Method for transmitting a communications packet in a wireless communications network

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.

FIELD OF THE INVENTION

The present invention relates generally to wireless communications, and more particularly to preambles in communication packets.

BACKGROUND OF THE INVENTION

As shown inFIG. 2, a typically prior art wireless packet200includes of a synchronization header (SHR)210, a physical layer header (PHR)220, and a payload230of data. The SHR210contains a preamble240and start of frame delimiter (SFD)250.

The SHR is used to achieve signal acquisition, signal synchronization and ranging. The SFD250is used to detect the end of the preamble240, the end of the SHR header210, and the start of the PHR220. That is, the SFD250serves as a delimiter between the SHR and the PHR.

The Task Group for the emerging IEEE 802.15.4a standard for an alternative physical layer is standardizing the structure of the preamble240and the SFD250. According to the IEEE Draft P802.15.4a/D2, April, 2006, incorporated herein by reference, the preamble240has a repetition of eight identical, fixed length perfectly balanced ternary sequences (PBTS)110of ternary symbols (Si), as shown inFIG. 1, i.e., all X are the same.

As shown inFIG. 3, the ternary symbols (Si)110can be a pulse with positive (+) polarity320, a pulse with negative (−) polarity330, or a pulse with a zero (0) multiplier340. Each pulse is separated from the next by a pulse repetition interval (PRI)310. The PBTS110inFIG. 3have perfect periodic autocorrelation properties in a sense that side lobes around an autocorrelation peak are zero as shown inFIG. 3for symbols S1.

FIG. 2shows the SFD250specified 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 X260. The repetition of the base pattern X260within the SFD250generates periodicity. The periodicity helps to achieve statistical multiplexing gain from the repetitions.

However, the SFD should have an arbitrary, non-periodic pattern to help the receiver to determine what section of the SFD250is 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

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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

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;

IEEE Draft P802.15.4a/D2, incorporated herein by reference in its entirety.

As shown inFIG. 4, a packet400includes a synchronization header (SHR)410, a physical layer header (PHR)420, and a payload430of data. The SHR420includes a preamble440and start of frame delimiter (SFD)450.

The preamble440includes repetitions of a selected PBTS110as shown inFIG. 1.

The SFD structure according to an embodiment of the invention differs from that specified in IEEE Draft P802.15.4a/D2 standard. The SFD450includes N repeated pairs of constant cores and varying suffixes,460and465,470and475,480and485, . . . ,490and495. The suffix can also be called a counter.

If there are N repetitions of the constant core, then there are N different suffixes or counters.

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.

As shown inFIG. 5, the first suffix can be C1={0, 0}465, the second suffix C2={0,−Si}475, the third suffix C3={0−Si}485, and the fourth suffix C4={Si,−Si}495.

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 SFD450without any ambiguity.

Furthermore, the receiver can still obtain statistical multiplexing gain from the repetitions of the constant cores of the SFD450.

Improved Autocorrelation

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.

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 packet600includes a synchronization header (SHR)610, a physical layer header (PHR)620and payload630.

The SHR610includes a preamble640and a start of frame delimiter (SFD)650. The preamble640contains repetitions of a selected one of the perfectly balanced ternary sequences (PBTS)110as described above. That is, the multiple PBTS110in the preamble are all identical.

The structure of the SFD650differs from that in the IEEE Draft P802.15.4a/D2, April, 2006. The SFD according to this embodiment includes an arbitrary sequence of codes660, e.g., 64 codes. Each code may represent a fixed length 2n−1 ternary sequence, e.g., length of 31, 63, or 127. If the code is 0, then the fixed length sequence661is all zeros. If the code is +1, then the sequence110is Sias in the preamble, and if the code is −1, then the sequence110is −Si, a negation of the PBTS110in the preamble.

The construction of the SFD650can be described as the Kronecker product of two codes as inFIG. 7. Specifically in this embodiment, the SFD650is the Kronecker product of an arbitrary sequence of ternary code Su700, e.g., 64 codes, and a selected fixed length ternary code701, 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
SFD=Su{circle around (x)}Si,
where the symbol {circle around (x)} represent the Kronecker product and the overall length of the SFD650is
64*31=1984.

In one embodiment, an order and arrangement of the first 8 codes660are 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.

The advantage of having an SFD650with the arbitrary structure as defined above is that the sequence650has a high peak to side-lobe ratio, which leads to improved detection performance.

In one embodiment, optimal ternary codes that maximize detection performance can be any of the following arbitrary sequences:

These arbitrary sequences are determined experimentally.

EFFECT OF THE INVENTION

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).