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Experimental demonstration of the transmission performance for LDPC-coded multiband OFDM ultra-wideband over fiber system
Optical Fiber Technology 21 (2015) 122–127
Contents lists available at ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Experimental demonstration of the transmission performance for LDPC-coded multiband OFDM ultra-wideband over fiber system Jing He a,b,⇑, Xuejie Wen a, Ming Chen a, Lin Chen a, Jinshu Su b a b
College of Computer Science and Electronic Engineering, Hunan University, Changsha, Hunan, China State Key Laboratory of Parallel and Distributed Processing, and School of Computer, National University of Defense Technology, Changsha, Hunan, China
a r t i c l e
i n f o
Article history: Received 5 May 2014 Revised 22 September 2014 Available online 16 October 2014 Keywords: Multiband ultra-wideband (MB-UWB) Low-density parity-check (LDPC) Orthogonal frequency-divisionmultiplexing (OFDM) Receiver sensitivity
a b s t r a c t To improve the transmission performance of multiband orthogonal frequency division multiplexing (MBOFDM) ultra-wideband (UWB) over optical fiber, a pre-coding scheme based on low-density parity-check (LDPC) is adopted and experimentally demonstrated in the intensity-modulation and direct-detection MB-OFDM UWB over fiber system. Meanwhile, a symbol synchronization and pilot-aided channel estimation scheme is implemented on the receiver of the MB-OFDM UWB over fiber system. The experimental results show that the LDPC pre-coding scheme can work effectively in the MB-OFDM UWB over fiber system. After 70 km standard single-mode fiber (SSMF) transmission, at the bit error rate of 1 103, the receiver sensitivities are improved about 4 dB when the LDPC code rate is 75%. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction As the rapid growth of the communication capacity demand such as high definition television and cloud computing, ultra-wideband (UWB), which shares the spectrum resources with existing radio communications system, is recognized as a potential solution to meet the bandwidth requirements [1]. For the high speed applications, multiband orthogonal frequency division multiplexing (MB-OFDM) in IEEE 802.15.3a has some advantages such as high spectral flexibility by dynamically shaping the spectrum, robustness to RF interference and multi-path effects [2]. However, due to the low power spectral density regulated by the Federal Communications Commission (FCC), the communication distance of a MB-OFDM UWB wireless system is limited only to several meters. Recently, MB-OFDM UWB over fiber (UWBoF) technology, developed to distribute the MB-OFDM UWB signal over a long distance, was proposed to increase the area of coverage [3,4]. For MB-OFDM UWB signal transmitted over optical fiber, there is the degradation of the signal quality due to non-linear effect and dispersion impairments, in particular, accumulated optical amplifier noise, chromatic dispersion (CD) and polarization mode dispersion (PMD) [5]. In the past, in order to avoid the performance degradation due to adverse channel propagation conditions, a proper forward error correction (FEC) scheme was used in optical communications ⇑ Corresponding author at: College of Computer Science and Electronic Engineering, Hunan University, Changsha, Hunan, China. Fax: +86 73188822417. E-mail address: [email protected] (J. He). http://dx.doi.org/10.1016/j.yofte.2014.09.006 1068-5200/Ó 2014 Elsevier Inc. All rights reserved.
[6]. As an error correcting code, low-density parity-check (LDPC) code was introduced to increase the system performance of a radio-over-fiber system and MB-OFDM UWB wireless system [7,8]. In addition, the short block length LDPC code in the IEEE 802.15.3.c standard was proposed as a low complexity solution for impulse response (IR)-UWB systems [9]. However, the investigation of the LDPC-coded MB-OFDM UWB signal transmitting over optical fiber has been seldom reported. In this paper, for the first time to our knowledge, a pre-coding scheme based on LDPC in the MB-OFDM UWBoF system is investigated. To avoid inter-symbol interference (ISI) or inter-channel interference (ICI), a symbol synchronization and pilot-aided channel estimation scheme is implemented on the LDPC-coded MB-OFDM UWBoF system. After transmission over 70 km standard single-mode fiber (SSMF), compared MB-OFDM UWB signal with and without LPDC-coded, when the LDPC code rate is 75%, the receiver sensitivities are improved about 4 dB at a BER of 1 103. The experiment results show that short block length LDPC is effective for MB-OFDM UWBoF system. 2. Principle 2.1. The structure of the IM/DD optical MB-OFDM UWB system The transmitter and receiver of the LDPC-coded MB-OFDM UWBoF system are shown in Fig. 1. In the transmitter, pseudo-random binary sequences (PRBS) are encoded using LDPC code. The LDPC code we used is based on WiMAX 802.16e standard [10].
