(12) United States Patent
`Jarvinen et al.
`
`USOO647047 OB2
`(10) Patent No.:
`US 6,470,470 B2
`(45) Date of Patent:
`*Oct. 22, 2002
`
`(54) INFORMATION CODING METHOD AND
`DEVICES UTILIZING ERROR CORRECTION
`AND ERROR DETECTION
`
`(*) Notice:
`
`(75) Inventors: Rattle,ESSES
`Jala, lamp
`s
`s
`Saaksjarvi (FI)
`(73) Assignee: Nokia Mobile Phones Limited, Espoo
`(FI)
`This patent issued on a continued pros-
`ecution application filed under 37 CFR
`1.53(d), and is subject to the twenty year
`patent term provisions of 35 U.S.C.
`154(a)(2).
`Subject to any disclaimer, the term of this
`tent
`tended
`diusted under 35
`ps g 5), o's JuSled under
`a --
`(21) Appl. No.: 09/019,656
`(22) Filed:
`Feb. 6, 1998
`O
`O
`(65)
`Prior Publication Data
`US 2002/0035713 A1 Mar. 21, 2002
`Foreign Application Priority Data
`(30)
`Feb. 7, 1997 (FI) ................................................. 970554
`51) Int. Cl."
`H03M 13700
`(51) Int. Cl." ...............................................
`
`(52) U.S. C. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 714/755; 714/758
`
`(58) Field of Search ................................. 714/746, 758,
`714/704, 755; 358/133; 341/95; 375/299,
`346; 370/337
`
`(56)
`
`References Cited
`
`EP
`(i.
`WO
`
`9/1996 Heikkila et al. ............ 375/224
`5,557,639 A
`5,675,372 A * 10/1997 Aguayo et al. ............... 348/12
`5,675,972 A * 10/1997 Aguayo et al. ............... 348/12
`5,701,390 A * 12/1997 Griffin et al. .......
`... 395/2.15
`5,737,331 A * 4/1998 Hoppal et al. .............. 370/349
`5,822,315 A * 10/1998 Seze et al. .................. 370/337
`5,901,186 A
`5/1999 Jamal et al. ................ 375/346
`6,170,073 B1 * 1/2001 Jarvinen et al. ............ 714/758
`FOREIGN PATENT DOCUMENTS
`O 651531 A2
`5/1995
`g
`A. gE.
`WO 93/O5502
`3/1993
`OTHER PUBLICATIONS
`GSM recommendation 06.11 “Substitution and muting of
`lost frames for full-rate speech traffic channels”, ETSI/GSM
`Jan. 1991.
`GSM recommendation 06.10 “GSM full rate speech
`transcoding”, ETSI/GSM Jan. 1991.
`PCT International Search Report.
`* cited by examiner
`Primary Examiner David Ton
`(74) Attorney, Agent, or Firm-Perman & Green, LLP
`(57)
`ABSTRACT
`Focused error correction and/or focused error detection is
`used in the information coding System. A speech encoding
`method, in which the number of speech parameter bits on
`which error correction coding and/or error detection coding
`
`focuses is automatically adjusted in relation tO the number of
`
`total Speech parameter bits as a function of the quality of the
`information transfer connection. There is no need to reduce
`the number of bits used for speech encoding. Thus the voice
`quality of the Speech remains high. The error correction
`and/or error detection is focused on the bits most important
`for the voice quality e.g., as a function of the C/I (Channel
`to Interference)13 parameter describing the quality of the
`information transfer connection. The muting of Speech Syn
`thesizing occuring in prior Systems on poor information
`transfer connection is reduced by using focused error detec
`tion.
`
`16 Claims, 12 Drawing Sheets
`
`U.S. PATENT DOCUMENTS
`5,115,469 A 5/1992 Taniguchi et al. ............ 381/36
`5.243,419 A * 9/1993 Faryar et al. .....
`... 358/133
`5.243,629 A 9/1993 Wei..............
`... 375/299
`5.245,339 A * 9/1993 Cideciyan .................... 341/95
`5,251.237 A * 10/1993 Baier ...........
`... 375/346
`5,491719 A * 2/1996 Sellin et al. ...
`... 375/213
`5,553,080 A * 9/1996 Fujiwara ..................... 714/704
`
`216
`
`---
`
`25
`
`DETECTION
`o, So
`SELECTING
`of SSINGA (s/no NSS
`CONNECTION
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`10-y
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`201 -
`
`A.
