WC PPT UNIT-II.ppt

2.1 Introduction to Cellular Systems
 Solves the problem of spectral congestion and user capacity.
 Offer very high capacity in a limited spectrum without major
technological changes.
 Reuse of radio channel in different cells.
 Enable a fix number of channels to serve an arbitrarily large
number of users by reusing the channel throughout the
coverage region.

Goals
1. Low power transmitter system
2. Increase network capacity
3. Frequency reuse

Idea!
# Partition the region into smaller regions called
cells.
# Each cell gets at least one base station or
tower
# Users within a cell talks to the tower
How can we divide the region into cells?

Properties of Cell structure
Advantages of cell structures:
1. More capacity due to frequency reusage
2. Less transmission power needed
3. Deals interference, transmission area locally
Problems:
1.Fixed network needed for the base stations
2.Handover (changing from one cell to another)
necessary
3.Interference with other cells

2.2 Frequency Reuse
• Each cellular base station is allocated a group of radio channels within
a small geographic area called a cell.
• Neighboring cells are assigned different channel groups.
• By limiting the coverage area to within the boundary of the cell, the
channel groups may be reused to cover different cells.
• Keep interference levels within tolerable limits.
• Frequency reuse or frequency planning
•seven groups of channel from A to G
•footprint of a cell - actual radio
coverage
•omni-directional antenna v.s.
directional antenna

• Consider a cellular system which has a total of S duplex channels.
• Each cell is allocated a group of k channels, .
• The S channels are divided among N cells.
• The total number of available radio channels
• The N cells which use the complete set of channels is called cluster.
• The cluster can be repeated M times within the system. The total
number of channels, C, is used as a measure of capacity
• The capacity is directly proportional to the number of replication M.
• The cluster size, N, is typically equal to 4, 7, or 12.
• Small N is desirable to maximize capacity.
• The frequency reuse factor is given by
S
k 
kN
S 
MS
MkN
C 

N
/
1
CAPACIY EXPANSION BY FREQUENCY REUSE

Frequency Reuse
11
F1
F2
F3
F4
F5
F6
F7 F1
F2
F3
F4
F5
F6
F7
F1
F2
F3
F4
F5
F6
F7 F1
F2
F3
F4
F5
F6
F7
F1
F1
F1
F1
Fx: Set of frequency
7 cell reuse cluster

Reuse Distance
12
F1
F2
F3
F4
F5
F6
F7
F1
F2
F3
F4
F5
F6
F7
F1
F1
• For hexagonal cells, the reuse distance
is given by
R
N
D 3

R
where R is cell radius and N is the
reuse pattern (the cluster size or the
number of cells per cluster).
N
R
D
q 3


• Reuse factor is
Cluster

Reuse Distance (Cont’d)
13
 The cluster size or the number of cells per cluster is given by
2
2
j
ij
i
N 


where i and j are integers.
 N = 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, 28, …, etc.
The popular value of N being 4 and 7.
i
j
60o

14
(b) Formation of a cluster for N = 7
with i=2 and j=1
60°
1 2 3 … i
j direction
i direction
(a) Finding the center of an adjacent cluster
using integers i and j (direction of i and j can
be interchanged).
i=2
i=2
j=1
j=1
j=1
j=1
j=1
j=1
i=2
i=2
i=2
i=2

15
(c) A cluster with N =12 with i=2 and j=2
i=3
j=2
i=3 j=2 i=3
j=2
i=3
j=2
i=3
j=2
i=3
j=2
(d) A Cluster with N = 19 cells with i=3
and j=2
j=2
j=2
j=2
j=2
j=2
j=2
i=2
i=2
i=2
i=2
i=2
i=2

2.3 Channel Assignment Strategies
• Frequency reuse scheme
– increases capacity
– minimize interference
• Channel assignment strategy
 fixed channel assignment
 dynamic channel assignment

• Fixed channel assignment
– each cell is allocated a predetermined set of voice channel
– any new call attempt can only be served by the unused
channels
– the call will be blocked if all channels in that cell are
occupied
2.3 Fixed Channel Assignment
• Borrowing Strategy
• Cell can be allowed to
borrow channels from
neighboring cell if all
its channels are
already occupied
• MSC supervises such
borrowing procedures

Dynamic Channel Assignment
• Dynamic channel assignment
– channels are not allocated to cells permanently.
– allocate channels based on request.
– reduce the likelihood of blocking, increase capacity.
– Reduces call blocking which in turn increases the trunking
capacity
– DCA requires the MSC to collect real time data
– Channel Occupancy
– Traffic distribution
– Radio signal quality of all channels on continuous basis
– Data collection is done to manage Handoff

2.4 Handoff Strategies
• When a mobile moves into a different cell while a conversation
is in progress, the MSC automatically transfers the call to a new
channel belonging to the new base station.
• Handoff operation
– identifying a new base station
– re-allocating the voice and control channels with the new base
station.

