AP P E N D IX F
C AR R IE R T O N O IS E R AT IO F O R 64-Q AM D IG IT AL C H AN N E LS (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 BE R (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 E
b
/N
o
terms. The relation between E
b
/N
o
and the C/N is given by the following equation:
Where :
E
b
=
Energy per bit
N
o
=
Noise power in a 1 Hz bandwidth
R
s
=
Symbol
rate
M
=
Number of points of the constellation
BW
=
Bandwidth
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.
09/97
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