J. He et al. / Optical Fiber Technology 21 (2015) 122–127
After the LDPC encoder, the output bit streams are mapping into M-PSK or M-QAM symbols. The complex-valued signal points are passing through a serial-to-parallel (S/P) converter. For the purpose of channel tracking, several sub-carriers are used to transmit pilot symbols in the transmitter. After inverse fast Fourier transforms (IFFT), a certain length of cyclic prefix (CP) is added to combat inter-symbol interference (ISI) in the fiber channel. To mitigate the nonlinearity of the optical transmission and reduce the highdynamic range requirement of digital-to-analog conversion (DAC), the peak-to-average power ratio (PAPR) of OFDM signal should be reduced. Thus, clipping is used to reduce the PAPR of OFDM signal before DAC. Then, a training sequence (TS) is inserted at the front of the OFDM symbols. And symbol synchronization at the receiver is realized by using the inserted TS. Subsequently, the baseband OFDM signal with LDPC code is digital up-conversion with time–frequency code (TFC) [2] to generate an MB-OFDM UWB signal with LDPC code. Then the generated LDPC-coded MB-OFDM UWB signal is converted to an analog one by the DAC. After up-conversion and DAC, the LDPC-coded MB-OFDM UWB signal is used as a radio frequency (RF) to driven a Mach–Zehnder modulator (MZM). And the generated optical LDPC-coded MBOFDM UWB signal is transmitted over optical fiber. At the receiver, the optical LDPC-coded MB-OFDM UWB signal is converted into electrical signal by a photo-detector after optical amplifier. The electrical LDPC-coded MB-OFDM UWB signal is digitized by an analog to digital converter (ADC). The digitalized signal is further down-converted to the baseband signal with the corresponding TFC. And the baseband signal is sent to the TS-based symbol synchronization function. Once the starting point of the data-carrying OFDM symbol is located, CP will be removed. After serial to parallel conversion, fast Fourier transforms (FFT) function is utilized to demodulated OFDM signal. In addition, the pilotaided channel equalization method is used to compensate the response of channel. Then the outputs of the M-PSK or M-QAM demodulator are fed to a LDPC decoder and it is implemented
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based on the sum-product algorithm [11]. Meanwhile, the bit error rate (BER) is calculated over de-mapped bits. 2.2. A symbol synchronization and pilot-aided channel estimation scheme Since MB-OFDM UWB systems are much more sensitive to synchronization errors than single carrier systems, the timing synchronization of MB-OFDM UWB is necessary to find the correct symbol start. Therefore, in the paper, the TS are used to implement the timing synchronization. In order to reduce the signal–signal beating interference (SSBI) in the direct-detection optical OFDM receiver [12], the TS is generated by transmitting BPSK symbols on the odd subcarriers. Meanwhile, the even subcarriers and the subcarriers at high frequencies are filled with zeroes. In addition, all the data (including BPSK symbols and zeros) on the subcarriers are constrained to have Hermitian symmetry, and then the result of the inverse fast Fourier transform (IFFT) will produce the realvalued time-domain sequence as shown in Eq. (1). The TS can be expressed as
TSpro ¼ ½AN=4
BN=4 AN=4 BN=4
ð1Þ
where AN=4 and BN=4 are real-value, N is the size of the IFFT. AN=4 means samples of length generated by the IFFT of a real-valued sequence, while BN=4 is designed to be symmetric with AN=4 so as to obtain an impulse-shaped timing metric. The basic form of the TS is similar to Park et al.’s method [13]. Therefore, Park et al.’s timing metric function for symbol synchronization can also be applied to the TS. However, it is observed that the timing metric of Park’s method has large sidelobes at the positions ±N with samples spaced from the correct starting point of OFDM symbols, which will affect the accurateness of timing offset estimation [14]. Here, the property that negative-valued samples at the second half of training symbol is used to overcome the effect of the sidelobes. Thus, the timing metric is defined as
Fig. 1. The structure of IM-DD MB-OFDM UWB over fiber system.
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J. He et al. / Optical Fiber Technology 21 (2015) 122–127
M pro ðmÞ ¼ signðPðmÞÞ
jPðmÞj
2
jRðmÞj
ð2Þ
2
PðmÞ ¼
tðlÞ rðm þ lÞ
3. Experimental setup and results
ð4Þ
l¼0
where sign½ stands for sign function, t(l) is the transmitted TS and r(l) is the received signal. Then, the estimation of timing offset can be written as
^e ¼ arg max ðM pro ðmÞÞ;
ð5Þ
m
Unlike the variable wireless channel environment, the channel response in the optical fiber can be considered approximately as a constant within a short period of time and the difference of channel frequency responses over several adjacent subcarriers is small [15]. Therefore, pilot-aided scheme has been used to estimate channel response [15] and sample frequency offset [16] in optical OFDM system. Here, a pilot-aided channel estimation scheme is used in the receiver of MB-OFDM UWBoF system. At the known pilot subcarriers, the low-complexity least-square (LS) estimation technique is adopted to calculate the channel transfer function
Y p ðkÞ ðk ¼ 0; . . . ; Np 1Þ X p ðkÞ
ð6Þ
where Y p ðkÞ is the received kth pilot subcarrier, X p ðkÞ is the transmitted kth pilot subcarrier and N p is the number of pilot sub-carriers. These LS estimates Hp ðkÞ are then interpolated with the time domain DFT interpolation to get the channel response at the non-pilot sub-carriers. The time domain DFT interpolation is a high-resolution interpolation based on zero-padding and DFT/IDFT [17]. After obtaining the estimated channel, it can be converted to the time domain by IDFT
Gp ðnÞ ¼
NX p 1
jN2p kn
Hp ðkÞe
p
ðk ¼ 0; . . . ; Np 1Þ
ð7Þ
k¼0
Then, the signal Gp ðnÞ is transformed into N points by padding with N N p zeros, and it becoming as
GN ðnÞ ¼
ð9Þ
ð3Þ
N1 X 2 rðm þ lÞ
Hp ðkÞ ¼
ðk ¼ 0; . . . ; N 1Þ
where HN ðkÞ denotes the channel response on the kth sub-carrier of all sub-carriers.
