GSM Air Interface

What is the Air Interface?

The air interface is how your phone communicates with the cell tower over radio waves.

GSM defines exactly how this works:

• Which frequencies to use
• How to share them among users
• How to send and receive at the same time

Think of it as the language your phone and the tower speak to each other.

FDD: Talking and Listening at Once

When you’re on a call, you need to send and receive simultaneously. You talk while the other person talks.

FDD (Frequency Division Duplex) solves this by using two different frequencies:

The two directions:

By using separate frequencies, your phone’s transmitter doesn’t interfere with its own receiver.

GSM Frequency Bands

In GSM 900 (the original band):

The 45 MHz gap keeps uplink and downlink cleanly separated.

Carriers: Dividing the Spectrum

GSM divides each band into carriers (individual channels).

• Band = the full chunk of spectrum (e.g., 25 MHz for uplink)
• Carrier = one narrow slice of that band (200 kHz) that carries one channel

Each carrier is 200 kHz wide.

Simple math:

With 25 MHz of spectrum:

$\text{Number of carriers} = \frac{25 \text{ MHz}}{0.2 \text{ MHz}} = 125 \text{ carriers}$

Each carrier is like a lane on a highway. More carriers = more lanes = more capacity.

Full Duplex = A Pair of Carriers

A full duplex carrier is actually a pair:

• One 200 kHz carrier for uplink
• One 200 kHz carrier for downlink

When someone says “one GSM carrier,” they usually mean the pair.

TDMA: Sharing a Carrier Among Users

One 200 kHz carrier could handle one call. But that’s wasteful.

TDMA (Time Division Multiple Access) lets 8 users share one carrier by taking turns.

How TDMA Works

Time is divided into frames. Each frame has 8 time slots.

Each user gets one slot per frame. They transmit their burst, then wait for the next frame.

Why this works:

The switching happens so fast (every 4.615 ms) that it feels continuous to humans.

Your voice is digitized, compressed, and sent in short bursts. The gaps are imperceptible.

It’s like a conversation where 8 people take very fast turns. So fast it sounds like everyone’s talking at once.

The Benefit of TDMA

Without TDMA: 1 carrier = 1 call

With TDMA: 1 carrier = 8 calls

This 8x improvement is huge for network capacity.

Time Slot Structure

Each 577 μs slot contains a burst of data:

The training sequence is like a “known pattern” that helps the receiver tune in properly.

Putting It All Together

GSM Air Interface Summary

Capacity Example

Given: 7 MHz paired spectrum, 7-cell cluster

Minus a few slots reserved for control channels.

This is how operators calculate network capacity and plan cell deployments.