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Interference & Co-existence
Ryszard Struzakwww.ryszard.struzak.com
ICTP-ITU-URSI School on Wireless Networking for DevelopmentThe Abdus Salam International Centre for Theoretical Physics ICTP, Trieste (Italy), 6 to 24 February 2006
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What is the purpose?
To review basic physics of interferenceissues in microwave radio links
Communication range
Coverage area
Service degradation
Interference mechanism
How to avoid interference
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Outline
Basic concepts
Physical models
Summary
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Radio link: basic concepts
Transmitter Receiver Radiator-Receptor; Source-Sink
Long distance short range Fixed Transportable Mobile
Terrestrial Satellite Space Simplex: Transmission in one direction (e.g. TV)
A simplex link = 1 transmitter & 1 receiver
Duplex: Transmission in both directions
A duplex link = 2 transmitters & 2 receivers Full-duplex (FDX) circuit: Simultaneous transmission in
both directions
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PTP & PMT network topologies
PTP (point-to-point): Onestation (node)communicating with anotherone
PMP (point-to-multipoint): One nodecommunicating with twoor more other nodes Broadcasting
IEEE 802.11Basic Service Set -a set of stations is controlled/coordinated by a commonCoordination Function
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Mesh network
Mesh networktopology (fullyconnected):there is a directcommunicationpath between
any two nodes The principle is
similar to the waypacketstravelaround the wiredInternet: withdynamic routing
data hop from onedevice to anotheruntil thedestination isreached.
.
Source: http://research.microsoft.com/mesh/
http://en.wikipedia.org/wiki/Routinghttp://en.wikipedia.org/wiki/Routinghttp://en.wikipedia.org/wiki/Internethttp://en.wikipedia.org/wiki/Internethttp://en.wikipedia.org/wiki/Packethttp://en.wikipedia.org/wiki/Packet8/12/2019 InterferCoexist eBook
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What is interference ?
Effect of unwanted energy uponreception of the wanted signal
manifested by performance degradation,misrepresentation, or loss of information
which would not happen in the absenceof that unwanted energy
May be unacceptable or harmful !
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Events involved
The probability of interference P(I) during the smalltime period:
P(I) = P(A and B and C and D*) A: The desired transmitter is transmitting".
B: The wanted signal is satisfactorily received inthe absence of unwanted energy
C: Another equipment is producing unwanted energy
D: The wanted signal is satisfactorily received in thepresence of the unwanted energy
D* is the negation (opposite) of the event D
All these refer to the same small time period.
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What causes interference?
Radio waves are unguided Propagate freely in the space
Cannot be confined to any specific volume, unlessspecial screens are applied
Radio interference may be intentional Jamming
Most often they are unintentional, due to Faulty spectrum management (wrong use of frequencies)
Wrong deployment of the equipment Spurious emissions
Spurious receiver responses
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What is coexistence?
Term known from politics. Popularized by IEEEsTask Group TG2-802.15
TG2 Coexistence deals with radio interference betweenWireless Local Area Networks (802.11) and Personal AreaNetworks (Bluetooth)
Not defined in major telecommunicationstandards (e.g. American National Standard -Telecom Glossary T1.523-2001).
We use it here as a synonym of electromagneticcompatibility (EMC) defined internationally since1970s
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What is EMC ?
Electromagnetic compatibility (EMC); the abilityof a system (equipment, device) to operate in itsintended operational environment
without suffering unacceptable degradation and
without causing unintentional degradation to theenvironment
because of electromagnetic radiation or response
It requires interference-free operation achievedby design, deployment and exploitation application of sound EMC-related policy, concepts,
regulations, standards, etc.
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Outline
Basic concepts
Physical models
Summary
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Spurious vs. regular links
Regular
Useful / Wanted
Designed before
deployment Checked/ controlled
Regulated nationallyand internationally
Require individual orgroup license (exceptISM-type bands)
Spurious
Useless/ Unwanted/Harmful
Not designed/ Random Unchecked, often
unnoticed until harmfulinterference appear
Regulations?
Interference can be modeled as interaction among regular and spurious radio links
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How many spurious links?
Maximum fully connected net
n (designed) simplex links= n transmitters & n receivers Each receiver may receive n
signals. One of these signals is wanted;
the remaining (n-1) signals areunwanted.
Each wanted signal may be inpotential conflict with one or moreunwanted signals
Thus, the maximum number ofpotential spurious links = n(n-1).
5 receivers5 transmitters5 wanted links20 potential spurious links
1
2
3
4
5
T
R
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How many potential conflicts?
