Bit-alignment testing for obtaining FEC code lock
Methods and apparatus disclosed herein may be used to establish framing more efficiently in communication protocols with block-coded forward error correction. Such protocols generally involve the check of different bit-alignments searching for positions that yield zero syndromes. The search can be undesirably slow, particularly in the presence of received errors. The presently-disclosed bit-alignment testing technique reduces this search time by checking the syndrome at each data word as if that data word was the last of a code. In other words, the word positions are effectively checked without a prior assumption as to which words are the first and last of the code. This reduces the task to the number of different bit-alignments possible within a single data word, rather than the number of bit-alignments possible in a complete FEC code. In one implementation, the lock time is reduced by approximately 50 times when compared to a straightforward solution.
1. A decoder for decoding an input data stream, the decoder comprising:
a data input for receiving data words;
a code lock module within control logic of a syndrome evaluator that is configured to receive the data words from the data input;
registers that hold values that are accessible by the code lock module; and
memory that holds array values that are accessible by the code lock module,
wherein the code lock module is configured to initially set a count to zero, and
wherein the code lock module is configured, for each data word of w symbols received, to set a contribution register value to be a contribution to a syndrome from the w symbols in the data word; set a syndrome shift register value to be a syndrome work register value shifted by w symbol positions; set the syndrome work register value to be a result of an exclusive-or operation applied to the syndrome shift register value and the contribution register value; and set an array value at a location in the memory associated with the count to the syndrome work register value.
2. The decoder of claim 1 , wherein the code lock module is further configured to set a syndrome register value to be the syndrome work register value modified as if additional zero'ed data had been received up to the maximum of 2 m −1 symbols, and wherein m is a number of bits per symbol.
3. The decoder of claim 2 , wherein the code lock module is further configured to:
determine whether the count is greater than or equal to n divided by w, where n is the number of symbols in a forward error correction code;
when the count is determined to be greater than or equal to n divided by w, apply the exclusive-or operation to the syndrome register value and the array value to obtain a match result; and
when the match result is all zeroes, indicate that a match is found for a bit-alignment such that the bit-alignment is to be further checked.
4. The decoder of claim 3 , wherein the code lock module is further configured to:
determine whether the count is greater than or equal to twice n divided by w;
when the count is determined to be greater than or equal to twice n divided by w, indicate that no match is found for the bit-alignment, apply a bit-slip to obtain a new bit-alignment, and test the new bit-alignment; and
otherwise, when the count is not determined to be greater than or equal to twice n divided by w, incrementing the count before processing a next received data word.
5. The decoder of claim 1 , wherein the code lock module is further configured to:
set a temporary register value to be the syndrome work register value padded up to a full length of the syndrome;
descramble the temporary register value using a scrambling syndrome register value to obtain a descrambled value; and
set a syndrome register value to be the descrambled value.
6. The decoder of claim 5 , wherein the code lock module is further configured to:
determine whether the count is greater than or equal to n divided by w, where n is the number of symbols in a forward error correction code;
when the count is determined to be greater than or equal to n divided by w, apply the exclusive-or operation to the syndrome register value and the array value to obtain a match result; and
when the match result is all zeroes, indicate that a match is found for a bit-alignment such that the bit-alignment is to be further checked.
7. The decoder of claim 6 , wherein the code lock module is further configured to:
determine whether the count is greater than or equal to twice n divided by w; and
when the count is determined to be greater than or equal to twice n divided by w, indicate that no match is found for the bit-alignment, apply a bit-slip to obtain a new bit-alignment, and test the new bit-alignment; and
otherwise, when the count is not determined to be greater than or equal to twice n divided by w, incrementing the count before processing a next received data word.
8. A method for testing a bit-alignment for code lock, the method comprising:
initially setting a count to zero;
for each data word of a number w of forward error correction (FEC) symbols received, setting a contribution register value to be a contribution to a syndrome from the w symbols in the data word;
after setting the contribution register value, setting a syndrome shift register value to be a syndrome work register value shifted by w symbol positions;
after setting the syndrome shift register value, setting the syndrome work register value to be a result of an exclusive-or operation applied to the syndrome shift register value and the contribution register value; and
after setting the syndrome work register value, setting an array value at a location in memory associated with the count to the syndrome work register value.
