Patent Publication Number: US-10333552-B2

Title: Methods and apparatuses for constructing punctured polar code

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of International Application No. PCT/CN2014/079020, filed on May 30, 2014, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     Embodiments of the present disclosure relate to the encoding and decoding field, and in particular, to methods and apparatuses for constructing a punctured polar code. 
     BACKGROUND 
     In a communications system, channel encoding is generally used to improve data transmission reliability, so as to ensure communication quality. The polar code is the first code that is theoretically proved that it can achieve the Shannon capacity and has low encoding-decoding complexity. The polar code is a linear block code, and decoding of the polar code can be performed by using successive-cancellation (SC) or a successive-cancellation list (SC-List). 
     In the prior art, a conventional random puncturing hybrid automatic repeat request (HARQ) technology is used for the polar code, that is, a puncture location is selected randomly. However, by using this conventional technology, a frame error rate is relatively high and HARQ performance is relatively poor. 
     In addition, when a code length is excessively long, SC decoding can achieve good performance that is close to the Shannon limit. However, when the code length is relatively short or is an intermediate length, SC decoding of the polar code has relatively poor performance, and a punctured polar code has a high frame error rate. Compared with the performance of SC decoding, performance of List decoding is improved, but complexity of List decoding is higher than complexity of SC decoding. 
     SUMMARY 
     Embodiments of the present disclosure provide a method and an apparatus for constructing a punctured polar code, which can improve performance of a punctured polar code, and reduce a frame error rate of the punctured polar code. 
     According to a first aspect, an apparatus for constructing a punctured polar code is provided. The apparatus includes a processor and a memory storage accessible to the processor. The processor is configured to compare a phase sequence number of a current bit channel with a period index of a puncturing pattern to obtain a comparison result. The processor is further configured to obtain a transition probability of the bit channel according to the comparison result and bit parity conditions of a sequence number of the bit channel in each phase. The processor is further configured to obtain a reliability value of each bit channel according to the transition probability. The processor is configured to determine an information bit index set according to the reliability values. 
     According to a second aspect, a method for constructing a punctured polar code is provided. The method may include: comparing a phase sequence number of a current bit channel with a period index of a puncturing pattern to obtain a comparison result; obtaining a transition probability of the bit channel according to the comparison result and bit parity conditions of a sequence number of the bit channel in each phase; obtaining a reliability value of each bit channel according to the transition probability; and determining an information bit index set according to the reliability values. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       According to the methods and the apparatuses for constructing a punctured polar code provided in the embodiments of the present disclosure, a phase sequence number of a current bit channel is compared with a period index of a puncturing pattern to obtain a comparison result, where the puncturing pattern is an input parameter for constructing a punctured polar code; a transition probability of the bit channel is obtained according to the comparison result and bit parity conditions of a sequence number of the bit channel in each phase; a reliability value of each bit channel is obtained according to the transition probability; and an information bit index set is determined according to the reliability values. In this way, the input parameter of the punctured polar code is determined according to the reliability value of the channel, which can improve performance of the punctured polar code, and reduce a frame error rate of the punctured polar code. 
       To describe the technical solutions in the embodiments of the present disclosure more clearly, the following briefly describes the accompanying drawings required for describing the embodiments. Apparently, the accompanying drawings in the following description show merely some embodiments of the present disclosure, and a person of ordinary skilled in the art may still derive other drawings from these accompanying drawings without creative efforts. 
         FIG. 1  is a schematic diagram of a wireless communications system according to embodiments of the present disclosure; 
         FIG. 2  is a schematic diagram of a system in which a method for constructing a punctured polar code of the present disclosure is performed in a wireless communications environment; 
         FIG. 3  is a schematic flowchart of a method for constructing a punctured polar code according to an embodiment of the present disclosure; 
         FIG. 4  is a schematic diagram of a frame error rate FER curve comparison between random puncturing and an algorithm for constructing a punctured polar code in an embodiment of the present disclosure according to an embodiment of the present disclosure; 
         FIG. 5  is a block diagram of an apparatus for constructing a punctured polar code according to an embodiment of the present disclosure; 
         FIG. 6  is a schematic diagram of an access terminal, which is helpful in executing a method for constructing a punctured polar code, in a wireless communications system; 
         FIG. 7  is a schematic diagram of an exemplary system, in which a method for constructing a punctured polar code is performed, in a wireless communications environment; and 
         FIG. 8  is a schematic diagram of a system in which a method for constructing a punctured polar code can be used in a wireless communications environment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following clearly describes the technical solutions in the embodiments of the present disclosure with reference to the accompanying drawings in the embodiments of the present disclosure. Apparently, the described embodiments are merely a part rather than all of the embodiments of the present disclosure. All other embodiments obtained by a person of ordinary skilled in the art based on the embodiments of the present disclosure without creative efforts shall fall within the protection scope of the present disclosure. 
     Terminologies such as “component”, “module”, and “system” used in this specification are used to indicate computer-related entities, hardware, firmware, combinations of hardware and software, software, or software being executed. For example, a component may be, but is not limited to, a process that runs on a processor, a processor, an object, an executable file, a thread of execution, a program, and/or a computer. As shown in figures, both a computing device and an application that runs on a computing device may be components. One or more components may reside within a process and/or a thread of execution, and a component may be located on one computer and/or distributed between two or more computers. In addition, these components may be executed from various computer-readable media that store various data structures. The components may communicate by using a local and/or remote process and according to, for example, a signal having one or more data packets (for example, data from two components interacting with another component in a local system, a distributed system, and/or across a network such as the Internet interacting with other systems by using the signal). 
     In addition, each embodiment is described with reference to an access terminal. An access terminal may also be referred to as a system, a subscriber unit, a subscriber station, a mobile station, a mobile, a remote station, a remote terminal, a mobile device, a user terminal, a terminal, a wireless communications device, user agent, a user apparatus, or UE (user equipment). The access terminal may be a cellular phone, a cordless phone, a SIP (Session Initiation Protocol) phone, a WLL (wireless local loop) station, a PDA (personal digital assistant), a handheld device having a wireless communications function, a computing device, or another processing device connected to a wireless modem. In addition, each embodiment is described with reference to a base station. A base station can be used to communicate with a mobile device; and the base station may be a BTS (base transceiver station) in GSM (Global System of Mobile Communications) or CDMA (Code Division Multiple Access); may be an NB (NodeB) in WCDMA (Wideband Code Division Multiple Access); or may be an eNB or eNodeB (evolved NodeB) in LTE (Long Term Evolution), a relay station or an access point, a base station device in a future 5G network, or the like. 
     In addition, aspects or features of the present disclosure may be implemented as a method, an apparatus or a product that uses standard programming and/or engineering technologies. The term “product” used in this application covers a computer program that can be accessed from any computer-readable component, carrier or medium. For example, the computer-readable medium may include but is not limited to: a magnetic storage component (for example, a hard disk, a floppy disk or a magnetic tape), an optical disc (for example, a CD (compact disk), a DVD (digital versatile disk), a smart card and a flash memory component (for example, EPROM (erasable programmable read-only memory), a card, a stick, or a key drive). In addition, various storage media described in this specification may indicate one or more devices and/or other machine-readable media that is used to store information. The term “machine-readable media” may include but is not limited to a radio channel, and various other media that can store, contain and/or carry an instruction and/or data. 
     Now, refer to  FIG. 1 , which shows a wireless communications system  100  according to embodiments described in this specification. The wireless communications system  100  includes a base station  102 , and the base station  102  may include multiple antenna groups. Each antenna group may include one or more antennas. For example, an antenna group may include antennas  104  and  106 , another antenna group may include antennas  108  and  110 , and an additional group may include antennas  112  and  114 .  FIG. 1  shows two antennas for each antenna group, but more or fewer antennas may be used for each group. The base station  102  may additionally include a transmitter chain and a receiver chain. A person of ordinary skilled in the art may understand that both the transmitter chain and the receiver chain can include multiple components (such as processors, modulators, multiplexers, demodulators, demultiplexers, or antennas) related to signal transmission and reception. 
     The base station  102  may communicate with one or more access terminals (such as an access terminal  116  and an access terminal  122 ). However, it may be understood that the base station  102  may communicate with any quantity of access terminals similar to the access terminal  116  or  122 . The access terminals  116  and  122  may be, for example, cellular phones, smartphones, portable computers, handheld communications devices, handheld computing devices, satellite radio apparatuses, global positioning systems, PDAs, and/or any other appropriate devices used for communication in the wireless communications system  100 . As shown in the figure, the access terminal  116  communicates with the antennas  112  and  114 , where the antennas  112  and  114  send information to the access terminal  116  by using a forward link  118 , and receive information from the access terminal  116  by using a reverse link  120 . In addition, the access terminal  122  communicates with the antennas  104  and  106 , where the antennas  104  and  106  send information to the access terminal  122  by using a forward link  124 , and receive information from the access terminal  122  by using a reverse link  126 . In an FDD (frequency division duplex) system, for example, the forward link  118  may use a frequency band different from that used by the reverse link  120 , and the forward link  124  may use a frequency band different from that used by the reverse link  126 . In addition, in a TDD (time division duplex) system, the forward link  118  and the reverse link  120  may use a same frequency band, and the forward link  124  and the reverse link  126  may use a same frequency band. 
     Each antenna group and/or area designed for communication is referred to as a sector of the base station  102 . For example, the antenna group may be designed to communicate with an access terminal in a sector of an area covered by the base station  102 . In a process that the base station  102  respectively communicates with the access terminals  116  and  122  by using the forward links  118  and  124 , a transmit antenna of the base station  102  may use beamforming to improve a signal-to-noise ratio of the forward links  118  and  124 . In addition, when the base station  102  uses beamforming to send signals to the access terminals  116  and  122  that are distributed randomly in a related coverage area, a mobile device in a neighboring cell receives less interference, compared with a manner in which a base station uses a single antenna to send signals to all access terminals served by the base station. 
     In given duration, the base station  102 , the access terminal  116  and/or the access terminal  122  may be a transmit wireless communications apparatus and/or a receive wireless communications apparatus. When sending data, the transmit wireless communications apparatus may encode data for transmission. Specifically, the transmit wireless communications apparatus may have (such as generate, obtain, and store in a memory) a specific quantity of information bits that are to be sent to the receive wireless communications apparatus by using a channel. This information bit may be included in a transport block (or multiple transport blocks) of the data, and the transport block may be segmented to produce multiple code blocks. In addition, the transmit wireless communications apparatus may use a polar encoder (not shown) to encode each code block. 
       FIG. 2  shows a system to which a method for constructing a punctured polar code of the present disclosure is applicable in a wireless communications environment. The system includes a wireless communications apparatus  202 . It is shown that the wireless communications apparatus  202  performs sending by using a channel. Although it is illustrated that the wireless communications apparatus  202  sends data, the wireless communications apparatus  202  may further receive data by using the channel (for example, the wireless communications apparatus  202  may send and receive data at the same time, the wireless communications apparatus  202  may send and receive data at different moments, or a combination thereof). For example, the wireless communications apparatus  202  may be a base station (such as the base station  102  in  FIG. 1 ) or an access terminal (such as the access terminal  116  in  FIG. 1  or the access terminal  122  in  FIG. 1 ). 
     The wireless communications apparatus  202  may include a module  204  for constructing a punctured polar code, a rate matching apparatus  205 , and a transmitter  206 . 
     The module  204  for constructing a punctured polar code may be a polar encoder, configured to encode to-be-transmitted data to obtain a corresponding polar code. Alternatively, another module may be configured to construct a punctured polar code, which is not limited herein. The rate matching apparatus  205  is configured to determine output bits according to the corresponding polar code, and the transmitter  206  may subsequently transmit the output bits over the channel. For example, the transmitter  206  may send related data to another different wireless communications apparatus. 
     The following provides detailed description of a specific processing process of constructing the foregoing punctured polar code. 
       FIG. 3  is a schematic flowchart of a method for constructing a punctured polar code according to an embodiment of the present disclosure. The method shown in  FIG. 3  may be performed by a module, (for example, a polar encoder) for constructing a punctured polar code, in a wireless communications device. The method includes: 
       301 . Compare a phase sequence number of a current bit channel with a period index of a puncturing pattern to obtain a comparison result. 
       302 . Obtain a transition probability of the bit channel according to the comparison result and bit parity conditions of a sequence number of the bit channel in each phase, where the bit parity condition indicates whether the sequence number of the bit channel is odd or even. 
       303 . Obtain a reliability value of each bit channel according to the transition probability. 
       304 . Determine an information bit index set according to the reliability values. 
     The polar code is the first code that can be theoretically proved that it can achieve the Shannon capacity and has low encoding-decoding complexity (encoding-decoding complexity is O(NlogN), where N is a code length). A brief description of encoding-decoding of the polar code is as follows: 
     The polar code is a linear block code, a generator matrix thereof is G N , and an encoding process thereof is X 1   N =u 1   N G N , where G N =B N F ⊗n . u 1   N ={u 1 , u 2 , . . . , u N } is a binary vector and represents a vector before encoding, X 1   N  represents a vector after encoding, and N represents a vector length, that is, the code length is N. For the code length N=2 n , n≥0, herein 
               F   =     [         1       0           1       1         ]       ,         
and B N  is a transposed matrix, for example, a bit reversal matrix. F ⊗n  is a Kronecker power of F and is defined as F ⊗n =F⊗F ⊗(n-1) .
 
