# A Cross-Layer Optimized Opportunistic Routing Scheme for Loss-and-Delay Sensitive WSNs
## 3 Preliminary Knowledge and Research Motivation
### Research Motivation  
Maintaining a reliable and low delay data transmission in wireless sensor networks is achieved by multiple factors. It is not only affected by the network routing layer, but also related to the sending power. Therefore, in order to effectively improve the network performance, a cross-layer optimization method is proposed in this paper. The research motivation is illustrated through the following experiments. The values of wireless sensor network parameters are given collectively in Table 1.  
The Packet Acceptance Rate (_PAR_) and _SNR_ have a progressive correlation (which can be seen from Figure 2) and the greater signal-to-noise ratio, the higher data packet acceptance rate. Figure 1: Illustration of opportunistic routing scheme. Figure 3 further shows the relationship between \(SNR\) and transmit power \(Pt\) with different distance \(d\). Obviously improving the \(SNR\) requires increasing the transmission power or reducing the transmission distance. Figures 4 and 5 respectively show the relationship between \(PAR\) and transmission power \(Pt\) as well as distance \(d\). The \(PAR\) will improve with the increase of the transmission power \(Pt\). Its growth \begin{table}
\begin{tabular}{c c c} \hline \hline
**Symbol** & **Description** & **Value** \\ \hline \(R_{D}\) & Sending rate/Receiving rate & 19.2 kbps \\ \(B_{N}\) & Noise bandwidth & 30 kHz \\ \(P_{n}\) & Background noise & \(-115\) dBm \\ \(d_{0}\) & Reference distance & 1 m \\ \(n\) & path loss exponent & 2 \\ \(L^{s}\) & Length of data packet & 128 Byte \\ \(L^{ACK}\) & Length of ACK packet & 8 Byte \\ \(L^{CTS}\) & Length of CTS packet & 8 Byte \\ \(N\) & Size of candidate set & 2 \\ \(R\) & Network radius & 1200 m \\ \(r\) & Transmission radius & 60 m \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Network parameters. Figure 3: Signal-to-noise ratio in different transmit power with different distance. Figure 2: Packet acceptance rate and signal-to-noise ratio. relationship can be divided into three segments. It can be seen that the value of _PAR_ is kept at 0 until it reaches a certain value and starts to grow rapidly, and finally it stays almost constant after reaching 1. In contrast, the _PAR_ decreases with the longer transmission distance \(d\). The descent relationship can also be divided into three similar segments (which can be seen in Figure 5). From the information in Figure 5, the impact of increasing the transmission power on the transition region can be generated as a farther and larger transition area with same one-hop reliability. If the transition region is fixed, the same communication link will become more reliable. However, increasing the transmission power requires more energy consumption. Unfortunately, the energy of a node in wireless sensor network is limited, so the improvement of transmission power is limited. After many experiments, we find that although the energy of wireless sensor networks is limited, the energy consumption of each node is very uneven. Figure 4: Packet acceptance rate and transmit power. Figure 5: Packet acceptance rate and transmission distance. As shown in Figure 6, since the nodes in the near-sink area need to relay the data packets from the nodes in far-sink area, the data volume of near-sink nodes is larger. This results in a very high energy consumption of the nodes near the sink, and a large amount of energy remaining in the nodes far away from the sink (which can be seen in Figure 7). Network performance can be improved by taking full advantage of the remaining energy. Based on these circumstances, two different methods to improve WSNS performance in COOR strategy are proposed in this paper. COOR(R) method: This keeps the transmission radius of each hop constant but improves the transmission power of each node in the far-sink region to a certain extent according to its energy surplus, which is related to the distance to the sink. Directly, the transmission reliability is improved greatly.