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1
Cellular Concept
Cellular telephone systems must accommodate a large number of users
over a large geographic area with limited frequency spectrum, i.e., with
limited number of channels.
Cellular concept is based on the process of substituting high power
transmitters with low power transmitters to support many users
This is achieved by dividing the coverage area into small segments, called
cells utilizing low power transmitters
Cell cluster: Refers to a group of cells adjacent to one another to provide
mobile coverage in a given area. Cells within a cluster use different radio
channels to avoid interference
This cluster can repeat itself and hence the same set of channels can be
used again and again (Frequency Reuse)
Each cell has a low power transmitter with a coverage area equal to the
area of the cell.
Cell Shape
The design of cells requires that they take regular shapes (polygons)
namely equilateral triangle, square and regular hexagon to ensure that
an entire area is covered without any overlaps or gaps.
A cell must be designed such that it is most reliable too, i.e., it supports
even the weakest mobile with occurs at the edges of the cell. For any
distance between the center and the farthest point in the cell from it, a
regular hexagon covers the maximum area.
Therefore, regular hexagonal geometry is normally adopted as the cells
in mobile communication
Macro-cellular nets, with cell radius 1 - 30 km
Micro-cellular net, with cell radius 200 - 2000 m
Pico-cellular nets, with cell radius 4 - 200 meter
2
Types of cells
3
4
Operational Channels in a cell
In each cell, there are four types of channels that take active part during a
mobile call:
Forward Voice Channel (FVC): This channel is used for the voice
transmission from the BS to the MS.
Reverse Voice Channel (RVC): This is used for the voice transmission from
the MS to the BS.
Forward Control Channel (FCC): Control channels are generally used for
controlling the activity of the call, i.e., they are used for setting up calls and
to divert the call to unused voice channels. Hence these are also called setup
channels. These channels transmit and receive call initiation and service
request messages. The FCC is used for control signalling purpose from the
BS to MS.
Reverse Control Channel (RCC): This is used for the call control purpose
from the MS to the BS. Control channels are usually monitored by mobiles.
Frequency Reuse
Frequency reuse, or, frequency planning, is a technique of reusing
frequencies and channels within a communication system to improve
capacity and spectral efficiency
Frequency reuse in mobile cellular systems means that frequencies
allocated to the service are reused in a regular pattern of cells, each
covered by one base station.
The repeating regular pattern of cells is called cluster. Two cells using
the same frequencies in different clusters is known as Co-channel cells
The reuse of frequencies enables a cellular system to handle a huge
number of calls with a limited number of channels
5
Frequency Reuse/planning cont’d
6
Frequency Reuse/planning cont’d
Consider a cellular system with S duplex channels available for use and
let N be the number of cells in a cluster
If each cell is allotted k duplex channels (where k<S), The total number
of available radio channels in the system is S=k N
The N cells which use the complete set of channels is called cluster.
If the cluster are repeated M times within the total area, the total
number of duplex channels, or, the total number of users in the system
(total system capacity) would be;
C = kMN = MS
If k and N remain constant, then the system capacity is directly
proportional to the number of times a cluster is repeated;
Cα M
if C and k remain constant, then
N α 𝟏 𝑴
The smaller the number of cells, the larger the larger the number of clusters
and hence the higher the capacity
7
Frequency Reuse/planning cont’d
However for small N, co-channel cells are located much closer and hence
more interference. The value of N is determined by calculating the amount
of interference that can be tolerated in the system.
The frequency reuse factor is given by 1/N
The cluster size N is given by the following formula;
where i and j are integer numbers
8
Assignment 2
Derive the equation of the cluster size of a mobile cellular system,
N=i2+ij+j2
Find the relationship between any two nearest co-channel cell distance
D and the cluster size N (Show that the Frequency Reuse distance
𝐷
𝑅 = √(3𝑁) where R is the cell radius
9
Improving Coverage and Capacity in Cellular Systems
There are two techniques used;
Sectoring
Cell splitting
Cell Splitting
Cell splitting is used in an already functioning network where
network expansion is required in certain regions as opposed to the
entire network
A cell (or multiple cells) can be split into smaller cells and
frequencies are redistributed in a way that does not cause additional
interference
A cell area is proportional to R2 (cell radius) hence splitting the cell
radius by a half reduces the cell area by a quarter. Theoretically, 4 of
the smaller cells can fit into 1 of the large cells but since it is not
possible to fit 4 quarter‐size hexagonal cells completely into 1 full‐size
hexagonal cell, some regions will have to be covered by adjacent cells
10
Cell Splitting cont’d
11
Original cell that has reached its capacity
Cell A has been split into 3 smaller cells
Cell Splitting cont’d
After cell splitting, the new small cells are reassigned new frequencies
that do not cause co‐channel interference with adjacent cells
The power transmitted in the small cells is reduced compared to the
power transmitted in the large cells as it would require much less power
to cover the cell compared to the large cells
The power is reduced by a factor of;
n = path loss exponent
Advantages of cell splitting;
Increased system capacity
Reduced transmitted power which increases battery life of mobile phones
Disadvantages;
Requires the construction of new network towers which is very costly
12
Sectoring
Sectoring
Involves the dividing of a cell into sectors and using 3 or 6 directional
antennas to provide coverage to a sector of the hexagon
When 3 directional antennas are used, 120° sectoring is achieved (each
antenna covers 120°), and when 6 directional antennas are used, 60° sectoring is achieved (each antenna covers 60°).
