WO1998025384A2 - Procede de codage en treillis de la repartition logarithmique de points de constellation - Google Patents

Procede de codage en treillis de la repartition logarithmique de points de constellation Download PDF

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Publication number
WO1998025384A2
WO1998025384A2 PCT/IE1997/000080 IE9700080W WO9825384A2 WO 1998025384 A2 WO1998025384 A2 WO 1998025384A2 IE 9700080 W IE9700080 W IE 9700080W WO 9825384 A2 WO9825384 A2 WO 9825384A2
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WIPO (PCT)
Prior art keywords
trellis
index
point
generated
code
Prior art date
Application number
PCT/IE1997/000080
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English (en)
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WO1998025384A3 (fr
Inventor
Michael Mclaughlin
Original Assignee
Fernway Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Publication date
Application filed by Fernway Limited filed Critical Fernway Limited
Priority to AU54078/98A priority Critical patent/AU5407898A/en
Publication of WO1998025384A2 publication Critical patent/WO1998025384A2/fr
Publication of WO1998025384A3 publication Critical patent/WO1998025384A3/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
    • H04L1/0056—Systems characterized by the type of code used
    • H04L1/0059—Convolutional codes
    • H04L1/006—Trellis-coded modulation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/38—Synchronous or start-stop systems, e.g. for Baudot code
    • H04L25/40—Transmitting circuits; Receiving circuits
    • H04L25/49—Transmitting circuits; Receiving circuits using code conversion at the transmitter; using predistortion; using insertion of idle bits for obtaining a desired frequency spectrum; using three or more amplitude levels ; Baseband coding techniques specific to data transmission systems
    • H04L25/4917—Transmitting circuits; Receiving circuits using code conversion at the transmitter; using predistortion; using insertion of idle bits for obtaining a desired frequency spectrum; using three or more amplitude levels ; Baseband coding techniques specific to data transmission systems using multilevel codes
    • H04L25/4927—Transmitting circuits; Receiving circuits using code conversion at the transmitter; using predistortion; using insertion of idle bits for obtaining a desired frequency spectrum; using three or more amplitude levels ; Baseband coding techniques specific to data transmission systems using multilevel codes using levels matched to the quantisation levels of the channel
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
    • H04L1/0041—Arrangements at the transmitter end
    • H04L1/0043—Realisations of complexity reduction techniques, e.g. use of look-up tables

