Abstract:
A soft decision method for determining a soft decision coordinate associated with a constellation is provided. The soft decision coordinate includes a first soft decision sub-coordinate and a second soft decision sub-coordinate. The method includes receiving an input signal including a coordinate value; defining a first coordinate range on a coordinate axis in the constellation, the first coordinate range having a first limit and a second limit; obtaining the first soft decision sub-coordinate according to the first coordinate range; defining a second coordinate range on the coordinate axis in the constellation, the second coordinate range having a third limit and a fourth limit; and obtaining the second soft decision sub-coordinate according to the second coordinate range; wherein the first and the third limit do not simultaneously equal to the second and the fourth limit.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATIONS 
     This patent application claims priority from Taiwan Patent Application No. 098136961, filed in the Taiwan Patent Office on Oct. 30, 2009, entitled “Soft Decision Method and Associated Signal Receiving System”, and incorporates the Taiwan patent application in its entirety by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a data processing method in a communication system, and more particularly to a soft decision demapping method in a communication system. 
     BACKGROUND OF THE PRESENT DISCLOSURE 
       FIG. 1  is a block diagram of a conventional signal receiving system  10 . The signal receiving system  10  comprises a signal retriever  140 , a demapper  160  and a decoder  180 . The demapper  160  comprises a mapping function mapping apparatus  164  and a quantizer  167 . 
     The signal retriever  140  receives an input signal and transforms a time-domain input signal to two corresponding signals namely a frequency-domain inphase signal (I signal) and quadrature signal (Q signal). The demapper  160  generates the digital data corresponding to I and Q signals according to constellations applied to the input signal. For example, the constellations applied to the modulation, such as binary phase shift keying (BPSK), 16 quadrature amplitude modulation (16QAM) and 64QAM, are different, so the I and Q signals corresponding to the digital data are different. Lastly, the decoder  180  transforms the digital data to an output data. 
     Theoretically, the I and Q signals generated by the signal receiving system  10  should map correctly on the constellations to two integers of a Gray code, which is a coding method and is a set of a sequence. Each number of the Gray code is represented by binary, and there is only one different bit between any two of Gray code. However, the signal processed by the signal receiving system  10  may be interfered by the noise such that the I and Q signal generated by the signal retriever  140  may not be an integer, i.e., the I and Q signal may not map exactly to the Gray code on the constellation, such that one needs other methods for mapping the non-integer I and Q signals to the Gray code. 
     One of the solutions to solve the above problem is a soft decision method.  FIG. 2  is a conventional 64QAM constellation, wherein the I-axis represents the I signal, and the Q-axis represents the Q signal. Each point on the constellation maps to a 6-bit value (0 to 2 6 −1), of which the first three bits represent the I part, and the last three bits represent the Q part. If the signal receiving system  10  uses the 64QAM, the soft decision method is to map the coordinates (I, Q) of the I and Q signals received by the demapper  160  to a soft coordinate (I 0 , I 1 , I 2 , Q 0 , Q 1 , Q 2 ). For example, the I coordinate of 5.3 maps to (I 0 *, I 1 *, I 2 *)=(5.3, −1.3, 0.7) by the mapping function mapping apparatus  164 . The corresponding mapping function is as follows: 
     
       
         
           
             { 
             
               
                 
                   
                     
                       
                         I 
                         0 
                         * 
                       
                       = 
                       I 
                     
                   
                 
                 
                   
                     
                       
                         I 
                         1 
                         * 
                       
                       = 
                       
                         
                           - 
                           
                              
                             I 
                              
                           
                         
                         + 
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                         I 
                         2 
                         * 
                       
