Error detection/correction and fault detection/recovery – Pulse or data error handling – Digital data error correction
Reexamination Certificate
1997-08-14
2001-02-20
De Cady, Albert (Department: 2784)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Digital data error correction
C714S780000, C714S755000
Reexamination Certificate
active
06192503
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to communications systems and methods, in particular, to communications systems and methods employing error correction.
BACKGROUND OF THE INVENTION
In a typical communications system, information is transmitted from a sender in the form of a communications signal representing the information. The communications signal typically is communicated to a receiving unit over a communications medium such as a radio, optical fiber, coaxial cable or similar link, which may introduce disturbances such as noise, delay, and distortion in the communications signal. These disturbances can induce errors when recovering the original information from the communicated communications signal at the receiving unit.
Conventional responses to overcome this problem include increasing the power level of the transmitted communications signal in order to increase the probability that the original information may be recovered. However, the ability to increase transmitter power may be limited due to power limitations of transmitter electronics, regulations on peak signal power levels and constraints on the power available for transmitting, for example, power supply limitations in devices such as mobile radiotelephones and satellites.
Redundancy may be introduced into a communications signal by using error control coding techniques. Redundant symbols supplied in a code such as block or convolutional code can provide an additional “separation” between the words of the set of code words, thus allowing a receiver which receives a group of symbols over a noisy communications channel to more easily discriminate between words of the set of code words, typically by determining which member of the set of code words most closely resembles the received group of symbols.
Many error control codes are effective at correcting random errors, e.g., errors which affect individual symbols in a random distributed fashion, while others are effective at compensating for so-called “burst” errors, e.g., errors which persist over several consecutive symbols. To compensate for burst errors, many systems employ interleaving which reorders symbols in a stream such that burst errors are more randomly distributed, for example, by using a device which stores the symbol stream in a matrix by rows and then retrieves the stored symbols by columns, such that the sequence retrieved from the device represents a reordering of the original input sequence. To combat random and burst errors, a system may employ a combination of random error correction encoding and interleaving, for example a cascade of a binary convolutional code and an interleaver, or a so-called “turbo coding” scheme, as described in U.S. Pat. No. 5,446,747 to Berrou et al. Turbo coding schemes typically employ a first code to encode a source data stream and a second code to encode an interleaved version of the source data stream to produce first and second encoded streams which are multiplexed and communicated over a channel. The received data stream typically is demultiplexed and decoded by first and second decoders employing the first and second codes, with appropriate interleaving and deinterleaving, with the output of one decoder being used to aid the other decoder in decoding the demultiplexed sequences in an iterative fashion.
While techniques such as turbo-coding are generally effective at reducing error rates for information communicated over a channel, conventional decoding schemes may not optimally decode the received information under various channel conditions. Turbo-coding can provide improved power efficiency, but may involve a large number of computations which may be unnecessary under favorable channel conditions and which may unnecessarily consume power.
SUMMARY OF THE INVENTION
In light of the foregoing, it is an object of the present invention to provide communications systems and methods which more efficiently decode communications signals representing parallel encoded source sequences.
This and other objects features and advantages are provided according to the present invention by communications systems and methods in which a communications signal representing a parallel encoded source sequence is selectively recursively decoded to produce estimates of a symbol in the source sequence based on a respective reliability associated with a respective revised estimate of the symbol. Preferably, the communications signal is processed to produce first and second sequences corresponding to the first and second error correction codes used to produce the communications signal, and then the received sequences are decoded in respective first and second soft output decoders. In a respective soft output decoder, a sequence is decoded according to the corresponding error correction code, augmented by previous estimates produced by the other decoder. Preferably, one of the first and second soft output decoders is chosen to first estimate the symbol based on a signal characteristic, e.g., signal strength, associated with the first and second received sequences. A selected group of symbols or bits of the source sequence, for example, a group of less significant symbols or bits, may be nonrecursive decoded, while another group of symbols or bits, for example, a group of more significant symbols or bits, may be selectively recursively decoded. Efficient techniques for decoding parallel encoded signals are thereby provided.
In particular, according to the present invention, a communications system includes encoding means for encoding a source sequence according to respective first and second error correction codes to produce respective first and second encoded sequences of symbols. Communications symbol processing means are responsive to the encoding means for processing the first and second encoded sequences to produce a communications signal. Communications signal communicating means are responsive to the communications symbol processing means for communicating the communications signal over a communications medium, and communications signal processing means are responsive to the communications signal communicating means for processing the communicated communications signal to produce first and second received sequences of symbols corresponding to the first and second encoded sequences, respectively. Selective recursive decoding means are responsive to the communications signal processing means for selectively recursively decoding the first and second received sequences according to the respective first and second codes augmented by previous estimates of a symbol of the source sequence to repeatedly produce revised estimates of the symbol until a revised estimate satisfying a predetermined reliability criterion is obtained.
The encoding means may include first encoding means for encoding the source sequence according to the first error correction code to produce the first encoded sequence, and second encoding means for encoding the source sequence according to the second error correction code to produce the second encoded sequence. The communications symbols processing means may include multiplexing means, responsive to the first and second encoding means, for multiplexing the first and second encoded sequences to produce a multiplexed sequence of symbols. Means may be provided, responsive to the multiplexing means, for processing the multiplexed sequence to produce the communications signal. According to one aspect, the encoding means further includes interleaving means for interleaving the source sequence to produce an interleaved source sequence, and one of the first and second encoding means is responsive to the encoding means for encoding the interleaved source sequence.
According to one embodiment, the selective recursive decoding means includes first soft output decoding means, responsive to the communications signal processing means, for decoding the first received sequence according to the first error correction code, and second soft output decoding means, responsive to the communications signal processing means,
Chennakeshu Sandeep
Hassan Amer A.
Cady Albert De
Ericsson Inc.
Lamarre Guy
Myers Bigel & Sibley & Sajovec
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