Hindawi Publishing Corporation
International Scholarly Research Notices
Volume 2014, Article ID 984157, 8 pages
http://dx.doi.org/10.1155/2014/984157
Research Article
Effects of Modulation Techniques (Manchester Code,
NRZ or RZ) on the Operation of Hybrid WDM/TDM Passive
Optical Networks
Kumbirayi Nyachionjeka1 and Wellington Makondo2
1
2
Department of Electronic Engineering, Harare Institute of Technology, Harare, Zimbabwe
Department of Information Technology, Harare Institute of Technology, Harare, Zimbabwe
Correspondence should be addressed to Kumbirayi Nyachionjeka; kuuh29@gmail.com
Received 19 March 2014; Revised 2 July 2014; Accepted 3 July 2014; Published 30 October 2014
Academic Editor: Meiyong Liao
Copyright © 2014 K. Nyachionjeka and W. Makondo. This is an open access article distributed under the Creative Commons
Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is
properly cited.
In this paper, the performance and feasibility of a hybrid wavelength division multiplexing/time division multiplexing passive
optical network (WDM/TDM PON) system with 128 optical network units (ONUs) is analysed. In this system, triple play services
(video, voice and data) are successfully communicated through a distance of up to 28 km. Moreover, we analysed and compared
the performance of various modulation formats for different distances in the proposed hybrid WDM/TDM PON. NRZ rectangular
emerged as the most appropriate modulation format for triple play transmission in the proposed hybrid PON.
1. Introduction
Currently, telecoms operators are adapting their broadband
access networks so as to enable these networks to support
high bandwidth demanding services such as high-definition
TV, interactive gaming and video conferencing. The exponential spread of internet into the remotest of areas in the
world has led to hunger for larger data capacity. Recent
increase in the demand for internet has brought focus on
high bandwidth last mile access networks as these are the
final hurdle to the consumer. Research into access networks
has largely focused on ways to improve how customers
can receive high definition television, videoconferencing and
many other real-time services. As access networks and metro
networks continue to converge, and research has begun to
focus on dense and cost-effective optical access networks.
The advent of WDM and TDM systems, as a solution to
increase the density of customers, is being realised, and the
coming in of these systems has led to reduced cost as there
is sharing of equipment in the optical distribution network
and the optical line terminal (ONU). To further reduce the
overall cost, the optic fibre network is continuously being
made passive., In TDM passive optical networks, the TDM
scheme is used to distribute the overall capacity among many
users and because of this, each user’s average bandwidth
becomes limited to less than 100 Mb/s for GPONs and
EPONs. In comparison, wavelength division multiplexing
(WDM) PON is a strong alternative for future applications for
access services requiring broadband. WDM PON provides
dedicated transmission paths between each ONU and the
OLT located at the CO, thus ensuring very fast connection for
each subscriber with user localized bandwidth up to 10 Gb/s.
A WDM PON has a maximum number of wavelengths that
can be used, thus reducing the number of users that can
be serviced, normally up to 64 users. To minimise these
shortcomings of both the WDM PONs and TDM PONs. It is
understood that the combination of these two PONs results in
a hybrid network that is able to increase the number of users
that an OLT can support with adequate bandwidth to every
user. Figure 1 [1] shows a PON set up with N users connected
to the OLT.
Hybrid pons together with various modulation techniques can lead to improved systems in terms of performance and distance reached by the access networks. Hence,
modulation techniques can thus enhance the performance of
the hybrid system. Passive optical networks have shown that
2
International Scholarly Research Notices
ONU
1
𝜆a
ONU
2
Optic fibre
OLT
𝜆u
ONU
3
RN
ONU
4
..
.
ONU
N
𝜆a
𝜆u
OLT
RN
ONU
Downstream wavelength
Upstream wavelength
Optical line terminal
Remote node
Optical network unit
Figure 1: Passive optical network.
various reasons, chief among them low cost, high bandwidth
support, simple operation, and maintenance, make them the
mostly likely networks that are used for next generation
access PONs.
