Page 216 - GuideFWA

value of K for interference calculations, where the factor ? accounts for the fact that the UPCS
receiver may not “see” FWA transmissions originating from some directions, or from some

antenna orientations. Clearly, ? = 1.

5.1.1.2.4 Interference Model

The actual number of transmissions within the circle on a given channel at a given time is of

course random, and can be modeled as a Poisson-distributed random variable with a mean value

of Keff , in which case the probability that there are J transmissions on a given frequency/timeslot

is:

P( J ) = e -Keff KJ . (5.3)
eff
J!

If the distribution of transmitters over the area within the circle is uniform, then the probability

density function (pdf) of the distance rj between the jth FWA transmitter and the UPCS receiver
is:

f rj (r) = 2r . rmin = r = d (5.4)

d2 - r2
min

The power into the UPCS receiver, in dBm, is:

( )I j,dBm = Pt + Gt + Gr - 10log L rj - 15dB (5.5)

where Pt is the RF power output of the FWA transmitter, Gt is the power gain of the FWA

antenna, and Gr is the power gain of the UPCS antenna. The 15 dB additional loss accounts for

attenuation into the building.

The total interference power from FWA transmissions into the UPCS receiver on a given FWA

channel is:

J (5.6)

I = ? I j (mW).
j =1

Clearly, the aggregate interference power I must be modeled as a random variable, since it is the

sum of interference contributions from a random number of randomly-positioned transmitters.

Therefore, to understand the impact of the FWA interference on the UPCS receiver, it is

necessary to determine the cumulative distribution function (CDF) of I, which is denoted by:

FI (x) = Pr{I < x} . (5.7)

Assuming that there are N channels (for DECT, N = 120), and the received interference power

levels on different channels are statistically independent, the CDF of the interference power I min

on the least-interference channel is:

{ } [ ]FImin ( x) = Pr I min < x = 1 - 1 - FI ( x) N . (5.8)

That is, I min < x if and only if In < x for all n, where In is the total power from the FWA
transmissions at the UPCS receiver, on the nth FWA channel. Not surprisingly, the CDF for I min

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