# Cross-Layer-Aided Opportunistic Routing for Sparse Underwater Wireless Sensor Networks
## 2 Relate Works  
Traditional routing protocols establish a complete transmission path before forwarding data. However, these protocols are inefficient for UWSNs due to the distinctive nature of the underwater environment. In contrast, OR protocols establish an online route for each transmission, adapting well to time-varying underwater channels. Typical OR protocols are composed of two procedures: Candidate Set Selection (CSS) and Candidate Set Coordination (CSC) [16; 17]. The first procedure is responsible for selecting a set of neighbor nodes as potential next-hop forwarders. The second procedure coordinates the transmission between the current sender and next hop forwarder. In this paper, we review recent OR protocols by categorizing the CSS into receiver-side-based, sender-side-based, and hybrid approaches. In addition, we review the cross-layer aided OR protocols in the last categories, since these protocols exploit hybrid approaches to accomplish CSC. In receiver-side-based CSS, the receiver decides whether to forward a packet. The node that receives the packet will check whether it belongs to the candidate set by the CSS algorithm. Then, only the nodes in the candidate set perform the candidate set coordination. A typical protocol that has been earlier proposed is vector-based forwarding (VBF) [18]. In VBF, each source node establishes a virtual three-dimensional (3D) pipe toward the sink. The neighbor nodes in the pipe and on the route to the sink will be added to the candidate set. Each candidate node that has received the data packet evaluates the priority according to its location information, and then sends the data packet in the order of priority. A protocol based on VBF, called hop-by-hop vector-based forwarding (HH-VBF), adds more potential forwarders to the candidate set by adjusting the size of pipes [19]. Depth-based routing (DBR) is a typical OR protocol using depth information to divide candidate sets [20]. In DBR, the neighbor nodes with lower depths are added to the candidate set. Energy-efficient cooperative opportunistic routing (EECOR) also determines the candidate set based on the depth information [21]. In addition, it introduces a fuzzy logic-based relay selection scheme to select an optimal next hop. In EECOR, only the energy consumption and packet deliver rate are considered in fuzzy function. To further optimize the candidate algorithm, energy-efficient optimal relay selection (Co-EEORS) selects the next-hop forwarder using physical distance to the sink together with the depth information of each potential forwarder [22]. A coding-aware opportunity routing method for sparse UWSNs (CORS) develops a forwarding with opportunistic coding (FOC) strategy based on network coding [23]; a sliding window-based coding algorithm provides effective coding gains with low coding overhead, and the coding process is triggered by an incoming packet within a valid coding window. For the protocols in this category, the position information is required to be updated periodically, which may be energy inefficient and may degrade the network performance. Due to the fact that the nodes in candidate sets only forward packets rather than being responsible for interacting with the sender, these protocols will suffer from void problems. Moreover, due to the limitation of integrating with contention-based MAC protocol, the hidden terminal problem cannot be effectively resolved. Furthermore, the network performance may degrade heavily in higher traffic rates and in networks with non-uniform topology. In sender-side-based CSS, only the sender executes the CSS algorithm. The candidate set of each node is established before each transmission by using the history knowledge of the neighboring nodes in the network. In Hydraulic pressure-based anycast routing (HydroCast), the neighbor nodes with lower depths are added into a candidate set according to the candidate algorithm [24]. Then, the candidate set will be divided into many sub-candidates where the node in the same sub-candidate can overhear each other.