## 1 Introduction  
Secure transmission is a fundamental problem in wireless communications due to the broadcast nature of the wireless medium. Along with the rapid advancement of information technology, the higher information transmission rate has called for a stricter standard of information transmission security. For a long time, the primary method of guaranteeing the secure transmission of information has been via encryption technology. Encryption technology utilizes the limitation in computing speed to prevent the eavesdropper from deciphering all encrypted information in a limited time. However, as computer technology advances with faster computation, the decryption of information becomes more straightforward. In theory, no encrypted information is indecipherable if the computer's calculation speed is fast enough. This indeed is the inherent flaw in the current information encryption technology. Therefore, the physical layer security technology has been proposed to solve the problems of secure information transmission. The physical layer security technology differs substantially from the information encryption technology. Unlike encryption technology, which relies on the limitation in computation speed, the physical layer security technology has its basis in the randomness of the wireless communication channel. The physical layer security technology tries to prevent eavesdroppers from decoding information, regardless of the amount of time or the computing speed. One of the most innovative physical layer security technologies is artificial noise (AN). AN adds extra noise to the information. This noise solely impacts the eavesdropper's channel but does not affect the legitimate receiver channel. That is, only the signal received by the eavesdropper is reduced in this method. The effectiveness of the physical layer technology is then evaluated by secrecy capacity. The study of physical layer security begins from [1]. This paper proposes unconditional secure transmission as the ultimate goal of physical layer security technology study. After [1,2] is the first paper to study the secure transmission of information from the perspective of information theory. In [2], wiretap communication model with the eavesdropping channel is proposed, and the aforementioned secrecy capacity was also first proposed in this paper. Paper [3] studies the physical layer security technology based on [2]. In [3], a broadcast channel model with confidential messages is proposed to extend [PERSON]'s work. Currently, the physical layer security technology has not been at the center of public attention, primarily due to a strict restriction that the eavesdropper's channel must be strictly worse than the legitimate channel. Considering the following cases: the eavesdropper is closer to the transmitter, or the eavesdropper has more antennas than the transmitter. These mentioned conditions will make the eavesdropper's channel better than the legitimate channel and thus reduces the effectiveness of the physical layer security technology. To help with the issue above, AN technology is introduced. The proposal of AN technology reduces the difficulty of applying the physical layer security technology in the multiple-input, multiple-output (MIMO) communication system [4]. AN is in the null space of the legitimate channel, which mean the legitimate channel is not affected. There is no need to employ any additional signal processing device to the legitimate receiver. Meanwhile, the eavesdropper's channel capacity is reduced significantly. To show this result quantitatively, let \(A\) denote the channel capacity of the legitimate receiver and \(B\) denote the channel capacity of the eavesdropper. The principle of AN is to increase the difference \(A-B\) by reducing \(B\) and keeping \(A\) constant. There have been many outstanding works in the realm of AN technology. In [5,6], AN and the interference alignment technology are creatively merged to introduce AN featuring interference alignment.