Texas Instruments TMS320C6457 DSP manual Output Decision Data Format, Stopping Criteria

Standalone (SA) Mode									www.ti.com
		Figure 15. Rate 3/4 EN = 0 (Big-Endian Mode) Rate = 3/4
		Word					Word
		N					N + 1
SP4	SP3	SP2	SP1	SP0	SP9	SP8	SP7	SP6	SP5
0	0	0	A0	X0	0	0	0	0	X1
		Word					Word
		N + 2					N + 3
SP4	SP3	SP2	SP1	SP0	SP9	SP8	SP7	SP6	SP5
0	0	0	0	X2	0	A3'	0	0	X3
		Word					Word
		N + 4					N + 5
SP4	SP3	SP2	SP1	SP0	SP9	SP8	SP7	SP6	SP5
0	0	0	0	X4	0	0	0	0	X5

4.1.2Interleaver Indexes

Each index is a 15-bit value being effectively saved as 16 bits right-justified. Given an index j, an interleaver table t, and a data x, the interleaved data x is given as x' = x[t(j)]. Table 2 shows how data must be organized in the memory. The base address must be double-word aligned. For big-endian configurations, see the TCP2 endian register (TCPEND) in Section 6.22.

Table 2. Interleaver Data

Little_big_endian	Endian_intr	Description (MSB to LSB)
0	0	1,0,3,2 ⇒ 3,2,1,0 (halfword)
0	1	0,1,2,3 ⇒ 3,2,1,0 (halfword)
1	0	Endianness manager has no effect
		3,2,1,0 ⇒ 3,2,1,0 (halfword)
1	1	Endianness manager has no effect
		3,2,1,0 ⇒ 3,2,1,0 (halfword)

4.2Output Decision Data Format

Hard decisions for TCP2 are 32-bit word-packed. The bit ordering within the 32-bit hard-decision word is programmable, such that the oldest bit can be either in the MSB or the LSB position. Their destination storage base address must be double-word aligned. Moreover, the buffer length must contain an even number of words.

4.3Stopping Criteria

The turbo decoder has an iterative structure, and the number of iterations that are performed for each frame is either a deterministic number or it depends on a test performed on the turbo decoder output after each iteration. In the first case, you decide how many iterations should be performed prior to decoding a frame. In the second case, the turbo decoder performs tests after each iteration to determine whether the iterative process should continue. In this case, the boundary conditions are programmed (for example, the minimum and the maximum number of iterations that should be performed). The tests performed are the SNR stopping criterion and cyclic redundancy check (CRC) iterations passed.

This SNR stopping criterion on TCP2 can be used by setting the SNR threshold from 1 to 100 (0 disables the SNR threshold check). The stopping criteria is met and a TCPREVT is generated when the SNR threshold is met, the minimum iterations have been processed, and sufficient CRC iterations have passed, if CRC is enabled. This indicates that decisions are ready for the EDMA3 to access.

Larger thresholds improve bit-error rate (BER) performance, but require more iterations. Smaller thresholds require fewer iterations, but may yield poorer BER performance. The actual number of iterations run can be read from the output parameters.

16 TMS320C6457 Turbo-Decoder Coprocessor 2 SPRUGK1–March 2009

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