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CHAPTER ONE

INTRODUCTION

1.1 Background of the study

For the past few years, demands for high-speed internet access and multimedia service for residential and business customers has increased greatly [1]. This has led to internet service providers looking to other ways in which to deliver their service to the consumer (last mile). This has led to a re-haul in thinking between the traditional methods of wired technology and wireless technology.

Because wireless systems have the capacity to address broad geographic areas without the costly infrastructure required to deploy cable links to individual sites, the technology may prove less expensive to deploy and should lead to more ubiquitous broadband access [3].The forum describes WiMAX as "a standards based technology enabling the delivery of last mile wireless broadband access as an alternative to cable and DSL [2]. Knowing that WiMAX has been developed for this very purpose there is need to test the capabilities of the technology.

The original idea of WiMAX is to provide users in rural areas with high speed communications as an alternative for fairly expensive wired connections (e.g. cable or DSL). These so called last mile connections are not the only purpose for which WiMAX systems are thought to be used. WiMAX standard includes utilization of adaptive modulation and coding, which makes it possible to provide users with high connection speeds close to the BS (Base Station) and lower speeds when the radio channel is not as good. Thus, WiMAX can offer home and business users high data rates and QoS (Quality of Service) on dense areas and moderate connection speeds and still good QoS on rural areas. It is also designed to enable LANs (Local Area Networks) to communicate with each other through a WMAN (Wireless Metropolitan Area Network).

This project aims to find out just how robust WiMAX is in terms of scalability. By pushing the protocol to its threshold and seeing how it performs.

1.2 Motivation

Broadband Wireless Access (BWA) has emerged as a promising solution for last mile access technology to provide high speed internet access in the residential as well as small and medium sized enterprise sectors. At this moment, cable and digital subscriber line (DSL) technologies are providing broadband service in this sectors. But the practical difficulties in deployment have prevented them from reaching many potential broadband internet customers.

Many areas throughout the region currently are not under broadband access facilities. Even many urban and suburban locations may not be served by DSL connectivity as it can only reach about three miles from the central office switch [3]. On the other side many older cable networks do not have return channel which will prevent to offer internet access and many commercial areas are often not covered by cable network. But with BWA this difficulties can be overcome. Because of its wireless nature, it can be faster to deploy, easier to scale and more flexible, thereby giving it the potential to serve customers not served or not satisfied by their wired broadband alternatives.

IEEE 802.16 standard for BWA and its associated industry consortium, Worldwide Interoperability for Microwave Access (WiMAX) forum promise to offer high data rate over large areas to a large number of users where broadband is unavailable. This is the first industry wide standard that can be used for fixed wireless access with substantially higher bandwidth than most cellular networks [2]. Wireless broadband systems have been in use for many years, but the development of this standard enables economy of scale that can bring down the cost of equipment, ensure interoperability, and reduce investment risk for operators.

The first version of the IEEE 802.16 standard operates in the 10–66GHz frequency band and requires line of sight (LOS) towers. Later the standard extended its operation through different PHY specification to 2-11 GHz frequency band enabling non line of sight (NLOS) connections, which require techniques that efficiently mitigate the impairment of fading and multipath [4]. Taking the advantage of OFDM technique the PHY is able to provide robust broadband service in hostile wireless channel.

The OFDM based physical layer of the IEEE 802.16 standard has been standardized in close cooperation with the European Telecommunications Standards Institute (ETSI) High performance Metropolitan Area Network (HiperMAN) [5]. Thus, the HiperMAN standard and the OFDM based physical layer of IEEE 802.16 are nearly identical. Both OFDM based physical layers shall comply with each other and a global OFDM system should emerge [4]. The WiMAX forum certified products for BWA comply with the both standards.

1.3 Problem statement

WiMAX is a promising new technology with much theoretical standards and performance expectations. However real life performance does not conform to these standard specifications.

This research seeks to uncover the WiMAX performance limitations in a typical real life deployment.

In real life is meant that WiMAX systems are set up in real life field trails, and will be used for performance testing. Real life also suit to the analysis performed over a WiMAX certified system deployment, which operates as it is in real life. The systems tested are as delivered by a specific vendor.

Performance is defined as “A major factor in determining the overall productivity of a system, performance is primarily tied to availability, throughput and response time” [5]. A range of different system characteristics can be considered to determine the overall productivity of a WiMAX system. To place a limitation on this study, the performance of WiMAX was limited to throughput and coverage at different distances under various conditions and sight capabilities.

Throughput and coverage are tightly related to the link quality and conditions. By link quality the researcher means physical parameters such as received signal strength (RSSI), signal to noise ratio (SNR), transmitted power, modulation rate and multipath effects. By conditions the researcher means distance, interfering objects and sight capabilities. All these parameters and the relation between them will be studied.

WiMAX is a complex system, thus it will be important to relate the measured performance attributes to the WiMAX system characteristics when analyzing.

1.4 Objectives

The goal of the thesis is to determine the performance of WiMAX through real life experiments with field deployment analysis. Especially performance regarding throughput, coverage and link quality measured and analyzed.

1.5 Research Questions

  1. How does WiMAX perform in real life according to the theoretical limitations given by the standard specifications?
  2. What is the effect of terrain variations on performance of WiMAX?
  3. What are the drawbacks of typical deployments and performance of WiMAX in sub-Sahara Africa?

1.6 Scope of the study

The study covered a typical deployment of a WiMAX network and related infrastructure (from server farm, base stations up to the Customer Premise Equipment (CPE)) putting into consideration the different environments on which a performance analysis can be made. 

1.7 Significance of the study

The main contribution of this research is to present measurement results from a real life fixed WiMAX deployment and in depth analysis of the physical performance. Secondly, the research will give an insight on considerations for a typical WiMAX deployment in sub-Sahara Africa. The research will also give guidance to RF Engineers, Data carriers and Internet Service providers on how well they can plan their networks for wireless transmission.

CHAPTER TWO

LITERATURE REVIEW

2.1 Overview

This chapter discusses the WiMAX protocol in depth, why there is a need for the protocol. Looks at related literature and ways in which the literature tackles the challenges presented. How best to go about this project looking at past information and relevant observations need to be made in this chapter so that the following chapters can build upon this foundation of knowledge.

WiMAX is a certification mark for products that pass conformity and operability test for the IEEE 802.16 standard [3]. It is a technology enabling the delivery of last mile wireless broadband access (BWA) as an alternative to cable and DSL. It is a point-to-multipoint (PMP) architecture, which resemble our traditional mobile telephone system, though it differs in specification and properties. Mainly WiMAX defines an all-IP architecture, which supports high throughput over long distances. Two layers in the OSI-stack are defined by WiMAX, which are the Physical (PHY) and the Medium Access Control (MAC) layer. These will be described in later sections.

By arguing that WiMAX is all-IP, this is meant that a common IP network core is used, where the circuit core network is absent in favor of the IP packet network. Overhead and complexity is reduced by deploying a single protocol. The fact that WiMAX is all IP makes it extremely attractive in that it is well suited for IP-services like among others VoIP, IPTV, streaming applications and Internet surfing. Services already available at the internet will be available for use over WiMAX. This is realized since IP works outside and inside the access network.

