WO2001017187A1 - Procede et appareil permettant de decoder des messages a codage convolutionnel et codes en continu - Google Patents

Procede et appareil permettant de decoder des messages a codage convolutionnel et codes en continu Download PDF

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Publication number
WO2001017187A1
WO2001017187A1 PCT/US1999/020036 US9920036W WO0117187A1 WO 2001017187 A1 WO2001017187 A1 WO 2001017187A1 US 9920036 W US9920036 W US 9920036W WO 0117187 A1 WO0117187 A1 WO 0117187A1
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WIPO (PCT)
Prior art keywords
encoder
states
message
decoding
decoder
Prior art date
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PCT/US1999/020036
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English (en)
Inventor
Stash Czaja
Sanguoon Chung
Xiaojun Li
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Sony Electronics, Inc.
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Publication date
Application filed by Sony Electronics, Inc. filed Critical Sony Electronics, Inc.
Priority to AU59064/99A priority Critical patent/AU5906499A/en
Priority to PCT/US1999/020036 priority patent/WO2001017187A1/fr
Publication of WO2001017187A1 publication Critical patent/WO2001017187A1/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
    • H04L1/0045—Arrangements at the receiver end
    • H04L1/0052—Realisations of complexity reduction techniques, e.g. pipelining or use of look-up tables
    • H04L1/0053—Realisations of complexity reduction techniques, e.g. pipelining or use of look-up tables specially adapted for power saving
    • 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
    • H03M13/37—Decoding methods or techniques, not specific to the particular type of coding provided for in groups H03M13/03 - H03M13/35
    • H03M13/39—Sequence estimation, i.e. using statistical methods for the reconstruction of the original codes
    • H03M13/3994—Sequence estimation, i.e. using statistical methods for the reconstruction of the original codes using state pinning or decision forcing, i.e. the decoded sequence is forced through a particular trellis state or a particular set of trellis states or a particular decoded symbol
    • 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
    • H03M13/37—Decoding methods or techniques, not specific to the particular type of coding provided for in groups H03M13/03 - H03M13/35
    • H03M13/39—Sequence estimation, i.e. using statistical methods for the reconstruction of the original codes
    • H03M13/41—Sequence estimation, i.e. using statistical methods for the reconstruction of the original codes using the Viterbi algorithm or Viterbi processors
    • 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
    • H03M13/6356—Error control coding in combination with rate matching by repetition or insertion of dummy data, i.e. rate reduction
    • 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
    • H04L1/0045—Arrangements at the receiver end
    • H04L1/0054—Maximum-likelihood or sequential decoding, e.g. Viterbi, Fano, ZJ algorithms

Definitions

  • the present invention relates generally to communications systems, and more particularly, to decoding convolutionally encoded messages such as paging messages transmitted within a code division multiple access (CDMA) wireless telecommunications system.
  • CDMA code division multiple access
  • a paging channel is used by a base station to communicate to mobile stations (radiotelephones) when they are not already assigned to a traffic channel.
  • the primary purpose of the paging channel is to convey pages, i.e., notifications of incoming calls, to the mobile stations.
  • the mobile station operates in an idle mode while awaiting notification of an incoming call over the paging channel .
  • the mobile station operates in a slotted mode, in which the mobile station periodically activates its receiver electronics to "listen" for a page during a periodic time slot allocated to at least that mobile station. If no page intended for that mobile station is received during the allocated time slot, the mobile station "goes to sleep", essentially turning off its components and conserving power, until it reactivates itself for the next assigned slot.
  • the percentage of time that the mobile station is allowed to sleep in the idle mode is proportional to the power conservation.
  • paging messages are convolutionally encoded in the paging channel. This technique reduces the likelihood of errors upon reception of the message due to multipath fading, noise or other factors.
  • convolutional encoding involves sequencing an original data stream through a shift register and summing the bits of predetermined stages of the shift register to generate a larger number of bits for transmission.
  • Mobile stations are permitted to operate either in a slotted mode as mentioned above to receive synchronized pages, or in a non-slotted mode to receive unsynchronized or synchronized pages.
  • the pages transmitted by the base station are of variable length, up to 80ms in duration.
  • decoding of the page message is preferably performed with a Viterbi decoder.
