ATHMANDU UNIVERSITY K ATHMANDU
SCHOOL OF E NGINEERING
DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING
FINAL YEAR PROJECT REPORT
Modeling and Simulation Of WiMAX/IEEE 802.16e Physical Layer
A final year project report submitted in partial fulfilment of the requirements for the degree of Bachelor of Engineering
Submitted By: Amar Shrestha (Regd. No.010617-09) Shashi Raj Pandey (Regd. No. 010608-09)
June, 2013
CERTIFICATION
FINAL YEAR PROJECT REPORT On MODELING AND SIMULATION OF WIMAX/IEEE 802.16e PHYSICAL LAYER by:
Amar Shrestha (Regd. No. 010617-09) Shashi Raj Pandey (Regd. No. 010608-09)
Approved by:
1. Project Supervisor
___________________ ___________________
_______________________ ____________________________ _____
(Signature)
(Name)
__________ (Date)
2. Head/In-Charge of the Department
___________________ ___________________ (Signature) (Signatur e)
_______________________ ____________________________ _____ (Name)
__________ (Date)
CERTIFICATION
FINAL YEAR PROJECT REPORT On MODELING AND SIMULATION OF WIMAX/IEEE 802.16e PHYSICAL LAYER by:
Amar Shrestha (Regd. No. 010617-09) Shashi Raj Pandey (Regd. No. 010608-09)
Approved by:
1. Project Supervisor
___________________ ___________________
_______________________ ____________________________ _____
(Signature)
(Name)
__________ (Date)
2. Head/In-Charge of the Department
___________________ ___________________ (Signature) (Signatur e)
_______________________ ____________________________ _____ (Name)
__________ (Date)
ABSTRACT WiMAX is a wireless transmission infrastructure that allows a fast deployment as well as low maintenance costs. Based on the IEEE 802.16-2004 standard, WiMAX allows for an efficient use of bandwidth in a wide frequency range, and can be used as a last mile solution for broadband internet access. To increase data rate of wireless medium with higher performance; better spectral efficient Orthogonal Frequency Division Multiplexing (OFDM) technique is used. Modulation schemes such as 16-QAM, 32-QAM, 64-QAM and 128-QAM (Quadrature amplitude modulation) have been used in the developed OFDM system for FFT based model. This research report discusses the model building of the WiMAX Physical layer using Simulink in Matlab. This model is a useful tool for performance evaluation of the WiMAX Physical layer under different modulation schemes and channel conditions. And utilizing tools such as BER and SNR, this research helps to find suitable modulation schemes for different channel condition regarding WiMAX.
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ACKNOWLEDGEMENT We would like to express sincere gratitude to our project coordinator, Mr. Brajesh Mishra, for granting us the permission to work on this project “MODELING AND SIMULATION OF WIMAX/IEEE 802.16e PHYSICAL LAYER ”. ”. We are grateful to have Mr. Subodh Ghimire as our project supervisor. It is due to his constant support, supervision and guidance we have been able to successfully complete the project. Our special thanks to all the faculty members of the Department of Electrical and Electronics Engineering for their provision of a sound ambience and of all the facilities required for the working of our project. Last but not the least; we would like to thank all our friends for the help they have given us in innumerable way.
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ABBREVIATIONS AWGN
Additive Gaussian White Noise
BER
Bit Error Rate
BPSK
Binary Phase Shift Keying
BS
Base Station
BWA
Broadband Wireless Access
CP
Cyclic Prefix
CRC
Cyclic Redundancy Check
DSL
Digital Subscribers Line
FDD
Frequency Division Duplexing
FEC
Forward Error Correction
FFT
Fast Fourier Transform
GT
Guard Time
HCS
Header Check Sequence
IFFT
Inverse Fast Fourier Transform
ISI
Inter symbol Interference
ICI
Inter carrier Interference
NLOS
Non Line of Sight
OFDM
Orthogonal Frequency Division Multiplexing
OFDMA
Orthogonal Frequency Division Multiple Access
PHY
Physical
QAM
Quadrature Amplitude Modulation
QPSK
Quadrature Phase Shift Keying
SC
Single Carrier iii
SNR
Signal to Noise Ratio
SS
Subscriber Station
TDD
Time Division Duplexing
WiMAX
Worldwide Interoperability for Microwave Access
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LIST OF FIGURES
Figure1.1: Basic Communication System....................................................................................... 2 Figure1.2 Simulation Setup ............................................................................................................ 2 Figure1.3: Simulation Overview ..................................................................................................... 3 Figure2.1 Digital Modulation Principle .......................................................................................... 9 Figure2.2 The BPSK and QPSK constellation ............................................................................... 9 Figure2.3 Possible Phase Value for QPSK modulation ............................................... ................... 9 Figure2.4 A 64-QAM constellation .............................................................................................. 10 Figure2.5 Time and frequency representation of the SC and OFDM ........................................... 12 Figure2.6 Generation of an OFDM signal (simplified) ............................................... ................. 13 Figure2.7 Presentation of the OFDM subcarrier frequency.......................................................... 13 Figure2.8 Cyclic Prefix insertion in an OFDM symbol............................................... ................. 14 Figure2.9 WiMAX OFDM subcarriers types ............................................................................... 15 Figure2.10 OFDM PHY data rates in Mb/s. ................................................................................. 17 Figure2.11 Model of an OFDM system ........................................................................................ 18 Figure3.1 BER for different modulation scheme in AWGN channel ........................................... 24 Figure3.2 Effect of Doppler shift on BER in QPSK ..................................................................... 25 Figure3.3 BER for QPSK for different values of Doppler shift ................................................... 26 Figure3.4 BER for QPSK for different delay spread ................................................... ................. 27 Figure3.5 BER for QPSK for different multipath gains ............................................................... 28
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LIST OF TABLES
Table 1 Simulation Parameters ............................................... ........................................................ 4 Table 2 Normalization Factors...................................................................................................... 11 Table 3 Modulation alphabet for the constellation map ............................................................... 11
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TABLE OF CONTENTS ABSTRACT............................................... ..................................................................................... i ACKNOWLEDGEMENT............................................................................................................ ii ABBREVIATIONS ...................................................................................................................... iii LIST OF FIGURES ...................................................................................................................... v Chapter 1 ....................................................................................................................................... 1 INTRODUCTION......................................................................................................................... 1
