OSA's Digital Library

Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 1 — Jan. 14, 2013
  • pp: 1203–1208
« Show journal navigation

Demonstration of 575-Mb/s downlink and 225-Mb/s uplink bi-directional SCM-WDM visible light communication using RGB LED and phosphor-based LED

Yuanquan Wang, Yiguang Wang, Nan Chi, Jianjun Yu, and Huiliang Shang  »View Author Affiliations


Optics Express, Vol. 21, Issue 1, pp. 1203-1208 (2013)
http://dx.doi.org/10.1364/OE.21.001203


View Full Text Article

Acrobat PDF (1710 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

We propose and experimentally demonstrate a novel full-duplex bi-directional subcarrier multiplexing (SCM)-wavelength division multiplexing (WDM) visible light communication (VLC) system based on commercially available red-green-blue (RGB) light emitting diode (LED) and phosphor-based LED (P-LED) with 575-Mb/s downstream and 225-Mb/s upstream transmission, employing various modulation orders of quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM). For the downlink, red and green colors/wavelengths are assigned to carry useful information, while blue chip is just kept lighting to maintain the white color illumination, and for the uplink, the low-cost P-LED is implemented. In this demonstration, pre-equalization and post-equalization are also adopted to compensate the severe frequency response of LEDs. Using this scheme, 4-user downlink and 1-user uplink transmission can be achieved. Furthermore, it can support more users by adjusting the bandwidth of each sub-channel. Bit error rates (BERs) of all links are below pre-forward-error-correction (pre-FEC) threshold of 3.8x 10−3 after 66-cm free-space delivery. The results show that this scheme has great potential in the practical VLC system.

© 2013 OSA

1. Introduction

In this paper, we proposed and experimentally demonstrated a novel full-duplex bi-directional VLC system using RGB LED and a commercially available phosphor-based LED in downlink and uplink, respectively. In this demonstration, subcarrier multiplexing (SCM) and wavelength division multiplexing (WDM) are adopted to realize the bi-directional transmission; quadrature amplitude modulation (QAM) and orthogonal frequency division multiplexing (OFDM) modulation are also employed to increase the data rate. Additionally, pre- and post-equalizations are both implemented to compensate the severe channel response of LEDs. For downlink, signals are only modulated on red and green chips, while the blue chip is just lighted by direct current (DC) voltage to maintain white colour illumination. Each LED chip has two SCM channels without channel guardband. A downstream at 575 Mb/s and an upstream at 225 Mb/s after 66-cm free-space transmission are achieved, and the measured bit error rates (BERs) for all channels are under hard-decision FEC limit of 3.8x10−3 [10

10. R. Elschner, T. Richter, T. Kato, S. Watanabe, and C. Schubert, “Distributed coherent optical OFDM multiplexing using fiber frequency conversion and free-running lasers,” in Proc. OFC, Los Angeles, CA, PDP5C.8 (2012).

]. We also discuss the interference caused by bi-directional transmission. Moreover, this scheme has good scalability for supporting more terminals and advantage of dynamic traffic reconfiguration by adjusting different bandwidth and modulation orders for uplink and downlink transmissions.

2. Experimental setup

At the transmitter, the input binary sequences are modulated using QAM format, and then passed to OFDM encoder. Then, the QAM-OFDM signals are up-converted to different subcarriers with center frequency at f1 = 18.75MHz (sub1), f2 = 43.75MHz (sub2) without SCM channel guardband in radio frequency (RF) domain and added up. The bandwidths of all sub-channels are 25MHz. From DC to 5MHz, the transfer curve is not good in this demonstration, and the 25dB bandwidth point is around 50MHz, therefore we choose the signal frequency band from 6.25MHz to 56.25MHz. Moreover, the center frequency and bandwidths of sub-channels can be adjusted to meet the demands of different users. Subsequently, the multiplexed QAM-OFDM signals came from AWG are filtered by a low-pass filter (LPF) and amplified by EA. The electrical QAM-OFDM signals and DC-bias voltage are combined via bias tee, and applied to different LEDs serving as the transmitter. In red colour chip, 64QAM is applied both in sub1 and sub2. However, in green colour chip the modulation format of sub2 is 32QAM, and 64QAM is used in sub1. For uplink, the modulation formats of sub1 and sub2 are 32QAM and 16QAM, respectively. As each sub-channel is independent, it can be used to support multiple users for upstream and downstream transmission.

In this experiment, QAM-OFDM signals which consist of 64 subcarriers are generated by an arbitrary waveform generator (AWG). Up-sampling by a factor 20 is employed, and the sample rate of AWG is 500MS/s. Pre-equalization is used before inverse fast Fourier transform (IFFT) to compensate the distortions of AWG, LED, EA and free-space channel, while training-symbols-based post-equalization is used for other channel impairments such as phase noise. At the receiver, the electrical QAM-OFDM signals are detected by low-cost PDs and recorded by a digital real-time oscilloscope (OSC) with 500MS/s sampling rate. Additionally, in front of each PD, the corresponding optical filter is implemented to filter out the undesired wavelength. Then the received signals are down-converted to baseband and further offline processing which is an inverse procedure of QAM-OFDM encoder.