  • the mobile station In the slotted mode, to demodulate and decode the message contained within its allocated time slot, the mobile station "wakes up" one frame ahead of time to synchronize the Viterbi decoder within the mobile station. This early wake-up is necessary because the paging messages are continuously encoded (i.e., the base station encoder shift register is not reset in between page messages) and the decoder has no knowledge of the initial encoder state at the start of the allocated slot. Usually, the decoder requires at least a 13 -bit transition to synchronize; however, since the transmitted message is interleaved over a 20ms frame, the synchronization takes 20ms. For most cases, there is no message transmitted to a particular mobile station, which then goes back into the sleep mode.
  • the mobile station spends 40ms in the active mode, i.e., 20ms for the wake-up period and 20ms thereafter in its allocated slot attempting to receive a message. This represents twice as much time in the active mode as necessary (40ms vs. 20ms), thus significantly reducing the mobile station's idle time capacity.
  • the method involves at least partially flushing (i.e., partially resetting) the encoder just prior to the onset of a synchronized paging slot, by inserting at least four padding bits (e.g., four zeroes) immediately prior to the onset of the synchronized page message.
  • padding bits e.g., four zeroes
  • an 8-bit encoder register is employed such that nine (K-l) consecutive known padding bits are necessary to reset the encoder to a known state.
  • the method takes advantage of the fact that, due to the variability in the length of the previous message, padding bits are usually added to the end of the message anyway in accordance with the standard.
  • the method then assumes that the initial encoder state at the onset of the allocated time slot is all zeroes (eight zeroes) since, by guaranteeing at least four zeroes, there will be more zeroes at the tail end of the previous slot most of the time.
  • the Viterbi decoding process would then proceed based on the assumption of an all zeroes state. Consequently, the Viterbi decoder need not be turned on one frame early to synchronize, thereby conserving power.
  • a shortcoming of this approach is that the assumption of the encoder being in an all zeroes state with only four consecutive zeroes guaranteed is not valid in most cases. Indeed, simulations have shown that the message error probability with this approach is unacceptably in the range of 15 to 25%.
  • an object of the present invention to provide an improved decoder for decoding a message transmitted in a channel in which continuous convolutionally encoded messages are transmitted without an encoder reset operation between messages.
  • a decoding method for use in a communications system employing a communication channel in which a message is convolutionally encoded by a base station encoder and transmitted to a remote terminal during a time slot allocated to at least that remote terminal .
  • the encoder is not completely reset immediately prior to the allocated time slot such that the encoder is in an unknown state at the onset thereof.
  • the decoding method includes the steps of assigning a most likelihood probability for an initial encoder state to a number of predetermined encoder states; and, convolutionally decoding succeeding bits of the message based on an assumption that the initial encoder state is one of the predetermined states.
  • the method has particular utility when used to decode paging channel messages in a CDMA wireless telecommunications system.
  • a Viterbi decoder is used to perform the decoding, and the predetermined encoder states assigned a most likelihood probability are sufficient to enable the trellis of the decoder to converge within a small number of transitions, less than the minimum free distance of the code.
  • This technique results in error-free decoding, provided that there are no consecutive bit errors in the communication channel.
  • the illustrative method and decoders embodying the same beneficially afford low error rate decoding of convolutionally encoded messages without the necessity of resetting the encoder on the transmit side in between messages.
  • the invention can be employed to lower power consumption within a mobile station operating within the slotted mode, since the mobile station need not awaken substantially before the onset of its allocated time slot to synchronize its decoder.
  • FIG. 1 is a timing diagram illustrating slotted mode paging channel message structure in accordance with an illustrative embodiment of the invention
  • FIG. 2 is a block diagram depicting electronics within a mobile station
  • FIG. 3 schematically illustrates a base station encoder
  • FIG. 4 is a trellis diagram depicting operation of an illustrative Viterbi decoder in accordance with the invention
  • FIG. 5 is a flow chart illustrating a routine within a Viterbi decoder in accordance with the invention.
  • FIG. 6 is a graph illustrating simulated performance of an embodiment of the invention.
  • Decoders in accordance with the invention and their method of operation may be employed to decode convolutionally encoded messages transmitted in other wireless or wireline communication systems, such as time domain multiplexed (TDM) and/or frequency division multiplexed (FDM) systems.
