In radio communication, which is the phenomenon in which waves reflected by buildings and other objects arrive late because of differing path lengths, interfere with the directly arriving wave, and cause the received power to drop sharply at certain locations or frequencies?

Answer: B. Multipath fading

Radio waves are reflected and diffracted by buildings and the ground, arriving over multiple paths of different lengths (multipath). Because the phase differs from path to path, there are places where the waves reinforce each other and places where they cancel, and the received power can drop by several tens of dB. This is called multipath fading, and when a terminal moves, the received power changes rapidly over time. Countermeasures include diversity reception with multiple antennas, schemes such as OFDM that split transmission across many frequencies, and error-correcting codes.

• A. This is the shift in received frequency caused by motion of the transmitter or receiver. It affects how fast fading changes, but it is not the drop caused by interference itself.
• B. Correct. Waves that traveled over multiple paths interfere, and the received power drops.
• C. This is the bending of radio waves into the shadow of an obstacle. It is one reason waves reach places out of line of sight, but it is not the drop caused by interference itself.