- Email: [email protected]
Performance of color-independent OFDM visible light communication based on color space
Optics Communications 324 (2014) 264–268
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Performance of color-independent OFDM visible light communication based on color space Pankaz Das, Youngil Park, Ki-Doo Kim n School of Electronics Engineering, Kookmin University, Seoul 136-702, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 13 December 2013 Received in revised form 24 February 2014 Accepted 22 March 2014 Available online 4 April 2014
In this paper, we propose an orthogonal frequency division multiplexing (OFDM)-based visible light communication (VLC) system that can be color independent by using a color-space-based modulation. Along with all of the promising advantages of OFDM, the proposed system will be applicable for all colors in the visible range. Through the simulation results, we show that the proposed OFDM-VLC system is robust to intersymbol interference (ISI) and a large peak-to-average power ratio (PAPR), and is also color independent. & 2014 Elsevier B.V. All rights reserved.
Keywords: Color space Color independent (compatible) Orthogonal frequency division multiplexing (OFDM) Optical wireless communication (OWC) Visible light communication (VLC) Light-emitting diode (LED)
1. Introduction Presently, the optical wireless communication (OWC) technology is being considered as a strong candidate for ubiquitous, highspeed wireless communication applications [1,2]. Through the extensive use of light-emitting diodes (LEDs) over the course of the last few years and the anticipated development of a widespread LED lighting and signaling infrastructure, visible light communication (VLC) has become the mainstream in the current OWC fields [1,2]. VLC can be applied to a short-range communication system using visible light and a solid-state light (SSL) source, such that the system can serve the dual purposes of communication together with illumination. A VLC system utilizes all of the notable advantages of visible light since it has aesthetically pleasing, an unregulated huge bandwidth with no electromagnetic interference, no known health risks, security, and ubiquitous nature (can be used in RF-prohibited areas such as aircraft, space shuttles, and hospitals). Hence, by using visible light for data transmission, many problems related to radio and infrared communications are avoided. In addition to the advantages over RFand IR-based communications, LEDs are more advantageous than other light sources because of their faster switching time, higher efficiency, smaller size, higher directivity, longer lifetime, and cheaper transmitter components as compared to expensive RF
n
Corresponding author. Tel.: þ 82 2 910 4707; fax: þ82 2 910 4449. E-mail address: [email protected] (K.-D. Kim).
http://dx.doi.org/10.1016/j.optcom.2014.03.060 0030-4018/& 2014 Elsevier B.V. All rights reserved.
units. Therefore, with all the advantages of visible light and LEDs, VLC systems are an attractive, alternative technology for indoor wireless communications [3,4]. A number of modulation schemes have been considered for VLC systems, which include variable on-off keying (VOOK), variable pulse position modulation (VPPM), multiple PPM (MPPM), pulse dual slope modulation (PDSM), orthogonal frequency division multiplexing (OFDM), and subcarrier modulation [5–10]. Among them, thus far, OFDM is found to be the most advantageous for VLC systems and OFDM for VLC was first introduced in [11]. Parallel data transmission by orthogonal subcarriers offers overall high data rates (i.e., rates similar to conventional single carrier modulation schemes), high bandwidth efficiency, and reduced complexity in equalizers. Owing to its long symbol duration, OFDM is inherently very robust against multipath induced intersymbol interference (ISI), which is a major concern in indoor OWC. OFDM has a relatively large peak-to-average power ratio (PAPR), which brings a reduced power efficiency of RF power amplifier. As a means of color-space-based modulation, color-shift keying (CSK) and generalized color modulation (GCM) have been proposed thus far [12,13]. While CSK is currently applicable for only white visible light, GCM is a color-independent modulation scheme that can be used for communication under varying target color conditions. Furthermore, GCM can provide flicker-free operation, dimming control, and the ability to function irrespective of the number of LEDs at the transmitter or photo detectors (PDs) at the receiver [13–15]. Most of the previous research on OFDM-based VLC has utilized only white light (LEDs) and did not consider the color issue of
P. Das et al. / Optics Communications 324 (2014) 264–268
visible light [9,10]. Therefore, no systematic way of coping with the color changing conditions of visible light has been suggested thus far. However, the color of visible light is a vital issue for a VLC system, and VLC should be compatible with all colors in the visible range. Although the possibility of color-independent OFDM-based VLC was introduced in [16], the simulation was not concrete and main results were not included there. In this paper, we propose a color-independent OFDM-VLC system in a complete form. Therefore, our proposed OFDM-VLC system will be applicable for all visible colors. The remainder of this paper is organized as follows. In Section 2, our proposed system model for the OFDM-VLC is introduced and explained in detail. The simulation results are presented and discussed in Section 3. Finally, Section 4 presents the conclusions of the paper.
