Thesis on Wimax

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Enhancing Execution and Cost Effectively Deploying
of Relay Stations (RS) in IEEE 802.16m (WiMAX-2)
A Dissertation Report Submitted in the Partial Fulfilment of
The Award of the Degree of
MASTER OF TECHNOLOGY
IN
COMPUTER SCIENCE AND ENGINEERING
Under Guidance of: Submitted By:
Name of Internal Guide Name of Students
(Designation) Roll No
LOGO
DEPARTMENT OF COMPUTER SCIENCE AND ENGINEERING

ABSTRACT
The relay stations are widely used in major wireless technologies such as WiMAX (Worldwide
Interoperability for Microwave Access) and LTE (Long term evolution) which provide cost
effective service to the operators and end users. It is quite challenging to provide guaranteed
Quality of Service (QoS) in WiMAX networks in cost effective manner.
In this thesis the WiMAX RS (relay station) is investigated for the purpose of saving overall cost
by decreasing the number of RS to cover the territory of base station and also to provide the
services to mobile users out of the range of base station. Secondly, the throughput and delay
matrices have been taken to enhance the system performance. In addition to cost effective
deployment of RS and evaluation of throughput and delay using relay station, the third factor
which is with comparison of QoS classes is also made in order to see the overall performance of
WiMAX network. As a technical challenge, radio resource management, RS selection, and QoS
parameters are also primarily considered.
The main objective is to decrease the overall deployment cost in relay stations and utilize the
available spectral resources as efficiently as possible to minimize the delay and improve
throughput for end users with high demanding applications such as voice and video. Having in
mind the cost and the increasingly more demanding applications with ever growing number of
subscribers, main consideration of this thesis have set the parameters and contribute to the
technology in cost effective way to improve QoS. Within the pool of scheduling algorithms and
for the purpose of achieving efficient radio resource management, link adaptation methods,
AMC scheme, cell sectoring and directional antenna have been studied in detail. Some of the
IEEE802.16m standard parameters are not supported in current version of OPNET 16.0 due to
new amendment and evolution of new techniques applied in WiMAX2.

CHAPTER. 1
INTRODUCTION
1.1 Introduction to WiMAX and WiMAX2:
In the field of telecommunication, cable or wired broadband connections are very commonly
used by average internet users because they are affordable, fast and reliable. WiMAX has the
potential to allow the broadband service providers to provide fast and reliable wireless
broadband. WiMAX was first established as a Standard for wireless Metropolitan Area Networks
by IEEE and based on 802.16 protocol family. The first WiMAX protocol was developed for
fixed wireless broadband access and later approved by IEEE in 2005 with mobility support and
named IEEE 802.16e [2]. The first WiMAX operate the range of 10-66 GHz and lower band
operates in frequency range from 2-11 GHz. WiMAX technology is based on point to multi point
technology. WiMAX2 or IEEE 802.16m is the advance version of WiMAX which is based on its
previous version IEEE 802.16e with added features such as it supports 300 Mbps data rates with
mobility whereas 802.16.2-2004 supports data rate of 100 Mbps. Therefore, IEEE 802.11 can
increase VoIP capacity with low latency to meet the requirement of 4G (International
telecommunication union). WiMAX forum has name IEEE 802.16m as WiMAX2. WiMAX2
uses the OFDM (orthogonal frequency division multiplex) and other advance antenna technology
like MIMO (multiple inputs and multiple outputs) for better performance. The main purpose of
IEEE 802.16m WiMAX standard is to improve spectral efficiency, improve VoIP capacity,
handover, and speed coverage range. The IEEE 802.16m works with the radio frequency range
from 2 to 6 GHz as well as it also supports scalable bandwidth of range 5 to 20MHz.
The main features of WiMAX2 are [1]:
 The peak and channel spectral efficiency has been increased which helps and provides
better spectral efficiency for the users at the cell edge.
 The overall VoIP capacity has also increased with the help of user plane latency, also the
handover drawback also decreased. The available channel bandwidth in WiMAX2 is
scalable to 40MHz.

 Throughput supposes to be at least three times more than the existing IEEE 802.16e or
mobile WiMAX.
 Mobility support should extend to 350 km/h
 Single user and multi user MIMO for throughput enhancement
 New and enhance RS which provides better throughput capability with MIMO
 It support multi cast and broadcast services
 Enhanced energy efficiency enabled for power savings
 It supports femtocells which are low power base station (BS) to enhance the coverage.
1.2 Relay Station in WiMAX
Relay stations enhance the capacity, throughput and coverage area of BS (Base station) in the
technologies like WiMAX and LTE. At early stages, relay stations were used to work as
repeaters, and their primary task was to boost the signals received from BS. However, the
booster did not have the capability to remove errors, increase throughput for long distance
communication and also cause inter cell interference. But, after the introduction of IEEE 802.16j
which is the first standard for relay station, various new features are added in RS to enhance the
functionality of the relay stations making them much more intelligent devices to work well with
BS and provide better performance to end users. The RS is capable of boosting the signal and
also it has some extra features like compression and decompression, error correction, and DF
(decode and forward). In WiMAX relay stations are either deployed at the cell edge to extended
coverage area or they are deployed within the cell to relay the BS signal into coverage holes.
Relay stations provide a cost effective, low coverage and easy to install solution for coverage
area extension and to eliminate coverage holes. Multi-hop wireless networks use two or more
relays to provide services to the users which are out of the range of BS. Instead of installing
multiple BS, use of multiple relay stations is a very cost effective solution. Relay stations are
very useful to ensure QoS in WiMAX as they increase coverage area, eliminate coverage holes,
increase throughput and capacity of the network.

