nal Decode UE2 signal Received SINR Figure 3.36 Illustration of the principle of downlink power-domain NOMA [52]. 364 Chapter 3 inter-cell interference power, that is, |hil2/Noi. Based on this order, we assume that any user can correctly decode the signals of other users whose decoding order comes before the cor- responding user. Therefore, the ith UE can remove the inter-user interference from the jth user whose channel gain is less than |hil2/Noi- In the example with two UEs, assuming that UE2 does not have to perform interference cancellation since it comes first in the decoding order. UE1, on the other hand, has to first decode X2 and subtract that component from received signal y1, then to decode X1 without interference from X2. Assuming successful decoding and no error propagation, the throughput of the ith UE, Ri, is given as follows [52]: It can be seen that power allocation for each UE greatly affects the user throughput and thereby the modulation and coding scheme used for data transmission of each UE. By adjusting the power allocation ratio P1/P2, the base station can effectively control the throughput of each UE. The overall cell throughput, cell-edge throughput, and user fairness are closely related to the adopted power allocation scheme [52]. In a system that uses orthogonal MA scheme and hypothetically serves two UEs, if normal- ized bandwidth 0<B<1 is assigned to the first UE, the remaining bandwidth 1 - B will be assigned to the second UE to maintain orthogonality between the users. The throughput of the ith UE Ri is given as follows: In power-domain NOMA, the performance gain relative to orthogonal MA increases when the difference in channel gains, for example, path loss between UEs, is large. The uplink capacity can be calculated in the similar manner as the downlink, although the formula is somewhat different. Defining Pr1 and P2 as the received powers at the base sta- tion from UE1 and UE2, respectively, the rate of each user in the case of non-orthogonal uplink access can be written as follo