l¼0
RðmÞ ¼
N1 X 2p GN ðnÞej N kn k¼0
where N1 X
HN ðkÞ ¼
Gp ðnÞ; 0 n Np 0; Np n N
ð8Þ
Again, the estimated channel transfer function is obtained by DFT of GN ðnÞ, and it can be expressed as
The experimental setup of LDPC-coded MB-OFDM UWB signals over fiber system is shown in Fig. 2. In the central station, continuous wave (CW) light is generated from a commercial external cavity laser (ECL) source at the wavelength of 1565.38 nm. And the launch power from ECL is 7 dB m. Then, the CW light is injected into a single-drive Mach–Zehnder modulation (MZM), which is driven by the LDPC-coded MB-OFDM UWB signal. In the paper, the generated MB-OFDM UWB signal is based on ECMA 368 standard [2]. The OFDM symbol interval is 312.5 ns. The total number of subcarriers is 128, where the subcarriers used for data, pilot, guard and zero are 100, 12, 10 and 6, respectively. The data rate can be expressed as Rdata ¼ log2 ðKÞ ð1=312:5Þ 100ðMb=sÞ, where the K is the modulation order. For the 16QAM coded MB-OFDM UWB signal, the data rate is equal to 1.28 Gb/s. In this way, for the 75% code rate LDPC coded MB-OFDM UWB signal, the data rate is 0.96 Gb/s. And Table 1 shows the parameters of LDPC-coded MBOFDM UWB signal in the experiment. Meanwhile, the polarization controller (PC) is rotated to maximize the MZM output optical power. The LDPC-coded MB-OFDM UWB signal is generated offline using MATLAB, and then loaded into a Tektronix Arbitrary Waveform Generator (AWG) operating at 10.561 GSps. The DC bias voltage is biased at negative quadrature point. And the generated LDPC-coded MB-OFDM UWB signal drives the single-drive MZM to modulate the optical carrier. The output optical LDPC code MB-OFDM UWB signal is then sent
Table 1 The parameters of LDPC-coded MB-OFDM UWB signal in the experiment. Description
Value
Symbol interval Total number of subcarriers (FFT size) Number of data subcarriers Number of pilot subcarriers Number of guard subcarriers Number of zero subcarriers Bandwidth (for one subband) Constellation Data rates with LDPC code LDPC code rate Cyclic Prefix
312.5 ns 128 100 12 10 6 528 MHz 16-QAM 0.96 Gb/s 75% 32
Fig. 2. Experimental setup of the LDPC-coded MB-OFDM UWBoF system.
J. He et al. / Optical Fiber Technology 21 (2015) 122–127
through up to 70 km standard single-mode fiber (SSMF), with fiber loss of a = 0.22 dB/km and chromatic dispersion of 17 ps/(nm km). At the base station, the optical signal is amplified to 6.3 dB m by an erbium-doped fiber amplifier (EDFA) with a noise figure (NF) of 5 dB to compensate the fiber and connector losses. The attenuation of the variable optical attenuator (VOA) is varied so that it can enable the BER performance of the system to be analyzed as a function of the received optical power. Then, the optical electrical conversion is obtained by a commercial photo-diode with a 3 dB bandwidth of 10 GHz. After photo-detection, the electrical LDPCcoded MB-OFDM UWB signal is filtered by a commercial electrical band-pass filter (EBPF) with a bandwidth of 3 GHz. The signal is
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then sampled by the Tektronix, bandwidth of 8 GHz, digital storage oscilloscope (DSO) operated at a sampling rate of 20 GSps. Moreover, there is a direct-current (DC) block between EBPF and DSO. And it is used to remove DC component. Subsequently, the received LDPC code MB-OFDM UWB signal is performed in MATLAB by offline processing. And the offline processing includes synchronization, channel equalization, demodulation OFDM, de-mapping 16QAM signal and LDPC decode. In the paper, TS is used to implement the timing synchronization. According to Eq. (2), the timing synchronization metric of LDPC-coded MB-OFDM UWB signal is shown in Fig. 3(a). It can be seen that one strong peak of the timing synchronization metric
Fig. 3. Timing synchronization metric of the experiment data after 70 km SSMF. (a) The MB-OFDM UWB signal timing metric of the experiment data frame. (b) Detailed impulse-shaped timing metric.
Fig. 4. The time domain waveform (a) and electrical spectrum (b) of LDPC code MB-OFDM UWB signal.
