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We proposed a cascaded amplitude equalizer used for high speed visible light communications (VLC) system. With the cascaded pre-equalization circuit, the −3dB bandwidth of VLC system can be extended from 17MHz to 366MHz using a commercially available phosphorescent white LED, a blue filter and a differential outputs PIN receiver. The data rate is 1.60Gbit/s exploiting 16QAM-OFDM with 400MHz modulation bandwidth over 1m free-space transmission under pre-forward error correction (pre-FEC) limit of 3.8 × 10−3. To our knowledge, this is the highest data rate ever achieved by using a commercially available phosphorescent white LED in VLC system.
© 2015 Optical Society of America
1. Introduction
Light-emitting diodes (LEDs) have been considered to be the most promising device for next generation illumination. LEDs have many advantages, such as high efficiency, long life, low cost, low power consumption and security. Using the LEDs for visible light communications (VLC), VLC has been an area of growing interest because of the numerous advantages, including free licensed bandwidth, no electromagnetic interference and security.
The limited bandwidth, usually from about 3MHz [1] to 20MHz [2] of the commercial LED, is the main challenge of achieving high speed VLC transmission. Although LEDs have low modulation bandwidth, many approaches have been used to extend the −3dB modulation bandwidth of LED and to increase the data rate, such as blue light filter, pre-equalization and post-equalization. Pre-equalization can be used to enhanced the relative power of high frequency and attenuate the low frequency. In 2008, 80 Mbit/s OOK-NRZ data rate using multiple-resonant equalization has been realized with a phosphorescent white LED operating over a very short distance of 10 cm and the pre-equalized −3dB bandwidth is 45MHz [3]. In 2012, Nobuhiro Fujimoto and Hikari Mochizuki perform 614 Mbit/s OOK-based transmission using the duobinary technique with a red chip of a RGB LED by adopting a pre-equalized and a post-equalized circuit at the distance of 40cm and the −3dB bandwidths is more than 150MHz [4]. They have improved the data rate to 662 Mbit/s by a blue chip of a RGB LED at the distance of 15 cm using NRZ-OOK with −3dB bandwidths of 180MHz [5]. In 2014, a 550 Mbit/s NRZ-OOK VLC transmission and −3dB bandwidths of 233MHz have been achieved by a commercial phosphorescent white light LED using analog pre-emphasis circuit and post-equalization circuit at the distance of 60 cm [6].
High order quadrature amplitude modulation (QAM) with spectrally efficient modulation techniques such as orthogonal frequency division multiplexing (OFDM) or discrete multitone (DMT) and single carrier frequency domain equalization (SC-FDE), can be used to greatly increase the data rate. Using high order QAM and wavelength division multiplexing (WDM), the data rates of 575 Mbit/s and 2.5 Gbit/s with OFDM [7,8], 3.4 Gbit/s and 5.6 Gbit/s (RGBA LED) [2,9] with DMT and 4.22 Gbit/s with SC-FDE [10] has been realized based on RGB LED. Compared with the triple-chip RGB type LEDs, the phosphor-based white LEDs are more attractive for general illumination due to their lower complexity and lower cost. Y. Wang and et al reported a 225 Mb/s data transmission with OFDM using a commercially available phosphorescent white LED [7]. In [11], a data rate of 1 Gbit/s VLC link has been realized using a phosphorescent white LED by optimized DMT modulation and adaptive bit and power loading algorithms at the distance of 10cm.
In this paper, we proposed a novel cascaded bridged-T amplitude equalizer circuit used in high speed VLC system. Using the proposed amplitude equalizer, the −3dB bandwidth of VLC system can be extended from 17 MHz to 366 MHz (starting frequency at 10 MHz) with a commercially available phosphorescent white LED, a blue filter and a differential outputs low-cost PIN receiver. Based on the system above, we successfully realized 1.6 Gbit/s VLC transmission using 16QAM-OFDM over 1m free-space transmission with 400MHz modulation bandwidth and 0.7Gbit/s employing quadrature phase shift keying (QPSK) OFDM over 2m with 350MHz modulation bandwidth. The bit error rate (BER) results are under pre-forward error correction limit of 3.8 × 10−3. The BER performance can be improved by 1 order of magnitude when compared with the system without using the equalizer. To our knowledge, this is the highest data rate ever achieved by using a commercially available phosphorescent white LED in VLC systems reported.
