## 3 Receiver Design for NOMA
### Cooperative NOMA  
With conventional NOMA, higher power users are unable to cancel the interference generated by lower power users, representing residual interference. As seen in Figure 3, Cooperative NOMA aims to mitigate this limitation by making lower power users relaying, over the air, the signals of higher power users, clean of interference. The higher power users can use any algorithm to combine such relayed signal(s) with the one received directly from the base station [18]. We assume the relay of type decode and forward. As seen in Figure 3, assuming that user 1 is the reference one (higher power user) and that user 2 is an interfering user (lower power user), as in the case of conventional NOMA, user 2 employs the SIC for subtracting the signal of user 1 from the overall received signal (using the SIC), before user 2 is detected. Cooperative NOMA considers that interfering users (user 2, in the case of Figure 3) relay, over the air, the signals detected by the SIC (user 1, in the case of Figure 3). As depicted in Figure 5, this allows that a user with higher received power level (user 1, in the case of Figure 3) receives several copies of its signal: (a) the signal received directly from the base station, where the interference created by lower power users cannot be cancelled by the SIC; (b) the signal relayed by the other interfering users (user 2, in the case of Figure 3), which can be clean of interference of lower power users. Let us focus on Figure 3. User 2 signal is assumed presenting lower power and, once detected, can be regenerated and cancelled from the overall signal of user 1, before it is relayed, as this is a matter of introducing an additional iteration. The signal of user 1 (higher power user) can then be relayed over the air. The several copies of signals of user  
Figure 4: Block Diagram of Conventional NOMA. 1 (higher power) can then be combined at the data symbol level \(\hat{b}\), using any combining algorithm. As previously described, the SIC introduces a delay in the signal corresponding to one symbol period for each cancellation stage, i.e., for each cancelled user. In the case of the cooperative NOMA, the number of parallel SIC employed correspond to the number of less powerful interfering users than the reference user, added with the SIC of the reference user. The worst case, in terms of delay, corresponds to the less powerful interfering user (one of the interfering users that relay the detected signal), which cancels all the other  
Figure 5: Block Diagram of Cooperative NOMA. users, introducing a delay of one symbol period for each user. This delayed and relayed signal is then combined with the signal of the reference user (clean of the interference of higher power users). Therefore, cooperative NOMA introduces a delay corresponding to the worst-case scenario, which is a delay of one symbol period for each of the users that share the spectrum with the NOMA technique minus one. Assuming that the number of NOMA users is \(U\), the delay introduced by cooperative NOMA corresponds to \(U\)-1 symbol periods. Comparing the delay of the NOMA with that of the cooperative NOMA, it is viewed that while NOMA only introduces a delay corresponding to the number of higher power users, cooperative NOMA introduces a delay corresponding to the number of NOMA users that share the spectrum minus one. * In terms of complexity, while conventional NOMA has a single receiver with the SIC to cancel the higher power interfering users, cooperative NOMA must incorporate a similar receiver added with multiple parallel receivers to detect those signals that have been relayed over-the-air. In the worst case, this corresponds to the number of interfering users with power lower than the reference one. Naturally that a trade-off can be achieved between complexity and performance. In this context, a cooperative NOMA receiver can only incorporate one or two parallel receivers to detect the signals that have been relayed over-the-air by one or two less powerful users.