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Abstract
Visible light communication (VLC), combining wireless communication with white lighting, has many advantages. It is free of electromagnetic interference, is rich in spectrum resources, and has a gigabit-per-second (Gbps) data rate. Laser diodes (LDs) are emerging as promising light sources for high-speed VLC communication due to their high modulation bandwidth. In this paper, we demonstrate a red/green/blue (R/G/B) LDs based VLC system with a recorded data rate of 46.41 Gbps, employing discrete multitone (DMT) and adaptive bit-loading technology to achieve high spectral efficiency (SE). The emission characteristics and transmission performance of R/G/B-LDs are discussed. The optimal data rates of R/G/B-LDs channels are 17.168/14.652/14.590 Gbps, respectively. The bit-error-ratio (BER) of each channel satisfies the 7% forward-error-correction (FEC) threshold (3.8×10−3) and greatly approaches the channel Shannon limit.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent years, wireless communication technology encounters bandwidth bottlenecks in the development of beyond fifth-generation wireless communication, with rapid growing data rate demands from mobile Internet, artificial intelligence (AI) and the Internet of things (IoT) applications [1]. Visible light communication (VLC) is an emerging optical communication technology, with wide application prospects [2]. VLC technology can support high-speed data link and rich spectrum resources, and is expected to become one of the next-generation broadband communication techniques [3]. The high-speed transmission characteristics of VLC can also satisfy many futuristic applications, such as artificial intelligence (AI) technology [4], virtual reality (VR) and augmented reality (AR) [5], vehicle to everything (V2X) communication [6], intra-satellite network [7], etc.
The laser diode (LD) based VLC system has attracted much attention because the GHz modulation bandwidth of LD is not limited by the carrier lifetime. Since 2015, researchers studying on VLC in free space with LDs as the light sources have made great progress. In 2015, Lee et al. demonstrated a direct non-return-to-zero on-off-keying (NRZ OOK) modulation system using a 450 nm blue laser diode (BLD), and achieved a data transmission rate of 4 Gbps in a free space of 0.15 m [8]. In the same year, Retamal of KAUST proposed a 4 Gbps visible light communication system adopting BLD with 16 quadrature amplitude modulation (16QAM) orthogonal frequency division multiplexing (OFDM) technology [9]. Chun proposed a VLC system that uses a combination of BLD and remote phosphors to achieve a data rate of about 6 Gbps by employing a fixed rate and OFDM-compatible adaptive bit loading method [10]. By 2017, Shaun Viola utilized OFDM and bit loading technology to modulate a 450 nm gallium nitride (GaN) based laser diode over a 15 cm free space to promote data rate to 15 Gbps [11].
With the application of wavelength division multiplexing (WDM) technology in VLC systems, high-speed LD-based VLC can further increase the speed. In 2018, WDM technology is embodied in a 20.231 Gbps red/green/blue (R/G/B)-LDs based VLC system over 1 m free space link, reported by Wei et al. [12]. In 2019, Wei’s team used R/G/B-LDs compatible polarization multiplexing technology to demonstrate a VLC system with the highest tricolor WDM data rate of 40.665 Gbps, which can realize more than 2 m free space transmission [13]. Chun used quadrature-color red/green/blue/violet (R/G/B/V) LDs to generate white light and adopt WDM technology and OFDM bit loading modulation for indoor VLC. A transmission data rate of 35 Gbps has been achieved at a free space channel of 4 m [14]. In 2020, Lee et al. synthesized white light with two BLDs and yellow phosphors, combined with QAM-OFDM modulation to achieve a rate of 22.45 Gbps within 3 m VLC link [15]. Wang et al. used QAM discrete multitone (DMT) modulation for R/G/V LDs through pre-equalization of channels, and got a 34.8 Gbps data rate in a free space of 0.3 m [16]. Chow et al. demonstrated a high-speed VLC system using quadrature-color LDs transmitter, and a data rate of 26.228 Gbps at a single polarization is achieved with free-space transmission distance of 2 m [17]. However, lots of previous work adopting WDM light sources are based on simple combinations of independent laser modules. Their large size and independent temperature control make them complex to operate, which is not conducive to applications and limits further data rate improvements.
