1. Cellular Concepts
المحاضرة السادسة
03/07/2015
Omar Abu-Ella

2. Introduction
The cellular concept was a major breakthrough in solving the problem of spectral congestion and user capacity.
High capacity in a limited spectrum.
The cellular concept has the following system level ideas
Replacing a single, high power transmitter with many low power transmitters, each providing coverage to only a small area.
Neighboring cells are assigned different groups of channels in order to minimize interference.
The same set of channels is then reused at different geographical locations.
03/07/2015
Omar Abu-Ella

3. Frequency Reuse
Reuse if only total interference from all users in the cells using the same frequency (co-channel cell) is sufficiently attenuated.
Factors need to be considered include:
Geographical separation (path loss)
Shadowing effect
03/07/2015
Omar Abu-Ella

4. Cell Footprint
For systematic cell planning, a regular shape is assumed for the footprint.
Coverage contour should be circular, but it is impractical because it provides ambiguous areas with either multiple or no coverage.
Hexagon shape chosen for economic reasons, because it has the maximum coverage area.
03/07/2015
Omar Abu-Ella

5. A cellular system which has a total of S duplex channels
A cellular system which has a total of S duplex channels. S channels are divided among N cells, with each cell uses unique and disjoint channels. If each cell is allocated a group of k channels, then S = k N .
03/07/2015
Omar Abu-Ella

6. Terminology
Cluster size: The N cells which collectively use the complete set of available frequency is called the cluster size.
Co-channel cell: The set of cells using the same set of frequencies as the target cell.
Interference tier: A set of co-channel cells at the same distance from the reference cell is called an interference tier. The set of closest co-channel cells is called the first tier. There is always 6 co-channel cells in the first tier.
03/07/2015
Omar Abu-Ella

7. Coordinates for hexagonal cellular geometry
With these coordinates, an array of cells can be laid out so that the center of every cell falls on a point specified by a pair of integer coordinates.
03/07/2015
Omar Abu-Ella

8. Designing a cellular system
(i=3, j=2)
03/07/2015
Omar Abu-Ella

9. Designing a cellular system
The cluster size must satisfy: N = i2 + ij + j2 where i, j are non-negative integers.
03/07/2015
Omar Abu-Ella

10. Designing a cellular system
Can also verify that
where Q is the co-channel reuse ratio
What is the proper cluster-size for the system?
03/07/2015
Omar Abu-Ella

11. Handover / Handoff
Occurs as a mobile moves into a different cell during an existing call, or when going from one cellular system into another.
It must be user transparent, successful and not too frequent.
Not only involves identifying a new BS, but also requires that the voice and control signals be allocated to channels associated with the new BS.
03/07/2015
Omar Abu-Ella

12. Handover / Handoff
Once a particular signal level Pmin is specified as the minimum usable signal for acceptable voice quality at the BS receiver, a slightly stronger signal level PHO is used as a threshold at which a handover is made.
03/07/2015
Omar Abu-Ella

13. =handoff threshold  too small: Insufficient time to complete handoff
before call is lost
More call losses
 too large:
Too many handoffs
Problem for MSC
03/07/2015
Omar Abu-Ella

14. Dwell Time
The time over which a user remains within one cell is called the dwell time.
Important parameter for the practical system design.
Depends on:
Speed of the user
Type of radio coverage
03/07/2015
Omar Abu-Ella

15. Umbrella Cells

16. Hard and Soft Handoff
Hard handoff- break before make, in this type connection with the source channel/BTS/BSC is first broken before making connection with target channel/BTS/BSC.
soft handoff- make before break, this type is one in which the channel in the source cell is retained and used for a while in parallel with the channel in the target cell.
03/07/2015
Omar Abu-Ella

17. Handover indicator
Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS. This information is forwarded to the MSC who makes decisions regarding handover.
Mobile assisted handover (MAHO): The mobile station MS measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS.
03/07/2015
Omar Abu-Ella

18. Interference and System Capacity
In a given coverage area, there are several cells that use the same set of frequencies. These cells are called co-channel cells.
03/07/2015
Omar Abu-Ella