3. Experimental results and discussions

The frequency characteristics of the electro-optical-electro channel are measured for all LED chips as shown in Fig. 4
Fig. 4 Channel response of individual LED.
. As we can observe, the frequency responses of blue and red chip of RGB LED are almost the same, and the green chip and the P-LED behave similarly. The bandwidths around 20dB point of the two groups are about 25MHz and 30MHz, respectively. Noting that the higher frequency is fast fading, equalization at frequency domain is needed. According to the channel knowledge, pre-equalization has been designed and applied. The amplitudes of the 64 subcarriers are appropriately pre-equalized. The electrical spectra of the received signals with (w) and without (w/o) equalizations at each wavelength are measured by Spectrum Analyzer HP8562A depicted in Fig. 5
Fig. 5 Electrical spectra of different wavelengths: (a) P-LED (w/o pre-) . (b) red (w/o pre-). (c) green (w/o pre-). (d) P-LED (w pre-). (e) red (w pre-) . (f) green (w pre-). (g) P-LED(w post). (h) red (w post). (i) green (w post).
(the spectra with post-equalization are offline processed).We could find that the spectra of each sub-channel are much more flatten after using pre-equalization and post-equalization. The power ratio of sub1 to sub2 is precisely assigned to obtain an optimal performance.

Then, we observe the nonlinearity effect introduced by LED chips. As these two types of LEDs behave similarly on nonlinearity, we take P-LED for discussion. In this demonstration, we just utilize sub-channel1 of P-LED. The input power of P-LED is varied from 8dBm to 20dBm with 2-dB step, and the results are presented in Fig. 6
Fig. 6 Measured BER versus input power of P-LED.
. As we can see, the optimal input power is 12dBm. A lower input power will reduce the signal-to-noise-ratio (SNR) and cause low modulation depth, while a higher one will cause nonlinearity and clipping. We can also get the same conclusion from the constellation diagrams inserted in Fig. 6. The nonlinearity should be addressed by current source driver instead of voltage source driver or adopt nonlinearity compensation. In this experiment, the input power of P-LED is fixed at 12dBm. The modulation indexes calculated as in Ref [2

2. A. M. Khalid, G. Cossu, R. Corsini, P. Choudhury, and E. Ciaramella, “1-Gb/s Transmission Over a Phosphorescent White LED by Using Rate-Adaptive Discrete Multitone Modulation,” IEEE Photon. J. 4(5), 1465–1473 (2012). [CrossRef]

]. of these three chips are all 1.

4. Conclusion

In this paper, we have reported a bi-directional VLC system based on a commercially available RGB-LED, a P-LED and a low-cost photodiode. A data rate of 225 Mb/s upstream and 575Mb/s downstream transmissions enabled by SCM, WDM and QAM-OFDM has been achieved. Pre- and post-equalization at frequency domain has been adopted to compensate the distortions. A four-user access for downlink and one-user access for uplink can be achieved. The crosstalk of bi-directional transmission is also analyzed. BERs of all channels are under pre-FEC limit of 3.8x10−3 after 66-cm free-space transmission. Moreover, the capacity of downlink and uplink can be easily dynamically reconfigured by adjusting the bandwidths and modulation formats of sub-channel. The results show that this scheme is a good candidate for bi-directional transmission in the real VLC system.

Acknowledgments

This work was partially supported by the STCSM (No.12dz1143000), and NHTRDP (973 Program) of China (Grant No. 2010CB328300), NNSF of China (No. 61107064, No. 61177071), NHTRDP (863 Program) of China (2011AA010302, 2012 AA011302).

References and links

1.

D. O’Brien, H. L. Minh, L. Zeng, G. Faulkner, K. Lee, D. Jung, Y. Oh, and E. T. Won, “Indoor visible light communications: challenges and prospects,” Proc. SPIE 7091, 709 106–1 –709 106–9 (2008).

2.

A. M. Khalid, G. Cossu, R. Corsini, P. Choudhury, and E. Ciaramella, “1-Gb/s Transmission Over a Phosphorescent White LED by Using Rate-Adaptive Discrete Multitone Modulation,” IEEE Photon. J. 4(5), 1465–1473 (2012). [CrossRef]

3.

H. Le Minh, D. O’Brien, G. Faulkner, L. Zeng, K. Lee, D. Jung, and Y. J. Oh, “High-speed visible light communications using multiple-resonant equalization,” IEEE Photon. Technol. Lett. 20(14), 1243–1245 (2008). [CrossRef]

4.

J. Vucic, C. Kottke, S. Nerreter, K. Langer, and J. W. Walewski, “513 Mbit/s visible light communications link based on DMT-modulation of a white LED,” J. Lightw. Technol. 28(24), −3512–3518 (2010).

5.

