UNIT - III

MOBILE RADIO PROPAGATION – SMALL

SCALE FADING AND MULTIPATH

Dr.S.Praveen Chakkravarthy
 INTRODUCTION
• Large scale fading - Large-scale fading is the result of signal attenuation due
to signal propagation over large distances and diffraction around large objects in the
propagation path.
• (The LARGE SCALE fading is used to describe the signal level at the
receiver after traveling over a large area (hundreds of wavelengths).
• Small scale fading or simply fading is used to describe the rapid fluctuations
of the amplitudes, phases, or multi path delays of radio signal over a short period of
time or travel distance, so that large scale path loss effects may be ignored.
• Small scale fading is used to describe the signal level at the receiver after
encountering obstacles near (several wavelengths to fractions of
wavelengths) the receiver.
SMALL SCALE FADING

• Describes rapid fluctuations of the amplitude, phase of multipath delays

of a radio signal over short period of time or travel distance Caused by
interference between two or more versions of the transmitted signal
which arrive at the receiver at slightly different times.
• These waves are called multipath waves and combine at the receiver
antenna to give a resultant signal which can vary widely in amplitude
and phase.
SMALL SCALE MULTIPATH
PROPOGATION

• Effects of multipath
• Rapid changes in the signal strength
• Over small travel distances, or
• Over small-time intervals
• Random frequency modulation due to varying Doppler shifts on different multiple signals
• Time dispersion (echoes) caused by multipath propagation delays
• Multipath occurs because of
• Reflections
• Scattering
MULTIPATH

• At a receiver point
• Radio waves generated from the same transmitted signal may come
from different directions
• with different propagation delays
• with (possibly) different amplitudes (random)
• with (possibly) different phases (random)
• with different angles of arrival (random).
• These multipath components combine vectorially at the receiver antenna and cause
the total signal
• to fade
• to distort
SMALL SCALE MULTIPATH PROPOGATION
SMALL SCALE MULTIPATH
PROPOGATION

• In Built up Urban areas – NLOS Occurs due to the fact that height of the
mobile antenna is well below the height of the surrounding structure.
• Even if LOS exists , MULTIPATH occurs due to reflections from ground
structures and surrounding structures
• Therefore the wave arrives the receiver from different directions with
different propagation delay.(have randomly distributed amplitude ,
phase and angle of arrival.)
• These Multipath signals c, which leads to Fading.
• Even when the mobile receiver is Stationary, the received signal may
fade due to the movement of surrounding objects in the channel.
DEMONSTRATION AND
ANALYSIS OF SMALL SCALE
FADING EFFECTS

• SMALL SCALE FADING EFFECTS

FACTORS INFLUENCING SMALL
SCALE FADING

• Multipath Propagation
• Speed of the Mobile
• Speed of Surrounding Objects
• Transmission Bandwidth of the Signal
• B.W of Tx Signal > B.W of the Multipath channel – SIGNAL DISTORTS
but don’t FADE over local area
• COHERENCE BANDWIDTH – Maximum frequency difference for which
signals are strongly correlated in amplitude
• If the transmitted signal has a narrow bandwidth compared to
compared to the channel , the amplitude of the signal will change
rapidly , but the signal will not be distorted in time.
DOPPLER SHIFT
 DOPPLER SHIFT
• The Phase change in the received signal due to difference in path
lengths is therefore
 DOPPLER SHIFT

The apparent change in Doppler Shift or frequency is given by

IMPULSE RESPONSE MODEL OF A
MULTIPATH CHANNEL

• The small-scale variations of a mobile radio signal can be directly related to the impulse
response of the mobile radio channel.
• The impulse response is a wideband channel characterization and contains all information
necessary to simulate or analyze any type of radio transmission through the channel.
• This stems from the fact that a mobile radio channel may be modelled as a linear filter
• with a time varying impulse response, where the time variation is due to
• receiver motion in space.
• The filtering nature of the channel is caused by the summation of amplitudes and delays of
the multiple arriving waves at any instant of time.
• The impulse response is a useful characterization of the channel, since it may be used to
predict and compare the performance of many different mobile communication systems and
transmission bandwidths for a particular mobile channel condition.
From above equation it is clear that the mobile radio channel can be
modelled as a linear time varying channel, where the channel changes with
time and distance.
PARAMETERS OF MOBILE
MULTIPATH CHANNELS
 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.
• Expressed in decibels.
• For outdoor channels –wavelength – 6m
• For Indoor channels –wavelength – 2m
A SAMPLE PDP
 TIME DISPERSION
PARAMETERS

• To develop guidelines for design of Multipath Channel , following parameters are considered
• Mean Excess Delay - The relative delay of the ith multipath component as compared to the first arriving component and is. denoted by i.
• RMS Delay spread -measured relative to the first detectable signal to at the receiver at t 0=0.
• Excess Delay Spread - defines the temporal extent of the multipath that is above a particular threshold.
DELAY SPREAD
 COHERENCE
BANDWIDTH

• It is the statistical measure of the range of frequencies over

which the channel can be considered ‘FLAT’ ( i.e.. A channel
which passes all spectral components with approximately
Equal Gain and Linear Phase)
• Used to characterize channel in frequency domain
• 2 cases of correlation btw 2 sinusoids (0.9 & 0.5)
 DOPPLER SPREAD AND COHERENCE
TIME

• DOPPLER SPREAD - Doppler spread BD is a measure of the

spectral broadening caused by the time rate of change of the
mobile radio channel .

