EE 132A, Spring 2007
Communication Systems
Prof. John Villasenor
Handout # 18
TA:
Choo Chin (Jeffrey) Tan
Computer Assignment 4
Assigned: Monday, May 21, 2007
Due: Wednesday, May 30, 2007
In this computer assignment, we will learn about Orthogonal Frequency Division Multiplexing (OFDM) by
using Matlab to look at some examples. OFDM is a popular modulation technique used in several
technologies such as ADSL (G.DMT), IEEE 802.11a/g (WiFi), UWB, and IEEE 802.16 (WiMAX). We
will explore OFDM generation and detection, its ability to reduce ISI, and its performance in noise.
We will start by generating a binary PAM signal and sending it through a channel that introduces ISI. Then
we will send the same information using OFDM with 4 subcarriers and an appropriate cyclic prefix, and
observe how ISI is eliminated. Finally we will send information through a noisy channel and compute bit
error probabilities.
What to turn in:
•
Answers to questions and required plots.
•
Source file for programs (.m files).
Suggestions:
•
Start working on this assignment early. Do not wait until the last moment.
•
Make your code parametric. You will be required to design an OFDM modulator and
demodulator, and then change the different parameters to observe different situations. It is easier
to do this if you define your modules in terms of these parameters.
Introduction to OFDM
The basic principle of OFDM is to split a highrate data stream into a number of lower rate streams that are
transmitted simultaneously over a number of subcarriers. Because the symbol duration increases for lower
rate parallel subcarriers, the relative amount of dispersion in time caused by multipath delay spread is
decreased. Inter Symbol Interference (ISI) is eliminated almost completely by introducing a guard time at
the beginning of every OFDM symbol.
The frequency spacing between these subcarriers is chosen to be equal to the inverse of the OFDM symbol
interval. When no cyclic prefix is added, this causes the subcarriers to be orthogonal over the symbol
interval, and the spectrum of each of the subcarrier signals to be zero at the peak frequency of all signals in
the other subcarrier frequencies, thus eliminating Inter Carrier Interference (ICI).
The basic diagram of the OFDM system is shown in Figure 1.
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View Full DocumentFigure 1: OFDM system
In an OFDM system with
N
subcarriers, input (information) symbols are grouped in blocks of
N
symbols
each. Each one of these blocks of N symbols are transmitted into what we will call an
OFDM symbol
. We
will assume there exist
M
blocks, so we are transmitting
MN
input symbols. For the
m
th block, the
transmitted symbols are {
x
0,
m
,
x
1,
m
,
…
,
x
N1,
m
}, as shown in Figure 1.
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 Spring '08
 Walker
 cyclic prefix, OFDM symbol

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