Interference and system capacity

Interference And System Capacity

AJAL. A. J
Assistant Professor –Dept of ECE,
Federal Institute of Science And Technology (FISAT) TM
16/9/2011
16/02/2012
MAIL: ec2reach@gmail.com
Free Powerpoint Templates

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

DVD Player Dual WiFi/cell PDA
camera phone MP3 Player

WiFi
phone
HDTV AP

Desktop

Multimedia
games

Laptop

Printer
Camcorder
Camera

Interference Defined

- Unwanted signals

either entering your equipment
or
getting into equipment of other parties but generated by
you.

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

Received Signals:
Line-of-sight:

Reflected:

The symbols add up on the
channel Delays
 Distortion!

5

Interference : Flavours

- 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

Solving RFI Problems

- - - 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 EMI Problems

- Hard to track down appliance causing interference

- Microprocessors often generate EMI

- Enclose in grounded box

- Ferrite cores on cables

Interference and system capacity

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

MAN-MADE MAN-MADE
NATURAL
NON-COMMUNICATION COMMUNICATION

TERRESTRIAL RF RADAR

ATMOSPHERIC INDUSTRIAL BROADCAST

PRECIPITATION SCIENTIFIC POINT-TO-POINT

STATIC MEDICAL POINT-TO-MULTIPOINT

EXTRA-TERRESTRIAL HOUSEHOLD MOBILE

SUN NON-RF LAND

COSMIC VEHICLES, TRACTION AERONAUTICAL

RADIO STARS TOOLS, COMPUTERS SATELLITE

THERMAL NOISE POWER LINES GEOSTATIONARY

ESD N-GEOSTATIONARY

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?
Emission Immunity
(Offending EM Coupling (Victim
apparatus) apparatus)

Given interference criteria, EMI
effects depend on
1. System emissions
2. System immunity
3. Degree of coupling

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

CREDENCE TECHNOLOGIES www.credencetech.com ©2002
and 1800MHz
• GSM phones produce
emission in bursts
• High emission levels
(~10V/m)
• Easily creates disruption in
sensitive equipment in
immediate proximity
577µS Carrier: 900/1800MHz

4.6mS
GSM Phone Transmission Pattern

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 DESIGN
METHODS 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"

VICTIM

SOURCE
PATH

System & Environment

For tests we separate the
SYSTEM & ITS ENVIRONMENT system from its
environment.

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

CONDUCTED EMISSIONS
TESTING
• Measure Noise on Power Line

Product
Spectrum Analyzer

Power
Cord

LISN

RADIATED EMISSIONS TESTING
• Test Site: Measure Radiated Spectrum
• Noise from Equipment Case Analyzer

• and Cables Open Area Test Site

Product

3 m or 10 m
Turntable

Measuring Antenna

RADIATED EMISSIONS TESTING
• Test Site: Measure Radiated Spectrum
• Noise from Equipment Case Analyzer

• and Cables Open Area Test Site

Product

3 m or 10 m
Turntable

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

TEST CONFIGURATION

Chamber Configuration

27

Chamber Configuration

28

AMS_02
Main AIR FLOTATION
door PLATFORM

CLEAN ROOM 1 Entry
box
Floor
panels

door

CLEAN ROOM 2

29

Tests from the air
This 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-controlled
airplanes and helicopters

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 bands
2. 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 Overlap

Rooms in Party (11 rooms)

• Blue – noise from room 1
• Red – noise from room 6
• Yellow – noise from room 11
• Only 3 quite rooms available; 1, 6, and 11

802.11b Channel Overlap

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

Types of Channel Interference

• 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 Rx

point-to-point (Shannon 48)

Tx 1 Rx1

Rx Tx

Tx 2
Rx 2
multiple-access broadcast

What We Don’t Know

Unfortunately we don’t know the capacity of most
other Gaussian networks.
Tx 1 Rx 1

Tx 2 Rx 2
Interference

Relay

S D
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 Stations
2. Neighboring Cells
3. 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 FACTOR for Cellular System
performance is the INTERFERENCE

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

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

Interference

1. CO-CHANNEL INTERFERENCE

2. ADJACENT CHANNEL INTERFERENCE

CO-CHANNEL INTERFERENCE

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

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

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

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

Co-Channel Interference

Cell Site-to-Mobile Interference (Downlink)

Mobile-to Cell-Site Interferences (Uplink)

 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
R
Second tier
1 Interfering Cell
First tier
D 1
1
1 1 1
1 1

1

1 1

Cell Geometry

R
D R

R
D
= q = 3N
R

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

INFERENCE

 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


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

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

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

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

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

Example:
a) NI =6 => cluster size N= 7, and when κ=4
The co-channel reuse ratio is q=D/R=sqrt(3N)=4.583
κ
S q
= = 1 ( 4.583) 4 = 75.3
6
I NI

Or 18.66 dB  greater than the minimum required
level  ACCEPT IT!!!
b) N= 7 and κ=3 κ
S q
= = 1 (4.583)3 = 16.04
6
I NI
Or 12.05 dB  less than the minimum required level
 REJECT IT!!!

