Cellular phones use cellular networks that are divided into hexagonal cells to improve capacity and spectrum efficiency. Each cell uses a different set of frequencies to avoid interference between adjacent cells. As users move between cells, their connection must be handed off seamlessly to the new cell. Network components like base stations and mobile switching centers work together to manage connectivity and roaming across the cellular network.

2. Speakers
Hasan Mahmood 061-19-324
Shamim Jahan 061-19-346
Nafiur Rahman 061-19-347

3. What’s a Cellular Phone ?
3

4. Why it Calls Cellular ?
Uses hexagonal Cells
4

5. Why Use Cells ?
If there was no cell
 Limited
frequency spectrum for available Mobile
Communication.
 Frequency utilization is inefficient.
 Single high power antenna causes to be the
limited number of users.
5

6. Why Use Cells ?
 Advantages of cell structures:
More capacity due to frequency reuse.
Less transmission power needed.
Deals interference, transmission area locally.
 Problems:
Fixed network needed for the base stations.
Handover necessary.
Interference with other cells.
6

7. Frequency Reuse
 Adjacent cells are assigned different frequencies
to avoid interference or crosstalk.
 The same frequency is reused in different areas
for after a certain distance.
 InGSM networks a frequency reuse pattern with
k = 3, 4, 7, 12 or 21. (k= Frequency reuse factor)
 Each ‘k’ number of cells make a cluster.
7

8. Different Frequency Reuse
 Fixed frequency assignment:
certain frequencies are assigned to a certain cell.
problem: different traffic load in different cells.
 Dynamic frequency assignment:
base station chooses frequencies depending on
the frequencies already used in neighbor cells.
more capacity in cells with more traffic.
8

9. Frequency Reuse 3 & 4
Reuse factor = 3 Reuse factor = 4
3 cell Cluster 4 cell Cluster
9

10. Frequency Reuse 7
Reuse factor = 7
7 cell Cluster
10

11. Adjacent Channel Interference
 Comes from imperfect filters that allow frequency
leakage into the band
 Serious problem if interferer is nearby, near-far
effect
Nearby mobile transmits on a frequency near to
that of a weak mobile
 Base station receivers need high-Q filters to reject
adjacent channel interference.
11

12. Adjacent Channel Interference
12

13. Co Channel Interference
One of the primary forms of man-made signal
degradation associated with digital radio, co-channel
interference occurs when the same carrier frequency
reaches the same receiver from two separate
transmitters.
In a fully equipped hexagonal-shaped cell system,
there is always 6 co channel cell.
13

14. Co Channel Interference
F1 F1
F1
F1 F1
MS
14

15. 6 Co Channel Cell in 1st Tier
15

16. Co Channel Interference
16

17. MS Handoff
When the mean signal (over some predetermined
time) is below some threshold
Max Handoff Gap: 150 ms in the case of inter cell.
17

18. Handoff Procedure
 BTS provides list to MS of available channels in
neighboring cells via BCCH (Broadcast Control
Channel)
 MS monitors RSS from the BCCHs of the neighboring
cells and reports it to MSC using SACCH
 BTS also monitors RSS of MS.
 Proprietary algorithms are used to decide when to
handoff.
 MSC negotiates new channel with the new BSS and
indicates to the MS to initiate a handoff .
 Upon completion of handoff MS informs the MSC.
 Old BSS is informed to release the resources.
18

19. Signal Strength
Signal Strength
(in dB)
Cell i Cell j
-60 -60
-70 -70
-80 -80
-90
-90 -100
-100
19

20. Problems of Handoff
 High speed vehicles can cross many “small” cells
in a short time.
Umbrella cell. Large cell with a powerful tower
to handle high speed vehicles
 Another problem is called cell dragging.
Happens when the user moves slowly away from
the cell and the tower didn’t recognize it due to
strong average signal.
20

21. Types of Handoff
 Between two BTS under same BSS
 Between two BTS under different BSS but same
MSC
 Between two BSS under different MSCs
(Old MSC continues to handle the call)
 Between two MSCs in different networks
(providers)
21

22. Next Speaker
Shamim Jahan
22

23. Sectored Cells
 To reduce the adjacent channel interference & co-
channel interference a cell can be divided into 3
or 6 sectors according to the demand.
Large cell (low
density)
Small cell (high
density)
Smaller cell
(higher density)
23

