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Channel Assignment

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Channel Assignment
• Objective to increaqse capacity and minimizing interference.
• Need to assign frequencies to users/cell
• Fixed
– Each cell has predetermined number of channels
– If all channels used, cell is “blocked”, no service
– Cells can borrow channels from neighboring cells
– The mobile MSC supervisees borrowing procedure and ensure that does
not disrupt or interfere with any of the calls in progress in the donor cell
• Dynamic
– No permanent allocation
– Frequency band requested from MSC, assigned using reuse distance,
blocking probabilities, use of channel, signal strength
– Advantages:
• Increases capacity
• Increases channel utilization
– Disadvantages:
• Increased computational load
1
Interference
• Major limiting factor in the performance of cellular
radio system
• Sources are:
–
–
–
–
Another mobile in the same cell
A call in progress in a neighboring cell
Other BS operating in the same freq. band
other systems which inadvertently leak energy into the
cellular frequency band
• Voice channel  cross talk ( subscriber hears interference in
the background due to undesired transmission)
• Control channel missed/blocked calls (due to errors in
digital signaling)
• Interference is more in urban areas, due to the greater RF noise floor.
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Interference and Capacity
• Two Major type of interference
• Co-channel interference
• Interference from cells using the same frequency group in
a cluster
• Cannot simply increase SNR to combat it
• Co-channel cells have to be physically separated to provide
isolation
• Adjacent channel interference
– Signal in adjacent frequency band
– Signals from other cell companies
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Co-Channel Interference
• In given coverage area there are several cells that use the
same set of frequencies.
• These cells are called co-channel cells.
• Interference from cells using the same frequency group in
a cluster
• Cannot simply increase SNR to combat it.
• Due to increase in transmit power increase the
interference to neighboring co-channel cells.
• Co-channel cells have to be physically separated to
provide isolation
• The minimum distance that allows the same frequency to
be used depends on many factor
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Re-use distance
Such as number cells in the vicinity of the centre cell, the type
of geographic terrain, antenna height, and transmitted
power at each cell
It is a function of cell radius (R) and distance to the center of
the nearest cell (D)
The mean distance (D) between cells using the same
frequencies.
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Hexagon Cell
Consider a single hexagon cell
Hexagon cell
Radius R
120o
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Hexagon Cell
The distance D is the mean distance between
cells using the same frequency sets. The
ratio of the D to R is expressed in
terms of cluster size N.
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Co-Channel Reuse Ratio
• The Co-channel Reuse Ratio, Q, is defined as:
D
Q   3N
R
• Increasing Q increases the spatial separation between
co-channel cells; however, it also increase N thereby
decreasing capacity
• A small value of Q provides large capacity since the
cluster size N is small,
• Large value of Q improve the quality, due smaller
level of co-channel interference
• Tradeoff must be made between Q and N
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Smaller N is greater capacity
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