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

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 phi(1/2) of the LED i.e. 30 degrees, 50 degrees and 70 degrees. In the two-user system, the LOS simulation result shows that the error probability is 7.49 x 10(-3), 4.88 x 10(-3) and 1.03 x 10(-1) when phi(1/2) equals to70 degrees, 50 degrees and 30 degrees respectively for user-1. And for user-2, the BER is found to be 4.89 x 10(-2), 5.84 x 10(-2) and 3.73 X 10(-1) using phi(1/2) equals to70 degrees, 50 degrees and 30 degrees. Thus, the optimal semi-angle phi(1/2) for user-1 and user-2 using LOS signal is 50 degrees, 70 degrees 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 degrees and for user-2 & user-3, it is 70 degrees. Moreover, in two-user system with L-R1 signal, the optimal value is 70 degrees 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 phi(1/2) equals to 50 degrees 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 phi(1/2) i.e. 20 degrees and 30 degrees. Finally, for the semi-angle value of 30 degrees the outage probability (for target rate beyond 3bps/Hz) reaches to 100 percent for both users.