EELE 5490, Fall, 2009EELE 5490, Fall, 2009

Wireless CommunicationsWireless Communications

Ali S. Afana

Department of Electrical Engineering

Class 5

Dec. 4th, 2009

                                                           

Small-Scale FadingSmall-Scale Fading Rapid fluctuations of radio signal amplitude, phase, or delays Occurs on short time period or short travel distance Large-scale path loss effects can be ignored Caused by arrival of two or more waves from the source

combining at the receiver Resultant detected signal varies widely in amplitudes and phase Bandwidth of transmitted signal is important factor

                                                           

Experimental record of received signal envelope in an urban area

                                                           

Multipath radio propagation in urban areas

                                                           

Determining the impulse response of a channel

Transmit a narrowband pulse into the channel

Measure replicas of the pulse that traverse different paths between transmitter and receiver

                                                           

Small-scale Multipath Propagation Fading: The rapid fluctuation of the amplitude of a radio signal over

a short period of time or travel distance. Fading is caused by interference between two or more versions of the

transmitted signal, which arrive at slightly different times. Multipath in the radio channel creates small-scale fading effects. Phenomenon : 1. Rapid changes in signal strength over a small travel distance

or time interval. 2.  Random frequency modulation due to varying Doppler shifts

on different multipath signals. 3.  Time dispersion caused by multipath propagation delays. If objects in the radio channel are static, and motion is considered to

be only due to that of the mobile, then fading is purely a spatial phenomenon.

Antenna space diversity can prevent deep fading nulls.

                                                           

Factors influencing Small-scale fading Multipath propagation: multipath propagation often lengthens the

time required for the baseband portion of the signal to reach the receiver which can cause signal smearing due to inter-symbol interference.– Draw a figure to explain ISI

Speed of the mobile: generate random Doppler shifts.– Train passing

Speed of surrounding objects: if the surrounding objects move at a greater rate than the mobile, then this effect dominates the small-scale fading.

The transmission bandwidth of the signal: if signal’s bandwidth bandwidth of the multipath channel received signal will be distorted. – The coherent bandwidth is a measure of the maximum

frequency difference for which signals are still strongly correlated in amplitude.

                                                           

Comparison of the BER for a fadingand non-fading channel

                                                           

Doppler ShiftDoppler Shift

                                                           

Illustration of Doppler effectIllustration of Doppler effect

                                                           

Doppler Shift Distance difference Phase difference Doppler frequency shift

Frequency shift is positive when mobile moves toward source

In a multipath environment, frequency shift for each ray may be different, leading to a spread of received frequencies.

For example, for pure sinusoid, the signal blurred in frequency.

Example 5.1

                                                           

Impulse Response Model of a Multipath Channel

Model radio channel as a linear filter with a time-varying impulse response

Time variation due to motion of receiver and/or objects in the environment

“Filtering” is caused by the summation of the amplitudes and delays of multiple arriving waves at an instant in time

d=vt

                                                           

Fading due to two incoming signals combine with different phases

                                                           

Small-scale Multipath Measurements

Three methods of wideband channel sounding techniques Direct RF Pulse System Spread Spectrum Sliding Correlator Channel Sounding

Frequency Domain Channel Sounding Direct RF Pulse System

Determine the power delay profile of any channel by using pulse signal with pulse width bb. The main problem with this system is that it is subject to interference and noise.

Another disadvantage is that the phases of the individual multipath components are not received.

                                                           

Questions?Questions?