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

7

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

8

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