MT Demodulate PSK VI

LabView Digital Modulation

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MT Demodulate PSK VI

Demodulates a PSK-modulated complex baseband waveform and returns the time-aligned oversampled complex waveform, the demodulated bit stream, and measurement results obtained during demodulation. This VI attempts to remove carrier and phase offset by locking to the carrier signal.

Note This VI assumes that the sample rate of the input complex waveform is exactly samples per symbol × the symbol rate. If this relationship does not apply to your application, use the MT Resample (Complex Cluster) VI to resample the waveform to the desired sample rate.

Note Matched filtering and/or waveform realignment performed during symbol timing recovery may lead to the apparent loss of bits. Refer to Filter Delay for more information about this effect. You can use the MT Detect PSK VI if your application requires only the demodulated bit stream output and not the recovered complex waveform or measurements.

Details

MT Demodulate PSK

input complex waveform specifies the PSK-modulated complex baseband waveform data.

	t0 specifies the trigger (start) time of the Y array. The default is 0.0.
	dt specifies time interval between data points in the acquired Y array. The default is 1.0.
	Y specifies the complex-valued signal-only baseband modulated waveform. The real and imaginary parts of this complex data array correspond to the in-phase (I) and quadrature-phase (Q) data, respectively.

PSK system parameters specifies parameter values defining the PSK system. Wire the PSK system parameters cluster of the PSK (M) or PSK (Map) instance of the MT Generate System Parameters VI to this cluster. Do not alter the values.

samples per symbol specifies an even, positive number of samples dedicated to each symbol. Multiply this value by the symbol rate to determine the sample rate. The default is 16.

Note The Modulation Toolkit demodulation and detector VIs use timing recovery, which is optimized for four or more samples per symbol.

symbol map specifies an ordered array that maps each Boolean symbol to its desired coordinates in the complex plane. The number of PSK states in the array must be 2^N, where N is the number of bits per symbol.

differential PSK specifies the status of the differential PSK.

disable (0)	Does not allow bit sequence encoding.
enable (1)	Allows bit sequence encoding.

PSK type specifies the type of PSK modulation.

normal	Sets the modulation type to regular PSK. This is the default value.
shifted	Rotates the constellation by /M each symbol.
offset	Sets the modulation type to offset quadrature phase-shift keying (OQPSK). This modulation scheme is a form of phase-shift keying in which four different phase angles are used. This scheme is sometimes referred to as staggered quadrature phase-shift keying (SQPSK). For offset PSK, the ideal symbol timing for Q is offset by 1/2 of a symbol period from the ideal symbol timing for I. offset is currently only supported for M = 4.

matched filter coefficients specifies an ordered array containing the desired matched filter coefficients. Wire the matched filter coefficients parameter of the MT Generate Filter Coefficients VI to this parameter. When generating the filter coefficients, ensure that the value of the matched samples per symbol parameter of the MT Generate Filter Coefficients VI is equal to the value of the samples per symbol element of the PSK system parameters cluster passed to this VI.

Note When reset? is set to TRUE, there is a transient response of half the filter length at the start of the demodulated signal, and the returned data is shortened by approximately half the filter length. When reset? is set to FALSE, the VI uses data from the previous iteration to eliminate the transient.

Tip When reset? is set to TRUE, the number of trailing symbols that are carried over to the next iteration during demodulation is upper bounded by [L + 4(13 + K)]/K , where L is the matched filter length in taps and K is the number of samples per symbol. For typical values of L = 25 (4 samples per symbol, filter length of 6 symbols) and K = 4, this value equals 23.25 symbols. Therefore when reset? is set to TRUE, a total of 1,024 PSK symbols must be input to the demodulator to obtain at least 1,000 symbols at the output. This formula does not account for truncation due to any specified synchronization parameters.

synchronization parameters specifies parameter values describing the synchronization sequence and the range of bits over which to search for this sequence. Wire the PSK synchronization parameters cluster returned by the PSK bit array or number array instances of the MT Generate Synchronization Parameters VI to this cluster.

