# UCMAC: A Cooperative MAC Protocol for Underwater Wireless Sensor Networks
## 1 Introduction
Existing cooperative MAC protocols for terrestrial wireless networks are largely based on the distributed coordination function (DCF) of the IEEE 802.11 standard [22], known as carrier sense multiple access with collision avoidance (CSMA/CA). In addition, for cooperator selection, these protocols largely rely on channel state information (CSI) between nodes. However, due to intrinsic channel characteristics and ever-changing channel states, these protocols do not work well in the underwater environment. In other words, utilizing the carrier sensing technique and keeping the CSI between all neighboring nodes up to date in the underwater channel are difficult. Until now, relevant research has taken into account such constraints to deal with several important issues such as cooperative signaling strategies [23], criteria for best cooperator selection [24, 25, 26] and cooperative routing protocols [27, 28]. To the best of our knowledge, however, researchers have not yet actively examined the cooperative MAC protocol in the underwater channel. Unlike the terrestrial case, underwater networking suffers from so-called _space-time uncertainty_ problem due to intrinsic channel characteristic of the long propagation delay [29]. The space-time uncertainty opens up new aspect on packet collision in the underwater channel, whereas this is irrelevant to terrestrial networks as the propagation delay is negligibly small and can be ignored. Therefore, we are motivated to devise a novel underwater-specific cooperative MAC protocol that can perform efficiently even with the space-time uncertainty. Figure 1: Cooperative communication. Based on our previous work [30; 31], we propose a reactive-type cooperative MAC protocol for the UWSNs, named Underwater Cooperative MAC (UCMAC). A source identifies cooperators and provides the destination with a list of cooperators along with information about their proximity to the destination. For erroneous reception of data packets, the destination then requests retransmission to the cooperators in a closest-one-first manner. A designated cooperator transmits the buffered data packet stored just in case to the destination. If no available cooperators exist, UCMAC follows the conventional MACA-U protocol [32] that basically uses 3-way handshaking (RTS-CTS-DATA) and optionally adds ACK without cooperation process. We evaluate the proposed scheme in terms of system throughput, latency, single-hop packet delivery ratio (PDR), and energy efficiency, comparing it to MACA-U and CD-MACA [12]. The novelty of this paper lies in the design of signaling procedure and in the calculation of the appropriate timer lengths of nodes to benefit from the cooperative diversity gains while overcoming the space-time uncertainty problem in the underwater channel. Also, we escape potential collisions between cooperators by making them cooperate one at a time and define a cooperative region to involve only qualified cooperators in cooperation process. The remainder of this paper is organized as follows: in Section 2, we review some related work. After describing a system model in Section 3, we explain how the UCMAC works in Section 4. Section 5 analyzes UCMAC's performance through computer simulation. Finally, we conclude the paper by presenting additional work in Section 6.