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Optimal Scheme of DCO-OFDM for Optical Frequency-selectivity
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Procedia Computer Science 131 (2018) 1074–1080
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8th International Congress of Information and Communication Technology (ICICT-2018) 8th International Congress of Information and Communication Technology (ICICT-2018) Optimal Scheme of DCO-OFDM for Optical Frequency-selectivity Optimal Scheme of DCO-OFDM for Optical Frequency-selectivity Ying-Dong Zanga一, Jian Zhanga
Ying-Dong Zang 一, Jian Zhang
a
a a Zhengzhou (450000), China National Digital Switching System Engineering and Technological Research Center,
a
National Digital Switching System Engineering and Technological Research Center, Zhengzhou (450000), China
Abstract For visible light communication (VLC), the optical channel of light emitting diode (LED) has the frequency-selectivity. In the Abstract conventional RF communication, orthogonal frequency division multiplexing (OFDM) is employed to resist the frequencyselectivity thecommunication channel. However, there few research OFDM diode (O-OFDM) the frequency-selectivity For visible of light (VLC), theisoptical channelofofoptical light emitting (LED)considering has the frequency-selectivity. In the conventional communication, frequency division multiplexing (OFDM) and is employed to resist thescheme. frequencycausing by theRF LED. Therefore, weorthogonal focus on the power and bit loading of the O-OFDM discover the optimal We select a common DC-biased optical OFDM (DCO-OFDM) for a universality. simulations indicate that the selectivity of the O-OFDM, channel. However, there is few research of optical OFDM (O-OFDM)Extensive considering the frequency-selectivity minimum detection and the aboveand 1.5bit dBloading at the bitoferror rate 10-4 for numerous causing bydistance the LED. Therefore, we attained focus ongain the ispower the O-OFDM and discoverscenarios. the optimal scheme. We select a common DC-biased opticalcommittee OFDM (DCO-OFDM) for a universality. simulations indicate that the Peer-review underO-OFDM, responsibility of organizing of the 8th International CongressExtensive of Information and Communication © 2018 The Authors. Published Technology (ICICT-2018). minimum distance detection andby theElsevier attainedLtd. gain is above 1.5 dB at the bit error rate 10-4 for numerous scenarios. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of organizing committee of the 8th International Congress of Information and Communication Selection and peer-review under responsibility of the scientific committee of the 8th International Congress of Information and Keywords: Visible light communication; OFDM; frequency-selectivity, power and bit loading Technology (ICICT-2018). Communication Technology. Keywords: Visible light communication; OFDM; frequency-selectivity, power and bit loading
1. Introduction
1. Introduction Visible light communication (VLC) has emerged as an eco-friendly green technology using visible light spectrum in provision of wireless access1-3. This advantage makes VLC technology and its spectrum resources attract lots of Visible VLC light communication (VLC) has emerged as anspeed eco-friendly green technology using visible light spectrum attention. system is mainly achieved by high light emitting diodes (LEDs) as transmitters and 4-6. This advantage makes VLC technology and its spectrum resources attract lots of in provision of wireless access1-3 photodiodes (PDs) as receivers . Compared to the radio frequency (RF) systems which could use complex valued attention. VLC system mainly can achieved high speed emitting (LEDs) as transmitters and bi-polar signals, VLCissystems only usebyreal-valued andlight positive signalsdiodes for data modulation because of and the 4-6 photodiodeslight (PDs) as receivers . Compared to the modulation radio frequency systems which could complex valued incoherent output of the LED. Thus, intensity (IM)(RF) and direct detection (DD) use systems are used in 7 and bi-polar VLC systems canmodulation only use real-valued and positive data modulation because of and the VLC system signals, to accomplish the data . On-off keying (OOK),signals pulse for amplitude modulation (PAM) incoherent output of the LED. Thus,ofintensity modulation (IM) schemes and direct detectionin(DD) systemswith are IM/DD used in pulse widthlight modulation (PWM) are some the common modulation employed conjunction VLC system to accomplish the data modulation7. On-off keying (OOK), pulse amplitude modulation (PAM) and transmission. pulse width modulation (PWM) are some of the common modulation schemes employed in conjunction with IM/DD transmission. * Corresponding author. Tel.:+86-15890098742. E-mail address:[email protected]. * Corresponding author. Tel.:+86-15890098742. E-mail 2018 Theaddress:[email protected]. Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection andAuthors. peer-review underby responsibility of This the scientific committee of theunder 8th International Congress of Information and Communication 2018 The Published Elsevier B.V. is an open access article the CC BY-NC-ND license Technology https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the 8th International Congress of Information and Communication Technology 1877-0509 © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the 8th International Congress of Information and Communication Technology 10.1016/j.procs.2018.04.261
