flatfading

Flat fading refers to wireless channels in which the channel’s complex gain is effectively constant across the signal bandwidth. This occurs when the multipath delay spread is small relative to the symbol duration or when the channel’s coherence bandwidth exceeds the signal bandwidth. In such cases all frequency components of the transmitted signal experience roughly the same attenuation and phase rotation.

The baseband model can be written as y(t) = h(t) x(t) + n(t), where h(t) is a complex gain with time variation and n(t) is additive noise. If the channel impulse response has negligible delay spread, it acts as a single tap and the received signal is a scaled and rotated version of the transmitted signal. The time variation of h(t) leads to fading: slowly varying h(t) causes slow fading, while rapidly changing h(t) results in fast fading. The statistical nature of h(t) is often modeled as Rayleigh fading (no dominant line of sight) or Rician fading (presence of a strong LOS path).

Flat fading is contrasted with frequency-selective fading, where different frequency components experience different gains due to a channel with multiple significant delays, causing intersymbol interference. In practice, systems such as OFDM use the flat fading assumption on each subcarrier when the subcarrier bandwidth is smaller than the coherence bandwidth. Channel estimation and simple scalar equalization are typically sufficient, though fast fading or noncoherent scenarios may require coding and pilot-assisted techniques.