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J. He et al. / Optical Fiber Technology 21 (2015) 122–127
is very obvious. The detailed impulse-shaped timing synchronization metric is shown in Fig. 3(b). It shows that the sidelobes is reverse polarity. The reason is the property that negative-valued samples at the second half of TS can overcome the effect of the sidelobes. In this way, the effect of the sidelobes can be negligible in the timing synchronization. In addition, the shape of the timing synchronization metric is very sharp so that the start point of LDPC-coded MB-OFDM UWB signal can be estimated accurately. The measured time domain waveform and electrical spectrum of LDPC code MB-OFDM UWB signal is shown in Fig. 4. In the ECMA 368 standard, the first band group has three bands. In fact, current efforts from semiconductor companies for the implementation of integrated UWB devices focus on the first band group of three bands [18]. From Fig. 4(a), it can be seen that the waveform of Band #1, Band #2 and Band #3 is responding to the first band group in the ECMA 368 standard, respectively. Meanwhile, the center frequency of the first three bands are f1 = 3.432 GHz, f2 = 3.960 GHz and f3 = 4.488 GHz, as shown in Fig. 4(b). Error vector magnitude (EVM) is a measurement of modulator or demodulator performance in the impaired signal. The EVM is defined as [19]
0"
Ns 1X EVM ðdBÞ ¼ 20log10 @ jer j2 Ns r¼1
,
#12 1 Ns 1X 2 A jX r j Ns r¼1
ð10Þ
where, X r denotes the reference symbol, Y r is distorted version, Ns is the number of the transmitted symbols and er ¼ Y r X r is the error signal. The error vector magnitude (EVM) of first three bands are shown in Fig. 5. At an EVM of 16 dB, after 70 km SSMF transmission, the required receiver optical powers are 15.2 dB m for Band #1, 14.7 dB m for Band #2 and 13.8 dB m for Band #3, respectively. Compared with Band #1, the power penalty of Band #3 is 1.4 dB at an EVM of 16 dB. The loss of high frequency component in Band #3 is due to the low-pass filter effects. It is mainly caused by the electrical or optical devices, such as the roll-off of DAC and the limited bandwidth of AWG. After transmission over 70 km SSMF, we measured BER curves on different conditions such as 1.28 Gb/s signal without LDPC code and 0.96 Gb/s signal with LDPC code. And the constellations of the received baseband OFDM signal with/without LDPC-coded corresponding to the received optical power of 19 dB m, 16 dB m and 13 dB m, respectively, as shown in Fig. 6. It can be seen that the constellation of baseband OFDM signal without LDPC-coded
Fig. 6. The total BER of MB-OFDM UWB using 16-QAM format modulated as a function of received optical power for a fixed SSMF length of 70 km.
will be deterioration as the received optical power decreases. At the BER of 1 103, the required receive optical power of 1.28 Gb/s signal without code and 0.96 Gb/s signal with code is 14.9 dB m and 19 dB m, respectively. However, when the received optical power is less than 19.6 dB m, the experiment results indicate that the BER of LDPC-coded MB-OFDM UWB will be even worse than that of without LDPC-coded. Meanwhile, for free-error ðBER 1 104 Þ transmission, the received optical power with the LPDC coded MB-OFDM UWB is at least 18 dB m. Moreover, based on LDPC pre-coding techniques, the receiver sensitivity has been improved about 4 dB. 4. Conclusion In this paper, a LDPC pre-coding scheme was experimentally demonstrated in the intensity-modulation and direct-detection MB-OFDM UWBoF system. In addition, the symbol synchronization and pilot-aided channel estimation schemes were used to combat with the inter-symbol interference or inter-channel interference of the MB-OFDM UWB signal, so as to improve the performance of MB-OFDM UWBoF system. After transmission over 70 km SSMF, at the BER of 1 103, the experimental results showed the received optical power of 1.28 Gb/s signal without LDPC code and 0.96 Gb/s signal with LDPC code, is 14.9 dB m and 19 dB m, respectively. And the receiver sensitivity has been improved about 4 dB when the LDPC code rate is 75%. Therefore, it can be seen that the LDPC pre-coding scheme can work effectively in the intensity-modulation and direct-detection MB-OFDM UWBoF system. Acknowledgments This work is supported by National Natural Science Foundation of China (61307087, 61377079), by Hunan Provincial Natural Science Foundation of China (12JJ3070) and by the Fundamental Research Funds for the Central Universities and Young Teachers Program of Hunan University. References
Fig. 5. The EVM of first three bands after 70 km SSMF transmission.
[1] Federal Communications Commission, In the Matter of Revision of Part 15 of the Commission’s Rules Regarding Ultra-Wideband Transmission Systems, First Report and Order in ET Docket 98-153, 2002.
J. He et al. / Optical Fiber Technology 21 (2015) 122–127 [2] Alliance, WiMedia, Standard ECMA-368 High Rate Ultra Wideband PHY and MAC Standard, second ed., ECMA International, 2008. [3] M.L. Yee, V.H. Pham, Y.X. Guo, L.C. Ong and B. Luo, Performance evaluation of MB-OFDM ultra-wideband signals over single mode fiber, in: Proc. Int. Conf. Ultra Wideband (ICUWB), Singapore, 2007, pp. 674–677. [4] M.N. Sakib, B. Hraimel, X. Zhang, M. Mohamed, W. Jiang, K. Wu, D. Shen, Impact of optical transmission on multiband OFDM ultrawideband wireless system with fiber distribution, J. Lightw. Technol. 27 (18) (2009) 4112–4123. [5] T. Alves, A. Cartaxo, Performance degradation due to OFDM-UWB radio signal transmission along dispersive single-mode fiber, IEEE Photon. Technol. Lett. 21 (3) (2009) 158–160. [6] I.B. Djordjevic, O. Milenkovic, B. Vasic, Generalized low-density parity-check codes for optical communication systems, J. Lightw. Technol. 23 (5) (2005) 1939–1946. [7] S.M. Kim, J. Tang, K.K. Parhi, Quasi-cyclic Low-density Parity-check Coded Multi-band-OFDM UWB Systems, IEEE ISCAS Circuits and Systems, Kobe, Japan, 2005, pp. 65–68. [8] G. Dziwoki, M. Kucharczyk and W. Sulek, Transmission over UWB channels with OFDM system using LDPC coding, in: Photonics Appl. Proc. SPIE 7502, (2009) 75021Q. [9] M.N. Sakib, T. Huang, W.J. Gross, O. Liboiron-Ladouceur, Low-density paritycheck coding in ultra-wideband-over-fiber systems, IEEE Photon. Technol. Lett. 23 (20) (2011) 1493–1495. [10] M.N. Khan, and S. Ghauri, The WiMAX 802.16e physical layer model, in: Proc. IET Int. Conf. Wireless, Mobile Multimedia Networks’ 08, Mumbai, India, 2008, pp. 117–120.
