Patent Publication Number: US-8976774-B2

Title: Radio link performance prediction in wireless communication terminal

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is related to U.S. Application No. 61/112,515 filed on 7 Nov. 2008, the contents of which are hereby incorporated by reference and from which benefits are claimed under 35 U.S.C. 119. 
    
    
     FIELD OF THE DISCLOSURE 
     The present invention relates to a wireless communications and more particularly to predicting radio link performance based on a predicted decoder error rate in a wireless communication terminal. 
     BACKGROUND 
     In some wireless communication systems, the decoding of an information-bearing codeword (CW 2 ) requires that another information-bearing codeword (CW 1 ) be decoded correctly. CW 2  may thus be denoted by the term “compound codeword” as decoding of CW 1  is essential for the correct decoding of CW 2 . For example, CW 1  could contain information regarding transmission parameters used in transmitting CW 2  which are essential for the decoding of CW 2 . The transmission parameters may include the number of OFDM symbols on which CW 2  is transmitted, or the time-frequency sub-carrier mapping used for carrying CW 2 , (e.g., start and range of resource elements in the time-frequency grid on to which the codeword is mapped), or coding scheme (e.g., block code, convolutional code, turbo-code, etc.), or a code-rate, or block size, or encoded information bit length, or modulation type, or a redundancy version number of the codeword in a hybrid ARQ transmission using incremental redundancy, or transmit antenna type (e.g., SIMO, Tx diversity, spatial multiplexing, etc.), or the precoding used, or the transmission rank, etc. 
     CW 1  and CW 2  may correspond to a block code (linear or otherwise) or a convolution code or a turbo-code or an uncoded transmission. Generally, a receiver decodes CW 1  first and then tries to decode to CW 2 . Suppose a receiver wants to predict the practical decoder performance of CW 2 , then it has to jointly consider this with the fact that decoding of CW 1  can be erroneous. In E-UTRA standard, one application of the above method is for obtaining an estimate of overall error probability of PDCCH. In this example, CW 1  corresponds to a physical control formatting indicator channel (PCFICH) which contains information about the PDCCH codeword transmission parameters like the number of OFDM symbol containing control information in the subframe under different deployment configurations as specified in Table 6.7-1 of 36.211 and Table 5.3.4-1 of 36.212 reproduced below: 
     
       
         
           
               
             
               
                 TABLE 6.7-1 
               
             
            
               
                   
               
               
                 Number of OFDM symbols used for PDCCH 
               
            
           
           
               
               
               
            
               
                   
                 Number of OFDM 
                 Number of OFDM 
               
               
                   
                 symbols for PDCCH 
                 symbols for PDCCH 
               
               
                 Subframe 
                 when N RB   DL  &gt; 10 
                 when N RB   DL  ≦ 10 
               
               
                   
               
               
                 Subframe 1 and 6 for frame 
                 1, 2 
                 2 
               
               
                 structure type 2 
               
               
                 MBSFN subframes on a 
                 1, 2 
                 2 
               
               
                 carrier supporting both 
               
               
                 PMCH and PDSCH for 1 
               
               
                 or 2 cell specificc 
               
               
                 antenna ports 
               
               
                 MBSFN subframes on a 
                 2 
                 2 
               
               
                 carrier supporting both 
               
               
                 PMCH and PDSCH for 4 
               
               
                 cell specific antenna 
               
               
                 ports 
               
               
                 MBSFN subframes on a 
                 0 
                 0 
               
               
                 carrier not supporting 
               
               
                 PDSCH 
               
               
                 All other cases 
                 1, 2, 3 
                 2, 3, 4 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 5.3.4-1 
               
             
            
               
                   
               
               
                 CFI Codewords 
               
            
           
           
               
               
               
            
               
                   
                   
                 CFI codeword 
               
               
                   
                 CFI 
                 &lt;b 0 , b 1 , . . . , b 31 &gt; 
               
               
                   
                   
               
               
                   
                 1 
                 &lt;0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 
               
               
                   
                   
                 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1&gt; 
               
               
                   
                 2 
                 &lt;1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 
               
               
                   
                   
                 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0&gt; 
               
               
                   
                 3 
                 &lt;1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 
               
               
                   
                   
                 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1&gt; 
               
               
                   
                 4 
                 &lt;0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 
               
               
                   
                 (Reserved) 
                 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0&gt; 
               
               
                   
                   
               
            
           
         
       