0
500
1000
0 5 10 15 20 25 30
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Communication range
The distance from atransmitter at which the signalstrength remains above theminimum usable level for a
particular antenna andreceiver combination. The usable signal level depends on
required system performance (e.g.BER) and is associated with noisepower N
With constant noise power N, the S/N and the range decrease with thedistance squared (in free space)
Min S/N required
S/N
Max communicationdistance for given BER
m
S/N
m
S
N
Distance, m
W
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Coverage area
Coverage = geographical area withinwhich service from a radio communicationfacility can be delivered under specified
conditions E.g. BER < 10-4 ; S/N > 30 dB, etc.
In broadcasting, one uses population coverage
The coverage concept may be useful in analyzesof financial efficiency: costs per unit area or costper user
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Example: ILS service volume
Instrument landing system(ILS) is a radio-navigationsystem which provides aircraftwith horizontal and verticalguidance just before and duringlanding and, at certain fixedpoints, indicates the distance tothe reference point of landing.[NTIA] [RR]
Aeronautical radio services use108-130 MHz frequency band. Ifnot properly coordinated, theymay suffer interference from FMbroadcast stations operating in
the adjacent band of 88-108MHz.
-15
-13
-10
-8
-5
-3
0
3
5
8
10
13
15
0 10 20 30 40
0
500
1000
1500
2000
0 10 20 30 40
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Signal protection
Over the coverage/ service area, the wantedsignal is to be protected against interferingsignal
The degree of protection is known as protectionratio
Protection ratio is the minimum value of the wanted-to-unwanted signal ratio at the receiver input, determined underspecified conditions (Analogous to signal-to-noise ratio)
Specified performance (reception quality of the wanted signal)is assumed at the receiver output
The ratio of the carrier to the interference is also called carrier-to-interference ratio (C/I)
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Coverage: Noise/ Interference-limited
Isolated transmitter
A minimum signal S/Nmin set
A test receiver & test pointsspecified
The receiver moved form onetest point to another and themeasured S/N compared withthe S/Nmin
The potential, or noise-limitedcoverage is the set of the test
points at which S/N >= S/Nmin Note: S/Nmin defines minimal
signal level
Transmitter + >1 interferer
A minimum protection ratio S/Imin set
A test receiver & test pointsspecified
The receiver moved form onetest point to another and themeasured S/I compared withthe S/Imin
The interference-limitedcoverage is the set of the testpoints at which S/N >= S/Imin
Note: Thermal noise isdisregarded
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Coverage map (simulation 0i) Potential coverage of an
isolated transmitter(omnidirectional)
Blue line: border ofcoverage area (noise-limited)
Test points
blue if the T0-R linkdoes operate correctly
cross if the T0-R linkdoes not operatecorrectly
Coverage Loss = 0
How close can we putneighboring stations?
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Coverage map (simulation 1i)
1 additional (identical
omnidirectional transmitters infree-space with tangentpotential coverage areas
Blue line: Potentialcoverage (the othertransmitter switched-off)
Red line: actual coverage
Brown line: interference-limited coverage (noise-freereceiver)
Coverage loss = 33%
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Coverage map (simulation 2i)
3 identical omnidirectionaltransmitters in free-space withtangent potential coverage areas
Blue line: Potential coverage (theother transmitters switched-off)
Red line: actual coverage
Brown line: interference-limitedcoverage (noise-free receiver)
Coverage loss = 52%
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Coverage map (simulation 6i)
7 identical omnidirectionaltransmitters in free-space withtangent potential coverage areas
Blue line: Potential coverage (theother transmitters switched-off)
Red line: actual coverage
Brown line: interference-limitedcoverage (noise-free receiver)
Coverage loss = 76%
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Coverage loss
May be expressed in terms of Volume,Surface, Population, Costs, etc. (absoluteor relative)
It was proposed as an objective characteristics in
evaluation of operation of radio systems incongested environment
Struzak R: Simulation model for evaluating interference threat to radiocommunicationsystems; Telecommunication Journal, Vol. 57 XII/1990, p. 827-839
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Transmission system
A message, generated by a source of messages, is to bedelivered from the source to a distant destination viatelecommunication channel consisting of a transmitter,propagation path and receiver.
Message in its most general meaning is the object of communication. Depending on the
context, the term may apply to both the information contents and its actual presentation,or signal.
The baseband signal usually consist of a finite set of symbols.E.g. text message is composed of words that belong to a finitevocabulary of the language used. Each word in turn is composedby letters of a (finite) alphabet. (Analog-to-digital conversion)
The transmitter and receiver process the signal using acommon communication protocol under a commoncommunication policy.