1.1 Background and Objectives................................................ ............................................... 1 1.2 System Overview .............................................................................................................. 2 1.3 Methodology ..................................................................................................................... 3 1.4 Overview of Report ................................................... ........................................................ 7 Chapter 2 ....................................................................................................................................... 8 LITERATURE REVIEW ......................................................................................... ................... 8
2.1 Literature Survey ............................................................................................................... 8 2.2 Technological Survey ................................................ ...................................................... 22 Chapter 3 ................................................... .................................................................................. 24 SIMULATION RESULTS AND ANALYSIS .......................................................................... 24
3.1 AWGN ............................................................................................................................. 24 3.2 Multipath Fading: ............................................................................................................ 25 Chapter 4 ................................................... .................................................................................. 29 DISCUSSION AND CONCLUSION ................................................................................ ........ 29
4.1 Discussion and Conclusion ............................................................................................. 29 4.2 Recommendations ........................................................................................................... 29 Bibliography ................................................................................................................................ 30 Appendix...................................................................................................................................... 31
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Chapter 1 INTRODUCTION 1.1 Background and Objectives 1.1.1 Background
Broadband Wireless Access (BWA) has emerged as a promising solution for last mile access technology to provide high speed internet access in the residential as well as small and medium sized enterprise sectors. At this moment, cable and digital subscriber line (DSL) technologies are providing broadband service in this sectors. But the practical difficulties in deployment have prevented them from reaching many potential broadband internet customers. Many areas throughout the world currently are not under broadband access facilities. Even many urban and suburban loations may not be served by DSL connectivity as it can only reach about three miles from the central office switch. On the other side many older cable networks do not have return channel which will prevent to offer internet access and many commercial areas are often not covered by cable network. But with BWA this difficulties can be overcome. Because of its wireless nature, it can be faster to deploy, easier to scale and more flexible, thereby giving it the potential to serve customers not served or not satisfied by their wired broadband alternatives. IEEE 802.16e standard for BWA and its associated industry consortium, Worldwide Interoperability for Microwave Access (WiMAX) forum promise to offer high data rate over large areas to a large number of users where broadband is unavailable. This is the first industrywide standard that can be used for fixed as well as mobile wireless access with substantially higher bandwidth than most cellular networks. Wireless broadband systems have been in use for many years, but the development of this standard enables economy of scale that can bring down the cost of equipment, ensure interoperability, and reduce investment risk for operators. 1.1.2 Objectives
This research project mainly focuses on the following topics
Analyze the basic concept of WiMAX including its standards and relationship with other technologies.
Modeling and Simulation of WiMAX/IEEE 802.16e Physical Layer and its performance evaluation implementing various modulation techniques.
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1.2 System Overview Physical layer set up the connection between the communicating devices and is responsible for transmitting the bit sequence. It also defines the type of modulation and demodulation as well as transmission power. WiMAX 802.16e PHY-layer considers two types of transmission techniques OFDM and OFDMA. Both of these techniques have frequency band below 11GHz and use TDD and FDD as its duplexing technology. WiMAX physical layer is based on the orthogonal frequency division multiplexing (OFDM). OFDM is a good choice of high speed data transmission, multimedia communication and digital video services. It even can maintain very fast data rate in a non-line of sight (NLOS) condition and multipath environment. The role of the PHY-layer is to encode the binary digits that represent MAC frames into signals and to transmit and receive these signals across the communication media. The basic communication system is shown in Figure1.1. . Channel
Transmitter
Receiver
Figure1.1: Basic Communication System
We have implemented the transmitter and receiver of the baseband part of the PHY-layer as in Figure1.2. This structure corresponds to the phyical layer of the standard. In this setup we have just implemented the mandatory features of the specification while leaving the implementation of optimal features for future work.The complementary operations are applied in the reverse order at channel decoding in the receiver end.
Random Data
Output
Mapping
IFFT
Cyclic Prefix Insertion
De-Mapping
FFT
Cyclic Prefix Removal
Figure1.2 Simulation Setup
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1.3 Methodology 1.3.1 Simulation strategy
Execute Initializing Matlab File (final.m)
Execute Simulink Model (.mdl extension file)
Output
Graph Plotting (Matlab)
Bertool (.ber file)
Figure1.3: Simulation Overview
Our simulation study is based on the standard of OFDM model with parameters based on IEEE 802.16e with changes in the channel according to the environment and as per the requirement for analysis. And Simulink being a block based design tool, it seemed the best option to implement the model. So the simulation model was developed model by module in Simulink based on the basic structure of an OFDM system .Then the standard parameters are provided to the models to direct the simulation in the direction that the standard permits. After gaining knowledge about the simulation platform, the model was simulated following the procedures given below.
At first we perform initialization of the model parameters.
The models are run via Bertool function or directly through Simulink model.
The results are plotted out in number of ways: BER versus SNR plot when AWGN channel is involved and BER versus Doppler spread/Doppler shift/Multipath gain vector for Rayleigh fading channel. Then on the basis of plots and our literature survey analysis is made.
