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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
are in different phases when hitting the receiving antenna. They may add-up, they may cancel each
other out.
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
are in different phases when hitting the receiving antenna. They may add-up, they may cancel each
other out.
TX antenna
Fig. 3.3: Multipath propagation
What makes things worse is that the path length changes over time. Since half the wavelength – e.g.
0.3 m at 450 MHz - makes all the difference between summation and cancellation, a 0.001% change
of a path length (10 cm per 10 km) is often significant. And a small change of air temperature gradient
can do that. Well, that is why we have to have a proper fade margin. Now, what makes things really
bad is that the path length depends also on frequency. Normally this dependency is negligible within
the narrow channel. Unfortunately, because of the phase combinations of multiple waveforms, the
resulting signal may get so distorted, that even the sophisticated demodulating techniques cannot read
the original data. That is the situation known to RF data network engineers – signal is strong enough
and yet “it” does not work.
0.3 m at 450 MHz - makes all the difference between summation and cancellation, a 0.001% change
of a path length (10 cm per 10 km) is often significant. And a small change of air temperature gradient
can do that. Well, that is why we have to have a proper fade margin. Now, what makes things really
bad is that the path length depends also on frequency. Normally this dependency is negligible within
the narrow channel. Unfortunately, because of the phase combinations of multiple waveforms, the
resulting signal may get so distorted, that even the sophisticated demodulating techniques cannot read
the original data. That is the situation known to RF data network engineers – signal is strong enough
and yet “it” does not work.
That is why RipEX reports the, somewhat mystic, figure of DQ (Data Quality) alongside the RSS. The
software demodulator uses its own metrics to assess the level of distortion of the incoming signal and
produces a single number in one-byte range (0–255), which is proportionate to the “quality” of the signal.
Though it is very useful information, it has some limitations. First, it is almost impossible to determine
signal quality from a single packet, especially a very short one. That results in quite a jitter of DQ values
when watching individual packets. However when DQ keeps jumping up and down it indicates a serious
multipath problem. In fact, when DQ stays low all the time, it must be noise or permanent interference
behind the problem. The second issue arises from the wide variety of modulation and data rates RipEX
supports. Though every attempt has been made to keep the DQ values modulation independent, the
differences are inevitable. In other words, experience is necessary to make any conclusions from DQ
reading. The less experience you have, the more data you have to collect on the examined link and
use other links for comparison.
software demodulator uses its own metrics to assess the level of distortion of the incoming signal and
produces a single number in one-byte range (0–255), which is proportionate to the “quality” of the signal.
Though it is very useful information, it has some limitations. First, it is almost impossible to determine
signal quality from a single packet, especially a very short one. That results in quite a jitter of DQ values
when watching individual packets. However when DQ keeps jumping up and down it indicates a serious
multipath problem. In fact, when DQ stays low all the time, it must be noise or permanent interference
behind the problem. The second issue arises from the wide variety of modulation and data rates RipEX
supports. Though every attempt has been made to keep the DQ values modulation independent, the
differences are inevitable. In other words, experience is necessary to make any conclusions from DQ
reading. The less experience you have, the more data you have to collect on the examined link and
use other links for comparison.
The DQ value is about proportional to BER (bit error ratio) and about independent of the data rate and
modulation used. Hence some rule-of-thumb values can be given. Values below 100 mean the link is
unusable. For a value of 125, short packets should get through with some retransmissions, 150–200
means occasional problems will exist (long term testing/evaluation of such link is recommended) and
values above 200 should ensure reliable communication.
modulation used. Hence some rule-of-thumb values can be given. Values below 100 mean the link is
unusable. For a value of 125, short packets should get through with some retransmissions, 150–200
means occasional problems will exist (long term testing/evaluation of such link is recommended) and
values above 200 should ensure reliable communication.
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