參數(shù)資料
型號(hào)： STV0196
廠商： 意法半導(dǎo)體
英文描述： QPSK/BPSK DEMODULATOR AND FEC IC
中文描述： 調(diào)制/ BPSK調(diào)制解調(diào)器和FEC集成電路
文件頁(yè)數(shù)： 19/23頁(yè)
文件大?。?/td> 236K
代理商： STV0196
The C/N indicator register permanently reports a
value Swhich dependsontheC/Nlevelattheinput
of the STV0196B.
The C/Nindicatoroffersaprogrammablesensitivity
which allows a reliable C/N estimation over a wide
Eb/No range (4dB to 16dB typically) ; this is par-
ticularly useful to optimizethe dish positioning.
Remark :
In this note, we have assumed that :
C
N
=
E
b
The sensitivityof theC/Nindicatorisdependanton
the SN bits of the register VSEARCH(Hex06) and
on theAGC functionreference level ”m”.
N
o
2
(
PR
)
, PR : Puncture Rate
A - SUGGESTEDPROCEDURE TO RELIABLY
ESTIMATE THE ACTUAL C/N
As no simple mathematical low ensumes a good
matchingbetweenthe 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 referencelevel m).
Basically :
- The valuereportedbytheC/Nindicator ispropor-
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-
rameterhas to be usedwith some care.
Procedure :
Before to make an estimation, the
VSTATUS register (internal addressHex 08) must
be checkedto makesure that :
- a carrieris actuallypresent (bit 7)
- puncturerate is found (bit 4)
- puncturerate is known (bits 0-1-2)
Remark : Optionally, it is possible to make an
estimationwithout informationsaboutthepuncture
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 overflowdetec-
tion,so itisnecessarytostartthemeasurewiththe
lowest sensitivity (SN = 00) and to gradually in-
creaseit(usingSN bits).Duetothenoise,theresult
S of the measure may have a lot of dispersion,
consequently it is recommended to measure S
severaltimes (typically100 times) and to calculate
the average value.
Remark : The requred duration t
W
between two
readingsof the register mustbe higherthan :
t
W
(
Min.
)
=
BC
BR
BR = 2 (Fs) x (PR)
with
BC : Bit Count (selected by SNbits)
Fs : Symbol Rate
PR : Puncture Rate
When the currentaverage value of the measure S
is lowerthan 63, the measure is doneagainwith a
higher sensitivity.
With this care the new C/N
measure S does not overflow the counter
(the countingtime is multiplied by 4 at each step).
In practicesome margin is given to this threshold:
a higher sensitivity is selected when the average
value of S is lower than 60.
When 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
(parameter
m
, internal address Hex11,
bit 0 to bit 5).
Remark : There is the need to change the AGC
referencelevel only in case of highC/N conditions,
thentochangethereferencelevelhasnoimportant
influenceonthebiterrorrate(BER).Inotherwords,
a completeteC/N estimationcan be runduring the
operationof thereceiver.
When the highest possible sensitivity is found
the result S (average value) is ready for further
process.
ANNEXE 1 :
C/N ESTIMATION
STV0196B
19/23
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