INTERFERENCE REDUCTION IN FDD AND TDD COEXISTENCE ...wimax-smartantennas.weebly.com/uploads/5/0/3/9/...INTERFERENCE REDUCTION IN FDD AND TDD COEXISTENCE SCENARIOS IN WIMAX SYSTEMS

INTERFERENCE REDUCTION IN FDD AND TDD COEXISTENCE SCENARIOS IN

WIMAX SYSTEMS

By

Ubair Ahmed Salahria

Usman Ali

Usman Akram

Muhammad Bilal Amin

Project DS: Gp Capt ( R) Muzaffar Ali

Submitted to the Faculty of Electrical Engineering National University of Sciences and Technology, Rawalpindi in partial fulfillment for the requirements of a B.E Degree in Telecommunication

Engineering MARCH 2008

ABSTRACT

INTERFERENCE REDUCTION IN FDD AND TDD COEXISTENCE SCENARIOS IN WIMAX SYSTEMS

The demand for broadband services is growing exponentially. Traditional solutions

that provide high-speed broadband access use wired access technologies, such as

traditional cable, digital subscriber line, Ethernet, and fiber optic. WiMAX

(Worldwide Interoperability for Microwave Access) is based on the IEEE 802.16

standard for Metropolitan Area Networks (MAN). Its goal is to deliver wireless

broadband access to customers using base stations with coverage distances in the

order of miles. The frequencies allocated for WiMAX span the 2-66 GHz range. As

WiMAX and its standards are relatively new, they have some issues which need to be

resolved over time. One such problem is the interference received by different

operator using different duplexing methods, when working in close proximity to each

other on adjacent channels. The solution we implement is to use adaptive antennas, as

opposed to conventional antennas currently being used. Adaptive antennas provide an

efficient means for minimizing channel interferences by directing the antenna beam

towards the desired signal/transmitter and placing a null towards the interfering

signal. Adaptive Beamforming is a technique in which an array of antennas is

exploited to achieve maximum reception. This is achieved by varying the weights of

each of the sensors (antennas) used in the array. The physical layer of WiMAX

system has been implemented and then a performance evaluation has been carried out

between a system using conventional antennas and another one using conventional

antennas. A decision directed algorithm has been used for frequency domain

Beamforming at the receiver.

ii

DECLARATION

No portion of the work presented in this dissertation has been submitted in

support of any other award or qualification either at this institution or

elsewhere.

iii

DEDICATION

In the name of Allah, the Most Merciful, the Most Beneficent

To our families, without whose unflinching support and unstinting cooperation,

a work of this magnitude would not have been possible

iv

ACKNOWLEDGEMENTS

We are eternally grateful to Almighty Allah for bestowing us with the

strength and resolve to undertake and complete the project.

We gratefully recognize the supervision and motivation provided to us

by our Project Supervisor, Gp Capt. MUZAFFAR ALI. Our gratitude goes to

Mr. CARLOS BATLLES (AT4 Wireless, Sweden) who helped us in taking the

initial steps of the project and in the implementation of Physical Layer of

Wimax. He kept correspondence with us throughout the course of the project.

We are thankful to him for selflessly helping us. We are also grateful to Dr.

ALI KHAYAM (NIIT - NUST) for helping us in the initial stages of the project.

We are also grateful to Mr. MUHAMMAD FARYAD (Quaid-e-Azam University,

Islamabad ) who helped us in understanding smart antennas and provided us

with tutorials and related material.

We are deeply obliged to our families for their never ending patience

and support for our mental peace and to our parents for the strength that they

gave us through their prayers. We deeply treasure the unparallel support and

tolerance that we received from our colleagues, friends and the Faculty of

Electrical Engineering Department (MCS-NUST) for their useful suggestions

that helped us in the completion of this project.

A word of thanks to the MILITARY COLLEGE OF SIGNALS (MCS) as

it has been our foundation and has made us capable to undertake the project.

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We would also like to express our deepest gratitude to all the authors,

researchers, engineers and analysts whose work has enabled us to

accomplish this task, a special note of thanks to all those who helped us

personally with their expertise.

We could be forgetting some vital contributors but we appreciate and regard

the efforts of everybody who lend a helping hand to us in the project.

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TABLE OF CONTENTS

ABSTRACT ........................................................................................................ I

DECLARATION ................................................................................................ II

DEDICATION ................................................................................................... III

ACKNOWLEDGEMENTS ................................................................................ IV

LIST OF FIGURES ......................................................................................... VIII

LIST OF TABLES ............................................................................................ IX

CHAPTER 1 INTRODUCTION ......................................................................... 1

1.1 Introduction to Wimax ....................................................................................................................... 1 1.1.1 What is Broadband Wireless? ....................................................................................................... 1 1.1.2 What is Wimax? ............................................................................................................................ 1 1.1.3 Salient Features of WiMAX .......................................................................................................... 3 1.1.4 How WiMAX Works? ................................................................................................................... 7 1.1.5 Types of Wimax .......................................................................................................................... 10

1.2 Duplex Methods ................................................................................................................................. 12 1.2.1 Frequency Division Duplex (FDD) ............................................................................................. 13 1.2.2 Time Division Duplex (TDD) ..................................................................................................... 14

CHAPTER 2 COEXISTENCE SCENARIOS IN WIMAX ................................ 15

CHAPTER 3 WIMAX PHYSICAL LAYER ...................................................... 19

3.1 Channel Coding ................................................................................................................................. 22 3.1.1 Randomization ....................................................................................................................... 23 3.1.2 Forward Error Correction ...................................................................................................... 24

3.1.2.1 Reed-Solomon Coding ........................................................................................................ 26 3.1.2.2 Convolutional Coding ......................................................................................................... 30 3.1.1.4 Convolutional Encoding in 802.16d ................................................................................... 33 3.1.1.5 Interleaving ................................................................................................................... 35

3.2 Symbol Mapping ................................................................................................................................ 36 3.2.1 QPSK ........................................................................................................................................... 37 3.2.2 16-QAM ....................................................................................................................................... 38 3.2.3 64-QAM ....................................................................................................................................... 39

3.4 OFDM Symbol creation .................................................................................................................... 44 3.4.1 What is OFDM ............................................................................................................................ 44 3.4.2 OFDM symbol structure in WiMAX .......................................................................................... 46 3.4.3 OFDM Symbol parameters .................................................................................................... 48

CHAPTER 4 ADAPTIVE ANTENNAS & BEAMFORMING ........................... 49

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4.1 Antenna Arrays ................................................................................................................................. 49 4.1.1 Uniformly Spaced Linear Array .................................................................................................. 50

4.2 Adaptive Array Systems ................................................................................................................... 51 4.2.1 Basic Working Mechanism ......................................................................................................... 52

4.3 Adaptive Beamforming ..................................................................................................................... 52 4.3.1 Null steering Beamforming ......................................................................................................... 53 4.3.2 Frequency Domain Beamforming ............................................................................................... 53

4.4 Adaptive Algorithms for Beamforming .......................................................................................... 54 4.4.1 Decision Directed Adaptive algorithms ...................................................................................... 54

4.5 LEAST MEAN SQUARE ALGORITHM (LMS) ......................................................................... 54 4.5.1 Introduction ................................................................................................................................. 54 4.5.2 LMS Algorithm and Adaptive Arrays ........................................................................................ 55 4.5.3 LMS Algorithm Equations .......................................................................................................... 56 4.5.4 Convergence and Stability of the LMS algorithm ...................................................................... 56

CHAPTER 5 SIMULATION RESULTS & FUTURE ENHANCEMENTS ....... 57

5.1 Results with Adaptive Antennas ...................................................................................................... 57

5.2 Simulink Results ................................................................................................................................ 61

5.3 Future Enhancements ....................................................................................................................... 61

