Patent Publication Number: US-8532201-B2

Title: Methods and apparatus for identifying a preamble sequence and for estimating an integer carrier frequency offset

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to wireless communication systems. More specifically, the present disclosure relates to methods and apparatus for identifying a preamble sequence and for estimating an integer carrier frequency offset in a wireless communication system. 
     BACKGROUND 
     Wireless communication devices have become smaller and more powerful in order to meet consumer needs and to improve portability and convenience. Consumers have become dependent upon wireless communication devices such as cellular telephones, personal digital assistants (PDAs), laptop computers, and the like. Consumers have come to expect reliable service, expanded areas of coverage, and increased functionality. Wireless communication devices may be referred to as mobile stations, stations, access terminals, user terminals, terminals, subscriber units, user equipment, etc. 
     A wireless communication system may simultaneously support communication for multiple wireless communication devices. A wireless communication device may communicate with one or more base stations (which may alternatively be referred to as access points, Node Bs, etc.) via transmissions on the uplink and the downlink. The uplink (or reverse link) refers to the communication link from the wireless communication devices to the base stations, and the downlink (or forward link) refers to the communication link from the base stations to the wireless communication devices. 
     Wireless communication systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, and orthogonal frequency division multiple access (OFDMA) systems. 
     As indicated above, the present disclosure relates generally to wireless communication systems. More specifically, the present disclosure relates to methods and apparatus for identifying a preamble sequence and for estimating an integer carrier frequency offset in a wireless communication system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example of a wireless communication system; 
         FIG. 2  illustrates an example of a transmitter and an example of a receiver for an OFDM/OFDMA system; 
         FIGS. 3A through 3D  illustrate an example of a frame structure for an OFDM/OFDMA system; 
         FIG. 4  illustrates an example of an OFDM/OFDMA receiver that is configured to identify a preamble sequence and to estimate an integer carrier frequency offset (CFO); 
         FIGS. 5A and 5B  illustrate examples of preamble sequences that may be defined for an OFDM/OFDMA system; 
         FIG. 5C  shows a frequency domain representation of a downlink preamble for an IEEE802.16e OFDM/OFDMA system; 
         FIG. 6  illustrates another example of an OFDM/OFDMA receiver that is configured to identify a preamble sequence and to estimate an integer carrier frequency offset (CFO); 
         FIG. 7  illustrates a method for identifying a preamble sequence and for estimating an integer CFO; 
         FIG. 8  illustrates means-plus-function blocks corresponding to the method shown in  FIG. 7 ; 
         FIG. 9  illustrates an example of a virtual segment table; and 
         FIG. 10  illustrates various components that may be utilized in a wireless device. 
     
    
    