13
Sectoring Cont’d
Dividing the cells into sectors actually reduces the network capacity
because the channels allocated to a cell are now divided among the
different sectors
The gain in network capacity is achieved by reducing the number of
interfering co‐channel cells
Example
A cellular system uses a frequency reuse factor N = 4 ( i = 0, j = 2 ). If the
path loss exponent n = 4 , and cell radius R = 5 km. Find in dB the following
quantities:
The SIR for the system with no cell sectoring.
The SIR for the system when 120° cell sectoring is used (note that
worst case occurs when mobile phone is at the furthest point from the
interfering towers).
The SIR for the system when 60° cell sectoring is used (note that worst
case occurs when mobile phone is at the furthest point from the
interfering towers). 14
Channel Assignment strategies
With the rapid increase in number of mobile users, the mobile service
providers had to follow strategies which ensure the effective utilization
of the limited radio spectrum
In addition to Frequency Reuse, a variety of Channel Assignment
strategies are adopted to improve capacity and lower interference
Channel assignment strategies are classified into two types: Fixed and
Dynamic
15
Channel Assignment strategies cont’d
Fixed Channel Assignment (FCA)
Each cell is allocated a fixed number of voice channels. Any
communication within the cell can only be made with the designated
unused channels of that particular cell
The call will be blocked if all channels in that cell are occupied and the
user has to wait for channels to be released by other users
FCA is simplest of the channel assignment strategies as it requires very
simple circuitry but provides worst channel utilization
Borrowing strategy; Another approach in which the channels were
borrowed from adjacent cell if all of its own designated channels were
occupied. In such cases the MSC supervises the borrowing process and
ensures that none of the calls in progress are interrupted
16
Channel Assignment strategies cont’d
Dynamic Channel Assignment (DCA)
Channels are temporarily assigned for use in cells for the duration of the
call. Each time a call attempt is made from a cell the corresponding BTS
requests a channel from MSC. The MSC then allocates a channel to the
requesting the BTS. After the call is over the channel is returned and kept
in a central pool.
To avoid co-channel interference any channel that in use in one cell can
only be reassigned simultaneously to another cell in the system if the
distance between the two cells is larger than minimum reuse distance
DCA reduces the likelihood of blocking and increases the capacity of the
network as all of the channels are available to all cells.
However, it results in heavy load on the MSC during heavy traffic
conditions
17
Handover 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
The handover process involves identifying a new base station and re-
allocating the voice and control channels with the new base station.
18
Once a signal level is set as the minimum
acceptable for good voice quality Pr(min),
then a slightly stronger level is chosen as
the threshold PrH at which handover has to
be made
A handover margin, ∆= PrH − Pr(min) is
defined and it cannot be too large or too
small
If ∆ is too large, unnecessary handovers will
burden the MSC
If ∆ is too small, there may be insufficient
time to complete handover before a call is
lost.
19
Factors affecting Handover process
Transmitted power: The transmission power is different for different
cells and therefore, the handover threshold or the power margin varies
from cell to cell.
Received power: The received power mostly depends on the Line of
Sight (LoS) path between the user and the BS. Especially when the user
is on the boundary of the two cells, the LoS path plays a critical role in
the handover procedure and therefore the power margin depends on the
minimum received power value from cell to cell.
Area and shape of the cell: Apart from the power levels, the cell
structure also a plays an important role in the handover process
Mobility of users: The number of mobile users entering or going out of
a particular cell, also fixes the handover strategy of a cell
20