265
2. OFDM-VLC system model The proposed OFDM-VLC system model is shown in Fig. 1; this system contains two main blocks: (a) the transmitter and (b) the receiver. We combined the OFDM and GCM schemes together in the implementation of a color-independent VLC system using OFDM. The operation of the entire OFDM-VLC block system is described as below:
2.1. Transmitter blocks The transmitter mainly consists of a channel coder, a colorspace-based modulator, and an OFDM modulator. Several of the
Fig. 1. Complete system model for the OFDM-VLC system. (a) Transmittar and (b) Receiver.
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P. Das et al. / Optics Communications 324 (2014) 264–268
blocks within the OFDM-VLC transmitter are implemented as follows: Coding and interleaving: First, binary input data is encoded by a forward error correction (FEC) code. The encoded data is then interleaved. These blocks can be optional depending on the channel condition. Color-space-based modulator: Color space is a tool which can be used to specify, create, and visualize color. For example, a computer may describe a color using the amounts of red, green, and blue phosphor emission required to match a color. We used GCM, which is a color-space-based modulation scheme, to transmit data symbols by varying the light intensities of multiple LEDs, irrespective of the target colors of VLC signals [13–15]. GCM constructs a constellation diagram in a color space and then a data symbol is represented by a constellation point ðx; yÞ. Each constellation point in a color space represents a corresponding color, and target color is the average of all the appropriate constellation points. At the transmitter, the target color can be determined by a “target color” input from outside. ðx; yÞ-to-RGB: A color can be generated by mixing light beams from multiple LEDs. The color point ðx; yÞ can be transformed into RGB by [17].
OFDM demodulator: It demodulates the received OFDM signal by down-conversion with sub-carriers as follows: R t þT i rðtÞ exp j2π ðt t s Þ dt ; for 1 r ir N Ri ¼ t ss N Z ts þ T i Gi ¼ gðtÞ exp j2π ðt t s Þ dt; for 1 r ir N N ts Z ts þ T i bðtÞ exp j2π ðt t s Þ dt; for 1 r i r N Bi ¼ N ts
X ¼ 2:7689R þ 1:7517G þ 1:1302B Y ¼ R þ 4:5907G þ 0:0601B Z ¼ 0:0565G þ 5:5943B X Y ; y¼ x¼ X þY þZ X þY þZ
ð1Þ
where R, G, and B are the colors of three monochromatic light sources with wavelengths 700 nm, 546.1 nm, and 435.8 nm, respectively, and X; Y; and Z are the tristimulus values of a particular color. OFDM modulator: The RGB intensities corresponding to each symbol (Ri ; Gi ; Bi for the ith symbol) undergo a serial-to-parallel conversion. After the serial-to-parallel conversion, the inputs to each OFDM modulator are R ¼ ½R1 ; R2 ; …; RN T for the redcolored channel, G ¼ ½G1 ; G2 ; …; GN T for the green-colored channel, and B ¼ ½B1 ; B2 ; …; BN T for the blue-colored channel, where N is the number of the OFDM subcarriers. Each low rate sub-intensity is modulated by an orthogonal sub-carrier. Then, an OFDM signal consists of a sum of modulated subcarriers and one OFDM symbol starting at t ¼ t s can be expressed as i rðtÞ ¼ ∑N i ¼ 1 Ri exp j2π N ðt t s Þ ; t s r t r t s þ T i gðtÞ ¼ ∑N i ¼ 1 Gi exp j2π ðt t s Þ ; t s r t rt s þ T N i N ð2Þ bðtÞ ¼ ∑i ¼ 1 Bi exp j2π ðt t s Þ ; t s rt r t s þT N
