CPFSK Demodulator Baseband

Demodulate CPFSK-modulated data

Library

CPM, in Digital Baseband sublibrary of Modulation

Description

The CPFSK Demodulator Baseband block demodulates a signal that was modulated using the continuous phase frequency shift keying method. The input to this block 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 2K for some positive integer K.

This block supports multi-h Modulation index. See CPM Modulator Baseband for details.

Integer-Valued Signals and Binary-Valued Signals

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

When you set the Output type parameter 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.

This block accepts a scalar-valued or column vector input signal with a data type of single or double.

Single-Rate Processing

In single-rate 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.

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 that the algorithm uses 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 variable-step solver or a fixed-step 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 single-rate 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 "five-times-the-constraint-length" rule, which corresponds to 5·log2(numStates).

For the definition of the number of states, see CPM Demodulator Baseband Help page.

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

Specify the modulation index {hi}. The default is 0.5. The value of this property must be a real, nonnegative scalar or column vector.

This block supports multi-h Modulation index. See CPM Modulator Baseband for details.

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 Upsample Signals and Rate Changes in Communications System Toolbox™ User's Guide.

Rate options

Select the rate processing method for the block.

  • Enforce single-rate 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 Inherit from input (this choice will be removed - see release notes) will be removed in a future release. See Frame-Based Processing in the Communications System Toolbox Release Notes for more information.

For more information, see Single-Rate Processing and Multirate Processing in the Description section of this page.

Traceback depth

The number of trellis branches that the CPFSK Demodulator Baseband block uses to construct each traceback path.

Output datatype

The output data type can be boolean, int8, int16, int32, or double.

Supported Data Types

PortSupported Data Types

Input

  • Double-precision floating point

  • Single-precision floating point

Output

  • Double-precision floating point

  • Boolean (When Output type set to Bit)

  • 8-, 16-, and 32-bit signed integers (When Output type set to Integer)

References

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

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