2D tier is sufficiently high, femtoBS can always find a cooperating DTx (or CDT). The femtoBS then pairs the available EU with the cooperating DTx and sends the information intended for the offloaded macroU to the cooperating DTx. As explained earlier, the D2D pair with the selected cooperating DTx is transformed to D2D group (which comprise the DRx and the offloaded macroU). This D2D group uses NOMA to transmit the intended message received from the femtoBS to the offloaded macroU. This way even when suitable pairing user is unavailable at the femtoBS tier under Scenario-II, the offloaded user does not experience outage. Furthermore, whether the offloaded user is served as a CU or EU in the D2D group is decided by the NC probability given in (5.3). It must be pointed here that the D2D-Coop is used only when the offloaded macroU is an EU and corresponding CU is unavailable for pairing with the offloaded macroU. Under the situation when the offloaded macroU is CU and the femtoBS tier could not find the appropriate pairing user, D2D-Coop leads to unnecessary P. Swami and V. Bhatia (OmacroU) (OmacroU) femtoBS femtoBS (Available pairing user) P-1: Direct path from femtoBS to OmacroU P-2-> P-3: Path using D2D-Coop Fig. 5.4 Reason for not using D2D-Coop when the offloaded macroU is CU power wastage. To make this justification simpler and clearer, we have used figures to explain the underlying concept of not using D2D-Coop when the offloaded macro user (or offloaded user (OmacroU)) is a cell center user (near user) and corresponding pairing user is not available. Figures 5.4 and 5.5 of the chapter show in detail the following case: Case-1 (as shown in Fig. 5.4) holds the following conditions: 1. Offloaded user is a cell center user (near user), denoted by "CU 1." 2. Available pairing user should be a cell edge user (far user). 3. However, the available pairing user is another cell center user (and not a cell edge user), denoted by "CU : 2." Thus, CU 2 cannot be used for pairing. Note: It should be noted that we do not p