     SUMMARY 
     A method for identifying a preamble sequence and for estimating an integer carrier frequency offset is disclosed. The method may include determining a reduced set of integer carrier frequency offset (CFO) candidates corresponding to a received signal that includes a preamble sequence from a set of possible preamble sequences. The method may also include performing correlation operations with respect to the received signal and multiple candidate transmitted signals. Each candidate transmitted signal may include one of the set of possible preamble sequences. Each candidate transmitted signal may correspond to one of the reduced set of integer CFO candidates. Correlation values may be determined as a result of the correlation operations. The method may also include using the correlation values to identify the preamble sequence and to estimate the integer CFO. 
     A wireless device that is configured to identify a preamble sequence and to estimate an integer carrier frequency offset is also disclosed. The wireless device may include a processor and memory in electronic communication with the processor. Instructions may be stored in the memory. The instructions may be executable to determine a reduced set of integer carrier frequency offset (CFO) candidates corresponding to a received signal that includes a preamble sequence from a set of possible preamble sequences. The instructions may also be executable to perform correlation operations with respect to the received signal and multiple candidate transmitted signals. Each candidate transmitted signal may include one of the set of possible preamble sequences. Each candidate transmitted signal may correspond to one of the reduced set of integer CFO candidates. Correlation values may be determined as a result of the correlation operations. The instructions may also be executable to use the correlation values to identify the preamble sequence and to estimate the integer CFO. 
     An apparatus that is configured to identify a preamble sequence and to estimate an integer carrier frequency offset is also disclosed. The apparatus may include means for determining a reduced set of integer carrier frequency offset (CFO) candidates corresponding to a received signal that includes a preamble sequence from a set of possible preamble sequences. The apparatus may also include means for performing correlation operations with respect to the received signal and multiple candidate transmitted signals. Each candidate transmitted signal may include one of the set of possible preamble sequences. Each candidate transmitted signal may correspond to one of the reduced set of integer CFO candidates. Correlation values may be determined as a result of the correlation operations. The apparatus may also include means for using the correlation values to identify the preamble sequence and to estimate the integer CFO. 
     A computer-program product for identifying a preamble sequence and for estimating an integer carrier frequency offset is also disclosed. The computer-program product includes a computer readable medium having instructions thereon. The instructions may include code for determining a reduced set of integer carrier frequency offset (CFO) candidates corresponding to a received signal that includes a preamble sequence from a set of possible preamble sequences. The instructions may also include code for performing correlation operations with respect to the received signal and multiple candidate transmitted signals. Each candidate transmitted signal may include one of the set of possible preamble sequences. Each candidate transmitted signal may correspond to one of the reduced set of integer CFO candidates. Correlation values may be determined as a result of the correlation operations. The instructions may also include code for using the correlation values to identify the preamble sequence and to estimate the integer CFO. 
     DETAILED DESCRIPTION 
     The methods and apparatus of the present disclosure may be utilized in a broadband wireless communication system. The term “broadband wireless” refers to technology that provides high-speed wireless, voice, Internet, and data network access over a wide area. 
     WiMAX, which stands for the Worldwide Interoperability for Microwave Access, is a standards-based broadband wireless technology that provides high-throughput broadband connections over long distances. There are two main applications of WiMAX today: fixed WiMAX and mobile WiMAX. Fixed WiMAX applications are point-to-multipoint enabling broadband access to homes and businesses. Mobile WiMAX offers the full mobility of cellular networks at broadband speeds. 
     Mobile WiMAX is based on OFDM (orthogonal frequency division multiplexing) and OFDMA (orthogonal frequency division multiple access) technology. OFDM is a digital multi-carrier modulation technique that has recently found wide adoption in a variety of high-data-rate communication systems. With OFDM, a transmit bit stream is divided into multiple lower-rate sub-streams. Each sub-stream is modulated with one of multiple orthogonal sub-carriers and sent over one of a plurality of parallel sub-channels. OFDMA is a multiple access technique in which users are assigned sub-carriers in different time slots. OFDMA is a flexible multiple-access technique that can accommodate many users with widely varying applications, data rates, and quality of service requirements. 
     IEEE 802.16x is an emerging standard organization to define an air interface for fixed and mobile broadband wireless access (BWA) systems. IEEE 802.16x approved “IEEE P802.16-REVd/D5-2004” in May 2004 for fixed BWA systems and published “IEEE P802.16e/D12 October 2005” in October 2005 for mobile BWA systems. Those two standards defined four different physical layers (PHYs) and one medium access control (MAC) layer. The OFDM and OFDMA PHY of the four PHYs are the most popular in the fixed and mobile BWA areas respectively. 
     Certain aspects of the present disclosure will be described in relation to BWA systems based on OFDM/OFDMA technology. However, the scope of the present disclosure is not limited to such systems. The methods and apparatus disclosed herein may be utilized in other types of wireless communication systems. 
       FIG. 1  illustrates an example of a wireless communication system  100 . The wireless communication system  100  may be a broadband wireless communication system  100 . The wireless communication system  100  provides communication for a number of cells  102 , each of which is serviced by a base station  104 . A base station  104  may be a fixed station that communicates with remote stations  106 . The base station  104  may alternatively be referred to as an access point, a Node B, or some other terminology. 
       FIG. 1  shows various remote stations  106  dispersed throughout the system  100 . The remote stations  106  may be fixed (i.e., stationary) or mobile. The remote stations  106  may alternatively be referred to as user terminals, access terminals, terminals, subscriber units, mobile stations, stations, etc. The remote stations  106  may be wireless devices, cellular phones, personal digital assistants (PDAs), handheld devices, wireless modems, laptop computers, personal computers, etc. 
     A variety of algorithms and methods may be used for transmissions in the wireless communication system  100  between the base stations  104  and the remote stations  106 . For example, signals may be sent and received between the base stations  104  and the remote stations  106  in accordance with OFDM/OFDMA techniques. If this is the case, the wireless communication system  100  may be referred to as an OFDM/OFDMA system  100 . 
     A communication link that facilitates transmission from a base station  104  to a remote station  106  may be referred to as a downlink  108 , and a communication link that facilitates transmission from a remote station  106  to a base station  104  may be referred to as an uplink  110 . Alternatively, a downlink  108  may be referred to as a forward link or a forward channel, and an uplink  110  may be referred to as a reverse link or a reverse channel. 
     A cell  102  may be divided into multiple sectors  112 . A sector  112  is a physical coverage area within a cell  102 . Base stations  104  within an OFDM/OFDMA system  100  may utilize antennas that concentrate the flow of power within a particular sector  112  of the cell  102 . Such antennas may be referred to as directional antennas. 
