Demodulate CPMmodulated data
CPM, in Digital Baseband sublibrary of Modulation
The Continuous Phase Modulation (CPM) Demodulator Baseband block demodulates a signal that was modulated using continuous phase modulation. The input is a baseband representation of the modulated signal:
$$\begin{array}{l}s(t)\text{\hspace{0.17em}}=\text{\hspace{0.17em}}\mathrm{exp}\left[j\text{\hspace{0.17em}}2\pi \text{\hspace{0.17em}}{\displaystyle \sum _{i\text{\hspace{0.17em}}=\text{\hspace{0.17em}}0}^{n}{\alpha}_{i}{h}_{i}q(tiT)}\right],\\ nT<\text{\hspace{0.17em}}t<(n+1)T\end{array}$$
See the CPM Modulator Baseband block reference page for the definition of {α_{i}}, {h_{i}}, and q(t).
This block accepts a scalarvalued or a column vector input signal with a data type of single or double. CPM is a modulation method with memory. The optimum receiver consists of a correlator followed by a maximumlikelihood sequence detector (MLSD) that searches the paths through the state trellis for the minimum Euclidean distance path. When the Modulation index h is rational, i.e., h = m/p, there are a finite number of phase states and the block uses the Viterbi algorithm to perform MLSD.
{h_{i}} represents a sequence of modulation indices that moves cyclically through a set of indices {h_{0}, h_{1}, h_{2}, ….,h_{H1}}.
h_{i} = m_{i}/p_{i} represents the modulation index in proper rational form
m_{i} represents the numerator of modulation index
p_{i} represents the denominator of modulation index
m_{i} and p_{i} are relatively prime positive numbers
The Least Common Multiple (LCM) of {p_{0}, p_{1}, p_{2}, ….,p_{H1}} is denoted as p
h_{i }= m'_{i}/p
{h_{i}} determines the number of phase states:
$$numPhaseStates=\left\{\begin{array}{l}p,\text{\hspace{0.17em}}\text{\hspace{0.17em}}for\text{\hspace{0.17em}}\text{\hspace{0.17em}}all\text{\hspace{0.17em}}\text{\hspace{0.17em}}even\text{\hspace{0.17em}}\text{\hspace{0.17em}}m{\text{'}}_{i}\\ 2p,\text{\hspace{0.17em}}\text{\hspace{0.17em}}for\text{\hspace{0.17em}}\text{\hspace{0.17em}}any\text{\hspace{0.17em}}\text{\hspace{0.17em}}odd\text{\hspace{0.17em}}\text{\hspace{0.17em}}m{\text{'}}_{i}\end{array}\right\}$$
and affects the number of trellis states:
numStates = numPhaseStates*M^{(L1)}
where
L represents the Pulse length
M represents the Mary number
When you set the Output type parameter
to Integer
, then the block produces odd
integers between (M1) and M1. When you set the Output
type to Integer
, you cannot
set Output datatype to boolean
.
When you set the Output type parameter
to Bit
, then the block produces groupings
of K bits. Each grouping is called a binary word.
When you set the Output type to Bit
,
the Output datatype can only be double
or boolean
.
In binary output mode, the block first maps each input symbol
to an intermediate value as in the integer output mode. Then, the
block maps the odd integer k to the nonnegative integer (k+M1)/2.
Finally, the block maps each nonnegative integer to a binary word,
using a mapping that depends on whether you set the Symbol
set ordering parameter to Binary
or Gray
.
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.
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
, 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 $$5\cdot \mathrm{log}2(numStates)$$.
For a binary raised cosine pulse shape with a pulse length of 3, h=2/3, this rule $$(5*\mathrm{log}2(3*{2}^{2})=18)$$ gives a result close to the optimum value of 20.
The size of the alphabet.
Determines whether the output consists of integers or groups of bits.
Determines how the block maps each integer to a group of output
bits. This field applies only when you set Output type to Bit
.
Specify the modulation index {h_{i}}. The
default is 0.5
. The value of this property must
be a real, nonnegative scalar or column vector.
Specify the type of pulse shaping that the corresponding modulator uses to smooth the phase transitions of the modulated signal. You can select from the following pulse shapes:
Rectangular
Raised Cosine
Spectral Raised Cosine
This option requires an additional parameter, Rolloff. The Rolloff parameter, which affects the spectrum of the pulse, is a scalar between zero and one.
Gaussian
This option requires an additional parameter, BT product. The BT product parameter, which represents bandwidth multiplied by time, is a nonnegative scalar. It is used to reduce the bandwidth at the expense of increased intersymbol interference.
Tamed FM
(tamed frequency
modulation)
Number of symbol intervals of the largest lobe of the spectral
raised cosine pulse. This field is active only when Frequency
pulse shape is set to Spectral Raised Cosine
.
The rolloff factor of the specified raised cosine filter. This
field appears only when you set Frequency pulse shape to Spectral
Raised Cosine
.
The product of bandwidth and time. This field appears only when Frequency
pulse shape is set to Gaussian
.
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. 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 CPM Demodulator 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 

CPFSK Demodulator Baseband, GMSK Demodulator Baseband, MSK Demodulator Baseband, Viterbi Decoder
[1] Anderson, John B., Tor Aulin, and CarlErik Sundberg. Digital Phase Modulation. New York: Plenum Press, 1986.