本研究中，我們探討一個多天線輸入單天線輸出(multi-input single-output, MISO)之異質網路架構下的無線通訊傳輸安全。我們考慮一個宏蜂巢(macrocell)，在其範圍之內有數個毫微蜂巢(femtocell)。在femtocell中，毫微單位基地站(femto base station, FBS)服務一個使用者(FU)，且femtocell中還有其他潛在竊聽者以及竊聽者。我們目標為最佳化系統的secure EE，其定義為系統的保密傳輸速率總和除以系統的總功率消耗。在本篇論文我們將分三大部分進行探討。

在第一部分中，我們探討FBS擁有與接收端之間的完整通道資訊(perfect CSI)下的最佳化問題，然而此最佳化問題是一個non-convex 問題、並且非常難以求解。為了解決這個問題，我們使用變數變換、泰勒一階近似等方法，將其近似成convex optimization problem。我們更近一步提出連續凸化近似演算法來改善系統的secure EE。在模擬結果中，我們可以觀察到當潛在竊聽者的能量獵取門檻分別設定為-30 dBm 和-15 dBm 時，我們提出的演算法比最大比率傳輸方法(MRT)還要分別高105%和717%。除此之外，我們探討使用者挑選與secure EE之間的交互關係。我們在模擬結果中觀察到：當透過使用者的距離來挑選使用者時，得到的secure EE會比隨機挑選使用者還要高出33.1%。

第二部分中，我們延伸第一部分的研究，探討基地台只擁有與接收端之間的不完整通道資訊(imperfect CSI)。當存在通道估計誤差的時候，最佳化問題變得更為複雜，我們進一步利用S-procedure來求解最佳化問題。除此之外，若透過使用者的距離來挑選使用者，得到的secure EE會比隨機挑選使用者還要高出35%。這也證明使用者挑選在 perfect CSI 以及 imperfect CSI 情境下皆為非常重要的議題。

因此，在第三個部分，我們延伸前有的研究成果，改為考慮FBS同時服務多個FU (multi-FU case)，並進一步考慮使用者挑選的最佳化問題。我們提出一個挑選使用者的最佳化問題，透過疊代求解此最佳化問題，我們可以獲得最佳使用者組合。在模擬結果中，我們發現我們所提出的使用者挑選演算法與最佳挑選組合(exhaustive search)只有不到5%的差距，除此之外，透過我們的演算法，將可以得到的比 NearU 還要高5.9%的secure EE、更比隨機挑選使用者所得到之secure EE還要高出16.4%。這也證明我們提出的演算法的確能夠挑選出更佳的使用者組合以達更高的secure EE。

總結來說，我們發現beamforming向量設計以及使用者挑選皆為非常重要的議題，若只考慮其中一個因素是不夠的。除此之外，在multi-cell的系統下，套用我們所提出之使用者挑選演算法能夠挑選出更佳之使用者組合、並達到更高的secure EE。

In this work, we consider a heterogeneous cellular network with a macrocell and several femtocells. In each femtocell, the femto base station (FBS) serves one femto user (FU) (single-FU case) with the presence of several potential eavesdroppers (PEs) and eavesdroppers (EVEs). We aim to maximize secure energy efficiency (secure EE), defined as the secrecy rate divided by the total power consumption, via jointly optimizing beamforming vector and artificial noise (AN).

We first solved the optimization problem under perfect channel state information (CSI). The optimization problem is non-convex, which makes it difficult to solve. To resolve this issue, we applied change of variables and first-order approximation to approximate it by a convex problem. We further proposed a successive convex approximation algorithm to achieve better performance. Our simulation results showed that our algorithm improved secure EE by as much as 105% and 717% over the baseline, maximum ratio transmission (MRT) scheme, when PE EH threshold is -30 and -15 dBm, respectively. Furthermore, we consider user selection under the single-FU case. We compared the secure EE achieved by random selection and other heuristic selection methods. Our simulation results showed that applying heuristic user selection improved secure EE by 33.1% in the perfect CSI case.