• COHERENCE TIME - Coherence time Tc is the time domain dual

of Doppler spread and is used to characterize the time varying
nature of the frequency dispersiveness of the channel in the
time domain.
DOPPLER SPREAD

• When a pure sinusoidal frequency fc is transmitted ,it will have

components fc+fd and fc-fd where fd is the doppler shift
• The amount of broadening depends on fd (where fd is a function of
relative velocity of the mobile and the angle between the direction of
motion of the mobile and direction of arrival of scattered waves)
• If the bandwidth of the baseband signal is much greater than BD , the
effect of Doppler Spread is negligible at the receiver .This is SLOW
FADING Channel.
 COHERENCE TIME

• It is actually a measure of the time duration over which the

channel impulse response is essentially invariant and quantifies
the similarity of the channel response at different times.

• It is the time duration over which two received signals have a

strong potential for amplitude correlation.
TYPES OF SMALL-SCALE FADING

1. Based on MULTIPATH TIME DELAY SPREAD

A . Flat Fading

B.W of signal < B.W of Channel

Delay Spread < Symbol Period)

B . Frequency Selective Fading

B.W of signal > B.W of Channel

Delay Spread > Symbol Period
TYPES OF SMALL SCALE FADING

2. Based on Doppler Spread

A. Fast Fading
High Doppler Spread
Coherence Time < Symbol Period
Channel variations faster than base band signal variations
B. Slow Fading
Low Doppler Spread
Coherence Time > Symbol Period
Channel variations slower than base band signal variations
FADING EFFECTS DUE TO MULTIPATH
DELAY SPREAD – FLAT FADING

• If mobile radio channel has a Constant Gain and Linear Phase response
over a bandwidth > bandwidth of transmitted signal – FLAT FADING.

• Here Multipath structure of the channel is such that the SPECTRAL

CHARACTERISTICS of the transmitted signal is preserved at the receiver.

• But the strength of the received signal changes with time , due to
fluctuations in the Gain of the Channel caused by Multipath.
FLAT FADING CHARACTERISTICS
FLAT FADING CHARACTERISTICS
• Also called as

• Amplitude Varying Channels or Narrowband

Channels

• Need 20 to 30 dB more power at the Transmitter end to

achieve Low BER.

• Rayleigh Fading
RAYLEIGH FADING
FLAT FADING CHARACTERISTICS
FADING EFFECTS DUE TO MULTIPATH DELAY
SPREAD – FREQUENCY SELECTIVE FADING

• If mobile radio channel has a Constant Gain and Linear

Phase response over a bandwidth < bandwidth of
transmitted signal – FREQUENCY SELECTIVE FADING.
• Here Channel Impulse response has a Multipath Delay
Spread > Reciprocal Bandwidth of the Transmitted Signal.

• Attenuation occurs in this case – ISI (Inter Symbol

Interference).

• Design of Frequency Selective Fading Channel is difficult

as it involves modelling of the channel to act as a
LINEAR FILTER.
FREQUENCY SELECTIVE FADING CHARACTERISTICS
FREQUENCY SELECTIVE FADING
CHARACTERISTICS
• WIDEBAND CHANNELS.
• Gain of each SPECTRAL COMPONENT varies when compared with the other.
FADING EFFECTS DUE TO DOPPLER
SPREAD- FAST FADING
• FAST FADING CHANNEL – Channel Impulse Response changes rapidly within the Symbol
Duration.
• Coherence time of Channel > Symbol Period of Transmitted Signal
• FREQUENCY DISPERSION occurs here – Signal Distortion.
FAST FADING CHARACTERISTICS

• The Amplitude , Phase and Time Delay of any one

of the Multipath components vary faster than the
rate of change of the transmitted signal.

• It occurs in very low data rates.

FADING EFFECTS DUE TO DOPPLER
SPREAD- SLOW FADING
• FAST FADING CHANNEL – Channel Impulse Response changes very slow than the
than the transmitted baseband signal.
• Channel - STATIC
• Doppler Spread of the Channel < Bandwidth of Transmitted Signal
STATISTICAL MODELS FOR MULTIPATH
FADING CHANNELS
STATISTICAL MODELS FOR MULTIPATH
FADING CHANNELS
TWO RAY INDEPENDENT RAYLEIGH
CHANNEL MODEL