Diversity reception has long been recognized as a viable im — pairment-mitigation technique in telecommunication systems (1,2), and applied in a variety of modes to practical systems (next section). Diversity reception has been used in high-fre­quency (HF) communications since the 1920s, when spaced receive antennas were found to yield partially decorrelated fading signals that could be used to improve path availability. Diversity operation is virtually mandatory in modern tropo — scatter communication systems, for which 4-channel (quadru­ple) diversity operation is common, implemented with dual spaced antennas at both ends of the link, each capable of cross-polarized reception or some other form of antenna-pat — tern diversity (3).

Vertical space diversity (often combined with angle diver­sity) improves the performance of terrestrial microwave links (4), and has been evaluated as a countermeasure against low-
angle refractive fading on satellite links (5). Protection against severe frequency-selective (notch) fading is achieved by reserving an alternate frequency-diversity channel to pro­tect several other channels that suffer notch fading (6). (In general, however, frequency diversity is considered wasteful of spectrum, and not recommended for many applications.) Space diversity using separated base-station antennas has proven valuable for mobile and cellular radio systems (7).


Figure 1. Annual statistics of 11.6 GHz rain attenuation for two earth-space paths, compared to diversity statistics obtained by al­ways selecting the smaller single-path attenuation. (Copyright 1979 COMSAT Corp. All rights reserved by COMSAT Corp. Used by per­mission.)

Site diversity reception is used on earth-satellite links at small path elevation angles to decrease the effects of severe low-angle fading (8), as well as to improve performance dur­ing rain impairments for high-reliability earth terminal in­stallations (9). Antennas with small horizontal separations can be used to decorrelate tropospheric scintillation fading on earth-space paths (10). Since at frequencies above about 10 GHz, rain impairments are often severe for significant per­centages of the time at many locations, site diversity may find wide application in earth-satellite systems at Ka-band (11), especially to protect feeder links that carry information from a central ‘‘hub’’ earth terminal to a satellite for eventual dis­tribution to user terminals such as mobiles.

11.6 GHz rain attenuation (dB)

Updated: 17.03.2014 — 20:45