Which technique adjusts the phase (and amplitude) of the signal sent from each of several antenna elements so that the radio waves reinforce one another in a particular direction?

Answer: B. Beamforming

In beamforming, the antenna elements in an array transmit with shifted phases so that the waves reinforce in the desired direction and cancel in others. Changing the phase differences electronically steers the beam without moving the antenna (a phased array). Concentrating the waves toward the terminal raises the received power and reduces interference to other terminals. Reception works on the same principle, strengthening waves arriving from a particular direction.

• A. This technique selects or combines the better of the signals received by several antennas to reduce the effect of fading.
• B. Correct. The phase of each element is adjusted to form the beam direction.
• C. This technique switches the transmit frequency at short intervals to spread out the effects of interference and fading.