It is envisioned, that WiMAX will provide the last mile internet access to residential users. This will be particularly useful in regions where wire lined infrastructure does not exist or cannot be setup, such as rural areas and remote mountainous areas for instance. It is interesting to note, that WiMAX proved its importance during the devastating December 2004 Tsunami in Aceh, Indonesia which completely destroyed the existing infrastructure, and thus crucial communication took through WiMAX stations deployed rapidly on urgent basis. For small and medium enterprises, WiMAX will create an economical alternative to expensive leased line solutions. [1] [2]

As broadband internet becomes the standard in homes and business around the world. The demand has never been higher. Originally broadband was delivered by wired connection only. This is meant to implement broadband in homes that did not have the capability, physical changes to the network would have to occur. Streets may have to be dug up, new wires laid etc. This led to a major rethinking in the way that last mile service is provider. The idea that broadband could be delivered with QoS over a wireless protocol was devised, and as such the WiMAX protocol was born.

Over the last 5-6 years there have been many revisions to the WiMAX protocol, adding improvements and making it more robust.

WiMAX is designed to deliver wired network performance over wireless. Wireless MAN’s are created by

WiMAX base stations are metropolitan area networks covering up to 30 miles [14] which means that broadband can be delivered to clients within this radius.

2.2 WiMAX Definition

WiMAX which stands for World Wide interoperability of Microwave Access is a standards-based technology which serves as a wireless extension or alternative to DSL or cable for “broadband” (i.e., faster than 1.5Mbps) access to IP-based networks and the Internet. It utilizes microwave communication in the 2 – 66GHz range to connect WiMAX-enabled fixed, portable, and mobile computers to a “base station” PC which in turn connects to an IP network (e.g., the Mukono University network) and then to the Internet.

Communication can take place over much longer distances than with WiFi (miles vs. hundreds of feet), and by utilizing an array of antennas, each supporting one or more base stations, user PC’s distributed across a very large geographical area (e.g., the entire Town) can all have a link and Internet access without requiring a “wired” DSL or cabled connection. The communication occurs either on licensed frequencies (lower likelihood of interference with other WiMAX service providers in the area) or in an unlicensed part of the 2-66 GHz radio spectrum (perhaps free of cost, but possibly subject to lower radiated power restrictions and/or higher level of interference).

WiMAX is a broadband wireless access system which offers high throughput, great coverage, flexible Quality of Service (QoS) support and extensive security. WiMAX is certified by the WiMAX forum [26], which is a certification mark based on the IEEE 802.16 standard [27] that pass conformity and interoperability tests.

There are two main classes of WiMAX systems called fixed WiMAX and mobile WiMAX. Fixed WiMAX is targeted for providing fixed and nomadic services, while mobile WiMAX will also provide portable and (simple and full) mobile connectivity. The system studied here is a fixed WiMAX system. It uses an air interface based on orthogonal frequency division multiplexing (OFDM), which is very robust against multi-path propagation and frequency selective fading. An adaptive modulation technique is used to enhance performance when the link characteristics vary.

2.2.1 Fixed WiMAX

The fixed WiMAX profile, based on the IEEE 802.16d specification, is mainly targeted for fixed wireless access, but may also support nomadic and portable access.

Fragmentation and packing is used at both the MAC and PHY layers. A variable length Protocol Data Unit (PDU) is used at the MAC layer. A PDU is the data exchanged between peer entities of the same protocol layer (Fig. 1). Multiple MAC PDUs may be concatenated into a single burst at the physical layer to save overhead and increase throughput. Similarly, multiple Service Data Units (SDU) from the same network service, may be concatenated into a single MAC PDU. A SDU is the data unit exchanged between two adjacent protocol layers. Thus overhead is saved by both the MAC and PHY layer. TCP/IP is the Transmission Control Protocol (TCP) on top of the Internet Protocol (IP), which is a group of protocols that specify how computers communicate over the Internet. All computers on the Internet need TCP/IP software.

2.3 Evolution of WiMAX

IEEE 802.16 physical layer has evolved much since its first version was completed in October 2001. The first version operated between 10-66 GHz and specified a single carrier for a fixed Point-to-Multipoint (PMP) communication. The second version, 802.16a, extended the frequency band to below 11 GHz. This enabled non line of sight communication by employing the benefits of diffraction which are available only at lower frequencies. In this version, two OFDM based air interfaces; 256-carrier Orthogonal Frequency Division Multiplex (OFDM) and 2048-carrier Orthogonal Frequency Division Multiple Access (OFDMA) were also provided. This version also allowed mesh based topology in addition to the existing PMP communication. This version was followed by 802.16d published in June 2004. It incorporates all the previous versions to provide fixed BWA. Then came 802.16e, concluded in 2005, which supports full mobility at speed up to 70-80 m/s. [2] [6]

2.3.1 Evolution in Detail

IEEE 802.16 Working Group:

In 1998, the IEEE 802.16 working group focused to develop WMAN solution for Line of Sight (LOS) based point to point and point to multipoint wireless broadband systems. It was also decided that the frequency range for IEEE 802.16 will be 10 GHz to 66 GHz. The first standard of WiMAX was completed in December 2001 which employs single carrier physical (PHY) layer with burst Time Division Multiplexed (TDM) on MAC layer.

IEEE 802.16a:

In January 2003, the working group produced another standard, IEEE 802.16a, after some amendments in the earlier standard including Non-Line of Sight (NLOS) applications in the frequency range of 2 GHz to 11 GHz band. It uses Orthogonal Frequency Division Multiplexing (OFDM) on physical layer with

Orthogonal Frequency Division Multiple Access (OFDMA) on the MAC layer.

IEEE 802.16-2004:

IEEE introduced the new standard, IEEE 802.16-2004, replacing all the previous versions. The main focus is to target fixed applications. It is also called as Fixed WiMAX or IEEE 802.16d.

IEEE 802.16e-2005:

In December 2005, IEEE approved and launched its new standard IEEE 802.16e- 2005. This new standard has come after some amendments in IEEE 802.16-2004 that is the support of mobility. This system gives the concept of nomadic and mobility services to WiMAX technology. It is also referred as Mobile WiMAX.

Moreover, the WiMAX forum defines the subset of mandatory and optional physical and MAC layer features for fixed and mobile WiMAX standards and they are known as System Prole. The system proles based on IEEE 802.16- 2004, OFDM PHY, are called as fixed system proles. The system proles of IEEE 802.16e-2005 scalable OFDMA PHY are known as mobile system profiles. The details of operating frequencies, channel bandwidth, modulation and multiplexing techniques

2.3.2 WiMAX Architecture

The basic architecture is a Base station, which is basically an antenna, which subscriber stations connect to. Subscriber stations will be antenna’s mounted on buildings that will connect to a router inside the building so the delivered broadband can be distributed. Originally both BS and SS were fixed implementations but with the introduction of 802.16e SS now have mobility options. WiMAX has no standard for routing and as such it can only deliver its service. The distribution once it gets to its destination has to be dealt with by another protocol. [23]

802.16 WiMAX protocol can be broken down into layers.

2.3.2.1 PHY Layer

The physical or PHY layer can be broken down into two layers, these are the physical medium dependent which is concerned with the actually transmission medium over which the wireless information is broadcast. The other sub layer is the Transmission convergence layer which deals with merging the physical layer with the MAC Layer [23]

The PHY layer can support bandwidths up to 28 MHz which OFDM efficiency of 3.6 Bytes/Hz can offer bandwidth of over 100Mbp/s for a single base station. [20]

Although this looks very impressive, because as a subscriber station gets further away from the base station the signal to noise ratio increases rapidly with this type of signal. As such adaptive modulation is used to try and counter this. The fact is that it becomes very difficult for a base station to utilize 100% of its bandwidth capabilities [15].

