Communications Blockset™

CPM Modulator Baseband - Modulate using continuous phase modulation

Library

CPM, in Digital Baseband sublibrary of Modulation

Description

The CPM Modulator Baseband block modulates using continuous phase modulation. The output 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.

Continuous phase modulation uses pulse shaping to smooth the phase transitions of the modulated signal. Using the Frequency pulse shape parameter, you can choose these types of 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)

For the exact definitions of these pulse shapes, see the work by Anderson, Aulin, and Sundberg among the references listed below. Each pulse shape has a correponding pulse duration. The Pulse length parameter measures this quantity in symbol intervals.

The Modulation index parameter times π radians is the phase shift due to the latest symbol when that symbol is the integer 1. The Phase offset parameter is the initial phase of the output waveform, measured in radians.

The Symbol prehistory parameter is a scalar or vector that specifies the data symbols used before the start of the simulation, in reverse chronological order. If it is a vector, then its length must be one less than the Pulse length parameter.

Inputs and Symbol Sets

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

If the Input type parameter is set to Bit, then the block accepts groupings of K bits. Each grouping is called a binary word. The input vector length must be an integer multiple of K.

In binary input mode, the block maps each binary word to an integer between 0 and M-1, using a mapping that depends on whether the Symbol set ordering parameter is set to Binary or Gray. The block then maps the integer k to the intermediate value 2k-(M-1) and proceeds as in the integer input mode. For more information, see Binary-Valued and Integer-Valued Signals in Communications Blockset User's Guide.

The input can be either a scalar or a frame-based column vector. If Input type is Bit, then the input can also be a vector of length K.

Upsampling the Modulated Signal

This block can output 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 Communications BlocksetUser's Guide.

Dialog Box

M-ary number: The size of the alphabet.
Input type: Indicates whether the input consists of integers or groups of bits.
Symbol set ordering: Determines how the block maps each group of input bits to a corresponding integer. This field is active only when Input type is set to Bit.
Modulation index: The number of half-revolutions of phase shift due to the latest symbol when that symbol is the integer 1.
Frequency pulse shape: The type of pulse shaping that the block 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 before the start of the simulation, in reverse chronological order.
Phase offset (rad): The initial phase of the output waveform.
Samples per symbol: The number of output samples that the block produces for each integer or binary word in the input.
Output data type: This block supports double and single data types.

Pair Block

CPM Demodulator Baseband

References

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