dvbrcs2CPMDemodConfig

R2026b

DVB-RCS2 CPM demodulator configuration object

Since R2026b

Description

The dvbrcs2CPMDemodConfig object sets the configuration parameters for the dvbrcs2CPMDemodulate function, which demodulates signals modulated using the continuous phase modulation (CPM) method, as specified in the Digital Video Broadcasting Second Generation Return Channel over Satellite (DVB-RCS2) standard ETSI EN 301 545-2 V1.4.1 [1]. For more information on demodulation, filtering applied, and iterative decoding, see the More About section.

Creation

Syntax

cpmDemodCfg = dvbrcs2CPMDemodConfig

cpmDemodCfg = dvbrcs2CPMDemodConfig(PropertyName=Value)

Description

cpmDemodCfg = dvbrcs2CPMDemodConfig creates a default DVB-RCS2 CPM demodulator configuration object.

cpmDemodCfg = dvbrcs2CPMDemodConfig(PropertyName=Value) sets properties using one or more name-value arguments. For example, dvbrcs2CPMDemodConfig(ModulationIndex=0.4) sets the modulation index to 0.4.

Properties

ModulationIndex — Modulation index

0.5 (default) | nonnegative scalar

Modulation index, specified as a nonnegative scalar. For more information, see CPM Method.

Data Types: double

PhaseResponseCoefficient — Phase response coefficient

0.98 (default) | scalar in the range [0, 1]

Phase response coefficient, specified as a scalar in the range [0, 1]. This value represents the weighting of the raised cosine component in the weighted average CPM pulse shape defined by the DVB-RCS2 standard. For more information, see Pulse Shape Filtering.

Data Types: double

SamplesPerSymbol — Number of samples per symbol

8 (default) | nonnegative scalar

Number of samples per symbol, specified as a positive integer. This value represents the number of samples per CPM symbol in the input signal to the dvbrcs2CPMDemodulate function. The length of the input signal must be an integer multiple of this property.

Data Types: double

TracebackDepth — Traceback depth

16 (default) | nonnegative scalar

Traceback depth for CPM demodulation, specified as a nonnegative scalar. The demodulator uses a trellis-based algorithm, and a larger traceback depth can improve demodulation accuracy at the cost of increased computation. The demodulator appends termination symbols internally to flush the trellis.

Data Types: double | int8

OutputType — Type of demodulator output

"integer" (default) | "bit" | "approxLLR"

Type of demodulator output, specified as one of these options.

• "integer" — Output demodulated CPM symbols from the set {-3, -1, 1, 3}. Use this option for symbol error rate performance simulations.

• "bit" — Output demodulated binary bits obtained by demapping the CPM symbols using Gray mapping. Use this option for bit error rate performance simulations.

• "approxLLR" — Output approximate log-likelihood ratios (LLRs) for iterative decoding within the DVB-RCS2 serially concatenated convolutional code (SCCC) structure. The LLRs enable exchange of extrinsic information between the CPM demodulator and the outer convolutional decoder. For more information, see Iterative Decoding.

Data Types: char | string

OutputDataType — Data type of output

"double" (default) | "single" | "int8" | "logical"

Data type of the demodulator output, specified as "double", "single", "int8", or "logical".

Dependencies

To enable this property, set the OutputType property to "bit" or "integer".

When you set the OutputType property to "approxLLR", the output inherits the data type of the input signal to the dvbrcs2CPMDemodulate function.

Data Types: char | string

Object Functions

Specific to This Object

dvbrcs2CPMDemodulate

Demodulate signal using CPM method specified in DVB-RCS2 standard

Examples

Demodulate DVB-RCS2 CPM Signal

Create a DVB-RCS2 CPM demodulator configuration object and demodulate a CPM modulated signal.

