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Modeling and analysis of multi-channel gigabit class CWDM-VLC system
Journal Pre-proof Modeling and analysis of multi-channel gigabit class CWDM-VLC system M.T. Rahman, Rajendran Parthiban
PII: DOI: Reference:
S0030-4018(19)31137-X https://doi.org/10.1016/j.optcom.2019.125141 OPTICS 125141
To appear in:
Optics Communications
Received date : 19 August 2019 Accepted date : 19 December 2019 Please cite this article as: M.T. Rahman and R. Parthiban, Modeling and analysis of multi-channel gigabit class CWDM-VLC system, Optics Communications (2019), doi: https://doi.org/10.1016/j.optcom.2019.125141. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
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Modeling and analysis of Multi-Channel Gigabit Class CWDM-VLC System M.T Rahman, Rajendran Parthiban
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Electrical and Computer Systems Engineering, Monash University, Malaysia *Corresponding author: [email protected]
ABSTRACT
Article history: Received Received in revised form Accepted Available online
Wavelength Division Multiplexing (WDM) is one possible solution to increase the data rate for Visible Light Communication (VLC). Coarse WDM (CWDM) was used to increase data rate in traditional optical fiber systems. In this work, we propose CWDM for a VLC system using multiple LEDs with different monochromatic colors in the full visible spectrum. Besides, the optical tunable filters were used to get the signal from each channel and maximize its performance, while minimizing crosstalk at the receiver. Channel spacing between the channels was 20nm, and the contribution of channels produce white light for indoor illumination. The simulation results for received power and bit error rate (BER) show very promising improvement of data rate for an indoor VLC link. This VLC-CWDM system can achieve a data rate of 3.2 Gb/s and 7.2 Gb/s for 10 and 20 channels respectively and compared with current WDM-VLC system which can achieve 1.97 Gb/s data rate with the same simulation parameters.
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Keywords: Visible light communication CWDM Optical filter High-speed communication. WDM-VLC
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ARTICLE INFO
Introduction
Recently, visible light communication (VLC) has become an alternative solution for wireless communication. The increased interest in this area was the result of its energy efficiency, durability, high security and high modulation bandwidth [1]. The intensity of LEDs can be modulated to provide high-speed data communication and white light simultaneously. The research in visible light for communication is advancing rapidly to match the data rate promised by future 5G internet technologies [2]. Potential applications of VLC are vehicle to vehicle communication, robots in hospitals, Internet of Things (IoT) applications, underwater communication, information display on signboards and indoor communications [3].
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Wavelength division multiplexing (WDM) is considered as the key technology to increase the number of communication channels in VLC, and it can transmit multi steam data using different wavelengths. The data rate can be enhanced with the number of optical
@2019 Elsevier Ltd. All rights reserved.
wavelengths used in WDM. Compared to phosphorcoated (PC) LEDs, multicolor (three or four) LED chips are promising solutions for high-speed VLC systems, since they allow for WDM, have higher modulation bandwidth and can be modulated separately [4]. RGBLEDs were used to achieve high data rate ranging from 3.2 Gb/s to 8 Gb/s [5] [6] [7] [8] typically over 1-m indoor free space. Some other techniques can be combined with WDM to increase the data rate, such as 3.375 Gbps achieved by WDM employing PAM-8 modulation with phase shifted (PS) Manchester coding [9]. Another work achieved 4.5 Gb/s WDM VLC system utilizing recursive least square (RLS) equalizer with carrier less amplitude/phase (CAP) modulation in which 1.5Gb/s data rate was achieved per channel [10]. Cossu et al. first time demonstrated bi-directional WDM-VLC link using four-color integrated LED board and Discrete Multitone (DMT) modulation to achieve an aggregate data rate of 5.6Gbit/s within the range of 1.5m to 4m without noticeable channel crosstalk [11]. In this paper, we utilize a CWDM grid in the visible light range of
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The horizontal luminance Ehor at point (x, y) is given by (4)
CWDM is a special WDM technology, where channel spacing is 20 nm as standardized by ITU-T G.694.2 and G695, for traditional transport networks in metropolitan area networks [12]. To the best of our knowledge, the conventional WDM VLC system uses multi-colors version (e.g., RGB, RGBA, RGBY, etc.), the combination of which can give different shades of white or no white color as a result [13]. We introduce a CWDM grid for VLC and slice the whole visible spectrum (380nm-780nm) into a maximum of 20 different channels. Moreover, we compare the performance of this CWDM with a conventional WDM system with a high number LED channels (RGBYA) as reported in the literature [14] using On-Off Keying (OOK) modulation technique for simplicity. Besides, we also analyze the performance of three different optical filters (Gaussian, Bassel, and Rectangular) at the receiver. The amount of light received by the filters can be controlled using filter order to get better BER. For comparison pupsoses, the data rate and total output power of 10 and 20 channels CWDM-VLC system were investigated within the indoor environment.
where r is the line of side (LOS) distance in the free space between the LED and photodiode (PD). Modulated output signal power from LED, P0 (t), can be described as: (5)
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where PLED is the launched optical power of LEDs, mi is the modulation index, and x(t) is the non-return-to-zero (NRZ) on-off-keying (OOK) signal. The modulated signal depends on the electron lifetime, RC constant, and the materials of the diode. After modulating the signal, the 3dB modulation bandwidth of the LED can be measured using the following equation [3] (6)
where is electron lifetime, and is the RC constant of parasitic capacitance. On the other hand, if the calculated peak wavelength radiated power of the LED is 1W, then the power factor can be described as follows [15]:
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System Model Analysis
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electromagnetic (EM) spectrum to increase the number of channels.
