## 9 Simulation Results and Discussion
It can be clearly noticed that as the number of users in the cell increases, our proposed DL channel estimation scheme remains superior in showing higher rates compared to other schemes. These results indicate that reliability can be guaranteed by the proposed scheme even when the cell capacity is increased. On the other hand, it is worth mentioning that as the total number of users keeps increasing in the cell, the interference will also increase, and consequently, the performance will degrade, and the sum rate will start to decrease. Figure 9: Outage probability vs. power-based DL and conventional NOMA (Rician channel). In Figure 12, two different simulation scenarios are conducted here to generate this figure; the first one when a fixed power allocation (FPA) scheme is applied for each user in the system. The other scenario is the optimized power scheme that is implemented according to the analytical power factors derived earlier. Both scenarios are employed in combination with the proposed DL for the channel estimation scheme. Simulation results for far and middle users prove the superiority of the power-optimized structure over the FPA structure in terms of BER. For the near user results, the proposed DL-based channel estimation jointly with FPA provides little enhancement in terms of the received bits error over the optimized power scheme, which could be justified in that for the near user scenario, a good channel condition is more beneficial than the allocated power. Figure 11: Sum rate vs. number of users for conventional NOMA, joint DL-NOMA, and the proposed DL-NOMA (Rayleigh Channel). Figure 10: Individual capacity vs. power-based DL and conventional NOMA (Rician Channel). Figure 13 illustrates the outage probability results against the power transmitted for far, middle, and near users when the optimized power scheme and FPA schemes are applied, and both scenarios are conducted in combination with the proposed DL-based channel estimation in NOMA cell. Far user results show an enhancement in outage probability and the power saving is approximately 5-6 dBm when both the DL and optimized scheme are applied compared to the FPA results. Similarly, for the middle user case, both the DL and optimized scheme provide a noticeable improvement in the outage probability, but with less power saving, i.e., 2-3 dBm approximately. Alternatively, the near user with the joint DL channel estimation scenario and FPA scheme show considerable outage improvement compared to the optimized power case. These results also confirm the results obtained for the outage propagability metric, which indicates that FPA coefficients, jointly with high channel gain, are more sufficient for the near user than the power optimization scheme. In Figure 14, the simulation results for the sum rate for the three examined users in NOMA cell are shown. Each of the optimized power scheme and FPA scheme is incorporated with the proposed DL algorithm utilized for estimating the channel coefficients prior to calculating the rate for each user. On the basis of the simulation outcomes, it can be clearly noticed that the channel estimation based on DL combined with the optimized power scheme show little improvement in the sum rate compared to the FPA scenario when the applied power level is low. Starting from 15 dBm, both the optimized power and FPA schemes provide a comparable sum rate when our proposed DL channel estimation scheme is implemented. Figure 12: BER vs. power for DL-based optimized and FPA power schemes (Rayleigh Channel). In Figure 15, the achievable user's rates are simulated separately against the transmitted power when the optimized power and fixed power schemes are implemented, where the proposed DL is employed for channel parameter prediction for each user in the cell. The simulation outcomes for far and middle users indicate that both the FPA scheme and the optimized power scheme provide comparable rates, even when the DL algorithm is considered.