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Abstract
High speed visible light communication (VLC) is a technology with great potential for future mobile and wireless communication. Here, we report and demonstrate a 2.705 Gbit/s white-light VLC and illumination system supporting indoor transmission distance of 1.5 m, corresponding a illumination of 545 lux. We also study the performance tolerance offset ranges in both x- and y-directions.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recently using white-light light-emitting-diodes (LED) for both illuminance and visible light communication (VLC) has attracted significant attentions [1–5]. A 3-Gbit/s VLC system using a single Gallium-Nitride (GaN) µLED was reported; however the transmission distance is limited to a few centimeters [6]. A white-light 1.68 Gbit/s VLC using a blue GaN µLED and fluorescent polymer color converter was reported; however the transmission distance is also limited to a few centimeters [7]. An red-green-blue (RGB) LED VLC system with data rate > 10 Gbit/s was demonstrated by using red resonant-cavity (RC)-LED and blue and green µLEDs [8]; however, the system cannot be applied for lighting. White-light laser diode (LD) can provide a highly directional light beam with better lumens per target area per watt of power consumption for solid-state lighting when compared with white-light LED [9]; and white-light LD illumination technology is being commercialized in applications such as specialty lighting and automotive lighting. Besides, to avoid the LED efficiency droop [10], LD could be a promising candidate for solid-state lighting. 9 Gbit/s and 10.6 Gbit/s orthogonal frequency division multiplexing (OFDM) based VLC systems were demonstrated using a 450 nm LD [11] and 682 nm LD [12] respectively. To provide VLC and lighting simultaneously, using tricolor R/G/B LDs [13–15] were proposed. However, these schemes may be complicated and costly as they required separated R/G/B LDs, wavelength combiner and filters. 11.2 Gbit/s R/G/B LDs based VLC was achieved using OFDM; however, the transmission distance is about 0.5 m [15]. Besides using tricolor LDs, 5.2 Gbit/s VLC based on a blue LD with yellow phosphor VLC was demonstrated to provide white-light; however, the transmission distance is 0.6 m [16]. Recently a 1.25 Gbit/s and 14.8 Gbit/s VLC systems using blue LD with yellow phosphor at transmission distances of 1 m [17] and 0.5 m [18] were also reported.
In this work, we demonstrate a 2.705 Gbit/s white-light VLC and illumination system supporting indoor transmission distance of 1.5 m, corresponding 545 lux. A 16-quadrature amplitude modulation (QAM) OFDM signal is used to direct modulate a white-light LD.
2. Experiment
Figure 1 shows the experiment of the phosphor-based white-light and blue-filter-free VLC system. The white-light LD consists of a 460 nm LD and a yellow phosphor diffuser. The white-light LD is direct-current (DC) biased through a current driver. The 16-QAM OFDM signal is generated by using offline Matlab program, which is sent from an arbitrary waveform generator (AWG, Tektronix 7082C) to a bias-T circuit. The OFDM encoding includes serial-to-parallel (S/P) conversion, symbol mapping, inverse fast Fourier transform (IFFT), parallel-to-serial (P/S) conversion, cyclic prefix (CP) insertion. The FFT size and the CP used are 512 and 1/64 respectively. The OFDM subcarriers are 256, and the sampling rate is 8 GSample/s. Then the bias-T circuit combines 16-QAM OFDM data and the DC signal to drive the white-light LD for direct modulation. The operating DC bias is < 1 V. The white-light LD has a divergent angle of about 5°. An optical lens with focal length of 30 mm is used in front of the PD for focusing. The PD (EOT ET-2030A) is silicon-based, it has a 3-dB bandwidth of 1.2 GHz, active area diameter of 400 µm, sensitivity of 450 V/W. Finally, the 16-QAM OFDM signal is received by a real-time oscilloscope (RTO, Tektronix DPO7354C). The RTO performs the function of analog-to-digital conversion (ADC). The received and ADC converted OFDM signal will be decoded, including signal re-sampling, synchronization, CP removal, S/P conversion, FFT, one-tap equalization, symbol mapping, and error analysis. Optical blue-filtering is usually regarded as a critical component to remove the slow response yellow light from the phosphor-based white LD or LED to enhance the data rate of VLC. However, the optical blue-filtering plays different roles when using different modulations [19]. For the digital signal processing (DSP)-based modulation, such as OFDM, optical blue-filtering is unnecessary since the optical equalization could be implemented in DSP directly. As optical blue filter will remove the yellow light from the white light, it will significantly reduce the Rx power and shorten the transmission distance.