Fig.1.1 Operation of relay stations in a WiMAX network
The figure above shows the operation of relay stations in a WiMAX network. Here RS of NTR-
RS (non transparent relay station) is used to extend the coverage are as it installed at the edge of
the cell and relay stations of TR-RS (transparent relay station) are used to eliminate the coverage
hole as they are deployed within the cells where signal are obstructed, possibly by tall building
or mountains or base signal signals are not strong enough to communicate. The link from BS to
Rs called relay link and from SS (subscriber station) to RS called access link.
1.3 Problem Statement
In a WiMAX network there are two main entities involved in communication which are
Subscriber Station (SS) and a BS. A BS is typically a service provider which has backhaul
connectivity and SS subscribes to the BS for the service. A BS exchange control messages and
negotiate the connection parameters with SS before setting up the communication link with it.
These parameters may vary during the communication depending on the requirements and

availability of resources between the two entities. When a BS try to create link with a SS and if
the SS is within the range then BS communicate directly with SS. Otherwise, if SS station is out
of the range of the BS or there is coverage limitations or no LOS (line of sight) between the BS
and SS then RS is a cost effective solution to overcome this problem.
There are two approaches applied in the research towards improving the WiMAX network
performance. Firstly the placement method should need to be determined in order to cut down
the cost as well as maintain the QoS standard. The second scenario is based on the performance
evaluation of WiMAX2 network using relay station with in depth analysis of how to increase
throughput and reduce delay parameters to improve overall network performance. The QoS
class’s comparison also will be included for network flow and its resource usage. In the course of
research, various issues have been addressed by providing solutions based on selection of RS and
using different modes of RS. WiMAX nodes are incorporated to produce useful functionalities;
communication models, antennas and other devices are technically enhanced. And using these
ideas and products WiMAX communication is brought to an advanced level, where multi-hop
scenarios were successfully simulated and studied. OPNET Modeller, version 16.0 is used for
simulations and all the models used in this research are based on features available or added to
OPNET Modeller.
The performance of a WiMAX communication system is also based on some assumptions as
IEEE 802.16m relay station which support advance antenna technology like MIMO (Multiple
input multiple output) and directional antennas and it also will have the capability to work as full
fledge BS (Base Station). These enhanced features are not supported by OPNET Modeller 16.0.
To make the WiMAX relay system more competitive and applicable to meet the QoS demands,
WiMAX RS has been considered as a promising solution for throughput and coverage
enhancement. There are many open issues regarding cost effective deployment and enhanced
QoS need to be considered including: The main responsibility of RSs to work as middle node
and regulate the data transmission between the BS and Subscriber Stations (SSs). As discussed
earlier, RS are used to extend coverage of BS by placing RS at cell edge or boundary where BS
signals start to fades and there is no direct link between BS and SS or link quality for the user out
of the boundary is not very strong to communicate. To cover the cell area, normally four relay

stations are used to provide services to the users out of the range of BS, however four relay
stations can provide better QoS but overall cost also increase. In order to get better QoS as well
minimize overall cost, RS should need to be placed at in cost effective manner so better results
could be achieved as well as save the overall cost.
Another important aspect should need to consider for network performance evaluation
measurement by improving the QoS standards in different RS usage scenarios such as multihop,
with three and with four RS in order to compare the performance with throughput and delay
parameters to maximize the overall system capacity.
1.4 Aim and Objectives
The aim and objectives of the thesis are described below.
1.4.1 Aim
The aim of the thesis is to cost effectively deploy the RS in a WiMAX network and also to takes
measures to enhance the QoS and conduct an analysis
1.4.2 Research Objectives
 To acquire detail knowledge of WiMAX and WiMAX2 technology
 To investigate different methods and techniques for RS deployment in order to cut down
the costs.
 To understand the different problems in maintaining cost effective deployment of RS.
 To investigate and analyse different QoS characteristics such as throughput, delay, SNR
(signal to noise ratio) and network load.
 To investigate and evaluate different techniques to improve overall system performance
which provides guaranteed QoS.
 To assess published major approaches (through literature review) on WiMAX RS
planning and optimization.
 To investigate advance antenna technology and MIMO to further improve coverage and
throughput in WiMAX2.
 To investigate and implement an efficient way to reduce delay and enhance throughput to
meet the QoS standard in 802.16m.

CHAPTER 2
LITERATURE REVIEW
2. Literature Review
The RS helps to improve coverage and throughput for better performance of WiMAX network.
Relay is cost effective technology to achieve high data rate, enhance performance and throughput
and increase cell coverage. The RS may be deployed in the following scenarios
 Signal reception is not very good such as in dense urban areas.
 The BS deployment cost is too much.
 During mobility, the power requirement at subscriber stations with high speed
communication and at distance.
RS also plays vital role to enhance throughput and coverage for better performance of WiMAX
system. All the above mentioned scenarios depends on the deployment and relay usage type as
there are three types of relay usage which can be classified as fixed, nomadic or mobile. The
fixed RS are deployed at fixed locations to enhance the throughput and coverage and the
nomadic RS can be deployed temporarily but at fixed location. However, the mobile RS are
deployed at trains, buses or any other moving objects for the users to access the service while on
move.
2.1 Cost effective Deployment of Multi-hop Relay Networks
There are different types of challenges in planning and optimization of RS in order to get better
QoS with cost effective deployment. The cost is the main factor for any type of technology.
Therefore a cost effective deployment solution could provide better performance results as well
as save the overall deployment cost.
2.1.1 Cost Analysis of Relay station

Generally four RS cover the territory of the BS in order to get guaranteed QoS for the users out
of the range of BS. However, BS planning and placement is another major factor in wireless
industry. Generally a site can be divided into three parts consist of backhauling, BS equipment
and overall site infrastructure. The backhauling is the connection of BS to the core network with
point to point or leased line. The BS equipment can be antennas, material for tower height and
infrastructure can consist of number of equipments like back up power units. The RS does not
have any connection with backhaul as it connected with nearest BS to provide services to EN
(end nodes). The position of the RS is also an important issue for RS placement in the area where
SNR (signal to noise ratio) is high and link budget is good. The table below shows the elements
needs to be considered as CAPEX (Capital Expenditure) and OPEX (Operational Expenditure)
for BS and RS deployment. In the table the one of cost for spectrum licence, research and
marketing has not been considered. The CAPEX and OPEX may be different dependent on the
scenario type such as urban, dense urban or in rural areas.
2.1.2 Relay station Placement
The design and implementation of WIMAX2 relay station model based on non transparent
modes. The approaches and techniques used can improve the operation of non transparent mode.
Whether WiMAX operators could provide better services to end users depends on available
resources. The more capacity which is made available within cell or region, the large amount of
data can be delivered. The critical aspect of this drawback is the type of services the end users
can access e.g. video, voice or data. This could be more complex in multihop scenarios where
more than one RS connected and providing services to the users out of the range of BS and
primary RS. Therefore, to satisfy end users requirements and meet QoS standard, it is very
important to determine some key issues like the end users requirements, overall load and what
type of requirements end users are demanding e.g. video streaming, audio or data as the
applications like online gaming or video streaming consume too much bandwidth when
compared with voice and data applications. In order to achieve better QoS standards, the
placement of RS should be carefully examined with site location, placement methods and area
zone where RS can perform better. In [59], writer deployed RS with AMC (adaptive modulation
and coding) by dividing into zone based on QPSK (Quadrature phase shift keying), 16QAM
(Quadrature amplitude modulation) and 64 QAM. The writer explained the advantages of AMC

