Accepted Manuscript

Analysis of radio over fiber system for mitigating Four-Wave Mixing effect

Namita Kathpal, Amit Kumar Garg

PII: S2352-8648(18)30055-5
DOI: https://doi.org/10.1016/j.dcan.2019.01.003
Reference: DCAN 152

To appear in: Digital Communications and Networks

Received Date: 12 March 2018

Revised Date: 4 January 2019
Accepted Date: 8 January 2019

Please cite this article as: N. Kathpal, A.K. Garg, Analysis of radio over fiber system for mitigating
Four-Wave Mixing effect, Digital Communications and Networks (2019), doi: https://doi.org/10.1016/
j.dcan.2019.01.003.

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Digital Communications and Networks(DCN)
ACCEPTED MANUSCRIPT

journal homepage: www.elsevier.com/locate/dcan

Analysis of Radio over Fiber System for

mitigating Four-Wave Mixing effect

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Namita Kathpal∗a , Amit Kumar Garga
a Departmentof Electronics Communication Engineering, Deenbandhu Chhotu Ram University of Science and Technology,
Murthal, Sonepat, Haryana- India

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Abstract

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In this paper, an efficient 8-channel 32Gbps RoF system incorporating Bessel Filter (8/32 RoF-BF) has been demonstrated
to reduce the impact of non-linear transmission effects specifically Four-Wave Mixing (FWM). The simulation results indicate
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that the proposed (8/32 RoF-BF) system provides an optimum result w.r.t. channel spacing (75GHz), input source power
(0dBm) and number of input channels (8). On comparison with existing RoF system, the proposed 8/32 RoF-BF system has
been validated analytically also and it is found that the performance of the proposed system seems to be in close proximity
particularly in FWM sideband power reduction of the order of 4dBm for 8-channel 32Gbps RoF system.
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c 2015 Published by Elsevier Ltd.

KEYWORDS:
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Dispersion Compensating Fiber, Four-Wave Mixing, Radio over Fiber, Single Mode Fiber, Wavelength Division Multiplexer
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1. Introduction network solution regarding high-speed communica-

tion systems is dependent on optical fiber for trans-
In recent years, a constantly increasing demand for mission of radio signals known as Radio over Fiber
wired and wireless networks has led to a significant (RoF) [4]. RoF technology uses optical fiber that has
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rise in data rates [1]. To satisfy this growing demand low attenuation, immunity to electromagnetic inter-
for high capacity and high-speed broadband wireless ference and superior signal integrity. Therefore, it
access, the microcellular system was discovered. This enables the transmission of the signal over long dis-
system consists of many small cells that have allured tances, thus improving the mobility and ubiquity of
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attention as a productive method for attaining high wireless networks [5, 6]. A general RoF architecture
speed and high capacity communication by enhanc- is shown in Figure 1. RoF network comprises all the
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ing frequency utilization. But this system has cer- equipment mandatory to convert electrical to an op-
tain limitations as it requires a huge investment to im- tical signal and vice-versa. Traditional optical com-
plement numerous base stations (BSs) to shroud the munication links operate at 1310/1550 nm wavelength
whole service area. The inevitability for complicated in order to increase their data transmission capabili-
channel control techniques among BSs for spectral de- ties. At the transmitter, the incoming RF signal is
livery and the handoff procedure inflates the invest- modulated by using direct or external modulation. In
ment. This huge increase in data traffic can be con- the downlink transmission from CS to BS, the input
trolled by implementing system architecture in which RF signal modulates the optical source which gener-
complicated functions are performed at a control sta- ates output wavelength that has an amplitude which
tion (CS) rather than at BS [2, 3]. One main access changes in accordance with the change of laser DC
biasing current. The optical wavelengths generated
by laser diode are coupled into WDM which in turn
∗ Namita Kathpal (Corresponding author) transported to the BS via optical fiber. At the BS,
(email:namitakathpal2016@gmail.com).
1 Amit Kumar Garg (email:garg amit03@yahoo.co.in). the optical detector is employed to receive the multi-
2 ACCEPTED MANUSCRIPT Namita Kathpal, Amit Kumar Garg

wavelength signal from the fiber and convert it back effect and the proposed algorithm works efficiently
into the original RF signal. The extracted RF signal at low input power. The three-channel code (TCC)
is transmitted to the MU via BS antenna. WDM sys- [22] has been described to reduce the impact of FWM
tems are implemented in an optical link to increase the on DWDM system for various kinds of fiber. The
number of wavelengths transmitted through a single TCC eliminates the inband FWM term and provides an
fiber [7]. It is observed that the inelastic scattering and improvement in signal-to-crosstalk ratio (SXR) upto
variation in the refractive index of the fiber core with 4dB. Novel Optical Burst Switching (OBS) architec-
the optical intensity produces non-linear impairments ture [23] has been developed using just-enough-time
in RoF system [8]. These non-linear impairments are signalling protocol to reduce burst losses in fast op-
broadly classified into two categories; the impairments tical networks. Based on the literature survey, it is