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1.3.2 Simulation Model Scenario:
For the simulation scenario, we have considered basic models available. As we are working under wireless communication network for WiMAX, we have taken into account AWGN (Additive White Gaussian Noise) channel in which noise components just adds up, fading channel due to multipath environment and a combination of both AWGN and fading channels for the purpose of simulation. Similarly for the simulation model, we have chosen different parameters as per the IEEE 802.16e standard. Table 1 Simulation Parameters Parameter
Value
Ndata
192
N pilot
8
Ntrain
3
BW nf
Variable,from1.25 to 20 MHz being an integer multiple of 1.25, 1.5, or 1.75 MHz 8/7
G
1/4, 1/8, 1/16, 1/32
Number of lower frequency guard subcarriers Number of higher frequency guard subcarriers Frequency offset indices of guard subcarriers Frequency offset indices of pilot carriers
28
Tframe(msec)
27 -128,-127,…, -101 +101,+102, …, +127 -88,-63,-38,-13 +13,+38,+63,+88 2.5,4,5,8,10,12.5,20
Beside the parameters that describe the OFDM symbol, other parameters are required in order to define parameters for the transmission, such as the frame duration, the packet size, or the total number of transmitted OFDM frame duration, the packet size, or the total number of transmitted OFDM symbols. The five primitive parameters that characterize the OFDM symbol are:
BW : nominal channel bandwidth.
N data: number of data subcarriers.
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N pilot : number of pilot subcarriers. n f : sampling factor, used with BW and Nused (number of non-zero subcarriers) to determine the subcarrier spacing and the useful symbol time.
G: ratio of CP time to useful time.
Next, derived parameters, which are dependent of the primitive parameters, are listed:
N used : number of used non-zero subcarriers. N used
=
N data + N pilot
N FFT : number of points used to perform the FFT. It is specified to be the smallest power of two, and greater than Nused. N FFT
F s: sampling frequency. F s
= 2[()]
∆
= 8000
: subcarrier spacing.
∆= : useful symbol time. = ∆ CP time. =
T b
T b
T g:
T g
T sym: OFDM symbol time. T sym
=
T s=
T b+ T g
T s: sampling time.
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1.3.3 Performance Metrics
The following different performance metrics are evaluated to analyze the performance of OFDM in different channels with different modulation scheme for WiMAX. Signal to Noise Ratio (SNR)
SNR is the ratio of signal power to noise power. SNR value provides the general information about the channel condition and helps in evaluating the channel capacity for the provided bandwidth.
= = = Where, Fs = symbol rate (1/sec) B= bandwidth (Hz=1/sec)
≥
Fs
No = noise power spectral density Es = Energy per symbol (Joules) E b = Energy per bit (Joules) N b = bits per symbol Bit Error Rate (BER)
Bit error rate (BER) also called as bit error probability simply is the ratio of lost bits to the total number of transmitted bit in a channel.
= For AWGN channel the BER can be calculated using following equations. BPSK:
= 2 6
4-QAM:
M-QAM:
≈ 2 2 ≈ 4 3−12
Where
1 () = √ 2 = √ 1.4 Overview of Report This project is divided into four different chapters, each one illustrating the description of the project in detail. The report highlights the system overview and the technology and literature review conducted for the completion of the project. Chapter 1 discusses the introduction part regarding the project. The background of the project and its objective initiate the report. The system overview defines the components of the system for simulation. The simulation strategy and different parameters considered for simulation are classified under methodology heading. Chapter 2 deals with the Literature Survey and Technological survey where we have tried to put all necessary details that help to understand technical terminologies of the project. The research and survey done during the project time are also included in the survey which could be referred to understand the project. Chapter 3 focuses on Simulation Results and Analysis. Chapter 4 emphasizes on the conclusion of the project. The outcome of the project under different parameter is highlighted under this section.
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Chapter 2 LITERATURE REVIEW 2.1 Literature Survey 2.1.1 Digital Modulation
As for all recent communication systems, WiMAX/802.16 uses digital modulation. The now well-known principle of a digital modulation is to modulate an analogue signal with a digital sequence in order to transport this digital sequence over a given medium: fiber, radio link, etc (Figure2.1). This has great advantages with regard to classical analogue modulation: better resistance to noise, use of high-performance digital communication and coding algorithms, etc. Many digital modulations can be used in a telecommunication system. The variants are obtained by adjusting the physical characteristics of a sinusoidal carrier, the frequency, phase or amplitude, or a combination of some of these. Four modulations are supported by the IEEE 802.16 standard: BPSK, QPSK, 16-QAM and 64-QAM. In this section the modulations used in the OFDM and OFDMA PHYsical layers are introduced with a short explanation for each of these modulations.
Binary Phase Shift Keying (BPSK)
The BPSK is a binary digital modulation; i.e. one modulation symbol is one bit. This gives high immunity against noise and interference and a very robust modulation. A digital phase modulation, which is the case for BPSK modulation, uses phase variation to encode bits: each modulation symbol is equivalent to one phase. The phase of the BPSK modulated signal is π or – π according to the value of the data bit. An often used illustration for digital modulation is the constellation. Figure2.2 shows the BPSK constellation; the values that the signal phase can take are 0 or π.
Quadrature Phase Shift Keying (QPSK)
When a higher spectral efficiency modulation is needed, i.e. more b/s/Hz, greater modulation symbols can be used. For example, QPSK considers two-bit modulation symbols. Figure 6 shows the possible phase values as a function of the modulation symbol. Many variants of QPSK can be used but QPSK always has a four-point constellation (Figure 5). The decision at the receiver, e.g. between symbol ‘00’ and symbol ‘01’, is less easy than a decision between ‘0’ and ‘1’. The QPSK modulation is therefore less noise- resistant than BPSK as it has a smaller immunity 8
against interference. A well-known digital communication principle must be kept in mind: ‘A greater data symbol modulation is more spectrum efficient but also less robust.’
