WiMax point-to-multipoint and mesh mode network planning

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Universidade de Aveiro Departamento deElectrónica, Telecomunica¸cões e Informática, 2009

Andr´

e Amorim de

Faria Cardote

Planifica¸c˜

ao de Redes Wimax Ponto-Multiponto e

em Malha

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Universidade de Aveiro Departamento deElectrónica, Telecomunica¸cões e Informática, 2009

Andr´

e Amorim de

Faria Cardote

WiMAX Point-to-Multipoint and Mesh mode

Network Planning

Disserta¸cão apresentada à Universidade de Aveiro para cumprimento dos requesitos necessários à obten¸cão do grau de Mestre em Engenharia Electrónica e de Telecomunica¸cões, realizada sob a orienta¸cão cient´ıfica da Prof. Dra. Susana Sargento, Professora auxiliar do Departamento de Electrónica, Telecomunica¸cões e Informática da Universidade de Aveiro e do Eng.o _{Sérgio Pires, à data de in´ıcio responsável pelo departamento de} investiga¸cão da Celfinet e actualmente Colaborador do Institudo de Teleco-munica¸cões - Pólo de Aveiro.

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`

A mem´oria de

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o j´uri / the jury

presidente / president Prof. Dr. At´ılio Manuel Silva Gameiro

Professor associado do Departamento de Electrónica, Telecomunica¸cões e In-formática da Universidade de Aveiro

vogais / examiners committee Prof. Dra. Susana Sargento

Professora auxiliar do Departamento de Electrónica, Telecomunica¸cões e In-formática da Universidade de Aveiro (orientador)

Prof. Dr. Manuel Alberto Pereira Ricardo

Professor Associado da Faculdade de Engenharia da Universidade do Porto (ar-guente)

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agradecimentos / acknowledgements

Neste desfecho de mais uma etapa da minha vida ´e com muito gosto que agrade¸co a todos os que me acompanharam ao longo destes anos e sem os quais n˜ao me conseguiria imaginar a terminar esta fase. Alguns presentes desde o primeiro dia em que integrei o curso, outros que fui conhecendo ao longo do tempo, mas todos igualmente importantes.

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A minha familia que sempre me apoiou incondicionalmente em todas as decis˜oes que tomei, bem como no decorrer da minha forma¸c˜ao.

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A professora Susana Sargento, pela incondicional disponibilidade que sempre teve para ouvir todos os problemas que surgiram no decorrer do trabalho, dando sempre os melhores conselhos de forma a que os pudesse resolver. A todos os meus amigos, que preenchem a minha vida dando-me motivos para sorrir a cada dia.

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palavras-chave IEEE 802.16, IEEE 802.16-2004, IEEE 802.16e-2005, WiMAX, OFDM, OFDMA, MIMO, SIMO, Antenas Adaptativas, Planeamento, Celular, Malha, Redes M´oveis

resumo Numa sociedade em que, crescentemente, utilizamos a Internet como meio de trabalho, lúdico ou simplesmente informativo, são necessários novos meios de levar esta tecnologia até todos, de maneira a que possamos tirar partido e evoluir com ela.

Sendo a falta de cobertura dos actuais operadores de telecomunica¸cões, em certas partes do território mais recônditas, um dos impedimentos para que toda a popula¸cão possa ter liga¸cão à Internet a pre¸cos acess´ıveis, o WiMAX surge como uma solu¸cão sem fios de banda larga com grande cobertura e baixo custo de implementa¸cão, que possibilita tanto o acesso directo por parte dos utilizadores como o funcionamento em modo de backhaul para redes Wi-Fi ou de outro tipo.

O objecto de estudo desta disserta¸cão é o planeamento de redes WiMAX ao n´ıvel fisico, ou seja, a disposi¸cão dos diversos elementos constituintes da rede da melhor forma poss´ıvel para maximizar a cobertura da rede ao menor custo.

No âmbito deste trabalho foram efectuados diversos estudos ao n´ıvel de propaga¸cão de ondas para a tecnologia WiMAX e planeamento de redes ponto-multiponto e em malha. Foram desenvolvidos algoritmos que per-mitem o planeamento de redes em diversos cenários, tais como: acesso em zonas de grande densidade populacional, acesso em zonas long´ınquas, até onde se torna dif´ıcil a passagem de um cabo, ou mesmo cenários em que seja mais proveitoso utilizar o Wi-Fi como tecnologia de acesso para os utilizadores, ficando o WiMAX como tecnologia de backhaul. Foi também criado um mechanismo de posicionamento automático e optimizado dos componenentes de uma rede em malha.

Como resultado do estudo realizado foram desenvolvidas com sucesso, a partir dos algoritmos estudados, duas aplica¸c˜oes para planeamento de redes WiMAX, uma em modo ponto-multiponto e outra em modo de malha, que ser˜ao devidamente apresentadas ao longo do texto.

Utilizando as aplica¸cões desenvolvidas, foi poss´ıvel obter vários resultados que permitirão uma melhor compreensão e avalia¸cão de redes WiMAX.

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keywords IEEE 802.16, IEEE 802.16-2004, IEEE 802.16e-2005, WiMAX, OFDM, OFDMA, MIMO, SIMO, Adaptive Beamforming, Planning, Cellular, Mesh, Mobile Networks

abstract In a society where the Internet is increasingly used for working, playing or simply watching the news, new ways to get this technology to everyone are required, so that each and everyone can take part on this great global community.

One of the major barriers for the widespread of the low-cost Internet is the unlikelihood to reach very remote locations of the territory. WiMAX appears as a promising broadband wireless access technology with low deployment cost and high coverage, allowing users to access directly to the network using this technology or providing backhaul connections for other technologies, such as Wi-Fi.

The aim of this MSc thesis is the physical WiMAX network planning, i.e. the placement of the various elements of a network in order to maximize de coverage with the lowest cost.

In the scope of this work, several studies in wave propagation for the WiMAX technology point-to-multipoint and mesh mode network planning were per-formed. Some algorithms that allow network planning in various scenarios, such as access in highly populated areas, isolated places, where it is hard to run a wire or even in scenarios where it is preferable to make the last mile access in Wi-Fi were developed, as well as a mechanism to select the positions of the elements in a WiMAX mesh network.

As a result of this study two applications for WiMAX network planning were successfully developed, based on the formulated algorithms: one for point-to-multipoint mode and the other for mesh mode operation, which will be properly presented through this text.

Using the developed applications, it has been possible to perform several essays that will allow a better comprehension and evaluation of WiMAX networks.