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Ying-Dong Zang et al. / Procedia Computer Science 131 (2018) 1074–1080 Author name / Procedia Computer Science00 (2018) 000–000
1075
To achieve high-speed data transmission, orthogonal frequency division multiplexing (OFDM) is used in order to get closer to the channel capacity by applying power and bit loading. The conventional OFDM employed in RF communications cannot used in VLC directly, because real-valued nonnegative symbols are needed in optical OFDM (O-OFDM)8. This can be accomplished by employing Hermitian symmetry on the information frame before the inverse fast Fourier transform (IFFT) operation. The common O-OFDM schemes have been achieved in VLC such as DC-biased optical OFDM (DCO-OFDM), asymmetrically clipped optical OFDM (ACO-OFDM), non-DCbiased OFDM (NDC-OFDM) and unipolar OFDM (U-OFDM)9-12. The optical channel of a LED has frequency-selectivity. In RF communications, OFDM is a practical scheme to resist the frequency-selectivity of the channel13-15. However, according the research on O-OFDM at present, there is few considering the frequency-selectivity of the LED and it is no doubt that the O-OFDM didn't exploit the advantages to the full. In this paper, we employ power and bit loading in O-OFDM symbols and select DCO-OFDM for a universality16-18. The optimal scheme is discovered and the exhaustive simulations have been made to examine the results of the optimal scheme comparing to the conventional DCO-OFDM in this paper. The framework of this paper is as follows: the second section describes the conventional DCO-OFDM. Then the optimal scheme is introduced in the third section. The forth section presents the performance comparison between the conventional and the optimal schemes with the discussion and analysis. Finally, summary of the full paper is made in fifth section. 2. System model
Fig. 1. The structure of DCO-OFDM system.
In this section, we will introduce the DCO-OFDM system in [7]. A DCO-OFDM is shown in Fig. 1. The bit stream is encoded into complex symbols, X (k ) , by the M -QAM modulator, where M is the constellation size. The symbols are distributed on to N subcarriers, X (k ) , 0 k N 1 , where N means the size of IFFT/FFT. According to the set in [7], we employ the same constellation size to every subcarriers in the convention scheme. In DCO-OFDM, the complex data signal is constrained to have Hermitian symmetry, as defined below,
X (m) X * ( N m) , 0 m N 1 , and X (0) X ( N / 2) 0 , where X (m) is the m th subcarrier of *
signal and X (m) denotes the conjugate of the X (m) . Then the signal is input into the inverse fast Fourier transform (IFFT). Because of the Hermitian symmetry of the input, the output signal of the IFFT, x , is real. The
k th of x , x(k ) is given by 1 x(k ) N
N 1
j2 km N
X (m) exp
m0
\* MERGEFORMAT (1)
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Ying-Dong Zang et al. / Procedia Computer Science 131 (2018) 1074–1080 Author name / Procedia Computer Science00 (2018) 000–000
3
Because of the Hermitian symmetry and X (0) X ( N / 2) 0 , the number of unique data carrying subcarriers is N/2-1 and x(k) is a real-valued signal. The electrical power normalization is employed here and the operation of the electrical power normalization is
x(k )
xnormalized (k )
\* MERGEFORMAT (2)
E x 2 ( k )
Then, a suitable DC-bias is added to xnormalized (k )
to make the signal nonnegative approximately,
xbiased (k ) xnormalized (k ) B , where the B denotes the DC-bias. Because of the electrical power normalization in (1), the DC-bias is set as a constant. xbiased (k ) will be clipped and any remaining negative peaks are clipped resulting in signal, as showed below,
1 xclipped (k ) ( xbiased (k ) xbiased (k ) ) 2
\* MERGEFORMAT (3)
In the simulations, the clipped signal will be transmitted by the LED. The signal will be detected by the PD in the receiver and the PD will converts the optical intensity signal back to an electrical signal with the additive white Gaussian noise (AWGN). After the equalizer, the recovered OFDM symbols will be obtained by the fast Fourier transform (FFT). In DCO-OFDM, we only care about the N / 2 1 symbols from the corresponding subcarriers to constitute a QAM symbol frame. Finally, the detected QAM symbols are decoded to obtain the output bit stream. 3. Optimization scheme In this section, we will introduce the optimal scheme for the distributions of the power and the constellation sizes on different subcarriers in DCO-OFDM. According to the Parseval's theorem,
1 N 1 2 \* MERGEFORMAT (4) C (m) X 2 (m) N m0 k 0 th where C (m) , is the coefficient of the symbol on the m subcarrier, and C (m) 0 . Therefore, N 1 2 1 N 1 2 E x ( k ) E C ( m) X 2 ( m) \* MERGEFORMAT (5) k 0 N m0 N 1
x
then,
2
(k )
N 1
E x 2 (k ) k 0
1 N 1 2 C (m)E X 2 (m) . N m0
\* MERGEFORMAT (6)
2
Because x (k ) , 0 k N 1 follow the same probability distribution function, E x ( k ) , 0 k N 1 , are equal. And then,
NE x 2 (k ) Therefore the expectation of electrical power is
E x 2 ( k ) When the square QAM is employed, there is
E x 2 ( k )
1 N
1 N2 1 N2
N 1
(m)E X 2 (m) .
