Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
  • H03M13/00—Coding, decoding or code conversion, for error detection or error correction; Coding theory basic assumptions; Coding bounds; Error probability evaluation methods; Channel models; Simulation or testing of codes
  • H03M13/25—Error detection or forward error correction by signal space coding, i.e. adding redundancy in the signal constellation, e.g. Trellis Coded Modulation [TCM]
  • H03M13/256—Error detection or forward error correction by signal space coding, i.e. adding redundancy in the signal constellation, e.g. Trellis Coded Modulation [TCM] with trellis coding, e.g. with convolutional codes and TCM
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
  • H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
  • H04L27/38—Demodulator circuits; Receiver circuits
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
  • H03M13/00—Coding, decoding or code conversion, for error detection or error correction; Coding theory basic assumptions; Coding bounds; Error probability evaluation methods; Channel models; Simulation or testing of codes
  • H03M13/63—Joint error correction and other techniques
  • H03M13/635—Error control coding in combination with rate matching
  • H03M13/6362—Error control coding in combination with rate matching by puncturing

Definitions

• Trellis-coded modulation combines channel coding and modulation 5 at a transmitter.
• Information bits are sent into a TCM encoder in parallel.
• the least- significant bits (LSBs) are convolutionally encoded.
• the most-significant bits (MSBs) are usually left uncoded and combined with the output of the convolutional coder to form a symbol on a signal constellation through a signal mapper.
• One advantage of TCM is that although no error control coding is performed on any bit
• the decoder is able to provide error correction on all bits. Therefore, significant coding gain over uncoded modulation can be achieved.
• Punctured Pragmatic TCM Punctured Pragmatic TCM
• P2TCM Punctured Pragmatic TCM
• the core of the trellis coder is a punctured version of the industry standard, rate Vz convolutional code. This approach was shown to lead to a wide class of high-rate pragmatic punctured trellis codes of rate n/n+1 for both phase-shift keying (PSK) and quadrature amplitude modulation (QAM) modulations.
• PSK phase-shift keying
• QAM quadrature amplitude modulation
• a trellis diagram can be used to represent the response of a convolutional coder to an input stream.
• Each trellis stage includes a fixed number of nodes that represent the possible states of the coder.
• the states are connected such that each state has inputs from disjoint states in the preceding stage and outputs to disjoint states in the succeeding stage.
• the connections represent possible state transitions and are more commonly referred to as branches.
• the decoding of TCM signals can be performed efficiently using the well- known Viterbi algorithm.
• the Viterbi algorithm selects the best path along the branches of thertrellis diagram to determine the originally transmitted input stream from a received sequence of symbols.
• the algorithm bases this selection on a measure of the accumulated squared (Euclidean) distance between the received symbols and possible points on a signal constellation.
• the Euclidean distance measurement is made for each branch of the trellis diagram and is referred to as a branch metric.
• the branch metric is added to the accumulated error for each state. This accumulated error is also called a state metric.
• a comparison is made between the two incoming branches and the branch with the smaller accumulated error is selected as the survivor branch for each state of the trellis stage.
• a common technique is to increase the order of modulation, for example, by upgrading from quarternary phase-shift keying (QPSK) to 8-ary phase-shift keying (8PSK).
• QPSK quarternary phase-shift keying
• 8PSK 8-ary phase-shift keying
• high order modulation can result in inferior performance, since the transmitter has to increase the signal power to maintain the same bit error rate as that of the low order modulation.
• a typical approach is to use TCM at the encoder and "soft decision" values input to a Viterbi decoder designed for the particular high order TCM.
• the Viterbi decoder engine is typically the most complex element.
• QPSK modulation there exists many commercially available cores and ASICs that provide a QPSK decoder as a whole package.
• QPSK decoders cannot be applied directly to higher order TCM.
• a need exists for a decoding capability which can take advantage of the symmetric properties of QPSK in decoding higher order modulations while also taking advantage of commercially available QPSK Viterbi decoder engines.
• the present invention provides a method and apparatus for decoding a high order trellis-coded modulated (TCM) signal using a QPSK Viterbi decoder.
• the invention avoids the need to modify a standard QPSK Viterbi decoder by including a pre-processing unit that transforms a received signal to an equivalent signal vector or soft value in the QPSK signal constellation.