REFERENCE .................................................................................................. 63

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LIST OF FIGURES

Fig 1.1: WiMAX has the potential to impact all forms of telecommunications [3] ...... 3 Fig 1.2: WiMAX network. ............................................................................................ 8 Fig 1.3: WiMAX base station. ..................................................................................... 9 Fig 1.4: Indoor WiMAX CPEs. .................................................................................... 9 Fig 1.5: Fixed WiMAX offers cost effective point to point and point to multi-point solutions ....................................................................................................................... 11 Fig 1.6: Mobile WiMAX allows any telecommunications to go mobile ..................... 12 Fig 1.7: Downlink and Uplink ..................................................................................... 13 Fig 1.8: Frequency Division Duplex (FDD) - full duplex mode Downlink and uplink sub-frames are transmitted at the same time in two adjacent channels. ...................... 13 Fig 1.9: Time Division Duplex (TDD) Downlink and uplink sub-frames are transmitted at different time slots in one channel. ....................................................... 14 Fig 2.1: The Sources of adjacent channel interference for the various FDD/TDD coexistence scenarios [1]. ............................................................................................ 16 Fig 3.1: WiMAX physical layer................................................................................... 21 Fig 3.2: PRBS for Data Randomization ....................................................................... 23 Fig 3.3: OFDM randomizer DL initialization vector ................................................... 24 Fig 3.4: OFDM randomizer UL initialization vector ................................................... 24 Fig 3.5: Data block disturbed by 25-bit noise burst [7] ............................................... 27 Fig 3.6: Connection Diagram for convolutional encoder ............................................ 31 Fig 3.7: Next and Present state of encoder .................................................................. 32 Fig 3.8: State Diagram of convolutional encoder ........................................................ 32 Fig 3.9: Trellis Diagram of convolutional encoder ...................................................... 33 Fig 3.10: 802.16 d Convolution Encoder[12] .............................................................. 34 Fig 3.11: QPSK Constellation Diagram ....................................................................... 37 Fig 3.12: 16-QAM Constellation ................................................................................. 38 Fig 3.13: 64-QAM Constellation Diagram .................................................................. 40 Fig 3.14: PRBS ............................................................................................................ 43 Fig 3.15: Frequency domain representation of an OFDM symbol .............................. 46 Fig 3.16: Time domain representation of an OFDM symbol ....................................... 47 Fig 4.1: A uniformly spaced linear antenna array [2] .................................................. 50 Fig 4.2: Beam formation for adaptive array antenna system [5] ................................. 52 Fig 4.3: LMS adaptive beamforming network [5] ....................................................... 55 Fig 5.1: BER vs. SNR comparison of both conventional and smart systems. ............. 57 Fig 5.2: Array plot of adaptive antennas with desired user at 15® ............................. 58 Fig 5.3: Data generation and coding ............................................................................ 59 Fig 5.4: OFDM Symbol level processing at transmitter side....................................... 59 Fig 5.5: OFDM Symbol level processing at receiver side ........................................... 60 Fig 5.6: Bit-level processing at the receiver side ......................................................... 60 Fig 5.7: Simulink model Results .................................................................................. 61

ix

LIST OF TABLES

Table 3.1: Some Primitive polynomials. [8] ................................................................ 28 Table 3.2: RS parameters for different modulation schemes [9] ................................. 29 Table 3.3: Puncturing Pattern [13] ............................................................................... 34 Table 3.4: Symbol mapping for QPSK ........................................................................ 37 Table 3.5: Symbol mapping for 16-QAM .................................................................... 39 Table 3.6: Symbol mapping for 64-QAM .................................................................... 42 Table 3.7: Primitive Parameters for OFDM Symbol [6] ............................................. 48

Chapter 1 Introduction to Wimax

1

Chapter 1 Introduction

1.1 Introduction to Wimax

1.1.1 What is Broadband Wireless?

Broadband wireless sits at the confluence of two of the most remarkable growth

stories of the telecommunications industry in recent years. Both wireless and

broadband have on their own enjoyed rapid mass-market adoption. Wireless mobile

services grew from 11 million subscribers worldwide in 1990 to more than 2 billion in

2005 [1]. During the same period, the Internet grew from being a curious academic

tool to having about a billion users. This staggering growth of the Internet is driving

demand for higher-speed Internet-access services, leading to a parallel growth in

broadband adoption. In less than a decade, broadband subscription worldwide has

grown from virtually zero to over 200 million [2].

According to the 802.16-2004 standard, broadband means 'having instantaneous

bandwidth greater than around 1 MHz and supporting data rates greater than about 1.5

Mbit/s.

1.1.2 What is Wimax?

WiMAX or “Worldwide Interoperability for Microwave Access” is a standards-based

wireless technology based on IEEE 802.16-2004 (fixed wireless applications) and

802.16e-2005 (mobile wire-less) that provides high throughput broadband

connections over long distance. The industry trade group WiMAX Forum TM has

defined WiMAX as a "last mile" broadband wireless access (BWA) alternative to


2

cable modem service, Telephone Company Digital Subscriber Line (DSL) or T1/E1

service.

The IEEE wireless standard has a range of up to 30 miles, and can deliver broadband

at around 75 megabits per second. This is theoretically, 20 times faster than a

commercially available wireless broadband. The 802.16, WiMax standard was

published in March 2002 and provided updated information on the Metropolitan Area

Network (MAN) technology. The extension given in the March publication, extended

the line-of-sight fixed wireless MAN standard, focused solely on a spectrum from 10

GHz to 60+ GHz.

This extension provides for non-line of sight access in low frequency bands like 2 - 11

GHz. These bands are sometimes unlicensed. This also boosts the maximum distance

from 31 to 50 miles and supports PMP (point to multipoint) and mesh technologies.

WiMAX has the potential to replace a number of existing telecommunications

infrastructures. In a fixed wireless configuration it can replace the telephone

company's copper wire networks, the cable TV's coaxial cable infrastructure while

offering Internet Service Provider (ISP) services. In its mobile variant, WiMAX has

the potential to replace cellular networks [3].


3

Fig 1.1: WiMAX has the potential to impact all forms of telecommunications [3]

1.1.3 Salient Features of WiMAX

WiMAX is a wireless broadband solution that offers a rich set of features with a lot of

flexibility in terms of deployment options and potential service offerings. Some of the

more salient features that deserve highlighting are as follows [4]:

OFDM-based physical layer: The WiMAX physical layer (PHY) is based on

orthogonal frequency division multiplexing, a scheme that offers good resistance to

multi-path, and allows WiMAX to operate in NLOS conditions. OFDM is now widely

recognized as the method of choice for mitigating multi-path for broadband wireless.

Chapter 4 provides a detailed overview of OFDM.

Very high peak data rates: WiMAX is capable of supporting very high peak data

rates. In fact, the peak PHY data rate can be as high as 74Mbps when operating using

a 20MHz2 wide spectrum. More typically, using a 10MHz spectrum operating using

TDD scheme with a 3:1 downlink-to-uplink ratio, the peak PHY data rate is about


4

25Mbps and 6.7Mbps for the downlink and the uplink, respectively. These peak PHY

data rates are achieved when using 64 QAM modulation with rate 5/6 error-correction

coding. Under very good signal conditions, even higher peak rates may be achieved

using multiple antennas and spatial multiplexing.

Scalable bandwidth and data rate support: WiMAX has scalable physical-layer

architecture that allows for the data rate to scale easily with available channel

bandwidth. This scalability is supported in the OFDMA mode, where the FFT (fast

fourier transform) size may be scaled based on the available channel bandwidth. For

example, a WiMAX system may use 128-, 512-, or 1,048-bit FFTs based on whether

the channel bandwidth is 1.25MHz, 5MHz, or 10MHz, respectively. This scaling may

be done dynamically to support user roaming across different networks that may have

different bandwidth allocations.

Adaptive modulation and coding (AMC): WiMAX supports a number of

modulation and forward error correction (FEC) coding schemes and allows the

scheme to be changed on as per user and per frame basis, based on channel

conditions. AMC is an effective mechanism to maximize throughput in a time-varying

channel. The adaptation algorithm typically calls for the use of the highest modulation

and coding scheme that can be supported by the signal-to-noise and interference ratio

at the receiver such that each user is provided with the highest possible data rate that

can be supported in their respective links.

Link-layer retransmissions: For connections that require enhanced reliability,

WiMAX supports automatic retransmission requests (ARQ) at the link layer. ARQ-

enabled connections require each transmitted packet to be acknowledged by the

receiver; unacknowledged packets are assumed to be lost and are retransmitted.


5

WiMAX also optionally supports hybrid-ARQ, which is an effective hybrid between

FEC and ARQ.

Support for TDD and FDD: IEEE 802.16-2004 and IEEE 802.16e-2005 supports

both time division duplexing and frequency division duplexing, as well as a half-

duplex FDD, which allows for a low-cost system implementation. TDD is favored by

a majority of implementations because of its advantages: (1) flexibility in choosing

uplink-to-downlink data rate ratios, (2) ability to exploit channel reciprocity, (3)

ability to implement in non-paired spectrum, and (4) less complex transceiver design.

All the initial WiMAX profiles are based on TDD, except for two fixed WiMAX

profiles in 3.5GHz.

Orthogonal Frequency Division Multiple Access (OFDMA): Mobile WiMAX uses

OFDM as a multiple-access technique, whereby different users can be allocated

different subsets of the OFDM tones. OFDMA facilitates the exploitation of

frequency diversity and multi-user diversity to significantly improve the system

capacity.

Flexible and dynamic per user resource allocation: Both uplink and downlink

resource allocation are controlled by a scheduler in the base station. Capacity is

shared among multiple users on a demand basis, using a burst TDM scheme. When

using the OFDMA-PHY mode, multiplexing is additionally done in the frequency

dimension, by allocating different subsets of OFDM sub carriers to different users.

Resources may be allocated in the spatial domain as well when using the optional

advanced antenna systems (AAS). The standard allows for bandwidth resources to be

allocated in time, frequency, and space and has a flexible mechanism to convey the

resource allocation information on a frame-by-frame basis.


