# RF Energy Harvesting Wireless Communications:
## 6 Secure Communications
### Artificial Noise Aided Method  
The secrecy rate of wireless communications is determined by the difference in mutual information of the legitimate channel and eavesdropping channel. By adding the AN to the eavesdropping channel, the mutual information of the eavesdropping channel can be considerably reduced in defense of eavesdropping attack [86].  
In the AN aided communication, based on a power-splitting coefficient, the transmission power is split into sending the confidential message and transmitting AN signal. The legitimate receiver removes the artificial interference from the received data via the noise cancellation based on the generation method of AN shared by the HBS. Through jointly optimizing the total transmission power and the power-splitting coefficient of the HBS, different objectives, such as a delay-limited secrecy information transmission or a no-delay-limited secrecy information transmission can be achieved. (For a delay-limited secrecy information transmission, the HBS aims to minimize the probability that the achievable secrecy rate at certain channel fading state below a specific threshold. For a no-delay-limited secrecy information transmission, the HBS aims at maximizing the ergodic secrecy capacity.)  
When the eavesdropper is an active transmitter, the HBS is able to measure the CSI of the eavesdropper. If the eavesdropper is passive but not completely silent, the HBS can also estimate the eavesdropping channel through measuring a local oscillator signal leaked from the front end of the eavesdropper's circuitry [87]. The spatially selective AN can be performed to efficiently jam the information channel of the eavesdropper, as shown in Figure 19b. However, a perfect CSI is usually unavailable due to incorrect channel estimation, measurement quantization, and untimely channel feedback, especially in a large-scale RF-EHWC network [88]. As a result, many compromised transmission strategies are developed to balance the requirements between the CSI and the secrecy performance [89]. Consider a case where the eavesdropper is completely silent and thus no CSI of the eavesdropper can be perceived by the HBS. In this case, the HBS needs to jam eavesdroppers possibly at any possible location. The isotropic AN will be transmitted to make man-made noises be uniformly spread in the space except in the direction to the legitimate IR [90], as demonstrated in Figure 19a.  
Figure 19: AN-aided secure communication in an RF-EHWC network: (**a**) without the knowledge of CSI; and (**b**) with the knowledge of CSI.  
In [91], both perfect and imperfect CSI situations are investigated to optimize the secrecy rate. With perfect CSI, the secrecy rate maximization problem is non-convex and can be relaxed and reformulated into a series of convex subproblems by using the semi-definite relaxation (SDR) and successive convex approximation methods. If the CSI is imperfect, the AN-aided transmission vector can be designed based on the worst-case model. The objective becomes maximizing the secrecy rate under any channel estimation error in the uncertainty region. To reach the best performance, the optimization problem is first converted into a quadratic matrix inequality problem. Afterward, by converting the constraints of the semi-infinite quadratic matrix inequality into that of the linear matrix inequality using the S-procedure [92] and the Schur complement [93], solvable semidefinite programs can be eventually constructed. In [94], the AN transmission combined with the cognitive radio and the non-orthogonal multiple access (NOMA) technologies in a multi-user network is studied.