IMPLEMENTATION & PERFORMANCE
ANALYSIS OF BIDIRECTIONAL FSO
CHANNEL IN HYBRID TDM/WDM GIGABIT
PASSIVE OPTICAL NETWORK
Kehkashan A. Memon
Department of Electronics, Mehran U.E.T Jamshoro (Pakistan)
E–mail: kehkashan@faculty.muet.edu.pk
A.W. Umrani
Department of Telecommunication, Mehran U.E.T Jamshoro (Pakistan)
E–mail: waheed.umrani@faculty.muet.edu.pk
M.A. Unar
Department of Computer Systems, Mehran U.E.T Jamshoro (Pakistan)
E–mail: mukhtiar.unar@faculty.muet.edu.pk
Wajiha Shah
Department of Electronics, Mehran U.E.T Jamshoro (Pakistan)
E–mail: chairman.es@admin.muet.edu.pk
Recepción: 05/03/2019 Aceptación: 27/03/2019 Publicación: 17/05/2019
Citación sugerida:
Memon, K. A., Umrani, A. W., Unar, M. A. y Shah, W. (2019). Implementation &
Performance Analysis of Bidirectional FSO channel in Hybrid TDM/WDM Gigabit
Passive Optical Network. 3C Tecnología. Glosas de innovación aplicadas a la pyme. Edición
Especial, Mayo 2019, pp. 166–181. doi: http://dx.doi.org/10.17993/3ctecno.2019.
specialissue2.166–181
Suggested citation:
Memon, K. A., Umrani, A. W., Unar, M. A. & Shah, W. (2019). Implementation &
Performance Analysis of Bidirectional FSO channel in Hybrid TDM/WDM Gigabit
Passive Optical Network. 3C Tecnología. Glosas de innovación aplicadas a la pyme. Special
Issue, May 2019, pp. 166–181. doi: http://dx.doi.org/10.17993/3ctecno.2019.
specialissue2.166–181
3C Tecnología. Glosas de innovación aplicadas a la pyme. ISSN: 2254–4143
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ABSTRACT
In this paper, experimental work is performed on hybrid TDM/WDM Gigabit
Passive Optical Network by replacing the last mile optical ber with Free Space
Optical channel. A Hybrid GPON system is a more intelligent choice of today’s
passive optical networks because it provides benets of both TDM as well as
WDM technologies at Giga b/s. And the use of Free Space Optical channel will
bring the last mile network to its peak performance. This paper demonstrates
the performance of hybrid TDM/WDM GPON system utilizing FSO channel
in terms of Quality of factor and Bit error rate along with eye diagrams. This
system is a full bidirectional system working on 2.5Gb/s downstream and
1.244Gb/s upstream transmission. The system is using varying lengths of FSO
channel to analyze the GPON network performance. The simulation results using
Optisystem v.15 verify the fully functional bidirectional transmission of FSO link
between OLT and ONU and achieve BER of the order of 10
–16
and 10
–12
at a
distance of 100 m for both upstream and downstream respectively.
KEYWORDS
Passive Optical Networks, Hybrid TDM/WDM GPON, Free Space Optics,
Optisystem.
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1. INTRODUCTION
The growth of sophisticated and intelligent devices which brings the need of
virtually always stay connected with anything and everything we can think of
is creating a demand of high data rates which could support the triple play.
So there is a massive deployment of FTTH (Fiber −To–The −Home) to serve
more number of users with increased bandwidth (Kaur, Kaur & Singh, 2017).