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The transmitting station:
1. Generates a RF carrier and combines it withthe baseband signal into a RF signal throughmodulation
2. Performs additional operations E.g. analog-to-digital conversion, formatting, coding,
spreading, adding additional messages/ characteristicssuch as error-control, authentication, or locationinformation
3. Radiates the resultant signal in the form of a
radio waveShortly - it maps the original message into the
radiated radio-wave signal
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Some characteristics of RF wave
Quantity UnitNo. of
dimensions
Frequency Hz, MHz, GHz 1
Time ms, s, hr, year 1
Spatial location (geographicallongitude and latitude and altitude)
Degree, m, km, 3
Elevation angle of launch/ arrival Degree 1
Azimuth angle of launch/ arrival Degree 1
Polarization Sense (left, right) 1
The radio-wave signal has a number of characteristics/variables that generate a multidimensional space
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Visualization of radio-wave signals
+
x3: Frequency
x2:No
rthdis
tance
x1:Eastdistance
Coveragearea
Time
Frequency
Power
Message Length
Frequency
BandOccupied
Only projections on a plane (or cuts) possible
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Spectrum masks
Spectrum masks forWiFi examples ofthe projection of
multidimensionalsignal solid onto theFrequency-Powerplane
Morrow R: Wireless network coexistence; McGraw-Hill 2004 p. 201 & 221
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Propagation process:
Transforms, or maps, the radio-wavesignal radiated by the transmitter into theincident radio wave at the receiver
The propagation mapping involves extravariables (e.g. distance, latency),additional radio waves (e.g. reflected),
random uncertainty (e.g. noise, fading)and distortions
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Receiver:
Filters the incident signals : rejectsunwanted signals and extract the wantedsignal The receivers response defines a solid
window in the signal hyperspace
Recovers the original message throughreversing the transmitter operations(demodulation, decoding),compensating propagationtransformations, and correctingtransmission distortions
Shortly: Maps the incident signals intothe recovered message
Receiver+
Wanted signal
Unwanted signals
Noise
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How the whole system operates?
A series of mappings Following the algorithm/ protocol/ policy
Mapping errors = effects of
interference, noise, distortions, etc. Incomplete (distorted) recovery of the original
message, or its loss -- the recoveredmessage differs from the original
What errors are acceptable?
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How big is small error?
BER
Subjective effect (voice)
10-6 Not audible interference
10-5 Barely audible
10-4 Audible, but not disturbing
10-3 Disturbing, but speech still understandable
10-2 Most disturbing, speech difficult to understandSource: Townsend AAR: Digital line-of-sight radio links, Prentice Hall, p.570
It is application-dependent. In computer communications itis BER. People prefer subjective criteria.
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How to keep error small?
Every component of the wantedsignal must fit exactly into the
receiver reaction window (RRW)in signal hyperspace
For each unwanted signal, at
least one component must falloutside the RRW
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How to implement it?
By filtering/ separing in signalhyperspace
Frequency separation (FMDA) Time separation (TDMA)
Code separation (CDMA)
Other filters (direction, distance,polarization, etc.)
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Frequency filter
The receiverfrequency-windowmay consist of aseries of non-
contiguousopenings (whiterectangles)
The receiverrejects red
frequencies Regular sampling
Irregular sampling
Frequency
Power
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Time filter
The receiver time-window may consistof a number ofseparate openings atdiscrete timeinstances (whiterectangles)
The receiver rejectsred impulses
Regular sampling Irregular sampling
Time
Power
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Direction filter
The receiver rejectssignals arriving at anglesoutside its direction-of-
arrival window Usually the azimuth and
elevation assumed to be
independent
Azimuth angle
Elevat
ion
angle
zy
x
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How BER relates to energy (SNR)?
BER versusSNR for the fourdata rates ofWiFi (IEEE802.11b) along
with values forBluetooth (IEEE802.15.1),WiMedia (IEEE802-15.3) andZigBee (IEEE802.15.4), alloperating in the2.4 GHz band
Morrow R: Wireless network coexistence; McGraw-Hill 2004 p. 192
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Multiple interferers
Probabilistic approach Resultant probability of interference due to a set of
independent interferers equals the product ofprobabilities of interference due to individualinterferers: Probtot= !(Probi)
Energetic approach Resultant power of a number of unwanted signals
equals the sum of powers of individual interferingsignals Itot= "(Ii)
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Outline
Basic concepts
Physical models
Summary
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Thank you for your attention
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Important notes
Copyright 2006 Ryszard Struzak. This work is licensed under theCreative Commons Attribution License (http://creativecommons.org/licenbses/by/1.0) and may be used freely for individual study,research, and education in not-for-profit applications. Any other use
requires the written authors permission. These materials and anypart of them may not be published, copied to or issued from anotherWeb server without the author's written permission. If you cite thesematerials, please credit the author.
Beware of misprints!!! These materials are preliminary notes for mylectures and may contain misprints. If you notice some, or if you havecomments, please send these to [email protected].
mailto:[email protected]:[email protected]:[email protected]://creativecommons.org/licenbses/by/1.0http://creativecommons.org/licenbses/by/1.0http://creativecommons.org/licenbses/by/1.0http://creativecommons.org/licenbses/by/1.0