 prior probability of code-word x; is uniform, hence: (5.8) First, we obtain the f,,function: (5.9) Step 2: Iterative Message Exchange Firstly, passing information from function nodes to Variable nodes: Codeword Priori Probabilities (APP) Function Node Updating Block CALCULATION CALCULATION Variable Node Updating Block FIGURE 5.11 Diagram for message passing algorithm (MPA). Non-Orthogonal Multiple Access Transmission (5.10) {ieN(k)/j} Then, passing information from variable nodes to function nodes: (5.11) Then, normalization: (5.12) Step 3: LLR Calculation A posteriori probability of the code-word xj is defined as Pr(xj): (5.13) The log-likelihood ratios (LLRs): (5.14) Finally, each of the user bits, bi with {1,2,...,log2 N are estimated by: (5.15) 5.4.2.3 Simulation Results The bit error rate performance of the SCMA system shown in Figure 5.10 is implemented in MATLAB software. The SCMA system we are working with depends on MPA to extract the signals for the users. But on the other hand the complexity is high because of the exponential operations. The simulation result in Figure 5.12 shows the bit error rate for SCMA Downlink system under the Rayleigh fading channel for 6 users sharing 4 resources. 5G Wireless Communication System in Healthcare Informatics 6 users tranmitted over SCMA Eb/No (dB) FIGURE 5.12 BER performance of an SCMA downlink. 5.4.3 REALISTIC TRANSMISSION OVER THE RAYLEIGH FADING CHANNEL In this section a realistic transmission over the Rayleigh fading channel is studied when using PD-NOMA. 5.4.3.1 System Models Consider a single cell downlink network with one base station serving a set of users. The users are deployed randomly within the cell as shown in Figure 5.13. Then, the BS will compare two nearby of users' angles, the two users with angle differences of less than /12 are considered as potential NOMA users. After that, the BS allocates to each of the pair a power according to their distance from the BS. In this scenario, we deployed 50 users randomly and we found the number of users conf