# Orthogonal Subblock Division Multiple Access for OFDM-IM-Based Multi-User VLC Systems
## 3 Results and Discussions
### Experimental Results  
We further experimentally investigate the performance of the OSDMA-MU-VLC system. The experimental setup of a two-user OSDMA-VLC system is shown in Figure 5a, where the transmitted signal is firstly generated offline by MATLAB and then sent to an arbitrary waveform generator (AWG, Tektronix AFG31102) with a sampling rate of 100 MSA/s. As a result, the bandwidth of the AWG output signal is 42 MHz and the data rate is 105 Mbits/s for all the schemes. The peak-to-peak voltage (Vpp) of the AWG output signal is 3 V. After that, the AWG output signal is combined with a 35-mA DC bias current via a bias-tee (bias-T, Mini-Circuits ZFBT-6 GW+) to drive the infrared LED. At the receiver side, each user is equipped with an avalanche photo-diode (APD, Hamamatsu C12702-12) to detect the optical signal and perform optical-to-electrical (O/E) conversion. It can be seen from Figure 5a the two users have the same vertical offset of 10 cm from the center of the LED while the horizontal distance is in the range between 40 to 80 cm. Subsequently, the received electrical signal of each user is captured by a digital storage oscilloscope (DSO, Tektronix MDO32) with a sampling rate of 500 MSA/s, which is further processed offline by MATLAB. Moreover, the square 16-QAM constellation for OFDM-SM-IM/DFT-S-OFDM-SM-IM and the circular (7,1)-QAM constellation for OFDM-DM-IM/DFT-S-OFDM-DM-IM are shown in the insets (i) and (ii) of Figure 5a, respectively. The photo of the overall experimental system is given in Figure 5b. Figure 6 shows the measured frequency response of the VLC system which is obtained by using the OFDM-based channel estimation method. As we can see, the system exhibits a typical low-pass characteristic and the -3 dB bandwidth is 15.6 MHz. Figure 4: Simulation BER vs. SNR for **(a)** OFDM-SM-IM and DFT-S-OFDM-SM-IM and **(b)** OFDM-DM-IM and DFT-S-OFDM-DM-IM without and with interleaving over the low-pass VLC channel (w/\(\alpha\): without, w/: with, interl. : interleaving). Figure 7 shows the measured average received SNR of each user without and with interleaving versus the length of IFFT/FFT, where the transmission distance is 60 cm. As we can observe, as the length of IFFT/FFT is increased from 16 to 256, the average received SNR of each user is gradually increased. Moreover, the average received SNR of each user becomes stable when the length of IFFT/FFT is larger than 256. For the case without interleaving, the average received SNRs of 17.4 and 9.4 dB are obtained by user 1 and user 2, respectively, when the length IFFT/FFT is 256. Hence, there exists an SNR difference of up to 8 dB for two users which is mainly caused by the low-pass frequency response of the bandlimited VLC system. In contrast, for the case with interleaving, the two users can achieve comparable SNRs when the length of IFFT/FFT reaches 256, indicating that interleaved subcarrier multiplexing is an efficient approach to mitigate user SNR unbalance. In our following experiments, the length of IFFT/FFT is fixed as 256. Figure 5: (**a**) Experimental setup of a two-user OSDMA-VLC system and (**b**) the photo of the experimental system. Figure 6: EOE frequency response of the experimental VLC system. Figure 8a,b show the measured BER versus the transmission distance for DFT-S-OFDM-SM-IM and DFT-S-OFDM-DM-IM, respectively. For DFT-S-OFDM-SM-IM without and with interleaving, as shown in Figure 8a, the BERs of all schemes can reach the 7% FEC coding limit of \(3.8\times 10^{-3}\) within the distance range of 40 to 70 cm. It can be clearly seen that the BERs of the two users for DFT-S-OFDM-SM-IM without interleaving are quite different and the maximum distances that can be transmitted by user 1 and user 2 below BER = \(3.8\times 10^{-3}\) are over 70 cm and about 45.3 cm, respectively. In contrast, for DFT-S-OFDM-SM-IM with interleaving, the two users can achieve comparable BERs and nearly the same maximum distances of about 61 cm can be achieved.