# A Cross-Layer Optimized Opportunistic Routing Scheme for Loss-and-Delay Sensitive WSNs
## 3 Preliminary Knowledge and Research Motivation  
### Details of Opportunistic Routing  
As a basis for this paper, more detailed features of opportunistic routing are explained in this section. The progress of opportunistic routing are as follows: before sending a packet, a node first selects a candidate set \(G\) based on local information, and then broadcasts the packet to all candidate nodes. When the transmission is accomplished, the candidate set \(G\) determines an optimal node \(i\) to be the current forwarding node according to some specific metrics. After reaching an agreement, the optimal node \(i\) continues routing, while the other nodes in the collection \(G\) delete their local packets. The above steps are repeat until the destination node receives the packet.  
As shown in the Figure 1, the node \(S\) is the source node that need to route packets to the sink. According to the reliability model, the transmission range of node \(S\) is divided into three areas [56]: (1) Connected region. Nodes in this area are close to the node \(S\), so the reliability of transmission between them is high. (2) Disconnected region. Far from the node \(S\), nodes in this domain have a rather low success rate of receiving packets from the node. (3) Transition area, where nodes have a specific range of success rates. If the node in the connected region is selected as the receiving node, the communication link is reliable, but the forward distance per hop is small. If we turn to the disconnected area, the reception rate is rather low. Therefore, the opportunistic routing selects multiple nodes in the transition region and broadcasts. In this way, unless all chosen nodes fail to receive data, the current transmission is successful. Obviously, it is an efficient routing method with high reliability, and maintains a certain distance per hop.  
Most research [29] results show that the appropriate regional division is based on the quality of communication link between nodes. If the Packet Reception Rate (\(PRR\)) is less than 0.1, the node belongs to a disconnected area. Otherwise, if the \(PRR\) of the transition area is between 0.1 and 0.9, and the \(PRR\) of the connected area is greater than 0.9.  
For instance, node \(A\), \(B\) and \(C\) are the three candidate nodes selected by node \(S\) in its transition region. As can be seen in Figure 1, node \(A\) fails to receive data, while node \(B\) and node \(C\) succeed. Then, the candidate nodes sequentially confirm the reception status according to the priority order. First, Node \(A\) sends a Not Acknowledge (NAK) packet to node \(B\) and node \(C\), informing them that it has failed to receive the current data packet. Then, the next priority node \(B\) checks and sends an ACK packet to inform node \(C\) and the sending node \(S\) that its reception is successful. Finally, node \(C\) deletes the current local packet and one-hop transmission is successfully finished. This process is repeated until the packet reaching the sink.  
Assuming the number of candidate nodes is \(N\) and the transmission success rate between the send node and the receive node is \(p\), the one-hop reliability can be computed as:  
\[\zeta=1-(1-p)^{N} \tag{1}\]  
Furthermore, if the probability that any node in the \(A\), \(B\) and \(C\) successfully receives a packet from node \(S\) is equal to 0.7, the traditional routing method chooses one of them as the forwarding node directly, which renders that the one-hop reliability \(\zeta\) is equal to 0.7. In the same situation, however, the one-hop reliability of the opportunistic routing is \(\zeta=1-(1-0.7)^{3}=0.973\). From this, it can be seen that the opportunistic routing can improve the transmission reliability to a certain extent. The more the candidate nodes selected, the more obvious the improvement of the reliability.