       FIG. 2  illustrates an example of a transmitter  202  for an OFDM/OFDMA system  100 . The transmitter  202  may be implemented in a base station  104 , for transmitting data to a remote station  106  on a downlink  108 . The transmitter  202  may also be implemented in a remote station  106 , for transmitting data to a base station  104  on an uplink  110 . 
     Data  206  to be transmitted is shown being provided as input to a serial-to-parallel (S/P) converter  208 . The S/P converter  208  splits the transmission data into N parallel data streams  210 . 
     The N parallel data streams  210  may then be provided as input to a mapper  212 . The mapper  212  maps the N parallel data streams  210  onto N constellation points. The mapping may be done using some modulation constellation, such as binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), 8 phase-shift keying (8PSK), quadrature amplitude modulation (QAM), etc. Thus, the mapper  212  outputs N parallel symbol streams  216 , each symbol stream  216  corresponding to one of the N orthogonal sub-carriers. These N parallel symbol streams  216  are represented in the frequency domain, and may be converted into N parallel time domain sample streams  218  by an inverse fast Fourier transform (IFFT) component  220 . 
     The N parallel time domain sample streams  218  may be converted into a serial stream of OFDM/OFDMA symbols  222  by a parallel-to-serial (P/S) converter  224 . A guard insertion component  226  may insert a guard interval between successive OFDM/OFDMA symbols in the OFDM/OFDMA symbol stream  222 . The output of the guard insertion component  226  may then be upconverted to a desired transmit frequency band by a radio frequency (RF) front end  228 . An antenna  230  may then transmit the resulting signal  232 . 
       FIG. 2  also illustrates an example of a receiver  204  for an OFDM/OFDMA system  100 . The receiver  204  may be implemented in a remote station  106 , for receiving data from a base station  104  on a downlink  108 . The receiver  204  may also be implemented in a base station  104 , for receiving data from a remote station  106  on an uplink  110 . 
     The transmitted signal  232  is shown traveling over a wireless channel  234 . When a signal  232 ′ is received by an antenna  230 ′, the received signal  232 ′ may be downconverted to a baseband signal by an RF front end  228 ′. A guard removal component  226 ′ may then remove the guard interval that was inserted between OFDM/OFDMA symbols by the transmitter  202 . 
     The output of the guard removal component  226 ′ may be provided to an S/P converter  224 ′. The S/P converter  224 ′ may divide the OFDM/OFDMA symbol stream  222 ′ into the N parallel time-domain sample streams  218 ′. A fast Fourier transform (FFT) component  220 ′ converts the N parallel time-domain sample streams  218 ′ into the frequency domain, and outputs N parallel frequency-domain (modulation) symbol streams  216 ′. 
     A demapper  212 ′ performs the inverse of the symbol mapping operation that was performed by the mapper  212 , thereby outputting N parallel data streams  210 ′. A P/S converter  208 ′ combines the N parallel data streams  210 ′ into a single data stream  206 ′. Ideally, this data stream  206 ′ corresponds to the data  206  that was provided as input to the transmitter  202 . 
       FIGS. 3A through 3D  illustrate an example of a frame structure for an OFDM/OFDMA system  100 . Referring initially to  FIG. 3A , an OFDM/OFDMA frame  306  is shown with respect to a time axis  308 . The OFDM/OFDMA frame  306  may be transmitted from a base station  104  to a remote station  106  on a downlink  108 . 
     The OFDM/OFDMA frame  306  is shown with one preamble symbol  310  and multiple data symbols  312 . Although just one preamble symbol  310  is shown in  FIG. 3A , an OFDM/OFDMA frame  306  may include multiple preamble symbols  310 . 
       FIGS. 3B and 3C  illustrate examples of frequency domain representations of a preamble symbol  310 . These frequency domain representations are shown with respect to a sub-carrier axis  316 . A used sub-carrier region  318  is shown. Two guard regions  320  are also shown. 
     In  FIG. 3B , the used sub-carrier region  318  includes pilot sub-carriers  314   a  alternated with unmodulated sub-carriers  314   b . In  FIG. 3C , each sub-carrier in the used sub-carrier region  318  is a pilot sub-carrier  314   a.    
       FIG. 3D  illustrates an example of a frequency domain representation of a data symbol  312 . The data symbol  312  includes both data sub-carriers  314   c  and pilot sub-carriers  314   a . A receiver  204  may perform channel estimation using pilot sub-carriers  314   a  of a preamble symbol  310  and/or pilot sub-carriers  314   a  of a data symbol  312 . 
       FIG. 4  illustrates an OFDM/OFDMA receiver  404  that is configured to identify a preamble sequence  406  and to estimate an integer carrier frequency offset (CFO)  408 . The receiver  404  may be implemented in a remote station  106  in an OFDM/OFDMA system  100 . In addition to the components that are shown in  FIG. 4 , the receiver  404  may also include the components that are shown in connection with the OFDM/OFDMA receiver  204  of  FIG. 2 . 
     The receiver  404  is shown receiving a signal  432  that was transmitted by an OFDM/OFDMA transmitter  202 . The received signal  432  includes a preamble sequence  406 . The received signal  432  is shown being processed by the OFDM/OFDMA receiver  404  for purposes of preamble sequence identification, integer carrier frequency offset (CFO) estimation, and segment identification. The receiver  404  is shown with a preamble sequence identification component  416 , an integer CFO estimation component  418 , and a segment identification component  420 . 
     Multiple preamble sequences  406  may be defined for an OFDM/OFDMA system  100 . Preamble sequence identification is the process of determining which preamble sequence  406 , out of all possible preamble sequences  406 , is included in the received signal  432 . 
     Carrier frequency offset (CFO) refers to the difference in frequency between the sub-carriers of the receiver  404  and the sub-carriers of the transmitter  202 . Integer CFO estimation is the process of estimating the integer CFO  408 . Integer CFO estimation may be performed in order to improve the performance of the receiver  204 . 
     Each preamble sequence  406  that is defined for an OFDM/OFDMA system  100  may be associated with a segment  410 . Segment identification is the process of determining which segment  410  the preamble sequence  406  is associated with. 
     A segment  410  may correspond to a sector  112 . For example in the case of a three sector-based network configuration, BS 0  (sector  0 ) may use segment  0 , BS 1  (sector  1 ) may use segment  1  and BS 2  (sector  2 ) may use segment  2 . 
     Preamble sequence identification, integer CFO estimation, and segment identification may be performed in a “cold start” situation, i.e., a situation where a remote station  106  is powered on but the remote station  106  has not yet associated with a segment  410  of a base station  104 . In order to associate with a segment  410  of a base station  104 , a remote station  106  may attempt to detect a specific preamble sequence  406  in a signal  432  that is transmitted by the base station  104  and received by the remote station  106 . Preamble sequence identification, integer CFO estimation, and segment identification may be performed concurrently. 
       FIGS. 5A and 5B  illustrate examples of preamble sequences  506   a ,  506   b  that may be defined for an OFDM/OFDMA system  100 . These preamble sequences  506   a ,  506   b  are defined in the standard specification for an IEEE.16e OFDM/OFDMA system  100 . The preamble sequences  506   a  shown in  FIG. 5A  correspond to an OFDM/OFDMA system that uses 1024 sub-carriers. The preamble sequences  506   b  shown in  FIG. 5B  correspond to an OFDM/OFDMA system that uses 512 sub-carriers. 
     Each preamble sequence  506   a ,  506   b  is associated with a segment  510   a ,  510   b . Each preamble sequence  506   a ,  506   b  is also associated with a cell  102 , which is identified by a cell identifier (IDcell)  512   a ,  512   b . Each preamble sequence  506   a ,  506   b  is also associated with an index  516   a ,  516   b , which may be referred to as a preamble index  516   a ,  516   b.    
     Different sets of sub-carriers  220  may be assigned to different segments  410 . As used herein, the term PA cset  may refer to the set of sub-carriers  220  that is assigned to segment s (where s=0, 1, or 2) for transmission of a signal  432  that includes a preamble sequence  406 . PA cset  may be given as:
 