In this paper, we demonstrate a high-speed WDM VLC system based on a compact tricolor laser transmitter (Triser-Tx). The Tx consists of red, green and blue laser diodes, producing a white light. DMT bit-power-loading modulation with Levin-Campello (LC) algorithm [18] is used in the VLC system. As a proof-of-concept study, the channel distance is 0.3 m. The R/G/B-LDs show threshold currents of 50 mA, 100 mA, and 20 mA, and slope efficiencies of 0.86 W/A, 0.61 W/A, and 0.43 W/A, respectively. The -20 dB bandwidths of tricolor channels can achieve is up to 4.06 GHz, 3.11 GHz, and 3.43 GHz. The achieved maximum data rates of R/G/B LDs channels at optimal operating points are 17.168/14.652/14.590 Gbps, with corresponding bit error rates (BER) of 3.68×10−3, 3.72×10−3, and 3.46×10−3, which are all below the forward-error-correction (FEC) threshold (BER = 3.8×10−3). In total, the overall data rate of the Triser Tx based VLC system reaches up to 46.41 Gbps. As far as we know, it is the highest data rate for the R/G/B-LDs based free space VLC system. Our work paves the path towards future long-range, high-speed laser-based VLC transmission links.
2. Experimental setup
Figure 1 shows a tricolor R/G/B high-speed laser VLC system based on WDM technology actually built. Figure 1(a) demonstrates the structure and principle of the entire system. Firstly, the 4-QAM training sequence is used to estimate the signal-to-noise ratio (SNR) of the free space channel. The number of bits for each DMT format subcarrier (6 zero-pads, 250 subcarriers and 2 times up-sample) can be determined according to the SNR table. Next, according to the correspondence between the SNR of each subcarrier and the number of bits in the channel estimated by the training sequence, the bits are loaded with the DMT signal. Then, under a certain total power condition, the LC algorithm is used to optimize the power allocation and the number of bits to obtain the maximum spectral efficiency (SE) and approach the Shannon limitation of the channel. The demodulation and recovery of the received data and BER calculation are all carried out in the digital signal processing at the receiving end (Rx DSP). Since this experiment performed three single-channel measurements to obtain the maximum rate of the system, no demultiplexer was set at the receiving end for tricolor-channel receptions.
Fig. 1. Tricolor R/G/B high-speed laser-based VLC system with WDM technology (a) the structure of high-speed tri-color laser-based VLC system and the process of Tx and Rx DSP. (b) a specially designed 6×4.4×3 cm3 Triser-Tx (c) Triser-Tx module on a thermoelectrically cooled aluminum heat sink. (d) absorptive neutral density filter, lens and PIN. (e) a tricolor 0.3 m VLC link.
The system setup is depicted in Fig. 1(b)-(e). In the experiments, an arbitrary waveform generator (AWG, Agilent, M8190A) with a maximum sampling rate of 12 GSa/s is used to generate the modulated signal, and an electrical amplifier (EA, Mini-Circuits, ZFL-2500VH+) amplifies the signal with a specific multiple. Figure 1(b) and (c) show the Triser-Tx module with a small size of 6×4.4×3 cm3 on a thermoelectrically cooled base. The Triser module consist of R/G/B LDs, 3 collimating lens, 1 reflective mirror and 2 dichroic mirrors. The 3 LDs were arranged with a spacing of 2 cm, mounted on an aluminum heat sink. The temperature of the Triser module is controlled by a water-cooling system. The cooling head with a TEC and a thermistor is attached to the aluminum heat sink. The water circulation system is operating at a circulating flux at 1.3 L/min with single fan operating at 56.2 CFM. A PID controller is used to keep the Tx at room temperature. The RLD (USHIO, HL63603TG) shows an emission wavelength at ∼638 nm, GLD (OSRAM, PL520) at ∼520 nm, and BLD (OSRAM, PL450B) at ∼450 nm. The LDs are connected to a bias-Tee (Mini -circuit, ZFBT-4R2GW-FT+). After a 0.3 m indoor free space link, the laser will be attenuated by an absorptive neutral density filter for long-distance simulation, as shown in Fig. 1(d) and (e). A PIN-based photodetector (Newport, 818-BB-45A) with a cut-off frequency of 10 GHz is used to receive the optical signal, and a digital storage oscilloscope (OSC, Agilent, MSO9254A) connected to it capture the signal and use offline DSP to process the received signal. The size of the PD is estimated to be 3.8×3.8×3.2 cm3 and hence, a compact VLC transceiver can be realized.
