MT Calculate BER After Trigger (User Pattern)

Calculates the average bit error rate (BER) against a user-defined sequence. The sequence input in the input bit stream array must be the same as the sequence specified in the reference bit pattern parameter.

When you call this node for the first time, or whenever reset? is TRUE, the node attempts to locate a trigger within the input bit stream by comparing it to the reference bit pattern under the constraints set by the confidence and BER trigger threshold parameters. When the BER on the subsequent input bits is below the specified threshold of bits specified by confidence, the node begins reporting the BER.

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Inputs/Outputs

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input bit stream

The bit sequence over which BER is calculated. This bit stream must include only data bits of the PN sequence.

Note To avoid faulty triggering caused by synchronization and/or guard bits, NI recommends that the input bit stream be a phase-continuous PN sequence such as the sequence generated by MT Generate Bits.
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reference bit pattern

Sequence of bits that are compared to the input bit stream to calculate BER and accumulated BER. When you reset this node, it adjusts the starting bit of this sequence to find the minimum BER.

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BER trigger threshold

The BER threshold that must not be exceeded for the trigger found? decision. This value determines when the BER measurement begins; bits before this value and seed bits are not included in the BER measurement. Valid values are 0.0 to 1.0, inclusive.

Note The number of bits examined for the pretrigger BER which is used for comparison to the BER trigger threshold, is determined by the specified confidence and BER trigger threshold values. The following images show the range of required input lengths for a confidence input in the range [0.5,0.99] and a BER trigger threshold input in the range [0.1,0.5].

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Default value: 0.1

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confidence

Level of confidence that the actual BER of the bits following a trigger will be below the specified BER trigger threshold. Values between 0.0 and 1.0 represent proportional confidence.

Tip Specify a value of -1 to set the minimum number of bits required for a trigger to the length of the specified reference bit pattern. This option is useful when the input bit stream is very short.
Note The specified confidence can only be reached after processing a minimum number of bits in the input bit stream. This parameter indirectly specifies the minimum input bit stream length in which a trigger can occur. Specify a confidence value of -1 to allow triggering on input bit stream lengths equal to 3 × (PN sequence order).

Default value: 0.95

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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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reset?

A Boolean that determines whether the node continues bit error rate calculations from a previous iteration.

TRUE Resets the trigger and the accumulated BER measurement.
FALSE Maintains the previous synchronization and does not clear the existing accumulated BER value.

Default value: TRUE

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BER

The measured BER as the ratio of erroneous bits to the bits following a trigger in the current input bit stream.

The number of bits considered for this measurement depends on the conditions shown in the following table.

Conditions BER Output
  • reset? set to TRUE or first call, and
  • trigger not detected in the current input bit stream
 1.0
  • reset? set to FALSE, and
  • trigger not detected in the current input bit stream, or
  • trigger not detected in previous calls to the same instance of this node
1.0
  • reset? set to TRUE or first call, and
  • trigger detected in the current input bit stream
 BER calculation based on the length of bits following the set of N + 1 bits in which the trigger was detected, where N is the specified PN sequence order
  • reset? set to FALSE, and
  • trigger detected in previous calls to the same instance of this node
BER calculation based on the length of the input bit stream
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accumulated BER

The measured bit error rate as the ratio of erroneous bits to bits following a trigger.

The number of bits considered for this measurement depends on the conditions shown in the following table.

Scenario Conditions BER Output
1.
  • reset? set to TRUE or first call, and
  • trigger not detected in the current input bit stream
 1.0
2.
  • reset? set to FALSE, and
  • trigger not detected in the current input bit stream, or
  • trigger not detected in previous calls to the same instance of this node
1.0
3.
  • reset? set to TRUE or first call, and
  • trigger detected in the current input bit stream
 BER calculation based on the length of bits following the set of N + 1 bits in which the trigger was detected, where N is the specified PN sequence order
4.
  • reset? set to FALSE, and
  • trigger detected in previous calls to the same instance of this node
Accumulated BER calculation based on the length of the input bit stream
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trigger found?

A Boolean that indicates whether the node found a user-defined sequence synchronization that exceeded the specified BER trigger threshold.

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trigger found index

Index value of the first bit where the bit pattern matched the specified user-defined pattern.

This index value indicates the number of bits since the last reset that did not contribute to the BER calculation. For example, if the transmitter is off for 30 bits, this parameter returns 30.

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error out

Error information.

The node produces this output according to standard error behavior.

Standard Error Behavior

Minimum Number of Input Bits

The minimum number of input bits required to return a meaningful BER value is given by

Nmin⁡=int(y×threshold×(1−threshold)+120×threshold+y×threshold×(1−threshold)2×threshold)⁡2Nmin⁡=int(y×threshold×(1−threshold)+120×threshold+y×threshold×(1−threshold)2×threshold)⁡2

where

y is defined such that confidence=P(x≤y)

x is a zero-mean, unit-variance Gaussian variate

P(x≤y) denotes the probability that the variable x takes a value that is less than or equal to y.

Unit Under Test Synchronization

The BER trigger threshold input triggers this node, meaning that the node only reports BER on bits after a match is found. Hence, you do not need to synchronize your transmitter with your unit under test (UUT)—the test can begin before transmission begins.