                       = 
                       
                         
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                                 I 
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     Limited by the memory in the hardware, practically, one needs to quantize a decimal to a value acceptable to the hardware. Therefore, (I 0 *, I 1 *, I 2 *)=(5.3, −1.3, 0.7) is quantized to (I 0 , I 1 , I 2 )=(3, −2, 2) as shown in  FIG. 2  by quantizer  167 , where (I 0 , I 1 , I 2 )=(3, −2, 2) is very different to the original (I 0 *, I 1 *, I 2 *)=(5.3, −1.3, 0.7). The conventional soft decision method is to divide I 0 , I 1  and I 2  on the constellation into N equal parts (N=8 in  FIG. 2 ) without taking the distinct ranges of I 0 , I 1  and I 2  into consideration. That is, when I 0  is determined as being positive or negative, the total range of I 1  is only a half of the total range of I 0 , i.e., the total range of I 1  is only the positive region or the negative region of I 0 . Similarly, when I 1  is determined as being positive or negative, the total range of I 2  is only a half of the total range of I 1 . More specifically, respective absolute distances from I 0 =4 and I 1 =4 to the origin are not identical. In fact, from  FIG. 2 , the distance between origin and I 1 =4 is a half of the absolute distance between origin and I 0 =4. Hence, dividing all I 0 , I 1  and I 2  into N equal parts causes quantizing distortion to undesirably affect the determination of the decoder, such that not only the coding gain is reduced but also the bit error rate (BER) is increased from being unable to accurately correct erroneous bits. Therefore, it is urgently needed a better soft decision method and associated signal receiving system to increase the coding gain and to reduce the bit error rate. 
     SUMMARY OF THE PRESENT DISCLOSURE 
     It is one of the objectives of the present disclosure to provide a soft decision method and associated signal receiving system to increase the coding gain and to reduce the bit error rate for data processing of a communication system. 
     The present disclosure provides a soft decision method for determining a soft decision coordinate associated with a constellation, the soft decision coordinate comprising a first soft decision sub-coordinate and a second soft decision sub-coordinate. The soft decision method comprises receiving an input signal comprising a coordinate value; defining a first coordinate range on a coordinate axis in the constellation, and the first coordinate range having a first limit and a second limit; obtaining the first soft decision sub-coordinate according to the first coordinate range; defining a second coordinate range on the coordinate axis in the constellation, and the second coordinate range having a third limit and a fourth limit; and obtaining the second soft decision sub-coordinate according to the second coordinate range; wherein the first limit and the third limit and the second limit and the fourth limit are not equal simultaneously. 
     The present disclosure further provides a soft decision method for determining a soft decision coordinate associated with a constellation, the soft decision coordinate comprising a first soft decision sub-coordinate and a second soft decision sub-coordinate. The soft decision method comprises receiving an input signal comprising a coordinate value; defining a first coordinate range on a coordinate axis in the constellation, such that the first coordinate range has a plurality of equal first intervals; obtaining the first soft decision sub-coordinate according to the first intervals; defining a second coordinate range according to a portion of the first intervals, such that the second coordinate range has a plurality of equal second intervals; and obtaining the second soft decision sub-coordinate according to the second intervals; wherein a size of the first interval and a size of the second interval are substantially equal. 
     The present disclosure further provides a signal receiving system comprising: a signal retriever, for receiving an input signal and for transforming the input signal to a coordinate value; a demapper, coupled to the signal retriever, for demapping the coordinate value to a soft decision coordinate, the soft decision coordinate comprising a first soft decision sub-coordinate and a second soft decision sub-coordinate, the first soft decision sub-coordinate having a first limit and a second limit, and the second soft decision sub-coordinate having a third limit and a fourth limit, wherein the first and limit the third limit and the second limit and the fourth limit are not equal simultaneously; and a decoder, coupled to the demapper, for decoding the soft decision coordinate to output an output data. 
     The soft decision method and associated signal receiving system provided by the present disclosure can minimize the misjudgment of the signal receiving system on noise-interfered signal to increase the coding gain and to reduce the bit error rate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
         FIG. 1  is a block diagram of a conventional signal receiving system; 
         FIG. 2  is a conventional 64QAM constellation; 
