Design and Simulation of
Orthogonal Frequency Division Multiplexing
(OFDM) Signaling
Final Report
Study by:
Alan C. Brooks
Stephen J. Hoelzer
Department:
Electrical and Computer Engineering
Advisors:
Dr. Thomas L. Stewart
Dr. In Soo Ahn
May 15, 2001
Abstract:
A MATLAB program has been written to investigate Orthogonal Frequency Division
Multiplexing (OFDM) communication systems. This program is valuable for future researchers
simulating systems that are too theoretically complex to analyze. Single-carrier QAM and multi-
carrier OFDM are compared to demonstrate the strength of OFDM in multipath channels. Two
graphical user interface demonstrations show some of the basic concepts of OFDM.
1
Introduction
The Electrical Engineering Senior Capstone Project is intended to give each student
experience in completing a sophisticated design project that spans most of the senior year.
Planning, management of time, allocation of responsibility, documentation, and presentation of
the results are integrated with the technical design task. The students work with one or two
faculty advisors who have expertise in the project research area. The student is fully responsible
for the design project, with the advisor(s) acting as guide and mentor. Each student is expected to
work an eight-hour lab period each week from October through May.
A common problem found in high-speed communication is inter-symbol interference
(ISI). ISI occurs when a transmission interferes with itself and the receiver cannot decode the
transmission correctly. For example, in a wireless communication system such as that shown in
Figure 1, the same transmission is sent in all directions.
Figure 1: Multipath Demonstration
2
Because the signal reflects from large objects such as mountains or buildings, the receiver sees
more than one copy of the signal. In communication terminology, this is called multipath. Since
the indirect paths take more time to travel to the receiver, the delayed copies of the signal
interfere with the direct signal, causing ISI.
Theory
This project will focus on Orthogonal Frequency Division Multiplexing (OFDM)
research and simulation. OFDM is especially suitable for high-speed communication due to its
resistance to ISI. As communication systems increase their information transfer speed, the time
for each transmission necessarily becomes shorter. Since the delay time caused by multipath
remains constant, ISI becomes a limitation in high-data-rate communication [1]. OFDM avoids
this problem by sending many low speed transmissions simultaneously. For example, Figure 2
shows two ways to transmit the same four pieces of binary data.
Figure 2: Traditional vs. OFDM Communication
Suppose that this transmission takes four seconds. Then, each piece of data in the left picture has
a duration of one second. On the other hand, OFDM would send the four pieces simultaneously
as shown on the right. In this case, each piece of data has a duration of four seconds. This longer
3
duration leads to fewer problems with ISI. Another reason to consider OFDM is low-complexity
implementation for high-speed systems compared to traditional single carrier techniques [2].
Significance
With the rapid growth of digital communication in recent years, the need for high-speed
data transmission has increased. New multicarrier modulation techniques such as OFDM are
currently being implemented to keep up with the demand for more communication capacity.
Multicarrier communication systems “were first conceived and implemented in the 1960s, but it
was not until their all-digital implementation with the FFT that their attractive features were
unraveled and sparked widespread interest for adoption in various single-user and multiple
access (MA) communication standards” [2]. The processing power of modern digital signal
processors has increased to a point where OFDM has become feasible and economical.
Examining the patents, journal articles, and books available on OFDM, it is clear that this
technique will have an impact on the future of communication. See the references section
(starting on page 21) for a condensed bibliography and list of patents related to this topic. Since
many communication systems being developed use OFDM, it is a worthwhile research topic.
Some examples of current applications using OFDM include GSTN (General Switched
Telephone Network), Cellular radio, DSL & ADSL modems, DAB (Digital Audio Broadcasting)
radio, DVB-T (Terrestrial Digital Video Broadcasting), HDTV broadcasting, HYPERLAN/2
(High Performance Local Area Network standard), and the wireless networking standard IEEE
802.11 [1] [3] [4].
4
Simulation Design
This project consists of research and simulation of an OFDM communication system.
Figure 3 shows a simplified flowchart of the MATLAB simulation code.
Figure 3: OFDM Simulation Flowchart
The transmitter first converts the input data from a serial stream to parallel sets. Each set of data
contains one symbol, S
i
, for each subcarrier. For example, a set of four data would be [S
0
S
1
S
2
S
3
]. Before performing the Inverse Fast Fourier Transform (IFFT), this example data set is
arranged on the horizontal axis in the frequency domain as shown in Figure 4.
