# A Physical-Layer Watermarking Scheme Based on 5G NR
## 1 Introduction  
With human society entering the information age, information transmission has become an indispensable part of human life, and the security of information transmission has become the focus of attention. In a wireless communication system, the importance of security issues is more obvious, because the wireless users involved in the communication process always share the physical medium, and the openness of the shared medium provides more security loopholes for opponents to use to eavesdrop or impersonate users [1]. Traditional wireless security methods usually rely on data encryption and authentication at higher layers in the protocol stack [2]. However, due to heavy calculation and signaling load, these methods usually lead to prolonged communication waiting time, increased power consumption, and reduced system capacity [3]. For this reason, the research on physical-layer security mechanisms has attracted people's attention in recent years. Physical-layer authentication is an important implementation of physical-layer security [4; 5]. Compared to traditional authentication technology, the authentication on the physical layer can enable the legitimate receiver to quickly distinguish the legitimate transmitter from the malicious transmitter without completing the higher-level processing. Physical-layer watermarking technology artificially adds tag signals to transmission signals for identity authentication and should not interfere with the normal demodulation of transmission signals. A common framework for identity authentication by embedding a watermark is summarized in [6]. The sender uses the key and transmission bits to generate the authentication tag and then superimposes the tag signal on the transmission signal. By controlling the power ratio between the tag signal and the transmission signal, the receiver can demodulate the transmission signal normally, thereby obtaining the tag signal from the received signal for identity authentication. The basic principle of the framework is introduced in [7], which considers the system model when there are eavesdroppers and active attackers, in addition to legitimate communication parties, and then analyzes the performance of the scheme in detail from three aspects: stealth, robustness, and security. The above scheme for a multi-carrier system is extended in [8], which still superimposes the tag signal on the transmission signal. As it is a multi-carrier system, the position wherethe tag signal is added can be considered, and then the influences of power allocation and the number of tag signals on performance can be tested. The above scheme was extended to the MIMO system again, considering the influences of precoding and power allocation on authentication performance [9]. Most of the existing physical-layer watermarking technology adds the watermark signal to the user's signal [10; 11; 12], and the watermark information is not encrypted, so the security is not enough. In [13], a watermark signal carefully designed to be independent of the message signal of a transmitter was encrypted and embedded into the user's signal. Since the watermark signal is not changed in the communication process, the watermark signal is easily found by the attacker. In [14], the proposed scheme utilizes the random wireless channel gain between communication entities to generate the watermark signal. The watermark signal is quite random, but it depends on accurate channel estimation. In all the schemes [10; 11; 12; 13; 14], the watermark signal is a kind of interference to the user's signal, which reduces the signal-to-noise ratio (SNR) of the user's signal. In [15], the watermark is embedded into the pilot, and the interference of the watermark could be eliminated. However, the pilot is usually used to estimate the channel parameters, and the embedded watermark signal will affect the performance of the channel estimation. More research works could be found in [16], which summarizes the latest research results in this field.