Fig 1 shows the model of the VLC system that we propose. This system model is explained through mathematical equations below. We use these equations in our simulation.
Fig.1 CWDM-VLC grid model (10 Channels)
LEDs have Lambertian radiant intensity, which can be explained using equation (1). (1) (2)
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where is irradiance angle, ml is the order of Lambertian emission, which relates to the LED’s semi-angle at half power as shown in Equation (2). Luminance expresses the brightness of an illuminated surface. The luminous intensity at angle φ can be given by: (3)
(7) where erfc is the error function, λ0 is the peak wavelength, and Δλ is the full width half maximum (fwhm). The optical signal passes through the free space channel, and the DC gain of this channel is measured by [15] ]
(8)
where A is the physical surface area of the photodiode (PD) at the receiver, r is the distance between the LED and PD, Ts (ψ) is the gain of the optical filter at the receiver, ψ is the angle of incidence, and ψFOV is the field of view (FOV) of PD. On the receiver side, received optical power detected by PD can be given by (9) where R is the responsivity of PD, H(0) is the channel gain. To produce a narrow LED spectrum for higher bandwidth and rejecting the ambient light from the selected wavelength, optical filters are important. For the Gaussian filter, the frequency transfer function can be given as: (10) where H(f) is the filter transfer function, α is the parameter insertion loss, d is the parameter depth, fc is the filter center frequency, B is the bandwidth of the filter, and f is the frequency. Fig.2 shows the output spectral power density (SPD) using Gaussian filter transfer
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function, where 3dB bandwidth of the LED is 19nm and the power level achieved -14dBm. A guard band is introduced for reducing channel crosstalk. Therefore, the transfer function used for a rectangular optical filter is
3 dB
1nm guard Channel spacing
20 nm
All LED channels were modulated for transmitting data, producing standard white light. At the receiver end, merged white light signal can be detected by a human eye. This light is then separated into color signals and detected by a photodiode according to the original subchannels. This separation is achieved using specific optical filters as shown in Fig 3. We consider a distance of 3m between the LEDs and PD. The attenuation was fixed at 0.19 dB/km, which is an acceptable international attenuation for indoor applications [16].
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Central wavelength
For the Bessel filter, the transfer function is
Simulation Parameters and Process
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19 nm bandwidth
Fig 2: Spectral shape using Gaussian function
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(11)
(12)
where
and
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where N is the parameter order, and is a normalizing constant and BN(s) is the filtering order, which can be defined as: (13) (14)
(15)
Here, B is the filter bandwidth, and Wb denotes the normalized 3 dB bandwidth (for N≥10). Bandpass filters are used to transmit electromagnetic radiation within a selected spectral region, and filter order defines the amount of light passing through the filters. Narrower emission spectra are more power-efficient than the wider full-width half maximum (FWHM) ranges emission spectra, and these narrow FWHM Filters permit less ambient light and interference from neighboring frequency channel. For the receiving light signal, we use Gaussian, Bessel, and rectangular filters, but to observe the effect of filter order at the receiver, we use Gaussian and Bessel filter. The BER of the received signal using NRZ OOK can be measured through [1]: (16)
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Fig 3: Concept of double functionality of LED-based VLC system
The optical attenuator is used at the receiver, which is required for different biasing conditions and correlated color temperatures (CCTs). A pseudo-random bit sequence (PRBS) generator was used to generate 106 bits, and then it is encoded by an NRZ pulse generator. The LED, as an optical transmitter, transmits the light signal in free space. The photodiode was designed based on the Thorlabs PD10A datasheet, where the responsivities are different at different wavelengths. The PIN diode has the detection wavelength range from 350 to 900 nm, with a maximum responsivity of 0.65 A∕W, as shown in Fig 4. The channel capacity received optical power from an individual channel, and BER was measured by Optisystem and MATLAB toolbox. System parameters for the simulation of WDM or CWDM- VLC model have been tabulated in Table 1.
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signals from the PIN diode are stored using a real-time oscilloscope for offline demodulation.
