acroU (no offloading) offloading to FemtoBS (no NOMA) offloading to FemtoBS as CU offloading to FemtoBS as EU D2D-Coop as CU D2D-Coop as EU Transmit SNR Fig. 5.8 Total outage probability (Scenario-II, q = 0.5) [37] 5 NOMA for 5G and Beyond Wireless Networks offloaded macroU. Due to the uncertainty of finding a pairing user, the outage probability at the offloaded macroU degrades. To cover this degradation in the outage probability at the offloaded macroU, D2D-Coop is used. It can be noticed that using D2D-Coop achieves significant enhancement in the outage performance, which is an essential requirement for the 5G and beyond networks. Also, whether the offloaded macroU is served as CU or EU by the cooperating DTx, nearly same performance is achieved after transmit SNR of 10 dB. For transmit SNR less than 10 dB, the outage probability achieved by offloaded macroU served as CU by the cooperating DTx is higher as compared when served as EU. The reason is higher interference experienced by the CU due to good channel condition as compared to the EU. Using D2D-Coop in Scenario-II, the offloaded macroU observes a decrease in outage probability by nearly 43%, 55%, 74%, and 87% at transmit SNR of 10 dB, 16 dB, 22 dB, and 28 dB, respectively, when the offloaded macroU is served as an EU by the cooperating DTx, in comparison to when no D2D- Coop is used. Similarly, D2D-Coop outperforms the offloading to femtoBS tier without NOMA and achieves enhancement of 21.54% in the outage probability. The performance enhancement achieved using NOMA and D2D-Coop justifies the proposed system model for the 5G and beyond networks. Moreover, NOMA results in higher connectivity which is very essential for the 5G and beyond networks [5]. 5.8 Conclusion In this chapter, an analytical framework is developed and investigated for a three-tier HetNet with macroBS tier, femtoBS tier, and D2D tier under offloading. The spatial distribution of BSs follows PPP model. NOMA is employed at the femtoBS tier and at the D2D tier. At the D2D tier, the applic