`
`2CO
`
`
`
`SEPARATION
`OF SPEECH
`PARAMETER
`BITS
`
`FOCUSED
`
`OF CRC
`
`FOCUSED
`CONVOLUTION
`CODING
`
`\
`210
`
`i
`
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`U.S. Patent
`U.S. Patent
`
`Oct. 22, 2002
`
`Sheet 1 of 12
`
`US 6,470,470 B2
`US 6,470,470 B2
`
`110
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`109
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`108
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`CRCaCONVOLUTION-ADDITIONENCODING
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`BITSEPARATION
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`106
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`U.S. Patent
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`Oct. 22, 2002
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`Sheet 2 of 12
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`US 6,470,470 B2
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`PEECH
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`U.S. Patent
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`Oct. 22, 2002
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`Sheet 3 of 12
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`US 6,470,470 B2
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`U.S. Patent
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`Oct. 22, 2002
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`US 6,470,470 B2
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`U.S. Patent
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`Oct. 22, 2002
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`Sheet S of 12
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`US 6,470,470 B2
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`US 6,470,470 B2
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`Oct. 22, 2002
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`Oct. 22, 2002
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`Sheet 8 of 12
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`U.S. Patent
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`Oct. 22, 2002
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`Sheet 9 of 12
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`US 6,470,470 B2
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`U.S. Patent
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`Oct. 22, 2002
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`Sheet 10 of 12
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`U.S. Patent
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`Oct. 22, 2002
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`US 6,470.470 B2
`
`1
`INFORMATION CODING METHOD AND
`DEVICES UTILIZING ERROR CORRECTION
`AND ERROR DETECTION
`
`FIELD OF THE INVENTION
`The present invention relates to an information coding
`method utilizing focused error correction and/or error
`detection, in which method the quality of the data transfer
`connection is used for Selecting the coding mode for the data
`transfer connection. The invention also relates to a System
`and terminal devices applying the method. The invention is
`particularly Suitable for use in connection with data transfer
`connections realized by radio.
`BACKROUND OF THE INVENTION
`While transferring information, Such as Speech or data,
`using transfer connections Subject to transmission errors, the
`information to be transferred is in general protected using an
`error correction algorithm. Especially in digital connections
`an attempt is made to detect transmission errors, and to
`correct the erroneous information bits. How Successfully
`this is done depends among other things on the number of
`transmission errors and on the error correction algorithm
`used. In Speech coding Systems prior known to a perSon
`skilled in the art, a major part of the bits comprising speech
`information are protected using an error correction code.
`This is the procedure e.g. in the so called full rate (FR, Full
`Rate) speech codec of the GSM system.
`In the full-rate speech codec (which later is also called
`FR-speech codec) of the GSM system a RPE-LTP (Regular
`Pulse Excitation-Long term Prediction) based speech encod
`ing System is used. It produces 260 speech parameter bits for
`each 20 ms speech frame. Out of these 260 bits, the 182
`Subjectively most important bits are protected using an error
`correction code. AS the error correction code, 72-rate con
`volution encoding is used. The remaining 78 bits are trans
`ferred in the data transmission connection completely with
`out error correction.
`The number of transmission errors on a data transfer
`connection may temporarily exceed the error correction
`capacity of the 72-rate convolution coding used in the
`GSM-System. As a result, the important received speech
`parameter bits may contain transmission errors. It is impor
`tant to detect that these transmission errors occurred, even if
`it were not possible to correct them. If the Speech parameters
`which are the most important for Speech quality contain
`transmission errors, they shall not be used for Speech Syn
`thesizing in the receiver, but they must be rejected. In the full
`rate FR-speech codec of the GSM-system 3-bit CRC (Cyclic
`Redundancy Check)-error detection is used. CRC-error
`detection is focused on the 50 most important bits of speech
`coding. In a receiver the error detection code is used for
`verifying the correctness of the 50 most important bits of
`each 20 ms Speech frame. If they contain errors, the frame
`is classified as bad and it is not used in Speech Synthesizing.
`Instead, an attempt is made to Substitute the bad frame with
`an estimate, which is formed e.g. based upon chronologi
`cally preceding error-free frames.
