Abstract:
A method and apparatus for transmitting a packet in a wireless communications network is presented. A packet is constructed to include synchronization header, a physical layer header, and a payload. A preamble and a start of frame delimiter are inserted in the synchronization header. Multiple fixed length ternary sequences are inserted in the start of frame delimiter in an arbitrary order, and then the packet is transmitted.

Description:
RELATED APPLICATION 
       [0001]    This application is a Continuation-in Part Application of U.S. patent application MERL-1830, filed on Jun. 27, 2006 by Sahinoglu, which claims priority to U.S. Provisional Application No. 60/808,412, “Preamble Design for Improved Synchronization,” filed on May 25, 2006 by Sahinoglu et al. 
     
     FIELD OF THE INVENTION  
       [0002]    The present invention relates generally to wireless communications, and more particularly to preambles in communication packets. 
       BACKGROUND OF THE INVENTION  
       [0003]    As shown in  FIG. 2 , a typically prior art wireless packet  200  includes of a synchronization header (SHR)  210 , a physical layer header (PHR)  220 , and a payload  230  of data. The SHR  210  contains a preamble  240  and start of frame delimiter (SFD)  250 . 
         [0004]    The SHR is used to achieve signal acquisition, signal synchronization and ranging. The SFD  250  is used to detect the end of the preamble  240 , the end of the SHR header  210 , and the start of the PHR  220 . That is, the SFD  250  serves as a delimiter between the SHR and the PHR. 
         [0005]    The Task Group for the emerging IEEE 802.15.4a standard for an alternative physical layer is standardizing the structure of the preamble  240  and the SFD  250 . According to the IEEE Draft P802.15.4a/D2, April, 2006, incorporated herein by reference, the preamble  240  has a repetition of eight identical, fixed length perfectly balanced ternary sequences (PBTS)  110  of ternary symbols (Si), as shown in  FIG. 1 , i.e., all X are the same. 
         [0006]    As shown in  FIG. 3 , the ternary symbols (Si)  110  can be a pulse with positive (+) polarity  320 , a pulse with negative (−) polarity  330 , or a pulse with a zero ( 0 ) multiplier  340 . Each pulse is separated from the next by a pulse repetition interval (PRI)  310 . The PBTS  110  in  FIG. 3  have perfect periodic autocorrelation properties in a sense that side lobes around an autocorrelation peak are zero as shown in  FIG. 3  for symbols S 1 . 
         [0007]      FIG. 2  shows the SFD  250  specified in the IEEE Draft P802.15.4a/D2 standard. The SFD also has a periodic structure that includes a repetition of the identical base pattern X  260 . The repetition of the base pattern X  260  within the SFD  250  generates periodicity. The periodicity helps to achieve statistical multiplexing gain from the repetitions. 
         [0008]    However, the SFD should have an arbitrary, non-periodic pattern to help the receiver to determine what section of the SFD  250  is being received. Therefore, it is desired to improve the SFD by removing the periodicity without sacrificing the statistical multiplexing gain that is achieved by repetitions of the base pattern X. 
       SUMMARY OF THE INVENTION  
       [0009]    A method and apparatus transmits a packet in a wireless communications network. The transmitted packet includes a synchronization header, a physical layer header, and a payload. The synchronization header includes a preamble and a start of frame delimiter. The start of frame delimiter includes an arbitrary pattern of fixed length ternary sequences. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a prior art list of length 31 ternary symbols that have autocorrelation properties; 
           [0011]      FIG. 2  is a block diagram of a prior art packet structure for a wireless network designed according to an emerging IEEE 802.15.4a standard in IEEE Draft P802.15.4a/D2, April, 2006; 
           [0012]      FIG. 3  is a prior art timing diagram of a length-31 ternary symbol with perfect periodic autocorrelation in time domain, and a corresponding autocorrelation function; 
           [0013]      FIG. 4  is a block diagram of a packet structure according to an embodiment of the invention; 
           [0014]      FIG. 5  is detailed block diagram of a start of frame delimiter (SFD) according to an embodiment of the invention; 
           [0015]      FIG. 6  is a block diagram of a SFD according to another embodiment of the invention; and 
           [0016]      FIG. 7  is a block diagram of a Kronecker operation to construct a SFD according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]    The embodiments of the invention provide a structure for a packet structure in a wireless communications network designed according to an emerging IEEE 802.15.4a standard as described in IEEE Draft P802.15.4a/D2, April, 2006; 
         [0018]    IEEE Draft P802.15.4a/D2, incorporated herein by reference in its entirety. 
         [0019]    As shown in  FIG. 4 , a packet  400  includes a synchronization header (SHR)  410 , a physical layer header (PHR)  420 , and a payload  430  of data. The SHR  420  includes a preamble  440  and start of frame delimiter (SFD)  450 . 
         [0020]    The preamble  440  includes repetitions of a selected PBTS  110  as shown in  FIG. 1 . 
         [0021]    The SFD structure according to an embodiment of the invention differs from that specified in IEEE Draft P802.15.4a/D2 standard. The SFD  450  includes N repeated pairs of constant cores and varying suffixes,  460  and  465 ,  470  and  475 ,  480  and  485 , . . . ,  490  and  495 . The suffix can also be called a counter. 
