final M&PC_wireless_communication ppt1.ppt

Introduction
• The Term Wireless : The way of accessing
a network without wire.
• Any radio terminal that could be moved
during operation.
• Goal :
• 1. Terminal mobility
• 2. Personal mobility
• 3. Service mobility

Characteristics of communication
device
• Fixed and Wired
• Mobile and Wired
• Fixed and Wireless
• Mobile and Wireless

Mobile communication
• Wireless vs. mobile Examples
  stationary computer
  laptop in a hotel (portable)
  wireless LAN in historic buildings
  Personal Digital Assistant (PDA)
• Integration of wireless into existing fixed networks:
– Local area networks: IEEE 802.11, ETSI (HIPERLAN)
– Wide area networks: Cellular 3G, IEEE 802.16
– Internet: Mobile IP extension

Advantages of cellular system
• Higher capacity
• Less transmission power
• Local interference only
• Robustness: If one antenna fails, this only
influences communication with in a small area
• Disadvantages:
• Infrastructure Needed
• Hand over needed
• Frequency planning needed

FREQUENCY REUSE
 The frequency spectrum is a
limited resource. Therefore,
wireless telephony, like radio,
must reuse frequency
assignments.
 For example, two radio stations
might transmit at 98.3 FM. There
is no interference as long as the
radio stations are far enough
apart.
Problem: Limited frequency spectrum
Solution: Based on the idea of splitting up coverage region
into small areas referred to as cells

GSM Network Architecture
SOURCE: UWC
INTERFACE TO LAND
TELEPHONE NETWORKS
HIERARCHY
OF CELLS
CELL TRANSMITTER
& RECEIVER
PHONE
SIM:
IDENTIFIES A
SUBSCRIBER
LIST OF
ROAMING
VISITORS
LIST OF SUBSCRIBERS
IN THIS AREA
STOLEN, BROKEN
CELLPHONE LIST
ENCRYPTION,
AUTHENTICATION

Mobile Station (MS)
 Mobile Station consist of two units
Mobile Hand set Subscriber Identity Module
Mobile Hand set is one of the most complicated
GSM device. It provides user the access to the
Network. Each handset has unique identity no.
called IMEI.
SIM is a removable module that fits in the mobile
handset. Each SIM has unique number called
International Mobile Subscriber Identity (IMSI).
It has built in Micro-computer & memory into it.
It contains the ROM of 6 to 16KB,RAM of 128 to
256 bytes and EEPROM of 3 to 8KB

Base Station Controller (BTS)
 BTS is the official name of the cell site (nearest tower)
When cell phone is switched on, it connects with nearest
BTS.
The Base Transceiver Station is responsible for transmit
and receive the signal (to and from mobile station).
It provides the path between MS to BSC.
Group of fixed channels are allocated to each BTS.
BTS

Base Station Controller (BSC)
 BSC controls several BTSs.
 BSC manages channel allocation, & Handover
of calls from one BTS to another BTS.
 BSC has database for all of its BTS’s parameters.
 BSC provides path from MS to MSC.
BSC

Mobile Switching Centre (MSC)
 MSC is heart of the entire network connecting Fixed line network to
Mobile network.
 MSC manages all call related functions and
Billing information.
 MSC is connected to HLR & VLR for subscriber
identification & routing incoming calls.
 MSC capacity is in terms of no of subscribers.
 MSC is connected to BSC at one end and Fixed
Line network on other end.
 Call Detail Record (CDR) is generated for each & every
call in the MSC.
MSC

Visiting Location Register (VLR)
VLR
MSC
 Active Subscriber is registered in VLR.
 It has temporary data base of all the active
subscribers used for their call routing.
 HLR validates subscriber before registration.
 MSC asks VLR before routing incoming call. (VLR Location)

 Process of selecting & allocating channel groups for all of the cellular
base stations within a system is called frequency reuse or frequency
planning.
 To keep interference level within tolerable limits.
 Hexagon-why?
FREQUENCY REUSE…
FREQUENCY REUSE…

Home Location Register (HLR)
HLR
MSC
 All Subscribers data is stored in HLR.
 It has permanent data base of all the registered
subscribers.
 HLR has series of numbers for all subscribers.

Authentication Centre (AUC)
HLR
MSC
 Authentication is a process to verify the subscriber SIM.
 Secret data & verification algorithm are stored in to the
AUC.
 AUC & HLR combine to authenticate the subscribers.
 Subscriber authentication can be done on every call, if required.
AUC

Equipment Identity Register (EIR)
EIR
MSC
 All subscriber's mobile handset data is stored in EIR.
 MSC asks mobile to send its IMEI & then checks it with
the data available in EIR.
 EIR has different classifications for mobile handsets
like, White list, Grey list & Black list.
 According to category the MS can make calls or can be
stopped from making calls.

Operation & Maintenance Centre (OMC)
OMC
 All the network elements are connected to OMC.
 OMC monitors health of all network elements &
carry out maintenance operation, if required.
 OMC links to BTSs are via parent BSC.
 OMC keeps records of all the faults occurred.
 OMC can also do Traffic analysis.
 OMC prepares the MIS Report for the network.

GSM Variants
Variant Uplink
(MHz)
Downlink
(MHz)
Total
Bandwidth
Duplex-
frequency
Channels
GSM-400 451-458 and
479-486
461-468 and
489-496
Twice 14 MHz 10 MHz Twice 72
GSM-900
(primary
band)
890-915 935-960 Twice 25 MHz 45 MHz Twice 124
Extended
GSM-900
880-915 925-960 Twice 35 MHz 45 MHz Twice 174
GSM-R 876-880 921-925 Twice 4 MHz 45 MHz Twice 19
DCS-1800 1,710-1,785 1,805-1,880 Twice 75 MHz 95 MHz Twice 373
PCS-1900 1,850-1,910 1,930-1,990 Twice 60 MHz 80 MHz Twice 300

CELLULAR
ACTUAL RF COVERAGE KEY FACTORS : ?
HEIGHT OF ANTENNA,TYPE OF ANTENNA,RF POWER LEVEL EMITTED.

Frequency Reuse
• The cellular system makes an efficient use of
available channels by using low-power
transmitters to allow frequency reuse at much
smaller distances. Thus increasing the number
of times each channel may be reused in a given
geographical area.
• Frequency can be repeated after some distance
i.e. safe distance between two channels. This
results into efficient frequency spectrum
utilization.

• Distance, after which the frequency is repeated
between the cells, is called frequency reuse
distance.
• Frequency reuse can either in time domain or
frequency domain.
• In time domain, it is done by TDMA scheme i.e.
allocation of different time slot to the frequency
reuse scheme. In frequency domain, it is done
by using FDMA scheme, i.e. repeat carrier
frequency after some time and frequency reuse
distance.

CELL SHAPE
IDEAL ACTUAL DIFFERENT CELL MODELS

Why Hexagonal cells?
• It avoids dead spots.
• For a given distance between the centre of a hexagon
and its farthest border points, the hexagon has the
largest area of the three. Thus by using the hexagon
geometry, full area coverage is achieved.
• Hexagon closely approximates a circular radiation
pattern which would occur for an omni directional base
station antenna and free space propagation.
• It requires fewer cells.
• It requires less transmitter sites.
• It is less expensive.

Ares of different shapes
• Triangle = 1bh = S2
2 2
=0.43 S2
• Hexagon
= n sin (180/n)cos(180/n) S2
= 3 S2
2
= 2.59 S2
Square =
S2
3
1
2
3

• When using hexagon to model coverage area,
base station transmitters are placed as either
being centre of the cell or on three of the six
cell vertices.
• Normally omni directional antennas are used
in the centre excited cell and sectored
directional antennas are used in corner-
excited cells.
• Practical consideration do not allow base
stations to be placed exactly as they appear in
the hexagonal layout. Most system designs
permit a base station to be positioned up to one
forth the cell radius away from the ideal
location.

