TABLE OF CONTENTS Signal performance that is degraded by severe fading can be improved by increasing transmitter power, antenna size and height, etc., but these solutions are costly in mobile-radio communications and sometimes impractical. The alternative is to use simultaneous or selective diversity-combining transmission over several channels, to reduce the probability of excessively deep fades at the receiving end. In Chap. 9, several schemes for providing signal diversity at the receiving end were described. In this chapter, the combining techniques for macroscopic diversity and microscopic diversity are analyzed.
In macroscopic diversity, as previously discussed in Sec. 9.2 of Chap. 9, only the local means of the received signal are considered. The local means may vary as a result of long-term fading when the mobile unit is traveling in an extensive area and when the terrain contour in that area is not flat. Diversity reception provides the advantage of being able to receive two signals whose local mean fades rarely occur below a certain level during the same time intervals.
As previously discussed in Sec. 9.1 of Chap. 9, selective diversity combining is chosen primarily to reduce long-term fading. Reducing the effects of long-term fading by combining two signals received from two different-sited transmitting antennas is possible because the local means of the two signals at any given time interval are rarely the same. It will be shown later that to effectively reduce fading requires the combining of two fading signals that have equal mean...