         FIG. 3  is a 64QAM constellation according to one embodiment of the present disclosure; 
         FIG. 4  is a block diagram of a signal receiving system according to one embodiment of the present disclosure; 
         FIG. 5  is a block diagram of the demapper  460  according to one embodiment of the present disclosure; 
         FIG. 6  is a 64QAM constellation according to one embodiment of the present disclosure; 
         FIG. 7  is a block diagram of the demapper  460  according to another embodiment of the present disclosure; 
         FIG. 8  is a quantizing function according to one embodiment of the present disclosure; 
         FIG. 9  is a block diagram of the demapper  460  according to another embodiment of the present disclosure; 
         FIG. 10  is a flowchart of soft decision method according to one embodiment of the present disclosure; and 
         FIG. 11  is a flowchart of a soft decision method according to another embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 3  is a 64QAM constellation according to one embodiment of the present disclosure.  FIG. 4  is a block diagram of a signal receiving system  40  according to one embodiment of the present disclosure. The signal receiving system  40  comprises a signal retriever  440 , a demapper  460  and a decoder  480 . The signal retriever  440  receives an input signal and converts the input signal to a complex signal 5.3+4.5j. The complex signal signifies a coordinate value representing a coordinate (5.3, 4.5) on the constellation in  FIG. 3 , where 5.3 is the I coordinate value, and 4.5 is the Q coordinate value. 
     The demapper  460 , coupled to the signal retriever  440 , demaps I coordinate value and Q coordinate value to the coordinate (I 0 , I 1 , I 2 , Q 0 , Q 1 , Q 2 ), wherein I 0 , I 1 , I 2 , Q 0 , Q 1  and Q 2  all can have limits. In a preferred embodiment, since the signal received by signal retriever  440  is noise-interfered, the amplitude of the input signal may be large, resulting in a large coordinate value. Therefore, to prevent the large coordinate value from affecting calculation results and to reduce hardware costs, the coordinate is limited within a predetermined range by a clipper. For example,  FIG. 3  shows the predetermined range is set from −8 to 8. Any coordinate value greater than 8 is regarded as 8, and any coordinate value smaller than −8 is regarded as −8. For example, a coordinate value of 9.8 is regarded as 8. 
     In the embodiment, the I-axis is defined as having 8 equal intervals on the constellation in  FIG. 3 , numbered from −4 to 4. That is, the range of the I 0  coordinate is from −4 to 4, with −4 and 4 being the limits of the I 0  coordinate. When the range of the I 0  coordinate is set from 0 to 8, the limits of the I 0  coordinate are then 0 and 8. Similarly, the range can be set from −8 to 0, and −8 and 0 are accordingly the limits of the I 0  coordinate. The above coordinate range can be user-defined based on performance requirements and the hardware costs. As the greater coordinate range is defined, the lower the bit error rate is achieved however with more hardware costs. 
     Since the I-axis is defined into 8 equal parts by the I 0  coordinate, to reflect the importance of the absolute value of the I 0  coordinate and the absolute value of the I 1  coordinate, the I 1  coordinate defines the positive part of the I 0  coordinate into four equal intervals numbered from 1 to 4, respectively. Similarly, I 1  coordinate defines  4  the negative part of the I 0  coordinate into four intervals numbered from −1 to −4, respectively. Hence, the interval length of the I 0  coordinate and the interval length of I 1  coordinate are equal. That is, total range of four parts of I 1  coordinate is a half of total range of the I 0  coordinate, i.e., −4 to −1 and 4 to 1 of I 0  coordinate both map to −2 to 2 of the I 1  coordinate. Therefore, the limits of the I 1  coordinate are −2 and 2. Similarly, the limits can be 0 and 4 or −4 and 0. From the above, two limits of the I 0  coordinate and the two limits of the I 1  coordinate are not identical simultaneously. 
     The decoder  480 , coupled to the demapper  460 , decodes the coordinate (I 0 , I 1 , I 2 , Q 0 , Q 1 , Q 2 ) to output an output data. In a preferred embodiment, the decoder can be Viterbi decoder. 
       FIG. 5  is a block diagram of the demapper  460  according to one embodiment of the present disclosure. The demapper  460  comprises a mapping function mapping apparatus  461  and a multiplier  462 . The mapping function mapping apparatus  461  is coupled to the signal retriever  440  and comprises mapping function mapping units  4612 ,  4614  and  4616 . The mapping function mapping units  4612 ,  4614  and  4616  comprises a quantizing unit (not shown) respectively. The I coordinate is taken as an example in the description below, whereas the Q coordinate have similar principles as the I coordinate.  FIG. 6  is a 64QAM constellation according to one embodiment of the present disclosure. The mapping function mapping units  4612 ,  4614  and  4616  receive an I coordinate value and map the I coordinate value to I 0 * coordinate, I 1 * coordinate and I 2 * coordinate according to the following mapping function, respectively: 
     
