SSBI mitigation at 60GHz OFDM-ROF system based ... - OSA Publishing

SSBI mitigation at 60GHz OFDM-ROF system based on optimization of training sequence Xin Wang,1 Jianjun Yu,1,2,3 Zizheng Cao,1 J. Xiao,1 and Lin Chen1,* 1

Key Laboratory for Micro/Nano Optoelectronic Devices of Ministry of Education, School of Information Science and Engineering, Hunan University, Changsha 410082, China 2 ZTE USA Inc., Iselin, New Jersey 08830, USA 3 ZTE Inc., Beijing 100876, China *[email protected]

Abstract: We have theoretically and experimentally investigated the effect of the interference between subcarrier-signal beat interference (SSBI) in 60 GHz orthogonal frequency division multiplexing - radio-over-fiber (OFDMROF) system. In order to reduce the influence of SSBI, we compared four kinds of OFDM frames with different training sequences as all-real, allcomplex, complex-zero and real-zero training. The experimental results show the power penalty of all-real, all-complex, complex-zero and real-zero training is 2.5, 5.5, 4 and 1dB at BER of 1x10-3 after 20km standard single mode fiber (SMF) transmission, respectively. The real-zero training OFDM frame with interleave structure and lower modulation order signal suffered from the least SSBI shows the best performance. ©2011 Optical Society of America OCIS codes: (060.4080) Modulation; (060.4230); Multiplexing; (060.2330) Fiber optics communication; (060.2360) Fiber optics links and subsystems.

References and links W.-J. Jiang, C.-T. Lin, P.-T. Shih, L.-Y. Wang He, J. Chen, and S. Chi, “Simultaneous Generation and Transmission of 60-GHz Wireless and Baseband Wireline Signals With Uplink Transmission Using an RSOA,” IEEE Photon. Technol. Lett. 22(15), 1099–1101 (2010). 2. H. S. Chung, S. H. Chang, J. D. Park, M.-J. Chu, and K. Kim, ““Transmission of Multiple HD-TV Signals Over a Wired/Wireless Line Millimeter-Wave Link With 60 GHz,” Lightwave Technology,” Journalism 25, 3413– 3418 (2007). 3. A. J. Lowery, L. B. Du, and J. Armstrong, ““Performance of Optical OFDM in Ultralong-Haul WDM Lightwave Systems,” Lightwave Technology,” Journalism 25, 131–138 (2007). 4. F. Vacondio, M. Mirshafiei, J. Basak, A. Liu, L. Liao, M. Paniccia, and L. A. Rusch, “A Silicon Modulator Enabling RF Over Fiber for 802.11 OFDM Signals,” IEEE J. Sel. Top. Quantum Electron. 16(1), 141–148 (2010). 5. C.-T. Lin, J. Chen, P.-T. Shih, W.-J. Jiang, and S. Chi, ““Ultra-High Data-Rate 60 GHz Radio-Over-Fiber Systems Employing Optical Frequency Multiplication and OFDM Formats,” Lightwave Technology,” Journalism 28, 2296–2306 (2010). 6. P.-T. Shih, C.-T. Lin, W. J. Jiang, Jr., Y.-H. Chen, J. J. Chen, and S. Chi, “Full duplex 60-GHz RoF link employing tandem single sideband modulation scheme and high spectral efficiency modulation format,” Opt. Express 17(22), 19501–19508 (2009). 7. L. Chen, J. G. Yu, S. Wen, J. Lu, Z. Dong, M. Huang, and G. K. Chang, ““A Novel Scheme for Seamless Integration of ROF With Centralized Lightwave OFDM-WDM-PON System,” Lightwave Technology,” Journalism 27, 2786–2791 (2009). 8. Z. Cao, Z. Dong, J. Lu, M. Xia, and L. Chen, “Optical OFDM signal generation by optical phase modulator and its application in ROF system,” in Optical Communication, 2009. ECOC '09. 35th European Conference on(German National Library, Vienna, Austria, 2009), pp. 1–2. 9. Z. Dong, Z. Cao, J. Lu, Y. Li, L. Chen, and S. Wen, “Transmission performance of optical OFDM signals with low peak-to-average power ratio by a phase modulator,” Opt. Commun. 282(21), 4194–4197 (2009). 10. J. Yu, J. Hu, D. Qian, Z. Jia, G. K. Chang, and T. Wang, “Transmission of microwave-photonics generated 16Gbit/s super broadband OFDM signals in radio-over-fiber system,” in Optical Fiber communication/National Fiber Optic Engineers Conference, 2008. OFC/NFOEC 2008. Conference on(Optical Society of America, San Diego,USA 2008), pp. 1–3. 11. Z. Jia, J. Yu, Y.-T. Hsueh, H.-C. Chien, and G.-K. Chang, “Demonstration of a symmetric bidirectional 60-GHz radio-over-fiber transport system at 2.5-Gb/s over a single 25-km SMF-28,” in Optical Communication, 2008. ECOC 2008. 34th European Conference on(2008), pp. 1–2. 1.

