International Journal of Electronics Engineering Research.
ISSN 0975-6450 Volume 9, Number 8 (2017) pp. 1345-1359
© Research India Publications
http://www.ripublication.com
Design and Analysis of 8 Data Channel WDM-PON
with Free Space Optics Device for Wireless Support
using NRZ coding
Khant Shailesh1,* and Patel Atul2
1 Assistant Professor, A D Patel Institute of Technology,
New V V Nagar, Gujarat, India. 2 Principal and Professor, CMPICA, Charusat, University,
Changa, Gujarat, India.
Abstract
Passive Optical Network has made great progress in recent era. It is worthy to
design Passive Optical Network using WDM and TDM Technology. This
paper includes simulations of Wavelength Division Multiplexing types of
Passive Optical Network with wireless support. For wireless support Free
Space Optics device is used. NRZ types of coding is used to represent data
signal which is generated using bit sequence generator. Nonlinear fiber is used
for optical data transmission between server and client. Each channel is
dropped one by one at desired point and if any new user wants to join the
network then it can be added using optical adder block using any of the
wavelength which is previously dropped by other user. Analysis is made in
terms of received signal strength which can be measured for specific
wavelength in spectrum analyzer, eye diagram for the user and signal
analyzer. This analysis helps researchers to develop powerful PON Network
which is efficient in terms of received power. As no active components like
any other LASER or amplifiers are present, the cost is reduced in great extent.
Keywords: PON, OLT, ONT, Fiber optics, WDM, FSO, OADM, FITL,
INTRODUCTION
The access network connects the central office to individual subscribers. The needs of
such networks are high bandwidth, rich Internet services, low power and comparable
prices with existing network. PON is a type of access mechanisms to fulfill the above
requirements. Other such mechanisms are DSL, VDSL, Dial-up connections and
ISDN. In FTTH applications CO can be connected directly via independent lines to
1346 Khant Shailesh and Patel Atul
subscribers similar to mesh topology or it can use Curb switch. Both this technologies
are costlier. This expensive technologies are replaced by inexpensive Optical filters
which are passive in construction. No active elements are involved from source to
destination. Day by day the requirements of bandwidth increases due to new varieties
of applications such as image and video transfer in conventional internet services.
Above method can help to fulfill these modern generation requirements. Local loops
created in access network is known as Fiber in the Loop (FITL). Fiber access systems
are generally known as Fiber-to-the-x (FTTX) system where x can be referred as
home, premises and curbs. Here fiber is connected from service provider to home
locations.
Fig.1. WDM-PON Block diagram
WDM system is very much popular as it supports multiple wavelengths on single
fiber. It uses different wavelengths for upstream and downstream channels. The
primary advantage of such system is that it supports virtually unlimited bandwidth.
WDM-PON system includes OLT, OADM and ONU blocks. Extra FSO block is
added for wireless support
Fig 2. OLT Diagram
Fig 3. FSO Transmitter and Receiver
Design and Analysis of 8 Data Channel WDM-PON with Free Space Optics… 1347
.
Fig 4. OADM and ONU
LITERATURE SURVEY
WDM is an approach which exploits huge optoelectronic bandwidth mismatch by
requiring that each end user’s equipment operate only at electronic rate, but multiple
WDM channels from different end-users. WDM is turning out to be more cost
effective alternative compared to laying more fibers. OADM component which is
used for adding and dropping wavelength at desired location includes multiplexer,
demultiplexer and 2X2 bar switch. More than one wavelength can be easily added or
dropped if OADM has hardware/software and processing capability. To design a
network with multi wavelength capability fiber interconnection device is required,
which can be of three type passive star device, passive router device and active switch
device. In simulation a direct device is available for adding and dropping desired
wavelength.
First generation of WDM optical network is having direct point to point physical link
similar to mesh topology. The network is manually configured. Second generation of
WDM is capable of establishing connection oriented end-to-end lightpaths in the
optical layer by introducing optical add/drop elements (WADM or OADM) and
optical crossconnects (OXC). The ring and mesh topologies can be implemented
using these OADMs and OXCs. The lightpaths are operated and managed based on a
virtual topology over the physical fiber topology, and the virtual topology can be
reconfigured dynamically in response to traffic changes. The technical issues of
second-generation WDM include the development of OADM and OXC, wavelength
conversion, routing and wavelength assignment (RWA), interoperability among
WDM networks, network control and management and so on. The third generation of
optical network is having connection less optical data transfer support. So the key
issues are designing optical access network and PON.
In WDN-PON wavelength routing is achieved by Array waveguide Grating (AWG).