\* MERGEFORMAT (7)
2
( m)E X 2 ( m) .
\* MERGEFORMAT (8)
2
( m)E X 2 ( m) ,
\* MERGEFORMAT (9)
C
2
m0 N 1
C
m0
N 1
C
m0
4
Ying-Dong et al. / Computer Procedia Computer 131 (2018) 1074–1080 Author nameZang / Procedia Science00Science (2018) 000–000
where M (m) denotes the constellation size employed on the m and (9), the expectation of the electrical power is N
th
1077
subcarrier. Considering the Hermitian symmetry
1
4 2 E x 2 (k ) C 2 (n )(M (n ) 1) . 2 3N n 0 th Therefore the Euclidean distance for the m subcarrier at the receiver is 2 H ( m)C ( m) , d Eud (m) 2 E x (k )
\* MERGEFORMAT (10)
\* MERGEFORMAT (11)
th
where H (m) means the amplitude-frequency response of the LED on the m subcarrier. Because of the channel noise is white Gaussian distributed, the optimal maximum likelihood (ML) detection follows the minimum Euclidean decision, whose complexity is very low. Thus, to obtain the optimal scheme for the distributions of the power and the constellation sizes on different subcarriers, the minimal Euclidean distance of (11) should be maximized. The optimization problem can be formulated as
2 H ( m)C ( m)
max min d Eud ( m) 4 3N 2
N 1 2
C
2
(n)( M (n) 1)
n 0
s.t. log 4 M (n) Z , N 1 2
M (n ) 2
DN
\* MERGEFORMAT (12)
,
i 1
C (n) 0, where DN donates the numbers of the bits transmitted by a OFDM symbol.
(n) , 0 n N / 2 1 , and the QAM symbols obtained Let us assume the result of the (12) are C (n) and M
(n) , are X (n) , considering the Hermitian symmetry, the symbols input to the IFFT from the constellation sizes M are X (n) C (n) X (n) ,
\* MERGEFORMAT (13)
and
X ( N n) C (n) X * (n) , where 0 n N / 2 1 and X (0) X ( N / 2) 0 .
\* MERGEFORMAT (14)
4. Simulation results In this section, we examine the error performance of the optimal scheme by comparing with the conventional DCO-OFDM in [7]. We compare the average bit error rate (BER) performance of the optimal scheme by comparing with the conventional DCO-OFDM and show the simulations in Figs. 2, 3, 4 and 5. The result of the optimal scheme is obtained by the exhaustive search in our simulations. We employ one-order Butterworth filter to fit the amplitudefrequency characteristic of the LED. The size of the FFT/IFFT is N 256 , and the numbers of the bits transmitted
by a OFDM symbol, DN , are the same for different schemes in the comparisons. Because of the Hermitian symmetry, the number of the effective subcarriers is N / 2 1 127 . When we employ 16QAM, 64QAM and
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Ying-Dong Zang et al. / Procedia Computer Science 131 (2018) 1074–1080 Author name / Procedia Computer Science00 (2018) 000–000
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256QAM for the conventional DCO-OFDM, DN will be 508, 762 and 1016 respectively. Fig. 2 presents the error performance of different DN with B 4 . As illustrated by Fig. 2, for B 4 , substantial gain can be attained by the optimal scheme over the conventional DCO-OFDM. For example, when DN 508 , the attained gain is about 1.5 dB at the BER of 10-4.
Fig. 2. Error performance of different
DN
with
B 4.
Furthermore, for a fixed DN , we show the error performance comparison for different B in Figs. 3, 4 and 5. From these figures, we observe that for the presented B , our optimal scheme has lower BER for the same SNR as a whole. For example, when DN 762 and B 4 , the performance gain is about 1.5 dB. In addition, we can see that a proper DC selection is of much significance for a satisfactory performance since if the values of B is too small or too large, the performance will be degraded. For example, when DN 1016 and B 2 , there exists an error floor, for which the main reason is that when B is too small, the distortion resulting from the clipping processing of nonnegative components will worsen the error performance. If B is too large, the energy-efficiency is not guaranteed as indicated by the case of DN 1016 and B 6 . Therefore, a proper trade-off with respect to B is required. It is a crucial topic and we will consider it in our future research.
Fig. 3. Error performance of
DN 508
with different
B.