Definitions

  • the present invention relates to a method for trellis coding a logarithmic distribution of constellation points, and in particular, though not limited to a method for applying trellis coding to such a system of constellation points for a pulse code modulation (PCM) modem which uses A-Law or ⁇ -Law codecs for coding voltage levels of analog signals outputted by the modem.
  • PCM pulse code modulation
  • A-Law and ⁇ -Law codecs have 256 analog output levels. These levels are divided into a number of groups of levels with nominally equal intervals between successive levels in a group. The group of lowest levels have the smallest intervals between the respective successive levels, and the intervals between the levels in the group of lowest levels are equal.
  • the group of lowest levels is followed by a group of the next levels above the levels of those of the group of lowest levels, and the intervals between the respective successive levels in this group are also equal and of value twice the value of the interval between the levels in the lowest levels group.
  • the group of second lowest levels is then followed by a group of third lowest levels and so on, in each case, the intervals between the levels in the respective groups being similar, but the intervals between the levels in the respective groups in each case is twice the intervals between the levels in the immediately lower group.
  • the groups of levels contain similar numbers of levels.
  • n-dimensional trellis coding In conventional n-dimensional trellis coding, the number of points in the constellation is doubled in n-dimensions , for example, a 32 point 2-dimensional constellation becomes a 64 point 2-dimensional constellation for 2-dimensional code, or a 1,024 point 4-dimensional constellation becomes a 2,048 point 4-dimensional constellation for a 4-dimensional code.
  • the following example illustrates how a conventional application of trellis coding is not an ideal method for increasing performance in, for example, A-Law and/or ⁇ -Law codec.
  • the top 128 levels are divided into four groups of 32 levels. For a 16 state 4-dimensional trellis code used in V.34 modems, if the top 128 levels were used as constellation coordinates at a sample rate of 8kHz, the following holds :
  • a 128 point uncoded 1-dimensional constellation gives a bitrate of 56kbit/sec.
  • a 128 point conventionally coded 4-dimensional constellation gives 54kbits/sec with approximately 4dB signal to noise ratio (SNR) coding gain.
  • SNR signal to noise ratio
  • the present invention is directed towards providing such a method .
  • a method for trellis coding a logarithmic distribution of constellation points wherein the trellis coding is applied only to the lower level points of the constellation whose interval from their respective next immediate lower level point is less than a predetermined value .
  • the points are initially in the form of an input bit stream, and the method comprises the step of generating an index (I n ) from the input bit stream for each point for determining if the point is to be trellis coded.
  • the generated index (I n ) lies between zero and a maximum index (I max ), where the maximum index (I max ) is equal to one less than the highest code value divided by two .
  • a maximum trellis code index (I maxt ) is generated, the maximum trellis code index being a function of the predetermined value of the interval between the levels below which the points are to be trellis coded.
  • the maximum trellis code index (T- m ) is equal to one less than the highest trellis code divided by two.
  • the generated index (I n ) is compared with the maximum trellis code index (I maxt ) for determining if the point from which the generated index (I n ) has been generated is to be trellis coded.
  • one bit is selected from the input bit stream and passed to a reflection means when the generated index (I n ) is determined to be greater than the value of the maximum trellis code index (I maxt ), and the point is not to be trellis coded.
  • the trellis coding means generates two 2-dimensional subsets or four 1-dimensional reflections from each three bits received from the input bit stream, the two 2-dimensional subsets or the four 1-dimensional reflections are then fed to the reflection means .
  • the generated index (I n ) is transformed to form a transform number
  • the 1-dimensional reflections generated by the trellis coding means is applied to the transform number (P n ) to generated a selector number (Q n ) for selecting an output level corresponding to the point to be transmitted.
  • the four 1-dimensional reflections outputted by the trellis coding means are sequentially applied to the transform number in the reflection means for generating respective selector numbers for the point corresponding to the generated index (I n ) and for the next three points, the generated index (I n ) of which is less than or equal to the maximum trellis code index
  • the transform number (P n ) is equal to twice the value of the generated index (I n ) less twice the sum of twice the generated index (I n ) + 1 multiplied by the modulo 2 of the generated index (I n ) where the modulo 2 of the generated index (I n ) is the remainder when the generated index (I n ) is divided by two.
  • the output level to be transmitted is determined by the selector number from a look-up table .
  • the reflection means generates at least one reflection of each point.
  • the reflection means generates four reflections of each point one in each subset.
  • the logarithmic distribution of constellation points are voltage levels to be transmitted by a pulse code modulation modem.
  • the invention provides a method for trellis coding a logarithmic distribution of constellation points using a convolutional coder, wherein each bit of the convolutional code or output is used to determine a trellis code subset via a one dimensional reflection
  • the invention provides apparatus for trellis coding a logarithmic distribution of constellation points, wherein the apparatus operates according to the method of the invention.
  • Fig. 1 illustrates a block representation of the method for trellis coding a system with a logarithmic distribution of constellation points, which in this case is a system used in a PCM modem which uses ⁇ -Law codec .