It is a type of passive device with fixed routing matrix. It routes all wavelength which
is separated by fixed amount of interval. Based on upstream and downward direction
different types of WDM architectures are proposed. One of such architecture is CPON
(Composite PON). Downstream configuration supports normal WDM structure while
upstream configuration uses single wavelength burst receiver. DFB laser is required at
each ONU as upstream transmitter .Both upstream and downstream signal can share
single fiber. Another architecture is Local Access Remote Network (LARNET). It
uses the concept of broadband signal and as upstream source each ONU uses LED
source. Next architecture is Remote Integrated Terminal Network (RITENET) which
avoids transmitter at ONU side. Instead of that it modulates downstream signal and
1348 Khant Shailesh and Patel Atul
send it back. All these architecture uses single multi wavelength laser source at optical
line terminal side.
To support wireless environment between OLT and ONT wireless transmitter and
receiver is used. In simulation it is modeled by including noise adder and attenuation
adder block and specifying beam parameters. Wireless communication is also known
as free space optical communication. FSO Transmitter convert electrical signal to
optical signal and send it to free space. FSO Receiver converts optical signal back to
electrical signal. It is cost effective and high bandwidth technique. It acts as a bridge
between end user and fiber optics infrastructure. Attenuation and scattering are the
two major parameters which can be characterized for wireless optical support.
Scintillation is another important parameter which is defined as temporal and spatial
variation in light intensity. These variations are created by atmospheric turbulence
such as wind and temperature gradients. Scintillation is characterized by scintillation
index (SI).It is also known as normalized variance of Intensity. In weak turbulence
regime the scintillation index is always less than 1.
(1)
Where,
In is the intensity of light,
SI is scintillation index.
The accurate statistical model for fading effect due to atmoshpheric turbuence is
developed by considering the gamma and lognormal probability functions. This
model can be applied to estimate link margin and availability. A compound
component FSO is developed based on above model. FSO considers signal
attenuation and all other geometrical apsects including link range. The received signal
power at FSO receievr end is characterized by PR.
(2)
Where,
PR is received power,
AC is Receiver capture area
Ө is beam divergence angle,
L is Link range,
T is Transceiver efficiency,
is attenuation
PT is transmitted power,
PBG is optical power of background radiation
Design and Analysis of 8 Data Channel WDM-PON with Free Space Optics… 1349
RESULTS
The simulation parameters need to be set in prior, to get the desired output analysis in
terms of signal spectrum, received signal, Eye diagram etc. Simulation includes input
data in terms of bits (conversion of audio and video data in digital form). Pseudo
random bit sequence generator is included which generates the desired input for
simulation. Bitrate can be specified at this stage only. Conversion to electrical form is
made by electrical signal generator where user can select type of coding method. NRZ
coding is selected for this simulation. Signal is intensity modulated with CW LASER
and transmitted to 10 km fiber. Fiber can be characterized by Nonlinearity parameters,
Attenuation over the length, Dispersion, scattering etc. Free space Optics device is
included which simulates wireless portion of the network. Again signal can pass from
fiber to reach at desired premises. Signal analysis can be made using various analysis
blocks at input stage like signal analyzer which display multiplexed signal and
spectrum analyzer which display spectrum of all wavelengths at OLT side.
Fig 5 : Optical Line Treminal schematic
Table 1 OLT Parameters
Test Bitrate 1.25 Gbps
Pattern Type Alternating
Pattern Length 7
CW LASER
wavelength
1550 nm, 1551 nm,
1552nm, 1553nm,
1554nm, 1555nm,
1556nm, 1557nm
Modulation Type
Fiber Length 10 km
Fiber Loss 0.25 dB/km
Dispersion present Yes
Nonlinearity Yes
1350 Khant Shailesh and Patel Atul
ONT block includes the drop line at user ends for getting separate FTTH receiver
connection. Optical drop multiplexer is used which is having two output channels.
One channel includes dropped signal while other includes remaining wavelength
signal which is to be carried ahead for remaining FTTH users. User needs to specify
dropped signal wavelength as well as filter bandwidth of dropped signal. Filter type
can also be specified. Optical filter is further used to get smooth signal. Optical
normalizer block is used to increase or decrease power level to desired one. Receiver
is having photo detector, electrical amplifier and electrical filter to get original data
signal back. Photo detector can be either PIN detector or APD detector. For
simulation PIN type of photo detector is used. The remaining signal wavelengths can
be further carry forwarded to next user by means of fiber where next channel to be
dropped is specified by other optical drop multiplexer. Here different topologies can
be used such as mesh, star, bus, ring or hybrid topology. In above example hybrid
topology is used where the front end is mesh and at ONT side bus type of topology is
used, where each ONTs are connected in series. Due to such connections if link
problem arises at initial stage than it affects all remaining users and data connection
will be shutdown. Even it is difficult to find fault easily as all remaining users have
lost data connection.
