# A Review of Physical Layer Security Techniques for Internet of Things: Challenges and Solutions
## 4 Promising PLS Solutions for IoT
### Anti-Eavesdropping Signal Design via Constellation Rotation  
Constellation rotation was originally proposed in [65] as a diversity approach over fading channels. It has been also utilized as a powerful solution in interference cancellation [66; 67] and cooperative spectrum sharing [68; 69]. In [70], we developed a constellation-rotation based anti-eavesdropping method to secure the two-way untrusted relaying systems, where two users exchange information bidirectionally with an untrusted relay node. The key issue in designing PLS schemes for this system is to ensure no information leakage to the untrusted relay while still enlisting the relay's ability in facilitating the two-way information delivery. Our key idea is to rotate the signal constellations. As is exhibited in Figure 2, the constellations employed at both terminal users are first rotated such that there exists a one-to-one mapping between the rotated signal constellation and its real or imaginary component. Then, only one dimension of the complex-valued signal is used to carry the user's information, and the other dimension is used to send the artificial noise. By careful design as shown below, the AN from one user aligns with the information-bearing signal from the other user at the untrusted relay, which decreases the detection performance of the relay dramatically and keeps the users' data confidential from the relay. From the terminal users' perspective, the desired signal and the AN lie in different directions at either terminal user, and thus the signal detection can be realized without being affected by the interference. The constellation-rotation based secure transmission scheme is elaborated on in more detail as follows:  
Let \(x_{0}\) be the original constellation point taken from an alphabet \(\mathcal{X}\). Then, after constellation rotation, the resulting constellation point is \(x=e^{i\theta}x_{0}\), with \(\theta\) being the rotation angle. The choice of \(\theta\) satisfies the condition that for any \(i\
eq k\),  
\[\Re\{x^{i}\}\
eq\Re\{x^{k}\},\ \ \Im\{x^{i}\}\
eq\Im\{x^{k}\},\ \forall x^{i},x^{k} \in e^{i\theta}\mathcal{X}. \tag{3}\]  
That is, both the real and imaginary parts of any two symbols in the rotated constellation are different. In our proposed scheme, constellation rotation is first applied to every original symbol prior to data transmission. As is shown from Figure 2, every cooperation period between user A and user B is composed of two phases. During the 1 st phase, user A as well as user B first selects the information-bearing symbol from the rotated constellation set, and introduces the artificial noise to constitute the composite signal. Then, the composite signal is pre-coded with a coefficient to compensate the phase distortion in the transmission procedure. To be specific, the transmitted signals from user A and user B are expressed by  
\[S_{\text{A}}=\sqrt{\frac{P}{2}}\left(\Re\{x_{\text{A}}\}+jw_{\text{A}}\right) e^{-j\angle h_{\text{AR}}} \tag{4}\]  
and  
\[S_{\text{B}}=\sqrt{\frac{P}{2}}\left(w_{\text{B}}+j\Re\{x_{\text{B}}\}\right) e^{-j\angle h_{\text{BR}}}, \tag{5}\]  
respectively. In Labels (4) and (5), \(x_{\text{A}}\) and \(w_{\text{A}}\) (\(x_{\text{B}}\) and \(w_{\text{B}}\)) represent the information-bearing symbol and the AN from user A (B), respectively, and \(h_{\text{AR}}\) (\(h_{\text{BR}}\)) is the channel coefficient from user A (B) to relay R. The received signal at the relay node is expressed as  
\[ \begin{array}{rcl}y_{\text{R}}&=&h_{\text{AR}}S_{\text{A}}+h_{\text{BR}}S_{ \text{B}}+n_{\text{R}}\\ &=&\sqrt{\frac{P}{2}}\left(|h_{\text{AR}}|\Re\{x_{\text{A}}\}+|h_{\text{BR}}|w _{\text{B}}\right)+j\sqrt{\frac{P}{2}}\left(|h_{\text{BR}}|\Re\{x_{\text{B}} \}+|h_{\text{AR}}|w_{\text{A}}\right)+n_{\text{R}},\end{array}  \tag{6}\]  
where \(n_{\text{R}}\) denotes the additive noise at R.  
Upon the reception of \(y_{\text{R}}\), the relay might attempt to decode the signals sent from user A and user B.