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Author nameZang / Procedia Science00Science (2018) 000–000 Ying-Dong et al. / Computer Procedia Computer 131 (2018) 1074–1080
Fig. 4. Error performance of
Fig. 5. Error performance of
DN 762
DN 1016
with different
with different
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B.
B.
5. Conclusions In this paper, we have research the power and bit loading of DCO-OFDM with the frequency-selectivity of the LED and discovered the optimal scheme. The optimal scheme for the distributions of the power and the constellation sizes on different subcarriers is obtained by maximizing the minimal Euclidean distance of the constellations. The exhaustive search is employed to solve the optimization problem of the optimal scheme which drive us to look for a sub-optimal scheme in our future work. Comprehensive simulation results by using the minimum Euclidean distance demodulation have shown that the optimal scheme has substantial performance gain over the conventional scheme. References 1. Zhang, Y. Y., Yu, H. Y., Zhang, J. K., Zhu, Y. J., Wang, J. L., & Wang, T. (2015). Space codes for MIMO optical wireless communications: error performance criterion and code construction. IEEE Transactions on Wireless Communications, PP(99), 1-1. 2. Zhang, Y. Y., Yu, H. Y., Zhang, J. K., Zhu, Y. J., Wang, J. L., & Ji, X. S. (2016). On the optimality of spatial repetition coding for MIMO optical wireless communications. IEEE Communications Letters, 20(5), 846-849.
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Ying-Dong Zang et al. / Procedia Computer Science 131 (2018) 1074–1080 Author name / Procedia Computer Science00 (2018) 000–000
3. Zhang, Y. Y., Yu, H. Y., Zhang, J. K., & Zhu, Y. J. (2016). Signal-cooperative multilayer-modulated VLC systems for automotive applications. IEEE Photonics Journal, 8(1), 1-9. 4. Jamali, M. V., & Salehi, J. A. (2015). On the BER of multiple-input multiple-output underwater wireless optical communication systems. International Workshop on Optical Wireless Communications (pp.26-30). IEEE. 5. Liu, X., Gong, C., Li, S., & Xu, Z. (2016). Signal characterization and receiver design for visible light communication under weak illuminance. IEEE Communications Letters, 20(7), 1349-1352. 6. Yichen Li, Majid Safari, Robert Henderson, & Harald Haas. (2015). Optical OFDM with single-photon avalanche diode. IEEE Photonics Technology Letters, 27(9), 943-946. 7. He, C., & Armstrong, J. (2016). Clipping noise mitigation in optical OFDM systems. IEEE Communications Letters, PP(99), 1-1 8. Wang, Q., Wang, Z., Dai, L., & Quan, J. (2016). Dimmable visible light communications based on multilayer ACO-OFDM. IEEE Photonics Journal,8(3), 1-11. 9. Zhang, T., Ghassemlooy, Z., Ma, C., & Guo, S. (2017). PAPR reduction scheme for ACO-OFDM based visible light communication systems. Optics Communications, 383, 75-80. 10. Aly, B. (2016). Performance analysis of adaptive channel estimation for U-OFDM indoor visible light communication. Radio Science Conference(pp.217-222). IEEE. 11. Zhou, J., & Qiao, Y. (2015). Low-PAPR asymmetrically clipped optical OFDM for intensity-modulation/direct-detection systems. IEEE Photonics Journal, 7(3), 1-8. 12. Alaka, S. P., Narasimhan, T. L., & Chockalingam, A. (2016). Coded Index Modulation for Non-DC-Biased OFDM in Multiple LED Visible Light Communication. Vehicular Technology Conference (pp.1-5). IEEE. 13. Long, S., Khalighi, M. A., Wolf, M., & Bourennane, S. (2016). Investigating channel frequency selectivity in indoor visible-light communication systems. Iet Optoelectronics, 10(3), 80-88. 14. Yesilkaya, A., Miramirkhani, F., Basar, E., Panayirci, E., & Uysal, M. (2016). Performance of MIMO enhanced unipolar OFDM with realistic indoor visible light channel models. Wireless Communications and NETWORKING Conference Workshops. IEEE. 15. Chen, H., Xu, Z., Gao, Q., & Li, S. (2017). A 51.6 mbps experimental VLC system using a monochromic organic led. IEEE Photonics Journal,PP(99), 1-1. 16. Zhang, Y., Huang, Y., Wan, L., Zhou, S., Shen, X., & Wang, H. (2016). Adaptive OFDMA with partial CSI for downlink underwater acoustic communications. Journal of Communications & Networks, 18(3), 387-396. 17. Hilton, C. D., Mehta, M., & Baker, M. P. (2016). Communications system and bit-loading method used in a communications system. 18. Vo, T. N., Amis, K., Chonavel, T., & Siohan, P. (2016). A low-complexity bit-loading algorithm for OFDM systems under spectral mask constraint. IEEE Communications Letters, 20(6), 1076-1079.