Features (Advantages) of Handoff

2.4 Handoff Strategies
• Handoff Threshold
– Minimum usable signal for acceptable voice quality (-90dBm to -
100dBm)
– Handoff margin cannot be too large or
too small.
– If is too large, unnecessary handoffs burden the MSC
– If is too small, there may be insufficient time to complete
handoff before a call is lost.
usable
minimum
,
, r
handoff
r P
P 





Handoff Scenario at cell boundary

• Handoff must ensure that the drop in the measured signal is not
due to momentary fading and that the mobile is actually moving
away from the serving base station.
• Running average measurement of signal strength should be
optimized so that unnecessary handoffs are avoided.
– Depends on the speed at which the vehicle is moving.
– Steep short term average -> the hand off should be made quickly
– The speed can be estimated from the statistics of the received
short-term fading signal at the base station
Handoff (Contd…)

• Dwell time: the time over which a call may be maintained
within a cell, without handoff.
• Dwell time depends on
– propagation
– interference
– distance
– speed
Dwell Time

Types of Handoffs
 Hard handoff: “break before make” connection
 Intra and inter-cell handoffs
Hard Handoff between the MS and BSs

Types of Handoffs
 Soft handoff: “make-before-break” connection.
 Mobile directed handoff.
 Multiways and softer handoffs
Soft Handoff between MS and BSTs

Types of protocols [Handoff
Methods]
 4 types of handoff protocols which help in
providing continuous and QOS-guaranteed
service.
 Network-controlled handoff (NCHO)
 Mobile-assisted handoff (MAHO)
 Soft handoff (SHO) and
 Mobile-controlled handoff (MCHO)

Handoff Prioritization:
 Two basic methods of handoff prioritization are
:
 Guard Channels
 Queuing of Handoff

Practical Handoff Consideration
• Different type of users
– High speed users need frequent handoff during a call.
– Low speed users may never need a handoff during a call.
• Microcells to provide capacity, the MSC can become burdened if
high speed users are constantly being passed between very
small cells.
• Minimize handoff intervention
– handle the simultaneous traffic of high speed and low speed users.

Practical Handoff Consideration (contd…)
Umbrella cell approach.
• Is used to provide large area coverage to high speed user while
providing small area coverage to low speed users
• Large and small cells can be located at a single location
– different antenna height
– different power level
Advantages
Increases radio coverage
Reduces number handoffs
Provides more no. of
channels
Less MSC intervention

Practical Handoff Consideration (contd…)
Cell dragging problem
 Pedestrian users provide a very strong signal to
the base station
 Occurs in urban environment – when LOS exists
between the subscriber and BS
 Average signal strength doesn’t decay rapidly
received signal at the BS > handoff threshold
thus Handoff may not be made
 Creates Potential Interference and Traffic
Management – when the user may travel deep
within a neighboring cell
Solution
Handoff Threshold and Radio coverage

2.5 Interference and System Capacity
• Sources of interference
– another mobile in the same cell
– a call in progress in the neighboring cell
– other base stations operating in the same frequency band
– noncellular system leaks energy into the cellular frequency band
– Intracell interference
– Intercell interference
• Two major cellular interference
– co-channel interference
– adjacent channel interference

2.5.1 Co-channel Interference and
System Capacity
• Frequency reuse - there are several cells that use the same set
of frequencies
– co-channel cells
– co-channel interference
• To reduce co-channel interference, co-channel cell must be
separated by a minimum distance.
• When the size of the cell is approximately the same
– co-channel interference is independent of the transmitted power
– co-channel interference is a function of
• R: Radius of the cell
• D: distance to the center of the nearest co-channel cell
• Increasing the ratio Q=D/R, the interference is reduced.
• Q is called the co-channel reuse ratio

• For a hexagonal geometry
• A small value of Q provides large capacity
• A large value of Q improves the transmission quality - smaller
level of co-channel interference
• A tradeoff must be made between these two objectives
N
R
D
Q 3



• Let be the number of co-channel interfering cells. The signal-
to-interference ratio (SIR) for a mobile receiver can be
expressed as
S: the desired signal power
: interference power caused by the ith interfering co-channel
cell base station
• The average received power at a distance d from the
transmitting antenna is approximated by
or
n is the path loss exponent which ranges between 2 and 4.
0
i


 0
1
i
i
i
I
S
I
S
i
I
n
r
d
d
P
P










0
0










0
0 log
10
)
dBm
(
)
dBm
(
d
d
n
P
Pr
close-in reference point
TX
0
d
0
P :measued power