The Cellular Concept
Cluster 1
Cluster 2

Frequency Reuse
Frequency Reuse
• The design process of selecting and allocating
The design process of selecting and allocating
channel groups for all of the cellular base
channel groups for all of the cellular base
stations within a system is called
stations within a system is called frequency
frequency
reuse
reuse or
or frequency planning
frequency planning
• Let us consider that total
Let us consider that total S (700)
S (700) duplex
duplex
channels are available for use.
channels are available for use.
• If each cell is allocated group of
If each cell is allocated group of k (100)
k (100)
channels then
channels then
S=kN (700=100 X 7)
S=kN (700=100 X 7)
Where
Where N (7) = Number of cell in one cluster
N (7) = Number of cell in one cluster
which utilize collectively complete set of
which utilize collectively complete set of
available frequencies.
available frequencies.
Cluster : Group of cells which collectively use the
Cluster : Group of cells which collectively use the
complete set of available frequencies.
complete set of available frequencies.

Frequency Reuse
Frequency Reuse
• The N cells which collectively use the complete set
The N cells which collectively use the complete set
of available frequencies is called a cluster. If a
of available frequencies is called a cluster. If a
cluster is replicated M times within the system, the
cluster is replicated M times within the system, the
total number of duplex channels, C can be used as
total number of duplex channels, C can be used as
a measure of capacity and is given by
a measure of capacity and is given by
C = MkN = MS
C = MkN = MS
C
C represents system capacity
represents system capacity
• Thus capacity of the system is directly proportional to the
Thus capacity of the system is directly proportional to the
number of times a cluster is replicated.
number of times a cluster is replicated.

Frequency Reuse
Frequency Reuse
Exercise
Frequency band 980 MHz to 990 MHz is allocated to
GSM operator who use 200 KHz per channel. In his
frequency planning, he use cluster size 7 and
cluster is replicated 10 times in his coverage area.
Find out number of channels per cell and total
capacity of GSM cellular operator

• Ans:
Ans: No. of channels per cell
No. of channels per cell = 7 for 6
= 7 for 6
cells and 8 for 1 cell.
cells and 8 for 1 cell. Total capacity
Total capacity = 500
= 500

Exercise
Exercise
• Total 33 MHz of bandwidth is allocated to particular FDD cellular
Total 33 MHz of bandwidth is allocated to particular FDD cellular
system which utilise two 25 KHz simplex channels to provide full
system which utilise two 25 KHz simplex channels to provide full
duplex voice and control channels. Compute number of
duplex voice and control channels. Compute number of
channels available per cell if a system uses
channels available per cell if a system uses
– 4 cell reuse
4 cell reuse
– 7 cell reuse
7 cell reuse
– 12 cell reuse
12 cell reuse
• If 1 MHz is allocated to control channels then determine
If 1 MHz is allocated to control channels then determine
distribution of voice channel and control channels in all systems
distribution of voice channel and control channels in all systems
Answer:
Answer:
Without using control channels
Without using control channels
• Total channels : 660
Total channels : 660
• Total number of channels available per cell 660/4 = 165 channels
Total number of channels available per cell 660/4 = 165 channels
– For 4 cell reuse : 165
For 4 cell reuse : 165
– 7 cell reuse : 95
7 cell reuse : 95
– 12 cell reuse : 55
12 cell reuse : 55

Exercise
Exercise
Answer:
Answer:
While using control channels
While using control channels
Total 32 MHz is available for voice channels hence
Total 32 MHz is available for voice channels hence
Number of voice channels are :
Number of voice channels are : 640
640
When 1 MHz is allocated for control channels total control channels = 20
When 1 MHz is allocated for control channels total control channels = 20
(In practice only one control channel is needed per cell)
(In practice only one control channel is needed per cell)
• For N=4, 5 control channel and 160 voice channel per cell (Only one control channel is
For N=4, 5 control channel and 160 voice channel per cell (Only one control channel is
needed in practice)
needed in practice)
• For N=7, 3 control channels and 92 voice channels for 4 cell &
For N=7, 3 control channels and 92 voice channels for 4 cell &
• 3 control channels and 90 voice channels for 2 cell &
3 control channels and 90 voice channels for 2 cell &
• 2 control channels and 92 voice channel or 1 cell
2 control channels and 92 voice channel or 1 cell
• This is also possible: 91*4 + 92*3
This is also possible: 91*4 + 92*3
• For N=12
For N=12
• 2 Control channels and 53 voice channels in 8 cell
2 Control channels and 53 voice channels in 8 cell
• 1 Control channels and 54 voice channels in 4 cell
1 Control channels and 54 voice channels in 4 cell

Channel assignment strategies
Channel assignment strategies
• Fixed
Fixed
• Dynamic
Dynamic
Fixed channel assignment strategies:
Fixed channel assignment strategies:
• Each cell is allocated a predetermined set of
Each cell is allocated a predetermined set of
voice channels. Any call attempt within the
voice channels. Any call attempt within the
cell can only be served by the unused
cell can only be served by the unused
channels in that particular cell.
channels in that particular cell.
• If all the channel in that cell are occupied the
If all the channel in that cell are occupied the
call is blocked and the subscriber does not
call is blocked and the subscriber does not
receive service.
receive service.

• Borrowing channel :
Borrowing channel :
• A cell allowed to borrow channel from a
A cell allowed to borrow channel from a
neighboring cell.
neighboring cell.
• The MSC supervises such borrowing procedures
The MSC supervises such borrowing procedures
and ensures that the borrowing of a channel does
and ensures that the borrowing of a channel does
not interfere with any of the call in progress in the
not interfere with any of the call in progress in the
donor cell.
donor cell.
• Dynamic channel assignment:
Dynamic channel assignment:
• In this strategy, voice channels are not allocated
In this strategy, voice channels are not allocated
to different cells permanently. Instead each time,
to different cells permanently. Instead each time,
call request is made, the serving base station
call request is made, the serving base station
request a channel from the MSC.
request a channel from the MSC.
• The switch then allocates a channels to the
The switch then allocates a channels to the
requested cell.
requested cell.

FREQUENCY REUSE
FREQUENCY REUSE
 The frequency spectrum is a
The frequency spectrum is a
assignments.
assignments.
 For example, two radio stations
For example, two radio stations
apart.
apart.
Problem: Limited frequency spectrum
Solution: Based on the idea of splitting up coverage region
into small areas referred to as cells

• In dynamic channel assignment,
In dynamic channel assignment, a central pool of
a central pool of
all channels is used
all channels is used. A channel is borrowed form
. A channel is borrowed form
the pool by a base station for use on a call.
the pool by a base station for use on a call.
• When the call is completed, the channel is
When the call is completed, the channel is
returned to the pool.
returned to the pool.
• In the call setup phase, the base station
In the call setup phase, the base station
assignment is done on the strongest signal from
assignment is done on the strongest signal from
neighboring base stations. The channel assignment
neighboring base stations. The channel assignment
is based on interference consideration. The
is based on interference consideration. The
interference level of the idle channel is measured
interference level of the idle channel is measured
and by means of the signal level form the
and by means of the signal level form the
preferred BS, the resulting S/I ratio is estimated.
preferred BS, the resulting S/I ratio is estimated.
• If the S/I ratio exceeds the selected threshold value,
If the S/I ratio exceeds the selected threshold value,
the channel is considered a suitable channel.
the channel is considered a suitable channel.
• If no suitable channel is found, the call is blocked.
If no suitable channel is found, the call is blocked.
• Different DCA algorithms vary in the selection of
Different DCA algorithms vary in the selection of
the preferred channel among the suitable
the preferred channel among the suitable
channels.
channels.