3. Results and discussions
3.1 Emission characteristics of the R/G/B-LDs
The measured emission spectra of R/G/B LDs under different driven currents (Id) are shown in Fig. 2(a)-(c). The results indicate that RLD and GLD can perform narrow full width at half maximum (FWHM) only above 40 mA and 50 mA, while BLD has a stable laser characteristic at lower driven currents. With the increase of Id, there is a slow blue shift of the center wavelength of RLD, which is mainly caused by the band filling effects [19]. GLD and BLD exhibit a small redshift of its center wavelength owing to the thermal effect [20]. The average peak emission wavelengths of R/G/B LDs are 636.96 nm/516.72 nm/448.89 nm. In Fig. 2 (d), the optical power responses to the injection current (Id) are plotted. The threshold currents of R/G/B LDs are 50 mA, 20 mA and 100 mA, the slope efficiencies are 0.86 W/A, 0.43 W/A and 0.61 W/A respectively.
Fig. 2. The spectra and optical power of R/G/B LDs under different driven currents (Id). (a) the RLD’s spectrum. (b) GLD’s spectrum. (c) BLD’s spectrum. (d) optical power response vs. Id.
The small signal modulation bandwidth of the tricolor channels is measured utilizing a network analysis (Agilent, N5230C). The S21 curves of the network analyzer show the system frequency responses and the variations of the -20 dB (royal dash line) bandwidth of the system under different Id are shown in Fig. 3(a)-(c). Since -3dB bandwidth of the system is limited by the low frequency response of devices, the frequency responses are shown here starting from 1GHz. The results indicate that the bandwidths of R/G/B channels are 4.06/3.11/3.43 GHz when the driven currents Id are gradually augmented to 150/240/60 mA. Higher injection currents lead to reduced frequency response, which is likely to be associated with the RF heating or roll-off effect [21]. With more than 3 GHz bandwidth measured in the Triser-Tx based WDM VLC system shown in Fig. 3(d), the system is expected to support high-speed data communication with DMT bit-loading modulation technology.
Fig. 3. Frequency responses of R/G/B channels under different Id. (a) RLD channel. (b) GLD channel. (c) BLD channel. (d) optimal frequency responses of tricolor channels.
3.2 Transmission performance of the R/G/B-LDs
In terms of the transmission performance of R/G/B-LDs, the maximum transmission data rate of the system depends mainly on the driven current Id, the peak-to-peak voltage (Vpp) of the signal and the modulation bandwidth. Since inappropriate operating points may cause nonlinearity, it is particularly necessary to find the optimal Id and Vpp of the R/G/B LDs, which generally represent the maximum data rate (Rb) under the certain bandwidth. And the maximum communication capacity of tricolor channels can be obtained by determining the optimal signal bandwidth on this basis.
