UNAOHM
EP2500 (Rev. 0)
51
7) Operating Mode or Number of Carriers (for OFDM).
This parameter is used only in OFDM. Actually, OFDM does not indicate a particular type of
modulation, but the fact that thousands of carriers are transmitted at the same time, each one with its
own modulation, which may be QPSK, 16QAM or 64QAM. Currently the standards are: two thousand
(2k) or eight thousand (8k) carriers per channel.
12.3
BER
It is fundamental and essential in the digital transmission business to adopt new methods for signal
quality evaluation, where signal quality depends mostly on the installation’s. Unlikely analogue
terrestrial Teletext –which appears to be either noise-free or missing at all, with no conditions in-
between where increasing impairments may suggest the installer to take preventive actions before
complete signal failure– the quality of the picture and sound of a digital transmission give no indication
at all of the quality of the signal, only of its presence. While the deterioration of an analogue signal in
the form of noise, interference, distortion, is gradual and, therefore, visually noticeable –a welcome
condition to the installer, as we just said, for any preventive action he wishes to take– a digital signal
remains perfect until the decoder is capable of reconstructing it on reception after which it goes astray
completely and abruptly. The measurement for digital signal quality is, therefore, quite difficult and
must be based on a method which is very sensitive to noise. This method is called BER or Bit Error
Ratio. BER is the measurement of the ratio between the number of bits in error found and the total
quantity of bits received. The BER is indicated by the meter in exponential form; i.e., 1 (bit in error) out
of 10 (bits total) is indicated as 1E-1, 2 out of 10 becomes 2E-1, 7.5 out of 10000 becomes 7.5E-4, etc.
The DVB Standard allows BER measurements to be taken in different parts of the demodulating chain.
We have two measuring points
for QPSK and OFDM transmissions
(see Figure 24 and Figure 26)
where the so called Channel BER (CH BER) and post-Viterbi BER measurements are taken. As
for the QAM, only a measuring point exists
(see Figure 25)
since CH BER coincides with the BER
pre Reed-Solomon, the Viterbi correcting stage being missing.
Figure 24
Simplified block diagram of an IRD DVB-S (QPSK) demodulating chain
Tuner
Serialize
Digitizer
RF Signal
QPSK-coded
I
Q
Channel BER
Viterbi error
correction
Reed-Solomon
error
c
orrection
post-Viterbi BER
Data output
MPEG coded
RU
Reception chain DVB-S (QPSK)
