Wireless channel effects from signal reflections
Multipath: Signal reaches receiver via multiple paths (direct, reflected, diffracted). Each has different delay and amplitude.
Constructive/Destructive: Paths can add (strong signal) or cancel (deep fade) depending on phase relationships.
Frequency Selective: Some frequencies fade while others don't. Creates notches in spectrum.
Coherence Bandwidth: B_c ≈ 1/(delay spread). Frequencies within B_c fade together.
Doppler: Motion causes frequency shifts. Creates time-varying fading (fast fading).
Mitigation: OFDM, equalization, diversity, MIMO.