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EKT 450 Mobile Communication System
Chapter 6: The Cellular Concept
Prof Dr. Sabira Khatun,
Dr. Muzammil Jusoh, Dr. Norsuhaida Ahmad
School of Computer and Communication Engineering
1
Introduction
2
• Introduction to Cellular System
• Frequency Reuse
• Channel Assignment Strategies
• Handoff Strategies
• Interference + System Capacity
• Trunking + Grade of Service
• Improving Capacity in Cellular System
Introduction to Cellular System
3
Traditional mobile service was structured in a fashion
similar to television broadcasting: One very powerful
transmitter located at the highest spot in an area would
broadcast in a radius of up to 50 kilometers.
Drawbacks:
• High power consumption
• Impossible to reuse same
frequencies throughout the
system - Low capacity
Introduction to Cellular System
4
• 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.
Frequency Reuse
5
• The design process of selecting and allocating channel
groups for all the cellular base stations within a system
is called frequency reuse or frequency planning.
• 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 limit.
Frequency Reuse
6
• Seven groups of channel (into different cells) from A to
G
• Actual radio coverage is called footprint
determined from field measurements or propagation
prediction models.
• Omni-directional antenna versus directional antenna.
Why Hexagonal Cell?
7
• Hexagonal cell shape has been universally adopted,
since it permits easy and manageable analysis of a
cellular system.
• For a given distance between the center of a polygon
and its farthest perimeter points, the hexagon has the
largest area among other sensible geometric cell
shapes.
• By using the hexagon geometry:
o The fewest number of cells can cover a geographic
region
o Closely approximates a circular radiation pattern
which would occur for an Omni-directional base
station antenna and free space propagation. What other sensible geometric cell
shapes?
Why Hexagonal Cell?
8
• When using hexagons to model coverage areas, base
station transmitters are depicted as either being:
o In the center of the cell (center-excited), or
o On three of the six cell vertices (edge-excited).
• Normally:
o Omni-directional antennas are used in center-
excited cells
o Sectored directional antennas are used in edge-
excited cells.
Concept of Frequency Reuse
9
• Consider a cellular system which has a total of S
duplex channels.
• Each cell is allocated a group of channels, k (k < S).
• The S channels are divided among N cells.
• The total number of available radio channels, S = kN
• 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
MSMkNC
Concept of Frequency Reuse
10
• The capacity is directly proportional to the number of
replication M.
• The cluster size, N, is typically equal to 4, 7, or 12.
• The frequency reuse factor is given by 1/N.
• Hexagonal geometry has:
• exactly six equidistance neighbors.
• the lines joining the centers of any cell and each
of its neighbors are separated by multiples of 60
degrees.
How to maximize
capacity?
solution
Concept of Frequency Reuse
11
• The total number of channels, C, is used as a
measure of capacity
• If k and N remain constant,
• If C and k remain constant,
MSMkNC
MC
MN
1
Concept of Frequency Reuse
12
• Only certain cluster sizes and cell layout are possible.
• The number of cells per cluster, N, can only have
values which satisfy:
where i and j are non-negative integers.
22 jijiN
Concept of Frequency Reuse
13
To find the nearest co-
channel of a neighboring
cell:
1. Move i cells along any
chain of hexagons.
2. Turn 60 degrees
counter clockwise.
3. Move j cell.
Method of locating co-channel cells in a cellular system. In this example, N = 19 (i.e., i = 3, j = 2). (Adapted from [Oet83] IEEE.)
Example 1
14
If a total of 33 MHz of bandwidth is allocated to
a particular FDD cellular telephone system
which uses two 25 kHz simplex channels to
provide full duplex voice and control channels,
compute the number of channels available per
cell if a system uses:
a. Four-cell reuse
b. Seven-cell reuse
c. Twelve-cell reuse
solution
Channel Assignment Strategies
15
• The choice of channel assignment strategy impacts
the performance of the system, particularly as to how
cells are managed when a mobile user is handed off
from one cell to another.
• Fixed channel assignment:
– Each cell is allocated a predetermined set of voice
channels.
– Any call attempt within the cells can only be
served by unused channels in that particular cell.
– If all the channels in the cell are occupied, the call
is blocked and the subscriber does not receive
service.
Channel Assignment Strategies
16
• Dynamic channel assignment: - Voice channels are not allocated to the cells permanently
- Each time a call request is made, the serving base station
requests a channel from the MSC.
- It reduces the likelihood of blocking, which increases the
trunking capacity of the system, since all the available
channels in a market are accessible to all of the calls.
- But, it requires the MSC to collect real-time data on
channel occupancy, traffic distribution, and radio signal
strength indications (RSSI) of all channels on a
continuous basis.