2. Proposed amplitude equalizer in VLC system
In Fig. 1, we present the proposed cascaded constant-resistance bridged-T amplitude equalizer used in VLC system. The principle of the single equalizer Equalizer1 is shown in [12], Z11 is the equivalent impedance of R1, C1 and L1, and Z22 is the equivalent impedance of R4, C2 and L2, while R2 is equal to R3 and both are equal to R0, and ZL is the load of the circuit.
In constant resistance equalizer, the product of Z11Z22 must be a constant chosen to be R02, where R0 is equal to the desired iterative impedance of the network [13] and that is 50Ω for equalizer between the arbitrary waveform generator (AWG, 50Ω output impedance) and electrical amplifier (EA, 50Ω input impedance) used in VLC system.
Z11, Z22 can be respectively expressed as
where ω is the angular frequency. To analyze and realize easily, we set L1 = L2, C1 = C2. When the output impedance of the signal generator ZS and the impedance of load ZL are both equal to R0, the forward transmission gain through the network S21Equalizer1 of single equalizer isFor the two cascaded equalizer consisting of two single equalizers shown in Fig. 1, the forward transmission gain through the network S21 is
where L1 = L2, C1 = C2, L3 = L4 and C3 = C4. The lower frequency response is decided by R4 and R8.When ZS = ZL = R0, the forward transmission gain through the network S21single of whole equalizer can be expressed aswhere Vin is the output voltage of the signal generator, Vout is the voltage of load and Hequalizer is the frequency response of the cascaded equalizer.3. Bandwidth measurements
The block diagram of frequency response measurement for VLC system is shown in Fig. 2. In this scheme, the driving signal from Port1 of vector network analyzer (VNA) is equalized by the proposed amplitude equalizer. After amplified by EA1 (Minicircuits, 25-dB gain), the resulting signal coupled with direct current (DC) by bias Tee is applied to a phosphorescent white LED. The transmitted signal is carried by a commercially available phosphorescent white light LED (OSRAM, LCWCRDP.EC-KULQ-5L7N-1, luminous flux about 120lm at 350mA). At the receiver, the lens (55-mm diameter and 18 mm focus length) is utilized to capture a high proportion of light in order to improve the signal to noise ratio (SNR) of the system. The distance between the LED and receiver is 100cm. A blue filter is placed in front of the PIN photodiode to filter out the slow-responding phosphor component. The blue filter has very high transmittance average 97.5% in the blue signal range of 430–485 nm and wide stopband from 500 to1050 nm [14]. The commercial PIN photodiode (HAMAMATSU S10784, 0.45 A/W sensitivity with −3dB bandwidth of 300MHz at 660 nm) is used to convert optical signal to electrical signal. Then, the photo current is amplified by a differential outputs trans-impedance amplifier (TIA) designed with MAX3665 (gain of 8KΩ and −3dB bandwidth of 470MHz). The differential output signal has many advantages, such as reduced sensitivity to system noise and generation of transient noise, improved system stability and linearity, and increased transmission speed. The differential outputs of TIA are amplified by EA2 and EA3 (Minicircuits, 25-dB gain), respectively. The resulting signals of the EA2 and EA3 are connected separately with Port2 of VNA as the input signals.
The parameters used for proposed bridged-T amplitude equalizer are R1 = 249 Ω, R2 = R3 = R6 = R7 = 49.9 Ω, R4 = 10 Ω, R5 = 1000 Ω, R8 = 2.5 Ω, C1 = C2 = C3 = C4 = 8.5 pF, L1 = L2 = L3 = L4 = 22 nH. The S-parameters are measured by the VNA (Agilent, N5230C) operating from 10MHz to 40GHz and the output power of the VNA is fixed at −25dBm. In Fig. 3, we show the measured forward transmission gains of the Equalizer1, Equalizer2 and the cascaded equalizer circuits, ranging from the lowest frequency 10MHz to the highest 352MHz with about 13.7dB, 19.4dB and 32.9dB dynamic magnitude, respectively.