After integrating the signal over the symbol period (T), we can obtain the exact RGB (Ri ; Gi ; Bi for the ith symbol) intensity for each symbol. The OFDM demodulator also averages all of the sub-intensities ðB1 ; B2 ; …; BN Þ in a channel to obtain BT . In the same manner, we obtain RT ; GT , and the resultant target color can be used for constructing the constellation diagram. RGB-to-ðx; yÞ: This block converts the received intensity (Ri ; Gi ; Bi ) to a point ðxi ; yi Þin the applied color space by using Eq. (1). In addition, this block generates the target color point ðxT ; yT Þ by converting the ðRT ; GT ; BT Þ. Color-space-based demodulation: First, this block generates a constellation diagram by using the target color information from the previous block. Note that finding the proper target color and constellation diagram results in a higher demapping accuracy. After that, the nearest constellation point (based on the minimum Euclidian distance) from ðxi ; yi Þ is the recovered data symbol point ðx; yÞ. The resultant constellation point is demapped to binary values. Deinterleaving and decoding: The resultant binary values may be deinterleaved and decoded to produce the desired output data.
3. Simulation results Table 1 lists the simulation parameters. Since the VLC channel has a considerably greater channel bandwidth and typical lighting levels provide a communication channel with a very high signalto-noise ratio (SNR) [1], we did not consider the SNR and bandwidth limitation seriously. A PD combined with an optical bandpass filter (OBPF) and an optical concentrator measures the intensity of each color. An OBPF is a device that selectively passes light in a particular band of wavelengths, that is, colors, while blocking the remainder. A silicon-based PIN photodiode was assumed to be the visible light detector. In addition, parameters of an LED driver circuit were adjusted to compensate for the different responsivities of different colors [14].
where T is the duration of one OFDM symbol.
Digital-to-analog converter (DAC) and light-emitting module: The OFDM signal is converted to an analog signal and transmitted by three LEDs, each having its own spectral distribution or a color component (RGB).
2.2. Receiver blocks
Several of the blocks within the OFDM-VLC receiver are implemented as follows: Photo detector (PD) and analog-to-digital converter (ADC): Each PD measures the light intensity corresponding to its wavelength or color component (RGB) and outputs its magnitude. The outputs of the PD are converted to digital signals by the ADC for further processing.
ð3Þ
Table 1 The simulation parameters. Parameter
Value
Number of LEDs and PDs Number of constellation points (a 2-bit data symbol is assumed) Total intensity Total number of data bits transmitted Target color point coordinates in CIE1931color space 3 LED positions (or points) at the transmitter (and the PDs at the receiver) in CIE1931color space Number of FFT points Number of data sub carriers
3 4 3 AU 213 (0.4,0.325) R: (0.7347, 0.2653) G: (0.2738, 0.7174) B: (0.1666, 0.0089) 256 256
P. Das et al. / Optics Communications 324 (2014) 264–268
267
Fig. 2 shows the generation of constellations for the target color at the transmitter in the CIE1931 color space. Gray coding is used to represent equiprobable data symbols. Figs. 3 and 4 depict the generated constellation diagram at the receiver for the GCM and OFDM schemes, respectively, for the same delay spread (1 ms). In the case of GCM, the ISI is so large that the constellation is seriously blurred, causing a high bit error rate
Fig. 5. BER vs. SNR of the GCM-VLC system for different delay spreads.
Fig. 2. Four-point constellation for the target color in CIE1931 color space.
Fig. 6. BER vs. SNR of the OFDM-VLC system for different delay spreads.