6

Support for advanced antenna techniques: The WiMAX solution has a number of

hooks built into the physical-layer design, which allows for the use of multiple-

antenna techniques, such as beamforming, space-time coding, and spatial

multiplexing. These schemes can be used to improve the overall system capacity and

spectral efficiency by deploying multiple antennas at the transmitter and/or the

receiver.

Quality-of-service support: The WiMAX MAC layer has a connection-oriented

architecture that is designed to support a variety of applications, including voice and

multimedia services. The system offers support for constant bit rate, variable bit rate,

real-time, and non-real-time traffic flows, in addition to best-effort data traffic.

WiMAX MAC is designed to support a large number of users, with multiple

connections per terminal, each with its own QoS requirement.

Robust security: WiMAX supports strong encryption, using Advanced Encryption

Standard (AES), and has a robust privacy and key-management protocol. The system

also offers very flexible authentication architecture based on Extensible

Authentication Protocol (EAP), which allows for a variety of user credentials,

including username / password, digital certificates, and smart cards.

Support for mobility: The mobile WiMAX variant of the system has mechanisms to

support secure seamless handovers for delay-tolerant full-mobility applications, such

as VoIP. The system also has built-in support for power-saving mechanisms that

extend the battery life of handheld subscriber devices. Physical-layer enhancements,

such as more frequent channel estimation, uplink sub-channelization, and power

control, are also specified in support of mobile applications.

IP-based architecture: The WiMAX Forum has defined reference network

architecture


7

that is based on an all-IP platform. All end-to-end services are delivered over IP

architecture relying on IP-based protocols for end-to-end transport, QoS, session

management, security, and mobility. Reliance on IP allows WiMAX to ride the

declining costcurves of IP processing, facilitate easy convergence with other

networks, and exploit the rich ecosystem for application development that exists for

IP.

1.1.4 How WiMAX Works?

Let us take a quick glance at the working of a basic WiMAX system. A WiMAX base

station is connected to public networks using optical fiber, cable, microwave link, or

any other high-speed point-to-point (P-P) connectivity, referred as a backhaul. In few

cases such as mesh networks, point-to-multi-point (P-MP) connectivity is also used as

a backhaul. Ideally, WiMAX should use point-to-point antennas as a backhaul to

connect aggregate subscriber sites to each other and to base stations across long

distances. A base station serves subscriber stations (also called customer premise

equipment [CPE] for obvious reasons) using non-line-of-sight (NLOS) or line-of-

sight (LOS) point-to-multi-point connectivity, and this connection is referred to as the

last mile. Ideally, WiMAX should use NLOS point-to-multi-point antennas to connect

residential or business subscribers to the base station. A subscriber station typically

serves a building (business or residence) using wired or wireless LAN [5].


8

Fig 1.2: WiMAX network.

WiMAX has been designed to address challenges associated with traditional wired

and wireless access deployments. Although the backhaul connects the system to the

core network, it is not an integrated part of WiMAX system as such. Typically, a

WiMAX system consists of two parts’ a WiMAX base station and a WiMAX receiver

(also referred as CPE).

WiMAX Base Station

A WiMAX base station consists of indoor electronics and a WiMAX tower.

Typically, a base station can cover up to 6 mi radius (theoretically, a base station can

cover up to 50 km radius or 30 mi, but practical considerations limit it to about 10 km

or 6 mi). Any wireless node within the coverage area would be able to access the

Internet. The WiMAX base stations would use the media access control layer defined

in the standard (a common interface that makes the networks interoperable) and

would allocate uplink and downlink bandwidth to subscribers according to their

needs, on an essentially real-time basis.


9

Fig 1.3: WiMAX base station.

WiMAX Receiver

A WiMAX receiver, which is also referred as CPE, may have a separate antenna (i.e.,

receiver electronics and antenna are separate modules) or could be a stand-alone box

or a PCMCIA card that sits in a laptop or computer. Access to a WiMAX base station

is similar to accessing a wireless access point (AP) in a Wi-Fi network, but the

coverage is more.

Fig 1.4: Indoor WiMAX CPEs.


10

So far one of the biggest deterrents to the widespread acceptance of broadband

wireless access (BWA) has been the cost of CPE. This is not only the cost of the CPE

itself, but also that of installation. Historically, proprietary BWA systems have been

predominantly LOS, requiring highly skilled labor and a truck role to install and “turn

up” a customer. The concept of a self-installed CPE has been the Holy Grail for BWA

from the beginning. With the advent of WiMAX, this issue seems to be getting

resolved.

Backhaul

Backhaul refers both to the connection from the AP back to the provider and to the

connection from the provider to the core network. A backhaul can deploy any

technology and media provided it connects the system to the backbone. In most of the

WiMAX deployment scenarios, it is also possible to connect several base stations

with one another by use of high-speed backhaul microwave links. This would also

allow for roaming by a WiMAX subscriber from one base station coverage area to

another, similar to roaming enabled by cellular phone companies.

1.1.5 Types of Wimax

Wimax is generally classified into two main types, fixed and mobile wimax [3].

Fixed Wimax

This is a phrase frequently used to refer to systems built using 802.16-2004

('802.16d') and the OFDM PHY as the air interface technology.

Fixed WiMAX deployments do not cater for handoff between Base Stations, therefore

the service provider cannot offer mobility.


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WiMAX provides fixed, portable or mobile non-line-of sight service from a base

station to a subscriber station, also known as customer premise equipment (CPE).

Some goals for WiMAX include a radius of service coverage of 6 miles from a

WiMAX base station for point-to-multipoint, non-line-of-sight (see following pages

for illustrations and definitions) service. This service should deliver approximately 40

megabits per second (Mbps) for fixed and portable access applications. That WiMAX

cell site should offer enough bandwidth to support hundreds of businesses with T1

speeds and thousands of residential customers with the equivalent of DSL services

from one base station.

Fig 1.5: Fixed WiMAX offers cost effective point to point and point to multi-point solutions

Mobile Wimax

A phrase frequently used to refer to systems built using 802.16e-2005 and the

OFDMA PHY as the air interface technology. "Mobile WiMAX" implementations

can be used to deliver both fixed and mobile services.

Mobile WiMAX takes the fixed wireless application a step further and enables cell

phone-like applications on a much larger scale. For example, mobile WiMAX enables

streaming video to be broadcast from a speeding police or other emergency vehicle at


12

over 70 MPH. It potentially replaces cell phones and mobile data offerings from cell

phone operators such as EvDo, EvDv and HSDPA. In addition to being the final leg

in a quadruple play, it offers superior building penetration and improved security

measures over fixed WiMAX. Mobile WiMAX will be very valuable for emerging

services such as mobile TV and gaming.

Fig 1.6: Mobile WiMAX allows any telecommunications to go mobile

1.2 Duplex Methods

Duplexing refers to the way downlink and uplink data is arranged in a two-way

wireless transmission. The downlink carries information from a Base Station (BS) to

Subscriber Stations (SSs). Downlink is also known as forward link. The uplink carries

information from a SS to a BS. It is also called reverse link. There are two types of

duplexing scheme [6],

1) Time Division Duplex (TDD).

2) Frequency Division Duplex (FDD).


13

Fig 1.7: Downlink and Uplink Downlink and uplink traffic in a 2-way communication.

1.2.1 Frequency Division Duplex (FDD)

FDD (Frequency Division Duplex) requires two distinct channels for transmitting

downlink sub-frame and uplink sub-frame at the same time slot. FDD is suitable for

bi-directional voice service since it occupies a symmetric downlink and uplink

channel pair. FDD is commonly used in cellular networks (2G and 3G). Meanwhile,

WiMAX supports full-duplex FDD and half-duplex FDD (HFDD or HD-FDD). The

difference is in full-duplex FDD a user device can transmit and receive

simultaneously, while in half-duplex FDD a user device can only transmit or receive.

Fig 1.8: Frequency Division Duplex (FDD) - full duplex mode Downlink and uplink sub-frames are transmitted at the same time in two adjacent channels.

FDD is inefficient for handling asymmetric data services since data traffic may only

occupy a small portion of a channel bandwidth at any given time. at any given

moment.


14

1.2.2 Time Division Duplex (TDD)

TDD (Time Division Duplex) is another duplexing scheme that requires only one

channel for transmitting downlink and uplink sub-frames at two distinct time slots.

TDD therefore has higher spectral efficiency than FDD. Moreover, using TDD

downlink to uplink (DL/UL) ratio can be adjusted dynamically. TDD can flexibly

handle both symmetric and asymmetric broadband traffic.

Fig 1.9: Time Division Duplex (TDD) Downlink and uplink sub-frames are transmitted at different time slots in one channel.

Most WiMAX implementations either on licensed or license-exempt bands will most

likely use TDD. The reasons are TDD uses half of FDD spectrum hence saving the

bandwidth, TDD system is less complex and thus cheaper, and WiMAX traffic will be

dominated by asymmetric data. The first release of Fixed WiMAX profiles support

both TDD and FDD, while Mobile WiMAX profiles only include TDD.