PDF10A(/M) Responsivity
0.8 0.6 0.4 0.2
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Responsivity (A/W)
0 320
520
720
920
Wavelength (nm)
Different types of optical filters were placed before PD to make a narrow spectrum and electrical amplifier is used to amplify the received signal. Table.1: System parameters for VLC model
RECEIVER
VALUE
(a)
460,525,560,610,645
For 10 CH
390, 430, 470, 510,550 590,630,670,710,750
For 20 CH
390,410, 430,450, 470, 490,510,530,550,570, 590,610,630,650,670, 690,710,730,750,770
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CHANNEL
Wavelength(nm) For 5 CH
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PARAMETERS TRANSMITTER
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Fig.4: Responsivity curve correspondent wavelength
Electron life time,Tn(ns) RC constant, TRC (ns)
1 ns
Transmitted power
32 dBm
Distance
3m
FOV concentrator
45 deg
Tx half-angle
60 deg
Irradiance angle
20 deg
Incidence angle
20 deg
Dark current
10 nA
Responsivity (A/W)
0.65
Thermal power density (W/Hz Load resistance
100x10-24 W/Hz
Absolute temperature
298 K
(b)
Fig 5 (a): SPD for10 channels LED (b) and 20 channels
IV.
Results and Discussion
1 ns
Bit Error Rates (BER) of each channel of the WDM system was investigated using different types of filters, as mentioned before - Bessel, Rectangular, and Gaussian. The result shows that the Low Pass Bessel filter has the lowest BER and adjacent channel crosstalk. 3dB bandwidth of the LED spectrum and received power level are shown in Fig.6.
50 Ohm
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The optical focusing lens is used to collect more light and to focus it onto the PIN active area, and filters are used in front of each PD to separate the WDM or CWDM signals with the different wavelengths. Multichip LEDs VLC system simulated using a Gaussian function [17] and spectral power distribution (SPD) of CWDM 10 channels and 20 channels were modeled as shows in Fig 5 (a) and (b). For 20 channels VLC model, the visible spectrum was considered from 390nm up to 770nm with FWHM 20nm, and channel spacing was considered ~1nm between 2 consecutive peak wavelengths. The output Fig.6: LED spectrum using a) Gaussian b) Bessel c) Rectangular filter.
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Fig.8: Effects of filter order into BER performance
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The BER increased with the related filter order. Different types of filter and filter factors were investigated to measure the BER and proper communication signal quality. WDM VLC system uses multi-colors version as Red, Green, Blue, Yellow and Amber (RGBY) with different full width half maximum of those LEDs. Different color LEDs emits different spectral power and using five different wavelength for conventional WDMVLC model and total data rate achieved 1.97 Gb/s. Related BER for those channels have shown in Table 2. CWDM-VLC model has specific channel spacing between different color LEDs. Using full visible spectrum, total 20 channel system was designed and measured the performance. BER analyzer was used to calculate the BER, the Q factor, and the data rate, which is summarized in Table 3. Table 2: Total data rate of conventional WDM-VLC systems
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The filter width was adjusted to 19nm, 18nm, 15nm, 10nm, and 5nm with properly optimized filter spacing to improve the received power of the signals. As a result, a BER of 10-19 was achieved by the corresponding Bassel filter with 19nm spacing. On the other hand, when the wavelength spacing reduced up to 5 nm, the BER of the rectangular filter decreased to 10-3 but still under FEC limit, which is explained in Fig 7. Bit Error Rates (BER) of the VLC link was investigated using different types of filters such as Bessel, Rectangular and Gaussian filters for each channel of WDM system. Bessel filter shows the best performance getting lower BER which is more than 10-15 and rectangular filter shows best performance when channel spacing 19nm its around 10-16 but it reduced it up to 10-3 when channel spacing was 5nm and passes more adjacent channel crosstalk. This system will provide opportunity to fully utilize the available bandwidth of visible light range and this narrow band filter is required to reduce the crosstalk between the LEDs.
Peak Wavelength 460 525 560 610 645
BER 3.29 x10-6 2.35 x10-8 1.91 x10-9 1.15 x10-8 9.88 x10-8
Data rate (Mb/s) 250 430 510 480 300
Initially, 10 channels are used for communication and others 10 channels keep on just for illumination. The total data rate of 3.2 Gb/s was achieved for 10 channels CWDM-VLC system.
Fig.7: Filter wavelength space Vs BER graph.
Filter factor states the multiplicative amount of light into the filter blocks. It dependents on the spectral response curve for maintaining daylight color temperature (CCT). The filter order was considered from 1-10, which indicates the multiplicity of filter factors with current bandwidth. When filter order was increased from 1 to 10 with Gaussian and Bessel optical filters, the portion of light reduced gradually, as shown in Fig 8.
