Communications Blockset

GMSK Modulator Passband

Modulate using the Gaussian minimum shift keying method

Library

CPM, in Digital Passband sublibrary of Modulation

Description

The GMSK Modulator Passband block modulates using the Gaussian minimum shift keying method. The output is a passband representation of the modulated signal.

This block uses the baseband equivalent block, GMSK Modulator Baseband, for internal computations and converts the resulting baseband signal to a passband representation, using FIR interpolation and then upconversion. The following parameters in this block are the same as those of the baseband equivalent block:

• Input type
• BT product
• Pulse length
• Symbol prehistory

The input must be sample-based. If the Input type parameter is Bit, then the input must be a vector of length log₂(M). If the Input type parameter is Integer, then the input must be a scalar.

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 Symbol period parameter must equal the sample time of the input signal, while the Output sample time parameter determines the sample time of the output signal.

This block uses a baseband representation of the modulated signal as an intermediate result during internal computations. The Baseband samples per symbol parameter indicates how many baseband samples correspond to each integer or binary word in the input, before the block converts them to a passband 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 Modulator 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

Input type

Indicates whether the input 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 before the start of the simulation, in reverse chronological order.

Symbol period (s)

The symbol period, which must equal the sample time of the input.

Baseband samples per symbol

The number of baseband samples that correspond to each integer or binary word in the input, before the block converts them to a passband output.

Carrier frequency (Hz)

The frequency of the carrier.

Carrier initial phase (rad)

The initial phase of the carrier in radians.

Output sample time (s)

The sample time of the output signal.

Pair Block

GMSK Demodulator Passband

See Also

GMSK Modulator Baseband

References

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

GMSK Modulator Baseband

Gold Sequence Generator