     
     CW 2  corresponds to a physical downlink control CH (PDCCH) codeword. The correct decoding of PCFICH is necessary for correctly decoding the PDCCH codeword. The channel state information corresponding to the PCFICH transmission can be used to estimate the block error rate using a mapping function that uses the subcarrier level SINR information. Another mapping function that uses the subcarrier level SINR information can be used to obtain the conditional probability of error in decoding the PDCCH under the assumption that PCFICH has been decoded correctly. 
     In another example, in an E-UTRA link, suppose that a physical downlink shared channel (PDSCH) codeword is scheduled by DCI information embedded in a PDCCH codeword. Then correct decoding of the PDSCH codeword is dependent on correct decoding of both PDCCH that contains scheduling information and the PCFICH codeword. 
     Methods for estimating BLER corresponding to a coded packet transmission from the subcarrier SINR information in an OFDM system are known generally. Two of the well-known methods, effective exponential-sum-of-SINR mapping (EESM) and mean mutual information per bit (MMIB) mapping, use the principle that the average BLER function corresponding to a packet transmission with a fixed set of parameters such as encoding type, codeword length, information size (or alternately code rate), modulation type, etc. can expressed in terms of basis functions of the appropriate type. A third method is to map instead the first few moments of the sample sub-carrier SINR distribution to BLER. The EESM, MMIB and the third approach are listed below as applied to OFDM systems. 
     Suppose that two codewords CW 1  and CW 2  are transmitted. Correct decoding of CW 1  is necessary for the correct decoding of CW 2  as transmission parameters associated with CW 2  are embedded in CW 1 . Now, suppose that a receiver wants to estimate the block error rate of decoding CW 2 . The probability of correct decoding CW 2  conditioned on the correct decoding of CW 1  might be different from the probability of correct decoding of CW 2 . This can happen due to one of more of the following side conditions: 1. Difference in code-rates, block-sizes of the different codewords; 2. Coding schemes used for the encoding of the information embedded in the two codewords; and 3. Operating SINR-point, interference statistics, etc. In the prior art, the problem of predicting the block error rate of a codeword when such dependencies exist has not been addressed. 
     The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a communication system. 
         FIG. 2  illustrates a possible configuration of a computing system to act as a base station. 
         FIG. 3  illustrates in a UE block diagram. 
         FIG. 4  is a process flowchart. 
     
    
    
