the secrecy capacity of wireless transmissions from sensors to the sink [6]. In this paper, we develop a practical countermeasure for passive attacks and propose a physical layer security communication scheme for CPS applications. A well-designed secure wireless link should have LPD and LPI properties with respect to illegal users [7; 8]. The concept of perfect secrecy was first introduced in [PERSON]'s fundamental paper [9]. He also proposed that security of communication could be guaranteed only when the transmitter and receiver have a certain degree of cooperation, and perfect secrecy could be achieved if a one-time pad protocol were employed. Traditional encryption techniques are based on the complexity of mathematical tasks, such as the computation of discrete logarithms in large finite fields. With the rapid development of computer hardware and computing technologies, such as distributed computing and cloud computing, the security of traditional encryption techniques has become questionable [10]. Quantum communication can provide almost perfect security through the use of quantum laws to detect any possible information leak [11]. However, its application to wireless and mobile communications is confined because the line of sight for the transmission of optical quantum is not always available, particularly in urban areas crowded by large buildings. The classical spread spectrum communication systems have good LPD, and LPI characteristics and are widely used. However, the random and noise-like properties of pseudo-noise spreading sequences are usually deterministic and periodical in actual systems. With the rapid development of blind signal detection techniques [12], the spreading sequences may be cracked by illegal users. Then, the traditional spread spectrum techniques are also not as secure as expected. Physical layer security is to develop a secure transmission that exploits the physical properties of transceivers without relying on source encryption [13]. [PERSON] introduced the concept of secrecy capacity over wire-tap channels [14]. In [PERSON]'s model, the wire-tap channel is a degraded version of the main channel; thus, the eavesdropper can only receive a noisy version of the signal received at the intended receiver. [PERSON]'s work was extended to single input multiple output (SIMO) systems in the presence of one eavesdropper [15]. [PERSON] proposed an information theoretical framework to investigate information security in wireless multiple-input multiple-output links [16]. Another important line of research is the design of a practical system to achieve near-optimal physical layer security performance [17]. [PERSON] proposed a low-complexity polar-coded cooperative jamming scheme for the general two-way wire-tap channel, without any constraint on channel symmetry or degradation [18; 19; 20; 21; 22]. The research mentioned above is unexceptionally confined to the information-theoretic perspective, which only focuses on the LPI performance. Therefore, the main contribution of our work is to design an LPD signal waveform and investigate its application in physical layer security. Motivated by achieving an LPD signal waveform, we previously proposed an under-sampling spectrum-sparse signal based on active aliasing [23]. In this work, we extend our earlier work to a more practical scenario. Application of the LPD signal for physical layer security is investigated, and a typical wire-tap channel model with three users, namely, the transmitter ([PERSON]), the intended receiver ([PERSON]), and the eavesdropper ([PERSON]), is considered. Since the under-sampling method may be effective only when the sub-band signals are accurately aligned after the sampling process, [PERSON] can shift the central frequencies of the transmitted sub-band signals according to the clock offset between [PERSON] and [PERSON], to make sure that [PERSON] can collect the signal power on all sub-carriers coherently. Furthermore, a precoding technique based on CSI can be employed to maximize [PERSON]'s SNR at the sampling stage.