# SOUNET: Self-Organized Underwater Wireless Sensor Network
## 1 Introduction  
With the advancements in underwater communication technologies such as acoustic modems, hydrophones, and digital signal processing (DSP), underwater wireless sensor networks (UWSNs) have drawn considerable attention in various applications such as tactical surveillance, oceanographic observation, disaster prevention, undersea exploration, and assisted navigation [1; 2; 3]. In general, UWSNs consist of multiple fixed sensor nodes that monitor a specific area collaboratively and forward data to one or more data-gathering points known as the sink by multi-hop relaying or occasionally with the aid of mobile machines referred to as autonomous underwater vehicles (AUVs). Either the wireless communication links between the sensor nodes are preprogrammed prior to placement or a remote user can centrally assign them after placement. However, to configure the number of communication links manually, the centralized method requires substantial efforts. Furthermore, it is necessary to reconfigure the links whenever a sensor node is lost or added. To eliminate such manual efforts required to maintain the network, _self-organizing network_ (SON) technology is studied. In SON, individual sensor nodes discover network topology in a best-effort manner with local information only or sometimes via probabilistic approaches [4]. Along with the elimination of complexity, SON provides three strong points: _adaptability_, _robustness_, and _scalability_. Adaptability means that when a node is lost, the child nodes connected to the lost node as data senders try to find new parent nodes by themselves. Such an adaptive reconfiguration avoids unexpected data loss, which may occur for a long time or permanently in centralized networks. Additionally, a newly deployed sensor node is capable of joining the network using the limited information acquired throughlocal observations. The active response to circumstances increases the network's robustness in the sense that the network can handle the unexpected losses and additions of sensor nodes while maintaining the network's performance up to a certain level. Lastly, scalability means that the network functions properly regardless of variations in the number of sensor nodes. However, the local decision-making cannot adequately meet the requirements for the global optimization of network performances because the sensor nodes are shortsighted without global information about the network. Thus, SONs have to be designed in such a way that the distributed local behaviors provide the closest optimum solution [5]. Clustering is an energy-efficient method to organize wireless sensor networks. For example, [PERSON] et al. proposed a clustering architecture to integrate different types of devices into several clusters [6]. Adjacent devices form a cluster group as cluster-members, which are connected to a cluster-head and to the same type of cluster-members in other clusters. Self-adaptive algorithms for newly added or lost nodes are also considered. However, the architecture is not for underwater environment. Furthermore, we consider homogeneous sensor nodes and non-clustered networks. Many previous works on SONs for UWSNs have also adopted a clustered architecture where the whole network is divided into several smaller groups for saving energy consumption during data gathering [7; 8; 9; 10; 11]. [PERSON] et al. proposed a clustering head selection algorithm (CHSA) [9] that reduces overall energy consumption by organizing direction-sensitive clusters. A sensor node, which satisfies specific criteria--direction toward a sink node, is elected as the cluster-head. In addition, CHSA adopts a self-healing mechanism to recover link holes [12], which is also based on direction awareness. However, it is costly to set up separate devices such as the ultrashort baseline (USBL) system in each node. On the other hand, [PERSON] et al. proposed two clustering algorithms, k-means cluster based energy efficient routing algorithm (KEER) and its enhanced version (EKEER) for mobile UWSNs [11].