WiMAX also uses adaptive burst profiling to try to deal with this problem, each SS is looked at individually, depending on range, modulation and coding schemes, this can be adjusted frame by frame. [26]

There are two channels, an Uplink channel and a downlink channel. The downlink channel is a broadcast channel. In that it broadcasts to many different subscriber stations, and it can contain downlink information, grant information, bandwidth request replies, packets sent to individual SS’s. All SS’s receive all bursts that they are robust enough to receive, i.e. they have enough bandwidth to receive all packets, and for example when a SS at a lower modulation scheme will not receive as many bursts as one at a higher modulation scheme. The uplink channel, is distributed in a shared manner by the base station, the base station grants time allocations to each SS in which the SS can send packets to the BS. When a SS connects to a BS it grants it a certain time allocation, this time allocation can then be used by the SS to request more bandwidth and hence more time allocations. [26]

2.3.2.2 MAC Layer

There are two types of WiMAX architectures; point-to-multipoint (PMP) and mesh. PMP consists of a base station (BS) which serves all the subscriber stations (SS) in its range. There is no communication between SSs. They all communicate through the BS. The BS is concerned with the setting up and management of the connections when an SS sends a request. The BS acts as a network gateway.

In case there is communication between SSs as well, then it forms mesh architecture. The mesh architecture allows a connection over several hops and a tree network topology can be formed. The mesh and PMP are incompatible because PMP is only capable of single hop transmission. PMP has a lower signaling overhead than the mesh mode [14], [15]

For data transfer in WiMAX, downlink and uplink sub frames are duplexed using either frequency-division duplex (FDD) or time-division duplex (TDD). [1] It should be noted that WiMAX is a connection oriented network BS schedules the uplink and downlink grants at the start of each frame in order to meet the negotiated QoS requirements. Each SS finds the boundaries of its allocated uplink sub frame by decoding the UL-Map message. The DL-Map message contains information about the downlink grants in the forthcoming sub frame. Both maps are transmitted by the BS at the beginning of each downlink sub frame. This is done for both FDD and TDD modes. [1]

Providing quality of service (QoS) simultaneously to services with different requirements is a much more difficult task in wireless mediums as compared to wired networks because of its highly variable and unpredictable nature in terms of time-dependence as well as location dependence. To cope with such issues, QoS in wireless networks is handled at the medium access control (MAC) layer. [1]

An exciting feature of WiMAX is its support for QOS. It classifies all traffic according to four types:

  • Unsolicited Grant Services (UGS): because of a constant bit rate requirement, this category needs constant bandwidth allocation.
  • Real-time Polling Services (rtPS): because of real time variable bit rate requirements, these applications need minimum bandwidth granted and have to request transmission resources by polling. Contention and piggybacking are not allowed.
  • Non-real-time Polling Services (nrtPS): because of non-real time flows, this category requires traffic polling. Bandwidth requests are allowed when minimum bandwidth requirements are needed, otherwise contention and piggybacking are used.
  • Best Effort Services (BES): best effort flows can make bandwidth request only with contention and no minimum resources allocation is granted. [16]

2.3.2.3 TDD and FDD Duplexing

The current implementation of WiMAX supports two types of duplexing. Time division duplexing and

Frequency division duplexing. In TDD mode same channel is shared by the uplink and downlinks. Each

frame is separated into downlink and uplink. With the downlink transmitted first then the uplink

transmitted second.

Figure 3: example of a TDD frame.

2.3.2.4 Downlink Sub Frame

The downlink sub frame contains a DL-MAP for the current sub frame, and UL-MAP for the uplink channel. It can also contain DCD and UCD messages. DL-MAP is used to specify parameters that concern the downlink. These include frame duration, frame time and also downlink channel ID. UL-MAP defines specific bandwidth grants to SS’s the channel ID. DCD contains the downlink channel descriptor which describes the physical downlink channel, UCD contains the uplink channel descriptor which does the same for the uplink physical channel. [23]

2.3.2.5 Uplink Sub Frame

The uplink sub frame can contain one of three different types of bursts, these are either bursts that are transmitted within the ranging contention slots, bursts that are transmitted in bandwidth request contention slots or bursts that are transmitted in the assigned slot to specific SS’s. The last is basically data transfer in the granted allocation time. [23]

2.3.2.6 MAC PDU

The MAC protocol data unit, a MAC level data unit that the base station and its subscriber stations transfer between their MAC layers. The MAC PDU can contain a bandwidth request, a packing request, or a fragmentation sub header. A bandwidth request, requests more allocated time from the base station to the SS. A packing request is when two fragments can be packed together. A fragmentation sub header indicates that the packet has been fragmented, it also identifies the location of the fragments. [23]

2.3.2.7 WiMAX QoS

WiMAX 802.16 describes 5 quality of service schedulers that it can use for different traffic types. These schedulers are unique in that they associate with what the SS needs and act in the appropriate manner to what a connection needs. In this way VoIP traffic will receive priority over say web traffic. The following are the QoS classes; [23]

Table 1: QoS classes [20]

QoS Class

Application

QoS Specifications

BE

Best Effort Service

Web browsing, data transfer

Maximum sustained rate

Traffic Priority

nrTPS

Non-real-time Polling Service

File Transfer Protocol

Minimum reserve rate

Traffic Priority

erTPS

Extended real Time Polling Service

Voice with activity detection

Minimum reserve rate

Maximum sustained rate

Traffic Priority

Jitter tolerance

Maximum latency tolerance

UGS

Unsolicited Grant Service

VoIP

Max sustained rate

Max latency tolerance

Jitter tolerance

rTPS

real Time Polling Service

Streaming audio and video

Minimum reserve rate

Maximum sustained rate

Traffic Priority

Maximum latency tolerance

2.3.2.8 Bandwidth Request and Grants

WiMAX deals with requests and grants in two ways. Either grant per connection or grant per subscriber station. GPC type grants work by having the base station schedule each connection individually. As such each connection will only be able to transmit during its transmit time, this time is allocated by the base station. GPSS on the other hand takes all connections from one subscriber station as a set, and allocated all connections a service time, this type of grant means that a scheduler at the subscriber station has to

employed so that it can schedule its own service order. Generally GPC is less saleable than GPSS. [23][26]

2.3.2.9 SS initialization

Channel Acquisition occurs when a SS scans for downlink channels and waits for a downlink control message. Once it has acquired a channel, it waits for a UCD from the base station before it will begin

broadcasting.