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Table 3: Total data rate from 10 channels of CWDM-VLC systems
Peak Wavelength 390 430 470 510 550 590 630 670 710 750
BER 1.50 x10-3 1.93 x10-3 1.04 x10-8 3.77 x10-6 1.32 x10-7 2.21 x10-8 1.73 x10-8 3.06 x10-5 7.45 x10-4 5.51 x10-3
Data rate (Mb/s) 120 180 250 300 570 590 430 300 260 200
This data rate is relatively good with OOK modulation, maintaining the bit error rate is below the FEC limit 2.8x10-3. The variety of data rate depends on the SPD of a different wavelength. Table 4 shows the total aggregate data rate for 20 channels with full utilization of 400nm visible spectrum by CWDM-VLC model. The data rate
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Conclusion
In this paper, a new CWDM-VLC system has been proposed for the first time with the optimum number of channels to get the maximum data rate. The results indicate that narrower emission spectra increased system performance by reducing channel overlapping and crosstalk, because narrower emission spectra permit less ambient light and interference from the neighboring channel. Moreover, with the OOK modulation scheme, the data rate achieved was 7.19 Gb/s using the total 20 channel CWDM grid with acceptable BER and 3 m distance between transmitter and receiver. The system can have up to 10-20 channels and channel spacing and filter space can be modified to achieve an increase in data-rate. If the spectra of each channel are optimized using suitable filters and higher-order modulation techniques, the total data rate of the system may increase further in the future.
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According to ITU CWDM grid, channel spacing of 20nm (2.5THz) can be implemented within multiband frequency range from 1270nm to 1675nm (i.e. 400nm range) [18] and same concept of CWDM has implemented in VLC system with its 400nm visible range from 380nm to780nm. Within this visible range maximum of 20 channels can fit in the LED-based VLC model. Each channel has different BER and data rate, because of the LED spectra and transmitted powers. For CWDM VLC model, 20 LEDs optical power was detected, and maximum power differences between high and low-intensity level were around 12 dB as shown in Fig.9.
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that could be achieved is 7.20 Gb/s maintaining high power LED’s configuration in the model.
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References
Table 4: Total data rate from 20 channels of VLC systems
[1]
BER 3.29 x10-3 2.35 x10-5 1.91 x10-5 3.85 x10-6 7.88 x10-4 7.81 x10-4 1.09 x10-4 5.75 x10-6 5.74 x10-5 5.79 x10-6 9.87 x10-8 1.26 x10-6 5.29 x10-4 8.31 x10-6 1.59 x10-6 1.05 x10-4 2.32 x10-5 2.21 x10-6 6.28 x10-4 3.28 x10-4
Data rate (Mb/s) 120 150 200 250 260 270 310 510 610 600 620 580 510 490 440 350 280 250 220 180
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Peak Wavelength 390 410 430 450 470 490 510 530 550 570 590 610 630 650 670 690 710 730 750 770
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Fig.9: Output power level of the 20 LED channels.
S. Rajbhandari, J. J. McKendry, J. Herrnsdorf, H. Chun, G. Faulkner, H. Haas, et al., "A review of gallium nitride LEDs for multi-gigabit-per-second visible light data communications," Semiconductor Science and Technology, vol. 32, p. 023001, 2017. [2] L. Hanzo, H. Haas, S. Imre, D. O'Brien, M. Rupp, and L. Gyongyosi, "Wireless myths, realities, and futures: from 3G/4G to optical and quantum wireless," Proceedings of the IEEE, vol. 100, pp. 1853-1888, 2012. [3] D. Karunatilaka, F. Zafar, V. Kalavally, and R. Parthiban, "LED Based Indoor Visible Light Communications: State of the Art," IEEE communications surveys and tutorials, vol. 17, pp. 16491678, 2015. [4] R. Jiang, Z. Wang, Q. Wang, and L. Dai, "Multi-user sum-rate optimization for visible light communications with lighting constraints," Journal of Lightwave Technology, vol. 34, pp. 39433952, 2016. [5] H. M. Oubei, J. R. Duran, B. Janjua, H.-Y. Wang, C.-T. Tsai, Y.C. Chi, et al., "4.8 Gbit/s 16-QAM-OFDM transmission based on compact 450-nm laser for underwater wireless optical communication," Optics express, vol. 23, pp. 23302-23309, 2015. [6] M. Zhang, M. Shi, F. Wang, J. Zhao, Y. Zhou, Z. Wang, et al., "4.05-Gb/s RGB LED-based VLC system utilizing PSManchester coded Nyquist PAM-8 modulation and hybrid timefrequency domain equalization," in Optical Fiber Communications Conference and Exhibition (OFC), 2017, 2017, pp. 1-3. [7] Y. Wang, L. Tao, X. Huang, J. Shi, and N. Chi, "8-Gb/s RGBY LED-based WDM VLC system employing high-order CAP modulation and hybrid post equalizer," IEEE Photonics Journal, vol. 7, pp. 1-7, 2015. [8] Y. Wang, R. Li, Y. Wang, and Z. Zhang, "3.25-Gbps visible light communication system based on single carrier frequency domain equalization utilizing an RGB LED," in Optical Fiber Communication Conference, 2014, p. Th1F. 1. [9] N. Chi, M. Zhang, Y. Zhou, and J. Zhao, "3.375-Gb/s RGB-LED based WDM visible light communication system employing PAM-8 modulation with phase shifted Manchester coding," Optics express, vol. 24, pp. 21663-21673, 2016. [10] Y. Wang, X. Huang, L. Tao, J. Shi, and N. Chi, "4.5-Gb/s RGBLED based WDM visible light communication system employing CAP modulation and RLS based adaptive equalization," Optics express, vol. 23, pp. 13626-13633, 2015. [11] G. Cossu, A. Wajahat, R. Corsini, and E. Ciaramella, "5.6 Gbit/s downlink and 1.5 Gbit/s uplink optical wireless transmission at