`The full-rate speech coding method of the GSM-system
`briefly presented above, operates reasonably well, provided
`that the relative share of transmission errors does not grow
`too high. Under these conditions the error correction algo
`rithm is capable of correcting transmission errorS Sufficiently
`for obtaining a Satisfactory transfer connection and through
`it a Satisfactory Speech quality. When the proportion of
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`transmission errors grows to medium or high level, the error
`correcting capability of convolution coding having /2-rate
`coding ratio is exceeded. In this case a more efficient error
`correction algorithm would be needed, Such as e.g. a con
`volution coding having /3-rate coding ratio. In this case,
`however, the total Speech encoding efficiency will be
`reduced essentially, because more error correction informa
`tion bits must be included in the data transfer connection.
`This naturally increases the data transfer rate required of the
`data transfer connection. Accordingly, this approach cannot
`be used for codecs with fixed line speed. Instead, the above
`presented method based upon making the error correction
`algorithm more efficient is suitable for systems with variable
`line Speed.
`For instance, the total bit rate of the data transfer System
`used for transferring speech can be kept constant, provided
`that at the same time when the number of bits used for the
`error correction of Speech parameter bits is increased, the
`number of bits used for Speech encoding itself is reduced.
`This in turn requires using Several different Speech codecs
`with different line speeds in both the transmitter and the
`receiver, which makes the Structure of the System more
`complicated. Further, the lower the number of bits used for
`Speech encoding, the more calculation capacity is normally
`required of the various components of the System. The above
`presented disadvantages increase the cost of the System. In
`addition to the above, the deterioration of Speech quality
`cannot be avoided when more bits are used for error
`correction, because the fewer bits there are available for
`Speech encoding, the more one has to compromise the Voice
`quality. The Voice quality deterioration due to the reduction
`of the number of bits used for Speech encoding is particu
`larly important in a case where there is background noise to
`Speech, e.g. the noise from a car engine.
`One problem occurring in Speech coding methods accord
`ing to prior art is the complete muting of the Speech
`Synthesizing in a receiver when data transfer connections
`containing a large number of transfer errors are used. This is
`due to the fact that when an error detection algorithm detects
`transfer errors in Speech frames, it too easily mutes the
`Speech Synthesizer. This leads to the loSS of Speech infor
`mation.
`AS is evident from the above description, there is a need
`to develop a better method of protecting information param
`eters on data transfer connections containing numerous
`transfer errors. In addition, there is a need to develop a
`system, the receiver of which better tolerates information
`parameter frames containing errors. In the following the
`information coding method according to the invention and
`the System utilizing it and the terminal devices are explained
`primarily using the Speech coding in a mobile communica
`tion System as an example. Nothing however limits using the
`information coding System according to the invention for
`coding of data other than speech data. For the Sake of clarity
`the invention is in the following also called a speech coding
`method, because it best describes one of the most important
`fields of application of the invention. It is possible to utilize
`the invention instead of a radio connection, also e.g. in
`connection with information transfer Systems realized using
`wireline connections.
`
`SUMMARY OF THE INVENTION
`Now an information coding method utilizing focused
`error correction and error detection System has been
`invented, by use of which the above described problems can
`be reduced. One of the purposes of the present invention is
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`3
`to present a speech coding method which will be automati
`cally adjusted as a function of the quality of a data transfer
`connection optimizing the Speech quality on data transfer
`connections of any quality. The quality of the data transfer
`connection used is analyzed by measuring the parameters
`describing the quality of a data transfer connection, Such as
`e.g. C/I (Carrier to Interference) ratio, S/N (Signal to Noise)
`ratio or bit error rate (Bit Error Rate, BER) as known to a
`person skilled in the art. In the information coding method
`according to the invention there is no need to reduce the
`number of bits used for Speech coding in relation to the total
`bit rate used in the information transfer connection, in which
`case the Voice quality of the Speech preferably remains good.
`In the information coding method according to the invention
`error correction and/or error detection is focused on the bits
`most essential for voice quality as a function of the C/I-ratio
`or of Some other parameter describing the quality of the data
`transfer connection. The muting of Speech Synthesizing
`occurring in prior art Systems on data transfer connections of
`poor quality is reduced in the information coding method
`according to the invention by using the focused error
`detection, in other words using focused detection of bad
`frames.
