AdvancedTopic 92 of 101Signals, Control & Comms
Fading and multipath
Signal strength varies due to reflections and movement.
In depth
Multipath produces constructive and destructive combination of delayed copies: fast fading (Rayleigh/Rician), slow shadowing and path loss. Delay spread causes ISI; Doppler spread causes time variation. Mitigation: diversity, equalisation, OFDM, spread spectrum, MIMO and coding.
Key points
- Coherence bandwidth and time
- Rayleigh distribution
- Rake receiver
ExampleSignal strength fluctuates quickly as a car moves through a city.
What is Fading and multipath?
Signal strength varies due to reflections and movement.
Why is it important to understand fading and multipath?
Fading and multipath make the received signal vary with location and time, so designers must include margin and diversity.
What is multipath?
Reflections of a signal that arrive by different paths, causing constructive and destructive combinations at the receiver.
What is diversity?
Using several independent signal paths, such as antennas or frequencies, so that a fade on one path is unlikely to affect all of them.
How is Fading and multipath used in practice?
Typical applications include explaining why a phone signal drops when walking around a building, designing diversity antennas for a base station and choosing a robust modulation for a vehicle link in an urban area.
What should I learn before Fading and multipath?
Study "Link budget" first (topic 91 of 101 in Signals, Control & Comms). It covers the groundwork this topic builds on.
What are the key points of Fading and multipath?
Coherence bandwidth and time. Rayleigh distribution. Rake receiver.
Can you give an example of Fading and multipath?
Signal strength fluctuates quickly as a car moves through a city.
What comes after Fading and multipath?
The next topic is "MIMO", which builds directly on this one.
How difficult is Fading and multipath?
It is classed as Advanced in Signals, Control & Comms (topic 92 of 101). It is an advanced topic. It assumes you are comfortable with the intermediate topics in Signals, Control & Comms.