Fig. 1. Experimental setup of the phosphor-based white-light and blue-filter-free VLC system. AWG: arbitrary waveform generator, RTO: real time oscilloscope, PD: photo-diode.
3. Results and discussions
Figure 2(a) shows the measured optical spectra of the white-light LD at different operating currents. The white-light LD consists of a 460 nm blue LD with yellow phosphor layer in front of it. The shorter wavelength blue light (higher energy blue photons) will be down-converted by a luminescent material (phosphor) to longer wavelengths. As shown in Fig. 2(a), the yellow color with wavelength of 570 - 580 nm will be generated. The combination of that yellow light with the remaining blue light will generate the observed white light. The yellow phosphor used in this demonstration not only be used to generate white-light for illumination, but also acts as a diffuser to provide a wide lighting zone. We also measure the CIE value under the 1931 CIE chromaticity coordinates when the white-light LD is operated at driving currents as shown in Fig. 2(b). We can observe that the LD used in the experiment has a cool white light color temperature (6000 K-9000 K). Figure 3(a) illustrates the measured P-I-V curve of the white-light LD. Figure 3(b) illustrates the measured its frequency response. The 3-dB bandwidths at 0.5 A and 1.5 A driving currents are about 800 MHz and 1.1 GHz respectively.
Fig. 2. (a) Measured optical spectra of the white-light LD at different currents (0.5, 1.0, 1.5, 2.0 A). (b) Measured CIE chromaticity coordinates when the white-light LD are at different operating currents.
The measured signal-to-noise ratios (SNRs) and their corresponding 16-QAM constellation diagrams, photographs of the experiments, and photographs of the lighting zones at different transmission distances are shown in insets (i), (ii), (iii) of Figs. 4(a)–4(c) respectively. The measured average SNRs of the 255 OFDM subcarriers are 18.73 dB, 18.11 dB, and 17.08 dB at transmission distances of 1 m, 1.5 m, and 2.15 m respectively. As also shown in insets (iii) of Figs. 4(a)–4(c), the light beam diameters are 9 cm, 12 cm, and 18 cm at distances of 1 m, 1.5 m, and 2.15 m respectively. The corresponding illuminance measured are 754 lux, 545 lux, and 305 lux at the center of the lighting zone. In the experiment, the PD is located at the center of lighting zone, and the achievable data rates are 2.898 Gbit/s, 2.705 Gbit/s, and 2.556 Gbit/s at 1 m, 1.5 m, and 2.15 m. The bit-error rates (BERs) of three distances are 2.364 × 10−3, 3.177 × 10−3, and 3.396 × 10−3 respectively; showing that all the measurements can satisfy the 7% forward-error-correction (FEC) requirement. The data rates are measured by changing the baud rate with nearly fixed BER threshold (FEC BER < 3.8 × 10−3). A schematic illustrating the white-light illumination and the achievable data rates is shown in Fig. 4(d). For example, in the lighting zone of 754 lux (typical lighting condition provided by desktop lamp), achievable data rate of 2.898 Gbit/s can be achieved. In the lighting zone of 545 lux (typical lighting condition in living room), achievable data rate of 2.705 Gbit/s can be achieved. As mentioned before, the yellow phosphor also acts as a diffuser to provide a wide lighting zone. By removing the yellow phosphor, the light diffusing effect is removed. In our previous studies, without using the yellow phosphor diffuser [14], 7.670 Gbit/s data rate was achieved at 2 m transmission distance.
