## 3 Cooperative Communication Algorithms  
### Cooperative Communication  
#### 3.1.1 Beaconless Routing Protocol  
In the positioning of WSNs, geographic routing has become an attractive solution due to its scalability and efficiency. Most existing geographic routing is stateful. That is, each node needs to maintain the location information of its immediate neighbors. General Packet Radio Service (GPSR) is the most representative geographic routing to get the location of neighbors by broadcasting beacon information regularly. Based on neighbor information table, the next hop is chosen by stateful routing. In highly dynamic networks, it has the following shortcomings: (1) excessive memory and communication overhead are caused by maintaining neighbor information, which leads to energy consumption; (2) the collected neighbor information is quickly out of date. Moreoever, data packets need to be sent repeatedly, which leads to excessive energy consumption.  
In order to overcome the shortcomings of traditional geographic routing, a stateless geographic routing protocol (beacon-free routing protocol) is proposed. In stateless routing, each node does not need to maintain neighbor node information and send beacons regularly. The next hop node is selected according to the competition mechanism. Neighbor nodes start a certain countdown. The node whose countdown first ends has the forwarding right. Therefore, with high reliability, the forwarding nodes are chosen by beacon-free geographic routing according to the actual topology. However, the node countdown time is too close, which may cause a conflict between nodes. Furthermore, the energy efficiency, delay and reliability may be affected. In order to avoid conflicts between potential forwarding nodes, a competitive timer is needed. However, how to set a timer to avoid conflicts and delays is still controversial. To solve this problem, the area where potential forwarding nodes are located is limited in some protocols, such as sector, triangle and circle. Reliable and Energy-Efficient Routing (REER) specifies forwarding nodes through the nodes in a circular overlap area. The two principles of setting forwarding areas are as follows: (1) the nodes in the area can receive messages from neighboring potential forwarders (PE); (2) to reduce or avoid conflicts, the nodes in the region set a dynamic countdown delay.  
Nodes in cooperative communication cooperate with each other, and multiple forwarding paths can be formed without multiple antennas. Wireless media has the characteristics of broadcasting and spatial distribution. Thus, the performance of WSN, especially the reliability of islanding network, can be improved by cooperative communication. The cooperative communication transmission mode is shown in Figure 1. Source nodes send data to Sink, and multiple forwarding nodes were selected for each hop to improve reliability.  
In most existing cooperative communication research, nodes are accessed through orthogonal channel (FDMA, TDMA, CDMA). Channel is shared among participating nodes (source, target, cooperative node), and only one hop of communication is studied. In this paper, a new multi-hopcooperative communication algorithm KCN is proposed, and fixed k cooperative nodes per hop are chosen.