Cellular Wireless Networks part2.pptx

CELLULAR WIRELESS
NETWORKS
Dr. Loai Bani Melhim 1

Principles of Cellular Networks
• Cellular Network Organization
• Frequency Reuse
• Increasing Capacity
• Operation of Cellular Systems
• Mobile Radio Propagation Effects
• Handoff
• Power Control
• Traffic Engineering

Capacity Problem
• In time, as more customers use the system
• traffic may build up
• so that there are not enough frequency bands (channels)
assigned to a cell
• to handle its calls.
• It is a Capacity Problem.

Approaches to Cope with Increasing
Capacity
Following approaches are used.
1. Adding new channels
2. Frequency borrowing
3. Cell splitting
4. Cell sectoring
5. Microcells
6. A Microcell Zone Concept
7. Femto Cell
[Rappa P-86]

Adding new channels
Typically, when a system is set up in a region,
• Not all of the channels are used, and
• Growth and expansion can be managed in an orderly
fashion
• by adding new channels.

Frequency borrowing
1. Frequencies are taken from adjacent cells by
congested cells
2. Dynamic assignment of frequencies to different cells

Cell splitting (1)
• Process of subdividing a congested cell into
smaller cells (Same concept as of micro-cells),
• each with its own base station and
• corresponding reduction in antenna height and
transmitter power.
• It increases the capacity of a cellular system
since
• it increases the number of times that channels are
reused.
• The additional number of channels per unit area

Cell splitting (2)
• The increased number of cells would 
increase the number of clusters over the
coverage region,  which in turn would
increase the number of channels,  and the
capacity, in the coverage area.
• It allows a system to grow by
• replacing large cells with smaller cells,
• while not upsetting the channel allocation scheme
required to maintain the minimum co-channel reuse
ratio Q between co-channel cells.
N
R
D
Q 3


Dr. Loai Bani Melhim

Cell splitting (3)
• A small value of Q means small N  provides large
capacity
• A large value of Q means large N  improves the
transmission quality due to a smaller level of co-channel
interference.
A trade-off must be made b/w these two objectives in actual
cellular design

Cell splitting (4)
• Base stations are placed at
the corners of the cells
• Assume that area served by
BS A is saturated with traffic.
• New BSs are needed to
increase the No. of channels
in the area and to reduce the
area served by single BS
• BS A has been surrounded
by 6 new cells

Cell splitting (5)
• Cells addition also preserves the
frequency reuse plan.
• Micro cell G has been placed half
way b/w two larger stations
utilizing the same channel set G.
• This is also case of other
microcells. L
L/2

Cell splitting (6)
Conclusion:
• By decreasing the cell radius R, and
• keeping the co-channel reuse ratio D/R unchanged,
• cell splitting increases the number of channels per unit area.

Example
• Consider Figure 3.9. Assume each base
station uses 60 channels, regardless of cell
size. If each original cell has a radius of 1
Km and each microcell has a radius of 0.5
Km, find the number of channels contained
in a 3 Km by 3 Km square centered around
A under the following conditions:
a) without the use of microcells;
b) when the lettered microcells as shown in
Figure 3.9 are used, also calculate increase
in capacity; and
c) if all the original base stations are replaced
by microcells. Assume cells on the edge of
the square to be contained within the
square.

Solution
• (a) 5x60 = 300 channels
• (b) 5+6 = 11
11 x 60 = 660 channels
• (c) 5 + 12 = 17 channels
17 x 60 = 1020 channels

Solution
• a - without the use of microcells:
• A cell radius of 1 km implies that the sides of the larger
hexagons are also 1 km in length. To cover the 3 km by 3
km square centered around base station A, we need to
cover 1.5 km (1.5 times the hexagon radius) toward the
right, left, top, and bottom of base station A. This is shown
in Figure 3.9. From Figure 3.9, we see that this area
contains five base stations. Since each base station has
60 channels, the total number of channels without cell
splitting is equal to 5 × 60 = 300 channels.