Secondly, we addressed the secure EE maximization problem under imperfect CSI. In the imperfect CSI case, the optimization problem becomes even more complicated. We applied S-procedure to handle these non-convex constraints. In simulation results, we observed that applying heuristic user selection can improve secure EE by 35% under the imperfect CSI case. This indicates that user selection is an important issue for both the perfect and imperfect CSI case.

In the last part of this work, we addressed the secure EE maximization problem and user selection optimization when multiple FUs are served simultaneously (multi-FU case). Specifically, we proposed a user selection algorithm by solving an optimization problem iteratively. In simulation results, it was shown that the gap between the exhaustive search and our proposed algorithm is less than 5%. Besides, by applying our proposed user selection algorithm, secure EE is improved by 5.9% over NearU and 16.4% over Random Selection. This indicates that our proposed algorithm indeed improves secure EE.

In conclusion, both beamforming design and user selection are important issues. Only addressing one of the two issues is not enough. Furthermore, in the multi-cell scenario, user selection methods effect secure EE significantly and our proposed user selection algorithm indeed achieves better secure EE.

. [1] V. N. Ha and L. B. Le, “Joint coordinated beamforming and admission control for fronthaul constrained cloud-RANs,” 2014 IEEE Global Communications Conference, pp. 4054–4059, 2014.

. [2] D. H. N. Nguyen, L. Bao Le, and T. Le-Ngoc, “Multiuser Admission Control and Beamforming Optimization Algorithms for MISO Heterogeneous Networks,” IEEE Access, vol. 3, pp. 759–773, 2015.
. [3] X. Liu, F. Gao, G. Wang, and X. Wang, “Joint Beamforming and User Selection in Multicast Downlink Channel under Secrecy-Outage Constraint,” IEEE Communications Letters, vol. 18, no. 1, pp. 82–85, Jan. 2014. 

. [4] T. S. Rappaport, Y. Xing, G. R. MacCartney, A. F. Molisch, E. Mellios, and J. Zhang, “Overview of Millimeter Wave Communications for Fifth-Generation (5G) Wireless Networks—With a Focus on Propagation Models,” IEEE Transactions on Antennas and Propagation, no. 12, pp. 6213– 6230, Dec. 2017. 

. [5] W. Xia, J. Zhang, S. Jin, C.-K. Wen, F. Gao, and H. Zhu, “Large System Analysis of Resource Allocation in Heterogeneous Networks With Wireless Backhaul,” IEEE Transactions on Communications, vol. 65, no. 11, pp. 5040–5053, Nov. 2017. 

. [6] N. Wang, E. Hossain, and V. K. Bhargava, “Joint Downlink Cell Association and Bandwidth Allocation for Wireless Backhauling in Two-Tier HetNets With Large-Scale Antenna Arrays,” IEEE Transactions on Wireless Communications, vol. 15, no. 5, pp. 3251–3268, 2016.
. [7] A. Damnjanovic, J. Montojo, Y. Wei, T. Ji, T. Luo, M. Vajapeyam, T. Yoo, O. Song, and D. Malladi, “A Survey on 3GPP Heterogeneous Networks,” IEEE Wireless Communications, vol. 18, no. 3, 2011. 

. [8] H. H. Yang, G. Geraci, and T. Q. Quek, “Energy-Efficient Design of MIMO Heterogeneous Networks with Wireless Backhaul,” IEEE Transactions on Wireless Communications, vol. 15, no. 7, pp. 4914–4927, 2016. 

. [9] R. F. Schaefer, H. Boche, and H. V. Poor, “Secure Communication Under Channel Uncertainty and Adversarial Attacks,” Proceedings of the IEEE, vol. 103, no. 10, pp. 1796–1813, Oct. 2015. 

. [10] W. Mei, Z. Chen, J. Fang, and B. Fu, “Secure D2D-Enabled Cellular Communication Against Selective Eavesdropping,” 2017 IEEE International Conference on Communications (ICC), 2017.
. [11] P. Zhao, M. Zhang, H. Yu, H. Luo, and W. Chen, “Robust Beamforming Design for Sum Secrecy Rate Optimization in MU-MISO Networks,” IEEE Transactions on Information Forensics and Security, vol. 10, no. 9, pp. 1812–1823, Sep. 2015. 

