April 1, 2014 / Vol. 39, No. 7 / OPTICS LETTERS
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Gigabit polarization division multiplexing in visible light communication Yuanquan Wang, Chao Yang, Yiguang Wang, and Nan Chi* Fudan University, Shanghai 200433, China *Corresponding author: [email protected] Received November 28, 2013; revised January 13, 2014; accepted February 14, 2014; posted February 14, 2014 (Doc. ID 202241); published March 20, 2014 In this Letter, polarization division multiplexing is proposed and experimentally demonstrated for the first time that we know of, in visible light communication systems based on incoherent light emitting diodes and two orthogonal groups of linear polarizers. Spectrally efficient 16-ary quadrature amplitude modulation Nyquist single carrier frequency domain equalization is employed to obtain a maximum spectral efficiency. We achieve an aggregate data rate of 1 Gb∕s, with bit error rate results for two polarization directions both below the 7% pre-forward-errorcorrection threshold of 3.8 × 10−3 after 80 cm free-space transmission. Moreover, the cross talk between x and y polarization is also discussed and analyzed. © 2014 Optical Society of America OCIS codes: (060.2605) Free-space optical communication; (060.4510) Optical communications; (230.3670) Light-emitting diodes. http://dx.doi.org/10.1364/OL.39.001823
Recently, there has been constantly gaining interest in visible light communication (VLC) motivated by the dramatic development of LED technologies and increasingly scarce spectrum resources [1–7]. Widespread use, cost effective, high brightness, larger bandwidth compared with other typical radio frequency (RF) based devices make it the most promising candidate for simultaneous illumination and communications, especially in some specific areas like hospitals, aircraft, underwater, and high security requirement environments. However, the relatively low intrinsic modulation bandwidth of commercially available LED is the main technical challenge in VLC system. Many research efforts have been dedicated to overcoming this limitation, such as digital signal processing [2], equalizations [3], wavelength division multiplexing (WDM) [4], and multiple inputs multiple outputs (MIMO) [5–7]. But the transmission data rate is rapidly reaching its increasing limit of commercial LED due to the extensive use of a combination of the aforementioned techniques. Thus, looking for a new growth point is critical to further increase the system capacity in the limited bandwidth. In this Letter, we propose and experimentally demonstrate a VLC system based on polarization division multiplexing (PDM). The polarization characteristic of visible light [8] brings another degree of freedom that can multiply the transmission capacity, compared to the aforementioned works. Two orthogonal groups of polarizers and incoherent red-green-blue (RGB) LEDs are implemented to realize PDM. Due to the limited experimental conditions, we just utilize the red LED chip of the RGB LED. Additionally, in this demonstration, spectrally efficient 16-ary quadrature amplitude modulation (16QAM) Nyquist single carrier frequency domain equalization (SC-FDE) [9] is employed to obtain a high spectral efficiency (SE). We successfully achieved an aggregate data rate of 1 Gb∕s. The measured bit error rates (BERs) for two polarizations are both below 7% preforward error correction (preFEC) limit of 3.8 × 10−3 after an 80 cm free-space transmission. Moreover, the cross talk between x and y polarizations is also discussed in this Letter. To the best 0146-9592/14/071823-04$15.00/0
of our knowledge, this is the first demonstration to realize PDM signal delivery in VLC. Figure 1 shows a block diagram of this proposed PDM Nyquist SC-FDE VLC system. As the lights emitting from incoherent LEDs are natural lights all polarization directions are included and they can be decomposed as two orthogonal bases of x polarization and y polarization, respectively. A linear x-polarizer that can only allow components in the x polarization direction passing through is implemented at transmitter1 (TX1); meanwhile, a linear y polarizer that can only allow components in the y polarization direction passing through is employed at TX2, as shown in the red box in Fig. 1. After passing through xpolarizer1 and y-polarizer1, we can obtain linearly polarized light, but they will be mixed up after free-space transmission. At the receiver (RX), two corresponding polarizers should be implemented to filter out the unwanted polarized lights, thus obtaining the transmitting signals. Assuming the offset angles between x-polarizer1 and x-polarizer2, x-polarizer1 and y-polarizer2, y-polarizer1 and x-polarizer2, and y-polarizer1 and y-polarizer2 are α11 , α12 , α21 and α22 , respectively, according to the Malus Law, the received optical intensity of each avalanche photodiode (APD) can be expressed as
Y1 Y2
H 1 2
I1 I2
N
cos2 α11 cos2 α21
cos2 α12 cos2 α22
I1 I2
N;
(1)
of which Y 1 and Y 2 represent the received optical intensity of RX1 and RX2; meanwhile, I 1 and I 2 represent the emitted optical intensity from TX1 and TX2. H and N denote the channel matrix and noise, respectively. In this theoretical model, we just consider the distortions induced by PDM. The transmitted signals can be recovered once the channel matrix satisfies the following equation: cos2 α11 cos2 α22 ≠ cos2 α12 cos2 α21 : © 2014 Optical Society of America
(2)
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OPTICS LETTERS / Vol. 39, No. 7 / April 1, 2014
Fig. 1. Block diagram and experimental setup of the proposed PDM Nyquist SC-FDE VLC system. (AWG, arbitrary waveform generator; P/S, parallel to serial; EA, electrical amplifier; LPF, low-pass filter; OSC, real-time oscilloscope; CP, cyclic prefix.)
