Abstract:
A method and an apparatus for carrying out a turbo decoding by utilizing the channel information are disclosed. When using a turbo-decoder in a wireless communication system, in order to reduce the decoding delay, the channels are estimated by using the information such as a pilot channel information or a pilot symbol information, and the number of iterative decoding is controlled by utilizing the signal-to-interference ratio (SIR), thereby shortening the decoding delay, and making the quality of the high speed multi-media service better and reliable. The turbo code decoding apparatus utilizing a channel information according to the present invention includes a channel estimator for estimating a channel status based on reception signals to measure a SIR. Further, an iterative decoding times controller controls the number of iterative decoding by comparing the SIR with a pre-set threshold value. Further, a decoder carries out the iterative decoding as many times as controlled by the iterative decoding times controller. The present invention is applied to communication systems.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a method and an apparatus for performing an iterative decoding of turbo code; and, more particularly, to a method and an apparatus for controlling the number of iterative decoding of turbo code based on channel information, which reduce decoding delay and maintain the required quality of service in a wireless communication system, and a recording medium for achieving the method and apparatus.  
         DESCRIPTION OF THE PRIOR ART  
         [0002]    Generally, in the wireless communication field such as mobile communication, significant loss is occurred due to the wireless channel impairments such as propagation path loss, multi-path components and shadowing effect. The impairments cause random errors and burst errors.  
           [0003]    In order to correct the errors such as random errors and burst errors, error correction codes are used as a method of improving the reliability.  
           [0004]    Recently many studies are continued for applying the turbo code to the 3rd generation mobile communication (IMT-200). The turbo code is known to have powerful performance compared with the convolutional code. The convolutional code is being currently used in the mobile communication system.  
           [0005]    The turbo code is constituted &#39;such that the RSC (recursive systematic convolutional) codes are connected in parallel, so as to make it possible to decode through iterative decoding (which is a sub-optimal decoding method).  
           [0006]    Further, the turbo code shows an excellent performance which approaches to the Shannon limit in case of the interleaver having a large size and sufficient iterative decoding in AWGN channel.  
           [0007]    [0007]FIG. 1 shows the constitution of a turbo decoding scheme.  
           [0008]    As shown in this drawing, the encoding apparatus includes: an interleaver  111 ˜ 11 ( n - 1 ) for reducing the correlations by interleaving the input transmission information bits; an encoder  121  for encoding the input transmission information bits; encoders  122 ˜ 12   n  for encoding the interleaved signals through relevant interleavers  111 ˜ 11 ( n - 1 )) and a multiplexer  130  for selectively transmitting the encoded parity bits and information bits after encoding by the encoders  121 ˜ 12   n.    
           [0009]    As shown in the drawing, the transmission information bits may be transmitted without being encoded, or may be transmitted after being encoded by the encoders  121 ˜ 12   n  into parity bits.  
           [0010]    The encoded parity bits and information bits are selectively transmitted by the multiplexer  130 , and therefore, it is possible to change the code rate from 1 to 1/n.  
           [0011]    Further, in the wireless communication system, if the above described apparatus is used, its performance depends on the number of iteration. If a high quality of service (QoS) is required under sever fading channel, the number of iteration has to be increased in order to meet QoS requirement.  
           [0012]    However, if additional information on the channels is not available for controlling the number of iteration, the decoding is carried out at the fixed number of iteration.  
           [0013]    Under this condition, if the status of the wireless communication channels has been severely degraded, the fixed number of iteration is insufficient and the degradation of the performance cannot be avoided. On the other hand, if the channel environment is good and the signal-to-interference ratio (SIR) is high the fixed large number of iteration results in decoding delay.  
         SUMMARY OF THE INVENTION  
         [0014]    The present invention is intended to overcome the above described disadvantages of the conventional technique which is based on the fixed number of iteration.  
           [0015]    Therefore, it is an object of the present invention to provide a method and an apparatus for carrying out a turbo decoding, and a recording medium for achieving the method and apparatus in order to reduce the decoding delay in a high SIR and maintain the required quality of service in a low SIR. The SIR is measured by the given channel information such as pilot channel or pilot symbol.  
