## 3 Proposed Algorithm
## 4 Simulation Results

In this section, the simulation results are provided to validate the performance of our proposed SCA-SPCA-based iterative algorithm. The system simulation parameters that we have considered for the simulations are summarized in Table 2. We assume that one smallcell is in the coverage area of a macrocell, where we set the radius of the macrocell and the smallcell to 20 m and 10 m, respectively [17]. Thus, the channel \(\mathbf{g}_{m}\), \(\mathbf{g}_{c,k}\), \(\mathbf{G}_{e,l}\)\(\mathbf{h}_{m}\), \(\mathbf{h}_{c,k}\), and \(\mathbf{H}_{e,l}\) are modelled as \(\mathbf{g}_{m}=H(g_{m})\overline{\



In Figure 3, we illustrate the convergence performance of the proposed SCA-SPCA-based iterative algorithm with respect to (w.r.t.) iteration numbers. We set \(\bar{R}^{c}=1\) bps\(/\)Hz, \(\bar{E}^{c}=10\) dBm, \(\bar{E}^{c}=10\) dBm, \(P=60\) dBm and \(P_{1}=40\) dBm. It is easily seen from Figure 3 that the convergence of all cases can be quickly achieved within three iterations. It is observed that the average minimal secrecy capacity (MSC) at the M-SN becomes great as \(P_{2}\) increases. Figure 4: Average MSC at the M-SN with respect to the total transmit power at macrocell sink node. Figure 3: Average MSC at the M-SN with respect to iteration number. Figure 5: Average MSC at the M-SN with respect to the target harvested power at HP-SN. Figure 6: Average MSC at the M-SN with respect to the number of transmit antennas at the macrocell sink node. Figure 4 illustrates the average MSC at the M-SN in terms of different target transmit power at macrocell sink node with \(\bar{R}^{c}=1.5\) bps/Hz, \(\bar{E}^{c}=5\) dBm, \(\bar{E}^{e}=5\) dBm, \(P=60\) dBm and \(P_{1}=40\) dBm over 500 feasible channel realizations. It is observed that the average MSC increases with the target transmit power at macrocell sink node. In addition, the proposed SCA-SPCA-based algorithm outperforms all the conventional baseline schemes. Moreover, we can check that the proposed SCA-SPCA-based algorithm performs better than the non-AN MSCM, null- AN MSCM and the MSCM scheme with \(\rho_{k}=0.5\), and the performance gap is always gradually increasing regardless of the target transmit power at macrocell sink node. This reveals that optimizing the PS ratio \(\rho_{k}\) is important. In Figure 5, we compare the average MSC at the M-SN w.r.t. the different target-harvested power at HP-SN by fixing \(\bar{R}^{c}=1\) bps/Hz, \(\bar{E}^{e}=10\) dBm, \(P=70\) dBm, \(P_{1}=50\) dBm and \(P_{2}=60\) dBm. In this figure, we can observe that all schemes decrease with the increase of \(\bar{E}^{c}\). Moreover, the proposed SCA-SPCA-based algorithm always perform better in average MSC than all three conventional baseline schemes. Compared with the non-AN MSCM and the MSCM scheme with \(\rho_{k}=0.5\), the averageMSC of the proposed AN-aidedalgorithm is 1.2 bps/Hz and 0.8 bps/Hz higher at 0 dBm. It is observed that the proposed algorithm outperforms the null-AN MSCM. In Figure 6, we plot the average MSC at the M-SN in terms of different number of transmit antennas at the macrocell sink node with \(\bar{R}^{c}=0.5\) bps/Hz, \(\bar{E}^{c}=5\) dBm, \(\bar{E}^{e}=10\) dBm, \(P=60\) dBm, \(P_{1}=40\) dBm and \(P_{2}=60\) dBm. We can check that the curves of the proposed SCA-SPCA-based algorithm and the MSCM scheme with fixed \(\rho_{k}=0.5\) increase with the same slope. Moveover, when \(N_{M}\) increases, the performance gap between the non-AN MSCM scheme and the proposed algorithm becomes wider. This indicates that the AN technology is essential in obtaining the performance gains. Furthermore, the proposed algorithm outperforms the null-AN MSCM scheme by 0.8 bps/Hz. In Figure 7, in terms of the average central processing unit (CPU) running time, the computational complexities of the proposed method and the conventional one-dimensional line search (1-D) method are compared for a channel realization under various MF on a computer (The CPU is Intel Core i7-8500U 1.8 GHz, and the size of random access memory (RAM) is 8 GB).