     The polar code may be represented as (N, K, A, u A     C   ) by using a coset code, and an encoding process thereof is x 1   N =u A G N. (A)⊕u A     c   G N. (A x ), where A herein is an information bit index set, G N. (A) is a submatrix, in G N. , obtained from a row corresponding to an index in the set A, G N. (A C ) is a submatrix, in G N. , obtained from a row corresponding to an index in a set A C , and selection of the set A decides performance of the polar code. u A     C    is a frozen bit, a quantity thereof is (N−K), and u A     C    is a known bit. For simplicity, these frozen bits may be set to 0. 
     In step  301 , the puncturing pattern is an input parameter for constructing the polar code. To represent the puncturing pattern more generally, a puncturing pattern of a broad-sense period may be used, and may be defined as follows: It is assumed that the code length of the polar code is N= 2m , and the puncturing pattern indicates that the puncturing pattern is a vector of a length 2 t ; and if i∈{0, 1, . . . , 2 t −1} is indicated, it indicates that locations of code symbols c j  and j=i mod 2 t  are punctured, that is, the puncturing pattern is periodic, and the period is 2 t , where j herein represents a phase sequence number. 
     The puncturing pattern of the broad-sense period may also be used to represent a non-periodic puncturing pattern; for example, if t=m, it may indicate that a puncturing pattern is non-periodic. 
     In an algorithm for constructing a punctured polar code, a phase sequence number j of the current bit channel, that is, an i th  bit channel, may be compared with a period index t of the puncturing pattern to obtain the comparison result. 
     According to the comparison result of step  301 , in step  302 , a transition probability W of the i th  bit channel is recursively obtained according to a first formula or a second formula and according to bit parity conditions of a binary representation of a sequence number i of the current bit channel in the phases, if the comparison result is that the phase sequence number j is less than or equal to the period index t. 
     The transition probability of the bit channel is obtained according to the first formula, if a bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 0; or the transition probability of the bit channel is obtained according to the second formula, if a bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 1. 
     The first formula herein includes: 
     
