IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 2, JANUARY 15, 2014
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High Bandwidth Visible Light Communications Based on a Post-Equalization Circuit Honglei Li, Xiongbin Chen, Beiju Huang, Member, IEEE, Danying Tang, and Hongda Chen, Member, IEEE Abstract— A research in extending bandwidth of the visible light communication (VLC) system that uses phosphorescent white LED has been reported in this letter. Slow response of the phosphorescent component limits the modulation bandwidth of white LED to the lower MHz range. In this letter, we present a post-equalization circuit that contains two passive equalizers and one active equalizer. With blue-filtering and the post-equalization circuit, a bandwidth of 151 MHz has been achieved in our VLC system, which allows OOK-NRZ data transmission up to 340 Mb/s. The VLC link operates at 43 cm using a single one Watt white LED, and the bit-error-ratio was below 2 × 10−3 , which is within the forward error correction limit. Index Terms— Visible light communication, phosphorescent white LED, bandwidth, post-equalization.
I. I NTRODUCTION
H
IGH-POWER white LEDs are being widely used in many areas due to its potential for high-efficiency, long lifetime and low cost. Additionally, because white LEDs have a better modulation bandwidth than other lighting sources such as incandescent and fluorescent lamps, they can combine illumination and data transmission functions together. VLC is becoming an emerging technology for high-speed and shortrange wireless optical communication that stimulates worldwide research and global standardization efforts [1]. White LEDs can be classified into three main types: (1) devices that use separate red-green-blue (RGB) emitters, (2) blue emitters used in combination with a phosphor that emits yellow light and (3) ultraviolet emitters used in combination with RGB phosphor. The second approach is more attractive for general illumination as well as communication due to its higher efficiency and lower complexity. However, one of the main challenges for VLC is providing high datarate communications using the limited modulation bandwidth of these phosphor-based emitters, typically 3∼5 megahertz. There are a number of approaches to improve the modulation bandwidth including using a blue-filter at the receiver to filter out the slow yellow component [2], pre-equalization Manuscript received September 2, 2013; revised October 16, 2013; accepted November 4, 2013. Date of publication November 7, 2013; date of current version December 26, 2013. This work was supported in part by the National Key Basic Research Program of China (973 Program) under Grants 2013CB329205 and 2011CBA00608; in part by the National High Technology Research and Development Program of China (863 Program) under Grants 2013AA013602, 2013AA013603, 2013AA03A104, and 2013AA031903; and in part by the National Natural Science Foundation of China under Grants 61036002 and 61178051. The authors are with the State Key Laboratory of Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China (e-mail:
[email protected];
[email protected];
[email protected];
[email protected];
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2013.2290026
Fig. 1. VLC post-equalization circuit using RC components and a wideband operational amplifier.
of the driving circuitry [3], [4], and post-equalization of the receiver [5]. By using spectrally efficient modulation techniques such as discrete multitone modulation (DMT) and MIMO-OFDM, 513 Mbps and 1.1 Gbps VLC systems have been demonstrated in [6]–[8]. However, both of their system 3-dB bandwidth was only 35–40 MHz, which was limited by the white LED module. In this letter, we show a post-equalization circuit. With blue-filtering and the post-equalization circuit, a bandwidth of 151MHz has been achieved in our VLC system, which allows OOK-NRZ data transmission up to 340 Mbit/s. The resultant BER was below 2 × 10−3 , which is within the limits of Forward Error Correction (FEC). II. VLC P OST-E QUALIZATION C IRCUIT In Fig. 1, we present a post-equalization circuit using RC components and a broadband operational amplifier (OPA847). The equalization circuit is consisted of two passive equalizers and one active equalizer. A. Passive Equalizer The passive equalizer comprises a capacitor in parallel with a resistor, and a load resistor RL , here the RL is R2 or R7 . In Passive equalizer 1(PEQ1), for instance, the transfer function is expressed by 1 + j ω ω1 R2 R2 = × (1) HP ( j ω) = 1 R1 + R2 1 + j ω ω2 R2 + R 1 // j ωC1 where ω1 = R11C1 and ω2 = equalizer response is
R1 +R2 R1 R2 C 1 .
R2 |HP ( j ω)| = × R1 + R2
The magnitude of the
1 + ω2 ω12 . 1 + ω2 ω22
(2)
We can calculate the magnitude response of the equalizer through equation (2) in theory. The frequency f1 = 2πR11 C1 is matched with the corner frequency of signal from the preamplifier. In the experiment, we always set the R2 (1 k
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Fig. 2. Experimental setup for measuring the bandwidth of visible light communication system.
