Doc. no. SSY135/ext:05, rev. B
Project 2
Design and Simulation of a Communication
System over Wireless Fading Channel
Erik Str¨om and Kasra Haghighi
February 4, 2010
Contents
1 Project purpose 3
2 Project background 3
3 Uncoded OFDM over Time-Varying Frequency-Selective Rayleigh
Channels 3
4 Coded OFDM over Time-Varying Frequency-Selective Rayleigh
Channels 9
5 Project deliverable 10
A Appendix: guidelines, tips and hints 11
1
2 (11)
Project 2
Design and Simulation of a Communication System over Wireless Fading Channel
Doc. no.: SSY135/ext:05, rev.: B, date: February 4, 2010, file: project2_rev2.tex
Project 2
Design and Simulation of a Communication System over Wireless Fading Channel 3 (11)
1 Project purpose
1.1 Objectives
The objective of this project is to understand the design methodology for an
OFDM system over time-varying frequency-selective channels.
1.2 Learning outcomes
After completing the project, the student should be able to
• simulate a frequency-selective channel and quantify the time and frequency
variations in terms of coherence time and coherence bandwidth .
• design and simulate a communication system to transmit over simulated
channel and choose the design parameters based on channel properties and
system requirements.
• analyze and compare the BER results with theory.
2 Project background
In this project we will study communication over time-varying Rayleigh fading
channels with Clarke’s Doppler spectrum. In the first project, a fading channel
is simulated and different parameters of this channel and their influences on vari-
ation along time and frequency axes reviewed. In second project, the channel
in the first project will be used, analyzed and certain properties of it like co-
herence time and delay spread will be extracted. Based on these properties and
certain requirements on bandwidth, power, bit rate and bit error rate, a multi-
carrier communication system for ISI and ICI free transmission over specified
time-varying frequency-selective channel should be designed.
3 Uncoded OFDM over Time-Varying Frequency-
Selective Rayleigh Channels
The system under consideration is seen in Figure 1. As seen, the information bits
b[k] ∈ {±1} are first transformed into the symbols s[p] using QPSK,
s[p] =
r
E
2
(b[2p − 1] + jb[2p]). (1)
The symbols are then blocked into blocks of N symbols to form the transmitted
vector s[m]. We will view the elements of s[m] as the samples of the transmitted
Doc. no.: SSY135/ext:05, rev.: B, date: February 4, 2010, file: project2_rev2.tex
4 (11)
Project 2
Design and Simulation of a Communication System over Wireless Fading Channel
[ ]
b k
( )
x t
[ ]
m
s
!!
( )
n t
"
!
[ ]
m
r
ˆ
[ ]
m
C
ˆ
[ ]
b k
ˆ
[ ] [ ]
∗
m m
C r
[ ]
s p
[ ]
m
z
[ ]
m
y
( )
s
x nT
Figure 1: Block diagram of uncoded OFDM system over a frequency-selective
channel
signal in the frequency domain. To get the time-domain samples, we use a (scaled)
inverse FFT. Consider the transmitted vector
s[0] =
s[0] s[1] ··· s[N − 1]
T
The vector z[0] is then
z[0] =
z[0] z[1] ··· z[N − 1]
T
where
z[n] =
1
√
NT
s
N−1
X
k=0
s[k] exp
j2π
nk
N
, n = 0, 1, . . . , N − 1 (2)
and T
s
is the sample interval used in the D/A in Figure 1.
In MATLAB, this would be done as
1
>> z = sqrt(N/Ts)*ifft(s);
The scale factor 1/
√
NT
s
in (2) is needed solely for normalization purposes, and
will allow us to interpret E in (1) as the energy per transmitted sample.
From (2) we see that z[n] consists of N complex exponentials that are scaled
with the QPSK symbols and truncated to 0 ≤ n ≤ N − 1. The frequency
associated with the symbol s[k] is f
k
= k/N. Hence, all exponentials are goes
Doc. no.: SSY135/ext:05, rev.: B, date: February 4, 2010, file: project2_rev2.tex
