Tektronix RFA300A, Руководство пользователя

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Reference
RFA300A Measurement Set 8VSB
3- 15
Some noise burst durations are longer than the RS encoder can manage. For
further protection, a time diversity scheme called data interleaving is used. Refer
to Figure 3--15.
c3, c2, c1, b3, b2, b1, a3, a2, a1
c3, b3, a3, c2, b2, a2, c1, b1, a1
Non-interleaved
Interleaved
Noise
burst
Figure 3- 15: Data interleaving
A noise burst could delete all the b bytes of a non-interleaved signal resulting in
a loss that the RS decoder in a receiver cannot recover. However, if the data is
interleaved, only one byte is lost in each of the a, b and c byte segments of the
interleaved signal. This leaves enough data for the decoder to determine the
correct value. In reality, 8VSB data is interleaved to a depth of 52. This allows
the decoder to determine the correct value of data that has been damaged by a
193
µ
s noise burst.
A Trellis Encoder is another Forward Error Correction method. However, where
as the RS encoder dealt with the entire data packet as a block, the Trellis encoder
affects the stream of bits as they develop through time. Trellis encoding is
accomplished by Trellis-Coded Modulation (TCM). TCM is a combination of
coding and modulation that does not require additional bandwidth expansion
although there are extra bits for FEC.
The Trellis encoder takes an eight-bit byte and splits it into four two-bit words.
Every two-bit word is compared with previous words. A three-bit code is
assigned to each two-bit word that describes the transition from the previous
word to the current one. Therefore, for every two bits in, the encoder produces
three bits out. The Trellis decoder (Viterbi) in a receiver uses the three bits to
reconstruct the evolution of the data stream from one two-bit word to the next.
The Trellis and Viterbi coding scheme is very effective in managing white noise.
Data Interleaver
Trellis Encoder