GMSK Demodulator Baseband
Demodulate GMSKmodulated data
Library
CPM, in Digital Baseband sublibrary of Modulation
Description
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.
IntegerValued Signals and BinaryValued Signals
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.
SingleRate Processing
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.
Multirate Processing
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.
Traceback Depth and Output Delays
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 toSingleTasking
, 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 5log_{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
Parameters
 Output type
Determines whether the output consists of bipolar or binary values.
 BT product
The product of bandwidth and time.
 Pulse length (symbol intervals)
The length of the frequency pulse shape.
 Symbol prehistory
A scalar or vector value that specifies the data symbols the block uses before the start of the simulation, in reverse chronological order.
A scalar value expands to a vector of length L_{P} – 1. L_{P} represents the pulse length, which is specified by the Pulse length (symbol intervals) parameter.
For a vector, the length must be L_{P} – 1.
 Phase offset (rad)
The initial phase of the modulated waveform.
 Samples per symbol
The number of input samples that represent each modulated symbol, which must be a positive integer. For more information, see Signal Upsampling and Rate Changes in Communications Toolbox™ User's Guide.
 Rate options
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 toInteger
).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.
 Traceback depth
The number of trellis branches that the GMSK Demodulator Baseband block uses to construct each traceback path.
 Output data type
The output data type can be
boolean
,int8
,int16
,int32
, ordouble
.
Supported Data Types
Port  Supported Data Types 

Input 

Output 

Pair Block
References
[1] Anderson, John B., Tor Aulin, and CarlErik Sundberg. Digital Phase Modulation. New York: Plenum Press, 1986.
Extended Capabilities
Version History
Introduced before R2006a