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indoor distances (≥ 1.5 m)," in Optical Communication (ECOC), 2014 European Conference on, 2014, pp. 1-3. S. Al-Rubaye, A.-D. Anwer, and H. Al-Raweshidy, "Next generation optical access network using CWDM technology," International Journal of Communications, Network and System Sciences, vol. 2, p. 636, 2009. R. Bian, I. Tavakkolnia, and H. Haas, "15.73 Gb/s Visible Light Communication with off-the-shelf LEDs," Journal of Lightwave Technology, vol. 37, pp. 2418-2424, 2019. R. K. Vs and I. B. Djordjevic, "MIMO-WDM Visible Light Communications Based on Commercial RGBA LEDs," in 2018 20th International Conference on Transparent Optical Networks (ICTON), 2018, pp. 1-5. L. Cui, Y. Tang, H. Jia, J. Luo, and B. Gnade, "Analysis of the multichannel WDM-VLC communication system," Journal of Lightwave Technology, vol. 34, pp. 5627-5634, 2016. F. Zafar, D. Karunatilaka, and R. Parthiban, "Dimming schemes for visible light communication: the state of research," IEEE Wireless Communications, vol. 22, pp. 29-35, 2015. H. Huang, Y. Tang, L. Cui, Q. Zhu, and J. Luo, "Capacity analyze of WDM indoor visible light communication based on LED for standard illumination," in 2015 International Conference on Optical Instruments and Technology: Optoelectronic Devices and Optical Signal Processing, 2015, p. 96190U. M. Nebeling, "CWDM: lower cost for more capacity in the shorthaul," Fiber Network Engineering: Livermore, CA, USA, 2002.
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PII: DOI: Reference:
S0030-4018(19)31137-X https://doi.org/10.1016/j.optcom.2019.125141 OPTICS 125141
To appear in:
Optics Communications
Received date : 19 August 2019 Accepted date : 19 December 2019 Please cite this article as: M.T. Rahman and R. Parthiban, Modeling and analysis of multi-channel gigabit class CWDM-VLC system, Optics Communications (2019), doi: https://doi.org/10.1016/j.optcom.2019.125141. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Journal Pre-proof *Manuscript Click here to view linked References
Optics Communications journal homepage: www.elsevier.com
of
Modeling and analysis of Multi-Channel Gigabit Class CWDM-VLC System M.T Rahman, Rajendran Parthiban
pro
Electrical and Computer Systems Engineering, Monash University, Malaysia *Corresponding author: [email protected]
ABSTRACT
Article history: Received Received in revised form Accepted Available online
Wavelength Division Multiplexing (WDM) is one possible solution to increase the data rate for Visible Light Communication (VLC). Coarse WDM (CWDM) was used to increase data rate in traditional optical fiber systems. In this work, we propose CWDM for a VLC system using multiple LEDs with different monochromatic colors in the full visible spectrum. Besides, the optical tunable filters were used to get the signal from each channel and maximize its performance, while minimizing crosstalk at the receiver. Channel spacing between the channels was 20nm, and the contribution of channels produce white light for indoor illumination. The simulation results for received power and bit error rate (BER) show very promising improvement of data rate for an indoor VLC link. This VLC-CWDM system can achieve a data rate of 3.2 Gb/s and 7.2 Gb/s for 10 and 20 channels respectively and compared with current WDM-VLC system which can achieve 1.97 Gb/s data rate with the same simulation parameters.
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Keywords: Visible light communication CWDM Optical filter High-speed communication. WDM-VLC
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ARTICLE INFO
Introduction
Recently, visible light communication (VLC) has become an alternative solution for wireless communication. The increased interest in this area was the result of its energy efficiency, durability, high security and high modulation bandwidth [1]. The intensity of LEDs can be modulated to provide high-speed data communication and white light simultaneously. The research in visible light for communication is advancing rapidly to match the data rate promised by future 5G internet technologies [2]. Potential applications of VLC are vehicle to vehicle communication, robots in hospitals, Internet of Things (IoT) applications, underwater communication, information display on signboards and indoor communications [3].
