Download Third Order Nonlinear Distortion Reduction in Broadband ASE Source - Based Full-Duplex ROF Transport Systems

NCRAMT|June 24-26, 2011|Haldia, India
International Journal of Soft Computing and Engineering (IJSCE)
ISSN: 2231-2307, Volume-1, Issue-NCRAMT2011, July 2011
Third Order Nonlinear Distortion Reduction in
Broadband ASE Source - Based Full-Duplex
ROF Transport Systems
Akinchan Das, Banibrata Bag, Hai-Han Lu, A. S. Patra*
network architecture and upgrade the deployment of BS since
they enable full-duplex operation on one fiber.
In this paper, we proposed and demonstrated a technique to
reduce third order nonlinear distortion in full-duplex
radio-on-fiber (ROF) transport system which is based on
broadband ASE source employing RF amplifier predistorter,
wavelength-division-multiplexing (WDM), optical add-drop
multiplexing techniques. A data rate of 70 Mbps/10GHz
(WiMAX) signal is externally modulated with broadband
ASE source by an external modulator and transmitted over
120-Km long-haul fiber link. A good eye diagram and low bit
error rate (BER) values were obtained in our proposed
full-duplex ROF transport systems.
Abstract— A technique to reduce third order nonlinear
distortion in full-duplex radio-on-fiber (ROF) transport system
which is based on broadband ASE source employing RF
amplifier
predistorter,
wavelength-division-multiplexing
(WDM), and optical add-drop multiplexing techniques is
proposed and demonstrated. A data stream of 70 Mbps/10GHz
(WiMAX) transmitted over 120 Km long-haul fiber link and
good bit error rate (BER) performances with better
eye-diagram were achieved in our proposed full-duplex ROF
transport system. Such proposed full-duplex ROF transport
systems is suitable for long-haul microwave optical link.
Index Terms— Full-duplex, optical add-drop multiplexing,
radio-on-fiber, third order nonlinear distortion.
II. EXPERIMENTAL SETUP
I. INTRODUCTION
Fig. 1 shows the schematic architecture of our proposed
full-duplex ROF transport system. For down-link
transmission, the CS is composed of a microwave signal
generator, a broadband ASE source, an external modulator,
Arrayed waveguide gratings (AWG), and two erbium-doped
fiber amplifiers (EDFAs). At CS, a 70 Mbps data stream is
mixed with 10 GHz microwave carrier to generate data signal.
The resulting microwave data signal is externally modulated
with a broadband ASE source. The modulated signals are
demultiplexed by AWG to select the appropriate
wavelengths. To avoid the crosstalk between the channels of
AWG, we choose the signals from the channels 1, 3, 5 and 7
of AWG and multiplexed them by another AWG. The central
wavelengths from the channels of AWG are 1549.96 nm
(
λ1), 1550.76 nm (λ2), 1551.56 nm (λ3) and 1552.36 nm (λ4),
respectively. In order to transmit optical wavelengths over a
long-haul fiber link, the optical power is amplified by EDFA
and fed into fiber backbone.
Signal is generated at the CS and then distributed to the
remote BSs by using cascaded EDFAs. All the EDFAs have
an output power of ~ 16 dBm and a noise figure of ~ 4.3 dB at
an input power of 0 dBm. Each BS deals with by individual
wavelength for an optical add-drop multiplexer (OADM) of
an in-out insertion loss of 4.2 dB and >40 dB add/drop
channel isolation. The down-link optical carrier is dropped to
the BS, and the up-link optical carrier is added to the fiber
backbone by OADM. The 70-Mbps down-link data signal is
detected by photodiode (PD), passes through RF amplifier
predistorter, demodulated and lastly fed into a BER tester for
BER analysis at the BSs. The up-link data signal is added to
the fiber backbone and transmitted to the CS. At CS, the
up-link data signal is passing through an optical tunable
band-pass filter (TBPF) to select the desired wavelength and
the optical power level of the signal is adjusted by a variable
A
s the high speed optical access network technology are
being rapidly developed, the interests and demands are
increasing in the area of Radio-on-fiber (ROF) transport
system. ROF transport system, the integration of optical fiber
and wireless systems [1], is the most acceptable solution to
increase the capacity and mobility of future communications
with low cost because of broad bandwidth and low attenuation
characteristics [2, 3]. ROF transport system is capable to
simplify the configuration of the transport system due to its
flexibility to link the central station (CS) and the base stations
(BSs) [4, 5]. For long-distance transmission, the nonlinear
distortion takes a vital role to degrade the system
performances.
In order to improve system performances, it is necessary to
use techniques and schemes to mitigate the nonlinear
distortions [6, 7]. Furthermore, to realize full-duplex ROF
transport systems, a simple and cost-effective architecture is
required. Considering the architecture of ROF transport
systems, Wavelength-division-multiplexing (WDM) and
optical add-drop multiplexing techniques can simplify the
Manuscript received June 10, 2011.