       
         
           
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                         I 
                         0 
                         * 
                       
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                       I 
                     
                   
                 
                 
                   
                     
                       
                         I 
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     For example, the coordinate value I=5.3 maps to another coordinate (I 0 *, I 1 *, I 2 *)=(5.3, −1.3, 0.7). The quantizers in the mapping function mapping units  4612 ,  4616  and  4616  define the I-axis into the I 0 ′, I 1 ′ and I 2 ′ coordinates respectively having a length of 1, ½ and ¼ according to a first step, a second step and a third step. Then, (I 0 *, I 1 *, I 2 *)=(5.3, −1.3, 0.7) is quantized to 6 as the I 0 ′ coordinate, −3 as the I 1 ′ coordinate and 3 as the I 2 ′ coordinate by the quantizers. In the embodiment, the first, second and third steps are 1, ½ and ¼ respectively. 
     The multiplier  462 , coupled to the mapping function mapping apparatus  461 , comprises multiplying units  4622 ,  4624  and  4626  for multiplying a first coefficient, a second coefficient and a third coefficient (i.e. k 0 , k 1  and k 2 ) by the I 0 ′ coordinate, the I 1 ′ coordinate and the I 2 ′ coordinate respectively to obtain the I 0  coordinate, the I 1  coordinate and the I 2  coordinate. For example, k 0 , k 1  and k 2  are set to ½, ¼ and ⅛ respectively. The I 0 ′ coordinate (6), the I 1 ′ coordinate (−3) and the I 2 ′ coordinate (3) are multiplied by k 0 , k 1  and k 2  respectively and quantized (such as unconditionally carried) to obtain 3, −1 and 1 as the I 0 , I 1  and I 2  coordinates, respectively. 
     In another preferred embodiment,  FIG. 6  shows the first, second and third steps are all set to 1, i.e., the unit lengths of the I 0 ′, I 1 ′ and I 2 ′ coordinates on the I-axis are all defined as 1. Then (I 0 *, I 1 *, I 2 *)=(5.3, −1.3, 0.7) is quantized to 6 as the I 0 ′ coordinate, −2 as the I 1 ′ coordinate and 1 as the I 2 ′ coordinate by the quantizer. k 0 , k 1  and k 2  are all set to ½. The I 0 ′ coordinate (6), the I 1 ′ coordinate (−2) and the I 2 ′ coordinate (1) are multiplied by k 0 , k 1  and k 2  respectively and quantized to obtain 3, −1 and 1 as the I 0 , I 1  and I 2  coordinates, respectively. From the above, it is concluded that the same results are achieved regardless that the first, second and third steps are equal, or the first, second and third coefficients are different. 
       FIG. 7  is a block diagram of the demapper  460  according to another embodiment of the present disclosure. The demapper  460  comprises a mapping function mapping apparatus  464 , a multiplier  465  and a quantizer  467 . The mapping function mapping apparatus  464 , coupled to the signal retriever  440 , comprises mapping function mapping units  4642 ,  4644  and  4646 . The mapping function mapping units  4642 ,  4644  and  4646  receive an I coordinate value and map the I coordinate to an I 0 * coordinate, an I 1 * coordinate and an I 2 * coordinate respectively according to a mapping function. 
     For example, I=5.3. The mapping function mapping units  4642 ,  4644  and  4646  map according to the following function: 
     
