# A 40 Mb/s VLC System Reusing an Existing Large LED Panel in an Indoor Office Environment
## 3 Challenges and Results
### Results and Discussion  
The BER was measured at varying distances from 1 to 2 m, see Figure 11. For reference purposes, a light level (in lux) curve is also provided. The experimental results show that, with the proposed flicker removal DSP solution, a 40 Mb/s link can be supported up to  
Figure 10: (**a**) Decoded constellation diagram at the 1 st carrier before flicker removal; (**b**) Decoded constellation diagram at the 1 st carrier after flicker removal.  
Figure 9: (**a**) Received signal \(r(t)\) (blue) and the corresponding estimated flicker signal \(e(t)\) (red); (**b**) a zoom-in view of a segment of \(r(t)\) and \(e(t)\); (**c**) filtered signal \(y(t)\); (**d**) a zoom-in view of a segment of \(y(t)\).  
2 m with an illuminance level of \(\sim\)300 lux (minimum illuminance for indoor workplaces in accordance with EN 12464-1 [43]), maintaining a BER below the 7% FEC limit. When the flicker removal algorithm was not applied, the system performance was mainly limited by the BER performance of the 1 st sub-band and the overall BER level was close to the FEC limit with a narrow margin. The effectiveness of the flicker mitigation was validated by the improvement in the system performance and robustness.  
In order to better demonstrate the system performance, the measured spectra of the received signal and background noise at a distance of 1.4 m (with a light level of 600 lux measured) are shown in Figure 12. The ten carriers can be clearly observed, with every recovered constellation diagram from each carrier included in Figure 13. The corresponding SNR curve is given in Figure 14 with the bit-loading pattern being plotted. As mentioned earlier, the bit-loading pattern is determined at the longest distance and, therefore, the SNR profile at the distance of 1.4 m can support quite clear constellation diagrams, as shown in Figure 13. A video demonstrating the successful demodulation of the 10 th carrier is also provided as Supplementary Materials for interested readers.  
Figure 11: Bit error rate (BER) performance and light level vs. distance.  
Figure 12: Measured signal spectrum and background noise spectrum at the distance of 1.4 m.  
Two major limiting factors for achieving higher \(R_{b}\) using the off-the-shelf LED panel are observed. Firstly, the modulation bandwidth is limited to 10 MHz, because severe distortion, which might be attributed to the LED's nonlinearity and the parasitic in the electronics, is incurred beyond 10 MHz. It could be a big issue when trying to reuse the existing lighting sources in the applications of high-speed VLC standards, like G.9991 [18] requiring a minimum bandwidth of 50 MHz. Secondly, the flicker that is caused by the LED driver inevitably reduces the modulation depth or the transmitted optical power for VLC signals, imposing a limit on the SNR and the achievable \(R_{b}\). An improvement in data rates is expected if the LED driver is optimised for VLC.  
Figure 14: The measured signal-to-noise ratio (SNR) profile and the bit-loading pattern at the distance of 1.4 m.  
Figure 13: Demodulated constellation diagrams at the distance of 1.4 m for the (**a**) 1 st carrier; (**b**) 2 nd carrier; (**c**) 3 rd carrier; (**d**) 4 th carrier; (**e**) 5 th carrier; (**f**) 6 th carrier; (**g**) 7 th carrier; (**h**) 8 th carrier; (**i**) 9 th carrier; (**j**) 10 th carrier.