The Cellular Concept (2)

8/13/2019 The Cellular Concept (2)

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GUNEET KAUR


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To solve problems of spectral congestion anduser capacity.Replacing a single, high power transmitterwith many low power transmitters.

Neighboring base stations are assigneddifferent groups of channels so that theinterference between base stations isminimized.

Available Channels are distributed throughoutthe geographic region and may be reused asmany times as necessary.


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It is the design process of selecting andallocating channel groups for all the cellularbase stations within a system


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Hexagonal cell shape has been universally adopted. Actual radio coverage of a cell - footprintCircles leave gaps

Other Sensible choices are Equilateral Triangle, Square, Hexagon

Base stations can be placed at: The cell center center-excited cells omni-

directionalantennas. The three cell vertices edge-excited cells sectored

directional antennas.


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Fixed Channel Allocation

Dynamic Channel Allocation


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Each cell is allocated a predetermined set ofvoice channels.Any call attempt within the cell can only beserved by the unused channels in thatparticular cell.Probability of blocking is high.


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Channels are not allocated to different cellspermanently.Each time a call request is made, the servingbase station requests a channel from theMSC.Channel allocation algorithm considers Future blocking within a cell Frequency of use

Reuse distance Cost functions.


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DEMERITSMSC has to continuously collect realtime data on

Channel occupancy Traffic distribution RSSI radio signal strength indication) of all

channels

Increased storage and computational load onthe system


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Interference is the major limiting factor in the performanceof cellular radio systems. It is a major bottleneck inincreasing capacity and is often responsible for droppedcalls. Interference is more severe in urban areas, due to thegreater RF noise floor and the large number of basestations and mobiles.

Interference on voice channels causes cross talk, where thesubscriber hears interference in the background due to

an undesired transmission. On control channels, interference leads to missed and

blocked calls due to errors in the digital signaling.


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Another mobile in the same cell

A call in progress in a neighboring cell

Any non-cellular system which inadvertently leaksenergy into the cellular frequency band.

Other base stations operating in the same frequencyband. ( the transmitters from competing cellular

carriers are often a significant source of out- of-bandinterference, since competitors often locate theirbase stations in close proximity to one another inorder to provide comparable coverage to customers.)


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Co-channel cells: cells that use the same set of frequencies in acoverage area (Frequency reuse concept)

Co-channel interference: The interference between signals fromco-channel cells.

Unlike thermal noise which can be overcome by increasing thesignal-to- noise ratio (SNR), co-channel interference cannot becombated by simply increasing the carrier power of atransmitter.

This is because an increase in carrier transmit power increasesthe interference to neighboring co-channel cells.

To reduce co-channel interference, co-channel cells must bephysically separated by a minimum distance to provide sufficientisolation due to propagation.


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The co-channel interference ratio dependson:

The radius of the cell (R) The distance between

centers of the nearestco-channel cells.

Q = D/R = co channel reuse ratio

= the spatial separation betweenco-channel cells relative to the coveragedistance of a cell.


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Q=D/R=6N=12

Q=D/R=4.6N=7


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Q is related to the cluster size(N)for a hexagonal geometry

A large cluster size (N) indicates thatthe ratio between the cell radiusand the distance between co-channelcells is large.Conversely, a small cluster size (N)indicates that co-channel cells arelocated much closer together.

The value for N is a function of howmuch interference a mobile or basestation can tolerate while maintaininga sufficient quality ofcommunications.


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A small value of Q provides larger capacitysince the cluster size N is small.(Smaller N is greater capacity)

A large value of Q higher transmission quality(less co-channel interference)

A trade-off must be made between these twoobjectives in actual cellular design.


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The Cellular Concept (2)

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