       
         
           
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                         I 
                         0 
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                         1 
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     The coordinate value I=5.3 is mapped to I 0 *=5.3, I 1 *=−1.3 and I 2 *=0.7 respectively. 
     The multiplier  465 , coupled to the mapping function mapping apparatus  464 , comprises multiplying units  4652 ,  4654  and  4656 . The multiplying units  4652 ,  4654  and  4656  multiply the first coefficient, the second coefficient and the third coefficient (i.e. k 0 , k 1  and k 2 ) by the I 0 * coordinate, the I 1 * coordinate and the I 2 * coordinate respectively to obtain an I 0 ′ coordinate, an I 1 ′ coordinate and an I 2 ′ coordinate. For example, when k 0 , k 1  and k 2  are all set to 1, the I 0 ′, I 1 ′ and I 2 ′ coordinates are also 5.3, −1.3 and 0.7 respectively. 
     The quantizer  467 , coupled to the multiplier  465 , comprises quantizing units  4672 ,  4674  and  4676 . The quantizing units  4672 ,  4674  and  4676  respectively quantize the I 0 ′, I 1 ′ and I 2 ′ coordinates to I 0 , I 1  and I 2  coordinates according to the first, second and third steps. For example,  FIG. 3  shows the first, second and third steps are all set to 2, i.e., I-axis is defined to 8 equal parts with an interval length of 2. 3, −1 and 1 as the I 0 , I 1  and I 2  coordinates can be obtained respectively after quantizing by the quantizer  467 . 
       FIG. 8  is a quantizing function according to one embodiment of the present disclosure. In another preferred embodiment,  FIG. 8  shows k 0 , k 1  and k 2  are set to ½, 1 and 2 respectively. An I 0 * coordinate (5.3), an I 1 * coordinate (−1.3) and an I 2 * coordinate (0.7) are multiplied by k 0 , k 1  and k 2  respectively to obtain 2.65, −1.3 and 1.4 as I 0 ′, I 1 ′ and an I 2 ′ coordinate, respectively. The first, second and third steps are set to be 1, 2 and 4, that is, the quantizing steps are set to be 1, 2 and 4. After quantizing by the quantizer  467 , 3, −1 and 1 are obtained as the I 0 , I 1  and I 2  coordinates, respectively. From the above, it is concluded that the same results are achieved regardless that the first, second and third steps are equal, or the first, second and third coefficients are different. 
       FIG. 9  is a block diagram of the demapper  460  according to another embodiment of the present disclosure. The demapper  460  comprises the mapping function mapping apparatus  464 , a quantizer  468  and a multiplier  469 . The embodiment is similar to the embodiment in  FIG. 7 . A main difference is that positions of the quantizer  468  and the multiplier  469  are swapped with those of the multiplier  465  and the quantizer  467  in  FIG. 7 . However, operation details are similar and shall not be repeated for brevity. 
       FIG. 10  is a flowchart of soft decision method according to one embodiment of the present disclosure. In Step  1010 , an input signal is received. In Step  1020 , a first coordinate range having a first limit and a second limit is defined on a coordinate axis in a constellation. In Step  1040 , a first soft decision sub-coordinate (such as I 0 ) is obtained according to the first coordinate range. In Step  1060 , a second coordinate range having a third limit and a fourth limit is defined on the coordinate axis in the constellation. In Step  1080 , a second soft decision sub-coordinate (such as I 1 ) is obtained according to the second coordinate range. Further, the first limit and the third limit and the second limit and the fourth limit are not equal simultaneously. 
     The embodiment can be applied to BPSK, QPSK or 16QAM. In an application to 64QAM, the above steps can be repeated. For example, referring to  FIG. 3 , for the coordinate value I=5.3, one can obtain I 2 =1 from  FIG. 3 . 
