1. Mobile Radio Environment – Propagation Phenomena
M. Schwartz
Mobile Wireless Communications
Chapter 2
2007
WNRL/KWU

2. Propagation Behaviors of Radio Waves
Ground wave (< 2MHz)
Sky wave (2-30MHz) : reflected at the ionosphere
LOS (> 30MHz) : most communication systems considered in this course
→ Why do mobile phones work even without an LOS?
2007
WNRL/KWU

3. Additional Signal Propagation Effects
In real life, we rarely have a LOS between sender and receiver.
Receiving power additionally influenced by obstacles and objects.
Due to large obstacles (compared to the wavelength)
‘Particle’ behavior of radio signals
Blocking (or shadowing)
Reflection at big obstacles
Refraction depending on the density of a medium: LOS bent to the earth
Due to small obstacles
‘Wave’ character of radio signals
Scattering at small obstacles
Diffraction at edges
shadowing
reflection
refraction
scattering
diffraction
2007
WNRL/KWU

4. Signal Fading
Time variation of received signal power caused by changes in the transmission medium or path
Large-scale fading
Long-term fading
Average power: Attenuation due to the distance
Shadow fading: Due to hills, buildings
Small-scale fading
Short-term or multi-path fading
Due to receiver mobility
2007
WNRL/KWU

5. 2007
WNRL/KWU

6. Fading Example
2007
WNRL/KWU

7. Received Power area-mean power local-mean power instantaneous power
2007
WNRL/KWU

8. 2007
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9. 2007
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10. 2007
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11. 2007
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12. 2007
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13. 2007
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14. 2007
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15. Multipath Fading for Large Cell
2007
WNRL/KWU

16. Multipath Fading for Small Cell
2007
WNRL/KWU

17. Instantaneous Received Power
2007
WNRL/KWU

18. 2007
WNRL/KWU

19. Doppler Shift for a Star
If objects are moving away from observer, light will be redshifted
2007
WNRL/KWU

20. 2007
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21. Fading on Modulated (Information-bearing) Signal
2007
WNRL/KWU

22. Information-Bearing Signal: Received Signal
Two received scattered signals s1(t) and s2(t)
x1, y1 – Gaussian RV
a1 – signal envelop; Rayleigh or Rician RV; functions of tk and fm
Delay of kth ray =
fixed large-scale delay (t1) + additional (incremental) random delay (tk)
2007
WNRL/KWU

23. 2007
WNRL/KWU

24. Normalized Correlation Function
Measure of the connectedness between two RP of s1(t) and s2(t)
Assuming Rayleigh fading and exponential incremental delay
When r → 1,
Two RV spaced t apart are completely correlated
The fading channel is non-distorting
2007
WNRL/KWU

25. 2007
WNRL/KWU

26. Frequency-selective Fading
2007
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27. Flat Fading
2007
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28. Frequency-Selective Fading
2007
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29. Time-selective Fading
2007
WNRL/KWU

30. Small-Scale Fading Fading
The distortion of signal that channel produces
Depends on both signal and channel
Signal parameters
symbol interval (symbol rate)
bandwidth
Channel parameters
delay spread
coherence bandwidth
Doppler spread
coherence time
2007
WNRL/KWU

31. 2 Independent Fading Issues: 4 Basic Types
2007
WNRL/KWU

32. Four Composite Fadings: Symbol Rate
(note) σ – delay spread, Tc – coherence time
2007
WNRL/KWU

33. Four Composite Fadings: Bandwidth
(note) Bd – Doppler spread, Bc – coherence bandwidth
2007
WNRL/KWU