Passive Optical Network (PON) architecture is adopted across the globe in order
to provide higher bandwidth support (Ahsan, Lee, Newaz & Asif, 2011). More
specically, the Gigabit PON or GPON provide a powerful point–to multipoint
solution to satisfy the increasing capacity demand in the access part of the
communication infrastructure, between service provider central oce (CO) and
a number of Optical Network Units (ONUs) at the consumer premises (Skubic,
Chen, Ahmed, Wosinska & Mukherjee, 2009).The hybrid GPON is a hybrid
passive optical network, where WDM–GPON and TDM–GPON are integrated
into a single passive optical network (Liu, Zhang & Li, 2011). Hybrid TDM/
WDM means for downstream we are using wavelength division multiplexing
only and in the upstream we are using Wavelength Division along with Time
Division so that multiple Optical Network Units (ONUs) which are using the
same wavelength do not send overlapping data. At the user end the deployment
of Free Space Optical (FSO) channel instead of Optical ber is the main focus of
this work. FSO can be considered as an alternative to the Optical Fiber cable or
RF/Microwave systems especially when the physical connections are impractical
due to several considerations and it has become the ideal choice for the access
technology (Memon, Umrani, Unar, Shah & Chowdhry, 2018).
Pla (2011) has worked on the TDM/WDM GPON utilizing a 2.5Gbps rate for
downstream and 1.244GBps rate for upstream communication. The author
has taken ve areas and the system is completely optical ber based. The main
bidirectional optical ber connecting the Optical Line Terminal (OLT) to Optical
Distribution Network (ODN) is of 5Km in length. At the access network side
the lengths of bidirectional optical ber in each area are dierent depending
upon the requirements of that area. In this work, we are implementing the FSO
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channel at the ODN/ONU side. The main backbone ber cable has a length of
50Km. and two dierent areas are simulated namely hospital block and building
block. Each block has an FSO channel for upstream and downstream. The FSO
channel length is one of the major issues which can impact the performance
of the system. So the objective of this work is to take ve dierent ranges, from
50m to 250m and analyze the system performance. Considering the tropical
environment such as of Pakistan, this system is going to provide performance in
terms of Q factor and BER along with eye diagrams.
The remaining sections are organized as follows. In section II system architecture
is discussed highlighting separately the downstream and upstream components,
its communication system and parameters. Section III discusses the experimental
results and gives the performance analysis in the form of eye diagrams, Quality of
factor and BER with respect to the multiple ranges of FSO channel. Section IV
concludes the paper. This is followed by section V and VI for acknowledgement
and references.
2. SYSTEM ARCHITECTURE
Figure 1 shows the complete system design of a hybrid TDM/WDM GPON.
For understanding, rst the downstream connectivity is explained which is from
OLT to ONUs and then upstream connectivity is explained i–e from ONUs to
OLT. As seen in Figure 1 the system comprises of three main sections, rst is
OLT, followed by backbone channel and nally the Access network. The bit rate
used in this system is 2.5Gbps. The system is based on two areas namely Hospital
block and Building block. Therefore, two wavelengths will be used.
2.1. DOWNSTREAM ARCHITECTURE
Referring to Figure 1, the Optical Line Terminal (OLT) consist of WDM
Transmitter that can provide multiple wavelengths. These wavelengths are
multiplexed by WDM multiplexer. We are using 2 dierent wavelengths λ1=
1450nm and λ2=1470nm with frequency spacing of 20nm. The extinction
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ratio is set on 10dB. WDM Mux is required to combine the wavelengths to be
transmitted on a single ber. The WDM mux has a bandwidth of 80nm with
an insertion loss of 2dB. To ensure a good quality signal transmission an optical
amplier with a 15dB gain is also used. A Bidirectional circulator is used to
transmit downstream data to the ONUs and to receive upstream trac from
the ONUs. The downstream optical signal is passed through the main backbone
channel which is a bidirectional optical ber having a length of 50Km. Table 1
gives the parameters of bidirectional optical ber.
Next comes the access network / (ODN) which comprises of a splitter and two
units namely hospital and buildings blocks. The rst branch of the splitter is
connected to hospital block and the second is connected with the building block.
Figure 1. TDM/WDM GPON complete design.
Table 1. Parameters of Bidirectional Optical Fiber.
