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APPENDIX F.- CARRIER TO NOISE RATIO FOR 64-QAM DIGITAL
CHANNELS (C/N).
For a 64-QAM DBV-C digital signal with a bandwidth of 8 MHz, the C/N ratio
must be better than 20 dB. Measurements lower than this value will correspond to
signals with a no acceptable quality.
Justification and example
The basic parameter which describes the quality of a digital signal is the ratio of
the number of erroneous bits to the total number of bits transmitted. This parameter is
known as BER (Bit Error Rate).
The DBV Group (Digital Video Broadcasting) defines a 'quasi error free
transmission' (ETR290 Measurements Group ETR 290) as that with less than 1 error
event per transmission hour. For a 64 QAM DBV-C transmission, the pre-FEC error
(Forward Error Correction) must be <1.E-4.
In digital transmissions, as there is not one carrier, it is better to speak in Eb/No
terms. The relation between Eb/No and the C/N is given by the following equation:
Where:
E b
N o
R s
M
BW
For DVB-C, 64 QAM, Rs=6.875 Mbaud, BW=8 MHz,
C/N (dB) =E b / N o (dB) + 7.12.
For a BER of 1E-4, E b /N o ≈ 16 dB. Therefore C/N ≈ 23 dB.
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E
b
=
+
C
/
( N
dB
)
(
dB
)
10
N
o
= Energy per bit
= Noise power in a 1 Hz bandwidth
= Symbol rate
= Number of points of the constellation
= Bandwidth
USER'S MANUAL. PROMAX-10
log
(
M
)
2
*
log(
R
*
)
s
BW
04/2004
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