scheme with deployment of RS and differentiated the deployment in three zones. The available
SNR and useful bits per symbol can be calculated by modulation scheme and its coding rate [64].
The BS nearside zone can be assumed on higher modulation and coding rate where SNR is high
and high data rate can be sent and receive. However, the area nearside cell edge can be defined
as QPSK and depending on the coding rate data rate is not as much as in higher modulation
schemes. As an extension for PMP (point to multipoint) mode the MMR (mobile multi hop
relay) mode in IEEE 802.16j was introduced to fill the communication gaps. As far as the better
performance, coverage, capacity and considering some other major advantages of RS but we also
need to bear in mind some critical aspects of RS. For example it also cause interference and if
deploy more relays then it also exceeds the cost compare to BS as in [49, 51], the RS deployment
in cost effective manner and also by simulated work showed the reduction of cost. The authors in
[49] mentioned in detail and analyze the cost of BS and RS in order to achieve the guaranteed
QoS. The QoS standard is based on better throughput less delay and packet loss The location or
placement of relays station is also another problem as the network operators will always like to
have cost effective solution to provide satisfactory service. RS at the cell edges are better for
coverage extension and relays between the BS and the cell edge are better for capacity
enhancement.
2.1.3 Placement and Capacity Requirements for Relay station Deployment
Before the deployment of BS and RS, it is very important to measure the overall system capacity
then specify the capacity requirement as it’s good to investigate the target city or region based on
population density, population growth rate and customer distribution etc [48]. Different user
demand different applications and some applications require large bandwidth and spectrum in
order to fulfil the user requirement such as voice applications, video streaming, video
conferencing and all other multimedia applications require more bandwidth as compare to users
who just require only simple applications like emailing and surfing internet. Also it depends on
the zone where of RS based on AMC [59]. As compare with other multi-hop networks routings
issues of relay based networks are less challenging because of that if has purpose full effects on
achievable throughput of such type of systems. System capacity is been reduced during
transmission through RS in two different transmission phases comparing with a data duplication
over RS which may affect the capacity of system. In relay based system may be higher delay will

be occurred because of use of multi-hop networks as comparing with single-hop network. The
DF (decode and forward) has studied widely and has much research done on this technique as the
writer in [53]. In this paper the author developed an Omni-directional relay scheme with multiple
sources using DF relay scheme, in this scheme every node can transmit multiple messages in
different directions by combining them into a single signal. However by applying this Omni
directional relay technique it can cause interference and also can cause week signal strength by
spreading the signal around. In [18], the authors present a method for effective post processing
processes for throughput at the receiver, but some other factors should be taken in consideration
in addition to the previously mentioned issues. To sum up, the planning process in WIMAX can
be modeled as a multi objective optimization problem. The cost functions also to be considered:
 Cost
 Coverage
 Performance (Throughput)
 Interference
Using the multi-objective optimization framework, the time used for simulation may be a little
long. A combination of both analytical study and simulation could be used to improve the speed
of optimization, for example, theoretical analysis on network relay. Also new simulation
techniques using OPNET can be considered to increase the simulator efficiency.
2.2 Adapted Approaches to Improve WiMAX Relay Station Performance
There are so many key techniques used to improve the performance of WIMAX based RS
included radio resource allocation, Advance antenna techniques, relay protocols, link adaptation,
MIMO and frequency reuse etc.
2.2.1 QoS with Delay Minimization and Throughput Enhancement
AMC schemes used network for better performance [10, 11, 13]. The error correction techniques
can be applied to UL (uplink) and DL (downlink) transmission which is adjustable as the higher
modulation constellations can provide better throughput. However, the BS assigned higher
modulation constellations to the users allocated nearside of the BS. There are other physical
medium like advanced antenna systems can be uses to improve throughput and link reliability

[12]. WiMAX especially WiMAX2 allows multiple antennas to be used at the transmitter and the
receiver. In order to get enhanced results IEEE 802.16m use new antennas technologies
including MIMO, frequency reuse and Comp (Coordinated multipoint) etc. The frequency
planning and frequency reuse are another techniques used in WiMAX2. These techniques reduce
the interference and therefore increasing the capacity. [11]
 Optimum frequency assignments can be applied by considering
 Site locations
 Power levels
 User distribution
 Spectrum availability
 Geography and building characteristics.
In OFDMA (Orthogonal frequency division multiple access) based technologies, hexagonal cell
is used to denote the area covered by BSs and RS. The RS cluster in the following includes
single RS or several adjacent RS; the frequency reuse method follows four rules [8]: Each RS
cluster has an isolation band [9].
 All the users are served by the BS except those within the coverage of RS cluster.
 The RS in each RS cluster could reuse the resource out of its isolation band.
 RS in each RS cluster could reuse the resource in its isolation band selectively depending
on the interference measurement or throughput decreasing.
2.2.2 Coverage and Capacity Enhancement Using Relay station
In WIMAX, most efforts have been aimed to improve spectral efficiency. This can be achieved
using one or more of the following approaches: MIMO large increase in signals bandwidth and
cross-layer optimizations. In [15] they present results for different simulation scenarios and show
that RS can provide an improvement in SINR coverage and spectral efficiency. In [16] results for
the coverage extension and capacity enhancement of RS in a realistic scenario are presented. In
[17], the writers introduce the algorithm of coverage angle and coverage range to establish the
relation between the coverage extensions achieved with RS. In [18], the writers present an
analysis of coverage extension with mobile relays and in [19] they propose dynamic load
balancing schemes based on the integrated cellular and using point to multipoint point relaying

