QWhich of the following techniques is not used to improve received signal quality over small scale times and distance?

In wireless communication systems, equalization, diversity, and channel coding are commonly used to improve received signal quality and link performance over small scale times and distance. Modulation, however, is not used to improve signal quality directly, but rather to encode information onto the signal for transmission. Equalization is used to compensate for the effects of multipath propagation and other channel impairments. It aims to recover the transmitted signal as accurately as possible by removing distortion caused by the channel. There are two types of equalization: linear and non-linear. Linear equalizers include zero-forcing equalizers, minimum mean square error (MMSE) equalizers, and maximum likelihood (ML) equalizers. Non-linear equalizers include decision feedback equalizers (DFE) and maximum a posteriori (MAP) equalizers. Diversity is used to combat the effects of fading caused by multipath propagation. It involves transmitting the same information over multiple channels, each with different fading characteristics, and then combining the received signals at the receiver to improve signal quality. There are two types of diversity: spatial and temporal. Spatial diversity involves using multiple antennas at the transmitter and/or receiver, while temporal diversity involves transmitting the same signal at different times. Channel coding is used to protect the transmitted signal against errors caused by noise and interference. It involves adding redundancy to the signal before transmission, which allows the receiver to detect and correct errors. There are two types of channel coding: error detection and error correction. Error detection codes include parity check codes and cyclic redundancy check (CRC) codes, while error correction codes include convolutional codes and Reed-Solomon codes. In summary, while modulation is a critical component of wireless communication systems, it is not used directly to improve received signal quality over small scale times and distance. Instead, equalization, diversity, and channel coding are commonly used to achieve this goal.