# A Wireless Covert Channel Based on Dirty Constellation with Phase Drift
## 1 Introduction  
The increase in capacity is one feature of emerging communication systems, including the fifth (5G) and sixth generation (6G) systems. This is due to the use of wider radio channels, their aggregation, or the use of higher frequency bands, i.e., millimeter, terahertz, or optical waves. A wider band of transmitted signals also gives greater possibilities to implement covert data transmission. Hence, the greater interest in searching for new steganographic methods is more evident [1]. Steganography consists in transmitting information to make the act of transmission undetectable. Unlike cryptographic information, whose content is encrypted, the very existence of steganographic information is concealed. Steganographic information, which is also known as covert information, requires a carrier--or, in other words, a cover. The simplest carrier scenario uses photos [2], audio [3] or video signals [4] (multimedia steganography) to hide additional information. One of the important steganography applications includes creating covert channels. The term \"covert channel\" was coined by [PERSON] as: \"... any communication channel that can be exploited by a process to transfer information in a manner that violates the systems security policy... \" [5]. [PERSON] focused on the exchange of data between programs. Nowadays, it is assumed that any method of communication used to illegally transmit information, which violates the system security policy, is a covert channel. Data security is generally ensured by the flow control between the sender and authorized recipient. Though wired networks commonly use firewalls, security can be violated by covert channels. In this case, the information can be embedded by manipulating the packet timing information (i.e., covert timing channel) [6; 7; 8; 9; 10] or putting some bits into the packet headers (i.e., covert storage channel) [11; 12]. So-called network steganography understood in a broad sense can be applied to both physical layer symbol frames [13], protocols of medium access control (MAC) [14], routing [15], networks [16], or higher layers, e.g., Transmission Control Protocol/Internet Protocol (TCP/IP) [11], Hypertext Transfer Protocol (HTTP) [17], and Domain Name System (DNS) [18]. These methods can be applied in homogeneous wired and wireless networks (e.g., accordant with the IEEE 802.11 standard [12; 13]) as well as in heterogeneous ad hoc networks (e.g., [15]). Along with the development of network steganography techniques, we may notice novel solutions of network steganalysis, e.g., [19]. Contrary to the wired network, the wireless physical layer gives further possibilities for implementing the covert communication. In general, the wireless covert channel has its advantages and disadvantages [20]. In wired communications, it must be ensured that the channel is not distorted by network devices on either side of the covert channel. In wireless communication, the range between two points is limited by the transmitter power and the parameters of the receiver, e.g., its sensitivity. In this case, the steganography can relate to both radio and optical (e.g., [21; 22]) wired and wireless communications. The further analysis focuses on wireless radio communication steganography. In wireless communications, we deal with noise, interference, and fading that can seriously degrade transmission capabilities. In this case, it is worth introducing the terms \"premodulation\" and \"postmodulation\" steganography [23]. Pemodulation steganography is related to the bit structure change of the transmitted cover information. In contrast, postmodulation steganography refers directly to the physical parameter change of the transmitted waveforms. The proposed method presented in this paper is included in the category of wireless postmodulation steganography. In the literature, we can find many ways to implement covert channels based on the physical layer. For \(N\)-ary frequency-shift keying (FSK) signals, [PERSON] et al. proposed hiding information in a frequency drift and offset [24].