- This increases the storage and computational load on the
system but provides the advantage of increased channel
utilization and decreased probability of a blocked call.
Handoff Strategies
17
• 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.
Handoff Strategies
18
• Handoff Threshold:
– Specify minimum usable signal for acceptable
voice quality (normally -90 dBm to -100 dBm) as
– A slightly stronger signal level is used at which
handoff is made, as
– Handoff margin, cannot be too
large or too small.
o If Δ is too large, unnecessary handoffs burden
the MSC.
o If Δ is too small, there may be insufficient time
to complete handoff before a call is lost.
usablemin ,, rhandoffr PP
usablemin ,rP
handoffrP ,
Handoff Strategies
19
Handoff Strategies
20
• 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 Strategies
21
• Dwell time: the time over which a call may be maintained
within a cell without handoff, depends on:
– propagation
– interference
– distance
– speed
Handoff Measurement
22
• In the first generation (1G) analog cellular systems:
o Signal strength measurements are made by the
base station to determine the relative location of
each mobile user with respect to the base station.
o Additionally, a spare receiver in each base station,
called the location receiver, is used to determine
signal strengths of mobile users which are in
neighboring cells (and appear to be in need of
handoff.)
Handoff Measurement
23
• In second generation systems (TDMA technology):
o Handoff decisions are made mobile assisted
handoff (MAHO).
o Every mobile station measures the received
power from surrounding base stations and
continually reports the results of these
measurements to the serving base station.
o A handoff is initiated when the power received
from the base station 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.
o The MAHO performs at a much faster rate, and is
particularly suited for micro cellular environments.
Handoff Measurement
24
• Intersystem handoff:
o Moves from one cellular system to a
different cellular system controlled by a
different MSC.
o It may become a long-distance call and a
roamer.
o Compatibility between the two MSCs need
to be determined.
• Handoff requests is much important than
handling a new call.
Prioritizing Handoff
25
• Guard channel concept:
o A fraction of total available channels in a
cell is reserved exclusively for handoff
requests from ongoing calls which may
be handed off into the cell.
• Queuing of handoff requests
o To decrease the probability of forced
termination of a call due to lack of
available channels.
Practical Handoff Consideration
26
• 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
27
• Using different antenna heights and different power
levels it is possible to provide large and small cells which
are co-located at a single location. This technique is called
umbrella cell approach and is used to provide large area
coverage to high speed users while providing small area
coverage to users traveling at low speeds.
• The umbrella cell approach ensures that the number of
handoffs in minimized for high speed users and provides
additional microcell channels for pedestrian users.
Hard Handoff and Soft Handoff
28
• Hard handoff: when the signal strength of a neighboring cell exceeds that of the current cell, plus a threshold, the mobile station is instructed to switch to a new frequency band that is within the allocation of the new cell assign different radio channels during a handoff.
• For 1st generation analog systems, if takes about 10
seconds and the value for Δ is on the order of 6 dB
to 12 dB.
• For 2nd generation digital systems, typically requires
only 1 or 2 seconds, and Δ usually is between 0 dB
and 6 dB.
Hard Handoff and Soft Handoff
29
• Soft handoff: a mobile station is temporarily connected to more than one base station simultaneously. A mobile unit may start out assigned to a single cell. If the unit enters a region in which the transmissions from two base stations are comparable (within some threshold of each other), the mobile unit enters the soft handoff state in which it is connected to the two base stations.
• Consequently, handoff does not mean a physical change in the assigned channel, rather than a different base station handles the radio communication task.
• By simultaneously evaluating the receiver signals from a single subscriber at several neighboring base stations, the MSC may actually decide which version of the user’s signal is best at any moment in time.
Interference and System Capacity
30
• 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
– non-cellular system leaks energy into the cellular
frequency band
• Two major cellular interference:
– co-channel interference
– adjacent channel interference
Co- and Adjacent-Channel Cells
31
Co-channel
cells
Adjacent-
channel
cells
Co-channel
interference
Adjacent-
channel
interference
Interference and System Capacity
32
• Interference on voice channels causes cross talk, where
the subscriber hears interference in the background due to
an undesired transmission.
• On control channels, interference leads to missed and
blocked calls due to errors in the digital signaling.
• Interference is more severe in urban areas, due to the
greater RF noise floor and the large number of base
stations and mobiles.
• The interference are difficult to control in practice largely
due to random propagation effects.
• Even more difficult to control is out-of-band interference
mainly from the base stations of competing cellular
carriers (locating their base stations in close proximity.)
