L64780 LSI Logic Corporation, L64780 Datasheet - Page 34

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

2.11 Viterbi Metric Assignment and Quantization
2-16
PRBS generator) to obtain a snapshot of the response of the “channel”
(including any timing uncertainty) for the corresponding carrier at that
time instant.
The data cells that must be corrected lie between the scattered pilots, in
both frequency and time. This allows for appropriately generated
corrections for each data cell by using a suitable form of interpolation
applied to the measured values of the scattered pilots. As well as
obtaining “in-between” values of the channel response, the interpolator
also slightly reduces the effects of thermal noise on the scattered-pilot
measurements. The reduction in noise and the fact that scattered pilots
are transmitted with a power approximately 2.5 dB greater than data
cells, keeps the inevitable loss of performance due to scattered-pilot
noise within acceptable bounds.
This block takes in the sequence of complex numbers, each describing
the signal received on one of the COFDM carriers, after equalization.
These are the received versions of the complex numbers chosen at the
modulator from the points of the current constellation, according to the
coded bits to be sent. This block forms Viterbi metrics (“soft decisions”)
for each of the received bits. These metrics are quantized and passed
on, after reordering in the deinterleaver stages, to the associated Viterbi
convolutional decoder. The use of “soft” rather than “hard” decisions is
vital to obtaining the rugged properties of COFDM.
The associated Viterbi convolutional decoder, which resides on a
separate chip (for example, the LSI Logic L64724), is capable of
accepting soft decisions that are quantized to either three or four bits.
Three bits are required for acceptable performance, while four bits
improve the performance for higher-order modulation options of the DVB-
T specification, especially with non-uniform constellations. Decoder chips
designed primarily for QPSK or BPSK modulation commonly use three
bits because the advantage of using four bits is minimal.
The L64780 chip provides 4-bit soft decisions, but it also can be used to
drive 3-bit decoders.
Architectural Overview