acroBS tier or femtoBS tier by selecting suitable bias factor, explained in Sect. 5.6.2.1. D2D tiers aid in offloading by providing cooperation using NOMA. The transmission is assumed to be successful under non-outage condition, i.e., when the instantaneous rate is higher than the target rate at the user. 5 NOMA for 5G and Beyond Wireless Networks For detailed analysis of the considered HetNet, we consider two scenarios to cover all the possibilities as follows. Scenario-I: Under this scenario, it is assumed that when the user is offloaded from the macroBS tier, a pairing user is always available with which the offloaded macroU can be served by the femtoBS tier using NOMA. This is much simpler as compared to the next scenario, since the femtoBS tier does not have to find for a pairing user. The offloaded user can either be served as a cell center user (CU) or cell edge user (EU), depending on the channel gain condition with respect to the available pairing user. Scenario-II: This scenario differs from Scenario-I because here the pairing user has to be searched in accordance with the channel gain condition at the offloaded user. Therefore, there is also a possibility that an appropriate pairing user is not available. Hence, D2D-Cooperation (D2D-Coop) is used in this scenario only. 5.6.1 Cooperation from D2D Tier As already discussed, D2D-Coop is applicable only for Scenario-II. Consider the case when the offloaded user is an EU and the pairing user that the femtoBS tier can find for the offloaded macroU is also an EU. Under such circumstances it becomes difficult for the femtoBS tier to pair the offloaded macroU and serve it. This is because for the application of NOMA, channel difference between the two users paired must be significant. If the channel difference does not exist, it may lead to performance degradation [62]. Therefore, when such situation arises, D2D-Coop plays a vital role. The femtoBS searches for a DRx which is a CU, and is termed as cooperating DTx. Since, it is assumed that the density of the D