. [12] K. Cumanan, Z. Ding, M. Xu, and H. V. Poor, “Secrecy Rate Optimization for Secure Multicast Communications,” IEEE Journal of Selected Topics in Signal Processing, vol. 10, no. 8, pp. 1417–1432, Dec. 2016. 

. [13]  J. Choi, “A Robust Beamforming Approach to Guarantee Instantaneous Secrecy Rate,” IEEE Transactions on Wireless Communications, vol. 15, no. 2, pp. 1076–1085, Feb. 2016. 

. [14]  L. Liu, R. Zhang, and K.-C. Chua, “Secrecy Wireless Information and Power Transfer With MISO Beamforming,” IEEE Transactions on Signal Processing, vol. 62, no. 7, pp. 1850–1863, Apr. 2014. 

. [15] Q. Shi, L. Liu, W. Xu, and R. Zhang, “Joint Transmit Beamforming and Receive Power Splitting for MISO SWIPT Systems,” IEEE Transactions on Wireless Communications, vol. 13, no. 6, pp. 3269–3280, Jun. 2014. 

. [16] Q. Shi, C. Peng, W. Xu, M. Hong, and Y. Cai, “Energy Efficiency Optimization for MISO SWIPT Systems With Zero-Forcing Beamforming,” IEEE Transactions on Signal Processing, vol. 64, no. 4, pp. 842–854, Feb. 2016. 

. [17] G. Y. Li, Z. Xu, C. Xiong, C. Yang, S. Zhang, Y. Chen, and S. Xu, “Energy-Efficient Wireless Communications: Tutorial, Survey, and Open Issues,” IEEE Wireless Communications, vol. 18, no. 6, pp. 28–35, 2011. 

. [18] M. Sheng, L. Wang, X. Wang, Y. Zhang, C. Xu, and J. Li, “Energy Efficient Beamforming in MISO Heterogeneous Cellular Networks With Wireless Information and Power Transfer,” IEEE Journal on Selected Areas in Communications, vol. 34, no. 4, pp. 954–968, Apr. 2016. 

. [19] Q. Shi, C. Peng, W. Xu, M. Hong, and Y. Cai, “Overview of Millimeter Wave Communications for Fifth-Generation (5G) Wireless Networks—With a Focus on Propagation Models,” IEEE Transactions on Signal Processing, vol. 64, no. 4, pp. 842–854, Feb. 2016. 

. [20] Y. C. Hsiao, C. M. Chen, and C. Lin, “Energy Efficiency Maximization in Multi-User MISO Mixed RF/VLC Heterogeneous Cellular Networks,” 2018 15th Annual IEEE International Conference on Sensing, Communication, and Networking (SECON), pp. 1–9, 2018. 

. [21] M. Zhang, K. Cumanan, and A. Burr, “Secure Energy Efficiency Optimization for MISO Cognitive Radio Network with Energy Harvesting,” 9th International Conference on Wireless Communications and Signal Processing, 2017. 

. [22] Y. Jiang, Y. Zou, J. Ouyang, and J. Zhu, “Secrecy Energy Efficiency Optimization for Artificial Noise Aided Physical-Layer Security in OFDM-Based Cognitive Radio Networks,” IEEE Transactions on Vehicular Technology, vol. 67, no. 12, pp. 858–872, Dec. 2018. 

. [23] M. Xu and D. Lin, “Low-complexity user selection strategies in the downlink of multi-user channels,” 2006 8th International Conference Advanced Communication Technology, vol. 1, pp. 204– 206, 2006. 

. [24] G. O. Okeke, W. A. Krzymien, Y. Jing, and J. Melzer, “A Novel Low-Complexity Joint User-Relay Selection and Association for Multi-User Multi-Relay MIMO Uplink,” IEEE Wireless Communications Letters, vol. 4, no. 3, pp. 309–312, Jun. 2015. 

. [25] H. Deng, H.-M. Wang, J. Yuan, W. Wang, and Q. Yin, “Secure Communication in Uplink Transmissions: User Selection and Multiuser Secrecy Gain,” IEEE Transactions on Communications, vol. 64, no. 8, pp. 3492–3506, Aug. 2016. 

