Cellular networks divide a large geographic service area into smaller cellular regions or "cells" to improve spectrum efficiency and increase user capacity. Each cell uses a subset of available radio frequency channels and base stations operate at low power, reducing interference between cells using the same channel. By reusing the same set of frequencies in cells separated by a minimum distance, the available spectrum can be reused throughout the system. The ratio of the distance between co-channel cells to the cell radius is known as the frequency reuse ratio or factor.

1. CC531: Advanced Computer Networks
SECTION 5

2. Cellular Networks
• The cellular concept was a major breakthrough in solving the problem of spectral congestion and
user capacity.
• It offered very high capacity in a limited spectrum.
• Each cellular base station is allotted a group of radio channels to be used within a small geographic
area called cells.
• Use multiple low-power transmitters.
• Areas divided into cells. Each served by its own antenna
• Served by base station consisting of transmitter, receiver, and control unit
• Band of frequencies allocated
•Cells set up such that antennas of all neighbors are equidistant (hexagonal pattern)

3. Cellular Network architecture

4. Cellular Network architecture

5. Hexagonal cellular coverage
• Offers best possible non-overlapped cell radio coverage.
• Multiple hexagons can be arranged next to each other.
• The hexagons covers the large area.
• Simplifies the planning and design of the cellular system.

6. Hexagonal cellular Clusters
•A group of cells that use a different set of
frequencies in each cell.
• 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 with in it
and designated by N

7. Mobile communication
•Main limitations of a conventional mobile wireless communication system is:
• Minimum availability of FREQUENCY SPECTRUM!!!
•So the big challenge is, to serve large no. of mobile users within limited allocated frequency
spectrum with a specified system quality.
•How to increase the capacity and Spectrum utilization?
•We need optimum spectrum usage.
• More capacity.
• High quality of service.
• Low cost.

8. Frequency Reuse
•Reuse allocated RF spectrum or a given set of frequencies 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.
•Large coverage area, efficient spectrum utilization and enhanced system capacity are the major
attributes of cellular communications.
• 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.

9. Frequency Reuse

10. Frequency Reuse
Co-channel and adjacent cells
• Cells, which use the same set of frequencies are referred to as co-channel cells.
• Co-channel cells are located sufficiently physically apart, so as not to cause co-channel
interference.
• The space between adjacent co-channel cells filled with other cells that use different
frequencies to provide frequency isolation.
• Technical issues for proper design & planning of a cellular network:
• Selection of a suitable frequency reuse pattern.
• Analysis of the relationship between the capacity, cell size & the cost of the infra structure.

11. Frequency Reuse
Cluster size and cell capacity
• In a cellular system, the whole geographical service area is divided into a number of clusters
having finite number of cells.
• The K number of cells in a cluster use the complete set of available frequency channels, N.
• Each cell in the cluster contain J = (N/K) number of channels only.
∴ 𝑁 = 𝐽 × 𝐾; 𝐽 ≤ 𝑁
• The cluster can be replicated many times to cover the desired geographical area by a cellular
communication system.
• Let M be a number of clusters in the system, then overall system capacity, C is given as:
𝐶 = 𝑀 ∗ 𝑁 ⟺ 𝐶 = 𝑀 ∗ 𝐽 ∗ 𝐾
• When K is reduced, J is proportionally increased since N = J * K is constant.
• To increase co-channel interference!

12. Frequency Reuse
Co-channel cells
• Cells which use the same set of frequencies are referred to as co-channel cells.
• The interference between co-channel cells is referred to as co-channel interference.
• The space between adjacent co-channel cells are filled with cells using different frequencies.
• Reusing an identical frequency channel in different cells is limited by co-channel interference
between cells.
• The co-channel interference can become a major problem in cellular communication.
• It is desirable to find the minimum frequency reuse distance D in order to reduce this co-
channel interference.

13. Frequency Reuse
Co-channel cells
• Factors Which Influence ‘D’:
• The number of co-channels in the vicinity of the center cell.
• The antenna height
• The transmitted power at each cell site
•NOTE: As long as the cell size is fixed, co-channel interference is independent of transmitter
power of each cell.
• The frequency reuse ratio, q is also referred to as:
• The co-channel reuse ratio.
• The co-channel reuse factor.
• Co-channel interference reduction factor.

14. Frequency Reuse Ratio
• The real power of the cellular concept is that interference is not related to the absolute distance
between cells but related to ratio of the distance between co-channel (same frequency) cells to
the cell radius.
• The frequency reuse ratio is given as:
𝑞 =
𝐷
𝑅
= 3 𝑁

15. Frequency Reuse
• Advantages:
• The frequency reuse system can increase the spectrum efficiency, thereby increasing the system
capacity.
• Disadvantages:
• If the system is not properly designed , co-channel interference may occur due to the
simultaneous use of the same channel.

16. Co-channel interference
•The interference between signals from co-channel cells channel cells.
• Unlike thermal noise which can be overcome by increasing the signal-to- noise ration (SNR), co-
channel interference cannot be combated by simply increasing the carrier power of a
transmitter.
• This is because an increase in carrier transmit power increases the interference to neighboring
co-channel cells.
• Adjacent co-channel interference: Interference resulting from signals which are adjacent in
frequency to desired signal.
• This results from imperfect receiver filters which allow nearby frequencies to leak into the pass
band.

17. Co-channel interference
•Reducing co-channel interference:
• Careful filtering and channel assignment
• By keeping the frequency separation between each channel in a given cell as large as possible
• By sequentially assigning successive channels in the frequency band to different cells.
• signal to interference ratio is given as:
𝑆
𝐼
=
𝐷
𝑅
𝑛
𝑖0
=
3𝑁
𝑛
𝑖0

18. Sheet 3
Question 6:
•In a typical mobile phone system with hexagonal cells, using only 3 unique frequencies (A, B and
C), it is forbidden to reuse a frequency band in an adjacent cell. If 840 frequencies are available, how
many can be used in a given cell?