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which are caused due to the interaction of the field in- concluded that FWM is very crucial effect in WDM
tensity with the fiber refractive index are Self-Phase systems as this degrade the system performance by in-
Modulation (SPM), Cross-Phase Modulation (XPM) ducing crosstalk. Moreover, in WDM systems, chan-
and Four-Wave Mixing (FWM) and the impairments neling with unequal spacing was proposed to reduce

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which are due to stimulated scattering mechanisms, the FWM effect.In the present work, analytical model-
are Brillouin (SBS) and Raman (SRS) [9]. These im- ing is described to calculate FWM crosstalk power for
pairments result in signal broadening [10], undesirable three channel system and also proposed a 32Gbps sim-
signal modulation, attenuation and thereby, limits the ulation model for 8-channel system to reduce FWM

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transmission capability of the long-haul system. These effect by employing Bessel Filter. Thus, it is con-
non-linearities are very crucial for the deployment of cluded that reduction in FWM can be achieved by re-
the RoF system as they produce accumulated effect ducing input source power, increasing number of input
for long distance. The non-linear effects are observed channels as well as channel spacing.

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to be less stringent for the single optical channel as
compared to the WDM system [11]. A statistical anal-
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2. Impact and Control of FWM non-linearity
ysis [12] has been described for reducing the power
level of crosstalk generated by FWM. An optimiza- It is observed that the fiber nonlinearities directly
tion technique based upon performance factors (chan- contribute to refractive index changes in relationship
nel spacing, channel power and fiber area) has been to input optical power variations which limits the high-
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proposed [7] to reduce the FWM effect for deploy- speed link performance. In WDM systems, FWM
ment of RoF system. Noise compression method [13] leads to inter-modulation distortion and the interfer-
which is based on the wavelength of the laser pump ence affecting a given channel is vigorously related to
and the injected signal has been described for isolat-
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the relative phases of the mixing products. In N chan-

ing the FWM component. The impact of FWM on two nel system, FWM sideband products are M and given
channels system has been analyzed using backward as [8]
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pumped Fiber Raman Amplifier (FRA) by varying the N3 − N2

FRA parameter and the results mitigate the FWM ef- M= (1)
2
fect for Raman constant of 0.18 but this FRA works
In WDM, two channels propagating at λ x and λy are
efficiently for fiber length up to 10km [14]. The mul-
influenced by fiber nonlinearities and generate side-
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tiple FWM efficiency has been improved by the em-

band at λu and λv i.e. λu = 2λ x - λy and λv = 2λy -
ployment of dispersion tailored photonic crystal fibers
λ x . The FWM power PFWM produced by three con-
[15]. The FWM crosstalk power generated in differ-
tinuous wave channels of input power P x ,Py and Pz at
ent kinds of fiber i.e. dispersion shifted fiber, non-
wavelength λ x , λy and λz at the fiber output is given as
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zero dispersion shifted fiber and single mode fiber are

[18]
found to be 2.8mW, 0.125mW and 5W respectively
PFW M = ηD2F P x Py Pz e−αL Le f f 2 (2)
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[16] and Corning large effective area fiber (LEAF)

(1−exp(−αL)) 2πη α2
[17] reduces the FWM crosstalk power by 20dB.The where Le f f = α ,γ = λAe f f ,η = α2 +(∆β)2
[1 +
variance of Intra-channel FWM (σIFW M ) has been de- 4 exp(−αL) sin2 (∆βL)
, ∆β = − λcπ dD
4
dλ [( f x − fo )+( fy − fo )]( f x −
2 c
rived mathematically and the results shows the reduc- (1−exp(−αL))2
tion in σIFW M by 75% by the incorporation of pre- fz )( fy − fz ) where N is the number of input channel and
compensation scheme at the transmitter side [18]. The M is the number of FWM sideband products, α is at-
FWM effect has few merits too, as it finds applications tenuation coefficient, η is non-linear refractive index,
in optical packet generations and optical labelling by Ae f f is effective fiber core area, λ is wavelength, Le f f
combining the different frequency signal into quater- is the effective length of the fiber, η is FWM efficiency,
nary signal. Thereby, supports long-distance transmis- γ is non-linear coefficient, ∆β is phase matching fac-
sion upto 350km and improve spectral efficiency but tor [24] which represents the difference of propagation
this packet generation requires payload power offset constant of original signals and FWM generated sig-
too [19, 20]. Assign Shortest Path First (ASPF) algo- nals, Dc is chromatic dispersion, fo is zero dispersion
rithm [21] has been proposed to mitigate the FWM frequency, f x is input frequency of channel x, fy is in-
put frequency of channel y and fz is input frequency
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Analysis of RoF system for mitigating FWM effect MANUSCRIPT 3

