ing DTx (5.10) When the OmacroU is served as an EU by the cooperating DTx Pcoop = (1-poff)Om+poffPPj+poff(1-Pc)(1-Pcomp) (Pt) . (5.11) Note: pk stands for the outage probability at the D2D tier using NOMA. 5.7 Results and Discussions The outage probability is studied in this section in accordance with the simulation results. The parameters' values for the plots are given in Table 5.1. The outage probability performance for both the considered scenarios, i.e., Scenario-I and Scenario-II, is analyzed. Firstly, Fig. 5.6 depicts the outage probability with respect to the transmit SNR. The transmit SNR refers to the SNR at the source, and the concept has been used widely, e.g., [64, 65]. The outage probability is plotted individually for all the three considered tiers, i.e., the macroBS tier, the femtoBS tier, and the D2D tier. As discussed, the macroBS tier is deployed for providing coverage to a larger area; therefore, their coverage range is also large. Due to the large coverage range, the path loss experienced at the macroBS tier is also the largest as compared to the femtoBS tier and the D2D tier. Due to high path loss, the outage probability at the macroBS tier is highest as compared to the femtoBS tier and the D2D tier. However, the macroBS tier is capable of serving users covering a large area. Similarly, the outage probability at the D2D tier is the lowest as compared to the femtoBS tier and the macroBS tier. This is because the D2D tier has smallest coverage range. Due P. Swami and V. Bhatia MacroBS FemtoBS NOMA (as CU) FemtoBS NOMA (as EU) D2D NOMA (as CU) D2D NOMA (as EU) Transmit SNR Fig. 5.6 Outage probability at macroBS, femtoBS, and D2D tier [37] to the small coverage range, the path loss is also low, thereby resulting in lowest outage performance. However, it must be noted that the D2D tier can serve only limited coverage area. Figure 5.7 shows the total outage probability for Scenario-I wherein offloading takes place from the macroBS tier to the femtoBS tier, and it is pre-assumed that pairing user i