1. CSE 4215/5431: Mobile Communications Winter 2010
Suprakash Datta
Office: CSEB 3043
Phone: ext 77875
Course page:
Some slides are adapted from the book website
11/7/2018
CSE 4215, Winter 2010

2. Last class Introduction to mobile communications
Similarities and differences with wired communication
Review of the TCP/IP architecture
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CSE 4215, Winter 2010

3. Today The physical layer for mobile communications
Let’s start with the very basic notions
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CSE 4215, Winter 2010

4. Signals, channels and systems
What is a signal?
Baseband signal
Modulation
Bandwidth
Transmission/reception
What is a channel?
Noise
Loss?
What is a communication system?
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CSE 4215, Winter 2010

5. Types of signals (a) continuous time/discrete time
(b) continuous values/discrete values
analog signal = continuous time, continuous values
digital signal = discrete time, discrete values
Periodic signal - analog or digital signal that repeats over time
s(t +T ) = s(t ) -¥< t < +¥
where T is the period of the signal
signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift 
sine wave as special periodic signal for a carrier: s(t) = At sin(2  ft t + t)
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CSE 4215, Winter 2010

6. Sine Wave Parameters

7. Bandwidth
Of a signal
Of a channel
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CSE 4215, Winter 2010

8. The underlying mathematics
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
The underlying mathematics
Fourier representation of periodic signals 1 1 t t
ideal periodic signal
real composition
(based on harmonics)
What about aperiodic signals ?
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 8

9. Frequency domain
Fundamental frequency - when all frequency components of a signal are integer multiples of one frequency, it’s referred to as the fundamental frequency
Spectrum - range of frequencies that a signal contains
Absolute bandwidth - width of the spectrum of a signal
Effective bandwidth (or just bandwidth) - narrow band of frequencies that most of the signal’s energy is contained in
11/7/2018
CSE 4215, Winter 2010

10. Transmitting rectangular signals
Observations
Any digital waveform will have infinite bandwidth
BUT the transmission system will limit the bandwidth that can be transmitted
AND, for any given medium, the greater the bandwidth transmitted, the greater the cost
HOWEVER, limiting the bandwidth creates distortions
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CSE 4215, Winter 2010

11. Bit rates, channel capacity
Impairments, such as noise, limit data rate that can be achieved
For digital data, to what extent do impairments limit data rate?
Channel Capacity – the maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions
11/7/2018
CSE 4215, Winter 2010

12. Nyquist Bandwidth For binary signals (two voltage levels)
C = 2B
With multilevel signaling
C = 2B log2 M
M = number of discrete signal or voltage levels
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CSE 4215, Winter 2010

13. Signal-to-Noise Ratio
Ratio of the power in a signal to the power contained in the noise that’s present at a particular point in the transmission
Typically measured at a receiver
Signal-to-noise ratio (SNR, or S/N)
A high SNR means a high-quality signal, low number of required intermediate repeaters
SNR sets upper bound on achievable data rate
11/7/2018
CSE 4215, Winter 2010

14. Shannon Capacity Formula
Equation:
Represents theoretical maximum that can be achieved
In practice, only much lower rates achieved
Formula assumes white noise (thermal noise)
Impulse noise is not accounted for
Attenuation distortion or delay distortion not accounted for
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CSE 4215, Winter 2010

15. Example of Nyquist and Shannon Formulations
Spectrum of a channel between 3 MHz and 4 MHz ; SNRdB = 24 dB
Using Shannon’s formula
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CSE 4215, Winter 2010

16. Example of Nyquist and Shannon Formulations
How many signaling levels are required?
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CSE 4215, Winter 2010

17. Modulation
Why?
How?
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CSE 4215, Winter 2010

18. Frequencies for wireless communication
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Frequencies for wireless communication
VLF = Very Low Frequency UHF = Ultra High Frequency
LF = Low Frequency SHF = Super High Frequency
MF = Medium Frequency EHF = Extra High Frequency
HF = High Frequency UV = Ultraviolet Light
VHF = Very High Frequency
Frequency and wave length
 = c/f
wave length , speed of light c  3x108m/s, frequency f
twisted pair
coax cable
optical transmission
1 Mm
300 Hz
10 km
30 kHz
100 m
3 MHz
1 m
300 MHz
10 mm
30 GHz
100 m
3 THz
1 m
300 THz
VLF LF MF HF
VHF
UHF
SHF
EHF
infrared
visible light UV
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 18

19. Frequencies for wireless communication
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Frequencies for wireless communication
VHF-/UHF-ranges for mobile radio
simple, small antenna for cars
deterministic propagation characteristics, reliable connections
SHF and higher for directed radio links, satellite communication
small antenna, beam forming
large bandwidth available
Wireless LANs use frequencies in UHF to SHF range
some systems planned up to EHF
limitations due to absorption by water and oxygen molecules (resonance frequencies)
weather dependent fading, signal loss caused by heavy rainfall etc.
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 19