Initial Ranging is done by means of a RNG-REQ and RNG-RSP, the SS sends ranging requests of various power, until it receives a response. Once it receives a response it can extrapolate the distance from the base station. The base station answers with a RNG-RSP which contains power adjustments. Negotiate Basic Capabilities is when a SS sends its capabilities to the BS, this is when modulation, coding schemes and duplexing are configured.

The next stage is security based and is basic authentication of the SS, a simple key swap is performed.

Registration and IP are then served to the SS from BS. Then the connection is set up. Now the SS can receive its allocated grants and start requesting bandwidth etc.

2.4 Manufacturers specified WiMAX performance

2.4.1 UDP throughput PtP (using a MACRO BST)

Table 2: Alvarion specified Base station throughput [26]

Modulation

Packet Size

DL

(Mbps)

UL

(Mbps)

UL+DL

(Mbps)

QAM64 ¾

64

3.788

2.337

2.614

1518

3.961

4.030

7.991

QAM64 2/3

64

3.490

2.300

2.626

1518

3.613

3.671

7.018

QAM16 ¾

64

2.547

2.471

2.630

1518

2.710

2.733

5.235

QAM16 ½

64

1.715

1.692

2.619

1518

1.853

1.841

3.428

QPSK  ¾

64

1.296

1.311

2.641

1518

1.367

1.378

2.687

QPSK ½

64

0.830

0.830

1.704

1518

0.857

0.857

1.807

BPSK ½

64

0.427

0.412

0.796

1518

0.417

0.394

0.811

2.4.2 UDP Throughput PtMP (13 CPEs) associating with BST

Table 3: Alvarion specified throughput on a PtMP [26]

Modulation

Packet Size

DL

(Mbps)

UL

(Mbps)

UL+DL

(Mbps)

QAM64 ¾

64

4.232

3.700

7.626

1518

4.104

4.092

8.039

QAM64 2/3

64

3.658

3.247

6.639

1518

3.691

3.643

7.067

QAM16 ¾

64

2.662

2.848

5.592

1518

2.732

2.586

5.476

QAM16 ½

64

1.927

1.701

3.621

1518

1.845

1.748

3.424

QPSK  ¾

64

1.392

1.334

2.643

1518

1.360

1.408

2.732

QPSK ½

64

0.953

0.879

1.733

1518

0.922

0.898

1.724

BPSK ½

64

0.404

0.417

0.774

1518

0.437

0.412

0.801

2.4.3 UDP Throughput PtMP (highest modulation)

Table 4: Alvarion specified throughput with highest modulation [26]

QAM64 ¾

Packet size (bytes)

Number of

SU’s

64

128

1518

Down

Up

Down

Up

Down

Up

5

4.241

3.221

4.259

3.952

4.371

3.934

13

4.232

3.700

4.224

4.038

4.104

4.092

2.4.4 FTP Throughput per modulation (15 CPEs)

Table 5: Alvarion specified ftp throughput [26]

FTP Traffic (Mbps)

Rate QAM64 ¾

Modulation

DL (Mbps)

UL (Mbps)

DL + UL (Mbps)

QAM64 ¾

3.976

3.572

6.748

QAM64 2/3

3.541

3.114

6.029

QAM16 ¾

2.641

2.363

4.457

QAM16 ½

1.752

1.506

2.963

QPSK  ¾

1.305

1.095

2.114

2.4.5 FTP Throughput per number of CPEs

Table 6: Alvarion specified throughput per number of CPEs [26]

FTP Traffic (Mbps)

Rate QAM64 ¾

Channel Spacing 3.5MHz

Number of SUs

1

15

Downlink

3.385

3.976

Uplink

3.115

3.572

Downlink/Uplink

4.511

6.748

CHAPTER THREE

METHODOLOGY

3.1 Overview

In this chapter, WiMAX theories, models, tools and principles will be discussed in detail and in the end give an insight on the 802.16d standard performance behavior under varying environments using an appropriately chosen experiment method. When running any experiments it is very important to look at what has been done in the past. What has worked in other projects or journals and what hasn’t worked. To find a platform on which the experiments in this project can build upon.

To effectively evaluate WiMAX performance in relevance to the chosen parameters and performance

metrics, there were 3 possible ways of going about it.

Direct Experiments: This uses physical hardware available to test WiMAX, in most cases this method is not used because of lack of hardware to setup the desired environment, time and skill set but otherwise it can easily yield better results as there are a few dependences involved.

Mathematical Modeling: A mathematical model of how WiMAX works is created and is used to mathematically calculate outcomes. The downside to this methodology is it would need resources and skills that are not available which may require the project to take longer than expected.

Simulation: This uses a computer program that creates a virtual environment by which a network can be

setup in varying ways to test certain criteria. The simulator will then output the results. This methodology is widely used because there are various resources to implement it ranging from open source to commercial.

The down side of this methodology is that assumptions based on a model are made and applied to any simulation. Random number generation can play a part in random results. Also when using 3rd party modules you have to trust their implementation is correct.

So the method chosen for evaluating WiMAX was direct experiment  as it was the most flexible, allowed for greater amount of experiments of varying complexity to be run in a smaller time-scale and the researcher had access to the required resources such as WiMAX base stations, customer premise equipment (CPEs) and has the required skill set to set up the test environment.

Throughput measurements were performed with the protocols FTP, TCP and UDP. The TCP Windows Size was tuned to be high enough to achieve the maximum throughput on both the client and server side for TCP and FTP measurements. Simultaneous uplink (UL) and downlink (DL) throughput tests were performed for TCP and UDP. Several simultaneous TCP and UDP connections in each single direction from a single subscriber were also measured.

Link Quality tests were utilized to study the physical medium. These included Signal to Noise Ratio (SNR) and Received Signal Strength Indication (RSSI) for UL and DL as well as RX rate, TX rate and TX power. The reason for measuring TX power is that an Automatic Transmission Power Control (ATPC) was used by the SU, where different levels of power can be transmitted dependent on the present link quality.

Based on the field deployment measurements over the WiMAX certified equipment, the researcher performed extensive analysis. The RSSI measurements were related to distance and compared against well-known path loss models, where the researcher’s measurements ranges between the free space loss models and the Cost 231 Hata model for suburban areas. The SNR was related to the RSSI and a mathematical expression was derived for this relation.

3.2 Propagation Model

A radio propagation model is an empirical mathematical formulation for the characterization of radio wave propagation as a function of frequency, distance and other conditions. Propagation models are much used in network planning, particularly for conducting feasibility studies and during initial deployment. Empirical propagation models are based on observations and measurements, which are mainly used to predict the path loss. It would therefore be interesting to deduce a path loss model based on the data obtained from the measurements. Since the data obtained are based on measurements performed over a field deployment, this is more correctly stated as an experimental propagation model.

Path loss is the attenuation undergone by an electromagnetic wave in transmit from a transmitter to a receiver in a telecommunication system [28]. Path loss is usually expressed in decibels as the ratio of the transmitted to received power. Many effects affect the signal propagation, thus a loss in power is caused to the signal. These effects may be the free-space loss, reflection, refraction, diffraction, absorption and aperture-medium coupling loss. Calculation of a path loss model will not provide exact results, but a prediction based on measurements and observations.

