16/02/201216/9/2011

Free Powerpoint Templates

Interference And System Capacity

AJAL. A. J

Assistant Professor –Dept of ECE,

Federal Institute of Science And Technology (FISAT) TM 

MAIL: ec2reach@gmail.com

Proofs of Wave Nature

• Thomas Young's Double Slit Experiment (1807) bright (constructive) and dark (destructive)

fringes seen on screen • Thin Film Interference Patterns

• Poisson/Arago Spot (1820) • Diffraction fringes seen within and around a

small obstacle or through a narrow opening

Multi-channel real time environment

AP

WiFi phone

HDTV

Camcorder

Desktop

PDA

Laptop

Printer

Dual WiFi/cellcamera phone

Multimediagames

MP3 PlayerDVD Player

Camera

Interference Defined

- Unwanted signals

either entering your equipment

or getting into equipment of other parties but generated by

you.

5

Inter-Symbol-Interference (ISI) due to Multi-Path Fading

Transmitted signal:

Received Signals:Line-of-sight:

Reflected:

The symbols add up on the channel

Distortion!

Delays

- RFI - Radio Frequency Interference- - - Two or more signals competing for the same channel

- EMI - Electromagnetic Interference- - - Appliances that are overloaded by strong EMI from nearby RF sources

Interference : Flavours

- - - Disconnect components to localize problem area

- - - Check cable connections

- - - Check for grounded polarized plugs

- - - Ferrite cores around power cables

- - - Hipass filter on 300 ohm TV feedline

Solving RFI Problems

- Hard to track down appliance causing interference

- Microprocessors often generate EMI

- Enclose in grounded box

- Ferrite cores on cables

Solving EMI Problems

EMI / EMC/ EMS

• EMI is defined as the undesirable signal which causes

unsatisfactory operation of a circuit or device.

• EMC is defined as the ability of electronic and communication

equipment to be able to operate satisfactorily in the presence of

interference and not be a source of interference to nearby

equipment.

• EMS Electromagnetic susceptibility (EMS) is the capability of a

device to respond to EMI.

EM ENVIRONMENT COMPONENTS

NATURALMAN-MADE

NON-COMMUNICATIONMAN-MADE

COMMUNICATION

TERRESTRIAL

EXTRA-TERRESTRIAL

THERMAL NOISE

ATMOSPHERIC

PRECIPITATION

STATIC

SUN

COSMIC

RADIO STARS

RF

NON-RF

INDUSTRIAL

SCIENTIFIC

MEDICAL

HOUSEHOLD

VEHICLES, TRACTION

TOOLS, COMPUTERS

POWER LINES

ESD

RADAR

BROADCAST

POINT-TO-POINT

POINT-TO-MULTIPOINT

MOBILE

SATELLITE

LAND

AERONAUTICAL

GEOSTATIONARY

N-GEOSTATIONARY

MOBILE

SATELLITE

Electromagnetic Interference (EMI)

• The effect of unwanted energy due to one or a combination of emissions, radiations, or inductions upon reception in a radiocommunication system, or loss of information which could be extracted in the absence of such unwanted energy

EMI depends on what?

Given interference criteria, EMI effects depend on

1.System emissions2.System immunity 3.Degree of coupling

Emission(Offending apparatus)

EM CouplingImmunity(Victim

apparatus)

Electromagnetic Interference (EMI)

• EMI: ‘quantification’ of degradation of the quality of an observation due to unwanted emissions, radiations, or inductions upon reception in a radiocommunication system

• Information about EMI is obtained by inspection of observations

EMI in Cleanrooms – Example

• Wafers are charged to the limit• Cart is charged by the wafers via capacitive coupling• Wheels are insulators – cart cannot discharge• EMI propagates throughout the fab causing lockup of wafer handlers

EMI from Mobile Phones

• Frequency range: 800, 900 and 1800MHz

• GSM phones produce emission in bursts

• High emission levels (~10V/m)

• Easily creates disruption in sensitive equipment in immediate proximity

577µS

4.6mS

GSM Phone Transmission Pattern

CR

ED

EN

CE

TE

CH

NO

LO

GIE

S

ww

w.c

red

ence

tech

.co

m ©

2002

Carrier: 900/1800MHz

EMC: what is it?

