Demodulate using CPM method and Viterbi algorithm
The CPMDemodulator object demodulates a signal that was modulated using continuous phase modulation. The input is a baseband representation of the modulated signal.
To demodulate a signal that was modulated using continuous phase modulation:
H = comm.CPMDemodulator creates a demodulator System object™, H. This object demodulates the input continuous phase modulated (CPM) data using the Viterbi algorithm.
H = comm.CPMDemodulator(Name,Value) creates a CPM demodulator object, H, with each specified property set to the specified value. You can specify additional name-value pair arguments in any order as (Name1,Value1,...,NameN,ValueN).
H = comm.CPMDemodulator(M,Name,Value) creates a CPM demodulator object, H, with the ModulationOrder property set to M, and the other specified properties set to the specified values.
Size of symbol alphabet
Specify the size of the symbol alphabet. The value of this property requires a power of two, real, integer scalar. The default is 4.
Output data as bits
Specify whether the output consists of groups of bits or integer values. The default is false.
When you set this property to false, the step method outputs a column vector of length equal to N/SamplesPerSymbol and with elements that are integers between -(ModulationOrder-1) and ModulationOrder–1. Here, N, is the length of the input signal which indicates the number of input baseband modulated symbols.
When you set this property to true, the step method outputs a binary column vector of length equal to P(N/SamplesPerSymbol), where P = log2(ModulationOrder). The output contains length-P bit words. In this scenario, the object first maps each demodulated symbol to an odd integer value, K, between –(ModulationOrder–1) and ModulationOrder–1. The object then maps K to the nonnegative integer (K+ModulationOrder–1)/2. Finally, the object maps each nonnegative integer to a length-P binary word, using the mapping specified in the SymbolMapping property.
Specify the mapping of the demodulated symbols as one of Binary | Gray. The default is Binary. This property determines how the object maps each demodulated integer symbol value (in the range 0 and ModulationOrder–1) to a P-length bit word, where P = log2(ModulationOrder).
When you set this property to Binary, the object uses a natural binary-coded ordering.
When you set this property to Gray, the object uses a Gray-coded ordering.
This property applies when you set the BitOutput property to true.
Specify the modulation index. The default is 0.5. The value of this property can be a scalar, h, or a column vector, [h0, h1, …. hH-1]
where H-1 represents the length of the column vector.
When hi varies from interval to interval, the object operates in multi-h. When the object operates in multi-h, hi must be a rational number.
Frequency pulse shape
Specify the type of pulse shaping that the modulator has used to smooth the phase transitions of the input modulated signal as one of Rectangular | Raised Cosine | Spectral Raised Cosine | Gaussian | Tamed FM. The default is Rectangular.
Main lobe duration of spectral raised cosine pulse
Specify, in number of symbol intervals, the duration of the largest lobe of the spectral raised cosine pulse. This value is the value that the modulatorused to pulse-shape the input modulated signal. The default is 1. This property requires a real, positive, integer scalar. This property applies when you set the FrequencyPulse property to Spectral Raised Cosine.
Rolloff factor of spectral raised cosine pulse
Specify the roll off factor of the spectral raised cosine pulse. This value is the value that the modulator used to pulse-shape the input modulated signal. The default is 0.2. This property requires a real scalar between 0 and 1. This property applies when you set the FrequencyPulse property to Spectral Raised Cosine.
Product of bandwidth and symbol time of Gaussian pulse
Specify the product of bandwidth and symbol time for the Gaussian pulse shape. This value is the value that the modulator used to pulse-shape the input modulated signal. The default is 0.3. This property requires a real, positive scalar. This property applies when you set the FrequencyPulse property to Gaussian.
Specify the length of the frequency pulse shape in symbol intervals. The value of this property requires a real positive integer. The default is 1.
Specify the data symbols used by the modulator prior to the first call to the step method. The default is 1. This property requires a scalar or vector with odd integer elements between –(ModulationOrder–1) and (ModulationOrder–1). If the value is a vector, then its length must be one less than the value in the PulseLength property.
Initial phase offset
Specify the initial phase offset of the input modulated waveform in radians as a real, numeric scalar. The default is 0.
Number of samples per input symbol
Specify the expected number of samples per input symbol as a positive, integer scalar. The default is 8.
Traceback depth for Viterbi algorithm
Specify the number of trellis branches that the Viterbi algorithm uses to construct each traceback path as a positive, integer scalar. The default is 16. The value of this property is also the output delay, which is the number of zero symbols that precede the first meaningful demodulated symbol in the output.
Data type of output
Specify the output data type as one of int8 | int16 | int32 | double, when you set the BitOutput property to false. When you set the BitOutput property to true, specify the output data type as one of logical | double. The default is double.
|clone||Create CPM demodulator object with same property values|
|getNumInputs||Number of expected inputs to step method|
|getNumOutputs||Number of outputs from step method|
|isLocked||Locked status for input attributes and nontunable properties|
|release||Allow property value and input characteristics changes|
|reset||Reset states of CPM demodulator object|
|step||Demodulate using CPM method and Viterbi algorithm|
% Create a CPM modulator, an AWGN channel, and a CPM demodulator. hMod = comm.CPMModulator(8, 'BitInput', true, ... 'SymbolMapping', 'Gray'); hAWGN = comm.AWGNChannel('NoiseMethod', ... 'Signal to noise ratio (SNR)','SNR',0); hDemod = comm.CPMDemodulator(8, 'BitOutput', true, ... 'SymbolMapping', 'Gray'); % Create an error rate calculator, account for the delay caused by the Viterbi algorithm. delay = log2(hDemod.ModulationOrder)*hDemod.TracebackDepth; hError = comm.ErrorRate('ReceiveDelay', delay); for counter = 1:100 % Transmit 100 3-bit words data = randi([0 1],300,1); modSignal = step(hMod, data); noisySignal = step(hAWGN, modSignal); receivedData = step(hDemod, noisySignal); errorStats = step(hError, data, receivedData); end fprintf('Error rate = %f\nNumber of errors = %d\n', ... errorStats(1), errorStats(2))
Error rate = 0.004006 Number of errors = 120
This object implements the algorithm, inputs, and outputs described on the CPM Demodulator Baseband block reference page. The object properties correspond to the block parameters.