mobile communication basic

1. Principles of
Cellular
Communication

2. • Cellular Terminology
• ‰Cell Structure and Cluster
• ‰Frequency Reuse Concept
• ‰Cluster Size and System Capacity
• ‰Method of Locating Co-channel Cells
• ‰Frequency Reuse Distance
• ‰Co-channel Interference and Signal Quality
• ‰Co-channel Interference Reduction Methods

3. Conventional Mobile Communication
• Large coverage area.
• Single high powered transmitter on tall tower
• Had very limited capacity since frequencies could not be
reused due to interference.
 High power consumption
 Low capacity
 Large size of the mobile

4. System Design Problem?
• Main limitation of a conventional mobile
wireless communication system is:
 Limited availability of FREQUENCY
SPECTRUM !!!
So the big challenge is …
To serve large number of mobile users ..
 within limited allocated frequency
spectrum with a specified system
quality

5. Mobile Radio systems
• Replace single, high power transmitter
(large cell) with many low power
transmitters (small cells), each providing
coverage to only a small portion of
coverage area.

6. Cellular Telephone System

7. Characteristic
• Limited number of users
• Large Geographic area
• Limited frequency spectrum
• High capacity is achieved by limiting
coverage of each BS into small
geographical region called “cell’’.

8. Cellular Systems

9. Cellular concept
• High capacity is achieved by limiting coverage of
each BS into small geographical region called “cell’’.
• Each cellular base station is allocated a group of
radio channels within a small geographic area called
a cell.
• A cell is the basic geographic unit of a cellular
system.
• It is also called a footprint.
• Neighboring cells are assigned different channel
groups.

10. Cell Structure (Cell Shape)
 Determined by the desired received signal level
by the mobile subscribers from its base station
transmitter in its operating area.
o Ideal, actual and fictitious cell models

11. Cell with BS and MS

12. Cell Structure (Cell Shape)
 Determined by the desired received signal level
by the mobile subscribers from its base station
transmitter in its operating area.
o Ideal, actual and fictitious cell models

13. Geometric Cellular Structures

14. Hexagonal cells Geometry
• Offers best possible non-overlapped cell radio
coverage.
• Multiple hexagons can be arranged next to each other.
• For a given radius (largest possible distance between the
polygon center and its edge), the hexagon has the largest
area.
• Approximating a circular radiation pattern for an
omni-directional base station antenna and free space
propagation
• Simplifies the planning and design of a cellular
system
o

15. A Cellular Cluster
• A group of cells that use a different set of frequencies in
each cell
• The N cells which use the complete set of channels is
called cluster.
• Only selected number of cells can form a cluster.
• Can be repeated any number of times in a systematic
manner.
• The cluster size is the number of cells
within it, and designated by N
o

16. Hexagonal Cluster Patterns

17. System Capacity & Spectrum Utilization?

18. Possible Solution – Frequency Reuse
• Reuse allocated RF spectrum or a given set of
frequencies (frequency channels) in a given large
geographical service area without increasing the
interference.
• Divide the service area into a number of
small areas called cells.
• Allocate a subset of frequencies to each cell .
• Use low-power transmitters with lower height
antennas at the base stations

19. Frequency Reuse
 Frequency reuse is the core concept of cellular
communications
 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.
Two main factors to consider during frequency
planning:
–Interference b/w co-channels
–Capacity

20. Frequency Reuse
For Cluster size (N)=7

21. Frequency Reuse
For Cluster size (N)=7

22. Technical Issues
Technical issues for proper design and planning of a
cellular network:
 Selection of a suitable frequency reuse pattern
 Physical deployment and radio coverage modeling
 Plans to account for the expansion of the cellular
network
 Analysis of the relationship between the capacity,
cell size, and the cost of the infrastructure

23. Cluster Size and Cell Capacity
Consider a cellular system which has a total
of S duplex channels.
Each cell is allocated a group of k channels,
k<S.
The S channels are divided among N cells.
 The total number of available radio
channels
S=kN
The N number of cells in a cluster
(cluster size) use the complete set of
available frequency channels,S.

24. Cluster Size and Cell Capacity
• The cluster can be repeated M times
within the system. The total number of
channels, C, is used as a measure of
capacity.
C=MkN=MS (S=kN)

25. Cluster Size and Cell Capacity
Cluster size: N is typically 4, 7 or 12 for
hexagonal cell shape.
If N=4 and cellular system has total 660
channels available, so total number of
channels available per cell=660/4=165
channels.
For the same cell size at a given area.
If “N” is reduced while the cell size is kept
constant, more clusters are required to
cover a given area, and hence more

26. Method to Locate Co-channel Cells
• To locate the nearest co-channel cell of a
particular cell, one must do the following
operation:
 Step 1: Move i cells along any side of
a hexagon.
 Step 2: Turn 60 degrees anticlockwise.
 Step 3: Move j cells
• where i and j are shift parameters and can
have integer value 0, 1, 2, 3, and so on …

27. Co-channel Cells for i =3, j = 2

28. Re-use Distance

29. Re-use Distance
• The distance between the co-channel cell
in adjacent cluster is given by:
• The geometry of hexagons is such that the
number of cells per cluster, N, can only
have values which satisfy the following eqn
:
where i and j are non-negative integers.

30. Co-channel Re-use Factor (Q)
• The frequency reuse ratio, Q is also referred as
 Co-channel reuse ratio
 Co-channel reuse factor
 Co-channel interference reduction
factor
• For Hexagonal cells the re-use distance(D) is
given by