In Figs. 4(a) and 4(b), the forward transmission gains are measured in different cases of VLC systems with the differential outputs, including output + and output-. VLC system without blue filter and equalizer shows that the −3dB bandwidth is 17MHz from 10MHz to 27MHz of output + and 21MHz from 10MHz to 31MHz of output- respectively, and can be extended to 31MHz with blue filter filtering out the slow-responding phosphorescent light in both cases. The bandwidths of the VLC system with blue filter and Equalizer1, Equalizer2 and cascaded equalizer are respectively improved to 38MHz, 79MHz and 366MHz of output + , while 45MHz, 135MHz and 207MHz of output-. Using the cascaded equalizer, the bandwidth can be greatly improved when compared with the single equalizer. The different bandwidth results of output + and output- show that the differential outputs are not strictly the same, mainly affected by the TIA.
Fig. 4 Measured forward transmission gains (a) output + and (b) output- of the differential outputs in VLC system.
4. Data transmission experiment and results using OFDM
The experimental setup of VLC system is shown in Fig. 5 and only the cascaded equalizer is used in the data transmission experiment because of its wider bandwidth. In the experiment, the OFDM signals are generated by a computer using the Matlab software. The OFDM transmitter consists of QAM modulation, serial-to-parallel conversion, inverse fast Fourier transform (IFFT), cyclic prefix (CP) insertion, parallel-to-serial conversion, up-sample and up-conversion. Other detailed parameters of generated OFDM signals include: subcarrier number = 128, up-sampling factor = 4. Then, the OFDM signal is supplied to an AWG (Tektronix AWG710), which converts it to an analog signal as the input of the VLC system. At the receiver, the optical signal is detected by the PIN detector. After amplified by TIA and EAs, the final differential output signals are captured by channel1 and channel2 of a real-time digital oscilloscope (OSC, Agilent 54855A). Then, an offline digital signal processing program is used to demodulate the OFDM signal. First, differential signal channel1 (CH1) subtracts channel2 (CH2) after synchronizations. Then, the resulted signal is demodulated after the following steps, including down-conversion, fast Fourier transform (FFT), post-equalization and QAM decoding. At last, from the demodulated OFDM signal, we calculate the BER. The total data rate includes the 3% CP, 5% training sequence and 7% forward error correction (FEC) overhead.
Figure 6 shows the measured electrical spectra for original signal of AWG output and the received signal of Channel1 after 50cm free space transmission, using 400MHz bandwidth 16QAM-OFDM. The bias voltage is 3.2V. The spectra of original signal of −34.0dBm and −18.4dBm are shown in Figs. 6(a) and 6(d), respectively. Figures 6(b) and 6(c) show the spectra of −34dBm without and with equalizer, respectively, while Figs. 6(e) and 6(f) for −18.4dBm. Because of the nonlinear amplification, the signal without pre-equalizer will be saturated and distorted and the output power of AWG is below −34.0dBm. With the pre-equalizer, the output power can be increased to about −18.4dBm. Compared Figs. 6(b) and 6(f), the power of high frequency part is enhanced using the pre-equalizer and the nonlinear distortion of low frequency part is also limited.
Fig. 6 Measured electrical spectra: output of AWG (a) @-34.0dBm and (d) @18.4dBm; after VLC system (b) without equalizer @-34.0dBm; (c) with equalizer @-34.0dBm; (e) without equalizer @18.4dBm and (f) with equalizer @18.4dBm.
