chniques in cognitive radios. IEEE Communications Surveys & Tutorials, 15(3):1136-1159, 2013. doi: 10.1109/SURV.2012.100412.00017. 18 K. Bonawitz, V. Ivanov, B. Kreuter, A. Marcedone, H. B. McMahan, S. Patel, D. Ramage, A. Segal, and K. Seth. Practical secure aggregation for privacy-preserving machine learning. In Proceedings of the 2017 ACM SIGSAC Conference on Computer and Communications Security, pages 1175-1191, 2017. 19 E. Boshkovska, D. W. K. Ng, N. Zlatanov, and R. Schober. Practical non-linear energy harvesting model and resource allocation for SWIPT systems. IEEE Com- munications Letters, 19(12):2082-2085, 2015. doi: 10.1109/LCOMM.2015.2478460. 20 E. Boshkovska, A. Koelpin, D. W. K. Ng, N. Zlatanov, and R. Schober. Robust beamforming for SWIPT systems with non-linear energy harvesting model. In 2016 IEEE 17th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), pages 1-5, 2016. doi: 10.1109/SPAWC.2016.7536860. 21 E. Boshkovska, R. Morsi, D. W. K. Ng, and R. Schober. Power allocation and scheduling for SWIPT systems with non-linear energy harvesting model. In 2016 IEEE International Conference on Communications (ICC), pages 1-6, 2016. doi: 10 .1109/ICC.2016.7511403. 22 E. Boshkovska, D. W. K. Ng, N. Zlatanov, A. Koelpin, and R. Schober. Robust resource allocation for MIMO wireless powered communication networks based on a non-linear EH model. IEEE Transactions on Communications, 65(5):1984-1999, 2017. doi: 10.1109/TCOMM.2017.2664860. 23 E. Boshkovska, N. Zlatanov, L. Dai, D. W. K. Ng, and R. Schober. Secure SWIPT networks based on a non-linear energy harvesting model. In 2017 IEEE Wireless Bibliography Communications and Networking Conference Workshops (WCNCW), pages 1-6, 2017. doi: 10.1109/WCNCW.2017.7919062. 24 E. Boshkovska, D. W. K. Ng, L. Dai, and R. Schober. Power-efficient and secure WPCNs with hardware impairments and non-linear EH circuit. IEEE Transactions on Communications, 66(6):2642-2657, 2018. doi: 10.1109/TCOMM.2017.2783628. 25 S. Boyd and L. Vandenberghe