• Electromagnetic compatibility (EMC): ability of an equipment or system to

• (1) function satisfactorily in its EM environment(2) without introducing intolerable disturbance to anything in that environment

– Criteria of ‘satisfactory’, and ‘intolerable’ and the definition of ‘anything’ and “environment” are all situation-dependent

– Harmful (intolerable) interference - when the risk (probability) of interference and extent of its consequences exceed the acceptable levels

METHODS TO ELIMINATE EMI OR DESIGNMETHODS FOR EMC

The effective methods to eliminate EMI are

1. Shielding

2. Grounding

3. Bonding

4. Filtering

5. Isolation

6. Separation and orientation

7. Circuit impedance level control

8. Cable design

9. Cancellation techniques in frequency or time

domain

10. Proper selection of cables, passive components

11. Antenna polarization control

12. Balancing

Elements of an EMI Situation

–Source "Culprit"

–Coupling method "Path"

–Sensitive device "Victim"

SOURCEPATH

VICTIM

PATH

VICTIM

System & Environment

SYSTEM & ITS ENVIRONMENT

SYSTEM

ENVIRONMENT

In emission testing we replace the environment by test equipment that evaluate the level of emissions. In immunity tests we create a known EM stress and observe reactions.

For tests we separate the system from its environment.

CONDUCTED EMISSIONS TESTING

• Measure Noise on Power Line

Spectrum Analyzer

LISN

Product

Power Cord

RADIATED EMISSIONS TESTING• Test Site: Measure Radiated• Noise from Equipment Case• and Cables

Measuring Antenna

Product

3 m or 10 m

SpectrumAnalyzer

Open Area Test Site

Turntable

RADIATED EMISSIONS TESTING• Test Site: Measure Radiated • Noise from Equipment Case• and Cables

Measuring Antenna

Product

3 m or 10 m

Photos: EMC Test System, Austin, TX emctest.com

SpectrumAnalyzer

Open Area Test Site

Turntable

RADIATED EMISSIONS TESTING

• Test Site: Measure Radiated• Noise from Equipment Case• and Cables

Measuring Antenna

Product

3 m or 10 m

SpectrumAnalyzer

Open Area Test Site

Turntable

Anechoic Chamber

27

TEST CONFIGURATION

Chamber Configuration

28

Chamber Configuration

29

CLEAN ROOM 1

CLEAN ROOM 2

AIR FLOTATION PLATFORM

AMS_02

Main door

Entry box

Floor panels

door

Test room

EMC tests

•

EMC tests

EMC tests

Test antennas

Test antennas

Tests from the airThis photo shows a flying laboratory on manned helicopter I designed and supervised many years ago.

Modern technology allows such measurements to be made at distance, using miniature unmanned radio-controlledairplanes and helicopters

Near field test

Summary on EMC

• The aim of EMC is – to ensure the reliability of all types of

electronic devices wherever they are used – and thus to ensure the reliable and safe

operation of the systems in which they are employed.

• EMC concerns all of us

Interference

• Interference management is an central problem in wireless system design.

