# Multi-Antenna Jammer-Assisted Secure Short Packet Communications in IoT Networks
## 4 Simulation Results  
In this section, computer simulations by Matlab are provided to verify the analytical results of the three proposed transmission schemes and evaluate the secrecy throughput performance of the three proposed transmission schemes. In simulations, unless specified otherwise, we assume that \(N_{j}=5\), \(\delta=10^{-2}\), \(M_{1}=20\), \(M_{2}=1500\), \(\tilde{\tau}_{ab}=\tilde{\tau}_{ae}=\tilde{\tau}_{jb}=\tilde{\tau}_{je}=1\), and \(\sigma^{2}=0\) dBm. From the figures, we can see that the analytical results coincide well with the computer simulation points, which demonstrates the correctness of our analysis. Figures 2-4 plot the secrecy throughput of the ZFB, NAN, and TAS schemes with different values of \(B\), respectively. It is clear that the secrecy throughput first increases and then decreases as \(N\) increases, i.e., an optimal value for \(N\) exists to maximize the secrecy throughput. This phenomenon is explained as follows: when \(N\) is relatively small, increasing \(N\) is beneficial to improve the transmission reliability, which, of course, will lead to a higher secrecy throughput. However, when \(N\) is very large, the communication latency is large, which, consequently, will result in the degradation of the secrecy throughput. The second observation is that the optimal blocklength increases as \(B\) increases. This is because compared with communication latency, the decoding error at Bob becomes more obvious when the number of information bits gets larger. Figure 2: Secrecy throughput of the ZFB scheme versus the blocklength \(N\) with different values of \(B\) for \(P_{a}=5\) dBm and \(P_{j}=10\) dBm, where the dashed lines are (15) to represent the asymptotic secrecy throughput. Figure 5 investigates the impact of the transmit power \(P_{a}\) on the secrecy throughput with the three proposed schemes. Moreover, we present a benchmark scheme for comparison, i.e., without the jammer scheme, which consists of a source, a destination, and an eavesdropper, as in [20]. It is clear that the secrecy throughput of all the proposed schemes increases first and then stays constant as \(P_{a}\) increases. This is because the ratio of average channel gain on the legitimate link to the eavesdropping link becomes the bottleneck of the secrecy throughput. Figure 4: Secrecy throughput of the TAS scheme versus the blocklength \(N\) with different values of \(B\) for \(P_{a}=5\) dBm and \(P_{j}=10\) dBm, where the dashed lines are (21) to represent the asymptotic secrecy throughput. Figure 3: Secrecy throughput of the NAN scheme versus the blocklength \(N\) with different values of \(B\) for \(P_{a}=5\) dBm and \(P_{j}=10\) dBm, where the dashed lines are (18) to represent the asymptotic secrecy throughput. throughput in the high-SNR regime. In addition, we observe that the ZFB and NAN schemes always attain better performance than the TAS and benchmark schemes. This is because the jamming signals of the ZFB scheme and the NAN scheme only selectively interfere with the eavesdropper and have no impact on the signal reception of the legitimate user. Thus, the multi-antenna jammer can be employed to improve the system performance. Figure 6 shows the impact of the jamming power \(P_{j}\) on the secrecy throughput with the three proposed schemes. We first observe that when \(P_{j}\) is sufficiently large, the secrecy throughput converges to a constant for the ZFB and NAN schemes, whereas it goes to zero for the TAS scheme. This can be explained by the fact that the use of the TAS scheme not only degrades the channel condition of the eavesdropper, which is beneficial to secure transmission, but also deteriorates the channel condition of the legitimate user, which is adverse to secure transmission. It is also worth noting that the TAS scheme outperforms the benchmark scheme in certain regimes, i.e., when the jamming power is small. Therefore, the designer has to carefully choose the jamming power of the TAS scheme.