1. Field of the Invention
The invention relates to a method of decoding turbo-encoded data and a receiver for decoding turbo-encoded data both of which are suitable to a mobile communication system which operates in CDMA (Code Division Multiple Access).
2. Description of the Related Art
There has been suggested a method of encoding data, called a turbo code, in which an error rate close to Shannon limit can be accomplished in encoding data, by C. Berrou et al. This method is explained in detail, for instance, in Proceeding of International Conference of communication, pp. 1064–1070, May 1993.
The method of turbo-encoding data is characterized by the step of dividing a code having high complexity in decoding data, into a plurality of components each having low complexity in decoding data, to thereby successively enhance accuracy in decoding data by virtue of interaction among those components. Division of a code having high complexity in decoding data, into a plurality of components each having low complexity in decoding data is carried out by a maximum posterior probability (MAP) decoder which carries out soft-input and soft-output decoding.
BCJR (Bahl, Cocke, Jelinek and Raviv) algorithm is known as an algorithm for consistently accomplishing MAP decoding, but is accompanied with a problem of necessity of too much calculation. In order to reduce calculation, there have been suggested Max-Log MAP algorithm and SOVA (Soft-Output Viterbi Algorithm) both of which carry out approximate calculation. Herein, Max-Log MAP algorithm which carries out approximation of calculation in BCJR algorithm in log domain, and SOVA algorithm is a process to have soft-input and soft-output on the basis of Viterbi algorithm.
In CDMA mobile communication system, a control to power of a data transmitter is made in order to keep the power at minimum and increase system capacity as much as possible. In addition, since CDMA system can have a high gain in encoding data, by virtue of statistics multiple, enhancement in an ability of decoding data in a turbo decoder would bring a merit that the number of subscribers covered by the CDMA system can be increased.
However, the above-mentioned Max-Log MAP algorithm and SOVA algorithm are accompanied with a problem of degradation in characteristics thereof, though they can reduce calculation.
In order to solve the problem, there is known a method of carrying out calculation equivalent to BCJR algorithm, in log domain, with reference to a table in which a correction term fc (|δ1−δ2|) is defined as a function of (|1−β2|) in Max-Log MAP, based on Jacobian Logarithm.ln(eδ1+eδ2)=max(δ1,δ2)+ln(1+e−|δ2−δ1|)=max(δ1,δ2)+fc(|δ2−δ1|)  (1)
However, if the above-mentioned equation (1) were arranged into a table, it would be unavoidable for the table to become large in size.
For instance, hereinbelow is explained a process of updating alpha metric as an example. The alpha metric and above-mentioned beta metric and gamma metric correspond to α, β and γ, respectively, and are described in detail, for instance, in IEEE Transaction on Information Theory, pp. 284–287, March 1974.
First, it is assumed that two alpha metrics selected on a torelis at that time are expressed as α1 and α2, and values of the alpha metrics in a log domain are expressed as αlog 1 and αlog 2. That is, α1 and α2 are expressed as follows.α1=exp [αlog 1]α2=exp [αlog 2]
In addition, it is assumed that gamma metrics associated with the alpha metrics on a torelis are expressed as γ1 and γ2, and values of the gamma metrics in a log domain are expressed as γlog 1 and γlog 2. Unless explicitly expressed, a product of α1 and γ1 is equal to or greater than a product of α2 and γ2 (α1γ1≧α2γ2).
Herein, it is assumed that an alpha metric having been updated is expressed as α3, a value of the alpha metric α3 in a log domain is expressed as follows.