           [0016]    In order to reduce the decoding delay in a high SIR and maintain the required quality of service in a low SIR, the number of iteration is controlled based on the SIR, thereby shortening the decoding delay, and making the quality of the high speed multi-media service better and reliable.  
           [0017]    To achieve the above object, in accordance with an aspect of the present, invention the turbo code decoding apparatus utilizing channel information according to the present invention includes: a channel estimator for estimating a channel status based on a signal-to-interference ratio (SIR); an iterative decoding controller for controlling the number of iterative decoding by comparing the SIR with a pre-set threshold value; and a decoder for carrying out iterative decoding as many times as controlled by the number of iterative decoding.  
           [0018]    In accordance with another aspect of the present invention, the turbo code decoding apparatus utilizing a channel information according to the present invention includes: a channel estimator for estimating a channel status based on a signal-to-interference ratio (SIR); a power control bit generator for generating transmission power control (TPC) bits correspondingly with the SIR of the channel estimation means; an iterative decoding times controller for controlling the number of iterative decoding in accordance with the power control bits of the power control bit generating means; and a decoder for carrying out iterative decoding as many times as controlled by the number of iterative decoding.  
           [0019]    In accordance with still another aspect of the present invention, the turbo code decoding method utilizing channel information by using a turbo code decoding apparatus according to the present invention includes the steps of: measuring a signal-to-interference ratio (SIR) by estimating a channel status based on reception signals; deciding the number of iterative decoding by comparing the SIR with a pre-set threshold value; and carrying out iterative decoding as many times as has been decided.  
           [0020]    In accordance with still another aspect of the present invention, the turbo code decoding method utilizing channel information by using a turbo code decoding apparatus according to the present invention includes the steps of: measuring a signal-to-interference ratio (SIR) by estimating a channel status based on reception signals; generating transmission power control (TPC) bits correspondingly with the SIR measured; deciding the number of iterative decoding in accordance with the power control bits thus generated; and carrying out the iterative decoding as many times as has been decided.  
           [0021]    In accordance with still another aspect of the present invention, there is provided a computer readable recoding media storing instructions for executing a method for performing a turbo decoding based on channel information, the method comprising the steps of: a) measuring a signal-to-interference ratio (SIR) by estimating a channel status based on reception signals; b) deciding the number of iterative decoding by comparing the SIR with a pre-set threshold value; and c) carrying out iterative decoding as many times as has been decided,  
           [0022]    In accordance with still another aspect of the present invention, there is provided a computer readable recoding media storing instructions for executing a method for performing a turbo decoding based on channel information, the method comprising the steps of: a) measuring a signal-to-interference ratio (SIR) by estimating a channel status based on reception signals; b) generating transmission power control (TPC) bits correspondingly with the SIR measured; c) deciding the number of iterative decoding in accordance with the power control bits thus generated; and d) carrying out the iterative decoding as many times as has been decided. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    The above objects and other advantages of, the present invention will become more apparent by describing in detail the preferred embodiment of the present invention with reference to the attached drawings in which:  
         [0024]    [0024]FIG. 1 is a block diagram showing the constitution of a turbo encoding apparatus;  
         [0025]    [0025]FIG. 2 is a block diagram showing the constitution of the turbo decoding apparatus utilizing the channel information according to the present invention;  
         [0026]    [0026]FIG. 3 is a flow chart showing the constitution of the turbo decoding method utilizing the channel information according to the present invention;  
         [0027]    [0027]FIG. 4 is a block diagram showing the constitution of another embodiment of the turbo decoding apparatus utilizing the channel information according to the present invention; and  
         [0028]    [0028]FIG. 5 is a flow chart showing the constitution of another embodiment of the turbo decoding method utilizing the channel information according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]    The preferred embodiments of the present invention will be described in detail referring to the attached drawings.  
         [0030]    [0030]FIG. 2 is a block diagram showing the constitution of the turbo decoding apparatus utilizing the channel information according to the present invention. The signal-to-interference ratio (SIR) which has been estimated by utilizing channel information is compared with a pre-set threshold value so as to control the number of iterative decoding.  
