Multipath Fading
Multipath fading is when reflected components add up out of phase with the direct signal and cancel each other. In a deep fade the received level can drop by tens of dB. This is why the link suddenly comes back when the operator moves a few steps; the event is not a fault but the spatial distribution of the interference pattern.
Fading points form a pattern that repeats at intervals on the order of half a wavelength. This pattern is not fixed: when a forklift passes, a crane slews or a door opens, the reflection geometry changes and the pattern shifts.
- The symptom is typical: at a certain point the remote does not hold, half a metre to the side it works fine.
- Safety margin is the defence: a margin of 10–20 dB makes shallow fades invisible.
- Frequency hopping is an effective solution: since fading is frequency dependent, the same point may not be in a deep fade on another channel.
The XRF protocol used in AXI's HCT400 and HCT402 sets works with frequency hopping; this architecture provides a natural defence against both narrowband interference and frequency-dependent fading. On fixed-frequency systems the solution is to change the antenna position and height.
Context in which this term is used: Wireless remote control installation: pairing, relay, safety