Demodulate GMSKmodulated data
CPM, in Digital Baseband sublibrary of Modulation
The GMSK Demodulator Baseband block uses a Viterbi algorithm to demodulate a signal that was modulated using the Gaussian minimum shift keying method. The input to this block is a baseband representation of the modulated signal.
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 two times the number of input symbols.
When you set Output type to Integer
,
the output width is the number of input symbols.
For a column vector input signal, the width of the input equals the product of the number of symbols and the value for the Samples per symbol parameter.
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 $$5\cdot \mathrm{log}2(numStates)$$. 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 in 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 product of bandwidth and time.
The length of the frequency pulse shape.
The data symbols the modulator uses before the start of the simulation.
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 in Communications System Toolbox™ User's Guide.
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.
Note:
The option 
For more information, see SingleRate Processing and Multirate Processing in the Description section of this page.
The number of trellis branches that the GMSK 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.