Consider a simple high power transmitter that can support 100 voice channel s covering given a service area. Let the service area be divided into seven smaller area cells.

As shown in figure, each supported by lower power transmitters. The available spectrum of 100 voice channels is divided into 4 groups of 25 channels each. The cells (1,7)(2,4)(3,5) and six are assigned distinct channel groups. Show that the total no of channels that can be supported is enhanced to 175 to cover the same service area.

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1 Answer

Total no of Channels available, N=100

Case 1: A single high power transmission transmitter is used to cover the given service area

This implies that it is a non-cellular system

Therefore, No of channels in system are limited to 100 only.

Case 2: Service area is divided into seven cells.

No. of distinct cells = 7

No. of channel groups = 4.

No. of channels per channel group =25

Step1: Allocation of channel groups to cells

Let channel group I be allocated to cells 1 as well as 7, channel group 2 be allocated to cells 2 as well as 4; channel group 3 be allocated to cells 3 as well as 5 and channel group 4 be allocated to cell 6 as shown in figure

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Step2: Total no of channels available in the specified cellular system

Total no. of channels allocated to all cells is equal to no. of channels per channel group multiplied by the Number of distinct cells i.e.

Total no. of channels allocated to all cells = 25*7

Hence, total no. of channels available=175 channels

Total no. of channels that can be supported by given cellular system is increased to 175 from 100 in a Non-cellular system to cover the same service area.

Technical issues to address during design:

  • Selection of suitable frequency reuse pattern
  • Plans to account for expansion of cellular network
  • Physical deployment and radio coverage modelling
  • Analysis of relationship between capacity and cost of infrastructure.

By making cells smaller, frequencies can be reused by shorter distances. Once radius drop below 0.5km, handoffs occur so frequently that it is difficult to cope with a mobile moving at high speed. The flexibility of cell size allows for larger cells in less developed areas and smaller cells in areas of higher traffic.

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