The data rate Rb contours of tricolor channels calculated by traversing the operating points at 2 GHz signal bandwidth are shown in Fig. 4(a)-(c). In Fig. 4(a), the distribution of the RLD data rate reflects high Rb appearing at intermediate Id and Vpp, because lower operating points lead to insufficient driven signal and low system SNR. Moreover, higher operating points also produce stronger nonlinearities. The operating points within the black circle (115 mA < Id < 140 mA, 1.0 V < Vpp < 1.3 V) can all meet the data rate of 13.7 Gbps (Rb > 14 Gbps). The GLD data rate contours are demonstrated in Fig. 4(b). Similarly, its distribution shows the same concentric ring shape as RLD. The operating points in the area of black circle (180 mA < Id < 250 mA, 0.4 V < Vpp < 0.9 V) have great potential to achieve 11.7 Gbps data rate (Rb > 12 Gbps). In Fig. 4(c), BLD has a more concentrated distribution of high-speed areas than RLD and GLD. The areas above 12.2 Gbps require approximately 55-85 mA of driven current and 0.55-0.7 V of peak-to-peak voltage (55 mA < Id < 85 mA, 0.55 V < Vpp < 0.7 V). Afterward, the BERs of each operating point is measured by loading a fixed Rb signal on each of the tricolor channels to determine the optimal operating point. Besides, the transmission efficiency potential value Ep is defined to replace the BER, and their conversion relationship is Ep = lg(1/BER). The above equation illustrates that the larger BER, the smaller potential at the operation point, while the smaller BER, the larger interspace for raising Rb. Figure 4(d)-(f) present the distribution of Ep at all operating points of R/G/B channels. The results are consistent with the distribution of Rb. Points within the black circle are all satisfying the FEC threshold. The maximum Ep of R/G/B channels are 2.64 at (120 mA, 1.2 V), 2.89 at (240 mA, 0.7 V) and 2.53 at (70 mA, 0.6 V), respectively. The corresponding BERs are 2.29×10−3, 1.28×10−3 and 2.95×10−3. The R/G/B LDs at these optimal operating points can achieve above 13.7/11.7/12.2 Gbps data rates and still have the largest Ep for improvement.
Fig. 4. The data rate Rb and the transmission efficiency potential Ep contours of R/G/B channels at all operating points. The Rb contours of (a) RLD, (b) GLD, (c) BLD. The Ep contours of (d) RLD, (e) GLD, (f) BLD.
After obtaining the optimal operating point at 2 GHz signal bandwidth, the maximum data rates Rb at the optimal bandwidth of the R/G/B channels are explored by varying the signal bandwidth at this operating point. The obtained results are shown in Fig. 5. The maximum Rb of tricolor channels are 17.168 Gbps, 14.652 Gbps and 14.590 Gbps at 3 GHz, 2.75 GHz and 3 GHz, respectively. Due to the trade-off between total power and signal bandwidth or the limitation of some components in the system, continued increases in bandwidth lead a decrease in Rb.
Figure 6 shows the results of QAM-DMT signal bit allocation and power allocation for R/G/B channels at the maximum data rate. In Fig. 6(a), a SE of 5.86 bits/s/Hz of RLD channel at optimal operating point and bandwidth is obtained by utilizing the LC algorithm. The average measured SNR is 20.73 dB. The allocated bits fluctuate strictly with the SNR response. In low frequency region, 8 bits representing 256-QAM DMT (gray frame) can be allocated because of the large channel SNR. At about 3 GHz, the rapid decline in SNR causes a 4-QAM DMT signal (royal frame). The inserts in Fig. 6(a) represent the received constellation diagram with each bit allocation, which are 256-QAM, 128-QAM, 64-QAM, 32-QAM, 16-QAM, 8-QAM, and 4-QAM, respectively. The calculated BER is 3.68×10−3, satisfying 7% FEC threshold. In Fig. 6(b), the GLD channel has a high SE of 5.456 at optimal conditions. Its average measured SNR is 19.59 dB. The allocated bits also limited by the channel SNR response. The GLD channel has a low SNR response near zero frequency (marked by the violet dash circle) while the overall SNR curve slope is smaller than RLD. The received constellation diagram shown in the inserts of Fig. 6(b) are 256-QAM, 128-QAM, 64-QAM, 32-QAM, 16-QAM, 8-QAM, and 4-QAM, respectively. Due to the few subcarriers in 256-QAM (gray frame) and 4-QAM (violet frame), the constellation diagram points are thinner than others. The calculated BER is 3.72×10−3, satisfying 7% FEC threshold. In Fig. 6(c), at the optimal operating state of BLD channel, the SE obtained by LC algorithm is 4.98 bits/s/Hz, which is lower than the SE of GLD and RLD channels. The average measured SNR is 18.04 dB. The allocated bits depend on the SNR response. The received constellation diagram shown in the inserts of Fig. 6(c) are 256-QAM (gray frame), 128-QAM, 64-QAM, 32-QAM, 16-QAM, 8-QAM, 4-QAM and 2-QAM (purple frame), respectively. The BER is 3.46×10−3, satisfying the 7% FEC threshold. The results of R/G/B LDs loaded normalized power ratio are illustrated in the part (I), (II) and (III) of Fig. 6(d). The gray dashed line represents the initial allocated power value of 1.0. According to the LC algorithm, the subcarriers with power redundancy in each iteration contribute a certain power to the subcarrier with the minimum power required to boost one bit. The final power allocation after multiple iterations depends on the SNR and the initial bit loading. Power ratios of R/G/B channels fluctuate between 0.7 and 1.3.
