1.
By
Manish Srivastava

2.
Power delay profile
 The power delay profile (PDP) gives the intensity of
a signal received through a multipath channel as a
function of time delay.
 The time delay is the difference in travel time between
multipath arrivals.
 In the graph of the PDP abscissa represents units of
time whereas ordinate is usually in decibels. example.
 It can be measured empirically and can be used to extr
act certain channel's parameters such as the delay spre
ad.

3.
Power delay profile
 The power delay profile (PDP) gives the intensity of
a signal received through a multipath channel as a
function of time delay.
 The time delay is the difference in travel time between
multipath arrivals.
 In the graph of the PDP abscissa represents units of
time whereas ordinate is usually in decibels. example.
 It can be measured empirically and can be used to extr
act certain channel's parameters such as the delay spre
ad.

4.
Power delay profile graph….
-90
RMS Delay Spread ( ) = 46.4 ns
-90
Received Signal Level (dBm)
Mean Excess delay ( ) = 45 ns
-95
Maximum Excess delay < 10 dB = 110 ns
-100
Noise threshold
-105
0 50 100 150 200 250 300 350 400 450
Excess Delay (ns)

5.
FADING…
 It is the rapid fluctuations of received signal
strength over short time intervals and/or travel
distances
 Caused by interference from multiple copies of Tx
signal arriving at Rx at slightly different times.
 Three most important effects:
1. Rapid changes in signal strengths over small travel
distances or short time periods.
2. Changes in the frequency of signals.
3. Multiple signals arriving at different times. When added
together at the antenna, signals are spread out in time.
This can cause a smearing of the signal and interference
between bits that are received.

6.
Multipath Propagation - Fading
a b
No direct path
Diffracted
wave Reflected
wave
a a
Antenna y=a+b Antenna y=0
b b
a & b are in phase a & b are out of phase by
Complete fading when
2 d/ = n , d is the path difference
Z. Ghassemlooy

7.
 Even stationary Tx/Rx wireless links can experience
fading due to the motion of objects
(cars, people, trees, etc.) in surrounding environment
off of which come the reflections.
Transmitting a short pulse over a
(i) frequency-selective (delay-spread) fading channel:
Transmitted Received
t t
Tp Tp + dt
(ii) time-selective (Doppler-spread) fading channel:
Transmitted Received
t t
Tp Tp

8.
Small scale fading classification
Flat fading
Multi path time delay
Frequency selective fading
fading
Fast fading
Doppler spread
Slow fading

9.
Multipath time Delay Spread
Figure Explaining Multi-path Fading
 When the waves of multi-path signals are out of
phase, reduction of the signal strength at the receiver can
occur.
 The 2 types of delay spreads are-
 A) FLAT FADING
 B) FREQUENCY SELECTIVE FADING

10.
DELAY SPREAD
 The different signal paths between Tx and a Rx
corresponds to different transmission times. For an
identical signal pulse from the Tx, multiple copies of
the signals are received at the receiver at different
moments.
 The signal on the shortest path (typically LOS) reaches
first than those on longer paths. The direct effect of
these un-simultaneous arrivals of signal causes the
spread of the original signal in time domain.
 This spread is called the DELAY SPREAD.

11.
 In tele-communication, the delay spread is a measure
of the MULTI-PATH RICHNESS of a communication
channel.
 It is used most of the time in characterizing wireless
channel but applies to any other multipath channel
like multipath in optical fibres.
 Corresponding to the concept of delay spread, there is
a term called COHERENCE BANDWIDTH used to
measure the up-limit bandwidth that can be
transmitted for a channel to be free of ISI.
 Defn: Defined as the 10% of the recipocal of rms delay
spread. In this the channel passes all the
spectral components with approx. equal gain and
phase.

12.
Multipath Delay Spread
 First-arrival delay (τA)
 Mean excess delay e ( A ) P( )d
Z. Ghassemlooy

13.
Effects of delay spread:
 It causes-
INTER SYMBOL INTERFERENCE (ISI)
 if the bandwidth of a transmitter signal is less
than the channel coherence bandwidth, the channel
shows flat fading to be free of ISI.
 otherwise, the channel shows frequency selective
fading and may suffer from ISI.
•Delay spread varies with the terrain with typical values for rural, urban and
suburban areas:
0 .2 s rural 3 .0 s urban 0.5 s suburban

14.
DOPPLER SPREAD
 Defn : Time varying fading due to the motion of a
scatter or the motion of the transmitter or receiver
or both results in Doppler spread.
 It is caused by TIME SELECTIVE FADING.
i.e for a particular instance of time channel behaves as a
fading channel and for rest it behaves as FLAT channel.
TYPES OF FADING ON BASIS OF DOPPLER SPREAD:
Fast fading – channel impulse Slow fading – channel impulse
response changes rapidly within the response changes at a rate much slower than
symbol duration the transmitted symbol bandwidth

15.
EFFECT OF MOVEMENT
Here is a plot of the magnitude of fading
as a function of time and frequency.
In this case, the channel does not change
much over time. It is a slowly fading
channel.

16.
EFFECT OF MOVEMENT
Here is a plot of the magnitude of fading as a
function of time and frequency.
In this case, the channel does not vary with
frequency, it only varies over time.

17.
EFFECT OF MOVEMENT
Here is a plot of the magnitude of fading
as a function of time and frequency.
In this case, the channel varies both with
frequency and time.

18.
Doppler Effect
 When the receiver or transmitter are moving, the frequency
is shifted by f = v/ cos( ), v is velocity and is wave length
v c is the speed of light.
The maximum shift is f m fc
c
If the the signal is sent at fc and passed
through a fading channel, the spectrum of the
received signal is:
Thus, not only one frequency is received, but many.

19.
•If a sinusoidal signal is transmitted
(represented by a spectral line in the
frequency domain), after transmission over a
fading channel, we will receive a power
spectrum that is spread according to the
image.
Figure: Measured Doppler spread at 1800
MHz. Doppler spread = 60.3 Hz
•The frequency range where the power
spectrum is nonzero defines the Doppler
spread.

20.
The Doppler frequency
V
fD cos f m cos
The received signal
fr fc f m cos
frequency
• When = 0o (mobile moving away from the transmitter)
fr fc fm
• When = 90o (I.e. mobile circling around)
fr fc
• When = 180o (mobile moving towards the transmitter)
fr fc fm

21.
Doppler spread and coherence
time
 Doppler spread and coherence Tc
1
time (Tc) are inversely frms
proportional
frms is the rms value of maximum doppler shift
Given as, fm
frms
2
9
Tc
16 frms For 0.5 correlation

22.
Doppler Effect
 To mitigate the Doppler effect:
 Use low frequencies
 Transmit in bursts so the channel is constant during the
burst.
 Include training sequences on each frame so the channel
can be re-estimated for each transmission.
 Do move – indoor use only

23.
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