Wireless Transmission PartI
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Transcript of Wireless Transmission PartI
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8/6/2019 Wireless Transmission PartI
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CS 647 2.1
CS647: Advanced Topics in
Wireless Networks
Basics of Wireless Transmission
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CS 647 2.2
Frequencies Signals
Antennas
Signal propagation
Multiplexing
Spread spectrum
Modulation
Outline
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CS 647 2.3
Receiver
Earth
Sky wave
Space wave
Ground wave
Troposphere
(0 - 12 km)
Stratosphere(12 - 50 km)
Mesosphere
(50 - 80 km)
Ionosphere
(80 - 720 km)
Transm
itter
Types of Wave
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CS 647 2.4
10-7 m1015 HzUltraviolet light
15 mm20 GHzKa band satellite
37.5 cm800 MHzCellular
3 m100 MHzFM radio
5,000 km60 HzAC current
WavelengthFrequencySystem
Speed, Wavelength, Frequency
Frequency and wave length:
= c/fwave length , speed of light c 3x108m/s, frequency f
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CS 647 2.5
Ground/Sky wave
Space wave
Sky wave
Ground wave
Characteristics
300 GHz - 3000 GHzTHFTremendously high
30 GHz - 300 GHzEHFExtremely high
3 GHz - 30 GHzSHFSuper high
300 MHz - 3 GHzUHFUltra high
30 MHz - 300 MHzVHFVery high
3 MHz - 30 MHzHFHigh
300 kHz - 3 MHzMFMedium
30 kHz - 300 kHzLFLow
3 kHz - 30 kHzVLFVery low
300 Hz - 3 kHzILFInfra low
< 300 HzELFExtremely low
Frequency RangeInitialsClassification Band
Radio Frequency Bands
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CS 647 2.6
Frequencies for 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
1 Mm
300 Hz
10 km
30 kHz
100 m
3 MHz
1 m
300 MHz
10 mm
30 GHz
100 m3 THz
1 m300 THz
visible lightVLF LF MF HF VHF UHF SHF EHF infrared UV
optical transmissioncoax cabletwistedpair
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CS 647 2.7
Frequencies for mobile communication
VHF-/UHF-ranges for mobile radio simple, small antenna for cars
deterministic propagation characteristics, reliable connections
SHF and higher for directed radio links, satellitecommunication
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.
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CS 647 2.9
Signals I
physical representation of data
function of time and location
signal parameters: parameters representing the value of data
classification
continuous time/discrete time
continuous values/discrete values
analog signal = continuous time and continuous values
digital signal = discrete time and discrete values
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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CS 647 2.10
Fourier representation of periodic signals
)2cos()2sin(2
1)(
11
nftbnftactgn
n
n
n
=
=
++=
1
0
1
0
t t
ideal periodic signal real composition
(based on harmonics)
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CS 647 2.11
Different representations of signals amplitude (amplitude domain)
frequency spectrum (frequency domain)
phase state diagram (amplitude M and phase in polar coordinates)
Composed signals transferred into frequency domain using Fourier
transformation
Digital signals need
infinite frequencies for perfect transmission
modulation with a carrier frequency for transmission (analog signal!)
Signals II
f [Hz]
A [V]
I= M cos
Q = M sin
A [V]
t[s]
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CS 647 2.13
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)
side view (xy-plane)
x
y
side view (yz-plane)
z
y
top view (xz-plane)
x
z
simple
dipole
/4/2
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CS 647 2.14
Antennas: directed and sectorized
side view (xy-plane)
x
y
side view (yz-plane)
z
y
top view (xz-plane)
x
z
top view, 3 sector
x
z
top view, 6 sector
x
z
Often used for microwave connections or base stations for mobile
phones (e.g., radio coverage of a valley)
directed
antenna
sectorizedantenna
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CS 647 2.15
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
+
/4/2/4
ground plane
/2
/2
+
/2
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CS 647 2.16
Signal propagation ranges
distance
sender
transmission
detection
interference
Transmission range
communication possible
low error rate
Detection range
detection of the signalpossible
no communication
possible
Interference range signal may not be
detected
signal adds to the
background noise
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CS 647 2.18
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 propagation
signal at sender
signal at receiver
LOS pulsesmultipath
pulses
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CS 647 2.19
The received signal power at distance d:
where Pt is transmitting power,Ae is effective area, and Gt is thetransmitting antenna gain. Assume that radiated power is uniformly
distributed over the surface of the sphere.
Transmitter
Distance d
Receiver
hb
hm
2r 4P d
PGA tte
=
Free-space Propagation
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CS 647 2.20
The relationship between an effective aperture and received antenna gain Gr can begiven by:
Gr = 4Ae/2
where is the wavelength, andAe is the effective area covered by the transmitter.
By substitutingAe, in terms ofGr and , we obtain
Pr = GrGtPt / (4 d/)2
Free Space path loss is defined as
Lf = Pt/ P r = (1/GrGt) (4 d/)2
Lf indicates power loss in the free space.
When Gr= Gt=1,
Lf = (4 d/)2 = (4 fcd/c )
2
where c = fc (cis speed of light) and fc is the carrier frequency.
Antenna Gain