       
         
           
             
               
                 ( 
                 
                   
                     W 
                     0 
                   
                   ⊗ 
                   
                     W 
                     1 
                   
                 
                 ) 
               
               ⁢ 
               
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                       y 
                       0 
                     
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                       y 
                       1 
                     
                   
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                     u 
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                 ) 
               
             
             = 
             
               
                 ∑ 
                 
                   
                     u 
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                   ∈ 
                   x 
                 
               
               ⁢ 
               
                   
               
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                     W 
                     0 
                   
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                     ( 
                     
                       
                         y 
                         0 
                       
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                           u 
                           0 
                         
                         + 
                         
                           u 
                           1 
                         
                       
                     
                     ) 
                   
                 
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                     W 
                     1 
                   
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                     ( 
                     
                       
                         y 
                         1 
                       
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                         u 
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     and the second formula includes:
 
( W   0   ⊕W   1 )( y   0   y   1   u   0   |u   1 )= W   0 ( y   0   |u   0   +u   1 ) W   1 ( y   1   |u   1 )
 
     where W 0  and W 1  represent transition probabilities of two channels corresponding to a polarization transformation basic unit of a punctured polar code, y 0  and y 1  represent channel output, and u 0  and u 1  represent input bits before encoding. 
     A transition probability W of the i th  bit channel is obtained recursively according to a third formula or a fourth formula and according to bit parity conditions of a binary representation of a sequence number i of the bit channel in each phase, if the comparison result is that the phase sequence number j is greater than the period index t. 
     The transition probability of the bit channel is obtained according to the third formula, if a bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 0; or the transition probability of the bit channel is obtained according to the fourth formula, if a bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 1. 
     The third formula herein includes:
 
 W   0   (j)   ←Q   0   (j-1)   ⊗Q   0   (j-1)  
 
     and the fourth formula includes:
 
 W   0   (j)   ←Q   0   (j-1)   ⊕Q   0   (j-1)  
 
     where W 0   (j)  represents a transition probability for a ji th  phase, Q 0   (j-1)  represents a transition probability for a (j−1) th  phase, and j represents the phase sequence number. 
     The third formula and the fourth formula herein have a same formula for calculating a transition probability as a non-punctured polar code. 
     In this way, after a transition probability of each bit channel is obtained, in step  303 , because noise always exists on a channel, an error is inevitable in an information transmission process. After the transition probability of each bit channel is obtained, the reliability value of each bit channel can be obtained according to the transition probability. The reliability value may include the Bhattacharyya parameter, an error probability, or the Bhattacharyya parameter and an error probability. The error probability is an error probability of incorrectly shifting a bit 0 to 1 or incorrectly shifting 1 to 0. An implementation manner herein of obtaining the error probability according to the transition probability can be understood by a person skilled in the art, which is not repeatedly described. In addition, the Bhattacharyya parameter of each bit channel can further be obtained according to the transition probability. 
     An implementation manner of obtaining the Bhattacharyya parameter corresponding to each bit channel according to the transition probability W may be: according to a comparison method in step  301 , obtaining the Bhattacharyya parameter according to a fifth formula, if the comparison result is that the phase sequence number is less than or equal to the period index, and the bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 0; 
     obtaining the Bhattacharyya parameter according to a sixth formula, if the comparison result is that the phase sequence number is less than or equal to the period index, and the bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 1; 
     obtaining the Bhattacharyya parameter according to a seventh formula, if the comparison result is that the phase sequence number is greater than the period index, and the bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 0; or 
     obtaining the Bhattacharyya parameter according to an eighth formula, if the comparison result is that the phase sequence number is greater than the period index, and the bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 1. 
     The fifth formula includes:
 
 Z ( W   0   ⊗W   1 )≤ Z ( W   0 )+ Z ( W   1 )
 
     and the sixth formula includes:
 
 Z ( W   0   ⊕W   1 )= Z ( W   0 ) Z ( W   1 )
 
     where W 0  and W 1  represent transition probabilities of two channels corresponding to a polarization transformation basic unit of a punctured polar code, and Z represents the Bhattacharyya parameter; and 
     the seventh formula includes:
 
 Z   0   (f) ←min{ Z ( W   0   (j) ,2 Z   0   (j-1) −( Z   0   (j-1) ) 2 )}
 
     and the eighth formula includes:
 
