InterferCoexist eBook

8/12/2019 InterferCoexist eBook

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Property of R Struzak

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
http://www.ryszard.struzak.com/http://www.ryszard.struzak.com/


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Property of R Struzak 2

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/Packet


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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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10

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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12

Outline

Basic concepts

Physical models

Summary


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13

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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3 identical omnidirectionaltransmitters in free-space withtangent potential coverage areas

Blue line: Potential coverage (theother transmitters switched-off)


Brown line: interference-limitedcoverage (noise-free receiver)

Coverage loss = 52%


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7 identical omnidirectionaltransmitters in free-space withtangent potential coverage areas

Blue line: Potential coverage (theother transmitters switched-off)


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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Property of R Struzak X

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

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