1. Introduction

It is predicted that the wireless traffic is booming rapidly particularly for indoor applications due to the increase in popularity of smart-home applications and internet-of-things (IOT) [1–4]. As the traditional radio-frequency (RF) communication frequency bands are nearly fully occupied, visible light communication (VLC) has gained considerable attentions as a promising technology for high speed wireless communication [5–9]. Furthermore, VLC is applicable in different scenarios, such as providing indoor illuminance [10], high precision positioning and navigation [11], underwater communication [12], and light-panel and mobile-phone communication [13,14]. The high directionality nature of the VLC makes it suitable for high data rate and high security data transmission.

Pushing VLC to higher data rates is another important goal in research and development. A 1.1 Gbit/s VLC system applying multiple-input-and-multiple-output (MIMO) in phosphor white LED was reported [10]. Red, green, and blue (RGB) LED scheme can be used [15]. Ref [16]. reported a 3.4 Gbit/s VLC system using RGB LED with orthogonal-frequency-division-multiplexed (OFDM) signal. Ref [17]. demonstrated a 6.36 Gbit/s RGB LED VLC transmission using polarization multiplexing with transmission distance of 1 m. Micro-LED with a larger direct-modulation bandwidth can be used to increase the data rate [18]. Visible-light laser diode (LD) can be a promising choice for the high-speed VLC. A 2.5 Gbit/s LD VLC transmission based on a 422 nm gallium nitride (GaN) LD was reported [19]. Recently RGB LD VLC transmission was demonstrated [20,21]. A 1.25 Gbit/s white-light phosphor LD based VLC was reported [22] with a transmission distance of 1 m. Besides, a bit-directional LD VLC system using downstream OFDM and upstream signal remodulated OOK was reported, achieving a data rate of 10.6 Gbit/s in the downstream and 2 Mbit/s in the upstream [23].

In this work, we demonstrate a VLC system using tricolor RGB LDs achieving a recorded data rate of 40.665 Gbit/s. Here, a practical free-space transmission distance of 2 m is achieved. We believe the proposed work has potential applications in machine-to-machine (M2M) communication in areas that are sensitive to electromagnetic interference, such as in hospitals and power stations. The proposed VLC system can provide high bandwidth, high directionality, high security communication links. The implementation of bit-loading, power-loading and polarization multiplexing are discussed. Experimental bit-error-ratio (BER) results show that each of the 6 polarization and wavelength de-multiplexed channels can achieve the 7% pre-forward-error-correction (FEC) requirement (BER = 3.8 × 10⁻³).

2. Architecture and experiment

Figure 1(a) shows the architecture of the LD VLC system using tricolor RGB LDs. At the transmitter (Tx) side, two arbitrary waveform generators (AWGs) are used to provide electrical OFDM signals to the two sets of tricolor RGB LDs respectively. The two AWGs used in the experiment are Tektronix AWG70001 and Tektronix AWG7082C. Figure 1(b) illustrates the encoding and decoding processes of OFDM. They are carried out by using Matlab programs. In the OFDM signal encoding process, the randomly generated data is first serial-to-parallel (S/P) converted. After this, symbol mapping (SM) to different quadrature-amplitude-modulation (QAM) levels is implemented. Then, inverse fast Fourier transform (IFFT) is employed to construct the orthogonal subcarriers. Parallel-to-serial (P/S) conversion together with cyclic prefix (CP) application are executed. The FFT size and CP are 512 and 1/32 respectively. After the OFDM signals generation, they are used to directly modulated the tricolor RGB LDs. During the implementation of the OFDM IFFT, parallel data stream is used to design the data frame for the IFFT. The first element of the data frame represents the DC value and is selected to be zero. If the number of subcarriers is M, and the IFFT size is N, then elements from 2 to M + 1 of the frame consist of the parallel data and elements from N − M + 1 to N consist of the conjugate complex and mirrored version of the data. This is to produce real value output from the IFFT operation [24]. Then, the RGB channels at p-polarization and another RGB channels at s-polarization are polarization multiplexed via polarization beam splitters (PBS₁, PBS₂, PBS₃) as shown in Fig. 1(a). Finally, the polarization multiplexed 40.665 Gbit/s tricolor RGB optical signal is produced at the Tx via 3 dichroic mirrors (DMs). After a free-space transmission of 2 m, the tricolor optical signal is first de-multiplexed by using color filters (with center wavelengths of R: 660 nm, G: 520 nm, B: 450 nm) before the receiver (Rx) for BER measurements. Then, PBS is used to polarization de-multiplexed each color channel to obtain the p- and s-polarization. Finally, the de-multiplexed optical OFDM signal is detected by different PIN photodiodes (PDs). Real-time oscilloscope (RTO, Teledyne LeCroy 816ZI-B) with 80 GS/s sampling rate and 16GHz analog bandwidth is used for signal analysis. Figure 1(b) also shows the OFDM decoding process, including removal of CP, S/P conversion, FFT execution, equalization (EQ), symbol de-mapping (SDM), and P/S conversion. The BER performance is obtained by the measured signal-to-noise ratio (SNR) and the error vector magnitude (EVM) of the received signal. The modeling showing the relationship between the BER, SNR and EVM is discussed in [25].