Parameters Value
Reference wavelength 1450nm
Length 50km
Attenuation 0.2dB/km
Dispersion 16.75ps/nm/km
Dispersion slope 0.075ps/nm
2
/km
Effective area 80um
2
Figure 2 shows the hospital block, which contains the FSO channel for
downstream. The FSO channel parameters are given in Table 2. The length of
the FSO varies from 50m to 250m. This channel is connected to the ONU of
hospital block (gure 3) where downstream data is detected by PIN photodetector
which will transform the optical signal into electrical. This output is connected to
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a Low pass Bessel lter and nally to a 3R generator which is connected to the
BER analyzer for detecting the performance in terms of Q factor, eye diagram
etc. The other part of ONU is discussed in upstream architecture.
Figure 2. Hospital Block.
Figure 3. ONU for hospital block.
Table 2. FSO channel parameters.
Parameter Value
Range 50 to 250m
Attenuation 25dB/km
Transmitter aperture diameter 5cm
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Parameter Value
Beam divergence 2mrad
Receiver aperture diameter 20cm
The second branch of the splitter is connected to Building block which is shown
in Figure 4. This block contains two sections of buildings and both will receive
λ2=1470nm.
Figure 4. Building block.
In Figure 5 it is shown that FSO channel is used for downstream as well as for
upstream, and its output is again connected to a 1:2 splitter because there are
two ONUs or two sections of the building each receiving the same wavelength
in the downstream. Inside the building unit, there is again a 50m bidirectional
optical ber which is used control both upstream and downstream data from
both ONUs. The ONU structure is shown in Figure 5.
Figure 5. ONU for building block.
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Figure 5 also contains the same detection mechanism i–e PIN photodetector
which is connected to loss pass Bessel lter that is connected to the 3R generator
and BER analyzer. The other part is upstream which is discussed in the upstream
architecture.
2.2. UPSTREAM ARCHITECTURE
In the upstream direction, the optical signal will travel from each end user (ONU)
to the OLT, Pla (2011). So now the OLT is the receiver and each ONU will work
as a transmitter. Refer to Figure 3 for ONU of the hospital block. Here there is
only one ONU, the optical transmitter will transmit at 1270nm with the power
of 0dB. This data will be sent to the OLT via FSO channel having the same
parameters as shown in Table 2. The output of this FSO is connected to the
splitter which is now working as a multiplexer. This splitter output is connected to
the backbone bidirectional optical ber which will send the upstream data to the
OLT. In the OLT the circulator is bidirectional hence the upstream data will be
received separately in the OLT via PIN photodetector, LP lter, buer selector,
3R generator and nally BER analyzer.
In Figure 4 there are two sections for building block hence both will share the same
wavelength 1290nm. In this case, we can utilize the benet of TDMA by allotting
a specic time slot to each ONU so that the data don’t get overlapped. Therefore as
shown in Figure 5 the ONU of building block contains an optical transmitter whose
output is connected to dynamic y select which will allow the ONU to send its data in
its specic time duration. The output of the dynamic y select is connected to other
devices which are already explained and are shown in Figure 4.
3. EXPERIMENTAL RESULTS
With the above explanation of complete system architecture, the experimental
results are discussed here. The results are in shape of Eye diagrams, Max. Q
factor, Min. BER with respect to the range of FSO channel. Therefore in this
section, we determine the feasibility of the FSO channel in the last mile network.
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3.1. DOWNSTREAM PERFORMANCE ANALYSIS
In the case of ONU for hospital block, Figure 6 shows the eye diagram results
when the FSO channel range is 50m and 100m. After 100m the eye diagram
degrades signicantly. Table 3 gives the BER analyzer values for hospital block
which clearly shows that FSO channel is giving its peak performance from 50 to
100m range. Figure 7 and 8 shows the Min. log of BER and Q factor with respect
to the FSO channel range and it shows that after 100m the BER and Q factor
shows poor performance and hence FSO channel is best suited for range from
50 to 100m.
