works [54, 57]. Two types of relaying techniques using D2D communications exist: one is relay-assisted D2D communication [58] and other is cooperative D2D communication [54]. In the former, relays assist the transmission to improve D2D user's performance, while in 5 NOMA for 5G and Beyond Wireless Networks the latter, D2D users take part in cooperation/relaying for cellular users to improve their performance. One of the prime utilizations of SIC technique used in NOMA can be for cooperative communication. In NOMA, the superimposed signal transmitted to the users contains messages of both the weak user and the strong user. At the strong user, SIC indicates that the signal of the weak user will be decoded before its own message. Since the strong user has access to the weak user's message, therefore, strong user can be effectively used as a relay to enhance the performance at the weak user [59]. 5.6 System Model This chapter investigates a HetNet with three tiers, as shown in Fig. 5.3. The three tiers are, namely, the macroBS tier, the femtoBS tier, and the D2D tier. The D2D tier aids in cooperation for Scenario-II (explained later in the chapter) wherein an appropriate pairing user for the offloaded user could not be found. The femtoBS tier, macroBS tier, and D2D tier work in underlay fashion. The locations of the BSs of the three tiers are distributed in accordance with the PPP assumption. The densities of the three tiers are fixed as is given in Table 5.1. Furthermore, the BSs of the three tiers transmit using a constant power mentioned in Table 5.1. The coverage range of the three tiers is fixed but differs from each other. Since the macroBS tier is for macroU femtoBS (a) femtol macroBS femtoBS Case 1 Case II User offloaded as CU NOMA Transmisison with a. User offloaded as EU NOMA Transmisison with a2 Fig. 5.3 System model [37] P. Swami and V. Bhatia Table 5.1 Simulation parameters Value taken Number of simulations Power of macroBS, femtoBS, and D2D BSs 40 W, 1 W, 3 mW Density of macroBS, femtoBS, and D2D tiers 