2. Hybrid Passive Optical
Networks (Hybrid PONs)
The explosion in internet traffic and ever expanding bandwidth requiring applications has led to the need for optic
fibre networks to be deployed extensively in access networks
area. The growth in these areas has become exponential
and rapid that research is focussing on this area to provide
the necessary degree of quality service to the consumer. In
using purely TDM PON systems, speed can be the advantage
obtained, but there is a limitation as to the number of users
reached and also there is a limitation as to the amount of
traffic a given user can access; this is because TDM uses one
wavelength for downstream and another wavelength for the
upstream communication limiting the optimum bandwidth
for a given user, and thus the bandwidth for that particular
single fibre is wasted, while on the other hand specific
wavelength assignment for a given customer can be used in
WDM PON which is an ideal and clear way of increasing
capacity.
WDM is inefficient in its available spectrum utilisation
because it lacks sharing of wavelengths. To deal with this
inefficiency that arises from the purely WDM-PON and
purely TDM-PON, further research and analysis into alternative access networks have to be done. This can be realised
by combining the TDM PON layer with the WDM PON
technology, thus increasing the capacity of the TDM PON.
This increased capacity is due to the WDM PON, while
the TDM layer allows the efficient spread of capacity to a
larger number of customers. Therefore, the development of
Hybrid WDM/TDM PON systems concept is imperative;
hence, there is a need to enhance designs of a hybrid nature.
Hybrid PONs architectures provide cost-effective, long reach,
and large bandwidth to the customer. Therefore, research
into different hybrid PON architectures used in conjunction
with different modulation techniques and other performance
enhancing solutions should also be investigated to further
realise the best performance out of the hybrid PON.
2.1. Hybrid PONs with Predetermined Flexibility and Enhanced
Performance. Increasing the bandwidth of access networks,
serving a large number of users at minimum cost and fewer
equipment, and covering the largest possible distance have
become the most important goals in the last mile of fibre
networks. This has been necessitated by the increase in the
demand for triple play services like video, voice, and data.
Now the advent of even improved quality of these services
like 3D video, HD television, online gaming, an avalanche
of social platforms, soaring online shopping, and advertising
has led to a demand for larger bandwidth and fast speeds [2].
All the pure PON networks have been found to be inadequate
to fulfil the needs of a next generation access PON network
[3].
In [4], it was proposed a variety of hybrid WDM/TDM
PONs whose operation depended on the flexibility of a
predetermined remote node (RN). In a completely flexible
hybrid network, simultaneous routing of a wavelength to any
TDM PON or second remote node is done, thus providing
the possibility of getting access to every TDM PON by any
wavelength, which offered capacity on demand and reduced
traffic congestion at the same time. The flexible system
comes with constraints, such as the equipment cost due to
the introduction of the active WSS (wavelength selective
switch), that will increase the total cost and security issues
will also arise due to the broadcast and multicast nature
of this system. They also proposed partially flexible hybrid
WDM/TDM passive optical network which uses the strength
of both the flexible and the static systems. The static system
has fixed wavelengths for each TDM PON, but it has low
component count and increased security and suffers from
traffic congestion.
Various investigations are done to continuously improve
the aspects and marketability of hybrid pons, and in recent
times the survivability of these networks has come into focus
as a way of reducing data and business losses [5]. Also hybrid
PON networks with a combination excluding TDM PON
are also under investigation for future access network usage.
OCDMA (optical code division multiplexing access) is a wellstudied area and a pure OCDMA is valued for its unique
properties like fully asynchronous access capability, high
security, and soft capacity-on-demand, and this combined
with the properties of optic fibre and the qualities of WDM
PON leads to competitive option for optical access networks
[6]. A hybrid WDM/TDM GPON using radio over fibre
technique was reported in [1] and a hybrid GPON using
RoF with 2.5 Gbps, digital modulation (8DPSK), and 2.4 GHz
was implemented and this gave good performance over
25 km of fibre length for 32–64 users with a good OSNR.
The working of a hybrid WDM/TDM PON depended on
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3
1 × 8 splitters
EDFA 1
ASG
LD1
WDM MUX WDM DEMUX
ONU 8
16
channels
16 inputs
ESG
ONU
121
RN1
10–35
km
LD2
ONU
1
···
···
···
···
···
EDFA 2
RN2
···
···
···
···
ONU
128
ASG
ESG
LD
EDFA
RN
ONU
Analog signal generator
Electrical signal generator
Laser diode
Erbium doped fibre amplifier
Remote node
Optical network unit
Figure 2: Block-diagram for hybrid optical network.
the architecture of the network and so does its performance.