Chapter 2 Coexistence Scenarios in Wimax

15

Chapter 2 Coexistence scenarios in Wimax

With the onset of new broadband wireless technologies such as WiMAXTM

technology, technology neutral assignments are increasingly being considered (and

indeed required) to facilitate technology growth and deployment. Regulators across

the globe are recognizing the importance of technology neutrality. However, they are

faced with new questions regarding the ability of technologies with different

characteristics to coexist in shared frequency bands.

One of the main considerations to promote coexistence is to address the needs of

differing duplex methods, namely, time division duplex (TDD) or frequency division

duplex (FDD). Unchecked, operating systems with differing duplex methods in close

proximity to one another may cause unacceptable levels of inter-system interference,

when the base stations and terminals have very different characteristics.

To identify the coexistence scenarios that might be encountered if TDD and FDD

variants of WiMAX technology (and indeed any other wireless technology) are

deployed in adjacent frequency bands, refer to Fig 2.1 which shows the five adjacent

channel coexistence scenarios that would be possible if TDD and FDD variants of

WiMAX systems were deployed within a single frequency band. UL and DL

transmissions are identified by the green and blue arrows, respectively.

Fundamentally, interference problems may occur if equipment on one frequency is

trying to receive whilst nearby equipment on an adjacent frequency is transmitting. In

each scenario there are four paths to be considered, namely, BS-to-BS, BS-to-SS, SS-

to-BS and SS-to-SS. The potential interference paths are identified with the yellow,


16

orange and red arrows, where the colour represents the potential risk/severity of

interference related issues. Each scenario is discussed in greater depth in the

following sections [1].

Fig 2.1: The Sources of adjacent channel interference for the various FDD/TDD coexistence scenarios [1].

2.1 FDD-FDD

The first coexistence scenario is FDD-FDD. There will typically be two interference

‘zones’. These are between adjacent UL frequencies (as shown in Fig 2.1 (a)) and

between adjacent DL frequencies (as shown in Fig 2.1 (e)). Note that for the purposes

of this discussion we will assume that there is always a sufficient guard band between

UL and DL frequencies so that interaction between the two is negligible. This

assumption is typically valid in multi-licensee scenarios, in which the DL and UL

frequencies tend to grouped and ordered consistently.

As stated above, adjacent channel interference problems may occur if equipment on

one frequency is trying to receive whilst nearby equipment on an adjacent frequency


17

is transmitting. Therefore for the FDD-FDD coexistence scenario the primary

interference paths are SS-to-BS on the UL and BS-to-SS on the DL; BS-to-BS and

SS-to-SS interference will not generally be significant.

2.2 FDD-TDD

The second coexistence scenario is FDD-TDD. Again there are two interference

‘zones’, i.e. a TDD system operating in the band adjacent to the UL (as shown in Fig

2.1 (b)) and a TDD system operating in the band adjacent to the DL (as shown in Fig

2.1 (d)). The most obvious difference between this and the previous scenario is that

frequency discrimination cannot be relied upon to isolate the UL and DL. This

scenario includes the same interference paths found in the FDD-FDD scenario plus

potentially crippling BS-to-BS and SS-to-SS interference paths between the systems.

These paths are identified in Fig 2.1.

SS-to-SS problems are caused when one SS is transmitting in the close proximity of

another receiving in the adjacent channel. When the TDD system operates in a

channel adjacent to the FDD UL, the TDD SS suffers interference from the FDD SS,

but not necessarily vice versa, while if the TDD system operates in a channel adjacent

to the FDD DL, the FDD SS suffers interference from the TDD SS, but not

necessarily vice versa. In general, if the SSs are operated close enough to one another

there is nothing that can be done to mitigate this problem. However, we note that

affected SSs will generally be mobile so a) the problem will only continue whilst the

SSs are close together and b) the number of users affected by the problem is minimal.

Furthermore, the severity of the problem is a function of the transmit power of the SSs

and the level of cochannel interference received. Therefore we will not consider this

interference path further in this section.


18

BS-to-BS interference affects the FDD system on the UL and TDD systems adjacent

to the FDD DL band. Again this is caused when one BS transmits whilst the other

receives on the adjacent channel. Unlike the SS-to-SS case, BS-to-BS interference is

more deterministic (i.e. it will typically be a problem or it won’t), as BSs are active

continuously and they do not generally move. However, BS-to-BS interference

potentially affects all cell users and will typically be more serious than SS-to-SS

interference.

2.3 TDD-TDD

The final coexistence scenario is TDD-TDD, shown in Fig 2.1 (c). The TDD-TDD

scenario is very similar to that of the FDD-TDD. The interference in this scenario is

mainly due to different transmission and reception timing of the two operators and

can be catered for by synchronizing the reception and transmission of the the two

operators.

Chapter 3 WiMAX Physical Layer

19

Chapter 3 WiMAX Physical Layer The physical (PHY) layer of WiMAX is based on the IEEE 802.16-2004 and IEEE

802.16e-2005 standards and is based on the principles of Orthogonal frequency

division multiplexing (OFDM), which is a suitable modulation/access technique for

non–line-of-sight (LOS) conditions with high data rates.

The IEEE 802.16 suite of standards (IEEE 802.16-2004 / IEEE 802-16e-2005) defines

within its scope four PHY layers, any of which can be used with the media access

control (MAC) layer to develop a broadband wireless system. The PHY layers

defined in IEEE 802.16 are

Wireless MAN SC, a single-carrier PHY layer intended for frequencies beyond

11GHz which requires a Line of Sight (LOS) condition. This PHY layer is part of the

original 802.16 specifications.

Wireless MAN SCa, a single-carrier PHY for frequencies between 2GHz and 11GHz

for point-to-multipoint operations.

Wireless MAN OFDM, a 256-point FFT-based OFDM PHY layer for point-to-

multipoint operations in non-LOS conditions at frequencies between 2GHz and

11GHz. This PHY layer, finalized in the IEEE 802.16-2004 specifications, has been

accepted by WiMAX for fixed operations and is often referred to as Fixed WiMAX.

[1]


20

Wireless MAN OFDMA (Orthogonal Frequency Division Multiple Access), a

2,048-point FFT-based OFDMA PHY for point-to-multipoint operations in NLOS

conditions at frequencies between 2GHz and 11GHz. In the IEEE 802.16e-2005

specifications, this PHY layer has been modified to SOFDMA (scalable OFDMA),

where the FFT size is variable and can take any one of the following values: 128, 512,

1,024, and 2,048. The variable FFT size allows for optimum

operation/implementation of the system over a wide range of channel bandwidths and

radio conditions. This PHY layer has been accepted by WiMAX for mobile and

portable operations and is also referred to as Mobile WiMAX. [1]

The physical layer of WiMAX which has been implemented in this project is 256-

point FFT based ODFM PHY layer. Following figure shows various functional

stages of the physical layer [2].


21

Fig 3.1: WiMAX physical layer

The first set of the functional stages is related to processing of data at bit- level. It

includes Randomization, FEC Coding, interleaving and symbol mapping. The next set

of functional stages is related to the construction of the OFDM symbol in the

frequency domain. During this stage, data is mapped onto the appropriate subcarriers.

Pilot symbols are inserted into the pilot subcarriers, which allow the receiver to

estimate and track the channel state information. Finally, the conversion of the OFDM

symbol from the frequency domain to the time domain by taking the IFFT of the

symbol and cyclic prefix is added to cater for inter symbol interference. At the

receiver end, the reverse of the above mentioned processes is done to obtain the

digital data.


22

In this chapter, the above mentioned functional stages of the WiMAX PHY layer have

been explained. The implementation of Randomization, FEC coding, Interleaving,

symbol mapping and OFDM symbol level processing has been explained with the

help of figures and tables as specified in the IEEE 802.16-2004 standard.

3.1 Channel Coding

The idea of channel coding is to improve the capacity of a channel by adding some

carefully designed redundant information to the data being transmitted through the

channel. Digital modulation methods in communication and storage where the

communication media or storage media is viewed as a channel, the channel code is

used to protect data sent over it for storage or retrieval even in the presence of noise

(errors). By adding redundancy to the information symbol vector resulting in a longer

coded vector of symbols that are distinguishable at the output of the channel. These

additional bits will allow detection and correction of bit errors in the received data

stream and provide more reliable information transmission. The cost of using channel

coding to protect the information is a reduction in data rate or an expansion in

bandwidth.

In 802.16 channel coding is composed of three steps.

1) Randomization

2) Forward Error correction

a) Reed‐Solomon Coding

b) Convolution Coding

3) Interleaving

They are applied in the same order at transmission. The complementary operations are

applied in the reverse order at the reception.


23

3.1.1 Randomization

Randomization means manipulating a data stream before transmitting. The

manipulations are reversed by de-randomization at the receiving side. A randomizer

replaces sequences into other sequences without removing undesirable sequences, and

as a result it changes the probability of occurrence of vexatious sequences.