19. Interference and System Capacity
The interference between signals from these cells is called co-channel interference.
If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area, the transmit power of each BS is almost equal:
Then, worse case signal to co-channel interference is independent of the transmitted power.
It becomes a function of the cell radius R, and the distance to the nearest co-channel cell D’.
03/07/2015
Omar Abu-Ella

20. R D’ D
03/07/2015
Omar Abu-Ella

21. Received power at a distance d from the transmitting antenna is approximated by
Useful signal at the cell boundary is the weakest, given by Pr (R). Interference signal from the co-channel cell is given to be Pr (D’) .
03/07/2015
Omar Abu-Ella

22. System SIR
D’ is normally approximated by the base station separation between the two cells D, unless when accuracy is needed. Hence
Recall:
the co-channel reuse ratio Q is given by

23. System SIR
If only first tier co-channel cells are considered, then i0 = 6.
Unless otherwise stated, normally assuming Di ≈ D for all i.
03/07/2015
Omar Abu-Ella

24. System SIR
The probability that a mobile station does not receive a usable signal.
For GSM, this is 12 dB and for AMPS, this is 18 dB. If there is 6 co-channel cells, then
For n=4, a minimum cluster size of N=7 is needed to meet the SIR requirements for AMPS.
For n=4, a minimum cluster size of N=4 is required to meet the SIR requirements for GSM.
Exercise : verify that
03/07/2015
Omar Abu-Ella

25. Channel Assignment Strategies
Channel allocation schemes can affect the performance of the system.
Fixed Channel Allocation (FCA) :
Channels are divided in sets.
A set of channels is permanently allocated to each cell in the network.
Dynamic Channel Allocation (DCA) :
Voice channels are not allocated to any cell permanently. All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system.
03/07/2015
Omar Abu-Ella

26. System Expansion Techniques
As demand for wireless services increases, the number of channels assigned to a cell eventually becomes insufficient to support the required number of users. More channels must therefore be made available per unit area.
03/07/2015
Omar Abu-Ella

27. System Expansion Techniques
To increase SIR we should use higher reuse factors Q, i.e., bigger cluster-size N.
This will reduce the number of assigned channels per cell.
So, how to increase the number of allocated channels per unit area?
03/07/2015
Omar Abu-Ella

28. System Expansion Techniques
Accomplished by: Cell-Splitting: Each initial cell divided area into a number of smaller cells. Cell splitting increases the capacity of a cellular system since it increases the number of times that channels are reused.
03/07/2015
Omar Abu-Ella

29. Cell Splitting
cell splitting achieves capacity improvement by essentially rescaling the system. By decreasing the cell radius R and keeping the co-channel reuse ratio (Q=D/R) unchanged, cell splitting increases the number of channels per unit area.
03/07/2015
Omar Abu-Ella

30. Cell Splitting
03/07/2015
Omar Abu-Ella

31. Sectoring:
The co-channel interference in a cellular system may be decreased by replacing a single omnidirectional antenna at the base station by several directional antennas, each radiating within a specified sector. By using directional antennas, a given cell will receive interference and transmit with only a fraction of the available co-channel cells.
By dividing the cell into sectors, we can assign more channels per cell, i.e., by having different sectors in the cell co-channel interference can be reduced, then we can use smaller reuse factors.
03/07/2015
Omar Abu-Ella

32. Sectoring
03/07/2015
Omar Abu-Ella

33. 5G Requirements
03/07/2015
Omar Abu-Ella

34. 5G General Key performance indicators (KPIs)
Source: IEEE Communications Magazine • July 2015
03/07/2015
Omar Abu-Ella

35. New Technologies
03/07/2015
Omar Abu-Ella

36. Heterogeneous Small Cell Networks (HetNet)
03/07/2015
Omar Abu-Ella

37. What would be in a HetNet
03/07/2015
Omar Abu-Ella

38. Cellular Model for HetNet
03/07/2015
Omar Abu-Ella

39. Useful Links:
03/07/2015
Omar Abu-Ella