20. Frequencies and regulations
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Frequencies and regulations
ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences)
Examples
Europe
USA
Japan
Cellular phones
GSM , , ,
UMTS ,
AMPS, TDMA, CDMA, GSM ,
TDMA, CDMA, GSM, UMTS ,
PDC, FOMA ,
PDC ,
FOMA ,
Cordless phones
CT , CT
DECT
PACS ,
PACS-UB
PHS
JCT
Wireless LANs
802.11b/g
802.11b/g
802.11b
802.11g
Other RF systems
27, 128, 418, 433, 868
315, 915
426, 868
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 20

21. Multiplexing Multiplexing in 4 dimensions
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Multiplexing
Multiplexing in 4 dimensions
space (si)
time (t)
frequency (f)
code (c)
Goal: multiple use of a shared medium
Important: guard spaces needed!
channels ki k1 k2 k3 k4 k5 k6 c t c s1 t s2 f f c t s3 f
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 21

22. Frequency multiplexing
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Frequency multiplexing
Separation of the whole spectrum into smaller frequency bands
A channel gets a certain band of the spectrum for the whole time
Advantages
no dynamic coordination necessary
works also for analog signals
Disadvantages
waste of bandwidth if the traffic is distributed unevenly
inflexible k1 k2 k3 k4 k5 k6 c f t
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 22

23. Time division multiplexing
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Time division multiplexing
A channel gets the whole spectrum for a certain amount of time
Advantages
only one carrier in the medium at any time
throughput high even for many users
Disadvantages
precise synchronization necessary k1 k2 k3 k4 k5 k6 c f t
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 23

24. Time and frequency multiplex
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Time and frequency multiplex
Combination of both methods
A channel gets a certain frequency band for a certain amount of time
Example: GSM
Advantages
better protection against tapping
protection against frequency selective interference
but: precise coordination required k1 k2 k3 k4 k5 k6 c f t
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 24

25. Code multiplex Each channel has a unique code
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Code multiplex k1 k2 k3 k4 k5 k6
Each channel has a unique code
All channels use the same spectrum at the same time
Advantages
bandwidth efficient
no coordination and synchronization necessary
good protection against interference and tapping
Disadvantages
varying user data rates
more complex signal regeneration
Implemented using spread spectrum technology c f t
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 25

26. Example Lack of coordination requirement is an advantage. 11/7/2018
CSE 4215, Winter 2010

27. Aside: Digital Communications
What is coding?
What is source coding?
What are line codes?
What is channel coding?
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28. Transceivers
How are signals sent and received in wireless communications?
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CSE 4215, Winter 2010

29. Antennas: isotropic radiator
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Antennas: isotropic radiator
Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission
Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna
Real antennas always have directive effects (vertically and/or horizontally)
Radiation pattern: measurement of radiation around an antenna z y z
ideal
isotropic
radiator y x x
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 29

30. Antennas: simple dipoles
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Antennas: simple dipoles
Real antennas are not isotropic radiators but, e.g., dipoles with lengths /4 on car roofs or /2 as Hertzian dipole  shape of antenna proportional to wavelength
Example: Radiation pattern of a simple Hertzian dipole
Gain: maximum power in the direction of the main lobe compared to the power of an isotropic radiator (with the same average power)
/4
/2 y y z
simple
dipole x z x
side view (xy-plane)
side view (yz-plane)
top view (xz-plane)
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 30

31. Antennas: directed and sectorized
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Antennas: directed and sectorized
Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valley) y y z
directed
antenna x z x
side view (xy-plane)
side view (yz-plane)
top view (xz-plane) z z
sectorized
antenna x x
top view, 3 sector
top view, 6 sector
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 31

32. Antennas: diversity Grouping of 2 or more antennas Antenna diversity
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Antennas: diversity
Grouping of 2 or more antennas
multi-element antenna arrays
Antenna diversity
switched diversity, selection diversity
receiver chooses antenna with largest output
diversity combining
combine output power to produce gain
cophasing needed to avoid cancellation
/2
/2
/4
/2
/4
/2 + +
ground plane
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 32

33. Antenna Gain Antenna gain Effective area
Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna)
Effective area
Related to physical size and shape of antenna
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CSE 4215, Winter 2010

34. Antenna Gain Relationship between antenna gain and effective area
G = antenna gain
Ae = effective area
f = carrier frequency
c = speed of light (» 3 ´ 108 m/s)
 = carrier wavelength
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CSE 4215, Winter 2010

