Cellular Fundamentals

The Problem

Radio spectrum is expensive and limited. But mobile networks need to serve millions of users across entire cities.

You can’t just use one frequency for everyone. Too many people would interfere with each other.

The solution: Divide the coverage area into small cells, and reuse the same frequencies in cells that are far apart.

Cells and Base Stations

A cell is a geographic area served by one base station (tower).

Instead of one giant tower trying to cover everything, you use many smaller cells. Each cell handles users in its area.

Why cells?

• Smaller area = fewer users per cell = less congestion
• Lower power needed = longer battery life for phones
• Same frequencies can be reused in distant cells

The Cluster Concept

You can’t use the same frequency in neighboring cells. They’d interfere.

So you group cells into a cluster. Within a cluster, every cell uses different frequencies.

Once you finish one cluster, you repeat the pattern. Cells using the same frequency are now far apart.

A cluster is the smallest group of cells that uses the complete set of available frequencies.

Cluster Size

Common cluster sizes: 3, 4, 7, 12

Smaller cluster = more frequencies per cell = higher capacity

Larger cluster = frequencies reused farther apart = less interference

Co-Channel Interference

When two cells use the same frequency, their signals can overlap. This is co-channel interference.

The closer the cells, the worse the interference. Users hear noise, calls drop, data slows down.

Co-channel interference is the main factor limiting how small your cluster can be.

The Trade-off

You want the smallest cluster that still gives acceptable call quality.

• Too small: interference ruins calls
• Too large: wasting capacity

Signal-to-Interference Ratio (SIR)

SIR measures how strong your wanted signal is compared to interference.

$\text{SIR} = \frac{\text{Signal Power}}{\text{Interference Power}}$

Higher SIR = better quality. You need a minimum SIR for acceptable calls (typically 12-18 dB for voice).

The most important criterion for choosing cluster size is maintaining adequate SIR.

Calculating Capacity

Example: You have 7 MHz of spectrum. Each carrier needs 200 kHz. You use a 7-cell cluster.

More carriers per cell = more simultaneous calls.

Spectral Efficiency

Spectral efficiency = how well you use your spectrum.

If you compare cluster N and cluster M, where N > M:

Smaller clusters are more spectrally efficient, but only if interference stays acceptable.

Summary