The Problem
Multiple users want to make calls at the same time.
But there’s only one radio spectrum. How do we share it?
The Solution: Divide by Frequency
FDMA (Frequency Division Multiple Access) gives each user their own slice of the spectrum.
Think of radio stations. Each broadcasts on its own frequency:
- 101.1 FM
- 102.3 FM
- 104.5 FM
They all transmit simultaneously, but on different frequencies. No interference.
FDMA works the same way for phone calls.
How It Works
The total spectrum is divided into non-overlapping channels.
Each user gets one channel for their entire call.
Guard bands sit between channels to prevent overlap.
Key point: Users transmit continuously on their assigned frequency. The channel is theirs until the call ends.
Channel Allocation
How do we decide who gets which channel?
Fixed Assignment:
- Channels permanently assigned to specific users
- Simple, but wasteful if the user isn’t always transmitting
Demand Assignment:
- Channels assigned only when needed
- Released when the call ends
- More efficient use of spectrum
Advantages
- Simple to implement
- No synchronization needed between users
- Works well for continuous traffic (like voice calls)
Disadvantages
- Wastes bandwidth when user is silent (channel still reserved)
- Limited users (one per channel)
- Guard bands waste spectrum
- Inflexible (can’t easily give one user more bandwidth)
Real World Example
1G cellular (AMPS) used FDMA:
- 30 kHz channels
- One call per channel
- Analog voice transmission
Modern systems moved beyond pure FDMA because it wastes too much spectrum. But the concept of dividing by frequency is still used in combination with other techniques.