Fig. 3. Generated constellation at receiver for GCM (delay spread ¼1 ms).
Fig. 4. Generated constellation at receiver for OFDM (delay spread¼ 1 ms).
(BER). However, since OFDM is robust to the ISI, the interference is still tolerable enough to obtain a reasonable received constellation and hence shows superior BER performance. Figs. 5 and 6 show the BER performance of the VLC system for the GCM and OFDM schemes, respectively, by varying the delay spread. It is observed that the BER performance of the OFDM-VLC system shows almost similar performances for various delay spread values, while the BER performance of the GCM-VLC system varies considerably with delay spread variation. A large PAPR for an OFDM signal brings disadvantages such as increased complexity of the ADC/DAC process and reduced efficiency of the RF power amplifier [18]. To reduce the PAPR, we take the clipping operation such as a multiplication of the OFDM signal by 1 if the OFDM amplitude is below a threshold and smaller than 1 if the amplitude needs to be clipped. In RF communication, it was known that clipping used to reduce the PAPR results in degradation of the BER performance [18]. In VLC, the compensation method by using pre-distortion of modulation input signal of LED might be considered to alleviate the performance degradation due to the large PAPR [19]. Most importantly, in this paper, since our proposed OFDM-VLC system is very resistant to signal clipping, it does almost not distort the OFDM signal amplitude. The reason for this resistance is that although we reduced the signal level, the ratio among the three RGB signals still remains the same. Since the symbol on the color space is determined by the RGB ratio (that means color), the reduction of the signal level does not
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the Global Scholarship Program for Foreign Graduate Students at Kookmin University in Korea.
References
Fig. 7. BER vs. SNR for the OFDM-VLC system for different clipping levels.
significantly affect the BER performance. Fig. 7 shows the BER performance of the OFDM-VLC system with and without clipping. In this figure, SC(scaling factor) is defined as the ratio of threshold and peak value, and an OFDM signal is clipped if the OFDM amplitude is above a threshold. This shows that the clipping of the OFDM-VLC signal has a very minor effect on the BER performance. Hence, we can see from the figure that the BER curves for different clipping levels are overlapped. 4. Conclusion We proposed an OFDM-based VLC system that can be made color independent by using color-space-based modulation. Through simulations, we have shown that the proposed OFDM-VLC system is robust to ISI and high PAPR and is also color independent. Acknowledgements This work was supported by the Basic Science Research Program through NRF funded by the MEST (No. 2011-0007107) as well as by
[1] D. O'Brien, et al., Proc. SPIE 7091 (2008) 7091061. [2] D. O'Brien, Optical wireless communications and potential applications in space, in: Proceedings of the International Conference on Space Optical Systems and Applications (ICSOS) 2012, Corsica, France, October 9–12, 2012. [3] J. Grubor, J.-O Gaete, J. Waleski, S. Randel, K. Langer, ITG-Fachber. 198 (2007) 203. [4] T. Komine, M. Nakagawa, IEEE Trans. Consum. Electron. 50 (2004) 100 (Feb.). [5] K. Lee, H. Park, IEEE Photonics Technol. Lett. 23 (16) (2011) 1136. [6] M. Anand, P. Mishra. A novel modulation scheme for visible light communication, in: Proceedings of Annual IEEE India Conference (INDICON), Dec. 17– 19, 2010, pp. 1–3. [7] H. Sugiyama, S. Haruyama, M. Nakagawa, IEICE Trans. Commun. E89-B (12) (2006) 3393. [8] H. Park, J.R. Barry. Modulation analysis for wireless infrared communications, in: Proceedings of IEEE International Conference on Communication, vol. 2, pp. 1182–1186, June 18–22, 1995. [9] M.Z. Afgani, H. Haas, H. Elgala, D. Knipp. Visible light communication using OFDM, in: Second International Conference on Testbeds and Research Infrastructures for the Development of Networks and Communities, 2006. [10] H. Elgala, R. Mesleh, H. Haas, B. Pricope. OFDM visible light wireless communication, in: IEEE 65th Vehicular Technology Conference, pp. 2185–2189, 2007. [11] Y. Tanaka, T. Komine, S. Haruyama, M. Nakagawa. Indoor visible communication utilizing plural white LEDs as lighting, in: Twelfth IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC 2001), San Diego, CA, pp. F81–F85, Sept. 2001. [12] IEEE Standard for local and metropolitan area networks Part 15.7: Short-range Wireless Optical Communication Using Visible Light, IEEE Standard 802.15.7, June 2011. [13] P. Das, B.