Supporting millions of mobile users across a city with a single high-power antenna is impractical because: (1) mobile devices lack enough transmit power to reach a 50 km distant tower, and (2) the available radio spectrum can only support a limited number of simultaneous channels. A city-wide single-antenna system would have insufficient capacity. The solution is to divide the coverage area into small cells, each with a low-power base station.
A cellular mobile system divides a geographical area into small hexagonal cells, each served by a low-power base station. Because each cell covers a small area, devices only need to reach antennas within hundreds of meters. The same frequencies can be reused in non-adjacent cells, massively increasing network capacity as cells get smaller.
- The city is divided into small hexagonal cells (typically 1–35 km radius)
- Each cell has a Base Transceiver Station (BTS) with low-power radio equipment
- Mobile devices connect to the nearest BTS via the Um (air) interface
- Cells using the same frequencies (co-channel cells) are separated by the co-channel reuse distance D
- As a mobile moves between cells, the call is handed over from one BTS to another (handoff)
- Base Station Controllers (BSCs) manage multiple BTSs; Mobile Switching Centers (MSCs) manage routing
Key concepts: Frequency Reuse (same frequencies in distant cells), Cell Splitting (splitting a large overloaded cell into smaller micro-cells), Sectoring (using directional antennas to divide a cell into 120° or 60° sectors).
- Massive capacity scaling: total capacity = channels_per_cell × number_of_cells
- Limited antenna power means devices only need short-range transmission
- Frequency reuse enables near-infinite capacity scaling as cells shrink
- Cell radius can be 100 m (dense urban) to 35 km (rural)
- Handoff (hard vs soft) maintains connectivity during mobility
- Sectoring reduces co-channel interference per sector
- Built from: Frequency Reuse — the foundational principle of cellular architecture
- Built from: Handoff — what allows continuous communication during mobility
- Built into: GSM — the most widely deployed 2G cellular system
- Related: Cell Splitting — increasing capacity by shrinking cell size
- Related: Sectoring — directional antennas that improve cell efficiency
- Related: Co-Channel Interference — the interference that constrains frequency reuse
- Related: Frequency Management — managing channel assignments across the network
- Cell planning in urban areas is complex due to irregular terrain and building shadowing
- Cell boundaries are not clean hexagons — they overlap and change with traffic and conditions
- Very small cells (micro-cells) require more base stations, increasing infrastructure cost
- Too many small cells cause excessive handoffs, degrading quality