|  Coverage
contouring
Passive distribution systems in the form of radiating cables also offer
precise ‘contouring’ of the RF coverage. “We can better
control the coverage using a radiating cable than we can with a point
source antenna,” says Raabe. “RF contouring minimizes the
risk of co-channel interference between adjacent access points in larger
systems, by preventing overlap of coverage zones.”
The WLAN systems of large campuses—such as hotels, universities
and airports—require multiple access points to achieve the required
coverage and capacity of the network. Depending on channel allocation
and re-use, any overlap in coverage between zones or ‘cells’
will result in co-channel interference and increased bit error rates,
unless the channels are separated adequately.
Containing the RF signal to prevent interference is difficult using point
source antennas, which have ‘hot’ spots at their source with
decreasing RF signal strength further away. Radiating cable, on the other
hand, can be laid out where coverage is required, dramatically increasing
the number of effective RF emission points, and reducing the average distance
between the source and wireless client. The power distribution of the
WLAN is thereby tailored and kept optimally low to minimize the potential
for interference.
Figure 1 illustrates the RF power distribution throughout the engineering
quadrant of a major North American university. Utilizing about 330 metres
of RFS radiating cable, the passive distribution system was designed around
an existing access point, which formerly consisted of a point-source antenna
and amplifier. Interference issues with the previous system led the university
to adopt the RFS radiating cable solution.
The RF power distribution in Figure 1 extends from around -35 dBm (yellow)
at points nearest to the amplifier, to a minimum of around -80 dBm (blue/purple)
at the outside walls. Since minimum receiver sensitivity is typically
around -100 dBm, RFS designed the university WLAN system for a worst case
at the outside walls of -85 dBm. Tailoring the RF coverage to this extent
using radiating cable minimizes the potential for co-channel interference,
and provides the university with maximum flexibility in channel allocations.
Overall performance—for example, data rate—is also improved
using radiating cable, which provides more direct ‘line of sight’
coverage to client devices than point-source antennas. This minimizes
the impact of RF obstructions—such as steel filing cabinets and
bookcases. Combined with the low emitting power used in radiating cable,
the shorter signal path also reduces the effects of multi-path interference,
which occurs due to ‘out of phase’ reflections of the RF signal.
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