EELE 5490, Fall, 2009 Wireless Communications

1. EELE 5490, Fall, 2009Wireless Communications Ali S. Afana Department of Electrical Engineering Class 5 Dec. 4th, 2009

2. Small-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

3. Experimental record of received signal envelope in an urban area

4. Multipath radio propagation in urban areas

5. 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

6. 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.

7. 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.

8. Comparison of the BER for a fadingand non-fading channel

9. Doppler Shift

10. Illustration of Doppler effect

11. 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

12. 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

13. Fading due to two incoming signals combine with different phases

14. 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.

15. Questions?