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[11] F.R. Kschischang, B.J. Frey, H.A. Loeliger, Factor graphs and the sum-product algorithm, IEEE Trans. Inf. Theory 47 (2) (2001) 498–519. [12] W. Zhou, J. Yu, J. Xiao, X. Wang, L. Chen, Direct-detection optical orthogonal frequency division multiplexing system with new training sequence, Frequenz 66 (1) (2012) 27–32. [13] B. Park, H. Cheon, C. Kang, D. Hong, A novel timing estimation method for OFDM systems, IEEE Commun. Lett. 7 (5) (2003) 239–241. [14] Y. Guo, G. Liu, J. Ge, A novel time and frequency synchronization scheme for OFDM systems, IEEE Trans. Consum. Electron. 54 (2) (2008) 321–325. [15] M. Chen, J. He, L. Chen, Real-Time optical OFDM long-reach PON system over 100 km SSMF using a directly modulated DFB laser, J. Opt. Commun. Netw. 6 (1) (2014) 18–25. [16] M. Chen, J. He, J. Tang, L. Chen, Pilot-aided sampling frequency offset estimation and compensation using DSP technique in DD-OOFDM systems, Opt. Fiber Technol. 20 (3) (2014) 268–273. [17] Y. Zhao, A. Huang, A novel channel estimation method for OFDM mobile communication systems based on pilot signals and transform-domain processing, in: IEEE 47th Veh. Technol. Conf., Phoenix, AZ, USA, 1997, pp. 2089–2093. [18] A. Valdes-Garcia, C. Mishra, F. Bahmani, J. Silva-Martinez, E. Sánchez-Sinencio, An 11-band 3–10 GHz receiver in SiGe BiCMOS for multiband OFDM UWB communication, IEEE J. Solid State Circuits 42 (4) (2007) 935–948. [19] R.A. Shafik, S. Rahman, R. Islam, On the extended relationships among EVM, BER and SNR as performance metrics, in: Int. Conf. Electrical and Computer Engineering (ICECE), 2006, pp. 408–411.
Contents lists available at ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Experimental demonstration of the transmission performance for LDPC-coded multiband OFDM ultra-wideband over fiber system Jing He a,b,⇑, Xuejie Wen a, Ming Chen a, Lin Chen a, Jinshu Su b a b
College of Computer Science and Electronic Engineering, Hunan University, Changsha, Hunan, China State Key Laboratory of Parallel and Distributed Processing, and School of Computer, National University of Defense Technology, Changsha, Hunan, China
a r t i c l e
i n f o
Article history: Received 5 May 2014 Revised 22 September 2014 Available online 16 October 2014 Keywords: Multiband ultra-wideband (MB-UWB) Low-density parity-check (LDPC) Orthogonal frequency-divisionmultiplexing (OFDM) Receiver sensitivity
a b s t r a c t To improve the transmission performance of multiband orthogonal frequency division multiplexing (MBOFDM) ultra-wideband (UWB) over optical fiber, a pre-coding scheme based on low-density parity-check (LDPC) is adopted and experimentally demonstrated in the intensity-modulation and direct-detection MB-OFDM UWB over fiber system. Meanwhile, a symbol synchronization and pilot-aided channel estimation scheme is implemented on the receiver of the MB-OFDM UWB over fiber system. The experimental results show that the LDPC pre-coding scheme can work effectively in the MB-OFDM UWB over fiber system. After 70 km standard single-mode fiber (SSMF) transmission, at the bit error rate of 1 103, the receiver sensitivities are improved about 4 dB when the LDPC code rate is 75%. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction As the rapid growth of the communication capacity demand such as high definition television and cloud computing, ultra-wideband (UWB), which shares the spectrum resources with existing radio communications system, is recognized as a potential solution to meet the bandwidth requirements [1]. For the high speed applications, multiband orthogonal frequency division multiplexing (MB-OFDM) in IEEE 802.15.3a has some advantages such as high spectral flexibility by dynamically shaping the spectrum, robustness to RF interference and multi-path effects [2]. However, due to the low power spectral density regulated by the Federal Communications Commission (FCC), the communication distance of a MB-OFDM UWB wireless system is limited only to several meters. Recently, MB-OFDM UWB over fiber (UWBoF) technology, developed to distribute the MB-OFDM UWB signal over a long distance, was proposed to increase the area of coverage [3,4]. For MB-OFDM UWB signal transmitted over optical fiber, there is the degradation of the signal quality due to non-linear effect and dispersion impairments, in particular, accumulated optical amplifier noise, chromatic dispersion (CD) and polarization mode dispersion (PMD) [5]. In the past, in order to avoid the performance degradation due to adverse channel propagation conditions, a proper forward error correction (FEC) scheme was used in optical communications ⇑ Corresponding author at: College of Computer Science and Electronic Engineering, Hunan University, Changsha, Hunan, China. Fax: +86 73188822417. E-mail address: [email protected] (J. He). http://dx.doi.org/10.1016/j.yofte.2014.09.006 1068-5200/Ó 2014 Elsevier Inc. All rights reserved.