Central Station Base Station Mobile Unit

RF Mach-Zehnder Photo RF
Data Filter Data
Modulator Modulator Diode Demodulator
Fiber

Laser
Diode

Fig. 1: Downlink Architecture of Radio over Fiber Link

2a and 2b.jpg then PFW

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Po will 0.00005, 0.05, 0.5 and 50 respectively
as shown in Figure 2(b). Thus, as the input power of
the channel is increase, the ratio will also increase but
by decreasing the input power, ratio will decrease ac-

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cording to Equation 3 which in turn leads to decrease
in SNR. Further sufficient power should be applied to
all the channels which will not deteriorate the system
performance.

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3. Simulation Setup of proposed (8/32 RoF-BF)
system

U The schematic of the proposed (8/32 RoF-BF) setup

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has been shown and illustrated in Figure 3. The sim-
ulation parameters for the proposed system are tabu-
larized in Table. The proposed (8/32 RoF-BF) system
consists of a transmitter, transmission medium, filter,
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amplifiers and receiver. The transmitter is shown in

Figure 4(a) consists of driver circuit along with a light
source (which generates continuous wave millimeter-
wave optical signal that transmits at different wave-
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lengths) and Mach-Zehnder Modulator (MZM). The

driver circuit includes the line coding formats which
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codes the ON/OFF pulses generated by Pseudo Ran-

Fig. 2: (a)Input Channels versus FWM sideband products, (b)Input dom Bit Sequence (PRBS) generator at a bit rate of
Power(mW) vs PFW Po
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4Gbps into Non-Return to Zero (NRZ) format. The
NRZ coding format is employed due to its easy im-
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plementation and compact spectrum. This coded in-

of channel z. The three continuous waves propagat- formation signal is superimposed onto high frequency
ing through the same fiber generates nine new opti- optical carrier by MZM. The MZM in conjunction
cal waves due to FWM effect by Equation 1. New with Erbium Doped Fiber Amplifier (EDFA) enhances
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waves or products are seen to be generated when there the performance of long-distance communication as
is a large number of channels having uniform spacing MZM coupled higher optical output power into an
which further results into various detrimental effects
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optical fiber, supports higher transmission rates, pro-

particularly crosstalk and reduction in SNR because vide low chirp and provides high compatibility with
there is a match of frequency with the original chan- EDFA. The modulated optical output from different
nel frequency. Thus, as the numbers of input channels laser transmitters at different wavelengths is combined
are increased, the FWM sideband products are also in- by WDM to increase system capacity. The multi-
creased shown in Figure 2(a). When same input power plexed optical signal is transmitted through SMF. SMF
is applied to each channel with accurate phase match- carry high data rate signal and covers longer distance
ing, then the ratio of power generated due to FWM to as compared to Multi-Mode Fiber (MMF) but disper-
the existing power of one channel [25] given as sion and non-linearities produce by fiber limits the
PFW M transmission distance. Thus, optical amplifiers and
= (DF γLe f f )2 P2i (mW) (3) Dispersion Compensating Fiber (DCF) are utilized in
Po
a link to compensate for the loss and dispersion. The
When all the channels having different frequency, DF first amplifier used in this link work as an in-line am-
is considered to be 2. Thus, as Pi varies from -10dBm plifier as it amplifies the weak incoming signal to a
to 20dBm (0.1mW to 100mW) with a step size of 10, specified level. EDFA is incorporated into the design
4 ACCEPTED MANUSCRIPT Namita Kathpal, Amit Kumar Garg

as it provides the required optical amplification with a

minimum level of added noise. The amplified signal Layout Parameters Value
is fed to DCF which provides equal and opposite dis- Bit Rate 4Gbps
persion ranging from -70ps/nm-km to -90ps/nm-km to Sequence Length 256 bits
link dispersion so that chromatic dispersion is zero. Samples per bit 128
For higher receiver sensitivity and high SNR demand, Number of Samples 32768
a pre-amplifier must be incorporated in front of the Sensitivity -100dBm
optical receiver. Therefore, the second EDFA work as Resolution 0.1nm
pre-amplifier capable of providing high gain with low Transmitter Parmeters Value
noise levels. To curtail the fiber nonlinearities pro- PRBS Bit Rate 4Gbps