Different path loss models are suited for different frequencies and landscapes. Examples of noontime dispersive empirical models are ITU-R [29], Hata [30] and the COST-231 Hata model [31]. These models are designed for mobile systems, and uses great amounts of measurement results to predict their constants. The most used COST-231 Hata model is designed for systems operating in frequencies up to 2 GHz. The WiMAX system in used operates in the 3.5 GHz frequency range, but due to the simplicity and availability of correction factors, the COST-231 model have been widely used for path loss calculation at higher frequencies.

3.2.1 Cost-231 Hata and Free Space Loss Model

The Cost 231 Hata model is formulated as:

COST231 = 46.3 + 33.9log(f) – 13.82log(Hb) – Ch + (44.9-6.55loh(Hb))log(d) + Cm                           (1)

where ‘f’ is the frequency in MHz for the system in use, ‘Hb’ is the height of the BS antenna and ‘d’ is the distance between the BS and the SU. ‘Cm’ is a parameter defined as 0 dB for rural and suburban areas and 3 dB for urban areas. ‘Ch’ is a correction factor for the mobile station antenna height, which has one definition for urban areas (Eq. 2) and another for rural and suburban areas (Eq. 3) as follows:

Ch = 3.20(log(11.75Hr))2 – 4.97, U f > 400MHz  and         (2)

Ch = (1.1 log(f)-0.7)Hr – (1.56log(f)-0.8)     (3)

where ‘Hr’ is the height of the SU antenna above ground level.

When calculating the Cost-231 Hata model for WiMAX, the parameters applied is listed as follows:

  • Operating Frequency : 3.5 GHz
  • Channel Bandwidth: 3.5 MHz
  • BS Transmitted Power: 28 dBm
  • BS Antenna Gain: 14 dBi
  • BS Antenna Height: 32 meter
  • SU Transmitted Power: 20 dBm (ATPC)
  • SU Antenna Gain: 18 dBi (vertical polarization)
  • SU Antenna Height: 5 meter

A calculation was made for the Cost 231 Hata suburban and urban models with the parameters as specified above.

The Free Space Loss (FSL) model was calculated, because it was interesting in the comparison with a WiMAX Path Loss model. FSL is calculated as follows as follows:

FSL = 32.45 + 20log(f) + 20log(d)                                                                                                                 (4)

where ‘f’ is the system frequency in MHz and ‘d’ is distance in kilometers.

3.3 System Description

The system in use is a fixed WiMAX system operating in the 3.5 GHz frequency band. The system utilizes FDD with 3.5 MHz channels in both uplink and downlink.

Each BS sector has a 90° beam width, and 4 licensed frequencies are available for use. Each BS is configured to transmit at a 28 dBm maximum where the BS antenna gain is 14 dBi.

The SUs are fixed antennas, which are located outdoor at the house wall or roof. Automatic Transmission Power Control (ATPC) is enabled at all the SUs where the maximum transmitted power is 20 dBm. SU antenna gain is 18 dBi.

If possible, the SU is setup within Line of Sight (LOS) to the BS, but there are also SUs with Non Line of Sight (NLOS) conditions. The NLOS sites are mostly present in areas close to the BS, whereas LOS becomes more common and also more important at farther distances.

The area of deployment consists of one medium sized town named Entebbe where the population density is low and averagely low buildings and Kampala city with a high population density and 5 floor high buildings.

3.3.1 WiMAX system setup

The test bed was set up with Alvarion’s BreezeMAX 3500, a WiMAX system operating in the 3,5 GHz frequency band. An overview of the measurement setup is roughly a radio access system. Where a WiMAX Base Station (BS) was connected to a VLAN, and a Subscriber Unit (SU) connected to a subscriber as illustrated in Fig. 4.

Figure 4 Measurement Setup with BS and SU

The general Radio parameters in the system were as specified in the fixed WiMAX profile specification, where some parameters were specific to the equipment vendor used. A Frequency Division Duplexing (FDD) technique utilizing two 3.5 MHz channels was deployed, one for UL and one for DL. There was also an option to use 1.75 MHz channels. Adaptive modulations and convolutional coding is used. The parameters used in the system are shown in Table 7.

Table 7 General Radio Specifications [20]

The UDP throughput in 3.5 MHz channels when 10 CPEs are given in Table 5 as presented by the equipment manufacturer.

Table 8 UDP Throughput with 10 CPEs, packet size 1518 Bytes in 3.5 MHz channels as presented by Alvarion [20]

3.3.2 Base Station

The BS delivered by Alvarion, was connected to VLAN700, an Infocom internal network. Fig. 5 illustrates the configuration of the BS where three of the slots were used. Two slots were allocated to Access Units (AU-IDU) and one for Network Processing Unit (NPU). The AU-IDUs were connected to Access Unit Outdoor Units (AU-ODU), which again were connected to the antennae. This was the interface to the wireless domain as illustrated in Fig.5

The NPU was the heart of the BS where traffic were aggregated from the Access Units and transferred to the VLAN, or backbone, via a network interface. What is meant by expressing that the NPU is the heart of the BS, is that this is where most of the BS functionality is placed. This includes Traffic Classification, connection establishment, Policy Based data switching, Service Level Agreements (SLA), an agent that manages cell sites (AUs), registered SUs, alarms management and synchronization, including GPS, clock and IF reference and A Simple Management Network Protocol (SNMP) agent that enables In Band (IB) management of the Base Station and all its managed SUs. A dedicated 10/100 Base-T interface supports Out Of Band (OOB) management. There were allocated two slots for NPUs in the BS chassis to provide redundancy [24].

The IP-addresses are included for the BS, management system, Gateway (GW) and Dynamic Host Configuration Protocol (DHCP) server in Fig. 5. The Network Management System (NMS) uses SNMP and controls among other things the BSs, subscribers and their services. The DHCP server allocates IP-addresses to the VLAN700 and WiMAX subscribers. The GW is the route to internet where the firewall is controlled. A content server is also included, which will be used to host a server for throughput measurements.

The AU-IDU and AU-ODU was connected via an Inter-media Frequency (IF) cable. The IEEE 802.16 MAC and modem was handled by the AU-IDU. Wireless network connection establishment and bandwidth management was also performed by the AU-IDU. The AU-ODU was a high power, full duplex multi-carrier radio unit that connected to an external antenna. There was support for 14MHz bandwidths with high system gain and interference robustness utilizing high transmit power and low noise.

Directional antennae were used with azimuth beamwidth at 90° and a maximal gain of 14 dBi. The electrical and mechanical parameters are listed in Fig. 6. Two antennae that point in different directions were used as pictured in Fig. 7.

Fig. 6 Antenna Technical Specification (3.4 - 3.6GHz 90/V, P.N. 300067)

Fig. 7 Base Stations ODU & Antenna

`3.3.3 Subscriber Unit

The SU installed at the client side, comprised an Indoor Unit (SU-IDU) and an Outdoor Unit (SU-ODU). These are pictured in Fig. 8. The setup of the subscriber, which was the client side, is illustrated in Fig. 9. The “SU monitor” (PC) was configured with a static IP-address to create a local LAN between the PC and the SU. The purpose of the “SU-monitor” was that it should connect to the SU-IDU and carry out performance monitoring. Functionality for performance monitoring was provided by the SU-IDU.