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List of Figures

2.1 Mesh WiMAX to improve territory coverage . . . 8

2.2 IEEE 802.16 mesh mode frames . . . 9

2.3 Three-way-handshake process . . . 10

2.4 Service Flow transitions . . . 12

2.5 MAC Sublayers [5] . . . 12

2.6 Multipath example . . . 14

2.7 Signal at the receiver in a multipath scenario . . . 14

2.8 OFDM subcarriers . . . 15

2.9 OFDM transmission . . . 16

2.10 OFDMA access method . . . 17

2.11 WiMAX Base Station Planning Tool . . . 21

2.12 WiMAX Base Station Planning Tool: calculation result . . . 22

3.1 Comparison between Fixed and Mobile WiMAX . . . 24

3.2 Influence of the type of terrain in Fixed and Mobile WiMAX . . . 25

3.3 Horizontal and vertical radiation patterns . . . 25

3.4 Number of sectors and capacity for the same scenario . . . 26

3.5 Range vs. Capacity for the different types of scenarios . . . 27

3.6 Multiple antenna techniques . . . 27

3.7 Adaptive beamforming . . . 29

3.8 Enhancement techniques comparison . . . 30

3.9 Comparison between the implementation of the enhancement techniques in Fixed and Mobile WiMAX . . . 31

3.10 Range improvement of each technique for each type of terrain . . . 31

3.11 Range improvement of each technique for each type of terrain compared to SISO in % 32 3.12 Capacity improvement with MIMO 4x2 S.M. in a rural environment. . . 32

3.13 Capacity improvement with MIMO 4x2 S.M. for each type of terrain . . . 33

3.14 Capacity improvement with MIMO 4x2 S.M. related to SISO for each type of terrain in % . . . 33

4.1 Two hop interference-aware model . . . 36

4.2 Two hop interference-aware model with some possible values . . . 37

4.3 Interference-aware model considering all nodes’ traffic with some possible values . . 37

4.4 Scenario representation. . . 42

4.5 Algorithm scheme . . . 43

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4.7 Step 1 scheme . . . 47

4.8 Sorting method example . . . 48

4.9 Step 3 scheme . . . 50

4.10 Step 4 scheme . . . 52

4.11 Order for optimized CS placing . . . 53

4.12 Linear programming . . . 54

5.1 Traffic point configuration . . . 59

5.2 Dragging a TP to its position . . . 60

5.3 Wi-Fi hotspot configuration . . . 60

5.4 Candidate site configuration . . . 62

5.5 Options panel . . . 63

5.6 Scenario 1 and results for both tools . . . 65

5.9 Number of MRs and MAPs for both tools . . . 68

5.10 Time required for each tool. . . 69

5.11 Scenario 1 - Traffic Points . . . 70

5.12 Manual Placing CSs: case 1 . . . 70

5.13 Manual Placing CSs: case 2 . . . 71

5.14 Auto Placing CSs . . . 72

5.15 Randomly Placing CSs . . . 73

5.16 Reaching remote locations without wire - Traffic Points . . . 73

5.17 Reaching remote locations without wire . . . 74

5.18 WiMAX-Wi-Fi Integration - Traffic Points . . . 75

5.19 WiMAX-Wi-Fi integration . . . 75

5.20 Scenario 2 - Traffic Points . . . 76

5.21 Scenario 2 - Placed CSs . . . 77

5.22 Relation between the internal/external traffic rate and the number of MAPs and MRs 77 5.23 Fixed and Mobile WiMAX . . . 78

5.24 Urban Scenario . . . 79

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List of Tables

2.1 802.16 standards comparison . . . 7

2.2 SUI model parameters . . . 17

2.3 SN RRxfor each modulation . . . 18

2.4 Constants . . . 20

2.5 Typical usage values for the residential profile . . . 20

2.6 Typical usage values for the business profile . . . 20

3.1 Results for Fixed and Mobile WiMAX . . . 24

3.2 Results for the three types of terrain . . . 24

3.3 Capacity growth with the number of sectors in Fixed and Mobile WiMAX . . 25

3.4 Results for the various enhancement techniques . . . 29

3.5 Results for range improvement in Fixed and Mobile WiMAX . . . 30

4.1 Typical bandwidth usage for some services . . . 38

4.2 Traffic class for each considered service . . . 38

4.3 Typical usage values for the residential profile . . . 39

4.4 Typical usage values for the business profile . . . 39

4.5 Typical values for density of users . . . 39

4.6 Typical configuration values for a candidate site . . . 40

4.7 Capacities of the IEEE 802.11 set of standards . . . 54

5.1 Results for scenario 1 with both tools . . . 64

5.4 Results for both tools with the same random scenario . . . 68

5.5 Density of users . . . 69

5.6 Manual Placing CSs: case 1 - Results . . . 70

5.7 Manual Placing CSs: case 2 - Results . . . 71

5.8 Configuration of the Auto Place feature . . . 72

5.9 Auto Placing CSs - Results . . . 72

5.10 Randomly Placing CSs - Results . . . 72

5.11 Reaching remote locations without wire - Results . . . 74

5.12 WiMAX-Wi-Fi Integration - Results . . . 74

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Acronyms

AAS Adaptive Antenna Systems

AMPL A Mathematical Programming Language

ATM Asynchronous Transfer Mode

BMP Bitmap file format

BWA Broadband Wireless Access

CID Connection Identifier

CS Candidate Site or Convergence Sublayer CPLEX ILOG CPLEX optimization software package

CPS Common Part Sublayer

CRC Cyclic Redundancy Check

DSL Digital Subscriber Line

ertPS Extended Real-Time Polling Service

FDM Frequency Division Multiplexing

FDMA Frequency Division Multiple Access

FEQ Forward Error Correction

FFT Fast Fourier Transform

GIF Graphics Interchange Format

HiperMAN High Performance Radio Metropolitan Area Network IFFT Inverse Fast Fourier Transform

IP Internet Protocol

IPTV Internet Protocol Television IPv4 Internet Protocol version 4

ISI Inter Symbol Interference

JPEG Joint Photographic Experts Group LMDS Local Multipoint Distribution Service

LOS Line Of Sight

LP Linear Programming

MAP Mobile Access Point

MBS Multicast and Broadcast Services MIMO Multiple Input Multiple Output

MPS Mesh Planning Software

MR Mobile Router

MS Mobile Station

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MT Mobile Terminal

NLOS Non-Line of Sight

OFDM Orthogonal Frequency Division Multiplex OFDMA Orthogonal Frequency Division Multiple Access

PDU Protocol Data Unit

PHY Physical

PKM Privacy Key Management

PKMv1 Privacy Key Management version 1 PKMv2 Privacy Key Management version 2

PNG Portable Network Graphics

QoS Quality of Service

P2P Peer-to-Peer

PAPR Peak-to-Average Power Ratio

PMP Point-to-Multipoint

RF Radio Frequency

SC Single Carrier

SIMO Single Input Multiple Output SISO Single Input Single Output

SOFDMA Scalable Orthogonal Frequency Division Multiplex

SS Subscriber Station

SUI Stanford University Interim TDM Time Division Multiplexing TDMA Time Division Multiple Access

TP Traffic Point

VoIP Voice over Internet Protocol Wi-Fi Wireless Fidelity

WiMAX Worldwide Interoperability for Microwave Access

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Chapter 1

Introduction

1.1 Motivation

Over the last years, the Internet access has been spread all over the developed countries’ population. Although it may seem common to people living in cities, only 23.8% of the worldwide population has access to the Internet [1]. The most probable reason for this number is the poverty experienced by the population of many countries around the world, but not only. Isolated places, like small villages, far away from big cities in developed countries also contribute. Many and many will never get a wired Internet connection, just because the operators would spend more money getting the wire there than they would ever earn by selling some dozens or hundreds of service packs.

One of the possible solutions for this problem is the use of Worldwide Interoperability for Microwave Access (WiMAX). WiMAX is a Broadband Wireless Access (BWA) technology, which aims to provide long-distance and high-speed wireless connections, up to 50 Km and 70 Mb/s [2]. This way, installation costs can be truly reduced, which will lead to the expansion of the Internet Access towards these farther locations. This is one of the major interest points of WiMAX, but there are many others.

WiMAX will also be available at urban areas, once it is a promising technology for fixed and mobile broadband access. Having such high transmission rates, turns it able to handle Wi-Fi backhaul connections, thus allowing Wi-Fi access points to operate without a wired connection to the backbone. Telemetering1_{is also an interesting topic, which can be supported} by WiMAX, as almost the whole country’s territory is supposed to be covered. Moreover, WiMAX supports mesh mode operation, which allows base stations to fully operate without a wired connection to the backbone.