Fig. 4. Measured SNRs and their corresponding (i) 16-QAM constellation diagrams, (ii) photographs of the experiments, (iii) photographs of the lighting zone (light beam diameter) with different transmission distances of (a) 1 m, (b) 1.5 m, (c) 2.15 m. (d) Schematic between illumination and data rates.
Figures 5(a) and 5(b) show the measured SNRs and 16-QAM-OFDM constellation diagrams at different X-axis and Y-axis offsets when the transmission distance is 1 m. The measured averaged SNRs are 17.11 dB, 17.01 dB and 16.69 dB for the 1 cm, 2 cm and 2.6 cm X-axis offsets; and 18.09 dB, 17.58 dB and 17.13 dB for the 1 cm, 2 cm and 2.5 cm Y-axis offsets. Figures 5(c) and 5(d) show the results at transmission distance of 1.5 m. The measured averaged SNRs are 18.08 dB, 17.48 dB and 16.98 dB for the 1 cm, 2 cm and 2.3 cm X-axis offsets; and 17.06 dB, 16.67 dB and 16.46 dB for the 1 cm, 2 cm and 2.5 cm Y-axis offsets. Figures 5(e) and 5(f) show the results at transmission distance of 2.15 m. The measured averaged SNRs are 17.04 dB, 16.63 dB and 16.21 dB for the 1 cm, 1.5 cm and 1.8 cm X-axis offsets; and 17.01 dB, 16.51 dB and 16.05 dB for the 1 cm, 1.5 cm and 1.9 cm Y-axis offsets. All these results indicate that the lighting zones are quite circular with nearly the same SNRs in both X and Y offsets at the same transmission distance.
Figure 6(a) shows BER performances of different X-axis and Y-axis offsets at transmission distances of 1 m, 1.5 m and 2.15 m, respectively. When the transmission distance is 1 m, BER performances are 3.475 × 10−3, 3.563 × 10−3, and 3.731 × 10−3 at X-axis offsets of 1 cm, 2 cm and 2.6 cm. When the transmission distance is increased to 2.15 m, BER performances are 3.41 × 10−3, 3.55 × 10−3, and 3.788 × 10−3 at X-axis offsets of 1 cm, 1.5 cm and 1.8 cm; and they all satisfy the 7% FEC requirement. The reduction of the acceptable X-axis and Y-axis offsets at longer transmission distance is due to the reduction of the illuminance. The illuminance decreases from 754 lux to 305 lux when the distances increases from 1 m to 2.15 m. Figure 6(b) shows corresponding achievable data rates after different transmission distances with different X-axis and Y-axis offsets. At the transmission distance of 1 m, the achievable data rates are 2.125 Gbit/s, 2.028 Gbit/s, and 1.352 Gbit/s at X-axis offsets of 1 cm, 2 cm and 2.6 cm. At the transmission distance of 1.5 m, the achievable data rates are 2.108 Gbit/s, 2.008 Gbit/s, and 1.565 Gbit/s at X-axis offsets of 1 cm, 2 cm and 2.3 cm. The reduction of data rates at longer transmission distances and at larger offsets are due to the reduction of the illuminance, which reduces the received SNR.
Fig. 6. (a) BER of different X and Y offsets at distances of 1 m, 1.5 m, and 2.15 m. (b) Data rates at different X and Y offsets at distances of 1 m, 1.5 m, and 2.15 m.
4. Conclusion
In this work, a high speed VLC and simultaneously lighting with practical transmission distance of 1.5 m was reported. In the lighting zone of 545 lux (typical lighting condition in living room), achievable data rate of 2.705 Gbit/s was achieved. We also studied the performance tolerance offset ranges in both x and y-directions.
Funding
Ministry of Science and Technology, Taiwan (MOST-107-2221-E-009-118-MY3).
Disclosures
The authors declare no conflicts of interest.
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