UNAOHM
56
EP2500 (Rev. 0)
single one, in order to synthesize the final complex signal. In the receiver too, an algorithm is capable of
breaking down the incoming signal into the many carriers composing it and subsequently decoding it.
12.5.1
OFDM CH BER and Post Viterbi BER Measurement
It is possible to carry out measurements on the OFDM signal received in different points of the receiver,
whose simplified block diagram is shown in the above Figure 28. In particular, there are two strategic
points: just before the Convolutional Error Corrector (or Viterbi) and in that case called Channel BER
(or pre Viterbi BER), or right after first corrector and, in this case, called post Viterbi BER (or pre Reed
Solomon BER, the name of the second corrector).
Norms establish that when a pV BER value is better than 2E-4 (one error out of 10000 bytes) the system
is to be considered good and the threshold of 2E-4 is to be considered Quasi Error Free (QEF);
therefore, apparently, it should be sufficient to control this value to consider the signal quality
acceptable. On a practical level, keep in mind which type of constellation is being received:
•
In the QPSK mode the modulation system itself operates so as to reduce the number of reception
errors to a minimum; the Viterbi corrector is capable of correcting many of these errors, especially
with 1/2 and 2/3 Code Rate. As a result, the number of measurable errors following this phase is so
low that it becomes difficult or impossible to carry out a measurement (in fact, it would be quite
difficult to carry out a measurement in a reasonable length of time in the presence, for example, of
one error an hour). In this case it is more convenient to use the Channel BER
•
In the QAM mode the number of errors in reception is higher and consequently even the post
Viterbi measurement becomes possible. It will still however be necessary to keep a good margin
with respect to the QEF value of 2E-4.
In general, for a good installation that should resist variations throughout long periods of time (aging of
components, thermal shifts) and those provoked by changes in meteorological conditions (weakening
due to snow, rain) the initial conditions to be obtained could be:
⇒
Channel BER better than 1E-3 (better than one error out of 1,000)
⇒
Post Viterbi BER better than 1E-5 (better than one error out of 100,000).
Both values should be guaranteed. It will only be a matter of reading one or the other according to the
functioning mode.
The values indicated refer to the users’taps; at the exchange output the values must necessarily be better,
roughly ten times lower.
12.5.2
OFDM MER Measurements
MER
is an index for digital signal quality. The better the signal the higher the MER. MER can be
equated to baseband SNR (Signal to Noise Ratio) in an analogue signal. MER, therefore, should not be
confused with C/N (Carrier to Noise Ratio) which is taken at RF level.
12.5.3
OFDM RU Count (Reed-Solomon Uncorrected)
Another parameter, RU, gives an idea of the quality of the signal for long periods of time. In particular,
it is possible to observe the loss of signal “blocks” that appear on the TV-picture as corrupted pixels
(image pixellation) or mosaics effect.
RU (
R
eed Solomon
U
ncorrected -Identified errors that are not corrected-) is a counter that increases
precisely when these events occur.
Naturally, the worse the BER is, the faster the RU count.
The value of the counter covers from 0 to 65535 and is blocked at the maximum value. It is reset to zero
each time there is a loss of signal lock, a re-tuning or a change in the position of the attenuator.
This conter gives an idea of the quality of the installation if checked over long periods of time.
