Analysis of channel crosstalk in muliti-spectrum visible light communication system

In visible light communication, the wavelength division mutiplexing (WDM) technology can improve system data rate by increasing the number of channels. However, because the emission spectrum of the light emitting diode (LED) has a certain width, a phenomenon of spectral overlapping will occur when the number of channels increases and channelspacing decreases, which results in channel crosstalk although optical filters are adopted. The channel crosstalk will restrict the capacity of WDM-VLC (visible light communication) system, which has great research significance. In this paper, the channel crosstalk based on LED spectra overapping is disscussed. The LED emission spectrum is modeled by combining the physical mechanism of LED emission with real shape of LED spectrum. According to the literature, the LED shape can be fitted greatly by Gauss function, and the full-width at half-maximum ΔE is in a range from about 4.3 kBTj to 6.8 kBT_j when the peak wavelengths of InGaN and AlInGaN LEDs are both less than 560 nm, Δ E values range from 2.1 kBT_j to 3.3 kBT_j when the peak wavelength of InAlGaP LED is larger than 560 nm. In order to reduce the overall system complexity we use the following values: when the peak wavelength is less than 560 nm, ΔE = 5.5 kBT_j; when the peak wavelength is larger than 560 nm, ΔE = 3.0 kBT_j. Then, according to the overlapping spectra and VLC channel with considering optical filter transmittance and detector spectral response, the channel crosstalk formula is derived. Some quantities are given before simulation such as the semi-angle at half illuminance of an LED is 60° all LEDs are so closely arranged together to mix light in free space that spacing between LEDs can be ignored with respect to the propagating distance; the strongest signal situation is considered. The simulation result shows that although at the same channel spacing, different channels have different crosstalks because spectra are different. And the crosstalk from one adjacent channel will not exceed -13.6 dB when channel-spacing is larger than and equal to 28 nm, which means that when OOK modulation is used and the BER achieve 10^-6, the channel-spacing should not be less than 28 nm. Finally, an experiment of channel crosstalk with using two-channel WDM VLC system and LEDs with different wavelengths is conducted and the correctness of the crosstalk analysis is verified. The colors of red (635 nm), red-orange (620 nm) and amber (596 nm) LEDs are used and two of them are used each time. Two sine signals with different frequencies are launched by AWGs (Agilent 33250A) and through amplifiers and Bias-Tees, drive two LEDs. The signal analyzer (Agilent N9020A) is used to observe the signal power. The experimental results of channle crosstalk are close to theoretical results. The analysis of channel crosstalk in muliti-channel WDM-VLC system will give some guidance in increasing the number of channels for optical communication in the future.