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MT Calculate BER After Trigger (PN Sequence) (G Dataflow)

Last Modified: January 9, 2017

Calculates the average bit error rate (BER) against a Galois PN sequence. The sequence must be the same as the PN sequence generated by MT Generate Bits (Galois, PN Order) with a matching PN order. All subsequent iterations of the node use a continuation of the same PN sequence until reset? is set to TRUE.

When you call this node for the first time, or whenever reset? is TRUE, the node begins a blind search over the input bit stream for a trigger found index location in which the BER of a number of subsequent bits is below the specified BER trigger threshold. The confidence parameter specifies the number of bits after the trigger found index that are used in the trigger threshold calculation.

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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.

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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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PN sequence order

The order of the transmitted PN bit sequence. This parameter must match the PN sequence order of the bits transmitted to the device under test. Valid values are 5 to 31, inclusive.

Default: 9

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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.

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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].

Default: 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.

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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.

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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: 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.

Default: 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: 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 trigger.

When trigger found? is TRUE, the trigger was found in the input bit stream. When trigger found? is FALSE, the trigger was found in the current or a previous input bit stream.

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trigger found index

The index value of the first bit where the bit pattern matched the specified PN sequence.

This index value indicates the number of bits since the last reset that are not PN data. 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.

Minimum Number of Input Bits

The minimum number of input bits required to return a meaningful BER value is given by
N min = int ( y × t h r e s h o l d × ( 1 t h r e s h o l d ) + 120 × t h r e s h o l d + y × t h r e s h o l d × ( 1 t h r e s h o l d ) 2 × t h r e s h o l d ) 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.

Locating the Trigger

While stepping through the input bit stream one bit at a time, this node attempts to locate a trigger using the following procedure:

  1. Length-checking the input bit stream (excluding any bits already examined in previous attempts to find a trigger). If the available length is insufficient to meet the specified confidence, the VI buffers input bits and exits without locating a trigger.
  2. Generating a trial PN sequence seed (trial seed) based on the next N + 1 bits, where N is the expected PN sequence order.
  3. Generating a trial PN sequence based on the trial seed, with a length of the specified confidence.
  4. Calculating a trial BER value using the trial PN sequence as a reference and comparing this reference to the bits following the N + 1 bits that generate the trial seed. The node returns to step 1 if the trial BER is above the specified BER trigger threshold.
  5. Generating a second trial PN sequence seed, using the N + 1 bits immediately following the N + 1 bits that were used to generate the first trial PN sequence seed.
  6. Generating a trial PN sequence based on the trial seed with a length of the specified confidence - (N + 1) bits.
  7. Calculating a trial BER value using the trial PN sequence as a reference and comparing this reference to the bits following the N + 1 bits that were used to generate the trial seed. The node returns to step 1 if the second trial BER is above the specified BER trigger threshold.
  8. Returning a trigger found? value of TRUE.

BER calculation begins on the bit following the first set of N + 1 bits in which the trigger is found.

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.

Where This Node Can Run:

Desktop OS: Windows

FPGA: Not supported


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