“Subscriber 1” obtained a dynamic IP-address from a DHCP server on VLAN700 located behind the BS. The subscriber should generate traffic and measure the throughput achieved. Both the “SU monitor” and “Subscriber 1” ran the OS Windows XP (any other OS would have worked). The SU-ODU contained all the active components and an integral high flat antenna. The SU-IDU was powered and connected to the AU-ODU via a category 5 Ethernet cable. This cable carried both data and power between the two units. The AU-IDU acted as a data bridge between the wireless and wireline media, supporting up to 512 MAC addresses. It connected the subscriber’s data equipment via a standard IEEE 802.3 Ethernet 10/100-BaseT interface.

3.3.4 Services

A service is a virtual connection between a subscriber’s application and the Network Resource.

The Network Resource could be internet, Content Provider, Corporate Network, etc. A subscriber is an entity that may be associated with devices connected to SUs. The Service Domain is illustrated in Fig. 10

The Services were implemented as IEEE 802.16 connections within the wireless domain. Each

Service associates a certain Service Profile with a specific Subscriber’s device behind a specific

SU. A diagram of how Services are composed is illustrated in Fig. 11, where the entities that

constitute a service are listed and described shortly as follows:

Subscriber: subscribes to a Service Profile behind a specific SU.

Service Profile: defines the properties of the specific service, and is associated with a specific Forwarding Rule and Priority Classifier.

Forwarding Rule: defines the attributes that affect forwarding and switching of data.

Priority Classifier: defines the QoS Profiles to be allocated to users/sessions differentiated by DSCP or 802.1p priority classifiers. It defines Uplink and Downlink QoS Profiles.

QoS Profile: defines the Quality of Service parameters that are applicable when the QoS Profile is used.

3.3.4.1 Implementation of service used in measurement

A service named “Isaac-testsu” was implemented as illustrated in Fig. 12. It defines a Service Profile named “SPF1” for subscriber “MI7”s station behind the SU with MAC Address 00-10-e7-22-a6-14. The implementation of the service is described below.

The Service Profile “SPF1” was defined with service type L2, which transports Layer 2 (Ethernet) frames in the Service Domain. That is between the subscriber’s site and the Network Resource located in the Infocom’s backbone, and/or between the subscriber’s sites. “SPF1” was associated with the Service Profile “PC1-L2” and Forwarding Rule “FW1”. The Priority Marking Mode was chosen to be VLAN tagging of the type 802.1p. The Priority Marking Classifier was set to 7, which is the highest value used by 802.1p. VPL ID was set to NONE because no virtual private LANs are implemented.

The Forwarding Rule “FW1” was set to use Service Type L2. Multicast Relaying was disabled and Unicast Relaying enabled. The Unknown Forwarding Policy was set to “Forward”, meaning that transmission of all packets is enabled. It defined the Multicast QoS Profile “BE-12M” to be used for all multicast and broadcast messages.

The Priority Classifier “PC1-L2” was setup with 802.1p as priority type. A QoS Profile “BE-12M” was defined for both UL and DL. Upper Priority Limits was set to 7, thus all packets tagged with values between 0 and 7 would have priority as defined in the QoS profile for the respective direction.

The QoS Profile “BE-12M” was setup with QoS type BE (Best Effort). The Maximum Information Rate (MIR) the system will allow for the connection was set to 12 Mbps. The Committed Time (CT) was set to medium that will be 100 miliseconds. CT defines the time window over which the information rate is averaged to ensure compliance with the MIR parameter.

3.4 Implementation and Site preparations

3.4.1 Objective

Test the performance of a WiMAX network working at 3.5 GHz frequency spectrum, covering long distance and under line-of-sight (LOS) conditions.

3.4.2 Sample field locations chosen

  • Base station location: Kololo summit and Entebbe
  • SU location 1, approximately 11km, LOS, away from the base station will be selected as a CPE location with the subscriber unit installed.
  • SU location 2, approximately 1.5km, LOS, away from the base station will be selected as a CPE location with the subscriber unit installed.

3.4.3 Throughput, SNR and Modulation test setup

TCP and UDP throughput will be carried out in the network at two CPE locations

  • SU location 1
  • SU location 2

With the aid of a 3rd party software tool, the throughput, SNR, Modulation and RSSI figures were obtained and summarized as shown in the next sections.

Equipment list:

  • Subscriber unit (SU) with outdoor unit (ODU)
  • Indoor unit (IDU)
  • 100BaseT switching Hub
  • Notebook computer
  • PC server (linux loaded)  at the Base station

Software tools:

I-perf (version 1.7.0 or higher)- used to test local end-to-end tests

Speedtest.net – used to test international traffic

Table 9: Kololo summit RF sample test illustration

SU location

Kololo summit

Direction

LOS

LOS

Estimated distance

11km

1.5km

Modulation type

SNR

RSSI

No. of connections

TCP throughput

8.0Mbit/s

8.3Mbit/s

Round trip delay

35ms-40ms

39ms-47ms

TCP window

48Kbytes

48Kbytes

UDP window

64Kbytes

64Kbytes

Table 10: Entebbe summit RF sample test illustration

SU location

Entebbe

Direction

LOS

LOS

Estimated distance

11km

1.5km

Modulation type

SNR

RSSI

No. of connections

1

1

TCP throughput

8.0Mbit/s

8.3Mbit/s

Round trip delay

26ms-33ms

32ms-40ms

TCP window

48Kbytes

48Kbytes

UDP window

64Kbytes

64Kbytes

3.4.4 Internet Service Test

A client-server network structure was setup at both locations with a LINUX server loaded with HTTP, FTP, DNS, DHCP, POP3 and SMTP services. This simulated the company’s service operations on the WiMAX network.

Table 11: Kololo summit throughput  sample test illustration

Base station

Kololo

Direction

LOS

LOS

Estimated distance

11km

1.5km

Configured PIPE

Local

Internet

Table 12: Entebbe throughput sample test illustration

Base station

Entebbe

Direction

LOS

LOS

Estimated distance

11km

1.5km

Configured PIPE

Local

Internet

CHAPTER FOUR

PRESENTATION OF RESULTS

4.1 Kololo Summit results

The results presented below were collected from one of the locations identified as densely populated.

A measure of the link quality was performed at each location. At the locations Kololo summit (urban) and Entebbe (sub-urban), positions with Non Line of Sight (NLOS) showed varying signal quality, where at some points the signal quality was as weak that no IP connectivity was obtained. Throughput measurements could not be performed in these positions. Expected system behavior was observed, where signals in NLOS conditions performed better at short distances. Signals in LOS conditions performed good at both short and farther distances.

The overall FTP throughput results achieved way less than expected.  A comparison with the results presented by the equipment manufacturer gave results divided by three in general when higher order modulations were used. The throughput did not always confirm to the theoretical bitrate for the respective modulation rates. The reason for this was probably that the SU used some time to decide upon modulation rate, thus the notified result might have been read too early.