Although this is a very promising technology, one of the problems of deploying WiMAX networks is that, until the date, there have not yet been performed sufficient studies in resource placement: what we call planning. The purpose of this MSc thesis is to fill this gap, by providing mechanisms to evaluate scenarios, optimize deployment solutions, automatically place the necessary resources given the traffic requirements within a certain area or even

1_{Telemetering is the act of reading the water, energy, natural gas, and other meters remotely. This is}

an excellent way for the companies to monitor these services and keep the users daily informed about their consumption.

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build backhaul networks to provide connectivity between Wi-Fi hotspots in both Point-to-multipoint and mesh WiMAX operation modes.

1.2 Objectives

The main objective of this thesis is to develop algorithms to aid the design and deployment of WiMAX networks. The algorithms will then be included in planning tools also developed in the framework of this thesis. For this to be accomplished, several studies on the IEEE 802.16 standard must be done, mainly concerning the PHY layer.

The following tasks will be performed:

• Study of the IEEE 802.16-2004 and IEEE 802.16e-2005 standards, focusing on mobility, enhancement techniques and mesh mode operation.

• Implementation of these techniques in a previously developed base station planning tool for point-to-multipoint (PMP) operation.

• Improvement of a linear programming model, so that it could fit the desired planning features.

• Development of a WiMAX mesh network planning algorithm through comparison with the linear programming model.

• Development of an optimal base station placing algorithm.

• Integration of WiMAX mesh networks and Wi-Fi hotspots.

• Development of a mesh network planning tool for Microsoft WindowsTM_.

1.3 Contributions of the thesis

As all the proposed tasks were accomplished, this thesis contributions are:

• The enhancement of an existing base station planning mechanism and tool, which sim-plifies the deployment of PMP networks.

• The development of a WiMAX mesh planning tool, which optimizes the deployment of mesh networks, by restricting the network to the minimum number of base stations.

1.4 Outline

This thesis is structured as follows. Chapter 2 presents the IEEE 802.16 standard in terms of the concepts necessary to clearly understand the content of this work, distinguishing the various amendments and each one’s features. It also presents the state of the art, along with a previous work on the area: a base station planning tool for PMP. Chapter 3 presents a performance comparison between Fixed and Mobile WiMAX, some enhancement techniques described in the IEEE 802.16 standard, as well as its implementation in the base station planning tool. Some interesting results, concerning these techniques, obtained with the base

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station planning tool are also shown. Chapter 4 focuses on mesh mode planning, by explaining the biggest problem of this type of network planning – interference – and exhibiting an algorithm developed to solve this type of problems. A linear programming model is presented, as a way of accounting for the accuracy of the developed algorithm. Chapter 5 describes the implementation of the algorithm, presented in Chapter 4, in a mesh planning tool, for Microsoft WindowsTM, a comparison between solutions calculated by this one and by the linear programming model, as well as some interesting results and scenarios.

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Chapter 2

Background

The IEEE 802.16 standard has had great evolutions since the first version was released. These evolutions are presented in the form of amendments to the standard, as they are released. Section 2.1 will give an overview of these amendments and the evolution of the technology, as well as the different types of access methods it defines.

As the standard defines the MAC and PHY layers of the OSI model, there are several con-cepts of wave propagation and MAC layer constitution that must be learned before exploring the standard. Sections 2.2 and 2.3 will present them.

In Section 2.4 some related work is presented, along with the introduction to a previous work on the area.

Finally, Section 2.5 includes a summary of the chapter.

2.1 IEEE 802.16 Access Technologies

2.1.1 IEEE 802.16-2004 - Fixed WiMAX

In 1999, when the IEEE 802.16 group was created, it was intended to develop a line of sight (LOS) PMP wireless broadband communications system operating in the 10-66 GHz band. In 2001, the first version of the IEEE 802.16 standard was released as result of the work of this group. This version uses the 10-66 GHz spectrum band and only supports fixed line of sight (LOS) communication. Burst multiplexing is based on time division multiplexing (TDM) or time division multiple access (TDMA) and a single carrier (SC) is used. The antennas are supposed to be exterior and placed on the top of buildings and communications are point-to-multipoint (PMP) oriented, achieving data rates from 32 Mbps to 130 Mbps. Although this may seem to be a nice alternative to the current broadband access technologies, such as digital subscriber line (DSL) or cable, there are two main problems: the use of frequencies in the licensed spectrum and the absence of conformance with high performance radio metropolitan area network (HiperMAN) European standard.

In December 2002, the IEEE 802.16 task group C came up with the IEEE 802.16c amend-ment, which aims to ensure interoperability with the existing local multipoint distribution service (LMDS) and defines system profiling, in order to establish guidelines for vendors, to assure interoperability between them. Such profiles determine the mandatory and optional features of the equipments. Mandatory features include provisioned connections, IPv4 and

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fragmentation support. As optional features there are different levels of security, which allow the vendors to differentiate their products by price and functionality. Another specification is that 802.16c is network topology independent, what means that it can run under asyn-chronous transfer mode (ATM), frame relay or Internet protocol (IP). The big disadvantage is that, due to the large bandwidth, it has a low coverage, around 5 Km only.

The most important of all the amendments was released in 2003 under the name of IEEE 802.16a. As it works in lower frequencies (2-11 Ghz), it is able to reach up to 50 Km with up to 75 Mbps bitrates; this feature also enables it to work under non line of sight (NLOS) conditions. Mesh mode operation is another of the main points of this amendment that eases the communication between subscriber stations (SS).

In June 2004, a compilation of the original standard and its amendments until that date was released and some new features introduced. It is called IEEE 802.16-2004 or IEEE 802.16d and defines the operation in the 2-66Ghz band [3], providing support at the MAC and PHY layers for distinct operation at low and high frequencies due to the different propagation properties. It includes fixed LOS and NLOS communications, as well as PMP, mesh mode operation and quality of service (QoS) support.

With all these amendments to the standard and configuration possibilities, the need for a certification entity grows, in order to assure the compatibility between all the equipments on the market. This is where the WiMAX word takes meaning. WiMAX is a certification entity, similar to Wi-Fi, that ensures that every two WiMAX certified products, running IEEE 802.16, can work together.

2.1.2 IEEE 802.16e-2005 - Mobile WiMAX

The 802.16-2004 document was surely useful as the gathering of a lot of documents talking about different parts and corrections to the same technology. However, there were, indeed, some features, such as mobility support, to be introduced and corrections to be made, so in December 2005 a new amendment – IEEE 802.16e – was approved.

The major differences between 802.16-2004 and 802.16e-2005 are [2]:

• The establishment of the concept of mobility into the technology and, therefore, the introduction of mobile stations (MS). From this point on the WiMAX subscriber stations (SS) do not need anymore to be fixed, so this is why 802.16e is often named as Mobile WiMAX.

• Handover procedures were introduced in the MAC layer, once again to support mobil-ity/nomadism.

• Power save modes were introduced: sleep and idle mode.

• Scalable orthogonal frequency division multiple access (SOFMA) was introduced or, in other words, the OFDM PHY layer was modified.

• Security methods were updated.

• Multiple input multiple output (MIMO) and Adaptive Antenna Systems (AAS) were enhanced. (This topic will be covered later on Section 3.2.1).

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• Multicast and broadcast services (MBS) were added.

• New QoS class was introduced: Extended Real-Time Polling Service (ertPS), which is designed for real-time traffic with variable data rate, such as VoIP with silence suppres-sion.

Table 2.1 summarizes the 802.16 standards family.