-Y. Kim, Y. Park, K.-D. Kim, Adv. Electr. Comput Eng. 12 (4) (2012) 11. [14] P. Das, B.-Y. Kim, Y. Park, K.-D. Kim, Opt. Commun. 286 (2013) 69 (Jan.). [15] P. Das, Y. Park, K.-D. Kim, Opt. Commun. 303 (2013) 1 (August). [16] P. Das, Y. Park, K.-D. Kim, Color-independent OFDM-based visible light communication, in: International Conference on Ubiquitous and Future Networks (ICUFN) (2013) 672673 (Vietnam). [17] A. Yokoi, Samsung Yokohama Research Institute, Color Multiplex Coding for VLC, IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs), Nov. 2008. [18] R.V. Nee, R. Prasad, OFDM for Wireless Multimedia Communications, Artech House, Boston, 2000. [19] D. Lee, K. Choi, K.-D. Kim, Y. Park, Opt. Commun. 285 (7) (2012) 1767–1770.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Performance of color-independent OFDM visible light communication based on color space Pankaz Das, Youngil Park, Ki-Doo Kim n School of Electronics Engineering, Kookmin University, Seoul 136-702, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 13 December 2013 Received in revised form 24 February 2014 Accepted 22 March 2014 Available online 4 April 2014
In this paper, we propose an orthogonal frequency division multiplexing (OFDM)-based visible light communication (VLC) system that can be color independent by using a color-space-based modulation. Along with all of the promising advantages of OFDM, the proposed system will be applicable for all colors in the visible range. Through the simulation results, we show that the proposed OFDM-VLC system is robust to intersymbol interference (ISI) and a large peak-to-average power ratio (PAPR), and is also color independent. & 2014 Elsevier B.V. All rights reserved.
Keywords: Color space Color independent (compatible) Orthogonal frequency division multiplexing (OFDM) Optical wireless communication (OWC) Visible light communication (VLC) Light-emitting diode (LED)
1. Introduction Presently, the optical wireless communication (OWC) technology is being considered as a strong candidate for ubiquitous, highspeed wireless communication applications [1,2]. Through the extensive use of light-emitting diodes (LEDs) over the course of the last few years and the anticipated development of a widespread LED lighting and signaling infrastructure, visible light communication (VLC) has become the mainstream in the current OWC fields [1,2]. VLC can be applied to a short-range communication system using visible light and a solid-state light (SSL) source, such that the system can serve the dual purposes of communication together with illumination. A VLC system utilizes all of the notable advantages of visible light since it has aesthetically pleasing, an unregulated huge bandwidth with no electromagnetic interference, no known health risks, security, and ubiquitous nature (can be used in RF-prohibited areas such as aircraft, space shuttles, and hospitals). Hence, by using visible light for data transmission, many problems related to radio and infrared communications are avoided. In addition to the advantages over RFand IR-based communications, LEDs are more advantageous than other light sources because of their faster switching time, higher efficiency, smaller size, higher directivity, longer lifetime, and cheaper transmitter components as compared to expensive RF
n
Corresponding author. Tel.: þ 82 2 910 4707; fax: þ82 2 910 4449. E-mail address: [email protected] (K.-D. Kim).
http://dx.doi.org/10.1016/j.optcom.2014.03.060 0030-4018/& 2014 Elsevier B.V. All rights reserved.
