In order to solve this problem you have to build the Noma transmission model with its constellations. By adding the noise and interference signal to each constellation point one can determine the bit error rate probability.
For the complete solution of the problem please follow the paper in the link:
https://arxiv.org/pdf/1804.03847.pdf
You may ask the authors for more help if you need it.
Naturally this is not the only paper calculating the bit error rate for Noma.
There is a more simplified paper for none orthogonal multiple access NOMA , where BER - SNR is plotted for different cases, please refer to the following file: