What is NOMA?

NOMA is highly promising for 5G radio access. The fundamental concept of NOMA is supporting multiple users in the power domain.
In contrast to the conventional multiple access (MA) techniques, NOMA uses a new dimension to perform
multiplexing within one of the classic time/frequency domains. 
NOMA can be regarded as an “add-on” technique, which has the promising potential of facilitating integration with the existing legacy-solutions. 

Basic NOMA concept:

Downlink NOMA

NOMA exploits superposition coding at the transmitter and successive interference cancellation (SIC) at the receiver, thus multiplexing users in the power domain. 
The Site sends the superposed signals to two users, where User 1 has higher channel gain than User 2. In NOMA, the user with higher channel gain and the user with lower channel gain are usually referred to as the Strong User and the Weak User, respectively.
The Strong User first subtracts the signal of the weak user through SIC and then decodes its own signal.
The Weak User considers the signal of the strong user as noise and detects its own signal directly. With worse channel gain and more interference, the weak user is assigned more power in NOMA to ensure fairness.

Benefits of NOMA

  1. It achieves spectral efficiency by serving multiple users at the same time and with the same frequency resource and mitigating the interference through SIC.
  2. It increases the number of simultaneously served users, and thus, it can support massive connectivity.
  3. Due to the simultaneous transmission nature, a user does not need to go through a scheduled time slot to transmit its information, and hence, it experiences lower latency.
  4. NOMA can maintain user-fairness and diverse quality of service by flexible power control between the strong and weak users.