`The information coding method according to the inven
`tion tolerates data transfer errors well. The high tolerance of
`data transfer errorS has been achieved by monitoring the
`quality of a data transfer connection, and by optimizing the
`focusing of the error correction and error detection of Speech
`parameters. Both the error correction coding (e.g. convolu
`tion coding) and the error detection coding (e.g. checking of
`the cyclic redundancy) are adjusted to match the error
`conditions of the data transfer connection.
`When few data transfer errors occur, all or almost all
`Speech parameter bits are protected with an error correction
`code in a System according to the invention. When more data
`transfer errors occur, the error correction is focused more on
`the Speech parameter bits most important for Speech quality
`and intelligibility (when data other than speech is
`transferred, on the most important bits for the information).
`Alternatively, or additionally, when few data transfer errors
`occur, all or almost all Speech parameter bits are protected
`with an error detection code in a System according to the
`invention, and when more data transfer errors occur, the
`error detection is focused more on the Speech parameter bits
`most important for Speech quality and intelligibility. The
`focusing of error detection bits can be done by keeping the
`number of error detection bits constant (e.g. 3 CRC bits) but
`performing error detection on a different amount of infor
`mation bits depending on data transfer quality. The focusing
`of error detection bits can be done by varying the number of
`error detection bits depending on data transfer quality.
`Which bits that are the most important for speech quality is
`determined based upon the Speech coding method used. For
`example, when a simple PCM (Pulse Code Modulation)
`coding is used, it is unambiguous that the most significant
`bits (MSB, Most Significant Bits) are more important and
`they must be protected carefully. The least Significant bits
`(LSB, Least Significant Bits) can be left unprotected if
`needed, because their effect on the intelligibility of Speech is
`small. In the FR-Speech codec of the GSM-system, known
`to a perSon Skilled in the art, the relative significance of the
`bits has been defined in the GSM-specification. The final
`decision of which bits are the most important for Speech
`quality has been made Subjectively, based upon listening
`tests. When in the method according to the invention, the
`error detection code is focused on the most important bits,
`the error correction code is at the same time changed to be
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`more efficient, or more error correction information is
`included in relation to protected Speech parameter bits. This
`is realized e.g. by using a convolution coding having a lower
`coding ratio.
`When the quality of an information transfer connection
`becomes poor, as happens in a mobile communication
`System when the quality of a radio connection between a
`mobile Station and a base Station deteriorates, in a System
`according to the invention a Smaller and Smaller part of all
`Speech parameter bits is protected Selectively, but using an
`enhanced error correction code. The error correction coding
`is focused on the bits most important for Speech quality in
`Such a way that the most important bits are always protected
`and the leSS important bits are protected within the limits Set
`by the quality of the information transfer connection and the
`number of bits available for error correction allow. As a
`result of the enhancement of the error correcting code, it is
`possible to decode speech in the receiving end even on
`information transfer connections containing numerous of
`errors, in other words, a System utilizing the Speech coding
`method according to the invention will not “collapse', i.e.,
`the output of the Speech decoder is not muted. Bits trans
`ferred without the error correction code may reduce the
`Speech quality due to transmission errors, but well protected,
`most important Speech parameter bits Still guarantee the
`intelligibility of the Speech. AS to Speech quality, this
`method is far better than trying to protect all Speech param
`eter bits or a major part of them using a poor error correction
`code. A poor error correction code on information transfer
`connections with plenty of interference results in a situation,
`in which the error correction code no longer is capable of
`correcting the transmission errors. In this case actually all
`bits used for error correction have been wasted. When the
`quality of the information transfer connection improves, or
`the number of transmission errorS is reduced, the Speech
`coding method according to the invention adapts itself
`correspondingly to the new situation and increases the Share
`of Speech parameter bits protected with the error correction
`code. Thus the error correction operates efficiently in all data
`transfer conditions.
`It is possible to analyze the quality of an information
`transfer connection using a number of methods. Among the
`methods are the above mentioned ones known to a perSon
`skilled in the art, C/I (Channel to Interference)- and S/N
`(Signal to Noise)-ratios measured in an information transfer
`connection. It is possible to analyze the quality of an
`information transfer connection in a receiver also as a
`function of the frequency of occurrence of Speech parameter
`frames rejected in the Speech Synthesizing due to errors
`contained in the most important bits, as it is explained below
`in more detail in connection with an embodiment of the
`invention. It is possible to perform the detection of the
`quality of an information transfer connection itself in both a
`transmitter and a receiver, but the information about the
`Selected speech coding mode (or how the error correction
`and/or error detection bits are focused according to the
`invention) must always be transferred to the Speech encoder
`of the transmitter.
`A System according to the invention typically uses the
`Same Speech codec operating at a fixed line Speed. Only the
`focusing of error correction coding and error detection
`coding is adapted to correspond with the current data
`transfer conditions. This facilitates the operation of the
`System above a “collapse point'. A collapse point means a
`Situation in which a data transfer connection contains So
`many data transfer errors that a receiver no longer is capable
`of interpreting the received information. In other words, an
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`information coding method utilizing the focused error cor
`rection and error detection according to the invention lowers
`the collapse point of a data transfer connection, which in
`practice means e.g. that a data transfer connection between
`a mobile Station and a base Station can be Successfully
`established on radio connections with poorer Signal to
`Noise-ratioS than previously.
`A System utilizing the Speech coding method according to
`the invention is automatically adapted according to the
`current data transfer conditions, and thus minimizes the
`effect of data transfer errors on the Speech quality. Because
`the focusing of the error correction code on certain speech
`bits is a part of the error correction code itself, it is possible
`to realize, utilizing the invention, Such a System operating at
`fixed line Speed, which provides high Speech quality. It is
`possible to use the invention on all kinds of data transfer
`connections without a need to go over to using a codec with
`a lower line Speed. In Such data transfer Systems which use
`variable line Speed or which change the ratio between
`Speech parameter bits and error correction information, the
`invention can equally be utilized for improving the data
`transfer connection and through it the Speech quality.
`In addition to that in the Speech coding method according
`to the invention the error correction code is focused in
`relation to the most important speech parameter bits, also the
`error detection code (for example CRC) is focused more to
`protect the most important Speech parameter bits, the more
`data transfer errors the current data transfer connection
`contains. This further improves the probability of the infor
`mation coming through and reduces the need to mute the
`Speech Signal in the receiver. This preferably improves the
`Speech quality and intelligibility. When data transfer con
`nections containing a very large number of data transfer
`errors are used, it is possible to accept data transfer errors in
`the less important bits of the Speech frames and to use these
`Speech frames for Synthesizing Speech in the receiver,
`because the quality of the Speech is reduced anyhow if there
`has been a need to reject Speech frames due to errors in the
`most important bits of the Speech frames.
`According to a first aspect of the invention there is
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`provided a digital information transfer System, which com
`prises a transmitter and a receiver and an information
`transfer connection between said transmitter and receiver,
`Said transmitter and receiver comprise means for the pro
`cessing of information and for transferring it from Said
`transmitter to Said receiver using Said information transfer
`connection, Said transmitter comprises a channel encoder for
`processing the information, Said information processing
`means comprise first dividing means for dividing the infor
`mation into at least two parts, a first part and a Second part,
`and that to Said information of the first part in Said channel
`encoder one of the following operations is performed: error
`correction coding for the correction of transfer errors occur
`ring on the information transfer connection at reception,
`error detection coding for the detection of transfer errors
`occurring on the information transfer connection at
`reception, and a combination of them, the System being
`characterized in that it comprises means for analyzing the
`quality of Said information transfer connection and for
`yielding a response describing the quality, and that Said
`transmitter has been arranged to adjust the ratio between
`Said first information and Said Second information based
`upon the response received from Said information transfer
`connection quality analysis means.
`According to a Second aspect of the invention there is
`provided a mobile Station comprising a transmitter and a
`receiver, Said transmitter and receiver comprise means for
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`processing the information, Said transmitter comprises a
`channel encoder for processing the information, Said infor
`mation processing means comprise first dividing means for
`dividing the information into at least two parts, a first part
`and a Second part, and that one of the following operations
`is performed in Said channel encoder on Said information of
`the first part: error correction coding for the correction at
`reception of transfer errors which occur in the information
`transfer connection, error detection coding for the correction
`at reception of transfer errors which occur in the information
`transfer connection, and a combination of them, the mobile
`Station being characterized in that it comprises means for
`analyzing the quality of Said information transfer connection
`and for yielding a response describing the quality, and that
`Said transmitter has been arranged to adjust the ratio
`between Said first information and Said Second information
`based upon the response received from Said information
`transfer connection quality analyzing means.
