Sub-Block And Sliding Window Segmentation; Map Unit Block Diagram - Texas Instruments TURBO-DECODER COPROCESSOR 2 TMS320C6457 DSP User Manual

Architecture
Data
from
memory
The TCP2 can enable or disable the max star function by modifying the E_MAX_STAR bit in the TCPIC3
register.
E_MAX_STAR = 0 = Enable max star
E_MAX_STAR = 1 = Disable max star
Log-map algorithm is implemented in a highly paralleled manner using the sliding window principle. The
max-log-map algorithm is implemented with apriori scaled prior to the map decoder.
8.1

Sub-block and Sliding Window Segmentation

The MAP unit splits the frame (in standalone mode) or the sub-frame (in shared processing mode) into
sub-blocks.
If the size of the frame is n and the number of states in the encoder trellis is S, then beta states must be
stored in the beta memory. To reduce the amount of memory required to store the beta states, the sliding
window technique is used. This technique divides the frame size n into several smaller pieces of size r
(reliability section). Because the starting state is unknown, a prolog section is added. The initial state of
the starting location of the prolog section is unknown. After processing the prolog states and the constraint
length, the values of the last prolog states have a high probability of being in the same states as if the
states were known from the start. Each sub-block consists of 1or 2 sliding windows. Each sliding window
consists of three portions:
Alpha prolog portion
Reliability portion
Beta Prolog portion
60 TMS320C6457 Turbo-Decoder Coprocessor 2
Figure 95. MAP Unit Block Diagram
Beta
Beta
memory
Scratch
Alpha
memory
Extrinsic
Extrinsic
signals
SPRUGK1 – March 2009
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