     DETAILED DESCRIPTION 
     In the present disclosure, the block error rate (BLER) of codeword  2  (CW 2 ) can be estimated from BLER of codeword  1  (CW 1 ) and the conditional error probability of decoding CW 2  upon correct decoding of CW 1 . This would provide for better estimates of the error rate of CW 2  than that achievable using an estimator that uses the conditional error probability of decoding CW 2  upon correct decoding of CW 1 . 
       FIG. 1  illustrates a communication system  100  including a communications network  102  comprising a base station  104  and user equipment (UE)  106 . Various communication devices may exchange data or information through the network. The network may be an evolved universal terrestrial radio access (E-UTRA) or other type of telecommunication network. In one embodiment, the base station may be a distributed set of servers in the network. The UE  106  may be one of several types of handheld or mobile devices, such as, a mobile phone, a laptop, or a personal digital assistant (PDA). In one embodiment, the UE  106  may also be a WIFI capable device, a WIMAX capable device, or other wireless devices. 
       FIG. 2  illustrates a possible configuration of a computing system to act as a base station comprising a controller/processor  210 , a memory  220 , a database interface  230 , a transceiver  240 , input/output (I/O) device interface  250 , and a network interface  260 , connected through bus  270 . The base station may implement any operating system, such as Microsoft Windows®, UNIX, or LINUX, for example. Client and server software may be written in any programming language, such as C, C++, Java or Visual Basic, for example. The server software may run on an application framework, such as, for example, a Java® server or .NET® framework. 
     In  FIG. 2 , the controller/processor  210  may be any programmable processor. The subject of the disclosure may also be implemented on a general-purpose or a special purpose computer, a programmed microprocessor or microcontroller, peripheral integrated circuit elements, an application-specific integrated circuit or other integrated circuits, hardware/electronic logic circuits, such as a discrete element circuit, a programmable logic device, such as a programmable logic array, field programmable gate-array, or the like. In general, any device or devices capable of implementing the decision support method as described herein may be used to implement the decision support system functions of this invention. 
     In  FIG. 2 , the memory  220  may include volatile and nonvolatile data storage, including one or more electrical, magnetic or optical memories such as a random access memory (RAM), cache, hard drive, or other memory device. The memory may have a cache to speed access to specific data. The memory  220  may also be connected to a compact disc-read only memory (CD-ROM), digital video disc-read only memory (DVD-ROM), DVD read write input, tape drive, or other removable memory device that allows media content to be directly uploaded into the system. 
     Data may be stored in the memory or in a separate database. In  FIG. 2 , the database interface  230  may be used by the controller/processor  210  to access the database. The database may contain any formatting data to connect the UE to the network. The transceiver  240  may create a data connection with the UE. The I/O device interface  250  may be connected to one or more input devices that may include a keyboard, mouse, pen-operated touch screen or monitor, voice-recognition device, or any other device that accepts input. The I/O device interface  250  may also be connected to one or more output devices, such as a monitor, printer, disk drive, speakers, or any other device provided to output data. The I/O device interface  250  may receive a data task or connection criteria from a network administrator. 
     The network connection interface  260  may be connected to a communication device, modem, network interface card, a transceiver, or any other device capable of transmitting and receiving signals from the network. The network connection interface  260  may be used to connect a client device to a network. The network connection interface  260  may be used to connect the teleconference device to the network connecting the user to other users in the teleconference. The components of the base station may be connected via an electrical bus  270 , for example, or linked wirelessly. 
     Client software and databases may be accessed by the controller/processor  210  from memory  220 , and may include, for example, database applications, word processing applications, as well as components that embody the decision support functionality of the present invention. The base station may implement any operating system, such as Microsoft Windows®, LINUX, or UNIX, for example. Client and server software may be written in any programming language, such as C, C++, Java or Visual Basic, for example. Although not required, the invention is described, at least in part, in the general context of computer-executable instructions, such as program modules, being executed by the electronic device, such as a general purpose computer. Generally, program modules include routine programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Moreover, those skilled in the art will appreciate that other embodiments of the invention may be practiced in network computing environments with many types of computer system configurations, including personal computers, hand-held devices, multi-processor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, and the like. 
       FIG. 3  illustrates in a block diagram one embodiment of a telecommunication apparatus or electronic device configured as the UE. The UE comprises a transceiver  302 , which is capable of sending and receiving data over the network  102 . The UE includes a processor  304  that executes stored programs. The UE may also include a volatile memory  306  and a non-volatile memory  308  which are used by the processor  304 . The UE may include a user input interface  310  that may comprise elements such as a keypad, display, touch screen, and the like. The UE also typically includes a user output device that may comprise a display screen and an audio interface  312  that may comprise elements such as a microphone, earphone, and speaker. The UE also may include a component interface  314  to which additional elements may be attached, for example, a universal serial bus (USB) interface, and a power supply  316 . 
     Consider a transmission in a wireless system such that at least two codewords are part of the transmission, wherein one of the codewords (denoted as “primary codeword”) is decodable correctly only if one or more of the remaining codewords transmitted are decoded correctly. These other codewords contain some essential information on the transmission parameters used for the primary codeword. 
     The receiver needs to estimate the block error rate (BLER) associated with the decoding of the primary codeword. This can be accomplished by using the equations listed below. The receiver would use the channel state information to estimate the BLER of the primary codeword based on some error probabilities associated with the decoding of the codewords whose correct decoding is necessary for the correct decoding of the primary codeword. Alternatively, the receiver could use an estimate of the channel state information obtained from the reference signal or pilot transmission in addition to the estimate of the interference/noise statistics to estimate the BLER of the primary codeword based on some error probabilities associated with the decoding of the codewords whose correct decoding is necessary for the correct decoding of the primary codeword. The channel state information includes for example, an SINR profile, or interference statistics (variance), estimates of channel coefficients among other channel information. 
       FIG. 4  illustrates a flow diagram for a process in a wireless communication terminal for predicting the performance of a radio link. At  410 , the terminal hypothesizes a first codeword. At  430 , the terminal hypothesizes a second codeword including information associated with the first codeword. In one embodiment, the first codeword corresponds to a control channel and the second codeword corresponds to a control format indicator channel carrying information related to the transmission parameters of the control channel. In another embodiment, the first codeword corresponds to a data payload and the second codeword corresponds to a control channel necessary for determining transmission parameters and scheduling information of a data payload. 
     In one implementation, the information, in the second code word, associated with the first codeword is a transmission parameter corresponding to any one of: a number of symbols on which the first codeword is mapped, or time-frequency resources on which the first codeword is mapped, or an encoding method used for generating the first codeword, or an information size of a payload of the first codeword, or a block length of the first codeword, or a rate of the first codeword, or a redundancy version number of the first codeword, or a transmit antenna configuration used for the first codeword, or pre-coding used for the first codeword. 
     In  FIG. 4 , at  430 , the terminal obtains channel state information from a received signal. At  440 , the terminal estimates a decoder error rate of the first codeword under a condition that the second codeword may not be decoded correctly, wherein the decoder error rate is estimated using the channel state information. In some embodiments, the terminal determines a synchronization status of the radio link based upon the estimated decoder error rate of the first codeword. 
     In another embodiment, the terminal hypothesizes a third codeword including information associated with the second codeword. The decoder error rate of the first codeword is estimated under a condition that the second and third codewords may not be decoded correctly, wherein the decoder error rate is estimated using a mapping function that includes channel state information. In a particular implementation, the first codeword corresponds to a data payload and the second codeword corresponds to a control channel, wherein the information of the second codeword includes transmission parameters and scheduling information of the data payload, and the third codeword corresponds to a control format indicator channel wherein the information of the third codeword includes information related to a transmission parameters of the control channel. Here too, channel quality indication reports may be generated based on the estimated decoder error rate for the first codeword. The information in the third code word is a transmission parameter corresponding to any one of: a number of symbols on which the second codeword is mapped, or time-frequency resources on which the second codeword is mapped, or an encoding method used for generating the second codeword, or an information size of a payload of the second codeword, or a block length of the second codeword, or a rate of the second codeword, or a redundancy version number of the first codeword, or a transmit antenna configuration used for the second codeword or pre-coding used for the second codeword. 
     In one embodiment, suppose a receiver attempts decoding of CW 1  (and uses the embedded information) and then attempts to decode CW 2 . The probability of error in decoding of CW 2  can be written as,
 