Figure2.1 Digital Modulation Principle
Figure2.2 The BPSK and QPSK constellation
Figure2.3 Possible Phase Value for QPSK modulation
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Quadrature Amplitude Modulation (QAM): 16-QAM and 64-QAM
The QAM changes the amplitudes of two sinusoidal carriers depending on the digital sequence that must be transmitted; the two carriers being out of phase of π/2, this amplitude modulation is called quadrature. It should be mentioned that according to digital communication theory, QAM4 and QPSK are the same modulation (considering complex data symbols). Both 16-QAM (4 bits/modulation symbol) and 64-QAM (6 bits/modulation symbol) modulations are included in the IEEE 802.16 standard. The 64-QAM is the most efficient modulation of 802.16 (Figure2.4). Indeed, 6 bits are transmitted with each modulation symbol. The 64-QAM modulation is optional in some cases: •license-exempt bands, when the OFDM PHYsical Layer is used •for OFDMA PHY, yet the Mobile WiMAX profiles indicate that 64-QAM is mandatory in the downlink.
Figure2.4 A 64-QAM constellation
The modulation mapping is built in the simulator by a Simulink block implemented as a Matlab m-file. The normalization factors and the symbol alphabet (A s) that represent the coordinate points in the constellation map are defined in Table 2 and Table 3 respectively.
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Table 2 Normalization Factors
Modulation scheme BPSK
Normalization constant for unit average power
=1 = 1/√ 2 = 1/√ 10 = 1/42 Cm
QPSK
Cm
16-QAM
Cm
64-QAM
Cm
Table 3 Modulation alphabet for the constellation map
Modulation scheme
Symbol alphabet
BPSK
As= (1, −1)
QPSK
As = (1 + j, 1 − j, −1 + j, −1 − j )
16-QAM
A = (j, 3j, −j, −3j ) As= (A + 1, A + 3, A − 1, A − 3) A = (j, 3j, 5j, 7j − j, −3j, −5j, −7j ) As= (A + 1, A + 3, A + 5, A + 7, A − 1, A − 3, A − 5, A− 7
64-QAM
2.1.2 OFDM
OFDM is a very powerful transmission technique. It is based on the principle of transmitting simultaneously many narrow-band orthogonal frequencies, often also called OFDM subcarriers or subcarriers. The number of subcarriers is often noted N. These frequencies are orthogonal to each other which (in theory) eliminates the interference between channels. Each frequency channel is modulated with a possibly different digital modulation (usually the same in the first simple versions). The frequency bandwidth associated with each of these channels is then much smaller than if the total bandwidth was occupied by a single modulation. This is known as the Single Carrier (SC) (Figure2.5). A data symbol time is N times longer, with OFDM providing a much better multipath resistance. Having a smaller frequency bandwidth for each channel is equivalent to greater time periods and then better resistance to multipath propagation (with regard to the SC). Better resistance to multipath and the fact that the carriers are orthogonal 11
allows a high spectral efficiency. OFDM is often presented as the best performing transmission technique used for wireless systems.
Basic Principle: Use the IFFT Operator
The FFT is the Fast Fourier Transform operator. This is a matrix computation that allows the discrete Fourier transform to be computed (while respecting certain conditions). The FFT works for any number of points.
Figure2.5 Time and frequency representation of the SC and OFDM
The operation is simpler when applied for a number N which is a power of 2 (e.g. N =256). The IFFT is the Inverse Fast Fourier Transform operator and realizes the reverse operation. OFDM theory shows that the IFFT of magnitude N, applied on N symbols, realizes an OFDM signal, where each symbol is transmitted on one of the N orthogonal frequencies. The symbols are the data symbols of the type BPSK, QPSK, QAM-16 and QAM-64 introduced in the previous section. Figure2.6 shows an illustration of the simplified principle of the generation of an OFDM signal. In fact, generation of this signal includes more details that are not shown here for the sake of simplicity.
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Figure2.6 Generation of an OFDM signal (simplified)
Figure2.7 Presentation of the OFDM subcarrier frequency
If the duration of one transmitted modulation data symbol is T d, then Td=1/∆f, where ∆f is the frequency bandwidth of the orthogonal frequencies. As the modulation symbols are transmitted simultaneously, Td=duration of one OFDM symbol =duration of one transmitted modulation data symbol. 13
This duration, ∆f, the frequency distance between the maximums of two adjacent OFDM subcarriers, can be seen in Figure 2.7. This figure shows how the neighboring OFDM subcarriers have values equal to zero at a given OFDM subcarrier maximum, which is why they are considered to be orthogonal. In fact, duration of the real OFDM symbol is a little greater due to the addition of the Cyclic Prefix (CP).
Time Domain OFDM Considerations
After application of the IFFT, the OFDM theory requires that a Cyclic Prefix (CP) must be added at the beginning of the OFDM symbol (Figure2.8). Without getting into mathematical details of OFDM, it can be said that the CP allows the receiver to absorb much more efficiently the delay spread due to the multipath and to maintain frequency orthogonality.
Figure2.8 Cyclic Prefix insertion in an OFDM symbol
The CP that occupies a duration called the Guard Time (GT), often denoted T G, is a temporal redundancy that must be taken into account in data rate computations. The ratio T G/Td is very often denoted G in WiMAX/802.16 documents. The choice of G is made according to the following considerations: if the multipath effect is important (a bad radio channel), a high value of G is needed, which increases the redundancy and then decreases the useful data rate; if the multipath effect is lighter (a good radio channel), a relatively smaller value of G can be used. For OFDM and OFDMA PHY layers, 802.16 defined the following values for G: 1/4, 1/8, 1/16 and 1/32. For the mobile (OFDMA) WiMAX profiles presently defined, only the value 1/8 is mandatory. The standard indicates that, for OFDM and OFDMA PHY layers, SS searches, on initialization, for all possible values of the CP until it finds the CP being used by the BS. The SS then uses the same CP on the uplink. Once a specific CP duration has been selected by the BS for 14
operation on the downlink, it cannot be changed. Changing the CP would force all the SSs to resynchronize to the BS.