  • TDM time domain multiplexed
  • FDM frequency division multiplexed
  • the shown message structure is similar to that required by the IS-95-A standard, but with one key difference -- a predetermined sequence of padding bits PB are inserted prior to a Synchronized Capsule Indicator (SCI) bit whenever a synchronized message is to immediately follow.
  • SCI Synchronized Capsule Indicator
  • Mobile stations can receive pages by operating in either a slotted or non-slotted mode.
  • a mobile station operating in the slotted mode is assigned one periodic paging channel slot. That mobile station will "listen" for a synchronized paging message intended therefor only during the assigned slot, such as by determining whether a transmitted message contains a unique 32 -bit electronic serial number (ESN) identifying that mobile station.
  • ESN electronic serial number
  • a mobile station operating in the non-slotted mode may receive a synchronized or unsynchronized paging message transmitted within any slot.
  • Each paging channel slot such as slot "i" 102 (shown expanded) is composed of four 20ms long paging channel frames 106, with each frame 106 composed of two paging channel half frames 108, each 10ms long.
  • a paging channel half frame consists of a single SCI bit and a paging channel half frame body 109. Whenever the base station transmits the SCI bit as a "1", it also transmits a synchronized paging message immediately thereafter, i.e, in the succeeding paging channel half frame body 109. A synchronized message may be followed by one or more unsynchronized messages. If an SCI bit is a "0", no message is transmitted in the succeeding half frame body 109.
  • a paging channel message is composed of an 8 -bit length field MSG_LENGTH, a message body field and an error detection field (CRC) of 30 bits.
  • the length field code is determinative of how long the message body field is.
  • the paging channel messages are of variable length.
  • the base station may transmit an unsynchronized message 131 immediately following the synchronized message. Contrarily, if there are fewer than eight bits remaining before the next SCI bit, or if no unsynchronized message is transmitted following a paging channel message (synchronized or unsynchronized) , then the base station includes sufficient terminal padding bits at the end of that message to extend it through the bit preceding the next SCI bi .
  • a mobile station in accordance with the invention will power up its receiver electronics just prior to slot (N-l) 103.
  • the mobile station decodes the succeeding synchronized page message to determine whether it contains the electronic serial number (ESN) identifying the same. If the mobile determines the ESN was not transmitted, it powers down its components until just prior to the next slot (N-l) 103', where the process is repeated. If the ESN is received, the mobile station maintains power to its components for further communications in a traffic channel to be assigned by the base station.
  • ESN electronic serial number
  • mobile stations of the prior art power up their receiver/decoder electronics at least one frame ahead of time to synchronize the Viterbi decoder.
  • a prior art mobile station were assigned slot (N-l) in the paging channel structure of FIG. 1, it would turn on about 20ms prior to the onset of slot (N-l) 103, then power down when no message is received, and then power up again approximately 20ms prior to the onset of slot 103'.
  • the mobile station assigned to slot (N-l) does not require any time for synchronizing the Viterbi decoder, and will turn on its electronics immediately prior to the start of slot 103 in this example. (Of course, it will need to turn on a minimum amount of time prior to the commencement of the slot to correctly demodulate the first bit of the slot) .
  • a predetermined number "N B " of known padding bits PB are convolutionally encoded and transmitted by the base station immediately prior to the SCI bit.
  • These padding bits PB are annexed to whatever bit stream immediately precedes them, as part of the original data stream.
  • the padding bits PB are shifted into the encoder register to be convolutionally encoded with the original data stream.
  • the bit sequence immediately preceding the padding bits PB may be either the end of a paging message or other padding bits added to the tail end of the previous message.
  • the mobile station's Viterbi decoder can perform error-free decoding based on an unknown encoder state in a manner to be described.
  • the latter error-free decoding is achieved provided that there are no consecutive bit errors (e.g., caused by fading, noise, etc.) between the transmitted and received data, of a length exceeding the code minimum free distance.
  • Such error-free decoding is performed without the necessity of synchronizing the Viterbi decoder ahead of time -- the decoder may be turned on immediately prior to the time that the first SCI bit of the assigned slot is transmitted.
  • error- free decoding of the page message is accomplished by assigning a maximum likelihood probability to predefined encoder states which are selected in accordance with the predetermined number N B of padding bits transmitted immediately prior to the message, and the number of encoder register bits.