 PA   cset   =s+ 3 z   (1)
 
     The term z represents a running index starting from 0 to M−1, where M is the length of the preamble sequence  406 . Thus, if the number of sub-carriers  220  is equal to 1024 (M=284), then the following sub-carriers  220  may be assigned to segment  0 :  0 ,  3 ,  6 ,  9 , . . . ,  849 . The following sub-carriers  220  may be assigned to segment  1 :  1 ,  4 ,  7 ,  10 , . . . ,  850 . The following sub-carriers  220  may be assigned to segment  2 :  2 ,  5 ,  8 ,  11 , . . . ,  851 . (In these numerical examples, the first sub-carrier in the used sub-carrier region  318  is designated sub-carrier  0 .) 
     A frequency offset index (FOI) based format of PA cset  may be defined as follows:
 
 i   s,m =convert_to_FOI_index_format( PA   cset ), m=1,2, . . . , M  (2)
 
     The term i s,m  is the m th  sub-carrier index (FOI based) of the preamble that is associated with segment s. The resulting preamble after assigning sub-carriers as described above is shown in  FIG. 5C . Assuming an N-point FFT (or IFFT), there are N sub-carriers from the first sub-carrier to the Nth sub-carrier. In FOI-based numbering, the first sub-carrier is associated with the lowest frequency, the Nth sub-carrier is associated with the highest frequency, and the DC sub-carrier is positioned in the center. 
     In the example of  FIG. 5C , the sub-carriers are numbered SC( 1 ) to SC(N). Alternatively, these sub-carriers may be numbered SC( 0 ) to SC(N−1). 
     As used herein, the term N pn  refers to the total number of preamble sequences  406  that are defined for a particular OFDM/OFDMA system  100 . The term N pnseg  refers to the total number of preamble sequences  406  that correspond to a specific segment  410 . The term N seg  refers to the number of segments  410 . The standard specification for an IEEE802.16e OFDM/OFDMA system  100  defines the following values for OFDM/OFDMA systems  100  that use 1024 sub-carriers: N pn =114, N pnseg =38, and N seg =3. 
     The set of preamble sequences  406  that are defined for a particular OFDM/OFDMA system  100  may be expressed as:
 
set of preamble sequences=[ PA   1   ,PA   2   , . . . ,PA   j   , . . . ,PA   N     pn   ]
 
 PA   j   ;j   th  preamble sequence
 
 j= 1,2 , . . . ,N   pn ; index of preamble sequence  (3)
 
     Each preamble sequence PA j  includes length M pseudo-noise (PN) codes. This is expressed in equation (4) below. As expressed in equation (5), each preamble sequence  406  has its own segment number&#39;s and sub-carrier set ‘i s,m ’ depending on the segment number.
 
