# A Cross-Layer Optimized Opportunistic Routing Scheme for Loss-and-Delay Sensitive WSNs
## 5 The Design of the COOR Scheme
### Transmission Power Optimization
Therefore, the reliability of the transmission can be improved by increasing the transmission power of the node to different degrees, and the farther the node is from the sink, the higher the transmission power is. The core part of the COOR scheme is to calculate the transmission power of nodes at different distances. Considering the COOR(R) method, since the receiving and forwarding structure of the network is mainly determined by the transmission radius, as long as it remains unchanged, the forwarding region assumed by each region in a round of transmission will not change. However, when the transmission power of the nodes is increased, the reliability of the network is improved. As the amount of data in the network increases, the energy consumption of the nodes is bound to raise. In view of this situation, when calculating the transmission power, it is necessary to estimate the amount of data in the optimized network and select the reference threshold energy. **Theorem 4**. : _Suppose the value of reference threshold energy is \(E_{\theta}^{TOT}\). In COOR (R) method, the transmit power of a node \(n_{l}\) with a distance \(l\) to the sink is:_  
\[S_{l,k}=\rho s=\rho ld_{\theta}d_{x}\]7  
_where \(Q_{l}^{S},\ Q_{l}^{R},\ Q_{l}^{C},\ Q_{l}^{A}\) respectively indicates the number of send, receive, \(CTS\), \(ACK\) packets estimated for the node \(n_{l}\)._  
Proof. : Putting Equations (6)-(10) together, the estimated energy consumption of the node \(n_{l}\) under unsteady power can be expressed as:  
\[S_{l,k}=\rho s=\rho ld_{\theta}d_{x}\]8 \[S_{l,k}=\rho s=\rho ld_{\theta}d_{x}\]9  
Figure 10:0 Energy residual degree of node at different distance. Figure 10:1  
Figure 10:2  
When \(P_{t}(l)\) equals \(\frac{R_{D}\cdot E_{l}^{TOT}+PL(r)\cdot L^{r}Q_{l}^{R}}{L^{s}Q_{l}^{s}+L^{r}Q_{l }^{s}+L^{ACK}Q_{l}^{r}}\), if the estimated amount of data is completely accurate, then the energy consumptions of all the nodes will be exactly balanced (values \(E_{\theta}^{TOT}\)). However, the data volume of the node is influenced by the transmission success rate, and the transmission success rate is directly related to the transmit power. Therefore, a heuristic method for estimating data volume is provided to obtain an approximate optimal solution of transmission power. \(\square\)  
According to Theorem 2, to estimate the data volume of the node one only needs to evaluate the reliability of transmission. Obviously, the transmission success rate of nodes near the sink is basically unchanged, and it reaches the upper limit 1 as the distance gradually increases. Therefore, the binary search method can be used to quickly enumerate the distance \(l\), at which the reliability of transmission reaches 1. Assuming that the transmission success rate changes linearly and remains at 1 after \(l\), then the predicted value and the calculated value can be obtained. Comparing the two values, the estimation is unacceptable unless the error is less than 3%. By this way, all nodes' transmit power are determined. As mentioned above, the improvement of the transmit power of the node increases with its distance to the sink, and the transmission distance of the node is fixed. Therefore, the reliability of transmission will increase with the distance to the sink. At the same time, according to the energy formula, if the amount of data carried by the node is overestimated, the node will have an energy surplus. The reference threshold energy can select the value of maximum energy consumption in the traditional routing method, that is \(E_{\theta}^{TOT}=max(E_{l}^{TOT})\). When the reliability of the network is relatively low, the reference energy should be properly increased. Figure 14 indicates the transmission power under the COOR (R) method when the initial value of the nearest sink node is \(P_{t}=-3\) dBm. The result of packet quantity difference (predicted value--calculated value) is given in Figure 10:3 In the COOR (P) method, since the transmission distance is increased, the original network structure is changed.