MT Despread Symbols

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

1378

Inputs/Outputs

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input chip stream

The sequence of data chips to despread.

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spreading code

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

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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 value: 0

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error in

Error conditions that occur before this node runs.

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

Standard Error Behavior

Default value: No error

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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 value: 0

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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 value: TRUE

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output bit stream

The despread series of bits returned by this node.

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

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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.

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error out

Error information.

The node produces this output according to standard error behavior.

Standard Error Behavior

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=0L−1an⁢cmg(T−nT−mTc)y(t)=∑n=−∞∞∑m=0L−1an⁢cmg(T−nT−mTc)

where

y(T) is the transmitted DSSS signal

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

a n is the nth information bearing symbol

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

T c is the chip period

T = L * Tc is the symbol period