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Procedia Computer Science 131 (2018) 1074–1080
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8th International Congress of Information and Communication Technology (ICICT-2018) 8th International Congress of Information and Communication Technology (ICICT-2018) Optimal Scheme of DCO-OFDM for Optical Frequency-selectivity Optimal Scheme of DCO-OFDM for Optical Frequency-selectivity Ying-Dong Zanga一, Jian Zhanga
Ying-Dong Zang 一, Jian Zhang
a
a a Zhengzhou (450000), China National Digital Switching System Engineering and Technological Research Center,
a
National Digital Switching System Engineering and Technological Research Center, Zhengzhou (450000), China
Abstract For visible light communication (VLC), the optical channel of light emitting diode (LED) has the frequency-selectivity. In the Abstract conventional RF communication, orthogonal frequency division multiplexing (OFDM) is employed to resist the frequencyselectivity thecommunication channel. However, there few research OFDM diode (O-OFDM) the frequency-selectivity For visible of light (VLC), theisoptical channelofofoptical light emitting (LED)considering has the frequency-selectivity. In the conventional communication, frequency division multiplexing (OFDM) and is employed to resist thescheme. frequencycausing by theRF LED. Therefore, weorthogonal focus on the power and bit loading of the O-OFDM discover the optimal We select a common DC-biased optical OFDM (DCO-OFDM) for a universality. simulations indicate that the selectivity of the O-OFDM, channel. However, there is few research of optical OFDM (O-OFDM)Extensive considering the frequency-selectivity minimum detection and the aboveand 1.5bit dBloading at the bitoferror rate 10-4 for numerous causing bydistance the LED. Therefore, we attained focus ongain the ispower the O-OFDM and discoverscenarios. the optimal scheme. We select a common DC-biased opticalcommittee OFDM (DCO-OFDM) for a universality. simulations indicate that the Peer-review underO-OFDM, responsibility of organizing of the 8th International CongressExtensive of Information and Communication © 2018 The Authors. Published Technology (ICICT-2018). minimum distance detection andby theElsevier attainedLtd. gain is above 1.5 dB at the bit error rate 10-4 for numerous scenarios. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of organizing committee of the 8th International Congress of Information and Communication Selection and peer-review under responsibility of the scientific committee of the 8th International Congress of Information and Keywords: Visible light communication; OFDM; frequency-selectivity, power and bit loading Technology (ICICT-2018). Communication Technology. Keywords: Visible light communication; OFDM; frequency-selectivity, power and bit loading
1. Introduction
1. Introduction Visible light communication (VLC) has emerged as an eco-friendly green technology using visible light spectrum in provision of wireless access1-3. This advantage makes VLC technology and its spectrum resources attract lots of Visible VLC light communication (VLC) has emerged as anspeed eco-friendly green technology using visible light spectrum attention. system is mainly achieved by high light emitting diodes (LEDs) as transmitters and 4-6. This advantage makes VLC technology and its spectrum resources attract lots of in provision of wireless access1-3 photodiodes (PDs) as receivers . Compared to the radio frequency (RF) systems which could use complex valued attention. VLC system mainly can achieved high speed emitting (LEDs) as transmitters and bi-polar signals, VLCissystems only usebyreal-valued andlight positive signalsdiodes for data modulation because of and the 4-6 photodiodeslight (PDs) as receivers . Compared to the modulation radio frequency systems which could complex valued incoherent output of the LED. Thus, intensity (IM)(RF) and direct detection (DD) use systems are used in 7 and bi-polar VLC systems canmodulation only use real-valued and positive data modulation because of and the VLC system signals, to accomplish the data . On-off keying (OOK),signals pulse for amplitude modulation (PAM) incoherent output of the LED. Thus,ofintensity modulation (IM) schemes and direct detectionin(DD) systemswith are IM/DD used in pulse widthlight modulation (PWM) are some the common modulation employed conjunction VLC system to accomplish the data modulation7. On-off keying (OOK), pulse amplitude modulation (PAM) and transmission. pulse width modulation (PWM) are some of the common modulation schemes employed in conjunction with IM/DD transmission. * Corresponding author. Tel.:+86-15890098742. E-mail address:[email protected]. * Corresponding author. Tel.:+86-15890098742. E-mail 2018 Theaddress:[email protected]. Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection andAuthors. peer-review underby responsibility of This the scientific committee of theunder 8th International Congress of Information and Communication 2018 The Published Elsevier B.V. is an open access article the CC BY-NC-ND license Technology https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the 8th International Congress of Information and Communication Technology 1877-0509 © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the 8th International Congress of Information and Communication Technology 10.1016/j.procs.2018.04.261
