# Link and System-Level NOMA Simulator: The Reproducibility of Research
## 4 Coded MC-NOMA in the Presence of Non-Gaussian Channels  
_Structure of MC-NOMA_  
In this section, the proposed MC-NOMA model is based on channel coding and LP, as shown in Figure 6, is discussed. Information bits from both the NEU and FEU are passed through the channel coding block to introduce redundancy for error correction. Convolution coding as forward error correction (FEC) is considered in the proposed system owing to its easy implementation and better performance in noisy channels [35; 36]. The encoder output is modulated and then power allocation is applied depending on the channel gains. The multiplexed signal x(t) from both NEU and FEU after SC is directed to a linear precoding block, which is implemented through the Walsh Hadamard Transform (WHT) owing to its simplicity [36]. WHT spreads information either in the entire system or half of the system and thus improves the BER performance in the frequency-selective channel. The superimposed signal is multiplied with the WHT kernel \(W_{H}\) to obtain the precoded signal \(X_{p}\) such that \(X_{p}=XW_{H}\) where \(X\) is the matrix representation of \(x(t)\) and \(W_{H}=\frac{1}{\sqrt{H}}\bigg{[}\begin{array}{cc}+W_{H-1}&+W_{H-1}\\ +W_{H-1}&-W_{H-1}\end{array}\bigg{]}\). LP breaks the autocorrelation among symbols, thus resulting in a significant improvement in the selectivity of the channels. The IFFT is applied to the incoming precoded stream to convert the frequency-domain symbols into the time-domain one. CP is also appended after an inverse transformation to circumvent inter-symbol interference. The transmitted signal is then passed through the Rayleigh fading channel, which is corrupted by both AWGN and impulse noise to provide a more realistic simulation scenario. Impulse noise can be modeled through Middleton noise model A, the explanation of which can be obtained in [37; 38; 39]. Moreover, inter-cell interference is considered a limiting factor in obtaining lower BER. Thus, the received signal \(r(t)\) at each \(UE_{l}\) (\(l=1,2,3,\ldots,k\)) can be expressed as,  
\[n_{l}=h_{l}x+\mu_{G}+\sigma_{l}+\rho_{ICI} \tag{6}\]  
where \(h_{l}\) represents the channel coefficients, and \(\mu_{G}\) denotes AWGN with zero mean and variance \(\phi_{G}^{2}\). \(\sigma_{l}\) denotes impulse noise, whereas intercell interference is represented by \(\rho_{ICI}\). At the receiver, all processing steps were performed in the reverse order. The CP is removed, and FDE is applied to undo the channel effects. In the following step, FFT is applied to convert the signal from the time domain to the frequency domain and then provided to the inverse WHT to obtain the signal. SIC is applied at this stage to separate multiplexed signals. Demodulation and channel decoding processes are performed to extract the transmitted bits for both users.  
As our study comprises NOMA as multiple access for two users to present the potential it holds for 5G wireless networks and more than two users, the research is at the infancy stage; thus extending the proposed work for multiple numbers of users is not possible. User pairing based on channel gains and power distribution among different users is a key issue, and several studies are underway to solve this restriction.