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a distance below 30cm by using discrete multi-tone (DMT) modulation and avalanche photo-diode (APD) [7]. However, bi-directional VLC transmission is still a main technique limit due to the lack of uplink transmission method. There have been few reports on bidirectional VLC transmission. Employing a retro-reflect-ing transceiver has been demonstrated in [8], but it cannot achieve a high modulation rate. Implementing LED VLC for bi-directional transmission via time-division-duplex has been proposed in [9], however, the synchro-nization is a big problem and the data rate of uplink and downlink is only 2.5Mb/s.

We have proposed a bidirectional sys-tem based on WDM, whose data rates are 800-Mb/s [10] and 875-Mb/s [11] before. Compared with our previous work, this scheme which based on WDM and SCM can provide more flexible bandwidth allocation, hence to obtain higher transmission data rate. In this demonstration, each of the wave-lengths at the downlink are divided into three sub-channels, while the uplink has one sub-channel. The bandwidths of all sub-channels are 25MHz, and the modulation orders are assigned according to the channel characteristics, ranging from 16QAM to 64QAM. The data rate of uplink and downlink are 300-Mb/s and 1.15-Gb/s, respectively. All BERs are under the pre-FEC limit of 3.8×10-3. As far as we know, this is the highest data rate

Abstract: In this paper, we propose and experimentally demonstrate a bi-directional indoor communication system based on visible light RGB-LED. Spectrally efficient modulation formats (QAM-OFDM), advanced digital signal processing, pre- and post-equalization are adopted to compensate the severe frequency response of indoor channel. In this system, we utilize red-green-blue Light emitting diodes (LEDs), of which each color can be used to carry different signals. For downlink, the low frequencies of each color are used while for uplink, the high frequencies are used. The overall data rate of downlink and uplink are 1.15-Gb/s and 300-Mb/s. The bit error ratios (BERs) for all channels after 0.7 m indoor delivery are below pre-forward-error-correction (pre-FEC) threshold of 3.8×10-3. To the best of our knowledge, this is the highest data rate in bi-directional visible light communication system.Key words: visible light communication; bi-directional transmission; orthogonal frequency division multiplexing; light emitting diode

I. INTRODUCTIONS

Visible light communication (VLC) based on light emitting diodes (LEDs) is receiving more and more attention [1-2]. It has been widely investigated [3–6]. The record data rate of uni-directional transmission is 3.4Gb/s, which is at

A High-Speed Bi-Directional Visible Light Communication System Based on RGB-LEDWANG Yuanquan and CHI Nan

Department of Communication Science and Engineering, Fudan University, Shanghai 200433, China

COMMUNICATIONS SYSTEM DESIGN

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process of OFDM encoder and decoder is also shown in Fig. 1. Cyclic prefix (CP) is used to mitigate the inter-symbol interference caused by multipath propagation.

In this scheme, a two-stage frequency allocation method jointly employing WDM and SCM is carried out. The first fold is to use different colors/wavelengths for coarse frequency allocation, and the second fold is to use different subcarriers for a more precise frequency allocation. For the downlink, red, green, and blue wavelengths are all used to carry useful information, and each one includes three SCM sub-channels. Subsequently, three QAM-OFDM signals with a bandwidth of 25MHz are modulated individually with different frequency at 18.75MHz (sub1), 43.75MHz (sub2), and 56.25 MHz (sub3) without SCM channel guard band in RF domain and combined. For the uplink, only one sub-channel is modulated at individual color, but occupied at

in an bi-directional VLC system.

II. CONFIGURATION OF PROPOSED

SYSTEM

The architecture and principle of the proposed full-duplex SCM-WDM VLC system is shown Fig. 1. RGB LEDs (Cree Xlamp MC-E) are used as the VLC transmitters (TXs) for both uplink and downlink, which generate a lumi-nous flux of about 106lm (red: 30.6lm, green: 67.2 and blue: 8.2lm) at 350mA bias currents with 110° Lambertian emission. It consists of four chips radiating in 625nm (red), 530nm (green), 455nm (blue) and white color. Low-cost PIN PDs (Hamamatsu S6968, 50MHz bandwidth) are adopted for the receivers (RXs). At the transmitters, the 16/32/64-ary quadrature amplitude modulation (16/32/64 QAM) OFDM signals which consist of 64 subcarriers are generated by an arbitrary waveform generator (AWG). The detailed

1=455nm 2=530nm 3=625nm1 2 31+fm 1+fn k1+f i1+f

Fig.1 Diagram of bi-directional transmission system. LPF: low pass filter; EA: electrical amplifier; DC: direct current; PD: photo detector; P/S: parallel to serial; TX: transmitter; RX: receiver

In this paper, a novel bi-directional trans-m i s s ion s y s tem i s proposed based on commercial RGB-LEDs and low-cost PINs.

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7122C with the maximum sampling rates of 24 GS/s and bandwidths of 6 GHz. The electrical signals from the RX are recorded by OSC (Tektronix TDS 6604) with the max-imum bandwidth of 6GHz and the maximum sampling rate of 20GSa/s. The luminous flux of LED chips at 350mA bias currents is 30.6lm (red), 67.2 (green) and 8.2lm (blue). The distance between the TX and RX in the different side is 70cm, and the distance in the same side can be neglected. As for the limited experiment condition, the three different OFDM signals of each link are shared by the one AWG output via power splitter.

According to the channel response, the modulation formats of three different colors are 64QAM (sub1), 32QAM (red, green, sub2), 64QAM (blue, sub2), 16QAM (sub3), and the modulation format of uplink is 16QAM. So the overall data rate of downlink is 1.15-Gb/s and uplink is 300-Mb/s. The measured electrical spectra of red LED are shown in Fig. 2. It can be easily seen that the spectra in each sub-channel are much more flatten by using pre-equalization. A coarse power allocation is made between different sub-channels, to satisfy the demand of different modulation orders.

