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15
16
x
FRAMING
(if any)
FORWARD
ERROR
CORRRECTION
(FEC)
REED-
SOLOMON
OUTER FEC
CODEC
(if any)
MODULATOR IF STAGES
Terrestrial
Data Rate
Data Rate
+ Framing
Data Rate
+ Framing + RS
Data Rate
+ Framing + RS
+ FEC
Baseband signal
at Symbol Rate
IF Signal Modulated
at Symbol Rate
D
ata Rate
Data Rate
I
ncrease:
Increase:
IBS/SMS:
IDR: +96kbps
2
3
x
x
7
8
x
3
4
x
Specified as (N.K)
e.g. (219,201)
N
K
x
1/2 Rate:
2/3 Rate:
3/4 Rate:
7/8 Rate:
Uncoded: x 1
2 BPSK:
QPSK:
8PSK:
1
2
3
Occupied Bandwidth:
1.2 x Symbol Rate.
Intelsat Recommended Carrier
Intelsat Recommended Carrier
Spacing: 1.4 x Symbol Rate
E
xample:
Example:
6
4 kbps
64 kbps
+ IBS/SMS framing:
=68.266kbps
+ (126,112) RS Coding:
=76.8kbps
+ 1/2 Rate FEC:
=153.6kbps
+ QPSK:
=76.8kSym/s
Occupied b/w= 92.16KHz
Carrier Spacing: 107.52KHz
Clock
Data
P300H P300 Series Modem Installation and Operating Handbook Page 145
For some applications, particularly when operating with lower data rates, a reduction in the processing delay
associated with Reed-Solomon FEC is the prime goal. In such circumstances use the `Custom Features`
feature and select shorter codewords to reduce the encoder / decoder & interleaver / de-interleaver delay.
For example switching from INTELSAT standard values of n, k, t = 126, 112, 7 to 64, 56, 4 provides
approximately the same correction ability (7 in 126 = 5.5% and 4 in 64 = 6.25% respectively), with similar
overheads (126/112=12.5%, 64/56=14.3%), but with interleaving & decoder delays reduced from 5632 to
3104 bits (156ms to 85ms at 32kbps). If using Sequential FEC, see also `Fault Mode = 3` in Appendix E.
8.5 NOTES ON DATA RATES & SYMBOL RATES
The normal constraint for a modem is the range of Symbol Rates over which the modem can operate
correctly. The Symbol Rate range however is not of initial interest to most modem users, as they first need
to know if the modem can operate at the terrestrial Data Rate they require. This section briefly explains the
relationship between Data Rates and Symbol Rates.
The Data Rate is normally the frequency of the clock used on the terrestrial port however if Drop/Insert
is active the Data Rate is 64kbps times the number of timeslots dropped/Inserted.
As the P300 Series Modem use PSK (Phase Shift Keying) the Symbol Rate is the rate of phase
changes on the I.F. signal. Each phase change represents a new symbol to the demodulator, and
depending on the modulation scheme that symbol may convey one (BPSK), two (QPSK), or three
(8PSK) bits of information.
The diagram below shows how the symbol rate is built up from the data rate.
You will see that with ½ rate FEC and QPSK (a common combination), the Symbol Rate is the same as
the Data Rate (assuming no Framing or Reed-Solomon). This is a useful combination to check on any data
sheet, as this will give you the Symbol Rate limits of the modem, from which the Data Rate limits in all other
modes and combinations can normally be derived.