• This inventive approach is referred to as the "transformed I-Q" approach where I-Q refers to the inphase (I) and quadrature (Q) components of the received signal at the decoder.
• a method of decoding includes receiving a signal having inphase (I) and quadrature (Q) signal components which correspond to a signal constellation of a first modulation type, such as a high order trellis code modulation or punctured QPSK.
• the received I, Q signal components are transformed to I 1 , Q 1 signal components corresponding to a QPSK signal constellation.
• the I', Q 1 signal components are decoded in a QPSK decoder such as a QPSK Viterbi decoder engine.
• the transforming includes selecting four signal points and determining branch metrics corresponding to the four selected signal points.
• the branch metrics correspond to a distance between the received signal and the selected signal points in the first modulation signal constellation. From among the selected signal points that signal point having the largest branch metric is determined and an offset is added thereto. The branch metrics of the remaining selected signal points are matched to corresponding QPSK branch metrics in the QPSK signal constellation.
• the transformed I-Q approach of the present invention is applicable to various high order TCM schemes, including one and two coded bits per symbol formats.
• FIGs. 1 A and IB show block diagrams for a conventional TCM encoder and decoder, respectively.
• FIG. 2 shows a conventional rate Vi convolutional encoder.
• FIG. 3 illustrates a QPSK signal constellation according to the DVB standard.
• FIG. 4 shows a block diagram for a conventional QPSK decoder.
• FIG. 5 shows a block diagram of a decoder according to the present invention.
• FIG. 6 illustrates a trellis diagram. -5-
• FIG. 7 illustrates a signal constellation for 16 QAM.
• FIG. 8 illustrates an 8PSK signal constellation.
• FIG. 9A illustrates a rate 8/9 8 PSK signal constellation based on a rate 5/6 convolutional code
• FIGs. 9B and 9C illustrate two possible bit-to-symbol 5 mappings.
• FIGs. 10A and 10B illustrate a unit vector on respective different signal constellations for 8PSK modulation.
• the encoder structure in FIG. 1 A includes three elements: differential precoder 100, convolutional encoder 102 of rate n/(n+l) with
• the input bit a is called an "encoded bit” and bits b,, b 2 , ..., b k are called “uncoded bits”.
• the output y of the convolutional encoder 102 provides "coded bits”. If the convolutional encoder 102 is the de facto rate Vz convolutional encoder 102' as shown in FIG. 2, then the coding is called pragmatic trellis code modulation (PTCM).
• PTCM pragmatic trellis code modulation
• mapping can be viewed as a lookup table that assigns an address to a signal point in the constellation. If there are two coded bits within the address of a symbol (not necessarily from the same transition), this coding is referred to as “two coded bits per symbol” (2CBPS). On the other hand, if there is only one coded bit, such coding is referred to as “one coded bit per symbol” (1CBPS). -6-
• FIG. IB shows the decoder structure comprising five elements: branch metric calculator 106, uncoded bit detector 108, convolutional re-encoder 110, Viterbi decoder engine 112 and differential decoder 114.
• branch metric calculator 106 uncoded bit detector 108
• convolutional re-encoder 110 convolutional re-encoder 110
• Viterbi decoder engine 112 differential decoder 114.
• the output of the bit-to-symbol mapping block 104 is denoted as s(b, b 2 ...b k y, yj for 2CBPS and s(b, b 2 ...b k y,) for 1CBPS.
• s(b, b 2 ...b k y,) for 1CBPS.
• d 00 ,d 01 ,d l0 ,d u are Euclidean distances between the received signal and the signal constellation.
• the Viterbi decoder engine 112 receives d go ,d ol ,d 10 ,d ⁇ as inputs and determine an optimal path in the trellis diagram.
• the output a of the Viterbi decoder engine is re-encoded by the convolutional re-encoder 110 to provide output y which is used to determine uncoded bits &,, b .... b by the uncoded bit detector 108.
• the branch metric calculator 106 needs to find two signals Sg,s ⁇ and branch metrics dg, d, such that
• a QPSK soft value calculator 200 calculates the soft values v 0 , v, as described above for Equations (21)(22).
• the Viterbi decoder engine is typically the most complex element. There are many commercially available cores and ASICs that provide a QPSK decoder as a whole package.