9. The method of claim 8 further comprising:
after setting the array value, setting a syndrome register value to be the syndrome work register value modified as if additional zero'ed data had been received up to the maximum of 2 m −1 symbols, where m is the number of bits per FEC symbol.
10. The method of claim 9 further comprising:
for each data word received, after setting the syndrome register value, determining whether the count is greater than or equal to n divided by w, where n is the number of symbols in a forward error correction code, and n is a multiple of w.
11. The method of claim 10 further comprising:
when the count is determined to be greater than or equal to n divided by w, applying the exclusive-or operation to the syndrome register value and the array value to obtain a match result, and,
when the match result is all zeroes, indicating that a match is found such that the bit-alignment is to be further checked.
12. The method of claim 11 further comprising:
determining whether the count is greater than or equal to twice n divided by w;
when the count is determined to be greater than or equal to twice n divided by w, indicating that no match is found for the bit-alignment, applying a bit-slip to obtain a new bit-alignment, and testing the new bit-alignment; and
otherwise, when the count is not determined to be greater than or equal to twice n divided by w, incrementing the count before processing a next received data word.
13. The method of claim 8 further comprising:
after setting the array value, setting a temporary register value to be the syndrome work register value modified as if additional zero'ed data had been received up to the maximum of 2 m −1 symbols, undoing effect of scrambling using a scrambling syndrome register value to obtain a descrambled value, and setting a syndrome register value to be the descrambled value, wherein m is a number of bits per FEC symbol.
14. The method of claim 13 further comprising:
for each data word received, after setting the syndrome register value, determining whether the count is greater than or equal to n divided by w, where n is the number of symbols in a forward error correction code.
15. The method of claim 14 further comprising:
when the count is determined to be greater than or equal to n divided by w, applying the exclusive-or operation to the syndrome register value and the array value to obtain a match result, and, when the match result is all zeroes, indicating that a match is found such that the bit-alignment is to be further checked.
16. The method of claim 15 further comprising:
determining whether the count is greater than or equal to twice n divided by w; and
when the count is determined to be greater than or equal to twice n divided by w, indicating that no match is found for the bit-alignment, applying a bit-slip to obtain a new bit-alignment, and testing the new bit-alignment; and
otherwise, when the count is not determined to be greater than or equal to twice n divided by w, incrementing the count before processing a next received data word.
17. The method of claim 8 , wherein the method is performed by a code lock module of a syndrome evaluator in a decoder.
18. An integrated circuit that receives a stream of data words, the integrated circuit comprising:
a data input for receiving the data words;
a code lock module within control logic of a syndrome evaluator that is configured to receive the data words from the data input;
registers that hold values that are accessible by the code lock module; and
memory means that holds array values that are accessible by the code lock module,
wherein the code lock module is configured to initially set a count to zero, and
wherein the code lock module is configured, for each data word of w symbols received, to:
set a contribution register value to be a contribution to a syndrome from the w symbols in the data word;
set a syndrome shift register value to be a syndrome work register value shifted by w symbol positions;
set the syndrome work register value to be a result of an exclusive-or operation applied to the syndrome shift register value and the contribution register value; and
set an array value in the memory means at a location associated with the count to the syndrome work register value.
19. The integrated circuit of claim 18 , wherein the code lock module is further configured to set a syndrome register value to be the syndrome work register value padded up to a full length of the syndrome.
20. The integrated circuit of claim 19 , wherein the code lock module is further configured to:
determine whether the count is greater than or equal to n divided by w, where n is the number of symbols in a forward error correction code;
when the count is determined to be greater than or equal to n divided by w, apply the exclusive-or operation to the syndrome register value and the array value to obtain a match result; and
when the match result is all zeroes, indicate that a match is found for a bit-alignment such that the bit-alignment is to be further checked.