• MSC only allocates a given frequency if that
MSC only allocates a given frequency if that
frequency not presently in use in the cell or any
frequency not presently in use in the cell or any
other cell which falls
other cell which falls within the minimum
within the minimum
restricted distance of frequency reuse to avoid
restricted distance of frequency reuse to avoid
co-channel interference.
co-channel interference.
• Dynamic channel assignment strategies require
Dynamic channel assignment strategies require
the MSC to collect real time data on channel
the MSC to collect real time data on channel
occupancy, traffic distribution and radio signal
occupancy, traffic distribution and radio signal
strength indications of all the channels on a
strength indications of all the channels on a
contentious basis.
contentious basis.
• This increase the storage and computational
This increase the storage and computational
load on the system but provides the advantage
load on the system but provides the advantage
of increased channel utilization and decreased
of increased channel utilization and decreased
probability of a blacked call.
probability of a blacked call.

In practice????????
In practice????????
• Hybrid Channel Assignment
Hybrid Channel Assignment :
:
• Some channels are permanently
Some channels are permanently
assigned to each BS as in FCA, and
assigned to each BS as in FCA, and
other are kept in a central pool for DCA.
other are kept in a central pool for DCA.

•CS 515
CS 515 •©
© İ
İbrahim K
brahim Kö
örpeo
rpeoğ
ğlu, 2002
lu, 2002 •47
47
Multipath Channel Model
Building
Building
Building
B
uilding Multipath
Channel
Multipath
Channel
Mobile 1
Mobile 2
Base
Station
1st
MC
2nd
MC
3rd
MC
(Multipath Component)
4th
MC
1st
MC
2nd
MC

Handoff Strategies:
Handoff Strategies:
Why?
Why?
• Once a call is established by the mobile user,
Once a call is established by the mobile user,
the calling and called user are on a voice
the calling and called user are on a voice
channel. If either of the mobile unit moves
channel. If either of the mobile unit moves
out of the coverage area of cell site, the
out of the coverage area of cell site, the
reception becomes weak and hence the
reception becomes weak and hence the
handoff is requested.
handoff is requested.
• Definition
Definition:
:
• Handoff is the procedure to switch the call to
Handoff is the procedure to switch the call to
a new frequency channel in a new cell site
a new frequency channel in a new cell site
without interrupting a call or alerting the user.
without interrupting a call or alerting the user.

• Handoff procedure is
Handoff procedure is essential in two situations
essential in two situations
where the cell site receives weak signals from the
where the cell site receives weak signals from the
mobile unit.
mobile unit.
1.
1. When the mobile unit is at the cell boundary –here,
When the mobile unit is at the cell boundary –here,
the signal level is going bellow the level set form
the signal level is going bellow the level set form
requesting handoff. (say -90dBm typically)
requesting handoff. (say -90dBm typically)
2.
2. When mobile unit is reaching the edging area or
When mobile unit is reaching the edging area or
gapes within the cell sites,-here fringe area in
gapes within the cell sites,-here fringe area in
valleys, river banks, hilly terrain where the signal
valleys, river banks, hilly terrain where the signal
level is very weak.
level is very weak.
Basic Requirements:
Basic Requirements:
• Handoff must be performed successfully and as
Handoff must be performed successfully and as
infrequently as possible.
infrequently as possible.
• Unnoticeable to the users.
Unnoticeable to the users.

• In order to meet the handoff requirements, the
In order to meet the handoff requirements, the
system designer generally specify an optimum
system designer generally specify an optimum
signal level at which the handoff is to initiate.
signal level at which the handoff is to initiate.
• Once a particular level specified as the minimum
Once a particular level specified as the minimum
usable signal for acceptable voice at the base
usable signal for acceptable voice at the base
station receiver, a slightly stronger signal is used as
station receiver, a slightly stronger signal is used as
a threshold at which a handoff is made.
a threshold at which a handoff is made.
• This margin is given by,
This margin is given by,


= Pr (Handoff) – Pr
= Pr (Handoff) – Pr
(minimum usable)
(minimum usable)
Delta cannot be too large or
Delta cannot be too large or
too small.
too small.

Handoff Margin
Handoff Margin

• What happen if delta is too large????
What happen if delta is too large????
Unnecessary handoff may occur, which is a
Unnecessary handoff may occur, which is a
burden to switching office.
burden to switching office.
• What happen if delta is too Small????
What happen if delta is too Small????
There may by insufficient time to complete a
There may by insufficient time to complete a
handoff before a call is lost due to weak
handoff before a call is lost due to weak
signal
signal
conditions.
conditions.

When to Handoff? Or handoff
When to Handoff? Or handoff
based on signal Strength.
based on signal Strength.
• It is important to ensure that the drop in the
It is important to ensure that the drop in the
measured signal level is not due to momentary fading
measured signal level is not due to momentary fading
and the mobile is actually moving away from the
and the mobile is actually moving away from the
serving base station.
serving base station.
• In order to ensure this, the base station monitors the
In order to ensure this, the base station monitors the
signal level for a certain period of time before a hand-
signal level for a certain period of time before a hand-
off is initiated.
off is initiated.
• The receive signal strengths includes Interference (I)
The receive signal strengths includes Interference (I)
and Carrier signal power (C)
and Carrier signal power (C)
RSS = C + I
RSS = C + I
Now suppose if we setup a threshold level for RSS, then,
Now suppose if we setup a threshold level for RSS, then,

• Case 1 :
Case 1 : sometimes, because of the very
sometimes, because of the very
strong ‘I’ the RSS level is tend to increase far
strong ‘I’ the RSS level is tend to increase far
above the handoff threshold level. At that
above the handoff threshold level. At that
situation if the voice quality is poor (i.e. C is
situation if the voice quality is poor (i.e. C is
less), the handoff is theoretically expected to
less), the handoff is theoretically expected to
take place but does not due to high RSS.
take place but does not due to high RSS.
• Case 2 :
Case 2 : When I is very low, but RSS is also
When I is very low, but RSS is also
low, so even if voice quality is good
low, so even if voice quality is good
unnecessary handoff takes place because
unnecessary handoff takes place because
RSS is low.
RSS is low.
• Hence even though handoff based on signal
Hence even though handoff based on signal
strength is an easy method, it is not an
strength is an easy method, it is not an
accurate method of determining handoffs.
accurate method of determining handoffs.

• The running average measurement of signal
The running average measurement of signal
strength should be optimized so that
strength should be optimized so that
unnecessary handoffs are avoided while
unnecessary handoffs are avoided while
ensuring that necessary handoffs are
ensuring that necessary handoffs are
completed before a call is terminated due to
completed before a call is terminated due to
poor signal level.
poor signal level.
• Information about the vehicle speed, which
Information about the vehicle speed, which
can be useful in handoff decisions, can also
can be useful in handoff decisions, can also
be computed from the statistics of the
be computed from the statistics of the
received short-term fading signal at the base
received short-term fading signal at the base
station.
station.

Dwell time
Dwell time
• The time over which the call may be
The time over which the call may be
maintained the cell, without handoff is called
maintained the cell, without handoff is called
dwell time. The dwell (Stay) time is governed
dwell time. The dwell (Stay) time is governed
by number of factors such as propagation,
by number of factors such as propagation,
distance between the subscriber and the base
distance between the subscriber and the base
station, interference and time varying effects.
station, interference and time varying effects.
The statistics of dwell time vary greatly,
The statistics of dwell time vary greatly,
depending upon the speed of the user and
depending upon the speed of the user and
the type of radio coverage.
the type of radio coverage.

Handoffs in first generation
Handoffs in first generation
analog cellular systems
analog cellular systems
• The signal strength measurements are
The signal strength measurements are
made by the base station and supervise
made by the base station and supervise
by the MSC.
by the MSC.
• Each base station constantly monitors
Each base station constantly monitors
the signal strength of all of its reverse
the signal strength of all of its reverse
voice channels to determine the relative
voice channels to determine the relative
location of each mobile user with
location of each mobile user with
respect to the base station tower.
respect to the base station tower.

