Lucent Technologies DSP1611, Information Manual, Page: 330 / 443

DSP1611/17/18/27/28/29 DIGITAL SIGNAL PROCESSOR Information Manual
Error Correction Coprocessor (DSP1618/28 Only) April 1998
14-4
DRAFT COPY
Lucent Technologies Inc.
14.2 Hardware Architecture
(continued)
14.2.1 Branch Metric Unit (continued)
Six 8-bit soft symbols S(0),
. . .
, S(5) are loaded into the ECCP. The incremental branch metrics associated with all
2
C
state transitions are calculated as indicated in Table 14-1 .
The received 8-bit signals S(5),
. . .
, S(0) are stored in the S5H5 through S0H0 registers. The generating polyno-
mials G(1) and G(0) are stored in the upper and lower bytes of the ZIG10 register, respectively. The generating
polynomials G(3) and G(2) are stored in the upper and lower bytes of the ZQG32 register, respectively. The gener-
ating polynomials G(5) and G(4) are stored in the upper and lower bytes of the G54 register, respectively.
14.2.2 Update Unit
The add-compare-select operation of the Viterbi algorithm is performed in this unit. At every time instant, there are
2
C
state transitions of which 2
C – 1
state transitions survive. The update unit selects and updates 2
C – 1
surviving
sequences in the traceback RAM that consists of the fourth bank of the internal RAM (RAM4). The accumulated
cost of the path p at the Jth instant (ACC(J, p)) is the sum of the incremental branch metrics belonging to the path
p up to the time instant J:
The update unit computes and stores full-precision 24-bit resolution path metrics of the bit sequence. To assist the
detection of a near overflow in the accumulated path cost, an internal vectored interrupt (EOVF) is provided.
14.2.3 Traceback Unit
The traceback unit selects a path with the smallest path metric among 2
C – 1
survivor paths at every instant. The
last signal of the path corresponding to the maximum likelihood sequence is delivered to the decoder output. The
depth of this last signal is programmable at the symbol rate. The traceback decoding starts from the minimum cost
index associated with the state with the minimum cost:
If the end state is known, the traceback decoding can be forced in the direction of the right path by writing the
desired end state into the minimum cost index register (MIDX).
Table 14-1. Incremental Branch Metrics
Distance Measure Code Rate 16-bit Incremental Branch Metric
Euclidean 1/1
Euclidean 1/2
Manhattan 1/1
Manhattan 1/2
Manhattan 1/3 or 1/4
Manhattan 1/5 or 1/6
S 0
( )
E 0
( )
–
( )
2
Σ
S i
( )
E i
( )
–
( ) 2 [ ]
>> 1 i 0 = 1 , ,
S i
( )
E i
( )
–
[ ]
<< 8 i , 0 =
Σ
S i
( )
E i
( )
–
( ) [ ]
<< 7 i 0 = 1 , ,
Σ
S i
( )
E i
( )
–
( ) [ ]
<< 6 i 0 = 1 2 or 3 , , , ,
Σ
S i
( )
E i
( )
–
( ) [ ]
<< 5 i 0 = 1
…
4 or 5 , , , , ,
ACC J p ,
( ) Σ
BM j p ,
( )
j 1 =
…
J , , , =
min ACC j p
1
,
( ) …
ACC j p
2
C 1 –
,
( )
, ,
{ }