• the present invention provides a modification to the conventional QPSK decoder by introducing a pre-processing unit which transforms a received signal to an equivalent signal vector or soft value in the QPSK signal constellation. This allows a high order TCM signal to be decoded using a conventional QPSK Viterbi decoder engine.
• This inventive approach is referred to as the "transformed I-Q" -10-
• I-Q refers to the components I and Q of the received signal at the decoder.
• the decoder shown in FIG. 5 includes the following conventional decoder elements: QPSK soft values calculator 302, QPSK Viterbi decoder engine 304, convolutional re-encoder 306, uncoded bit detector 308 and differential decoder 310.
• the decoder further includes an IQ transformer block 300 which provides a transform of the I,Q components such that a general TCM decoder is provided which uses the conventional QPSK Viterbi decoder engine 304 to produce a very good approximation for higher order modulation schemes.
• invention can simplify the decoding algorithm and leave the Viterbi decoder engine unchanged.
• the transformed I-Q approach of the present invention can be applied to punctured QPSK.
• Table 1 shows the corresponding function table of the branch metric calculator where x denotes a puncture.
• a more popular implementation method for punctured QPSK TCM involves insertion of an erasure. This method can be interpreted as a form of the transformed I-Q approach.
• This method can be interpreted as a form of the transformed I-Q approach.
• the four Euclidean distances chosen as the branch metrics can be highly irregular compared to the d' tone which are measured from the received signal to four corners of a square.
• dominates the difference between the conventional decoding method and the transformed I-Q method of the present invention.
• the optimal decision and the transformed I-Q method For any signal constellation that satisfies
• 0 for all received signal points, there is no degradation between the optimal decision and the transformed I-Q method.
• the box 402 around the two rightmost bits of a symbol 400 indicates the two coded bits per symbol.
• the 16QAM constellation satisfies
• 0 because any four closest points chosen form four corners of a square.
• the transformed I-Q method for 16QAM has no degradation compared with the conventional method.
• the V and Q ' in Equations (29)(31)(33)(35) are based on the difference of the Euclidean distance instead of the absolute distance. Using a triangle inequality,
• is finite. Since most of the Viterbi decoder is using fixed point computation, a finite dynamic range will not only be easy to determine the number of quantization bits, but also will reduce the quantization noise due to finite precision. For example, for an 8PSK constellation as shown in FIG. 8,
• is about three times smaller than a typical I-Q signal which requires a dynamic range of at least 1.5. If a 2CBPS scheme is based on a punctured convolutional code, two coded bits may come from the same transition, from two consecutive transitions or from two far away transitions. Even at the transition where neither yl nor y2 is punctured, -19-
• FIG. 9A shows rate 8/9 8PSK based on a rate 5/6 convolutional code.
• FIGs. 9B and 9C illustrate two possible bit-to-symbol mapping patterns with transitions indicated as 420-1,. . ., 420-6 and 422-1,. . .,422-6, respectively.
• the conventional decoding method for punctured TCM can be very complicated.
• coded bits y2 and y5 are carried by the one symbol. Since the four closet signals never form four corners of a rectangle, the 1-d metric method cannot be directly applied. This is because the Euclidean distance d, cannot be split into two individual / and Q terms.
• the optimal decoding method has to wait until the third transition and then perform the standard add-compare-select operation.
• the transformed I-Q method of the present invention provides a very good solution.
• the components (I, Q) are transformed to an equivalent component set (V, Q') in the QPSK signal constellation
• the 1-d metric method in Table 1 or the transformed I-Q method in Table 2 can be applied.
• the bit-to-symbol mapping is as complicated as shown in FIG. 9B, a very "clean" decoding method can still be applied.
• Table 3 shows the transform I-Q for different conditions, where the index in d 0 i , d l t indicates a different received symbol.
• Equations (10) (11) and (12) can also be expressed as a function of
• These soft values can be used as the soft values input to the QPSK Viterbi decoder engine 304 described above with reference to FIG.5.

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• Physics & Mathematics (AREA)
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• 1999
  • 1999-04-16 EP EP99918600A patent/EP1072099A1/de not_active Withdrawn
  • 1999-04-16 WO PCT/US1999/008346 patent/WO1999055010A1/en not_active Application Discontinuation
  • 1999-04-16 AU AU36474/99A patent/AU3647499A/en not_active Abandoned

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