Communications Blockset

GMSK Demodulator Baseband

Demodulate GMSK-modulated data

Library

CPM, in Digital Baseband sublibrary of Modulation

Description

The GMSK Demodulator Baseband block demodulates a signal that was modulated using the Gaussian minimum shift keying method. The input is a baseband representation of the modulated signal.

The BT product, Pulse length, Symbol prehistory, and Phase offset parameters are as described on the reference page for the GMSK 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.

Inputs and Outputs

The input can be either a scalar or a frame-based column vector. If the Output type parameter is set to Integer, then the block produces values of 1 and -1. If the Output type parameter is set to Bit, then the block produces values of 0 and 1.

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

Output type

Determines whether the output consists of bipolar or binary values.

BT product

The product of bandwidth and time.

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

GMSK Modulator Baseband

See Also

CPM Demodulator Baseband, Viterbi Decoder

References

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

General QAM Modulator Passband

GMSK Demodulator Passband