Frequency Domain OFDM Considerations
All the subcarriers of an OFDM symbol do not carry useful data. There are four subcarrier types (Figure2.9): •Data subcarriers: useful data transmission. •Pilot subcarriers: mainly for channel estimation and synchronization. For OFDM PHY, there are eight pilot subcarriers. •Null subcarriers: no transmission. These are frequency guard bands. •Another null subcarrier is the DC (Direct Current) subcarrier. In OFDM and OFDMA PHY layers, the DC subcarrier is the subcarrier whose frequency is equal to the RF center frequency of the transmitting station. It corresponds to frequency zero (Direct Current) if the FFT signal is not modulated. In order to simplify Digital-to-Analogue and Analogue-to-Digital Converter operations, the DC subcarrier is null.
Figure2.9 WiMAX OFDM subcarriers types
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OFDM Symbol Parameters and Some Simple Computations
The main WiMAX OFDM symbol parameters are the following: •The total number of subcarriers or, equivalently, the IFFT magnitude. For OFDM PHY, NFFT=256, the number of lower-frequency guard subcarriers is 28 and the number of higher frequency guard subcarriers is 27. Considering also the DC subcarrier, there remains Nused, the number of used subcarriers, excluding the null subcarriers. Hence, Nused=200 for OFDM PHY, of which 192 are used for useful data transmission, after deducing the pilot subcarriers. •BW, the nominal channel bandwidth •n, the sampling factor. The sampling frequency, denoted fs, is related to the occupied channel bandwidth by the following (simplified) formula: f s=nBW. This is a simplified formula because, according to the standard, f s is truncated to an 8 kHz multiple. According to the 802.16 standard, the numerical value of n depends of the channel bandwidths. Possible values are 8/7, 86/75, 144/125, 316/275 and 57/50 for OFDM PHY and 8/7 and 28/25 for OFDMA PHY.
Duration of an OFDM Symbol
Based on the above-defined parameters, the time duration of an OFDM symbol can be computed: OFDM symbol duration = useful symbol time + guard time (CP time) =1/ (one subcarrier spacing) +G × useful symbol time = (1/∆f) (1+G) = [1/ ( f s/ NFFT)] (1+G) = [1/ ( nBW / NFFT)] (1+G). The OFDM symbol duration is a basic parameter for data rate computations.
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Data Rate Values
In OFDM PHY, one OFDM symbol represents 192 subcarriers, each transmitting a modulation data symbol (see above). One can then compute the number of data transmitted for the duration of an OFDM symbol (which value is already known). Knowing the coding rate, the number of uncoded bits can be computed. Figure2.10 shows the data rates for different Modulation and Coding Schemes (MCSs) and G values. The occupied bandwidth considered is 7 MHz and the sampling factor is 8/7 (the value corresponding to 7 MHz according to the standard).Consider the following case in Figure 12: 16-QAM, coding rate 3/4 and G 1/16. It can be verified that the data rate is equal to: Data rate =number of uncoded bits per OFDM symbol/OFDM symbol duration =192 ×4 × (3/4)/ {[256/ (7 MHz ×8/7)] (1 + 1/16)} =16.94 Mb/s.
Figure2.10 OFDM PHY data rates in Mb/s
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The OFDM Model
Figure2.11 Model of an OFDM system
OFDM signals are typically generated digitally due to the difficulty increasing large banks of phase lock oscillators and receivers in the analog domain. Figure2.11 shows the block diagram of such an OFDM system .In the transmitter, the incoming data stream is grouped in blocks of N
c
data symbols, which are the OFDM symbols, and can be represented by a vector X m. Next, an IFFT is performed on each data symbol block and acyclic prefix of length Ng is added. The received signal is, generally, the sum of a linear convolution with the discrete channel impulse response, h (n), and an additive white Gaussian noise, w (n). It has to be said that it is implicitly assumed that the channel fading is slow enough to consider it constant during one symbol, and both, transmitter and receiver, are p erfectly synchronized. At the receiver, the cyclic prefix is removed, and then, the data symbol y k,m(frequency index k, OFDM symbol m) is obtained by performing the FFT operation. 2.1.3 CHANNEL
AWGN
Additive white Gaussian noise (AWGN) is a channel model in which the only impairment to communication is a linear addition of wideband or white noise with a constant spectral density (expressed as watts per hertz of bandwidth) and a Gaussian distribution of amplitude. The model does not account for fading, frequency selectivity, interference, nonlinearity or dispersion.
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However, it produces simple and tractable mathematical models which are useful for gaining insight into the underlying behavior of a system before these other phenomena are considered.
Multipath Fading Channel
A fading channel is a communication channel comprising fading. In wireless systems, fading may either be due to multipath propagation, referred to as multipath induced fading, or due to shadowing from obstacles affecting the wave propagation, sometimes referred to as shadow fading. The presence of reflectors in the environment surrounding a transmitter and receiver create multiple paths that a transmitted signal can traverse. As a result, the receiver sees the superposition of multiple copies of the transmitted signal, each traversing a different path. Each signal copy will experience differences in attenuation, delay and phase shift while travelling from the source to the receiver. This can result in either constructive or destructive interference, amplifying or attenuating the signal power seen at the receiver. Strong destructive interference is frequently referred to as a deep fade and may result in temporary failure of communication due to a severe drop in the channel signal-to-noise ratio. Doppler spread and Delay spread
When the receiver and the transmitter are in relative motion, the received signal is subject to a constant frequency shift, called the Doppler shift Therefore, as it occurs in the time domain, the Doppler spread is defined as the difference between the largest and the smallest among these frequency shifts, f d = f M cos ϕ Where • f M = f c( v/c) is the maximum Doppler shift, • v is the vehicle speed, • f c is the carrier frequency, • c is the speed of light, and • ϕ is the arrival angle of the received signal component.