In WiMAX, Data randomization is performed on each burst of data on the uplink as

well as the downlink. The randomization is performed on each allocation (downlink

or uplink), which means that for each allocation of a data block (sub channels on the

frequency domain and OFDM symbols on the time domain) the randomizer is used

independently.

The shift register used for randomization is initialized for each new allocation. The

PRBS generator is 1 + X14 + X15 as shown in Fig.2. [3]. Each data byte to be

transmitted is entered sequentially into the randomizer, MSB first. Preambles are not

randomized. The seed value is used to calculate the randomization bits, which are

combined in XOR operation with the serialized bit stream of each burst. The

randomizer sequence is applied only to information bits.

Fig 3.2: PRBS for Data Randomization


24

The bits issued from the randomizer are applied to the encoder.

On the downlink, the randomizer is re-initialized at the start of each frame with the

sequence: 100101010000000. The randomizer is not reset for start of burst#1. at the

start of subsequent bursts, the randomizer should be initialized with the vector shown

in fig 3.2 [3]. The Frame number used for initialization refers to the frame in which

the downlink burst is transmitted.

Fig 3.3: OFDM randomizer DL initialization vector

On the Uplink, the randomizer is initialized with the vector shown in figure 3.3[3].

The Frame number used for initialization refers to the frame in which the Uplink burst

is transmitted.

Fig 3.4: OFDM randomizer UL initialization vector 3.1.2 Forward Error Correction

Forward error correction (FEC) is a system of error control for data transmission,

whereby the sender adds redundant data to its messages, also known as an error

correction code. This allows the receiver to detect and correct certain errors without


25

the need to ask the sender for additional data. The advantage of forward error

correction is that a back-channel is not required, or that retransmission of data can

often be avoided, at the cost of higher bandwidth requirements on average. FEC is

therefore applied in situations where retransmissions are relatively costly or

impossible.

FEC is accomplished by adding redundancy to the transmitted information using a

predetermined algorithm. Each redundant bit is invariably a complex function of

many original information bits. The original information may or may not appear in the

encoded output; codes that include the unmodified input in the output are systematic,

while those that do not are nonsystematic.

Types of FEC

The two main categories of FEC are:

a) Block coding

b) Convolutional coding.

Block codes work on fixed-size blocks (packets) of bits or symbols of

predetermined size.

Convolutional codes work on bit or symbol streams of arbitrary length.

In 802.16d Forward Error Correction (FEC) consists of a Reed-Solomon outer code

and a rate compatible convolution inner code. Both are supported on uplink and

downlink. The encoding is performed by first passing the data block format through

the RS encoder and then passing it through a zero-terminating convolutional encoder.


26

3.1.2.1 Reed-Solomon Coding

Reed Solomon codes are error correcting codes that are used to care for burst errors

that occur at the channel due to various unwanted reasons.

RS codes are systematic linear non-binary block codes. They block because the

original message is split into fixed length blocks and each block is split into m bit

symbols, linear because each m bit symbol is a valid symbol and systematic because

the transmitted information contains the original data with extra CRC or 'parity' bits

appended.

These codes are specified as RS (n, k), with m bit symbols. This means that the

encoder takes k data symbols of m bits each, appends n - k parity symbols, and

produces a code word of n symbols (each of m bits). The RS (n, k) codes are able to

correct burst errors in correcting any combination of “t” or fewer errors, where,

t= (n-k)/2

How R-S codes perform well against Burst Errors

Consider an (n, k) = (255, 247) R-S code, where each symbol is made up of m = 8 bits

(such symbols are typically referred to as bytes). Since n - k = 8, above indicates that

this code can correct any four symbol errors in a block of 255. Imagine the presence

of a noise burst, lasting for 25-bit durations and disturbing one block of data during

transmission, as illustrated in Figure.


27

Fig 3.5: Data block disturbed by 25-bit noise burst [7]

In this example, notice that a burst of noise that lasts for a duration of 25 contiguous

bits must disturb exactly four symbols. The R-S decoder for the (255, 247) code will

correct any four-symbol errors without regard to the type of damage suffered by the

symbol. In other words, when a decoder corrects a byte, it replaces the incorrect byte

with the correct one, whether the error was caused by one bit being corrupted or all

eight bits being corrupted. Thus if a symbol is wrong, it might as well be wrong in all

of its bit positions. This gives an R-S code a tremendous burst-noise advantage over

binary codes, even allowing for the interleaving of binary codes. In this example, if

the 25-bit noise disturbance had occurred in a random fashion rather than as a

contiguous burst, it should be clear that many more than four symbols would be

affected (as many as 25 symbols might be disturbed). Of course, that would be

beyond the capability of the (255, 247) code. [7].

Encoder

Reed Solomon codes are based on a specialized area of mathematics known as Galois

fields (finite fields). For any prime number, p, there exists a finite field denoted GF

(p) that contains p elements. It is possible to extend GF (p) to a field of pm elements,

called an extension field of GF (p), and denoted by GF (pm), where m is a nonzero

positive integer. Note that GF (pm) contains as a subset the elements of GF (p).


28

Symbols from the extension field GF (2m) are used in the construction of Reed-

Solomon (R-S) codes. [8]

A class of polynomials called primitive polynomials is used to define the finite fields

GF (2m) that in turn are needed to define R-S codes. The following condition is

necessary and sufficient to guarantee that a polynomial is primitive. An irreducible

polynomial f(X ) of degree m is said to be primitive if the smallest positive integer n

for which f(X ) divides X n + 1 is n = 2m - 1.

Some primitive polynomials are given as under:

Table 3.1: Some Primitive polynomials. [8]

The Reed-Solomon encoder reads in k data symbols, computes the n - k parity

symbols, and appends the parity symbols to the k data symbols for a total of n

symbols. The encoder is essentially a 2t tap shift register where each register is m bits

wide.

The degree of the generator polynomial is equal to the number of parity symbols. The

multiplier coefficients are the coefficients of the RS generator polynomial. The

general idea is the construction of a polynomial the coefficients produced will be


29

symbols such that the generator polynomial will exactly divide the data/parity

polynomial.

RS encoder in 802.16d

The Reed-Solomon encoding in 802.16d is derived from a systematic RS

(n=255,k=239,t=8) using GF(2 8) .

Following polynomials are used for the systematic code:

Different Reed-Solomon parameters for various modulation types are specified in the

standard and are shown as under:

Table 3.2: RS parameters for different modulation schemes [9] Decoder

The Reed-Solomon decoder tries to correct errors and/or erasures by calculating the

syndromes for each codeword. Based upon the syndromes the decoder is able to

determine the number of errors in the received block. If there are errors present, the


30

decoder tries to find the locations of the errors using the Berlekamp-Massey algorithm

by creating an error locator polynomial. The roots of this polynomial are found using

the Chien search algorithm. Using Forney's algorithm, the symbol error values are

found and corrected. For an RS (n, k) code where n - k = 2T, the decoder can correct

up to T symbol errors in the code word. Given that errors may only be corrected in

units of single symbols (typically 8 data bits), Reed-Solomon coders work best for

correcting burst errors. [10]

3.1.2.2 Convolutional Coding

Convolution Coding is deployed in digital communication in order to care for bit

errors that occur due to the effects of the channel.

A convolutional code is a type of error-correcting code in which each n-bit

information symbol to be encoded is transformed into an k-bit symbol, where n/k is

the code rate (k ≥ n) and the transformation is a function of the last “K” information

symbols, where “K” is the constraint length of the code.

Convolution coding is specified by three parameters (n, k, K)

Where,

n= number of input bits

k= number of output bits

K = number of stages in the encoding shift register

To convolutionally encode data, start with K memory registers, each holding 1 input

bit. Unless otherwise specified, all memory registers start with a value of 0. The

encoder has “k” modulo-2 adders, and “k” generator polynomials.

Connection Diagram


31

To understand the main concept of convolution encoding, consider the following

connection diagram.

Fig 3.6: Connection Diagram for convolutional encoder In this connection diagram, each input bit is being transformed into 2 bits depending

upon the operations being performed at the bits stored in the encoding shift register.

The functions performed over the bits in the register depend upon the generator

polynomials of the encoder. A generator polynomial tells about the bits over which

modulo-2 operation is to be performed to get the corresponding output.

Two outputs at n1 and n2 are used to encode a particular bit sequence. Here [n1n2]

will be the output. When the next bit comes into the register at the left most, a new

output will be shown there at n1 and n2 and this will go on until the end of the

sequence.

So by this the output of the encoder is made dependent not only on the current input

bits but also on the previous entries in the register. And by using this dependency at

the receiver end error can be caught and thus corrected.


32

State Diagram

State diagram represents the outputs when a input is applied into the register. For this

we have two states

Present state

Next state

Both states are shown in the following diagram

Fig 3.7: Next and Present state of encoder

Depending upon the present and next states we construct the state diagram of the

coding scheme as shown below. Here when shown 1/10 means that output 10 when

input bit 1 is entered to the register having memory of previous bits.

In this case we start from the register entries as 000.

