Also known as: multipath, multipath propagation

Multipath propagation occurs when a signal reaches the receiver by several paths at once — directly and via reflections off buildings, terrain, and vehicles.¹ The copies arrive slightly out of step and add or cancel, so the received amplitude and phase are the vector sum of many delayed echoes rather than a single clean wave.

Multipath: copies arrive by direct and reflected paths and interfere, causing fading and decode errors.

How it works

Each reflected path travels a different distance, so its copy arrives with a different delay and phase. Where copies arrive in phase they reinforce; where they arrive out of phase they cancel, and the result is fading — deep, position-dependent dips in signal strength. Because the phase relationships depend on wavelength, the fading pattern is finely grained: moving a receive antenna a fraction of a wavelength can turn a dead null into a strong signal, which is why nudging the antenna a few centimetres sometimes fixes reception.

Two numbers characterize a multipath channel:

• Delay spread — the time between the earliest and latest significant echo. When the delay spread is comparable to a symbol period, one symbol’s echoes smear into the next, producing intersymbol interference that a decoder sees as errors.²
• Fading rate — how fast the fades come and go as the receiver, transmitter, or reflectors move. Motion also imposes a Doppler shift on each path, and the spread of those shifts sets how quickly the channel changes.

When many comparable echoes arrive with no dominant direct ray, the summed amplitude follows Rayleigh fading statistics; when a strong line-of-sight path is present, the milder Rician distribution applies instead.

In practice

Multipath is worst in dense urban and indoor settings and mildest with a high, clear line-of-sight path. Digital systems fight it in several ways: equalizers such as a CMA equalizer estimate and undo the channel’s smearing; robust clock recovery keeps symbol timing locked through fades; and multi-antenna techniques exploit it rather than merely tolerating it. MIMO famously turns rich multipath into a benefit, using the independent paths to carry parallel data streams, while simpler antenna diversity just picks whichever of two antennas is not in a fade.

Relevance to SDR

Multipath is a common reason a strong signal still won’t decode; the meter reads plenty of power, yet symbols arrive smeared and the BER stays high. An equalizer and good clock recovery help combat it, and moving or re-siting the antenna can change the multipath picture dramatically. GopherTrunk’s C4FM/CQPSK demodulators include timing recovery and equalization stages precisely because land-mobile signals so often arrive through this kind of reflective channel; when decoding fails on a strong carrier, multipath is one of the first suspects.

Sources

1. Multipath propagation — Wikipedia, on multiple-path arrival, fading, and intersymbol interference. ↩

2. Delay spread — Wikipedia, on how echo delay relative to the symbol period produces intersymbol interference. ↩

See also

• Radio propagation
• Rayleigh fading
• Doppler shift
• MIMO
• CMA equalizer
• Clock recovery
• Radio horizon

Learn it step by step

• How signals travel