In [7], it was reported how the performance of a hybrid PON
can be improved by using a tunable wavelength converter
based switching ring converter structure (TWCSS). It is
demonstrated that 64 ONUs × 16 wavelength system worked
properly with reduced loss of packets; the average packet
delay was the shortest and increased access capacity was also
realised.
A spectrally efficient design [8] was demonstrated with
a centralized light source hybrid WDM/TDM-PON that
supported 160 ONUs with a data rate of 625 Mbit/s downstream and 156 Mbit/s upstream for each customer, and it
was shown that the spectral efficiency of 50 GHz channel
spacing can be implemented over a distance of 20 km. A
host of advantages were noticed such as good constellation
diagrams, clear and wide eye diagrams, and low transmission
power penalties on receiver sensitivity in both upstream
and downstream transmissions. Study has also been done in
using mathematical algorithms to aid in the design of the
most effective hybrid PON in [9]. They proposed a location/
allocation algorithm (LAPON) whose nature is three phases.
This algorithm provided ways to physically arrange the
equipment without naming the type of the equipment. It also
provided insight into how the ONUs should be physically
cascaded into PON network architecture and it also provides
the probable location of the splitters or AWG.
Kim et al. [10] demonstrated a hybrid PON that uses
colourless RSOA-based 32-channel loopback WDM PON
that satisfies backward compatibility and this was able to
support a reach of about 50 Km and 128 split ratios for a
given wavelength and error-free upstream and downstream
communication was achieved at injection power in excess
of −17 dBm. A hybrid WDM/TDM EPON using a dynamic
wavelength and bandwidth-allocation algorithm (DWBA)
was discussed in [11]. In this system the ONUs are required
to report their bandwidth requirement status with respect to
their priority queues. An adaptive linear prediction method
was investigated to compute the average arrival rate of
variable bit rate flow during the next waiting interval for a
given ONU. This enabled the procedure to assign bandwidth
and wavelength effectively. It was shown that using this algorithm significantly improved the delay jitter performance,
thus improving the quality of video streaming sent to the
customer. Different modifications have been carried out to
try and improve the performance of hybrid PONS, and the
target was to increase the split ratio and achieve the maximum
possible reach by these access networks. One such proposal
was the designing of a WDM-PON/TDM-PON with selfhomodyne system employing R-SOA as the transmitter in
the colourless domain. Balancing the receiver eliminates the
beating noise which is due to seed light reflected; also the
variation in phase due to the coherent detection is done
by the balanced receiver. Thus, it was seen that power loss
budget was much improved due to the total suppression of
the reflection noise and this impacted well on the overall
performance of the hybrid PON [12].
Wavelength specified laser can be used. It offers an
advantage by generating multiple wavelengths since there
is no probable gain that will be taken advantage of by the
different wavelengths supporting ONUs, it also helps in the
probable multiplexing of flow from every ONU leading to
improved performance of the system, enabling the colourless
characteristic of the ONUs greatly aids with added simplicity
4
International Scholarly Research Notices
Time (bit period)
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Figure 3: Eye openings for a hybrid network system with 128 customers using the nonreturn-to-zero format at (a) 19 km, (b) 22 km, (c) 25 km,
(d) 28 km, and (e) 31 km distance, respectively.
the management of the inventory, minimises costs for spares
and enhances the automated provisioning of wavelength [13].
In [14] signals were communicated over 26 km. Each
signal was 1.25 Gb/s and this was done over a single mode
fibre promoting dual opposing traffic to the user. This analysis
was done on a hybrid WDM/TDM PON access network
which was built using low cost equipment. A polymer Bragg
reflector fitted to a TECL with the modes spaced at 0.8 nm has
25 channels directly modulated at 2.5 Gb/s for use as sources
with low cost in WDM-PON systems. It was reported that the
tunable external laser with directly modulated transmission
at 2.5 Gb/s over a single mode fibre of length 20 km was
implemented with success [15]. GEPON design utilising a
1-to-8 optical splitter was used in [16]. The design used
the elements of the passive optical network to connect the
CO and different customers. The design was investigated
for different lengths between the CO and the PON up to
a distance of 15 km and the parameter under observation was
the BER.
