# Link and System-Level NOMA Simulator: The Reproducibility of Research
## 2 The NOMA Link-Level Simulator
### Transmitter  
A detailed description of the transmitter for the downlink link-level NOMA simulator is presented in Figure 2a. Unlike time-division multiple access and frequency-division multiple access, the power-domain NOMA multiplexes multiple users in the power domain. Conventional multiple access systems, as standardized for 4G systems, are based on OFDMA, where resources are exclusively allocated to users [22]. No two users can share the same resources simultaneously, which reduces the overall capacity of the system for high data rate communication. Therefore, multiplexing users in a non-orthogonal fashion in the form of NOMA is one of the many potential solutions to increase the system capacity for FRA. In NOMA, users are free to utilize the entire bandwidth based on the difference in their power levels. The flexibility of subcarrier reuse within the same cell, as well as its combination with other technologies, has attracted the interest of the research community with regard to NOMA standardization.  
Three important processes performed at the transmitter side that make NOMA different from OMA-based systems are as follows,  
* User pairing;
* Power allocation;
* Superposition coding.  
Figure 1: NOMA link-level simulator overall structure.  
NOMA-based systems exploit the difference in channel conditions to pair users. Users close to the base station, i.e., near-end users (NEUs), are assumed to have better channel conditions; thus, low power is assigned for successful communication. On the contrary, users at a distance far away from the base station, i.e., the far-end users (FEUs), experience bad channel conditions, and consequently, high power is assigned to them. The greater the channel gain difference between users, the higher the sum-rate capacity to be achieved. Power allocation to different users based on their channel conditions is one of the many critical factors in NOMA-based systems. Although different power allocation techniques have been proposed in the literature; fractional transmit power allocation (FTPA) is considered in most research articles owing to its simplicity [23; 24; 25]. Another important process to be performed at the transmitter is SC. It is used to multiplex users at different power levels. Thus, a combination of NEU and FEU signal \(x(t)\) after SC and power allocation is given as [19];  
\[x(t)=\sqrt{P_{1}}x_{1}(t)+\sqrt{P_{2}}x_{2}(t) \tag{1}\]  
where \(x_{1}\) and \(x_{2}\) represent the data, and \(P_{1}\) and \(P_{2}\) denote the power allocated to NEUs and FEUs, respectively. For the MC-NOMA-based system, the superimposed signal must pass through further processing blocks before its final transmission through the channel. One of these significant processes is the mapping of a super-coded signal on the sub-carriers present in the system. This mapping is performed using the inverse fast Fourier transform (IFFT). Subsequently, a cyclic prefix (CP) was inserted to reduce the effect of inter-symbol interference. CP is redundant information and comprises the last samples of the symbol, appended at the start of the symbol. The signal is now ready to transmit and propagate through the multipath channel.