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.
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.
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. At 125 short packets should get through with some retransmissions, 150 and above can be
considered “good” values.
3.4.1. How to battle with multipath propagation?
The first step is the diagnosis. We have to realize we are in trouble and only a field measurement can
tell us that. We should forget about software tools and simply assume that a multipath problem may
appear on every non-LOS hop in the network.
These are clear indicators of a serious multipath propagation problem:
•
directional antennas "do not work", e.g. a dipole placed at the right spot yields a better RSS than
a long Yagi, or rotating the directional antenna shows several peaks and troughs of the signal and
no clear maximum
•
RSS changes rapidly (say 10 dB) when antenna is moved by less than a meter in any direction
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