2
Ying-Dong Zang et al. / Procedia Computer Science 131 (2018) 1074–1080 Author name / Procedia Computer Science00 (2018) 000–000
1075
To achieve high-speed data transmission, orthogonal frequency division multiplexing (OFDM) is used in order to get closer to the channel capacity by applying power and bit loading. The conventional OFDM employed in RF communications cannot used in VLC directly, because real-valued nonnegative symbols are needed in optical OFDM (O-OFDM)8. This can be accomplished by employing Hermitian symmetry on the information frame before the inverse fast Fourier transform (IFFT) operation. The common O-OFDM schemes have been achieved in VLC such as DC-biased optical OFDM (DCO-OFDM), asymmetrically clipped optical OFDM (ACO-OFDM), non-DCbiased OFDM (NDC-OFDM) and unipolar OFDM (U-OFDM)9-12. The optical channel of a LED has frequency-selectivity. In RF communications, OFDM is a practical scheme to resist the frequency-selectivity of the channel13-15. However, according the research on O-OFDM at present, there is few considering the frequency-selectivity of the LED and it is no doubt that the O-OFDM didn't exploit the advantages to the full. In this paper, we employ power and bit loading in O-OFDM symbols and select DCO-OFDM for a universality16-18. The optimal scheme is discovered and the exhaustive simulations have been made to examine the results of the optimal scheme comparing to the conventional DCO-OFDM in this paper. The framework of this paper is as follows: the second section describes the conventional DCO-OFDM. Then the optimal scheme is introduced in the third section. The forth section presents the performance comparison between the conventional and the optimal schemes with the discussion and analysis. Finally, summary of the full paper is made in fifth section. 2. System model
Fig. 1. The structure of DCO-OFDM system.
In this section, we will introduce the DCO-OFDM system in [7]. A DCO-OFDM is shown in Fig. 1. The bit stream is encoded into complex symbols, X (k ) , by the M -QAM modulator, where M is the constellation size. The symbols are distributed on to N subcarriers, X (k ) , 0 k N 1 , where N means the size of IFFT/FFT. According to the set in [7], we employ the same constellation size to every subcarriers in the convention scheme. In DCO-OFDM, the complex data signal is constrained to have Hermitian symmetry, as defined below,
X (m) X * ( N m) , 0 m N 1 , and X (0) X ( N / 2) 0 , where X (m) is the m th subcarrier of *
signal and X (m) denotes the conjugate of the X (m) . Then the signal is input into the inverse fast Fourier transform (IFFT). Because of the Hermitian symmetry of the input, the output signal of the IFFT, x , is real. The
k th of x , x(k ) is given by 1 x(k ) N
N 1
j2 km N
X (m) exp
m0
\* MERGEFORMAT (1)
1076
Ying-Dong Zang et al. / Procedia Computer Science 131 (2018) 1074–1080 Author name / Procedia Computer Science00 (2018) 000–000
3
Because of the Hermitian symmetry and X (0) X ( N / 2) 0 , the number of unique data carrying subcarriers is N/2-1 and x(k) is a real-valued signal. The electrical power normalization is employed here and the operation of the electrical power normalization is
x(k )
xnormalized (k )
\* MERGEFORMAT (2)
E x 2 ( k )
Then, a suitable DC-bias is added to xnormalized (k )
to make the signal nonnegative approximately,
xbiased (k ) xnormalized (k ) B , where the B denotes the DC-bias. Because of the electrical power normalization in (1), the DC-bias is set as a constant. xbiased (k ) will be clipped and any remaining negative peaks are clipped resulting in signal, as showed below,
1 xclipped (k ) ( xbiased (k ) xbiased (k ) ) 2
\* MERGEFORMAT (3)
In the simulations, the clipped signal will be transmitted by the LED. The signal will be detected by the PD in the receiver and the PD will converts the optical intensity signal back to an electrical signal with the additive white Gaussian noise (AWGN). After the equalizer, the recovered OFDM symbols will be obtained by the fast Fourier transform (FFT). In DCO-OFDM, we only care about the N / 2 1 symbols from the corresponding subcarriers to constitute a QAM symbol frame. Finally, the detected QAM symbols are decoded to obtain the output bit stream. 3. Optimization scheme In this section, we will introduce the optimal scheme for the distributions of the power and the constellation sizes on different subcarriers in DCO-OFDM. According to the Parseval's theorem,
1 N 1 2 \* MERGEFORMAT (4) C (m) X 2 (m) N m0 k 0 th where C (m) , is the coefficient of the symbol on the m subcarrier, and C (m) 0 . Therefore, N 1 2 1 N 1 2 E x ( k ) E C ( m) X 2 ( m) \* MERGEFORMAT (5) k 0 N m0 N 1
x
then,
2
(k )
N 1
E x 2 (k ) k 0
1 N 1 2 C (m)E X 2 (m) . N m0
\* MERGEFORMAT (6)
2
Because x (k ) , 0 k N 1 follow the same probability distribution function, E x ( k ) , 0 k N 1 , are equal. And then,
NE x 2 (k ) Therefore the expectation of electrical power is
E x 2 ( k ) When the square QAM is employed, there is
E x 2 ( k )
1 N
1 N2 1 N2
N 1
(m)E X 2 (m) .