Meanwhile, the BER performances vs. luminous flux of LED are shown in Fig. 3 (a)-(d). Fig. 3(a)-(c) represent the three sub-channels in downlink of red, green and blue color LED chips, respectively. And Fig. 3(d) shows the uplink sub-channel of each color LED. The transmission data rate at downlink of red, green and blue colors are 375Mb/s,

a higher frequency at 81.25MHz (sub4) this time in order to realize a seamless frequency allocation and avoid overlapping at frequency domain. Thus the SCM-WDM scheme can successfully simultaneously support 9-user downlink and 3-user uplink transmission. The principle of frequency allocation is also shown in Fig. 1. The data rates and numbers of uplink and downlink can be dynamically reconfigured by changing the center frequency and modula-tion orders of SCM subcarriers, and adjusting the bandwidths of baseband OFDM signals. Furthermore, the subcarriers of OFDM signals can also be assigned to different users. Addi-tionally, SCM used in this configuration cannot only provide flexible frequency allocation, but also offer RF for QAM modulation. Moreover, it can avoid low-frequency noise for the sig-nals are up-converted to RF frequency.

In this experiment, up-sampling by a factor 20 is employed, and the sample rate of AWG is 500MS/s. The multiplexed QAM-OFDM signals from the AWG output serve as the signal source and driver combined with DC-bias voltage. Before modulated on the optical source, the signals are passing through LPF and EA. The electrical amplifier is operated below saturation to eliminate the nonlinearity. After 70 cm free space delivery, the modulated light is focused onto the PD through a focusing lens. In front of each PD, a corresponding red/green/blue dichroic filter is implemented to filter out the unwanted signals. The transmission bands of red, blue, and green filter are 605nm-730nm, 390nm-480nm, and 500nm-580nm, respectively. Then, the detected electrical QAM-OFDM signals are recorded by OSC with 500MS/s sampling rate and off-line processed.

III. EXPERIMENTAL RESULTS AND

DISCUSSION

The system is a point-to-point one in this demonstration, and the PD should be placed on or near the focus plane of lens. The OFDM signals are generated by Tektronix AWG

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1.15-Gb/s downlink and 300-Mb/s uplink transmission data rates enabled by higher order modulation, SCM, WDM, pre- and post-equalizations. The BERs for all channels and sub-channels are below pre-FEC limit of 3.8x10-3, which shows it has great potential in bi-directional transmission and multiple-access in VLC system.

ACKNOWLEDGEMENT

This work was part ial ly supported by the NNSF of China ( No . 61177071 , No.61250018), the Key Program of Shang-hai Science and Technology Association (12dz1143000).

References[1] D. O’Brien, H. L. Minh, L. Zeng, G. Faulkner, K.

Lee, D. Jung, Y. Oh, and E. T. Won, “Indoor visi-ble light communications: challenges and pros-pects,” Proc. SPIE 7091, 709 106–1 –709 106–9 (2008).

375Mb/s, and 400Mb/s. Therefore, the overall transmission data rate of downlink is 1.15Gb/s. and the data rate of uplink at each color is 100Mb/s, hence to obtain the overall uplink data rate is 300Mb/s. The BERs for all channels and sub-channels can be below the 7% pre-FEC threshold of 3.8x10-3. The constellations of different sub-channels at 50% of maximum luminous flux are also inserted in Fig. 3. From the experimental results and discussions, we can validate the feasibility of this two-stage bi-directional transmission scheme.

IV. CONCLUSIONS

In th i s paper, we have proposed and experimentally demonstrated a novel bi-directional transmission system, based on commercial RGB-LEDs and low-cost PINs. A two-stage color/frequency allocation scheme has been proposed. We have achieved

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Fig.3 BER performance vs. luminous flux (a) downlink (red); (b) downlink (green); (c) downlink (blue); (d) uplink.

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“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-direction-al LED visible light communication using TDD traffic with mitigation of reflection interference,” Opt. Express 20(21),23019–23024 (2012).

[10] Y. Wang, Y. Wang, N. Chi, J. Yu, and H. Shang, “Demonstration of 575-Mb/s downlink and 225-Mb/s uplink bi-directional SCM-WDM vis-ible light communication using RGB LED and phosphor-based LED,” Opt. Express21(1), 1203-1208(2013).

[11] Y. Wang, Y. Shao, H. Shang, X. Lu, Y. Wang, J. Yu, and N. Chi, “ 875-Mb/s Asynchronous Bi-direc-tional 64QAM-OFDM SCM-WDM Transmission over RGB-LED-based Visible Light Communi-cation System,” Optical Fiber Communication Conference (OFC) 2013 paper: OTh1G.3.

BiographiesWANG Yuanquan, Ph.D student, speedboy_yq@163.com,No.220,Handan Road, Fudan University, Shang-hai

CHI Nan,Professor,nanchi@fudan.edu.cn,No.220, Handan Road, Fudan University, Shanghai

[2] M. Zhang, Y. Zhang, X. Yuan, and J. Zhang, “Mathematic models for a ray tracing method and its applications in wireless optical com-munications,” Opt. Express 18(17), 18431-18437(2010).

[3] A. M. Khalid, G. Cossu, R. Corsini, P. Choud-hury, 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).

[4] 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).

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

[6] A. H. Azhar, T. Tran, and D. O Brien, ”Demonstra-tion of high-speed data transmission using MI-MO-OFDM visible light communications,” Proc. of Globecom Workshops, 1052–1056 (2010).

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

[8] T. Komine, S. Haruyama, and M. Nakagawa,