  • N PES (K-1-N B ) 2
  • the base station were to always convolutionally encode and transmit a sufficient number of padding bits to completely reset the encoder immediately prior to the SCI bit, then the decoding process could be commenced based on the encoder being in a known state.
  • this approach would require eight consecutive known padding bits before the SCI bit.
  • the encoder is guaranteed to be only partially reset (partially flushed) whereby it is permissible to transmit less than eight consecutive padding bits for the 8-bit register case. Consequently, more bits of the paging channel are allocated for messages and overhead, thereby providing a more efficient channel .
  • a battery 226 supplies power to the various components of the mobile station through a switch unit 223.
  • processor 222 controls the switching state of switch unit 223.
  • Processor 222 operates in conjunction with a timer 224 synchronized to the base station clock to periodically "wake up" the processor just before the onset of the assigned paging channel slot.
  • processor 222 wakes up the receive electronics in sufficient time prior to the start of the assigned slot to enable the initial bits of the slot to be properly received and demodulated.
  • a paging channel signal S R (forward signal) transmitted by the base station is received at the mobile station.
  • signal S R is convolutionally encoded, modulated by the Walsh function, and direct spread by a PN sequence.
  • Signal S R is received and routed through duplexer 212, then down-converted, filtered, phase demodulated and gain controlled (AGC) within RF demodulator 214.
  • a frequency synthesizer 217 operating under the control of processor 222 provides a precise local oscillation frequency to RF demodulator 214 to allow the latter to isolate the frequency components of signal SR during down-conversion and filtering.
  • the RF demodulated output signal is digitized by A/D converter 215, despread by CDMA despreader 216 and then demodulated with a Walsh sequence by Walsh demodulator 218.
  • the Walsh demodulated signal is then decoded by Viterbi decoder 220 which provides decoded output data to processor 222.
  • Processor 222 determines whether the decoded paging message indicates a call notification to the mobile station, and if so, initiates activation of the transmit path components for ensuing traffic mode operation. In the traffic mode, receive signals are likewise decoded by Viterbi decoder 220 and outputted to processor 222, which separates decoded overhead data from audio data.
  • the audio data is supplied to digital to analog (D/A) converter 228 for conversion to an analog signal which is output to a speaker.
  • D/A digital to analog
  • an analog input signal is digitized by A/D converter 228, and then Walsh and long code modulated within modulator 230.
  • the modulator 230 output signal is spread with a pseudonoise sequence and converted to an analog signal by CDMA spreader and D/A converter 231.
  • RF modulator 232 phase modulates the analog output signal of spreader/converter 231.
  • Frequency synthesizer 217 supplies a synthesized frequency to RF modulator 232 to be modulated (or up- converted and then modulated) .
  • the modulated output is amplified by RF power amplifier 234 and routed through duplexer 212 as a reverse transmit signal S ⁇ .
  • Encoder 300 includes an 8 -bit shift register 301 for receiving and temporarily storing original data to be coded.
  • Adder 302a modulo-2 adds the incoming bit with bits from stages 304 !
  • Adder 302b modulo-2 adds the incoming bit and bits from stage 304 2 , 304 3 , 304 4 , and the last stage 304 8 of register 301 to generate the second bit of the encoded output code.
  • Adder 302b modulo-2 adds the incoming bit and bits from stage 304 2 , 304 3 , 304 4 , and the last stage 304 8 of register 301 to generate the second bit of the encoded output code.
  • N B of padding bits PB is four and that these padding bits are all zeroes.
  • the contents of register 301 are shown in FIG. 3 at a time t 0 immediately prior to the placement of an SCI bit into the original data stream.
  • the contents of register stages 304- L to 304 4 are all zeroes while the contents of stages 304 5 -304 8 are unknown (represented as an "X"), i.e., unknown to the mobile station decoder.
  • These bit values are unknown because mobile station 210, when operating in the slotted mode, wakes up immediately prior to the transmission of the encoded SCI bit.
  • FIG. 4 is a portion of a trellis diagram depicting path memories in the operation of the illustrative
  • Viterbi decoder 220 in accordance with the invention, with an assumption of an 8 -bit encoder register and four padding bits PB as just described in conjunction with FIG. 3.