\* MERGEFORMAT (7)
2
( m)E X 2 ( m) .
\* MERGEFORMAT (8)
2
( m)E X 2 ( m) ,
\* MERGEFORMAT (9)
C
2
m0 N 1
C
m0
N 1
C
m0
4
Ying-Dong et al. / Computer Procedia Computer 131 (2018) 1074–1080 Author nameZang / Procedia Science00Science (2018) 000–000
where M (m) denotes the constellation size employed on the m and (9), the expectation of the electrical power is N
th
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subcarrier. Considering the Hermitian symmetry
1
4 2 E x 2 (k ) C 2 (n )(M (n ) 1) . 2 3N n 0 th Therefore the Euclidean distance for the m subcarrier at the receiver is 2 H ( m)C ( m) , d Eud (m) 2 E x (k )
\* MERGEFORMAT (10)
\* MERGEFORMAT (11)
th
where H (m) means the amplitude-frequency response of the LED on the m subcarrier. Because of the channel noise is white Gaussian distributed, the optimal maximum likelihood (ML) detection follows the minimum Euclidean decision, whose complexity is very low. Thus, to obtain the optimal scheme for the distributions of the power and the constellation sizes on different subcarriers, the minimal Euclidean distance of (11) should be maximized. The optimization problem can be formulated as
2 H ( m)C ( m)
max min d Eud ( m) 4 3N 2
N 1 2
C
2
(n)( M (n) 1)
n 0
s.t. log 4 M (n) Z , N 1 2
M (n ) 2
DN
\* MERGEFORMAT (12)
,
i 1
C (n) 0, where DN donates the numbers of the bits transmitted by a OFDM symbol.
(n) , 0 n N / 2 1 , and the QAM symbols obtained Let us assume the result of the (12) are C (n) and M
(n) , are X (n) , considering the Hermitian symmetry, the symbols input to the IFFT from the constellation sizes M are X (n) C (n) X (n) ,
\* MERGEFORMAT (13)
and
X ( N n) C (n) X * (n) , where 0 n N / 2 1 and X (0) X ( N / 2) 0 .
\* MERGEFORMAT (14)
4. Simulation results In this section, we examine the error performance of the optimal scheme by comparing with the conventional DCO-OFDM in [7]. We compare the average bit error rate (BER) performance of the optimal scheme by comparing with the conventional DCO-OFDM and show the simulations in Figs. 2, 3, 4 and 5. The result of the optimal scheme is obtained by the exhaustive search in our simulations. We employ one-order Butterworth filter to fit the amplitudefrequency characteristic of the LED. The size of the FFT/IFFT is N 256 , and the numbers of the bits transmitted
by a OFDM symbol, DN , are the same for different schemes in the comparisons. Because of the Hermitian symmetry, the number of the effective subcarriers is N / 2 1 127 . When we employ 16QAM, 64QAM and
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Ying-Dong Zang et al. / Procedia Computer Science 131 (2018) 1074–1080 Author name / Procedia Computer Science00 (2018) 000–000
5
256QAM for the conventional DCO-OFDM, DN will be 508, 762 and 1016 respectively. Fig. 2 presents the error performance of different DN with B 4 . As illustrated by Fig. 2, for B 4 , substantial gain can be attained by the optimal scheme over the conventional DCO-OFDM. For example, when DN 508 , the attained gain is about 1.5 dB at the BER of 10-4.
Fig. 2. Error performance of different
DN
with
B 4.
Furthermore, for a fixed DN , we show the error performance comparison for different B in Figs. 3, 4 and 5. From these figures, we observe that for the presented B , our optimal scheme has lower BER for the same SNR as a whole. For example, when DN 762 and B 4 , the performance gain is about 1.5 dB. In addition, we can see that a proper DC selection is of much significance for a satisfactory performance since if the values of B is too small or too large, the performance will be degraded. For example, when DN 1016 and B 2 , there exists an error floor, for which the main reason is that when B is too small, the distortion resulting from the clipping processing of nonnegative components will worsen the error performance. If B is too large, the energy-efficiency is not guaranteed as indicated by the case of DN 1016 and B 6 . Therefore, a proper trade-off with respect to B is required. It is a crucial topic and we will consider it in our future research.
Fig. 3. Error performance of
DN 508
with different
B.
