In most NOMA studies, power allocation is fixed. However, in reality, the optimal power allocation varies depending on the environment in which it is used. Therefore, the goal of this paper is to automatically perform the optimal power allocation in NOMA. SIC is a technique for separating signals using the power difference, and it is known that signals fail to separate if the power difference is small. However, there are few researches to discuss how much power difference is required for successful separation. In addition, IM-NOMA enables flexible communication by adding index modulation to conventional NOMA. In this paper, we investigate the success of signal separation in SIC in terms of BER, SNR, Throughput, Sumrate, and Fairness Index by varying the power ratio in NOMA-based multi-user system. In addition, in comparison of conventional NOMA scheme, IM-NOMA with flexible power allocation were examined in terms of BER, SNR, Throughput, Sumrate, and Fairness Index. It was found that it was easier to maintain communication quality to some extent even for terminals with low power allocation, which are prone to relatively poor communication quality.