# Underlying Security Transmission Design for Orthogonal Time Frequency Space (OTFS) Modulation
###### Abstract  
With the aim of ensuring secure transmission in high-mobility wireless scenarios, this paper proposes a 2D permutation-aided Orthogonal Time Frequency Space (OTFS) secure transmission scheme, which uses the Gosudarstvennyi Standard (GOST) algorithm to perform disturbance control on the OTFS modulation domain. Furthermore, we develop an improved SeLective Mapping (SLM) algorithm, which can significantly improve the Peak-to-Average Power Ratio (PAPR) problem with very low complexity. In addition, we carry out the security analysis, investigating the proposed scheme's resistance performance to a range of effective attacks. Finally, our numerical results show that our proposed transmission scheme can guarantee the underlying security property of OTFS. secure transmission; OTFS; security analysis 202222279191390/s2220791390/12  
The OTFS technique has drawn a lot of research attention, such as [7; 8; 9; 10; 11; 12; 13]. In particular, refs. [7; 8; 9] were devoted to researching the Peak-to-Average Power Ratio (PAPR) of the OTFS system, wherein [7] proposed a MuLaw companding technique to reduce the PAPR of the OTFS, and [9] proposed an effective PAPR reduction method based on an iterative limiting and filtering framework. The channel estimation methods designed for OTFS transmission were studied in [10; 11; 12]. In a little more detail, ref. [10] proposed an algorithm based on Orthogonal Matching Pursuit (OMP) and Modified Subspace Pursuit (MSP) for the DD channel estimation in OTFS multiple access (OTFS-MA) systems on the uplink. In Multiple-Input Multiple-Output (MIMO) scenarios based on the OTFS, ref. [11] proposed an iterative signal detection algorithm based on message passing and a channel estimation scheme in the DD domain. An embedded pilot-assisted channel estimation scheme was proposed in [12]. Under the OTFS scheme, especially for future high-mobility scenarios, secure transmission is particularly important and still faces huge challenges. As is known, the future 6G mobile network contains a range of high-mobility scenarios, such that UAV communications are deployed to supplement the terrestrial networks, building an integrated air-space-earth-sea network to cover a much wider area. In this case, the probability of eavesdropping on the communication links significantly increases, and thus there is a greater risk of information exposure. Nevertheless, as far as we know, very limited studies [13; 14] have been devoted to the security issue of OTFS transmissions. To ensure the security of wireless communications, traditional security mechanisms based on cryptography are mainly deployed on multilayer protocols above the physical layer, and the main idea is to improve the security of the wireless system by using a variety of upper layer authentication mechanisms. For example, at the Medium Access Control (MAC) layer, the MAC address is authenticated so that access is only granted to legitimate users. At the network layer, two authentication protocols for the wireless Local Area Network (LAN) security access WPA and WPA2 were adopted [15; 16]. Although upper layer encryption can enhance the security of the wireless system, the corresponding techniques are completely dependent on the confidentiality of the key and the computational complexity. However, with the continuous development of quantum computing technology and breakthroughs in computing power, traditional cryptographic schemes may be easily broken, and they face great challenges. Moreover, one of the prerequisites for upper layer encryption is that the physical layer can actively provide smooth and error-free transmission, which is difficult to achieve in practical scenarios [17]. In contrast, physical layer security techniques can compensate well for the shortcomings of traditional encryption schemes and have gradually gained widespread attention in recent years.