GMSK Modulator

Modulate data bits using Gaussian minimum shift keying (GMSK) method

Since R2024a

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Libraries:
Wireless HDL Toolbox / Modulation

Description

The GMSK Modulator block modulates data bits using the Gaussian minimum shift keying (GMSK) method. The block supports scalar inputs and outputs modulated complex-valued scalar or column-vector data symbols along with valid and ready controls signals. The block enables you to use your bandwidth more effectively and helps reduce the problems caused by multipath fading and signal distortion.

You can use this block in applications such as global system for mobile communications (GSM), satellite communications, Bluetooth, military communications, tactical radios, drone control/command and missile communications. The block provides an interface and architecture suitable for HDL code generation and hardware deployment.

Examples

Modulate Data Bits Using GMSK

Use GMSK Modulator block to modulate data bits to symbols using Gaussian minimum shift keying (GMSK) method.

• Open Script

Ports

Input

data — Data bits

scalar

Data bits, specified as a Boolean scalar or ufix1.

The block supports double and single data types for simulation, but not for HDL code generation.

For HDL code generation, the input data type must be ufix1 or Boolean and the maximum output word length the block supports is 16 bits.

Data Types: single | double | Boolean | fixdt(0,1,0)

valid — Indicates valid input data

scalar

Indicates valid input data, specified as a Boolean scalar.

This port is a control signal that indicates when the sample from the data input port is valid. When this value is 1, the block captures the values on the data input port. When this value is 0, the block ignores the values on the data input port.

Data Types: Boolean

reset — Clear internal states

scalar

Clear internal states, specified as a Boolean scalar. When this value is 1, the block stops the current calculation and clears all internal states.

Dependencies

To enable this port, select the Enable reset input port parameter.

Data Types: Boolean

Output

data — Modulated complex output data symbols

complex scalar | complex column vector

Modulated complex output data symbols, returned as a complex-valued scalar or column vector.

• When you set the Output type parameter to Vector, the output is a vector of number of samples per symbol.

• When you set the Output type parameter to Scalar, the output is a scalar.

Data Types: single | double | signed fixed point
Complex Number Support: Yes

valid — Indicates valid output data

scalar

Indicates valid output data, returned as a scalar.

This port is a control signal that indicates when the data output port is valid. The block sets this value to 1 when the data samples are available on the data output port.

Data Types: Boolean

ready — Indicates block is ready

scalar

Indicates block is ready, returned as a scalar.

This is a control signal that indicates when the block is ready for new input data. When this value is 1, the block accepts input data in the next time step. When this value is 0, the block ignores input data in the next time step.

Dependencies

To enable this port, set the Output type parameter to Scalar.

Data Types: Boolean

Parameters

Main

Bandwidth time product — Product of the bandwidth and symbol time for the Gaussian pulse shape

0.3 (default) | positive scalar value

Product of the signal bandwidth and symbol time for the Gaussian pulse shape, specified as a positive scalar value.

Pulse length — Pulse length

8 (default) | positive integer in the range from 1 to 16

Pulse length, specified as a positive integer. The pulse length value represents the length of the Gaussian pulse shape in symbol intervals.

Symbol prehistory — Symbol prehistory

1 (default) | -1 | vector of –1s or 1s with length equal to L – 1

Symbol prehistory, specified as a scalar or vector of –1s or 1s that the block uses before the start of the simulation.

• For a scalar value, the block expands it to a vector of length L – 1, where L represents the pulse length. You can specify the L value using the Pulse length parameter.

  For example, if you specify a scalar value -1 and if the Pulse length parameter is set to 4, the block expands this value as [-1 -1 -1].

• For a vector value, the block considers the elements in the vector in a reverse chronological order. The vector length must be L – 1.

  For example, if you specify a vector value [-1 1 1] and if the Pulse length parameter is set to 4, the block considers this value as [1 1 -1].

Phase offset (rad) — Phase offset of modulated waveform

0 (default) | numeric scalar in the range from -2*π to 2*π.

Phase offset of the modulated waveform in radians, specified as a numeric scalar in the range from –2*π to 2*π.

Number of samples per symbol — Number of samples per symbol

4 (default) | in the range 1 to 64

Number of samples per symbol, specified as a positive integer in the range 1 to 64. The number of samples per symbol represents the upsampling factor from input samples to output samples.

