Introduction to OFDM
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Transcript of Introduction to OFDM
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John W. Thomas
PhD Candidate - University of Texas at Dallas
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OFDM
Speaker Biography
Introduction to OFDM
OFDM System Model
OFDM Mathematical Model
Characteristics of OFDM
Advantages
Disadvantages
Usage of OFDM
Presentation Contents
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Speaker Biography
• Morehouse College
o Atlanta, GA
o B.S - Mathematics and Physics, 2000
o Minor - Economics
• Columbia University
o New York City, NY
o M.S. - Electrical Engineering, 2002
o Concentration - Wireless Communications
• University of Texas at Dallas
o Richardson, TX
o PhD - Electrical Engineering, 2013
o Dissertation - Element Management System for a Telemedicine
Sensor Environment
AcademicProfessional
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Speaker Biography
• Motorola - R&D Lab - Communications Research Lab
o Schaumburg, IL - 2002-2008
o Medium Access - standards work (3GPP HSPA+, IEEE WiMax, 3GPP
LTE)
o PHY/MAC development of proprietary OFDM-based Antenna
Array Platform
o Multiple published articles
• Texas Instruments, Inc. - R&D Lab intern
o Dallas, TX - Summer 2009 and 2010
o Simulation work for 3GPP LTE SON ICI compression standar
o Simulation work for WiFi Alliance WiFi Direct standard
o Multiple patents filed
• TCT Mobile - Alcatel One Touch - Validation Engineering Consultant
o Irvine, CA - 2010-present
o Device RF lab manager/tester
Academic
Professional
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OFDM
Biography
Introduction to OFDM
OFDM System Model
OFDM Mathematical Model
Characteristics of OFDM
Advantages
Disadvantages
Usage of OFDM
Presentation Contents
![Page 6: Introduction to OFDM](https://reader033.fdocuments.in/reader033/viewer/2022052315/556c8803d8b42a7a738b46c9/html5/thumbnails/6.jpg)
Introduction to OFDM
• a method of encoding digital data on multiple carrier frequencies
o Wikipedia
• an FDM modulation technique for transmitting large amounts of
digital data over a radio wave
o Webopedia
• a method of digital modulation in which a signal is split into
several narrowband channels at different frequencies.
o SearchNetworking
• A digital transmission technique that uses a large number of
carriers spaced apart at slightly different frequencies
o PC Magazine Encyclopedia
DefinitionsFDM - Frequency Division MultiplexingOrthogonality
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Introduction to OFDM
• Definition
o Total bandwidth available within a communications system is divided into
smaller non-overlapping frequency sub-bands
o Usually a separate data signal is associated to each frequency sub-band
o Passband filter at receiver extracts requested frequency sub-band / data signal
• Concerns
o Sub-band spacing – guard bands
o Interference
o Tuned filters at receiver
o Spectral Efficiency – Max Baud/Data rate
• Usage Examples
o Cable television (channel, video, color, audio multiplexing) - NTSC
o Satellite Communications
Definitions
FDM - Frequency Division MultiplexingOrthogonality
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Introduction to OFDM
• Definitiono In terms of real functions, the integral of the product of any two different functions is zero.
�� � � �� � cos 2��� � � � , � ∈ ��, 0 � � � ��� � � �� � cos 2��� � � � , � ∈ ��, 0 � � � �
Az(t) – Amplitude of z signal at time t
Φz(t) – Phase of z signal at time t
fz – Frequency of z signal
T – symbol duration – useful duration
If and only if,
�� � �� � �� , � ∈ ��
Then,
�� � , �� � � � ������
���
� � � 0
• Concerns• How do you ensure signals are orthogonal? How complex is the process?
