# A Physical Layer Security Enhancement Scheme under the Ambient Backscatter System
## 4 Numerical Simulation Results  
In this section, in order to evaluate the performance of the IANT scheme proposed in this paper, we compare it with the multi-tag physical layer security scheme proposed by [PERSON] team [18] in the ABC system. The latter only selected a single tag with optimal channel gain according to the achievable secrecy rate but did not select other tags to generate artificial noise. In addition, from the point of view of achievable secrecy rate, the difference of noise removal effect caused by different channel estimation errors on the physical layer security scheme in this paper is shown in the following. Figure 3 shows the BER performance of the IANT scheme when the channel quality between tags and reader changes. We set the number of tags K to different values to study the influence of the number of tags on the system BER. As can be seen from the figure above, when the number of tags increases, the BER difference between the IANT scheme and the contrast maximum power scheduling scheme is gradually decreasing. From the point of view of BER, the performance of IANT scheme is worse than that of the security scheme without noise removal method. Even if we adopt the noise removal method, the BER of the IANT scheme is only close to that of the conventional scheduling scheme, but the artificial noise of the IANT scheme has a positive impact on the achievable secrecy rate of the system, that is, we choose to sacrifice certain BER performance to improve the physical layer security of the communication process. Figure 4 shows the change in the achievable secrecy rate of the system when the number of tags K and the channel quality between tags and reader change. We can draw a conclusion from Figure 4 that in the case of a small number of tags, the artificial noise will greatly affect the achievable secrecy rate. Furthermore, when the number of tags is set to 5, the IANT scheme has no advantage over the contrast security scheme in terms of the achievable secrecy rate. However, on the whole, the achievable secrecy rate of the system has improved with the increase in the number of tags. In addition, after the noise cancellation method is used to remove the influence of artificial noise, the achievable secrecy rate is correspondingly improved again. Therefore, the IANT scheme proposed in this paper is superior to the contrast security scheme in terms of the achievable secrecy rate of the system. In addition, regardless of the number of tags, the IANT scheme proposed in this paper using the noise cancellation method still achieves a higher secrecy rate. In order to further research the relationship between the number of tags and the achievable secrecy rate of the system, in the case of a fixed channel SNR, we show the changes in the achievable secrecy rate of IANT scheme and the contrast scheme, as shown in Figure 5. We can observe that the IANT scheme with noise cancellation method can always get a better secrecy rate than the contrast scheme, and the secrecy rate of the former is about twice that of the latter, which implies that the IANT scheme proposed in this paper has better security performance when the number of tags is relatively large. Figure 6 shows the effect of noise removal methods with different channel estimation errors on achievable secrecy rate when the number of tags K = 8. In this paper, we draw lessons from the channel estimation expression adopted by [PERSON] team [19]:  
\[\hat{d}_{k}=d_{k}+\Delta,\Delta\sim\mathcal{CN}(0,\sigma_{c}^{2}) \tag{23}\]  
Figure 4: The influence of the channel quality change between tags and reader on achievable secrecy rate. Figure 3: The influence of the channel quality change between tags and reader on the system BER (Bit Error Rate). We can clearly observe from Figure 6 that as the quality of the channel between tags and reader increases, the achievable secrecy rate of the system also increases.