Fig 3.8: State Diagram of convolutional encoder


33

Trellis Diagram

In the trellis diagram the x-axis is discrete and all possible states are shown on the y-

axis. We move horizontally through the trellis with the passage of time. Each

transition means new bits have arrived. The trellis diagram is drawn by lining up all

the possible states (2K) in the vertical axis. Then we connect each state to the next

state by the allowable codewords for the state. There are only two possible choices at

each state. These are determined by the arrival of either 1 or 0 bit. The trellis diagram

for a (1, 2, 3) code is shown as under:

Fig 3.9: Trellis Diagram of convolutional encoder

3.1.1.4 Convolutional Encoding in 802.16d

The Convolutional encoder uses a constituent encoder with a constraint length 7 and a

code rate ½.

The connection diagram for convolutional encoder used in 802.16d is show as under:


34

Fig 3.10: 802.16 d Convolution Encoder[12]

The output of the data randomizer is encoded using this constituent encoder. In order

to initialize the encoder to the 0 state, each FEC block is padded with a byte of 0x00

at the end in the OFDM mode.

Puncturing

In 802.16 d the convolutional encoder has a native code rate of ½. In order to achieve

code rates higher than ½ the output of the encoder is punctured using the puncturing

pattern mentioned in the standard. The puncturing pattern is shown here as

Table 3.3: Puncturing Pattern [13]

Here R is the overall rate that you want to achieve after encoding and puncturing.


35

3.1.1.5 Interleaving

Interleaving is a way to arrange data in a non-contiguous way in order to increase

performance. Interleaving is used in digital data transmission to protect the

transmission against burst errors. These errors overwrite a lot of bits in a row, so a

typical error correction scheme that expects errors to be more uniformly distributed

can be overwhelmed. Interleaving is used to help stop this from happening.

Data is often transmitted with error control bits that enable the receiver to correct a

certain number of errors that occur during transmission. If a burst error occurs, too

many errors can be made in one code word, and that codeword cannot be correctly

decoded. To reduce the effect of such burst errors, the bits of a number of code words

are interleaved before being transmitted. This way, a burst error affects only a

correctable number of bits in each codeword, and the decoder can decode the code

words correctly.

This method is popular because it is a less complex and cheaper way to handle burst

errors than directly increasing the power of the error correction scheme.

In WiMAX, all encoded bits are interleaved by a block interleaver with a block size

corresponding to the no. of coded bits per OFDM symbol, Ncbps. The interleaver is

defined by a two step permutation. The first step ensures that the adjacent coded bits

are mapped onto nonadjacent subcarriers. The second permutation ensures that

adjacent bits are alternately mapped to less and more significant bits of the

modulation constellation, thus avoiding long runs of lowly reliable bits [3].

Let Ncpc be the no. of coded bits per subcarrier i.e. 1, 2, 4 or 6 for BPSK, 16-QAM, or

64-QAM respectively. Let s= ceil (Ncpc /2). Within a block of Ncbps bits at

transmission, let k be the index of the coded bit before the first permutation; mk be


36

the index of the coded bit after the first and before the second permutation; and let jk

be the index after the second permutation, just prior to the symbol mapping.

The first permutation is defined by the equation [3]

mk = (Ncbps /12).kmod 12 + floor (k/12) k=0, 1, 2,….,

Ncbps-1

The second permutation is defined by the equation [3]

jk = floor (mk /s) + (mk + Ncbps - floor (12. mk / Ncbps )) mod (s) k=0, 1, 2,….,

Ncbps-1

The de-interleaver, which performs the inverse operation, is also defined by two

permutations. Within a block of Ncbps bits at reception, let j be the index of the coded

bit before the first permutation; mj be the index of the coded bit after the first and

before the second permutation; and let kj be the index after the second permutation,

just prior to delivering the block to decoder.

The first permutation is defined by the equation [3]

mj = s. floor (j/s) + (j + floor (12.j / Ncbps )) mod(s) j=0, 1, 2,…., Ncbps-1

The second permutation is defined by the equation [3]

kj = 12. mj – (Ncbps -1).floor (12. mj / Ncbps) j=0, 1, 2,…., Ncbps-1

The first permutation in the de-interleaver is the inverse of the second permutation in

the interleaver, and vice versa.

3.2 Symbol Mapping

During the symbol mapping stage, the sequence of binary bits is converted to a

sequence of complex valued symbols. The mandatory constellations are QPSK and

16 QAM, with an optional 64 QAM constellation also defined in the standard.


37

Although the 64 QAM is optional, most WiMAX systems will likely implement it, at

least for the downlink.

Each modulation constellation is scaled by a number c, such that the average

transmitted power is unity, assuming that all symbols are equally likely. The value of

c is 1/sqrt(2) , 1/sqrt(10) and 1/sqrt(42) for the QPSK, 16 QAM, and 64 QAM

modulations respectively.

The number of bits per symbol depends upon the modulation type. For QPSK, n=2;

for 16-QAM, n=4 and for 64-QAM, n=6.

3.2.1 QPSK

The bits to symbol mapping and constellation diagram for QPSK is shown as

under.[13]

Fig 3.11: QPSK Constellation Diagram

Table 3.4: Symbol mapping for QPSK


38

3.2.2 16-QAM

The bits to symbol mapping and constellation diagram for 16-QAM is shown as

under.[13]

Fig 3.12: 16-QAM Constellation


39

Table 3.5: Symbol mapping for 16-QAM

3.2.3 64-QAM

The bits to symbol mapping and constellation diagram for 64-QAM is shown as

under.[13]


40

Fig 3.13: 64-QAM Constellation Diagram


41


42

Table 3.6: Symbol mapping for 64-QAM


43

3.3 Pilot modulation

Pilot subcarriers shall be inserted into each data burst in order to constitute the symbol

and they shall be modulated according to their carrier locations within the OFDM

symbol. The PRBS depicted in fig. 3.14 [3] is used to produce a sequence wk . The

polynomial used is X11 + X9 +1.

Fig 3.14: PRBS

The value of the pilot modulation is derived from wk. On the downlink, the index k

represents the symbol index relative to beginning of the downlink sub frame. On the

uplink, the index k represents the symbol index relative to beginning of the burst. For

each pilot, (indicated by the frequency offset index), the BPSK modulation shall be

derived as shown in the following equations.[3]

DL: c-88 = c-38 = c63 =c 88 = 1-2 wk and c-63 = c-13 = c13 =c 38 = 1-2 wk (inverse)

UL: c-88 = c-38 = c13 =c 38 = c63 =c 88 = 1-2 wk and c-63 = c-13 = 1-2 wk (inverse)


44

3.4 OFDM Symbol creation

3.4.1 What is OFDM

Orthogonal frequency division multiplexing (OFDM) is a multicarrier modulation

technique that has recently found wide adoption in a widespread variety of high-data-

rate communication systems, including digital subscriber lines, wireless LANs

(802.11a/g/n), digital video broadcasting, and now WiMAX and other emerging

wireless broadband systems such as the proprietary Flash-OFDM developed by

Flarion (now QUALCOMM), and 3G LTE and fourth generation cellular systems.

OFDM’s popularity for high-data-rate applications stems primarily from its efficient

and flexible management of inter symbol interference (ISI) in highly dispersive

channels.[4]

In an OFDM system, a high-data-rate sequence of symbols is split into multiple

parallel low-data rate-sequences, each of which is used to modulate an orthogonal

tone, or subcarrier. The transmitted base band signal, which is an ensemble of the

signals in all the subcarriers, can be represented as [4]

where s[i] is the symbol carried on the ith subcarrier; Bc is the frequency separation

between two adjacent subcarriers, also referred to as the subcarrier bandwidth; ∆f is

the frequency of the first subcarrier; and T' is the total useful symbol duration

(without the cyclic prefix). At the receiver, the symbol sent on a specific subcarrier is

retrieved by integrating the received signal with a complex conjugate of the tone

signal over the entire symbol duration T'. If the time and the frequency


45

synchronization between the receiver and the transmitter is perfect, the orthogonality

between the subcarriers is preserved at the receiver. When the time and/or frequency

synchronization between the transmitter and the receiver is not perfect, the

orthogonality between the subcarriers is lost, resulting in intercarrier interference

(ICI). Timing mismatch can occur due to misalignment of the clocks at the

transmitter and the receiver and propagation delay of the channel. Frequency

mismatch can occur owing to relative drift between the oscillators at the transmitter

and the receiver and nonlinear channel effects, such as Doppler shift. The flexibility

of the WiMAX PHY layer allows one to make an optimum choice of various PHY

layer parameters, such as cyclic prefix length, number of subcarriers, subcarrier

separation, and preamble interval, such that the performance degradation owing to ICI

and ISI (intersymbol interference) is minimal without compromising the performance.

