# A Novel Cooperative Opportunistic Routing Scheme for Underwater Sensor Networks
## 1 Introduction
In the underwater environment, forwarding set selection without a hidden terminal and prioritizing them are affected by features like a high error bit rate, energy consumption, node movement and slow propagation speed. Furthermore, some terrestrial opportunistic protocols are GPS-based, which make them inappropriate for the GPS-denied underwater environment. The redundant packet transmission issue is one of the influential factors on the opportunistic routing performance. When a group of candidate nodes are selected to collaboratively forward a packet while placed out range of each other, they cannot notice the transmission of any packet by other candidates. Thus, each forwarding node sets its forwarding timer and forwards the packet separately, resulting in more collisions and energy consumption. If the forwarding nodes are selected within the transmission range of each other (without any hidden node), this increases the chance of hearing the packet transmission by other higher priority candidate nodes, although there is no absolute guarantee, because of other factors, like shadow zone occurrence [21]. Nevertheless, some underwater routing protocols (e.g., Adaptive Hop-by-Hop Vector-Based Forwarding (AHH-VBF) [22], HydroCast [14], Void-Aware Pressure Routing (VAPR) [8]) take advantage of a group of forwarding nodes in the vicinity of each other with a timer-based coordination to eliminate the duplicated packet problem in the routing layer. It should be noticed that the hidden terminal problem still may exist in the other layers of the network, which is out of the scope of this work. In this paper, we propose a new opportunistic void avoidance routing (OVAR) protocol in order to increase the throughput and reliability in the sparse and lossy underwater environment while imposing less overhead in comparison to those protocols using high cost localisation to obtain their geographic coordinates in this environment. Furthermore, unlike the stateful protocols, which require global topology information, OVAR only depends on the information provided by one-hop neighbouring nodes. Each forwarding node selects its forwarding set with the aid of information obtained from the distributed beaconing mechanism initiated from the sink node. OVAR is able to bypass void areas before being stuck in a void node and simultaneously selects a group of candidate nodes with the highest advancement towards the sink. The forwarding set is selected in such a way that its members can hear each other and suppress duplicate transmissions, which leads to a decrease in energy consumption and congestion. In order to prevent energy wasting in a high-density forwarding set, the number of receiving nodes can be appropriately adjusted. The rest of this paper is organised as follows: In Section 2, we review the related work in this field. In Section 3, the details of the OVAR protocol are presented after introducing the network architecture and void problem. In Section 4, we make theoretical analyses about the energy consumption and reliability. Section 5 evaluates the performance of OVAR through simulations. In Section 6, we conclude the paper and discuss future work.