Demodulate differentially encoded MSKmodulated data
CPM, in Digital Baseband sublibrary of Modulation
The MSK Demodulator Baseband block demodulates a signal that was modulated using the differentially encoded minimum shift keying method. The block expects the input signal to be a baseband representation of a coherent modulated signal with no precoding. The Phase offset parameter represents the initial phase of the modulated waveform.
Differentially encoded minimum shift keying modulation uses pulse shaping to smooth the phase transitions of the modulated signal. The function q(t) is the phase response obtained from the frequency pulse, g(t), through this relation: $$q(t)={\displaystyle {\int}_{\text{\hspace{0.17em}}\infty}^{t}g(t)dt}$$.
The specified frequency pulse shape corresponds to this rectangular pulse shape expression for g(t).
Pulse Shape  Expression 

Rectangular  $$g(t)=\{\begin{array}{cc}\frac{1}{2LT},& 0\le t\le LT\\ 0& \text{otherwise}\end{array}$$ 
L_{main} is the main lobe pulse duration in symbol intervals.
The duration of the pulse, LT, is the pulse length in symbol intervals.
This block accepts a scalarvalued or column vector input signal with a data type
of single
or double
. If you set the
Output type parameter to
Integer
, then the block produces values of 1 and 1. If
you set the Output type parameter to
Bit
, then the block produces values of 0 and
1.
In singlerate processing mode, the input and output signals have the same port sample time. The block implicitly implements the rate change by making a size change at the output when compared to the input. The input width must be an integer multiple of the Samples per symbol parameter value, and the input can be a column vector.
When you set Output type to
Bit
, the output width is K
times the number of input symbols.
When you set Output type to
Integer
, the output width is the number of
input symbols.
In multirate processing mode, the input and output signals have different port sample times. The input must be a scalar. The output symbol time is the product of the input sample time and the Samples per symbol parameter value.
When you set Output type to
Bit
, the output width equals the number of
bits per symbol.
When you set Output type to
Integer
, the output is a scalar.
Internally, this block creates a trellis description of the modulation scheme and uses the Viterbi algorithm. The Traceback depth parameter, D, in this block is the number of trellis branches used to construct each traceback path. D influences the output delay, which is the number of zero symbols that precede the first meaningful demodulated value in the output.
When you set the Rate options parameter to
Allow multirate processing
, and the model
uses a variablestep solver or a fixedstep solver with the
Tasking Mode parameter set to
SingleTasking
, then the delay consists of D+1
zero symbols.
When you set the Rate options parameter to
Enforce singlerate processing
, then the
delay consists of D zero symbols.
The optimal Traceback depth parameter value is dependent on minimum squared Euclidean distance calculations. Alternatively, a typical value, dependent on the number of states, can be chosen using the “fivetimestheconstraintlength” rule, which corresponds to 5×log2(numStates). The number of states is determined by the following equation:
$$numStates=\left\{\begin{array}{l}p\cdot {2}^{(L1)},\text{\hspace{0.17em}}for\text{\hspace{0.17em}}even\text{\hspace{0.17em}}m\\ 2p\cdot {2}^{(L1)},\text{\hspace{0.17em}}for\text{\hspace{0.17em}}odd\text{\hspace{0.17em}}m\end{array}\right\}$$
where:
h = m/p is the modulation index proper rational form
m = numerator of modulation index
p = denominator of modulation index
L is the Pulse length
Determines whether the output consists of bipolar or binary values.
The initial phase of the modulated waveform.
The number of input samples that represent each modulated symbol, which must be a positive integer. For more information, see Upsample Signals and Rate Changes.
Select the rate processing method for the block.
Enforce singlerate processing
—
When you select this option, the input and output signals have the
same port sample time. The block implements the rate change by making
a size change at the output when compared to the input. The output
width is the number of symbols (which is given by dividing the input
length by the Samples per symbol parameter value
when the Output type parameter is set to Integer
).
Allow multirate processing
—
When you select this option, the input and output signals have different
port sample times. The output period is the same as the symbol period
and equals the product of the input period and the Samples
per symbol parameter value.
For more information, see SingleRate Processing and Multirate Processing in the Description section of this page.
The number of trellis branches that the MSK Demodulator Baseband block uses to construct each traceback path.
The output data type can be boolean
,
int8
, int16
,
int32
, or double
.
Port  Supported Data Types 

Input 

Output 

[1] Anderson, John B., Tor Aulin, and CarlErik Sundberg, Digital Phase Modulation, New York, Plenum Press, 1986.