The correct answer is D. Feedback diversity.
Space diversity is a technique used to improve the reliability of a communication link by using multiple antennas at the transmitter and receiver. This is done in order to combat the effects of multipath fading, which is a phenomenon that occurs when a signal is received at the receiver via multiple paths, each of which may have a different delay and attenuation. By using multiple antennas, the receiver can combine the signals from the different paths in such a way as to cancel out the effects of fading.
There are three main categories of space diversity techniques:
- Time diversity: This technique involves transmitting the same signal over multiple time slots. This is done in order to combat the effects of fading, which is a time-varying phenomenon. By transmitting the signal over multiple time slots, the receiver can average out the effects of fading and improve the reliability of the communication link.
- Frequency diversity: This technique involves transmitting the same signal over multiple frequencies. This is done in order to combat the effects of multipath fading, which is a frequency-dependent phenomenon. By transmitting the signal over multiple frequencies, the receiver can avoid the paths that are experiencing fading and improve the reliability of the communication link.
- Space diversity: This technique involves using multiple antennas at the transmitter and receiver. This is done in order to combat the effects of multipath fading, which is a phenomenon that occurs when a signal is received at the receiver via multiple paths, each of which may have a different delay and attenuation. By using multiple antennas, the receiver can combine the signals from the different paths in such a way as to cancel out the effects of fading.
Feedback diversity is not a category of space diversity technique. It is a technique that is used to improve the performance of a communication link by using feedback from the receiver to the transmitter. This feedback can be used to adjust the transmit power, the modulation scheme, or the coding rate in order to improve the reliability of the communication link.