Performance analysis of downlink multipath multi-user NOMA-VLC system

Abstract

In the present paper, we have investigated performance of Non-Orthogonal Multiple Access based downlink multipath multi-user Visible Light Communication System. The Bit Error Rate (BER), sum rate and outage probability have been simulated using Line-Of-Sight (LOS) and LOS plus first reflection (L-R1) signal for two-user and three-user system. The simulation has been carried out for different values of semi-angle ${\mathrm{\varphi }}_{1/2}$ of the LED i.e. $30^\circ , 50^\circ$ and 70$^\circ$. In the two-user system, the LOS simulation result shows that the error probability is 7.49 × 10⁻³, 4.88 × 10⁻³ and 1.03 × 10⁻¹ when ${\mathrm{\varphi }}_{1/2}$ equals to70$^\circ$, 50° and $30^\circ$ respectively for user-1. And for user-2, the BER is found to be 4.89 × 10⁻², 5.84 × 10⁻² and 3.73 × 10⁻¹ using ${\mathrm{\varphi }}_{1/2}$ equals to70$^\circ$, $50^\circ$ and $30^\circ$. Thus, the optimal semi-angle ${\mathrm{\varphi }}_{1/2}$ for user-1 and user-2 using LOS signal is $50^\circ$, 70$^\circ$ respectively. However, for the three-user system, the error probability increases when compared to two-user system and the optimal values of semi-angle for user-1 changes to $30^\circ$ and for user-2 & user-3, it is 70$^\circ$. Moreover, in two-user system with L-R1 signal, the optimal value is 70$^\circ$ for both user-1 and user-2. The SIC method fails to decode the user’s signal when number of users increases to three in L-R1 signal case. Further, with the far user power allocation coefficient equal to 0.8, the average BER is found to be minimum for two-user system. The sum rate vs. normalized offset has been simulated for different values of semi-angle of the LED. The investigation shows that the maximum sum rate of 278.84 Mbps is achieved at ${\mathrm{\varphi }}_{1/2}$ equals to $50^\circ$ for the two-user system using LOS signal. We have also examined the effect of shadowing (blocked LOS signal) and found that sum rate is almost zero for small values of ${\varphi }_{1/2}$ i.e. $20^\circ$ and $30^\circ$. Finally, for the semi-angle value of $30^\circ$ the outage probability (for target rate beyond 3bps/Hz) reaches to 100 percent for both users.