# Secure Beamforming in 5G-Based Cognitive Radio Network
## 2 Related Work and Contributions  
### Zero-Forcing Beamforming (ZFBF) Related Works  
A downlink cascaded transmitting ZFBF technique [12] was proposed to secure the communications in a two-cell MIMO-NOMA-based CRN. This technique was combined with the proposed method for better security in the MIMO-NOMA network. It should be noted that this article uses an existing method in which the proposed technique has improved greatly.  
In [13], a beamforming (BF) scheme was proposed to use the interference as a green source for the enhancement of PLS in the satellite network. The ZFBF technique was used as a sub-method for the secure communication in the cognitive satellite terrestrial network (CSTN) with a sphere decoder algorithm (SDA).  
The joint design of the beamforming vector and an artificial noise covariance matrix was investigated [14] for the multiple-input-single-output-multiple eavesdropper simultaneous wireless information and power transfer (MISOME-SWIPT) systems. In the MISOME-SWIPT system, the base station (BS) sends an information signal to the honest user equipment and transmits a jamming signal to the eavesdropper. A secret energy efficiency (SEE) maximization problem was proposed for the MISOME-SWIPT system with an imperfect channel state information (CSI). A ZFBF method was used in the MISOME-SWIPT system to perform the task, but did not show better improvement than SEE.  
In [15], a uniform planar array (UPA) at the BS was applied for the robust security of 5G cellular networks coexisting with a satellite network. Then, a known imperfect angle of arrival (AoA)-based CSI was assumed of a multiple Eves. The constrained optimization problem was formulated to maximize the secrecy rate (SR) of the cellular user under the limitations of the transmit power at the BS. Two robust BF methods were proposed to solve the complex optimization problem for both an uncoordinated and a coordinated Eves. The authors then converted the non-convex problem into a convex one, and further proposed an iterative penalty function (IPF)-based algorithm to obtain the optimal beamforming weight vectors. ZFBF methods were also used as one of the methods to solve the same problem, but the performance was not higher than that of an IPF-based algorithm.  
In [16], an optimal multiuser multiple-input single-output (MISO) beamforming for power splitting SWIPT CRN was studied. A multi-antenna secondary trasnsmitter (ST) sent a data stream to multiple single antenna single receivers (SRs) equipped with a power splitting (PS) structure for an information decoding and an energy harvesting. A non-convex problem occurred in this structure, which was solved by an optimal method. First, the problem with fixed PS ratios was solved using the software-defined ratio (SDR) technique and the optimal PS ratios were found using the particle swarm optimization (PSO) method. The overall EPS technique was used to solve the problem, which showed a better performance than all other methods. A ZFBF technique was also proposed to solve the problem. It showed a better performance than the optimal methods, but a slightly worse performance compared with the equal power splitting (EPS) method.