Solution
• b - with the use of the microcells as shown in Figure 3.9:
• In Figure 3.9, the base station A is surrounded by six
microcells. Therefore, the total number of base stations in
the square area under study is equal to 5 + 6 = 11. Since
each base station has 60 channels, the total number of
channels will be equal to 11 × 60 = 660 channels. This is
a 2.2 times increase in capacity when compared to case
(a).

Solution
• if all the base stations are replaced by microcells:
• From Figure 3.9, we see there are a total of 5 + 12 = 17
base stations in the square region under study. Since
each base station has 60 channels, the total number of
channels will be equal to 17 × 60 = 1020 channels. This is
a 3.4 times increase in capacity compared to case (a).
• Theoretically, if all cells were microcells having half the
radius of the original cell, the capacity increase would
approach four.

Sectoring
• cell splitting achieves capacity improvement by
decreasing the cell radius R and keeping the co-channel
reuse ratio D/R unchanged, cell splitting increases the
number of channels per unit area.
• Another way to increase capacity is to keep the cell radius
unchanged and seek methods to decrease the D/R ratio.
Sectoring increases SIR so that the cluster size may be
reduced. First the SIR is improved using directional
antennas, and then capacity improvement is achieved by
reducing the number of cells in a cluster, thus increasing
the frequency reuse. However, in order to do this
successfully, it is necessary to reduce the relative
interference without decreasing the transmit power.

Cell sectoring
1. Cells are divided into a number of wedge-shaped
sectors,
 Typically three or six sectors per cell
 Each with their own set of channels
 Each sector is assigned a separate subset of the cell’s
channels
 Directional antennas are used at the base station
• to focus on each sector.

Cell sectoring
• The technique
• for decreasing co-channel interference and
• thus increasing system performance by using directional antennas
is called sectoring.

Cell sectoring
• To increase the capacity is
• to keep the cell radius unchanged and
• seek methods to decrease the D/R ratio.
• Sectoring increases SIR (Signal to Interference Ratio) so
that cluster may be reduced.
• In this approach:
• First the SIR is improved using directional antennas,
• Then capacity improvement is achieved by reducing the number of
cells in a cluster,
• Thus increasing the frequency reuse.
• However, in order to do this successfully, it is necessary to
reduce the relative interference without decreasing the
transmit power.

Sectoring improves S/I
A cell is normally partitioned into
• three 120 sectors or
• six 60

http://www.awsolutions.net/servicesimages/basestation.jpg

A Nortel Networks base station deployed for the network.
http://www.mobilecomms-technology.com/projects/sk-telecom/sk-telecom3.html

Cell sectoring
• During sectoring
• The channels used in a particular cell are broken down into
sectored groups and
• are used only within a particular sector
• Assume
• Seven cell reuse
• 120 sectors

Sectoring improves S/I
I
II
III
IV
V
VI
Only two cells have sectors
with antenna patterns which
radiate into the center cell.
Compare it with situation of 6
omni directional antennas
S/I has been increased from
17dB to 24.2 dB.
S/I improvement allows the
wireless engineer to
•then decrease the cluster size N
• in order to improve the
frequency reuse
•and thus system capacity.

Conclusion:
• Initially co-channels were placed at a distance D due to
possible interference.
• Now we have reduced interference and increased SIR.
• So we can decrease D and bring co-channels near to
each other
• Thus we decrease N and increase channel capacity.
N
R
D
Q 3



Microcells
1. Microcells are used in
• city streets
• congested areas,
• along highways, and
• inside large public buildings
2. As cells become smaller,
3. antennas move from the tops of tall buildings or hill 
to the top of small buildings or  the sides of large
buildings, and finally  to lamp posts, where they
form microcells.
• Decrease in cell size
• Decrease in radiated power by BS and mobile station

Typical Parameters
• The average delay spread refers to multipath
delay spread that is
• Same signal follows different paths and
• There is a time delay between the earliest and latest
arrival of the signal at the receiver.

Problem
• Assume a system of 32 cells with a cell
radius of 1.6km, a total of 32 cells, a total
frequency bandwidth that supports 336
traffic channels, and a reuse factor of N=7.
• What is the geographic area covered?
• How many channels are per cell?
• What is the total number of concurrent calls that
can be handled?
• Repeat with 0.8km and 128 cells.

