wireless transmission parti

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.


8/20


9/20

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]


12/20


13/20

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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18/20

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

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