∼10 k) firstly, and calculate the R1 value according to the dynamic range of the signal, then determine C1 . B. Active Equalizer In active equalizer, the capacitor C3 is not considered firstly to simplify the complexity of analysis. Then the active equalizer frequency response is expressed by HA ( j ω) = 1 +
R4 1 R3 // j ωC2
=1+
R4 (1 + j ω R3C2 ). R3
The magnitude of the equalizer response is R4 2 |HA ( j ω)| = (1 + ) + R42 C22 ω2 . R3
(3)
(4)
Note that 1+ RR34 is the gain of amplifier when the C2 =0. While C2 is considered, the 3-dB point above the 1+ RR34 is computed as √ (1 + RR43 ) 1.91(1 + RR43 ) 0.5 + 2 ≈ . (5) ω3dB = R4 C 2 R4 C 2 From equation (5), we can see that when the R4 , R3 and 3-dB cutoff frequency is known, the C2 can be calculated. However, the model is so simple that the experiment results may be a little different especially in high-frequency operation. It is necessary to use C3 to filter out high frequency interference and ensure the stability of the receiver. The value of C2 and C3 need to compromise and be carefully chosen. C4 is a compensation capacitor to improve phase margin and avoid self-oscillating. To sum up, post-equalization circuit can be divided into three parts: PEQ1, AEQ and PEQ2. PEQ1 and PEQ2 decay the low-frequency signal amplitude and AEQ can compensate for the amplitude of the high-frequency signal. They are integral and interdepend, and the order of these three parts cannot be exchange. Both the magnitude of signal and bandwidth could be extended if more AEQ and PEQ were properly used. III. E XPERIMENTAL S ETUP There are two apparatus that we used to measure the bandwidth of VLC system. The first one is measuring the amplitude frequency response using network analyzer (Agilent E5071B) which is presented in Fig. 2. By this way, the 3-dB cutoff frequency of the receiver can be measured accurately, while the received signal is unable to be read. Fig. 2 also
Fig. 3.
VLC Experiment Link.
shows another way to measure the bandwidth of VLC system that the magnitude of the channel’s frequency response is measured by varying the frequency of a sine wave provided by the analog signal generator (Agilent N5181A), and directly monitoring the receive amplitude at the oscilloscope (Agilent MSO6104A). We can know more details about the signal of the receiver from the real-time storage oscilloscope. The two methods can be complementary, mutually verified. Therefore, we utilize both of the methods in our experiment to ensure the accuracy of measuring the bandwidth of VLC system. The output signal from the network analyzer or analog signal generator is directly (without amplifier) superimposed onto the LED DC power supply via a Bias-T (Aeroflex 8810). And the output of Bias-T was directly supplied to white LED. The light source was a commercial phosphorescent white LED (OSRAM LUW W5AM), devised for general lighting. A 120° full opening angle lens and was fixed to make sure the light transmits along the regular direction. The analogue receiver consisted of several key components. An optical blue-filter (400 nm–500 nm, transmittivity is 90%) which can suppress the phosphorescent component of the white LED and an optical convex lens (30cm focal length) was mounted in front of the PIN photodiode (HAMAMATU, S10784). The optical signal from the LED was converted to electrical current signal through the PIN photodiode and then the current signal was amplified to voltage signal by a lownoise transimpedance amplifier (TIA) circuit. The high-pass filter can filter out 50 Hz power frequency signal and reduce the low-frequency noise. In order to extend the bandwidth of VLC system based on phosphorescent white LED and improve the signal amplitude, a post-equalization circuit was designed. IV. R ESULTS AND D ISCUSSIONS Fig. 3 shows the visible light communication experimental link. An operational amplifier OPA847 was used to design the tansimpedance amplifier circuit. Two convex lenses were used and the link operates over a distance of 43 cm with a single 1W white LED. The configuration of the VLC experimental link parameters is presented in Table I. Fig. 4 shows the frequency response of white light, blue component and yellow component emitted by a white LED. We can see that the bandwidth of white-light response is only 3MHz (the 3-dB cutoff frequency of TIA), whereas the blue component response is 12 MHz. The bandwidth of yellow component response is approximately 3MHz, which is consistent with white light response. However, yellow
LI et al.: HIGH BANDWIDTH VLCs BASED ON A POST-EQUALIZATION CIRCUIT
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TABLE I VLC C OMMUNICATION S YSTEM PARAMETERS
Fig. 5. EOE system frequency response of different cases (with PEQ1, PEQ1 and Amplifier, PEQ1, Amplifier and PEQ2, normalized response of PEQ1).