Example: Worst Case
Cochannel Interference (2)
 A cellular system that requires an S/I
ratio of 18dB. (a) if cluster size is 7,
what is the worst-case S/I? (b) Is a
frequency reuse factor of 7 acceptable
in terms of co-channel interference? If
not, what would be a better choice of
frequency reuse ratio?
 Solution
(a) N=7  q = 3 N = 4.6. If a path loss component of κ=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.

 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

Carrier to Interference Ratio C/I
C C
C/I is calculated as: = KI KI = # of interfering cells
I
∑ Ik
k =1
The maximum number of K in the first tier is 6 and knowing that
−γ −γ
C∝R = αR Wanted signal

I ∝ D −γ = αD −γ Interfering signal

−γ
The above equation becomes: C R
= KI
I
∑ Dk−γ
k =1

Rearranging:

C 1 1
= −γ
= KI
I KI
 Dk 
∑( q )
−γ
∑ R ÷
k =1   k =1
k

and
Dk
qk =
R
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

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
D
Q= = 3N
R

• 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

• Let i0 be the number of co-channel interfering cells. The signal-to-
interference ratio (SIR) for a mobile receiver can be expressed as
S S
= i0
I
∑I
i =1
i

S: the desired signal power
I i : 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
−n
d  close-in reference point
Pr = P0  
d 
 0 d0
or
 d  P0 :measued power
Pr (dBm) = P0 (dBm) − 10n log 
d  TX

 0
n is the path loss exponent which ranges between 2 and 4.

• When the transmission power of each base station is equal, SIR for a
mobile can be approximated as
S R −n
= i0
I
∑ ( Di ) −n
i =1
• Consider only the first layer of interfering cells
S ( D / R)n
= =
( 3N ) n

i0 = 6
I i0 i0

• Example: AMPS requires that SIR be
greater than 18dB
– N should be at least 6.49 for n=4.
– Minimum cluster size is 7

• 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

S R −4
=
I 2( D − R ) − 4 + ( D − R / 2 ) − 4 + ( D + R / 2 ) − 4 + ( D + R ) − 4 + D − 4

2.Adjacent channel interference

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

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

NEAR FAR EFFECT:
 Adjacent channel user is transmitting in very close range to a
subscriber’s receiver, while the receiver attempts to receive a base
station on the desired channel.
 Near far effect also occurs, when a mobile close to a base station
transmits on a channel close to one being used by a weak mobile.
 Base station may have difficulty in discriminating the desired mobile
user from the “bleedover” caused by the close adjacent channel
mobile.

ADJACENT CHANNEL INTERFERENCE

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

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

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

signal on adjacent channel signal on adjacent channel

desired signal

FILTER
interference
interference desired signal

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 interference
Receiver filter

f1 f2 f3
interference

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/ level
I

(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.

Near-Far Effect: Case 1

Uninte
nded
Tx
Strong “bleed
over” BS as Tx

Mobile User Weaker signal
Rx

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

Near-Far Effect: Case 2

BS as Rx

Weaker signal
Strong “bleed
over”
Desired Mobile
Tx Adjacent
Channel
Mobile Tx
The Base Station faces difficulty in recognizing the actual
mobile user, when the adjacent channel bleed over is too
high.

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.

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 CONTROL
o 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 Control

In 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

Thanks
“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.

Channel A Channel B Channel C Channel D

Fixed Block of Radio Frequency Spectrum

Ideal usage of channel bandwidth
• Should use entire range of freqs spanning a channel
• Usage drops down to 0 just outside channel boundary

Power

Frequency

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

Real Usage
Wastage of spectrum

• PROBLEM:
– Transmitted power of some freqs. < max. permissible limit
– Results in lower channel capacity and inefficient usage of the
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

Link A Ch 1 Ch 1 Ch 3 Ch 6

Link B Ch 3

Link C Ch 6

Amount of Interference

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

Empirical proof of benefits of partial overlap

Link A Ch 1 Ch 1 Ch 3 Ch 6

Link B Ch 3

Link C Ch 6

Virtually non-overlapping

• Typically partially overlapped channels are avoided
• With sufficient spatial separation, they can be used

Interference and system capacity

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Interference and system capacity

Editor's Notes