Distortion or loss of received signal quality caused by the coherent or partially coherent superposition of two or more delayed replicas of the transmitted waveform arriving via different propagation paths; observable when relative path delays and phases produce constructive and destructive combination at the receiver.

Principle

The receiver’s instantaneous complex amplitude is the vector sum of all path replicas; when replica delays are comparable to the symbol interval or their phases differ, the sum produces amplitude/phase fluctuations and intersymbol interference proportional to relative delay spread and phase differences.

Demonstration

Illustrative scenario: A mobile handset receives a direct line-of-sight component and a reflection from a building delayed by a fraction of a symbol period. Recognition: the received waveform shows superposed echoes and reduced eye opening. Action: the receiver applies a channel estimator and equalizer. Consequence: equalization mitigates symbol errors caused by the interfering replica; without it, bit-error rate rises.

Misapplication

Treating any signal attenuation or fading as multipath interference is incorrect; the semantic error is conflating simple path loss or noise with the specific phenomenon of multiple delayed replicas summing at the receiver.

Consequence

Multipath interference can produce frequency-dependent spectral notches, time-domain intersymbol interference, and increased bit-error probability; it motivates design choices such as equalization, diversity combining, RAKE reception (for resolvable echoes), or modulation that tolerates delay spread.

Reversal

When path delays are large enough to be resolved as separate arrivals by the receiver (delay significantly greater than symbol duration), the effect can be exploited (e.g., RAKE combining) rather than treated solely as harmful interference; conversely, when replicas are wholly incoherent, the phenomenon reduces to power-adding or fading statistics rather than deterministic interference.

Boundary

Clearly within: two or more replicas with relative delays up to about a symbol period causing observable ISI. Boundary case: relative delay close to one symbol period, where effect depends on pulse shape and receiver timing. Clearly outside: single-path attenuation or additive white noise without delayed replicas.

Semantic Tension

Receiver complexity (equalization, channel estimation) versus spectral/temporal design choices (narrowband signaling, OFDM, symbol duration): simpler waveforms reduce processing but increase susceptibility to multipath interference.

Synthesis

Multipath interference is a superposition phenomenon whose practical effect depends on the timing and phase relation of replicas; the same physical multipath can be damaging or useful depending on whether the receiver resolves and exploits separate arrivals.