 p   e (CW 2 )=1−(1− p   e (CW 1 ))(1− p   e (CW 2 |CW 1  correctly decoded)),
 
     where, p e  (CW 1 ) is the probability of decoding error associated with CW 1 , and p e (CW 2 |CW 1  correctly decoded) is the conditional probability of decoding of CW 2  is in error when CW 1  has been correctly decoded. 
     Suppose that the receiver estimates p e  (CW 2 ) based on channel state information. Then, using the channel state information, it can estimate p e  (CW 1 ) and p e (CW 2 |CW 1  correctly decoded) and then use the above equation to estimate p e  (CW 2 ). 
     This concept can be generalized to estimating the BLER of a codeword CWn, whose decoding is conditional on the correct decoding of several other codewords CW 1 , CW 2 , . . . , CW(n−1). The probability of decoding error of CWn can be expressed as,
 
 p   e (CW n )=1−(1− p   e (CW 1 | . . . |CW n-1 ))(1− p   e (CW n |CW 1 , . . . , CW n-1  correctly decoded)),
 
     where p e (CW 1 | . . . |CW n-1 ) is the probability that any of the codewords CW 1 , CW 2 , . . . , CW(n−1) has been decoded incorrectly, and p e (CW n |CW 1 , . . . , CW n-1  correctly decoded) is the probability of correct decoding of CWn given that CW 1 , . . . , CW(n−1) have been decoded correctly. The interdependencies between the codewords CW 1 , . . . , CWn can be used to further simplify the above equation. 
     In a second embodiment, a method is presented for estimating the BLER of the compound codeword directly from the channel state information corresponding to the time-frequency resources onto which the two codewords are mapped. First, we list some methods for the two codeword case (i.e., decoding of one codeword is conditioned on the correct decoding of one other codeword) and then list methods for the multiple codeword case (i.e., decoding of one codeword is conditioned on the correct decoding of two or more other codewords). 
     Two codeword case: Suppose a receiver attempts to decode CW 1  (and uses the embedded information) and then attempts to decode CW 2 . Alternately, the two sets of subcarrier information {γ k } k−1   N     1    and {η k } k=1   N     2    can be used to obtain the relevant metrics. By modifying either of the three methods, Effective Exponential Sum of SINR Mapping (EESM) approach, Mean Mutual Information per Bit (MMIB) approach or SINR moments approach, we can jointly obtain the overall PDCCH BLER as follows. 
     EESM approach. Suppose that CW is the transmitted codeword and p e  (CW) represents the probability that CW is decoded in error. Then, in the EESM method, a function ƒ eesm (●) maps the effective SNR defined as 
                 γ   eff     =       -   β     ⁢           ⁢     ln   (       1   N     ⁢       ∑     k   =   1     N     ⁢     ⅇ       -     γ   k       /   β           )         ,         
where N is the codeword length, β is a parameter that is derived using a suitable curve-fitting criterion (eg. min-max, least-squares, etc.), and {γ k } k−1   N  is the bitwise SINR obtained from the subcarrier SINR information for the encoded bits. The map ƒ eesm (●) is calibrated using simulations and subsequently, the approximation p e (CW)≈ƒ eesm (γ eff ) can be used for estimating the BLER using the subcarrier SINR information. A modified EESM approach is as follows:
 