Fig. 6. The bit allocation and power allocation of R/G/B channels by utilizing LC algorithm. (a) RLD, (b) GLD, (c) BLD bit allocation, SNR response and the received constellation diagram of each subcarrier. (d) (I) RLD, (II) GLD, (III) BLD loaded power ratio of each subcarrier.
Based on the above experimental results, the measured communication parameters of R/G/B-LDs in the optimal state are summarized in Table 1. Employing the LC algorithm and adjusting the operating point to approximate the Shannon limit of the system, the synthetic white light can reach a maximum recorded data rate of 46.41 Gbps.
Table 1. R/G/B-LDs Carried DMT LC-bit-loading Transmission Parameters
To further investigate the transmission properties of the VLC system, the spectra of R/G/B-LDs received signal (Rx) and transmitted signal (Tx) are compared and analyzed. Figure 7(a)-(c) show the Tx and Rx spectra of the R/G/B LDs at data rates of 17.168 Gbps, 14.652 Gbps and 14.590 Gbps, respectively. In the high frequency region, the SNR of the RLD Rx signal is significantly reduced by ∼18 dB compared to the Tx signal due to the high frequency noise and the bandwidth limitation of the device. Nonetheless, a fair SNR of ∼10 dB can still be obtained, proving that the RLD channel has a stable spectrum with high SE. The GLD Rx signal also has a stable spectrum for ∼12 dB attenuation and ∼14 dB remnant SNR in high frequency region. BLD Rx signal only has ∼5 dB SNR existing and ∼19 dB attenuation in high frequency region due to its relatively weak high frequency stability.
Fig. 7. The spectra of R/G/B-LDs’ received signal (Rx) and transmitted signal (Tx). (a) RLD, (b) GLD, (c) BLD.
4. Conclusion
This work demonstrates a compact Triser-Tx based WDM VLC system with a record data rate of 46.41 Gbps utilizing QAM-DMT modulation and LC bit-loading algorithm. The emission characteristics, system bandwidth, optimal operating points, optimal signal bandwidth, implementation of bit and power allocation and transceiver spectra of modulated signals are analyzed. The achieved maximum data rates of R/G/B channels are 17.168 Gbps, 14.652 Gbps and 14.590 Gbps, respectively. The calculated BERs satisfying the 7% FEC threshold are 3.68×10−3, 3.72×10−3 and 3.46×10−3 respectively, which approximate the Shannon limit. The transceiver spectra of RLD and GLD show a certain SNR redundancy at high-frequency region. The work presents a novel approach for building high-speed VLC data links exceeding tens of Gbps with small form-factor transceivers.
Funding
Natural Science Foundation of Shanghai (21WZ2500600, 21ZR1406200); National Natural Science Foundation of China (61925104, 62031011); Peng Cheng Laboratory project (PCL2021A14); Joint project of China Mobile Research Institute & X-NET.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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