• Within same system (eg. adjacent cells in a cellular system) or across different systems (eg. multiple WiFi networks)

• Two basic approaches:1. orthogonalize into different bands2. full sharing of spectrum but treating

interference as noise

Interference• Sources of interference

– another mobile in the same cell

– a call in progress in the neighboring cell

– other base stations operating in the same frequency band

– noncellular system leaks energy into the cellular frequency band

• Two major cellular interference

–co-channel interference

–adjacent channel interference

802.11b Channel Overlap802.11b Channel Overlap

• Blue – noise from room 1

• Red – noise from room 6

• Yellow – noise from room 11

• Only 3 quite rooms available; 1, 6, and 11

• Blue – noise from room 1

• Red – noise from room 6

• Yellow – noise from room 11

• Only 3 quite rooms available; 1, 6, and 11

Rooms in Party (11 rooms) Rooms in Party (11 rooms)

802.11b Channel Overlap802.11b Channel Overlap

Only 3 non-overlapping channels: 1, 6, and 11.Only 3 non-overlapping channels: 1, 6, and 11.

Types of Channel InterferenceTypes of Channel Interference

• Adjacent channel interference: inversely proportional to the

distance

• Co-channel interference: directly proportional to the co-

channel interference factor

• Adjacent channel interference: inversely proportional to the

distance

• Co-channel interference: directly proportional to the co-

channel interference factor

Gaussian Network Capacity: What We Know

Tx

Rx1

TxRx

Rx

Tx 1

Tx 2Rx 2

point-to-point (Shannon 48)

C = log2(1+ SNR)

multiple-access broadcast

Real time process

What We Don’t Know

Unfortunately we don’t know the capacity of most other Gaussian networks.

D

Tx 1

Relay

S

Tx 2 Rx 2

Rx 1

Interference

relay

Multiuser Opportunistic Communication

Multiple users offer new diversity modes, just like time or frequency or MIMO channels

Interference scenario : Real Time

It’s the model.

• Shannon focused on noise in point-to-point communication.

• But many wireless networks are interference rather than noise-limited.

• We propose a deterministic channel model emphasizing interaction between users’ signals rather than on background noise.

• Far more analytically tractable and can be used to determine approximate Gaussian capacity

Interference

• So far we have looked at single source, single destination networks.

• All the signals received is useful.

• With multiple sources and multiple destinations, interference is the central phenomenon.

• Simplest interference network is the two-user interference channel.

Main message:

If something can’t be computed exactly, approximate.

• Similar evolution has happened in other fields:

– fluid and heavy-traffic approximation in queueing networks

– approximation algorithms in CS theory

• Approximation should be good in engineering-relevant regimes.

Interference It is a major limiting factor in the performance of cellular radio systems. (In comparison with wired comm. Systems, the amount and sources of interferences in Wireless Systems are greater.)

Creates bottleneck in increasing capacity

Sources of interference are:1. Mobile Stations2. Neighboring Cells3. The same frequency cells

4. Non-cellular signals in the same spectrum

Interference in Voice Channels: Cross-Talk

Urban areas usually have more interference, because of:a)Greater RF Noise Floor, b) More Number of Mobiles

MAJOR LIMITING FACTORMAJOR LIMITING FACTOR for Cellular System performance is the for Cellular System performance is the INTERFERENCEINTERFERENCE

Interferences can cause:Interferences can cause: CROSS TALKCROSS TALK Missed and Blocked Calls.Missed and Blocked Calls.

SOURCES OF INTERFERENCE?SOURCES OF INTERFERENCE? Another mobile in the same cellAnother mobile in the same cell (if distance & frequency are close) (if distance & frequency are close) A call in progress in A call in progress in neighboring cell (if frequency is close)neighboring cell (if frequency is close).. Other base stations operating in the same frequency band (from co-channel cells)Other base stations operating in the same frequency band (from co-channel cells) Non-cellular systems leaking energy into cellular frequency bandNon-cellular systems leaking energy into cellular frequency band

InterferenceInterference

1. CO-CHANNEL INTERFERENCE

2. ADJACENT CHANNEL INTERFERENCE

1.Co-Channel Interference

CO-CHANNEL INTERFERENCECO-CHANNEL INTERFERENCE

Frequency ReuseFrequency Reuse Given coverage area Given coverage area cells using the same set of frequenciescells using the same set of frequencies co-channel cell !!! co-channel cell !!!