Parameters

Downlink

Uplink

Configured PIPE

256

256

Modulation scheme

QAM643/4

QAM643/4

SNR

34

23

RSSI

64

70

Local throughput

239

144

Internet throughput

60

29

Parameters

Downlink

Uplink

Configured PIPE

256

256

Modulation scheme

QAM643/4

QAM643/4

SNR

50

76

RSSI

-70

-56

Local throughput

120

100

Internet throughput

100

90

Parameters

Downlink

Uplink

Configured PIPE

256

256

Modulation scheme

QAM643/4

QAM643/4

SNR

35

23

RSSI

-56

-74

Local throughput

230

211

Internet throughput

58

22

Parameters

Downlink

Uplink

Configured PIPE

256

256

Modulation scheme

QAM643/4

QAM643/4

SNR

67

72

RSSI

-64

-68

Local throughput

200

134

Internet throughput

145

101

Parameters

Downlink

Uplink

Configured PIPE

256

256

Modulation scheme

QAM643/4

QAM643/4

SNR

34

25

RSSI

-64

-70

Local throughput

239

144

Internet throughput

150

120

Parameters

Downlink

Uplink

Configured PIPE

256

256

Modulation scheme

QAM643/4

QAM643/4

SNR

18

22

RSSI

-56

-64

Local throughput

200

189

Internet throughput

140

110

Parameters

Downlink

Uplink

Configured PIPE

256

256

Modulation scheme

QAM643/4

QAM643/4

SNR

34

24

RSSI

-64

-74

Local throughput

248

124

Internet throughput

198

200

Parameters

Downlink

Uplink

Configured PIPE

256

256

Modulation scheme

QAM643/4

QAM643/4

SNR

32

30

RSSI

-67

-74

Local throughput

247

242

Internet throughput

152

93

Parameters

Downlink

Uplink

Configured PIPE

256

256

Modulation scheme

QAM643/4

QAM643/4

SNR

32

24

RSSI

-67

-74

Local throughput

249

209

Internet throughput

150

24

Parameters

Downlink

Uplink

Configured PIPE

256

256

Modulation scheme

QAM643/4

QAM643/4

SNR

35

23

RSSI

-58

-73

Local throughput

230

213

Internet throughput

146

49

The parameters in the tables above are the minimum required for optimal WiMAX performance as specified by the 802.16d standard.

4.2 Entebbe results

Parameters

Downlink

Uplink

Configured PIPE

256

256

Modulation scheme

QAM643/4

QAM643/4

SNR

35

33

RSSI

-62

-74

Local throughput

256

253

Internet throughput

200

213

Parameters

Downlink

Uplink

Configured PIPE

256

256

Modulation scheme

QAM643/4

QAM643/4

SNR

23

19

RSSI

-85

-73

Local throughput

233

204

Internet throughput

211

219

Parameters

Downlink

Uplink

Configured PIPE

256

256

Modulation scheme

QAM643/4

QAM643/4

SNR

34

35

RSSI

-60

-70

Local throughput

246

250

Internet throughput

198

190

Parameters

Downlink

Uplink

Configured PIPE

256

256

Modulation scheme

QAM643/4

QAM643/4

SNR

31

22

RSSI

-65

-33

Local throughput

231

245

Internet throughput

169

200

Parameters

Downlink

Uplink

Configured PIPE

256

256

Modulation scheme

QAM643/4

QAM643/4

SNR

246

250

RSSI

-60

-70

Local throughput

246

202

Internet throughput

198

190

Parameters

Downlink

Uplink

Configured PIPE

256

256

Modulation scheme

QAM643/4

QAM643/4

SNR

34

33

RSSI

-65

-74

Local throughput

210

235

Internet throughput

183

180

Parameters

Downlink

Uplink

Configured PIPE

256

256

Modulation scheme

QAM643/4

QAM643/4

SNR

27

26

RSSI

-78

-73

Local throughput

250

202

Internet throughput

200

250

Parameters

Downlink

Uplink

Configured PIPE

256

256

Modulation scheme

QAM643/4

QAM643/4

SNR

34

21

RSSI

-70

-69

Local throughput

228

200

Internet throughput

200

187

Parameters

Downlink

Uplink

Configured PIPE

256

256

Modulation scheme

QAM643/4

QAM643/4

SNR

24

29

RSSI

-81

-76

Local throughput

228

220

Internet throughput

200

213

Parameters

Downlink

Uplink

Configured PIPE

256

256

Modulation scheme

QAM643/4

QAM643/4

SNR

30

24

RSSI

-81

-73

Local throughput

240

237

Internet throughput

200

199

Parameters in the tables above are the minimum required for optimal performance as specified in the 802.16d standard.  The values are gathered from different locations but in a specific area.

CHAPTER FIVE

DISCUSSION OF RESULTS

5.1 Overview

This research used an empirical research method for analysis performed over measurement data extracted from a fixed WiMAX system deployed in real life. Analytical models and conclusions will be based on these collected measurement data presented in the previous section.

A Network Management System (NMS) is used by the researcher for administrating the BSs and SUs. The functionality in the BSs and SUs logs performance attributes. These performance attributes are DL and UL RSSI, DL and UL SNR, transmit (Tx) and receive (Rx) modulation rate and Tx power for the SU which is important due to the use of ATPC.

5.2 Link Performance

5.2.1 Received Signal Strength Indicator (RSSI)

The system has a Received Signal Strength Indicator (RSSI) which reports a value in dBm. The RSSI is specified in the IEEE 802.16-2004 standard, sect. 8.3.9. When analyzing signal strength it is useful to relate the RSSI to the distance between the BS and SU.

The RSSI is measured for both uplink and downlink, and will be analyzed and compared to well-established models in the following subsections.

The well-established models are Free Space Loss (FSL) and the Cost 231 Hata models for suburban and urban environments which were discussed in chapter 3.

5.2.2 Downlink Signal Strength versus Distance

The DL RSSI for each subscriber is plotted in Figure 15 together with the well-established models FSL and the Cost 231 Hata models for suburban and urban environments.

Most of the plotted subscribers are expected to perform similar to the FSL since they were installed with LOS conditions to the BS if possible, but this is not always possible when deploying a wireless communication system in cities with obstacles as high buildings. This is illustrated by the divergence in Figure 15.

Fig. 15 shows that all signal strength measurements classified as typical are better than the Cost-231 models for Path Loss in urban and suburban environments. The reason for this is that the terrain around the BS was very favorable with respect to radio propagation since it was first flat and then gradually increased in height as the distance from the BS increased.

Some of the subscribers very close to the BS perform equal to or worse than the Cost 231 Hata models. This is mainly due to the fact that subscribers close to the BS are more frequently under NLOS conditions than subscribers farther away from the BS. The reason for the greater performance of this system than the Cost 231 Hata models is that this is a fixed system rather than nomadic or mobile as used when constructing the Cost 231 Hata models.

5.2.3 Uplink Signal Strength versus Distance

As for DL RSSI, the UL RSSI values for each subscriber are plotted in figure 16 together with the models FSL and the Cost 231 Hata suburban and urban models.

Since Automatic Transmission Power Control (ATPC) is used by the SU, normalization is performed on the RSSI values where the corresponding transmission power is below the maximum of 20 dBm. This is done by adding the transmission power back-off in dBm as follows:

RSSIULnorm = RSSIUL + (20-TxPower)     (5)

The UL RSSI versus distance plot is similar to the DL RSSI versus distance plot with the exception that lower RSSI values are observed. This was expected due to the fact that the SU transmits with 8 dBm less power than the BS.