units. Therefore, with all the advantages of visible light and LEDs, VLC systems are an attractive, alternative technology for indoor wireless communications [3,4]. A number of modulation schemes have been considered for VLC systems, which include variable on-off keying (VOOK), variable pulse position modulation (VPPM), multiple PPM (MPPM), pulse dual slope modulation (PDSM), orthogonal frequency division multiplexing (OFDM), and subcarrier modulation [5–10]. Among them, thus far, OFDM is found to be the most advantageous for VLC systems and OFDM for VLC was first introduced in [11]. Parallel data transmission by orthogonal subcarriers offers overall high data rates (i.e., rates similar to conventional single carrier modulation schemes), high bandwidth efficiency, and reduced complexity in equalizers. Owing to its long symbol duration, OFDM is inherently very robust against multipath induced intersymbol interference (ISI), which is a major concern in indoor OWC. OFDM has a relatively large peak-to-average power ratio (PAPR), which brings a reduced power efficiency of RF power amplifier. As a means of color-space-based modulation, color-shift keying (CSK) and generalized color modulation (GCM) have been proposed thus far [12,13]. While CSK is currently applicable for only white visible light, GCM is a color-independent modulation scheme that can be used for communication under varying target color conditions. Furthermore, GCM can provide flicker-free operation, dimming control, and the ability to function irrespective of the number of LEDs at the transmitter or photo detectors (PDs) at the receiver [13–15]. Most of the previous research on OFDM-based VLC has utilized only white light (LEDs) and did not consider the color issue of
P. Das et al. / Optics Communications 324 (2014) 264–268
visible light [9,10]. Therefore, no systematic way of coping with the color changing conditions of visible light has been suggested thus far. However, the color of visible light is a vital issue for a VLC system, and VLC should be compatible with all colors in the visible range. Although the possibility of color-independent OFDM-based VLC was introduced in [16], the simulation was not concrete and main results were not included there. In this paper, we propose a color-independent OFDM-VLC system in a complete form. Therefore, our proposed OFDM-VLC system will be applicable for all visible colors. The remainder of this paper is organized as follows. In Section 2, our proposed system model for the OFDM-VLC is introduced and explained in detail. The simulation results are presented and discussed in Section 3. Finally, Section 4 presents the conclusions of the paper.
265
2. OFDM-VLC system model The proposed OFDM-VLC system model is shown in Fig. 1; this system contains two main blocks: (a) the transmitter and (b) the receiver. We combined the OFDM and GCM schemes together in the implementation of a color-independent VLC system using OFDM. The operation of the entire OFDM-VLC block system is described as below:
2.1. Transmitter blocks The transmitter mainly consists of a channel coder, a colorspace-based modulator, and an OFDM modulator. Several of the
Fig. 1. Complete system model for the OFDM-VLC system. (a) Transmittar and (b) Receiver.
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P. Das et al. / Optics Communications 324 (2014) 264–268
blocks within the OFDM-VLC transmitter are implemented as follows: Coding and interleaving: First, binary input data is encoded by a forward error correction (FEC) code. The encoded data is then interleaved. These blocks can be optional depending on the channel condition. Color-space-based modulator: Color space is a tool which can be used to specify, create, and visualize color. For example, a computer may describe a color using the amounts of red, green, and blue phosphor emission required to match a color. We used GCM, which is a color-space-based modulation scheme, to transmit data symbols by varying the light intensities of multiple LEDs, irrespective of the target colors of VLC signals [13–15]. GCM constructs a constellation diagram in a color space and then a data symbol is represented by a constellation point ðx; yÞ. Each constellation point in a color space represents a corresponding color, and target color is the average of all the appropriate constellation points. At the transmitter, the target color can be determined by a “target color” input from outside. ðx; yÞ-to-RGB: A color can be generated by mixing light beams from multiple LEDs. The color point ðx; yÞ can be transformed into RGB by [17].