35. Back to modulation Digital modulation Analog modulation Motivation
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Back to modulation
Digital modulation
digital data is translated into an analog signal (baseband)
ASK, FSK, PSK - main focus in this chapter
differences in spectral efficiency, power efficiency, robustness
Analog modulation
shifts center frequency of baseband signal up to the radio carrier
Motivation
smaller antennas (e.g., /4)
Frequency Division Multiplexing
medium characteristics
Basic schemes
Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation (PM)
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 35

36. Modulation and demodulation
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Modulation and demodulation
analog
baseband
signal
digital
data
digital
modulation
analog
modulation
radio transmitter
radio
carrier
analog
baseband
signal
digital
data
analog
demodulation
synchronization
decision
radio receiver
radio
carrier
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 36

37. Digital modulation Modulation of digital signals known as Shift Keying
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Digital modulation
Modulation of digital signals known as Shift Keying
Amplitude Shift Keying (ASK):
very simple
low bandwidth requirements
very susceptible to interference
Frequency Shift Keying (FSK):
needs larger bandwidth
Phase Shift Keying (PSK):
more complex
robust against interference 1 1 t 1 1 t 1 1 t
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 37

38. Advanced Frequency Shift Keying
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Advanced Frequency Shift Keying
bandwidth needed for FSK depends on the distance between the carrier frequencies
special pre-computation avoids sudden phase shifts  MSK (Minimum Shift Keying)
bit separated into even and odd bits, the duration of each bit is doubled
depending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosen
the frequency of one carrier is twice the frequency of the other
Equivalent to offset QPSK
even higher bandwidth efficiency using a Gaussian low-pass filter  GMSK (Gaussian MSK), used in GSM
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 38

39. Example of MSK 1 1 1 1 data bit even 0 1 0 1 even bits odd 0 0 1 1
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Example of MSK 1 1 1 1
data
bit
even
even bits
odd
signal h n n h value
odd bits
low frequency
h: high frequency
n: low frequency
+: original signal
-: inverted signal
high frequency
MSK
signal t
No phase shifts!
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 39

40. Advanced Phase Shift Keying
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Advanced Phase Shift Keying
BPSK (Binary Phase Shift Keying):
bit value 0: sine wave
bit value 1: inverted sine wave
very simple PSK
low spectral efficiency
robust, used e.g. in satellite systems
QPSK (Quadrature Phase Shift Keying):
2 bits coded as one symbol
symbol determines shift of sine wave
needs less bandwidth compared to BPSK
more complex
Often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK (IS-136, PHS) Q I 1 Q I 11 01 10 00 A t 11 10 00 01
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 40

41. Quadrature Amplitude Modulation
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Quadrature Amplitude Modulation
. Quadrature Amplitude Modulation (QAM)
combines amplitude and phase modulation
it is possible to code n bits using one symbol
2n discrete levels, n=2 identical to QPSK
Bit error rate increases with n, but less errors compared to comparable PSK schemes
Example: 16-QAM (4 bits = 1 symbol)
Symbols 0011 and 0001 have the same phase φ, but different amplitude a and 1000 have different phase, but same amplitude
0000
0001
0011
1000 Q I
0010 φ a
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 41

42. Hierarchical Modulation
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Hierarchical Modulation
DVB-T modulates two separate data streams onto a single DVB-T stream
High Priority (HP) embedded within a Low Priority (LP) stream
Multi carrier system, about 2000 or 8000 carriers
QPSK, 16 QAM, 64QAM
Example: 64QAM
good reception: resolve the entire 64QAM constellation
poor reception, mobile reception: resolve only QPSK portion
6 bit per QAM symbol, 2 most significant determine QPSK
HP service coded in QPSK (2 bit), LP uses remaining 4 bit Q 10 I 00
000010
010101
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 42

43. Signal propagation basics
Many different effects have to be considered
11/7/2018
CSE 4215, Winter 2010

44. 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
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 44

45. 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
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 45

46. Real world example 11/7/2018 CSE 4215, Winter 2010
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Real world example
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 46

47. Propagation Modes Ground-wave propagation Sky-wave propagation
Line-of-sight propagation
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48. Ground Wave Propagation
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49. 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 2010

50. Sky Wave Propagation
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51. 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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52. Line-of-Sight Propagation
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53. 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
11/7/2018
CSE 4215, Winter 2010

54. 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
11/7/2018
CSE 4215, Winter 2010

55. 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 2010

56. 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 2010

57. 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 2010

58. 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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59. Free Space Loss Free space loss equation can be recast: 11/7/2018
CSE 4215, Winter 2010

60. 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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61. Free Space Loss
Free space loss accounting for gain of other antennas can be recast as
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62. Multipath Propagation

63. 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
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 63

64. 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…
11/7/2018
CSE 4215, Winter 2010

65. 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
11/7/2018
CSE 4215, Winter 2010
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 65

66. Next Channel effects (e.g. fading), noise Spread spectrum
Cellular system basics
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