Frequency Selective Fading
If signal copies with varying propagation delays overlay, for example, as a result of multipath propagation with different path lengths, a communications channel is frequency-selective. A Tap or Cluster is defined as all multipath components (MPCs) arriving at the receiver at the same time. Although there may theoretically be an infinite number of Taps, we only consider a finite number of Taps for mobile communication. It is obvious that the signal received at various Taps has varied frequencies. So we only select a few Taps, we are indirectly considering only a few frequencies or multipath that take less time or cover the shortest paths between TX and RX, or are usually stronger multipath. Modeling can be done using the system-theoretical description of the mobile radio channel as a linear, time-variant (LTV), and causal system.