Solution
 What is the geographic area covered?
• Find the area for each cell
area of hexagonal=6.65k
total geographic area= 6.65*32=213k
• How many channels are per cell?
• For N=7 the number of channels per cell is 336/7=48.
48 channels per cell.

Solution
• What is the total number of concurrent calls that
can be handled?
• 48 channels per cell* 32 cells=1536 channels

The area of hexagon 3
5
.
1 2
R
A hexagon of radius 1.6Km has an area 6.65 km2.
Total area covered = 6.65 km2 per cell x 32 cells= 213 Km2.
No. of channels per cell = 336/7 = 48 channels per cell
Total channel capacity = 48 channels per cell x 32 cells = 1536 channels
For second fig same 213 Km2 is covered but Channel Capacity = 6144 channels

The Micro Cell Zone Concept

Repeaters for Range Extension
• Often a wireless operator needs to provide
dedicated coverage for hard-to-reach areas, such
as within building, or in valleys or tunnels.
• Radio re-transmitters, known as repeaters are
often used to provide such range extension
capabilities.
• In practice, directional antennas or distributed
antenna system (DAS) are connected to the
inputs or outputs of repeaters for localized spot
coverage, particularly in tunnels or buildings.

• Frequency Reuse
• Handoff
• Power Control

CELLULAR TELEPHONE SYSTEM

[Rappa P-10]

Principal Elements of a Cellular systems
• Mobile Telecommunications Switching Office (MTSO)
• The MSC (Mobile Switching Centre ) and MTSO are one and the
same
• Base Transceiver Station (BTS)
• Public Telecommunications Switching Network

Base Station (BS/BTS)
In the center of each cell there is BS that includes
• An antenna,
• A controller,
• Processes call b/w the mobile unit (MU) and the rest of the network
• Number of transceivers,
• For communicating on the channels assigned to that cell.

Cellular System Overview
MTSO

MTSO
• One MTSO serving multiple BSs
• Typically the link b/w MTSO and BS is by wire
line, although a wireless link is also possible
• It connects call b/w MUs
• It connects public telephone or
telecommunication network
• It assigns the voice channel to each call,
• performs handoffs and
• monitors the call for billing information.

Operation of Cellular Systems
Channels Type
Two types of channels b/w BS and MU
1. Control channels
• To exchange information having to do with setting up and
maintaining calls and with establishing a relationship b/w MU
and nearest BS
2. Traffic channels
• Carry a voice or data connection b/w users

Steps in an MTSO Controlled Call
between Mobile Users
• Mobile unit initialization
• Mobile-originated call
• Paging
• Call accepted
• Ongoing call
• Handoff

Mobile unit initialization
1. MU scans/selects the strongest
setup control channel.
2. MU selects the BS
3. Handshake takes place b/w
MU and the MTSO through BS
4. Handshake is used to identify
the user and to register its location
5. Handshake is repeated
periodically to care for handoff

Mobile-originated call
A MU originates a call by
sending the number of the
called unit on the pre-selected
setup channel.
MU checks forward channel
(from the BS)  transmit on
corresponding reverse
channel (to BS)

ROAMING
• Two fundamental operations are associated with
Location Management; location update and paging.
When a Mobile Station (MS) enters a new Location
Area, it performs a location updating procedure by
making an association between the foreign agent and
the home agent.
• One of the BSs, in the newly visited Location Area is
informed and the home directory of the MS is updated
with its current location. When the home agent receives
a message destined for the MS, it forwards the
message to the MS via the foreign agent. An
authentication process is performed before forwarding
the message.

Paging
The MTSO sends a paging
message to certain BSs
depending on the called
mobile unit number.
Each BS transmits the paging
signal on its own assigned
setup channel

Call accepted
The called MU recognizes its
number on the setup channel
being monitored and
responds to that BS.
BS sends the response to MTSO
and circuit is setup
The MTSO selects an available
traffic channel and notifies.

MOBILITY MANAGEMENT
• A MS is assigned a home network, commonly
known as location area. When an MS migrates out
of its current BS into the footprint of another, a
procedure is performed to maintain service
continuity, known as Handoff management.
• An agent in the home network, called home agent,
keeps track of the current location of the MS. The
procedure to keep track of the user’s current
location is referred to as Location management.
• Handoff management and location management
together are referred to as Mobility management.