The concept of independently modulating multiple orthogonal frequency tones with

narrowband symbol streams is equivalent to first constructing the entire OFDM signal

in the frequency domain and then using an Inverse Fast Fourier transform (IFFT) to

convert the signal into the time domain. The IFFT method is easier to implement, as it

does not require multiple oscillators to transmit and receive the OFDM signal. In the

frequency domain, each OFDM symbol is created by mapping the sequence of

symbols on the subcarriers. WiMAX has three classes of subcarriers.

1. Data subcarriers are used for carrying data symbols.

2. Pilot subcarriers are used for carrying pilot symbols. The pilot symbols

are known a priori and can be used for channel estimation and channel

tracking.


46

3. Null subcarriers have no power allocated to them, including the DC

subcarrier and the guard subcarriers toward the edge. The DC subcarrier is not

modulated, to prevent any saturation effects or excess power draw at the

amplifier. No power is allocated to the guard subcarrier toward the edge of the

spectrum in order to fit the spectrum, of the OFDM symbol within the

allocated bandwidth and thus reduce the interference between adjacent

channels.

3.4.2 OFDM symbol structure in WiMAX

Frequency domain representation

Figure 3.15 shows a typical frequency domain representation of an OFDM symbol

containing the data subcarriers, pilot subcarriers, and null subcarriers. The power in

the pilot subcarriers, as shown here, is boosted by 3 dB, allowing reliable channel

tracking even at low-SNR conditions. [4]

Fig 3.15: Frequency domain representation of an OFDM symbol Time domain representation

By taking the Inverse- Fast-Fourier transform of the frequency domain symbol, an

OFDM waveform in time domain is obtained. This time duration is referred to as the


47

useful symbol time Tb . A copy of the last Tg of the useful symbol period, termed

Cyclic Prefix (CP) is added to the time domain signal to cater for multipath effects,

while maintaining the orthogonality of the tones. Fig.7 illustrates this structure. [5]

Fig 3.16: Time domain representation of an OFDM symbol

The transmitter energy increases with the length of the cyclic prefix, while the

receiver energy remains the same (cyclic prefix is discarded at the receiver ), so there

is a 10log (1-Tg /(Tb +Tg ))/log(10) dB loss in Eb /No. The CP overhead fraction and

resultant SNR loss could be reduced by increasing the FFT size, which would,

however, among other things, adversely affect the sensitivity of the system to phase

noise of the oscillators. Using the cyclic prefix, the samples required for performing

the FFT at the receiver can be taken anywhere over the length of the extended symbol.

This provides multipath immunity as well as tolerance for symbol time

synchronization errors.


48

3.4.3 OFDM Symbol parameters

Primitive parameters

Table 3.7: Primitive Parameters for OFDM Symbol [6]

Derived parameters

The following parameters are defined in terms o the above mentioned primitive

parameters.

• NFFT : Smallest power of 2 greater than no. of data subcarriers

• Sampling frequency : Fs= floor(n . BW/8000) x 8000.

• Subcarrier spacing: ∆f= Fs / NFFT

• Useful Symbol time: Tb = 1/ ∆f

• CP time: Tg = G. Tb

• OFDM Symbol time: Ts =Tb + Tg

• Sampling time: Tb / NFFT

Chapter 4 Adaptive Antennas & Beam Forming

49

Chapter 4 Adaptive Antennas & Beamforming Adaptive antenna systems are one of the most promising technologies that will enable

a higher capacity in wireless networks by effectively reducing multipath and inter-

channel interference [1]. This is achieved by focusing the radiation only in the desired

direction and adjusting itself to changing traffic conditions or signal environments.

The objective of beamforming is to separate the desired signal from interfering

signals, given that they have the same frequencies but different spatial locations.

Interfering signals can be the delayed version of the desired signal originated from the

multipath environment or signals generated by other users. A digital beam former

samples the propagated wave field at the input of each antenna element, weights them

based on a certain performance criterion and then combines them at the output of the

beamformer[2].

4.1 Antenna Arrays

An antenna Array is a configuration of individual radiating elements that are arranged

in space and can be used to produce a directional radiation pattern. Single-element

antennas have radiation patterns that are broad and hence have a low directivity that is

not suitable for long distance communications. A high directivity can still be achieved

with single-element antennas by increasing the electrical dimensions (in terms of

wavelength) and hence the physical size of the antenna. When increasing the

electrical size of an antenna is not possible, high directivity can be achieved by using

multiple antenna elements that are spatially arranged in a geometric configuration and


50

electrically connected [3]. These configurations are called antenna arrays and are

available as linear array, circular array, and planar array.

4.1.1 Uniformly Spaced Linear Array

Figure 3.1 shows a uniformly spaced linear array with K identical isotropic elements,

with the rightmost element as reference element.

Fig 4.1: A uniformly spaced linear antenna array [2]

A single source is assumed to be transmitting from a far distance so that the received

signal at the antenna array is considered to be a plane wave incident at an angle q with

respect to the array normal. According to Figure 3.1, the plane wave first reaches the

first antenna element, which is the reference element, and propagates all the way to

the K-th antenna element.

The received signal at the first antenna element can be expressed as

}]2exp{)(Re[)( 11

~tfjtxtx cπ= (3.1)

where x1(t) is the complex envelope representation of the received signal and fc is the

carrier frequency. The propagation delay from the first to second antenna element is

c

d θτ sin= (3.2)

d sin θ

Incident Plane wave

d d

θ

3 2 1k

Reference element

Array normal Plane wave


51

where c is the speed of the light. Therefore, the received signal at the second antenna

element with respect to the received signal at the first antenna can be expressed as

}])2exp{)(Re[( 12

~tfjtxtx cπτ −−= (3.3)

If the carrier frequency is large compared with the bandwidth of the signal, the

narrowband signal model can be applied here, in which a small time delay can be

modeled as a simple phase shift. In this case, equation 2.3 can be rewritten as

)}](2exp{Re[)( 12 τπ −−= tfjxtx c (3.4)

The received signal at the kth element can then be expressed in terms of vector

notation as:

[ ]Tk txtxtxtx )(),...,(),()( 21= (3.5)

TdKjdj

eea ⎥⎦

⎤⎢⎣

⎡=

−−− θλπ

θλπ

θsin)1(2sin2

,...,,1)( (3.6)

where a(Ө) is known as the array response vector or the steering vector.

4.2 Adaptive Array Systems

The early adaptive antenna systems were designed for use in military applications to

suppress interfering or jamming signals from the enemy [4]. Since interference

suppression was a feature in this system, this technology was borrowed to apply to

personal wireless communications where interference was limiting the number of

users that a given network could handle.

Great performance improvements can be achieved by implementing advanced signal

processing techniques to process the information obtained by the antenna arrays. The

adaptive array systems are smart because they are able to dynamically react to the

changing RF environment.

An adaptive array, is controlled by signal processing. This signal processing steers the

radiation beam towards a desired mobile user, follows the user as he moves, and at the


52

same time minimizes interference arising from other users by introducing nulls in

their directions. This is illustrated in a simple diagram shown below in figure 3.1

Fig 4.2: Beam formation for adaptive array antenna system [5]

4.2.1 Basic Working Mechanism

An adaptive antenna system can perform the following functions: Firstly, the

direction of arrival of all the incoming signals including the interfering signals and the

multipath signals are estimated. Secondly, the desired user signal is identified and

separated from the rest of the unwanted incoming signals. Lastly a beam is steered in

the direction of the desired signal and the user is tracked as he moves while placing

nulls at interfering signal directions by constantly updating the complex weights.

The direction of radiation of the main beam in an array depends upon the phase

difference between the elements of the array. Therefore it is possible to continuously

steer the main beam in any direction by adjusting the progressive phase difference

between the elements. The same concept forms the basis in adaptive array systems in

which the phase is adjusted to achieve maximum radiation in the desired direction.

4.3 Adaptive Beamforming Adaptive Beamforming is a technique in which an array of antennas is exploited to

achieve maximum reception in a specified direction by estimating the signal arrival


53

from a desired direction (in the presence of noise) while signals of the same frequency

from other directions are rejected. This is achieved by varying the weights of each of

the sensors (antennas) used in the array. It basically uses the idea that, though the

signals emanating from different transmitters occupy the same frequency channel,

they still arrive from different directions. This spatial separation is exploited to

separate the desired signal from the interfering signals[5].

Beamforming is generally accomplished by phasing the feed to each element of an

array so that signals received or transmitted from all elements will be in phase in a

particular direction. The phases (the inter-element phase) and usually amplitudes are

adjusted to optimize the received signal.

4.3.1 Null steering Beamforming

Null steering algorithms are a type of Beamforming algorithms that do not look for

the signal presence and then enhance it, instead they examine where nulls are located

or the desired signal is not present and minimize the output signal power. One

technique based on this approach is to minimize the mean squared value of the output.

4.3.2 Frequency Domain Beamforming

A frequency-domain beamformer first accumulates the sampled signal at each antenna

element to form N-point data blocks. It then takes an N-point Discrete Fourier

Transform of the data at each antenna element using the Fast Fourier Transform

(FFT) algorithm and performs beamforming on individual frequency components to

accommodate different phase shifts experienced by each of them. At the output of the

beamformer, an N-point Inverse Discrete Fourier Transform is performed to convert

the data back to the time domain.


