TDMA

The Problem With FDMA

FDMA gives each user their own frequency. Simple, but wasteful.

When you’re listening (not talking), your channel sits idle. Nobody else can use it.

What if we could share a single frequency among multiple users?

The Solution: Take Turns

TDMA (Time Division Multiple Access) gives everyone the same frequency, but different time slots.

Think of a shared microphone at a meeting:

• Only one person speaks at a time
• Everyone takes turns
• Rotate fast enough, and everyone gets to talk

How It Works

Time is divided into repeating frames.

Each frame contains multiple time slots.

Each user gets one slot per frame to transmit their data.

Key point: Users transmit in short bursts, then wait for their next turn. The rotation is so fast you don’t notice.

Frame Structure

A TDMA frame looks like this:

|←―――――――――― One Frame ―――――――――→|
┌──────┬──────┬──────┬──────┬──────┐
│Slot 1│Slot 2│Slot 3│Slot 4│Slot 5│  ...
└──────┴──────┴──────┴──────┴──────┘
│User 1│User 2│User 3│User 4│User 5│

The frame repeats continuously. Each user transmits during their assigned slot, every frame.

Guard Times

Between slots, there’s a tiny guard time (a gap).

This prevents overlap if one user’s timing is slightly off.

Similar to guard bands in FDMA, but in time instead of frequency.

TDMA vs FDMA

Advantages

• More users per frequency channel
• Flexible slot allocation (give busy users more slots)
• No guard bands wasting spectrum
• Battery saving: phone can sleep between slots

Disadvantages

• Precise timing required (synchronization is critical)
• Buffering needed (must store data between transmission slots)
• Multipath delay can cause inter-slot interference

Real World Example

GSM (2G) uses TDMA:

• 8 time slots per frame
• Frame duration: 4.615 ms
• Each slot: ~577 microseconds
• One frequency supports 8 simultaneous calls

This is why GSM could handle more users than 1G (which used FDMA only).