dr. farid farahmand. wired vs. wireless communication wiredwireless each cable is a different...
TRANSCRIPT
Dr. Farid Farahmand
Wired Wireless
Each cable is a different channelOne media (cable) shared by all
Signal attenuation is low High signal attenuation
No interference High interferencenoise; co-channel interference; adjacent channel interference
Why go wireless ?
Advantageso Sometimes it is impractical
to lay cableso User mobilityo Cost
Limitationso Bandwidtho Fidelityo Powero (In)security
Broadcast (analog)
2-way communication(digital)
2-way communication(analog)
Wireless Systems: ExamplesAM, FM RadioTV BroadcastSatellite Broadcast2-way RadiosCordless PhonesSatellite LinksMobile Telephony SystemsWireless Local Loop (WLL)Microwave LinksWireless LANs Infrared LANs
SatelliteLinks
SWRadio
MWRadio
FMRadio
MobileTelephony,WLL
WLANsBlueoothIR
1,000 Km100 Km10 Km1 Km100 m10 m1 m
Propagation characteristics are different in each frequency band
UV
1 MHz1 kHz 1 GHz 1 THz 1 PHz 1 EHz
infraredvisible
X raysGamma rays
AM ra
dio
S/W
radi
oFM
radi
oTV TV cellu
lar
LF HF VHF UHF SHF EHFMF
30kHz 300kHz 3MHz 30MHz 300MHz 30GHz 300GHz
10km 1km 100m 10m 1m 10cm 1cm 100mm
3GHz
902 – 928 Mhz
2.4 – 2.4835 Ghz
5.725 – 5.785 Ghz
ISM band
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RADIO IR VISIBLE UV X-RAYS GAMMA RAYS
VLF LF MF HF VHF UHF SHF EHF
3k 30k 300k 3M 30M 300M 3G 30G 300GHz
VLF: Very Low Frequency LF: Low FrequencyMF: Medium Frequency HF: High FrequencyVHF: Very High Frequency UHF: Ultra High FrequencySHF: Super High Frequency EHF: Extremely High Frequency
Frequency Band Allocations
RADIO
VLF, LF long wavesMF medium wavesHF, VHF short wavesUHF, SHF microwaves
EHF millimeter waves
Above microwave region, only certain windows of frequencies propagate freely through air, rain, etc.
Infrared and visible light will not penetrate wallsX-rays and gamma rays interact with matter
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Wavelengths of Frequency Bands
c
fmeters sec
cycles secmeters
cycle
Propagate well beyond line of sight
The distance the signal travels decreases as the frequency increases
Electromagnetic Signals Electromagnetic Signals are emitted and received in wireless systems
Requires a transmitting and receiving antenna
The EM signal goes through the unguided medium Free space (vacuum) Earth’s atmosphere
EM propagation is also referred to radio frequency propagation Wireless communications examples
Terrestrial radio Microwave radio Broadband radio Mobile radio Cellular phone
What is EM?EM involves both a varying electric field (E)
and a varying magnetic field (H)E and H appear at right angles to each other
and to the direction of travel of the wave (Z-axis)
The power passing a given signal is called the power density (P) P (Watt/m2)= H.E
http://info.ee.surrey.ac.uk/Teaching/Courses/EFT/transmission/html/TEMWave.html
EM Propagation Electromagnetic waves are invisible We use the concept of rays to describe them When radiating uniformly over a spherically
we refer to it as isotropic radiation Power Density (W/m2) = P_radiated / Area of
sphere As we get further from the source the radiation
(received power) becomes smaller
Example of Power DensityAssume the isotropic radiated power from an
antenna is 100 W. Assuming the receiving antenna is 100 m away, calculate the received power density (assume vacuum).
R=100 m
RXTX
P(density) = 100W / 4(100)2
=0.796 W/m2
Free Space LossThe signal disperses with distance Free space loss, ideal isotropic antenna
Pt = signal power at transmitting antenna (watt) Pr = signal power at receiving antenna (Watt) = 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)
2
2
2
2 44
c
fdd
P
P
r
t
Example of Power RadiationAssume the isotropic radiated power from an antenna is
100 W. Assuming the receiving antenna is 100 m away, calculate the received power (assume vacuum and frequency of radiation is 100 MHz).
R=100 m
RXTX
P(received) =Pr =100W / (100x106x4(100)/3x108)2
=0.057 W very little power received!
2
2
2
2 44
c
fdd
P
P
r
t
AttenuationStrength of signal falls off with distance
over transmission mediumAttenuation 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
d
P
PL
r
tdB
4log20log10
Note: Attenuation in dB can be calculated by
Signal Loss and Attenuation Pulse spreading in free space Attenuation in non-vacuum
Attenuation due to particles absorbing the EM energy
Called “wave absorption”Remember: Attenuation = 10 log (Pout/Pin)
Other ImpairmentsMultipath – obstacles reflect signals so that
multiple copies with varying delays are received
Refraction – bending of radio waves as they propagate through the atmosphere
Atmospheric absorption – water vapor and oxygen contribute to attenuation
The Effects of Multipath PropagationMultiple copies of a signal may arrive at
different phasesIf phases add destructively, the signal level
relative to noise declines, making detection more difficult
Intersymbol interference (ISI)One or more delayed copies of a pulse may
arrive at the same time as the primary pulse for a subsequent bit
Multipath Propagation & FadingReflection – occurs when signal encounters a
surface that is large relative to the wavelength of the signal
Diffraction - occurs at the knife-edge of an impenetrable body that is almost the same compared to wavelength of radio wave
Scattering – occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less
Multipath Propagation & Fading
Three basic propagation mechanisms (D is the size of the material)
Reflectionλ << D
Diffraction
λ DScattering
λ >> D
RefractionRefraction – bending of
microwaves by the atmosphereVelocity 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
ThinAir
Dense Air
References Narayan Mandayam, Tomasi