  • Viterbi decoder 220 assigns a most likelihood probability to predetermined initial encoder states, and calculates trellis path memories accordingly. As a result, the trellis converges within a short time interval.
  • prior art decoders decoding from an unknown initial encoder state assign equal probability to all possible initial encoder states, thereby requiring a substantial amount of time for the decoder to converge.
  • a trellis diagram corresponding to operation of a Viterbi decoder illustrates the different possible data sequences outputted by a convolutional encoder.
  • Each path of the trellis diagram represents a different data sequence, with the paths being drawn between different encoder states.
  • the unknown encoder state at time t 0 it is possible for the unknown encoder state at time t 0 to be 00000000 (state 0) .
  • the state will change to 10000000 (state 128 decimal) .
  • trellis line 412 is drawn from state 0 at time t 0 to state 128 at time t to represent that possible case.
  • the two bit output code of the encoder for this case would be 11 due to the modulo-addition of adders 302a, 302b (for the polynomials of FIG. 3) .
  • the encoder state would remain at state 0, whereby trellis line 410 is drawn as shown.
  • the two bit encoder output code at time t x would be 00.
  • trellis line 416 represents the case of the original data bits at times t x and t 2 both being ones, which would produce an encoder output code of 01 at time t 2 .
  • Trellis line 414 represents the case of the bit at time t 1 being 1 and the bit at time t 2 being 0, which would yield an encoder output code of 10 at time t 2 .
  • trellis lines 418 and 420 are drawn to correspond to the cases of the bit at time t 2 being 0 and 1, respectively. Accordingly, trellis paths are computed and stored in memory within the Viterbi decoder.
  • the survivor path combination of lines 412 and 414 would be stored in a memory table as 10 corresponding to the original data sequence it represents.
  • the combination of paths 412 and 416 would be stored to correspond to the original bit sequence of 01. The process continues until the bit sequences, and associated original data of all possible paths of a predetermined length are initially stored in the decoder memory.
  • the received data sequence is compared to the sequences of the various path memories. Paths are then eliminated based on the likelihood of the received data to the respective paths, until the decoder converges and only one path survives. This elimination process is known as maximum likelihood estimation.
  • the surviving path corresponds to the most likely transmitted data sequence. Most errors between the transmitted encoded data and the received data will be corrected with this process.
  • the decoder when turned on, assumes that the encoder can be in any one of 256 states of equal likelihood.
  • the decoder rather than beginning the decoding procedure by assuming all 256 possible states and defining all possible trellis paths from those states, the decoder assigns a relatively small number of initial encoder states of most likelihood probability, and the least likely probability to all other states. As a consequence, the trellis is able to converge within a short time interval. In the example of FIGS. 3 and 4, encoder states 0 to 15 are assigned a maximum likely probability, while states 16 to 255 are assigned a lowest likely probability. The trellis is then "drawn" from states 0 to 15 at time t 0 . With this approach, all possible paths through the trellis will converge with the most likely path at time t s . Since convergence occurs within five transitions, which is less than the minimum free distance of the code, error-free decoding is achieved provided that no consecutive bit errors are present between the transmitted and received data.
  • the Viterbi decoder states of maximum likelihood i.e., states 0 to 15 corresponding to binary codes 00000000 to 00001111, respectively, are established based on knowledge of the precise number of padding bits (and their values) inserted immediately prior to the SCI bit and the synchronized message. For instance, if five zeroes were always inserted prior to the SCI bit, the decoder would assign initial encoder states 0 to 7, whereas if only three consecutive zeroes were inserted, initial encoder states 0 to 31 would be assigned. The smaller the number of known padding bits employed, however, the longer it takes for the Viterbi decoder to converge. Preferably, the states are selected such that convergence occurs within the minimum free distance of the code to guarantee error-free decoding as just mentioned.
  • Step 502 When the decoder is turned on, it assigns most likelihood probabilities to predetermined encoder states (step 502) in accordance with the protocol of the communication system, i.e., in accordance with the number and values of the padding bits and the number of encoder register stages. The remaining decoding process may then be performed in a manner well known to those skilled in the art.
  • Steps 504 to 508 depict a typical decoding process in accordance with the Viterbi algorithm.
  • a Branch Matrix transitional probability
  • the Branch Matrix is defined as the square of the difference between the received symbol code and the optimum code value.