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Wavelength division multiplexing (WDM) is considered as the key technology to increase the number of communication channels in VLC, and it can transmit multi steam data using different wavelengths. The data rate can be enhanced with the number of optical
@2019 Elsevier Ltd. All rights reserved.
wavelengths used in WDM. Compared to phosphorcoated (PC) LEDs, multicolor (three or four) LED chips are promising solutions for high-speed VLC systems, since they allow for WDM, have higher modulation bandwidth and can be modulated separately [4]. RGBLEDs were used to achieve high data rate ranging from 3.2 Gb/s to 8 Gb/s [5] [6] [7] [8] typically over 1-m indoor free space. Some other techniques can be combined with WDM to increase the data rate, such as 3.375 Gbps achieved by WDM employing PAM-8 modulation with phase shifted (PS) Manchester coding [9]. Another work achieved 4.5 Gb/s WDM VLC system utilizing recursive least square (RLS) equalizer with carrier less amplitude/phase (CAP) modulation in which 1.5Gb/s data rate was achieved per channel [10]. Cossu et al. first time demonstrated bi-directional WDM-VLC link using four-color integrated LED board and Discrete Multitone (DMT) modulation to achieve an aggregate data rate of 5.6Gbit/s within the range of 1.5m to 4m without noticeable channel crosstalk [11]. In this paper, we utilize a CWDM grid in the visible light range of
Journal Pre-proof
The horizontal luminance Ehor at point (x, y) is given by (4)
CWDM is a special WDM technology, where channel spacing is 20 nm as standardized by ITU-T G.694.2 and G695, for traditional transport networks in metropolitan area networks [12]. To the best of our knowledge, the conventional WDM VLC system uses multi-colors version (e.g., RGB, RGBA, RGBY, etc.), the combination of which can give different shades of white or no white color as a result [13]. We introduce a CWDM grid for VLC and slice the whole visible spectrum (380nm-780nm) into a maximum of 20 different channels. Moreover, we compare the performance of this CWDM with a conventional WDM system with a high number LED channels (RGBYA) as reported in the literature [14] using On-Off Keying (OOK) modulation technique for simplicity. Besides, we also analyze the performance of three different optical filters (Gaussian, Bassel, and Rectangular) at the receiver. The amount of light received by the filters can be controlled using filter order to get better BER. For comparison pupsoses, the data rate and total output power of 10 and 20 channels CWDM-VLC system were investigated within the indoor environment.
where r is the line of side (LOS) distance in the free space between the LED and photodiode (PD). Modulated output signal power from LED, P0 (t), can be described as: (5)
pro
where PLED is the launched optical power of LEDs, mi is the modulation index, and x(t) is the non-return-to-zero (NRZ) on-off-keying (OOK) signal. The modulated signal depends on the electron lifetime, RC constant, and the materials of the diode. After modulating the signal, the 3dB modulation bandwidth of the LED can be measured using the following equation [3] (6)
where is electron lifetime, and is the RC constant of parasitic capacitance. On the other hand, if the calculated peak wavelength radiated power of the LED is 1W, then the power factor can be described as follows [15]:
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System Model Analysis
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II.
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electromagnetic (EM) spectrum to increase the number of channels.
Fig 1 shows the model of the VLC system that we propose. This system model is explained through mathematical equations below. We use these equations in our simulation.
Fig.1 CWDM-VLC grid model (10 Channels)
LEDs have Lambertian radiant intensity, which can be explained using equation (1). (1) (2)
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where is irradiance angle, ml is the order of Lambertian emission, which relates to the LED’s semi-angle at half power as shown in Equation (2). Luminance expresses the brightness of an illuminated surface. The luminous intensity at angle φ can be given by: (3)
(7) where erfc is the error function, λ0 is the peak wavelength, and Δλ is the full width half maximum (fwhm). The optical signal passes through the free space channel, and the DC gain of this channel is measured by [15] ]
(8)
where A is the physical surface area of the photodiode (PD) at the receiver, r is the distance between the LED and PD, Ts (ψ) is the gain of the optical filter at the receiver, ψ is the angle of incidence, and ψFOV is the field of view (FOV) of PD. On the receiver side, received optical power detected by PD can be given by (9) where R is the responsivity of PD, H(0) is the channel gain. To produce a narrow LED spectrum for higher bandwidth and rejecting the ambient light from the selected wavelength, optical filters are important. For the Gaussian filter, the frequency transfer function can be given as: (10) where H(f) is the filter transfer function, α is the parameter insertion loss, d is the parameter depth, fc is the filter center frequency, B is the bandwidth of the filter, and f is the frequency. Fig.2 shows the output spectral power density (SPD) using Gaussian filter transfer
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function, where 3dB bandwidth of the LED is 19nm and the power level achieved -14dBm. A guard band is introduced for reducing channel crosstalk. Therefore, the transfer function used for a rectangular optical filter is
3 dB
1nm guard Channel spacing
20 nm
All LED channels were modulated for transmitting data, producing standard white light. At the receiver end, merged white light signal can be detected by a human eye. This light is then separated into color signals and detected by a photodiode according to the original subchannels. This separation is achieved using specific optical filters as shown in Fig 3. We consider a distance of 3m between the LEDs and PD. The attenuation was fixed at 0.19 dB/km, which is an acceptable international attenuation for indoor applications [16].
