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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 theGMSK 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.

The optimal Traceback length parameter value choice is dependent on minimum squared Euclidean distance calculations. Alternatively, a typical value can be chosen using the "five-times-the-constraint-length" rule which depends on the number of states given by:

h = m/p is the modulation index proper rational form
- m = numerator of modulation index
- p = denominator of modulation index
L is the Pulse length
M is M-ary number

Inputs and Outputs

The input can be either a scalar or a frame-based column vector and must be of type single or double. 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 Communications Blockset User's Guide.

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 GMSK Demodulator Baseband block uses to construct each traceback path.
Output data type: The output data type can be boolean, int8, int16, int32, or double.