• In addition to measuring the RSSI of calls in
In addition to measuring the RSSI of calls in
progress within the cell, a spare receiver in
progress within the cell, a spare receiver in
each base station called lacator receiver, is
each base station called lacator receiver, is
used to scan and determine signal strength
used to scan and determine signal strength
of mobile user which are in neighboring cells.
of mobile user which are in neighboring cells.
• The locator receiver is controlled by the MSC
The locator receiver is controlled by the MSC
and it used to monitor the signal strength of
and it used to monitor the signal strength of
user in neighboring cells which appear to be
user in neighboring cells which appear to be
in need of handoff and reports all RSSI values
in need of handoff and reports all RSSI values
to the MSC.
to the MSC.
• Based on the locator receiver signal strength
Based on the locator receiver signal strength
information from each base station, the MSC
information from each base station, the MSC
decides if a handoff is necessary of not.
decides if a handoff is necessary of not.

Mobile Assisted Handoff
Mobile Assisted Handoff
(MAHO)
(MAHO)
• In today’s second generation systems, handoff
In today’s second generation systems, handoff
decision are mobile assisted. In mobile assisted
decision are mobile assisted. In mobile assisted
handoff (MAHO), every mobile station measures
handoff (MAHO), every mobile station measures
the received power from surrounding base
the received power from surrounding base
station and continuously report the results of
station and continuously report the results of
these measurements to the serving base station.
these measurements to the serving base station.
• A handoff is initiated when the power received
A handoff is initiated when the power received
from the base station of a neighboring cell
from the base station of a neighboring cell
begins to exceed the power received from the
begins to exceed the power received from the
current base station by certain level,
current base station by certain level,

• The MAHO method enables the call to be
The MAHO method enables the call to be
handed over between base stations at a
handed over between base stations at a
much faster rate than in first generation
much faster rate than in first generation
analog systems since the handoff
analog systems since the handoff
measurements are made by each mobile,
measurements are made by each mobile,
and MSC no longer constantly monitors
and MSC no longer constantly monitors
signal strengths.
signal strengths.
• Intersystem Handoff:
Intersystem Handoff:
• During the period of call, if mobile user
During the period of call, if mobile user
moves from one cellular system to a
moves from one cellular system to a
different cellular system, controlled by a
different cellular system, controlled by a
different mobile telephone switching office
different mobile telephone switching office
(MSCs), an intersystem handoff takes place.
(MSCs), an intersystem handoff takes place.

• An MSC engages in an intersystem handoff
An MSC engages in an intersystem handoff
when a mobile signal becomes weak in a
when a mobile signal becomes weak in a
given cell land MSC cannot find another cell
given cell land MSC cannot find another cell
with in its system to which it can transfer the
with in its system to which it can transfer the
call in progress.
call in progress.
• There are many issues that must be
There are many issues that must be
addressed when implementing an
addressed when implementing an
intersystem handoff. For Instance, a local call
intersystem handoff. For Instance, a local call
may become a long distance call as the
may become a long distance call as the
mobile out of its home system and becomes
mobile out of its home system and becomes
a roamer in neighboring system.
a roamer in neighboring system.
• Also compatibility between the two MSCs
Also compatibility between the two MSCs
must be determined before implementing
must be determined before implementing
an intersystem handoff.
an intersystem handoff.

Prioritizing Handoffs
Prioritizing Handoffs
• One method for giving priority to handoffs
One method for giving priority to handoffs
is called the guard channel concept, where
is called the guard channel concept, where
by a fraction of the total available channels
by a fraction of the total available channels
in a cell is reserved exclusively for handoff
in a cell is reserved exclusively for handoff
requests from going calls which may be
requests from going calls which may be
handed off in to the cell.
handed off in to the cell.
• Drawback :
Drawback :
• Reduce the total carried traffic. As fewer
Reduce the total carried traffic. As fewer
channels are allocated to originating calls.
channels are allocated to originating calls.
Where to use????
Where to use????

Ans. Dynamic channel assignment strategies.
Ans. Dynamic channel assignment strategies.
• Queuing of handoff request:
Queuing of handoff request:
• Decrease the probability of forced termination of a call
Decrease the probability of forced termination of a call
due to lack of available channels.
due to lack of available channels.
Tradeoff
Tradeoff
• Decrease in probability of forced termination of total
Decrease in probability of forced termination of total
carried traffic and total carried traffic.
carried traffic and total carried traffic.
• Because queuing of handoffs is possible due to the
Because queuing of handoffs is possible due to the
fact that there is a finite time interval between the
fact that there is a finite time interval between the
time during which received signal level drops below
time during which received signal level drops below
the handoff threshold and the time the call is
the handoff threshold and the time the call is
terminated due to insufficient signal level.
terminated due to insufficient signal level.
• Therefore it should be noted that queing does not
Therefore it should be noted that queing does not
guarantee a zero probability of forced termination,
guarantee a zero probability of forced termination,
since large delays will cause the received signal level
since large delays will cause the received signal level
to drop below the minimum required level to maintain
to drop below the minimum required level to maintain
communication and hence lead to force termination.
communication and hence lead to force termination.

Practical Handoff Considerations-
Practical Handoff Considerations-
Umbrella cell approach
Umbrella cell approach
Practical Problem
Practical Problem
• For example, consider two person using mobile
For example, consider two person using mobile
phone. One person is on high speed vehicle and
phone. One person is on high speed vehicle and
other person is pedestrian (walker).
other person is pedestrian (walker).
• High speed vehicles passes through the coverage
High speed vehicles passes through the coverage
region of one base station to other base station
region of one base station to other base station
within a few seconds.
within a few seconds.
• Where pedestrian user may never need a handoff
Where pedestrian user may never need a handoff
during a cell.
during a cell.
• Various schemes have been developed to handle
Various schemes have been developed to handle
the simultaneous traffic of high speed and low
the simultaneous traffic of high speed and low
speed users while minimizing the handoff
speed users while minimizing the handoff
intervention from the MSC
intervention from the MSC

• We know that cellular system provides
We know that cellular system provides
additional capacity through the addition of
additional capacity through the addition of
cell sites, but it is very difficult for cellular
cell sites, but it is very difficult for cellular
service providers to obtain a new physical
service providers to obtain a new physical
cell site location in urban area and
cell site location in urban area and
residential areas. By using different antenna
residential areas. By using different antenna
heights and different power level, it is
heights and different power level, it is
possible to provide large and small cells
possible to provide large and small cells
which are co-located at a single location.
which are co-located at a single location.
• The umbrella cell approach ensures that the
The umbrella cell approach ensures that the
n umber of handoffs is minimized for high
n umber of handoffs is minimized for high
speed user and provides additional micro
speed user and provides additional micro
cell channels for pedestrian users.
cell channels for pedestrian users.

• The speed of each user may be calculated by
The speed of each user may be calculated by
the base station or MSC by evaluating how
the base station or MSC by evaluating how
rapidly the short-term overage signal
rapidly the short-term overage signal
strength on the reverse voice channel
strength on the reverse voice channel
changes over time.
changes over time.
• If a high speed user in the large umbrella
If a high speed user in the large umbrella
cell is approaching the base station and its
cell is approaching the base station and its
velocity is rapidly decreasing the base
velocity is rapidly decreasing the base
station may decide to hand the user in to the
station may decide to hand the user in to the
co-located micro cell without MSC
co-located micro cell without MSC
interference.
interference.

Cell dragging
Cell dragging
• One handoff problem may result when a
One handoff problem may result when a
mobile user travels away from the base
mobile user travels away from the base
station at a very slow speed, the average
station at a very slow speed, the average
signal strength does not decay rapidly. Even
signal strength does not decay rapidly. Even
when the user has traveled well beyond the
when the user has traveled well beyond the
designed range of the cell, the receive signal
designed range of the cell, the receive signal
at the base station may be above the handoff
at the base station may be above the handoff
threshold, thus a handoff may not be made.
threshold, thus a handoff may not be made.
• Why????
Why????

