Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Patent
1996-06-25
2000-02-22
Chin, Wellington
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
375262, 371 43, H04L 2706
Patent
active
06028899&
DESCRIPTION:
BRIEF SUMMARY
The invention relates to digital transmission systems and, more particularly, a radio a decoding device for forming a decoded digital signal from a received signal via an estimate for the probability with which the respective symbol was sent, the estimate being assigned to the respective symbols of the decoded digital signal, forward state metrics and backward state metrics to be stored being used for calculating the estimates.
BACKGROUND OF THE INVENTION
The decoding of convolutional codes with soft-input and soft-output values is often performed according to the principle of the symbol-by-symbol MAP algorithm (MAP=Maximum A posteriori Probability). The a posteriori probability for the decoded symbols is maximized subject to the received sequence. The symbol-by-symbol MAP decoding algorithm can be realized by the trellis diagram of the convolutional code when a forward and backward recursion is used. Both the forward recursion and the backward recursion are very similar to the Viterbi algorithm but for the recursion direction. The accumulated metrics calculated during the backward recursion are to be stored, because they are necessary in the forward recursion for calculating the soft-output values. The memory requirement for this is N.2.sup.L-1 words (in currently customary fixed point digital signal processors (DSP's) a word usually comprises 16 bits), where N is the block length and L the influence length of the convolutional code. Typical values for L lie in the range [5, . . . 7]. Already with moderate block lengths N of several hundred bits, this implies a large memory requirement which cannot be satisfied in currently available digital signal processors. In view of the backward recursion and the storage of the metrics, the algorithm is primarily suitable for signal sequences which have a block structure. The exact symbol-by-symbol MAP algorithm is basically unsuitable for fixed point DSP's, because the algorithm needs as soft-input values probabilities whose combinations in the algorithm (multiplication and addition) will rapidly lead to numerical problems. Therefore, a sub-optimum variant is to be used in currently available fixed point DSP's, which variant uses either logarithmic probabilities or so-called log-likelihood ratios as soft-input values while the combinations in the algorithm then consist of additions and maximization.
SUMMARY OF THE INVENTION
It is an object of the invention to reduce the memory requirement of the decoding device.
This object is achieved in that the backward state metrics are stored only in each L.sup.th step, where L is the influence length of the convolutional code used in the decoding device.
This object is also achieved in that the decoding device comprises means for storing the backward state metrics only in each L.sup.th step, where L is the influence length of the convolutional code used in the decoding device.
The invention comprises reducing the necessary memory requirement for the accumulated metrics from the backward recursion by the factor L, by modifying the base algorithm already known. This holds both for the exact and for the sub-optimum algorithm. For the sub-optimum algorithm it is possible to achieve a further saving on both memory requirement and computation effort if soft-output values are needed only for part of the bits of a block. These bits may then be re-sorted at the beginning and/or end of a block, so that there is then the possibility of limiting the whole soft-output algorithm to only these bits. For the remaining bits, the storage of the accumulated metrics of the backward recursion and also the backward recursion itself may be omitted. Instead, a survivor or path memory is to be used during the forward recursion, which corresponds to the conventional Viterbi algorithm and requires only N.2.sup.L-1 /16 words of memory locations.
The invention is based on a transmission system which comprises the components of binary source, convolutional encoder, channel and convolutional decoder. More particularly, the channel is to contain not on
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Chin Wellington
Tran Congvan
U.S. Philips Corporation
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