Define modulation parameters for DVB-RCS2 reference waveform ID 9.

payloadBitLen = 2052; % Payload length in bits
preambleLen = 64;     % Preamble length in bits
sps = 4;              
modulationIndex = 2/5;
alphaRC = 0.98;       % phase response coefficient

Create a CPM modulator object.

modulatorObj = dvbrcs2CPMModulator(ModulationIndex=modulationIndex, ...
    PhaseResponseCoefficient=alphaRC, ...
    SamplesPerSymbol=sps, ...
    OutputDataType="double");

Generate and interleave the input bits.

numBits = preambleLen+payloadBitLen;
bits = randi([0,1],numBits,1);
% Specify the initial state for random number generator
seed = 4831; 
inpBits = randintrlv(bits,seed);

Map interleaved bits to CPM input symbols using Gray mapping.

bit1 = inpBits(1:2:end);
bit2 = inpBits(2:2:end);
modIn = -3 + 4*bit1 + 2*(xor(bit1,bit2));

Generate the CPM-modulated waveform.

modOut = modulatorObj(modIn);

Add noise to the modulated signal.

rxIn = awgn(modOut,10,"measured");

Create a demodulator configuration object with matching parameters and demodulate the received CPM-modulated signal.

cpmDemodCfg = dvbrcs2CPMDemodConfig(ModulationIndex=modulationIndex, ...
    PhaseResponseCoefficient=alphaRC, ...
    SamplesPerSymbol=sps, ...
    OutputType="integer");

Demodulate the received waveform.

demodOutSymbols = dvbrcs2CPMDemodulate(rxIn,cpmDemodCfg);

Verify the demodulation by comparing the output symbols to the original modulator input.

isequal(demodOutSymbols(1:end-2),modIn(1:end-2))

ans = logical
   1

More About

CPM Method

The demodulator recovers data symbols from a received CPM signal. The CPM modulated signal has the baseband representation:

$$\begin{array}{l}s(t)=\exp \left[j2\pi h{\displaystyle \sum _{i=0}^n{\alpha }_{i}q(t-i{T}_s)}\right],\text{ and}\\ n{T}_s<t<(n+1)T_s\end{array}$$

where:

• h is the modulation index.

• {α _i } is a sequence of M-ary data symbols, which is the input to the modulator, and must be from the set {±1, ±3}. Hence, M is equal to 4.

• q is the CPM phase response.

Pulse Shape Filtering

The CPM method uses pulse shaping to smooth the phase transitions of the modulated signal. DVB-RCS2 standard ETSI EN 301 545-2 V1.4.1 defines the pulse shape, known as weighted average CPM pulse shape, to be a linear combination of the raised cosine and rectangular pulse shapes. The weighted average g_AV is calculated as:

g_AV(t) = α_RCg_RC(t) + (1 - α_RC)g_REC(t)

where:

• α_RC is the phase response coefficient of the raised cosine pulse shape and 0 ≤ α_RC ≤ 1.

• g_RC is the raised cosine phase pulse.

  $$g_{RC}(t)=\{\begin{array}{cc}\frac{1}{2L{T}_s}\left[1-\cos (\frac{2\pi t}{L{T}_s})\right],& 0\le t\le L{T}_s\\ 0& \text{otherwise}\end{array}$$

  where L is the memory of modulation and is set to L=2.

• The value of (1 – α_RC ) gives the phase response coefficient of the rectangular phase pulse.

  For example, if α_RC = 0.75, the coefficient of the rectangular phase pulse is 0.25 = (1 – 0.75).

• g_REC is the rectangular phase pulse.

  $$g_{REC}(t)=\{\begin{array}{cc}\frac{1}{2L{T}_s},& 0\le t\le L{T}_s\\ 0& \text{otherwise}\end{array}$$

Iterative Decoding

The DVB-RCS2 standard specifies a SCCC structure in which the CPM modulator acts as the inner encoder and a convolutional code acts as the outer encoder. The demodulator functions as the inner decoder in this structure. The iterative decoding process operates as follows:

1. The CPM demodulator computes approximate LLRs for the coded bits using the Bahl-Cocke-Jelinek-Raviv (BCJR) algorithm.

2. The LLR output is deinterleaved and passed to the outer decoder (convolutional decoder).

3. The outer decoder produces extrinsic information, which is interleaved and fed back to the CPM demodulator as extrinsic input for the next iteration.

4. The process repeats for multiple iterations to improve demodulation performance.

References

[1] ETSI Standard EN 301 545-2 V1.4.1(2024-01). Digital Video Broadcasting (DVB); Second Generation Interactive Satellite Systems (DVB-RCS2); Part-2: Lower Layers for Satellite Standard.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.

Version History

Introduced in R2026b