• Consider one person walking with very low
Consider one person walking with very low
speed and traveling from point A to B.
speed and traveling from point A to B.
• Ideally when person is at intermediate point
Ideally when person is at intermediate point
X, handoff must occur. Because due to slow
X, handoff must occur. Because due to slow
speeded of mobile,
speeded of mobile, MSC----NOT ABLE TO
MSC----NOT ABLE TO
DETECT THE rapid change in short term
DETECT THE rapid change in short term
power level. (May Be Due To Line Of Sight)
power level. (May Be Due To Line Of Sight)
• Hence call is not transferred to the other base
Hence call is not transferred to the other base
station and user call is terminated at point B
station and user call is terminated at point B
due to minimum strength of mobile signal.
due to minimum strength of mobile signal.
• This problem is known as Cell Dragging.
This problem is known as Cell Dragging.
• Cell dragging results from pedestrian users
Cell dragging results from pedestrian users
that provide a very strong signal to the base
that provide a very strong signal to the base
station.
station.

• Also cell dragging creates a potential
Also cell dragging creates a potential
interference and traffic management
interference and traffic management
problem, since the user has traveled deep
problem, since the user has traveled deep
within a neighboring cell and no handoff
within a neighboring cell and no handoff
occur.
occur.
• To solve this problem, handoff threshold and
To solve this problem, handoff threshold and
radio coverage parameters must be adjusted
radio coverage parameters must be adjusted
carefully.
carefully.

• Cellular radio mobile also rely on trunking to accommodate large number of users in a limited radio
Cellular radio mobile also rely on trunking to accommodate large number of users in a limited radio
spectrum (In basic phone, large no. of users for limited switches)
spectrum (In basic phone, large no. of users for limited switches)
• Traffic analysis is required for designing cost effective network which provides desired grade of service.
Traffic analysis is required for designing cost effective network which provides desired grade of service.
• Traffic analysis exploits statistical behavior of users and determine number of channels necessary to
Traffic analysis exploits statistical behavior of users and determine number of channels necessary to
provide desired GOS.
provide desired GOS.
Truninkg

Grade of Service
Grade of Service
• Grade of service is measure of the ability of a user to access trunk
Grade of service is measure of the ability of a user to access trunk
system during busiest hour.
system during busiest hour.
• It is wireless designer’s job to allocate the proper number of
It is wireless designer’s job to allocate the proper number of
channels in order to meet the GOS.
channels in order to meet the GOS.
• GOS is typically given as likelihood of call blocking or likelihood of
GOS is typically given as likelihood of call blocking or likelihood of
call experiencing a delay greater than certain queuing time.
call experiencing a delay greater than certain queuing time.
• In a loss system, GOS is described as the ratio of calls that are lost
In a loss system, GOS is described as the ratio of calls that are lost
due to congestion in the busy hour.
due to congestion in the busy hour.
• In other words it is ratio of lost traffic to offered traffic
In other words it is ratio of lost traffic to offered traffic
A
A
-
A
GOS 0

Where A = Offered traffic
Where A = Offered traffic
A
A0
0 = Carried traffic
= Carried traffic
A-Ao is lost traffic

• In order to maintain the value within
In order to maintain the value within
reasonable limits, initially the network is
reasonable limits, initially the network is
sized to have much smaller GOS value
sized to have much smaller GOS value
than the recommended, so that the GOS
than the recommended, so that the GOS
value continue to be within limits as the
value continue to be within limits as the
network traffic grows.
network traffic grows.
• Smaller the value of GOS, better is the
Smaller the value of GOS, better is the
service. The recommended value of GOS
service. The recommended value of GOS
in India is 0.002 which means that 2 calls
in India is 0.002 which means that 2 calls
in every 1000 calls may be lost.
in every 1000 calls may be lost.

Grade of Service
Grade of Service
• If GOS is
If GOS is 0.02 (2%)
0.02 (2%) then
then 2
2 out of
out of 100
100 calls will
calls will
be
be blocked
blocked due to channel occupancy during
due to channel occupancy during
busiest hour.
busiest hour.
Exercise:
Exercise:
• Find out number of blocked calls per busiest hour if
Find out number of blocked calls per busiest hour if
grade of service is 0.05 and number of call attempts
grade of service is 0.05 and number of call attempts
are 500 during busiest hour in particular cellular area.
are 500 during busiest hour in particular cellular area.
Solution:
Solution:
5 out of 100 calls are blocked hence total 25 calls will be blocked
5 out of 100 calls are blocked hence total 25 calls will be blocked

Terminology used in traffic
Terminology used in traffic
engineering
engineering
• BUSY HOUR : continuous 1 hour period lying wholly
BUSY HOUR : continuous 1 hour period lying wholly
in the time interval concerned, for which the traffic
in the time interval concerned, for which the traffic
volume or the number of call attempts is greatest.
volume or the number of call attempts is greatest.
• PEAK BUSY HOUR : The busy hour each day : it usually
PEAK BUSY HOUR : The busy hour each day : it usually
varies from day to day or over a number of days.
varies from day to day or over a number of days.
• TIME CONSISTENT BUSY HOUR : The 1 hour period
TIME CONSISTENT BUSY HOUR : The 1 hour period
starting at the same time each day for which the
starting at the same time each day for which the
average traffic volume or the number of call attempts
average traffic volume or the number of call attempts
is greatest over the days under consideration.
is greatest over the days under consideration.

• Traffic Intensity, A
Traffic Intensity, A0
0
= Period for which a server is occupied /
= Period for which a server is occupied /
total period of observation
total period of observation
• Unit : Erlang
Unit : Erlang

Key Definitions for Trunked
Key Definitions for Trunked
Radio
Radio

Capacity Estimation
Capacity Estimation

Traffic intensity offered by each user is
equal to the No of call initiated multiplied
equal to the No of call initiated multiplied
by holding time.
by holding time.

Thus each user generates traffic intensity:
A
Au
u =
= 
H
H

Where H = average duration of call &

 = average number of call per unit time
= average number of call per unit time

For the system containing
For the system containing U users
U users total traffic
total traffic
intensity is:
intensity is:
A = UA
A = UAu
u

For C channel system, traffic is uniformely distributed then
traffic intensity per channel is:
A
Ac
c = UA
= UAu
u/C
/C

Major Points to be noted.
Major Points to be noted.

The offered traffic is not necessarily the
The offered traffic is not necessarily the
traffic which is carried by the trunked
traffic which is carried by the trunked
system.
system.

When the offered traffic exceeds the
When the offered traffic exceeds the
maximum capacity of the system, the
maximum capacity of the system, the
carried traffic be becomes limited due to
carried traffic be becomes limited due to
the limited number of capacity or
the limited number of capacity or
channels.
channels.

If the cellular system has a GOSof 2%
If the cellular system has a GOSof 2%
blocking, it means that 2 out of 100 calls
blocking, it means that 2 out of 100 calls
will be blocked due to channel occupancy
will be blocked due to channel occupancy
during busiest hour.
during busiest hour.

Traffic intensity
Traffic intensity
Exercise:
Exercise:
 In a group of 10 radio channels each occupied for
In a group of 10 radio channels each occupied for
30 minutes in an observation of two hours.
30 minutes in an observation of two hours.
Calculate traffic carried by radio channels.
Calculate traffic carried by radio channels.
Solution:
Solution:
Traffic carried per channel = occupied duration/total
Traffic carried per channel = occupied duration/total
duration=0.25 E
duration=0.25 E
Total traffic carried by group of radio channels =
Total traffic carried by group of radio channels = 0.25x10 =
0.25x10 =
2.5 E
2.5 E

Problem
Problem
 Two cellular operators X &Y provides services in some urban
Two cellular operators X &Y provides services in some urban
area. System X has 100 cells with 20 channels in each and
area. System X has 100 cells with 20 channels in each and
system Y has 150 cells with 10 channels in each. Find number
system Y has 150 cells with 10 channels in each. Find number
of users supported considering total carried traffic
of users supported considering total carried traffic 12 Erlang in
12 Erlang in
system X
system X and
and 10 Erlang in System Y
10 Erlang in System Y. Each user generates
. Each user generates
(average) two call per hour at average call duration 3 minute.
(average) two call per hour at average call duration 3 minute.
Calculate % penetration if both users are operated at
Calculate % penetration if both users are operated at
maximum capacity
maximum capacity.
.
Solution:
Solution:
Probability of blocking = 2% =0.02
Probability of blocking = 2% =0.02
For system X:
For system X:
No. of channels per cell = 20
Traffic intensity per user = A
Traffic intensity per user = Au
u =
= 
H = 2*3/60 = 0.1 E
H = 2*3/60 = 0.1 E
Total carried traffic A=12
Number of users supported U = A/Au = 12/0.1 = 120 per cell
Total subscribers = 100 cells * 120 = 12000