Wason and Kaler [17] came up with an algorithm for
efficient assigning of wavelengths in light path dynamic
provisioning. This system was based on the concept of the
most frequently wavelength used, to effectively reduce the
probability of blocking. The system was then compared with
those systems that were already available and it was found that
it gave more accurate results.
2.2. Methodology. The simulation setup for the hybrid network is as shown in Figure 2. In the envisaged system, 128
customers can utilise video and voice/data for a given distance
of 28 km in the absence of a repeater. OLT components are
placed at the central system, while at the customers end
ONT components are placed there. Branch architecture is
employed for fibre distribution. 128 customers are connected
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Figure 4: Eye openings for a hybrid network system with 128 customers using the return-to-zero format at (a) 19 km, (b) 22 km (c) 25 km,
and (d) 28 km distance, respectively.
to the OLT by splitting the fibre to 16 × 8 times. A quick
glance at the literature survey suggests that video, data, and
internet are also known as triple play. The CO unit is where
video, data, and voice signals are realised. The downstream
data components are obtained by a data link of 1.25 Gb/s
bandwidth and the signals are generated by pseudo random
data generator (PBRS) and an electrical signal generator.
Voice is represented as voice over IP (VOIP), packet switched
protocol. Voice and data are combined and transmitted using
either Manchester code, RZ or NRZ modulation formats.
Eight wavelengths with spacing of 0.8 nm are used within the
range 1480–1500 nm and transmitted, each through a direct
modulated laser and a booster amplifier. RF SCM system
with eight channels is used to represent video components at
0.8 Gb/s bandwidth; these channels are within the frequency
range 55.25 MHz–547.25 MHz and psk modulation is used in
these channels for video transmission. Two analogue signal
generators, electrical adder, direct modulated lasers and a
preamplifier, are used to generate each of the video signals
which are then transmitted within the 1550 nm–1560 nm
range. A preamplifier is then used to strengthen the signals
before transmission and in this case an EDFA is employed.
An optical combiner is further used to combine the video
and data/voice to come up with a signal that is launched into
the channel. The signal has to go through two remote nodes
before it gets to the ONU. The first remote node is equipped
with a fiber trunk that varies in length between 10 km and
35 km, and the fiber is also split by using a 1-to-16 splitter.
The 16 outputs travel to the second remote node over a fiber
of 1 km. The RN2 uses 1-to-8 splitter and is used to supply 8
separate channels. The second RNs are at least 300 m from the
users (ONUs), while the second remote nodes are 1 km from
the first remote nodes. The first remote node is connected to
the transmitter at the central office by a 10–35 km varying
fiber trunk. A power splitter, delay blocks, and pulse train
generator are found at the second RN. A delay of 10 ps is
instituted in the second RN for every signal arriving there,
while those arriving at the first node have zero delay, and so
forth. At the ONU, these signals are further separated by a
demultiplexer into a video signal and data/voice signal by way
of a Fabry-Perot optical filter, whose centre wavelength varies
depending on the service the user requires. Finally, an APD
is used to detect the signals.
2.3. Results. Different modulation techniques were used to
transmit video and voice/data over a fiber cable, so simulation is done for different distance and produces various
waveforms. The BER and quality factor (𝑄) for different fiber
distances are computed and eye opening graphs are seen.
Figures 3(a)–3(e) show the eye openings for 128 customers
simulated with nonreturn-to-zero for 19 km, 22 km, 25 km,
and 28 km. Clearly from the eye openings, it is seen that communication distance is improved to 28 km, and going above
this distance, noise increases drastically and transmission is
inhibited. Hence, the services can be utilised for 28 km only.
Figures 4(a)–4(d) show the eye openings for 128 customers simulated with return-to-zero for distances of 19 km,
22 km, 25 km, and 28 km. Clearly from the eye openings, it
is seen that communication distance is improved to 25 km,
and going above this distance, noise increases drastically and
6
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Figure 5: Eye openings for a hybrid network system with 128 customers using the Manchester code at (a) 10 km, (b) 16 km, (c) 19 km, (d)
22 km, and (e) 25 km distance.
transmission is inhibited. Hence, the services can be utilised
for 25 km only.