Table 2.1: 802.16 standards comparison

802.16 802.16-2004 802.16e-2005

Release date December 2001 June 2004 December 2005

2-11GHz for fixed

Frequency band 10-66GHz 2-11GHz _{2-6GHz for mobile}

Application Fixed LOS Fixed NLOS Fixed and mobile

NLOS

MAC architecture PMP, mesh PMP, mesh PMP, mesh

Data rate 32-135Mbps 1-75Mbps 1-75Mbps Multiplexing Burst TDM/TDMA Burst TDM/TDMA/ OFDMA Burst TDM/TDMA/ OFDMA

Duplexing TDD and FDD TDD and FDD TDD and FDD

Channel bandwidths 20Mhz, 25MHz, 28MHz 1.75MHz, 3.5MHz, 7MHz, 14MHz, 1.25MHz, 5MHz, 10MHz, 15MHz, 8.75MHz 1.75MHz, 3.5MHz, 7MHz, 14MHz, 1.25MHz, 5MHz, 10MHz, 15MHz, 8.75MHz

2.1.3 IEEE 802.16 Mesh mode

Mesh networks are networks where each node can connect either to all of its neighbors (full mesh) or a part of them (partial mesh). These networks offer a set of benefits, mainly the capability to self-heal when a node goes down, due to the redundancy introduced by multiple connections. Another main point of mesh networking is that not all the nodes of a mesh network need to be connected to the backbone, once they can communicate with each other or, at least, some others and appropriately transfer the data to its destination. This property of mesh networks leads to the conception of two types of devices: mesh routers (MR), which can communicate with each other and with other devices, and mesh access points (MAP) which are connected to the backbone and thus can communicate with MRs and with the wired backbone. These devices have the same physical attributes as PMP base stations.

Applying this to reality, imagine that somewhere there is a small village far away from everything. Its inhabitants could never dream of having a wired Internet connection just because the cost of running there a wire for the operator would be too big comparing to the benefits it would bring. Now imagine that, close to that village, there is another one. Running there a wire is equally expensive, but we can try doing it with a wireless mesh technology, such as WiMAX. Figure 2.1 illustrates this scenario.

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Figure 2.1: Mesh WiMAX to improve territory coverage

By using one MAP and two MRs, it is possible to grant Internet connection to these two villages at a low cost. Using WiMAX mesh networking, only MAP1 needs to be connected to the wired backbone.

Oppositely to Wi-Fi mesh, WiMAX is topology aware, what means that it uses information from nodes 2 or 3 hops away to take decisions. This fact turns it more robust in terms of minimizing the hidden and exposed node problems.

WiMAX mesh mode uses a channel for control communications and another for data. This reduces the risk of collision, once the control messages do not interfere with data.

Although it is a very promising technology, IEEE 802.16e mesh mode is not widely scal-able. In single radio architectures, it is proved that the signal degrades some hops away [4] with the increasing number of nodes. There are some ways to solve or reduce this problem, mainly:

• The usage of multi-radio and multi-channel systems. Using a radio for control traffic and another for data traffic can be a solution, though it is expensive.

• Locating techniques to prevent control packets, for instance, from traveling across the whole network.

• Implementation of a hierarchical network scheme by deploying fixed nodes to act as relay candidates.

The IEEE 802.16 standard defines the mesh frames as having two subframes: control subframe and data subframe. The length of the control subframe is given by MSH-CTRL-LEN × 7, where MSH-CTRL-MSH-CTRL-LEN has 4 bits, so it ranges from 0 to 105 OFDM symbols. The data subframe is divided into minislots.

Figure 2.2 shows the two types of control subframes: network control (2.2(a)) and schedule control (2.2(b)). The first one occurs in the frames sent periodically with a well defined period for each network. This type of subframe is intended for nodes gaining synchronization and

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joining the network. Schedule control subframes are defined for centralized or distributed scheduling, which will be explained next.

The data subframe is used for the PHY transmission frames. It starts with a long pream-ble, for synchronization, followed by several MAC protocol data units (PDUs). Each MAC PDU has a 6-byte MAC header, a 2-byte mesh subheader with the node ID, a variable length MAC payload field and an optional 4-byte cyclic redundancy check (CRC).

(a) Network control subframe

(b) Network schedule subframe

Figure 2.2: IEEE 802.16 mesh mode frames

As said before, there are two types of scheduling: centralized and distributed. The dis-tributed scheduling can be further divided into uncoordinated and coordinated.

Centralized Scheduling

In centralized scheduling, a designated node, the mesh BS, coordinates the data subframe scheduling, resource allocation and grants for the other nodes in the mesh network. The process for resource allocation is the following:

• Each SS determines the traffic estimation for him and its children and sends a message up to its parent, until it reaches the mesh BS.

• When the mesh BS gathers all the necessary information, it determines the amount of granted resources and broadcasts this information in a message to all its neighbors, which pass it down to their children.

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Figure 2.3: Three-way-handshake process Uncoordinated Distributed Scheduling

Uncoordinated distributed scheduling is used for temporary communications between two nodes. A three-way-handshake process, depicted in Figure 2.3, is used, based on mesh dis-tributed scheduling messages (MSH-DSCH), which contain information about slot availabil-ity, scheduling, requests and grants. In this type of scheduling, the MSH-DSCH messages are transmitted in the data subframe. The three-way-handshake process works in the following way:

• When a node wants to communicate, randomly selects an idle slot and sends a MSH-DSCH: Request message to acquire the resource. If there is a collision, it enters a random backoff time and then sends the request again.

• When the granter receives the message requesting the resources, it evaluates the request through a slot allocation algorithm. If the granter is able to allocate the resource, sends out a MSH-DSCH: Grant to the requester.

• The requester copies the message and sends it as an acknowledge to the granter through a MSH-DSCH: Grant Confirmation message.

By listening to this message exchange, all the nodes are aware of the free resources. There is, currently, no slot allocation algorithm defined in the IEEE 802.16 standard. This gives the deployer the opportunity to select an appropriate algorithm according to its network needs.

Coordinated Distributed Scheduling

In the same way as in uncoordinated distributed scheduling, the MSH-DSCH message plays an important role in coordinated distributed scheduling. The difference here is that

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it is transmitted in the control subframe. In this case, the three-way-handshake process (Figure 2.3) is used to create a communication link with a neighbor node. When this link is successfully created, the two nodes can communicate in the reserved slots.

Also, in this type of scheduling, no slot allocation algorithm is defined.

2.2 MAC Layer

The IEEE 802.16-2004 standard defines three sublayers for the MAC layer: convergence sublayer (CS), common part sublayer (CPS) and security sublayer, which will be presented in this section. First we describe some fundamental concepts.

Connection Identifiers

A connection is a unidirectional MAC connection between a BS and a SS/MS or vice-versa. The purpose of a connection is to transmit the traffic of a service flow. Each connection can only serve one type of service and is identified through a connection identifier (CID).

The CID is a 16-bit value, thus providing 64 000 connections for each downlink and uplink channel. There are many CIDs defined in the standard [5] with specific meanings.

Service Flows (SF)

A service flow (SF) is a unidirectional MAC transport service that defines the QoS pa-rameters for the PDUs exchanged on a connection. There are three types of SFs:

• Provisioned - is know by provision from the network management, for example.

• Admitted - the standard supports a two-phase activation model, just like in telephony. So, the resources are first admitted and then activated, once the negotiation process ends. This is the SF for admitted connections.

• Active - service flow with the resources consigned by the BS. A SF can transit from one state to another, according to Figure 2.4.