. [26] W.-C. Liao, T.-H. Chang, W.-K. Ma, and C.-Y. Chi, “QoS-Based Transmit Beamforming in the Presence of Eavesdroppers: An Optimized Artificial-Noise-Aided Approach,” IEEE Transactions on Signal Processing, vol. 59, no. 3, pp. 1202–1216, 2011. 

. [27] D. W. K. Ng, E. S. Lo, and R. Schober, “Robust Beamforming for Secure Communication in Systems With Wireless Information and Power Transfer,” IEEE Transactions on Wireless Communications, vol. 13, no. 8, pp. 4599–4615, Aug. 2014.
. [28] D. H. N. Nguyen, L. Bao Le, and T. Le-Ngoc, “Multiuser Admission Control and Beamforming Optimization Algorithms for MISO Heterogeneous Networks,” IEEE Access, vol. 3, pp. 759–773, 2015. 

. [29] M. O. Ojo and O. O. Aramide, “Various Interference Models for Multicellular Scenarios: A Comparative Study,” 2015 Fifth International Conference on Digital Information and Communication Technology and its Applications (DICTAP), pp. 54–58, 2015. 

. [30] M. Sheng, L. Wang, X. Wang, Y. Zhang, C. Xu, and J. Li, “Energy Efficient Beamforming in MISO Heterogeneous Cellular Networks With Wireless Information and Power Transfer,” IEEE Journal on Selected Areas in Communications, vol. 34, no. 4, pp. 954–968, Apr. 2016. 

. [31] H. Deng, H.-M. Wang, W. Wang, and M. H. Lee, “Dual User Selection for Security Enhancement in Uplink Multiuser Systems,” IEEE Communications Letters, vol. 20, no. 9, pp. 1900–1903, Sep. 2016. 

. [32] M. Liyanage, I. Ahmad, A. B. Abro, A. Gurtov, and M. Ylianttila, Eds., A Comprehensive Guide to 5G Security. John Wiley & Sons, 2018. 

. [33]  M. Grant and S. Boyd, “CVX: Matlab Software for Disciplined Convex Programming, version 2.1,” May 2014. [Online]. Available: http://cvxr.com/cvx 

. [34] B. R. Marks and G. P. Wright, “Technical Note—A General Inner Approximation Algorithm for Nonconvex Mathematical Programs,” Operations Research, vol. 26, no. 4, pp. 681–683, Aug. 1978. 

. [35] A. Beck, A. Ben-Tal, and L. Tetruashvili, “A Sequential Parametric Convex Approximation Method with Applications to Nonconvex Truss Topology Design Problems,” Journal of Global Optimization, vol. 47, no. 1, pp. 29–51, May 2010. 

. [36] W.-C. Li, T.-H. Chang, C. Lin, and C.-Y. Chi, “Coordinated Beamforming for Multiuser MISO Interference Channel Under Rate Outage Constraints,” IEEE Transactions on Signal Processing, vol. 61, no. 5, pp. 1087–1103, Mar. 2013. 

. [37] S. P. Boyd and L. Vandenberghe, Convex optimization. Cambridge, UK; New York: Cambridge University Press, 2004. 

. [38] S. A. Vorobyov, A. B. Gershman, and Z.-Q. Luo, “Robust Adaptive Beamforming Using Worst-Case Performance Optimization: A Solution to the Signal Mismatch Problem,” IEEE Transactions on Signal Processing, vol. 51, no. 2, pp. 313–324, 2003. 

. [39] D. W. K. Ng, E. S. Lo, and R. Schober, “Robust Beamforming for Secure Communication in Systems With Wireless Information and Power Transfer,” IEEE Transactions on Wireless Communications, vol. 13, no. 8, pp. 4599–4615, Aug. 2014.
. [40] W. Xu, Y. Cui, H. Zhang, G. Y. Li, and X. You, “Robust Beamforming with Partial Channel State Information for Energy Efficient Networks,” IEEE Journal on Selected Areas in Communications, vol. 33, no. 12, pp. 2920–2935, 2015.