[6]. As an error correcting code, low-density parity-check (LDPC) code was introduced to increase the system performance of a radio-over-fiber system and MB-OFDM UWB wireless system [7,8]. In addition, the short block length LDPC code in the IEEE 802.15.3.c standard was proposed as a low complexity solution for impulse response (IR)-UWB systems [9]. However, the investigation of the LDPC-coded MB-OFDM UWB signal transmitting over optical fiber has been seldom reported. In this paper, for the first time to our knowledge, a pre-coding scheme based on LDPC in the MB-OFDM UWBoF system is investigated. To avoid inter-symbol interference (ISI) or inter-channel interference (ICI), a symbol synchronization and pilot-aided channel estimation scheme is implemented on the LDPC-coded MB-OFDM UWBoF system. After transmission over 70 km standard single-mode fiber (SSMF), compared MB-OFDM UWB signal with and without LPDC-coded, when the LDPC code rate is 75%, the receiver sensitivities are improved about 4 dB at a BER of 1 103. The experiment results show that short block length LDPC is effective for MB-OFDM UWBoF system. 2. Principle 2.1. The structure of the IM/DD optical MB-OFDM UWB system The transmitter and receiver of the LDPC-coded MB-OFDM UWBoF system are shown in Fig. 1. In the transmitter, pseudo-random binary sequences (PRBS) are encoded using LDPC code. The LDPC code we used is based on WiMAX 802.16e standard [10].
J. He et al. / Optical Fiber Technology 21 (2015) 122–127
After the LDPC encoder, the output bit streams are mapping into M-PSK or M-QAM symbols. The complex-valued signal points are passing through a serial-to-parallel (S/P) converter. For the purpose of channel tracking, several sub-carriers are used to transmit pilot symbols in the transmitter. After inverse fast Fourier transforms (IFFT), a certain length of cyclic prefix (CP) is added to combat inter-symbol interference (ISI) in the fiber channel. To mitigate the nonlinearity of the optical transmission and reduce the highdynamic range requirement of digital-to-analog conversion (DAC), the peak-to-average power ratio (PAPR) of OFDM signal should be reduced. Thus, clipping is used to reduce the PAPR of OFDM signal before DAC. Then, a training sequence (TS) is inserted at the front of the OFDM symbols. And symbol synchronization at the receiver is realized by using the inserted TS. Subsequently, the baseband OFDM signal with LDPC code is digital up-conversion with time–frequency code (TFC) [2] to generate an MB-OFDM UWB signal with LDPC code. Then the generated LDPC-coded MB-OFDM UWB signal is converted to an analog one by the DAC. After up-conversion and DAC, the LDPC-coded MB-OFDM UWB signal is used as a radio frequency (RF) to driven a Mach–Zehnder modulator (MZM). And the generated optical LDPC-coded MBOFDM UWB signal is transmitted over optical fiber. At the receiver, the optical LDPC-coded MB-OFDM UWB signal is converted into electrical signal by a photo-detector after optical amplifier. The electrical LDPC-coded MB-OFDM UWB signal is digitized by an analog to digital converter (ADC). The digitalized signal is further down-converted to the baseband signal with the corresponding TFC. And the baseband signal is sent to the TS-based symbol synchronization function. Once the starting point of the data-carrying OFDM symbol is located, CP will be removed. After serial to parallel conversion, fast Fourier transforms (FFT) function is utilized to demodulated OFDM signal. In addition, the pilotaided channel equalization method is used to compensate the response of channel. Then the outputs of the M-PSK or M-QAM demodulator are fed to a LDPC decoder and it is implemented
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based on the sum-product algorithm [11]. Meanwhile, the bit error rate (BER) is calculated over de-mapped bits. 2.2. A symbol synchronization and pilot-aided channel estimation scheme Since MB-OFDM UWB systems are much more sensitive to synchronization errors than single carrier systems, the timing synchronization of MB-OFDM UWB is necessary to find the correct symbol start. Therefore, in the paper, the TS are used to implement the timing synchronization. In order to reduce the signal–signal beating interference (SSBI) in the direct-detection optical OFDM receiver [12], the TS is generated by transmitting BPSK symbols on the odd subcarriers. Meanwhile, the even subcarriers and the subcarriers at high frequencies are filled with zeroes. In addition, all the data (including BPSK symbols and zeros) on the subcarriers are constrained to have Hermitian symmetry, and then the result of the inverse fast Fourier transform (IFFT) will produce the realvalued time-domain sequence as shown in Eq. (1). The TS can be expressed as
TSpro ¼ ½AN=4
BN=4 AN=4 BN=4
ð1Þ
where AN=4 and BN=4 are real-value, N is the size of the IFFT. AN=4 means samples of length generated by the IFFT of a real-valued sequence, while BN=4 is designed to be symmetric with AN=4 so as to obtain an impulse-shaped timing metric. The basic form of the TS is similar to Park et al.’s method [13]. Therefore, Park et al.’s timing metric function for symbol synchronization can also be applied to the TS. However, it is observed that the timing metric of Park’s method has large sidelobes at the positions ±N with samples spaced from the correct starting point of OFDM symbols, which will affect the accurateness of timing offset estimation [14]. Here, the property that negative-valued samples at the second half of training symbol is used to overcome the effect of the sidelobes. Thus, the timing metric is defined as
Fig. 1. The structure of IM-DD MB-OFDM UWB over fiber system.