Fig 6 : Optical Network Unit
Various simulation components included are shown in Fig 5, Fig 6 and Fig 7. These
blocks are PRBS generator, Electrical signal generator, CW Laser, Modulator, optical
Design and Analysis of 8 Data Channel WDM-PON with Free Space Optics… 1351
multiplexer, nonlinear fiber, compound component FSO at OLT side. At ONT side
optical drop multiplexer, optical add multiplexer, optical filter and receiver is used.
Table 2 ONT Parameters
Optical Filter bandwidth 1 X 10-10
Optical Filter Type Gaussian
Filter center Wavelength to be dropped
Power Normalized -10dB
Receiver Noise Included Yes
Photodiode Type PIN Diode
Quantum efficiency 0.8
Fig 7: User for OADM
Fig 8: Spectrum at OLT (Transmitter)
1352 Khant Shailesh and Patel Atul
Fig 9: Multiplexed signal
Fig 10: Spectrum after Fiber
Fig 11 : Receiver signal with drooped 1550 nm wavelength
Design and Analysis of 8 Data Channel WDM-PON with Free Space Optics… 1353
Fig 12 : Receiver signal with drooped 1550 nm and 1551 nm wavelength
Fig 13 : Dropped Line signal for 1551 nm wavelength
Fig 14: Received signal with drooped 1551 nm wavelength
1354 Khant Shailesh and Patel Atul
Fig 15: Eye diagram for drooped 1551 nm wavelength
Fig 16: All dropped signal except 1557 nm wavelength
Fig 17: Received signal with 1550 nm wavelength added at final stage
Design and Analysis of 8 Data Channel WDM-PON with Free Space Optics… 1355
Fig 18 : BER vs Attenuation in FSO (atten_add)
Fig 19 : BER vs FSO distance
Fig 20 : BER vs Bitrate
1356 Khant Shailesh and Patel Atul
Fig 21 : Q2 vs Attenuation in FSO (atten_add)
Fig 22 : Q2 vs FSO distance
DISCUSSION
Various relationships between signal parameters, signal spectrum and signals are
shown in above results. Fig 8 shows signal spectrum at OLT side which includes all
eight different wavelengths multiplexed using optical multiplexer. Fig 9 shows all
multiplexed signals in time domain representation using signal analyzer.
Design and Analysis of 8 Data Channel WDM-PON with Free Space Optics… 1357
Fig 23 : Q2 vs Bitrate
Fig 10 shows power level in spectrum analyzer after signal travels significance
distance from fiber and FSO device. It shows that significant attenuation is there in
each signal strength. Fig 11 shows the signal after first optical drop multiplexer where
1550 nm wavelength is dropped. In spectrum it shows low power level compared to
other undropped wavelength. Similarly Fig 12 shows spectrum after next 1551 nm
wavelength is dropped. Fig 13 indicates spectrum of dropped 1551 nm wavelength
where significant power level is shown. Fig 14 and Fig 15 shows received signal
strength and eye diagram of 1551 nm wavelength signal. Fig 17 shows how 1550 nm
wavelength is added to spectrum. Fig 18 to Fig 23 shows relationship between various
parameters like bitrate, FSO attenuation and FSO distance with BER and Q-factors.
CONCLUSIONS AND FUTURE WORK
WDM-PON system with upstream data from OLT to ONT is simulated and analyzed
in this paper. Downstream data can also be simulated using the bidirectional nonlinear
fiber. Due to wireless optical data transfer, power level is significantly reduced.
Power level can be raised by introducing power normalizer component at receiver
end. As the distance between FSO transmitter and FSO receiver increases the BER
increases significantly and Q-factor decreases. Same phenomena happens for
attenuation. As the attenuation in wireless medium increases the BER increases. Even
with increase in bitrate the BER increases. Further analysis can be made in terms of
power budget at each stage. These results can be compared with other coding
techniques like RZ coding and Manchester coding. Even by introducing another free
space optics model the results can be optimized.
ACKNOWLEDGEMENTS
The authors would like to thanks Charutar Vidya Mandal, Principal ADIT and Head
EC Department for providing facilities and resources to complete the research. The
authors would also like to thank CHARUSAT for their constant support and
motivation.
1358 Khant Shailesh and Patel Atul
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