A comprehensive study guide covering frequency reuse, cluster patterns, capacity planning, and interference management in cellular systems using hexagonal geometry.
GSM networks use hexagonal cells because they provide the most efficient coverage with minimal overlap. Unlike circles (which leave gaps) or squares (which have varying distances to neighbors), hexagons offer:
The fundamental principle of cellular systems is frequency reuse. By dividing the coverage area into cells, we can reuse the same frequencies in non-adjacent cells, dramatically increasing system capacity.
To locate co-channel cells, move i cells along any chain of hexagons, then turn 60° counter-clockwise and move j cells.
GSM 900 typically has 124 frequency channels (ARFCN 1-124). With FDMA/TDMA, each channel carries 8 timeslots.
The Erlang B model calculates blocking probability in trunked systems with infinite users and blocked calls cleared.
Subdividing congested cells into smaller cells to increase capacity. New cells have reduced power and radius.
Using directional antennas (typically 120° or 60° sectors) to reduce interference and enable tighter reuse.
The dominant interference in cellular systems comes from co-channel cells. The signal-to-interference ratio (SIR) determines voice quality and system reliability.
Interference from adjacent frequency channels due to imperfect receiver filters. Managed by careful channel allocation.
Comprehensive tool for GSM network dimensioning
The cellular concept enables frequency reuse by dividing coverage into hexagonal cells. The cluster size N determines the reuse distance D = R√(3N).
Use Erlang B to determine required channels. Trade-off between capacity (small N) and quality (large N). Sectoring improves SIR without reducing capacity.
Maintain C/I ≥ 9 dB minimum. Use sectoring (120° or 60°) to reduce interferers. Cell splitting increases capacity in hotspot areas.