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Author nameZang / Procedia Science00Science (2018) 000–000 Ying-Dong et al. / Computer Procedia Computer 131 (2018) 1074–1080
Fig. 4. Error performance of
Fig. 5. Error performance of
DN 762
DN 1016
with different
with different
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B.
B.
5. Conclusions In this paper, we have research the power and bit loading of DCO-OFDM with the frequency-selectivity of the LED and discovered the optimal scheme. The optimal scheme for the distributions of the power and the constellation sizes on different subcarriers is obtained by maximizing the minimal Euclidean distance of the constellations. The exhaustive search is employed to solve the optimization problem of the optimal scheme which drive us to look for a sub-optimal scheme in our future work. Comprehensive simulation results by using the minimum Euclidean distance demodulation have shown that the optimal scheme has substantial performance gain over the conventional scheme. References 1. Zhang, Y. Y., Yu, H. Y., Zhang, J. K., Zhu, Y. J., Wang, J. L., & Wang, T. (2015). Space codes for MIMO optical wireless communications: error performance criterion and code construction. IEEE Transactions on Wireless Communications, PP(99), 1-1. 2. Zhang, Y. Y., Yu, H. Y., Zhang, J. K., Zhu, Y. J., Wang, J. L., & Ji, X. S. (2016). On the optimality of spatial repetition coding for MIMO optical wireless communications. IEEE Communications Letters, 20(5), 846-849.
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Ying-Dong Zang et al. / Procedia Computer Science 131 (2018) 1074–1080 Author name / Procedia Computer Science00 (2018) 000–000
3. Zhang, Y. Y., Yu, H. Y., Zhang, J. K., & Zhu, Y. J. (2016). Signal-cooperative multilayer-modulated VLC systems for automotive applications. IEEE Photonics Journal, 8(1), 1-9. 4. Jamali, M. V., & Salehi, J. A. (2015). On the BER of multiple-input multiple-output underwater wireless optical communication systems. International Workshop on Optical Wireless Communications (pp.26-30). IEEE. 5. Liu, X., Gong, C., Li, S., & Xu, Z. (2016). Signal characterization and receiver design for visible light communication under weak illuminance. IEEE Communications Letters, 20(7), 1349-1352. 6. Yichen Li, Majid Safari, Robert Henderson, & Harald Haas. (2015). Optical OFDM with single-photon avalanche diode. IEEE Photonics Technology Letters, 27(9), 943-946. 7. He, C., & Armstrong, J. (2016). Clipping noise mitigation in optical OFDM systems. IEEE Communications Letters, PP(99), 1-1 8. Wang, Q., Wang, Z., Dai, L., & Quan, J. (2016). Dimmable visible light communications based on multilayer ACO-OFDM. IEEE Photonics Journal,8(3), 1-11. 9. Zhang, T., Ghassemlooy, Z., Ma, C., & Guo, S. (2017). PAPR reduction scheme for ACO-OFDM based visible light communication systems. Optics Communications, 383, 75-80. 10. Aly, B. (2016). Performance analysis of adaptive channel estimation for U-OFDM indoor visible light communication. Radio Science Conference(pp.217-222). IEEE. 11. Zhou, J., & Qiao, Y. (2015). Low-PAPR asymmetrically clipped optical OFDM for intensity-modulation/direct-detection systems. IEEE Photonics Journal, 7(3), 1-8. 12. Alaka, S. P., Narasimhan, T. L., & Chockalingam, A. (2016). Coded Index Modulation for Non-DC-Biased OFDM in Multiple LED Visible Light Communication. Vehicular Technology Conference (pp.1-5). IEEE. 13. Long, S., Khalighi, M. A., Wolf, M., & Bourennane, S. (2016). Investigating channel frequency selectivity in indoor visible-light communication systems. Iet Optoelectronics, 10(3), 80-88. 14. Yesilkaya, A., Miramirkhani, F., Basar, E., Panayirci, E., & Uysal, M. (2016). Performance of MIMO enhanced unipolar OFDM with realistic indoor visible light channel models. Wireless Communications and NETWORKING Conference Workshops. IEEE. 15. Chen, H., Xu, Z., Gao, Q., & Li, S. (2017). A 51.6 mbps experimental VLC system using a monochromic organic led. IEEE Photonics Journal,PP(99), 1-1. 16. Zhang, Y., Huang, Y., Wan, L., Zhou, S., Shen, X., & Wang, H. (2016). Adaptive OFDMA with partial CSI for downlink underwater acoustic communications. Journal of Communications & Networks, 18(3), 387-396. 17. Hilton, C. D., Mehta, M., & Baker, M. P. (2016). Communications system and bit-loading method used in a communications system. 18. Vo, T. N., Amis, K., Chonavel, T., & Siohan, P. (2016). A low-complexity bit-loading algorithm for OFDM systems under spectral mask constraint. IEEE Communications Letters, 20(6), 1076-1079.
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