 PA   j   =[c   1   ,c   2   , . . . ,c   m   , . . . ,c   M ]
 
 c   m   ;m   th  code of preamble sequence  (4)
 
 i   s,m ;FOI based index of segment(PA subcarrier set) s  
 
 m= 1,2 , . . . ,M  
 
 s= 0,1,2; segment(PA subcarrier set)  (5)
 
     For purposes of the present discussion, let X(k;j) be a frequency domain representation of a transmitted signal  232  that includes the j th  preamble sequence  406  from the set of all possible preamble sequences  406 . Let x(n;j) be the corresponding time domain signal of X(k;j). Let y(n;j) be the received signal  432 , in the time domain, corresponding to x(n;j). Let Y(k;j) be the corresponding frequency domain signal of y(n;j). For purposes of the present discussion, it will be assumed that X(k;j) and Y(k;j) are ordered in FOI (frequency offset index).
 
 X ( k;j )=preamble signal in frequency domain,  k= 1,2 , . . . ,N   (6)
 
 x ( n;j )= ifft{fftshif ( X ( k;j ))}, n= 1,2 , . . . ,N,k= 1,2 , . . . ,N   (7)
 
 y ( n;j )=received signal in time domain, n= 1,2 , . . . ,N=x ( n;j )* h ( n )+η( n )  (8)
 
 Y ( k;j )= fftshift ( fft ( y ( n;j ))), n= 1,2, . . . , Nk= 1,2 , . . . ,N   (9)
 
     In the case of the “cold start” situation described above, one approach for preamble sequence identification might be to search for the preamble sequences  406  for all possible integer CFO candidates. As indicated above, there may be a relatively large number of possible preamble sequences  406  (e.g., 114 possible preamble sequences in OFDM/OFDMA systems that utilize 1024 or 512 sub-carriers). For each preamble sequence  406 , 2×Z i  integer CFO candidates are possible, where Z i  is the maximum allowable integer CFO value. Thus, searching for the preamble sequences  406  for all possible integer CFO candidates may include a significant number of computations. 
     Both preamble sequence identification and integer CFO estimation may be done concurrently as the following cross correlation process: 
     
       
         
           
             
               
                 
                   
                     
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     In equation (10), the term Z i  is the maximum allowable integer CFO value, the term M is the length of a preamble sequence  406 , and the term i s,m  is the m th  sub-carrier index that is associated with segment s, in frequency offset index (FOI) format. 
     Using the above results, it may be possible to estimate the integer CFO  408  normalized by sub-carrier frequency spacing. It may also be possible to identify the preamble sequence  406  (or, more specifically, the preamble index  516   a ,  516   b  corresponding to the preamble sequence  406 ). This is shown in equations (11) through (14) below. Once the preamble sequence  406  is known, the segment  410  may also be extracted from the appropriate table of preamble sequences  406  (e.g., the tables shown in  FIGS. 5A and 5B ). 
                     [       z   c     ,     j   c       ]     =         arg   ⁢           ⁢   max       z   ,   j       ⁢     {          C   ⁡     (     z   ;   j     )            }               (   11   )               Δ f   int   N   =z   c   (12)
 
 J   PAindex   =j   c   (13)
 
 s= from  J   PAindex   (14)
 
     Equation (10) for determining the cross-correlation may not work properly in some environments where there is an imperfect symbol timing or channel effects. To mitigate effects of the phase rotation caused by channel or symbol timing offset, a partial correlation scheme may be used as follows: 
     
       
         
           
             
               
                 
                   
                     
                       
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                         ⁢ 
                         
                             
                         
                         ⁢ 
                         range 
                       
                     
                     ⁢ 
                     
                       
 
                     
                     ⁢ 
                     
                       
                         j 
                         = 
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                       , 
                       
                         
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                         ; 
                         
                           Possible 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           preamble 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           sequences 
                         
                       
                     
                     ⁢ 
                     
                       
 
                     
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                         ⁢ 
                         
                             
                         
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                         = 
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                     ⁢ 
                     
                       
 
                     
                     ⁢ 
                     
                       
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                         = 
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                       , 
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                       , 
                       
                         2 
                         ; 
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                     ⁢ 
                     
                       
 
                     
                     ⁢ 
                     B 
                     = 
                     
                       ceil 
                       ⁡ 
                       
                         ( 
                         
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                         ) 
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
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                       N 
                       b 
                     
