ns Ltd. 4 NOMA in Relay and loT for 5G Wireless Networks In this chapter, two NOMA relay schemes are presented and evaluated, namely NOMA cooperative scheme and NOMA TDMA scheme. In NOMA cooperative scheme, the com- pletion of one round information transmission consists of two time slots. In the first slot the BS uses NOMA to send the superimposed signal to two relays. Upon receiving the signal, these two relays will decode the signals by using SIC and then form a cooperative commu- nication pair to send the precoded signals to the respective recipients in the second time slot. Dirty paper code (DPC) is used as precoding at relays to eliminate the inter-user inter- ference in the second time slot. As a comparison, NOMA TDMA scheme uses three times slots to complete one round information transmission. The first time slot does the same as in scheme one. After relays decode the message, the first relay sends one signal to user one in the second slot and the second relay send another signal to user 2 in the following slot. Analytical models on outage performance are derived for both schemes in this chapter. 4.1.2 System Model The study considers a downlink wireless communication system that consists of one access point (AP) and a number of UEs [175]. Each UE can either function as a relay when needed or as a regular UE. The transmit powers of AP and UE are Ps and Pr, respectively. With NOMA, the AP can communicate with two UEs simultaneously. In the case that the channels between AP and these two UEs are poor, two other UEs are selected as relays for multi-hop cooperative transmission. Relays operate in a half-duplex decode-and-forward (DF) mode. The AP and UEs in the system are equipped with a single antenna. For notational simplicity, we denote AP, relay 1, relay 2, UE 1, and UE 2 with subscripts b, r1, r2, ul, and u2 in the equations, respectively. Furthermore, it is assumed that channels between the AP and two relays are two independent random variables (RVs) following a complex Gaussian distribution with zero me