SVP HDT-70, Manual

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HDT-70_H.264 High Definition Camera Transmitter
MANUAL V9.0
An OFDM modulated signal, since uses a low symbol rate modulation
scheme ( i.e. where the symbols are relatively long compared to the channel
time characteristics) suffer less from intersymbol interference caused by
multipath propagation. As the duration of each symbol is long, it is feasible
to insert a guard interval between the OFDM symbols, thus eliminating the
intersymbol and co-channel interference. So, if one carrier’s information is
lost, it would only be lost a small part of the whole information.
Besides, in OFDM, the sub-carrier frequencies are chosen so that the sub-
carriers are orthogonal to each other, meaning that cross-talk, interference,
between the sub-channels is eliminated. The orthogonality allows high
spectral efficiency.
On the other hand, OFDM system is invariably used in conjunction with
channel coding (forward error correction). The error correction code used in
this equipment is Reed-Solomon coding, which is concatenated with LDPC,
and there is an additional interleaving between the two layers of coding.
Error correcting codes build redundancy into the transmitted data stream.
This redundancy allows bits that are in error or even missing to be corrected
in the receiver.
The European ETSI EN 300755 standard defines the following LDPC coding
rates: 1/2, 3/5, 2/3, 3/4, 4/5, 5/6. There is a compromise between the
coding rate (signal robustness) and the transmitted bit rate. If the coding
rate is higher the signal transmission is more robust (1/2 is the most
robust) but the bit rate that the system is capable to transmit is lower.
Used modulation scheme of each OFDM sub-carrier, QPSK, 16QAM, 64QAM
and 256 QAM is also connected with signal robustness and transmitted bit
rate. QPSK is the most robust and 256QAM is able to transport a higher bit
rate.
Besides, the system can define 7 guard intervals: 1/4, 19/128, 1/8, 19/256,
1/16, 1/32 and 1/128. The guard interval is used to reduce intersymbol
interferences due to the multipath propagation.
In addition, it also provides several bandwidths: 1.7, 5, 6, 7 and 8 MHz in
case there are needed for different applications.
To summarize, with all of these characteristics, the maximum bit rate
achieved is 44.6 Mbps.