Wireless communication channel
Signal degradation can be classified by type :
–
Path Loss
happen during distance covered by the radio signal, it is called “Free
space
path loss “, it can be calculated by
LFS = 32.44 + 20 log F (MHz) +20 log d (Km)
–
Signal attenuation
Resulting from shadowing effects introduced by the obstacles
between transmitter and receiver
–
Fading of the signal
Caused by numerous effects all of which are related to the Radio
propagation phenomenon
Effects on Radio Communication
Wireless Multipath Channel
One of the most problem in communication channel is
fading
Fading Problems
1.
Shadowing (Normal fading):
The
reason
for
shadowing
is
the
presence
of
obstacles
like
large
hills
or
buildings
in
the
path
between
the
site
and
the
mobile
.
The signal strength received fluctuates around a
mean value while changing the mobile position
resulting in undesirable beats in the speech
signal.
Fading Problems
2.
Raleigh Fading (Multi

path Fading):
The received signal is coming from different
paths due to a series of reflection on many
obstacles. The difference in paths leads to a
difference in paths of the received components.
Parameters of multi

path channel
Time Domain Frequency
Domain
1

Max delay spread:
2

Coherence BW:
3

Coherence Time :
4

Doppler Shift:
𝜏
𝑇
𝐵
=
1
5
𝜏
𝑇
=
9
16
𝜋
2
𝐵
Doppler Shift
Phase change due to path length difference
Doppler shift (apparent change in freq.)
∆
𝜑
=
2
𝜋
∆
𝐿
𝜆
=
2
𝜋𝜈
Δ
𝜆
𝑜 𝜃
=
1
2
𝜋
.
Δ
𝜑
Δ
=
𝜈
𝜆
𝑜 𝜃
X
Y
S
𝜃
𝜃
d
Δ
𝐿
𝜈
Types of fading
𝜏
𝜏
At High Data Rate
•
High data rate transmission
short symbol time compared to the delay spread.
𝑇
𝑦𝑜
<
𝑇
𝑦
𝑇
𝐷
𝑇
= Delay spread
= Symbol period
Problems
ISI
1

2

𝐵
𝐵
= signal BW
= coherence BW
Orthogonal frequency division multiplexing
(OFDM)
•
OFDM
was
introduced
in
1950
but
was
only
completed
in
1960
’s
Originally
grew
from
Multi

Carrier
Modulation
used
in
High
Frequency
military
radio
.
•
Patent
was
granted
in
1970
’s
•
Earlier
OFDM
wasn’t
popular
Large
arrays
of
sinusoidal
generators
and
coherence
demodulator
Too
expensive
and
complex
.
•
Later
when
DFT
and
IDFT
became
a
known
solution
to
the
arrays
of
generators
and
demodulators
.
•
It
was
still
not
popular
as
there
is
no
efficient
method
to
perform
the
IFFT
and
FFT
operation
.
•
Advances
in
VLSI
technology
allows
implementation
of
fast
and
cheap
FFT
and
IFFT
operation
drive
OFDM
popularity
.
OFDM
Orthogonal
Frequency Division Multiplexing
•
Frequency Division Multiplexing

Divide the information over several carriers
Instead of using one big truck
When
The
truck is lost…
All is lost!
Use several small trucks
When
one
truck is lost…
Only a portion of the shipment is lost!
Concept of an OFDM signal
Ch.1
Ch.2
Ch.
3
Ch.4
Ch.
5
Ch.
6
Ch.7
Ch.8
Ch.9
Ch.10
Saving of bandwidth
Ch.3
Ch.
5
Ch.
7
Ch.9
Ch.2
Ch.4
Ch.6
Ch.
8
Ch.10
Ch.
1
Conventional multicarrier techniques
Orthogonal multicarrier techniques
50% bandwidth saving
frequency
frequency
OFDM changes Frequency Selective Fading to Flat Fading
Channel
𝑇
.
.
.
𝑁𝑇
.
.
.
N number of subcarrier
Solution to Frequency Selective Fading
When the data rate is lower
𝐵
𝐵
= Delay spread
= Symbol period
= signal BW
= coherence BW
Frequency Selective => Flat Fading
In flat fading, the amplitude varies but there is no ISI
Multicarrier Modulation
•
Divide broadband channel into narrowband subchannels
–
No ISI in
subchannels
if constant gain in every
subchannel
and if ideal
sampling
•
Orthogonal Frequency Division Multiplexing
–
Based on the fast Fourier transform
–
Standardized for DAB, DVB

