The Problem
You have digital data: 1 0 1 1 0 1
You want to send it wirelessly through the air. But you can’t just transmit and directly. The air doesn’t understand bits.
You need to encode your bits onto a wave that can actually travel.
The Carrier Wave
A carrier is just a simple sine wave oscillating at a fixed frequency.
By itself, it carries no information. It’s just a steady hum. Think of it as a blank canvas, a vehicle that will carry your data.
But a sine wave has three properties you can change:
| Property | What it means |
|---|---|
| Amplitude | The height of the wave (how loud) |
| Frequency | How fast it oscillates (how high-pitched) |
| Phase | Where in the cycle it starts (shifted left or right) |
The Three Modulation Types
To encode bits, you change one of these properties based on whether you’re sending a 0 or 1.
| Modulation | What changes |
|---|---|
| ASK | Amplitude |
| FSK | Frequency |
| PSK | Phase |
ASK: Amplitude Shift Keying
Change the amplitude to represent bits.
- 1 = full amplitude (wave present)
- 0 = low/zero amplitude (wave weak or absent)
Simple idea: Turn the wave ON for 1, OFF (or quiet) for 0.
The problem: Noise in the real world affects amplitude. If interference weakens your signal, a 1 might look like a 0.
ASK is simple but vulnerable to noise.
FSK: Frequency Shift Keying
Change the frequency to represent bits.
- 1 = higher frequency (faster oscillation)
- 0 = lower frequency (slower oscillation)
Why is this better? Noise typically affects amplitude, not frequency. Even if the wave gets weaker, you can still count how fast it’s oscillating.
FSK is more robust than ASK.
The tradeoff: You need two different frequencies, which uses more bandwidth.
PSK: Phase Shift Keying
Change the phase to represent bits.
Phase is where the wave “starts” in its cycle. Shifting the phase by 180° flips the wave upside down.
- 1 = normal phase (0°)
- 0 = inverted phase (180°)
Notice how at each bit transition, the wave either continues normally or flips. That flip is the 180° phase shift.
PSK is very robust. Phase is harder for noise to corrupt than amplitude.
This is why BPSK (Binary PSK) is widely used in WiFi, cellular, and satellite systems.
Comparison
| Type | What changes | Advantage | Disadvantage |
|---|---|---|---|
| ASK | Amplitude | Simple to implement | Vulnerable to noise |
| FSK | Frequency | Robust to noise | Uses more bandwidth |
| PSK | Phase | Very robust | More complex receiver |
The Key Insight
All three are doing the same fundamental thing:
Mapping bits to wave properties.
- ASK: bit → amplitude
- FSK: bit → frequency
- PSK: bit → phase
In binary versions (BASK, BFSK, BPSK), you have 2 possible symbols, so you encode 1 bit per symbol.
What’s Next?
What if we used 4 phases instead of 2? Then each symbol could represent 2 bits.
That’s QPSK (Quadrature PSK), and it leads us to even more efficient modulation schemes like QAM.