Introduction• A transmitted signal undergoes changes while traveling through the

propagation path to the receiver. The effect of these changes is commonly called fading.

• In free-space, a signal follows one path and arrives at the receiver with little attenuation. This is not the case for a signal that encounters obstacles in the propagation path.

• Instead, the signal is reflected, diffracted, and scattered from objects that are present in the path. Each path can have a different amount of attenuation, delay, and fading type.

• The combination of these different paths is termed multipath fading or multipath propagation. Simulating different numbers of paths with different fading configurations is essential for thoroughly testing receiver performance.

• At the receiver, the signals can add constructively or destructively, causing random and rapid fluctuations in the received amplitude when the receiver or the transmitter is moving. Due to the Doppler effect, this situation also causes the signal to be spread in the frequency domain.

• The fading phenomena can be classified into two main groups known as large scale fading and small scale fading.

• 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 is used to describe the signal level at the receiver after encountering obstacles near (several wavelengths to fractions of wavelengths) the receiver.

Small-Scale Multipath Propagation

• The three most important effects– Rapid changes in signal strength over a small travel distance or

time interval– Random frequency modulation due to varying Doppler shifts on

different multipath signals– Time dispersion caused by multipath propagation delays

• Factors influencing small-scale fading– Multipath propagation: reflection objects and scatters– Speed of the mobile: Doppler shifts– Speed of surrounding objects– Transmission bandwidth of the signal

• The received signal will be distorted if the transmission bandwidth is greater than the bandwidth of the multipath channel.

• Coherent bandwidth: bandwidth of the multipath channel.

Fading Channel

Large-scale: Path lossMedium-scale: ShadowingSmall-scale: Multipath fading

d

Pr/Pt

6

From Signals to Packets

Analog Signal

“Digital” Signal

Bit Stream 0 0 1 0 1 1 1 0 0 0 1

Packets0100010101011100101010101011101110000001111010101110101010101101011010111001

Header/Body Header/Body Header/Body

ReceiverSenderPacket

Transmission

NORMAL SHADOWING

Tx Rx0 Rx

d0

d

Small Scale Fading• Causes of small scale fading

– Multipaths

• Coherence distance, time, and Doppler spread– Coherence distance: the signal strength experiences change from

largest to smallest traveling the distance– Coherence time: the time a moving Rx travels the coherence distance– Doppler spread: coherence time expressed in time domain

• Coherence bandwidth and delay spread– Coherence bandwidth: the frequency band causing signal changes

from largest to smallest– Delay spread: coherence bandwidth expressed in time domain

Small scale fading

Rapid fluctuations of the signal over short period of time

Invalidates Large-scale path loss Occurs due to multi-path waves

Two or more waves (e.g: reflected/diffracted/scattered waves)

Such waves differ in amplitude and phase

Can combine constructively or destructively resulting in rapid signal strength fluctuation over small distances

Example of Multipath

Phase difference between original and reflected wave

Small Scale Fading

• The signal variation over a short period of time or a short distance

Large Scale Fading

• Large scale fading is due to the shadowing effect of large size objects (buildings, mountains)

• It is the mean signal strength (or power) vs. (large) distance between Tx and Rx

• Path loss and path gain– The ratio of total transmitted power over the received

power– The path gain is 1/path loss

                                                           

ECE6331 Spring 2009

Multipathradio propagation in urban areasMultipathradio propagation in urban areas

Fading Channel

Large-scale Fading Small-scale Fading

Path Loss Shadowing Effect Multipath Delay Spread

DopplerSpread

Flat Fading

FrequencySelectiveFading

Fast Fading

Slow Fading

Signal BW << Channel BWSymbol period >> Delay spread

Signal BW > Channel BWSymbol period < Delay spread

High Doppler spreadSymbol period > Coherence TimeSignal variation < Channel variation

Low Doppler spreadSymbol period << Coherence TimeSignal variation >>Channel variation

Mobile SpeedPropagation Environment

Small scale fading

Multi path time delay

Doppler spread

Flat fading BC

BS

Frequency selective fading BC

BS

TC

TSSlow fading

Fast fading TC

TS

Small scale fading