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                       : 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     # 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     samples 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     of 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     a 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     partial 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     correlation 
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
     In equation (15), the term N b  is the number of samples of partial correlation. The term M is the length of a preamble sequence  406 . The term B is the number of partial correlation. The value of N b  may fall within the range of 4 to 16 for partial correlation. 
       FIG. 6  illustrates another OFDM/OFDMA receiver  604  that is configured to identify a preamble sequence  606  and to estimate an integer carrier frequency offset (CFO)  608 . The receiver  604  is an example of an implementation of the receiver  404  shown in  FIG. 4 . The receiver  604  may be implemented in a remote station  106  in an OFDM/OFDMA system  100 . 
     The receiver  604  is shown receiving a signal  632  that was transmitted by an OFDM/OFDMA transmitter  202 . In a cold start situation, the receiver  604  may initially perform signal detection and preamble detection with respect to the received signal  632 . Signal detection involves determining whether there is an incoming signal  632  or not, and preamble detection involves determining whether the incoming signal  632  includes a preamble sequence  606  or not. The receiver  604  is shown with a signal detection component  618  and a preamble detection component  620 . 
     After signal detection and preamble detection are performed, symbol boundary detection may be performed. Symbol boundary detection involves detecting the OFDM/OFDMA symbol boundary. The receiver  604  is shown with a symbol boundary detection component  622 . 
     Once signal detection, preamble detection, and symbol boundary detection are performed, then fractional carrier frequency offset (CFO) compensation may be performed in the time domain. The receiver  604  is shown with a fractional CFO compensation component  624 . 
     The output of the fractional CFO compensation component  624  may be converted from the time domain into the frequency domain. This may be performed by a fast Fourier transform (FFT) component  626 . The output of the FFT component  626  may be referred to as a processed received signal  628 . 
     As indicated above, the received signal  632  may include a preamble sequence  606 . Transmission of the preamble sequence  606  may have been achieved by modulating the preamble sequence  606  onto multiple orthogonal sub-carriers. The power of the sub-carriers may be determined in accordance with equation (16) below.
 
 P ( k )=| Y ( k )| 2   , k=K   min :1 :K   max  
 
 K   min =min( i   s,m=1 )− Z   i  
 
 K   max =max( i   s,m=M )+ Z   i  
 
 z=−Z   i :1: Z   i ; possible int eger CFO range  (16)
 
     The receiver  604  is shown with a power measurement component  630  that receives the processed received signal  628  as input, and that outputs power values  634  corresponding to the sub-carriers. The processed received signal  628  may correspond to Y(k) in equation (16). The power values  634  may correspond to P(k) in equation (16). 
     Various alternatives to equation (16) are possible. For example, to reduce complexity, only some of the samples may be used instead of all possible samples. As another example, instead of determining the power of the sub-carriers, the absolute value of the processed received signal  628  may be determined. 
     A virtual segment  636  may be determined based on the power values  634  of the sub-carriers. The virtual segment  636  indicates the offset position of the most active sub-carriers starting from K min  (as K min  is defined in equation (16) above). The virtual segment  636  may be determined in accordance with equations (17) and (18).
 
 P ( v )=sum( P ( K   min   +v: 3 :K   max ))
 
 v= 0,1,2; virtual segment  (17)
 
 v   s =arg max( P ( v )); decided virtual segment
 
     
       
         
           
             
               
                 
                   
                     
                       
                         v 
                         s 
                       
                       = 
                       
                         
                           
                             arg 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             max 
                           
                           v 
                         
                         ⁢ 
                         
                           ( 
                           
                             P 
                             ⁡ 
                             
                               ( 
                               v 
                               ) 
                             
                           
                           ) 
                         
                       
                     
                     ; 
                     
                       decided 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       virtual 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       segment 
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       v 
                       = 
                       0 
                     
                     , 
                     1 
                     , 
                     
                       2 
                       ; 
                       
                         virtual 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         segment 
                       
                     
                   
                 
               
               
                 
                   ( 
                   18 
                   ) 
                 
               
             
           
         
       
     
     The receiver  604  is shown with a virtual segment detection component  638  that receives the power values  634  as input, and that outputs the virtual segment  636 . The virtual segment  636  may correspond to v s  in equation (18). 
     A reduced set of integer CFO candidates  640  (i.e., a set of integer CFO candidates that is smaller than a full set of integer CFO candidates  642 ) may be determined. The reduced set of integer CFO candidates  640  may be determined based on the virtual segment  636  that is determined. A virtual segment table  644  may also be used to determine the reduced set of integer CFO candidates  640 . An example of a virtual segment table  644  is shown in  FIG. 9  and will be discussed below. 
     The receiver  604  is shown with a possible integer CFO extraction component  646 . The possible integer CFO extraction component  646  may be configured to determine the reduced set of integer CFO candidates  640  based on the virtual segment  636  that is determined, and also based on the virtual segment table  644 . 
     Cross-correlation operations may be performed with respect to the received signal  632  and multiple candidate transmitted signals  648 . Each candidate transmitted signal  648  may include a particular preamble sequence  606  selected from the set of all possible preamble sequences  650 . Additionally, each candidate transmitted signal  648  may correspond to a possible integer CFO candidate selected from the reduced set of integer CFO candidates  640 . 
     The cross-correlation operations may be performed in accordance with equation (19). 
     