systems. The BS and RS transmit signals with a certain power so that the average received power
at the border of the cell is reaching to the end users without path loss and shadowing. The main
factors in path loss are the frequency band and the distance from source to destination as the path
loss and attenuation caused by higher frequencies used by neighboring cell. Also shadowing is
caused by obstacles between the source and the destination which cause reflection and scattering.
The increase in the required received power results in the decrease of the coverage. As more
users increase in the cell or in the case of load, the coverage area decreases. The coverage and
the capacity in a cell have both advantage and disadvantage as higher frequencies are a
disadvantage for coverage, but it’s an advantage when it comes to capacity. Capacity is another
important factor which affects the WIMAX performance. In general term we can determine
capacity by the amount of data that can be delivered to the user and from the user [26]. In a
WIMAX system, user normally access internet for surfing net, video streaming and voice
applications and these applications or user requirements applies or request different demands on
the system depending on the applications type. Different applications require a higher data rate
and need more bandwidth for downloading purpose but not on upload. The authors of [47]
evaluate the performance of WiMAX using RS for the purpose of cost effective coverage
extension with link capacity model for 802.16 MMR and also address the scheduling schemes
for EN. However, they mentioned that with good RS antenna gain and power, RS can be
deployed further away from the cell coverage to increase the cell coverage but it is not
mentioned about the BS and RS link quality as placing the RS out of the cell where signal
strength normally very week can result in poor link or delay.
2.2.3 Optimization of Radio Resource Management in Relay station
The RRM (Radio Resource Management) in WiMAX network covers the management and
optimization of the radio resource utilization. The new developing standards like 802.16m
require better spectral efficiency with high data rates to fulfill user and QoS requirements [23].
There are so many ways to achieve better performance in IEEE 802.16m such as Link adaptation
techniques where different types of modulation scheme applied to get better results. Link
adaptation can be useful if before transmission the BS as transmitter has the knowledge about
channel state. To utilize the radio resources in WiMAX link adaptation plays an important role.
There are different approaches which help in good link adaptation. In [70], uplink scheduling

algorithm has been proposed for RS. The purposed algorithm enhances system capacity,
bandwidth efficiency and improves delay performance for real time applications. AMC
(Adaptive Modulation and Coding) plays an important role in wireless communication
technology for both fixed and mobile environments. The authors of [66] clearly defined and
implemented AMC scheme and its effects on QoS performance of WiMAX network. all the new
upcoming technology like LTE and 802.16m using advance antenna technologies such as MIMO
and directional which help to utilization of resources efficiently. MIMO has more than four
streams which are used in IEEE 802.16m [60, 52, 55]. In IEEE 802.16m, the enhanced MIMO
plays an important role for increasing the throughput [55]. The previous link adaptation
techniques based on MIMO can be classified into two general categories which are analytical
and heuristic which explain limitations of packet error rate. In [52], authors explain error rates
for link adaptation which is bit error rate (BER) or packet-error rate (PER) against SNR.
2.3 The QoS with Relay stations
The QoS based on MAC layer of IEEE 802.16m on the concept of connections as unidirectional
data flow from each side (from source to destination and from destination to source). The flow is
assigned a four bit flow ID also called FID. To generate the network-unique 16 bit identifier, the
FID can be combined with a 12 bit station ID (STID). As compare to IEEE 802.16m the existing
legacy model allowed full 16 bit connection ID for each connection which means almost 216
users can be connected per BS. But the disadvantage is, each of these connection IDs had to be
reestablish on handover which cause more overhead. Not much work is done on QoS in
WiMAX2 or IEEE 802.16m as compare to existing WiMAX networks. There has been related
work such as in [15], where the IEEE 802.16 QoS was simulated but mostly on BE (Best Effort)
services with limited scope and scenarios. Our research includes simulation and detailed analysis
of all the five service classes in varied conditions and scenarios. In [13] the authors, worked on
both physical and MAC layer and used NS-2 to simulate the scenario. However, the work is only
simulated for packet loss whereas there are different types of QoS characteristics such as delay,
network load and throughput to be pin pointed in order to improve the performance. In contrast,
the writers in [40] calculate the throughput to improve the performance of WiMAX non
transparent mode. The parameters chosen by writers in this work were very basic. However, the
idea was just based on non transparent mode where average throughput inside and outside the

coverage area of the BS is calculated. The simulation was made for UGS (unsolicited grant
service, BE (best effort) and rtPS (real time polling service) scheduler in [21], in order to
compare the results of all the mentioned above QoS scheduler, writer investigated and
implemented a new module to get and compare the results of all three QoS classes. In [42], the
writers present the flow management framework for multi-hop mobile systems and apply it to
QoS scheduling with different priorities. The writers mentioned that application sessions on the
Data Link Layer, flows are assigned priorities to distinguish QoS requirements and simulated
results are based on single and multi-hop scenarios. Writers in [43] evaluated on-demand
bandwidth allocation in RS. They develop new algorithm for spectrum efficiency based adaptive
resource allocation. The writers have in detail look and simulated the results of available
throughput, packet loss and delay but here it is needed to consider network load which the
writers did not mentioned. Because when the network load increases the QoS automatically
decreases [16]. The authors further describe in the paper about QoS and their problems in which
they considered the centralized scheduling using UL scheduling. They proposed an architecture
named as SQSA named as scheduling QoS scheduling architecture to ensure QoS and to find a
specific request for the quality of request. WiMAX forum worked on IEEE 802.16m bandwidth
request protocol for better performance [1]. Because in existing legacy system a five message
request was needed for bandwidth request but in 80.16m three messages grant request is
available by knocking off two to decreasing the latency. WiMAX channel bandwidth is 20MHz
and WiMAX2 bandwidth has doubled and varying bandwidth is used based on the traffic.
WiMAX uses OFDMA to allocate sub carriers or modulated carrier to the users. The available
sub carriers to allocate in the UL and DL (down link) are based on UL and DL transmits power
ratio, frame structure and size and available bandwidth as utilization of resources in OFDMA
relay network relay on BS. The efficient and simple resource algorithm proposed in [65] for
relay network to maintain the fairness among users while maximized data rate.
2.3.1 Relay stations Applications
RS can be used for different applications in WiMAX networks but it most commonly used for
three aspects which are coverage extension, capacity enhancement and throughput enhancement
[42]. The WiMAX2 have very challenging requirements for transmission rates and there is a
growing demand in WiMAX networks for coverage and capacity enhancement. RS have been