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Thus we have, f d = f c( v/c) cos ϕ Furthermore, a time-varying Doppler shift is induced on each multipath component if the reflecting objects and scatters in the propagation channel are in motion, causing frequency dispersion. The two manifestations of the channel time variations are the delay spread and the Doppler spread. Depending on their values, the signal transmitted through the channel will undergo flat or frequency selective fading. On one hand, the delay spread is a measure of the spreading time over which the multipath signals arrive. It is a measure of the time dispersion of a channel, and is very important in determining how fast the symbol rate can be in digital communications. One of the most widely used measurement for characterizing the delay spread of a multipath channel is the rms delay spread στ. Furthermore, the inverse of the delay spread defines the coherence bandwidth, Bcoh. It is the frequency separation at which two frequency components of the signal undergo independent attenuations and a measure of the range of frequencies over which the multipath fading channel frequency response can be considered to be flat or not. On the other hand, the Doppler spread Bd, is a measure of the spectral broadening caused by the time rate of change of the multipath components due to the relative motion between transmitter and receiver. Depending on how rapidly the multipath components change, the channel may be classified either as a fast or a slow fading channel. Inversely proportional to one another rare the Doppler spread and the coherence time. The coherence time Tcoh, is the time domain dual of Doppler spread and is used to characterize the time-varying nature of the frequency dispersiveness of the channel in the time domain. It is a statistical measure of the time duration over which the channel impulse response is essentially invariant quantifying the similarity of the channel response at different times. Rayleigh fading
Rayleigh fading models assume that the magnitude of a signal that has passed through such a transmission medium (also called a communications channel) will vary randomly, or fade, according to a Rayleigh distribution — the radial component of the sum of two uncorrelated Gaussian random variables. 20
Rayleigh fading is a reasonable model when there are many objects in the environment that scatter the radio signal before it arrives at the receiver. The central limit theorem holds that, if there is sufficiently much scatter, the channel impulse response will be well-modeled as a Gaussian process irrespective of the distribution of the individual components. If there is no dominant component to the scatter, then such a process will have zero mean and phase evenly distributed between 0 and 2π radians. The envelope of the channel response will therefore be Rayleigh distributed. Calling this random variable R, it will have a probability density function:
() = Ω /Ω ≥ 0 , r
Where
Ω=E ( ) Often, the gain and phase elements of a channel's distortion are conveniently represented as a complex number. In this case, Rayleigh fading is exhibited by the assumption that the real and imaginary parts of the response are modeled by independent and identically distributed zeromean Gaussian processes so that the amplitude o f the response is the sum of two such processes. Rician fading
Rician fading is a stochastic model for radio propagation anomaly caused by partial cancellation of a radio signal by itself — the signal arrives at the receiver by several different paths (hence exhibiting multipath interference), and at least one of the paths is changing (lengthening or shortening). Rician fading occurs when one of the paths, typically a line of sight signal, is much stronger than the others. In Rician fading, the amplitude gain is characterized by a distribution. Rayleigh is the specialized model for stochastic fading when there is no line of sight signal, and is sometimes considered as a special case of the more generalized concept of Rician fading. In Rayleigh fading, the amplitude gain is characterized by a Rayleigh distribution.
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2.2Technological Survey 2.2.1 Matlab
Matlab (Matrix Laboratory) is an interactive system whose basic data element is an ARRAY. It was intended for use in Matrix theory, Linear algebra and Numerical analysis. Later and with the addition of several toolboxes the capabilities of Matlab were expanded and today it is a very powerful tool at the hands of an engineer. Its typical uses include: •Math and Computation •Algorithm development •Modeling, simulation and prototyping •Data analysis, exploration and visualization •Scientific and engineering graphics •Application development, including graphical user interface building. 2.2.2 Simulink
Simulink (Simulation and Link) is an extension of MATLAB by Math works Inc. It is a platform for multi domain simulation and Model-Based Design of dynamic systems. It works with MATLAB to offer modeling, simulation, and analysis of dynamical systems under a graphical user interface (GUI) environment. The construction of a model is simplified with click-and-drag mouse operations. Simulink includes a comprehensive block library of toolboxes for both linear and nonlinear analyses. Models are hierarchical, which allow using both top-down and bottomup approaches. As Simulink is an integral part of MATLAB, it is easy to switch back and forth during the analysis process and thus, the user may take full advantage of features offered in both environments. Also, control logic can be tested and evaluated with-out the need to do time consuming hardware experiments. Key Features • Extensive and expandable libraries of predefined blocks • Interactive graphical editor for assembling and mana ging intuitive block diagrams • Ability to manage complex designs by segmenting models into hierarchies of design components
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• Model Explorer to navigate, create, configure, and search all signals, parameters, and properties of the model • Ability to interface with other simulation programs and incorporate hand-written code, including MATLAB algorithms • Option to run fixed- or variable-step simulations of time-varying systems interactively or through batch simulation • Functions for interactively defining inputs and viewing outputs to evaluate model behavior • Graphical debugger to examine simulation results and diagnose unexpected behavior in the design • Full access to MATLAB for analyzing and visualizing data, developing graphical user interfaces, and creating model data and parameters • Model analysis and diagnostics tools to ensure model consistency and identify modeling errors
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Chapter 3 SIMULATION RESULTS AND ANALYSIS 3.1 AWGN This section gives a comparison between the different modulation schemes used in the simulator. These results have been obtained in an AWGN channel.