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Central wavelength
For the Bessel filter, the transfer function is
Simulation Parameters and Process
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19 nm bandwidth
Fig 2: Spectral shape using Gaussian function
III.
of
(11)
(12)
where
and
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where N is the parameter order, and is a normalizing constant and BN(s) is the filtering order, which can be defined as: (13) (14)
(15)
Here, B is the filter bandwidth, and Wb denotes the normalized 3 dB bandwidth (for N≥10). Bandpass filters are used to transmit electromagnetic radiation within a selected spectral region, and filter order defines the amount of light passing through the filters. Narrower emission spectra are more power-efficient than the wider full-width half maximum (FWHM) ranges emission spectra, and these narrow FWHM Filters permit less ambient light and interference from neighboring frequency channel. For the receiving light signal, we use Gaussian, Bessel, and rectangular filters, but to observe the effect of filter order at the receiver, we use Gaussian and Bessel filter. The BER of the received signal using NRZ OOK can be measured through [1]: (16)
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Fig 3: Concept of double functionality of LED-based VLC system
The optical attenuator is used at the receiver, which is required for different biasing conditions and correlated color temperatures (CCTs). A pseudo-random bit sequence (PRBS) generator was used to generate 106 bits, and then it is encoded by an NRZ pulse generator. The LED, as an optical transmitter, transmits the light signal in free space. The photodiode was designed based on the Thorlabs PD10A datasheet, where the responsivities are different at different wavelengths. The PIN diode has the detection wavelength range from 350 to 900 nm, with a maximum responsivity of 0.65 A∕W, as shown in Fig 4. The channel capacity received optical power from an individual channel, and BER was measured by Optisystem and MATLAB toolbox. System parameters for the simulation of WDM or CWDM- VLC model have been tabulated in Table 1.
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signals from the PIN diode are stored using a real-time oscilloscope for offline demodulation.
PDF10A(/M) Responsivity
0.8 0.6 0.4 0.2
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Responsivity (A/W)
0 320
520
720
920
Wavelength (nm)
Different types of optical filters were placed before PD to make a narrow spectrum and electrical amplifier is used to amplify the received signal. Table.1: System parameters for VLC model
RECEIVER
VALUE
(a)
460,525,560,610,645
For 10 CH
390, 430, 470, 510,550 590,630,670,710,750
For 20 CH
390,410, 430,450, 470, 490,510,530,550,570, 590,610,630,650,670, 690,710,730,750,770
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CHANNEL
Wavelength(nm) For 5 CH
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PARAMETERS TRANSMITTER
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Fig.4: Responsivity curve correspondent wavelength
Electron life time,Tn(ns) RC constant, TRC (ns)
1 ns
Transmitted power
32 dBm
Distance
3m
FOV concentrator
45 deg
Tx half-angle
60 deg
Irradiance angle
20 deg
Incidence angle
20 deg
Dark current
10 nA
Responsivity (A/W)
0.65
Thermal power density (W/Hz Load resistance
100x10-24 W/Hz
Absolute temperature
298 K
(b)
Fig 5 (a): SPD for10 channels LED (b) and 20 channels
IV.
Results and Discussion
1 ns
Bit Error Rates (BER) of each channel of the WDM system was investigated using different types of filters, as mentioned before - Bessel, Rectangular, and Gaussian. The result shows that the Low Pass Bessel filter has the lowest BER and adjacent channel crosstalk. 3dB bandwidth of the LED spectrum and received power level are shown in Fig.6.
50 Ohm
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The optical focusing lens is used to collect more light and to focus it onto the PIN active area, and filters are used in front of each PD to separate the WDM or CWDM signals with the different wavelengths. Multichip LEDs VLC system simulated using a Gaussian function [17] and spectral power distribution (SPD) of CWDM 10 channels and 20 channels were modeled as shows in Fig 5 (a) and (b). For 20 channels VLC model, the visible spectrum was considered from 390nm up to 770nm with FWHM 20nm, and channel spacing was considered ~1nm between 2 consecutive peak wavelengths. The output Fig.6: LED spectrum using a) Gaussian b) Bessel c) Rectangular filter.
