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HDTV TRANSMODULATOR - Dual 8PSK to QAM
10. Distribution Bandwidth considerations
The bandwidth of a transmodulated QAM signal depends on the parameters
of the incoming satellite signal (baud rate, modulation and FEC) and on the
QAM constellation selected for the output carrier. For the same parameters of
the satellite signal, the higher the QAM order, the narrower the QAM signal is.
As an example, in automatic mode, an input baud rate and satellite modulation
signal (8PSK 2/3 21.5MBaud) is transmodulated to 256QAM carriers occupying
bandwidths up to 6.43MHz. Lower input baud rate signals are transmodulated
to 64QAM or 128QAM carriers with narrower bandwidths up to 5.75MHz.
64 / 128 QAM output
Bandwidth up to 5.75 MHz
Standard American CATV channels have 6 MHz of bandwidth, therefore,
256 QAM modulated signals with a bandwidth wider than 6 MHz can cause
interference to the adjacent channels. Two 256 QAM carriers on adjacent
channels can cause a 3 dB MER degradation.
256 QAM output
Bandwidth up to 6.43 MHz
In order to avoid this overlapping it advisable to alternate narrow 64/128 QAM
carriers with wider 256 QAM carriers in the distribution, as in the example
below:
While some satellite input signals may be transmodulated to 1024QAM
carriers that are less than 6 MHz, it is still recommended to alternate narrow
64/128 QAM carriers in the same fashion. It is also recommended to set the
output level of the 1024 QAM carrier to be 2-3 dB greater than the adjacent
carriers. These steps are recommended due to the increased sensitivity to
interference in 1024QAM signals.
The fully agile and fully independent QAM outputs of the HDTV
TRANSMODULATOR - Dual 8PSK to QAM greatly help to overcome this issue.
26
Whenever channel bandwidth
is wider than 6 MHz, there is an
overlap of the 2 adjacent channels
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