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duced by fiber distinctively FWM, optical fiber link CW Laser Power 20dBm to-10dBm
must incorporate Bessel filter into the design. Bessel CW Laser Frequency 193.1THz to 193.8THz
filter had been employed in design for features such Channel Spacing 25GHz to 100GHz
as maximally flat group delay, slow cut-off, overshoot CW Laser Line width 10MHz

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and most importantly it preserves the wave shape of
MZM Extinction Ratio 30dB
filtered signal in pass band which directly provides
SMF Parameters Value
best phase response. Bessel filter reduces the FWM
Length 100km

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sideband power by 4dBm discussed in the result. The
Attenuation 0.2dB/km
filtered optical signal is applied to the receiver. The
Dispersion 16.75ps/nm-km
receiver shown in Figure 4(b) consists of photo detec-
tor, filter and BER analyzer. The PIN photo detector Dispersion Slope 0.075
converts the incident optical signal into an electrical Effective Area 80µm2

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signal and this signal is filtered with the help of Low EDFA Parameters Value
Pass Bessel filter to remove the noise generated by the Gain 20dB
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link. The performance of the receiver is measured by Power 80dBm
BER analyzer in terms of Eye Diagram and Q-Factor. Noise Figure 2dB
The effect of nonlinearity in optical Fiber has been vi- DCF Parameters Value
sualized by three visualize components namely Opti- Length 20km
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cal Power Meter, WDM Analyzer, Optical Spectrum Attenuation 0.005dB/km

Analyzer. Dispersion -83.75ps/nm-km
Dispersion Slope 0.075
80µm2
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Effective Area
4. Results & Discussions
Receiver Parameters Value
PIN Responsitivity 1A/W
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As the evaluation of non-linear impairments via

non-linear Schrodinger equation (NLSE) method is in- Dark Current 10nA
tricate so, the optical Bessel filter is inculcated in in Thermal Noise 10−21 W/Hz
long-haul RoF system to reduce the non-linear effect Low Pass Bessel Filter 0.75? Bit Rate Hz
particularly FWM. By considerably increasing chan- cut off frequency
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nel spacing (25GHz to 100GHz), increasing number Low Pass Bessel Filter 0dB
of input channels (2, 4 & 8) with a simultaneously re- Insertion Loss
duction in channel input power (20dBm to -10dBm) in Low Pass Bessel Filter Order 4
the proposed system the FWM effect can be reduced.
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Tab. 1: Simulation Parameters employed in proposed (8/32 RoF-

BF) system
4.1. Effect of Channel Spacing of various CW Laser
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Source
In order to observe the effect of channel spacing
in proposed RoF system, the following simulation pa- Channel Power Power Q- BER
rameters were considered: input power level (0dBm), Spacing received obtained Factor
fiber length (120km), dispersion (16.75ps/nm-km). At (GHz) without with
the output, the effect of increasing channel spacing is filter filter
shown in Table 2. Using results of Fig. 5-8 it is con- (dBm) (dBm)
templated that the FWM sideband power is reduced 25 25.72 25.37 43.17 0
by 2dBm, 3dBm, 4dBm and 5dBm respectively when 50 25.72 24.93 47.05 0
the spacing between the channels is increased from 75 25.72 24.40 53.15 0
25GHz to 100GHz with a step size of 25. The Bessel 100 25.72 23.86 46.83 0
filter employed in the proposed (8/32 RoF-BF) sys-
Tab. 2: Impact of Channel Frequency on mitigating FWM effect
tem to reduce the sideband power which is generated
due to fiber nonlinearity. Table 2 shows the optical
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Analysis of RoF system for mitigating FWM effect MANUSCRIPT 5

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Fig. 3: Setup of proposed (8/32 RoF-BF) system
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Fig. 4: Transmitter and Receiver Section