Since Automatic Transmission Power Control (ATPC) is used by the SU, normalization is performed on the RSSI values where the corresponding SU transmission power is below the maximum of 20 dBm. This is done by adding the transmission power back-off in dBm as follows:

RSSIULnorm =RSSIUL + (20 -TxPower)    (6)

The UL RSSI versus distance plot is similar to the DL RSSI versus distance plot with the exception that lower RSSI values are observed. This was expected due to the fact that the SU transmits with 8 dBm less power than the BS.

5.3 Signal to Noise Ratio (SNR)

The Signal to Noise Ratio (SNR) is the power ratio between the signal and the background noise. SNR will give a better indication of the actual system conditions because interference and noise is revealed.

SNR and RSSI are measured at all locations and should be closely correlated, and a plot of RSSI versus SNR should give a smooth graph if the interference and background noise is absent. The following subsections analyses SNR for downlink and uplink.

5.3.1 Downlink Signal to Noise Ratio

The DL RSSI versus DL SNR is plotted for each subscriber in Fig. 17. The graph flatten off at around -65 dBm and outwards, which indicates that optimal performance could be achieved if RSSI is above -65 dBm and no interference or background noise is present. The results indicate that the maximum measurable SNR value at the SU is 36 dB.

Many of the subscribers vary from the linearity and the “flatten off” pattern, which indicates that interference is present. Since the system consists of 10 BSs, where 4 different frequencies are reused at adjacent BSs all sending with max power, there is a great possibility for CCI.

Even though the SU cannot measure higher SNR values than 36 dB, the RSSI continue to increase. The SNR values that layes below 36 dB as the RSSI increase may therefore not be valid, and not very influential as the RSSI increases.

Another observation is that the curve seems to decrease with 1 dB in SNR at around -50 dBm in RSSI and more, which may be due to saturation in the SU antenna.

5.3.2 Uplink Signal to Noise Ratio

Fig. 18 shows the uplink RSSI vs. SNR. A linear increase is observed, and the subscribers that deviate from this line are probably disturbed by interference.

The “flatten off” observation found in the downlink graph (Fig. 17) is not present in the uplink, which is because the SU transmits with less power than the BS. The maximum SNR value in the uplink is found at around -68 dBm RSSI which is a bit better than for the downlink.

It is interesting to compute the cumulative distribution of SNR for different distance intervals as shown in Fig. 19, which illustrates that the probability to obtain a better SNR value at shorter distances is naturally higher than at farther distances in a WiMAX deployment.

5.3.3 Path Loss Model

A Path Loss model could be derived based on the high amount of RSSI values obtained in the distance range up to 20 km. The high amount of RSSI values made it possible to construct a Path Loss model with great accuracy.

Only SUs with LOS conditions are considered in the model. As many of the SUs at shorter distances to the BS have NLOS or near LOS conditions, the subscribers within 2 km are excluded from the model. The rest of the subscribers are classified, where NLOS subscribers are excluded.

The researcher aimed at finding a model of the Path Loss model by the following equation:

PL = A + Blog(R)(7)

Fig. 20 plots all the RSSI values versus the logarithm of the distance between the BS and the SU. If the Path Loss confirm to a straight line with the form given in Eq. 7 a straight line should be drawn through the points in Fig. 20. The straight line was found by doing linear regression on the points:

RSSI = -50.11 – 21.29.log(r)  (8)

Fig. 21 Eq. 8 plotted together with all RSSI values

Eq. 8 is plotted together with all RSSI values in Fig. 21. The standard deviation for Eq. 8 was

found to be 7.17. The mean RSSI value was -66.45 dBm.

A Path Loss model can be derived from

PL = TP + Gbs + Gsu – RSSI    (9)

where ‘TP’ is Transmitted Power, ‘Gbs’ is BS antenna gain and ‘Gsu’ is SU antenna gain. The resulting Path Loss model for the measurements is given by

PLLOS = 110.11 + 21.29log10(r)                                                                                                                 (10)

where ‘d’ is the distance between BS and SU, and the Path Loss is denoted dB. It can be seen that the loss exponent in Eq. 10 is similar to the free space loss, which was as expected for the SUs with LOS conditions.

It is interesting to compare the Path Loss model to the Free Space Loss model (FSL) and the Cost 231 Hata [30] models for suburban and urban areas. These models are plotted together in Fig. 22.

As expected, the Path Loss model approaches the FSL model because most of the subscribers have LOS capabilities. The Cost 231 Hata models for suburban and urban environments have greater Path Loss because they are based on mobile systems, whereas the fixed WiMAX Path Loss model is based on a fixed system.

CHAPTER SIX

CONCLUSION

The main contribution and objective of this thesis was to uncover the WiMAX performance and conclude upon the hype and expectations, through extensive field trial measurements and real life deployment analysis. A detailed analysis and discussion on the throughput, coverage and physical system performance has been subject for research.

Theoretically, WiMAX technology can provide coverage in both LOS and NLOS conditions. NLOS has many implementation advantages that enable operators to deliver broadband data to a wide range of customers

It was noted that with the same base station configurations, varying results were obtained both on the signal quality and throughput. This was further proof that sparsely populated areas provide better WiMAX performance than densely populated areas.

As for the fixed WiMAX field trial in this thesis, with a 3.5 MHz channel bandwidth and LOS conditions, maximum 9.6 Mbps was obtained out of theoretical 12.71 Mbps (76 %). The variance is considered to be overhead from the transport protocol UDP and management traffic, and the results were similar to the system vendor specification. The throughput was difficult to determine because of much variance, where different distances and sight capabilities among others were determining.

The same applies to the coverage in WiMAX, where great distances are achieved under ideal conditions with line of sight to the base station and no interference. Sub-channelization, which is optional in fixed WiMAX showed to improve the coverage under difficult conditions with non-line of sight (NLOS) to the base station (BS). Diversity also showed to improve both coverage and throughput.

These results showed that performance is dependent on a range of factors. Throughput and coverage in WiMAX should be considered as dependent variables, and not constants as is more common in wireline systems. It is therefore necessary to explain throughput and coverage (path loss) as analytical expressions and models for the representation of performance, where these dependent variables take the underlying performance attributes into account. Without these considerations in mind, the hypothesis is easily falsified by the general public and the system may easily be abused due to hype and high expectations.

WiMAX-compliant equipment based on the IEEE 802.16-2004 Air Interface Standard will provide operators the technology necessary to deploy cost-effective wireless metro area networks with ubiquitous coverage offering broadband services to multiple types of customers. The examples described in this research point out some of the considerations that should be taken into account when planning a WiMAX-based network in the 3.5 GHz frequency band. For wireless access networks, accurately projecting present and future capacity requirements is important to ensure deployment of the most cost-effective base station infrastructure, particularly in areas where fixed base station costs are expected to be high or are already high. The minimum amount of spectrum for a cost-effective deployment varies with the demographics, the targeted market segment, the services being offered, and the cell frequency re-use factor.

Future works should look at the 2.5 GHz frequency band as this has not yet been implemented commercially in this region.

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