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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.
v
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.
vi
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
vii
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
viii
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
Chapter 1 Introduction to Wimax
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].
Chapter 1 Introduction to Wimax
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
Chapter 1 Introduction to Wimax
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.
Chapter 1 Introduction to Wimax
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.
Chapter 1 Introduction to Wimax
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
Chapter 1 Introduction to Wimax
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].
Chapter 1 Introduction to Wimax
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.
Chapter 1 Introduction to Wimax
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.
Chapter 1 Introduction to Wimax
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.
Chapter 1 Introduction to Wimax
11
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
Chapter 1 Introduction to Wimax
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).
Chapter 1 Introduction to Wimax
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.
Chapter 1 Introduction to Wimax
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,
Chapter 2 Coexistence Scenarios in Wimax
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
Chapter 2 Coexistence Scenarios in Wimax
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.
Chapter 2 Coexistence Scenarios in Wimax
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]
Chapter 3 WiMAX Physical Layer
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].
Chapter 3 WiMAX Physical Layer
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.
Chapter 3 WiMAX Physical Layer
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.
Chapter 3 WiMAX Physical Layer
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
Chapter 3 WiMAX Physical Layer
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
Chapter 3 WiMAX Physical Layer
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.
Chapter 3 WiMAX Physical Layer
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.
Chapter 3 WiMAX Physical Layer
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).
Chapter 3 WiMAX Physical Layer
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
Chapter 3 WiMAX Physical Layer
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
Chapter 3 WiMAX Physical Layer
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
Chapter 3 WiMAX Physical Layer
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.
Chapter 3 WiMAX Physical Layer
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
Chapter 3 WiMAX Physical Layer
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:
Chapter 3 WiMAX Physical Layer
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.
Chapter 3 WiMAX Physical Layer
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
Chapter 3 WiMAX Physical Layer
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.
Chapter 3 WiMAX Physical Layer
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
Chapter 3 WiMAX Physical Layer
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
Chapter 3 WiMAX Physical Layer
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]
Chapter 3 WiMAX Physical Layer
40
Fig 3.13: 64-QAM Constellation Diagram
Chapter 3 WiMAX Physical Layer
41
Chapter 3 WiMAX Physical Layer
42
Table 3.6: Symbol mapping for 64-QAM
Chapter 3 WiMAX Physical Layer
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)
Chapter 3 WiMAX Physical Layer
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
Chapter 3 WiMAX Physical Layer
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.
Chapter 3 WiMAX Physical Layer
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
Chapter 3 WiMAX Physical Layer
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.
Chapter 3 WiMAX Physical Layer
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
Chapter 4 Adaptive Antennas & Beam Forming
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
Chapter 4 Adaptive Antennas & Beam Forming
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
Chapter 4 Adaptive Antennas & Beam Forming
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
Chapter 4 Adaptive Antennas & Beam Forming
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.
Chapter 4 Adaptive Antennas & Beam Forming
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.
Chapter 4 Adaptive Antennas & Beam Forming
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.
Chapter 4 Adaptive Antennas & Beam Forming
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.
Chapter 5 Simulation Results & Future Enhancements
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
Chapter 5 Simulation Results & Future Enhancements
59
Fig 5.3: Data generation and coding
Fig 5.4: OFDM Symbol level processing at transmitter side
Chapter 5 Simulation Results & Future Enhancements
60
Fig 5.5: OFDM Symbol level processing at receiver side
Fig 5.6: Bit-level processing at the receiver side
Chapter 5 Simulation Results & Future Enhancements
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
Chapter 5 Simulation Results & Future Enhancements
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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