STV0196B Datasheet, PDF (19/23 Pages) STMicroelectronics – QPSK/BPSK DEMODULATOR AND FEC IC

STV0196B
ANNEXE 1 : C/N ESTIMATION
The C/N indicator register permanently reports a
value S which depends on the C/N level at the input
of the STV0196B.
The C/Nindicator offers a programmable sensitivity
which allows a reliable C/N estimation over a wide
Eb/No range (4dB to 16dB typically) ; this is par-
ticularly useful to optimize the dish positioning.
Remark : In this note, we have assumed that :
C
N
=
E_b
N_o
⋅
2
(PR),
PR : Puncture Rate
The sensitivity of the C/N indicator is dependant on
the SN bits of the register VSEARCH (Hex06) and
on the AGC function reference level ”m”.
A - SUGGESTED PROCEDURE TO RELIABLY
ESTIMATE THE ACTUAL C/N
As no simple mathematical low ensumes a good
matching between the C/N indicator and the actual
C/N, the method relies on a comparaison of the
value S (reported by the C/N indicator) with a
reference look-up table which has been realized
under well controlled conditions.
Basically there are 3 steps in the C/N estimation
software.
1. To collect C/N indication (under adapted
conditions).
2. Indication scaling and correction versus the
puncture rate
3. Comparaison with the look-up table
A.1 - To collect C/N Indication
The purpose of this first step is to collect the C/N
indicator with the appropriate sensitivity (SN bits
and AGC reference level m).
Basically :
- The value reported by the C/N indicator is propor-
tional to the Number of bits (at the output of the
VITERBI decoder) selected by the SN bits.
- The AGC reference level is only changed to ap-
preciate the high Eb/No ratios. This second pa-
rameter has to be used with some care.
Procedure : Before to make an estimation, the
VSTATUS register (internal address Hex 08) must
be checked to make sure that :
- a carrier is actually present (bit 7)
- puncture rate is found (bit 4)
- puncture rate is known (bits 0-1-2)
Remark : Optionally, it is possible to make an
estimation without informations about the puncture
rate (useful when the dish is still very far from
optimum position), in such case the puncture rate
is forced.
The C/N indicator register has no overflow detec-
tion, so it is necessary to start the measure with the
lowest sensitivity (SN = 00) and to gradually in-
crease it (using SN bits). Due to the noise, the result
S of the measure may have a lot of dispersion,
consequently it is recommended to measure S
several times (typically 100 times) and to calculate
the average value.
Remark : The requred duration tW between two
readings of the register must be higher than :
tW
(Min.)
=
BC
BR
BR = 2 (Fs) x (PR)
with BC : Bit Count (selected by SN bits)
Fs : Symbol Rate
PR : Puncture Rate
When the current average value of the measure S
is lower than 63, the measure is done again with a
higher sensitivity. With this care the new C/N
measure S does not overflow the counter
(the counting time is multiplied by 4 at each step).
In practice some margin is given to this threshold :
a higher sensitivity is selected when the average
value of S is lower than 60.
Wh en the maximum SN value is reached
(SN = 11 ⇔ to 65 536 bits at the output of the
VITERBI decoder), the sensitivity can be further
increased by lowering the AGC reference level
(p ara met er m, in t e rnal a dd res s He x11,
bit 0 to bit 5).
Remark : There is the need to change the AGC
reference level only in case of high C/N conditions,
then to change the reference level has no important
influence on the bit error rate (BER). In other words,
a completete C/N estimation can be run during the
operation of the receiver.
When the highest possible sensitivity is found
the result S (average value) is ready for further
process.
19/23