Ongoing call

TRASMITTING & RECEIVING
• TRASMITTING involves the following steps:
• A caller enters a 10-digit code (phone number) and presses the send
button.
• The MS scans the band to select a free channel and sends a strong
signal to send the number entered.
• The BS relays the number to the MSC.
• The MSC in turn dispatches the request to all the base stations in the
cellular system.
• The Mobile Identification Number (MIN) is then broadcast over all the
forward control channels throughout the cellular system. It is known as
paging.
• The MS responds by identifying itself over the reverse control channel.
• The BS relays the acknowledgement sent by the mobile and informs
the MSC about the handshake.
• The MSC assigns an unused voice channel to the call and call is
• established.

TRASMITTING & RECEIVING
•RECEIVING involves the following steps:
• All the idle mobile stations continuously listens to the
paging signal to detect messages directed at them.
When a call is placed to a mobile station, a packet is
sent to the receiver’s home MSC to find out where it
is.
• A packet is sent to the base station in its current cell,
which then sends a broadcast on the paging channel.
• The receiver MS responds on the control channel.
• In response, a voice channel is assigned and ringing
starts at the MS.

HANDOFF
• At any instant, each mobile station is logically in a
cell and under the control of the cell’s base station.
When a mobile station moves out of a cell, the base
station notices the MS’s signal fading away and
requests all the neighboring BSs to report the
strength they are receiving. The BS then transfers
ownership to the cell getting the strongest signal
and the MSC changes the channel carrying the call.
The process is called handoff..

HANDOFF TYPES
• Hard Handoff and Soft Handoff.
• hard handoff, which was used in the early systems, a MS
communicates with one BS. As a MS moves from cell A to cell B,
the communication between the MS and base station of cell A is
first broken before communication is started between the MS
and the base station of B. As a consequence, the transition is not
smooth.
• soft handoff For smooth transition from one cell (say A) to
another (say B), an MS continues to talk to both A and B. As the
MS moves from cell A to cell B, at some point the communication
is broken with the old base station of cell A. This is known as.

Handoff
While moving from one channel to another
The traffic channel has to change to one
assigned to the BS in the new cell.
The system makes this change
without either interrupting the call or
alerting the user.

Additional Functions in an MTSO
Controlled Call
1. Call blocking
 If all the traffic channels assigned to the nearest BS are busy
 MU makes a preconfigured number of repeated attempts.
 After a certain number of failed tries a busy tone is returned to
the user.
2. Call termination
 When one of two users hangs up, the MTSO is informed and
… two BSs are released

Additional Functions in an MTSO
Controlled Call
3. Call drop
 b/c of interference, weak signals
 BS can not maintain minimum required SNR for a certain
period of time
 The traffic channel to the user is dropped and the MTSO is
informed
4. Calls to/from fixed and remote mobile subscriber
 The MTSO connects to the PSTN.
 MTSO can set up a connection b/w MU and fixed subscriber.
 MTSO can connect to a remote MTSO via the telephone
network or via dedicated line and set up a connection

[Rappa P-12]

[Rappa P-14]

• CAI: Common Air Interface
• FVC: Forward Voice Channels
• RVC: Reverse Voice Channels
• FCC: Forward Control Channels
• RCC: Reverse Control Channels
• MIN: Mobile identification number, which is the
subscriber’s telephone number
• MSC: Mobile Switching Center. It is sometime called a
mobile telephone switching office (MTSO)
• NOTE: Control Channels are also called as Setup
channels because they are only involved in setting up a
call and moving it to an unused voice channel.
[Rappa P-13-15]

Figure 1.6: Timing diagram illustrating how a call to a mobile
user initiated by a landline subscriber is established.
[Rappa P-16]
Dr. Abid
Ali
Minhas

Figure 1.7 Timing diagram illustrating how a call initiated by a mobile is established.
[Rappa P-17]
Dr. Abid
Ali
Minhas