 

Fig. 1 (a) Architecture of the tricolor RGB LD VLC experiment. AWG: arbitrary waveform generator; LD: laser diode; DM: dichroic mirrors; PBS: polarization beam splitter; PD: photodiode; RTO: real-time oscilloscope. (b) Encoding and decoding processes of OFDM in Matlab.

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In order to achieve high data rate, proper bit-loading and power-loading are applied based on the SNRs of different OFDM subcarriers. Besides, during the RGB multiplexing, as the R, G, B LDs are not temperature control, each DM should have a large enough pass-band allowing the wavelength shift due to temperature change. In our case, the 10 nm pass-band DMs are used. Furthermore, the PBS should be placed slight offset from the optical path in order to avoid back-reflection launching into the LDs producing laser instability.

Figures 2(a) and 2(b) show the top-view and side-view photographs of the polarization multiplexing tricolor RGB VLC system. In the experiment, the tricolor RGB LDs are housed in an aluminum-package, and each R, G, B LD has a separated bias-tee and current driving circuit. In the next version, the two sets of tricolor RGB LDs together with the optics components, such as PBSs and DMs could be housed in the same package to minimize the size of Tx. The LDs used in experiment are not temperature controlled. The R, G, B LDs used in the experiment are commercially available with miniaturized TO38 packages.

 

Fig. 2 Photographs of (a) top-view and (b) side-view of the polarization multiplexed tricolor RGB VLC Tx. (b) Measured RGB optical spectra at the output of Tx.

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Figure 3(a) shows the measured optical spectra of the RGB LDs at the output of Tx. The peak emission wavelengths of 660 nm, 514 nm and 450 nm are observed. There spectral widths (full-width half-maximum, FWHM) are about 2 nm. Figure 3(b) shows the optical output powers of the R, G, B LDs at different biased currents. We can observe that the threshold currents are 60 mA (R), 45 mA (B) and 45 mA (B) respectively. The measured power conversion efficiencies are 0.913 W/A (R), 0.453 W/A (G) and 0.682 W/A (B) respectively.

 

Fig. 3 (a) Measured RGB optical spectra at the output of Tx. (b) Measured optical output powers of the R, G, B LDs at different biased currents.

