Digital Communications

12Diversity and Combining Techniques

In wireless communications, there is an increasing demand for reliable transmission, higher data rates and increased user mobility. These require more efficient transmission technologies, modulation, coding and signal processing techniques with comparable quality of service at similar cost as wireline technologies. However, the wireless channel is in general a non‐LOS channel and often suffers time‐ and frequency‐selective fading. Consequently, the received signals may not be successfully detected at all times. Performance degradation may be due to time selectivity (Doppler spread), frequency selectivity (delay spread), angular selectivity (angular spread) of the fading channel, and/or the interference. Use of FEC coding, increasing transmit power and/or antenna gains may not always be economical or sufficient for achieving reliable communications.

Diversity may provide an alternative to these approaches. If several replicas of the message signal can be made to simultaneously reach the receiver over independently fading (frequency, space, time, delay, direction or polarization) channels, then at least one of these signals will not be degraded by fading. This will increase the probability of successful detection of the received signals. Performance of diversity systems also depends largely on how these signals are combined and the correlation between them. Proper combination of received signals greatly reduces the severity of fading and improves ...