# A Biologically Inspired Self-Organizing Underwater Sensor Network
## 1 Introduction  
Underwater sensor networks are important tools in the marine environment. They can be applied to gather oceanography data, track marine animals, monitor the pollution of the environment, and defend harbors from intruders [1, 2]. In an emergency, a large number of sensor nodes can be deployed by planes or ships to cover a large arena, which forms a large-scale wireless underwater sensor network. Due to the ability to deploy and monitor large areas in a short period of time, such sensor networks have become a hot topic in the research field. A large-scale wireless underwater sensor network has the following features:
* Communication among nodes is achieved via acoustic communication. Radio signals attenuate fast in the water, and underwater laser communication is unreliable under the disturbance of water flow. As a result, acoustic communication becomes the only choice for wireless communication in wireless underwater sensor networks. * The communication routing should emerge in a self-organized way. In large-scale wireless underwater sensor networks, it is expected that a message can reach its destination node as soon as possible. However, as nodes are deployed randomly, users are unable to set a routing table for each node before deployment. The communication constraints of acoustic communication make it impossible to assign a central coordinator to control the communication process of the whole network. As a result, we can only seek a self-organized way to coordinate communication in the network. * Communication collision should be avoided as much as possible. For underwater acoustic communication, if one node hears multiple signals at the same time, it is unable to parse the information included in any signal. The same thing happens when it hears a signal while speaking at the same time. These phenomena are called communication collisions. When a communication collision occurs, the sender node needs to resend the message, extending the time for a message to reach its destination. The aforementioned features make currently available self-organizing schemes not applicable in an underwater environment. For example, under a time division multiple access (TDMA) framework [3] there are two kinds of schemes to coordinate the self-organizing communication process for radio communication. The first type of scheme is developed following the idea of \"collision detection and resend\", such as the famous Aloha protocol. In these schemes, a node tries to send a message in a slot, and once a collision happens, it will resend the message later in another slot. For radio communication, each slot lasts only several microseconds or less. Thus, it can endure wasting time. However, for acoustic communication, each slot lasts several seconds, and thus wasting time is unaffordable, making these protocols an unfavorable choice. Other schemes have been developed following the idea of \"constructing an optimized routing table via communication\", such as the method proposed in [4]. In these schemes, an extra message needs to be conveyed to coordinate the communication. Taking the method in [4] as an example, nodes need to send the IDs of all their neighbors, their second-order neighbors (i.e., neighbors' neighbors), and slots occupied by each of these nodes. As a result, a node can obtain information about all its first- and second-order neighbors to decide which slot it should occupy. The drawback of this method is that the extra data occupy much communication capability for acoustic communication. Consequently, a method to coordinate the acoustic communication process in a large-scale wireless underwater sensor network is necessary. In this paper, we propose a coordinator to coordinate communication for a large-scale wireless underwater sensor network. It is developed under the TDMA framework and consists of a slot distributor and a forwarding guide. The slot distributor is used to determine which slot the current node should occupy.