 Z   0   (j) ←( Z   0   (j-1) ) 2 }
 
     where W 0   (j)  represents a transition probability for a j th  phase, Z 0   (j)  represents the Bhattacharyya parameter for the i th  phase, and Z 0   (j-1)  represents the Bhattacharyya parameter for a (j−1) th  phase. 
     After the Bhattacharyya parameter and the error probability of each bit channel are obtained, in step  304 , the error probabilities may be sorted in ascending order, and a bit index set A is formed according to sequence numbers corresponding to bit channels of the first K error probabilities; the Bhattacharyya parameters are sorted in ascending order, and the information bit index set is formed according to sequence numbers corresponding to bit channels of the first K Bhattacharyya parameters; or the smaller one of the error probability and the Bhattacharyya parameter of each bit channel is selected, the selected smaller ones are sorted in ascending order, and sequence numbers of bit channels corresponding to the first K smaller ones are selected to form the information bit index set. For example, if an error probability of a bit channel is 1/1000, and the Bhattacharyya parameter is 2/1000, 1/1000 is selected for sorting the bit channel. 
     A value of K herein is preset and known, and K may be selected according to a code rate or a code length of channel encoding-decoding. 
     In this way, for the algorithm for constructing a punctured polar code, during obtaining of the information bit index set A, the error probability or the Bhattacharyya parameter of each bit channel is obtained, so that sequence numbers of bit channels of the first K error probabilities and/or Bhattacharyya parameters are selected as the information bit index set A, and a transmitter and a receiver in a communications device can perform encoding and decoding according to the information bit index set A. 
       FIG. 4  shows a frame error rate FER curve comparison between random puncturing and an algorithm for constructing a punctured polar code in this embodiment of the present disclosure, where SC decoding is used. The curve with * is for the case in which a conventional random puncturing method is used, and the curve with □ is for the case in which a puncturing method in this embodiment of the present disclosure is used. The horizontal axis indicates a bit signal-to-noise ratio (Eb/N0), and the vertical axis indicates an FER value. It can be seen that by using the algorithm for constructing a punctured polar code in this embodiment of the present disclosure, FER performance of a punctured polar code can be improved and a frame error rate of the punctured polar code can be reduced. For example, if a code length N=256, K=128, and a puncture number=64, when FER=10^(−2), performance of a punctured polar code constructed by using the algorithm in this embodiment of the present disclosure is 0.7 dB better than performance of a randomly punctured polar code, where the puncture number indicates the number of punctured bits. The FER reflects comprehensive forward link quality, and is a frame transmission error probability in a data transmission process. For example, for a base station and a mobile phone user, a smaller FER indicates that a link on which the mobile phone is located is better, and a signal received from the base station is better. 
     Therefore, in the method for constructing a punctured polar code provided in this embodiment of the present disclosure, a phase sequence number of a current bit channel is compared with a period index of a puncturing pattern to obtain a comparison result, where the puncturing pattern is used to produce an input parameter of the polar code; a transition probability of the bit channel is recursively obtained according to the comparison result and bit parity conditions of a binary representation of a sequence number of the bit channel in each phase; an error probability of each bit channel is obtained according to the transition probability, the error probabilities are sorted in ascending order, and sequence numbers corresponding to bit channels of the first K error probabilities are selected to form an information bit index set, so that a receiver of a terminal performs decoding according to the information bit index set constructed by a punctured polar code in this embodiment of the present disclosure, which can improve decoding performance of the polar code. 
       FIG. 5  is a block diagram of an apparatus  500  for constructing a punctured polar code according to an embodiment of the present disclosure. The apparatus may be located on a mobile user terminal, a base station, or the like. The apparatus may include a processor and a non-transitory storage medium configured to store units that include a comparison unit  501 , an obtaining unit  502 , and a determining unit  503 . 
     The comparison unit  501  is configured to compare a phase sequence number of a current bit channel with a period index of a puncturing pattern to obtain a comparison result. The obtaining unit  502  is configured to obtain a transition probability of the bit channel according to the comparison result and bit parity conditions of a sequence number of the bit channel in each phase. The obtaining unit  502  is further configured to obtain a reliability value of each bit channel according to the transition probability. The determining unit  503  is configured to determine an information bit index set according to the reliability values. 
     The reliability value herein may include the Bhattacharyya parameter, an error probability, or the Bhattacharyya parameter and an error probability. 
     Alternatively or additionally, the reliability value includes the Bhattacharyya parameter, and the determining unit  503  is configured to: 
     sort the Bhattacharyya parameters in ascending order, and form the information bit index set according to sequence numbers corresponding to bit channels of the first K Bhattacharyya parameters, where K is a positive integer less than or equal to a total quantity of the bit channels. 
     Alternatively or additionally, the reliability value includes the error probability, and the determining unit  503  is configured to: 
     sort the error probabilities in ascending order, and form the information bit index set according to sequence numbers corresponding to bit channels of the first K error probabilities, where K is a positive integer less than or equal to a total quantity of the bit channels. 
     Alternatively or additionally, the reliability value includes the Bhattacharyya parameter and the error probability, and the determining unit  503  is configured to: 
     select the smaller one of the error probability and the Bhattacharyya parameter of each bit channel, sort the selected smaller ones in ascending order, and select sequence numbers of bit channels corresponding to the first K smaller ones to form the information bit index set, where K is a positive integer less than or equal to a total quantity of the bit channels. 
     Alternatively or additionally, the obtaining unit  502  may be configured to: 
     obtain the transition probability of the bit channel according to a first formula or a second formula and according to bit parity conditions of a binary representation of the sequence number of the bit channel in each phase, if the comparison result is that the phase sequence number is less than or equal to the period index; or 
     obtain the transition probability of the bit channel according to a third formula or a fourth formula and according to bit parity conditions of a binary representation of the sequence number of the bit channel in each phase, if the comparison result is that the phase sequence number is greater than the period index. 
     Alternatively or additionally, if the comparison result is that the phase sequence number is less than or equal to the period index, the obtaining unit  502  may be configured to: 
     obtain the transition probability according to the first formula, if a bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 0; or 
     obtain the transition probability according to the second formula, if a bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 1. 
     The first formula includes: 
     
       
         
           
             
               
                 ( 
                 
                   
                     W 
                     0 
                   
                   ⊗ 
                   
                     W 
                     1 
                   
                 
                 ) 
               
               ⁢ 
               
                 ( 
                 
                   
                     
                       y 
                       0 
                     
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                       y 
                       1 
                     
                   
                   ❘ 
                   
                     u 
                     0 
                   
                 
                 ) 
               
             
             = 
             
               
                 ∑ 
                 
                   
                     u 
                     1 
                   
                   ∈ 
                   x 
                 
               
               ⁢ 
               
                   
               
               ⁢ 
               
                 
                   
                     W 
                     0 
                   
                   ⁡ 
                   
                     ( 
                     
                       
                         y 
                         0 
                       
                       ❘ 
                       
                         
                           u 
                           0 
                         
                         + 
                         
                           u 
                           1 
                         
                       
                     
                     ) 
                   
                 
                 ⁢ 
                 
                   
                     W 
                     1 
                   
                   ⁡ 
                   
                     ( 
                     
                       
                         y 
                         1 
                       
                       ❘ 
                       
                         u 
                         1 
                       
                     
                     ) 
                   
                 
               
             
           
         
       
     
     and the second formula includes:
 
( W   0   ⊕W   1 )( y   0   y   1   u   0   |u   1 )= W   0 ( y   0   |u   0   +u   1 ) W   1 ( y   1   |u   1 )
 
     where W 0  and W 1  represent transition probabilities of two channels corresponding to a polarization transformation basic unit of a punctured polar code, y 0  and y 1  represent channel output, and u 0  and u 1  represent input bits before encoding. 
     Alternatively or additionally, 
     if the comparison result is that the phase sequence number is greater than the period index, the obtaining unit  502  may be configured to: 
     obtain the transition probability according to the third formula, if a bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 0; or 
     obtain the transition probability according to the fourth formula, if a bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 1. 
     The third formula includes:
 