OFDM demodulator: It demodulates the received OFDM signal by down-conversion with sub-carriers as follows: R t þT i rðtÞ exp j2π ðt t s Þ dt ; for 1 r ir N Ri ¼ t ss N Z ts þ T i Gi ¼ gðtÞ exp j2π ðt t s Þ dt; for 1 r ir N N ts Z ts þ T i bðtÞ exp j2π ðt t s Þ dt; for 1 r i r N Bi ¼ N ts
X ¼ 2:7689R þ 1:7517G þ 1:1302B Y ¼ R þ 4:5907G þ 0:0601B Z ¼ 0:0565G þ 5:5943B X Y ; y¼ x¼ X þY þZ X þY þZ
ð1Þ
where R, G, and B are the colors of three monochromatic light sources with wavelengths 700 nm, 546.1 nm, and 435.8 nm, respectively, and X; Y; and Z are the tristimulus values of a particular color. OFDM modulator: The RGB intensities corresponding to each symbol (Ri ; Gi ; Bi for the ith symbol) undergo a serial-to-parallel conversion. After the serial-to-parallel conversion, the inputs to each OFDM modulator are R ¼ ½R1 ; R2 ; …; RN T for the redcolored channel, G ¼ ½G1 ; G2 ; …; GN T for the green-colored channel, and B ¼ ½B1 ; B2 ; …; BN T for the blue-colored channel, where N is the number of the OFDM subcarriers. Each low rate sub-intensity is modulated by an orthogonal sub-carrier. Then, an OFDM signal consists of a sum of modulated subcarriers and one OFDM symbol starting at t ¼ t s can be expressed as i rðtÞ ¼ ∑N i ¼ 1 Ri exp j2π N ðt t s Þ ; t s r t r t s þ T i gðtÞ ¼ ∑N i ¼ 1 Gi exp j2π ðt t s Þ ; t s r t rt s þ T N i N ð2Þ bðtÞ ¼ ∑i ¼ 1 Bi exp j2π ðt t s Þ ; t s rt r t s þT N
After integrating the signal over the symbol period (T), we can obtain the exact RGB (Ri ; Gi ; Bi for the ith symbol) intensity for each symbol. The OFDM demodulator also averages all of the sub-intensities ðB1 ; B2 ; …; BN Þ in a channel to obtain BT . In the same manner, we obtain RT ; GT , and the resultant target color can be used for constructing the constellation diagram. RGB-to-ðx; yÞ: This block converts the received intensity (Ri ; Gi ; Bi ) to a point ðxi ; yi Þin the applied color space by using Eq. (1). In addition, this block generates the target color point ðxT ; yT Þ by converting the ðRT ; GT ; BT Þ. Color-space-based demodulation: First, this block generates a constellation diagram by using the target color information from the previous block. Note that finding the proper target color and constellation diagram results in a higher demapping accuracy. After that, the nearest constellation point (based on the minimum Euclidian distance) from ðxi ; yi Þ is the recovered data symbol point ðx; yÞ. The resultant constellation point is demapped to binary values. Deinterleaving and decoding: The resultant binary values may be deinterleaved and decoded to produce the desired output data.
3. Simulation results Table 1 lists the simulation parameters. Since the VLC channel has a considerably greater channel bandwidth and typical lighting levels provide a communication channel with a very high signalto-noise ratio (SNR) [1], we did not consider the SNR and bandwidth limitation seriously. A PD combined with an optical bandpass filter (OBPF) and an optical concentrator measures the intensity of each color. An OBPF is a device that selectively passes light in a particular band of wavelengths, that is, colors, while blocking the remainder. A silicon-based PIN photodiode was assumed to be the visible light detector. In addition, parameters of an LED driver circuit were adjusted to compensate for the different responsivities of different colors [14].
where T is the duration of one OFDM symbol.
Digital-to-analog converter (DAC) and light-emitting module: The OFDM signal is converted to an analog signal and transmitted by three LEDs, each having its own spectral distribution or a color component (RGB).
