## 5 A Framework for SDN-SDR-Based Maritime Communications
### Performance Improvement of SDN-SDR over SATCOM  
The exact area occupied by a CSG can vary depending on numerous factors, such as the number and type of ships in the group, the mission of the group, the conditions of the ocean, and the operational orders for spacing, among other things. The US Navy has conducted emergent technology experiments in its annual Trident Warrior exercises [77]. In 2003, the Trident Warrior demonstrated the use of a line-of-sight (LOS) inter-battle-group wireless network to improve both the availability and data rate of the three ships in the experiment compared to SATCOM only [78]. With operations permitting LOS, operating frequencies from ship-to-ship can match the same frequencies used from ship-to-satellite and make use of the same amount of bandwidth, \(B\), as shown in the Shannon-Hartley formula,  
\[C=B\cdot\log_{2}\biggl{(}1+\frac{S}{N}\biggr{)}, \tag{1}\]  
where \(C\) is the data rate in bits per second, \(S\) is the received signal power, and \(N\) is the received noise power. Among ships in the CSG, the distance will be much less (approximately 20 km) than the distance between the ships and the satellite (approximately 35,000 km), and that is an advantage in the V2V case due to \(S\) reducing with the square of distance. In an AIS system, the satellite antenna gain is modeled at 6 dBi, whereas a shipboard dipole antenna may only have a gain of 2 dBi according to the ITU [79]. However, the 4 dBi of advantage to the SATCOM system is erased by distance, \(d\), in the Friis transmission equation,  
\[S^{[dB]}=P_{i}^{[dB]}+G_{i}^{[dBi]}+G_{i}^{[dBi]}+20\log_{10}\biggl{(}\frac{ \lambda}{4\pi d}\biggr{)}, \tag{2}\]  
where \(P_{i}\) is the transmitter power, \(G_{i}\) is the transmitter gain, \(G_{r}\) is the receiver gain, and \(\lambda\) is the signal wavelength. The additive noise to the signal, \(N\), is modeled as white Gaussian and is a function of noise temperature and bandwidth. Based on Equations (1) and (2), the potential data rate with the same carrier frequency is much greater for an LOS V2V system compared to a SATCOM scenario due to the distances involved. However, if ships are operating beyond LOS, lower frequencies, such as in the VHF to HF range, need to be used. Since the maximum operating frequency is lower, the potential bandwidth will be likewise reduced. Additionally, when using an SDN-SDR unified framework, an additional processing delay will be incurred. Processing time in SDN refers to the time required for the SDN controller to manage and make decisions about network traffic and operations. This includes tasks such as analyzing incoming packets, determining routing or forwarding instructions, generating and updating flow rules, enforcing network policies, and responding to changes in network conditions. The processing time in an SDN is closely tied to the control plane activities managed by the controller. Depending on the complexity of the decision-making process and the load on the controller, this can introduce some delay. The control centralization, while beneficial,can introduce additional delays compared to traditional networks, where devices make decisions independently and in parallel. There are two types of messages that need to be transmitted between the controller and switches in data planes: Packet-In message (for packets that require controller decision) and FlowMod message (for defining new flow rules). When a packet arrives at a switch or router without a matching flow entry, it triggers a Packet-In message to be sent to the SDN controller. The processing time includes the time the controller takes to make decisions about how to handle the packet. The controller determines and generates flow rules (FlowMod messages) for switches to apply to incoming packets. This includes analyzing network conditions, policies, and the desired behavior. It should be noted that even in SATCOM, which is the dominant mode of maritime communications, there is still a nodal delay, \(d_{nodal}\).