               γ   eff     =       -     β   1       ⁢           ⁢     ln   (       1     N   1       ⁢       ∑     k   =   1       N   1       ⁢     ⅇ       -     γ   k       /     β   1             )             
and
 
               η   eff     =       -     β   2       ⁢           ⁢     ln   (       1     N   2       ⁢       ∑     k   =   1       N   2       ⁢     ⅇ       -     η   k       /     β   2             )             
can be used jointly in the map p e  (CW 2 )≈g eesm (γ eff , η eff ), where g eesm (●) is a joint map that is calibrated in simulations.
 
     MMIB approach. In the MMIB approach, generally, the mutual information per bit function is expressed in terms of J-functions as the basis function set. The J-functions are modulation-dependent (e.g., QPSK, 16QAM, etc.) and map the bitwise SINR to a mutual information per bit metric. The average mutual information metric is then mapped to BLER, and mapping function ƒ mmib (●) is calibrated using simulations. Subsequently, the approximation p e (CW)≈ƒ mmib (I mean ), where I mean  is the mean of bitwise mutual information metric, can be used for estimating the BLER. A modified MMIB approach is as follows: I mean   (1)  and I mean   (2)  correspond to the MMIB metrics for CW 1  and CW 2  respectively, say, derived from {γ k } k=1   N     1    and {η k } k−1   N     2   . A joint map g mmib (●) can be calibrated to estimate the overall CW 2  BLER, using the approximation p e (CW 2 )≈g mmib (I mmib   (1) , I mmib   (2) ). 
     SINR moment approach: Alternately, the BLER can be estimated using the first few moments of the subcarrier SINR profile {γ k } k=1   N  as the input. Suppose  γ ,  γ 2   ,  γ 3   , . . . , etc. denote the first, second, third and higher central moments of SINR sequence {γ k } k=1   N , defined as 
                 γ   n     _     =       1   N     ⁢       ∑     k   =   1     N     ⁢              γ   k          n     .               
Then, a mapping function ƒ sinr (●) can be calibrated such that BLER is estimated using the expression p e (CW)≈ƒ sinr (  γ ,  γ 2   ,  γ 3   , . . . ). The modified SINR moments approach is as follows: The first few SINR moments corresponding to CW 1  and CW 2 ,  γ ,  γ 2   ,  γ 3   , . . . and  η ,  η 2   ,  η 3   , . . . derived from {γ k } k=1   N     1    and {η k } k=1   N     2    can be used to calibrate a joint map g sinr (●) to estimate the overall CW 2  BLER using the approximation p e (CW 2 )≈ƒ sinr (  γ ,  γ 2   ,  γ e   , . . . ;  η ,  η 2   ,  η 3 , . . .  ).
 
     Multiple codeword case: The mapping approaches above can be generalized to the case when BLER needs to be estimated for a codeword CWn, whose decoding is conditional on correctly decoding of several other codewords CW 1 , CW 2 , . . . , CW(n−1). The joint mapping function can be constructed for the three methods as below. 
     EESM approach: 
     p e (CW n )≈g eesm (γ eff   (1) , γ eff   (2) , . . . , γ eff   (n-1) ), where γ eff   (k)  corresponds to the effective SNR for codeword CWk. 
     MMIB approach: 
     p e (CW n )≈g mmib (I mmib   (1) , I mmib   (2) , . . . , I mmib   (n-1) ), where I mmib   (k)  corresponds to the mean mutual information per bit for codeword CWk. 
     SINR moments approach: 
     p e (CW 2 )≈ƒ sinr (  γ 1   ,  γ 1   2   ,  γ 1   3   , . . . ;  γ 2   ,  γ 2   2   ,  γ 2   3   , . . . ; . . . ;  γ   n-1 ,  γ n-1   2   ,  γ n-1   3   ), where  γ   k ,  γ k   2   ,  γ k   3   , . . . correspond to the SINR moments for subcarriers carrying codeword CWk. 
     While the present disclosure and the best modes thereof have been described in a manner establishing possession by the inventors and enabling those of ordinary skill to make and use the same, it will be understood that there are equivalents to the exemplary embodiments disclosed herein and that modifications and variations may be made thereto without departing from the scope and spirit of the inventions, which are to be limited not by the exemplary embodiments but by the appended claims.