designed to meet these requirements with guarantee QoS support. The QoS in relay technology
can be:
 Better throughput with less delay
 Coverage Extension for the user out of the coverage of BS
 Reduce signal overhead/Latency
 Higher bandwidth efficiency
 Less delay and packet loss during mobility
Together with all mentioned above better performance and QoS results in relay networks can be
achieved. In RS communication, the SS or EN can receive the signal from the BS or via RS
through different paths depends on the end user location. It can be through the multi hop relay
link (transparent) and the multi hop (non transparent) where direct link from BS is also possible.
IEEE802.16j and IEEE802.16m define two different types of modes in relay technology called
transparent mode and non transparent mode [9, 16, 25]. The transparent mode can provide better
QoS demands for end users as compared to non transparent mode because the transparent mode
basically works to extend the capacity of BS not coverage because the end users may access the
service directly from BS or through RS depending on the link quality. Also, it enhances the
throughput within the cell. However, covering end users QoS demands we need to enhance the
throughput and minimize the delay in order to WiMAX RS work well. The performance can be
improved in RS by taking all the necessary QoS characteristics such as delay, throughput, pack
loss and network load. Most of the work has been done on individual factor by focusing on
single term to show the improvement by enhancing the system performance in that specific
parameters like in [18], the writers focus on throughput and packet loss but delay has not been
simulated as it is clear from the title but there is no simulation found for delay analysis. The
writers done simple simulation with only one BS and one RS connected with mobile node out of
the range of BS. The critical aspect in this paper is the antenna height mentioned in simulation
parameters which is 10 meters. The normal antenna height of both BS and RS should be above
25 meters to get better performance and signal strength.
2.3.2 Performance of Relay stations
WiMAX like other wireless systems suffers from different propagation characteristics and
resource allocation in reasonable manner. The performance of RS can be affected by different

characteristics such as antenna height, distance from BS and distance from SS as the SNR (signal
to noise ratio) decreases when distance increase. Also NLOS (non line of sight) communication
where signal reflects with objects like tall buildings, forest and mountains can affect the signal
quality. Throughput enhancement, capacity and reliability can be achieved if the users have
better SNR especially in the area where BS signal fades at the edge of the cell. The RS enhance
the link quality, throughput and coverage extensions. There are two approaches defined by IEEE
802.16 standard which are centralized and distributed [10]. In centralized approach, the BS can
cover the cell radius where RS also deployed and the second approach called distributed scheme,
where RS coordinates the performance of the SSs. RS is also very useful in load balancing.
During congestion or high load within same cell RS transfers the traffic of one cell to
neighboring cell. The RS extends the coverage where there is no direct link between the BS and
the destination node.
2.3.3 Relay stations Selection in WiMAX System
In wireless networks such as IEEE 802.16m or 3GPP (Third Generation Partnership Project)
LTE, there are typically several fixed RS in the region deployed depending on the user’s access.
If source A as MS (mobile station) wants to send a message to Z (MS) as destination node and
there are several nodes (RS) in between A and Z then relay selection determines the best suited
RS for this communication. The selection process will operate in distributed manner in terms of
message complexity and delay. In the first step relay estimates the channel quality between itself
and source and itself and destination. For example A is source and z is destination and R is relay.
So it can be R and A and R and Z respectively. Source A send ready to send message to
destination Z or destination received this message. Also, all other neighbors of source A received
this message. When destination Z receives the RTS (request to send) message it then send CTS
(clear to send message) back to source A. When relay receive RTS message from source a, it
check or determine the channel state information (CSI) from source A to R (relay and R(relay to
destination Z. The main point we need to keep in mind on this stage that the Relay (R) assumes
the channels are the same from forward source (A) to relay(R) and backward relay(R) to
destination (Z) then each nodes or RS determines the best channel state information (CSI) value
and worst channel state information (CSI) value should served as relay. Relay selection plays an
important role in WIMAX network [29 – 30]. As discussed above, in congested wireless

networks there are different RS deployed in the region to fill the transmission gap and user
requirements. Determining from different relays which one should be selected for
communication is a difficult problem, because some RS may have a strong channel link or link
quality to the destination, but it may also be heavily loaded with traffic from other SS. In [29] the
authors proposed a relay selection algorithm to meet the QoS standard. However the writer did
not mention about the available throughput for each end user and their algorithm improved the
performance in accordance with signal to noise ratio and latency. Also the writer chooses very
simple
services like HTTP and voice to be checked and meet the demand of user. The writer suggests
through effective relay selection algorithm, RS can play an important role by considering the
QoS parameters in order to get better performance. There are different types of relay selection
methods mentioned in by the writer.
The main relay selection methods are:
 RS selection with physical distance
 RS selection with path loss
 RS selection based on SINR
 RS based on transmission power
However, there are some disadvantages of above mentioned selection’s methods e.g. Delay can
cause while selection suitable relay for communication, also path loss transmission’s delay can
occur. In [30] the author proposes a cross-layer design relay selection algorithm for two hop
relay networks. The authors introduce a novel function for relay and proposed algorithm by
considering both channel state information on physical layer and queue state information at data
link layer. As compare to this, the authors of [34] proposed a method based on geographical
information, aiming to minimize the symbol error probability (SEP). Also the suitable relay is
determined withthe aim of minimizing the symbol error probability so the proposed scheme can
achieve better performance in selection process. Relay mainly works as half duplex and DF
technique can be applied for error free communication through RS. However, the half duplex
DF, the transmission of RS can be divided into two time slots. In the first attempt, the source
transmits the data to the RS where it demodulates and decodes received information. In the
second phase, the RS encode again the received data and retransmit it to the EN. There is also an

important factor in the selection process which is that when relay send a message to the end users
with signaling message indicating his availability. Then the pilot sequence used by BS estimate
the instantaneous SNRs of that RS for selection process but this type of scenario can cause time
delay.
2.4 Background
IEEE 802.16m also called WiMAX2 is new and enhanced version of existing WiMAX with the
new and enhanced features. It works on peak rates of its capacity that is 300 Mbps that increase
VoIP capacity with low latency to meet the requirement of 4G (International telecommunication
union). IEEE 802.16m uses the OFDM and MIMO to achieve the performance, importance to
support advance services in featuring for emerging broadband mobile communication
applications. The main purpose of IEEE 802.16m WiMAX standard is to improve spectral
efficiency, improve VoIP capacity, and improve handover and coverage range. WiMAX physical
layer support both TDD (time division duplexing) and FDD (frequency division duplexing)
modes in to optimized multipoint application. The architecture of IEEE 802.16m works with the
radio frequency which ranges at same standard from 2 to 6 GHz as well as it also supports
scalable bandwidth of range 5 to 20MHz.
2.4.1 WiMAX Physical Layer
WiMAX2 or IEEE 802.16m is compatible with IEEE 802.16e 2005 specification and it’s define
three different physical layers characteristics Single carrier transmission.
 OFDM (“Orthogonal frequency division multiplexing”)
 OFDMA (“Orthogonal frequency division multiple access”)
 SCOFDMA (“Scalable orthogonal frequency division multiple access”)
2.4.2 Frequency Division Multiplexing (FDM)
As the name suggest, In FDM signal transmitted over different frequencies at the same time slot
or carrier and each sub carrier is modulated separately by different data stream. Figure 2.1 shows
five FDM carriers.

