## 6 Simulation Results and Discussions  
Three important metrics, namely the SER, secrecy rate, and the robustness of the proposed DM systems, were simulated. The detailed simulation parameters are provided in Table 2. Meanwhile, the locations of the LU, the potential Eve, and multiple transmit antenna arrays in the Cartesian coordinate system are shown in Figure 7.  
To simplify the model and analysis, assuming that there exist \(K=3\) transmit antenna arrays with \(N=21\) antennas for each array. As mentioned in Table 1, cost, receiver complexity and transmitter complexity of the proposed DM schemes all depend on the number \(K\) of the ULA. Therefore, the choosing of the number \(K\) of the ULA is a compromise between the security performance and the cost in practical applications. To avoid grating lobe effect and mutual coupling between the array elements, the inter-element spacing is set one-half of a wavelength for each ULA [52; 53; 54; 55; 56; 57; 58; 59; 60; 61]. For mm-wave system, applications for B5G and future wireless networks, the carrier frequencies can be set as 30 GHz and more. To distinguish different direction information for different antenna arrays, the antenna arrays should be away from each other, thus we set the inter-array spacing as far as \(100\lambda\). In the setup, one LU and one Eve in two potential locations are considered. In detail, for the first scenario, we assume for three transmit antenna arrays, the desired location \(B\) and eavesdropper \(E_{1}\) are all located in different directions, where \(\theta_{BA_{1}}=95.7392^{\circ}\), \(\theta_{BA_{2}}=90^{\circ}\), \(\theta_{BA_{3}}=84.2608^{\circ}\), \(\theta_{E_{1}A_{1}}=90^{\circ}\), \(\theta_{E_{1}A_{2}}=84.2608^{\circ}\), \(\theta_{E_{1}A_{3}}=78.6346^{\circ}\), with the distance \(d_{BA_{1}}=1000\lambda\), \(d_{BA_{2}}=994.9874\lambda\), \(d_{BA_{3}}=1000\lambda\), \(d_{E_{1}A_{1}}=994.9874\lambda\), \(d_{E_{1}A_{2}}=1000\lambda\), \(d_{E_{1}A_{3}}=1014.9\lambda\). Likewise, for the second scenario, \begin{table}
\begin{tabular}{l l} \hline \hline
**Parameters** & **Value** \\ \hline Number of ULA, \(K\) & 3 \\ Number of ULA elements, \(N\) & 21 \\ Single carrier frequency, \(f_{c}\) & 30 GHz \\ Inter-array spacing, \(D\) & 100\(\lambda\) \\ Inter-element spacing, \(d\) & 0.5\(\lambda\) \\ Multiple carrier frequencies, \(f_{c1}\), \(f_{c2}\), \(f_{c3}\) & 30 GHz, 30.1 GHz, 30.2 GHz \\ Total transmit power per symbol, \(P_{s}\) & 1 \\ Number of LU & 1 \\ Number of Eve & 1 \\ Location of the 1 st array \({}^{*}\), \((r_{1},\theta_{1})\) & (1000\(\lambda\), 95.7392\({}^{\circ}\)) \\ Location of the 2 nd array \({}^{*}\), \((r_{2},\theta_{2})\) & (994.9874\(\lambda\), 90\({}^{\circ}\)) \\ Location of the 3 rd array \({}^{*}\), \((r_{3},\theta_{3})\) & (1000\(\lambda\), 84.2608\({}^{\circ}\)) \\ Location 1 for the [PERSON] \({}^{*1}\), \((r_{1}^{\mathrm{E}},\theta_{1}^{\mathrm{E}})\) & (100\(\lambda\), 180\({}^{\circ}\)) \\ Location 2 for the [PERSON] \({}^{*2}\), \((r_{1}^{\mathrm{E}},\theta_{1}^{\mathrm{E}})\) & (500\(\lambda\), 84.2608\({}^{\circ}\)) \\ Modulation mode & QPSK \\ Information rate & 20 Mbps \\ \hline \hline \end{tabular}  
* The location of the LU is regarded as the reference point. \({}^{1}\) The case where the Eve is not in the same direction as the LU for any arrays. \({}^{2}\) The case where the [PERSON] is located in the same direction as the LU for the 3 rd array.  
\end{table} 
Table 2: Simulation parameters. we assume for the third transmit antenna array, the desired location \(B\) and eavesdropper \(E_{2}\) are located in the same direction, where \(\theta_{BA_{1}}=95.7392^{\circ}\), \(\theta_{BA_{2}}=90^{\circ}\), \(\theta_{BA_{3}}=84.2608^{\circ}\), \(\theta_{E_{2}A_{1}}=106.7787^{\circ}\), \(\theta_{E_{2}A_{2}}=95.7932^{\circ}\), \(\theta_{E_{2}A_{3}}=84.2608^{\circ}\), with the distance \(d_{BA_{1}}=1000\lambda\), \(d_{BA_{2}}=994.9874\lambda\), \(d_{BA_{3}}=1000\lambda\), \(d_{E_{2}A_{1}}=519.6152\lambda\), \(d_{E_{2}A_{2}}=500\lambda\), \(d_{E_{2}A_{3}}=500\lambda\).