# A Cross-Layer Optimized Opportunistic Routing Scheme for Loss-and-Delay Sensitive WSNs
## 2 Related Work
### Loss Sensitive Works in WSN
Especially when the data packets is lost during delivery, the sender needs to wait for a relatively long round trip time (RTT) before each retransmission. Furthermore, a send-and-wait (SW) protocol is an important mechanism to protect the reliability of data transmission [47]. In subsequent studies a variety of improved methods are proposed, such as the Goback-N (GBN) protocol, or the selective repeat(SR) [47] algorithm. Specifically, a strategy for high energy efficiency of wireless sensor networks to ensure data transmission reliability can be seen in [47]. (3) Data collection by broadcasting is an effective way to improve data transmission reliability. [PERSON] and [PERSON] [52] proposed a method of broadcast data transmission in a fusion network. In the network of [52], the data of an infinite number of nodes can be merged into one data packet when they meet, so the amount of data that the network needs to transmit is hardly increased. Thus, in the method of [52], each node routes during broadcast, and the receiver node collects data packets from all child nodes and merges them into one packet to continue broadcasting to the sink. Since the fused data packet contains all the information of the received data packets, it is equivalent to broadcasting each data packet and routing multiple copies to the sink along different paths. However, the biggest limitation of this method is that it requires multiple data packets be fused into one packet. Otherwise, a broadcast storm will be formed and the network energy will be quickly consumed. On this basis, for a more general data fusion network readers can refer to [29]. (4) Improve the reliability of data transmission by multi-layer optimization in [29]. The reliability of data transmission is directly determined by the signal-to-noise ratio (SNR). With the increase of the signal-to-noise ratio of the receiver, the probability of correctly receiving data increases and the bit error rate (BER) decreases. Although there are many factors that influence signal-to-noise ratio, the external environment of sensor nodes is not selectable after deployment, so the most effective way to improve SNR is to increase the transmission power of the sender. Unfortunately, with very limited energy of the wireless sensor node, increasing the transmission power will seriously affect the network lifetime. In wireless communication, a relatively low SNR means a quite unreliable communication link, but as long as the transmission power is slightly increased, the SNR will be promoted distinctly, and the success rate of data transmission will rise significantly. However, when the data transmission success rate reaches a certain value, even if the transmission power increases multiply (the lifetime reduces multiply), the improvement of the data transmission success rate is still rather small. It can be seen that instead of improving the transmission power of node alone, adopting multiple methods is more likely to achieve good performance, such as cross-layer optimization combined with the network layer [35]. This is also the starting point of this paper. Considering the nonlinear relationship between SNR and data receiving rate, another way to improve the reliability of data transmission in low power condition is by reducing the distance between sender and receiver. In wireless communication, the speed of signal fading is proportional to 2 or even 4 times the distance. Therefore, short-distance multi-hop transmission is more appropriate for wireless sensor networks. According to this principle, the optimization of data transmission can be achieved by controlling the distance between nodes during network deployment. Meanwhile it is easier to balance the energy consumption and reliability of data transmission by this method.