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3. Results and discussions

The measured SNRs, bit-loaded and power-loaded p- and s-polarization R, G and B color channels are illustrated in this section. The bit-loading and power-loading applied are based on the SNRs of different OFDM subcarriers received at the Rx. Figures 4(a) and 4(b) show the measured SNRs, bit-loaded and power-loaded of the p-polarization R channel respectively; while Figs. 4(c) and 4(d) show measured SNRs, bit-loaded and power-loaded of the s-polarization R channel respectively. The averaged measured SNRs of p-polarization and s-polarization R channels are 20.9 dB and 21.09 dB, respectively. The highest level of bit-loading applied is 7, and representing 128-QAM can be employed. The constellation diagrams are illustrated in the insets of Figs. 4(b) and 4(d), showing 128-QAM, 64-QAM, 32-QAM, 16-QAM and 8-QAM respectively. As the s-polarization R channel has a higher SNR than the p-polarization, higher data rate can be applied. The data rates of the p- and s-polarization channels are 6.621 Gbit/s and 7.024 Gbit/s respectively; and their corresponding BERs are 2.876 × 10⁻³ and 2.19 × 10⁻³, respectively, satisfying the 7% pre-FEC threshold (BER = 3.8 × 10⁻³).

 

Fig. 4 (a) Measured SNRs, bit-loaded and (b) power-loaded of the p-polarization and (c), (d) the corresponding s-polarization R channel. (e) Measured SNRs, bit-loaded and (f) power-loaded of the p-polarization and (g), (h) the corresponding s-polarization G channel. Measured (i) SNRs, bit-loaded and (j) power-loaded of the p-polarization, (k), (l) the corresponding s-polarization B channel.

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Figures 4(e) - 4(h) show the measured SNRs, bit-loaded and power-loaded for the p- and s-polarization G channel respectively. The averaged measured SNRs of p- and s-polarization G channels are 22.7 dB and 23.3 dB, respectively. We can also observe that the highest bit-loading level of 7 can be applied, and this means 128-QAM can be used. The data rates of the p- and s-polarization channels are 6.435 Gbit/s and 6.804 Gbit/s respectively; and their corresponding BERs are 2.514 × 10⁻³ and 2.832 × 10⁻³, respectively, satisfied 7% pre-FEC threshold.

Figures 4(i) - 4(l) show the measured SNRs, bit-loaded and power-loaded for the p- and s-polarization B channel respectively. The averaged measured SNRs of p- and s-polarization B channels are 21.96 dB and 21.93 dB, respectively. The data rates of the p- and s-polarization channels are 6.111 Gbit/s and 7.670 Gbit/s respectively; and their corresponding BERs are 2.579 × 10⁻³ and 3.092 × 10⁻³, respectively, satisfied 7% FEC threshold.

Then, the BER performances of the polarization and color de-multiplexed R, G, and B channels are evaluated. Figures 5(a) - 5(c) present the BER curves of the R, G, and B channels. The receiver sensitivities at FEC threshold for the R, G, and B channels are about 7.8 mW, 5.3 mW, and 5.7 mW respectively. The receiver sensitivity differences between the p- and s-polarization de-multiplexed channels are from 0.5 mW to 1 mW. This is due to the non-ideal orthogonal alignment of the p- and s-polarizations in the separated package of the two sets of RGB LDs.

 

Fig. 5 Measured BER curves of color and polarization de-multiplexed (a) R, (b) G and (c) B color channels.

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4. Conclusion

It is predicted that the wireless traffic is booming rapidly particularly for indoor applications in the near future. As VLC can provide many transmission merits and to relieve the congested RF wireless communication spectrum, it is attracting many attentions recently. Previously reported VLC systems are limited by transmission distances and data rates. Here we demonstrated a LD VLC system with a recorded data rate of 40.665 Gbit/s using tricolor RGB LDs and polarization multiplexing satisfying 7% pre-FEC threshold. A transmission distance of 2 m was achieved. The implementation of bit-loading, power-loading and polarization multiplexing were discussed. The aggregated 40.665 Gbit/s RGB visible light transmission was achieved by using the tricolor signal: R channel of 13.645 Gbit/s, G channel of 13.239 Gbit/s, and B channel of 13.781 Gbit/s. The receiver sensitivities at FEC threshold for the R, G, and B channels were about 7.8 mW, 5.3 mW and 5.7 mW respectively. The receiver sensitivity differences between the p- and s-polarization de-multiplexed channels were from 0.5 mW to 1 mW.