     In a preferred embodiment, the input signal can have coordinate limits. Since the received signal is noise-interfered, the amplitude of the input signal may be large. Therefore, to prevent undesired effects on the calculation results and to reduce the hardware costs, the coordinate range is limited to a fifth limit and a sixth limit. For example, the fifth limit and the sixth limit are set to −8 and 8 respectively. Any coordinate value greater than 8 is regarded as 8, and any coordinate value smaller than −8 is regarded as −8. For example, a coordinate value of 9.8 is regarded as 8. 
     From the above, in  FIG. 3 , the range of the I coordinate value can be viewed as from −8 (the fifth limit) to 8 (the sixth limit).  FIG. 3  shows the I-axis is defined to have 8 equal intervals numbered from −4 to 4. That is, the range of the I 0  coordinate is from −4 to 4, which are the limits of the I 0  coordinate. Hence, the first coefficient (such as k 0 ) is ½ or −½ and can be obtained by dividing 4 by 8 or dividing −4 by 8. That is, the first limit and the second limit are obtained by multiplying the fifth limit and the sixth limit by the first coefficient respectively. Similarly, the third limit (−2) and the fourth limit (2) of the I 1  coordinate are obtained by multiplying the fifth limit and the sixth limit by the second coefficient ( 2/8=¼, such as k 1 ) respectively. It is concluded from the above that the second coefficient is not equal to the first coefficient. 
       FIG. 11  is a flowchart of a soft decision method according to another embodiment of the present disclosure. In Step  1110 , an input signal is received. In Step  1120 , a first coordinate range having a plurality of equal first intervals is defined on a coordinate axis in a constellation. In Step  1150 , a first soft decision sub-coordinate (such as I 0 ) is obtained according to the first intervals. In Step  1170 , a second coordinate range having a plurality of equal second intervals is defined according to a portion of the first intervals. For example, the positive part of the first coordinate range (such as I 0 ), i.e. intervals from 1 to 4 from intervals 1 to 8, is defined as the second coordinate range such that the second coordinate range has four intervals with equal lengths. Consequently, the interval lengths of the first coordinate range and the second coordinate range are equal. Similarly, the negative part of the first coordinate range, i.e. intervals −1 to −4, can also be defined as the second coordinate range. In Step  1190 , a second soft decision sub-coordinate (such as I 1 ) is obtained according to the second intervals. Further, the size of the first interval and the size of the second interval are substantially equal. 
     The embodiment can be applied to BPSK, QPSK or 16QAM. In an application to 64QAM, the above steps can be repeated. For example,  FIG. 3  shows the coordinate value I=5.3, the two rightmost second intervals of the second coordinate range can be defined as the coordinate range of the I 2  coordinate, i.e. the range of the original intervals 4 to 8 on the I-axis, and I 2 =1 can then be obtained from  FIG. 3 . 
     With the above description, when the coordinate value I=5.3, the optimal coordinate value is (I 0 *, I 1 *, I 2 *)=(5.3, −1.3, 0.7). In the embodiment, (I 0 *, I 1 *, I 2 *)=(5.3, −1.3, 0.7) is mapped to (I 0 , I 1 , I 2 )=(3, −1, 1). However, the prior art maps (I 0 *, I 1 *, I 2 *)=(5.3, −1.3, 0.7) to (I 0 , I 1 , I 2 )=(3, −2, 2), which has larger errors compared to the embodiment, and such errors result in greater probabilities of misjudgment by the decoder. Therefore, the present disclosure provides a soft decision method and associated signal receiving system to improve the misjudgment of the signal receiving system on noise-interfered signals to increase coding gain and to reduce bit error rate. 
     While the present disclosure has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the present disclosure needs not to be limited to the above embodiments. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.