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CS 414

Wireless Propagation Basics

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Last Lecture

Radio waves 3KHz to 300 Ghz

Analog and Digita data and signals

Bandwidth and Data Rate

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Last Lecture

db relative measure

db = 10 log10

(ratio)

if Ps = 10 mW and Pr = 10mW ~ 0 db

1 mW ~ -10 db

100 mW ~ 10 db 1000 mW ~ 20 db

dbm, dbv, dbi, dbW ...

absolute measure

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dbW, dbm, dbi, dbv ...

db = relative maginutes Fix reference for to initial signal for absolute

compariton

dbm = 10 log (PowermW

/ 1mW)

1 mW = 0 dbm

10 mW = 10 dbm

30 dbm = ?

If P1 P2 = 10 dbm then, absolute differenceis ?

1 dbW = ? dbm

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Antennas

Essential wireless propogation component Entry and end-points of RF signals

Two funtions

Transmission Converts electric energy to electro-magnetic

Receiver

Converts received electro-magnetic energy to

electric signal Passive element

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Antenna Types

Omni-directional radiation in all directions

Directional

Focuses (more) power in certain direction

Does not amplify

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Types of Antennas

/4

Quarter-wave Antenna

/4

/2Half-wave Dipole

www.cellamericas.com

Parabolic Antenna

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Antenna Gain

Isotropic Antenna Idealized omni-directional antenna

Tranmits power in all directions uniformly

Antenna Gain (G)

ratio of transmit/receive power in a particulardirection w.r.t to an isotropic antenna

G = Pdirection

/Pisotropic

Measure of reception/tranmission strength ofantenna

Transmit Gain = Receive Gain

Typical units: dBi (dB gain w.r.t to isotropic)

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Antenna Gain

Isotropic G = 1 in all directions

Non-uniform/Directional

Ga

= ?

Gb = ?

Gc

= ?

A

B

C

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Antenna Parameters

Radiation Pattern Spread of transmitter

power

(Half-power) Beam width

Angle between half power point (direction) andthe point (direction) of maximum power

Assumption: Reception beyond beam width ispoor and unreliable

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Antenna Parameters

Antenna Polarization Orientation/plane of the electric field (E-plane)

w.r.t earth's surface

Horizontally polarizedVertically polarized

image source: wikipedia

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RF Propagation

Questions: Relation between RF propogation & distance

different environments

quantification metrics

Goals: Provide connectivity, Estimate linkquality (Is this link good?)

Solution Design:

Antenna placement

Link quality mapping and monitoring

Tranmist power, antenna height

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Propagation Phenomena

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Path Loss Estimation

Isotropic Antenna Free space loss

PtPt

d

Pdensity=Pt

4d2

Pr=PdensityEffective Antenna Area

Aeff=2

4

FreeSpacePathLoss=Pr

Pt= 4d

2

Pr=Pt 4d 2

Aeff

= product of physical area (mW/m2)

and conversion efficiency

Prdbm=Ptdbm[20logf20log d32.5 ] d in Km and f in MHz

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Free Space Loss

With antenna gains Gt and Gr

Pr=PtGtGr 4d 2

=PtGtGr c4df2

EIRP regulations to control radiated power

Prdbm=PtdbmGt dbGrdb[20logf20logd 32.5 ]

Effective isotropic radiated power (EIRP)

Free Space Loss

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Path Loss Example

Pt = 50 mW, f = 2.4 Ghz, d = 2Km, Pr = ?

Gt = 24 dBi, Gr = 24 dBi, Pr = ?

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Next Class

Loss proportional to d2 ... in reality ?

ReadingThe Mistaken axioms of wireless-network research, D. Kotz,C. Newport, C. Elliotthttp://pdos.csail.mit.edu/decouto/papers/kotz03.pdf

Most research on ad-hoc wireless networks makessimplifyingassumptions about radio propagation. The Flat Earthmodel of the world is surprisingly popular: all radioshave circularrange, have perfect coverage in thatrange, and travel on a two-dimensional plane.

...We then present a set of 802.11 measurements that clearlydemonstrate that these axioms are contrary to fact.

http://pdos.csail.mit.edu/decouto/papers/kotz03.pdfhttp://pdos.csail.mit.edu/decouto/papers/kotz03.pdf