The received signals can be recovered by zero forcing (ZF) algorithms neglecting the noise,
Y1 Y2
H −1
I1 : I2
(3)
In this Letter, as a proof of concept, the offset angle is set at α11 α22 0°, and α12 α21 90°. Therefore, the channel matrix can be simplified as a diagonal matrix, so the received optical intensity can be denoted as:
Y1 Y2
1∕2 0 0 1∕2
I1 I2
N:
(4)
Neglecting the noise term, N, the only difference between transmitted signals and received signals is a constant, so no extra demultiplexing algorithm is needed. The light illuminance versus offset angle between xpolarizer1 and x-polarizer2 are depicted in Fig. 2. In this measurement, x-polarizer1 is fixed while x-polarizer2 is rotated around the axis. At the point of maximum illuminance (44 lx), the offset angle can be regarded as 0°, while at the minimum illuminance (0.4 lx), the offset
illuminance(lx)
50 40 30 20 10 0 0
15
30
45
60
75
90
offset angle(degree) Fig. 2. Light illumination versus offset angle between x-polarizer1 and x-polarizer2.
angle can be considered as 90°, i.e., y polarization direction. From this figure, we can find the cross talk between x polarization and y polarization is lower than 1%. Additionally, the light intensity after x-polarizer1 will be reduced to 42% of its original intensity. The experimental setup is also shown in Fig. 1. The random binary data is generated in MATLAB and would be first split into two parallel streams, one for each transmitter (TX) channel. In each channel, the bit stream is mapped into 16-ary quadrature amplitude modulation (16-QAM) format and then the training sequences (TSs) are inserted into the signals. After adding cyclic prefix (CP), upsampling by a factor of 10, and filtering by a rectangular filter, upconversions are accomplished by multiplying the real part and image part of the signals with the sine function and cosine function, respectively, and then added. The SC-FDE waveform is then loaded into an arbitrary-waveform generator (Tektronix 7122C). The output of this generator is amplified by an electrical amplifier (EA) (Minicircuits, 25 dB gain), and combined with direct current (DC) bias via bias tee, and the resulting waveform is then applied to a red chip of the RGB LED, acting as an optical transmitter. Passing through x-/y-polarizer1, free-space, lens (50 mm diameter, 18 mm focus length), and x-/ypolarizer2, the signals are detected by two APDs. Then, the received signals from each of the 2 RXs are routed to a high speed oscilloscope (Lecroy) and are acquired for further digital signal processing (DSP). After synchronization, downconverting to baseband, and removing CP, the received data streams are processed at the frequency domain. The final streams are then passed through the QAM decoder to recover the original binary stream. The photographs of the experimental setup are depicted in Fig. 3. Two commercially available RGB LEDs (Cree, PLCC) are used as the TXs and two APDs (Hamamatsu, 0.5 A∕W sensitivity at 800 nm) are used as the receivers (RXs). The FFT size is 128, and the CP length is 1∕16. The sampling rate of the AWG and OSC is 1.25 GS∕s and 2.5 GS∕s, respectively. Thus, the available
April 1, 2014 / Vol. 39, No. 7 / OPTICS LETTERS
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0.014 0.012 0.010 0.008 0.006 0.004 0.002 0.000
Fig. 3. Photograph of PDM platform (a) transmitters and (b) receivers.
