MT Modulate PSK
- Updated2023-02-17
- 7 minute(s) read
MT Modulate PSK
Receives a sequence of data bits, performs PSK modulation, and returns the modulated complex baseband waveform in the output complex waveform parameter.
Inputs/Outputs
input bit stream
The sequence of information bits to be modulated.
PSK system parameters
Parameter values defining the PSK system. Wire the PSK system parameters cluster of MT Generate PSK System Parameters (M) or MT Generate PSK System Parameters (map) to this cluster. Do not alter the values.
samples per symbol
An even number of samples dedicated to each symbol. Multiply this value by the symbol rate to determine the sample rate.
Default value: 16
symbol map
An ordered array that maps each Boolean symbol to its desired coordinates in the complex plane. The number of states in the array must be 2N, where N is the number of bits per symbol.
differential PSK
Status of the differential PSK.
| disable |
Disables bit sequence encoding. |
| enable |
Enables bit sequence encoding. |
Default value: enable
PSK type
Type of PSK modulation.
| normal |
Sets the modulation type to regular PSK. |
| 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. |
Default value: normal
symbol rate
The desired symbol rate, in hertz (Hz).
Default value: 1.0
pulse shaping filter coefficients
An ordered array containing the desired pulse-shaping coefficients. Wire the pulse shaping filter coefficients parameter of MT Generate Filter Coefficients to this parameter. When generating the filter coefficients, ensure that the value of the pulse shaping samples per symbol parameter of MT Generate Filter Coefficients is equal to the value of the samples per symbol element of the PSK system parameters cluster which is passed to this node.
error in
Error conditions that occur before this node runs.
The node responds to this input according to standard error behavior.
Default value: No error
reset?
A Boolean that determines whether the node continues modulating using the previous iteration states. The node resets on the first call and when you configure reset? to TRUE. You must configure reset? to TRUE the first time this node is called and whenever you want to restart the modulator.
Default value: TRUE
flush buffers?
A Boolean that determines whether samples are forced out from the modulated waveform that are affected by the FIR pulse-shaping filter delay. Set this parameter to TRUE during single-shot operations and during the last iteration of continuous operations.
| TRUE | Destroys the internal states of the algorithms such that you cannot perform continuous processing on the signal during subsequent iterations. If flush buffers? is set to TRUE, you must set reset? to TRUE on the subsequent iteration. |
| FALSE | Stores the internal states of the algorithms so that you can perform continuous processing on the signal during subsequent iterations. |
Default value: FALSE
output complex waveform
The modulated complex baseband waveform data.
t0
Time of the first value in the Y array.
dt
Time interval between data values in the Y array.
Default value: 1.0
Y
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.
symbols out
The array of mapped symbols before pulse shaping is applied. The array represents the complex value of each mapped symbol.
error out
Error information.
The node produces this output according to standard error behavior.
Filter Delay
Finite impulse response (FIR) filters are used for different operations such as pulse-shaping, matched filtering, and downconversion filtering. For such filters, the output signal is related to the input signal as shown by the following equation:
where
P is the filter order
x[n] is the input signal
y[n] is the output signal
b i are the filter coefficients
The initial state for all samples in an FIR filter is 0. The filter output until the first input sample reaches the middle tap (the first causal sample) is called the transient response, or filter delay. For an FIR filter that has N taps, the delay is (N-1)/2 samples. This relationship is illustrated in the following figure, where a sine wave is filtered by an FIR filter with 50 taps.
Recovering Samples in Single-Shot Operations
In single-shot operations for modulators and demodulators, the filter delay is truncated before the signal is generated because these samples are not valid. Some samples at the end of the block do not appear at the modulator or demodulator output, and hence appear to have been lost.
- For modulation: Let
L be the pulse-shaping filter length,
m be the number of samples per symbol, and
M be the modulation order. The number of bits to be added to the input bit stream is given by the following formula:
- For demodulation: Demodulation use filters during matched filtering. Let
L be the length of the matched filter. The number of samples to be added to the input signal prior to filtering is given by the following formula: The N extra samples are obtained by repeating the last sample value of the input signal N times to ensure signal continuity.