WIRELESS COMMUNICATIONTHE CELLULAR CONCEPT- SYSTEM DESIGNFUNDAMENTALSLECTURE 4
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The Cellular Concept
1.Introduction
2.Frequency Reuse
3.Channel Assignment Strategies
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Early Mobile Telephone Systems
One high-power transmitter was used to cover a large area---approx. 50km. Located at a very high spot.
The mobiles were
simultaneously connected
using different Frequency
channels.
Capacity of such systems
was very limited.
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Basics
Early mobile telephony systems were not cellular. Coverage over a largearea was provided by a high powered transmitter mounted on a tall tower.Frequency reuse was not employed. That resulted in very low capacity.
The cellular concept arose from the need to restructure the radio telephonesystem with the increase in demand. The increase in demand could not besatisfied just by additional spectrum allocations.
Cellular Concept: Replace large transmitters with many smallertransmitters. Neighboring base stations (BS) are assigned different sets ofchannels. Capacity can be increased by additional partitions.
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Cell – a geographical area covered by a BS.
Frequency Reuse – the frequency channels allocation scheme.
For convenience, the cells are shown with a hex pattern. A hex pattern isthe simplest pattern that can tessellate an area.
In practice, cells are not hexagonal and BS are not exactly in the center ofthe cell.
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An Example of a Cellular Cluster
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Capacity Computations
Assume there are N cells, each allocated k different frequency channels.These N cells are said to form a cluster. Total number of channels percluster is given by
S = k N
Total capacity associated with M clusters:
C = M k N = M S
A cluster may be replicated more times in a given area if the cells are madesmaller (note that power needs to be reduced accordingly).
Capacity of cellular system is directly proportional to “M”, number of times acluster is replicated.
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Capacity versus interference forsame size cell
Decrease N for More Capacity: If Cluster Size, N is decreasedwhile cell size remains fixed, more clusters are required to cover thearea (M increases). Therefore, Capacity increases.
Increase N for Less Interference: On the other hand, if N isincreased (large cluster size) means that co-channels are nowfarther than before, and hence we have will have less interference.
Value of N is a function of how much interference a mobile or a basestation can tolerate.
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Frequency Reuse
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Frequency Reuse
Geometry of the hexagonal cells is such that to cover adjacent areascompletely, N can have only some of the values.
N should satisfy the relationship,
where i and j are positive integers. Typical values of N are 4,7,12etc.
To draw the cell pattern given i and j:
1.Move i cells across hexagons.
2.Turn 60º CCW and move j cells.
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Frequency Reuse (N=7, i=2, j=1)
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Frequency Reuse (N=19, i=3, j=2)
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Example:A total of 33 MHz are allocated to a system which uses 2x25 kHz forfull duplex (i.e., each channel is 50 kHz). What is the number ofchannels per cell?
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Number of channels per system
Now assume 1 MHz of the 33 MHz is allocated to control channels.Each control channel is still 50 kHz Total number of voice (traffic)channels is now
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Channel AssignmentStrategies
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Channel Allocation Techniques
To satisfy the user, a channel needs to be available on request.
Reasonable probability of call blockage (GOS) is 2%.
GOS fluctuate with location and time.
The goal is to keep a uniform GOS across the system.
Reduction of variations in GOS allow more users – an increase incapacity.
Three types of algorithms for channel allocation:
Fixed channel allocation (FCA)
Channel Borrowing
Dynamic channel allocation (DCA)
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Fixed Channel Allocation Techniques
Available spectrum is W Hz and each channel is B Hz. Total numberof channels:
Nc = W/B
For a cluster size N, the number of channels per cell:
Cc = Nc/N
To minimize interference, assign adjacent channels to different cells.
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Features of Fixed Channel AllocationTechniques
FCA is the optimum allocation strategy for uniform traffic across thecells.
A non uniform FCA strategy, when it is possible to evaluate GOS inreal time and adjust the FCA accordingly. This requires a morecomplex algorithm.
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Channel Borrowing
Borrow frequencies from low traffic cells to high traffic cells.
Temporary channel borrowing: channel is returned after call iscompleted.
If all the channels in a cell are occupied, channels are borrowedfrom neighboring cells.
The MSC supervises such borrowing procedures and ensuresdisruption free service.
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Dynamic Channel Allocation
All channels are placed in a pool, and are assigned to new callsaccording to the reuse pattern. Signal is returned to the pool, whencall is completed.
Issues related to channel allocation are still under research.
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Comparison of Channel AllocationTechniques
Fixed Channel Allocation
Advantages:
Less load on MSC
Simple
Disadvantages:
Blocking may happen
Dynamic Channel Allocation
Advantages:
Voice channels are not allocated permanently. That is shared on need-basis
Disadvantages:
Requires MSC for processing---burden on MSC
May be very complicated
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Hand off
HANDOFF: The process of transferring a call across the cellboundaries.
Handoffs are prioritized over new calls.
Handoffs need to be performed infrequently.
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Assignment # 1:
Due date – 02 February, 2012
Problems: 1.3, 1.9, 1.10, 1.13, and 1.18,
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