# Cooperative Friendly Jamming Techniques for Drone-Based Mobile Secure Zone
## 4 CFJ-DMZ Model  
This section introduces the CFJ-DMZ model that forms a secure zone by using the mobility of devices and the friendly jamming method. Three drones transmit jamming signals to the outside of the secure zone. These cooperative jamming signals reduce the eavesdropping probability of eavesdroppers and facilitate secure communication in the secure zone. Figure 1b shows the CFJ-DMZ model proposed in this paper. The CFJ-DMZ network consists of a source node (S), destination node (D), arbitrary eavesdropper (E), and three friendly jamming drones (J). The drones transmit jamming signals after forming a secure zone, as shown in Figure 1b. The source and destination nodes communicate D2D inside the secure zone. As it is difficult to use a multi-antenna communication interface for low-power lightweight drones, we considered a communication method using a single antenna in this study. Figure 2 shows the flowchart of a scenario for the CFJ-DMZ model, which consists of three stages. In the first stage, three drones are used to form a secure zone. When D2D communication is determined, the receiver (D) delivers its location coordinate information using GPS  
Figure 1: Network configuration for friendly jamming: (**a**) friendly jamming model and (**b**) CFJ-DMZ model. Figure 2: Simulation flow chart. to the sender (D). In addition, [PERSON] sends the determined location information to D and moves to the determined location, and [PERSON] uses the arriving coordinate information of S and the current coordinate information of D to calculate the locations of the drones (J). Moreover, [PERSON] creates a virtual circle around its position. Here, given that the maximum transmittable distance according to the strength of the transmitted jamming signals of J is x, the radius of the circle is as shown in Equation (2):  
\[\text{Radius}=\frac{2}{3}\sqrt{3}x \tag{2}\]  
Here, [PERSON] selects three arbitrary points to make an equilateral triangle on the virtual circle created by centering on the position of D and sending the selected location to J. Each J moves to the received location. The J that arrives at each location moves to D until their respective jamming signals are no longer caught while transmitting to adjust the size of the secure zone. After the secure zone is created, [PERSON] stops sending the jamming signals and waits until [PERSON] arrives at the final target coordinates. The second stage is the jamming signal transmission stage. After arriving at the final target coordinates, [PERSON] sends the jamming signal transmission start time and the jamming signal transmission maintenance time to both D and J. The three units of J transmit jamming signals simultaneously based on the time information, and [PERSON] and D can send secure data during the jamming signal transmission maintenance time. As such, [PERSON] sends jamming signals only when communication is made according to the data transmission time. If jamming signals are continuously transmitted irrespective of the data transmission, the security inside the secure zone improves. However, continuous jamming signals interfere with the communication of other nearby transmitting and receiving objects [27]. Furthermore, it is inefficient to transmit jamming signals continuously in terms of energy. In the CFJ-DMZ, [PERSON] therefore sends jamming signals only when [PERSON] and D are communicating to minimize the effects of such signals on other nearby sender and receiver nodes and to use their batteries efficiently. In addition, it is assumed that the secure zone is formed only when the legitimate nodes exchange confidential information with the help of surrounding UAVs used for other purposes. Therefore, the cost of friendly jamming drones for CFJ-DMZ was not considered in this paper. On the other hand, very small control logic can be added to friendly jamming drones and the ground user's hardware to implement the proposed method, but it is assumed that the added hardware cost is trivial.