an but different variances, i.e. hb.r1~CN(0,027), Without loss of generality, we assume 1hb.r1/2 >1hb.1212 and x < B B is satisfied to provide sufficient decoding capa- bility for NOMA weaker user. On the other hand, channels between relays and UEs can be modeled as independent complex Gaussian RVs with zero mean and unit variance, i.e.hi~CN(0,1).i{r1,r2},j={ul,u2}. 4.1.2.1 NOMA Cooperative Scheme Each round of NOMA cooperative transmission consists of two time slots. In the first time slot, the AP transmits a composite signal = VasPsX1 + VBSPXX2 according to the NOMA principle, where X1 and X2 are signals intended for users 1 and 2, respectively; as and Bs are the corresponding power allocation factors and satisfy as + Bs = 1. The received signals at two relays are, respectively, expressed as (4.1) (4.2) and Nb,r2 are additive white Gaussian noise (AWGN) and follow i = {r1,r2}. Both relays use SIC to decode the received signals. We first present the 4.1 Outage Probability Study in a NOMA Relay System analysis by assuming perfect SIC and the results with imperfect SIC will be presented later. For relay 1, X2 will be decoded first by treating X1 as interference and the achievable signal-to-interference-noise ratio (SINR) for X2 is (4.3) Relay 1 then subtracts X2 from the composite signal and decodes X1 with only AWGN. Thus, the achievable SINR becomes (4.4) Similarly, at relay 2, the SINR for X2 and X1 can be expressed as pspsh (4.5) (4.6) respectively. In the second time slot, relay 1 transmits X1 to user 1 while relay 2 transmits X2 to user 2 by using precoded cooperative transmission. The received signals at users 1 and 2 are expressed as yuh2+u = (4.7) where AWGN Ni ~ CN(0,No), = {u1, ,u2}. If we re-write the above equation in the matrix format, we can get y = HX + n and y = [yu1 ] is the precoded transmit- ted signal vector. The precoding mechanism will be discussed later. n = [nu1 Nu2.IT and (4.8) To further minimize inter-user interference, DPC is applied at relays as the precoding scheme. Assume H is a ful