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of each polarization is 500 Mb∕s, thus the overall data rate is 1 Gb∕s. The measured results are shown in Fig. 6. All BERs are under the preFEC limit of 3.8 × 10−3 after 80 cm transmission. And the BER performance will be slightly degraded when two polarization directions transmitted at the same time. In the case of both polarizations transmission, the electrical spectra of two RXs are also measured and illustrated in Fig. 7. From this figure, we can find that the power of noise floor is about −60 dBm, and gains of low frequency part and high frequency part are about 35 and 15 dB, respectively. Additionally, the power difference between frequencies can be preequalized using the method mentioned in [4], and the performance can be further enhanced.
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Fig. 5. Measured amplitude of channel matrix at frequency domain of (a) H11, (b) H12, (c) H21, and (d) H22.
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bandwidth is 125 MHz, ranging from 7.8125 to 132.8125 MHz. The distance between the two transmitters is 5 cm. In the VLC system, the LED shows significant nonlinearity due to the nonlinear characteristics of the current-voltage and output power–current curves. Additionally, the LED has a threshold value of about 2 V in this demonstration. The peak to peak voltage after the electrical amplifier is about 900 mV. When the lowest signal is below the threshold value, clipping will be introduced; meanwhile, it will work at its saturation area when the signal is too large, so it is necessary to render the LED work at the quasi-linear region. The BER performances versus bias voltages at 70 and 80 cm are measured as shown in Fig. 4. The bias voltage is varied from 2.1 to 2.7 V, with a step of 0.1 V. From the measured results at both distances, the optimal working points are at 2.5 V. The constellations of 2.2 and 2.5 V at 70 cm are also inserted in Fig. 4. It can be easily seen that the constellation of 2.5 V is much clearer than that of 2.2 V. The channel matrix of this system is measured via the time-multiplexed training sequences mentioned in [10]. The experimental results are shown in Fig. 5. The channel matrix elements H ij i 1; 2; j 1; 2 represent the gain from the jth transmitter to ith receiver. Between these four elements, H 11 and H 22 denote the signal gain of the x–x polarization link and the y–y polarization link, while H 12 and H 21 denote the cross talk between these two polarization branches. The cross talk can be neglected, as it is about 100 times smaller than the signal gain. Next, we measured the BER performance of two receivers in the case of only x-/y-polarization transmission and both polarizations transmission. The data rate
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Fig. 6. Measured BER performance versus different transmission distances in the case of only one polarization transmission and both polarization transmissions (a) RX1 and (b) RX2.
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OPTICS LETTERS / Vol. 39, No. 7 / April 1, 2014 0.015
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co-orthogonal polarizers. The cross talk between two orthogonal polarizers is demonstrated to be very small, and the data rate can be doubled at a sacrifice of a 10 cm shorter transmission distance by using polarizers. Meanwhile, the PDM scheme utilizing two mismatched polarizers are also theoretically investigated. The data rate can be further improved by adopting WDM and a larger bandwidth APD. Additionally, a 3 × 3 or 4 × 4 PDM scheme employing more polarizers can also be our future research direction.
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Distance(cm) Fig. 8. Measured BER performance versus different transmission distance, with and without polarizers.
