# Opportunistic Multipath Routing in Long-Hop Wireless Sensor Networks
## 2 Related Works  
In this section, we briefly summarize the existing research relating to improvements of the packet delivery ratio: multipath, opportunistic, and learning-based routing protocols. Multipath multi-SPEED protocol (MMSPEED) [6] is proposed to guarantee packet delivery services for reliability and real-time WSNs. To provide real-time services, the nodes select a network node with the proper speed of the packets and exploit the probabilistic multipath forwarding according to the requirements. It also performs without the global network information and could preserve various advantages, such as scalability for long-hop networks and flexibility with respect to network dynamics. However, to exploit this protocol in long-hop networks, it consumes massive amounts of energy to branch many paths to satisfy the packet delivery ratio. The robust and energy efficient multipath routing protocol (REER) [7] constructs the multipath paths in three phases: initial phase, main path construction, and sub-path construction. However, as REER only constructs two paths, without considering the size of the network or the success rate of the nodes, REER could potentially suffer from packet delivery ratio degradation in long-hop networks. [PERSON] et al. proposed a radio-disjoint geographic multipath routing protocol (RGMR) [8] that can determine the number of paths by the requirements. Thus, RGMR could satisfy the packet delivery ratio by transmitting the packet by multipath. However, it requires a high energy consumption because many paths are required to satisfy the required packet delivery ratio. In addition, as it should occupy a wide area to construct a radio-disjoint multipath, it may not be possible to construct a sufficient number of paths in a narrow network. Reliable and energy-efficient data collection protocol (RMER) [9] was proposed to meet the reliability and energy efficient requirements. In particular, it has the advantage of improving the network lifetime. To satisfy the reliability requirement and to reduce energy consumption, RMER evenly exploits the energy of the network nodes during packet transmission. In addition, the packets are aggregated near the hotspot area and transmitted to the sink. Therefore, RMER can maximize the network lifetime through the multipath. However, each packet is delivered through only one path and it may fail to transmit the packet close to the hotspot area when the scale of the network is increasing. Even the aggregated packet could fail to transmit to the sink. Energy-efficient reliable multipath data transmission (ERMDT) [10] is based on congestion avoidance. When congestion occurs during packet transmission on the main path, the packet is transmitted to the sink by an alternative path. In addition, it grades the priority of a packet. Thus, a packet with a high priority is transmitted first. In opportunistic routing [11], which is a receiver-based routing protocol, a source node broadcasts the packet to all of its neighbors. The neighbor node, which receives a packet from the source node, has a transmission opportunity with a timer. The duration of the timer is decided by certain criteria such as the distance between itself and the destination. When the timer expires, the node broadcasts the packet to all of its neighbors, similar to the source. The neighbor node has a transmission chance with a timer, and the timer of the competitors are canceled. Adaptive opportunistic routing (AOR) protocol [12] was proposed for reliable data transmission. AOR also grants transmission opportunities to all of the neighbors with the energy and distance of the criterion. The node is at a shorter distance, has a higher energy, and can acquire the highest priority. To ensure that all neighbors recognize the packet transmission from the high-priority node to avoid duplicate transmission, AOR computes the packet holding time. However, AOR does not significantly differ from existing OR.