CPM Demodulator Baseband (Communications Blockset)

Demodulate CPM-modulated data

Library

CPM, in Digital Baseband sublibrary of Modulation

Description

The CPM Demodulator Baseband block demodulates a signal that was modulated using continuous phase modulation. The input is a baseband representation of the modulated signal. The M-ary number parameter, M, is the size of the input alphabet. M must have the form 2^K for some positive integer K.

The input can be either a scalar or a frame-based column vector.

The Modulation index, Frequency pulse shape, Rolloff, BT product, Pulse length, Symbol prehistory, and Phase offset parameters are as described on the reference page for the CPM Modulator Baseband block.

Traceback Length and Output Delays

Internally, this block creates a trellis description of the modulation scheme and uses the Viterbi algorithm. The Traceback length 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.

If the input signal is sample-based, then the delay consists of D+1 zero symbols.
If the input signal is frame-based, then the delay consists of D zero symbols.

Outputs and Symbol Sets

If the Output type parameter is set to Integer, then the block produces odd integers between -(M-1) and M-1.

If the Output type parameter is set to Bit, then the block produces groupings of K bits. Each grouping is called a binary word.

In binary output mode, the block first maps each input symbol to an intermediate value as in the integer output mode. The block then maps the odd integer k to the nonnegative integer (k+M-1)/2. Finally, the block maps each nonnegative integer to a binary word, using a mapping that depends on whether the Symbol set ordering parameter is set to Binary or Gray. For more information about Gray and binary coding, see Binary-Valued and Integer-Valued Signals in Using the Communications Blockset.

Processing an Upsampled Modulated Signal

The input signal can be an upsampled version of the modulated signal. The Samples per symbol parameter is the upsampling factor. It must be a positive integer. For more information, see Upsampled Signals and Rate Changes in Using the Communications Blockset.

Dialog Box

M-ary number: The size of the alphabet.
Output type: Determines whether the output consists of integers or groups of bits.
Symbol set ordering: Determines how the block maps each integer to a group of output bits. This field is active only when Output type is set to Bit.
Modulation index: The number of half-revolutions of phase shift in the modulated signal after modulating the latest symbol of 1.
Frequency pulse shape: The type of pulse shaping that the corresponding modulator uses to smooth the phase transitions of the modulated signal.
Main lobe pulse duration (symbol intervals): 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.
Rolloff: The rolloff factor of the raised cosine filter. This field appears only when Frequency pulse shape is set to Spectral Raised Cosine.
BT product: The product of bandwidth and time. This field appears only when Frequency pulse shape is set to Gaussian.
Pulse length (symbol intervals): The length of the frequency pulse shape.
Symbol prehistory: The data symbols used by the modulator before the start of the simulation.
Phase offset (rad): The initial phase of the modulated waveform.
Samples per symbol: The number of input samples that represent each modulated symbol.
Traceback length: The number of trellis branches that the Viterbi Decoder block uses to construct each traceback path.

Pair Block

CPM Modulator Baseband

References

[1] Anderson, John B., Tor Aulin, and Carl-Erik Sundberg. Digital Phase Modulation. New York: Plenum Press, 1986.

CPFSK Modulator Passband CPM Demodulator Passband