Page 220 - GuideFWA

thermal noise (-174 dBm/Hz). This is perilously close to the “upper threshold” for UPCS
devices (Subpart D of FCC Part 15), which is 50 dB above thermal noise. A UPCS device is not
allowed to use a channel on which the monitored signal level is 50 dB or more above thermal
noise (kT, where k is Boltzman’s constant, 1.38 × 10-23 watts/Hz/°K and T is the reference
temperature, often taken as 290°K.

5.1.1.4.2 Effect of Interior Building Loss

For UPCS systems sufficiently far away from the exterior walls of the building, the interference
from the FWA transmissions will tend to be reduced somewhat, but the degree of the attenuation
will be highly dependent on the interior construction of the building. Many modern office
buildings use an interior “landscape” arrangement consisting largely of soft partitions which
often do not extend fully to the ceiling. In such cases, RF attenuation within the building will be
relatively small, as represented by the propagation model provided in [23] (page 45) for office
environments with “semi-high soft partitions, but without interior walls”:

L = 41 + 20log s + G × max[0,(s - 10)] dB, (5.9)

where G is 0.37 dB/m or 0.59 dB/m depending on the density of the partition. This model could

be applied to the current problem, with s representing the distance inside the building from the

nearest exterior wall. As can be seen, it is basically a free-space model with additional

loss/meter (after the first 10m) added to represent the effect of the partitions.

A relevant question of how the distance from the nearest exterior wall is distributed. If the
building is assumed to be a square D meters on a side, it is easily shown (see 5.1.1.6 Annex) that
if UPCS receivers are uniformly-distributed over area, the CDF for the distance s from a UPCS
receiver to the nearest exterior wall is:

Fs (x) = Pr{s < x} = 4??? x - x2 ? , 0= x = D (5.10)
D D2 ?? 2

and the mean value is

s= D . (5.11)
6

The table below shows the CDF of s for a few points on the distribution.

x/D Pr{s<x}
0.1 36%
0.15 51%
0.2 64%
0.3 84%
0.4 96%

As an example, consider the 100m × 100m reference building used in the coexistence analyses of
ETR 310 [23]. With D = 100m, the average distance to the nearest exterior wall is about 16m,

and half of the UPCS receivers will be within 15m of the nearest exterior wall. For these, the

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