Method and device for controlling the decoding of a LDPC encoded codeword, in particular for DVB-S2 LDPC encoded codewords
View Patent ↗This is a method for controlling the decoding of a LDPC encoded codeword composed of several digital data, said LDPC code being represented by a bipartite graph between check nodes (CN 1 ) and variable nodes (VN i ). Said method comprises updating messages exchanged iteratively between variable nodes (VN 1 ) and check nodes (CN 1 ). Said method comprises, at each iteration, calculating for each variable node a first sum (Λ n ) of all the incident messages (λ i ) received by said variable node and the corresponding digital data (λ ch ) and calculating a second sum (VNR new ) of all the absolute values of the first sums (Λ n ), and stopping the decoding process if the second sum (VNR new ) is unchanged or decreases within two successive iterations and if a predetermined threshold condition is satisfied.
1. A method for controlling decoding of a low density parity-check (LDPC) encoded codeword comprising digital data, an LDPC code being represented by a graph between check nodes and variable nodes, the method comprising:
updating messages exchanged iteratively between the variable nodes and the check nodes; and
at each iteration performing at least
calculating for each variable node a first sum of incident messages received by the variable node and the corresponding digital data,
calculating a second sum of absolute values of the first sums, and
stopping the decoding process if the second sum is unchanged or decreases within two successive iterations, and if a threshold condition is satisfied.
2. The method according to claim 1 , wherein the threshold condition is satisfied if a current second sum is less than a threshold.
3. The method according to claim 2 , wherein the threshold is dependent on a code rate of the LDPC code.
4. The method according to claim 2 , wherein the encoded codeword is received from a channel, and the threshold is independent of a signal-to-noise ratio of the channel.
5. The method according to claim 2 , wherein the threshold is dependent on a number of bits of the exchanged messages.
6. The method according to claim 2 , wherein the encoded codeword is received from a channel, and the threshold is defined by:
VNR
off
=
4
R
×
(
E
b
N
0
)
WR
×
N
wherein:
N is a number of bits of the exchanged messages,
E b is a mean energy for transmitting a bit information on the channel,
N 0 is a noise energy of the channel,
R is a code rate of the LDPC code, and
(
E
b
N
0
)
WR
is a value of a ratio
E
b
N
0
of a waterfall region of a curve representing a frame error rate (FER) as a function of the ratio.
7. The method according to claim 2 , wherein the threshold (VNR off ) is defined by: VNR off =2 (q−1) ×N
wherein:
N is a number of bits of the exchanged messages, and
q is a number of bits used for representing an absolute value of the message.
8. The method according to claim 1 , wherein the threshold condition is satisfied as long as the number of iterations is less than or equal to a predetermined number of iterations.
9. The method according to claim 1 , wherein the digital data of the encoded codeword and the messages comprise log-likelihood ratios (LLR).
10. The method according to claim 1 , wherein the LDPC code comprises a DVB-S2 LDPC code.
11. A decoder for decoding a low density parity-check (LDPC) encoded codeword comprising digital data, the decoder comprising:
a processor for updating messages exchanged iteratively between variable nodes and check nodes of a graph for an LDPC code, said processor comprising
a calculation module for calculating, at each iteration for each variable node a first sum of incident messages received by the variable node and the corresponding digital data, and for calculating a second sum of absolute values of the first sums, and
a control module for stopping the decoding process if the second sum is unchanged or decreases within two successive iterations, and if a threshold condition is satisfied.
12. The decoder according to claim 11 , wherein the threshold condition is satisfied if a current second sum is less than a threshold.
13. The decoder according to claim 12 , wherein the threshold is dependent on a code rate of the LDPC code.
14. The decoder according to claim 12 , wherein the encoded codeword is received from a channel, and the threshold is independent of a signal-to-noise ratio of the channel.
15. The decoder according to claim 12 , wherein the threshold is dependent on a number of bits of the exchanged messages.
16. The decoder according to claim 12 , wherein the encoded codeword is received from a channel, and the threshold (VNR off ) is defined by:
VNR
off
=
4
R
×
(
E
b
N
0
)
WR
×
N
wherein:
N is a number of bits of the exchanged messages,
E b is a mean energy for transmitting a bit information on the channel,
N 0 is a noise energy of the channel,
R is a code rate of the LDPC code, and
(
E
b
N
0
)
WR
is a value of a ratio
E
b
N
0
of a waterfall region of a curve representing a frame error rate (FER) as a function of the ratio.
17. The decoder according to claim 12 , wherein the threshold is defined by: VNR off =2 (q−1) ×N
wherein:
N is a number of bits of the exchanged messages, and
q is a number of bits used for representing an absolute value of the message.
18. The decoder according to claim 12 , further comprising a memory comprising a look-up table for storing values of the threshold according to a code rate of the LDPC code and a number of bits of the exchanged messages.
19. The decoder according to claim 11 , wherein the digital data of the encoded codeword and the messages comprise log-likelihood ratios (LLR).
20. A receiver comprising:
a demodulator; and
a decoder coupled to said demodulator for decoding a low density parity-check (LDPC) encoded codeword comprising digital data said decoder comprising:
a processor for updating messages exchanged iteratively between variable nodes and check nodes of a graph for an LDPC code, said processor configured for performing the following
calculating, at each iteration for each variable node a first sum of incident messages received by the variable node and the corresponding digital data, and for calculating a second sum of absolute values of the first sums, and
stopping the decoding process if the second sum is unchanged or decreases within two successive iterations, and if a threshold condition is satisfied, with the threshold condition being satisfied if a current second sum is less than a threshold.
21. The receiver according to claim 20 , wherein the encoded codeword is received from a channel, and the threshold (VNR off ) is defined by:
VNR
off
=
4
R
×
(
E
b
N
0
)
WR
×
N
wherein:
N is a number of bits of the exchanged messages,
E b is a mean energy for transmitting a bit information on the channel,
N 0 is a noise energy of the channel,
R is a code rate of the LDPC code, and
(
E
b
N
0
)
WR
is a value of a ratio
E
b
N
0
of a waterfall region of a curve representing a frame error rate (FER) as a function of the ratio.
22. The receiver according to claim 20 , wherein the threshold is defined by: VNR off =2 (q−1 )×N
wherein:
N is a number of bits of the exchanged messages, and
q is a number of bits used for representing an absolute value of the message.
23. The receiver according to claim 20 , further comprising a memory comprising a look-up table for storing values of the threshold according to a code rate of the LDPC code and a number of bits of the exchanged messages.
24. The receiver according to claim 20 , wherein the digital data of the encoded codeword and the messages comprise log-likelihood ratios (LLR).
25. The receiver according to claim 20 , wherein the LDPC code comprises a DVB-S2 LDPC code.