Figure 2.1 FDM (Frequency Division Multiplexing)
2.4.3 Orthogonal Frequency Division Multiplexing (OFDM)
To better understand OFDM or OFDMA technologies, it is useful to know FDM (frequency
division multiplexing) as discussed above. In OFDM the frequencies are combined and are
orthogonal with each other for data to be transmitted over a radio resource. The Figure 2.2
showing the multiple overlapped subcarriers combined with each other without causing
interference. The main advantage of using OFDM is the data stream can be divided into low rate
streams then each stream is converted to sub carrier with the help of adaptive modulation
scheme.
Figure 2.2 OFDM modulation techniques

Figure 2.3 below shows where five subcarriers are overlapped and not interfering with
each other at peak where it carries data.
2.4.4 Orthogonal Frequency Division Multiple Access (OFDMA)
As compare to OFDM, the OFDMA combined subcarriers into groups of sub carriers which is
also called sub channel and using sub channel all the user can send and receive data at same time
and all the users can be accommodated at the same channel. WiMAX uses OFDMA as different
FFT (fast Fourier transform) modes used in different standards of WiMAX e.g. WiBRO uses
1024 FFT whereas IEEE802.16d support 256 FFT.
2.4.5 Scalable OFDMA (SOFDMA)
Scalable OFDMA is widely used in new technologies like IEEE802.16m and LTE advance as it
has the extra features compared to OFDM and OFDMA. In this scheme there are multiple FFT
sizes supported such as 128 FFT, 512FFT and 2k FFT to address bandwidth up to 20MHz. From
all of the mentioned above technologies, WiMAX forum selected OFDMA, because as compare
to TDMA (time division multiple access) based technology, OFDMA based system leads to cell

range extension on the UL, however cell range extension can also be achieved and enhanced on
the DL if we allocate extra power to the carrier group assigned to users with high distance.
2.5 Modulation Scheme in WiMAX
In wireless communication system, the selection of modulation scheme that includes both
modulation and channel schemes depends on radio resource management. The WiMAX use
OFDM which is most efficient schemes used by advance wireless technologies [22]. One of the
major advantages of OFDM is frequency signals with data can be transmitted by using different
modulation schemes depending on available resources and SNR As it depends on SNR like if the
value of SNR is high then the powerful modulation can be used, however when the SNR is low
then the lower type of modulation scheme can be used. In WiMAX, there are four different
modulation schemes used which are as follows:
2.5.1 Quadrature Phase Shift Keying (QPSK)
It uses four different possible phases, making it possible to send two bits for every symbol. The
QPSK is popular scheme where two bits accommodate one symbol. These two bits send
information by changing the phase of the radio wave. In the constellation diagram of QPSK, we
have four different points showing in the figure 2.4. QPSK efficiently used spectrum as
compared to BPSK, however it cannot guarantee against noise.

Figure 2.4 QPSK constellation
2.5.2 Quadrature Amplitude Modulation (QAM)
WiMAX also uses QAM which is combination of phase shift keying and amplitude modulation
is the efficient and reliable scheme. In QAM, the amplitude and phase by adjusting signal wave
and by combining these two phases a symbol can be generated. In WiMAX, the area where have
high SNR, the QAM can be utilize for better performance and throughput... The figure 2.5 shows
the different region of AMC scheme as we can see the area near to BS can have better capacity
but less coverage and in area in 16QAM have less capacity and more coverage as compare to
64QAM. And in QPSK represent where it has large coverage area but less capacity.
Figure 2.5 Adaptive modulation and coding transmission of BS
2.5.3 Quality of Service in WiMAX and Relay Station:
WiMAX allows the network operators to provide better services which differentiate them from
operators using other technologies; this edge attracts a range of subscribers. It provides flow
types which allow the provider to provide optimized data, video and voice services. In WiMAX

traffic can be prioritized via four services classes, each class prioritizes specific traffic such as
voice, video or data. These classes are listed below.
 UGS (Unsolicited Grant Service)
 rtPS (real-time Polling Service)
 nrtPS (non-real-time Polling Service)
 BE (Best Effort) The second phase of WiMAX with the support of mobility has added
fifth class which is extended real time polling service (ertPS);
A. Unsolicited Grant Service (UGS): The UGS scheduling service is suitable when
the constant data stream is required hence it is suitable for VoIP. In UGS, fixed size
packets are sent with as low jitter and latency as possible. It is important to mention that
in UGS, packets are sent at persistent intervals. UGS packets have higher priority over
BE and nrtPS and system first transmit the UGS packets and then transmit the BE or
nrtPS packets.
B. Real-Time Polling Service (rtPS): This service supports real time service flows
where variable size data packets are generated. It is important to mention that these
packets are generated periodically. This service is suitable for video transmission, such as
MPEG (Moving Pictures Experts Group) videos.
C. Non-Real-Time Polling Service (nrtPS): The nrtPS supports data streams which
consist of variable size packets tolerate delay. This service guarantees minimum data rate.
This service is suitable for FTP.
D. Best Effort (BE): The basic service class of QoS does not guarantee minimum data rate,
meaning at one instance data rate can be very low or idle and as soon as network
becomes less congested data rates increases allowing the traffic to move faster. This type
of service is not suitable for voice and video as at low data rates it cause interruptions. It
is more suitable for data streams which can be dealt on best available basis. BE packets

have lowest priority over the network and these packets are only transmitted if no packets
of UGS, rtPS, nrtPS and ertPS are waiting for transmission.
2.6 Advance antenna technology
WiMAX2 supports advanced antenna technology including enhanced MIMO, directional
antenna with diversity techniques.
2.6.1 Directional Antennas
An antenna gives three fundamentals in WiMAX technology which are based on direction of
antenna, antenna gain and polarization. The antenna gain can be measure by increasing the
power to boost the signal and making the antenna direction in the shape where it directs the
antenna lobe for signal power and cover large area as shown in figure 2.6.
The larger the beam width can decrease the area and smaller beam width can increase area. The
beam width power can be measured in dBm where it increase or decrease the power with 3 or -3
dBm
A directional can enhance the throughput as it radiates power in one or more directions as
compared to an Omni directional antenna that radiates equal power in all direction.
The main Advantages of a directional antenna are Less interference
 Higher gain
 Higher adaptive modulation coding(AMC)

