Wireless LAN Planning

In a number of countries, it is now permitted to provide public access services in the
unlicensed industrial scientific medical bands. Bands of interest include the ISM band
at 900 MHz (between 900 and 930 MHz) used for digital cordless phones in the United
States, the ISM band at 2.4 GHz used for IEEE802 wireless LANs and Bluetooth, and
the 5 GHz band used for wideband HIPERLANs (high-performance radio local area
networks).
The availability of plug-in wireless LAN cards that include IEEE802 and Bluetooth
have begun to focus attention on the need to integrate wide area wireless planning
with in-building coverage planning.
One of the problems of planning in-building coverage is the unpredictability of inbuilding
propagation, particularly at 2.4 GHz and 5 GHz (propagation unpredictability
increases with frequency). Figure 11.18 shows a comparison between free space loss
in an empty room and the loss in a room with cubicles partitioned off from one another.
Signal levels received in the bare room are typically a few milliVolts. Signal levels
received in the room with partitioning quickly attenuate down to a few microVolts.
Losses are very dependent on the materials used in the partitioning. Fire-resistant silver
foil, for example, will provide a high degree of shielding.

In Figure 11.19 we show the minimum levels of attenuation normally experienced
in a building, typically 6 or 7 dB floor to floor and 3 dB through a wall (without fireresistant
foil cladding). The attenuation through the exterior wall of the building
will typically be 20 to 30 dB. This is good news and bad news. The good news is we
probably do not want signals from inside the building to be visible outside the building
and vice versa, both for security reasons and in order to deliver reasonable receive
sensitivity.
However, present wireless LAN standards (IEEE802) include handover protocols
that allow a user (theoretically) to move within a building (floor to floor) and into
and out of buildings and still remain in continuous coverage. In practice, given the
very substantial and rapid changes in signal level, it is very easy to drop a call
under these conditions. It has also been proposed that wireless LAN to cellular handover
should be supported. Again, in practice, this is hard to realize consistently
because of the rapid fluctuation in received signal strength in the wireless LAN
environment.

Designing a wireless LAN radio scheme is therefore a reasonably complex process
and depends on having knowledge of the building configuration and building materials
used. The process is not dissimilar to undertaking heat loss calculations/heat gain
calculations from buildings where the sizing of the heating and cooling system is
dependent on the building materials used, the size of windows, and whether windows
are double or triple glazed. There are also many similarities with lighting design.
Radio waves and light waves behave very similarly. In lighting calculations, we have
to take into account the polar diagrams (also known as ISO candela diagrams) describing
the light distribution available from the luminaire.
Unsurprisingly, this is very similar to looking at an RF antenna specification. Figure
11.20 shows a lighting product from Philips, and Figure 11.21 shows the related ISO
candela diagram.

It makes considerable sense to do a wireless coverage design at the same time as
lighting design and heat loss calculations are being done for a building, since many of
the inputs needed are common: the configuration and layout of the building and the
materials used in the construction of the building. In lighting, we calculate lux intensity
at various points in the room—the number of lumens on a desk, number of lumens
reflecting off wall surfaces. In RF design, we calculate signal voltages available to RF
receivers. The light output available from the luminaires (typically a few tens of Watts)
is directly analogous to the RF power available from the wireless LAN transmitter (a
few hundred milliWatts). Both lighting and RF design are directly affected by building
geometry, user geometry (where people are in the building and what they are doing in
the building), and the materials used in the building.
For further information on lighting design and integrated building services design,
go to the Chartered Institute of Building Services Engineers Web site (www.cibse.org).
In designing for Bluetooth or IEEE802 RF wireless LAN coverage, we find, because
of the wide variability of building geometry and building materials used, that range is
not included in the specification, though there are guidelines offered based on the
power output and receive sensitivity available. With Bluetooth, the guidelines suggest
a range of 10 meters and 100-meter figures based on 0 dB and +20 dBm power output
and assuming -70 dBm receiver sensitivity and -5 dBi antenna gain.
Although this may seem to be a reasonable assumption, in practice we have shown
that there are typically attenuation effects of several tens of dBs to take into account. It
is also in practice very difficult to deliver even moderate antenna efficiency because of
space and size constraints and capacitive effects in handheld devices.