Figures 5(a)–5(e) show the eye openings for 128 customers simulated with Manchester code for distances of
10 km, 16 km, 22 km, and 25 km. Clearly from the eye openings, it is seen that communication distance is improved
to 16 km, and going above this distance, noise increases
drastically and transmission is inhibited. Hence, the services
can be utilised for 16 km only.
BER and quality factor computed results at various
distances for the three modulation formats used in a hybrid
PON system for 128 users and this is given in Tables 1, 2, and
3.
The optic fiber length was varied between 10 km and
31 km. Figure 6 shows the quality factor (𝑄) versus distance
graph for hybrid PON system. It is seen that the minimum
𝑄 value for acceptable transmission to the user is reached
Table 1: Calculated BER and 𝑄 values for NRZ.
Distance (km)
16
19
22
25
28
31
𝑄 factor
10.64
9.77
9.31
8.53
6.69
5.60
BER
1.02𝑒 − 26
7.39𝑒 − 23
6.15𝑒 − 21
7.53𝑒 − 18
1.13𝑒 − 11
1.064𝑒 − 8
at 19 km, 25 km, and 28 km for all three modulation formats
for 128 users. Increasing the distance beyond 35 km reduces
the 𝑄 value and hence the ability of the system to transmit
the triple play services is reduced. Triple play services in
hybrid PON can best be transmitted by using the NRZ
International Scholarly Research Notices
7
Table 2: Calculated BER and 𝑄 values for RZ.
0
BER
2.089𝑒 − 26
16
8.74
1.0551𝑒 − 22
19
7.29
7.075𝑒 − 21
22
6.72
5.597𝑒 − 15
25
5.34
1.1221𝑒 − 10
28
4.79
3.419𝑒 − 9
4
6
8
10
10
11
10
Quality factor
𝑄 factor
9.57
Distance (km)
13
2
8
9
6
8
7
4
6
2
5
4
Table 3: Calculated BER and 𝑄 values for Manchester code.
BER
1.03𝑒 − 13
13
7.67
2.5𝑒 − 11
16
6.83
9.5𝑒 − 10
19
5.47
8.7𝑒 − 9
22
4.18
8.5𝑒 − 8
25
3.92
6.91𝑒 − 7
modulation format. Figure 7 shows the transmission distance
in kilometres versus Log BER for the hybrid passive optical
network for 128 users. A transmission distance range 10–
35 km is used in this graph. It is observed that the least BER is
obtained up to 19 km, 25 km and 28 km for Manchester code,
RZ and NRZ respectively. If distance is increased beyond
these the BER increases and the system fails to communicate
the triple play services.
Hybrid PON based FTTH with 128 customers was simulated.
The customers were able to receive triple play signals through
a single optic fiber. A range of wavelengths between 1480 nm
and 1500 nm is used to communicate voice/data signal at
1.25 Gb/s and a range of wavelengths between 1550 and
1560 nm is used for video signals with 0.8 Gb/s. Based on
these results, it is evident that NRZ rectangular provided
the longest transmission distance making it the most ideal
for the transmission of the signal. The split ratio of passive
networks can be greatly increased by hybrid optical networks
as evidenced in this work.
Improvement may still be done to better understand
hybrid networks such as (a) to improve the split ratio, (b)
to increase the reach of the PON, (c) to enhance component
performance, in particular power output of the source and
the sensitivity of modulation, and (d) to analyse the system
in terms of the wavelengths.
Conflict of Interests
The authors declare that there is no conflict of interests
regarding the publication of this paper.
0
35
Figure 6: Distance in kilometres versus quality factor, 𝑄.
−5
0
2
4
6
8
10
10
−10
8
−15
6
−20
4
−25
2
−30
3. Conclusion
15
20
25
30
Transmission distance (km)
Manchester code
RZ
NRZ
Log BER
𝑄 factor
8.01
Distance (km)
10
10
10
15
20
25
30
Transmission distance (km)
35
0
Manchester code
RZ
NRZ
Figure 7: Distance in kilometres versus Log of BER.
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