24. 3 sectors per cell
 3 cells per cluster
1 1
2 2 2
2 2
3 3 3
3 3
24

25.  3 sectors per cell
 4 cells per cluster
1 3 1
4 2 4
3 1 3
2 4 2
1 3 1
4 2 4
25

26.  6 sectors per cell
 2 cells per cluster
1 2
2 1 2
2 1
1 2 1
1 2
26

27. Original View of Sectors
27

28. FDM & FDMA
F3
F2
F1
yc ne uqer F
Time
28

29. TDM & TDMA
T1 T2 T3
y
nc
ue
eq
Fr
e du il p mA
Time
t
29

30. CDM & CDMA
Code3
Code2
Code1
y
e nc
qu
e do C
e
Fr
Time
30

31. Frequency Spectrum
z
~96 0MH
5
lin k 93
n
Dow
H z
0 ~9 15M
lin k 89
Up
BTS
MS
31

32. Channels
 GSM 900 uses 25 MHz frequency spectrum
with 124 channels for uplink & downlink.
 Each channels have 200 KHz bandwidth.
 Each two channels are separated by 1.6 KHz
guard band.
 Each channel have 8 time slots which allows 8
subscribers to use same frequency by TDMA.
32

33. Channels
Channels with 200 KHz bandwidth
1 2 3 4 5 6 123 124
1 2 3 4 5 6 7 8
Time slots
4.615 ms
33

34. Cell Types
Cells can be classified into three types
according to cell size
 Macro cell
 Micro cell
 Pico cell
34

35. Cell Types based on Range
35

36. Cell Types based on Range
36

37. Next Speaker
Nafiur Rahman
37

38. Types of Antenna
There are three Types of antennas:
 Omni-directional
 Semi-directional
 Highly-directional
38

39. Omni-Directional Antenna
The dipole is an omni-directional antenna, because
it radiates its energy equally in all directions
around its axis. The signal from an omni-
directional antenna radiates in a 360-degree
horizontal beam.
Dipole Doughnut Dipole Side
View
39

40. Omni-Directional Antenna
40

41. Semi-Directional Antenna
Semi-directional antennas often radiate in a
hemispherical or cylindrical coverage pattern. Semi-
directional antennas types frequently used with
wireless LANs are Patch, Panel, and Yagi antennas.
41

42. Usage of Semi-Directional Antenna
Semi-directional antennas are ideally suited for short
& medium range bridging. A corridor, or a large
room, a semi-directional antenna would be a good
choice to provide the proper coverage.
Coverage area of semi directional antenna.
42

43. Semi Directional Antenna
Semi directional antenna
43

44. Directional Antenna
44

45. Highly-Directional Antenna
Highly-directional antennas emit the most
narrow signal beam of any antenna type &
have the greatest gain of these three groups
of antennas. Highly-directional antennas are
typically concave, dish-shaped devices.
Highly Directional Parabolic Dish Antenna & Grid Antenna
45

46. Usage of Highly-Directional Antenna
These antennas are used for point-to-point
communication links, and can transmit at
distances up to 25 miles. Highly directional
antennas have a very narrow beam width and
must be accurately aimed at each other.
Radiation Pattern of Highly Directional Antenna
46

47. LOS (Line of Sight)
Directional antenna always propagate signal
through LOS. Basically LOS are two types:
1.Visual LOS
2.RF LOS
47

48. LOS
48

49. Fresnel Zone
Typically 40% fresnel zone blockage introduce a
manageable amount of interference into the link.
But more then 40% is means the link will be
unreliable.
Allowing no more then 20% blockage of fresnel
zone. When system is design that time we have to
keep it 0%.
49

50. Fresnel Zone
50

51. GSM 2G Architecture
NSS
BSS
E PSTN
Abis
A
PSTN
B
BSC C
MS MSC GMSC
D
BTS VLR
H
HLR
AuC
BSS  Base Station System NSS  Network Sub-System
BTS  Base Transceiver Station MSC  Mobile-service Switching Controller
BSC  Base Station Controller VLR  Visitor Location Register
MS  Mobile Station HLR  Home Location Register
AuC  Authentication Server
GMSC  Gateway MSC
51

52. Data Transfer Over GSM
Four enhancements to GSM for data
 HSCSD: High Speed Circuit Switched Data
 GPRS: General Packet Radio Service
 EGPRS: Enhanced GPRS
 EDGE: Enhanced Data rates for the GSM
Evolution
 Technically EGPRS and EDGE are same.
52