# Cooperative Opportunistic Pressure Based Routing for Underwater Wireless Sensor Networks
## 1 Introduction  
Recently, UWSNs have gained much attention due to high demand of these networks in many applications. Like prediction of natural disasters, security, measurement of water traits, exploration of underwater world, pollution control, exploration of natural resources in underwater, etc. However, unique characteristics of the aquatic environment have posed many challenges for underwater communication [1], like low bandwidth, high propagation delay, path loss, high bit error rate (BER), etc. resulting in high energy consumption, low reliability of data packet delivery, etc. [2]. In such conditions, opportunistic routing is a remarkable technique in sensor networks due to its redundant transmissions. However, the reliability is achieved by redundant packet transmission that is one of the influential factor on the performance of the opportunistic routing. As shown in Figure 1, let a source node (S) broadcasts a data packet to its neighbor nodes (\(PFN_{1}\), \(PFN_{2}\), \(PFN_{3}\), \(PFN_{4}\)), the \(PFN_{1}\) has highest priority because it is deployed near the sink. It will forward a data packet and acknowledge its neighbor nodes in its transmission range (\(PFN_{2}\), \(PFN_{3}\)). The \(PFN_{4}\) will not overhear the acknowledgement from \(PFN_{1}\) because it is deployed outside its transmission range. Therefore,after its specific holding time, it will consider that the data packet is not forwarded and it broadcasts the data packet resulting in redundant transmission and unnecessary energy will be consumed [3]. Although some pressure based routing protocols for underwater sensor networks [4] take advantage of the redundant transmissions to recover the data packets from void holes. Similarly, in [5], the authors proposed analysis of the cooperative routing with incremental best relay scheme. In this scheme, a single bit message received from a node that indicates whether the data packet transmitted directly, received successfully or not. If the data packet is not within the predefined threshold then a relay node broadcasts the data packet which is combined at the destination to obtain the error free packet. Authors [6] presented void aware pressure routing (VARP) algorithm to propagate periodic beacon messages in the entire network for establishing paths to the nearest sink. The data packets are relayed via multiple paths to the destination for reliable data delivery. Similarly, the authors in [7] suggested an energy efficient cooperative scheme for the selection of the best relay node amongst the successful candidates at each hop those received the data packet successfully. However, limitations of [4; 5; 6; 7] are: (1) long routing paths lead to the path loss and high propagation delay; (2) They are efficient in dense network conditions however when the network sparsity increases, the length of routing paths increases that increase the breakage probability of the path from the source to the destination resulting in high packet drop ratio. Therefore, Co-Hydrocast, improved Hydrocast and Co-improved Hydrocast opportunistic based routing protocols are proposed in this paper. These protocols are presented to increase the network throughput and reliability of the data transmissions in the dynamic nature of the aquatic environment by shortening the recovery path between the source and the destination. These routing techniques avoid void node occurrence by obtaining neighbors of a next relay node up to two hops. Each forwarding node selects its best relay node having at least one neighbor and minimum distance relative to the sink. The redundant transmissions are avoided while ensuring that each candidate node listens to each other before transmitting a data packet towards the destination that results in minimum energy consumption and high network throughput. The rest of the paper is organised as follows: In Section 2 related work is presented. Details of our proposed schemes are discussed in Section 3. Section 4 validates the performance through simulation results of our proposed techniques.