T, IEEE
802.11
a,
802.16
a,
HyperLAN
II
–
Considered for fourth

generation mobile communication systems
subchannel
frequency
magnitude
subcarrier
channel
Subchannels are
312
kHz wide in
802.11
a and HyperLAN II
Carrier
1
has a maximum
where
all other
carriers
are
0
OFDM
Frequency Spectrum
s
k
T
k
f
f
1
0
8192
4096
232
Hz
5
.
4312
or
N
s
T
f
s
s
Use many carriers that are
equally
spaced:
k =
0
,
1
, … , N

1
T
s
= Symbol Time
1
2
3
frequency
s
T
f
1
4
5
ISI
OFDM
Many carriers with small spacing => Long symbol time
But many carriers carry a lot of information!
Long symbol time is an
advantage!
•
Delay Spread (Multipath)
Symbol n

1
Symbol n
Symbol n+
1
Direct Path
Delayed Path
ISI = Inter Symbol Interference
ISI
Symbol n

1
Symbol n
Symbol n+
1
OFDM
Avoid ISI and preserve Orthogonality => Guard Interval
Direct Path
Delayed Path
Integration
Period
Symbol n

1
Guard
Symbol n
Guard
Symbol n
Guard
Symbol n+
1
Guard
Symbol n

1
Guard
Symbol n
Guard
Symbol n+
1
Guard
s
ol Time
Total Symb
s
Guard
ol Time
Total Symb
Guard
μs
e
Symbol Tim
290
58
232
58
4
232
Useful Symbol length
Guard
Total Symbol length
Symbol n is added constructively
or destructively according to
phase
Avoid ICI and preserve Orthogonality
=
>
cyclic prefix
N
samples
v
samples
CP: Cyclic Prefix
CP
CP
s y m b o l
i
s y m b o l
(
i
+
1
)
copy
copy
Discrete
versus
Fast
Fourier Transform
•
Discrete
(DFT):
–
For each frequency sample ‘k’ (
0
to N

1
) loop ‘n’ (over
0
to N

1
) => N
2
complex multiplications
•
Fast
(FFT, Cooley

Tukey
algorithm):
–
“An efficient algorithm to calculate a DFT”
–
N.log(N) complex multiplications
k
n
N
j
N
n
n
k
e
x
X
.
.
2
1
0
DFT
respect to
with
0,3%
4096
12
:
FFT
152
49
12
4096
Fast
tions
multiplica
216
777
16
4096
*
4096
Discrete
4096
:
Example
.
*
.
.
N
OFDM Block Diagram
•
High
spectral
efficiency
.
And
high
data
rate
.
•
Efficient
in
multipath
environments
.
•
Simple
digital
realization
by
using
the
FFT
operation
.
•
Low
complex
receivers
due
to
avoidance
of
ISI
.
•
Different
modulation
schemes
can
be
used
on
individual
sub

carriers
.
Main advantages
x Drawbacks
•
Large
Peak
to
Average
Ratio
(PAR)
.
Added
sinusoid
cause
large
PAR
and
issue
of
amplifier
non

linearity
arises
.
•
Accurate
frequency
and
time
synchronization
is
required
.
•
More
sensitive
to
Doppler
spreads
than
single

carrier
schemes
.
•
Sensitive
to
frequency
offset
and
phase
noise
caused
by
imperfections
in
the
transmitter
and
the
receiver
oscillators
.
•
Guard
interval
causes
loss
in
spectral
efficiency
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