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Received 1 Mar 2011; revised 14 Apr 2011; accepted 14 Apr 2011; published 21 Apr 2011

25 April 2011 / Vol. 19, No. 9 / OPTICS EXPRESS 8839

12. Z. Cao, J. Yu, M. Xia, Q. Tang, Y. Gao, W. Wang, and L. Chen, ““Reduction of Intersubcarrier Interference and Frequency-Selective Fading in OFDM-ROF Systems,” Lightwave Technology,” Journalism 28, 2423–2429 (2010). 13. X. Xin, L. Zhang, B. Liu, and J. Yu, “Dynamic λ-OFDMA with selective multicast overlaid,” Opt. Express 19(8), 7847–7855 (2011). 14. W.-R. Peng, X. Wu, V. R. Arbab, K.-M. Feng, B. Shamee, L. C. Christen, J.-Y. Yang, A. E. Willner, and C. Sien, ““Theoretical and Experimental Investigations of Direct-Detected RF-Tone-Assisted Optical OFDM Systems,” Lightwave Technology,” Journalism 27, 1332–1339 (2009). 15. W.-R. Peng, K.-M. Feng, A. E. Willner, and S. Chi, ““Estimation of the Bit Error Rate for Direct-Detected OFDM Signals With Optically Preamplified Receivers,” Lightwave Technology,” Journalism 27, 1340–1346 (2009). 16. J. Ma, J. Yu, C. Yu, X. Xin, J. Zeng, and L. Chen, ““Fiber Dispersion Influence on Transmission of the Optical Millimeter-Waves Generated Using LN-MZM Intensity Modulation,” Lightwave Technology,” Journalism 25, 3244–3256 (2007).

1. Introduction Recently, 60 GHz radio (with 7-GHz free bandwidth license) over fiber system (ROF) to provide an effective way for the next generation of ultra-broadband wireless access network has been paid much attention [1,2]. Due to its good resistance to the dispersion and highspectral efficiency, OFDM signal has been used in the wireless system and become the core technique of standards such as IEEE802.11 and 802.16 [3,4]. To use OFDM signal in the ROF system can provide ultra-wide bandwidth in the future of wireless communication networks [5–13]. In the past few years, people investigate on OFDM signals without the guard interval in direct detection (DD) OFDM-ROF system [7–12]. However, these systems mainly consider the architecture of the OFDM-ROF system, but neglect the performance limitation in the DDOFDM transmission system. Recently analysis of OFDM-ROF has been presented in Ref [12], and their conclusions show that interference between subcarrier signal beat interference (SSBI) and frequency-selective fading (FF) is the main impairment in the OFDM-ROF system. Ref [12]. also proposed to use turbo codes and bit interleaver technologies to mitigate error distribution caused by SSBI. However, the coding technology increases the system redundancy. Therefore, how to maintain the simple structure of the system while reducing the impact of SSBI becomes an important topic. In the previous research on DD-OFDM system, people proposed an interval structure to overcome the impairments of SSBI in [14,15]. In this paper we theoretically investigate SSBI influence on 60GHz DD OFDM-ROF system. To reduce the impact of SSBI in the OFDM-ROF system, we experimentally investigate four kinds of OFDM structures using different training sequences to mitigate these SSBI after transmission over 20km standard single mode fiber (SMF). The experimental results show that the OFDM signal with zero-real training sequence has the best performance with the least influence caused by SSBI. 2. Theoretical Analysis The configuration of 60GHz DD OFDM-ROF communication system based on external modulator is shown in Fig. 1. In the central office (CO), the CW lightwave is generated by one distributed feedback (DFB) laser, represented by Ein (t )  Ec cos(wc t ) , here Ec and

wc represents the amplitude and angular frequency of the optical carrier, respectively. Then the optical carrier is intensity-modulated via one Mach-Zehnder modulator (MZM) intensity modulator by a RF (radio frequency) with VRF (t )  VRF cos RF t , here VRF and  RF is the amplitude and the angular frequency of the RF, respectively. The output of the MZM is expressed as [16] Eout1  t   a1 cos c  RF  t  a0 cos c t  a1 cos c  RF  t.

