Transcript wirelessprop1.odp

CS 414
Wireless Propagation Basics
CS 414
Indian Institute of Technology, Bombay
Last Lecture
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Antenna Gain
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GdB = 10 log (Pdirection/Pisotropic)
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Transmit/Receive gain
Beamwidth
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Angle between half-power direction and direction
of maximum power
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Last Lecture
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Path Loss Estimation
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Isotropic Antenna
Pr

λ
FreeSpacePathLoss= =
Pt 4×π×d
 P r dbm = P t dbm −[ 20log f 20log  d32.5 ]
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
d in Km and f in MHz
P r dbm =P t dbm Gt dbGr db −[ 20log f 20 logd32.5 ]
CS 414
Indian Institute of Technology, Bombay
Path Loss Example
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Pt = 50 mW, f = 2.4 Ghz, d = 2Km, Pr = ?
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Path loss = 32.5 + 20 log (2400) + 20 log (2) =
106 dB
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Pt = 17 dBm
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Pr = 17dBm – 106 dB = -89 dBm
Gt = 24 dBi, Gr = 24 dBi, Pr = ?
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Pr = 17 dBm + 24 dBi + 24 dBi – 106 dB
Pr = -41 dBm
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Near Field & Far Field

λ
Pr=Pt×Gt ×Gr×
4×π×d
d
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Valid only when d is in far field
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Near field
2D
dd f=
λ
Far Field
D: largest dimension of antenna
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Angular effects at short distances
At large distances, angular/spherical power
density appears flat
D=1m, f=2.4 Ghz, df =?
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Path Loss Remars
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Free space path loss is idealistic
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Other losses
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Cable (dB/ft)
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Environment
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Simultaneous tranmissions
More realistically loss is proportional to d3 or
higher
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RF Propagaton Phenomena
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Reflection
λ<D
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Defraction
λ~D
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Scattering
λ>D
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Refraction
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Noise
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Change in signal due to random/unwanted
distrortions
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Medium variations
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Frequency interactions
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Interference
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Impulse noise
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Simultaneous transmissions
Lightning ...
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Fading
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Variation in received signal strength over time
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Short-term fading
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Rapidly changing
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Due to multipath
Long-term fading
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Due to mobility
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Slowly changing (degrading) due to distance
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Multipath
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Effect of reflection
Same signal reaches receiver by more than one
path
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Constructive or destrictive side-effects
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Inter-symbol interference
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Received Signal Strength
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Function of --–
Transmit power
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Path loss
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Short-term fading
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Long-term fading
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RF Propogation in reality
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“The Mistaken axioms of wireless-network research”, D. Kotz,
C. Newport, C. Elliott
http://pdos.csail.mit.edu/decouto/papers/kotz03.pdf
“Most research on ad-hoc wireless networks makes simplifying
assumptions about radio propagation. The “Flat Earth” model of
the world is surprisingly popular: all radios have circular range,
have perfect coverage in that range, and travel on a twodimensional plane.
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We then present a set of 802.11 measurements that clearly
demonstrate that these “axioms” are contrary to fact.”
CS 414
Indian Institute of Technology, Bombay
Assumptions of RF Propogation
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Isotropic
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radiation pattern is defined (circular for omni)
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if I can hear you, you can hear me
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if I can hear you, I can hear you perfectly
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Signal strength is a function of distance
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All similar radios have similar range
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World is flat
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Are omni-antennas isotropic?
Isotropic
Antenna
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Real radio radiation patterns
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Are omni-antennas isotropic?
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Link quality is binary!
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??
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Are Radio ranges same?
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With non-isotropic radiation patterns
difficult to estimate range
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Are links symmteric?
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No
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Signal Strength
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Simple function of distance ?
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NO
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To Summarize ...
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Radiation pattern not circular (spherical)
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Links not symmetric
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Link quality and Signal strength not a simple
function of distance
Radio range cannot be estimated
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Implications of non-isotropic?
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Implications of non-isotropic?
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Cannot estimate signal strength based on
distance
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does link exist? is it probabilistic?
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Probability of data/packet reception uncertain
Simplistic assumption of d^2 used in
simulations will not mimic reality
Routing decisions
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Next Class
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Loss proportional to d2
... in reality ?
Reading
“The Mistaken axioms of wireless-network research”, D. Kotz,
C. Newport, C. Elliott
http://pdos.csail.mit.edu/decouto/papers/kotz03.pdf
“Most research on ad-hoc wireless networks makes
simplifyingassumptions about radio propagation. The “Flat Earth”
model of the world is surprisingly popular: all radioshave circular
range, have perfect coverage in thatrange, and travel on a twodimensional plane.
...
We then present a set of 802.11 measurements that clearly
demonstrate that these “axioms” are contrary to fact.”
CS 414
Indian Institute of Technology, Bombay