2.2.1 Convergence Sublayer (CS)

As stated before, the IEEE 802.16 MAC layer is divided into three sublayers, as shown by Figure 2.5. The CS is the top sublayer of the IEEE 802.16 MAC layer. It is responsible for the communication with the upper layers, in other words, it is responsible for sending and receiving the PDUs to/from the upper layers. This sublayer also performs basic QoS operations, such as classifying and mapping the MAC layer service data units (MSDUs) into the proper CIDs.

As this sublayer deals with various types of technology on the upper layers, there must be different specifications for each type. Currently there are two types defined: ATM CS and packet CS, but more can be defined in the future. The first one is intended to deal with the asynchronous transfer mode (ATM) technology, whereas the latter is for packet-based technologies, such as IPv4, IPv6, PPP or IEEE 802.3 (Ethernet).

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Figure 2.4: Service Flow transitions

Figure 2.5: MAC Sublayers [5]

There is an optional function which is Payload Header Suppression. With this function, repetitive parts of the payload headers are suppressed at the sender and restored at the receiver.

2.2.2 Common Part Sublayer (CPS)

The Common Part Sublayer (CPS) is the middle sublayer of the MAC layer. It is respon-sible for the bandwidth allocation, connection establishment and connection maintenance.

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The IEEE 802.16 standard defines a set of messages that must be exchanged to negotiate and establish a connection. After the negotiation process is finished and the bandwidth is allocated for a certain connection, transfer messages can be exchanged in order to receive/send data.

2.2.3 Security Sublayer

The last sublayer, Security Sublayer, is responsible, as the name suggests, for the security of the connections. It provides authentication, secure key exchanging, encryption and integrity control across the whole system. Several procedures for data encryption are included in the standard. For secure key exchanging, the IEEE 802.16-2004 defines the Privacy Key Management (PKM) authentication protocol. The IEEE 802.16e-2005 amendment updates the encryption protocol, by defining the PKMv2, which is an evolution of the renamed PKMv1 protocol.

2.3 PHY Layer

2.3.1 Modulation Schemes: OFDM

Orthogonal Frequency Division Multiplex (OFDM) is a frequency division multiplexing scheme where data is transmitted simultaneously in many narrow-band orthogonal frequen-cies called subcarriers. As these frequenfrequen-cies are orthogonal to each other, the problem of interference between channels is reduced or even eliminated.

The number of subcarriers into which each wideband signal is break is usually called N. As each channel has smaller bandwidth than in single carrier (SC) transmission, by (2.1) we know that its transmission time will be as large as smaller its bandwidth is.

∆T = 1

∆f (2.1)

So, if the wideband signal is broke into N subcarriers, the transmission time for each sub-carrier will be N times longer, as shown by (2.2), thus providing better multipath resistance.

∆T = N

∆f (2.2)

Multipath

Multipath is one of the major problems of wave propagation. This phenomenon results in radio signals reaching the receiver by more than one path and, therefore, at different times. The causes of this phenomenon are various, and include refraction and reflection due to the ionosphere, the terrain conditions or buildings in the way, weather conditions and the presence of water bodies which cause reflection, among others. Figure 2.6 describes a multipath scenario: the base station is sending out a wave towards 1, but this wave also reaches building 2, which reflects it also towards 1. Both signals will reach the receiver, but at different times, as shown by Figure 2.7.

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Figure 2.6: Multipath example

Figure 2.7: Signal at the receiver in a multipath scenario FFT and IFFT

The fast Fourier transform (FFT) is a matrix computation that allows computing the discrete Fourier transform under certain mathematical conditions. The inverse fast Fourier transform (IFFT) is the inverse computation which makes it possible to divide the wideband signal into N orthogonal subcarriers. Although the FFT can be calculated for any number of points, the operation is easier for a number of points which is a power of 2. As shown by Figure 2.8, after the division of the wideband signal, each subcarrier has a null value at the maximum of the others.

Cyclic Prefix and the guard band

The cyclic prefix technique is also used in OFDM to avoid inter symbol interference (ISI). This technique consists of repeating the end of each symbol at its beginning. The purpose of this is to allow for multipath to settle before the main data arrives at the receiver.

To see how it works, let us consider a maximum channel delay spread of v + 1 samples. By adding a guard band of, at least, v samples between OFDM symbols we can assure that

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Figure 2.8: OFDM subcarriers

each symbol is independent from its neighbors. Let x denote an OFDM symbol with length N

x =£x1 x2 . . . xN

¤

(2.3) After applying a cyclic prefix of length v, we get:

xcp=

£

xN −v xN −v+1 . . . xN −1 x0 x1 . . . xN −1

¤

(2.4) If h is a v + 1 length vector which describes the impulse response of the channel during the OFDM symbol, we can calculate the output of the channel through (2.5)

ycp= h ∗ xcp (2.5)

Thus, the output has L + 2v samples: the first v samples contain interference from the preceding symbol and the last v from the last symbol, and so are discarded. This leaves exactly N samples, which is exactly what is needed to recover the N data symbols in x. These discarded symbols form the guard band, which guarantees the reduction of the Inter-Symbol Interference (ISI). We can assure that these resulting N symbols are equivalent to

y = h ⊗ x, (2.6)

however it is out of the scope of this text to prove it. Further readings on this topic can be found in [6].

Looking at Figure 2.9 we are now able to distinguish the various steps of a transmission in OFDM:

1. The IFFT of the X signal is calculated in order to break the wideband signal into various orthogonal subcarriers.

2. The parallel to serial (P/S) converts the resulting signal into a series of samples.

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Figure 2.9: OFDM transmission

4. The channel is traversed and noise is added to the signal.

5. The cyclic prefix is removed and the reverse operation of step 1 is done.

6. The FFT of the signal is calculated.

7. A forward error correction (FEQ) algorithm is applied in order to regenerate erroneous symbols.

One of the greatest advantages of OFDM is that it does not need complex equalization filters, once an OFDM symbol can be seen as many slowly modulated symbols, instead of one rapidly modulated. This is what makes it possible to introduce a guard band between symbols, making it unnecessary to use complex filters to separate them.

Another interesting point is the possibility of adapting the transmission to the channel conditions. This can be done by deactivating subcarriers exposed to high interference or attenuation or applying stronger modulation and error correction to them, which turns them slower, but more robust.

There are two main disadvantages of OFDM relating to SC:

• It is very sensitive to frequency synchronization problems because the orthogonality of the symbols relies on their being correctly distinguished in the frequency domain.

• The peak to average power ratio (PAPR) is high, what can be a hard constraint to some devices, namely RF amplifiers.

2.3.2 Modulation Schemes: OFDMA

OFDM is indeed a powerful modulation scheme, but in order to address the WiMAX needs, a multiple access technique is essential. Orthogonal Frequency Multiple Access (OFDMA) is the answer. Through the assignment of subsets of subcarriers, called subchannels, to each individual user, the multiple access issue can be solved.

OFDMA is a combination of two popular channel access methods: time division multiple access (TDMA) and frequency division multiple access (FDMA) as shown in Figure 2.10.

On the uplink, one or more subchannels can be assigned to the transmitter, while on the downlink a subchannel can be used for different receivers.

One major advantage of OFDMA is its potential to reduce the transmit power, because each user is only assigned a subset of subcarriers, thus reducing the PAPR, as it increases with the bandwidth.

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Figure 2.10: OFDMA access method

2.3.3 Propagation Model

In order to calculate the area covered by a radio, we must use models that usually rely on physical facts and statistics obtained from practical measures. For the NLOS case, especially, the latter are more accurate.