For system Y:
For system Y:
Traffic intensity per user = A
Traffic intensity per user = Au
u =
= 
H = 2*3/60 = 0.1 E
H = 2*3/60 = 0.1 E
Total number of subscribers supported = 15000+12000=27000
Total number of subscribers supported = 15000+12000=27000
% penetration of operator X = 12000/27000 = 0.44 44%
% penetration of operator X = 12000/27000 = 0.44 44%
% penetration of operator Y = 15000/27000 = 0.56 56%
% penetration of operator Y = 15000/27000 = 0.56 56%

Types of Trunking Systems
Types of Trunking Systems
1.Blocked calls cleared (BCC)
1.Blocked calls cleared (BCC)

It offers no queuing for call requests.
It offers no queuing for call requests.

For every user who requests services,
For every user who requests services,
there is no setup time and hence user is
there is no setup time and hence user is
given immediate access to a channel if one
given immediate access to a channel if one
is available.
is available.

If no channels are available, the requesting
If no channels are available, the requesting
user is blocked without access and is free
user is blocked without access and is free
to try again later.
to try again later.

This model is called Earlang –B model and
This model is called Earlang –B model and
it is based upon the following basic
it is based upon the following basic
assumptions.
assumptions.

 Call requests are memory less, implying
Call requests are memory less, implying
that all users, including blocked users
that all users, including blocked users
may request a channel at any time.
may request a channel at any time.
 All free channels are fully available for
All free channels are fully available for
servicing calls until all channels are
servicing calls until all channels are
occupied.
occupied.
 There are a finite number of channels
There are a finite number of channels
available in the trunking pool.
available in the trunking pool.
 Traffic requests are described by a
Traffic requests are described by a
possion distribution.
possion distribution.
 Intrarrival times of call requests are
Intrarrival times of call requests are
independent of each other.
independent of each other.


 C
k
k
C
k
A
C
A
Blocking
0 !
!
)
Pr(
Blocking probability is given
Blocking probability is given
by
by

Erlang B Trunking GOS
Erlang B Trunking GOS

How to use chart?
How to use chart?

Locate the number of channels on the top
Locate the number of channels on the top
portion of the graph.
portion of the graph.

Locate the traffic intensity of the system
Locate the traffic intensity of the system
on the bottom portion of the graph.
on the bottom portion of the graph.

The blocking probability is shown in the
The blocking probability is shown in the
figure.
figure.

If two of the parameters specified is
If two of the parameters specified is
known, it is easy to find the third
known, it is easy to find the third
parameter.
parameter.

Erlang- C System (BCD)
Erlang- C System (BCD)

It provides queuing to hold calls
It provides queuing to hold calls
which are blocked. If a channel is not
which are blocked. If a channel is not
available immediately, the call
available immediately, the call
request may be delayed until a
request may be delayed until a
channel becomes available.
channel becomes available.

Here, it is possible that a call is
Here, it is possible that a call is
blocked after waiting a specific
blocked after waiting a specific
length of time in the queue.
length of time in the queue.


First, the probability of a call not having
First, the probability of a call not having
immediate access to a channel in queue
immediate access to a channel in queue
is given as
is given as

PB =[Call delayed] =PB [delayed>0 Sec.]
PB =[Call delayed] =PB [delayed>0 Sec.]

If the delayed call is forced to wait form
If the delayed call is forced to wait form
more than t seconds, it probability is
more than t seconds, it probability is
given by th probability that a call is
given by th probability that a call is
delayed multiplied by the conditional
delayed multiplied by the conditional
probability that the delay is grater than ‘t’
probability that the delay is grater than ‘t’
seconds and is given by
seconds and is given by

PB [delay> t second] =
PB [delay> t second] =
PB [delay>0 second] X PB [Wait>t| call
PB [delay>0 second] X PB [Wait>t| call
delayed]
delayed]

Comparison of Erlang B & Erlang C
Comparison of Erlang B & Erlang C
BCC (Erlang-B)
BCC (Erlang-B)

No queuing for call
No queuing for call
requests.
requests.

No set-up time.
No set-up time.

User gets immediate
User gets immediate
access.
access.

User is free to try again
User is free to try again
later.
later.

Infinite number of users
Infinite number of users
and finite number of
channels.
channels.

Blocking probability-
Blocking probability-
BCD (Erlang-c)
BCD (Erlang-c)

Queuing to hold calls.
Queuing to hold calls.

Varying set-up time.
Varying set-up time.

User gets immediate
User gets immediate
access if free channel is
access if free channel is
available else delayed until
available else delayed until
a channel becomes free.
a channel becomes free.

Not required as the user
Not required as the user
is in queue.
is in queue.

Infinite number of user
Infinite number of user
channels.
channels.

Average delay to the
Average delay to the
queued calls-
queued calls-

Problem
Problem
How many users can be supported for 0.5%
How many users can be supported for 0.5%
blocking probability in block call clear
blocking probability in block call clear
system for 100 trunk lines assuming that
system for 100 trunk lines assuming that
each user generates 0.1 E of traffic?
each user generates 0.1 E of traffic?
Solution:
Solution:
We know that A=UA
We know that A=UAu
u
Trunk lines C=100
Trunk lines C=100
Traffic by each user A
Traffic by each user Au
u=0.1
=0.1
From the table (or graph/equation) A=80.9
From the table (or graph/equation) A=80.9
Number of users supported = U = A/Au = 80.9/0.1 =
Number of users supported = U = A/Au = 80.9/0.1 =
809
809

Cell Splitting
Cell Splitting

As the demand for wireless service
As the demand for wireless service
increases day by day, the number of
increases day by day, the number of
channels assigned to any particular cell
channels assigned to any particular cell
eventually becomes insufficient to support
eventually becomes insufficient to support
and provide the services of number of
and provide the services of number of
users.
users.

Hence some technique are required to
Hence some technique are required to
provide more channels per unit coverage
provide more channels per unit coverage
area.
area.

1. Cell Splitting
1. Cell Splitting

2. Sectoring
2. Sectoring

3. Zonal concept
3. Zonal concept

Coverage and capacity expansion
Coverage and capacity expansion
techniques in cellular system
techniques in cellular system
 There are basically four methods to
There are basically four methods to
expand the capacity of a cellular
expand the capacity of a cellular
network.
network.
1.
1. Obtain additional spectrum for new
Obtain additional spectrum for new
subscribers.
subscribers.
2.
2. Change the cellular architecture.
Change the cellular architecture.
3.
3. Change the frequency allocation
Change the frequency allocation
methodology.
methodology.
4.
4. Change the modem and access
Change the modem and access
technology
technology

Cell splitting
Cell splitting
 As the number of subscribers increase
As the number of subscribers increase
within a given area, the number of
within a given area, the number of
channels allocated to a cell is no longer
channels allocated to a cell is no longer
sufficient for supporting the subscriber
sufficient for supporting the subscriber
demand. It then becomes necessary to
demand. It then becomes necessary to
allocate more channels to the area that is
allocate more channels to the area that is
being covered by this cell.
being covered by this cell.
 This can be done by splitting cells into
This can be done by splitting cells into
smaller cells and allowing additional
smaller cells and allowing additional
channels the smaller areas.
channels the smaller areas.
 Hence, cell splitting is done when the area
Hence, cell splitting is done when the area
of a cell is further divided, thus creating
of a cell is further divided, thus creating
more cell areas.
more cell areas.