• Frequency Reuse
• Handoff
• Power Control

Mobile Radio Propagation Effects
• Signal strength (SS)
• Must be strong enough between base station and
mobile unit to maintain signal quality at the receiver
• Must not be so strong as to create too much co-channel
interference with channels in another cell using the
same frequency band
• Fading
• Signal propagation effects may disrupt the signal and
cause errors

Mobile Radio Propagation Effects
In designing a cellular layout
1. The desired maximum transmit power level at the BS
and the MU
2. The typical height of the MU antenna
3. The available height of the BS antenna
These factors determine the size of Cell

Channel (Path Loss) Model
• Propagation effects are dynamic and difficult to predict
• Find a model based on empirical data
• One of the most famous widely used models was
developed by Okumura et al.

• Frequency Reuse
• Handoff
• Power Control

Second Generation
• Each MU measures the received power from surrounding
BS and
• continually reports the results of these measurements to the
serving BS.
• A handoff is initiated when
• the power received from the BS of a neighboring cell begins to
exceed the power received from the current base station by a
certain level or for a certain period of time.

MAHO
• The MAHO method enables the call to be handed
over b/w BSs at a much faster rate than in first
generation analog systems
• Since the handoff measurements are made by each MU
and
• The MSC no longer constantly monitors SS
• MAHO is particularly suited for microcellular
environments
• where handoffs are more frequent.

Intersystem handoff
• If a mobile moves from one cellular system to a different
cellular system controlled by a different MSC 
Intersystem handoff becomes necessary
• Many issues
• Local call may become a long distance call
• Compatibility between two MSCs before handoff

Managing Handoff (1)
Different systems have different policies and
methods for managing handoff requests
• Some systems handle handoff request in the
same way they handle originating calls.
• In such systems, the probability that a handoff request
will not be served by a new BS is equal to the blocking
probability of incoming calls.
• However, from the users point of view, having a call
abruptly terminated while in the middle of a
conversation is more annoying than being blocked
occasionally on a new call attempt.

Managing Handoff (2)
To improve the QoS as perceived by the users,
various methods have been devised to
prioritize handoff requests over call initiation
requests when allocating voice channels.
1. Prioritizing Handoffs
i. Guard Channel Concepts
ii. Queuing of handoff requests
2. Practical Handoff Considerations
i. Umbrella Cell Approach  solution to a problem
ii. Cell Dragging  is a problem

Guard Channel Concepts
• Channel Reservation:
• Fraction of total available channels in a cell is reserved
exclusively for handoff requests
• Disadvantage:
• Reduces the total carried traffic (CT) 
• as fewer channels are allocated for originating calls.
• Possible Solution:
• Use of dynamic channel assignment strategies 
• Minimize the No. of required guard channels by efficient
demand based allocation
• Provides efficient spectrum utilization

Practical Handoff Considerations
During a call
• High speed vehicles need more handoff
• Pedestrians may never need a handoff
For micro-cells
• MSC become burdened if high speed users are constantly
being passed b/w very small cells.
Problem
• Handling of high speed and low speed traffic
simultaneously  while minimizing the handoff
intervention from MSC
Solution:
• Umbrella Cell approach

Umbrella Cell approach
“Large” and “Small” cells are
• Provided
• by using different antenna heights (often on the same
building and towers) and
• different power levels and
• Co-located at a single location
Provides
• large area coverage to high speed users while
• small area coverage to users traveling at low
speeds.

Umbrella Cell Approach

This approach ensures that
i. the number of handoffs is minimized for high speed
users and
ii. provides additional micro-cell channels for pedestrian
users.

Scenario:
A driver in a high speed car is attending a call on mobile
telephone, present in a large cell enters in a micro cell.
Discuss the responsibility of BS/MSC

The speed of each user may be estimated by the BS
or MSC by
• Evaluating how rapidly the short-term average
signal strength on the RVC changes over time,
OR
• More sophisticated algorithms may be used o
evaluate and partition users.
• If a high speed user in the large umbrella cell is
approaching the BS and its velocity is rapidly
decreasing,
• the BS may decide to hand the user into the co-
located micro-cell, without MSC intervention.

