Physics & RF
B17 Multipath & Fading — Study Guide
This guide covers Multipath & Fading as part of Physics & RF. Start with the vocabulary, then try to answer each question before revealing the answer. Connect these facts to a real radio link or electrical circuit and practice the subject in the arcade when you are ready.
Acronyms first
| Term | Meaning |
|---|---|
| OFDM | Orthogonal Frequency Division Multiplexing |
| TDOA | Time Difference of Arrival |
| SNR | Signal-to-Noise Ratio |
Key facts
Fading is…
variation in signal strength as received by an antenna
Multipath propagation is…
a wave taking multiple paths between transmitter and receiver
Multipath waves arriving IN phase…
reinforce — stronger signal
Multipath waves arriving OUT of phase…
cancel — weak or fading signal
Scenario: signal fades deeply every time you move the receiver a few feet. Cause?
Reflected signals alternately canceling and reinforcing the direct path (multipath)
Field fix for LOS multipath fading:
Use a directional antenna; raise it above reflectors; relocate
Antenna diversity means…
using different antenna positions or polarizations
Ionospheric absorption mostly happens in the…
D-layer
Scenario: signals below 10 MHz are much weaker during the day than at night. Type of fading?
Ionospheric absorption fading
Field fix for daytime D-layer absorption:
Switch to a higher frequency
The ionosphere is a dispersive medium, meaning…
different frequencies travel at different speeds/bend differently
Scenario: different hops, sporadic-E patches and mixed E & F reflections make your HF signal vary. Type of fading?
Ionospheric multipath fading
Mitigation for ionospheric multipath:
Frequency diversity, cross-fix geolocation, multiple receivers/TDOA
Because of ionospheric multipath, you should be skeptical of single-site bearings.
True
Fading where different frequency components of a wideband signal fade by different amounts:
Selective (frequency-selective) fading
Fading where ALL frequency components fade by the same amount:
Flat fading
Scenario: on a waterfall display you see drifting notches at specific frequencies. Type?
Selective fading
Scenario: signal strength rises and falls but the signal's spectrum shape stays the same. Type?
Flat fading
Scenario: a digital link shows bursty errors on particular subcarriers. Type?
Selective fading
Flat fading is most likely when the signal's bandwidth is…
narrow
How does OFDM beat selective fading?
Splits a wide channel into many narrow subcarriers; lost ones are rebuilt
Field fix for flat fading:
Diversity — spatial, frequency or time
Time diversity means…
sending the same info at different times
Spatial diversity means…
using multiple antennas
Frequency diversity means…
transmitting on different frequencies
Practice this module
Continue with a Physics & RF game. Choose this module in games that offer module selection.