       
         
           
             
               
                 
                   
                     
                       
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                         = 
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                         ; 
                         
                           Possible 
                           ⁢ 
                           
                               
                           
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                       : 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     # 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     samples 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     of 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     a 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     partial 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     correlation 
                   
                 
               
               
                 
                   ( 
                   19 
                   ) 
                 
               
             
           
         
       
     
     In equation (19), the term v s  refers to the virtual segment  636 . The possible integer CFO range (i.e., −Z i +v s −s:3:Z i ) corresponds to the reduced set of integer CFO candidates  640 . The term X( ) corresponds to a candidate transmitted signal  648 . The term Y( ) corresponds to the processed received signal  628 . 
     The receiver  604  is shown with a cross-correlation component  652  that receives the processed received signal  628  and candidate transmitted signals  648  as input, and that outputs correlation values  654 . The correlation values  654  may correspond to C(z;j) in equation (19). 
     The correlation values  654  may be used to identify the preamble sequence  606  within the received signal  632  and to estimate the integer CFO  608  of the received signal  632 . Once the preamble sequence  606  is identified, the segment  610  that corresponds to the preamble sequence  606  may also be identified. Preamble sequence identification, integer CFO estimation, and segment identification may be done in accordance with equations (11) through (14) above. 
     The receiver  604  is shown with a peak detection component  656 . The peak detection component  656  is shown receiving the correlation values  654  as input, and outputting a preamble sequence  606 , an estimated integer CFO  608 , and a segment  610  corresponding to the identified preamble sequence  606 . The preamble sequence  606  may be identified by the appropriate preamble index  516   a ,  516   b.    
     In equation (19) above, correlation is performed in the frequency domain. However, another correlation scheme may be used for the reduced candidates. For example, a time domain peak detection scheme may be used. 
       FIG. 7  illustrates a method  700  for identifying a preamble sequence  606  and for estimating an integer carrier frequency offset (CFO)  608 . The method  700  may be performed by a receiver  604 , which may be implemented in a remote station  106  in an OFDM/OFDMA system  100 . 
     In response to a signal  632  being received, signal detection may be performed  702  on the received signal  632 . Preamble detection may also be performed  704  on the received signal  632 . Symbol boundary detection may also be performed  706  on the received signal  632 . Fractional CFO compensation may also be performed  708  on the received signal  632 . A fast Fourier transform (FFT) operation may also be performed  710  on the received signal  632 . At this stage, the received signal  632  may be referred to as a processed received signal  628 . 
     As indicated above, the received signal  632  may include a preamble sequence  606 . Transmission of the preamble sequence  606  may have been achieved by modulating the preamble sequence  606  onto multiple orthogonal sub-carriers. The method  700  may include determining  712  the power of the sub-carriers. This may be accomplished in accordance with equation (16) above. 
     A virtual segment  636  may then be determined  714  based on the power of the sub-carriers. This may be done in accordance with equations (17) and (18) above. A reduced set of integer CFO candidates  640  may then be determined  716  based on the virtual segment  636 . 
     Cross-correlation operations may be performed  718  with respect to the received signal  632  and multiple candidate transmitted signals  648 . Each candidate transmitted signal  648  may include a particular preamble sequence  606  selected from the set of all possible preamble sequences  650 . Additionally, each candidate transmitted signal  648  may correspond to a possible integer CFO candidate selected from the reduced set of integer CFO candidates  640 . The cross-correlation operations may be performed in accordance with equation (19) above. 
     The correlation values  654  that are obtained as a result of performing the cross-correlation operations may be used to identify the preamble sequence  606  (e.g., by identifying a preamble index  516   a ,  516   b  corresponding to the preamble sequence  606 ) and to estimate the integer CFO  608  of the received signal  632 . Once the preamble sequence  606  is identified, the segment  610  that corresponds to the preamble sequence  606  may also be identified. Identifying the preamble sequence  606 , estimating the integer CFO  608 , and identifying the segment  610  that corresponds to the preamble sequence  606  may be performed concurrently. 
     The method  700  of  FIG. 7  described above may be performed by various hardware and/or software component(s) and/or module(s) corresponding to the means-plus-function blocks  800  illustrated in  FIG. 8 . In other words, blocks  702  through  720  illustrated in  FIG. 7  correspond to means-plus-function blocks  802  through  820  illustrated in  FIG. 8 . 
       FIG. 9  illustrates an example of a virtual segment table  944 . As indicated above, the virtual segment table  944  may be used to determine a reduced set of integer CFO candidates  640 . The virtual segment table  944  indicates relationships between virtual segments  636  and reduced sets of integer CFO candidates  640 . For example, the reduced set of integer CFO candidates  640  that corresponds to virtual segment zero are marked by an “O” within the highlighted portion  912  of the table. Although the virtual segment table  944  is shown in the form of a table, there are many other kinds of data structures that may be used to represent the information contained therein. 
     As indicated above in equation (19), the reduced set of integer CFO candidates for a given segment s is given by z=−Z i +v s −s:3:Z i . As shown in  FIG. 9 , the reduced sets of integer CFO candidates for different segments may be as follows: 
     v s =0 and s=0; z= . . . −3 0 3 6 . . . . 
