative communication technique. The basic idea of the cooperative communication technique is that neighbor nodes (cooperators) can provide an alternative path for other pairs of communicating nodes (source-destination), as shown in Figure 1. This generates spatial diversity by enabling the transmission of independent copies of the signal, thus allowing independently faded versions of the signal at the destination [6]. Earlier research on cooperative communication--mainly conducted in the terrestrial domain--focused on issues such as theoretical analysis based on information theory and its implementation aspects [7, 8]. Such analyses led to the creation of link layer protocols to gain benefits by coordinating nodes thereafter. Some studies [9, 10] proposed a cooperative ARQ scheme that simply describes the behavior of the nodes for cooperative retransmission, analyzing performance using their own analysis model, while others [11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21] proposed a so-called _cooperative MAC protocol_ that mostly combines the cooperative ARQ mechanism with the MAC protocol. Depending on the cooperative mechanism, the cooperative MAC protocols can be categorized as a _proactive_ type, a _reactive_ type, or a _hybrid_ type. In proactive-type protocols, the cooperation process mostly starts before a destination receives an initial data packet from a source, whereas reactive-type protocols are an on-demand method where the destination requests cooperation only when it fails to receive the data packet. A major problem with proactive-type protocols is that cooperators may send a redundant data packet to the destination even when the destination succeeds in receiving the initial packet from the source. This redundancy leads to a waste of energy but can decrease delay with a high chance of successful destination decoding. On the other hand, the reactive-type protocols can save energy at the expense of delay. Both features of the proactive- and reactive-type protocols combine to form hybrid-type protocols.