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(1)



Fig. 1. Principle of the proposed 60GHz OFDM-ROF architecture.

Here a1 , a1 and a 0 are the relative amplitude of the lower sidebands, upper sidebands and the central optical carrier, respectively. The higher harmonics of the optical lightwave are too small to be considered. After the interleaver, the central optical carrier is filter out. The optical mm-wave with two sidebands can be expressed as: Eout1  t   a1 cos c  RF  t  a1 cos c  RF  t. (2) So the frequency between the upper and lower sidebands is double repetitive frequency of the RF signal. The discrete-time domain electrical OFDM signal, used for driving the second MZM intensity modulator, can be described as

x(t ) 

1 N

N 1

 d (n) exp( n 0

j 2 nt ), N

(3)

where t is the discrete time index, N is the number of the sub-carrier, d (n) is the data symbol modulated on the n th sub-carrier. The modulator works in the linear range by adjusting the bias voltage and DC of the modulator. The expression of the OFDM signal after upconversion by the second intensity modulator is: 1  Eout 2  t   [a1 cos c  RF  t  a1 cos c  RF  t ]  1  m N 

N 1

 d (n) exp( n 0

j 2 nt ) N

 (4) , 

where m is the optical modulator index. When the optical mm-wave signal is distributed along the fiber, the sidebands and the OFDM signal are transmitted at different velocities because of the fiber chromatic dispersion. Here we assume that the propagation constant is  ( w) , the fiber loss is r , and the effects of other nonlinearities are neglected. The optical mm-wave after transmission over z length fiber, the optical signal can be expressed as:

Eout 2  z, t   a1e rz cos[c  RF  t   (c  RF ) z ]  a1e rz cos[c  RF  t   (c  RF ) z ]  a1e rz m x(t  c  RF   (c  RF ) z )cos[c  RF  t   (c  RF ) z ] 1

(5)

 a1e rz m x(t  c  RF   (c  RF ) z )cos[c  RF  t   (c  RF ) z ]. 1

Here we assume Ak  ak e rz x(t  (wc  kwRF )1  (wc  kwRF ) z )(k  1). Bk  ak e

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 rz

(k  1) (6)



After the detection by a PIN at the base station after transmission over optical fiber with a certain length, the output current of the PIN is: I (t )   Eout 2  z, t 

2

1  (mA1 B1  mA1 B1  m 2 A21  m 2 A21  B21  B21 ) 2 (7) 1 ' 2 ''   mA1 B1 cos[( wc  wRF )t  2  ( wc ) z  2wRF  ( wc ) z  2wRF  ( wc ) z ] 2 1 2   mA1 B1 cos[( wc  wRF )t  2 ( wc ) z  2wRF  ' ( wc ) z  2wRF  '' ( wc ) z ] 2 1   ( A1 B1  A1 B1  B1 B1  m 2 A1 A1 ) cos 2wRF [t   ' ( wc ) z ]  MM _ OFDM _ Signal. 2 

Here  is the conversion efficiency of the photon detector, and the Taylor’s expansion of the propagation constant can be shown as

1 2

2  (C  kRF )   (C )  kRF  (C )  k 2RF  (C ).

(8)

Here  (C ) is the phase shift,  (C ) is the group delay, and  (C ) is the first-order dispersion. In Eq. (7), the first item is DC component, the second and third one is the fundamental components, and the last one is the mm-wave OFDM signal. Here we only consider the mm-wave OFDM signal. The current of the OFDM signal can be written as: I MM _ OFDM (t ) 

1  ( A1 B1  A1 B1  B1 B1 ) cos 2wRF [t   ' (wc ) z ]  OFDM _ Signal 2 (9)

1   m2 A1 A1 cos 2wRF [t   ' ( wc ) z ]  SSBI . 2 SSBI can be expressed as:

1  m2 A1 A1 cos 2wRF [t   ' ( wc ) z ] 2 1   m2 a1e rz x(t  ( wc  wRF )1  ( wc  wRF ) z ) 2 a1e rz x(t  ( wc  wRF ) 1  ( wc  wRF ) z )  cos 2wRF [t   ' ( wc ) z ].

I SSBI 

(10)

It is shown that the beating between the sub-carriers of the upper and lower sideband OFDM signal generates SSBI when the OFDM signal is detected by the PIN after transmission over the fiber. The output current from the PIN includes not only the OFDM signal but also the SSBI, and the electrical OFDM signal is degraded by the SSBI signal. We describe the beating of the mm-wave OFDM signal as shown in Fig. 2.

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(a)

Optical spectrum

LSB

OFDM Subcarriers

USB

electrical spectrum

.