Figure 2.6 Beam width of directional antenna
2.7 Overview of WIMAX Relay station
The RS technology works as middle node as it transmits the BS data to SS which is can be out of
the range of BS or in the area where signal strength is very low. The RS are widely used in all
the main today’s wireless technologies such LTE advance and WiMAX2 A RS does not have
backhaul connectivity as it get the signal from BSs in line of sight connectivity and it can be
connected with a BS through a wired, leased cable or radio link [2]. There are two types of
connections in RS communication known as access link and relay link which can be further
define as, the communication path between RS and BS is called a relay link where

communication is possible from BS to RS or RS to BS. The second path can be described as the
communication path between RS and EN is called access link. The main advantage of RS is to
extend the coverage, throughput and minimize the coverage gaps. The BS usually covered a cell
territory, however in NLOS communication due to tall buildings, forest and mountains can cause
in coverage gap where RS can be used to fill the gap and improve overall system performance.
There are different types of scenarios in wireless communication where RS plays vital role to
overcome and provide better performance, some of the key factors are:
 Low coverage due to poor SNR at the cell boundary.
 Less coverage or very low signal reception in dense urban area
 Cost of BS deployment too high in rural area.
RS can be deployed at the edge of the cell to extend the coverage or top of the building
in NLOS communication of BS for EN.
2.7.1 Multi-hop Communications
Multi-hop communications is a way where users get the services from BS through different hops.
In IEEE 802.16a standard introduced multi-hop communication in WiMAX as mesh mode and
later in IEEE 802.16e in introduced mobile multi-hop relay topology. Figure 2.7 shows the
difference between PMP, Mesh and relay topologies.
Point to multipoint
In point to multi point communication is a topology where BS communicate with end users in
LOS and NLOS environment. The typical range of BS in PMP topology can be up to 8km.
Relay Topology
This is based on tree topology, where relay communicate as a middle node between BS and MS
where one end is connected with BS and other with MS. The BS provides resources to RS for the
MS out of the range of BS. Next generation mobile networks need very high data rates to
enhance the overall network performance. So, therefore relay is a cost effective topology.

Figure 2.7 Different WiMAX topologies
Mesh Topology
In mesh topology, all the devices can be connected with each other within the same network. In
mesh every node is connected to other nodes within the same topology or network. The mesh
topology can further extend into two categories called as partial full mesh which can be
described as if the all the nodes have a connection with each other then it will be full mesh..
However it is very expensive to implement. And the partial mesh topology which is less
expensive to implement as in this mode some node are organized as full mesh and some are
connected with one or two only in the network. The different topologies above shows the
different communication methods used in WiMAX system. Use of multi hop relays raises in MS
of Routing or “Relay Selection”. Because the relays operate at baseband layer, so the Power,
QoS and delay constraints should be taken into account for routing. The deployment of WiMAX
technology without RS can be more expensive as BSs cost is almost three times more than a RS.
The communications methods of RS are based on single hop or with multi-hop.
2.7.2 Relay stations Modes
RS can be further described in two different modes depends on its usage. The two modes are [2]
 Transparent mode

 Non-transparent mode
Transparent Relay
Transparent mode of RS can be used to extend the capacity of the network and to make the
communication possible in NLOS environment. [3] The BSs initial ranging request can be
possible due to BS coverage but still RS needed to cover the coverage gap within cell. However,
in the multi-hop scenario where the number of relays increases to further boost the signal but it
can decrease the overall system capacity. To maintain the QoS and end users demands
satisfactory based on transparent mode then are several key features to be discussed in detail in
order to understand the transparent mode which are
 In this mode single relays data traffic can be transmitted to the BS and vice versa
 Transparent mode only operate in centralized scheduling
 It can support multi-hop topologies
 Scheduling is not possible with transparent mode.
 Does not transmit preamble nor broadcast control message
Non Transparent Relay
The non transparent RS can be used to extend the coverage of the cell by placing them on the
cell boundary where BS signals fades and the signal quality is not very strong to cover the EN
out of the cell. The EN cannot get the signal directly from BS as compared to transparent mode,
so the RS has to send its control information to EN for connectivity. Following are the key
features of non-transparent mode.
 Non transparent RS can operate as a BS for EN.
 Both distributed and centralized scheduling can be used in this mode
 Suitable for multi-hop scenario.
 It can be used for scheduling
 Non transparent mode sends its own preamble, FCH and MAP messages to SS
 The purpose of this mode is to improve throughput and cell coverage enhancement.
 Communication using the same or different carrier frequencies
 Participate in bandwidth allocation in distributed scheduling mode

Multi-hop system where more than one RS is connected in non transparent modes can
communicate with the EN out of the range of BS.
2.8 Relaying Techniques
Based upon relaying or forwarding schemes Relays can be broadly classified in three categories
where each category have its own functionality depends on QoS demands and link adaptation.
The main techniques widely used in RS are
2.8.1 Amplify and Forward
In this technique, relays receive the signal, amplify it and retransmit it. It is the simplest form of
relaying and it requires minimum processing power at the RS. This is a non transparent
technique which means BS has no knowledge of RS. One major demerit of this technique is that,
since the relay amplifies the received signal, it also amplifies the noise received with the signal
which can degrade its performance.
2.8.2 Decode and Forward
This technique overcome the noise amplification problem by decoding the received data and
error correction before forwarding it hence only error free data is forwarded. This kind of
relaying is good if there is a good channel between BS and RS. If the channel is not good then
this causes ARQ overhead and degrades the performance.
2.8.3 Compress and Forward
In this technique RS compress the data before forwarding to EN or users. It is assumed that MS
also have direct transmission from BS. This technique can perform better if there is direct
transmission from BS to EN without using RS.

2.8.4 Adaptive Forwarding
This is additional technique used in new wireless standards such as 3GPP LTE and
IEEE802.16m. In this technique the methods of transmission can be changed depends on the
channel state information of both access link and relay link.
2.8.5 Pairing Schemes for Selection of Relay
There are two types of pairing schemes which can be used in selection process of RS when more
than two RS exist in the same cell.
2.8.6 Centralized Pairing Scheme
The BS collects information from all the neighboring RS and subscriber stations for paring of RS
with mobile stations because BS have full access to all the RS and subscriber stations within the
cell and range of BS. This scheme works with transparent RS mode and BSs updates pairing
information frequently.
2.8.7 Distributed Pairing Scheme
In this scheme, RS used two mechanisms for pairing with subscriber stations which are
 Contention based mechanism
 Local channel information In this pairing scheme BS has no fully access on all the
subscriber stations because
in this scheme paring scheme handled by non transparent RS for selection and communication.
2.8.8 Architecture of Relay Station
To understand the architecture of RS there are two basic fundamental can be used which are:
 Firstly, whether BS has awareness about nearest RS or not, if BS knows nothing about
RS then RS integration with service area is simpler, no change to the BS and no special
signaling between BS and RS are required. Here RS only act as a helper to the BS and it
poses no burden over BS. Earlier cellular systems such as GSM (global system for
mobile communication) used this kind of RS also called repeaters.