Cell Dragging (1)
Problem of Cell Dragging
• It occurs in an urban environment when there is a
LOS radio path b/w the subscriber and the BS.
• As the user travels away from the BS at very slow
speed, the average SS does not decay rapidly.
• Even when the user has traveled well beyond the
designed range of the cell, the RSS at the BS
may be above the handoff threshold,
• Thus a handoff may not be made

Cell Dragging (2)
• This creates a potential interference and traffic
management problem
• Since the user has meanwhile traveled deep within a neighboring
cell.
Solution:
• Handoff threshold and radio coverage parameters must
be adjusted carefully.

Handoff Time
1. First generation- Analog Cellular Systems
• 10 Seconds
• Requires that the value of  be on the order of 6 dB
to 12 dB
2. GSM
• MAHO
• 1or 2 Seconds, after decision is made
•  is usually on the order of 0 dB to 6 dB
• Advantage: Faster handoff  greater range of
options for handling high speed and low speed
users

Recent Trends in handoff
Two types
1. Hard handoff
 Channelized wireless systems that assign different radio
channels during a handoff
2. Soft handoff
 It is the ability to select b/w the instantaneous RSS from a
variety of BS.

Soft handoff
 Spread Spectrum mobiles (Code Division
Multiple Access, CDMA) share the same
channel in every cell.
 By simultaneously evaluating the RSS from a
single subscriber at several neighboring BSs,
The MSC may actually decide which version of the
user’s signal is the best at any moment in time.
 This technique exploits macroscopic space
diversity provided by the different physical
locations of the BS.

• Frequency Reuse
• Handoff
• Power Control

Power Control
• Design issues making it desirable to include
dynamic power control in a cellular system
• Received power must be sufficiently above the
background noise for effective communication
• Desirable to minimize power in the transmitted signal
from the mobile
• Reduce co-channel interference, alleviate health concerns, save
battery power
• In Spread Spectrum systems using CDMA, it’s desirable
to equalize the received power level from all mobile
units at the BS

Types of Power Control (1)
1. Closed-loop power control
• Adjusts signal strength in reverse channel (MU to
BS) based on some metric of performance in that
reverse channel, such as
• Received signal power level
• Received signal-to-noise ratio, or
• Received bit error rate
• BS makes power adjustment decision and
communicates to MU on control channel

• Closed loop power control is also used to adjust power in
the forward channel.
• In this case MU provides information about received
signal quality to the BS, which then adjusts transmitted
power.
• GSM standard  a TDMA standard defines, according to
power output
• Eight classes of BS channels and
• Five classes of MU

GSM Transmitter Classes
Adjustments in both directions are made using
closed loop power control

2. Open-loop power control
• Depends solely on mobile unit with no feedback from BS
 used in some spread spectrum systems (SSS).
• In SSS, the BS continuously transmits an unmodulated
signal known as a pilot.
• The pilot allows MU to acquire the timing of the forward
(BS to mobile) CDMA channel.

• It can also be used for power control.
• The MU monitors the RSS of the pilot and sets the
transmitted power in the reverse (mobile to BS) channel
inversely proportional to it.
• Assumption: Forward and Reverse link RSS are
correlated.

• Not as accurate as closed-loop,
• but can react quicker to fluctuations in signal strength
• E.g when MU emerges from behind a large building.
• This fast action is required in the reverse link of a CDMA
systems where the sudden increase in RSS at the BS
may suppress all other signals.

Trunking Theory (1)
http://www.signalharbor.com/ttt/00jan/index.html

Trunking Theory (2)
Trunked Radio Systems
• share a small pool of frequencies among a large
number of users
• The benefit of this technology to the agencies is
that
• many more channels are available for specialized traffic
than there are frequencies.
• Example, the Fort Worth trunked system has only 20
frequencies, but services over 400 channels.
http://wiki.radioreference.com/index.php/Trunking

Trunking Theory (3)
• Trunking is the aggregation of multiple user circuits into a
single channel.
• The aggregation is achieved using some form of
multiplexing.
• Trunking theory was developed by Agner Krarup Erlang,
• Erlang based his studies of the statistical nature of the arrival and
the length of calls.
• The Erlang B formula allows for the calculation of the
number of circuits required in a trunk based on
• the Grade of Service (GoS) and
• the amount of traffic in Erlangs the trunk needs cater for.
http://en.wikiversity.org/wiki/Trunking

