On the spectral efficient nonorthogonal multiple access schemes

In this paper, we introduce a nonorthogonal multiple access scheme, called relay-aided multiple access (RAMA), which is based on network coding. This scheme is compared with a well-known scheme, nonorthogonal multiple access (NOMA), which is based on superposition coding. Both the schemes can increase the spectral efficiency by supporting two users in the same frequency band and time slot. To compare them, we consider the sum rate, which is the sum of achievable rates of uplink and downlink transmissions of two users, and study a sum rate maximization problem with transmission power constraints. Based on sum rate analysis, we can show that each scheme can perform better than the other under different conditions. Thus, we can enjoy a degree of freedom to choose one of them depending on conditions to improve the spectral efficiency.