The performance degradation after using polarizers in a single input single output channel is also investigated. The measured BER results as a function of distance with and without polarizers are depicted in Fig. 8. From this figure we can find that, after using polarizers, the transmission distance will be about 10 cm shorter at the same BER level; in other words, the sensitivity of the RX will be reduced. But the transmission distance can become longer by employing multiple LEDs instead of only one chip in this demonstration. The constellations of these two cases are also inserted in Fig. 8. If the polarizers are removed it will become a traditional space diversity MIMO scheme, demonstrated in [5–7]. The cross talk will become very serious and, if no MIMO demultiplexing is utilized, the signals cannot be recovered. Figure 9 shows the received constellations of two RXs without using polarizers. In conclusion, we have experimentally demonstrated 1 Gb∕s PDM in VLC system for the first time by using spectrally efficient Nyquist SC-FDE and two groups of
This work was partially supported by the NNSF of China (Nos. 61250018 and 61177071), the Key Program of Shanghai Science and Technology Association (12dz1143000). References 1. M. Biagi, T. Borogovac, and T. D. C. Little, J. Lightwave Technol. 31, 3676 (2013). 2. Y. Wang, N. Chi, Y. Wang, R. Li, X. Huang, C. Yang, and Z. Zhang, Opt. Express 21, 27558 (2013). 3. H. Le Minh, D. C. O’Brien, G. Faulkner, L. Zeng, K. Lee, D. Jung, Y. Oh, and E. T. Won, IEEE Photon. Technol. Lett. 21, 1063 (2009). 4. Y. Wang, Y. Wang, N. Chi, J. Yu, and H. Shang, Opt. Express 21, 1203 (2013). 5. G. Ntogari, T. Kamalakis, and T. Sphicopoulos, IEEE J. Sel. Areas Commun. 27, 1545 (2009). 6. E. Baccarelli, M. Biagi, and C. Pelizzoni, IEEE Trans. Signal Process. 53, 2335 (2005). 7. W. O. Popoola, E. Poves, and H. Haas, IEEE Trans. Commun. 61, 1968 (2013). 8. B. Schnabel, E. Kley, and F. Wyrowski, Opt. Eng. 38, 220 (1999). 9. D. Falconer, S. L. Ariyavisitakul, A. Benyamin-Seeyar, and B. Eidson, IEEE Commun. Mag. 40(4) 58 (2002). 10. F. Li, Z. Cao, X. Z. Dong, and L. Chen, J. Lightwave Technol. 31, 2394 (2013).
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Gigabit polarization division multiplexing in visible light communication Yuanquan Wang, Chao Yang, Yiguang Wang, and Nan Chi* Fudan University, Shanghai 200433, China *Corresponding author: [email protected] Received November 28, 2013; revised January 13, 2014; accepted February 14, 2014; posted February 14, 2014 (Doc. ID 202241); published March 20, 2014 In this Letter, polarization division multiplexing is proposed and experimentally demonstrated for the first time that we know of, in visible light communication systems based on incoherent light emitting diodes and two orthogonal groups of linear polarizers. Spectrally efficient 16-ary quadrature amplitude modulation Nyquist single carrier frequency domain equalization is employed to obtain a maximum spectral efficiency. We achieve an aggregate data rate of 1 Gb∕s, with bit error rate results for two polarization directions both below the 7% pre-forward-errorcorrection threshold of 3.8 × 10−3 after 80 cm free-space transmission. Moreover, the cross talk between x and y polarization is also discussed and analyzed. © 2014 Optical Society of America OCIS codes: (060.2605) Free-space optical communication; (060.4510) Optical communications; (230.3670) Light-emitting diodes. http://dx.doi.org/10.1364/OL.39.001823
Recently, there has been constantly gaining interest in visible light communication (VLC) motivated by the dramatic development of LED technologies and increasingly scarce spectrum resources [1–7]. Widespread use, cost effective, high brightness, larger bandwidth compared with other typical radio frequency (RF) based devices make it the most promising candidate for simultaneous illumination and communications, especially in some specific areas like hospitals, aircraft, underwater, and high security requirement environments. However, the relatively low intrinsic modulation bandwidth of commercially available LED is the main technical challenge in VLC system. Many research efforts have been dedicated to overcoming this limitation, such as digital signal processing [2], equalizations [3], wavelength division multiplexing (WDM) [4], and multiple inputs multiple outputs (MIMO) [5–7]. But the transmission data rate is rapidly reaching its increasing limit of commercial LED due