         [0031]    As shown in FIG. 2, the turbo decoding apparatus utilizing the channel information according to the present invention includes: channel estimators  210  and  211  for measuring the SIR by estimating the wireless channel status based on the received pilot channel signals or the received pilot symbol signals; a comparator  220  for comparing the SIR measured by the channel estimators  210  and  211  with a pre-set threshold value so as to output a control information on the number of iterative decoding; an iterative decoding control counter  230  for counting up and down the number of iterative decoding in accordance with the output value of the comparator; a switch  240  for controlling iterative decoding in accordance with the number of iterative decoding of the counter  230 ; and a decoding part  250  for carrying out the iterative decoding as many times as controlled by the counter  230 .  
         [0032]    The decoding part  250  includes: an MAP1 (Maximum A Posteriori 1) decoder  251 ; an interleaver  252 ; an MAP2 (Maximum A Posteriori 2) decoder  253 ; and deinterleavers  254  and  255 .  
         [0033]    This as a known technology, and therefore, it will not be described more here.  
         [0034]    The turbo decoding apparatus of the present invention which is constituted as above will now be described as to its operations.  
         [0035]    [0035]FIG. 3 is a flow chart showing the constitution of the turbo decoding method utilizing the channel information according to the present invention. Here, the SIR is applied to control the number of iterative decoding.  
         [0036]    First, parameters are set up for controlling the number of iterative decoding for the turbo code at step  300 . The parameters include the maximum number of iteration (Nmax), the minimum number of iteration (Nmin) and the threshold of the comparator, which are determined according to the required quality of service and the limited decoding delay.  
         [0037]    Then in order to decide the number of iterative decoding, the channels are estimated using the channel estimators  220  and  211 . The SIR is measured at step  301 .  
         [0038]    Then the measured signal-to-interference ratio is compared with a pre-set threshold value at step  302 , and if the measured SIR is larger than the threshold value, the value N of a counter which counts the number of iterative decoding is decreased at step  303  On the other hand, if the measured SIR is smaller than the threshold value, it means the severe wireless channel environment, and therefore, the value N of the counter is increased at step  304  so as to maintain the required performance.  
         [0039]    Then, a comparison is carried out to know as to whether the value of counter N lies in the Nmax˜Nmin range (e.g., 2˜20) for satisfying the minimum performance requisite, thereby carrying out a confirmation.  
         [0040]    That is, at step  305 , it is determined whether the adjusted counter value N is larger or smaller than the maximum number Nmax of iteration or the number of minimum number of iteration Nmin. If N is larger than Nmax, then N is fixed to Nmax at step  306 , while if N is smaller than Nmax, then N is compared with Nmin to maintain the minimum performance level at step  307 .  
         [0041]    If N is found to be smaller than Nmin as a result of the comparison, then N is set to Nmin at step  308 , while if N lies between Nmin and Nmax, N is fixed without variation.  
         [0042]    When the value of N is decided through the above described procedure, the code words which have been encoded by the encoders  1  and  2  of FIG. 1 at the transmitting end (with the code rate of ⅓) are decoded by MAP1 and MAP2 at steps  309  and  310 .  
         [0043]    Under this condition, the decoding is carried out as many times as the value N of the counter.  
         [0044]    Each time when an iterative decoding is carried out, the value of the counter is diminished by 1 at step  312 . Thus the value N is checked at step  311 , and when N becomes 0, the decoding is terminated, thereby obtaining a decoded final data.  
         [0045]    The above described procedure is carried out for each slot, or for an integer-multiple of the basic slot, or for each frame.  
         [0046]    [0046]FIG. 4 is a block diagram showing the constitution of another embodiment of the decoding apparatus utilizing the channel information according to the present invention. Here, power control bits are generated correspondingly with the SIR which is obtained by utilizing the channel information, thereby controlling the number of iterative decoding.  
         [0047]    As shown in FIG. 4, the turbo decoding apparatus based on the channel information according to the present invention includes: channel estimators  410  and  411  for measuring the SIR by estimating the channel status of the pilot channel signals or the pilot symbol signals; a transmission power control bit generator  420  for generating the transmission, power control bits (TPC) correspondingly with the SIR of the channel estimators  410  and  411 ; an iterative decoding control counter  430  for counting the number of iterative decoding in accordance with the transmission power control bit generator  420  to output a control information on the number of iterative decoding; a switch  440  for controlling the number of iterative decoding in accordance with the value of counter  430 ; and a decoding part  450  for carrying out iterative decoding as many times as controlled by the switch.  
