Page 203 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

Cellular Systems - Basic Concepts
• Cellular system solves the problem of spectral congestion.
• Offers high capacity in limited spectrum.
• High capacity is achieved by limiting the coverage area of each BS to a small geographical area called cell.
• Replaces high powered transmitter with several low power transmitters.

• Each BS is allocated a portion of total channels and nearby cells are allocated completely different channels.
• All available channels are allocated to small no of neighboring BS.
• Interference between neighboring BS’s is minimized by allocating different channels.
• Same frequencies are reused by spatially separated BS’s.

• Interference between co-channels stations is kept below acceptable level.
• Additional radio capacity is achieved.
• Frequency Reuse-Fix no of channels serve an arbitrarily large no of subscribers.
Frequency Re-use:
The design process of selecting and allocating channel groups for all the cellular base stations within a system is
called frequency reuse or frequency planning.
• Cell labeled with same letter use the same set of frequencies.
• Cell Shapes - Circle, Square, Triangle and Hexagon.
• Hexagonal cell shape is conceptual; it is irregular in shape.

• In hexagonal cell model, BS transmitter can be in center of cell or on its 3 vertices.
• Centered excited cells use omni directional whereas edge excited cells use directional antennas.
• A cellular system having ‘S’ duplex channels, each cell is allocated ‘k’ channels (k<S)
• If S channels are allocated to N cells into unique and disjoint channels, the total no of available channel is
• S = kN.

Hand off Strategies
In cellular telecommunications, the terms handover or handoff refers to the process of transferring an ongoing
call or data connectivity from one Base Station to another Base Station. When a mobile moves into a different cell
while the conversation is in progress then the MSC (Mobile Switching Center) transfers the call to a new channel
belonging to the new Base Station.

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CITS : E & H - Electronics Mechanic - Lesson 118 - 126