AP01 HD rev.09 UK
36
7.3.
OFDM MEASUREMENTS
OFDM is the name of the transmission system adopted in Europe to transmit TV signals
over the air. The performance of this system can be briefly summarized as follows:
•
Taking up of a channel frequency equal to that of current analogue transmissions.
•
Transmission of several programs with different resolutions in a single channel, roughly
from four high-quality programs to eight low-quality ones, with eventual intermediate
combinations.
•
Decrease in power required from transmitters.
•
Particular resistance to echoes and multiple paths.
•
Possibility of transmitting in optimized way to be received by installations with fixed
antenna on the roof, internal antenna or by motor vehicles.
•
Easy transmission of any kind of data, such as program menus, several simultaneous audio
channels, subtitles in different languages.
The OFDM is characterized by the simultaneous transmission of a very high number of
carriers, two or eight thousands, each one with its own digital modulation. Actually carriers
are not generated and modulated individually: mathematical algorithms generate
modulation for the carrier, which is initially one, in order to synthetize the final complex
signal. Even in the receiver, an algorithm can separate the incoming signal into the
composing carriers and then decode them.
7.3.1.
CH BER AND POST VITERBI BER
Measures of the received OFDM signal are carried out immediately before the Convolutional
Error Corrector (Viterbi) or immediately after it, indicated respectively as Channel BER (or
pre-Viterbi BER ) and post-Viterbi BER.
As a rough guide, for an installation that keeps good over time (component ageing, thermal
drifts) and despite varying weather conditions (attenuation for snow, rain), initial conditions
could be:
1°
Channel BER better than 1E-3 (better than one error out of 1.000)
2°
Post Viterbi BER better than 1E-5 (better than one error out of 100.000)
These values refer to the user sockets. At the switchboard output, values must necessarily
be better, indicatively ten times lower.
7.3.2.
MER in OFDM
The
MER
(Modulation Error Ratio) measure is expressed in dB and refers to the quality of
the demodulated signal.
MER can be compared to the Signal-Noise ratio (C/N) of an analogue signal, measured in
bandbase.
Do not confuse it with C/N measure, which is executed on carrier in Radio Frequency.