2.2. Receiver blocks
Several of the blocks within the OFDM-VLC receiver are implemented as follows: Photo detector (PD) and analog-to-digital converter (ADC): Each PD measures the light intensity corresponding to its wavelength or color component (RGB) and outputs its magnitude. The outputs of the PD are converted to digital signals by the ADC for further processing.
ð3Þ
Table 1 The simulation parameters. Parameter
Value
Number of LEDs and PDs Number of constellation points (a 2-bit data symbol is assumed) Total intensity Total number of data bits transmitted Target color point coordinates in CIE1931color space 3 LED positions (or points) at the transmitter (and the PDs at the receiver) in CIE1931color space Number of FFT points Number of data sub carriers
3 4 3 AU 213 (0.4,0.325) R: (0.7347, 0.2653) G: (0.2738, 0.7174) B: (0.1666, 0.0089) 256 256
P. Das et al. / Optics Communications 324 (2014) 264–268
267
Fig. 2 shows the generation of constellations for the target color at the transmitter in the CIE1931 color space. Gray coding is used to represent equiprobable data symbols. Figs. 3 and 4 depict the generated constellation diagram at the receiver for the GCM and OFDM schemes, respectively, for the same delay spread (1 ms). In the case of GCM, the ISI is so large that the constellation is seriously blurred, causing a high bit error rate
Fig. 5. BER vs. SNR of the GCM-VLC system for different delay spreads.
Fig. 2. Four-point constellation for the target color in CIE1931 color space.
Fig. 6. BER vs. SNR of the OFDM-VLC system for different delay spreads.
Fig. 3. Generated constellation at receiver for GCM (delay spread ¼1 ms).
Fig. 4. Generated constellation at receiver for OFDM (delay spread¼ 1 ms).
(BER). However, since OFDM is robust to the ISI, the interference is still tolerable enough to obtain a reasonable received constellation and hence shows superior BER performance. Figs. 5 and 6 show the BER performance of the VLC system for the GCM and OFDM schemes, respectively, by varying the delay spread. It is observed that the BER performance of the OFDM-VLC system shows almost similar performances for various delay spread values, while the BER performance of the GCM-VLC system varies considerably with delay spread variation. A large PAPR for an OFDM signal brings disadvantages such as increased complexity of the ADC/DAC process and reduced efficiency of the RF power amplifier [18]. To reduce the PAPR, we take the clipping operation such as a multiplication of the OFDM signal by 1 if the OFDM amplitude is below a threshold and smaller than 1 if the amplitude needs to be clipped. In RF communication, it was known that clipping used to reduce the PAPR results in degradation of the BER performance [18]. In VLC, the compensation method by using pre-distortion of modulation input signal of LED might be considered to alleviate the performance degradation due to the large PAPR [19]. Most importantly, in this paper, since our proposed OFDM-VLC system is very resistant to signal clipping, it does almost not distort the OFDM signal amplitude. The reason for this resistance is that although we reduced the signal level, the ratio among the three RGB signals still remains the same. Since the symbol on the color space is determined by the RGB ratio (that means color), the reduction of the signal level does not
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the Global Scholarship Program for Foreign Graduate Students at Kookmin University in Korea.
References
Fig. 7. BER vs. SNR for the OFDM-VLC system for different clipping levels.
significantly affect the BER performance. Fig. 7 shows the BER performance of the OFDM-VLC system with and without clipping. In this figure, SC(scaling factor) is defined as the ratio of threshold and peak value, and an OFDM signal is clipped if the OFDM amplitude is above a threshold. This shows that the clipping of the OFDM-VLC signal has a very minor effect on the BER performance. Hence, we can see from the figure that the BER curves for different clipping levels are overlapped. 4. Conclusion We proposed an OFDM-based VLC system that can be made color independent by using color-space-based modulation. Through simulations, we have shown that the proposed OFDM-VLC system is robust to ISI and high PAPR and is also color independent. Acknowledgements This work was supported by the Basic Science Research Program through NRF funded by the MEST (No. 2011-0007107) as well as by
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