         [0048]    The decoding part  450  includes: an MAP1 (Maximum A Posteriori 1) decoder  451 ; an interleaver  452 ; an MAP2 (Maximum A Posteriori 2) decoder  453 ; and deinterleavers  454  and  455 .  
         [0049]    This is a known technology, and therefore, it will not be described any more here.  
         [0050]    The turbo decoding apparatus of the present invention which is constituted as above will now be described as to its operations by referring to FIG. 5.  
         [0051]    [0051]FIG. 5 is a flow chart showing the construction of another embodiment of the turbo decoding method utilizing the channel information according to the present invention. Here, power control bits are generated to control the number of iterative decoding for turbo code having a code rate of ⅓.  
         [0052]    First, parameters are set up for controlling the number of iterative decoding for the turbo code at step  501 .  
         [0053]    The parameters include the maximum number of iteration (Nmax), the minimum number of iteration (Nmin) and the threshold of the comparator, which are determined according to the required quality of service and the limited decoding delay.  
         [0054]    Then, in order to control the number of iterative decoding, the pilot channel information or the pilot symbol information is utilized to estimate the channel so as to obtain the SIR at step  502 . Then TPC (transmission power control) bits are generated correspondingly with the measured SIR at step  503 .  
         [0055]    Then a checking is carried out as to whether a power decrease command is inputted from the terminal to the base station or from the base station to the terminal at step  504 .  
         [0056]    Upon the checking, if it is found that a power increase command has been inputted from the terminal to the base station or from the base station to the terminal, then a closed loop power control is carried out, and the number of iterative decoding of the turbo decoder is increased in accordance with the power increase command at step  506 .  
         [0057]    Upon the checking, on the contrary, if it is found that a power decrease command has been inputted, then the number of iterative decoding is decreased at step  505 .  
         [0058]    The value of N which has been decided by the power control bits is subjected to a checking as to whether the minimum service quality requisite (value of Nmin) and the delay limitation (value of Nmax) is satisfied at steps  507  to  510 .  
         [0059]    That is, the adjusted value N of the counter is compared with the maximum repetition value Nmax at step  507 , and if N is larger than Nmax, then N is fixed to Nmax at step  508 . On the other hand, if N is smaller than Nmax, N is compared with Nmin which is the minimum repetition times at step  509 .  
         [0060]    Upon comparing N with Nmin, if it is found that N is smaller than Nmin, then N is set to Nmin at step  510 , while if N lies between Nmin and Nmax, then N is kept as it is.  
         [0061]    When N is decided through the above described procedure, the code words which have been encoded at the transmission end of FIG. 1 by the encoders  1  and  2  having a code rate of ⅓ are, decoded by MAP1 and MAP2 at steps  511  and  512 . Under this condition, the decoding is carried out as many times as N which has been decided through the above described procedure.  
         [0062]    Each tine when an iterative decoding is carried out, the counter value is diminished by 1 at step  514 , and checking is carried out on the value of N. Ultimately, when the value of N becomes zero, the decoding is terminated, thereby obtaining the finally decoded data.  
         [0063]    The above described procedure is carried out for each slot, or for an integer-multiple of the basic slot, or for each frame.  
         [0064]    Meanwhile, the method of estimating the optimum channel can be different depending on the pilot structure which is used. However, in the general pilot channels and the general pilot symbols, the estimation can be carried out by utilizing a moving average filter.  
         [0065]    According to the present invention as described above, the channels information which is given in the wireless communication system is utilized so as to efficiently control the number of iterative decoding for the turbo code. Thus the problem of the decoding delay can be solved in a high SIR by reducing the number of iteration keeping the required performance and also it is possible to afford a required quality, service in a low SIR by increasing the number of iteration within the range of iteration (Nmin˜Nmax).  
         [0066]    Further, as the decoding delay time is shortened, the power consumption can be saved, and therefore, the overall capacity of the system can be expanded.  
         [0067]    Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.