UNAOHM
50
EP2500 (Rev. 0)
12
APPENDIX B – DVB-S DVB-C DVB-T MODULATION
12.1
INTRODUCTON
The modulation systems adopted for the transmission of digital signals have been optimized for the
different types of transmission being used: the QPSK is used for satellite signals owing to its resistance
to noise; the QAM is used for cable signals owing to its extreme efficiency in exploiting the available
band; the OFDM for terrestrial signals, since it guarantees reception even in the presence of multiple
signal paths and in case of reception from moving vehicles.
12.2
DIGITAL MODULATIONS AND THEIR PARAMETERS
A digital transmission appears as noise which covers the entire channel, normally about 30 MHz in
QPSK, 7 MHz in QAM and OFDM. To decode the signal it is necessary to know some of the
transmission parameters. Many of them are standard, but some may vary from one channel to another,
and must be programmed on the receiver. They are:
1) Channel Frequency.
Tuning may seem common a common operation at first sight but is not. The tolerance allowed in tuning
is very narrow since the transmission appears as a channel containing only “noise” and, as a
consequence, the channel nominal frequency is difficult to identify. The QPSK and QAM cards are
fitted with an automatic frequency control (AFC). The tuning capture range is
±
3MHz for the QPSK
and 0.5 MHz for the QAM. The OFDM frequency must be accurately tuned.
2)
Spectrum.
Just as what happens passing from Ku to C band, the band spectrum may be either direct or inverted;
depending on whether the frequency is lower or higher than that of the reception frequency of the local
oscillator of converters that may be used on distribution. The transmodulators may also generate direct
or inverted spectra.
3) Symbol Rate (for QPSK and QAM).
This is the speed at which digital data are transmitted. Each symbol corresponds to two bits (equivalent
to the Baud Rate in telephone modems) in QPSK, from four to eight bits in QAM, and to a very high
number of bits (several thousands ) in OFDM environment. Currently, for example, the most used
Symbol Rates are 27.5 MS/s (Mega Symbol/second) in QPSK and 6.111 MS/s in QAM. The OFDM is a
particular transport coding, since a very low Symbol Rate, a few kS/s (kilo symbols per second) relates
to a high number of bits per symbol.
4) Modulation (for QAM and OFDM).
It is the number of bits transmitted each symbol, corresponding to 4 for QAM16, 5 for QAM32, 6 for
QAM64, 7 for QAM128, 8 for QAM256. The higher the number of bits per symbol transmitted, the
higher the transmission efficiency of the channel, but also the lower the robustness against any form of
interference.
5) Code Rate (for QPSK and OFDM).
This is also known as Viterbi rate (name of the correction circuit). Since the SAT signal is quite noisy,
control bits are interlaced with the actual transmission data bits to allow the receiver to identify those
received incorrectly and, if possible, to correct or eliminate them. The more control bits added, the safer
the transmission is but the lower its efficiency id est, the quantity of data that can be transmitted at the
same time. From the transponder’s point of view, this means fewer TV channels and/or a poorer MPEG
quality. Code Rates at 1/2, 2/3 and 3/4 are currently used, corresponding to one control bit following
one data bit, one control bit following two data bits, one control bit following three data bits. The main
purpose of the Viterbi corrector is to correct errors caused by noise, therefore it is not used in QAM
where signals distributed by cable are almost noise free.
6) Guard Interval (for OFDM).
This parameter is used only
in OFDM. It may vary from 1/4 to 1/32. It indicates the percentage of
transmission time dedicated to cancelling echoes and multiple signal paths.