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Fig.8: Effects of filter order into BER performance
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The BER increased with the related filter order. Different types of filter and filter factors were investigated to measure the BER and proper communication signal quality. WDM VLC system uses multi-colors version as Red, Green, Blue, Yellow and Amber (RGBY) with different full width half maximum of those LEDs. Different color LEDs emits different spectral power and using five different wavelength for conventional WDMVLC model and total data rate achieved 1.97 Gb/s. Related BER for those channels have shown in Table 2. CWDM-VLC model has specific channel spacing between different color LEDs. Using full visible spectrum, total 20 channel system was designed and measured the performance. BER analyzer was used to calculate the BER, the Q factor, and the data rate, which is summarized in Table 3. Table 2: Total data rate of conventional WDM-VLC systems
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The filter width was adjusted to 19nm, 18nm, 15nm, 10nm, and 5nm with properly optimized filter spacing to improve the received power of the signals. As a result, a BER of 10-19 was achieved by the corresponding Bassel filter with 19nm spacing. On the other hand, when the wavelength spacing reduced up to 5 nm, the BER of the rectangular filter decreased to 10-3 but still under FEC limit, which is explained in Fig 7. Bit Error Rates (BER) of the VLC link was investigated using different types of filters such as Bessel, Rectangular and Gaussian filters for each channel of WDM system. Bessel filter shows the best performance getting lower BER which is more than 10-15 and rectangular filter shows best performance when channel spacing 19nm its around 10-16 but it reduced it up to 10-3 when channel spacing was 5nm and passes more adjacent channel crosstalk. This system will provide opportunity to fully utilize the available bandwidth of visible light range and this narrow band filter is required to reduce the crosstalk between the LEDs.
Peak Wavelength 460 525 560 610 645
BER 3.29 x10-6 2.35 x10-8 1.91 x10-9 1.15 x10-8 9.88 x10-8
Data rate (Mb/s) 250 430 510 480 300
Initially, 10 channels are used for communication and others 10 channels keep on just for illumination. The total data rate of 3.2 Gb/s was achieved for 10 channels CWDM-VLC system.
Fig.7: Filter wavelength space Vs BER graph.
Filter factor states the multiplicative amount of light into the filter blocks. It dependents on the spectral response curve for maintaining daylight color temperature (CCT). The filter order was considered from 1-10, which indicates the multiplicity of filter factors with current bandwidth. When filter order was increased from 1 to 10 with Gaussian and Bessel optical filters, the portion of light reduced gradually, as shown in Fig 8.
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Table 3: Total data rate from 10 channels of CWDM-VLC systems
Peak Wavelength 390 430 470 510 550 590 630 670 710 750
BER 1.50 x10-3 1.93 x10-3 1.04 x10-8 3.77 x10-6 1.32 x10-7 2.21 x10-8 1.73 x10-8 3.06 x10-5 7.45 x10-4 5.51 x10-3
Data rate (Mb/s) 120 180 250 300 570 590 430 300 260 200
This data rate is relatively good with OOK modulation, maintaining the bit error rate is below the FEC limit 2.8x10-3. The variety of data rate depends on the SPD of a different wavelength. Table 4 shows the total aggregate data rate for 20 channels with full utilization of 400nm visible spectrum by CWDM-VLC model. The data rate
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Conclusion
In this paper, a new CWDM-VLC system has been proposed for the first time with the optimum number of channels to get the maximum data rate. The results indicate that narrower emission spectra increased system performance by reducing channel overlapping and crosstalk, because narrower emission spectra permit less ambient light and interference from the neighboring channel. Moreover, with the OOK modulation scheme, the data rate achieved was 7.19 Gb/s using the total 20 channel CWDM grid with acceptable BER and 3 m distance between transmitter and receiver. The system can have up to 10-20 channels and channel spacing and filter space can be modified to achieve an increase in data-rate. If the spectra of each channel are optimized using suitable filters and higher-order modulation techniques, the total data rate of the system may increase further in the future.
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According to ITU CWDM grid, channel spacing of 20nm (2.5THz) can be implemented within multiband frequency range from 1270nm to 1675nm (i.e. 400nm range) [18] and same concept of CWDM has implemented in VLC system with its 400nm visible range from 380nm to780nm. Within this visible range maximum of 20 channels can fit in the LED-based VLC model. Each channel has different BER and data rate, because of the LED spectra and transmitted powers. For CWDM VLC model, 20 LEDs optical power was detected, and maximum power differences between high and low-intensity level were around 12 dB as shown in Fig.9.
V. [[
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that could be achieved is 7.20 Gb/s maintaining high power LED’s configuration in the model.
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References
Table 4: Total data rate from 20 channels of VLC systems
[1]
BER 3.29 x10-3 2.35 x10-5 1.91 x10-5 3.85 x10-6 7.88 x10-4 7.81 x10-4 1.09 x10-4 5.75 x10-6 5.74 x10-5 5.79 x10-6 9.87 x10-8 1.26 x10-6 5.29 x10-4 8.31 x10-6 1.59 x10-6 1.05 x10-4 2.32 x10-5 2.21 x10-6 6.28 x10-4 3.28 x10-4
Data rate (Mb/s) 120 150 200 250 260 270 310 510 610 600 620 580 510 490 440 350 280 250 220 180
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Peak Wavelength 390 410 430 450 470 490 510 530 550 570 590 610 630 650 670 690 710 730 750 770
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Fig.9: Output power level of the 20 LED channels.
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