# Ambient LED Light Noise Reduction Using Adaptive Differential Equalization in Li-Fi Wireless Link
## 3 Experimental Setup  
Figure 2 shows the white LED-based optical wireless transmission link, which was implemented to experimentally verify the proposed ADE technique. The NRZ-OOK signal and the ADE technique were implemented using MATLAB(r) (the part in the dashed box). The NRZ-OOK signal was generated using a pseudo-random bit sequence (PRBS) signal with a length of \(2^{23}-1\). The number of sampling points per 1 bit was set to 10. The NRZ-OOK signal was loaded to an arbitrary waveform generator (Tektronix, Beaverton, USA, AWG 70002A) sampled at 100 MS/s. Therefore, the transmission rate of the NRZ-OOK signal was 10 Mb/s. The light from the white LED was modulated by the electrical NRZ-OOK signal using a bias-tee. Next, two kinds of light sources were used to generate noises from themselves. One was a fluorescent lamp (Philips Energy Saving CFL spiral lamp 12 W ES (E27)), and the other was a dimmable LED lamp (IKEA's LEDARE GU10 LED bulb) with adjustable brightness. The optical power of the white LED modulated by the NRZ-OOK signal and the optical powers of the fluorescent lamp, and dimmable LED was all set equal to \(-5\) dBm. A collimator (Edmund optics, LightPath 355561 115 mm Dia., 0.60 NA, BBAR (350-700 nm), molded aspheric lens) was used to uniformly transmit the lights generated from the three light sources to the avalanche photodiode (APD, Hamamatsu, Hamamatsu City, Japan, C12702-11, 100 MHz bandwidth). If the collimator is not used, the light from the light source will spread, and then the SNR of the received NRZ-OOK signal may be reduced. The important thing is that it is difficult to ascertain whether the amount of light emitted from the three different light sources is injected equally into the APD without a collimator. The optical wireless transmission distance was set to 1 m, and the received optical power was set to \(-10\) dBm using an attenuator (Thorlabs, Newton, USA, LCC 1620) to prevent the optical saturation of the APD and then aligned to maximize the received optical power. The attenuator was used to measure the BER while varying the received optical power. In an actual optical wireless environment, the transmission distance may be at least 1 m because it is highly likely that various types of light sources are installed on the ceiling. Therefore, the wireless transmission distance was set to 1 m in the experimental setup of Figure 2. For reference, since the optical power of the light source is inversely proportional to the square of the transmission distance, the optical SNR and the intensity of interference noise depend on the transmission distance. The transmitted light was converted into an electric signal by optical to electrical (OE) conversion in APD, and then the waveform of the converted electric signal was captured at a real-time oscilloscope (Tektronix, Beaverton, USA, MSO 71604C) sampled at 250 MS/s. After this, the NRZ-OOK signal was recovered after reducing the noise generated from the ambient light source using the proposed ADE technique offline. At this time, the regularization parameter, \(\lambda\) mentioned in Equation (11), is set to 9. The four  
Figure 2: Optical wireless transmission link based on white light—emitting diode (LED) for the verification of adaptive differential equalization (ADE) technique. Insets below experimental setup: (**A**) input non—return –to –zero –on –off keying (NRZ-OOK) waveform, (**B**) output NRZ—OOK waveform from arbitrary waveform generator (AWG), (**C**) NRZ–OOK waveform received at avalanche photodiode (APD), (**D**) NRZ–OOK waveform after using the proposed ADE technique, (**E**) RF(radio frequency) spectrum of fluorescent lamp, (**F**) RF spectrum of dimmable LED. different signal waveforms presented at the bottom of the experimental setup show the waveforms measured at points Figure 2A-D, and two RF spectra (Figure 2E,F) under the four different signal waveforms show the noises generated at the fluorescent lamp and the dimmable LED at the optical power of \(-5\) dBm.