Power Delay ProfileDelay Spread

Doppler Spread

ByManish Srivastava

Power delay profileThe 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 extract certain channel's parameters such as the delay spread.

Power delay profileThe 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 extract certain channel's parameters such as the delay spread.

Rec

eive

d S

igna

l Lev

el (

dBm

)

-105

-100

-95

-90

-90

0 50 100 150 200 250 300 350 400 450

Excess Delay (ns)

RMS Delay Spread () = 46.4 ns

Mean Excess delay () = 45 ns

Maximum Excess delay < 10 dB = 110 ns

Noise threshold

Power delay profile graph….

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.

Multipath Propagation - Fading

Z. Ghassemlooy

AntennaAntennaa

by = a + b

a & b are in phase a & b are out of phase by

Complete fading when 2d/ = n, d is the path difference

Diffractedwave Reflected

wave

No direct patha b

AntennaAntennab

y = 0a

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:

(ii) time-selective (Doppler-spread) fading channel:

t tTp Tp + dt

Transmitted Received

t tTp Tp

Transmitted Received

Small scale fading classification

Multi path time delay

Doppler spread

Flat fading

Frequency selective fading

Slow fading

Fast fading

fading

Multipath time Delay Spread

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

Figure Explaining Multi-path Fading

DELAY SPREADThe 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.

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.

Multipath Delay SpreadFirst-arrival delay (τA)Mean excess delay dPAe )()(

Z. Ghassemlooy

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:

( )ruralsm2.0» ( )urbansm0.3» ( )suburbansm5.0»

DOPPLER SPREADDefn : 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.Fast fading – channel impulse

response changes rapidly within the symbol duration

Slow fading – channel impulse response changes at a rate much slower than the transmitted symbol bandwidth

TYPES OF FADING ON BASIS OF DOPPLER SPREAD:

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.

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.

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.

Doppler EffectWhen the receiver or transmitter are moving, the

frequency is shifted by f = v/ cos(), v is velocity and is wave length

cm fc

vf isshift maximum The c is the speed of light.

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.

Figure: Measured Doppler spread at 1800 MHz. Doppler spread = 60.3 Hz

•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.

•The frequency range where the power spectrum is nonzero defines the Doppler spread.

• When = 0o (mobile moving away from the transmitter)

mcr fff • When = 90o (I.e. mobile circling around)

cr ff • When = 180o (mobile moving towards the transmitter)

mcr fff

The Doppler frequency

coscos mD fV

f

The received signal

frequency cosmcr fff

Doppler spread and coherence timeDoppler spread and

coherence time (Tc) are inversely proportional

frmsTc

1

rmsc fT

16

9

For 0.5 correlation

frms is the rms value of maximum doppler shift

2

mrms

ff Given

as,

Doppler EffectTo mitigate the Doppler effect:

Use low frequenciesTransmit 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

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