 W   0   (j)   ←Q   0   (j-1)   ⊗Q   0   (j-1)  
 
and the fourth formula includes:
 
 W   0   (j)   ←Q   0   (j-1)   ⊕Q   0   (j-1)  
 
     where W 0   (j)  represents a transition probability for a j th  phase, Q 0   (j-1)  represents a transition probability for a (j−1) th  phase, and j represents the phase sequence number. 
     Alternatively or additionally, 
     the obtaining unit  502  may be configured to: 
     obtain the Bhattacharyya parameter according to a fifth formula, if the comparison result is that the phase sequence number is less than or equal to the period index, and the bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 0; 
     obtain the Bhattacharyya parameter according to a sixth formula, if the comparison result is that the phase sequence number is less than or equal to the period index, and the bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 1; 
     obtain the Bhattacharyya parameter according to a seventh formula, if the comparison result is that the phase sequence number is greater than the period index, and the bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 0; or 
     obtain the Bhattacharyya parameter according to an eighth formula, if the comparison result is that the phase sequence number is greater than the period index, and the bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 1. 
     The fifth formula includes:
 
 Z ( W   0   ⊗W   1 )≤ Z ( W   0 )+ Z ( W   1 )
 
     and the sixth formula includes:
 
 Z ( W   0   ⊕W   1 )= Z ( W   0 ) Z ( W   1 )
 
     where W 0  and W 1  represent transition probabilities of two channels corresponding to a polarization transformation basic unit of a punctured polar code, and Z represents the Bhattacharyya parameter; and 
     the seventh formula includes:
 
 Z   0   (f) ←min{ Z ( W   0   (j) ,2 Z   0   (j-1) −( Z   0   (j-1) ) 2 )}
 
     and the eighth formula includes:
 
 Z   0   (j) ←( Z   0   (j-1) ) 2 }
 
     where W 0   (j)  represents a transition probability for a j th  phase, Z 0   (j)  represents the Bhattacharyya parameter for the j th  phase, and Z 0   (j-1)  represents the Bhattacharyya parameter for a (j−1) th  phase. 
     According to the apparatus for constructing a punctured polar code provided in this embodiment of the present disclosure, a phase sequence number of a current bit channel is compared with a period index of a puncturing pattern to obtain a comparison result, where the puncturing pattern is an input parameter for constructing the polar code; a transition probability of the bit channel is recursively obtained according to the comparison result and bit parity conditions of a binary representation of a sequence number of the bit channel in each phase; the Bhattacharyya parameter and an error probability of each bit channel are obtained according to the transition probability; and an information bit index set is determined according to the Bhattacharyya parameters and/or the error probabilities, so that a receiver performs decoding according to the information bit index set constructed by a punctured polar code in this embodiment of the present disclosure, which can improve decoding performance of the polar code. 
       FIG. 6  is a schematic diagram of an access terminal  600 , which is helpful in executing the foregoing method for constructing a punctured polar code, in a wireless communications system. The access terminal  600  includes a receiver  602 , where the receiver  602  is configured to receive a signal from, for example, a receive antenna (not shown), perform a typical action (such as filtering, amplifying, and down-conversion) on the received signal, and digitalize an adjusted signal to obtain a sample. The receiver  602  may be, for example, an MMSE (minimum mean square error) receiver. The access terminal  600  may further include a demodulator  604 , where the demodulator  604  may be configured to demodulate a received symbol and provide the symbol to a processor  606  for channel estimation. The processor  606  may be: a processor that is specially configured to analyze information received by the receiver  602  and/or generate information to be sent by a transmitter  616 ; a processor configured to control one or more components of the access terminal  600 ; and/or a controller that is configured to analyze information received by the receiver  602 , generate information to be sent by a transmitter  616 , and control one or more components of the access terminal  600 . 
     The access terminal  600  may additionally include a memory  608 . The memory  608  may be operatively coupled to the processor  606 , and store the following data: data to be sent, data received, and any other proper information related to execution of various actions and functions described in this specification. The memory  608  may additionally store a protocol and/or an algorithm that are/is related to polar code processing. 
     It can be understood that a data storage apparatus (such as the memory  608 ) described in this specification may be a volatile memory or a non-volatile memory, or may include both of a volatile memory and a non-volatile memory. Exemplary but not restrictive, the non-volatile memory may include: a ROM (read-only memory), a PROM (programmable read-only memory), an EPROM (erasable programmable read-only memory), an EEPROM (electrically erasable programmable read-only memory), or a flash memory. The volatile memory may include: a RAM (random access memory), which is used as an external cache. Exemplary but not restrictive, RAMs of many forms are applicable, such as an SRAM (static random access memory), a DRAM (dynamic random access memory), an SDRAM (synchronous dynamic random access memory), a DDR SDRAM (double data rate synchronous dynamic random access memory), an ESDRAM (enhanced synchronous dynamic random access memory), an SLDRAM (synchlink dynamic random access memory), and a DR RAM (direct rambus random access memory). The memory  608  in both a system and a method described in this specification aims to include but is not limited to these memories and a memory of another proper type. 
     In actual application, the access terminal  600  may include a polar encoder. The polar encoder  612  is basically similar to the polar encoder  204  in  FIG. 2 , and may be configured to: compare a phase sequence number of a current bit channel with a period index of a puncturing pattern to obtain a comparison result; obtain a transition probability of the bit channel according to the comparison result and bit parity conditions of a sequence number of the bit channel in each phase; obtain a reliability value of each bit channel according to the transition probability; and determine an information bit index set according to the reliability values. 
     The reliability value may include the Bhattacharyya parameter, an error probability, or the Bhattacharyya parameter and an error probability. 
     Alternatively or additionally, in an embodiment, a transition probability W of an i th  bit channel is obtained according to a first formula or a second formula and according to bit parity conditions of a binary representation of a sequence number i of the current bit channel in the phases, if the comparison result is that a phase sequence number j is less than or equal to a period index t. 
     The transition probability of the bit channel is obtained according to the first formula, if a bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 0; or the transition probability of the bit channel is obtained according to the second formula, if a bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 1. 
     The first formula herein includes: 
     
       
         
           
             
               
                 ( 
                 
                   
                     W 
                     0 
                   
                   ⊗ 
                   
                     W 
                     1 
                   
                 
                 ) 
               
               ⁢ 
               
                 ( 
                 
                   
                     
                       y 
                       0 
                     
                     ⁢ 
                     
                       y 
                       1 
                     
                   
                   ❘ 
                   
                     u 
                     0 
                   
                 
                 ) 
               
             
             = 
             
               
                 ∑ 
                 
                   
                     u 
                     1 
                   
                   ∈ 
                   x 
                 
               
               ⁢ 
               
                   
               