         [0022]    If there are N repetitions of the constant core, then there are N different suffixes or counters. 
         [0023]    One structure for the core includes ternary symbols {−Si, 0, 0, 0, Si, −Si }. If N is 4, then four different suffixes can be specified. 
         [0024]    As shown in  FIG. 5 , the first suffix can be C 1 ={0, 0}  465 , the second suffix C 2 ={0,−Si}  475 , the third suffix C 3 ={0−Si}  485 , and the fourth suffix C 4 ={Si,−Si}  495 . 
         [0025]    If each suffix has a different pattern of ternary symbols as described above, then the receiver can determine the number of repetitions received at a given time. This makes it possible to synchronize a clock of the receiver relatively within the SFD  450  without any ambiguity. 
         [0026]    Furthermore, the receiver can still obtain statistical multiplexing gain from the repetitions of the constant cores of the SFD  450 . 
         [0027]    Improved Autocorrelation 
         [0028]    The problem with the counter approach as described above is that the SFD as a whole does not have a good autocorrelation function. The autocorrelation function contains high side lobes. Therefore, it improves the detection performance compared to that in IEEE Draft P802.15.4a/D2, April, 2006. However, the improvement is marginal. 
         [0029]    This embodiment of the invention specifies a packet structure as an improvement to the packet structure described in the IEEE Draft P802.15.4a/D2, April, 2006 and also in the parent application. According to this embodiment, a packet  600  includes a synchronization header (SHR)  610 , a physical layer header (PHR)  620  and payload  630 . 
         [0030]    The SHR  610  includes a preamble  640  and a start of frame delimiter (SFD)  650 . The preamble  640  contains repetitions of a selected one of the perfectly balanced ternary sequences (PBTS)  110  as described above. That is, the multiple PBTS  110  in the preamble are all identical. 
         [0031]    The structure of the SFD  650  differs from that in the IEEE Draft P802.15.4a/D2, April, 2006. The SFD according to this embodiment includes an arbitrary sequence of codes  660 , e.g., 64 codes. Each code may represent a fixed length 2 n −1 ternary sequence, e.g., length of 31, 63, or 127. If the code is 0, then the fixed length sequence  661  is all zeros. If the code is +1, then the sequence  110  is S i  as in the preamble, and if the code is −1, then the sequence  110  is −S i , a negation of the PBTS  110  in the preamble. 
         [0032]    The construction of the SFD  650  can be described as the Kronecker product of two codes as in  FIG. 7 . Specifically in this embodiment, the SFD  650  is the Kronecker product of an arbitrary sequence of ternary code Su  700 , e.g., 64 codes, and a selected fixed length ternary code  701 , e.g., Si, −Si. If the ternary code in Su is 0, then the Kronecker product results in a sequence of all zeros. After the Kronecker operation, we have the following equation for the entire SFD sequence 
         [0000]      SFD=Su{circle around (×)}Si, 
         [0000]    where the symbol {circle around (×)} represent the Kronecker product and the overall length of the SFD  650  is 
         [0000]      64*31=1984. 
         [0033]    In one embodiment, an order and arrangement of the first 8 codes  660  are identical to the second 8 codes, e.g., {0, 0, +1, +1, −1, 0, −1, 0, 0, 0, +1, +1, −1, 0, −1, 0}, and the order and arrangement of the remaining codes are arbitrary. 
         [0034]    The advantage of having an SFD  650  with the arbitrary structure as defined above is that the sequence  650  has a high peak to side-lobe ratio, which leads to improved detection performance. 
         [0035]    In one embodiment, optimal ternary codes that maximize detection performance can be any of the following arbitrary sequences: 
         [0036]    Code-1: 
         [0000]    [0 0 −1 1 1 0 −1 1 −1 −1 −1 0 0 1 0 0 1 1 0 0 0 0 1 0 1 −1 0 1 0 1 0 0 −1 1 1 0 0 0 0 1 −1 0 1 0 1 0 0 1 0 0 0 1 0 1 1 −1 −1 −1 0 −1 1 0 0 0]; 
         [0037]    Code-2: 
         [0000]    [1 −1 0 1 0 1 0 0 0 0 1 0 1 0 1 1 −1 −1 −1 0 −1 1 0 0 −1 1 1 0 0 0 0 0 0 0 0 1 −1 0 0 −1 0 0 −1 1 1 1 1 0 1 −1 1 0 0 0 1 0 −1 0 1 1 0 −1 0 0]; 
         [0038]    Code-3: 
         [0000]    [1−1 0 1 0 1 0 0 1 −1 0 1 0 1 0 0 −1 −1 −1 0 −1 1 0 0 −1 1 1 0 0 0 0 0 0 0 0 1 −1 0 0 −1 0 0 −1 1 1 1 1 0 1 −1 0 0 0 1 0 −1 0 1 1 0 −1 0 0]; 
         [0039]    These arbitrary sequences are determined experimentally. 
       EFFECT OF THE INVENTION  
       [0040]    The embodiment of the invention improves synchronization of a receiver to a received packet and improves the detection of the start of frame delimiter (SFD). 
         [0041]    Although the invention has been described by way of examples of preferred embodiments, it is to be understood that various other adaptations and modifications may be made within the spirit and scope of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.