     v s =0 and s=1; z= . . . −4 −1 2 5 . . . . 
     v s =0 and s=2; z= . . . −5 −2 1 4 . . . . 
     Once the virtual segment is chosen, the possible integer CFOs are limited for each segment as shown in the table of  FIG. 9  (“O” indicates a possible candidate, while “x” indicates an impossible candidate). The actual segment is not known at this time, but all possible preamble sequences that are defined (see, e.g.,  FIG. 5A  or  5 B as appropriate) will be searched with the corresponding segment number. For example, assuming the virtual segment=0, a search may proceed as follows for preamble index  0  that corresponds to segment  0  from the table in  FIG. 9 : 
     Reference preamble sequence of index  0 : X(i s,m ;j), i s,m =87, 90, . . . (see  FIG. 5C ), j=0 (index  0 ) 
     Received preamble: Y(i s,m +z;j), z= . . . −3, 0, 3, . . . . 
     Correlation for z=−3; X*(87)×Y(84)+X*(90)×Y(87)+ . . . . 
     Correlation for z=0; X*(87)×Y(87)+X*(90)×Y(90)+ . . . . 
     Correlation for z=3; X*(87)×Y(90)+X*(90)×Y(93)+ . . . . 
     In this example, z= . . . −2, −1, 1, 2, . . . were not considered because in this example those positions are not allowed as a possible integer CFO if the virtual segment is “0” and the actual segment is “0” based on the table in  FIG. 9  and the preamble sequence definitions in  FIGS. 5A and 5B . 
     The partial cross-correlation scheme represented by equation (19) is used in this example. However, as mentioned above, other correlation schemes may be used. 
       FIG. 10  illustrates various components that may be utilized in a wireless device  1002 . The wireless device  1002  is an example of a device that may be configured to implement the various methods described herein. The wireless device  1002  may be a base station  104  or a remote station  106 . 
     The wireless device  1002  may include a processor  1004  which controls operation of the wireless device  1002 . The processor  1004  may also be referred to as a central processing unit (CPU). Memory  1006 , which may include both read-only memory (ROM) and random access memory (RAM), provides instructions and data to the processor  1004 . A portion of the memory  1006  may also include non-volatile random access memory (NVRAM). The processor  1004  typically performs logical and arithmetic operations based on program instructions stored within the memory  1006 . The instructions in the memory  1006  may be executable to implement the methods described herein. 
     The wireless device  1002  may also include a housing  1008  that may include a transmitter  1010  and a receiver  1012  to allow transmission and reception of data between the wireless device  1002  and a remote location. The transmitter  1010  and receiver  1012  may be combined into a transceiver  1014 . An antenna  1016  may be attached to the housing  1008  and electrically coupled to the transceiver  1014 . The wireless device  1002  may also include (not shown) multiple transmitters, multiple receivers, multiple transceivers and/or multiple antenna. 
     The wireless device  1002  may also include a signal detector  1018  that may be used to detect and quantify the level of signals received by the transceiver  1014 . The signal detector  1018  may detect such signals as total energy, pilot energy per pseudonoise (PN) chips, power spectral density, and other signals. The wireless device  1002  may also include a digital signal processor (DSP)  1020  for use in processing signals. 
     The various components of the wireless device  1002  may be coupled together by a bus system  1022  which may include a power bus, a control signal bus, and a status signal bus in addition to a data bus. However, for the sake of clarity, the various busses are illustrated in  FIG. 10  as the bus system  1022 . 
     As used herein, the term “determining” (and grammatical variants thereof) is used in an extremely broad sense. The term “determining” encompasses a wide variety of actions and, therefore, “determining” can include calculating, computing, processing, deriving, investigating, looking up (e.g., looking up in a table, a database or another data structure), ascertaining and the like. Also, “determining” can include receiving (e.g., receiving information), accessing (e.g., accessing data in a memory) and the like. Also, “determining” can include resolving, selecting, choosing, establishing and the like. 
     Information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals and the like that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles or any combination thereof. 
     The various illustrative logical blocks, modules and circuits described in connection with the present disclosure may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array signal (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any commercially available processor, controller, microcontroller or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core or any other such configuration. 
     The steps of a method or algorithm described in connection with the present disclosure may be embodied directly in hardware, in a software module executed by a processor or in a combination of the two. A software module may reside in any form of storage medium that is known in the art. Some examples of storage media that may be used include RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, a hard disk, a removable disk, a CD-ROM and so forth. A software module may comprise a single instruction, or many instructions, and may be distributed over several different code segments, among different programs and across multiple storage media. A storage medium may be coupled to a processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. 
     The methods disclosed herein comprise one or more steps or actions for achieving the described method. The method steps and/or actions may be interchanged with one another without departing from the scope of the claims. In other words, unless a specific order of steps or actions is specified, the order and/or use of specific steps and/or actions may be modified without departing from the scope of the claims. 
     The functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium. Computer-readable media includes both computer storage media and communication media including any medium that facilitates transfer of a computer program from one place to another. A storage media may be any available media that can be accessed by a computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer. Also, any connection is properly termed a computer-readable medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. 
     It is to be understood that the claims are not limited to the precise configuration and components illustrated above. Various modifications, changes and variations may be made in the arrangement, operation and details of the methods and apparatus described above without departing from the scope of the claims.