Interference between these cells is calledInterference between these cells is called CO-CHANNEL INTERFERENCE.CO-CHANNEL INTERFERENCE.

However, co-channel interference However, co-channel interference cannot be overcome just by increasing the carrier power of a transmitter. cannot be overcome just by increasing the carrier power of a transmitter. Because increase in carrier transmit power increases theBecause increase in carrier transmit power increases the interference.interference.

How to Reduce co-channel interference?How to Reduce co-channel interference? Co-channel cells must be physically separated by a minimum distanceCo-channel cells must be physically separated by a minimum distance to provide sufficient isolation. to provide sufficient isolation.

Co-Channel InterferenceCo-Channel Interference

Cell Site-to-Mobile Interference (Downlink)

Mobile-to Cell-Site Interferences (Uplink)

Co-Channel Co-Channel InterferenceInterference

Intracell Interference: interferences from other mobile terminals in the same cell.– Duplex systems – Background white noise

Intercell interference: interferences from other cells.– More evident in the downlink than uplink for reception– Can be reduced by using different set of frequencies

Design considerations:– Frequency reuse– Interference– System capacity

1.Co-Channel Interference

• Cells using the same frequency cause interference to each other

• Called co-channel interference (CCI)• CCI increases as the cluster size N

decreases• Important factor for signal quality is the

Carrier to Interference Ratio C/I• Most interference comes from the first tier of

co-channel cells

Co-Channel Interference…

1

1

1

1

1

1

1

11

1

1 1

1

Interfering CellFirst tier

Second tier

D

R

Cell Geometry

DR

R

R

NqR

D3

CALCULATION

• Let i0 be the number of co-channel interfering cells, then the signal-to-interference ratio for a mobile receiver which monitors a forward channel is

– where S is the desired signal power from desired

BS and Ii is the interference power caused by ith interfering co-channel cell

By increasing the ratio of D/R, ► separation between co-channel cells relative to coverage

distance of a cell is increased.► Thus interference is reduced.

The parameter Q (co-channel reuse ratio) is related to cluster size. Thus for a hexagonal geometry

A small value of Q provides larger capacity since N is cluster size

Large value of Q improves transmission quality due to smaller level of co-channel interference

A trade-off must be made between these two objectives

Let i0 be the number of co-channel interfering cells, then the signal-to-interference ratio for a mobile receiver which monitors a forward channel is

► where S is the desired signal power from

desired BS and Ii is the interference power caused by ith interfering co-channel cell

Average received signal strength at any point decays as a power law of the distance of separation between transmitter and receiver

Average received power Pr at a distance d from the transmitting antenna is approx

► Where Po is the power received at a close-in reference point at a small distance do from the transmitting antenna, n is path loss exponent ranging between 2 and 4

Now consider co-channel cell interference If Di is the distance of ith interferer from

the mobile, the received power will be proportional to (Di)-n

When the transmit power of each BS is equal and the path loss exponent is same throughout coverage then S/I can be approximated as

Considering only the first layer of interfering cells, which are equidistant D from the desired BS

Eqn 4 implies to

► It relates S/I to cluster size N, which in turn determines the overall capacity of the system

For US AMPS system, tests indicate that for sufficient voice quality S/I should be greater or equal to 18 dB.

By using Eqn 5, in order to meet this requirement, N should be at least 6.49 assuming n=4.

Thus a minimum cluster size of 7 is required to meet S/I requirement of 18 dB

It should be noted Eqn 5 is based on hexagonal cell geometry

INFERENCE

Co-Channel InterferenceCo-Channel Interference

An S/I of 18 dB is the measured An S/I of 18 dB is the measured value for the value for the accepted voice accepted voice qualityquality from the present day from the present day cellular mobile receivers.cellular mobile receivers.

Sufficient voice quality is provided Sufficient voice quality is provided when when S/IS/I is greater than or equal to is greater than or equal to 18dB.18dB.

