1. Unit 4 Cellular Telephony

2. Contents
history of cellular telephony design and principle of cellular operation cellular telephony operation analog cellular telephones digital cellular telephones digital networks personal communication systems the third generation recent events in cellular telephony

3. Cellular Telephony System

4. Timing diagram illustrating how a call to a mobile user initiated by a landline subscriber is established.

6. Timing diagram illustrating how a call initiated by a mobile is established

8. Cellular concept

9. Cellular telephone system

10. Cellular telephone system
Cellular telephone system provides a wireless connection to PSTN for any user location within the radio range
A cellular system offers
1.high quality of service
2.high capacity
3.same radio channel may be reused
4.handoff
Each BS have limited coverage area called cell

11. Communication between BS & MS is defined by use of two types of channels:
Control Channels
Voice channels

12. Cellular network Regions divided into “cells” modeled as hexagons.
Each cell has a base station that all cell phones within the cell communicate with.
Communication uses both data and control channel.

13. IDEAL ACTUAL DIFFERENT CELL MODEL
CELL SHAPE
IDEAL ACTUAL DIFFERENT CELL MODEL

14. Cellular network Why hexagon
1) it avoids dead spots & full area coverage is achieved
2) it requires fewer cell
3) it requires less transmitter sites
4) less expensive
Group of cells is called cluster

15. Clusters The cells are grouped into clusters.
The total number of channels are equally distributed among all cells in a cluster.
The channel allocation pattern is repeated in every cluster. 1 2 3 4 5 6 7
Cluster size
N = 7

16. Clusters (2) 1 2 4 1 3 1 2 4 2 4 3 3 1 4 2 4 3 1 3 2 4 3 1
Cluster size
N = 4 2

17. Clusters (3) We cannot select cluster size arbitrarily.
Cluster sizes must satisfy the following:
N = i2+ij+j2 where i and j are non-negative integers.
Thus N = 1, 3, 4, 7, 9, 12 …

18. 1.Frequency reuse
Users in different geographical areas (in different cells) may simultaneously use the same frequency
Frequency reuse drastically increases user capacity and spectrum efficiency

19. Frequency Reuse…
Process of selecting & allocating channel groups for all of the cellular base stations within a system is called frequency reuse or frequency planning.
To keep interference level within tolerable limits.

20. FREQUENCY REUSE
The frequency spectrum is a limited resource. Therefore, wireless telephony, like radio, must reuse frequency assignments.
For example, two radio stations might transmit at 91.3 FM. There is no interference as long as the radio stations are far enough apart.
Problem: Limited frequency spectrum
Solution: Based on the idea of splitting up coverage region
into small areas referred to as cells

21. Frequency reuse
Each cellular BS station allocated a group of radio channel to be used within small geographic area called as cell
Adjacent BS are allocated different group of channel
Antenna at BS are designed to achieve desired coverage in same cell
The same group of channel may be used to cover different cell that are separated from one another by distance large enough to keep interference level min
The process of selecting & allocating channel groups for all cellular BS is called frequency re-use

22. Frequency reuse distance

23. Frequency reuse

24. Fixed channel assignment strategies:
Each cell is allocated a predetermined set of voice channel
Any call attempt within the cell can only be served by the unused channel in that particular cell
If all the channel in that cell are occupied, the call is blocked & subscriber does not Rx call

25. 2.channel assignment
Why we need channel assignment ? - to utilize the spectrum efficiently - reuse the frequency - increase the capacity - minimize interference There are Two types of channel assignment strategies - fixed or dynamic - borrowing strategies

26. Borrowing strategies:
A cell is allowed to borrow channel from a neighboring cell if all of its own channel are already occupied
MSC supervised the borrowing procedure
Borrowing does not disrupt or interfere with any of the calls in progress in the donor cell

27. Dynamic channel assignment:
Voice channels are not allocated to different cell permanently
Instead each time a call request is made the BS request a channel from MSC
The switch then allocates a channel to be requested cell following the algorithm

28. 3.Handoff
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.

29. Hard Handoff :

30. Soft Handoff :

31. Umbrella Cell Approach
Low speed users can be handled by microcells or picocells.
High speed users are handled by a larger cell macrocell which is co-located with smaller cells.
Speed of MS can be estimated by MSC by observing the rate of change of signal strength.

32. Macro-cellular Micro-cellular Large cells(5-20km radius)
Higher TX power
BS is expensive
Lesser back-haul cost
Low capacity
Supports mobility
Low bit rates
Smaller cells(200m to 2km)
Lower TX power
BS is much cheaper
Higher back-haul cost
High capacity possible
Pedestrian & fixed use
High bit rates
Macrocell
Microcell

33. Interference and System Capacity
Sources of interference
another mobile in the same cell
a call in progress in the neighboring cell
other base stations operating in the same frequency band
Non-cellular system leaks energy into the cellular frequency band
Two major cellular interference
co-channel interference
adjacent channel interference

34. Cell Splitting Split congested cell into smaller cells.
Preserve frequency reuse plan.
Reduce transmission power.
Reduce R to R/2
microcell

35. Practical Handoff Consideration

36. Expanded Signal (Increased Bandwidth)
Applying Spread Spectrum Technique
Expanded Signal (Increased Bandwidth)
Original Signal (Narrow Bandwidth)
Recovering back Original Signal

37. Personal Communication System
Current state of cellular telephony
Cellular telephony
Utilizes frequency in range 800 MHz
Has both Analog and Digital capabilities
PCS
Operates in 1900 MHz range
It is purely digital
Need:
To deal with explosive growth of cellular telephony
To avoid overloading in 800 MHz range

38. Advantages:
PCS is developed as a digital tool that provides user with:
Voice communication
Paging service
Fax service
Mobile connection to internet

39. Contents
history of cellular telephony design and principle of cellular operation cellular telephony operation analog cellular telephones digital cellular telephones digital networks personal communication systems the third generation recent events in cellular telephony

40. Q. 1 ) Explain public address system in detail.
Q. 2 ) Explain the following terms related with detail N/W :
Signal regeneration
Echo control
Crosstalk.
Q. 3 ) Draw and explain the architecture of Digital Cellular Telephony.
Q. 4 ) Discuss the soft hand-off in brief.
Q. 5 ) What is cell? Explain the cell structure in detail.
Q. 6 ) Explain any three specification of 3G in details.
Q . 7 ) Explain the principle of operation of cellular telephony.
Q . 8 ) Explain the medium available for connecting the cell site to MSC.