The available TX output power and guaranteed RX sensitivity level for the given data rate have to be
declared by the radio manufacturer. RipEX values can be found in Table 4.6, “Technical parameters”
and Chap Section 4.4.1, “Detailed Radio parameters”. Antenna gains and directivity diagrams have to
be supplied by the antenna manufacturer. Note that antenna gains
against isotropic radiator (dBi)
are used in the calculation. The figures of feeder cable loss per meter should be also known. Note that
coaxial cable parameters may change considerably with time, especially when exposed to an outdoor
environment. It is recommended to add a 50-100 % margin for ageing to the calculated feeder loss.
3.3.1. Path loss and fade margin
The path loss is the key element in the signal budget. Not only does it form the bulk of the total loss,
the time variations of path loss are the reason why a fade margin has to be added. In reality, very often
the fade margin is the single technical figure which expresses the trade-off between cost and perform-
ance of the network. The decision to incorporate a particular long radio hop in a network, despite that
its fade margin indicates 90 % availability at best, is sometimes dictated by the lack of investment in a
higher tower or another repeater. Note that RipEXs Auto-speed feature allows the use of a lower data
rate over specific hops in the network, without the need to reduce the rate and consequently the
throughput in the whole network. Lower data rate means lower (= better) value of receiver sensitivity,
hence the fade margin of the respective hop improves. See the respective Application note to learn
more on the Auto-speed feature. Apart of Auto-speed, there is a possibility from fw ver. 1.6 to set certain
Radio protocol parameters individually for a specific radio hop (Individual link options). For more see
Section 7.3.2, “Radio”
When the signal path profile allows for LOS between the TX and RX antennas, the standard formula
for free-space signal loss (below) gives reliable results:
Path loss [dB] = 20 * log10 (distance [km]) + 20 * log10 (frequency [MHz]) + 32.5
In the real world the path loss is always greater. UHF radio waves can penetrate obstacles (buildings,
vegetation), can be reflected from flat objects, can bend over round objects, can disperse behind sharp
edges – there are numerous ways how a radio signal can propagate in non-LOS conditions. The addi-
tional loss when these propagation modes are involved (mostly combined) is very difficult to calculate.
There are sophisticated methods used in RF design software tools which can calculate the path loss
and its variations (statistical properties) over a computer model of terrain. Their accuracy is unfortunately
very limited. The more obstacles on the path, the less reliable is the result. Such a tool can be very
useful in the initial phase of network planning, e.g. to do the first network layout for the estimate of total
throughput, however field measurements of every non-LOS radio hop should be done before the final
network layout is designed.
Determining the fade margin value is even more difficult. Nevertheless the software tools mentioned
can give some guidance, since they can calculate the statistical properties of the signal. Generally the
fade margin (for given availability) is proportional to the difference between the real path loss and the
LOS path loss over the same distance. Then it is about inversely proportional to frequency (in the UHF
range at least). To give an example for 10 km, non-LOS, hop on 450 MHz, fade margin of 20 dB is a
bare minimum. A field test may help again, provided it is run for longer period of time (hours-days).
RipEX diagnostic tools (ping) report the mean deviation of the RSS, which is a good indication of the
signal stability. A multiple of the mean deviation should be added to the fade margin.
3.4. Multipath propagation, DQ
Multipath propagation is the arch-enemy of UHF data networks. The signal coming out of the receiving
antenna is always a combination of multiple signals. The transmitted signal arrives via different paths,
by the various non-LOS ways of propagation. Different paths have different lengths, hence the waveforms
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© RACOM s.r.o. – RipEX Radio modem & Router
Network planning
Summary of Contents for RipEX
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