Example: Example: Co-Channel InterferenceCo-Channel Interference

If If S/I = 15 dBS/I = 15 dB required for satisfactory performance for forward required for satisfactory performance for forward channel performance of a cellular system.channel performance of a cellular system.

a)a) What is the Frequency Reuse Factor q (assume K=4)?What is the Frequency Reuse Factor q (assume K=4)?b)b) Can we use K=3? Can we use K=3?

Assume 6 co-channels all of them (same distance from the mobile), I.e. N=7

Example: Example: Co-Channel InterferenceCo-Channel Interferencea)a) NNII =6 => cluster size N= 7 =6 => cluster size N= 7, and when , and when =4The co-channel reuse ratio is

q=D/R=sqrt(3N)=4.5833.75)583.4( 4

61

IN

q

I

S

Or 18.66 dB greater than the minimum required level ACCEPT IT!!!

b) N= 7 and N= 7 and =3

04.16)583.4( 361

IN

q

I

S

Or 12.05 dB less than the minimum required level REJECT IT!!!

Example: Example: Worst CaseWorst Case Cochannel Interference Cochannel Interference (2)(2)

A cellular system that requires an S/I ratio A cellular system that requires an S/I ratio of 18dB. (a) if cluster size is 7, what is the of 18dB. (a) if cluster size is 7, what is the worst-case S/I? (b) Is a frequency reuse worst-case S/I? (b) Is a frequency reuse factor of 7 acceptable in terms of co-factor of 7 acceptable in terms of co-channel interference? If not, what would be channel interference? If not, what would be a better choice of frequency reuse ratio?a better choice of frequency reuse ratio?

Solution (a) N=7 q = . If a path loss component of

=4, the worst-case signal-to-interference ratio is S/I = 54.3 or 17.3 dB.

(b) The value of S/I is below the acceptable level of 18dB. We need to decrease I by increasing N =9. The S/I is 95.66 or 19.8dB.

6.43 N

For 7-cell cluster, hexagonal cell geometry layout

Mobile is at the boundary of the cell

The worst case S/I ratio can be approximated using Eqn 4

The above Eqn can be rewritten in terms of co-channel reuse ratio Q as

For N=7, the value of Q is 4.6 The worst case S/I is approximated as 49.56 (17 dB) using

Eqn 7, where exact solution using Eqn 4 is 17.8 dB.

Example

If S/I is required 15 dB for satisfactory forward channel performance, what is the frequency reuse factor and cluster size that should be used for maximum capacity if path loss exponent n = 4 and n = 3? Assuming 6 co-channel cells in first tier at same distance from desired BS► n = 4, lets consider 7-cell reuse

• Using Eqn. 1, reuse ratio is 4.583• Using 5, S/I = 1/6 x (4.583)^4 = 75.3 = 18.66 dB• Since this is greater than min required, N=7 can be used

► n = 3, first consider 7-cell reuse• S/I = 1/6 x (4.583)^3 = 16.04 = 12.05 dB• Since this is less than min required, • Next possible value of N is 12-cell reuse (i = j = 2)• Using Eqn. 1, reuse ratio is 6.0• S/I = 1/6 x (6)^3 = 36 = 15.56 dB• Since this is greater than min required S/I, So N=12 is

used

IK

kkI

C

I

C

1

C/I is calculated as:

The maximum number of K in the first tier is 6 and knowing that

RRC

IK

kkD

R

I

C

1

DDI

Wanted signal

Interfering signal

The above equation becomes:

Carrier to Interference Ratio C/I

KI = # of interfering cells

11

1 1II

KKk

kkk

C

I D qR

Rearranging:

and

R

Dq k

k

The qk is the co-channel interference reduction factor with kth co-channel interfering cell.

Co-Channel Interference…

• As N decreases the number of frequency channels per cell increases but C/I decreases

• C/I is improved by different methods– Sectored antennas: reduces KI

– Beam tilting: Reduces power to co-channel cells

– Channel assignment: minimizes activation of co-channel frequencies, which reduces KI

Co-channel interference & system capacity

• Co-channel cells use the same set of frequencies in a given coverage area.