• When the transmission power of each base station is equal, SIR
for a mobile can be approximated as
• Consider only the first layer of interfering cells
 




 0
1
i
i
n
i
n
D
R
I
S
 
0
0
3
)
/
(
i
N
i
R
D
I
S
n
n


• Example: AMPS requires that SIR be
greater than 18dB
– N should be at least 6.49 for n=4.
– Minimum cluster size is 7
6
0 
i

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

Adjacent Channel Interference
• Adjacent channel interference: interference from adjacent in
frequency to the desired signal.
– Imperfect receiver filters allow nearby frequencies to leak into the
passband
– Performance degrade seriously due to near-far effect.
desired signal
receiving filter
response
desired signal
interference
interference
signal on adjacent channel
signal on adjacent channel
FILTER

Adjacent Channel Interference
 Interference from channels that are adjacent in frequency,
 The primary reason for that is Imperfect Receive Filters which cause the
adjacent channel energy to leak into your spectrum.
 Problem is severer if the user of adjacent channel is in close proximity.
Near-Far Effect
 Near-Far Effect: The other transmitter(who may or may not be of the
same type) captures the receiver of the subscriber.
 Also, when a Mobile Station close to the Base Station transmits on a
channel close to the one being used by a weaker mobile: The BS faces
difficulty in discriminating the desired mobile user from the “bleed over”
of the adjacent channel mobile.

Near-Far Effect: Case 1
 The Mobile receiver is captured by the unintended, unknown transmitter,
instead of the desired base station
42

Near-Far Effect: Case 2
 The Base Station faces difficulty in recognizing the actual mobile user,
when the adjacent channel bleed over is too high.
43

HOW TO MINIMIZE ACI:
• Adjacent channel interference can be minimized
through careful filtering and channel assignment.
• Keep the frequency separation between each channel in
a given cell as large as possible
• Sequentially assigning cells the successive frequency
channels
• A channel separation greater than six is needed to bring
the adjacent channel interference to an acceptable level.
• Also, secondary level of interference can be reduced by
not assigning adjacent channels to neighboring cells.
• For tolerable ACI, we either need to increase the
frequency separation or reduce the passband BW.

Power Control for Reducing Interference
• Ensure each mobile transmits the smallest
power necessary to maintain a good quality link
on the reverse channel
– long battery life
– increase SIR
– solve the near-far problem

Trunking and Grade of Service
(GOS)
Trunking:
 A means for providing access to users on demand
from available pool of channels.
 With trunking, a small number of channels can
accommodate large number of random users.
 Telephone companies use trunking theory to
determine number of circuits required.
 Trunking theory is about how a population can
be handled by a limited number of servers.

Terminology:
1. Traffic intensity is measured in Erlangs:
 One Erlang: traffic in a channel completely occupied. 0.5
Erlang: channel occupied 30 minutes in an hour.
2. Grade of Service (GOS): probability that a call is blocked
(or delayed).- measure of the ability of a user to access
trunked sys in busiest hour
3. Set-Up Time: time to allocate a channel.
4. Blocked Call: Call that cannot be completed at time of
request due to congestion. Also referred to as Lost Call.
5. Holding Time: (H) average duration of typical call.
6. Load: Traffic intensity across the whole system.
7. Request Rate: (λ) average number of call requests per unit
time.

Traffic Measurement (Erlangs)
48

Trunking and Grade of Service
• Each user genrates a traffic intensity of
H: average duration of a call.
: average number of call requests per unit time
• For a system containing U users and an unspecified
number of channels, the total offered traffic intensity A, is
given by
• For C channel trunking system, the traffic intensity, is
given as

u
UA
A 
c
A
C
UA
A u
c /

H
Au 

u
A

Erlang C Model –Blocked calls
cleared
 Channel is not available ---call request may be delayed until a
channel becomes available.
 It is a measure of GOS – Defined as the probability that a call
is blocked after waiting a specific length of queue.
 A type of trunked system queues blocked calls –Blocked Calls
Delayed. This is known as an Erlang C model.

Erlang C Formula
 The likelihood of a call not having immediate
access to a channel is determined by Erlang C
formula:

Improving Capacity in Cellular
Systems
• Methods for improving capacity in cellular systems
– Cell Splitting: subdividing a congested cell into smaller
cells.
– Sectoring: directional antennas to control the
interference and frequency reuse.
– Coverage zone : Distributing the coverage of a cell and
extends the cell boundary to hard-to-reach place.

Cell Splitting
• Split congested cell into smaller cells.
– Preserve frequency reuse plan.
– Reduce transmission power.
microcell
Reduce R to R/2

Cell Splitting
 Cell Splitting is the process of subdividing the congested cell
into smaller cells (microcells),Each with its own base station
and a corresponding reduction in antenna height and
transmitter power.
 Cell Splitting increases the capacity since it increases the
number of times the channels are reused.

