I. Introduction
Non-orthogonal multiple access (NOMA) is a promising technology to increase the spectral efficiency of 5G communication systems [1], [2]. Unlike the orthogonal multiple access (OMA), which transmits only one data in one resource block (RB), NOMA can transmit multiple data in one resource block at once. Therefore, NOMA is considered as a promising technology since more users can be supported using the same frequency, time and space resource [3]. The key techniques of the NOMA system are superposition coding (SC) and successive interference cancellaion (SIC) [4]. SC is a technique that combines two or more types of data with power allocation, and SIC is a technique that decodes the stronger signal first and removes the decoded signal from the combined signal to decode the weaker signal. In a NOMA system with multiple users, users with better channel conditions perform more SIC to decode their signals [5].