TABLE OF CONTENTS The use of antenna diversity for fading channels has the advantage of improving channel quality for a given radio spectral allocation, relative to that available from a single port antenna. In this section, the improvement of receive diversity is couched in terms of the improvement in signal to noise ratio (SNR) relative to the best single branch performance. The improvements in system spectral efficiency follow from this, but can be complicated to estimate because they also require statistical models of the communications system, multi user traffic, outage criterion and grade of service parameters, etc. Transmit diversity can have similar benefits and joint transmit receive diversity as well. Transmit receive diversity and the capacity improvements are looked at in Chapter 9. The basic requirement for efficient diversity action is to have uncorrelated antennas with equal gain. How these requirements can be implemented is covered here after a review of basic combining principles.
The diversity gain is defined here as the improvement in the time averaged SNR from the combined signals of a set of diversity elements, relative to the SNR from the best single antenna element. The single element is sometimes a reference antenna, such as a dipole, but usually it is the best of the branches in the diversity set. The definition of diversity gain is conditioned by a probability that the SNR is above a reference level. There is no standard for the probability value, but it is often taken as 0.5...
