bability After Offloading For total outage probability, we combine the outage probability, the offloading probability, and the NC probability and get the expressions as follows [37]: When the macroU is offloaded to femtoBS tier and served by the femtoBS tier which does not employ NOMA 0,=(1-0" (5.5) When macroU is offloaded to the femtoBS tier and served as a CU at femtoBS tier using NOMA (5.6) When the macroU is offloaded as an EU at femtoBS (with NOMA) (5.7) Note: Please note that 0m stands for outage probability at macroBS tier, 0 denotes the outage probability at the femtoBS tier when NOMA is not used, and is the outage probability at the kth when femtoBS employs NOMA. Next, for Scenario-II, we calculate the total outage probability when an appropri- ate pairing user is searched by the femtoBS for the offloaded macroU. The selection of the pairing users is decided on whether the offloaded macroU is an EU or CU. Therefore, the total probability is written as follows [37]: When the macroU is offloaded as a CU and the femtoBS tier ed for pairing (5.8) When the macroU is offloaded as an EU and a CU is searched for pairing (5.9) 5 NOMA for 5G and Beyond Wireless Networks 5.6.3.2 Total Outage Probability After D2D Cooperation Under Scenario-I, it is assumed that pairing user is always available. However, for Scenario-II, an appropriate pairing user needs to be searched for. Under situations of unavailability of corresponding pairing user for the offloaded macroU, D2D-Coop becomes useful SO that the offloaded user does not fall into outage. As explained in Sect. 5.6.1, D2D-Coop is used only when the offloaded macroU is an EU and a corresponding CU is unavailable. Therefore, the femtoBS searches for a DTx which can act as a CU. This selected DTx is termed as cooperating DTx, and the designated pair converts to a D2D group and performs cooperation as explained in Sect. 5.6.1. The total outage probability at the offloaded macroU after D2D-Coop is given as follows [37]. When the OmacroU is served as a CU by the cooperat