User's Manual

UtiliNet® Endpoint User Guide 11-1-2006
Page 23 of 30
© Cellnet 2005
hill or water tank, the terrain is considered relatively flat. The path loss adder term is -10 dB at 915 MHz,
according to the Okumura/Hata propagation prediction method using the suburban area model.
Receiver Antenna
The previously mentioned antenna considerations also apply here, including possible ground plane effects.
For this example, this is (again) a “3 dB omni-directional.”
Receiver Transmission Line Loss
The principles and goals here are the same as those for the Transmitter Transmission Line Loss term.
Receiver
Receive sensitivity for the UtiliNet product line is typically -107 to -112 dBm. Automated production tests
are performed on all deliverable radios at various frequencies, and with signal levels down to -104 dBm.
For this example, assume the receiver sensitivity is -107 dBm. This is a safe, conservative estimate for
current production radio sensitivity.
Noise Floor Factor
One of the factors that make the 800 to 900 MHz frequency band so attractive is the relatively low
equivalent noise temperature of man-made noise. For a noisy business environment, this has been estimated
to be 87 K at 915 MHz. Relative to an ambient antenna noise temperature of 298K, this translates to a noise
floor increase of 1.2 dB. Thus, we will allocate 1 dB (rounded to the nearest dB) of the link budget to
overcoming man-made noise.
Multipath Fade Factor
For most receive signal measurements, slight changes of antenna location (on the order of 1 wavelength),
or measurements at different times, will show a short-term variance of received signal level. This is
generally due to multipath. An allowance of approximately 8 dB should be made on most radio links at 915
MHz. This is because the difference between a median received signal level and a multipath null does not
exceed 8 dB for about 80% of the measurements you could make. This factor can be reduced by one or
more diversity schemes that are available. All Cellnet spread-spectrum radios inherently employ frequency
diversity; however, it doesn’t hurt to be conservative and still include this factor. Tabulated below are 915
MHz multipath factors for other values of percent coverage. This factor varies slightly (1 to 3 dB) for
various types of environments. Values shown are for 915 MHz urban area, small city.