Texas Instruments TMS320TCI648 Series, User Manual, Page: 34 / 48

State metric unit
Traceback unit
State
metrics
memory
Viterbi Processing Unit
Branch metrics
Decisions
SD and HD
traceback
memories
SM computation
F K−1
TB computation
Only output F decisions
Architecture
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8 Architecture
The VCP2 processing unit is shown in Figure 25 . The state metrics unit performs the Viterbi forward
recursion using branch metrics as inputs and updates the states metrics for all states (add/compare/select
or ACS operations) at every trellis stage. The state metrics memory is not accessible by the DSP. The
traceback unit performs the Viterbi backward recursion and generates hard decisions or soft decisions.
The traceback memories are not directly accessible by the DSP.
Figure 25. Processing Unit
8.1 Sliding-Windows Processing
The traceback hard-decision memory can store up to 32768 traceback bits and there are 2
(K-1)
bits stored
at each trellis stage. Therefore, the traceback hard-decision memory can store decisions of 32768/2
(K-1)
symbols. The traceback soft-decision memory can store up to 8192 traceback soft values and, therefore,
contain up to 8192 soft decisions of 8192/2
(K-1)
symbols. Assume a terminated frame of length F (excluding
tail bits) and a constraint length K, F and K determine whether all decisions can be stored in the traceback
memories.
If the decisions do not fit in the traceback HD/SD memory, then the convergent or mixed mode is used
and the original frame is segmented into sliding windows (SW); otherwise, the traceback mode is set to
tailed and no segmentation is required.
8.1.1 Tailed Traceback Mode
This mode is used when the frame is terminated and fits within the coprocessor traceback memory (see
Figure 26 ). The state metrics are computed over F + K - 1 symbols, the traceback is initialized to the
proper state and executed over F + K - 1 symbols. It should be noted that only F decisions are output.
They are output in reverse order and in blocks of user-defined size.
Figure 26. Tailed Traceback Mode
34 TMS320TCI648x/9x Viterbi-Decoder Coprocessor 2 SPRUE09E – May 2006 – Revised December 2009
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