to the extensive use of a combination of the aforementioned techniques. Thus, looking for a new growth point is critical to further increase the system capacity in the limited bandwidth. In this Letter, we propose and experimentally demonstrate a VLC system based on polarization division multiplexing (PDM). The polarization characteristic of visible light [8] brings another degree of freedom that can multiply the transmission capacity, compared to the aforementioned works. Two orthogonal groups of polarizers and incoherent red-green-blue (RGB) LEDs are implemented to realize PDM. Due to the limited experimental conditions, we just utilize the red LED chip of the RGB LED. Additionally, in this demonstration, spectrally efficient 16-ary quadrature amplitude modulation (16QAM) Nyquist single carrier frequency domain equalization (SC-FDE) [9] is employed to obtain a high spectral efficiency (SE). We successfully achieved an aggregate data rate of 1 Gb∕s. The measured bit error rates (BERs) for two polarizations are both below 7% preforward error correction (preFEC) limit of 3.8 × 10−3 after an 80 cm free-space transmission. Moreover, the cross talk between x and y polarizations is also discussed in this Letter. To the best 0146-9592/14/071823-04$15.00/0
of our knowledge, this is the first demonstration to realize PDM signal delivery in VLC. Figure 1 shows a block diagram of this proposed PDM Nyquist SC-FDE VLC system. As the lights emitting from incoherent LEDs are natural lights all polarization directions are included and they can be decomposed as two orthogonal bases of x polarization and y polarization, respectively. A linear x-polarizer that can only allow components in the x polarization direction passing through is implemented at transmitter1 (TX1); meanwhile, a linear y polarizer that can only allow components in the y polarization direction passing through is employed at TX2, as shown in the red box in Fig. 1. After passing through xpolarizer1 and y-polarizer1, we can obtain linearly polarized light, but they will be mixed up after free-space transmission. At the receiver (RX), two corresponding polarizers should be implemented to filter out the unwanted polarized lights, thus obtaining the transmitting signals. Assuming the offset angles between x-polarizer1 and x-polarizer2, x-polarizer1 and y-polarizer2, y-polarizer1 and x-polarizer2, and y-polarizer1 and y-polarizer2 are α11 , α12 , α21 and α22 , respectively, according to the Malus Law, the received optical intensity of each avalanche photodiode (APD) can be expressed as
Y1 Y2
H 1 2
I1 I2
N
cos2 α11 cos2 α21
cos2 α12 cos2 α22
I1 I2
N;
(1)
of which Y 1 and Y 2 represent the received optical intensity of RX1 and RX2; meanwhile, I 1 and I 2 represent the emitted optical intensity from TX1 and TX2. H and N denote the channel matrix and noise, respectively. In this theoretical model, we just consider the distortions induced by PDM. The transmitted signals can be recovered once the channel matrix satisfies the following equation: cos2 α11 cos2 α22 ≠ cos2 α12 cos2 α21 : © 2014 Optical Society of America
(2)
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OPTICS LETTERS / Vol. 39, No. 7 / April 1, 2014
Fig. 1. Block diagram and experimental setup of the proposed PDM Nyquist SC-FDE VLC system. (AWG, arbitrary waveform generator; P/S, parallel to serial; EA, electrical amplifier; LPF, low-pass filter; OSC, real-time oscilloscope; CP, cyclic prefix.)
The received signals can be recovered by zero forcing (ZF) algorithms neglecting the noise,
Y1 Y2
H −1
I1 : I2
(3)
In this Letter, as a proof of concept, the offset angle is set at α11 α22 0°, and α12 α21 90°. Therefore, the channel matrix can be simplified as a diagonal matrix, so the received optical intensity can be denoted as:
Y1 Y2
1∕2 0 0 1∕2
I1 I2
N:
(4)
Neglecting the noise term, N, the only difference between transmitted signals and received signals is a constant, so no extra demultiplexing algorithm is needed. The light illuminance versus offset angle between xpolarizer1 and x-polarizer2 are depicted in Fig. 2. In this measurement, x-polarizer1 is fixed while x-polarizer2 is rotated around the axis. At the point of maximum illuminance (44 lx), the offset angle can be regarded as 0°, while at the minimum illuminance (0.4 lx), the offset
illuminance(lx)
50 40 30 20 10 0 0
15
30
45
60
75
90
offset angle(degree) Fig. 2. Light illumination versus offset angle between x-polarizer1 and x-polarizer2.
