AdvancedTopic 80 of 101Signals, Control & Comms
Digital modulation
ASK, FSK, PSK and QAM map bits to carrier changes.
In depth
Digital modulation maps bits to carrier changes: ASK (amplitude), FSK (frequency), PSK (phase: BPSK, QPSK, 8-PSK), QAM (amplitude and phase: 16-, 64-, 256-QAM). Higher order carries more bits per symbol but needs higher SNR. Pulse shaping (raised cosine) limits bandwidth.
Key points
- Bits per symbol log₂M
- Symbol rate vs bit rate
- Coherent vs non-coherent detection
ExampleWi-Fi uses up to 1024-QAM for fast links at short range.
What is Digital modulation?
ASK, FSK, PSK and QAM map bits to carrier changes.
Why is it important to understand digital modulation?
Digital modulation maps bits onto carriers, which is how modern wireless links and cable modems send data.
What does QAM do?
It changes both the amplitude and phase of the carrier to represent several bits per symbol, which increases the data rate for a given bandwidth.
Why do higher-order modulations need better SNR?
They place points closer together in the constellation, so noise is more likely to move a point to the wrong decision region.
How is Digital modulation used in practice?
Typical applications include choosing QPSK for a robust low-rate link, using 64-QAM for a high-speed cable modem and selecting FSK for a simple low-power sensor radio.
What should I learn before Digital modulation?
Study "Sampling and PCM" first (topic 79 of 101 in Signals, Control & Comms). It covers the groundwork this topic builds on.
What are the key points of Digital modulation?
Bits per symbol log₂M. Symbol rate vs bit rate. Coherent vs non-coherent detection.
Can you give an example of Digital modulation?
Wi-Fi uses up to 1024-QAM for fast links at short range.
What comes after Digital modulation?
The next topic is "Constellation diagrams", which builds directly on this one.
How difficult is Digital modulation?
It is classed as Advanced in Signals, Control & Comms (topic 80 of 101). It is an advanced topic. It assumes you are comfortable with the intermediate topics in Signals, Control & Comms.