Co-Channel Interference and System
Capacity
33
• Frequency reuse - there are several cells that
use the same set of frequencies:
– co-channel cells
– co-channel interference
• To reduce co-channel interference, it cannot
be combated by simply increasing the
carrier power of a transmitter. Instead, co-
channel cell must be separated by a
minimum distance.
Co-Channel Interference and System
Capacity
34
• 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
• Q is called the co-channel reuse ratio
NR
DQ 3
Co-Channel Interference and System
Capacity
35
• A small value of Q (small N) provides large capacity
• A large value of Q (large N) improves the
transmission quality - smaller level of co-channel
interference
• A tradeoff must be made between these two
objectives
Co-Channel Interference and System
Capacity
36
Example 2
37
You are trying to design a cellular network that
will cover an area of at least 2800 km2. There are
300 available voice channels. Your design is
required to support at least 100 concurrent calls
in each cell.
If the co-channel cell centre distance is required
to be 9 km, how many base stations will you
need in this network?
solution
Co-Channel Interference and System
Capacity
38
• Let i0 be the number of co-channel interfering cells. The signal-
to-interference ratio (SIR) for a mobile receiver can be
expressed as:
S : the desired signal power
Ii : interference power caused by the i-th 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
1
i
i
iI
S
I
S
n
rd
dPP
0
0
0
0 log10)dBm()dBm(d
dnPPr Receive
r
d0
P0 : Measured power
Co-Channel Interference and System
Capacity
39
• When the transmission power of each base station
is equal and the path loss exponent same
throughout the coverage area, SIR for a mobile can
be approximated as
Example: for N=7, the first layer of interfering cell, i0 = 6.
• For simplification, assume all interferers have
equidistance,
0
1
i
i
n
i
n
D
R
I
S
00
3)/(
i
N
i
RD
I
Sn
n
which relates S/I to the cluster size,
and in turn determines the overall
capacity of the system
Example 3
40
If a signal-to-interference ratio of 15 dB is
required for satisfactory forward channel
performance of a cellular system, what is the
frequency reuse factor and cluster size that
should be used for maximum capacity if the path
loss exponent is (a) n = 4, (b) n = 3 ?
Assume that there are six co-channel cells in the
first tier, and all of them are at the same distance
from the mobile. Use suitable approximations.
solution
Adjacent-Channel Interference
41
• Adjacent channel interference: interference from
adjacent in frequency to the desired signal.
– Imperfect receiver filters allow nearby frequencies to
leak into the pass-band
– Performance degrade seriously due to near-far
effect.
desired signal
receiving filter response
desired signalinterference
interference
signal on adjacent channelsignal on adjacent channel
FILTER
Adjacent-Channel Interference
42
• Adjacent channel interference can be minimized
through :
o careful filtering and channel assignment.
o Keep the frequency separation between each
channel in a given cell as large as possible
o A channel separation greater than six is
needed to bring the adjacent channel interference
to an acceptable level.
Power Control for Reducing
Interference
43
• In practical cellular radio and personal communication systems, the power levels transmitted by every mobile unit are under constant control by the serving base stations.
• This is done to ensure that each mobile transmits the smallest power necessary on the reverse channel.
• Power control not only helps prolong battery life, also reduces the interference on the reverse channel.
• It is especially important for CDMA systems, because every user in every cell share the same radio channel. (to reduce the co-channel interference.)
Power Control for Reducing
Interference
44
Need for power control ?
Trunking and Grade of Service
45
• Trunking allows a large number of users to share the
relatively small number of channels in a cell by
providing access to each user, on demand, from a
pool of available channels.
• It exploits the statistical behavior of users so that a
fixed number of channels or circuits may
accommodate a large, random user community.
• The measure of traffic intensity, namely Erlang.
For example:
0.5 Erlangs of traffic =
a radio channel that is occupied for 30 minutes
during an hour.
Trunking and Grade of Service
46
Trunking and Grade of Service
47
The Grade of Service (GOS):
• A measure of the ability of a user to access
a trunked system during the busiest hour.
• It is typically given as the likelihood that a
call is blocked, or the likelihood of a call
experiencing a delay greater than a certain
queuing time.
Trunking and Grade of Service
48
• The traffic intensity generated by each user
Where H is the average duration of a call, λ is the average number of
call requests per unit time for each user.
• The total offered traffic intensity for U users
• In a C channel trunked system, if the traffic is equally
distributed among the channels, the traffic intensity per
channel is
HAu
C
UAA u
C
uUAA
Trunking and Grade of Service
49
• Example:
o AMPS cellular system is designed for a
GOS of 2% blocking.
o This implies that the channel allocations
for cell sites are designed so that 2 out of
100 calls will be blocked due to channel
occupancy during the busiest hour.