  • Table 1 illustrates a code, which is similar to a ⁇ -Law code with the exception that fewer groups of intervals and fewer intervals per group are provided. If for example :
  • the voltage levels to be coded are initially selected, and the following procedure sets out how to select the levels for coding. Starting at the highest level, all levels which are separated by an interval from their adjacent level, which is greater than an interval CI min from the previously accepted level are accepted. If the lowest level accepted is less than 0.5 CI min , it should also be discarded. This is because both positive and negative values of each level are used and need to be distinguished from each other. Thus, the smallest positive level should be at least CI min above its negative equivalent. It should however be noted that in the case of A-Law codes, an extra point near zero is discarded. It is possible to use this extra point by changing the mapping scheme, although this would require two different schemes, one for ⁇ -Law and one for A-Law codes .
  • the values of the levels which are to be used are arrived at.
  • the levels are divided into two categories, namely, the first category which takes all levels from the lowest level to the lowest level below the first interval greater than or equal to UI min .
  • Small adjustments to the value of the levels used may be applied, for example, to distribute the levels more widely, particularly, at the end points.
  • neither CI min for the lower category nor UI ⁇ for the upper category should be exceeded.
  • the codes are then assigned numbers consecutively starting at one for the lowest positive level and continuing in steps of two.
  • the pairs of positive and negative levels are assigned the same code except that one is the negative of the other. This is carried out for the example in Table 1.
  • Trellis coding is only applied to the lower category of levels, in other words the lower levels of which the intervals separating adjacent levels is less than or equal to a predetermined interval value .
  • Trellis coding is carried out by a trellis coding means, namely, a convolutional coder, illustrated as block 1.
  • a trellis coding means namely, a convolutional coder, illustrated as block 1.
  • a preferred method for carrying out the trellis coding is as follows .
  • a Bit-to-index generator which is illustrated by block 2 takes the number of bits it requires from an input bit stream fed on a line 3 through a switch 4 and generates an index I n which lies between zero and a maximum index I max .
  • the maximum index I max is equal to the (highest code -l)/2.
  • the Bit-to-Index generator 2 comprises a shell mapping scheme similar to that defined in International Telecommunications Union - Telecommunications Division recommendation V.34.
  • the Bit-to-Index generator may be a base conversion from the binary input base (base 2) to an (I mx plus 1) output base, or any other suitable generator.
  • n levels are taken together they constitute an n-dimensional point with each 1-dimensional level being a coordinate .
  • the first level that belongs to this category is considered to be the first of n-dimensions .
  • the next level that also belongs to this category is considered to be the second of n-dimensions and so on.
  • Block 6 compares the generated index I n with the maximum trellis code index I maxt .
  • block 6 determines that the generated index I n is greater than the maximum trellis code index I maxt one bit from the input bit stream on line 3 is fed through the switch 4 directly to a reflection means, illustrated by block 9, which is described below. The bit is transferred directly to the reflection block 9 through switches 7 and 8 under the control of block 6.
  • the switches 7 and 8 are thrown, and bits are taken from the bit stream on the input line 3 and are fed through the switch 4 and the switch 7 to the convolutional coder 1.
  • block 6 determines that the generated index I n is less than or equal to the maximum trellis code index I maxt , then for the first such point, and every fourth point thereafter three bits are taken from the input bit stream 3 and fed into the convolutional coder 1 and trellis coded, and are fed to the reflection block 9 through the switch 8 as will be described below.
  • any trellis coding scheme can be applied in the convolutional coder 1, for example, the 16 state Wei trellis code which is used in V.34 may be used to trellis code the lower category levels .
  • 2-dimensional points are divided into four subsets. All the points at this stage, when P n and the next trellis coded output P n+i are combined belong to a single 2-dimensional trellis code subset. Each point must then be moved to the required subset. The correct subset is determined by the convolutional coder 1.
  • the shell mapping algorithm in the Bit-to-Index generator 2 generates the index I n on the basis of the required power of the point . In order not to change the power of the point, the point is reflected in the X and Y axes to transform the point into the required subset. This is carried out in the reflection block 9.
  • a reflection in the X axis in 2-dimensions that is taking two consecutive lower category levels and considering them to be a 2-dimensional point and multiplying the first level by -1, brings the 2-dimensional point into the second trellis subset.
  • a 2-dimensional rotation could also be used to modify the subset, without changing the power, but rotations in 2-dimensions require both coordinates to be available at the time of rotation, which is more difficult to organise.
  • the generated index I n is less than or equal to the maximum trellis code index I maxt , in other words that the point is found to be from the lower category, then for the first such point, (and every fourth such point thereafter) three bits are taken from the input stream and fed into the convolutional coder 1, as discussed above. This generates two 2-dimensional subsets or four 1-dimensional reflections. The first of these reflections is applied to the transform number P n in block 9 to give a selector number Q n . The second is applied to the next index found to be from the lower category, the third to the third such and the fourth to the fourth such, and then the next index in the lower category generates a new set of rotations from three more input bits .