               ⁢ 
               
                 
                   
                     W 
                     0 
                   
                   ⁡ 
                   
                     ( 
                     
                       
                         y 
                         0 
                       
                       ❘ 
                       
                         
                           u 
                           0 
                         
                         + 
                         
                           u 
                           1 
                         
                       
                     
                     ) 
                   
                 
                 ⁢ 
                 
                   
                     W 
                     1 
                   
                   ⁡ 
                   
                     ( 
                     
                       
                         y 
                         1 
                       
                       ❘ 
                       
                         u 
                         1 
                       
                     
                     ) 
                   
                 
               
             
           
         
       
     
     and the second formula includes:
 
( W   0   ⊕W   1 )( y   0   y   1   u   0   |u   1 )= W   0 ( y   0   |u   0   +u   1 ) W   1 ( y   1   |u   1 )
 
     where W 0  and W 1  represent transition probabilities of two channels corresponding to a polarization transformation basic unit of a punctured polar code, y 0  and y 1  represent channel output, and u 0  and u 1  represent input bits before encoding. 
     A transition probability W is obtained according to a third formula or a fourth formula and according to bit parity conditions of a binary representation of a sequence number i of the bit channel in each phase, if the comparison result is that a phase sequence number j is greater than a period index t. 
     The transition probability of the bit channel is obtained according to the third formula, if a bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 0; or the transition probability of the bit channel is obtained according to the fourth formula, if a bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 1. 
     The third formula herein includes:
 
 W   0   (j)   ←Q   0   (j-1)   ⊗Q   0   (j-1)  
 
     and the fourth formula includes:
 
 W   0   (j)   ←Q   0   (j-1)   ⊕Q   0   (j-1)  
 
     where W 0   (j-1)  represents a transition probability for a j th  phase, Q 0   (j-1)  represents a transition probability for a (j−1) th  phase, and j represents the phase sequence number. 
     The third formula and the fourth formula herein have a same formula for calculating a transition probability as a non-punctured polar code. 
     Alternatively or additionally, after the polar encoder obtains the transition probability of each bit channel, because noise always exists on a channel, an error is inevitable in an information transmission process. After the transition probability of each bit channel is obtained, the error probability of incorrectly shifting a bit 0 to 1 or incorrectly shifting 1 to 0 of each bit channel can be obtained according to the transition probability. An implementation manner herein of obtaining the error probability according to the transition probability can be understood by a person skilled in the art, which is not repeatedly described. In addition, the Bhattacharyya parameter of each bit channel can further be obtained according to the transition probability. 
     An implementation manner of obtaining the Bhattacharyya parameter corresponding to each bit channel according to the transition probability W may be: according to a comparison method in step  301 , obtaining the Bhattacharyya parameter according to a fifth formula, if the comparison result is that the phase sequence number is less than or equal to the period index, and the bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 0; 
     obtaining the Bhattacharyya parameter according to a sixth formula, if the comparison result is that the phase sequence number is less than or equal to the period index, and the bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 1; 
     obtaining the Bhattacharyya parameter according to a seventh formula, if the comparison result is that the phase sequence number is greater than the period index, and the bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 0; or 
     obtaining the Bhattacharyya parameter according to an eighth formula, if the comparison result is that the phase sequence number is greater than the period index, and the bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 1. 
     The fifth formula includes:
 
 Z ( W   0   ⊗W   1 )≤ Z ( W   0 )+ Z ( W   1 )
 
     and the sixth formula includes:
 
 Z ( W   0   ⊕W   1 )= Z ( W   0 ) Z ( W   1 )
 
     where W 0  and W 1  represent transition probabilities of two channels corresponding to a polarization transformation basic unit of a punctured polar code, and Z represents the Bhattacharyya parameter; and 
     the seventh formula includes:
 
 Z   0   (f) ←min{ Z ( W   0   (j) ,2 Z   0   (j-1) −( Z   0   (j-1) ) 2 )}
 
     and the eighth formula includes:
 
 Z   0   (j) ←( Z   0   (j-1) ) 2 }
 
     where W 0   (j)  represents a transition probability for a j th  phase, Z 0   (j)  represents the Bhattacharyya parameter for the j th  phase, and Z 0   j-1)  represents the Bhattacharyya parameter for a (j−1) th  phase. 
     Alternatively or additionally, after the Bhattacharyya parameter and the error probability of each bit channel are obtained, the error probabilities may be sorted in ascending order, and a bit index set A is formed according to sequence numbers corresponding to bit channels of the first K error probabilities; the Bhattacharyya parameters are sorted in ascending order, and the information bit index set is formed according to sequence numbers corresponding to bit channels of the first K Bhattacharyya parameters; or the smaller one of the error probability and the Bhattacharyya parameter of each bit channel is selected, the selected smaller ones are sorted in ascending order, and sequence numbers of bit channels corresponding to the first K smaller ones are selected to form the information bit index set. 
     In addition, the access terminal  600  may further include a modulator  614  and the transmitter  616 , where the transmitter  616  is further configured to send a signal to a base station, another access terminal, or the like. Although it is illustrated that the polar encoder  612 , a rate matching device  610 , and/or the modulator  614  are independent of the processor  606 , it should be understood that the polar encoder  612 , the rate matching device  610 , and/or the modulator  614  may be a part of the processor  606  or multiple processors (not shown). In actual application, the receiver  602  and the transmitter  616  may also be integrated to constitute one transceiver. 
       FIG. 7  is a schematic diagram of an exemplary system, in which a method for constructing a punctured polar code is performed, in a wireless communications environment. A base station  702  (such as an access point, an NB, or an eNB) is included. The base station  702  has a receiver  710  that receives a signal from one or more access terminals  704  by using multiple receive antennas  706 , and has a transmitter  724  that transmits a signal to the one or more access terminals  704  by using a transmit antenna  708 . Generally, “a receive antenna” and “a transmit antenna” may be integrated to constitute one receive-transmit antenna. The receiver  710  may receive information from the receive antenna  706 , and may be operatively associated with a demodulator  712  that performs demodulation on the received information. A demodulated symbol is analyzed by using a processor  714 . The processor  714  is connected to a memory  716 , where the memory  716  is configured to store data that is to be sent to the access terminal  704  (or a different base station (not shown)), or data received from the access terminal  704  (or a different base station (not shown)), and/or any other proper information related to execution of various actions and functions described in this specification. The processor  714  may be further coupled to a polar encoder  726  to compare a phase sequence number of a current bit channel with a period index of a puncturing pattern to obtain a comparison result; recursively obtain a transition probability of the bit channel according to the comparison result and bit parity conditions of a binary representation of a sequence number of the bit channel in each phase; obtain the Bhattacharyya parameter and an error probability of each bit channel according to the transition probability; and determine an information bit index set according to the Bhattacharyya parameters and/or the error probabilities. 
     Alternatively or additionally, a reliability value includes the Bhattacharyya parameter; and 
     determining the information bit index set according to the reliability value includes: 
     sorting the Bhattacharyya parameters in ascending order, and forming the information bit index set according to sequence numbers corresponding to bit channels of the first K Bhattacharyya parameters, where K is a positive integer less than or equal to a total quantity of the bit channels. 
     Alternatively or additionally, a reliability value includes the error probability; and 
     determining the information bit index set according to the reliability value includes: 
     sorting the error probabilities in ascending order, and forming the information bit index set according to sequence numbers corresponding to bit channels of the first K error probabilities, where K is a positive integer less than or equal to a total quantity of the bit channels. 
     Alternatively or additionally, a reliability value includes the Bhattacharyya parameter and the error probability; and 
     determining the information bit index set according to the reliability value includes: 
     selecting the smaller one of the error probability and the Bhattacharyya parameter for each bit channel, sorting the selected smaller ones in ascending order, and selecting sequence numbers of bit channels corresponding to the first K smaller ones to form the information bit index set, where K is a positive integer less than or equal to a total quantity of the bit channels. 
     Alternatively or additionally, in an embodiment, a transition probability W of an i th  bit channel is obtained according to a first formula or a second formula and according to bit parity conditions of a binary representation of a sequence number i of the current bit channel in the phases, if the comparison result is that a phase sequence number j is less than or equal to a period index t. 
     The transition probability of the bit channel is obtained according to the first formula, if a bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 0; or the transition probability of the bit channel is obtained according to the second formula, if a bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 1. 
     The first formula herein includes: 
     
