A High-Speed Bi-Directional Visible Light Communication System Based on RGB-LED

Transcription

1 COMMUNICATIONS SYSTEM DESIGN A High-Speed Bi-Directional Visible Light Communication System Based on RGB-LED WANG Yuanquan and CHI Nan Department of Communication Science and Engineering, Fudan University, Shanghai , China 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 postequalization 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 (s) for all channels after 0.7 m indoor delivery are below pre-forwarderror-correction (pre-fec) threshold of 3.8. To the best of our knowledge, this is the highest data rate in bi-directional visible light communication system. Key words: visible light communication; bidirectional 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 unidirectional transmission is 3.4Gb/s, which is at a distance below 30cm by using discrete multitone (DMT) modulation and avalanche photodiode (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-reflecting 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 synchronization is a big problem and the data rate of uplink and downlink is only 2.5Mb/s. We have proposed a bidirectional system based on WDM, whose data rates are 800-Mb/s [] 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 wavelengths at the downlink are divided into three sub-channels, while the uplink has one sub-channel. The bandwidths of all subchannels 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 s are under the pre-fec limit of 3.8. As far as we know, this is the highest data rate China Communications March

2 In this paper, a novel bi-directional transmission system is proposed based on commercial RGB-LEDs and low-cost PINs. 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 luminous flux of about 6lm (red: 30.6lm, green: 67.2 and blue: 8.2lm) at 350mA bias currents with 1 Lambertian emission. It consists of four chips radiating in 625nm (red), 530nm (green), 455nm (blue) and white color. Lowcost 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 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 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 1 1+fm 1+fn 1+fk 1+fi 2 3 1=455nm 2=530nm 3=625nm 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 41 China Communications March 2014

3 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 modulation 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. Additionally, 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 signals 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 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 maximum 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 performances vs. luminous flux of LED are shown in Fig. 3 (a)-(d). Fig. 3(a)-(c) represent the three subchannels 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, Power(dBm) (a)frequency(mhz) (b)frequency(mhz) Fig.2 Measured spectra of red chip at (a) downlink; (b) uplink. Power(dBm) China Communications March

4 Sub1 of red chip,64qam Sub2 of red chip,32qam Sub3 of red chip,16qam 1E Sub1 of green chip,64qam Sub2 of green chip,32qam Sub3 of green chip,16qam 1E 1E-4 1E-5 1E-4 (a) Percentage of Maximum Luminous Flux(red-chip) Sub1 of blue chip,64qam Sub2 of blue chip,64qam Sub3 of blue chip,16qam (b) Percentage of Maximum Luminous Flux(green-chip) Sub4 of red chip,16qam Sub4 of green chip,16qam Sub4 of blue chip,16qam 1E 1E (c) Percentage of Maximum Luminous Flux(blue-chip) (d) Percentage of Maximum Luminous Flux(RGB-LED) Fig.3 performance vs. luminous flux (a) downlink (red); (b) downlink (green); (c) downlink (blue); (d) uplink. 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 0Mb/s, hence to obtain the overall uplink data rate is 300Mb/s. The s for all channels and sub-channels can be below the 7% pre-fec threshold of 3.8x. 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 this paper, we have proposed and experimentally demonstrated a novel bidirectional transmission system, based on commercial RGB-LEDs and low-cost PINs. A two-stage color/frequency allocation scheme has been proposed. We have achieved 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 s for all channels and sub-channels are below pre-fec limit of 3.8x, which shows it has great potential in bi-directional transmission and multipleaccess in VLC system. ACKNOWLEDGEMENT This work was partially supported by the NNSF of China ( No , No ), the Key Program of Shanghai Science and Technology Association (12dz ). 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, (2008). 43 China Communications March 2014

5 [2] M. Zhang, Y. Zhang, X. Yuan, and J. Zhang, Mathematic models for a ray tracing method and its applications in wireless optical communications, Opt. Express 18(17), (20). [3] 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), (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), (2008). [5] 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), (20). [6] A. H. Azhar, T. Tran, and D. O Brien, Demonstration of high-speed data transmission using MI- MO-OFDM visible light communications, Proc. of Globecom Workshops, (20). [7] 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). [8] T. Komine, S. Haruyama, and M. Nakagawa, Bidirectional visible-light communication using corner cube modulator, IEIC Tech. Report 2, (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), (2012). [] 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 visible light communication using RGB LED and phosphor-based LED, Opt. Express21(1), (2013). [11] Y. Wang, Y. Shao, H. Shang, X. Lu, Y. Wang, J. Yu, and N. Chi, 875-Mb/s Asynchronous Bi-directional 64QAM-OFDM SCM-WDM Transmission over RGB-LED-based Visible Light Communication System, Optical Fiber Communication Conference (OFC) 2013 paper: OTh1G.3. Biographies WANG Yuanquan, Ph.D student, speedboy_yq@163. com,no.220,handan Road, Fudan University, Shanghai CHI Nan,Professor,nanchi@fudan.edu.cn,No.220, Handan Road, Fudan University, Shanghai China Communications March