Key Definitions for Trunked Radio

• Frequency Reuse
• Handoff
• Power Control

Traffic Engineering
• The Danish mathematician Agner Krarup Erlang
is the founder of teletraffic engineering.
• Ideally, available channels would equal number of
subscribers active at one time
• In practice, not feasible to have capacity handle
all possible load
• For N simultaneous user capacity and L
subscribers
• L < N – nonblocking system
• L > N – blocking system

Blocking System Performance Questions
• Probability that call request is blocked? OR
• What capacity is needed to achieve a certain upper bound on
probability of blocking?
• If blocked calls are queued, what is the average delay?
OR
• What capacity is needed to achieve a certain average delay?

Traffic Intensity (1)
• The basic measure of traffic intensity is A.
• It is measured in a dimensionless unit, the erlang
•  = mean rate of calls attempted per unit time
• h = mean holding time per successful call
• A = average number of calls arriving during average holding period, for
normalized 
h
A 


Cell as a Multiserver Queuing System:
• No. of servers equal to channel capacity, N.
• The average service time at a server is h.
• A basic relationship in a multiserver queue is
h = N
• Where  is server utilization, or the fraction of
time that a server is busy.
• Note: A = N, is considered as a measure of the
average number of channels required.

Example:
• What should be capacity of the cell to meet average
demand, for average calling rate of 20 calls per minute
and average holding time 3 minutes?
• To meet the fluctuations, what do you suggest about the
capacity of the cell?

Example:
• A cell having 10 channels is busy over a period of one
hour as shown in the figure. Calculate mean number of
channels busy for this time.

Traffic Modeling (1)
• Upper limit of traffic intensity?
• ITU-T  “mean busy hour traffic”
• Average of the busy-hour-traffic on the 30 busiest day
• A is measure of busy-hour-traffic
• A is input to traffic model
• Traffic model answers the questions posed.

• Two key factors/elements that determines the nature
of traffic model
i. The manner in which blocked calls are handled
ii. Then number of traffic sources.
See detail on next slide

i. Manner in which blocked calls are handled
• Lost calls delayed (LCD) – blocked calls put in a
queue awaiting a free channel
• Blocked calls rejected and dropped
• Lost calls cleared (LCC) – user waits before another
attempt
• Lost calls held (LCH) – user repeatedly attempts calling
For Cellular systems, the LCC model is generally used
 most accurate
ii. Number of traffic sources
• Whether number of users is assumed to be finite or
infinite

Erlang B
• Infinite Source, LCC Model:
• The key parameter is the probability of loss, or
grade of Service (GoS).
• GoS = 0.01 means that, during a busy hour, the
probability that an attempted call is blocked is 0.01
• Values in the range 0.01 to 0.001 are generally
considered quite good.

Erlang B
• Following equation is known as Erlang B.
• P = Probability of blocking (grade of service)
• A = Offered traffic, erlangs
• N = Number of servers


 N
x
x
N
x
A
N
A
P
0 !
!

Two important points can be deduced from the
table:
1. A larger-capacity system is more efficient than
a smaller-capacity one for a given grade of
service.
2. A larger-capacity system is more susceptible
(open) to an increase in traffic.
Illustrations of these points on next slides

Illustrations:
1. First point
• Consider two cells,
• each with a capacity of 10 channels
• Joint capacity of 20 channels
• Look in column GoS=0.002
• Traffic intensity for 10 channels = 3.43
• Traffic intensity for 20 channels = 6.86
• For a single cell of 20 channels
• Traffic intensity = 10.07

2. Second point
• Consider a single cell of 10 channels,
• Traffic intensity/ Load = 3.43 Erlangs
• 30% increase in the traffic reduces the GoS to 0.01
• Consider a single cell of 70 channels,
• Traffic intensity/ Load = 51.0 Erlangs
• Only 10% increase in the traffic reduces the GoS
from 0.002 to 0.01

Effect of Handoff
.

Cellular Wireless Networks part2.pptx

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