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M pro ðmÞ ¼ signðPðmÞÞ
jPðmÞj
2
jRðmÞj
ð2Þ
2
PðmÞ ¼
tðlÞ rðm þ lÞ
3. Experimental setup and results
ð4Þ
l¼0
where sign½ stands for sign function, t(l) is the transmitted TS and r(l) is the received signal. Then, the estimation of timing offset can be written as
^e ¼ arg max ðM pro ðmÞÞ;
ð5Þ
m
Unlike the variable wireless channel environment, the channel response in the optical fiber can be considered approximately as a constant within a short period of time and the difference of channel frequency responses over several adjacent subcarriers is small [15]. Therefore, pilot-aided scheme has been used to estimate channel response [15] and sample frequency offset [16] in optical OFDM system. Here, a pilot-aided channel estimation scheme is used in the receiver of MB-OFDM UWBoF system. At the known pilot subcarriers, the low-complexity least-square (LS) estimation technique is adopted to calculate the channel transfer function
Y p ðkÞ ðk ¼ 0; . . . ; Np 1Þ X p ðkÞ
ð6Þ
where Y p ðkÞ is the received kth pilot subcarrier, X p ðkÞ is the transmitted kth pilot subcarrier and N p is the number of pilot sub-carriers. These LS estimates Hp ðkÞ are then interpolated with the time domain DFT interpolation to get the channel response at the non-pilot sub-carriers. The time domain DFT interpolation is a high-resolution interpolation based on zero-padding and DFT/IDFT [17]. After obtaining the estimated channel, it can be converted to the time domain by IDFT
Gp ðnÞ ¼
NX p 1
jN2p kn
Hp ðkÞe
p
ðk ¼ 0; . . . ; Np 1Þ
ð7Þ
k¼0
Then, the signal Gp ðnÞ is transformed into N points by padding with N N p zeros, and it becoming as
GN ðnÞ ¼
ð9Þ
ð3Þ
N1 X 2 rðm þ lÞ
Hp ðkÞ ¼
ðk ¼ 0; . . . ; N 1Þ
where HN ðkÞ denotes the channel response on the kth sub-carrier of all sub-carriers.
l¼0
RðmÞ ¼
N1 X 2p GN ðnÞej N kn k¼0
where N1 X
HN ðkÞ ¼
Gp ðnÞ; 0 n Np 0; Np n N
ð8Þ
Again, the estimated channel transfer function is obtained by DFT of GN ðnÞ, and it can be expressed as
The experimental setup of LDPC-coded MB-OFDM UWB signals over fiber system is shown in Fig. 2. In the central station, continuous wave (CW) light is generated from a commercial external cavity laser (ECL) source at the wavelength of 1565.38 nm. And the launch power from ECL is 7 dB m. Then, the CW light is injected into a single-drive Mach–Zehnder modulation (MZM), which is driven by the LDPC-coded MB-OFDM UWB signal. In the paper, the generated MB-OFDM UWB signal is based on ECMA 368 standard [2]. The OFDM symbol interval is 312.5 ns. The total number of subcarriers is 128, where the subcarriers used for data, pilot, guard and zero are 100, 12, 10 and 6, respectively. The data rate can be expressed as Rdata ¼ log2 ðKÞ ð1=312:5Þ 100ðMb=sÞ, where the K is the modulation order. For the 16QAM coded MB-OFDM UWB signal, the data rate is equal to 1.28 Gb/s. In this way, for the 75% code rate LDPC coded MB-OFDM UWB signal, the data rate is 0.96 Gb/s. And Table 1 shows the parameters of LDPC-coded MBOFDM UWB signal in the experiment. Meanwhile, the polarization controller (PC) is rotated to maximize the MZM output optical power. The LDPC-coded MB-OFDM UWB signal is generated offline using MATLAB, and then loaded into a Tektronix Arbitrary Waveform Generator (AWG) operating at 10.561 GSps. The DC bias voltage is biased at negative quadrature point. And the generated LDPC-coded MB-OFDM UWB signal drives the single-drive MZM to modulate the optical carrier. The output optical LDPC code MB-OFDM UWB signal is then sent
Table 1 The parameters of LDPC-coded MB-OFDM UWB signal in the experiment. Description
Value
Symbol interval Total number of subcarriers (FFT size) Number of data subcarriers Number of pilot subcarriers Number of guard subcarriers Number of zero subcarriers Bandwidth (for one subband) Constellation Data rates with LDPC code LDPC code rate Cyclic Prefix
312.5 ns 128 100 12 10 6 528 MHz 16-QAM 0.96 Gb/s 75% 32
Fig. 2. Experimental setup of the LDPC-coded MB-OFDM UWBoF system.
J. He et al. / Optical Fiber Technology 21 (2015) 122–127
through up to 70 km standard single-mode fiber (SSMF), with fiber loss of a = 0.22 dB/km and chromatic dispersion of 17 ps/(nm km). At the base station, the optical signal is amplified to 6.3 dB m by an erbium-doped fiber amplifier (EDFA) with a noise figure (NF) of 5 dB to compensate the fiber and connector losses. The attenuation of the variable optical attenuator (VOA) is varied so that it can enable the BER performance of the system to be analyzed as a function of the received optical power. Then, the optical electrical conversion is obtained by a commercial photo-diode with a 3 dB bandwidth of 10 GHz. After photo-detection, the electrical LDPCcoded MB-OFDM UWB signal is filtered by a commercial electrical band-pass filter (EBPF) with a bandwidth of 3 GHz. The signal is
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then sampled by the Tektronix, bandwidth of 8 GHz, digital storage oscilloscope (DSO) operated at a sampling rate of 20 GSps. Moreover, there is a direct-current (DC) block between EBPF and DSO. And it is used to remove DC component. Subsequently, the received LDPC code MB-OFDM UWB signal is performed in MATLAB by offline processing. And the offline processing includes synchronization, channel equalization, demodulation OFDM, de-mapping 16QAM signal and LDPC decode. In the paper, TS is used to implement the timing synchronization. According to Eq. (2), the timing synchronization metric of LDPC-coded MB-OFDM UWB signal is shown in Fig. 3(a). It can be seen that one strong peak of the timing synchronization metric
Fig. 3. Timing synchronization metric of the experiment data after 70 km SSMF. (a) The MB-OFDM UWB signal timing metric of the experiment data frame. (b) Detailed impulse-shaped timing metric.