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Physics & Mathematics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Digital Transmission Methods That Use Modulated Carrier Waves (AREA)
  • Error Detection And Correction (AREA)

Abstract

Cette invention concerne un procédé permettant de coder en treillis les niveaux de tension de signaux provenant d'un modem à modulation par impulsion et codage qui utilise le codage en treillis. Ce procédé consiste à ne coder en treillis que les points de niveaux réduits dans lesquels les intervalles entre des niveaux adjacents sont inférieurs ou égaux à une valeur prédéterminée. Un générateur bit-indice (2) va générer un indice (In) à partir d'un train binaire d'entrée sur une ligne d'entrée (3), lequel indice est ensuite comparé dans un bloc (6) à un indice de code en treillis maximal (Imaxt). Les points correspondant à l'indice (In) ainsi généré, lorsque ce dernier est supérieur à l'indice de code en treillis maximal (Imaxt), sont ainsi libres de passer sans effectuer de codage en treillis. En revanche, les points correspondant à l'indice (In) ainsi généré, lorsque ce dernier est inférieur ou égal à l'indice de code en treillis maximal (Imaxt), sont alors codés en treillis. Le codage en treillis se fait dans un codeur convolutif (1) dans lequel trois bits sont extraits du train binaire d'entrée lorsque l'indice (In) est inférieur ou égal à l'indice de code en treillis maximal (Imaxt). Deux sous-ensembles bidimensionnels ou quatre réflexions unidimensionnelles sont ensuite émis et envoyés vers un bloc de réflexion (9). Les réflexions unidimensionnelles sont appliquées à un nombre transformé (Pn) qui est dérivé de l'indice généré (In) dans un convertisseur (4), étant entendu que Pn=2.In+1-2(2.In+1).(In modulo 2). Cette application permet de générer un nombre sélecteur (Qn) qui est utilisé afin de choisir le niveau de sortie à émettre depuis une table de consultation.
PCT/IE1997/000080 1996-12-04 1997-12-04 Procede de codage en treillis de la repartition logarithmique de points de constellation WO1998025384A2 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU54078/98A AU5407898A (en) 1996-12-04 1997-12-04 A method for trellis coding a logarithmic distribution of constellation points

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IES960854 1996-12-04
IE960854 1996-12-04

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0919087A1 (fr) * 1997-01-17 1999-06-02 Motorola, Inc. Systeme, dispositif et procede de communication selon un code composite
GB2361853A (en) * 2000-04-28 2001-10-31 Mitel Corp Turbo encoding for trellis modulation
US6671327B1 (en) 2000-05-01 2003-12-30 Zarlink Semiconductor Inc. Turbo trellis-coded modulation

Citations (5)

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Publication number Priority date Publication date Assignee Title
EP0533363A2 (fr) * 1991-09-03 1993-03-24 AT&T Corp. Codeur et décodeur non-linéaire pour transmission d'information par canaux non-linéaires
EP0634856A2 (fr) * 1993-07-16 1995-01-18 AT&T Corp. Codage non linéaire pour canaux à interférence intersymbole
US5396519A (en) * 1993-10-22 1995-03-07 At&T Corp. Method and apparatus for adaptively providing precoding and preemphasis conditioning to signal data for transfer over a communication channel
EP0725508A1 (fr) * 1995-02-01 1996-08-07 AT&T Corp. Système de communication à suppression d'écho et de bruit améliorée dans un canal de communication avec quantiscation
EP0725487A2 (fr) * 1995-02-01 1996-08-07 AT&T Corp. Codage à niveaux multiples pour bits fractionnairs

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0533363A2 (fr) * 1991-09-03 1993-03-24 AT&T Corp. Codeur et décodeur non-linéaire pour transmission d'information par canaux non-linéaires
EP0634856A2 (fr) * 1993-07-16 1995-01-18 AT&T Corp. Codage non linéaire pour canaux à interférence intersymbole
US5396519A (en) * 1993-10-22 1995-03-07 At&T Corp. Method and apparatus for adaptively providing precoding and preemphasis conditioning to signal data for transfer over a communication channel
EP0725508A1 (fr) * 1995-02-01 1996-08-07 AT&T Corp. Système de communication à suppression d'écho et de bruit améliorée dans un canal de communication avec quantiscation
EP0725487A2 (fr) * 1995-02-01 1996-08-07 AT&T Corp. Codage à niveaux multiples pour bits fractionnairs

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
ASANO & KOHNO: "Serial unequal error-protection codes based on trellis-coded modulation" IEEE TRANSACTIONS ON COMMUNICATIONS., vol. 45, no. 6, June 1997, NEW YORK, US, pages 633-636, XP000659558 *

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0919087A1 (fr) * 1997-01-17 1999-06-02 Motorola, Inc. Systeme, dispositif et procede de communication selon un code composite
EP0919087A4 (fr) * 1997-01-17 2001-08-16 Motorola Inc Systeme, dispositif et procede de communication selon un code composite
GB2361853A (en) * 2000-04-28 2001-10-31 Mitel Corp Turbo encoding for trellis modulation
US6671327B1 (en) 2000-05-01 2003-12-30 Zarlink Semiconductor Inc. Turbo trellis-coded modulation

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WO1998025384A3 (fr) 1998-09-17
AU5407898A (en) 1998-06-29

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