6 ACCEPTED MANUSCRIPT Namita Kathpal, Amit Kumar Garg

Input Power Power Q- BER Number Power Power Q- BER

Power received obtained Factor of received obtained Factor
(dBm) without with Channels without with
filter filter filter filter
(dBm) (dBm) (dBm) (dBm)
20 45.52 43.83 3.21 0.00034 2 20.24 19.51 61.44 0
10 35.54 33.83 15.43 3.08e−45 4 22.89 22.06 51.95 0
0 25.72 23.86 46.83 0 8 25.72 23.86 46.83 0
-10 17.17 14.16 35.95 1.54e−283
Tab. 4: Impact of Number of Channels on mitigating FWM effect

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Tab. 3: Impact of Input Power Level on mitigating FWM effect

power will reduce the SNR as SNR is computed by

power received at the output of pre-amplifier and fil- Q-Factor.
ter along with Q-Factor and BER, it is clear that as

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spacing of channel is increased, power obtained at the
output of filter is also decreased but this power will 5. Conclusion
not reduce the SNR as SNR is computed by Q-Factor

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and Q-Factor is also increased but the optimum result In this paper, the feasibility of the proposed (8/32
is achieved at the channel spacing of 75GHz. RoF-BF) system has been tested and demonstrated.
The performance of the proposed (8/32 RoF-BF) sys-
4.2. Effect of transmitter power of Signal Source tem has been investigated as a function of channel

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spacing, power level and a number of channels. Based
The elemental requirement of a RoF system is to on the results, it has been observed that by increasing
transmit a signal of appropriate signal power to dis-
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the channel spacing as well as decreasing the power
tinguish it from the noise at the receiver. The per- level of the signal source; the FWM effect decreases.
formance of the RoF system is analyzed by Signal to The simulation results indicate that the impact of
Noise Ratio which is a measure of signal strength. The FWM effect is reduced by maintaining the channel
effects of FWM are governed by input power level,
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spacing at 75GHz, input power level at 0dBm for 8-

higher the power launched into the fiber rapidly leads channel system. The FWM sideband power seems to
to increase the level of FWM. In order to analyse this be decreased by 4dBm by the incorporation of Bessel
effect, other parameters were kept unchanged and the Filter, which symbolized momentous reduction. Fur-
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power level of the input sources has been varied from ther, it seems that the present work may contributes to
20dBm to -10dBm. Based on the result obtained in better understanding of non-linear effects in RoF sys-
Fig. 9-12, it is examined that as the input power level
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tem and as future prospects, the above-proposed sys-

is decreased from 20dBm to -10dBm in addition to tem can be extended using adaptive modulation for-
the employment of Bessel filter, the sideband power is mats by employing external modulation for the de-
decreased by 2dBm, 4dBm, 4.1dBm and 4.2dBm re- ployment of RoF system in future transport networks
spectively. Table 3 shows the optical power received
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at the output of pre-amplifier and filter along with Q-

Factor and BER. It is evident from the results obtained References
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Fig. 5: Optical Spectrum at the output of (a) EDFA, (b) Filter and (c) Eye Diagram at 25GHz Channel Spacing.

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Fig. 6: Optical Spectrum at the output of (a) EDFA, (b) Filter and (c) Eye Diagram at 50GHz Channel Spacing.
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Fig. 7: Optical Spectrum at the output of (a) EDFA, (b) Filter and (c) Eye Diagram at 75GHz Channel Spacing.
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Fig. 8: Optical Spectrum at the output of (a) EDFA, (b) Filter and (c) Eye Diagram at 100GHz Channel Spacing.
8 ACCEPTED MANUSCRIPT Namita Kathpal, Amit Kumar Garg

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Fig. 9: Optical Spectrum at the output of (a) EDFA, (b) Filter and (c) Eye Diagram at 20dBm Input Power.

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Fig. 10: Optical Spectrum at the output of (a) EDFA, (b) Filter and (c) Eye Diagram at 10dBm Input Power.
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Fig. 11: Optical Spectrum at the output of (a) EDFA, (b) Filter and (c) Eye Diagram at 0dBm Input Power.
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Fig. 12: Optical Spectrum at the output of (a) EDFA, (b) Filter and (c) Eye Diagram at -10dBm Input Power.
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Fig. 13: Optical Spectrum at the output of (a) EDFA, (b) Filter and (c) Eye Diagram for 2 Channel System.

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Fig. 14: Optical Spectrum at the output of (a) EDFA, (b) Filter and (c) Eye Diagram for 4 Channel System.
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Fig. 15: Optical Spectrum at the output of (a) EDFA, (b) Filter and (c) Eye Diagram for 8 Channel System.
10 ACCEPTED MANUSCRIPT Namita Kathpal, Amit Kumar Garg

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Namita Kathpal (Corresponding Author) Deenbandhu Chhotu Ram University
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