y. We showed how full-duplex system can help us in achieving better performance than its half-duplex counterpart. The input-output equation derived in this chapter is not only applicable to the considered system model, but the same concept can also be applied to many other 5G and beyond technologies, for example, full-duplex cell-free [43], full-duplex intelligent reflecting surfaces, and full-duplex non-orthogonal multiple access [44], among many others. In this chapter we considered ideal hardware and also ignored channel correlation. The future works can consider imperfect hardware, for example, low-cost low-quality radio- frequency (RF) chains at the relay and the users [45-47]. Due to limited scattering and small antenna spacing, channels between the different antennas of the relay will be correlated [41, 48]. All these practical constraints can be considered to create new research problems for the abovementioned 5G and beyond technologies. References 1. A. Zappone, E. Jorswieck, Energy efficiency in wireless networks via fractional programming theory. Found. Trends Commun. Inf. Theory 11(3-4), 185-399 (2014) 2. E. Sharma, R. Budhiraja, K. Vasudevan, L. Hanzo, Full-duplex massive MIMO multi-pair two- way AF relaying: energy efficiency optimization. IEEE Trans. Commun. 66(8), 3322-3340 (2018) E. Sharma and P. Singh 3. G. Auer, V. Giannini, C. Desset, I. Gódor, P. Skillermark, M. Olsson, M.A. Imran, D. Sabella, M.J. Gonzalez, O. Blume, A.J. Fehske, How much energy is needed to run a wireless network? IEEE Wirel. Commun. 18(5), 40-49 (2011) 4. T.L. Marzetta, Noncooperative cellular wireless with unlimited numbers of base station antennas. IEEE Trans. Wirel. Commun. 9(11), 3590-3600 (2010) 5. E.G. Larsson, O. Edfors, F. Tufvesson, T.L. Marzetta, Massive MIMO for next generation wireless systems. IEEE Commun. Mag. 52(2), 186-195 (2014) 6. F. Rusek, D. Persson, B.K. Lau, E.G. Larsson, T.L. Marzetta, O. Edfors, F. Tufvesson, Scaling up MIMO: opportunities and challenges with very large arrays. IEEE Signal Process. 