• Co-channel interference cannot be removed by increasing signal power.

• They must be physically separated by certain distance to provide sufficient isolation for propagation.

• Co-channel re-use factor is given by:

Q = D/R = √3N

where R – radius of the cell

D – distance to the center of the nearest co-channel cells

N – cluster size

Increasing D/R will give less interference, whereas decreasing Q value gives more capacity!

CCI Reduction: Cell Sectoring• Shown 120 sectored

antennas• Channel per cell are

divided among 3 sectors• CCI decreased. Sector 0

gets interference from sectors 4, 5 and 6 only

• 60 degrees sectored also possible

02

34

1

5

6

Co-channel Interference and System Capacity

• Frequency reuse - there are several cells that use the same set of frequencies – co-channel cells

– co-channel interference

• To reduce co-channel interference, co-channel cell must be separated by a minimum distance.

• When the size of the cell is approximately the same– co-channel interference is independent of the transmitted power

– co-channel interference is a function of • R: Radius of the cell

• D: distance to the center of the nearest co-channel cell

• Increasing the ratio Q=D/R, the interference is reduced.

• Q is called the co-channel reuse ratio

• For a hexagonal geometry

• A small value of Q provides large capacity

• A large value of Q improves the transmission quality - smaller level of co-channel interference

• A tradeoff must be made between these two objectives

NR

DQ 3

• Let be the number of co-channel interfering cells. The signal-to-interference ratio (SIR) for a mobile receiver can be expressed as

S: the desired signal power

: interference power caused by the ith interfering co-channel cell base station

• The average received power at a distance d from the transmitting antenna is approximated by

or

n is the path loss exponent which ranges between 2 and 4.

0i

0

1

i

iiI

S

I

S

iI

n

r d

dPP

00

00 log10)dBm()dBm(

d

dnPPr

close-in reference point

TX

0d

0P :measued power

• When the transmission power of each base station is equal, SIR for a mobile can be approximated as

• Consider only the first layer of interfering cells

0

1

i

i

ni

n

D

R

I

S

00

3)/(

i

N

i

RD

I

Snn

• Example: AMPS requires that SIR be greater than 18dB– N should be at least 6.49 for n=4.

– Minimum cluster size is 7

60 i

• For hexagonal geometry with 7-cell cluster, with the mobile unit being at the cell boundary, the signal-to-interference ratio for the worst case can be approximated as

44444

4

)()2/()2/()(2

DRDRDRDRD

R

I

S

2.Adjacent channel interference

2. ADJACENT2. ADJACENT CHANNEL INTERFERENCE CHANNEL INTERFERENCE

Interference resulting from signals Interference resulting from signals which are adjacent which are adjacent in frequency to the desired signalin frequency to the desired signal is called is called ADJACENT CHANNEL INTERFERENCE.ADJACENT CHANNEL INTERFERENCE.

WHY?WHY?From imperfect receiver filters (which allow nearby frequencies) to leak into the pass-band.From imperfect receiver filters (which allow nearby frequencies) to leak into the pass-band.

NEAR FARNEAR FAR EFFECT: EFFECT: Adjacent channel userAdjacent channel user is transmitting is transmitting in very close rangein very close range to a subscriber’s receiver, while the receiver attempts to receive a to a subscriber’s receiver, while the receiver attempts to receive a

base station on the desired channel.base station on the desired channel. Near far effect also occurs, when a mobile close to a base station transmits on Near far effect also occurs, when a mobile close to a base station transmits on a channel close to onea channel close to one being used by a weak being used by a weak

mobilemobile.. Base station may have difficulty in Base station may have difficulty in discriminating the desired mobile user from the “bleedover”discriminating the desired mobile user from the “bleedover” caused by the close adjacent caused by the close adjacent

channel mobile.channel mobile.

