Table Of Contents

MT Despread Symbols (G Dataflow)

Version:
    Last Modified: February 7, 2018

    Performs the direct sequence spread spectrum (DSSS) despreading operation on a spread sequence of data bits.

    connector_pane_image
    datatype_icon

    input chip stream

    The sequence of data chips to despread.

    datatype_icon

    spreading code

    The sequence of bits that determine how the bits in input bit stream are spread.

    datatype_icon

    sync confidence threshold

    The minimum correlation required for which you can consider input chip stream to be synchronized with the spreading code. Configure this parameter only if you set reset? to be TRUE. Valid values are between 0.0 (no correlation required) to 1.0 (perfect correlation required), inclusive.

    Default: 0

    datatype_icon

    error in

    Error conditions that occur before this node runs.

    The node responds to this input according to standard error behavior.

    Standard Error Behavior

    Many nodes provide an error in input and an error out output so that the node can respond to and communicate errors that occur while code is running. The value of error in specifies whether an error occurred before the node runs. Most nodes respond to values of error in in a standard, predictable way.

    error in does not contain an error error in contains an error
    If no error occurred before the node runs, the node begins execution normally.

    If no error occurs while the node runs, it returns no error. If an error does occur while the node runs, it returns that error information as error out.

    If an error occurred before the node runs, the node does not execute. Instead, it returns the error in value as error out.

    Default: No error

    datatype_icon

    sync search depth

    The desired number of bits (not chips) over which to search for a sync confidence metric greater than the sync confidence threshold. This parameter is only applicable when reset? is set to TRUE.

    Default: 0

    datatype_icon

    reset?

    A Boolean that determines whether this node synchronizes despreading at each call using the sync search depth and sync confidence threshold parameters.

    TRUE Synchronizes despreading at each call using the sync search depth and sync confidence threshold parameters.
    FALSE Ignores the values of sync search depth and sync confidence threshold and continues despreading from the previous iteration.

    Default: TRUE

    datatype_icon

    output bit stream

    The despread series of bits returned by this node.

    spd-note-note
    Note  

    output bit stream has (1/L × input chip stream) number of elements, where L is the length of the spreading code sequence.

    datatype_icon

    sync found index

    The synchronization index of the synchronization sequence found within the input chip stream. This parameter returns -1 if no sync is found.

    datatype_icon

    sync confidence metric

    The measured confidence metric when input chip stream is aligned using the sync found index. Valid values are 0.0 (0% measured confidence) to 1.0 (100% measured confidence), inclusive.

    datatype_icon

    error out

    Error information.

    The node produces this output according to standard error behavior.

    Standard Error Behavior

    Many nodes provide an error in input and an error out output so that the node can respond to and communicate errors that occur while code is running. The value of error in specifies whether an error occurred before the node runs. Most nodes respond to values of error in in a standard, predictable way.

    error in does not contain an error error in contains an error
    If no error occurred before the node runs, the node begins execution normally.

    If no error occurs while the node runs, it returns no error. If an error does occur while the node runs, it returns that error information as error out.

    If an error occurred before the node runs, the node does not execute. Instead, it returns the error in value as error out.

    Direct Sequence Spread Spectrum (DSSS)

    Direct Sequence Spread Spectrum (DSSS) is a process by which data is transmitted using a higher bandwidth signal as required by the data rate. Using DSSS allows multiple channels to occupy the same bandwidth, thus mitigating interference from other users at the expense of bandwidth expansion.

    DSSS spreads each bit of signal data at the transmitter into L chips using a pseudorandom L-chip spreading code called a code word. The length L of the pseudorandom spreading code is also known as the bandwidth expansion factor because the chips are transmitted at a rate equal to L * bit rate of the data. The spreading code appears random to all receivers except the intended one, which uses the knowledge of the spreading code to demodulate and recover the transmitted information. Thus, multiple channels can occupy the same portion of the frequency spectrum by using code words that have little or no correlation with one another, and little or no autocorrelation for any shift other than zero.

    Mathematically, a DSSS signal is described by
    y ( t ) = n = m = 0 L 1 a n c m g ( T n T m T c )

    where

    y(T) is the transmitted DSSS signal

    g(T) is the pulse-shaping signal of duration Tc

    an is the nth information bearing symbol

    cm is the mth element of the L-long pseudorandom spreading code (also known as the chip sequence)

    Tc is the chip period

    T = L * Tc is the symbol period

    Where This Node Can Run:

    Desktop OS: Windows

    FPGA: Not supported

    Web Server: Not supported in VIs that run in a web application


    Recently Viewed Topics