Why cell splitting????
Why cell splitting????

The purpose of cell splitting is to increase the
The purpose of cell splitting is to increase the
channel capacity and improve the availability
channel capacity and improve the availability
and reliability of a cellular telephone network.
and reliability of a cellular telephone network.

Splitting cell areas creates new cells, providing
Splitting cell areas creates new cells, providing
and increase in the degree of frequency reuse,
and increase in the degree of frequency reuse,
thus increasing the channel capacity of a
thus increasing the channel capacity of a
cellular network.
cellular network.

When traffic density starts to buildup and the
When traffic density starts to buildup and the
frequency channels F in each cell C cannot
frequency channels F in each cell C cannot
provide enough mobile calls, the original cell
provide enough mobile calls, the original cell
can be split into smaller cells. Usually the new
can be split into smaller cells. Usually the new
radius is one-half the original radius.
radius is one-half the original radius.

 New cell radius =
New cell radius = Old cell radius
Old cell radius
2
2
From above equation we can say that
From above equation we can say that
New cell area
New cell area =
= Old cell area
Old cell area
4
4
Each new cell carry the traffic load =
Each new cell carry the traffic load =
New traffic load
New traffic load =
= 4 x traffic load
4 x traffic load
Unit area
Unit area Unit area
Unit area
 Cell splitting provides for systematic
Cell splitting provides for systematic
growth in a cellular system.
growth in a cellular system.
 The major draw back of cell splitting is that it
The major draw back of cell splitting is that it
results in more base station transfers i.e.
results in more base station transfers i.e.
handoffs per call is required.
handoffs per call is required.

 Basically resizing or redistribution of
Basically resizing or redistribution of
cell areas is called cell splitting, or we
cell areas is called cell splitting, or we
can say that cell splitting is the process
can say that cell splitting is the process
of sub dividing highly congested cell in
of sub dividing highly congested cell in
to smaller cells each with their own
to smaller cells each with their own
base station and set of channel
base station and set of channel
frequencies.
frequencies.
 Cell splitting occurs when traffic level in
Cell splitting occurs when traffic level in
a cell reach the point where channel
a cell reach the point where channel
availability is limited.
availability is limited.
 If a new call is initiated in an area
If a new call is initiated in an area
where all the channels are in use, a
where all the channels are in use, a
condition called blocking occurs.
condition called blocking occurs.



Therefore cells are initially set up to
Therefore cells are initially set up to
cover relatively large areas, and then the
cover relatively large areas, and then the
cells are divided in to smaller areas when
cells are divided in to smaller areas when
the need arises.
the need arises.

The area is proportional to its radius
The area is proportional to its radius
squared. Therefore if the radius of cell is
squared. Therefore if the radius of cell is
divided in half, four times as many
divided in half, four times as many
smaller cells could be created to provide
smaller cells could be created to provide
service to the same coverage area.
service to the same coverage area.

Capacity is also increased by a factor of 4
Capacity is also increased by a factor of 4
since each new cell has the same number
since each new cell has the same number
of channels as the original cell.
of channels as the original cell.

Practical consideration
Practical consideration
 In practice, only a single small cell
In practice, only a single small cell
will be introduced such that it is
will be introduced such that it is
midway between two co-channel
midway between two co-channel
cells.
cells.
 Its logical to reuse the channels
Its logical to reuse the channels
allocated to those cells in the smaller
allocated to those cells in the smaller
cell to minimize the interference.
cell to minimize the interference.
 But some problems may rises with
But some problems may rises with
these approach.
these approach.

 Lets assume that the radius of the smaller split
Lets assume that the radius of the smaller split
cell is R/2.
cell is R/2.
 Let the TX power of the BTS of small cell be
Let the TX power of the BTS of small cell be
the same as the TX power of the larger cells.
the same as the TX power of the larger cells.
 For the smaller cell the S/I ratio is maintained
For the smaller cell the S/I ratio is maintained
because the maximum distance the mobile can
because the maximum distance the mobile can
be from the base station in this cell is R/2.
be from the base station in this cell is R/2.
 So though the distance between this cell and
So though the distance between this cell and
the co-channel cell A is reduced by half, the
the co-channel cell A is reduced by half, the
value of signal to noise ratio remain the same.
value of signal to noise ratio remain the same.
 On the other hand, this is not the case for the
On the other hand, this is not the case for the
large cells.
large cells.
 In order to maintain the same level of
In order to maintain the same level of
interferences the transmit power of the base
interferences the transmit power of the base
station in the smaller cell should be reduces.
station in the smaller cell should be reduces.


But this will increase the interference
But this will increase the interference
observed by the mobiles in the
observed by the mobiles in the
smaller cell.
smaller cell.

Solution???????????????
Solution???????????????


Divide the channel allocated to larger cells
Divide the channel allocated to larger cells
into two parts those used by smaller cell
into two parts those used by smaller cell
and those not used by smaller cell.
and those not used by smaller cell.

The channel used by smaller cell will be
The channel used by smaller cell will be
used in the larger cells only within a radius
used in the larger cells only within a radius
of R/2 from the center of the cell,
of R/2 from the center of the cell,

This is known as overlaid cell concept.
This is known as overlaid cell concept.

Cells are split to add channels
Cells are split to add channels
with no new spectrum usage
with no new spectrum usage

Sectoring
Sectoring

Cell splitting achieves capacity
Cell splitting achieves capacity
improvement by essentially rescaling the
improvement by essentially rescaling the
system.
system.

However, another way to increase capacity
However, another way to increase capacity
is to keep the cell radius unchanged.
is to keep the cell radius unchanged.

Basically, sectoring increases the S/I ratio
Basically, sectoring increases the S/I ratio
so that the cluster size may be reduced.
so that the cluster size may be reduced.


The co-channel interference in a cellular
The co-channel interference in a cellular
system may be decreased by replacing a
system may be decreased by replacing a
single omni directional antenna at the
single omni directional antenna at the
base station by several directional
base station by several directional
antennas, each radiating within a specific
antennas, each radiating within a specific
sector.
sector.

By using directional antennas, a given
By using directional antennas, a given
cell will receive interference and transmit
cell will receive interference and transmit
with only a fraction of the available co-
with only a fraction of the available co-
channel cells.
channel cells.

The factor by which the co-channel
The factor by which the co-channel
interference is reduced depends on the
interference is reduced depends on the
amount of sectoring used
amount of sectoring used

Sectoring improves S/I
Sectoring improves S/I

Practically……………
Practically……………
 For example 120 degree sectors or 60 degree
For example 120 degree sectors or 60 degree
sectors are used in practice.
sectors are used in practice.
 In the three sector configuration, three
In the three sector configuration, three
antennas would be placed in each 120 degree
antennas would be placed in each 120 degree
sector- One transmit antenna and two receive
sector- One transmit antenna and two receive
antennas.
antennas.
 Placing two receive antennas is called
Placing two receive antennas is called space
space
diversity
diversity.
.
 Space diversity improves the reception for
Space diversity improves the reception for
signals radiated from mobile units.
signals radiated from mobile units.
 The separation between two receive antennas
The separation between two receive antennas
depends on the height of the antennas above
depends on the height of the antennas above
the ground.
the ground.

•Antenna Diversity
Antenna Diversity
•Single Input Single Output Multiple Input Single
Single Input Single Output Multiple Input Single
Output
Output
•Single Input Multiple Output Multiple Input Multiple
Single Input Multiple Output Multiple Input Multiple
Output
Output

MISO and MIMO using Alamouti Coding Scheme

Example
Example

Assuming 7 cell reuse for 120 degree
Assuming 7 cell reuse for 120 degree
sectors, the number of interferers in the
sectors, the number of interferers in the
first tier is reduced form six to two.
first tier is reduced form six to two.