       
         
           
             
               
                 ( 
                 
                   
                     W 
                     0 
                   
                   ⊗ 
                   
                     W 
                     1 
                   
                 
                 ) 
               
               ⁢ 
               
                 ( 
                 
                   
                     
                       y 
                       0 
                     
                     ⁢ 
                     
                       y 
                       1 
                     
                   
                   ❘ 
                   
                     u 
                     0 
                   
                 
                 ) 
               
             
             = 
             
               
                 ∑ 
                 
                   
                     u 
                     1 
                   
                   ∈ 
                   x 
                 
               
               ⁢ 
               
                   
               
               ⁢ 
               
                 
                   
                     W 
                     0 
                   
                   ⁡ 
                   
                     ( 
                     
                       
                         y 
                         0 
                       
                       ❘ 
                       
                         
                           u 
                           0 
                         
                         + 
                         
                           u 
                           1 
                         
                       
                     
                     ) 
                   
                 
                 ⁢ 
                 
                   
                     W 
                     1 
                   
                   ⁡ 
                   
                     ( 
                     
                       
                         y 
                         1 
                       
                       ❘ 
                       
                         u 
                         1 
                       
                     
                     ) 
                   
                 
               
             
           
         
       
     
     and the second formula includes:
 
( W   0   ⊕W   1 )( y   0   y   1   u   0   |u   1 )= W   0 ( y   0   |u   0   +u   1 ) W   1 ( y   1   |u   1 )
 
     where W 0  and W 1  represent transition probabilities of two channels corresponding to a polarization transformation basic unit of a punctured polar code, y 0  and y 1  represent channel output, and u 0  and u 1  represent input bits before encoding. 
     A transition probability W is obtained according to a third formula or a fourth formula and according to bit parity conditions of a binary representation of a sequence number i of the bit channel in each phase, if the comparison result is that a phase sequence number j is greater than a period index t. 
     The transition probability of the bit channel is obtained according to the third formula, if a bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 0; or the transition probability of the bit channel is obtained according to the fourth formula, if a bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 1. 
     The third formula herein includes:
 
 W   0   (j)   ←Q   0   (j-1)   ⊗Q   0   (j-1)  
 
     and the fourth formula includes:
 
 W   0   (j)   ←Q   0   (j-1)   ⊕Q   0   (j-1)  
 
     where W 0   (j)  represents a transition probability for a j th  phase, Q 0   (j-1)  represents a transition probability for a (j−1) th  phase, and j represents the phase sequence number. 
     The third formula and the fourth formula herein have a same formula for calculating a transition probability as a non-punctured polar code. 
     Alternatively or additionally, after the polar encoder obtains the transition probability of each bit channel, because noise always exists on a channel, an error is inevitable in an information transmission process. After the transition probability of each bit channel is obtained, the error probability of each bit channel can be obtained according to the transition probability. The error probability is an error probability of incorrectly shifting a bit 0 to 1 or incorrectly shifting 1 to 0. An implementation manner herein of obtaining the error probability according to the transition probability can be understood by a person skilled in the art, which is not repeatedly described. In addition, the Bhattacharyya parameter of each bit channel can further be obtained according to the transition probability. 
     An implementation manner of obtaining the Bhattacharyya parameter corresponding to each bit channel according to the transition probability W may be: according to a comparison method in step  301 , obtaining the Bhattacharyya parameter according to a fifth formula, if the comparison result is that the phase sequence number is less than or equal to the period index, and the bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 0; 
     obtaining the Bhattacharyya parameter according to a sixth formula, if the comparison result is that the phase sequence number is less than or equal to the period index, and the bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 1; 
     obtaining the Bhattacharyya parameter according to a seventh formula, if the comparison result is that the phase sequence number is greater than the period index, and the bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 0; or 
     obtaining the Bhattacharyya parameter according to an eighth formula, if the comparison result is that the phase sequence number is greater than the period index, and the bit of the binary representation of the sequence number of the bit channel in each phase corresponds to 1. 
     The fifth formula includes:
 
 Z ( W   0   ⊗W   1 )≤ Z ( W   0 )+ Z ( W   1 )
 
     and the sixth formula includes:
 
 Z ( W   0   ⊕W   1 )= Z ( W   0 ) Z ( W   1 )
 
     where W 0  and W 1  represent transition probabilities of two channels corresponding to a polarization transformation basic unit of a punctured polar code, and Z represents the Bhattacharyya parameter; and 
     the seventh formula includes:
 
 Z   0   (f) ←min{ Z ( W   0   (j) ,2 Z   0   (j-1) −( Z   0   (j-1) ) 2 )}
 
     and the eighth formula includes:
 
