TANDBERG SM6610, Справочное руководство

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Equipment Description
Page 7-4 Reference Guide: SM6610, SM6615, SM6620 and SM6625 Satellite Modulators
ST.RE.E10152.2
Figure 7.2: 8PSK and 16QAM Constellations
7.2.3 Forward Error Correction and Interleaving
Noise and interference can cause some bits to be received in error. To
counter such errors, a concatenated forward error correction (FEC) coding
scheme is used, comprising an outer Reed-Solomon (204, 188) code,
convolutional interleaving and an inner convolutional code. In this scheme,
firstly 16 parity check bytes are added to the data in each packet by the
RS Encoder. The data are then passed through a convolutional interleaver
which shuffles the order of transmission of the data bytes. Finally, the data
are passed through a convolutional encoder which adds additional
redundant data bits. Different convolutional code rates can be used to
provide different levels of ruggedness:
1
⁄
2
,
2
⁄
3
,
3
⁄
4
,
5
⁄
6
,
7
⁄
8
and
8
⁄
9
, the values
available being dependent upon the modulation selected.
At the receiver the data are Viterbi decoded, de-interleaved and then RS
decoded. The inner convolutional coding together with Viterbi decoding
provides powerful error correction capability against errors induced by
additive white Gaussian noise (AWGN) in the channel, but any residual
errors at the output of the Viterbi Decoder tend to occur predominantly in
blocks. The de-interleaving process redistributes the errors so that they
occur more randomly as well as restoring the original order of the data
bytes, allowing the following RS Decoder to further correct the residual
errors to maximum effect.
7.3 Modulator Functional Description
7.3.1 Introduction
Each of the SM6600 Satellite Modulator Base Unit models contains three
main cards: a Baseband card, an Output card and a Host Controller card
(see Figure 7.3 and Figure 7.4). The Baseband card and the Output card
together form the core of the Modulator. The Host Controller card is
responsible for the communication, configuration and monitoring functions
performed by the Modulator. Additionally, an optional Input card (not
shown in Figure 7.3 and Figure 7.4) may be fitted.
The function of each card is described in greater detail throughout this
section.
8PSK
3 bits
0 0 0
16QAM
4 bits
0 0 0 0