Consider the interference experienced by a
Consider the interference experienced by a
mobile located in the right most sector in
mobile located in the right most sector in
the center cell labeled “5”
the center cell labeled “5”

Out of six co-channel cell, only two cells
Out of six co-channel cell, only two cells
have sectors with antenna pattern which
have sectors with antenna pattern which
radiated into the center cell, and hence a
radiated into the center cell, and hence a
mobile in the center cell will experience
mobile in the center cell will experience
interference on the forward link form only
interference on the forward link form only
these two sectors only.
these two sectors only.

19-cell reuse example (N=19)
19-cell reuse example (N=19)
Figure 3.2
Figure 3.2 Method of locating co-channel cells in a cellular system. In this example,
Method of locating co-channel cells in a cellular system. In this example, N
N = 19 (i.e.,
= 19 (i.e.,
I
I = 3,
= 3, j
j = 2). (Adapted from [Oet83] © IEEE.)
= 2). (Adapted from [Oet83] © IEEE.)
•N = i
N = i2
2
+ij +j
+ij +j2
2
•i=
i=
3
3
•j=
j=
2
2


Sectoring is defined as the technique for
Sectoring is defined as the technique for
decreasing co-channel interference and
decreasing co-channel interference and
thus increasing system performance by
thus increasing system performance by
using directional antennas.
using directional antennas.

Conclusion
Conclusion:
:

Use of sectoring increases the signal to
Use of sectoring increases the signal to
interference ratio at the terminal.
interference ratio at the terminal.

In this approach, first the S/I is improved
In this approach, first the S/I is improved
using directional antennas, then capacity
using directional antennas, then capacity
improvement is achieved by reducing the
improvement is achieved by reducing the
number of cells in a cluster, thus
number of cells in a cluster, thus
increasing the frequency reuse and hence
increasing the frequency reuse and hence
the capacity of system.
the capacity of system.

 Disadvantage:
Disadvantage:
 Increases hand over.
Increases hand over.
Cell splitting Vs. Sectoring
Cell splitting Vs. Sectoring:
:
 In sectoring, service providers needs to add antennas
In sectoring, service providers needs to add antennas
to the base station in desired area.
to the base station in desired area.
 Compared to cell splitting method, using sectoring
Compared to cell splitting method, using sectoring
directional antennas is less effective in increasing
directional antennas is less effective in increasing
capacity but installation cost is less.
capacity but installation cost is less.
 In cell splitting method, additional cell site required so
In cell splitting method, additional cell site required so
cost increases----Expensive compared with directional
cost increases----Expensive compared with directional
antennas.
antennas.
 Additional planning efforts are required in cell splitting
Additional planning efforts are required in cell splitting
to maintain interference level on the smaller cells.
to maintain interference level on the smaller cells.
 By using directional antennas, the transmitted signal
By using directional antennas, the transmitted signal
power from the mobile station will be reduced which
power from the mobile station will be reduced which
can potentially result in large battery life for the user.
can potentially result in large battery life for the user.

Capacity Estimation
Capacity Estimation

equal to the call request rate multiplied by
equal to the call request rate multiplied by
holding time.
holding time.

A
Au
u =
= 
H
H


 = average number of call per unit time
= average number of call per unit time

For the system containing
For the system containing U users
U users total traffic
total traffic
intensity is:
intensity is:
A = UA
A = UAu
u

A
Ac
c = UA
= UAu
u/C
/C

Lee’s Microcell zone concept
Lee’s Microcell zone concept


Disadvantage of Sectoring—Increases Handoff.
Disadvantage of Sectoring—Increases Handoff.

In Lee’s micro cell zone concept, only one base
In Lee’s micro cell zone concept, only one base
station per cell, but there are three “Zone
station per cell, but there are three “Zone
Sites” located at the corners of a cell.
Sites” located at the corners of a cell.

In this scheme all three zone sites act as
In this scheme all three zone sites act as
receivers for signal transmitted by mobile
receivers for signal transmitted by mobile
terminal and connected to a single base station
terminal and connected to a single base station
and share the same radio equipment.
and share the same radio equipment.

The zones are connected by fiber optic cable.
The zones are connected by fiber optic cable.

This multiple zone and a single base station
This multiple zone and a single base station
make up a cell.
make up a cell.

As mobile user travels within the cell it is
As mobile user travels within the cell it is
served by the zone with the strongest signal.
served by the zone with the strongest signal.

Advantages
Advantages
 This technique is superior to sectoring since
This technique is superior to sectoring since
antennas are placed at the outer edge of the cell
antennas are placed at the outer edge of the cell
or corner of the cell, and any channel may be
or corner of the cell, and any channel may be
assigned to any zone by the base station.
assigned to any zone by the base station.
 Because only a single zone site active at a time,
Because only a single zone site active at a time,
the interference faced by a mobile terminal from
the interference faced by a mobile terminal from
a co-channel zone site is smaller compared with
a co-channel zone site is smaller compared with
omni directional antenna.
omni directional antenna.
 As a mobile subscribers travels from one zone to
As a mobile subscribers travels from one zone to
another within the cell, it retains the same
another within the cell, it retains the same
frequencies so no handoff is required at the MSC
frequencies so no handoff is required at the MSC
when the mobile travels between zones within
when the mobile travels between zones within
the cell.
the cell.


The base station simply switches the
The base station simply switches the
channel to a different zone site. Due to
channel to a different zone site. Due to
this process, a given channel is active only
this process, a given channel is active only
in the particular zone in which mobile user
in the particular zone in which mobile user
is traveling and hence the base station
is traveling and hence the base station
radiation is limited on local zone so
radiation is limited on local zone so
interference is reduced.
interference is reduced.

Decrease co-channel interference
Decrease co-channel interference
improves the signal quality and also leads
improves the signal quality and also leads
to an increase in the system capacity.
to an increase in the system capacity.

Adjacent channel interference
Adjacent channel interference

ACI occurs when transmission from adjacent
ACI occurs when transmission from adjacent
channel (Channel next to one another in the
channel (Channel next to one another in the
frequency domain) interfere with each other.
frequency domain) interfere with each other.

ACI results from imperfect filters in receivers that
ACI results from imperfect filters in receivers that
allow nearby frequencies to enter the receiver.
allow nearby frequencies to enter the receiver.
ACI is most prevalent when an adjacent channel
ACI is most prevalent when an adjacent channel
is transmitting very close to mobile units receiver
is transmitting very close to mobile units receiver
at the same time the mobile unit is trying to
at the same time the mobile unit is trying to
receive transmission from the base station on an
receive transmission from the base station on an
adjacent frequency.
adjacent frequency.

This is called near-far-effect and it is most
This is called near-far-effect and it is most
prevalent when a mobile unit is receiving a weak
prevalent when a mobile unit is receiving a weak
signal from the base station.
signal from the base station.

ACI
ACI

Alternatively near-far-effect occurs
Alternatively near-far-effect occurs
when a subscriber mobile close to
when a subscriber mobile close to
base station transmits on a channel
base station transmits on a channel
close to one being used by a weak
close to one being used by a weak
mobile. The base station may have
mobile. The base station may have
difficulty in discriminating the desired
difficulty in discriminating the desired
mobile user.
mobile user.

Way and means to minimize the
Way and means to minimize the
ACI
ACI

Through careful filtering.
Through careful filtering.

By careful channel assignments.
By careful channel assignments.

By keeping the frequency separation
By keeping the frequency separation
between each cell in a given cell as
between each cell in a given cell as
large as possible.
large as possible.

By avoiding the use of adjacent
By avoiding the use of adjacent
channels in neighboring cell sites.
channels in neighboring cell sites.

• In cellular system, power control is very much essential. The base station continuously
In cellular system, power control is very much essential. The base station continuously
controls the power level transmitted by every subscriber’s mobile handset.
controls the power level transmitted by every subscriber’s mobile handset.
Power control is essential because:
Power control is essential because:
• Each mobile handset maintain the smallest power necessary to maintain good quality link
Each mobile handset maintain the smallest power necessary to maintain good quality link
over reverse channel.
over reverse channel.
• Power control helps in increasing talk time.
Power control helps in increasing talk time.
Power control

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