.

.

f c  f RF

.

(e)

f c  f RF

electrical spectrum

(b)

Optical spectrum



2 f RF

.

SSBI

.

detection

(c)

.

.





LSB X OFDM Subcarriers Optical spectrum

(f)

electrical (d) spectrum

.



.

. 2 f RF

All out

. 2 f RF Signal

OFDM Subcarriers X USB

Fig. 2. DD-ROF-OFDM signal detected at the PIN.

Figure 2(a) shows the electrical spectrum after the PIN. It includes the two sidebands of the optical mm-wave as upper sideband (USB) and lower sideband (LSB), and the subcarriers of OFDM signal are distributed in each sideband. The electrical OFDM signal after detection by a PIN is shown in Fig. 2(d), which is generated from beating between the optical sideband and sub-carriers of the other optical sideband as shown in Fig. 2(b) and (c). The SSBI in Fig. 2(e) is generated from beating between the sub-carriers of the OFDM signal in the USB and LSB when the optical signal is received by a square-law photon detector as shown in Fig. 2(a). So the output electrical OFDM signal from the PIN includes the SSBI which can degrade the electrical OFDM. Then the signal is down-converted by an electrical mixer with a LO signal to obtain the baseband data. The demodulation and analysis of OFDM signal are realized by using an offline program. 3. OFDM Signal with Different Training Sequences In order to decrease the effect of the SSBI when the optical OFDM signals are detected, we propose to equalize and estimate the received electrical signal by combining training sequence with the pilots in the data-OFDM-symbol. Figure 1 shows four kinds of OFDM structure with different types of training sequence. For each OFDM symbol, the IFFT size is 256, where 8 channels are used for pilot transmission. Then we apply a cyclic prefix per each OFDM data or training data to eliminate Inter Symbol Interference (ISI). The length of the cyclic prefix (CP) is 32. At last, by combing the training sequence with the data OFDM symbol, the OFDM frame is generated. Figure 3(a) is the OFDM frame with all-real training sequence. In this OFDM symbol, the training sequence is composed with random 1 and 1. Finally, these 256 sub-carriers are modulated by IFFT and add 32 CP for the synchronization and estimation. Figure 3(b) is the OFDM frame with all-complex training sequence. The training sequence is composed with random 1 + i 1-i 1 + i and-1-i. Figure 3(c) is the OFDM frame with complexzero training sequence. In this training sequence, all the odd channels are filled with random complex, but the even channels are filled with zero. Figure 3(d) is the OFDM frame with realzero training sequence. In this training sequence, we fill the odd channels with random real signal and let the even channels to be blank. In these interleave training sequence, as we predict, the SSBI generated by beating between sub-carriers will just fall into the even channels if we only fill the odd channels with signal. Therefore the information on the oddchannels will not be interfered. Hence we can use the information on the odd-channels to estimate the entire OFDM by interpolation. The frame with real date training sequence can be regarded as the data OFDM mapped with BPSK signal, and the frame with complex training #143348 - $15.00 USD

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can be regarded as the data OFDM mapped with QPSK signal. Although the BPSK signal has less spectral efficiency, it has higher transmission performance and it makes the probability of error even smaller when the OFDM signal detected at the receiver after a certain optical transmission length. Therefore, the OFDM frame with real training sequence has better performance. Therefore, we can reduce the impact of SSBI and improve the system performance by using this kind of training sequence.

Fig. 3. OFDM frame with different training sequence. (a) all-real training sequence, (b) allcomplex training sequence, (c) complex-zero training sequence, and (d) real-zero training sequence.

4. Experimental setup and results Figure 4 shows the experimental setup for 58 GHz OFDM-ROF system. In the optical transmitter the continuous-wave lightwave was generated by a DFB laser at 1542.8nm as shown in Fig. 4 as inset (a), and the power of the continuous-wave light is 8.25dBm. Then the lightwave was modulated by a single arm MZM intensity modulator driven by a 29GHz RF microwave signal. Double sideband (DSB) modulation signal is generated after the modulator, as shown in Fig. 4 as inset (b). The power of the higher-order sidebands (second-order and higher) are 40 dB lower than the first-order sidebands, and the power of the light after the first modulator is about 6.38dBm. An 50/100GHz optical interleaver (IL) was employed to separate the optical carrier and the first-order sidebands, and an EDFA was used to amplified the separated signal.. The power of the signal after IL and EDFA is 23.7dBm and 7.5dBm, respectively. The wavelength spacing between the first-order sidebands is 0.46nm (58GHz), as shown in Fig. 4 as inset (c). The optical mm-wave signal which was amplified by an EDFA was modulated by the second intensity modulator (IM) driven by the 2.5Gbit/s OFDM baseband signal which were generated offline by Matlab program. The OFDM waveform produced by the arrayed waveform generator (AWG) was continuously output at 4.0GSample/s. The OFDM frame with the structure has been discussed in the previous section. The OCS-OFDM signals with the power of 4.4dBm, as shown in Fig. 4 as inset (d), was amplified by the second EDFA. Then the signal were launched into 20km SMF with an input power of 8.6dBm before the OFDM signals were detected by a high-speed photodetector (>60GHz bandwidth). The converted electrical signals were amplified by an