19. Sheet 3
Question 6:
•In a typical mobile phone system with hexagonal cells, using only 3 unique frequencies (A, B and
C), it is forbidden to reuse a frequency band in an adjacent cell. If 840 frequencies are available, how
many can be used in a given cell?
•Solution:
•Each cell has six neighbors. If the central cell uses frequency group A, its six neighbors can use B, C, B, C, B,
and C, respectively.
•In other words, only three unique cells are needed. Consequently, each cell can have 280 frequencies.

20. Sheet 3
Question 7:
•Sometimes when a mobile user crosses the boundary from one cell to another, the current call is
abruptly terminated, even though all transmitters and receivers are functioning perfectly. Why?

21. Sheet 3
Question 7:
•Sometimes when a mobile user crosses the boundary from one cell to another, the current call is
abruptly terminated, even though all transmitters and receivers are functioning perfectly. Why?
•Solution:
•Frequencies cannot be reused in adjacent cells, so when a user moves from one cell to another, a new frequency
must be allocated for the call. If a user moves into a cell, all of whose frequencies are currently in use, the
user’s call must be terminated.
•This operation is known as frequency handover.

22. Sheet 3
Question 8:
• How were computers attached to a thinnet ethernet?

23. Sheet 3
Question 8:
• How were computers attached to a thinnet ethernet?
•Solution:
•The transceiver was integrated to the NIC and a thinner cable ran from one computer to another.

24. Sheet 4
Question 1:
• 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, and
(c) 12-cell reuse.
If 1 MHz of the allocated spectrum is dedicated to control channels, determine an equitable distribution of
control channels and voice channels in each cell for each of the three systems.

25. Sheet 4
Question 1:
• 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,
•Solution:
Cluster size (N) = 4
Total bandwidth = 33 MHz
Channel bandwidth = 25 kHz × 2 simplex channel = 50 kHz/duplex channel
Total available channels = (33,000 × 103)/(50 × 103) = 660 channels
Total number of available channel per cell = (Total available channels)/N = 660/4 = 165
we can have five control channels and 160 voice channels per cell, However in practice, one control channel
and 160 voice channels would be assigned to each cell.

26. Sheet 4
Question 1:
• 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:
(b) seven-cell reuse
•Solution:
For N = 7,
Total number of channels available per cell = 660/7 ≈ 95 channels.
We can have four cells with three control channels and 92 voice channels, two cells with three control channels
and 90 voice channels, and one cell with two control channels and 92 voice channels could be allocated.
In practice, however, each cell would have one control channel, four cells would have 91 voice channels, and
three cells would have 92 voice channels.

27. Sheet 4
Question 1:
• 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:
(c) 12-cell reuse.
Solution:
For N = 12,
Total number of channels available per cell = 660/12 ≈ 55 channels.
we can have eight cells with two control channels and 53 voice channels, and four cells with one control
channel and 54 voice channels each.
In an actual system, each cell would have one control channel, eight cells would have 53 voice channels, and
four cells would have 54 voice channels.

28. Sheet 4
Question 2:
• 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 6 co-channels cells in the first tier, and all of them are at the same distance
from the mobile. Use suitable approximations.

29. Sheet 4
Question 2:
• 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
Solution:
For the case with n = 4
First, let us consider a 7-cell reuse pattern.
Using equation 𝑄 =
𝐷
𝑅
= 3𝑁 ∴ the co-channel reuse ratio D/R = 4.583.
Using equation :
𝑆
𝐼
=
𝐷
𝑅
𝑛
𝑖0
=
3𝑁
𝑛
𝑖0
The Signal-to-Noise Interference ratio is given by: S/I = 4.583 4
6 = 75.3 = 18.06 dB.
Since this is greater than the minimum required S/I, N = 7 can be used.

30. Sheet 4
Question 2:
• 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
b) n = 3
Solution:
First, let us consider a 7-cell reuse pattern. Using 𝑄 = 𝐷
𝑅 = 3𝑁
the co-channel reuse ratio D/R = 4.583. Using
𝑆
𝐼
=
𝐷
𝑅
𝑛
𝑖0
=
3𝑁
𝑛
𝑖0
Hence, Signal-to-Noise Interference ratio is given by: S/I = 4.583 3
6 = 16.04 = 12.05 dB
Since this is less than minimum required S/I, we need to use a larger N. Using 𝑁 = 𝑖2
+ 𝑖𝑗 + 𝑗2
, the next possible value of N is 12,
(i=j=2).
The corresponding co-channel ratio is given by 𝑄 = 𝐷 𝑅 = 3𝑁 = 3 × 12 = 6.0
Then the signal-to-interference ratio is given by S/I = (1/6) x 63
= 36 = 15.56 dB
Since this is greater than the minimum required S/I, N = 12 can be used.

31. Sheet 3
Question 3:
• If a cellular system has p number of co-channel interfering cells, S is the desired power from serving
base station and Ip is the interference power from the pth interfering co-channel cell base station.
Then what is the signal to interference ratio for a mobile receiver in the functioning cell?

32. Sheet 3
Question 3:
• If a cellular system has p number of co-channel interfering cells, S is the desired power from serving
base station and Ip is the interference power from the pth interfering co-channel cell base station.
Then what is the signal to interference ratio for a mobile receiver in the functioning cell?
Solution:
The signal to interference ratio for a mobile receiver in the functioning cell is given by:
𝑆
𝑖=1
𝑃
𝐼𝑃
S : the desired signal power from the desired base station
Ii : interference power caused by the ith interfering co-channel cell base station
Ip : number of co-channel interfering cells