An Example
 The area covered by a circle with radius R is four times the
area covered by the circle with radius R/2
 The number of cells is increased four times
 The number of clusters the number of channels and the
capacity in the coverage area are increased
 Cell Splitting does not change the co-channel re-use ratio Q
=D/R
55

Transmit Power
 New cells are smaller, so the transmit power of the new cells
must be reduced
 How to determine the transmit power?
 The transmit power of the new cells can be found by
examining the received power at the new and old cell
boundaries and setting them equal
 Pr(at the old cell boundary) is proportional to
 Pr(at the new cell boundary) is proportional to

Application of cell splitting
 Not all cells are split at the same time.
 Larger transmit power
 Some of the channels would not be sufficiently separated
from the co-channel cells.
 Smaller transmit power --parts of the larger cells left
uncovered
 Two groups:
 one that corresponds to the smaller cell and the other for
larger cell reuse requirements
58

Application of cell splitting (cont.)
 The sizes of these two groups depend on the stage of the
splitting process
 At the beginning, fewer channels will be there in the smaller
power group. As the demand grows, smaller groups would
require more channels
 Cell splitting continues until all the channels are in the
smaller power group
 Antenna Down tilting
 To limit the radio coverage of microcells
59

• Transmission power reduction from to
• Examining the receiving power at the new and old cell boundary
• If we take n = 4 and set the received power equal to each other
• The transmit power must be reduced by 12 dB in order to fill in
the original coverage area.
• Problem: if only part of the cells are splited
– Different cell sizes will exist simultaneously
• Handoff issues - high speed and low speed traffic can be
simultaneously accommodated
1
t
P 2
t
P
n
t
r R
P
P 
 1
]
boundary
cell
old
at
[
n
t
r R
P
P 
 )
2
/
(
]
boundary
cell
new
at
[ 2
16
1
2
t
t
P
P 

Sectoring
• Decrease the co-channel interference and keep the cell radius R
unchanged
– Replacing single omni-directional antenna by several directional
antennas
– Radiating within a specified sector

• Interference Reduction
position of the
mobile
interference
cells

Sectoring Advantages
1. Reduce interference by reducing the number of interfering co
channels
2. Increase SIR (better call quality).
3. The increase in SIR can be traded with reducing the cluster
size (N) which increase the capacity.
Disadvantages
1. Cost: Increase number of antennas at each base station.
2. Next section: Decrease trunking efficiency due to channel
sectoring at the base station.
3. The available channels in the cell must be subdivided and
dedicated to a specific antenna.

Microzones
 Multiple zones and a base station make up a cell. Microcell zone
concept:. Large control base station is replaced by several lower
power transmitters on the age of cell. ... Since a given channel is
active only in a particular zone in which mobile is travelling, base
station radiation is localized and interference is reduced.
 As a mobile travels within the cell, it is served by the zone with the
strongest signal
 This technique is superior to sectoring because antennas are placed
at the outer edges of the cell, and any base station channel can be
assigned to any zone by the base station
65

Microcell Zone Concept
• Antennas are placed at the outer edges of the cell
• Any channel may be assigned to any zone by the base
station
• Mobile is served by the zone with the strongest signal.
• Handoff within a cell
– No channel re-
assignment
– Switch the channel
to a different zone
site
• Reduce interference
– Low power
transmitters are
employed

ADVANTAGES
 No handoffs is required at the MSC
 The base station radiation is localized and
interference is reduced. A given channel is active
only in the particular zone in which the mobile is
traveling
 The co-channel interference is also reduced

 Decreased co-channel interference improves signal
quality which leads to an increase in capacity
without any degradation in trunking efficiency
caused by sectoring
 For example
We know an (S/I) of 18dB is required for
satisfactory system performance in narrowband
FM

Repeaters for Range Extension
 Repeaters are radio re-transmitters used to provide coverage
for hard-to-reach areas, such as within buildings or in valleys
or tunnels
 Repeaters are bidirectional. Upon receiving signals from base
station, then amplifies and reradiates the base station signals
to the specific coverage region. Also it will send signals to the
serving base station.
 The repeaters do not add capacity to the system-it simply
serves to reradiate the base station signal into specific
locations

1.Incoming Call to
Mobile X is given to
MSC
2. MSC dispatches the
request to all base
station
3.. BS broadcast the
MIN, Telephone number
of MobileX,

WC PPT UNIT-II.ppt

More Related Content

Similar to WC PPT UNIT-II.ppt (20)

Recently uploaded (20)

WC PPT UNIT-II.ppt