Akinchan Das, Department of Electronics and Communication
Engineering, Haldia Institute of Technology, Haldia, India (e-mail:
[email protected]).
Banibrata Bag, Department of Electronics and Communication
Engineering, Haldia Institute of Technology, Haldia, India (e-mail:
[email protected]).
Wei-Yi Lin, Institute of Electro-Optical Engineering, National Taipei
University of Technology, Taiwan.
Hai-Han Lu, Institute of Electro-Optical Engineering, National Taipei
University of Technology, Taiwan,
A.S. Patra, Department of Applied Sciences, Haldia Institute of
Technology, Haldia, India (e-mail: [email protected]).
*Corresponding author
31
Third Order Nonlinear Distortion Reduction in Broadband ASE Source - Based Full-Duplex ROF Transport Systems
λ
1
λ
Broadband
ASE Source
External
Modulator
EDFA
2
AWG MUX/DEMUX
λ
λ
OADM
EDFA
40km SMF
3
EDFA
40km SMF λ
2
λ1
λ1
EDFA
OADM
λ2
4
10GHz/70Mbps
BS1
BS2
40km SMF
BER
Tester
RF Amplifier
Predistorter
Demodulator
PD
EDFA
VOA
CS
Optical
TBPF
40km SMF
OADM
λ4
λ4
EDFA
40km SMF
BS4
OADM
λ3
λ3
BS3
Fig. 1. The schematic diagram of our proposed full-duplex ROF systems
RF IN
+
RF Splitter
A1
Delay Line
2nd order
Nonlinearity
Generator
RF Combiner
appropriate nonlinear coefficient to eliminate the third order
nonlinear term:
a 1. b 3 = - a 3. b 1
(4)
Then equation (3) can be changed as
v0 = (a1. b1).m
(5)
From equation (5), it is clear that, third order nonlinear
distortion can be eliminated by proper adjusting the nonlinear
coefficient. Only linear term is left in equation (3), the
proposed RF amplifier predistorter is employed to cancel out
the third order nonlinear distortion.
RF OUT
–
A2
Attenuator
Fig.2. A schematic diagram of the RF amplifier predistorter.
optical attenuator (VOA). The received optical signal is
detected by a broadband PD and fed into RF amplifier
predistorter. The signal is also fed into a BER tester for BER
analysis after demodulation. The architecture for RF amplifier
predistorter is shown in Fig. 2. An appropriate nonlinear
co-efficient is used to cancel out the third order nonlinear
distortions.
III. EXPERIMENTAL RESULTS AND DISCUSSION
The optical spectrum of broadband ASE source and the
output of multiplexed signals are shown in Fig. 3(a). The
optical spectrum has a good free spectral range of 15 nm. We
selected the optical wavelengths λ1, λ2, λ3 and λ4 from the odd
channels of AWG to avoid the crosstalk among the channels.
The optical spectrum for the selected wavelengths is
illustrated in Fig. 3(b). A schematic diagram of the RF
amplifier predistorter is illustrated in Fig. 2. The RF amplifier
predistorter output in ROF band is given by
v0 = a1.vi + a3. vi3
(1)
where v0 is the output voltage, vi is the input voltage, and a1,
a3 are the amplitude coefficients (a3 is coefficient characterize
nonlinearity). The third order nonlinear distortion in ROF
transport systems can be expressed as
q = b1.m + b3. m3
(2)
where q is system’s output voltage detected from PD, m is
system’s input voltage, and b1, b3 are the amplitude
coefficients (b3 is coefficient characterize nonlinearity). q is
equal to vi, substituting equation (2) into equation (1) and
neglecting higher order nonlinear terms, then yields
v0 = (a1. b1).m + (a1. b3 + a3. b1). m3(3)In ROF transport
systems, IMD3 is the major nonlinear distortion induced in the
system. While achieving linearity means cancelling out the
nonlinear terms, a RF amplifier predistorter would have to
cancel out the third order nonlinear term. Setting the
Fig. 3 (a). The spectrum of broadband ASE source and the
multiplexed optical signals.
Fig. 3 (b). The optical spectrum of selected Wavelengths for
transmission.
The down-link BER curves of 10-GHz (f1)/70-Mbps
transmitted data signal from CS to BS3 (120 km fiber link) are
measured with and without the RF amplifier predistorter and
are presented in Fig. 4(a). For a BER of 10-9; the received
32
NCRAMT|June 24-26, 2011|Haldia, India
International Journal of Soft Computing and Engineering (IJSCE)
ISSN: 2231-2307, Volume-1, Issue-NCRAMT2011, July 2011
optical power is -7.6 dBm with the RF amplifier predistorter;
and without the predistorter, the received optical power is -2.4
dBm. Due to the employment of RF amplifier predistorter, the
receiver sensitivity is improved by 5.2 dB. And further, the
measured up-link BER curves of 10-GHz (f1)/70-Mbps data
channel from BS2 to CS (120 km fiber link) are presented in
Fig. 4(b). For a BER of 10-9; with and without the RF
amplifier predistorter, the received optical powers are -7.3
dBm and -2.2 dBm respectively. A 5.2 dB receiver sensitivity
is improved when the RF amplifier predistorter technique is
employed.
