## 6 Results and Discussion
### Transfer Learning Actor-Critic Solution for Energy-Efficient Data Protection Scheme
This is because the SU can encrypt data with a longer key size (e.g., \(Nk=256\)) by utilizing extra energy in the battery when the average harvested energy increases. Therefore, the policy would be updated to favor the action that gives a better reward in the future. Figure 14: Average reward according to harvested energy for different data protection schemes. POMDP: partially observable Markov decision process. Figure 16 depicts the total number of data packets transmitted from the SU to the fusion center based on harvested energy under three different data protection schemes. As can be seen from the figure, the SU can transmit more packets of data when using the TLAC algorithm, compared to the myopic scheme. The reason is that the proposed learning scheme can allocate the harvested energy more efficiently than the myopic one. Consequently, the SU can operate in transmission mode in more time slots, and thus, can transmit more encrypted data packets to the FC. Meanwhile, using the myopic scheme can cause the SU to be inactive due to lack of energy for future use. For that reason, the proposed TLAC framework can guarantee the security level, and can effectively utilize the limited energy resource. More specifically, in Figure 17, we present the detailed number of successfully transmitted data packets that are encrypted using the AES algorithm with different key lengths. We can see from the figure that the total number of data packets delivered under the TLAC algorithm is 10% higher than when using the myopic scheme. In particular, more packets are encrypted with longer key sizes (i.e., AES-192 or AES-256) with a rise in the arrival of harvested energy. Figure 16: The number of successfully transmitted data packets according to the harvested energy for different data protection schemes. Figure 15: Channel utilization according to the harvested energy for different data protection schemes. Finally, we examine the performance of the proposed TLAC by comparing it with that of AES algorithms with fixed key length. In the fixed key length schemes, the SU uses only one key size to encrypt data at each time step even when it has enough energy. In Figure 18, the rewards under the proposed TLAC and other schemes grow persistently with the increment in the harvested energy. While the proposed solution provides the highest average reward, the fixed encryption method with the shortest cipher key (AES-128) shows the lowest security level. The reason is that the proposed method can efficiently allocate the energy to every time slot by estimating expected reward in the future time slots. Meanwhile, the AES-128 algorithm always uses the lowest amount of energy for data encryption, and thus, does not utilize the redundant energy in the SU's battery to enhance the security level as the arrival speed of the harvested energy increases. On the other hand, the AES-256 uses maximum energy to encrypt data whenever the energy is sufficient to increase data security. However, this action reduces the chance for the SU to operate in transmission mode, which leads to low successful transmissions, as shown in Figure 19. From the figure, we can see that the proposed TLAC provides the SU with the highest channel utilization since the SU can transmit more data packets in comparison to other methods. This is because the fixed encryption techniques do not utilize the energy effectively for future use. Among those fixed  
Figure 17: Comparison between the proposed TLAC and the myopic schemes, based on harvested energy. Figure 18: Reward comparison between the proposed TLAC and the fixed key-length schemes according to the harvested energy. encryption methods, the AES-128 with lower energy consumption allows the SU to transmit more data packets than the AES-192 and the AES-256, but provides the SU with the lowest reward. Consequently, we can verify that the proposed TLAC algorithm can ensure effective data communications between the SU and the fusion center in terms of security level and channel utilization.