Altera Viterbi Compiler - Manual (Page 29)

Parameter Value Description
ACS Units (A) 1, 2, 4, 8, or 16 The number of ACS units, which
adds a degree of parallelism
(hybrid architecture only). The
range of values available depends
upon the value of maximum
constraint length L
MAX
.
Traceback (v) 8 (minimum) The traceback length, which is
the number of stages in the trellis
that are traced back to obtain a
decoded bit. It is typically set to 6
× L for unpunctured codes, and
up to 15 × L for highly
punctured codes.
Softbits (softbits) 1 to 16 The number of soft decision bits
per symbol. When softbits is set
to 1 bit, the decoder acts as a
hard decision decoder, and still
allows for erased symbols to be
entered using the eras_sym
input.
Bmgwide – The precision of the state metric
accumulation. The parameter
editor selects and displays the
optimum value, which depends
on N
MAX
, L
MAX
and, softbits.
Soft Symbol Input
The number of soft decision bits per symbol, softbits, represent 2 softbits – 1 soft 0s and 2 softbits – 1 soft 1s. The
input values represent received signal amplitudes. If the input is in log-likelihood format, a transforma‐
tion is required and you must use extra softbits to retain signal integrity. The decoder marks depunctured
values separately. The decoder allows a hard-decision input when softbits = 1.
Table 3-3: Soft Symbol Input Representation
softbits = 3
Soft Symbol Meaning
011 Strongest '0'
010 Strong '0'
001 Weak '0'
000 Weakest '0'
111 Weakest '1'
110 Weak '1'
101 Strong '1'
UG-VITERBI
2014.12.15
Soft Symbol Input 3-11
Viterbi IP Core Functional Description Altera Corporation
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