GMSK Demodulator Passband (Communications Blockset)

Demodulate GMSK-modulated data

Library

CPM, in Digital Passband sublibrary of Modulation

Description

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

This block converts the input to an equivalent baseband representation using downconversion and then FIR decimation. The block uses the baseband equivalent block, GMSK Demodulator Baseband, for internal computations. The following parameters in this block are the same as those of the baseband equivalent block:

Output type
BT product
Pulse length
Symbol prehistory
Traceback length

The input must be a sample-based scalar signal.

Parameters Specific to Passband Simulation

Passband simulation uses a carrier signal. The Carrier frequency and Carrier initial phase parameters specify the frequency and initial phase, respectively, of the carrier signal. The Input sample time parameter specifies the sample time of the input signal, while the Symbol period parameter equals the sample time of the output signal.

This block uses a baseband representation of the modulated signal as an intermediate signal during internal computations. The Baseband samples per symbol parameter indicates how many baseband samples correspond to each integer or binary word in the output.

The timing-related parameters must satisfy these relationships:

Symbol period > (Carrier frequency)^-1
Symbol period must be an integer multiple of the product of Output sample time and Baseband samples per symbol.
Baseband samples per symbol > 4
Output sample time < [2*Carrier frequency + 2*F_max)]^-1

where F_max is defined as follows:

F_max = [Frequency separation * (M-ary number - 1) / 2] + 1 / Symbol period

The Carrier frequency parameter is typically much larger than the highest frequency of the baseband signal.

The GMSK Demodulator Passband block creates a delay in signals that it processes. This delay is caused by FIR filters in the block, whose tap length depends on signal and simulation parameters.

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.
Symbol period (s): The symbol period, which equals the sample time of the output.
Baseband samples per symbol: The number of baseband samples that represent each modulated symbol, after the block converts the passband input to a baseband intermediary signal.
Carrier frequency (Hz): The frequency of the carrier.
Carrier initial phase (rad): The initial phase of the carrier in radians.
Input sample time (s): The sample time of the input signal.
Traceback length: The number of trellis branches that the Viterbi Decoder block uses to construct each traceback path.

Pair Block

GMSK Modulator Passband

References

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

GMSK Demodulator Baseband GMSK Modulator Baseband