# FL-SDUAN: A Fuzzy Logic-Based Routing Scheme for Software-Defined Underwater Acoustic Networks
## 1 Introduction  
The ocean is the largest ecosystem, which plays a significant role in the survival and development of human beings. With the shortage of resources, the exploitation of the ocean has become a hot topic [1]. An underwater acoustic network (UAN) consists of sensor nodes with acoustic communication and computational capabilities [2]. This system has the advantages of simple infrastructure, small size, and low cost compared to traditional underwater networks. In a conventional UAN, the routing function relies on the collaboration of underwater data nodes (DNs). The customer burns the specified protocols in advance into the read-only memory (ROM) of DNs. Routing technologies that rely on hardware infrastructure cannot be optimized regarding reliability and energy consumption [3]. Additionally, these technologies are complex, poorly converged, and inefficient. Software-defined networking (SDN) is an emerging paradigm that enables the separation of the data plane and control plane, greatly simplifying management and fostering innovation and growth.  
The software-defined underwater acoustic network (SD-UAN) consists of multiple DNs and surface buoys. The DN is set as an OpenFlow white switch, and the buoy is the controller [4]. In SD-UAN, the DN sends a packet-in message containing the routing request to the specified controller. At this point, the controller is triggered to execute a predesigned routing algorithm and sends the calculated routing decision to the DN as a flow table, which determines the path of data forwarding. Migrating routing tasks from performance- and resource-constrained underwater nodes to a performance-powered SDN controller is the most significant advantage of SD-UAN [5]. As a result, the routing computation is faster and more efficient. Based on SDN, it dramatically reduces the energy consumption of underwater nodes and extends the survival time of underwater nodes. The topology ofthe UAN is time-varying due to ocean currents. Therefore, it is not possible to evaluate the path weights accurately. The fuzzy logic approach provides a novel routing mechanism for uncertain UANs. This paper proposes a fuzzy logic-based software-defined routing scheme for underwater acoustic networks (FL-SDUAN). To the best of our knowledge, this is the first work to apply a combined SDN and fuzzy logic routing algorithm for UANs. The main contributions of this paper are summarized as follows.  
1. The architecture of FL-SDUAN was designed to perform routing protocols in a robust SDN control plane, reducing complexity and increasing the availability of underwater nodes.
2. Fuzzy logic was used to implement routing decisions, which improves the reliability of underwater acoustic networks.
3. Two minimum spanning tree algorithms, fuzzy path optimization (FPO-MST), and fuzzy truncated set optimization (FCO-MST) were proposed for different underwater network scenarios.
4. FPO-MST and FCO-MST are compared with state-of-the-art methods in terms of packet transmission rate, end-to-end latency, and throughput. Experiments showed that a trade-off between performance and complexity was achieved in our work.  
The rest of this paper is organized as follows. Section 2 describes the related work. In Section 3, we present the architecture of FL-SDUAN. Section 4 offers the FPO-MST and FCO-MST algorithms. Simulation experiments are performed in Section 5. In addition, in Section 6, we conclude the paper.