John W. Thomas

PhD Candidate - University of Texas at Dallas

OFDM

Speaker Biography

Introduction to OFDM

OFDM System Model

OFDM Mathematical Model

Characteristics of OFDM

Advantages

Disadvantages

Usage of OFDM

Presentation Contents

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

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

OFDM

Biography

Introduction to OFDM

OFDM System Model

OFDM Mathematical Model

Characteristics of OFDM

Advantages

Disadvantages

Usage of OFDM

Presentation Contents

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

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

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

OFDM

Biography

Introduction to OFDM

OFDM System Model

OFDM Mathematical Model

Characteristics of OFDM

Advantages

Disadvantages

Usage of OFDM

Presentation Contents

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

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

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

OFDM

Biography

Introduction to OFDM

OFDM System Model

OFDM Mathematical Model

Characteristics of OFDM

Advantages

Disadvantages

Usage of OFDM

Presentation Contents

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

OFDM

Biography

Introduction to OFDM

OFDM System Model

OFDM Mathematical Model

Characteristics of OFDM

Advantages

Disadvantages

Usage of OFDM

Presentation Contents

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

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

OFDM

Biography

Introduction to OFDM

OFDM System Model

OFDM Mathematical Model

Characteristics of OFDM

Advantages

Disadvantages

Usage of OFDM

Presentation Contents

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

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

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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