1. CSE 4215/5431: Mobile Communications Winter 2011
Suprakash Datta
Office: CSEB 3043
Phone: ext 77875
Course page:
Some slides are adapted from the book website
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CSE 4215, Winter 2011

2. Signal propagation basics
Many different effects have to be considered
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CSE 4215, Winter 2011

3. Signal propagation ranges
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Signal propagation ranges
Transmission range
communication possible
low error rate
Detection range
detection of the signal possible
no communication possible
Interference range
signal may not be detected
signal adds to the background noise
sender
transmission
distance
detection
interference
12/2/2018
CSE 4215, Winter 2011
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 3

4. Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Signal propagation
Propagation in free space always like light (straight line)
Receiving power proportional to 1/d² in vacuum – much more in real environments (d = distance between sender and receiver)
Receiving power additionally influenced by
fading (frequency dependent)
shadowing
reflection at large obstacles
refraction depending on the density of a medium
scattering at small obstacles
diffraction at edges
shadowing
reflection
refraction
scattering
diffraction
12/2/2018
CSE 4215, Winter 2011
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 4

5. Real world example 12/2/2018 CSE 4215, Winter 2011
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Real world example
12/2/2018
CSE 4215, Winter 2011
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 5

6. Propagation Modes Ground-wave propagation Sky-wave propagation
Line-of-sight propagation
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CSE 4215, Winter 2011

7. Ground Wave Propagation
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CSE 4215, Winter 2011

8. Ground Wave Propagation
Follows contour of the earth
Can Propagate considerable distances
Frequencies up to 2 MHz
Example
AM radio
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CSE 4215, Winter 2011

9. Sky Wave Propagation
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10. Sky Wave Propagation
Signal reflected from ionized layer of atmosphere back down to earth
Signal can travel a number of hops, back and forth between ionosphere and earth’s surface
Reflection effect caused by refraction
Examples
Amateur radio
CB radio
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CSE 4215, Winter 2011

11. Line-of-Sight Propagation
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CSE 4215, Winter 2011

12. Line-of-Sight Propagation
Transmitting and receiving antennas must be within line of sight
Satellite communication – signal above 30 MHz not reflected by ionosphere
Ground communication – antennas within effective line of site due to refraction
Refraction – bending of microwaves by the atmosphere
Velocity of electromagnetic wave is a function of the density of the medium
When wave changes medium, speed changes
Wave bends at the boundary between mediums
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CSE 4215, Winter 2011

13. Line-of-Sight Equations
Optical line of sight
Effective, or radio, line of sight
d = distance between antenna and horizon (km)
h = antenna height (m)
K = adjustment factor to account for refraction, rule of thumb K = 4/3
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CSE 4215, Winter 2011

14. Line-of-Sight Equations
Maximum distance between two antennas for LOS propagation:
h1 = height of antenna one
h2 = height of antenna two
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CSE 4215, Winter 2011

15. LOS Wireless Transmission Impairments
Attenuation and attenuation distortion
Free space loss
Atmospheric absorption
Multipath (diffraction, reflection, refraction…)
Noise
Thermal noise
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CSE 4215, Winter 2011

16. Attenuation
Strength of signal falls off with distance over transmission medium
Attenuation factors for unguided media:
Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal
Signal must maintain a level sufficiently higher than noise to be received without error
Attenuation is greater at higher frequencies, causing distortion
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CSE 4215, Winter 2011

17. Free Space Loss Free space loss, ideal isotropic antenna
Pt = signal power at transmitting antenna
Pr = signal power at receiving antenna
 = carrier wavelength
d = propagation distance between antennas
c = speed of light (» 3 ´ 10 8 m/s)
where d and  are in the same units (e.g., meters)
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CSE 4215, Winter 2011

18. Free Space Loss Free space loss equation can be recast: 12/2/2018
CSE 4215, Winter 2011

19. Free Space Loss Free space loss accounting for gain of other antennas
Gt = gain of transmitting antenna
Gr = gain of receiving antenna
At = effective area of transmitting antenna
Ar = effective area of receiving antenna
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CSE 4215, Winter 2011

20. Free Space Loss
Free space loss accounting for gain of other antennas can be recast as
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CSE 4215, Winter 2011

21. Multipath Propagation

22. Multipath propagation
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Multipath propagation
Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction
Time dispersion: signal is dispersed over time
interference with “neighbor” symbols, Inter Symbol Interference (ISI)
The signal reaches a receiver directly and phase shifted
distorted signal depending on the phases of the different parts
multipath
pulses
LOS pulses
signal at sender
signal at receiver
12/2/2018
CSE 4215, Winter 2011
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 22

23. Atmospheric absorption
Water vapor and oxygen contribute most
Water vapor: peak attenuation near 22GHz, low below 15Ghz
Oxygen: absorption peak near 60GHz, lower below 30 GHz.
Rain and fog may scatter (thus attenuate) radio waves.
Low frequency band usage helps…
12/2/2018
CSE 4215, Winter 2011

24. Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Effects of mobility
Channel characteristics change over time and location
signal paths change
different delay variations of different signal parts
different phases of signal parts
 quick changes in the power received (short term fading)
Additional changes in
distance to sender
obstacles further away
 slow changes in the average power received (long term fading)
long term
fading
power t
short term fading
12/2/2018
CSE 4215, Winter 2011
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 24

25. Fading channels Fading: Time variation of received signal power
Mobility makes the problem of modeling fading difficult
Multipath propagation is a key reason
Most challenging technical problem for Mobile Communications
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CSE 4215, Winter 2011

26. Types of Fading Short term (fast) fading Long term (slow) fading
Flat fading – across all frequencies
Selective fading – only in some frequencies
Rayleigh fading – no LOS path, many other paths
Rician fading – LOS path plus many other paths
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27. Fading models
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28. Dealing with fading channels
Error correction
Adaptive equalization
attempts to increase signal power as needed
can be done with analog circuits or DSP
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CSE 4215, Winter 2011