 Z   0   (j) ←( Z   0   (j-1) ) 2 }
 
     where W 0   (j)  represents a transition probability for a j th  phase, Z 0   (j)  represents the Bhattacharyya parameter for the j th  phase, and Z 0   (j-1)  represents the Bhattacharyya parameter for a (j−1) th  phase. 
     Alternatively or additionally, after the Bhattacharyya parameter and the error probability of each bit channel are obtained, the error probabilities may be sorted in ascending order, and a bit index set A is formed according to sequence numbers corresponding to bit channels of the first K error probabilities; the Bhattacharyya parameters are sorted in ascending order, and the information bit index set is formed according to sequence numbers corresponding to bit channels of the first K Bhattacharyya parameters; or the smaller one of the error probability and the Bhattacharyya parameter of each bit channel is selected, the selected smaller ones are sorted in ascending order, and sequence numbers of bit channels corresponding to the first K smaller ones are selected to form the information bit index set. 
     In addition, the access terminal may further include a modulator  722  and a transmitter  724 , where the transmitter  724  is configured to send a signal to a base station, another access terminal, or the like. Although it is illustrated that the polar encoder  726 , a rate matching device  720 , and/or the modulator  722  are independent of the processor  714 , it should be understood that the polar encoder  726 , the rate matching device  720 , and/or the modulator  722  may be a part of the processor  714  or multiple processors (not shown). 
     In addition, in a system, the modulator  722  may modulate a frame, and the transmitter  724  sends, by using the antenna  706 , the frame modulated by the modulator  722  to the access terminal  704 . Although it is illustrated that the polar encoder  726 , the rate matching device  720 , and/or the modulator  722  are independent of the processor  714 , it should be understood that the polar encoder  726 , the rate matching device  720 , and/or the modulator  722  may be a part of the processor  714  or multiple processors (not shown). 
     It may be understood that the embodiments described in this specification may be implemented by hardware, software, firmware, middleware, microcode or a combination thereof. For hardware implementation, a processing unit may be implemented in another electronic unit for executing the functions described in this application and a combination thereof, such as one or more ASICs (application specific integrated circuit), a DSP (digital signal processor), a DSPD (digital signal processing device), a PLD (programmable logic device), an FPGA (field-programmable gate array), a processor, a controller, a microcontroller, a microprocessor, and a chip. 
     When the embodiments are implemented in software, firmware, middleware, microcode, program code, or a code segment, they may be stored in a machine-readable medium of, for example, a storage component. The code segment may represent a process, a function, a subprogram, a program, a routine, a subroutine, a module, a software group, a class, or any combination of instructions, data structures or program statements. The code segment may be coupled to another code segment or hardware circuit by transmitting and/or receiving information, data, an independent variable, a parameter, or memory content. The information, the independent variable, the parameter, the data, and the like may be transferred, forwarded, or sent in any suitable manner including memory sharing, message transfer, token transfer, network transmission, and the like. 
     For software implementation, the technology described in this specification may be implemented by using modules for executing the functions (for example, a process or a function) described in this specification. Software code may be stored in a memory and executed by a processor. A memory unit may be implemented inside a processor or outside a processor, and in the latter case, the memory unit may be coupled to the processor in a communication manner through various means known in the art. 
     According to the apparatus for constructing a punctured polar code provided in this embodiment of the present disclosure, a phase sequence number of a current bit channel is compared with a period index of a puncturing pattern to obtain a comparison result, where the puncturing pattern is an input parameter for constructing the polar code; a transition probability of the bit channel is recursively obtained according to the comparison result and bit parity conditions of a binary representation of a sequence number of the bit channel in each phase; the Bhattacharyya parameter and an error probability of each bit channel are obtained according to the transition probability; and an information bit index set is determined according to the Bhattacharyya parameters and/or the error probabilities, so that a receiver performs decoding according to the information bit index set constructed by a punctured polar code in this embodiment of the present disclosure, which can improve decoding performance of the polar code. 
     Refer to  FIG. 8 , which shows a system  800  in which a method for constructing a punctured polar code may be used in a wireless communications environment. For example, the system  800  may at least partially reside on a base station. According to another example, the system  800  may at least partially reside on an access terminal. It should be understood, the system  800  may include a functional block, which may represent a functional block of functions implemented by a processor, software, or a combination thereof (for example, firmware). The system  800  includes a logical group  802  that has electronic components that can operate jointly. 
     For example, the logical group  802  may be configured to: compare a phase sequence number of a current bit channel with a period index of a puncturing pattern to obtain an electronic component  804  of a comparison result; recursively obtain an electronic component  806  of a transition probability of the bit channel according to the comparison result and bit parity conditions of a binary representation of a sequence number of the bit channel in each phase; obtain an electronic component  808  of the Bhattacharyya parameter and an error probability of each bit channel according to the transition probability; and determine an electronic component  810  of an information bit index set according to the Bhattacharyya parameters and/or the error probabilities. 
     In the method for constructing a punctured polar code provided in the present disclosure, a receiver performs decoding according to an information bit index set constructed by a punctured polar code in this embodiment of the present disclosure, which can improve decoding performance of the polar code. 
     In addition, the system  800  may include a memory  812 , and the memory  812  stores instructions for performing functions related to the electronic components  804 ,  806 ,  808 , and  810 . Although it is illustrated that one or more of the electronic components  804 ,  806 ,  808 , and  810  exist outside the memory  812 , it may be understood that, one or more of the electronic components  804 ,  806 ,  808 , and  810  may exist in the memory  812 . 
     A person of ordinary skill in the art may be aware that, in combination with the examples described in the embodiments disclosed in this specification, units and algorithm steps may be implemented by electronic hardware or a combination of computer software and electronic hardware. Whether the functions are performed by hardware or software depends on particular applications and design constraint conditions of the technical solutions. A person skilled in the art may use different methods to implement the described functions for each particular application, but it should not be considered that the implementation goes beyond the scope of the present disclosure. 
     It may be clearly understood by a person skilled in the art that, for the purpose of convenient and brief description, for a detailed working process of the foregoing system, apparatus, and unit, reference may be made to a corresponding process in the foregoing method embodiments, and details are not described herein. 
     In the several embodiments provided in the present application, it should be understood that the disclosed apparatus and method may be implemented in other manners. For example, the described device embodiment is merely exemplary. For example, the unit division is merely logical function division and may be other division in actual implementation. For example, a plurality of units or components may be combined or integrated into another system, or some features may be ignored or not performed. In addition, the displayed or discussed mutual couplings or direct couplings or communication connections may be implemented by using some interfaces. The indirect couplings or communication connections between the apparatuses or units may be implemented in electronic, mechanical, or other forms. 
     In addition, functional units in the devices and systems in the embodiments of the present disclosure may be integrated into one processing unit, or each of the units may exist alone physically, or two or more units are integrated into one unit. The units may be implemented in a form of hardware, or may be implemented in a form of hardware in addition to a software functional unit. 
     All or some of the steps of the method embodiments may be implemented by a program instructing relevant hardware. The program may be stored in a computer readable storage medium. When the program runs, the steps of the method embodiments are performed. The foregoing storage medium includes: any medium that can store program code, such as a USB flash drive, a removable hard disk, a read-only memory (Read Only Memory, ROM for short), a random access memory (Random Access Memory, RAM for short), a magnetic disk, or an optical disc. 
     The foregoing descriptions are merely specific implementation manners of the present disclosure, but are not intended to limit the protection scope of the present disclosure. Any variation or replacement readily figured out by a person skilled in the art within the technical scope disclosed in the present disclosure shall fall within the protection scope of the present disclosure. Therefore, the protection scope of the present disclosure shall be subject to the protection scope of the claims.