Omni-Directional Cell Sites
The omni-directional cell site is used typically in a low capacity area of the
network where the system is noise limited and not interference limited.The
omni-cell is typically used to cover uniformly in all directions, 360 degrees.
Under ideal conditions, the omni-cell site would have a circular pattern
when there are no obstructions and the coverage is purely line of sight.
With an omni site there are several methods that can be used for
antenna installations. The first is a simple installation on a monopole
shown in Figure 14-2. The transmit antenna is highest on the structure
with the receive antennas located under the platform, which was the standard
installation for 1G and typically is a hold over from that design era.
The distance between the receive antennas is usually determined for maximizing
spatial diversity so that the mobile signal arriving at both receive
antennas are somewhat decorrelated enabling for a diversity gain, or
rather, fade margin protection.
There are of course other variants to omni-cell site antenna installations,
and they involve installing on a building and when the amount of antennas
is limited. Figure 14-3 is an example of an antenna installation that occurs
on a building. Please note that the location of antenna needs to meet the
required set back rules. If the set back rules cannot be adhered to, then it is
possible to install the antennas near the edge of the roof; however, they may
become visible to the public at this point, the landlord may not wish this
type of installation to take place, or the local ordinances prohibit this from
Please note that the placement of the receive antennas for rooftop installation
should be such that if there is only one major road in the area for the cell to cover, then the horizontal diversity placement for the antennas
should be such that it is maximized in the direction toward the road. Lastly,
if the primary location is not achievable for mounting the antennas, then
moving them to the lower level is possible. However, based on the penthouse
size, significant blockage may occur in a direction, and this needs to be factored
into the design process. 587
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