For the frequencies at which WiMAX radios operate, the SUI (Stanford University In-terim) model is the most appropriate [7]. In this model, three types of scenarios are con-templated: A, B and C. Type A is the most lossy, being appropriate for bumpy zones with moderate to dense vegetation. Type C is applicable to plane places with light vegetation, whereas type B relies between these two.

Using the SUI model, the path losses can be calculated by (2.7): Lp = 20 log µ 4πd0 λ ¶ + 10γ log µ d d0 ¶ + Xf+ Xh+ s (2) (2.7)

where s ∈ [8.2, 10.6] dB is the statistical lognormal distribution which accounts for the fading due to obstacles, γ stands for the loss factor exponent, Xf for the frequency correction factor

and X_h the antenna height correction factor. These parameters can be determined by (2.8), (2.9) and (2.10) respectively:

γ = a − b · hb+ _hc

b (2.8)

where hb is the height of the antenna in meters and parameters a, b, and c depend on the type

of terrain and are given by Table 2.2.

Table 2.2: SUI model parameters Type A Type B Type C

a 4.6 4.0 3.6

b 0.0075 0.0065 0.0050

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X_f = 6.0 log µ f 2 ¶ (2.9) being f the frequency of operation in GHz.

Xh=      −10.8 log¡hr 2 ¢

, for type A and B −20 log¡hr

2 ¢

, for type C

(2.10)

where hr stands for the antenna height.

Now that we are able to quantify the path losses, Equation 2.11 can be used to determine the power at the receiver.

PR= PT + GT + GT − LS− LP [dBm] (2.11)

where PT is the transmitter power in dBm, GT and GR the transmitter and receiver gains,

both in dBi, L_Sthe inherent system losses in dB (transmitter + receiver) and L_P, as previously stated, the path losses, also in dB.

The next step is to calculate the receiver’s sensitivity SR using Equation 2.12:

S_R= −101 + SN R_Rx+ 10 log µ F s · Nused NF F T ·Nsubchannels 16 ¶ [dB] (2.12) being FS the sampling frequency in MHz, Nused the number of used subcarriers, NF F T the

total number of subcarriers, and Nsubchannelsthe number of subchannels. The minimum SN R

for each modulation is defined in the IEEE 802.16e-2005 standard [8] and presented in Table 2.3.

Table 2.3: SN RRx for each modulation

Modulation SN RRx[dB] BPSK1/2 3.0 QPSK1/2 6.0 QPSK3/4 8.5 16-QAM1/2 11.5 16-QAM3/4 15.0 64-QAM2/3 19.0 64-QAM3/4 21.0

It’s clear that for a receiver to be able to detect a signal (2.13) must be true.

PR≥ SR (2.13)

So now that we can calculate both PR and SR, we can determine the maximum distance for

each modulation using (2.14), which results from the combination of (2.7), (2.12) and (2.13).

d = d0· 10   µ PT −LS+GR+GT −SR−Xf −Xh−s(2)−20 log µ 4πd0 λ ¶¶ 10γ   (2.14) Using this information, we can set the maximum reachable distance of each cell, thus providing the needed information to determine the coverage area.

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2.4 Related Work

In what concerns PMP base station planning, in [7], R´es presents a solution for base station planning, explaining all the necessary aspects to take into account when planning a PMP base station, including user prediction, propagation models and antenna physical parameters. Our work in the area of PMP planning will be based in its assumptions, so Section 2.4.1 will present a software developed in the scope of it.

Although mesh network planning is a topic that has not yet been much explored, there are a few documents on the area. In [9], Alicherry et al. present an algorithm for slot allocation in multi-radio, multi-channel Wireless Mesh Networks (WMN). Chandra et al. have formulated in [10] the Internet gateway placement in WMNs under three wireless models. They have developed an algorithm to minimize the amount of these devices, through optimal placement and have also taken into account robustness in WMNs, by including fault tolerance in their algorithm. In [11], Kodialam and Nandagopal have worked on capacity analysis in WMNs by considering the problem of determining the achievable rates in multi-hop wireless networks with orthogonal channels. [12] addresses the problem of trading range for capacity in WMNs while exploring the benefits of using relay nodes in networks. In [13], Pabst et al. propose two heuristic algorithms to solve the problem of Internet gateways placement in WMNs.

In [14], Amaldi, et al. propose a linear programming formulation of the problem of placing Internet access nodes in a network, which will be used in this work to account for the accuracy of a developed algorithm.

Most of the work in WMNs has been developed thinking on sensor networks. In [15], Bogdanov et al. analyze the problem of positioning data collector stations in terms of data rate and power efficiency for mesh sensors. In [16], Cheng et al. propose two algorithms to minimize the number of relay sensors in a mesh sensor network by improving their placement. With the same purpose, Khanna et al. propose in [17] a genetic algorithm.

Finally, in [18], Poduri et al. perform a very interesting work by evaluating the problem of representing 3-D scenarios in 2-D and prove that sometimes this generalization is not valid. Although all the cited works in the area of mesh networks are very useful and contributed to the development of the algorithms and mechanisms presented in this work, none of them presents a heuristic capable of being implemented in a planning software, independent of other calculation platforms. Moreover, all the works in the area consider that the whole scenario is defined a priori, while our solution is able to evaluate a completely defined scenario or to propose the placement of the access devices, given the traffic parameters for a well defined zone.

2.4.1 WiMAX Base Station Planning Tool

This section will give a brief explanation about the WiMAX Base Station Planning Tool, whose development was started in the scope of [7] and its conclusion is one of the objectives of this thesis.

This is a flexible and very user-friendly application which, after the definition of some parameters, calculates the best solution for a WiMAX base station.

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This tool contains two main windows: one for configuration and parameter setting and the other to present the results. Within the first window there are three panels, as shown by Figure 2.11.

The first panel, “Constants” (Figure 2.11(a)) allows the configuration of VoIP, data, IPTV, media stream, online gaming, P2P and Video Conference bandwidth requirements and also the type of terrain where the BS is to be placed: rural, urban or suburban. According to this selection, an appropriate propagation model will be selected. Default values for each field are suggested and presented in Table 2.4. In the second panel (Figure 2.11(b)) the user can define

Table 2.4: Constants Service Bandwidth VoIP 80Kbps Data - Residential 1000Kbps Data - Business 2000Kbps IPTV 2000Kbps Media Stream 20Kbps Online Gaming 85Kbps P2P 500Kbps Videoconference 385Kbps

the clients’ needs in terms of utilizations of the services considered during two periods: day and night. There are two types of users: residential and business, that can coexist, having different bandwidth needs. For each type of users it is also possible to define the density of users. Typical values, suggested by the application, are also presented in Tables 2.5 and 2.6.

Table 2.5: Typical usage values for the residential profile

Day Night

Service Nr. of utiliz. Dur. (min) Nr. of utiliz. Dur. (min)

VoIP 7 5 2 5 Data 6 40 4 20 IPTV 2 60 1 100 Media Stream 1 20 0 0 Online Gaming 1 60 0 0 P2P 1 30 0 0 Video conference 1 30 0 0

Table 2.6: Typical usage values for the business profile

Day Night

Service Nr. of utiliz. Dur. (min) Nr. of utiliz. Dur. (min)

VoIP 30 2 5 2

Data 20 10 0 0

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(a) Constants

(b) Profiles

(c) Technology

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The third panel (Figure 2.11(c)) contains technology parameters that must be config-ured according to the type of network being planned and the physical characteristics of the equipment.

When the application is executed, the boxes are filled with the presented typical values which can and shall be modified by the user in order to accurately define his scenario.