Note If the synchronization parameters cluster is not wired, the demodulator does not attempt to synchronize, and the constellation of the demodulated waveform has a 180° carrier phase ambiguity.

	expected sync location specifies the expected location of the first symbol of the sync sequence. This value is an index to the input complex waveform. A value of -1 searches the entire input complex waveform and ignores the sync location uncertainty parameter.
	sync sequence specifies the mapped symbol pattern used to synchronize the bit stream. To prevent false synchronization, select this pattern such that there is a low probability of accidental correlation to nonsynchronized parts of the data stream. If this parameter is left empty (default), the signal is still demodulated, but there is a phase ambiguity in the recovered symbols.
	sync location uncertainty specifies the number of symbols before or after the expected sync location where the first symbol of the sync sequence may be located.
	sync indent specifies the distance that the sync sequence is indented into the information block. The distance is the number of demodulated symbols preceding the sync sequence. For example, a value of 10 indicates that the output bit stream consists of 10 data symbols, followed by the sync sequence, followed by the remaining data symbols.

pulse shaping filter coefficients specifies an ordered array containing the desired pulse shaping coefficients. This parameter is used internally to remodulate the bit stream to calculate measurements. Wire the pulse shaping filter coefficients parameter of the MT Generate Filter Coefficients VI to this parameter. When generating the filter coefficients, ensure that the value of the pulse shaping samples per symbol parameter of the MT Generate Filter Coefficients VI is equal to the value of the samples per symbol element of the PSK system parameters cluster passed to this VI.

reset? specifies whether the VI continues demodulating using the previous iteration states. reset? must be TRUE whenever you want to restart the demodulator. The VI resets on the first call and when reset? is set to TRUE. Set reset? to FALSE if the input complex waveform is contiguous with the input complex waveform from the previous iteration of this VI. The default is TRUE.

error in (no error) can accept error information wired from previously called VIs. Use this information to decide if any functionality should be bypassed in the event of errors from other VIs. Right-click the front panel error in control and select Explain Error or Explain Warning from the shortcut menu for more information about the error.

	status is TRUE (X) if an error occurred or FALSE (checkmark) to indicate a warning or that no error occurred. Right-click the front panel error in control and select Explain Error or Explain Warning from the shortcut menu for more information about the error.
	code identifies the error or warning code. Right-click the front panel error in control and select Explain Error or Explain Warning from the shortcut menu for more information about the error.
	source describes the origin of the error or warning. Right-click the front panel error in control and select Explain Error or Explain Warning from the shortcut menu for more information about the error.

recovered complex waveform returns the time-aligned and oversampled complex waveform data after matched filtering, frequency offset correction, and phase offset correction. The frequency offset and phase offset corrections are scalar values applied to the entire block.

	t0 returns the time of first value in Y array.
	dt returns the time between values in the Y array.
	Y returns the complex-valued signal-only baseband modulated waveform. The real and imaginary parts of this complex data array correspond to the in-phase (I) and quadrature-phase (Q) data, respectively.

output bit stream returns the demodulated information bit stream.

Note For PSK systems with more than 1 bit per symbol, such as 4-PSK, the symbols are converted to bits in least significant bit (LSB) first order. For example, if the detected symbols are 2,1,... the generated bits are 0,1,1,0...

measurements returns the results of measurements performed by the PSK demodulator.

	frequency offset returns the measured carrier frequency offset, in hertz. The measured frequency offset is removed from the recovered complex waveform.
	frequency drift returns the measured carrier frequency drift, in hertz. The measured frequency drift is not removed from the recovered complex waveform.
	phase offset returns the measured phase offset, in degrees. The measured phase offset is removed from the recovered complex waveform.

sync found index returns the symbol index within the input complex waveform where the peak correlation to the sync sequence was found. If no sync sequence is specified in the synchronization parameters cluster, this parameter returns the offset from the start of the input complex waveform to the first complete symbol.

error out passes error or warning information out of a VI to be used by other VIs. Right-click the front panel error out indicator and select Explain Error or Explain Warning from the shortcut menu for more information about the error.

	status is TRUE (X) if an error occurred or FALSE (checkmark) to indicate a warning or that no error occurred. Right-click the front panel error out indicator and select Explain Error or Explain Warning from the shortcut menu for more information about the error.
	code is the error or warning code. Right-click the front panel error out indicator and select Explain Error or Explain Warning from the shortcut menu for more information about the error.
	source describes the origin of the error or warning. Right-click the front panel error out indicator and select Explain Error or Explain Warning from the shortcut menu for more information about the error.

Details

Note Offset PSK mode is only supported for 4-PSK (OQPSK), and setting psk type to offset requires some form of band-limiting filtration. Raised cosine and root-raised cosine are the only valid filter types for OQPSK demodulation. Demodulator locking performance degrades with alpha values <0.3 in the raised cosine or root-raised cosine filter design. Set the PSK type parameter to offset in the PSK (M) or PSK (Map) instance of MT Generate System Parameters to access this mode.

Successful Locking

Successful locking depends on many factors, including signal quality, modulation type, filtering parameters, and acquisition size. Locking also requires a fairly uniform distribution of symbols in the signal. The demodulator lock rate increases (and failures decrease) as the number of symbols demodulated increases. In general, you can expect to achieve a better than 95% lock when demodulating 10 × M number of symbols, where M is 2^{bits per symbol}.

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