Number of data points — Number of data points

1024 (default) | positive integer within the range 65,536 and a power of 2

Number of data points to retrieve from the look-up table (LUT), specified as a positive integer and a power of 2. The maximum value the block supports is 65,536.

Output type — Output type

Vector (default) | Scalar

Specify the output type as Vector or Scalar.

Data Type

Output word length — Output word length

16 (default) | positive scalar value with word length less than 16

Output word length, specified as a positive scalar with word length less than 16.

Control Port

Enable reset input port — Reset signal

off (default) | on

Select this parameter to enable the reset input port.

Algorithms

The Bandwidth time product parameter represents bandwidth multiplied by time. This parameter reduces the bandwidth at the expense of increased intersymbol interference. The Pulse length parameter measures the length of the Gaussian pulse shape, in symbol intervals. B_b represents the bandwidth of the pulse and T is the symbol durations. Q(t) is the complementary cumulative distribution function.

$$\begin{array}{l}g(t)=\frac{1}{2T}\left\{Q\left[2\pi B_b\frac{t-\frac{T}{2}}{\sqrt{\ln (2)}}\right]-Q\left[2\pi B_b\frac{t+\frac{T}{2}}{\sqrt{\ln (2)}}\right]\right\}\\ Q(t)={\displaystyle \underset{t}{\overset{\infty }{\int }}\frac{1}{\sqrt{2\pi }}}{e}^{-{\tau }^2/2}d\tau \end{array}$$

The block generates the filter coefficients using these equations based on the parameters you select in the block mask of the example. The choice of Bandwidth time product parameter decides the performance of the modulator as the bit error rate can be expressed as a function of BT. For this block, an input symbol of 1 causes a phase shift of π/2 radians, which corresponds to a modulation index of 0.5.

Latency

The latency of the block is equal to Pulse length + 4 clock cycles.

This figure shows a sample output and latency of the GMSK Modulator block for default configuration. The latency of the block is eight clock cycles.

Performance

The performance of the synthesized HDL code varies with your target and synthesis options. The input data type used in this example for generating HDL code is fixdt(0,1,0).

This table shows the resource and performance data synthesis results of the block, when you set the Number of samples per symbol parameter to 4, the Pulse length parameter to 8, the Number of data points parameter to 1024, and the Output word length parameter to 16. The generated HDL is targeted to the AMD^® Zynq^®- 7000 ZC706 evaluation board.

Output Type	Slice LUTs	Slice Registers	DSPs	Block RAMs	Maximum Frequency in MHz
Scalar	462	389	0	2	259
Vector	476	245	0	2	276

This table shows the resource and performance data synthesis results of the block, when you set the Number of samples per symbol parameter to 4, the Pulse length parameter to 8, the Number of data points parameter to 16384, and the Output word length parameter to 16. The generated HDL is targeted to the AMD Zynq- 7000 ZC706 evaluation board.

Output Type	Slice LUTs	Slice Registers	DSPs	Block RAMs	Maximum Frequency in MHz
Scalar	557	397	0	32	240
Vector	576	253	0	32	254

Increasing the number of data points improves the precision of the results. However, it is important to note that this increment also leads to higher utilization of LUTs (Look-Up Tables) and BRAMs (Block RAMs), which could impact overall resource efficiency. To maintain a balance between precision and performance, it is recommended to limit the use of bits to no more than 25, as exceeding this can prolong simulation times and further increase resource usage. The standard LUT size is calculated as 2 raised to the power of (Output word length – 2). You can modify the value based on your requirements.

References

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

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using Simulink® Coder™.

This block supports C/C++ code generation for Simulink^® accelerator and rapid accelerator modes and for DPI component generation.

HDL Code Generation

Generate VHDL, Verilog and SystemVerilog code for FPGA and ASIC designs using HDL Coder™.

HDL Coder™ provides additional configuration options that affect HDL implementation and synthesized logic.

HDL Architecture

This block has one default HDL architecture.

HDL Block Properties

ConstrainedOutputPipeline	Number of registers to place at the outputs by moving existing delays within your design. Distributed pipelining does not redistribute these registers. The default is 0. For more details, see ConstrainedOutputPipeline (HDL Coder).
InputPipeline	Number of input pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see InputPipeline (HDL Coder).
OutputPipeline	Number of output pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see OutputPipeline (HDL Coder).

Restrictions

You cannot generate HDL for this block inside a Resettable Synchronous Subsystem (HDL Coder).

Version History

Introduced in R2024a