Page 214 - GuideFWA

One important question is whether FWA and UPCS can coexist in the same band, from an
interference perspective. The purpose of this contribution is to provide an analysis of the
interference from FWA systems that would be experienced by UPCS systems. For simplicity,
the analysis focuses on cochannel interference only, and assumes that the UPCS systems would
use some form of dynamic channel assignment (DCA) to select the least-interfered channel, or at
least one with an acceptably low interference level.

The purpose here is not to perform an exhaustive study of all possible combinations of FWA and
UPCS configurations, but rather to understand the potential magnitude of the interference and its
effect on the UPCS systems.

It is clear from the results that for geographic areas in which the demand for FWA is likely to be
high (i.e., urban and high-density suburban environments), the interference can be quite high,
severely impairing operation of UPCS systems. For a FWA traffic density of 300 Erlangs per
square kilometer,2 the interference power inside a building, on the least-interfered FWA
frequency/timeslot, is found to be roughly -65.6 dBm, which exceeds the thermal noise floor by
about 46 dB, and exceeds expected receiver noise (assuming a 10-dB noise figure) by 36 dB.
With such a high interference level, UPCS system performance would be severely impaired. As
a reference point, the UPCS rules in the U.S. prohibit a device from using a channel if the
monitored interference is 50 dB or more about the thermal noise floor.

The effective interference on the least-interfered channel becomes larger if the lack of
synchronization between UPCS and FWA, and the possibility of UPCS systems that use frames
structures different from that of the FWA system, are taken into account. Moreover, the analysis
does not account for adjacent-frequency interference or interference from the FWA uplink.
Including these factors would increase the calculated interference.

The distribution of UPCS users within a building is considered, and it is concluded that from
geometric considerations, most users will be relatively near an outside wall. Further, those at the
upper levels of corporate management hierarchies, who are most likely to be provided with
wireless handsets, are also most likely to have offices along external walls, with windows. For
these reasons, it seems unlikely that interior path loss within a building can be relied upon to
mitigate the interference problem.

It is concluded that spectrum-sharing between FWA and UPCS is generally impractical, due to
the interference problems that would result. An exception might be in very low-density (i.e.,
rural) areas, where the traffic density of both FWA and UPCS would likely be relatively low.

2 The 300 E/km2 figure is from ETSI Technical Report (ETR) 310, [23] page 28.

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