Figure3.1 BER for different modulation scheme in AWGN channel
The curves show the BER as a function of the bit energy to noise rate (E b/N0), which is a measure of the energy e fficiency of a modulation scheme. If a higher E b/N0 is needed to transfer data for a given modulation scheme, it means that more energy is required for each bit transfer. Low spectral e fficiency modulation schemes, such as BPSK and QPSK, require a lower E b/N0, and hence, are more energy e fficient and less vulnerable to bit errors than the high spectral efficiency modulation schemes, such as 16-QAM and 64-QAM. And it is clearly visible in the figure 3.1 which shows BPSK and QPSK with same curves and 16-QAM and 64-QAM with progressively higher BER curves. .
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3.2 Multipath Fading: As discussed earlier, the Rayleigh fading channel was used to implement the multipath fading. And the analysis for the three main parameters for the fading channel is done from the obtained simulation results.
Effect of Doppler shift
.
Figure3.2 Effect of Doppler shift on BER in QPSK
OFDM requires very accurate frequency synchronization between the receiver and the transmitter; with frequency deviation the sub-carriers will no longer be orthogonal, causing intercarrier interference (ICI) (i.e., cross-talk between the sub-carriers). Frequency offsets are caused by Doppler shift due to movement. While Doppler shift alone may be compensated for by the receiver, the situation is worsened when combined with multipath, as reflections will appear at various frequency offsets. This effect typically worsens as speed increases, and it can be easily seen from the simulation results in figure 3.2 and figure 3.3.
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Figure3.3 BER for QPSK for different values of Doppler shift
Keeping the delay spread and multipath gain constant, the above simulation results for the change in Doppler shift for QPSK modulation scheme was determined. In the figure 3.2, the bit error rate increases as the relative velocity increases. This is also presented in the BER vs. SNR plot which displays that with increasing Doppler shift, the curves have also shifted showing higher BER at certain SNR point for higher Doppler shifts.
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Effect of delay spread:
Figure3.4 BER for QPSK for different delay spread
The simulation result showing the effect of delay spread of the Rayleigh fading channel presented in the figure3.4. When the delay spread is less than the sample period, the signal bandwidth is smaller than the coherence bandwidth causing only flat fading and the delayed samples interfere only with the own sample itself. The result is that there is no Inter Symbol Interference (ISI). But when the delay spread increases such that it is higher than the sample period, and the signal bandwidth becomes larger than the coherence bandwidth causing the delayed samples interfere with the adjacent samples. This results in ISI and thus bit error. And this can easily be seen in the above figure. For a delay spread less than sample period, the BER vs. SNR curve shows no effect of multipath fading and almost represents the AWGN result. But for when delay spread is greater than sample period, the effect of multipath fading is clearly seen with the increase in BER.
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Effect of multipath gain
Figure3.5 BER for QPSK for different multipath gains
The figure3.5 shows the effect of the gains of the multipath components. The amount a multipath component interferes with a transmitting signal depends on the power that the component reaches the receiver. When the gain of the component is higher, the interference is higher causing loss in performance. Similarly, low multipath gain leads to less interference and thus higher performance. And this is clearly seen in the figure3.5.
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Chapter 4 DISCUSSION AND CONCLUSION 4.1 Discussion and Conclusion In this wireless communication industry, WiMAX is an attractive alternative for fulfilling needs higher range and data rates. And the physical layer defined in the standard IEEE 802.16e describes the use of the Orthogonal Frequency Division Multiplexing (OFDM) which is a technique based on multi carrier modulation (MCM) and frequency division multiplexing (FDM).
So this research helps to analyze the characteristic behavior of this technique in
different channels with different modulation schemes. BER and SNR are the main metrics used to perform the analysis. This research helps to find suitable modulation schemes for different channel conditions. This research discusses the introduction of the WiMAX physical layer, its components and its characteristics. The flexible and parameterizeable OFDM was studied, then simulated in Simulink and analyzed, deriving its characters at different parameters. Based on the analysis we can conclude that in AWGN channel, modulation schemes with low spectral efficiency like BPSK and QPSK are better. Similarly, the performance of the OFDM in multipath fading channel was seen to deteriorate at higher velocity, longer delay spread and higher multipath gain.
4.2 Recommendations The simulation analyzes and helps to find suitable modulation schemes for different channel conditions. And thus this research can be further extended to develop and analyze a system with adaptive modulation. Similarly, channel coding can be added to this model to add error protection in the system, and in future this model can be expanded to include the components of the MAC layer and a complete end to end WiMAX system could be built based on this model.
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Bibliography IEEE Computer Society and the IEEE Microwave Theory and Techniques Society , IEEE Standard for Local and metropolitan area networks, Part 16: Air Interface for Broadband Wireless Access Systems John Wiley & Sons, WiMAX: Technology for Broadband Wireless Access Loutfi Nuaymi © 2007, Ltd. ISBN: 0-470-02808-4 Chritian Bauer (Stuttguart), Introduction to WiMAX, Jeffrey G. Andrews, Ph.D.Arunabha Ghosh, Ph.D.AT&T Labs Inc,Rias Muhamed.AT&T Labs Inc., Fundamentals of WiMAX M.A. Mohamed, F.W. Zaki, R.H. Mosbeh, Simulation of WiMAX Physical Layer: IEEE 802.16e Univ.Prof. Dipl.-Ing. Dr.techn. Markus Rupp, Dipl.-Ing. Christian Mehlführer, Implementation of a WiMAX simulator in Simulink Roberto Cristi, Wireless Communications with Matlab and Simulink: IEEE802.16 (WiMax) Physical Layer Won Gi Jeon, Student Member, IEEE, Kyung Hi Chang, Senior Member, IEEE, and Yong Soo Cho, Member, IEEE, An Equalization Technique for Orthogonal Frequency-Division, Multiplexing Systems in Time-Variant Multipath Channels
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Appendix OFDM simulator block diagram In order to have a general view of the WiMAX simulator described through the thesis, a complete block diagram of the Simulink model file is given in this appendix.