DefinitionsFDM - Frequency Division Multiplexing
Orthogonality
Cross-correlation coefficient = 0Linearly Independent
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OFDM
Biography
Introduction to OFDM
OFDM System Model
OFDM Mathematical Model
Characteristics of OFDM
Advantages
Disadvantages
Usage of OFDM
Presentation Contents
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OFDM System Model
Serial to parellel conversion
• Series of bits divided into N parallel frames of size F
• N - # of subchannels, F - # bits per symbol = 4
!"#$
#%&� '(
QPSK modulation
• Differ signal by phase to transfer information
• Xn - Modulated symbol
s)�* � cos 2�� � 4 2, � 1 ,, � )14*
TransmitterReceiver
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OFDM System Model
Inverse Fast Fouier Transform
• Creates N carriers that are all orthogonal
DAC
• Integrated circuit that converts the digital signal to analog signal
TransmitterReceiver
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OFDM System Model
Low Pass Filter
• Eliminate unwanted signals
ADC
• Integrated circuit that converts the analog signal to digital signal
Fast Fourier Transform
Transmitter
Receiver
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OFDM
Biography
Introduction to OFDM
OFDM System Model
OFDM Mathematical Model
Characteristics of OFDM
Advantages
Disadvantages
Usage of OFDM
Presentation Contents
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OFDM Mathematical Model
Euler Formula
• Connects exponential functions to trigonometry
Previously defined data signal can also be expressed differently using the above formula
� � � � cos 2�� � � ./0� exp 4�2�� � � 5
Summation of all OFDM carriers
�6 � � 1 7⁄ ! �9 exp 4�2�Δ�� � 9�6;&
9%�If the continuous data signal is sampled at multiples of the symbol duration
t = kT
�6 <� � 1 7⁄ ! �9 exp 4�2�Δ�<� � 96;&
9%�5 � 1 7⁄ ! �9 exp 42�Δ�<� exp�4 9
6;&
9%��
If difference in frequency between carriers is a multiple of inverse of sample duration, then sN is equal to generalized IFFT equation
∆f = 1/NT
IFFT - IDFT
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OFDM
Biography
Introduction to OFDM
OFDM System Model
OFDM Mathematical Model
Characteristics of OFDM
Advantages
Disadvantages
Usage of OFDM
Presentation Contents
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Characteristics of OFDM
• Can easily adapt to severe channel conditions without complex time-domain
equalization.
• Robust against narrow-band co-channel interference.
• Robust against intersymbol interference (ISI) and fading caused by multipath
propagation.
• High spectral efficiency as compared to conventional modulation schemes, spread
spectrum, etc.
• Efficient implementation using Fast Fourier Transform (FFT).
• Low sensitivity to time synchronization errors.
• Tuned sub-channel receiver filters are not required (unlike conventional FDM).
• Facilitates single frequency networks (SFNs); i.e., transmitter macrodiversity.
AdvantagesDisadvantages
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Characteristics of OFDM
• Sensitive to Doppler shift.
• Sensitive to frequency synchronization problems.
• High peak-to-average-power ratio (PAPR), requiring linear transmitter
circuitry, which suffers from poor power efficiency.
• Loss of efficiency caused by cyclic prefix/guard interval.
Advantages
Disadvantages
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OFDM
Biography
Introduction to OFDM
OFDM System Model
OFDM Mathematical Model
Characteristics of OFDM
Advantages
Disadvantages
Usage of OFDM
Presentation Contents
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Usage of OFDM
• Wired
o Power Line Communications
o Modern DSL modems
• Wireless
o Modern WiFi access points – IEEE 802.11g
o HD Radio broadcasts
WiredWirelessOFDMA - Variation of OFDM
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Usage of OFDM
• Orthogonal Frequency Division Multiple Access protocol
• Variation of OFDM into a protocol for multiple users accessing multiple data
channels
• Multiple users are provided access to bandwidth via a subset of available
frequency sub-bands
WiMax – IEEE 802.16e
- Clearwire, Clear, Sprint
- Comcast/Time Warner Mobile
LTE – Long Term Evolution – 4G Cellular
- Verizon Wireless
- AT&T Wireless
- T-Mobile
WiredWireless
OFDMA - Variation of OFDM
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OFDM
Biography
Introduction to OFDM
OFDM System Model
OFDM Mathematical Model
Characteristics of OFDM
Advantages
Disadvantages
Usage of OFDM
Presentation Contents
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