Main Content


Demodulate using M-ary FSK method


The FSKDemodulator object demodulates a signal that was modulated using the M-ary frequency shift keying method. The input is a baseband representation of the modulated signal. The input and output for this object are discrete-time signals.

To demodulate a signal that was modulated using frequency shift keying:

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

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


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.


H = comm.FSKDemodulator creates a demodulator System object, H. This object demodulates an M-ary frequency shift keying (M-FSK) signal using a noncoherent energy detector.

H = comm.FSKDemodulator(Name,Value) creates an M-FSK demodulator object, H, with each specified property set to the specified value. You can specify additional name-value pair arguments in any order as (Name1,Value1,...,NameN,ValueN).

H = comm.FSKDemodulator(M,FREQSEP,RS,Name,Value) creates an M-FSK demodulator object, H. This object has the ModulationOrder property set to M, the FrequencySeparation property set to FREQSEP, the SymbolRate property set to RS, and the other specified properties set to the specified values.



Number of frequencies in modulated signal

Specify the number of frequencies in the modulated signal as a numeric, positive, integer scalar value that is a power of two. The default is 8.


Output data as bits

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

When you set this property to false, the step method outputs a column vector of length equal to N/SamplesPerSymbol. N is the length of the input data vector to the step method. The elements of the output vector are integers between 0 and ModulationOrder–1. When you set this property to true, the step method outputs a column vector of length equal to log2(ModulationOrder)×(N/SamplesPerSymbol). The property's elements are bit representations of integers between 0 and ModulationOrder–1.


Symbol encoding

Specify how the object maps an integer or group of log2(ModulationOrder) bits to the corresponding symbol as one of Binary | Gray. The default is Gray.

When you set this property to Gray, the object uses Gray-coded ordering.

When you set this property to Binary, the object uses natural binary-coded ordering.

For either type of mapping, the object maps the lowest frequency to the integer 0 and maps the highest frequency to the integer M–1. In baseband simulation, the lowest frequency is the negative frequency with the largest absolute value.


Frequency separation between successive tones

Specify the frequency separation between successive symbols in the modulated signal in Hertz as a positive, real scalar value. The default is 6 Hz.


Number of samples per input symbol

Specify the number of samples per input symbol as a positive, integer scalar value. The default is 17.


Symbol duration

Specify the symbol rate in symbols per second as a positive, double-precision, real scalar value. The default is 100. To avoid output signal aliasing, specify an output sampling rate, Fs = SamplesPerSymbol×SymbolRate, which is greater than ModulationOrder × FrequencySeparation. The symbol duration remain the same, regardless of whether the input is bits or integers.


Data type of output

Specify the output data type as one of logical | int8 | uint8 | int16 | uint16 | int32 | uint32 | double. The default is double. The logical type is valid only when you set the BitOutput property to false and the ModulationOrder property to two. When you set the BitOutput property to true, the output data requires a type of logical | double.


stepDemodulate using M-ary FSK method
Common to All System Objects

Allow System object property value changes


Reset internal states of System object


collapse all

Modulate and demodulate a signal using 8-FSK modulation with a frequency separation of 100 Hz.

Set the modulation order and frequency separation parameters.

M = 8;
freqSep = 100;

Create FSK modulator and demodulator System objects™ with modulation order 8 and 100 Hz frequency separation.

fskMod = comm.FSKModulator(M,freqSep);
fskDemod = comm.FSKDemodulator(M,freqSep);

Create an additive white Gaussian noise channel, where the noise is specified as a signal-to-noise ratio.

ch = comm.AWGNChannel('NoiseMethod', ...
    'Signal to noise ratio (SNR)','SNR',-2);

Create an error rate calculator object.

err = comm.ErrorRate;

Transmit one hundred 50-symbol frames using 8-FSK in an AWGN channel.

for counter = 1:100
    data = randi([0 M-1],50,1);
    modSignal = step(fskMod,data);
    noisySignal = step(ch,modSignal);
    receivedData = step(fskDemod,noisySignal);
    errorStats = step(err,data,receivedData);

Display the error statistics.

es1 = 'Error rate = %4.2e\n';
es2 = 'Number of errors = %d\n';
es3 = 'Number of symbols = %d\n';
fprintf([es1 es2 es3],errorStats)
Error rate = 1.40e-02
Number of errors = 70
Number of symbols = 5000


This object implements the algorithm, inputs, and outputs described on the M-FSK Demodulator Baseband block reference page. The object properties correspond to the block parameters, except:

  • The Symbol set ordering parameter corresponds to the SymbolMapping property.

  • The SymbolRate property replaces the block sample rate capability.

Extended Capabilities

Version History

Introduced in R2012a