ADJACENT CHANNEL INTERFERENCEADJACENT CHANNEL INTERFERENCE

How to reduce?How to reduce?• Careful filteringCareful filtering• Channel assignmentChannel assignment no channel assignment which are all adjacent in frequency. no channel assignment which are all adjacent in frequency.• Keeping frequency separationKeeping frequency separation between each channel in a given cell as large as possible. between each channel in a given cell as large as possible.

e.g., in e.g., in AMPS SystemAMPS System there are 395 voice channels which there are 395 voice channels which are divided into are divided into 21 subsets21 subsets each with 19 channels. each with 19 channels.

• In each subset, the closest adjacent channel is 21 channels away.In each subset, the closest adjacent channel is 21 channels away.• 7-cell reuse7-cell reuse -> each cell uses -> each cell uses 3 subsets of channels3 subsets of channels..• 3 subsets are assigned such that every channel in the cell is assured of being separated from every other 3 subsets are assigned such that every channel in the cell is assured of being separated from every other

channel channel by at least 7 channel spacings.by at least 7 channel spacings.

Adjacent Channel Interference

• Adjacent channel interference: interference from adjacent in frequency to the desired signal. – Imperfect receiver filters allow nearby frequencies to leak into the

passband

– Performance degrade seriously due to near-far effect.

desired signal

receiving filter response

desired signalinterference

interference

signal on adjacent channelsignal on adjacent channel

FILTER

Adjacent Channel Interference• Adjacent channel interference: interference from adjacent in frequency

to the desired signal. – Imperfect receiver filters allow nearby frequencies to leak into the

passband

– Performance degrade seriously due to near-far effect.

desired signal

receiving filter response

desired signalinterference

interference

signal on adjacent channelsignal on adjacent channel

FILTER

Adjacent channel interference can be minimized through

1. careful filtering and

2. channel assignment.• Keep the frequency separation between

each channel in a given cell as large as possible

• A channel separation greater than six is needed to bring the adjacent channel interference to an acceptable level.

Adjacent channel interferenceReceiver filter

f1 f3f2interference

Adjacent-site constraint: channels assigned to neighboring cells

Adjacent channel interference

Interference resulting from signals which are adjacent in frequency

It results from imperfect receiver filters which allow nearby frequencies to leak into passband

It is more serious if the transmitter is more close to the user’s receiver listening to desired channel

This is near-far effect► A nearby transmitter captures the receiver of

subscriber.► Or mobile close to BS transmits on adjacent channel

to one being used by a weak mobile

Adjacent channel interference can be minimized by careful filtering and channel assignment

A cell need not be assigned channels adjacent in frequency

By keeping frequency separation in a given cell between channels as large as possible, interference can considerably minimized

By sequentially assigning successive channels to different cells, channel allocation schemes are able to separate channels in a cell as many as N

Some assigning strategies also avoid use of adjacent channels in neighboring cell sites.

If reuse factor (1/N) is large i.e. N is small, the separation may not be sufficient to keep intf within tolerable limits.

For example if a close-in mobile is 20 times as close to BS as another mobile and energy has leaked to passband, S/I at BS for weak mobile is approx

S/I = (20)-n

For n-4, this is -52 dB If filter of BS receiver has a slope of 20 dB/octave

then intf must be displaced 6 times the passband bandwidth from the center to achieve 52 dB attenuation

This implies more than 6 channels separation are needed for an acceptable S/I level

(2) Adjacent Channel Interference Interference from channels that are adjacent in frequency,

The primary reason for that is Imperfect Receiver Filters which cause the adjacent channel energy to leak into your spectrum.

Problem is severer if the user of adjacent channel is in close proximity. Near-Far Effect

Near-Far Effect: The other transmitter(who may or may not be of the same type) captures the receiver of the subscriber.

