L64780 LSI Logic Corporation, L64780 Datasheet - Page 35

• Manufacturer Part Number: L64780
• Description: Dvb-t Cofdm Demodulator Technical Manual 2/00
• Manufacturer: LSI Logic Corporation

2.12 Symbol Deinterleaver
2.13 Bit Deinterleaver
From each received constellation point, the Viterbi metric assignment
block extracts up to six soft decisions. The symbol deinterleaver accepts
one COFDM symbol containing soft decisions and reorders them
according to an algorithm defined in the DVB-T specification. It then
passes this reordered data to the bit deinterleaver.
Echoes cause minima (“holes”) in the received spectrum, leading to
groups of carriers on which the data is unreliable. For optimum operation
of the Viterbi decoder, these groups of unreliable carriers must be split
up. The symbol deinterleaver fulfills part of this function; the remainder
is done by the bit deinterleaver.
In 2 k mode, there are 1705 COFDM carriers, of which 1512 in each
symbol carry data. Each of these carriers yields either 2 (QPSK), 4 (16
QAM), or 6 (64 QAM) soft decisions. Each of these soft decisions
consists of four bits of information. The effect of the operation of the
symbol deinterleaver is to write the possible 1512 24-bit words into a
RAM using one address sequence, and to read them using a different
address sequence. This means there is a delay of one COFDM symbol
between the input and the output. In the 8 k case, there are 6048 (out
of a total of 6817) COFDM carriers that convey data; thus, the L64780
provides a 6048 by 24-bit RAM.
In the presence of multipath or interference, some COFDM carriers
convey data less ruggedly than others. Each COFDM carrier supplies
either two, four, or six soft decisions; these would be impaired if the
carrier they came from were impaired. For optimum operation of the
Viterbi decoder, it is best if groups of unreliable soft decisions are
maximally spaced. The purpose of the bit deinterleaver is to split up
unreliable soft decisions caused by a single unreliable COFDM carrier.
The symbol deinterleaver separates groups of unreliable COFDM
carriers.
The bit deinterleaver accepts groups of 3-bit or 4-bit soft decisions
generated from the same COFDM carrier from the symbol deinterleaver.
Symbol Deinterleaver
2-17