After all the specifications have been introduced, the program is able to perform the necessary calculations to present an optimized solution to the proposed problem. When these calculations end, the second window (Figure 2.12) appears showing the results. The blue circles around the base station represent the different modulations allowed by the IEEE 802.16 standard and the distances covered by each of them. There are still some parameters that the user can modify, in order to choose the configuration that best suits his problem, such as the channel width and the number of sectors of the antenna. Whenever there is a field whose text is red, as happens in the figure, it means the solution is not feasible and the user must change the parameters.

Figure 2.12: WiMAX Base Station Planning Tool: calculation result

2.5 Summary

After an overview of the evolution of the IEEE 802.16 standard and the different access methods it defines, especially the mesh mode, as well as the basics of propagation, modulation and the IEEE 802.16 MAC layer, we are now ready to explore some features of the standard. The introduced topics on propagation and modulation schemes will be applied to the development of the necessary algorithms to solve WiMAX planning problems.

The explanation about the Base Station Planning Tool will be useful for the comprehension of the next chapter, once some of the studied/developed topics will be implemented in that tool.

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Chapter 3

IEEE 802.16 Performance and

Enhancement Techniques

As said before, one of the purposes of this work was to finish the development of a WiMAX base station planning tool. Mobile WiMAX along with the enhancement techniques discussed here were some of the features that were not yet implemented, so we performed the necessary studies to include them in the application.

First, Section 3.1 presents a performance comparison between Fixed and Mobile WiMAX by analyzing the type of terrain and sectoring influence in each technology and also inspecting the trade between range and capacity. Section 3.2 gives an overview of the enhancement techniques along with the necessary calculations to be performed and, Section 3.3 shows some results and comparisons in terms of range and capacity and their relation with the type of terrain.

Finally, Section 3.4, presents the chapter conclusions.

3.1 Performance Comparison: Fixed and Mobile WiMAX

As stated before, there are two distinct access technologies within the IEEE 802.16 stan-dard: Fixed and Mobile WiMAX. These two technologies have already been described and distinguished in the last chapter, so our purpose now is to demonstrate its implementation in the base station planning tool and the practical differences in terms of coverage and capacity.

3.1.1 Primary comparison

For a primary comparison, we left all the values in the tool as the default suggests (Tables 2.4, 2.5 and 2.6) except for the density of users which we considered to be 2.5 usr/Km2_{. Only} the residential traffic profile has been activated and the channel is 20 MHz wide.

The results are shown in Figure 3.1 and Table 3.1. As we can see, for the same scenario, Mobile WiMAX performs better than Fixed WiMAX, mainly due to the SOFDMA technique.

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(a) Fixed Wimax BS (b) Mobile Wimax BS

Figure 3.1: Comparison between Fixed and Mobile WiMAX

Table 3.1: Results for Fixed and Mobile WiMAX Number of users Range (Km)

Fixed WiMAX 55 2.91

Mobile WiMAX 62 3.11

3.1.2 Type of Terrain influence in Fixed and Mobile WiMAX

As explained in Section 2.3.3, the SUI model is used to account for the propagation path losses in three terrain types. This section compares the influence of the type of terrain in Fixed and Mobile WiMAX. We have considered the maximum range for each technology and type of terrain with a fixed 3.5 MHz wide channel regardless of the density of users. Table 3.2 and Figure 3.2 show the comparison between Fixed and Mobile WiMAX.

Table 3.2: Results for the three types of terrain

Type of Terrain Range for Fixed (Km) Range for Mobile (Km)

Rural 7.81 11.23

Suburban 5.36 7.52

Urban 3.60 4.88

As we can see, once again Mobile WiMAX proves to perform better, but the greatest difference is indeed in the rural type of terrain, where the difference is of 3.42 Km, which is almost the range of Fixed WiMAX performing in urban scenarios.

3.1.3 Sectoring in Fixed and Mobile WiMAX

Sectoring is a powerful antenna design technique, which allows dividing one antenna into various sectors, each one operating at a different frequency thus maximizing the transmitting power while minimizing interference. The radiation patterns of a sectorized antenna beam

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1 2 3 0 2 4 6 8 10 12

Type of Terrain and Max. Range

Max. Range (Km)

Type of Terrain

Fixed WiMAX Mobile WiMAX

Rural Surburban Urban

Figure 3.2: Influence of the type of terrain in Fixed and Mobile WiMAX

Figure 3.3: Horizontal and vertical radiation patterns are shown in Figure 3.3.

We used the same scenario to account for the growth of capacity of a base station with the number of sectors in Fixed and Mobile WiMAX. The results are shown in Table 3.3 and Figure 3.4.

Table 3.3: Capacity growth with the number of sectors in Fixed and Mobile WiMAX Nr. Sectors Cap. Fixed (Mbps) Cap. Mobile (Mbps)

1 5.68 14.40 2 11.36 28.80 3 17.04 43.20 4 22.72 57.60 5 28.40 72.00 6 34.09 86.40

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1 2 3 4 5 6 0 10 20 30 40 50 60 70 80 90 Number of sectors Capacity (Mbps)

Number of sectors and Capacity Fixed WiMAX

Mobile WiMAX

Figure 3.4: Number of sectors and capacity for the same scenario

As we can see, the capacity grows linearly with the number of sectors, so, as each Mobile WiMAX sector has more capacity than each Fixed WiMAX sector, it is clear to see that in implementations where sectoring is possible, Mobile WiMAX has better performance.

3.1.4 Range vs. Capacity

We will now account for the trade between range and capacity of a base station in three types of terrain (Rural, Suburban and Urban) for Fixed and Mobile WiMAX. We have con-sidered a 5 MHz wide channel and 1 sector only antennas to calculate the range and capacity of the cells. All the parameters remain the same during the simulation, except for the tech-nology, which can be Fixed or Mobile and the type of terrain. Figures 3.5(a), 3.5(b) and 3.5(c) show the results.

Watching the results, we can say that, although the differences in range and capacity are obvious for the two variants of WiMAX as we had seen before, the relation of the trade between range and capacity for Fixed and Mobile WiMAX does not vary with the type of terrain considered.

3.2 Enhancement Techniques

The IEEE 802.16 standard defines some techniques to enhance the WiMAX transmission. This section will start by describing two possible implementations of MIMO in Section 3.2.1 followed by SIMO in Section 3.2.2. Section 3.2.3 presents the adaptive beamforming approach and finally Section 3.3.1 shows some results and comparisons concerning these methods.

3.2.1 MIMO

MIMO stands for Multiple Input Multiple Output, what, in wireless communications, means multiple transmitters and multiple receivers. MIMO can be used to achieve higher

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2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 6 8 10 12 14 16 18 20 22

Range vs Capacity for Rural scenarios

Range (Km)

Capacity (Mbps)

(a) Rural scenario

2 2.5 3 3.5 4 4.5 5 6 8 10 12 14 16 18 20 22

Range vs Capacity for Suburban scenarios

Range (Km) Capacity (Mbps) Fixed WiMAX Mobile WiMAX (b) Suburban scenario 1.5 2 2.5 3 8 10 12 14 16 18 20 22

Range vs Capacity for Urban scenarios

Range (Km)

Capacity (Mbps)

(c) Urban scenario

Figure 3.5: Range vs. Capacity for the different types of scenarios

(a) MIMO scheme (b) SIMO scheme

Figure 3.6: Multiple antenna techniques

data rates or to fight adverse channel conditions depending on the chosen method.

Spatial Diversity

MIMO can be used to improve the receiver sensitivity through the spatial diversity scheme. By using multiple transmitters sending the same information and multiple receivers, we can

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achieve a gain in sensitivity called diversity gain. If NT and NRare, respectively, the number

of antennas at the transmitter and at the receiver, the theoretical gain can be calculated by (3.1) [2].