In the data transmission experiment, we first obtained the optimal working condition by studying the influence of different input power to VLC system and bias voltages of the phosphorescent white LED with and without pre-equalizer at the distance of 50cm. The BER results are measured based on 16QAM-OFDM with 300MHz modulation bandwidth. In Fig. 7, we show the measured BER results versus different input power from −40dBm to −17dBm with pre-equalizer and without equalizer. The electrical power is measured after the output of the AWG by spectrum analyzer HP 8562A and controlled by varying the signal driving peak-to-peak voltage (Vpp) of AWG. The bias voltages of the phosphorescent white LED is fixed at 3.3V. The optimal input power −18.4dBm and −34.0dBm of system with pre-equalizer and without pre-equalizer are obtained. The BER results are respectively 1.59 × 10−2 and 1.68 × 10−2 of channel1 and channel2 at −34.0dBm without equalizer, while 1.79 × 10−3 and 1.74 × 10−3 for system with equalizer at −18.4dBm. The BER performances of the single outputs are very similar to each other. Processing the differential signals by using channel1 to subtract channel2 after synchronizing, the BER results are respectively 4.24 × 10−3 and 6.64 × 10−4 without pre-equalizer and with pre-equalizer. When the BER performance of the single outputs deteriorates, the improvement of processing the differential signals together will be limited, especially when the BER results are very bad. The constellations of different cases are inserted in Fig. 7. The system with equalizer has better BER performance when compared with the optimal BER performance. Using the equalizer, the BER results of VLC system can be greatly reduced by an order of magnitude. The optimal input power to VLC system with pre-equalizer is higher than the one without pre-equalizer, so the systems with equalizer need more transmission power to achieve the best working condition. In Fig. 8, we show the BER results versus bias voltage from 2.9V to 3.5V with equalizer and without equalizer, and the input power are fixed at −18.4dBm and −34.0dBm, respectively. The bias voltage of white LED at 3.2V is the optimal point of system with and without pre-equalizer systems and the corresponding BER results are respectively 5.27 × 10−4 and 3.94 × 10−3 with constellations inserted in Fig. 8.
Fig. 7 Measured BER results versus different input power to VLC system with pre-equalizer and without equalizer.
Fig. 8 Measured BER results versus bias current with pre-equalizer @-18.4dBm and without equalizer @-34.0dBm.
At the optimal input power −18.4dBm and bias voltage 3.2V, we measured the BER results versus different bandwidths from 200MHz to 450MHz with 50MHz step, different modulation orders from QPSK to 32QAM, and different distances from 50cm to 200cm with 50cm step using the pre-equalizer, and distances at 100cm and 200cm are presented in Fig. 9. The total data rate can be calculated as R = B*log2(M), where B is the modulation bandwidth of the system and M is the constellation size of QAM. From Fig. 9, the BER performance degrades with the increase of modulation bandwidth, modulation order and transmission distance. The high frequency after about 300MHz fades fast, resulting to lower SNR in this part. Higher modulation orders require higher SNR to achieve the FEC limit. As the transmission distance becomes longer, the received optical power will decrease, especially after 100 cm. In Fig. 10, we show the data rate as a function of the distance from 50 cm to 200 cm and the corresponding illumination levels are also plotted in the secondary X axis. The illumination levels are measured after the blue filter, ranging (approximately) from 143 lx to 12.7 lx. The data rates are respectively 1.6 Gbit/s (16QAM, BER: 3.27x10−3), 1.6 Gbit/s (16QAM, BER: 3.62x10−3), 1.05 Gbit/s (8QAM, BER: 3.32x10−3) and 0.7 Gbit/s (QPSK, BER: 1.93x10−3) at the distance of 50 cm, 100 cm, 150 cm and 200 cm with BER under the pre-FEC limit of 3.8x10−3.
5. Conclusion
In this paper, we proposed a cascaded bridged-T amplitude pre-equalizer used in high speed VLC system. Using the proposed equalizer and a blue filter, the −3dB bandwidth of VLC system can be increased from 17MHz to 366MHz (starting frequency at 10 MHz) with a single commercial phosphorescent white light LED and a differential outputs low-cost PIN receiver. The data rate is 1.60 Gbit/s by 16QAM-OFDM of 400MHz modulation bandwidth and the BER is under the pre-FEC limit of 3.8 × 10−3 after 1m free-space transmission. With longer distance of 2m, 0.7 Gbit/s data rate can be achieved using QPSK-OFDM signal of 350MHz modulation bandwidth. The BER performance can be improved by 1 order of magnitude compared with the system without using the cascaded equalizer. To our best knowledge, this is the highest data rate ever achieved by employing a single commercially available phosphor-based white LED in VLC systems reported.
Acknowledgments
This work was partially supported by the NNSF of China (No. 61177071) and NHTRDP of China (2013AA013603).
References and links
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