Trunking and Grade of Service
50
Two types of trunked systems:
• Blocked Calls Cleared trunking :
– Offers no queuing for call requests.
– Calls arrive as determined by a Poisson distribution.
– There are an infinite number of users.
– There are memoryless arrivals of requests.
– The probability of a user occupying a channel is
exponentially distributed.
– A finite number of channels available.
– This is known as an M/M/m queue.
a memoryless poisson arrivals an exponential service time
the number of trunked channels
Trunking and Grade of Service
51
This leads to the derivation of the Erlang B
formula:
where C is the number of trunked channels, A
is the total offered traffic.
GOS
!
!][
0
C
k
k
c
r
k
A
C
A
blockingP
Trunking and Grade of Service
52
Trunking and Grade of Service
53
• Blocked Calls Delayed trunking :
– Queuing is provided to hold calls which are blocked.
– If no channel available, the call request may be
delayed until a channel becomes available.
– Measure of GOS : probability that a call is blocked
after waiting a specific length of time.
– A call not having immediate access to a channel is
determined by Erlang C:
1
0 !)1(!
]0[C
k
kc
c
r
k
A
C
ACA
AdelayP
Trunking and Grade of Service
54
The GOS of a trunked system where blocked
calls are delayed :
The average delay for all calls in a queued
system
)/)(exp(]0[
]0|[]0[][
HtACdelayP
delaytdelayPdelayPtdelayP
r
rrr
AC
HdelayPD r
]0[
Trunking and Grade of Service
55
The Erlang B formula plotted in graphical form
Trunking and Grade of Service
56
The Erlang C formula plotted in graphical form
Example 4
57
How many users can be supported for 0.5%
blocking probability for the following number of
trunked channels in a blocked calls cleared
system?
(a) 1, (b) 5, (c) 10, (d) 20 and (e) 100
Assume each user generates 0.1 Erlangs of
traffic.
solution
Example 5
58
Pauh has an area of 1300 square miles and is covered by CELCOM
using a seven-cell reuse pattern. Each cell has a radius of four miles
and the city is allocated 40 MHz of spectrum with a full duplex
channel bandwidth of 60 kHz. Assume a GOS of 2% for an Erlang B
system is specified. If the offered traffic per user is 0.03 Erlangs,
compute:
(a) The number of cells in the service area,
(b) The number of channels per cell,
(c) Traffic intensity of each cell,
(d) The maximum carried traffic,
(e) The total number of users that can be served for 2% GOS
(f) The number of mobiles per unique channel (where it is
understood that channels are reused)
(g) The theoretical maximum number of users that could be served
at one time by CELCOM.
solution
Example 6
59
An urban area has a population of two million residents.
Three competing trunked mobile networks (CELCOM,
MAXIS and DIGI) provide cellular service in this area.
CELCOM has 394 cells with 19 channels each, MAXIS has
98 cells with 57 channels each, and DIGI has 49 cells, each
with 100 channels.
Find number of users that can be supported at 2%
blocking if each user averages two calls per hour at an
average call duration of three minutes. Assume that all three
trunked systems are operated at maximum capacity,
compute the percentage market penetration of each
cellular provider.
solution
Improving Capacity in Cellular System
60
• Methods for improving capacity in cellular systems:
– Cell Splitting :
o Subdividing a congested cell into smaller cells.
o Allows an orderly growth of the cellular system
– Sectoring :
o Directional antennas to control the interference and
frequency reuse of channels.
– Microcell Zone Concept :
o Distributing the coverage of a cell and extends the cell
boundary to hard-to-reach place.
– Repeaters for range extension.
– More bandwidth – standards, country regulation etc.
– Borrow channel from nearby cells.
Improving Capacity in Cellular System
61
• Cell Splitting : Split congested cell into smaller cells.
– Preserve frequency reuse plan.
– Reduce transmission power.
Improving Capacity in Cellular System
62
microcell
Reduce R to R/2
Improving Capacity in Cellular System
63
Improving Capacity in Cellular System
64
• 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 splitted:
– Different cell sizes will exist simultaneously
• Handoff issues - high speed and low speed traffic can be
simultaneously accommodated
1tP 2tP
n
tr RPP 1]boundary cell oldat [
n
tr RPP )2/(]boundary cellnew at [ 2
16
12
tt
PP
Improving Capacity in Cellular System
65
• 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.
Improving Capacity in Cellular System
66
67
Base Station Antennas
Omnidirectional : broadcast
3600
Sector : broadcasts 600 /
900 / 1200
Panel / Dish : Point to point
68
600 Sectoring
69
1200 Sectoring
Improving Capacity in Cellular System
70
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
Improving Capacity in Cellular System
71
• Repeaters for range extension: