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comm.GMSKDemodulator System object

Package: comm

Demodulate using GMSK method and the Viterbi algorithm

Description

The GMSKDemodulator object uses a Viterbi algorithm to demodulate a signal that was modulated using the Gaussian minimum shift keying method. The input is a baseband representation of the modulated signal.

To demodulate a signal that was modulated using Gaussian minimum shift keying:

  1. Define and set up your GMSK demodulator object. See Construction.

  2. Call step to demodulate a signal according to the properties of GMSKDemodulator. The behavior of step is specific to each object in the toolbox.

    Note:   Starting in R2016b, instead of using the step method to perform the operation defined by the System object™, you can call the object with arguments, as if it were a function. For example, y = step(obj,x) and y = obj(x) perform equivalent operations.

Construction

H = comm.GMSKDemodulator creates a demodulator System object, H. This object demodulates the input Gaussian minimum shift keying (GMSK) modulated data using the Viterbi algorithm.

H = comm.GMSKDemodulator(Name,Value) creates a GMSK demodulator object, H. This object has each specified property set to the specified value. You can specify additional name-value pair arguments in any order as (Name1,Value1,...,NameN,ValueN).

Properties

BitOutput

Output data as bits

Specify whether the output is groups of bits or integer values. The default is false.

When you set the BitOutput property to false, the step method outputs a column vector of length equal to N/SamplesPerSymbol. N is the length of the input signal, which is the number of input baseband modulated symbols. The elements of the output vector are –1 or 1.

When you set the BitOutput property to true, the step method outputs a binary column vector of length equal to N/SamplesPerSymbol with bit values of 0 or 1.

BandwidthTimeProduct

Product of bandwidth and symbol time of Gaussian pulse

Specify the product of bandwidth and symbol time for the Gaussian pulse shape as a real, positive scalar. The default 0.3.

PulseLength

Pulse length

Specify the length of the Gaussian pulse shape in symbol intervals as a real positive integer. The default 4.

SymbolPrehistory

Symbol prehistory

Specify the data symbols used by the modulator prior to the first call to the step method. The default is 1. This property requires a scalar or vector with elements equal to -1 or 1. If the value is a vector, its length must be one less than the value you set in the PulseLength property.

InitialPhaseOffset

Initial phase offset

Specify the initial phase offset of the input modulated waveform in radians as a real, numeric scalar value. The default is 0.

SamplesPerSymbol

Number of samples per input symbol

Specify the expected number of samples per input symbol as a positive, integer scalar value. The default is 8.

TracebackDepth

Traceback depth for Viterbi algorithm

Specify the number of trellis branches that the Viterbi algorithm uses to construct each traceback path as a positive, integer scalar value. The value of this property is also the output delay, and the number of zero symbols that precede the first meaningful demodulated symbol in the output. The default is 16.

OutputDataType

Data type of output

Specify the output data type as one of int8 | int16 | int32 | double, when you set the BitOutput property to false.

When you set the BitOutput property to true, specify the output data type as one of logical | double. The default is double.

Methods

cloneCreate GMSK demodulator object with same property values
getNumInputsNumber of expected inputs to step method
getNumOutputsNumber of outputs from step method
isLockedLocked status for input attributes and nontunable properties
releaseAllow property value and input characteristics changes
resetReset states of the GMSK demodulator object
stepDemodulate using GMSK method and the Viterbi algorithm

Examples

expand all

% Create a GMSK modulator, an AWGN channel, and a GMSK demodulator. Use a phase offset of pi/4.
    hMod = comm.GMSKModulator('BitInput', true, 'InitialPhaseOffset', pi/4);
    hAWGN = comm.AWGNChannel('NoiseMethod', ...
                    'Signal to noise ratio (SNR)','SNR',0);
    hDemod = comm.GMSKDemodulator('BitOutput', true, ...
                    'InitialPhaseOffset', pi/4);
% Create an error rate calculator, account for the delay caused by the Viterbi algorithm
    hError = comm.ErrorRate('ReceiveDelay', hDemod.TracebackDepth);
    for counter = 1:100
      % Transmit 100 3-bit words
      data = randi([0 1],300,1);
      modSignal = step(hMod, data);
      noisySignal = step(hAWGN, modSignal);
      receivedData = step(hDemod, noisySignal);
      errorStats = step(hError, data, receivedData);
    end
    fprintf('Error rate = %f\nNumber of errors = %d\n', ...
      errorStats(1), errorStats(2))
Error rate = 0.000133
Number of errors = 4

This example illustrates the mapping of binary sequences of zeros and ones to the output of a GMSK modulator. The relationship also applies for MSK modulation.

Create a GMSK modulator that accepts binary inputs. Specify the pulse length and samples per symbol to be 1.

gmsk = comm.GMSKModulator('BitInput',true,'PulseLength',1,'SamplesPerSymbol',1);

Create an input sequence of all zeros. Modulate the sequence.

x = zeros(5,1);
y = gmsk(x)
y =

   1.0000 + 0.0000i
  -0.0000 - 1.0000i
  -1.0000 + 0.0000i
   0.0000 + 1.0000i
   1.0000 - 0.0000i

Determine the phase angle for each point. Use the unwrap function to better show the trend.

theta = unwrap(angle(y))
theta =

         0
   -1.5708
   -3.1416
   -4.7124
   -6.2832

A sequence of zeros causes the phase to shift by -π/2 between samples.

Reset the modulator. Modulate an input sequence of all ones.

reset(gmsk)
x = ones(5,1);
y = gmsk(x)
y =

   1.0000 + 0.0000i
  -0.0000 + 1.0000i
  -1.0000 - 0.0000i
   0.0000 - 1.0000i
   1.0000 + 0.0000i

Determine the phase angle for each point. Use the unwrap function to better show the trend.

theta = unwrap(angle(y))
theta =

         0
    1.5708
    3.1416
    4.7124
    6.2832

A sequence of ones causes the phase to shift by +π/2 between samples.

Algorithms

This object implements the algorithm, inputs, and outputs described on the GMSK Demodulator Baseband block reference page. The object properties correspond to the block parameters.

Introduced in R2012a

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