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Training sequence

Different Training bits

OFDM symbol modulation

Data OFDM

electrical amplifier (EA) with a bandwidth of 10GHz centered at 60GHz. An electrical local oscillator (LO) signal at 58GHz was generated by using a frequency quadruple from 14.5 to 58GHz. The electrical signal was down-converted by using an electrical mixer with the electrical LO to retrieve the baseband OFDM signal. The retrieved OFDM signal samples was sampled by a digital oscilloscope, then the demodulation and analysis of OFDM signal are realized by an offline program.

Data bit input

OFDM symbol modulation

OFDM-Tx

AWG

OFDM Waveform

MUX

LO

Optical Transmitter

ATT EDFA

Mixer

LO

EDFA IM

SSMF

EA MUX

EA

EDFA MZM

PD

CW

IL

Optical Receiver

(d)

(c)

(b)

(a)

LPF

(b)

Data bit output

Oscilloscope

(a)

OFDM-Rx OFDM symbol demodulation

(c)

(d)

Fig. 4. Experimental setup of the proposed 58 GHz OFDM-ROF system. AWG: arbitrary waveform generator; CW: continuous-wave; MZM: Mach-Zehnder modulator; EA: electrical amplifier; LO: local oscillator; IL: interleaver; IM: intensity-modulator; PD: photon-detector; LPF: low-pass filter; (a) optical spectrum of the source, (b) optical spectrum after DSB modulation, (c)optical spectrum after the optical carrier is removed, (d) optical spectrum after the second IM.

Figure 5 shows constellations at different kinds of training sequence for both BTB and after 20km optical downstream transmission. For the BTB case, it can be seen that constellations of the OFDM frame with continuous training (as all-real training and allcomplex training) is a little better than the frame with interleave training (as real-zero training and complex-zero training) and the frame with complex training’s constellations is better that the frame with real training. The reason is that the OFDM frame with continuous training and complex training has more information to estimate the channel; therefore the result of channel estimation is better. However, after 20km SMF transmission, all the sub-channels of the OFDM signal with continuous training are degraded by the SSBI, and it makes the dispersing of the constellations very seriously. But, the constellation of the OFDM with real-zero training sequence is the clearest, which means the little effect of the SSBI.

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Fig. 5. Constellations of BTB and 20km transmission at different kinds of training sequence. (a) All-real training, (b) all-complex training, (c) complex-zero training, and (d) real-zero training. all-real BTB all-real 20km all-complex BTB all-complex 20km complex-zero BTB complex-zero 20km real-zero BTB real-zero 20km

-log(BER)

2

3

4 5 6 -26

-25

-24

-23

-22

-21

-20

-19

-18

Received power Fig. 6. BER curves of OFDM frame with different training sequence.

The measured bit error rate (BER) curves of the OFDM frame are shown in Fig. 6. The receiver sensitivity of the OFDM signal with real-zero training is the highest. The power penalty of the OFDM signal with all-real, all-complex, complex-zero and real-zero training after transmission over 20km SMF at a BER of 1x10 3 is 2.5, 5.5, 4 and 1dB, respectively. 4. Conclusion We have theoretically and experimentally investigated the DD OFDM-ROF system with different training structures. The experimental results show that the power penalty of all-real, all-complex, complex-zero and real-zero training after transmission over 20km SMF at the BER of 1x103 is 2.5, 5.5, 4 and 1dB, respectively. It is shown that the real-zero training OFDM frame with interleave structure and lower modulation order signal has the least power penalty, and it provides a good solution for SSBI reduction. Acknowledgments This work is partially supported by the National Natural Science Foundation of China (No. 60977049), the National “863” High Tech Research and Development Program of China (No. 2009AA01A347), the Program for Hunan Provincial Science and Technology (No.2009FJ3131), and the Hunan Provincial Innovation Foundation For Postgraduate (No. CX2010B141). #143348 - $15.00 USD

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