Figure 6. Downstream for Hospital block FSO channel range 50m and 100m.
Table 3. Hospital block BER analyzer values.
Range (m) Max. Q Factor Min. BER Min. log of BER
50 22.58 3.40298e–113 –112.468
100 6.2209 2.46883e–010 –9.60751
150 2.14274 0.0159928 –1.79608
200 0 1 0
250 0 1 0
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Figure 7. Min. log of BER Vs FSO channel range for hospital block.
Figure 8. Max. Q Factor Vs FSO channel range for Hospital block.
For the Building block the eye diagrams are shown in Figure 9 and they appear
nearly the same as for hospital block hence all other graphs are deliberately not
shown for simplicity because both the blocks at the ONU side are providing
similar performance for FSO channel range from 50 to 100m. Table 4 is giving
the BER analyzer values for both building sections hence validating this point.
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Figure 9. Downstream for Building block FSO channel range 50m and 100m.
Table 4. Building block BER analyzers values.
Range (m) of
FSO channel
BER analyzer_2 BER analyzer_3
Max. Q
Factor
Min. BER
Min. log of
BER
Max. Q
Factor
Min. BER
Min. log of
BER
50 19.913 1.56954e–088 –87.8042 21.4066 5.78105e–102 –101.238
100 5.47108 2.2316e–008 –7.65138 5.57135 1.2635e–008 –7.89842
150 2.39446 0.00831056 –2.08037 2.67231 0.00376535 –2.42419
200 0 1 0 0 1 0
250 0 1 0 0 1 0
Also, Figure 10 shows the cumulative response of Q factor with respect to the
range of the FSO channel used building block. Note that the quality of signal
drops from 150m to onwards.
Figure 10. Cumulative response of Q factor in building block w.r.t FSO channel range.
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Finally, Figure 11 shows the min. log of BER vs total power in dBm comparison
of all users.
Figure 11. Min. log of BER vs Total power for all users.
3.2. UPSTREAM PERFORMANCE ANALYSIS
In the case of upstream communication, the OLT becomes receiver as shown
in Figure 1. Hence the results are shown in Figure 12 where eye diagrams for
upstream FSO channel range 50m and 150m is shown. It is to note here that in
the upstream the eye–opening at the 150m range is a little bit better as compared
to the transmission in downstream. But the peak performance is still from 50 to
100m.
Figure 12. Eye diagrams for upstream FSO channel 50m and 150m.
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Table 5 gives the complete set of BER analyzer values for upstream
communication, showing the ideal performance is up to 100m range. Figure 13
gives the graph of min log of BER at the OLT side for upstream at dierent
ranges of FSO channel. Note that at 50m the BER produces peak performance
as at this point the BER achieved is 5.75 x10–176. At 100m the BER value is
4.91x10–16 and after that BER degrades signicantly.
Table 5. BER analyzer values for upstream.
Range (m) Max. Q Factor Min. BER Min. log of BER
50 28.2572 5.75632e–176 –175.24
100 8.02883 4.91682e–016 –15.3083
150 3.14412 0.000830138 –3.08085
200 0 1 0
250 0 1 0
Figure 13. Min. log of BER at the OLT side for upstream at different ranges of FSO channel.
4. CONCLUSION
With the above discussion and results it can be concluded here that bidirectional
free–space optical channels are providing good quality data transmission in hybrid
TDM/WDM GPON system at the range from 50m to 100m. BER of the order
of 10
–16
and 10
–12
at a distance of 100 m for both upstream and downstream
is achieved which means it can be practically implemented. But if the range is
greater than 100m the system performance deteriorates signicantly. Therefore
careful considerations must be taken while installing the FSO channel.
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ACKNOWLEDGEMENTS
This work is supported by Mehran University of Engineering and Technology,
Pakistan.
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