Also, when a Mobile Station close to the Base Station transmits on a channel close to the one being used by a weaker mobile: The BS faces difficulty in discriminating the desired mobile user from the “bleed over” of the adjacent channel mobile.

Unintended

Tx

Mobile User Rx

BS as Tx

Weaker signal

Strong “bleed over”

The Mobile receiver is captured by the unintended, unknown transmitter, instead of the desired base station

Near-Far Effect: Case 1

Adjacent Channel

Mobile Tx

Desired Mobile Tx

BS as Rx

Weaker signal

Strong “bleed over”

The Base Station faces difficulty in recognizing the actual mobile user, when the adjacent channel bleed over is too high.

Near-Far Effect: Case 2

Minimization of ACI

(1) Careful Filtering ---- min. leakage or sharp transition(2) Better Channel Assignment Strategy

Channels in a cell need not be adjacent: For channels within a cell, Keep frequency separation as large as possible.

Sequentially assigning cells the successive frequency channels.

Also, secondary level of interference can be reduced by not assigning adjacent channels to neighboring cells.

For tolerable ACI, we either need to increase the frequency separation or reduce the pass band BW.

Power Control in Mobile Com

What is power control ?

Both the BS and MS transmitter powers are adjusted dynamically over a wide range.

Typical cellular systems adjust their transmitter powers based on received signal strength.

TYPES OF POWER CONTROLo Open Loop Power Control

It depends solely on mobile unit, not as accurate as closed loop, but can react quicker to fluctuation in signal strength. In this there is no feed back from BS.

o Closed Loop Power ControlIn this BS makes power adjustment decisions and communicates to mobile on control channels

Why power control ?

Near-far effect Mechanism to compensate for “channel

fading” Interference reduction, prolong battery life

Power Control for Reducing Interference

• Ensure each mobile transmits the smallest power necessary to maintain a good quality link on the reverse channel

– long battery life

– increase SIR

– solve the near-far problem

ThanksThanks

“You can't predict the future, but you can invent it.”

Spectral Bands and Channels• Wireless communication uses emag signals

over a range of frequencies• FCC has split the spectrum into spectral

bands• Each spectral band is split into channels

Example of a channel

Typical usage of spectral band

• Transmitter-receiver pairs use independent channels that don’t overlap to avoid interference.

Fixed Block of Radio Frequency Spectrum

Channel A Channel B Channel C Channel D

Ideal usage of channel bandwidth

• Should use entire range of freqs spanning a channel

• Usage drops down to 0 just outside channel boundary

Channel A Channel B

Frequency

Po

we

r

Channel C Channel D

Realistic usage of channel bandwidth

• Realistically, transmitter power output is NOT uniform at all frequencies of the channel.

• PROBLEM:– Transmitted power of some freqs. < max.

permissible limit– Results in lower channel capacity and inefficient

usage of the spectrum

Real Usage

Channel A Channel B

Po

we

r

Channel C Channel D

Wastage of spectrum

Consideration of the 802.11b standard

• Splits 2.4 GHz band into 11 channels of 22 MHz each– Channels 1, 6 and 11 don’t overlap

• Can have 2 types of channel interferences:– Co-channel interference

• Address by RTS/CTS handshakes etc.– Adjacent channel interference over partially overlapping

channels• Cannot be handled by contention resolution

techniques

Wireless networks in the past have used only non-overlapping channels

Focus

• To examine approaches to use partially overlapped channels efficiently to improve spectral utilization

Channel A Channel B

Channel A’

Empirical proof of benefits of partial overlap

• Can we use channels 1, 3 and 6 without interference ?

Ch 1 Ch 6Ch 3

Amount of Interference

Link A Ch 1

Link C Ch 6

Link B Ch 3

Empirical proof of benefits of partial overlap

• Typically partially overlapped channels are avoided

• With sufficient spatial separation, they can be used

Link A Ch 1

Link C Ch 6

Link B Ch 3

Ch 1 Ch 6Ch 3

Virtually non-overlapping