SRgain= 10 ∗ log(NT × NR) [dB] (3.1)

Spatial Multiplex

On the other hand, if there is no need to improve the sensitivity on the receiver, MIMO can be used to achieve high data rates through spatial multiplexing. Spatial diversity consists on sending, at the same time, different information on each antenna. Using this scheme we can improve the capacity of the link. Theoretically, the capacity grows linearly with the minimum of NT and NR [2], as shown by (3.2).

C_gain = min(N_T, N_R) (3.2)

Figure 3.6(a) shows an example of MIMO.

3.2.2 SIMO

Single input multiple output (SIMO) is a particular case of MIMO, represented in Figure 3.6(b), where the transmitter has multiple antennas and the receiver only has one. Spatial diversity is the only application of SIMO once the receiver only has one antenna and thus cannot receive two different symbols at the same time. For a SIMO system the theoretical gain can be determined by (3.3).

S_Rgain= 10 ∗ log(N_T) [dB] (3.3)

3.2.3 Adaptive Beamforming

Adaptive beamforming consists on directing the antenna beam to the best position in order to assure the best channel conditions, range and power saving at the SS/MS side. In order to perform beamforming, the antenna must have at least two elements. As shown in Figure 3.7, using an appropriate algorithm and some extra hardware the antenna can focus its power in a certain point, thus providing better channel conditions and higher range there. As the transmit power is increased, the MT’s effort to receive the signal is less, needing less power to operate. Gains in the uplink and downlink are different, but significant in both, and can be calculated based on the number of elements of the antenna (N ) by (3.4) and (3.5).

U L_gain= 10 ∗ log(N ) [dB] (3.4)

DLgain = 20 ∗ log(N ) [dB] (3.5)

3.3 Enhancement Techniques comparison: Scenarios and

Re-sults

Now that we are able to distinguish the various WiMAX enhancement techniques, and they are introduced in the planning tool, some results are shown here.

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Figure 3.7: Adaptive beamforming

3.3.1 Primary Comparison

We will start by comparing the benefits of each enhancement technique, so all the traffic parameters will be left with their default values, indicated in Tables 2.4, 2.5 and 2.6. The considered channel size is 20 MHz, the antennas only have one sector and the implementation is Mobile WiMAX in a rural environment. The results are presented in Table 3.4.

Table 3.4: Results for the various enhancement techniques

Enhancement Nr of users Cell radius (Km)

– 46 3.46

MIMO 4x2 Spatial Diversity 82 4.61

MIMO 4x2 Spatial Multiplex 93+1 _3.46

Adaptive Beamforming 3 el. 90 4.83

SIMO 2x1 54 3.72

By looking at Figure 3.8 and Table 3.4 we can see that, while with MIMO spatial multiplex only the number of users is greatly increased, the other enhancement techniques allow the increasing of the number of users and the radius of the cell. This happens because MIMO spatial multiplex only increases the capacity of the link, without interfering with the sensitivity of the transmitter/receiver. It is good if we have good channel conditions, but in case we have bad channel conditions, improving the sensitivity may be preferable, by using one of the other techniques. We can conclude that, for this general scenario, Adaptive Beamforming is the most effective technique, but the implementation heavily depends on the place where we are settling the BS.

3.3.2 Range Improvement for Fixed and Mobile WiMAX in Rural

Scenar-ios

Now that we have an idea of the improvements that each enhancement technique brings, we will compare the efficiency, in terms of range, of SIMO and MIMO spatial diversity in

1_{This number could be higher, once the resources are not pushed to the limit, but we would need to modify}

the parameters of simulation, what would not be good for the comparison of the techniques, so we preferred to represent the maximum number of users for this conditions.

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3.4 3.6 3.8 4 4.2 4.4 4.6 4.8 5 45 50 55 60 65 70 75 80 85 90 95

Enhancement Techniques comparison

Cell radius (KM)

Number of users

MIMO 4x2 SM

No enhancement

MIMO 4x2 SD Adaptive Beamforming 3 el.

SIMO 2x1

Figure 3.8: Enhancement techniques comparison

Fixed and Mobile WiMAX. Adaptive beamforming has not been considered, once it is only available for Mobile WiMAX, thus we could not compare the results with Fixed WiMAX. We have considered a rural environment, because in this type of terrain the differences are more evident, and a channel width of 3.5 MHz. The results are displayed in Table 3.5 and Figure 3.9.

Table 3.5: Results for range improvement in Fixed and Mobile WiMAX

Fixed WiMAX (Km) Mobile WiMAX (Km)

SISO 7.81 11.23

SIMO 2x1 8.53 13.46

MIMO 4x2 S.D. 23.21 33.36

As we had already seen, from the three techniques tested, MIMO 4x2 spatial diversity is the most effective in range improvement. For Mobile WiMAX the achievements in terms of range proved to be even higher than for Fixed WiMAX, for every case tested, although the results are more evident in MIMO.

3.3.3 Range Improvement vs. Type of Terrain

It is important to take into account the type of terrain where we are settling a BS before choosing an enhancement technique to improve the performance. We will now compare the various techniques performance according to the type of terrain. For this to be possible we used the Mobile WiMAX technology, which proved to achieve higher improvements, as stated in the last section with a 3.5 MHz wide channel. The antennas have 3 sectors.

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1 2 3 0 5 10 15 20 25 30 35

Range Improvement in Rural Scenarios

Enhancement Technique

Maximum Range (Km)

SISO SIMO 2x1 MIMO 4x2 S.D

Figure 3.9: Comparison between the implementation of the enhancement techniques in Fixed and Mobile WiMAX 1 2 3 0 5 10 15 20 25 30 35 Type of Terrain Maximum Range (Km)

Range improvement vs Type of Terrain SISO SIMO 2x1 MIMO 4x2 S.D. Adapt. Beamforming 3 el.

Rural Suburban Urban

Figure 3.10: Range improvement of each technique for each type of terrain

As we can see in Figure 3.10, it is in the rural type of terrain that the achievements in terms of range are higher. If we take a look at Figure 3.11, we can conclude that, although the achievements due to the implementations of SIMO 2x1 do not depend on the type of terrain, for MIMO 4x2 spatial diversity and Adaptive Beamforming with 3 elements, there are variations with the type of terrain, especially in MIMO. Therefore, these results must be taken into account when choosing the technique to implement.

At a first look, these results may seem contradictory when compared to the ones presented in Figure 3.8: in that figure, Adaptive Beamforming has higher improvement in terms of range than MIMO 4x2 S.D.; however, it is important to remember that in this section we present the maximum absolute ratings for each technique in terms of range, regardless of the number

WiMax point-to-multipoint and mesh mode network planning

Andr´

e Amorim de

Faria Cardote

Planifica¸c˜

ao de Redes Wimax Ponto-Multiponto e

em Malha

Andr´

e Amorim de

Faria Cardote

WiMAX Point-to-Multipoint and Mesh mode

Network Planning

Contents

List of Figures

List of Tables

Acronyms

Chapter 1

Introduction

1.1

Motivation

1.2

Objectives

1.3

Contributions of the thesis

1.4

Outline

Chapter 2

Background

2.1

IEEE 802.16 Access Technologies

2.2

MAC Layer

2.3

PHY Layer

2.4

Related Work

2.5

Summary

Chapter 3

IEEE 802.16 Performance and

Enhancement Techniques

3.1

Performance Comparison: Fixed and Mobile WiMAX

3.2

Enhancement Techniques

3.3

Enhancement Techniques comparison: Scenarios and

Re-sults