- Cellular networks address the problem of limited spectrum availability by using frequency reuse, where nearby base stations are assigned different channels to avoid interference. Cells are arranged in a hexagonal pattern and the same set of channels are reused in cells sufficiently far from each other. - There are two main channel assignment strategies - fixed assignment, where each cell has a predetermined set of channels, and dynamic assignment, where channels are allocated on demand by a central controller considering interference levels. Dynamic assignment helps improve spectrum utilization but requires more complex coordination. - Frequency reuse allows the available spectrum to be reused as needed across multiple cells as long as interference is kept at acceptable levels, increasing network capacity.

1. CELLULAR
CONCEPTS
- FREQUENCY REUSING
- CHANNEL ASSIGNMENT STARTEGIES

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3. Frequency Reusing
 The design objective of early mobile radio systems was to achieve a large coverage
area by using a Single high power transmitter on a tall tower.
Drawbacks:
 Only Less number of calls can be handled simultaneously over a large geographic
region.
 Regulatory organizations cannot allocate spectrum for the increasing demand of
mobile users.
 These drawbacks lead ways to restructure the cellular architecture. Thus the single high power antennas are replaced with
many low power antennas each providing coverage to small region called Cells.
 So each Base station will be allocated a set of channels to manage calls and the nearby base stations are allocated a different set
of channels so as to avoid interference.
 Doing so will result in spectrum scarcity within a small region. This problem is addressed by a concept called FREQUENCY
REUSING or FREQUENCY PLANNING.
“By systematically spacing the base stations and their channel groups throughout a market, the available channels can be reused as
many times as necessary so long as the interference between the channel stations are kept under acceptable levels”
 The figure represents the concept of cellular frequency reuse, where cells labelled with same letters use same frequency group.
 Hexagon cell shape is universally used for representing a cell because they closely approximates a circle and also they permit
easy and manageable analysis of cellular systems.
 The actual radio coverage in a cellular system is called as “Foot Prints” which are determined by field measurements and
Propagation Prediction models.
 These cells can be of two types
 Centre excited cell – Base stations are placed at the centre of the cell
 Edge Excited Cell – Base stations are placed at the edges of the cell.

4. Understanding Frequency Reusing Concept
Let us consider,
S – Total available duplex channels
k – Each cell is allocated a group of k channels (k<S)
When these S channels are to be divided among N Cells into
Unique and Disjoint channel groups, then the total number of
radio channels available is expressed as
S=k.N
The N is the collective number of cells uses a complete set of
available frequencies is called CLUSTER. If a cluster is repeated
for M number of times in a system, then the total number of
duplex channels C can be expressed as
C=MkN = MS
N is typically called as the Cluster size and is generally given
by
N=i2+ij+j2
Where, i and j are non negative integers. Usually N take the
value such as 4,7 or 12 etc.,
Large Cluster size indicates  ratio between the cell radius
and distance between the co channel cells are small
Small Cluster size indicates  the co channel cells are located
much closer to each other
To find the nearest co channel neighbours of a
particular cell:
1. Move i cells along any chain of hexagons
2. Turn 60degrees counter clock wise and move j cells
Following figure illustrates the co channel for i=3 and
j=2 (N=19)

5. Channel Assignment Strategies
• For efficient utilization of the radio spectrum, a frequency reuse scheme that is consistent with the objectives of
increasing capacity and minimizing interference is required.
• Channel assignment strategies can be classified in to
 Fixed channel Assignment Strategies.
 Dynamic Channel Assignment Strategies.
• The selection of channel assignment will have an higher impact on the performance of the system when the user
is handed off from one cell to another cell.
 Fixed Channel Assignment Strategies:
• Each cell is allocated with a predetermined set of voice
channels.
• Any attempt to place a call should be done within the
allocated spectrum of channel.
• If all the channels in that cells are occupied then the new call
is BLOCKED and the subscriber does not receive any
services.
• To overcome this drawback several variations in the fixed
channel assignment were proposed.
• One among the variation is called “Borrowing Strategy”
which allows the cell to borrow a channel from
neighbouring cell.
• MSC supervises such borrowing facilities without
introducing interference among the cells.
 Dynamic Channel Assignment Strategies:
• in dynamic channel assignment, channels are not not
permanently allotted.
• Instead, each time the call request is made, the
serving base station requests the channel to MSC.
• MSC assigns the channel using an algorithm which
considers
• Future blocking within the cell.
• Frequency of use of the candidate channel
• Reuse distance of the channel
• And other cost functions.