With WiFi systems, it's very difficult to predict the
propagation of radio waves and detect the presence of
interfering signals without the use of test equipment. Even if
you're using omni-directional antennas, radio waves don't
really travel the same distance in all directions. Instead
walls, doors, elevator shafts, people, and other obstacles
offer varying degrees of attenuation, which cause the Radio
Frequency (RF) radiation pattern to be irregular and
unpredictable. As a result, it is essential to perform a RF
site survey to fully understand the behavior of radio waves
within a facility or geographic area before installing
wireless network access points.
The ultimate goal of a RF site survey is to supply enough
information to determine the number and placement of access
points that provides adequate coverage throughout the facility
or location. In most implementations, "adequate coverage"
means support of a minimum data rate. A RF site survey also
detects the presence of interference coming from other sources
that could degrade the performance of an OFDM network or
OTA’s team can supply you with a Site Survey that will
identify and resolve all the technical issues involved in your
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How to Conduct a Successful Site Survey.
The need and complexity of a RF site survey will vary
depending on the facility. For example, a small three room
office may not require a site survey. This scenario can
probably get by with a single access point located anywhere
within the office and still maintain adequate coverage. If
this access point encounters RF interference from another
nearby wireless LAN, you can likely choose a different channel
and eliminate the problem.
A larger facility, such as an office complex, apartment
building, hospital, or warehouse, generally requires an
extensive RF site survey. Without a survey, users will
probably end up with inadequate coverage and suffer from low
performance in some areas. You certainly wouldn't want to
relocate and add access points to the facility after
installing and interconnecting 20 access points or more.
When conducting an RF site survey, consider these
- Obtain a facility diagram. Before getting too far
with the site survey, locate a set of building blueprints.
If none are available, prepare a floor plan drawing that
depicts the location of walls, walkways, etc.
- Visually inspect the facility. Be sure to walk
through the facility before performing any tests to verify
the accuracy of the facility diagram. This is a good time to
note any potential barriers that may affect the propagation
of RF signals. For example, a visual inspection will uncover
obstacles to RF such as metal racks and partitions, items
that blueprints generally don't show.
- Identify user areas. On the facility diagram, mark
the areas of fixed and mobile users. In addition to
illustrating where mobile users may roam, indicate where
they will not go. You might get by with fewer access points
if you can limit the roaming areas.
- Determine preliminary access point locations. By
considering the location of wireless users and range
estimations of the wireless LAN products you're using,
approximate the locations of access points that will provide
adequate coverage throughout the user areas. Plan for some
propagation overlap among adjacent access points, but keep
in mind that channel assignments for access points will need
to be far enough apart to avoid inter-access point
- Be certain to consider mounting locations, which
could be vertical posts or metal supports above ceiling
tiles. Be sure to recognize suitable locations for
installing the access point, antenna, data cable, and power
line. Also think about different antenna types when deciding
where to position access points. An access point mounted
near an outside wall, for example, could be a good location
if you use a patch antenna with relatively high gain
oriented within the facility.
- Take note of data rates and signal readings at
different points as you move to the outer bounds of the
access point coverage. In a multi-floor facility, perform
tests on the floor above and below the access point. Keep in
mind that a poor signal quality reading likely indicates
that RF interference is affecting the wireless LAN. This
would warrant the use of a spectrum analyzer to characterize
the interference, especially if there are no other
indications of its source. Based on the results of the
testing, you might need to reconsider the location of some
access points and redo the affected tests.
- Document findings. Once you're satisfied that the
planned location of access points will provide adequate
coverage, identify on the facility diagrams recommended
mounting locations. Of course the installers will need this
information. Also, provide a log of signal readings and
supported data rates near the outer propagation boundary of
each access point as a basis for future redesign efforts.
These steps will point you in the right direction in
conducting a useful and accurate RF Site Survey.