Simulation on AWGN channel
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Simulation on Rayleigh and AWGN channel
Simulation on Rayleigh channel
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CODES FOR SIMULATION AND ANALYSIS
Initialization Code: clear BW=input('Required channel bandwidth in MHz(max 20 MHz)='); disp('choose cyclic prefix to overcome delays spreads') disp(',1/4 for longest delay spread ,1/8 for long delay spreads ,') disp('1/16 for short delays spreads ,1/32 for very small delay spread channels') G=input('= ');
if ((G~=1/4)&(G~=1/8)&(G~=1/16)&(G~=1/32)) error('You have choosen a guard period thats not valid in the IEEE 802.16') end inputsize=192; Nfft=256; fs=floor((2)*BW*1e6); %sampling freqency freqspacing= fs/Nfft; %freqency spacing Tb= 1/freqspacing; %usfel symbol time Tg= G*Tb ;%Guard time Ts=Tb+Tg ;%symbol time samplingttime= Tb/Nfft;
CPsel=[(256-G*256+1):256 1:256]; CPremove=[(256*G+1):(256+G*256)];
SNR=input('Enter the channel SNR in dB='); 33
if SNR<0 error('not a valid SNR value') end a= input('Choose the modulation scheme :1=BPSK, 2=QPSK, 3 =16 QAM, 4=64QAM ');
if (a==1) %BPSK m=2; Ry=[+1 -1]; Iy=[0 0]; qamconst=complex(Ry,Iy); qamconst=qamconst(:); bitspersymbol=1;
disp('Modulation scheme of BPSK is chosen');
elseif (a==2) %QPSK m=4; Ry=ones(2,1)*[+1 -1];
Iy=([+1 -1]')*ones(1,2);
qamconst=complex(Ry,Iy); qamconst=qamconst(:)/sqrt(2); bitspersymbol=2;
disp('Modulation scheme of QPSK is chosen'); 34
elseif (a==3) %16-QAM m=16; Ry=ones(4,1)*[+1 +3 -1 -3]; Iy=([+1 +3 -3 -1]')*ones(1,4); qamconst=complex(Ry,Iy); qamconst=qamconst(:)/sqrt(10); bitspersymbol=4;
disp('Modulation scheme of 16-QAM is chosen');
elseif (a==4) %64-QAM m=64; Ry=ones(8,1)*[+3 +1 +5 +7 -3 -1 -5 -7 ]; Iy=([+3 +1 +5 +7 -3 -1 -5 -7 ]')*ones(1,8); qamconst=complex(Ry,Iy); qamconst=qamconst(:)/sqrt(42); bitspersymbol=6;
disp('Modulation scheme of 64-QAM is chosen');
end
Plotting code: Effect of doppler shift on BER in QPSK fd=[.1 1 10 15 20 30 40 50 60 70 80 90 100]; 35
ber=[0 0 .054 .104 .162 .2198 .2646 .3015 .3326 .3566 .3769 .391 .398] %ber_0=[1.6e-6 1.6e-6 .025 .049 .071]; plot(fd,ber,'b*-') grid on xlabel('Doppler Shift ') ylabel('Bit error rate') title('Effect of doppler shift on BER in QPSK') hold all %plot(fd,ber_0,'r+-') legend('QPSK','DQPSK', 3)
Bit error rate for QPSK for different values of doppler shift clc SNR=[0
3
6
ber=[ 0.2156
9 12
0.1330
15 18
0.0580
21 24
0.0132
27 30];
0.0008 0.00001
0
0
0
0
0];
ber_0=[0.2341
0.1654 0.1117
0.0574
0.0179
0.0032
0.0002
0.00001
0
0
0];
ber_1=[0.3141
0.1929
0.1104
0.0795
0.0469
0.0249
0.0115
0.0049
0.0018
0.2612
0.1947
0.1325
0.0854
0.0532
0.0333
0.0187
0.0077
0.0020
0.2863
0.2257
0.1644
0.1116
0.0686
0.0364
0.0191 0.0079
0.0022
0.2981
0.2370
0.1757
0.1221
0.0792
0.0466
0.0004
0.00005]; ber_2=[ 0.3212 0.0002]; ber_3=[ 0.3386 0.0002]; ber_4=[0.3499
0.0236
0.0095
0.0031
0.0005]; semilogy(SNR,ber,'b*-'); grid on xlabel('SNR ')
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ylabel('Bit error rate') title('Bit error rate for QPSK for different values of doppler shift') hold all semilogy(SNR,ber_0,'r+-'); semilogy(SNR,ber_1,'bo-') semilogy(SNR,ber_2,'gx-') semilogy(SNR,ber_3,'k*-') semilogy(SNR,ber_4,'cs-') legend('0.001Hz','1Hz','5Hz','10Hz','20Hz','100Hz', 3)
Bit error rate for QPSK for different delay spread SNR=[0
3
6
9 12
ber=[0.2551 0.1788 ber_0=[ 0.3596
15 18
0.1018
21 24
0.0422
0.3114 0.2548
27 30];
0.0112
0.1944
0.0016
0.1371
0.0001
0.0908
0.0000
0.0597
0
0 0]
0.0417 0.0322
0.0273
0.0249]; semilogy(SNR,ber,'b*-') grid on xlabel('SNR ') ylabel('Bit error rate') title('Bit error rate for QPSK for different delay spread') hold all semilogy(SNR,ber_0,'r+-') legend('1.7e-6 delay','2e-5 delay', 3)
Bit error rate for QPSK for different multipath gains SNR=[0
3
6
9 12
15 18
21 24
27 30];
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