angle can be considered as 90°, i.e., y polarization direction. From this figure, we can find the cross talk between x polarization and y polarization is lower than 1%. Additionally, the light intensity after x-polarizer1 will be reduced to 42% of its original intensity. The experimental setup is also shown in Fig. 1. The random binary data is generated in MATLAB and would be first split into two parallel streams, one for each transmitter (TX) channel. In each channel, the bit stream is mapped into 16-ary quadrature amplitude modulation (16-QAM) format and then the training sequences (TSs) are inserted into the signals. After adding cyclic prefix (CP), upsampling by a factor of 10, and filtering by a rectangular filter, upconversions are accomplished by multiplying the real part and image part of the signals with the sine function and cosine function, respectively, and then added. The SC-FDE waveform is then loaded into an arbitrary-waveform generator (Tektronix 7122C). The output of this generator is amplified by an electrical amplifier (EA) (Minicircuits, 25 dB gain), and combined with direct current (DC) bias via bias tee, and the resulting waveform is then applied to a red chip of the RGB LED, acting as an optical transmitter. Passing through x-/y-polarizer1, free-space, lens (50 mm diameter, 18 mm focus length), and x-/ypolarizer2, the signals are detected by two APDs. Then, the received signals from each of the 2 RXs are routed to a high speed oscilloscope (Lecroy) and are acquired for further digital signal processing (DSP). After synchronization, downconverting to baseband, and removing CP, the received data streams are processed at the frequency domain. The final streams are then passed through the QAM decoder to recover the original binary stream. The photographs of the experimental setup are depicted in Fig. 3. Two commercially available RGB LEDs (Cree, PLCC) are used as the TXs and two APDs (Hamamatsu, 0.5 A∕W sensitivity at 800 nm) are used as the receivers (RXs). The FFT size is 128, and the CP length is 1∕16. The sampling rate of the AWG and OSC is 1.25 GS∕s and 2.5 GS∕s, respectively. Thus, the available
April 1, 2014 / Vol. 39, No. 7 / OPTICS LETTERS
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0.016
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Channel Matrix
0.014 0.012 0.010 0.008 0.006 0.004 0.002 0.000
Fig. 3. Photograph of PDM platform (a) transmitters and (b) receivers.
70cm 80cm 1
0.5
BER
60
80
100
120
140
of each polarization is 500 Mb∕s, thus the overall data rate is 1 Gb∕s. The measured results are shown in Fig. 6. All BERs are under the preFEC limit of 3.8 × 10−3 after 80 cm transmission. And the BER performance will be slightly degraded when two polarization directions transmitted at the same time. In the case of both polarizations transmission, the electrical spectra of two RXs are also measured and illustrated in Fig. 7. From this figure, we can find that the power of noise floor is about −60 dBm, and gains of low frequency part and high frequency part are about 35 and 15 dB, respectively. Additionally, the power difference between frequencies can be preequalized using the method mentioned in [4], and the performance can be further enhanced.
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BER
bandwidth is 125 MHz, ranging from 7.8125 to 132.8125 MHz. The distance between the two transmitters is 5 cm. In the VLC system, the LED shows significant nonlinearity due to the nonlinear characteristics of the current-voltage and output power–current curves. Additionally, the LED has a threshold value of about 2 V in this demonstration. The peak to peak voltage after the electrical amplifier is about 900 mV. When the lowest signal is below the threshold value, clipping will be introduced; meanwhile, it will work at its saturation area when the signal is too large, so it is necessary to render the LED work at the quasi-linear region. The BER performances versus bias voltages at 70 and 80 cm are measured as shown in Fig. 4. The bias voltage is varied from 2.1 to 2.7 V, with a step of 0.1 V. From the measured results at both distances, the optimal working points are at 2.5 V. The constellations of 2.2 and 2.5 V at 70 cm are also inserted in Fig. 4. It can be easily seen that the constellation of 2.5 V is much clearer than that of 2.2 V. The channel matrix of this system is measured via the time-multiplexed training sequences mentioned in [10]. The experimental results are shown in Fig. 5. The channel matrix elements H ij i 1; 2; j 1; 2 represent the gain from the jth transmitter to ith receiver. Between these four elements, H 11 and H 22 denote the signal gain of the x–x polarization link and the y–y polarization link, while H 12 and H 21 denote the cross talk between these two polarization branches. The cross talk can be neglected, as it is about 100 times smaller than the signal gain. Next, we measured the BER performance of two receivers in the case of only x-/y-polarization transmission and both polarizations transmission. The data rate
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2.6
2.7
Bias Voltage(V) Fig. 4.
-1.5 -1.5
1E-5
-0.5
BER performance versus different bias voltages.
Fig. 6. Measured BER performance versus different transmission distances in the case of only one polarization transmission and both polarization transmissions (a) RX1 and (b) RX2.
