Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
1999-05-24
2001-10-16
Chin, Stephen (Department: 2734)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C375S265000, C375S241000, C714S792000, C714S795000
Reexamination Certificate
active
06304612
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a transmission system comprising a transmitter having a channel encoder for encoding sequences of source symbols into sequences of channel symbols, the channel encoder being initialized into a state corresponding to the final state of the channel encoder after having encoded the sequence of source symbols, the transmitter being arranged for transmitting a channel signal representing the channel symbols to a receiver, the receiver comprising a channel decoder for deriving the sequences of source symbols from a channel signal representing the channel symbols by keeping track of a plurality of state sequences with a corresponding likelihood measure representing the likelihood of the state sequences.
Such transmission systems are used for transmitted sequences of source symbols in a reliable way to a receiver. Application of such transmission systems include mobile telephony systems in which frames comprising an encoded speech signal have to be transmitted from a base station to a mobile terminal and vice versa.
In order to achieve a reliable transmission, the sequences of source symbols are encoded into sequences of channels symbols by a channel encoder. The channel encoder can e.g. be a convolutional encoder which is initialized in a predetermined state. This predetermined state is often the zero state corresponding to the &ngr; last symbols in the sequence of source symbols. A disadvantage of this is that the last symbols of the source symbols should be set to zero. This reduces the number of source symbols which can be transmitted in one sequence of channel symbols.
The reduction of the number of source symbols can be prevented by initializing the channel encoder to a state which is defined by the last source symbols in the sequence of source symbols. It is known to initialize the channel encoder in a state which is the same as the state of the channel encoder after the encoding of said last source symbols. Codes using the above mechanism are known as “tail biting” codes. Combinations of “zero tailing” codes and “tailbiting” codes are also conceivable.
It is however conceivable that the channel encoder is initialized in a state which is different from the state of the channel encoder after the encoding of said last source symbols. However in such a case there should be a one to one relationship between the state of the channel encoder after the encoding of said last source symbols, and the state to which the channel encoder is initialized before starting decoding.
A disadvantage of tailbiting codes is that the decoding complexity increases substantially, because the decoding should in principle be performed for each possible initial state. If the code has 32 possible states (&ngr;=5), the decoding complexity is increased with a factor 32. It has been shown that it is possible to reduce the complexity by decoding the channel signal repeatedly in a circular fashion, but the complexity is still a few times the complexity of an decoder for a code initialized in a state known to the channel decoder.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a transmission system according to the preamble in which the complexity of the channel decoder is substantially decreased.
To achieve said purpose, the transmission system according to the present invention is characterized in that the channel decoder is arranged for selecting a suitable state sequence after the channel signal has been processed at least once, determining from said suitable state sequence, an earlier state in said suitable state sequence, and in that the channel decoder is arranged for selecting as final state sequence the state sequence terminating in a state corresponding to said earlier state.
The present invention is based on the recognition that the earlier state determined from the suitable state sequence is almost always correct. By using the known relation between said earlier state and the proper terminating state, it becomes possible to select said terminating state with a high reliability. As is stated above, the terminating state can be the same as the earlier state, but it is also possible that there is a different predetermined relation between the terminal state and the earlier state. The suitable state sequence is preferably the most likely state sequence. This state has a likelihood measure indicating a largest likelihood. If the number of states between the final state and the earlier state is large enough, each state is a suitable state as starting point for searching the earlier state.
An embodiment of the invention is characterized in that the channel decoder is arranged for extending, on basis of a cyclically extended channel signal, the state sequence beyond a number of states equal to the number of source symbols in a sequence of source symbols and in that the channel decoder is arranged for selecting the final state sequence on basis of the terminating states of the extended state sequences.
By extending the state sequence, it becomes possible to increase the reliability of the detection substantially with only a small extension of the state sequence. Simulations have shown that in the transmission system according to the invention only a small extension of the state sequence is required to increase the detection reliability substantially. It is observed that without using the measures according to the invention the length of the extended state sequence should be a few times the length of the basic state sequence.
A further embodiment of the invention is characterized in that the channel decoder is arranged for determining the suitable state sequence when the state sequences comprise a number of states equal to the number of source symbols in a sequence of source symbols.
Simulations have shown that a suitable instant to determine the most likely state is when the number of states in the state sequence is equal to the number of source symbols.
A preferred embodiment of the invention is characterized in that the channel encoder is arranged for determining the suitable state sequence from terminating states of the extended state sequence.
Simulations have shown that by determining the most likely state from the terminating state of the extended state sequence, the performance of the decoder can be enhanced slightly without adding any complexity.
A still further embodiment of the invention is characterized in that the channel decoder is arranged for determining the state sequence by selecting the state sequence by comparing the likelihood measures of a plurality of candidate state sequences terminating in said state, in that the channel decoder is arranged for determining a difference measure between the likelihood measure of the selected candidate sequence and the likelihood measure of the rejected candidate sequences, in that the channel decoder is arranged for determining for the final state sequence the minimum value of the difference measure. In the channel decoder according to the invention, new states are added to the actual state sequences by creating from each state sequence a plurality of candidate state sequences, each of the candidate sequences terminating in one of the possible new states. By comparing for each of the new states the likelihood of the new state sequences leading to said new state, the new state sequence with the largest likelihood can be selected. This selected state sequence is called the survivor. A measure for the reliability of this selection is the difference between the likelihood of the survivor and the likelihood of the other state sequences.
By selecting the minimum value of said difference corresponding to the finally selected state sequence, a transmission quality measure for said sequence of channel symbols is obtained.
A still further embodiment of the invention is characterized in that the channel decoder is arranged for keeping track of the minimum difference measure for each of the state sequences.
By keeping track of the minimum difference measure, the minimum di
Baggen Constant P. M. J.
Koppelaar Arie G. C.
Chin Stephen
Ha Dac V.
Slobod Jack D.
U.S. Philips Corporation
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