                              ln          ⁡                      [                          α              3                        ]                          =                              ln            ⁡                          [                                                                    α                    1                                    ·                                      γ                    1                                                  +                                                      α                    2                                    ·                                      γ                    2                                                              ]                                =                                                    ln                ⁡                                  [                                                            (                                                                        α                          1                                                ·                                                  γ                          1                                                                    )                                        ·                                          (                                              1                        +                                                                                                            α                              2                                                        ·                                                          γ                              2                                                                                                                                          α                              1                                                        ·                                                          γ                              1                                                                                                                          )                                                        ]                                            ∴                              α                                  log                  ⁢                                                                          ⁢                  5                                                      =                                          α                                  log                  ⁢                                                                          ⁢                  1                                            +                              λ                                  log                  ⁢                                                                          ⁢                  1                                            +                              ln                ⁡                                  [                                      1                    +                                          exp                      ⁢                                              {                                                                              α                                                          log                              ⁢                                                                                                                          ⁢                              2                                                                                +                                                      γ                                                          log                              ⁢                                                                                                                          ⁢                              2                                                                                -                                                      α                                                          log                              ⁢                                                                                                                          ⁢                              1                                                                                -                                                      γ                                                          log                              ⁢                                                                                                                          ⁢                              1                                                                                                      }                                                                              ]                                                                                        (        2        )            
Accordingly, a term corresponding to the correction term fc (|δ1−δ2|) in the above-mentioned equation (1) is expressed as follows.fc(|δ1−δ2|)=ln [1+exp {αlog 2+γlog 2−αlog 1−γlog 1}]  (3)
Herein, the gamma metric is expressed as follows.
                                                        γ                              log                ⁢                                                                  ⁢                1                                      =                          ln              ⁢                              {                                                      Π                    i                                    ⁢                                      1                                                                                            2                          ·                          π                                                                    ·                      σ                                                        ⁢                  exp                  ⁢                                      ⌊                                          -                                                                                                    {                                                                                          y                                                                  1                                  ⁢                                  i                                                                                            -                                                                                                                                    K                                    s                                                                                                  ·                                                                  (                                                                                                            2                                      ·                                                                              x                                                                                  1                                          ⁢                                          i                                                                                                                                                      -                                    1                                                                    )                                                                                                                      }                                                    2                                                                          2                          ·                                                      σ                            2                                                                                                                                                                            ]                }                            (        4        )            
The equation (4) is substituted for the equation (3) to thereby cancel common terms. As a result, the following equation (5) is obtained.
                                          f            c                    ⁡                      (                                                                          δ                  1                                -                                  δ                  2                                                                    )                          -                  ln          ⁡                      [                          1              +                              exp                ⁢                                  {                                                                          ⁢                                                            α                                              log                        ⁢                                                                                                  ⁢                        2                                                              -                                          α                                              log                        ⁢                                                                                                  ⁢                        1                                                              +                                                                  ∑                        i                                            ⁢                                                                        {                                                                                    y                                                              2                                ⁢                                i                                                                                      ·                                                                                          K                                s                                                                                      ·                                                          (                                                                                                2                                  ·                                                                      x                                                                          2                                      ⁢                                      i                                                                                                                                      -                                1                                                            )                                                                                }                                                                          σ                          2                                                                                      -                                                                  ∑                        i                                            ⁢                                                                        {                                                                                    y                                                              1                                ⁢                                i                                                                                      ·                                                                                          K                                s                                                                                      ·                                                          (                                                                                                2                                  ·                                                                      x                                                                          1                                      ⁢                                      i                                                                                                                                      -                                1                                                            )                                                                                }                                                                          σ                          2                                                                                                      }                                                      ]                                              (        5        )            
In the equation (5), the correction term fc (|δ1−δ2|) contains noise variance σ2 and signal component Es. Hence, it is necessary to update values in Jacobian table by multi-pass fading each time noises and/or signal levels are varied.
However, since noise variance σ2 and signal component Es are contained also in a process of updating beta metric and an equation for computing likelihood, it would be necessary for a memory to have a great capacity.
In addition, it would be necessary for a circuit to include an additional memory to store noise variance σ2 and signal component Es associated with positions of bits in each of information sequence and parity sequence, resulting in an increase in a size of the circuit. This would make it impossible to fabricate the circuit in a small size, in low consumption of power, and in small fabrication costs. Furthermore, steps of measuring noise variance σ2 and signal component Es have to be additionally carried out.