IV.
CONCLUSION
We have proposed a novel technique to reduce third order
nonlinear distortion in full-duplex radio-on-fiber (ROF)
transport system which is based on broadband ASE source
employing
RF
amplifier
predistorter,
wavelength-division-multiplexing (WDM), optical add-drop
multiplexing techniques. We have experimentally observed
and demonstrated the feasibility of our proposed system with
WiMAX signal and obtained low BER values with very good
eye-diagram over a long-haul fiber link. Our proposed
full-duplex ROF transport systems have the potential to
transmit signal over long-haul microwave optical link.
with RF Amplifier Predistorter (λ3 )
without RF Amplifier Predistorter (λ3 )
10
-5
Down-Link BER
10 -6
10 -7
10 -8
10 -9
10 -10
10
(a)
-11
-10
-8
-6
-4
-2
0
Received Optical Power ( dBm )
Fig.4.a. The experimental BER for down-link transmission.
with RF Amplifier Predistorter (λ2 )
without RF Amplifier Predistorter (λ2)
10 -5
Up-Link BER
10 -6
(b)
Fig. 5. The eye-diagram for 120 Km SMF transmissions (a)
with and (b) without RF amplifier predistorter.
10 -7
10
-8
10 -9
ACKNOWLEDGMENT
10 -10
10 -11
-10
The authors would like to thank Haldia Institute of
technology and National Taipei University of technology.
-8
-6
-4
-2
0
Received Optical Power ( dBm )
Fig. 4.b. The experimental BER for up-link transmission.
REFERENCES
[1] H. H. Lu, A. S. Patra, W. J. Ho, P. C. Lai, M. H. Shiu, “A full-duplex
More than 5 dB BER curve power penalty improvement is
obtained for both the uplink and downlink transmissions when
the RF amplifier predistorter scheme is implemented. RF
power degradation improvement is observed for this
technique. So this scheme reduces the RF power degradation,
causing system with higher SNR value, finally leading to an
improvement BER performance. Cross talk from down-link
optical signal to up-link optical signal may influence the BER
performance because 10 GHz carrier is used both at CS and
BS in the experiment. However, since the OADM has a high
add/drop channel isolation (>40 dB), such a high channel
isolation characteristic provides excellent add/drop ability to
prevent crosstalk.
The clear eye-diagrams of RF data signal with and without
the RF preamplifier predistorter are observed in our
experiment and shown in Fig. 5(a) and (b) respectively.
radio-over-fiber transport system based on FP laser diode with OBPF
and optical circulator with fiber bragg grating,” IEEE Photon. Technol.
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Third Order Nonlinear Distortion Reduction in Broadband ASE Source - Based Full-Duplex ROF Transport Systems
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Akinchan Das has completed B.Sc. with Electronics
(Hons) from Vidyasagar University in 2005, M.Sc.
with Electronics from Vidyasagar University in 2007,
and M.Tech. with Modern Communication
Engineering Under WBUT (West Bengal University of
Technology)in 2009.Curently working as an Assistant
Professor(from 2010) in the Dept. of Electronic and
Communication Engineering, Haldia Institute of
Technology, Haldia , WB, India. His research interest
includes Wireless networking and Optical fiber communication. He is
working towards his Ph.D. degree.
Banibrata Bag has completed B.E. in Computer
Science and Engineering from Burdwan University
in 2004 and M.Tech. Under WBUT (West Bengal
University of Technology) in 2009. He has worked
as a software Developer (from 2005 to 2010), and
currently working as an assistant professor (from
2010) in the Dept. of Electronic and Communication
Engineering, Haldia Institute of Technology, Haldia,
WB, India. His research interest includes
networking, Optical fiber communication and processor architecture. He is a
professional member of ACM. He is working towards his Ph.D. degree.
Prof. Hai-Han Lu was born in Taipei, Taiwan, on
June 1963. He received the B.S. in the
Electro-Physics Department of the National
Chiao-Tung University, Taiwan, in 1987. He
received the M.S. and Ph.D. degree in the Institute
of Optical Sciences of the National Central
University, Taiwan, in 1991 and 2000, respectively.
He joined the Institute of Electro-Optical
Engineering, National Taipei University of
Technology as an Associate Professor in 2001. His
current research interests include the applications
of the lightwave communication and hybrid DWDM systems.
Dr. Ardhendu Sekhar Patra is working as an
Assistant professor at Department of Applied
Sciences, Haldia Institute of Technology,
Haldia, India. He received his Ph.D. degree
from Indian Institute of Technology. He has
worked as a post doctorate fellow at Institute of
Electro-optical Engineering, National Taipei
University of Technology, Taiwan and Center
for
semiconductor
technology
and
Optoelectronics
(ZHO),
University
of
Duisburg-Essen, Duisburg, Germany. His current research interests include
the applications of the lightwave communication and WDM-PON systems.
34