Fig. 4. The time domain waveform (a) and electrical spectrum (b) of LDPC code MB-OFDM UWB signal.
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J. He et al. / Optical Fiber Technology 21 (2015) 122–127
is very obvious. The detailed impulse-shaped timing synchronization metric is shown in Fig. 3(b). It shows that the sidelobes is reverse polarity. The reason is the property that negative-valued samples at the second half of TS can overcome the effect of the sidelobes. In this way, the effect of the sidelobes can be negligible in the timing synchronization. In addition, the shape of the timing synchronization metric is very sharp so that the start point of LDPC-coded MB-OFDM UWB signal can be estimated accurately. The measured time domain waveform and electrical spectrum of LDPC code MB-OFDM UWB signal is shown in Fig. 4. In the ECMA 368 standard, the first band group has three bands. In fact, current efforts from semiconductor companies for the implementation of integrated UWB devices focus on the first band group of three bands [18]. From Fig. 4(a), it can be seen that the waveform of Band #1, Band #2 and Band #3 is responding to the first band group in the ECMA 368 standard, respectively. Meanwhile, the center frequency of the first three bands are f1 = 3.432 GHz, f2 = 3.960 GHz and f3 = 4.488 GHz, as shown in Fig. 4(b). Error vector magnitude (EVM) is a measurement of modulator or demodulator performance in the impaired signal. The EVM is defined as [19]
0"
Ns 1X EVM ðdBÞ ¼ 20log10 @ jer j2 Ns r¼1
,
#12 1 Ns 1X 2 A jX r j Ns r¼1
ð10Þ
where, X r denotes the reference symbol, Y r is distorted version, Ns is the number of the transmitted symbols and er ¼ Y r X r is the error signal. The error vector magnitude (EVM) of first three bands are shown in Fig. 5. At an EVM of 16 dB, after 70 km SSMF transmission, the required receiver optical powers are 15.2 dB m for Band #1, 14.7 dB m for Band #2 and 13.8 dB m for Band #3, respectively. Compared with Band #1, the power penalty of Band #3 is 1.4 dB at an EVM of 16 dB. The loss of high frequency component in Band #3 is due to the low-pass filter effects. It is mainly caused by the electrical or optical devices, such as the roll-off of DAC and the limited bandwidth of AWG. After transmission over 70 km SSMF, we measured BER curves on different conditions such as 1.28 Gb/s signal without LDPC code and 0.96 Gb/s signal with LDPC code. And the constellations of the received baseband OFDM signal with/without LDPC-coded corresponding to the received optical power of 19 dB m, 16 dB m and 13 dB m, respectively, as shown in Fig. 6. It can be seen that the constellation of baseband OFDM signal without LDPC-coded
Fig. 6. The total BER of MB-OFDM UWB using 16-QAM format modulated as a function of received optical power for a fixed SSMF length of 70 km.
will be deterioration as the received optical power decreases. At the BER of 1 103, the required receive optical power of 1.28 Gb/s signal without code and 0.96 Gb/s signal with code is 14.9 dB m and 19 dB m, respectively. However, when the received optical power is less than 19.6 dB m, the experiment results indicate that the BER of LDPC-coded MB-OFDM UWB will be even worse than that of without LDPC-coded. Meanwhile, for free-error ðBER 1 104 Þ transmission, the received optical power with the LPDC coded MB-OFDM UWB is at least 18 dB m. Moreover, based on LDPC pre-coding techniques, the receiver sensitivity has been improved about 4 dB. 4. Conclusion In this paper, a LDPC pre-coding scheme was experimentally demonstrated in the intensity-modulation and direct-detection MB-OFDM UWBoF system. In addition, the symbol synchronization and pilot-aided channel estimation schemes were used to combat with the inter-symbol interference or inter-channel interference of the MB-OFDM UWB signal, so as to improve the performance of MB-OFDM UWBoF system. After transmission over 70 km SSMF, at the BER of 1 103, the experimental results showed the received optical power of 1.28 Gb/s signal without LDPC code and 0.96 Gb/s signal with LDPC code, is 14.9 dB m and 19 dB m, respectively. And the receiver sensitivity has been improved about 4 dB when the LDPC code rate is 75%. Therefore, it can be seen that the LDPC pre-coding scheme can work effectively in the intensity-modulation and direct-detection MB-OFDM UWBoF system. Acknowledgments This work is supported by National Natural Science Foundation of China (61307087, 61377079), by Hunan Provincial Natural Science Foundation of China (12JJ3070) and by the Fundamental Research Funds for the Central Universities and Young Teachers Program of Hunan University. References
Fig. 5. The EVM of first three bands after 70 km SSMF transmission.
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