Design and Analysis of 8 Data Channel WDM-PON with Free ... · Design and Analysis of 8 Data...

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


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


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


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


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)


Fig 9: Multiplexed signal

Fig 10: Spectrum after Fiber

Fig 11 : Receiver signal with drooped 1550 nm wavelength


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


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


Fig 18 : BER vs Attenuation in FSO (atten_add)

Fig 19 : BER vs FSO distance

Fig 20 : BER vs Bitrate


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.


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.


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