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OPTICS LETTERS / Vol. 39, No. 7 / April 1, 2014 0.015
0.015
RX1
RX2
-20
Power(dBm)
Power(dBm)
-20
-40
-60
-80
0.01
0.01
0.005
0.005
-40
-60
0
0
-80
-0.005
-0.005
-0.01
-0.01
-100
-100 20
40
60
80
100
120
20
140
40
(a) Frequency (MHz)
60
80
100
120
140
(b) Frequency (MHz)
-0.015 -0.015
Fig. 7. Measured electrical spectra of (a) RX1 and (b) RX2 in the case of both polarizations transmission. 0.01
SISO,with polarizers SISO,w/o polarizers
BER
1E-4 1.5
1E-5
1
1
0.5
0.5
0
0
-0.5
-0.5
-1
1E-6
-1.5 -1.5
70
-0.005
0
(a)
0.005
0.01
-0.01
-0.005
0
0.005
0.01
0.015
(b)
Fig. 9. Constellations of (a) RX1 and (b) RX2 without using polarizers.
co-orthogonal polarizers. The cross talk between two orthogonal polarizers is demonstrated to be very small, and the data rate can be doubled at a sacrifice of a 10 cm shorter transmission distance by using polarizers. Meanwhile, the PDM scheme utilizing two mismatched polarizers are also theoretically investigated. The data rate can be further improved by adopting WDM and a larger bandwidth APD. Additionally, a 3 × 3 or 4 × 4 PDM scheme employing more polarizers can also be our future research direction.
1E-3
1.5
-0.01
-0.015 0.015 -0.015
-1
-1
75
-0.5
0
0.5
1
-1.5 1.5-1.5
80
-1
-0.5
0
0.5
1
1.5
85
Distance(cm) Fig. 8. Measured BER performance versus different transmission distance, with and without polarizers.
The performance degradation after using polarizers in a single input single output channel is also investigated. The measured BER results as a function of distance with and without polarizers are depicted in Fig. 8. From this figure we can find that, after using polarizers, the transmission distance will be about 10 cm shorter at the same BER level; in other words, the sensitivity of the RX will be reduced. But the transmission distance can become longer by employing multiple LEDs instead of only one chip in this demonstration. The constellations of these two cases are also inserted in Fig. 8. If the polarizers are removed it will become a traditional space diversity MIMO scheme, demonstrated in [5–7]. The cross talk will become very serious and, if no MIMO demultiplexing is utilized, the signals cannot be recovered. Figure 9 shows the received constellations of two RXs without using polarizers. In conclusion, we have experimentally demonstrated 1 Gb∕s PDM in VLC system for the first time by using spectrally efficient Nyquist SC-FDE and two groups of
This work was partially supported by the NNSF of China (Nos. 61250018 and 61177071), the Key Program of Shanghai Science and Technology Association (12dz1143000). References 1. M. Biagi, T. Borogovac, and T. D. C. Little, J. Lightwave Technol. 31, 3676 (2013). 2. Y. Wang, N. Chi, Y. Wang, R. Li, X. Huang, C. Yang, and Z. Zhang, Opt. Express 21, 27558 (2013). 3. H. Le Minh, D. C. O’Brien, G. Faulkner, L. Zeng, K. Lee, D. Jung, Y. Oh, and E. T. Won, IEEE Photon. Technol. Lett. 21, 1063 (2009). 4. Y. Wang, Y. Wang, N. Chi, J. Yu, and H. Shang, Opt. Express 21, 1203 (2013). 5. G. Ntogari, T. Kamalakis, and T. Sphicopoulos, IEEE J. Sel. Areas Commun. 27, 1545 (2009). 6. E. Baccarelli, M. Biagi, and C. Pelizzoni, IEEE Trans. Signal Process. 53, 2335 (2005). 7. W. O. Popoola, E. Poves, and H. Haas, IEEE Trans. Commun. 61, 1968 (2013). 8. B. Schnabel, E. Kley, and F. Wyrowski, Opt. Eng. 38, 220 (1999). 9. D. Falconer, S. L. Ariyavisitakul, A. Benyamin-Seeyar, and B. Eidson, IEEE Commun. Mag. 40(4) 58 (2002). 10. F. Li, Z. Cao, X. Z. Dong, and L. Chen, J. Lightwave Technol. 31, 2394 (2013).
