Diversity Spacing

The diversity spacing for the receive antennas need to ensure that the
proper fade margin protection is designed into the system. Diversity spacing
is meant to achieve some de-correlation between the mobile received
signal. There have been numerous studies conducted on diversity reception
and the system performance improvements associated with the proper
implementation of a receive diversity scheme. The diversity scheme is typically
achieved through horizontally-placed antennas that are then fed to
the radio receiver at the base station.
The base station receiver typically would use either max ratio combing
or select diversity as the method of achieving the system performance
improvement.
However, for the diversity reception, the antennas for mobile communication
for 1G, 2G, 2.5G, and 3G involve horizontal diversity spacing. The initial
objective would be to place the receive antennas so that they were as
de-correlated. However there is a practical limit: the spacing between the
receive antennas when the feedline length between the antennas becomes
such that either the feedline loss exceeds the diversity advantage or the signals
are completely de-correlated as to eliminate any diversity combining
gain possible.
For a micro or pico-cell site, the use of diversity reception is usually a
forgone thought due to the antenna configuration—one omni antenna.
But when looking at a macro or even micro-cell with multiple antennas
for receive, the question arises about what spacing is needed between the antennas. The equation that follows should be used for a two-branch
receive system.
Diversity spacing (feet)
[(AGL of antenna (feet)/11) (835/fo)] where fo is
the center receive frequency in MHz.