 Secondly, Two kinds of characteristics are popular in relay types which are DF and
second one is amplify and forward (AF), each has their own merits and demerits and
hence the use. Generally, AF equipment is less expensive than decode and forward.
2.9 MIMO in Relay Station
Multiple-input multiple-output (MIMO) technique can increase the spectral efficiency of
wireless communication systems. MIMO can increase the throughput, capacity, extend the
coverage and maintain the link reliability.
Figure 2.8 MIMO communication with multiple source antenna and designations
Relay stations with MIMO provide high capacity with coverage extension and throughput
enhancement of relay transmission. The point to point MIMO channel or for the single antenna

relay channel to the MIMO relay channel is complicated task in WiMAX communication
networks and as compared to the single antenna relay channel, the MIMO relay channel
introduces additional advantages to make it possible to perform more sophisticated encoding and
decoding techniques to improve system performance. Figure 2.8 shows sending/receiving
multiple MIMO antennas.

CHAPTER 3
PROJECT ESSENITIALS
3. Design and Network Architecture
In this chapter detailed description of RS deployment with cost effective placement and QoS
classes with throughput enhancement and delay minimization is discussed. The placement of RS
based on AMC scheme replaces the four RS which covers the territory of BS with three RS to do
the same job and save overall deployment cost and other expenditures. The relay itself a cost
effective solution for BS as the deployment of BS is more expensive then RS whereas the RS can
be worked as fixed, nomadic and mobility environments.
The overall system performance based on deployment of RS and overall QoS can be depended
on operators, cost and user requirements. The QoS also discussed in detail by having the QoS
class’s comparison to analyze the performance of real time applications, also different scenarios
has been taken such as the performance without RS, multihop scenario where more than two RS
used to reach at MS which is far from BS coverage, QoS with four RS and finally QoS with three
RS. IEEE 802.16j also called RS supports many options to enhance the overall system
performance. In order to make some reasonable models to analyze the cost effective systems, it
is necessary to make it clear that we are going to take those functionalities adapted by both IEEE
802.16m and IEEE 802.16j RS. In this section the methodologies to determine the cell range, the
relay position, the transmit power at the RSs and the number of relays deployed are described.
The main step in the design of multi cell system is described. The main purpose in the
dimensioning of the cell size is to ensure that all the SSs in a cell are able to receive the framing
information from the BS.
The SINR at the cell edge is analyzed for different frequency reuse factors and by applying the
direction antenna. This work focuses on the deployment of non transparent relays which can
provide throughput capacity gain over WiMAX system and coverage extension.

3.1 OPNET Modeler Architecture
A commercial tool by MIL3, Inc., OPNET (Optimized Network Engineering Tools) [21] is an
engineering system capable of simulating large communication networks with detailed protocol
modeling and performance analysis.
Figure 3.1 OPNET workflow
Its features include graphical specification of models, a dynamic, event scheduled Simulation
Kernel, integrated data analysis tools and hierarchical, object based modeling. “It is a network
simulation tool that allows the definition of a network topology, the nodes, and the links that go

towards making up a network. The processes that may happen in a particular node can be user
defined, as can the properties of the transmission links. A simulation can then be executed, and
the results analyzed for any network element in the simulated network” [19]. The key features of
OPNET are summarized here as:
Modeling and Simulation Cycle OPNET provides powerful tools to help user to go
through three out of the five phases in a design circle (i.e. the building of models, the execution
of a simulation and the analysis of the output data), see Figure 3.1.
Hierarchical Modeling OPNET employs a hierarchical structure to modeling. Each level of
the hierarchy describes different aspects of the complete model being simulated.
Specialized in communication networks Detailed library models provide support for
existing protocols and allow researchers and developers to either modify these existing models or
develop new models of their own.
Automatic simulation generation OPNET models can be compiled into executable code.
An executable discrete-event simulation can be debugged or simply executed, resulting in output
data.
3.2 Modeling Domain:
OPNET defines a model using a hierarchical structure - at the top there is the network level,
which is constructed from the node level, which in turn is made from the process level. The
network level, node level and process level designs are implemented using the Network Editor,
Node Editor and Process Editor respectively. The Network level contains one Top Level
Network.
3.2.1Network Model
Network Editor is used to specify the physical topology of a communications network, which
define the position and interconnection of communicating entities, i.e., node and link. The

specific capabilities of each node are realized in the underlying model. A set of parameters or
characteristics are attached with each model that can be set to customize the node's behavior. A
node can either be fixed, mobile or satellite. Simplex (unidirectional) or duplex (bi-directional)
point-to point links connects pairs of nodes. A bus link provides a broadcast medium for an
arbitrary number of attached devices. Mobile communication is supported by radio links. Links
can also be customized to simulate the actual communication channels. The complexity of a
network model would be unmanageable where numerous networks were being modeled as part
of a single system. This complexity is eliminated by an abstraction known as a sub network. A
sub network may contain many sub networks, at the lowest level, a sub network is composed
only of nodes and links Communications links facilitate communication between sub networks.
3.2.2 Node Model
Communication devices created and interconnected at the network level need to be specified in
the node domain using the Node Editor. Node models are expressed as interconnected modules.
These modules can be grouped into two distinct categories. The first set is modules that have
predefined characteristics and a set of built-in parameters. Examples are packet generators, point-
to-point transmitters and radio receivers. The second group contains highly programmable
modules. These modules referred to as processors and queues, rely on process model
specifications. Each node is described by a block structured data flow diagram. Each
programmable block in a Node Model has its functionality defined by a Process Model. Modules
are interconnected by either packet streams or statistic wires. Packets are transferred between
modules using packet streams. Statistic wires could be used to convey numeric signals.
3.2.3 Process Model
Process models, created using the process editor, are used to describe the logic flow and behavior
of processor and queue modules. Communication between process is supported by interrupts.
Process models are expressed in a language called Proto-C, which consists of state transition
diagrams (STDs), a library of kernel procedures, and the standard C programming language. The
OPNET Process Editor uses a powerful state-transition diagram approach to support
specification of any type of protocol, resource, application, algorithm, or queuing policy.

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