Channel fading that varies significantly over frequency within the signal bandwidth such that different spectral components experience unequal attenuation and phase shift, typically caused by multipath delay spread comparable to or greater than the reciprocal of the signal bandwidth.

Principle

If the channel coherence bandwidth is smaller than the transmitted signal bandwidth, different frequency components fall into different fading states, producing frequency-dependent attenuation (selectivity) and possible spectral nulls.

Demonstration

Illustrative scenario: an OFDM transmitter sends many narrow subcarriers spanning the channel. Recognition: some subcarriers exhibit deep attenuation while others do not. Action: the receiver applies per-subcarrier channel estimates and adapts coding or uses bit-loading. Consequence: without adaptation, subcarrier errors produce packet loss; with adaptation or coding, performance is preserved using frequency diversity.

Misapplication

Labeling any amplitude fluctuation as frequency-selective fading is wrong; the error is failing to consider the relation between signal bandwidth and channel coherence bandwidth—narrowband fluctuations are flat fading, not selective.

Consequence

Frequency-selective fading causes unequal SNR across the spectrum, creates spectral notches that act like erasures on subbands, and requires equalization, multicarrier modulation, or channel coding exploiting frequency diversity to maintain reliability.

Reversal

When signal bandwidth is smaller than the channel coherence bandwidth the channel behaves as frequency-flat over that signal—i.e., selectivity disappears at that scale; conversely, extremely wideband systems may resolve multipath and allow explicit exploitation of multipath components.

Boundary

Clearly within: signal bandwidth substantially exceeds coherence bandwidth so spectral components see different gains. Boundary case: bandwidth similar to coherence bandwidth, where effect depends on exact channel tap structure. Clearly outside: signal bandwidth much smaller than coherence bandwidth (flat fading).

Semantic Tension

Design trade-off between increasing signal bandwidth (to gain multipath resolvability and frequency diversity) and the added complexity of equalization, synchronization and hardware capable of handling selective fading.

Synthesis

Frequency selectivity is not an intrinsic property of the medium alone but of the medium relative to the signal bandwidth; whether fading is selective or flat depends on that scale relation and dictates fundamentally different receiver strategies.