# A Cross-Layer Optimized Opportunistic Routing Scheme for Loss-and-Delay Sensitive WSNs
## 1 Introduction
The reliability strategies that have been proposed are mainly as follows: (1) Data retransmission mechanism. The send-wait protocol [47] is the most representative method, whose main feature is confirming the transmission status of the packet by another special message called acknowledgement (ACK). (2) Multipath routing approach [48]. In this approach, a node transmits multiple copies of data and routes to destinations along different paths, so data routing fails only if all routes fail. Compared to a data retransmission mechanism, multipath routing provides an advantage of smaller delays. (3) Data encoding method [46]. The mechanism of this method is that the data is encoded in advance of being routed. With the help of redundant code, the receiver can deduce the missing data in incomplete received information,thus maintaining a certain reliability in the volatile WSNs communication links. However the main drawback is that encoding increases the amount of data, which means more energy consumption and reduced network life. (4) Opportunistic routing scheme [49]. In this way, instead of directly determining the next hop node, sender nodes determine a set of candidate nodes with higher-priority selected from the neighbor nodes, and then broadcast packets to these candidate nodes. This kind of routing method avoids the missing data caused by unexpected anomaly of nodes in the path, and improves the transmission reliability. Similarly, delays can also be divided into one-hop delays and end-to-end delays [47]. The former indicates the period of time during the sender node begins sending data to the receiver until the receiver node receives that data. The latter is the time interval of transmission from source node to sink through multi-hop routing. Generally speaking, there are plenty of factors affecting delay, but the reliability of communication links plays a decisive role. The strategies adopted in reliability assurance often determine the delay. Thus, in many studies, delays are often studied in conjunction with reliability, but rarely studied separately. The reliable and low delay guarantee methods mentioned above mainly improve the communication reliability from the perspective of the network layer. Making full use of the characteristics of wireless broadcast transmission, the sender only needs to send data once and high reliability is ensured through an opportunistic routing scheme. However, optimizing only the network layer will provide a very limited performance improvement. Actually, the network reliability and delay can be optimized not only from the network layer, but also from many other aspects. Moreover, comprehensive optimization of multiple layers can always get better results. Based on the above analysis, a Cross-layer optimized Opportunistic Routing (COOR) scheme is proposed in this paper. Different from previous research, the main innovations of our work are as follows:  
(1) A Cross-layer optimized Opportunistic Routing (COOR) scheme is proposed to improve reliability of communication links and reduce delays for loss-and-delay sensitive WSNs. The main innovation of the COOR scheme is that rather than optimizing only the network layer, the performance of the network is improved by increasing the transmission power of the nodes with remaining energy and combining the routing method, so as to improve the performance of the network more effectively. (2) Two kinds of cross layer optimization methods are given to improve network performance effectively in this paper. The first one selects the same candidate node set as the previous strategy for routing after increasing the transmission power of the nodes with remaining energy. With the improvement of transmission power, the reliability of the links is promoted. As a result, even if the routing scheme is the same as the previous one, the routing is still more stable. Moreover, the increase in reliability reduces the chance of queuing and validating between candidate nodes, thus reducing latency.