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5 System Level Aspects for Multiple Cell Scenarios
the transmission function of the wireless channel remains nearly constant, if
there are no movements of persons or furniture. Long term measurements
of habitations have shown that the transmission function of the channel is
influenced significantly by persons moving around. However, in many mea-
surements the best channel remains the optimum choice even when habitants
changed their position. Even if adaptation to the best transmission frequency
is required, this can be organized with limited communication overhead, be-
cause changes happen comparably slow. The evaluation of a large number
of measured transmission channels has shown the potential of a dynamic fre-
quency selection in our use case. Measurements have been carried out for a
bandwidth of 100 MHz (2.4 - 2.5 GHz), thus covering the complete ISM band.
A WLAN channel can be characterized by a portion of 20 MHz within this
band. The 13 WLAN channels specified for Europe have carrier frequencies of
2.412 - 2.472 GHz with a 5 MHz distance. Therefore, the available channels
overlap. Calculating the average received power of these specified channels and
comparing it for the different possible choices, and evaluating this figure for
a large number of measurements, a statistical measure for improvements in a
single link can be given. The frequency selection gain illustrated in Fig. 5.4 is
specified as the difference of the received average power between the worst and
the best channel. In the 2.4 GHz band this gain is up to 20 dB. Furthermore, it
turns out that in 50% of all cases this gain is at least 5.4 dB for a line-of-sight
(LOS) transmission and 7.2 dB for a non-line-of-sight (NLOS) transmission.
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