Since a process of referring to a table comprised of a memory having a great capacity is carried out at a low rate, such a low rate would be a bottle neck for processing rates of ACS circuit and a comparison/selection circuit both of which cannot have a pipeline structure.
Japanese Unexamined Patent Publication No. 6-132936 has suggested a digital transmission system in which digital data to be transmitted is encoded by means of an encoding circuit at a transmitter, the thus encoded data is transmitted in the form of a modulated signal into a transmission path, and the modulated signal is decoded by means of a decoding circuit at a recipient. The encoding circuit is comprised of a first unit which groups channels in accordance with an importance of digital data to be transmitted, a second unit which weights the digital data in accordance with a predetermined weighting method, and a modulator which multiplexes the weighted digital data to thereby produce a modulated signal and transmits the thus produced modulated signal into a transmission path. The decoding circuit is comprised of a demodulator which receives the modulated signal and demodulates the received modulated signal, a third unit which checks receipt condition in each of channels in accordance with the thus demodulated digital data, and a data selector which selects the digital data in an order of highly weighted channels in accordance with the receipt condition in each of channels.
Japanese Unexamined Patent Publication No. 9-261203, based on U.S. patent application Ser. No. 08/617,462 filed on Mar. 18, 1996, has suggested a method of determining a weighting coefficient in CDMA radio-signal receiver, including the steps of receiving a first expression expressed in desired RF signals, transmitting a plurality of first data signals in accordance with the first expression, transmitting a plurality of first pilot signals in accordance with the first expression, measuring first total power of received signals, and determining a plurality of first weighting coefficients in accordance with the data signals, the pilot signals and the first total power.
Japanese Patent No. 2877248 (Japanese Unexamined Patent Publication No. 8-37515) has suggested a method of controlling power of a first transmission signal transmitted from a first station, in accordance with a control signal included in a second transmission signal transmitted from a second station and received at the first station. The method includes the steps of decoding the second transmission signal at the first station by means of a first Viterbi decoder including a path memory having a first predetermined length, decoding the second transmission signal at the first station by means of a second Viterbi decoder including a path memory having a second predetermined length shorter than the first predetermined length, extracting the control signal from output signals transmitted from the second Viterbi decoder, at the first station, controlling power of the first transmission signal in accordance with the control signal having been extracted from the output signals transmitted from the second Viterbi decoder, and extracting data other than the control signal, from output signals transmitted from the first Viterbi decoder, at the first station.
Japanese Unexamined Patent Publication No. 6-261021, based on U.S. patent application Ser. No. 07/991,841 filed on Dec. 16, 1992, has suggested a device used in CDMA system in which encoded user signals are transmitted to each of a plurality of users, the user signals are produced by processing user signals with associated sequence of encoding coefficients, and a received signal includes a combination of the encoded user signals. The device is comprised of means for receiving samples of the received signals received at a predetermined interval, and means for predicting users' symbols in response to the samples through the use of the extracted sequence of encoding coefficients. The sequence of encoding coefficients is defined as an interactive function of a sequence of encoding coefficients, associated with the users, and a sequence of encoding coefficients, associated with other user.
Japanese Unexamined Patent Publication No. 2000-4196, based on U.S. patent application Ser. No. 09/038,724, has suggested a multiple access system of communication across a wireless interface, including a turbo encoder for turbo coding signal representations of packets of information, a transmitter for transmitting a first signal representation of a first packet of information and a second signal including a re-transmission of part of the first signal and a new signal representation of a second packet of information, a receiver for receiving the signal representations, and a means for processing the signal representations by combining the transmitted signals with the re-transmitted signals to obtain an output signal representation of the packet of information the transmitted and re-transmitted signals being combined using rake processing.
The above-mentioned problems remain unsolved even in the above-mentioned Publications.