`According to a third aspect of the invention there is
`provided a transmitter comprising means for processing
`information and transferring it to an information transfer
`connection, Said transmitter comprises a channel encoder for
`processing the information, Said information processing
`means comprise a first dividing means for dividing the
`information into at least two parts, a first part and a Second
`part and that one of the following operations is performed on
`Said information of the first part in Said channel encoder:
`error correction encoding for the correction at reception of
`transfer errors occurring in the information transfer
`connection, error detection encoding for the detection at
`reception of transfer errors occurring in the information
`transfer connection, and a combination of them, the trans
`mitter being characterized in that it comprises means for
`analyzing the quality of Said the information transfer con
`nection and for yielding a response describing the quality,
`and that Said transmitter has been arranged to adjust the ratio
`between Said first information and Said Second information
`based upon the response received from Said information
`transfer connection quality analyzing means.
`According to a fourth aspect of the invention there is
`provided a receiver comprising means for receiving infor
`mation from a transmitter over an information transfer
`connection, and a channel decoder for performing one of
`channel decoding operations on the information received
`from the information transfer connection, the receiver being
`characterized in that it comprises means for analyzing the
`quality of Said information transfer connection and for
`forming a response based upon the performed analysis,
`means for determining the focused channel encoding mode
`to be used in Said information transfer connection based
`upon Said response, and means for transferring Said channel
`encoding mode to the transmitter using Said information
`transfer connection.
`According to a fifth aspect of the invention there is
`provided a method for transferring information in a digital
`information transfer System, in which
`the information to be transferred is divided for transmission
`into at least two parts, a first part and a Second part,
`on Said information of the first part one of channel encoding
`operations is performed, the method being characterized
`in that
`prior to the channel coding the quality of an information
`transfer connection is analyzed in order to form an
`analysis result, and
`the ratio between Said first part and Second part is adjusted
`based upon Said analysis result.
`BRIEF DESCRIPTION OF THE DRAWINGS
`The information coding method utilizing focused error
`correction and focused error detection according to the
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`7
`invention and its realization are explained in detail in the
`following, using Speech coding as an example, with refer
`ence to enclosed figures, of which
`FIG. 1A presents in form of a block diagram a speech
`encoder prior known to a person skilled in the art, and error
`detection and error correction parameters included in the
`data flow in connection with it,
`FIG. 1B presents in form of a block diagram a speech
`decoder corresponding with the Speech encoder presented in
`FIG. 1A and functional blocks, prior known to a perSon
`skilled in the art,
`FIG. 1C presents the focusing of convolution coding on
`the most important Speech parameter bits within a speech
`frame in the GSM-system,
`FIG. 2 presents as a diagram the ratio of convolution
`coding efficiency to the number of parameter bits on which
`error correction is focused in an information coding System
`according to the invention,
`FIG. 3 presents the focusing of error correction coding on
`Speech parameter bits in relation to the efficiency of the error
`correction coding used in a speech coding System according
`to the invention,
`FIG. 4 presents in form of a block diagram a transmitter
`utilizing the focused error correction and error detection
`according to the invention,
`FIG. 5 presents in form of a block diagram a receiver
`utilizing the focused error correction and error detection
`according to the invention,
`FIG. 6 presents the focusing of the focused error detection
`according to the invention on Speech parameter bits as a
`function of an error detection mode,
`FIGS. 7 and 8 present the realization of the focused error
`correction and error detection according to the invention in
`connection with a speech coder using Several Speech codecs
`with different bit rates,
`FIG. 9 presents the structure of a mobile station according
`to the invention as a block diagram, and
`FIG. 10 presents an information transfer System according
`to the invention.
`DETAILED DESCRIPTION OF THE
`INVENTION
`FIG. 1A presents as a block diagram the Structure and
`function of the transmitter of an FR-Speech codec prior
`known from the GSM-system. Speech signal 100 is coded in
`speech encoder 101 into speech parameters 102, which are
`further transferred to channel encoder 104. Channel encoder
`104 adds the error correction- and error detection bits in
`connection with Speech parameters 102. In bit Separation
`block 103 the speech parameters are divided into two
`importance classes. Error detection- and error correction
`parameters are formed for the most important 182 bits (Cl