54

4.4 Adaptive Algorithms for Beamforming

In order to obtain the optimum weight vector, one needs to know different statistics.

Since these statistics are usually unknown and change over time, adaptive

beamforming algorithms are employed to estimate them and update the weight vector

over time. As the weights are iteratively adjusted, the performance of beamformer is

closer to the desired criterion. The algorithm is said to be converged when such a

performance criterion is met.

4.4.1 Decision Directed Adaptive algorithms

These types of algorithms are based on minimization of the mean square error

between the received signal and the reference signal. Therefore it is required that a

reference signal be available which has high correlation with the desired signal. The

reference signal is not the actual desired signal, in fact it is a signal that closely

represents it or has strong correlation with it [6].

Examples: The Least Mean Square (LMS) algorithm and the Recursive Least Square

(RLS) algorithm

4.5 LEAST MEAN SQUARE ALGORITHM (LMS) 4.5.1 Introduction

The Least Mean Square (LMS) algorithm, introduced by Widrow and Hoff in 1959

[7] is an adaptive algorithm, which uses a gradient-based method of steepest decent

[8]. LMS algorithm uses the estimates of the gradient vector from the available data.


55

LMS incorporates an iterative procedure that makes successive corrections to the

weight vector in the direction of the negative of the gradient vector which eventually

leads to the minimum mean square error.

4.5.2 LMS Algorithm and Adaptive Arrays

Consider a Uniform Linear Array (ULA) with N isotropic elements, which forms the

integral part of the adaptive beamforming system as shown in the figure below. The

output of the antenna array is given by,

)()()()()()(1

0 tnatuatstx i

N

ii

u

++= ∑=

θθ (3.7)

Fig 4.3: LMS adaptive beamforming network [5]

As shown above the outputs of the individual sensors are linearly combined after

being scaled using corresponding weights such that the antenna array pattern is

optimized to have maximum possible gain in the direction of the desired signal and

nulls in the direction of the interferers. The weights here will be computed using LMS

algorithm based on Minimum Squared Error (MSE) criterion. Therefore the spatial

filtering problem involves estimation of signal s(t) from the received signal x(t) (i.e.


56

the array output) by minimizing the error between the reference signal d(t), which

closely matches or has some extent of correlation with the desired signal estimate and

the beamformer output y(t) (equal to w.x(t)). This is a classical Weiner filtering

problem for which the solution can be iteratively found using the LMS algorithm [5].

4.5.3 LMS Algorithm Equations

The LMS algorithm can be summarized by the following equations

Output, y(n) = whx(n) (3.8)

Error, e(n) = d*(n)-y(n) (3.9)

Weight, w(n+1)=w(n) + µx(n)e*(n) (3.10)

4.5.4 Convergence and Stability of the LMS algorithm

The LMS algorithm initiated with some arbitrary value for the weight vector is seen to

converge and stay stable for

0 ≤ µ ≤ 1/λmax (3.11)

where λmax is the largest eigenvalue of the correlation matrix R. If µ is chosen to be

very small then the algorithm converges very slowly. A large value of µ may lead to a

faster convergence but may be less stable around the minimum value [5].

Chapter 5 Simulation Results & Future Enhancements

57

Chapter 5 Simulation Results & Future Enhancements In this chapter, we present the results of the simulations of proposed algorithms for

beamforming in WiMAX systems in the presence of interference in coexistence

scenarios.

5.1 Results with Adaptive Antennas To observe the improvement in performance that can be achieved by the use of

adaptive antennas systems in comparison to conventional antennas, a physical layer of

WiMAX systems was developed. The performance of both systems was tested using

this layer as a simulation basis. An AWGN channel and a fading channel were

incorporated within the simulator and the systems were simulated. The results were

achieved for QPSK modulation scheme. These results can also be achieved for other

modulation schemes. The following results were achieved.

Fig 5.1: BER vs. SNR comparison of both conventional and smart systems.


58

From this result we can conclude that a very significant increase can be achieved with

the help of adaptive antenna systems, even for very low values of SNR.

In order to test the performance of the beamformer it was tested against various

combinations of Angle of Arrival of desired user and the angle of interference

sources.

Fig 5.2: Array plot of adaptive antennas with desired user at 15®

The above plot was obtained for a desired user at an angle of arrival of 15° and

interference sources at 0° and 75°.

Aside from these plots the results of the physical layer simulation of WiMAX were

achieved as follows


59

Fig 5.3: Data generation and coding

Fig 5.4: OFDM Symbol level processing at transmitter side


60

Fig 5.5: OFDM Symbol level processing at receiver side

Fig 5.6: Bit-level processing at the receiver side


61

5.2 Simulink Results The Physical layer of WiMax with LMS Equalization was also implemented on

simulink for comparison of results and ease in future implementation on hardware.

Fig 5.7: Simulink model Results

5.3 Future Enhancements

This project currently covers coexistence scenarios for fixed WiMAX (IEEE

802.16.d) access. It can be extended to cater for coexistence scenarios for mobile

WiMAX (IEEE 802.16e) as well.

This project currently uses the LMS algorithm for adaptive beam forming to reduce

interference by directing power in the desired location of the user. It can be modified


62

to include other algorithms or a combination of algorithms to further reduce

interference and ensure efficient use of spectrum.

For further research and development, WiMAX phy layer simulator can be

implemented on FPGA and other hardware tools.

63

REFERENCE

Chapter1 [1] ITU. Telecommunications indicators update—2004. [2] In-stat Report. Paxton. The broadband boom continues: Worldwide subscribers pass 200 million, No. IN0603199MBS, March 2006. [3] http://www.wimax.com/education/wimax/what_is_wimax.pdf [4] Jeffrey G. Andrews, Arunabha Ghosh, Rias Muhamed. Fundamentals of Wimax, Prentice – Hall. Februaury 2007. Chapter 2, Page 37. [5] Deepak Pareek, WiMAX - Taking Wireless to the MAX. Auerbach Publications, 2006. Chapter 8, Page 150 [6] http://www.conniq.com/WiMAX/tdd-fdd.pdf Chapter 2 [1] IEEE 802.16 Broadband Wireless Access Working Group, Adjacent Frequency Block TDD/FDD Coexistence Scenarios for BWA. WiMAX Forum . 10 April 2007

Chapter 3 [1] Jeffrey G. Andrews, Arunabha Ghosh, Rias Muhamed. Fundamentals of Wimax, Prentice – Hall. Februaury 2007. Chapter 8 [2] Deepak Pareek, WiMAX - Taking Wireless to the MAX. Auerbach Publications,

2006. Chapter 8

[3] WiMAX standard Chapter 8, Section 8.3

[4] Jeffrey G. Andrews, Arunabha Ghosh, Rias Muhamed. Fundamentals of Wimax, Prentice – Hall. Februaury 2007 Chapter 4 [5] WiMAX standard Chapter 8 Section 8.3

[6] Jeffrey G. Andrews, Arunabha Ghosh, Rias Muhamed. Fundamentals of Wimax, Prentice – Hall. Februaury 2007. Chapter 8 [7] Digital Communications by Bernard Sklar Chapter 8,Section 8.1.2 [8] Digital Communications by Bernard Sklar Chapter 8, Section 8.1.4 [9] WiMAX standard Chapter 8, Section 8.3

64

[10] Reed-Solomon Coding by Vocal Technologies LTD. [11] Convolution Coding Complex to Real tutorial [12] WiMAX standard Chapter 8, Section 8.3 [13] WiMAX standard Chapter 8, Section 8.3 [14] WiMAX standard Chapter 8, Section 8.1 Chapter 4 [1] Okamoto, Garret T. Smart Antenna Systems and Wireless Lans, New York

Kluwer Academic Publishers, 2002.

[2] Simulation of Adaptive Array Algorithms for OFDM and Adaptive Vector OFDM Systems by Bing-Leung Patrick Cheung, 2002 [3] C.A. Balanis, Antenna Theory: Analysis and Design, John Wiley and Sons, New York, 1997. [4] N G Chee, Desmond. Smart Antennas for Wireless Applications and Switched Beamforming, undergraduate thesis, University of Queensland, School of Information Technology and Electrical Engineering, 2001. [5] Kiran K. Shetty, “A Novel Algorithm for Uplink Interference Suppression Using Smart Antennas in Mobile Communications,” MS Thesis, FAMU-FSU College of Engineering, Florida State University, 2004. [6] Lal, C. Godara. Applications of Antenna Arrays to mobile Communications,Part II: Beam-Forming and Direction-of-Arrival Consideration, Proceedings of the IEEE, Vol. 85, No. 8, August 1997. [7] B. Widrow and M.E. Hoff. Adaptive Switch Circuits, IRE WESCOM, Conv. Rec., Part 4, 1960 [8] Simon Haykin. Introduction to Adaptive Filters. Macmillan Publishing Company, New York , 1985

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