  • a Path Matrix (joint probability) is calculated by determining the Euclidean distance between the most likely path (sequence) and all other paths (sequences) . This distance is computed by adding and subtracting the Branch Matrix from the state transitions leading to the current state and updating the current state Path Matrix. As the Path Matrix is computed, the binary decision (survivor bit) for each of the 2 K ⁇ 1 possible states is stored in a survivor path memory.
  • the Viterbi decoder then traces the most likely path through the trellis by tracing back the survivor path memory starting with the most likely state (step 508) .
  • the depth of the survivor path is long enough to guarantee that the "all state" trellis merges with the most likely path, the effects of channel noise are mitigated as the paths through the trellis converge.
  • the bits selected through the trace back operation are the maximum likely estimation of the originally transmitted bit sequence.
  • FIG. 6 is a graph showing simulated performance results of the message error rate (MER) as a function of signal to noise ratio (Eb/Nt) for an embodiment of a mobile station decoder in accordance with the invention.
  • MER message error rate
  • Eb/Nt signal to noise ratio
  • the simulated decoder receives pages in the slotted mode with four padding bits prior to the message as described above in connection with FIGS. 3-5. Operation in an Additive White Gaussian Noise (AWGN) channel at a data rate of 9600bps is simulated.
  • Curve 602 represents performance of the decoder initialized to start decoding from an unknown state (equal likelihood for all encoder states) .
  • Curve 604 represents the performance of the decoder in accordance with the invention.
  • AWGN Additive White Gaussian Noise
  • the present invention has been particularly shown and described in conjunction with preferred embodiments thereof, it will be readily appreciated by those of ordinary skill in the art that various changes and modifications may be made to the disclosed embodiments without departing from the spirit and scope of the invention.
  • the preferred embodiment has been described for use in a CDMA communication system
  • the invention may also be used for the encoding and decoding of messages in time domain multiplexed (TDM) and frequency division multiplexed (FDM) wireless telecommunication systems as well as in wireline systems.
  • TDM time domain multiplexed
  • FDM frequency division multiplexed
  • the convolutionally encoded messages to be decoded in accordance with the invention may be short communication messages rather than strictly paging messages.
  • encoder configurations may be employed as a function of the application, and the number of encoder states initially assigned the most likely probability for a given encoder configuration can be modified to achieve specific objectives.
  • another type of convolutional decoder such as a tree decoder may be substituted for the Viterbi decoder to decode the messages. Accordingly, all such changes and modifications are intended to be included within the scope of the invention as defined by the appended claims.

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  • Probability & Statistics with Applications (AREA)
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Abstract

L'invention concerne un procédé de décodage destiné à un système de communication utilisant un canal de communication dans lequel un message est codé de manière convolutionnelle par un codeur de station de base (300) et transmis à un téléterminal (210) pendant un intervalle de temps (102) attribué à au moins ce téléterminal. Le codeur n'est pas immédiatement remis à l'état initial avant l'intervalle de temps attribué, de sorte que l'état du codeur est inconnu lorsqu'il démarre. Ce procédé de décodage consiste à attribuer (502) une probabilité maximale à un état initial de codeur selon laquelle ledit état initial se trouve parmi un certain nombre d'états de codeur prédéterminés; et, à décoder de manière convolutionnelle (504, 506, 508) des bits consécutifs du message sur la base de l'hypothèse que l'état initial du codeur est un des états prédéterminés. Ce procédé fournit un décodage à faible taux d'erreur et permet à une station mobile de mieux gérer sa consommation, étant donné que la station mobile n'a pas besoin d'être sensiblement en éveil avant son intervalle de temps attribué. De préférence, on se sert d'un décodeur de Viterbi (220) afin d'effectuer le décodage convolutionnel. Ce procédé est particulièrement utile lorsqu'il est utilisé pour décoder des messages de page signalant des notifications d'appel dans le canal de recherche d'un système de télécommunication sans fil à accès multiple par répartition en code (CDMA).
PCT/US1999/020036 1999-09-01 1999-09-01 Procede et appareil permettant de decoder des messages a codage convolutionnel et codes en continu WO2001017187A1 (fr)

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AU59064/99A AU5906499A (en) 1999-09-01 1999-09-01 Method and apparatus for decoding continuously coded convolutionally encoded messages
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