Fig. 4. EOE system frequency response of white light, blue component and yellow component emitted by a white LED.
component response decreases rapidly as frequency increases. The experiment proves that the slow responding phosphor component response limits the bandwidth of white LED. In addition, Suitable values of RF and CF chosen can make the system response flat in the middle frequency region. Blue-filter can filter out the slow-responding phosphor component of emission, leaving the faster directly modulated blue LED emission. For the low-frequency region with a bluefilter, the light intensity of signal detected by the photodiode decreases and photocurrent of detector reduces, then the lowfrequency amplitude decline with 0∼13 dB as shown in Fig. 4. For the high-frequency region with a blue-filter, the main reason why the high-frequency amplitude of the signal can be enhanced with 0–2 dB is that sensitivity (Sλ ) of photodiode is improved due to the nonlinear characteristic of photodiode [9]. For the middle range frequency range, the light intensity and nonlinear characteristic of photodiode work together, and this is the reason why the blue component response between 45 MHz and 120 MHz is very flat with the blue-filter. Fig. 5 presents the frequency response of different cases. PEQ1 decays the low-frequency signal magnitude (0∼10 dB) Then a system bandwidth of 151 MHz could be achieved. However, the signal magnitude is very small approximately −44 dB (about 30 mV). We took a wideband amplifier to amplify the signal from the pre-amplifier. The signal from the amplifier is not as flat as what the theory indicates since impedance mismatching and other complex reasons. PEQ2 can solve the problem perfectly. Fig. 5 shows us that a bandwidth of 135 MHz of visible light communication system
Fig. 6. EOE system frequency response using post-equalization with different value of C2 ( network analyzer 0pF, 5pF, 10pF, 15pF, 20pF, and 27pF; oscilloscope 5pF, 10pF and 15pF).
was achieved with the PEQ1, amplifier and PEQ2. The signal magnitude was enhanced from −44 dB to −24 dB. As shown in Fig. 6, AEQ can compensate for the magnitude of high-frequency and keep the amplitude of low-frequency unchanged obviously. When the value of C2 is 5 pF, the 3-dB cutoff frequency of receiver is 143MHz; whereas when it becomes 10 pF, the 3-dB cutoff frequency of receiver is 148 MHz; and when the value of C2 is 15 pF, a system bandwidth of 151 MHz could be achieved; and when the value of C2 is 20 pF, 27pF or even larger, system bandwidth improves very little, but the response gain peak becomes too large (>3dB than low reference frequency), which severely affect the EOE system response flatness. Thus the value of C2 is proposed to be chosen between 0 pF to 15 pF. To verify the bandwidth of VLC system measured by network analyzer, we use analog signal generator and oscilloscope to measure the frequency response of receiver. The measurements took the same condition except that the output signal power of network analyzer is 0 dBm (modulation index is about 7%) and signal generator is 17 dBm (modulation index is about 30%). In Fig. 6, we can see that the bandwidth of the VLC system measured by oscilloscope is 152 MHz when C2 = 15 pF. In Fig. 7, system frequency response in terms of SNR is reported. On the whole, EOE system SNR is higher
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was below 2 × 10−3 , within the FEC limit. Fig. 9 shows the BER versus measured received blue-light optical power. We can read smallest received blue optical power at different data rate with BER below 2 × 10−3 . Higher data rate means higher received optical power to overcome the SNR penalty due to post-equalization. V. C ONCLUSION
Fig. 7.
EOE system SNR versus frequency.
Fig. 8. BER versus transmission data rate (white light and blue component without post-equalizer and equalized cases). BER below 10−9 is truncated to this threshold. Inset: Eye diagram at 150Mb/s and 200Mb/s.
The letter has described a VLC post-equalization circuit to improve the bandwidth of phosphor white-LED-based VLC system. With the blue-filtering and the post-equalization circuit, the VLC system bandwidth achieved is 151 MHz, which is 50 times than white light response. The OOK-NRZ VLC link can be operated at 340Mbit/s transmission data rate, the resulting BER was below 2 × 10−3 , within the FEC limit. Although the post-equalization circuit can extend the system bandwidth, the wireless communication distance of VLC link was limited. By using two convex lenses, the communication distance has been increased to 43cm. Longer distance could be achieved if more identical LEDs were used. The postequalization circuit just changes the signal magnitude without changing signal frequency information. To the best of our knowledge, VLC post-equalization circuit can be applied to OOK, PPM and other advanced modulation formats. Spectrally efficient modulation techniques combining post-equalization circuit was also suggested to improve the VLC link transmission data rate. ACKNOWLEDGMENT The authors thank Shaoxin Zhu, Zanyun Zhang, Junqing Guo, Xu Zhang, and Yuan Wang for valuable comments in the experiment. R EFERENCES
Fig. 9.
BER versus received blue optical power at different data rate.
than 40dB. The post-equalization circuit adds noise to the system mainly in low frequency region. Manchester coding for VLC system can be used to mitigate the optical noise [10]. Furthermore, BER measurements as a function of data rate and received blue optical power were performed. A pseudorandom binary sequence (PRBS)-15 (215 − 1) OOK-NRZ data stream with a peak-to-peak voltage swing of 2.5 V is used to modulate the emitted light. Fig. 8 shows the measured data rate achieved when detecting white light and blue component are approximately 9 and 50Mbit/s. Using post-equalization circuit, 340Mbit/s data rate could be achieved , the resulting BER
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