## 5 Simulation Results
### Comparison of BER Performance between [PERSON] and [PERSON]  
The objective of the proposed physical layer security communication scheme is to simultaneously guarantee the LPD and LPI properties of wireless links. On the one hand, [PERSON] can detect and demodulate the signals transmitted by [PERSON], while [PERSON] can hardly detect the presence of the transmitted signals. On the other hand, although [PERSON] can detect the transmitted signal, he can hardly extract useful information. Arguably, BER is an effective and useful measure for both reliability and security. We hope the BER at [PERSON]'s side is as low as possible; meanwhile, the BER at [PERSON]'s side is (very close to) 0.5, so he essentially cannot recover any information transmitted by [PERSON]. Simulation results demonstrate that the proposed scheme can guarantee that the BER at [PERSON] will always be unacceptably high regardless of the received SNR, while the BER at [PERSON] will be decreased significantly as the received SNR increases. For the AWGN channel, the security of the proposed communication system is mainly determined by the sampling clock frequency offset between Bob and Eve. According to the communication protocol proposed in Section III, Bob can increase the transmit power  
Figure 8: Signal detection performance of Bob and Eve over fading channel. or length of pilot signals in order to improve the estimation accuracy. In this way, the sampling clock offset can be estimated nearly perfectly as the SNR of the pilot signal is sufficiently high or the number of pilot symbols is sufficiently large. Meanwhile, the sampling frequency between Alice and Eve can hardly be synchronized because they have no negotiation for sampling frequency synchronization. The BER performance of [PERSON] and [PERSON] is shown in Figure 9. The parameters are set as \(D=5\) and \(N=4\); thus, the sampling frequency is 50 MHz. As the accuracy of the sampling clocks is always at PPM(parts per million) level, we can reasonably assume that the sampling clock offset between Alice and Eve is 1 Hz. The BER versus \(SNR_{y}\) at [PERSON] and Eve are illustrated in Figure 9. The sampling phase offset at Bob is set as \(\Delta=0,1/8,1/16\). A significant improvement in BER performance can be achieved when the sampling phase offset decreases. Simulation results show that the BER at [PERSON] decreases rapidly as the SNR increases. Meanwhile, the BER at [PERSON] stays at a high level, and decreases very slowly with the increase in SNR that he can hardly intercept useful information. When some artificial jamming signals are added to the LPD signal, simulation results in Figure 9 show that [PERSON] can still detect and demodulate the LPD signal. The parameter \(\gamma\) in the figure is defined as \(\gamma=P_{x}/P_{j}\), where \(P_{x}\) denotes the transmit power of useful signals and \(P_{j}\) denotes the transmit power of artificial jamming signals. As a result, the proposed secure communication scheme is proven effective in the AWGN channel. Next, the BER performance of the proposed secure communication scheme over fading channels is shown in Figure 10. For [PERSON], both perfect CSI and imperfect scenarios are investigated. For the imperfect CSI scenario, the estimation error \(\rho\) is assumed to be 0.2 and 0.4. It is not surprising that the BER performance loss is induced by the increase in estimation error under the same channel condition. The results have clearly demonstrated  
Figure 9: BER performance of [PERSON] and [PERSON] over AWGN channel for \(D=5\) and \(N=4\). that [PERSON] can detect and demodulate the LPD signal effectively. The BER performance at Eve with different \(\xi_{f}\) is also given in this figure, where \(\xi_{f}\) denotes the sampling frequency offset between [PERSON] and [PERSON]. Simulation results show that the BER at Eve is about 0.5 even \(\xi_{f}=0\), which means the sampling clock offset between [PERSON] and [PERSON] does not exist.