A. H. Azhar, T. Tran, and D. O Brien, ”Demonstration of high-speed data transmission using MIMO-OFDM visible light communications,” Proc. of Globecom Workshops, 1052–1056 (2010).

6.

G. Cossu, A. M. Khalid, P. Choudhury, R. Corsini, and E. Ciaramella, “3.4 Gbit/s visible optical wireless transmission based on RGB LED,” Opt. Express 20(26), B501–B506 (2012). [CrossRef] [PubMed]

7.

J. Vucic, C. Kottke, S. Nerreter, A. Buttner, K.-D. Langer, and J. W. Walewski, “White light wireless transmission at 200+ Mb/s net data rate by use of discrete-multitone modulation,” IEEE Photon. Technol. Lett. 21(20), 1511–1513 (2009). [CrossRef]

8.

T. Komine, S. Haruyama, and M. Nakagawa, “Bidirectional visible-light communication using corner cube modulator,” IEIC Tech. Report 102, 41–46 (2003).

9.

Y. F. Liu, C. H. Yeh, C. W. Chow, Y. Liu, Y. L. Liu, and H. K. Tsang, “Demonstration of bi-directional LED visible light communication using TDD traffic with mitigation of reflection interference,” Opt. Express 20(21), 23019–23024 (2012). [CrossRef] [PubMed]

10.

R. Elschner, T. Richter, T. Kato, S. Watanabe, and C. Schubert, “Distributed coherent optical OFDM multiplexing using fiber frequency conversion and free-running lasers,” in Proc. OFC, Los Angeles, CA, PDP5C.8 (2012).

OCIS Codes
(230.3670) Optical devices : Light-emitting diodes
(060.2605) Fiber optics and optical communications : Free-space optical communication

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: November 12, 2012
Revised Manuscript: December 20, 2012
Manuscript Accepted: December 28, 2012
Published: January 10, 2013

Citation
Yuanquan Wang, Yiguang Wang, Nan Chi, Jianjun Yu, and Huiliang Shang, "Demonstration of 575-Mb/s downlink and 225-Mb/s uplink bi-directional SCM-WDM visible light communication using RGB LED and phosphor-based LED," Opt. Express 21, 1203-1208 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-1-1203


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. D. O’Brien, H. L. Minh, L. Zeng, G. Faulkner, K. Lee, D. Jung, Y. Oh, and E. T. Won, “Indoor visible light communications: challenges and prospects,” Proc. SPIE 7091, 709 106–1 –709 106–9 (2008).
  2. A. M. Khalid, G. Cossu, R. Corsini, P. Choudhury, and E. Ciaramella, “1-Gb/s Transmission Over a Phosphorescent White LED by Using Rate-Adaptive Discrete Multitone Modulation,” IEEE Photon. J.4(5), 1465–1473 (2012). [CrossRef]
  3. H. Le Minh, D. O’Brien, G. Faulkner, L. Zeng, K. Lee, D. Jung, and Y. J. Oh, “High-speed visible light communications using multiple-resonant equalization,” IEEE Photon. Technol. Lett.20(14), 1243–1245 (2008). [CrossRef]
  4. J. Vucic, C. Kottke, S. Nerreter, K. Langer, and J. W. Walewski, “513 Mbit/s visible light communications link based on DMT-modulation of a white LED,” J. Lightw. Technol. 28(24), −3512–3518 (2010).
  5. A. H. Azhar, T. Tran, and D. O Brien, ”Demonstration of high-speed data transmission using MIMO-OFDM visible light communications,” Proc. of Globecom Workshops, 1052–1056 (2010).
  6. G. Cossu, A. M. Khalid, P. Choudhury, R. Corsini, and E. Ciaramella, “3.4 Gbit/s visible optical wireless transmission based on RGB LED,” Opt. Express20(26), B501–B506 (2012). [CrossRef] [PubMed]
  7. J. Vucic, C. Kottke, S. Nerreter, A. Buttner, K.-D. Langer, and J. W. Walewski, “White light wireless transmission at 200+ Mb/s net data rate by use of discrete-multitone modulation,” IEEE Photon. Technol. Lett.21(20), 1511–1513 (2009). [CrossRef]
  8. T. Komine, S. Haruyama, and M. Nakagawa, “Bidirectional visible-light communication using corner cube modulator,” IEIC Tech. Report102, 41–46 (2003).
  9. Y. F. Liu, C. H. Yeh, C. W. Chow, Y. Liu, Y. L. Liu, and H. K. Tsang, “Demonstration of bi-directional LED visible light communication using TDD traffic with mitigation of reflection interference,” Opt. Express20(21), 23019–23024 (2012). [CrossRef] [PubMed]
  10. R. Elschner, T. Richter, T. Kato, S. Watanabe, and C. Schubert, “Distributed coherent optical OFDM multiplexing using fiber frequency conversion and free-running lasers,” in Proc. OFC, Los Angeles, CA, PDP5C.8 (2012).

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited