40120140502002

International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976
– 6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 2, February (2014), pp. 10-20 © IAEME
10
OSNR CHALLENGE IN DWDM LINK
T. S. Khatavkar1
and Prof. (Dr). D. S. Bormane2
1
Electronics and Telecommunication Department of PVG’s College of Engineering &
Technology, affiliated to University of Pune; Research Scholar at SCOE, Wadgaon (Bk), Pune.
M.S. (India)
2
Principal, J.S.P.M’s Rajashree Shahu College of Engineering, Tathawade, Pune, (India)
ABSTRACT
Transmission rates for telemedicine are driven primarily by the need for full-motion video,
desire for extremely-high-quality images for pathology and radiology, and the ability to carry out
sophisticated surgery at nodes with desktop computers. Advanced medical systems demand higher
data rates of the order of 40 Gb/s to 100 Gb/s. Current fiber optic systems working at 10 Gb/s need
to be migrated to 100 Gb/s line rates; this calls for a theoretical increase in optical signal to noise
ratio by 10 dB in order to compensate for the ten times wider receiver bandwidth. This paper brings
out the challenges at 40 Gb/s and 100 Gb/s and analyses the possible approaches for enhancing the
OSNR performance in DWDM links at these rates.
Keywords: DWDM, EDFA, Fiber Raman Amplifiers, OSNR, PMD.
1. INTRODUCTION
With the tremendous increase in the bandwidth requirements for the development of very
high bandwidth and delay sensitive applications such as Video - driven IP traffic, Telemedicine,
Internet Games and network storage, there is a need to maximize the capacity that can be transported
by optical backbone networks. The Internet and mobile devices continue to grow as key utilities in
peoples’ lives, presenting the optical communications industry with new opportunities and challenges
in 2013 to ensure that the networks can keep up with the demand. Therefore the priorities for the
optical communications industry are to support the need for faster data rates, more powerful
switching, and smarter network architectures that can handle unpredictable and fast – changing traffic
patterns and improve cost efficiencies. The internet traffic is continuously growing, and around 2015-
2020, it is expected that the current transmission fibers would become inadequate [1].
INTERNATIONAL JOURNAL OF ELECTRONICS AND
COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)
ISSN 0976 – 6464(Print)
ISSN 0976 – 6472(Online)
Volume 5, Issue 2, February (2014), pp. 10-20
© IAEME: www.iaeme.com/ijecet.asp
Journal Impact Factor (2014): 3.7215 (Calculated by GISI)
www.jifactor.com
IJECET
© I A E M E

11
Application areas such as Telemedicine demand long reach and highest data rates from the
optical transport network. Transmission rates for telemedicine are driven primarily by the need for
full-motion video, desire for extremely-high-quality images for pathology and radiology, and the
ability to carry out sophisticated surgery at nodes with desktop computers. Currently, fiber optic
systems are being designed to support data rates as high as 40 Gb/s and 100 Gb/s dense wavelength
division multiplexing (DWDM) transport networks for advanced medical systems. The major
challenges encountered by the National Telemedicine Network include basically the connectivity/
bandwidth, reach, speed provision, & reliability along with the Telemedicine cost consideration. The
key challenge seen by the optical industry is the deployment of 40 Gb/s and 100 Gb/s transponders
using the existing 10 Gb/s link engineering rules over existing fiber DWDM setup. Therefore the
priorities for the optical communications industry are to support the need for long reach/ long haul
systems, faster data rates, more powerful switching, and smarter network architectures that can
handle unpredictable and fast – changing traffic patterns and improve cost efficiencies. Long reach or
long haul optical fiber transmission can be achieved by using regenerators or amplifiers. Two factors
that directly limit system reach and capacity are the signal-to-noise ratio and non-linearity. For fibers,
improving these parameters translates into reducing the attenuation and enlarging the effective area
(Aeff) [2]. The role of OSNR in DWDM systems is discussed in Section II. Section III throws light
on the challenges of 40 Gb/s and 100 Gb/s transmission. Section IV reviews the possible approaches
of enhancing the OSNR performance in DWDM links.
2. OSNR in DWDM Links
DWDM is a technology that combines large number of independent information carrying
wavelengths onto the same fiber and thereby increases the transmission capacity of fiber. The
“spectral bands” where the optical fiber and the transmission equipment can operate more efficiently
are specified by ITU-T as O, E, S, C, L and U bands (from 1260 nm to 1675 nm). While setting up
the transmission link, there is a need to ensure that the signal can be retrieved intelligibly at the
receiving end. This can be done preferably by using optical amplifiers that serve as the key
component of a DWDM system. When the signal is amplified by the optical amplifier (OA), like
EDFA, its optical signal to noise ratio (OSNR) is reduced, and this is the primary reason to have
limited number of OAs in a network. One of the mitigation is to use RAMAN amplifier but it also
has some intrinsic noise, though it is less than that of EDFA.
The OSNR values that matter the most are at the receiver, because a low OSNR value means
that the receiver will probably not detect or recover the signal. The OSNR limit is one of the key
parameters that determine how far a wavelength can travel prior to regeneration. OSNR serves as a
benchmark indicator for the assessment of performance of optical transmission systems. DWDM
networks need to operate above their OSNR limit to ensure error – free operation. There exists a
direct relationship between OSNR and bit error rate (BER), where BER is the ultimate value to
measure the quality of a transmission. Given the OSNR, the empirical formula to calculate BER for
single fiber is:
Log10 (BER) = 10.7-1.45 (OSNR) (1)
In DWDM links a rule of thumb would be to target an OSNR value greater than 15 dB to 18
dB at the receiver. OSNR requirements depend on:
• Location: The required OSNR will be different for different locations in the light path. The
OSNR requirement will be higher closer to the transmitter and lower closer to the receiver. This

12
is because optical amplifiers and reconfigurable add/drop modules (ROADMs) add noise, which
means that the OSNR value degrades after going through each optical amplifier or ROADM. To
ensure that the OSNR value is high enough for proper detection at the receiver, the number of
optical amplifiers and ROADMs needs to be considered when designing a network.
• Type of Network: For a metro network, an OSNR value of >40 dB at the transmitter might be
perfectly acceptable, because there are not many amps between the transmitter and the receiver.
For a submarine network, the OSNR requirements at the transmitter are much higher.
• Data Rate: With the increase in the data rate for a specific modulation format, the OSNR
requirement also increases.
• Target BER: A lower target BER calls for a higher OSNR value.
The exact requirements at the receiver will vary from one manufacturer to another. Table 2
displays a few average OSNR figures to guarantee a BER lower than 10-8
at the receiver [3]:
TABLE 2 Typical OSNR values
Data Rate (Gb/s) 10 40 40 100 100
Modulation Format NRZ NRZ DPSK NRZ DPSK
Approx. OSNR (dB) 11 17 14 21 18
A higher OSNR translates into a lower BER, which equals fewer errors in transmission and
higher quality of service (QoS). The relation and impact of OSNR on system performance is shown
in Fig.1 (a) and (b) [4].
Figure.1 (a) Relation between OSNR, BER and QoS; (b) Impact of poor OSNR
3. CHALLENGES OF 40 GB/S AND 100 GB/S TRANSMISSION
This section discusses the challenges of 40 Gb/s and 100 Gb/s transmission and the enabling
technologies for DWDM transmission at these line rates. The issues involved in DWDM systems and
their probable solution [5] are summarized in Table 1.

13
TABLE 1: Issues and relevant solution in WDM system
Sr.
No.
Issues Solution
1. Capacity
bottleneck
Optical amplifier repeaters
2. Bandwidth Wavelength division multiplexing
3. Channel
interactions
Dispersion management & modern
fibers
4. Data rates New transmission formats
5. Error
performance
Forward error correcting codes
6. Spectral
efficiency
Coherent Receivers
The major challenges include Optical Signal to Noise Ratio (OSNR), Chromatic Dispersion
(CD) and Polarization Mode Dispersion (PMD). One of the fundamental limitations in an optically
amplified transmission system is the signal-spontaneous beat noise at the receiver caused by
accumulated amplified spontaneous emission (ASE). This noise impairment can be characterized in
terms of the OSNR. It is a quantitative measurement of how much the signal has been corrupted by
noise, during the propagation in a fiber. OSNR is defined as the ratio of the optical signal power to
the power of the ASE in a specified bandwidth. This bandwidth is referred to as resolution bandwidth
(RBW) and is usually considered as 0.1 nm. OSNR is given by:
OSNR = 10 Log10 (Ps/Pn) = dB (signal) – dB (noise) (2)
Figure2. Definition of OSNR
OSNR is important because it suggests the degree of impairment when the optical signal is
carried by an optical transmission system that includes optical amplifiers. Usually values higher than
20dB are needed at the receiver to provide error free 10Gbps non FEC detection. The required OSNR
should have sufficient margin to include any impairments arising from CD, PMD, fiber nonlinearities
and transmitter and receiver induced distortions. When migrating from 10 Gb/s to 100 Gb/s line rate,
the required OSNR must theoretically increase by 10 dB in order to compensate for the ten times
wider receiver bandwidth [6].

14
In optically amplified transmission system, performance degrades by the presence of noise
from two fundamental sources; viz. ASE and Rayleigh scattering. Of these, ASE is more serious
type of noise. Every amplifier contributes ASE, and these contributions get added cumulatively
along a chain of amplifiers. Maintaining high OSNR is a challenge for 40 Gb/s and 100 Gb/s long-
haul WDM transmission. Chromatic Dispersion and Polarization Mode Dispersion are important
sources of distortion in long haul 40 Gb/s and 100 Gb/s systems. Transmission fibers with lower
dispersion require shorter dispersion compensation fiber (DCF) and thus tend to have lower PMD.
The key requirements to use the existing 10 Gb/s DWDM systems at 40 Gb/s and 100 Gb/s,
the spectral efficiency ηs and dispersion tolerance (both CD and PMD) must be increased [7].
Deployment of higher link rates can improve ηs and thus maximize the capacity on existing DWDM
systems and fiber pairs. The result of increase in CD and PMD tolerance would eliminate the
dispersion compensation units (DCUs) in the DWDM links. This in turn would improve the
performance of delay sensitive telecommunication applications such as internet gaming, network
storage and telemedicine. Literature review suggests that Coherent Polarization – Multiplexed
Quadrature Phase Shift Keying (PM – QPSK) modulation format is a critical enabler that meets the
100 Gb/s requirements [7]. Theoretically speaking, 100 Gb/s means 10 x higher capacity than 10
Gb/s, which indicates a 10 dB performance improvement. Practically 10 dB performance
improvement at 100 Gb/s can be achieved by:
a. Use of Coherent PM – QPSK (6 dB) [7]
b. Use of high coding gain Soft Decision Forward error correction (SD-FEC) scheme
(2 – 3 dB) [8].
c. Reduction in penalty allocations (1- 2 dB).
This would result in a total performance improvement of 9 – 11 dB, approaching the OSNR
sensitivity of 10 Gb/s direct detection OOK systems. From this analysis it can be concluded that for
transport of high quality full motion video in telemedicine application the aspects mentioned earlier
play a crucial role. The enhancement of channel bit-rate demands a higher OSNR for optical
transmission systems. Higher bit rates need higher optical signal-to-noise ratio (OSNR). As discussed
earlier, OSNR suggests the degree of impairment caused by optical amplifiers to the signal.
4. TECHNIQUES TO IMPROVE OSNR IN DWDM LINKS
The optically amplified DWDM networks form the backbone of the long haul and ultra - long
haul commercial terrestrial and submarine systems. This section discusses the various techniques to
enhance the OSNR parameter in DWDM links.
• Use of advanced fiber technology
The OSNR serves as the main constraint in impairment-aware optical routing and is
expressed in general by (3) and (4).
OSNR=
Pch
PASE
=
Pch
SSP BOP (3)
Where Pch is the signal power, SSP is the spectral density of ASE noise and BOP bandwidth of
optical filter [9].

15
OSNR≈
Pch
S (Pp.NF.Nspan) (4)
Per-channel power Pch is directly proportional to effective area Aeff of the transmission fiber
and the parameter S is directly proportional to the attenuation coefficient of the fiber. Reducing the
loss of the transmission fiber and components will increase OSNR. Also increasing the launch
power per span is yet another possible approach to enhance OSNR, while maintaining terrestrial span
lengths. However, this can increase the non-linear impairments such as self-phase modulation
(SPM), cross-phase modulation (XPM), four-wave mixing (FWM) and stimulated Raman scattering
(SRS) effects [10].
This suggests the approach of enhancing OSNR using advanced fiber technology wherein
fiber with large effective area and ultra-low loss is fabricated. UltraWaveTM
fibers with effective area
of 107 µm2
and attenuation of 0.187dB/km can be considered to improve OSNR for high data rate
systems. Lowest loss in fiber ensures longer distances between amplifiers in DWDM links and
increases the OSNR and large effective area suppresses non-linearity. Attenuation less than 0.185
dB/km and Aeff larger than 110 µm2
at 1550 nm, have been reported [2]. Yoshinori Yamamoto et
al. demonstrated the enhancement of OSNR by designing pure-silica-core fiber (PSCF) with large
effective area, Aeff of 134 µm2
and low loss coefficient of 0.169dB/km [11]. By using depressed
cladding profile they suppressed both the micro and macro bending loss of the PSCF. The newly
designed PSCF is expected to have the highest OSNR improvement among practical fibers by as
much as 3.4 dB in 80 km-span transmission systems compared to standard single mode fiber (SSMF)
[11]. In submarine networks, transmission lengths are long so transmission loss is of paramount
importance. Large effective area fibers like Corning’s Vascade EX2000 with pure silica core allows
ultra low loss and delivers significant OSNR gain up to 4.4 dB relative to a standard G.652 fiber over
a 100 km span [12]. Ultra low-loss-fibers with large effective areas and backward-pumped
Raman/EDFA amplifiers have demonstrated 112 Gb/s PM-QPSK transmission over 200 km hybrid
fiber spans [13]. As mentioned earlier larger effective areas enhance the linearity of fiber by virtue of
which the new PSCF would be best suited for the high-speed and long-haul transmission systems.
Solid single-core, single-mode fibers, if well designed and fabricated, can have Aeff of 160 µm2
with attenuations of even 0.150 dB/km at 1550 nm have been reported [2].
• Optical Amplifiers
Semiconductor optical amplifiers (SOAs) and fiber amplifiers have dominated their use in
the existing 10 Gb/s networks. Semiconductor optical amplifiers (SOAs) are attractive as they are
compact and can be integrated with other photonic components. However, research reveals that the
relatively high insertion loss and optical signal-to-noise ratio (OSNR) degradation hinder
commercialization of SOAs [3].
• Cascaded Amplifiers
Another parameter that one should consider in optically amplified transmission system is the
noise factor (NF) of the amplifiers in cascade. By definition
NF (dB) = dB (OSNR)in – dB (OSNR)out (5)

16
With NFi and Gi are noise figure and gain in linear units, the total noise factor NFt for a chain
of amplifiers can be written as:
NFt = NF1 + (NF2 – 1)/G1 + (NF2 – 1)/G1*G2 + ….. (6)
Figure 3. (a) Gain after Loss scheme (b) Gain before Loss scheme
It means the noise figure of 1st
amplifier stages and spans are the main contributors to total
NF. The scheme resulting in lower NF i.e. case (b) enhances the OSNR. Moving gain before loss (as
in case of a Booster instead of Preamplifier) improves the OSNR in optically amplified DWDM long
haul links. This is exactly what Raman amplifier does. Inderpreet Kaur et al have proposed that
when TDFA and EDFA are used in series configuration, then gain spectrum is broadened up to
100nm. The gain variation is less than ± 1.5% in the wavelength region of 1460-1580 nm and there is
a noticeable reduction in the Noise Figure correspondingly in the hybrid amplifier (not mentioned)
[14]. For a system containing N fiber spans, where each span is optically amplified, the simplified
OSNR of a 1550 nm signal channel at the end of the system can be expressed as [2]:
OSNR [in dB/0.1 nm RBW] = 58 + Pch – Lsp – NF – 10 log10 (N) (7)
where Pch is the per- channel power (in dBm) launched into the span; Lsp is the span loss (in dB).
The conditions applied to obtain this simplified expression are:
• NF is same for all OAs
• All amplifiers compensate for link loss (Gi = Li)
• All spans have same loss L.
From (7), it can be seen that increase in OSNR can be achieved by increasing Pch, decreasing
NF and decreasing Lsp. If the OSNR is increased by 3 dB, the length of the system can be doubled,
assuming that the amplifiers are at equal distances and operate in linear region. Reduction in NF calls
for enhancement of OSNR. From the above discussion it can be inferred that for telemedicine
application, enhancement of OSNR becomes the prime focus.
• Hybrid Amplifiers
In view of improving the quality of the transferred signal, [15] have proposed three typical
calculating models of terrestrial DWDM cascaded EDFAs fiber optic communication links using
Hybrid amplifier (HFA) at three locations viz. first, mid and last span. The authors have selected a
combination of Distributed Raman Amplifier (DRA) and EDFA and proposed general calculating
models and suggested algorithm charts to optimize parameters including signal power per channel
launched fiber, EDFAs gain and pump power of Raman amplifier for improving optical OSNR at the

17
end of link. The methodology used by them involves the use of algorithm-based numerical
calculating MathCAD program application in a typical system - WDM Nation-wide links in Vietnam.
The optimized results of OSNR are compared with the case of links where parameters are
conventionally chosen by experience way. The authors claim an enhancement in OSNR of 1-5 dB
over the non-optimized results [14]. The impact of hybrid EDFA/Raman amplification on a coherent
WDM multiplexed optical communication system has been studied and modeled. The authors have
experimentally demonstrated a 3 dB improvement in the delivered OSNR (as compared to an EDFA-
only amplification system) for a 2 Tb/s CoWDM system at 42.84 Gbaud over 124 km of field-
installed SMF using DRA. The Q factor improvement was 1.2 dB compared with EDFA
amplification only [15]. In this approach, bidirectional pumping cannot be used for performance
enhancement, because degradation in OSNR resulted if the launch power was reduced. Also forward
Raman amplification, that enhances the gain, degrades the Q factor due to increased nonlinearities at
the higher launch power. The feasibility of gain enlargement and equalization on extended reach
WDM-ring PON by means of hybrid Raman/EDFA amplification has been investigated by using
simulation tools [17]. Another solution to improve OSNR in long haul systems is to employ only
distributed Raman amplification (DRA) system. This involves the use of transmission fiber itself as
the gain media and high pump power lasers to provide enhanced link performance at 40 Gb/s and 100
Gb/s.
Based on the literature survey done so far, hybrid amplifiers enhance the transmission
capacity of broadband systems, upgrade the existing systems built with EDFA amplifiers with
broader/flatter bandwidth. They provide an ability to carry more wavelength-multiplexed optical
channels at given spacing among the channels. If Raman amplifiers are chosen for combination with
EDFA, it gives flexibility to the selected band amplification and is less sensitive to nonlinear effects.
Hybrid amplifiers are concerned with maximizing the span length and/or minimizing the
impairments of fiber nonlinearities, enhancing the EDFAs’ bandwidth and designing “optimal”
hybrid amplifiers in order to obtain flat and widest output gain performance. The gain balance
between Raman and EDFAs involves complex problem with several degrees of freedom
(Optimization technique); OSNR, gain-flatness, bandwidth; number of channels, number of spans
and maximum transmission capacity.
• Pumping Methods
A theoretical investigation about the characterization of RFAs with bidirectional or co-
propagating Raman pump so as to improve the performance of the amplifier has been proposed [18].
The paper provides a brief theoretical analysis and does not take into account the taxing effect of
large pump power requirement and also the issues associated with large pump powers.
• Macro Bending
Macro bending effect in optical amplifiers is yet another method to improve the doped fiber
amplifier Gain and Noise Figure [19]. Macro-bending is defined as a smooth bend of fiber with a
bending radius much larger than the fiber radius. Macro-bending modifies the field distribution in
optical fibers and thus changes the spectrum of the wavelength dependent loss. The macro bending
also reduces the noise figure of EDFA at wavelength shorter than 1550. Since keeping the amount of
noise low depends on a high population inversion in the input end of the erbium-doped fiber (EDF),
the backward ASE power P –ASE is reduced by the bending loss. Consecutively, the forward ASE
power PASE can be reduced when the pump power P is large at this part of the EDF which is
especially undesirable. This is attributed and can be described numerically by the following equation:

18
NF = 1/G + 2PASE/G*hf (8)
where G is the amplifier’s gain, PASE is the ASE power and hf is the photon energy. There
exists an inverse relation between the NF and the OSNR. The noise figure decreases appreciably due
to bending effect.
From the study it can be concluded that the use of advanced fiber technology to enhance
OSNR faces a limitation posed by the fabrication of fibers with highest possible effective areas and
lowest possible attenuation. For Raman amplification such a system would demand huge pump
powers. Also these fibers may pose incompatibility issues while splicing them with conventional
standard single mode fibers. Whereas macro-bending technique improves both gain and noise figure
by approximately 6 dB and 3 dB, respectively. The method is cost effective which needs 100mW
pump power and does not require any additional optical components to flatten the gain, thus enables
reduction in the system complexity.
• Multi-Level Modulation formats
Rapid progress has been achieved in transmission systems including multi-level modulation
formats and digital coherent detection techniques to reduce the OSNR requirement. Especially in
conjunction with DPSK modulation formats, use of more advanced amplification schemes leads to
significant improvement in OSNR performance over conventional EDF only amplification [20].From
the literature survey there is further scope to research in achieving enhancement in OSNR by an
optimum configuration of hybrid amplifiers with the support of the advanced fiber technology.
V. CONCLUSION
In conclusion, we state that to improve the OSNR there is a need to enhance the linearity in
optical fibers or to increase the input channel power level. An increase in the spectral efficiency
using advanced modulation formats, or use of novel fibers can upgrade the performance of the
DWDM systems. Next-generation systems and future upgrades of existing systems will benefit from
these new concepts emerging from system research. The proposed research is an effort to enhance
the optical signal-to-noise ratio by 1-5 dB in DWDM fiber optic transmission link using HFAs and
advanced fiber technology in comparison with standard single mode fiber (SSMF) links.
REFERENCES
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19
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AUTHOR’S DETAIL
BIBLIOGRAPHY
Tanuja Khatavkar received Bachelor and Master’s degree in Electronics
Engineering from Shivaji University, in 1991 and University of Pune in 2004
respectively. She has earned Master’s degree in Business Management with
Human Resource as her specialization as well. She is currently pursuing Ph.D. in
Electronics and Telecommunication Engineering from Sinhgad College of
Engineering, affiliated to University of Pune. She has 23 years of teaching
experience and is presently working as Assistant Professor in Electronics and
Telecommunication department of Pune Vidyarthi Griha’s College of Engineering and Technology,
Pune, Maharashtra (INDIA). She authored a book on Network Fundamentals and Analysis published
by WILEY and has also published papers in optical communication field.
Prof. Dr. D. S. Bormane- Completed B.E. Electronics from Marathwada
University, Aurangabad in 1987, M.E. Electronics from Shivaji University,
Kolhapur, Ph.D. Computer from Ramanand Tirth University, Nanded, and has a
vast teaching experience of 3 decades as a Lecturer, Assistant Professor,
Professor, Head of Department, and is currently working as Principal in
J.S.P.M’s Rajarshi Shahu College of Engineering, Pune, Affiliated to University
of Pune, Maharashtra (INDIA). He has published about 60+ papers at national
and international conferences and journals.

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