Communications Toolbox™

modem.pskdemod - Construct PSK demodulator object

Syntax

h = modem.pskdemod(M) h = modem.pskdemod(M, phaseoffset) h = modem.pskdemod(property1, value1, ...) h = modem.pskdemod h = modem.pskdemod(pskmod_object) h = modem.pskdemod(pskmod_object, property1, value1, ...)

Description

The modem.pskdemod function creates a demodulator object that you can use with the demodulate method to demodulate a signal. To learn more about the process for demodulating a signal, see Using Modem Objects.

h = modem.pskdemod(M) constructs a PSK demodulator object h for M-ary demodulation.

h = modem.pskdemod(M, phaseoffset) constructs a PSK demodulator object h whose constellation has a phase offset of phaseoffset radians.

h = modem.pskdemod(property1, value1, ...) constructs a PSK demodulator object h with properties as specified by the property/value pairs. If a property is not specified, it is assigned a default value. See the following section on properties.

h = modem.pskdemod constructs a PSK demodulator object h with default properties. It constructs a demodulator object for BPSK demodulation and is equivalent to:

h = modem.pskdemod('M', 2, 'PhaseOffset', 0, ...
    'SymbolOrder', 'binary', 'OutputType', 'integer', ...
    'DecisionType', 'hard decision')

h = modem.pskdemod(pskmod_object) constructs a PSK demodulator object h by reading the property values from the pskmod_object PSK modulator object. The properties that are unique to the PSK demodulator object are set to default values.

h = modem.pskdemod(pskmod_object, property1, value1, ...) constructs a PSK demodulator object h by reading the property values from the pskmod_object PSK modulator object. Additional properties are specified by the property/value pairs.

Modem Demodulation Method

This object has a method demodulate that is used to demodulate signals.

The syntax is y = demodulate(h, x), where h is the handle to a demodulator object and x is a signal. This syntax processes the binary words (bits) or symbols (integers) in signal x with the demodulator object and output the baseband signal y.

x can be a multichannel signal. The columns of x are considered individual channels, while the rows are time steps.

The demodulator object's property DecisionType should be set depending on whether you want hard or soft (LLR or approximate LLR) decisions. To allow for soft decisions, the demodulator object's property OutputType must be set to 'bit'.

For h.outputtype = ‘bit', an output y of size is computed for an input x of size , where nBits = log₂(h.M).

For h.outputtype = ‘integer', an output y of size is computed for an input x of size .

See Using Modem Objects for usage examples.

Properties

The following table describes the properties of the PSK demodulator object.

Property	Description
`Type`	Type of modulation object. This property is a fixed value, set to `'PSK Demodulator'`.
`M`	M-ary value. Default is `2`.
`PhaseOffset`	Phase offset of ideal signal constellation in radians. Default is `0`.
`Constellation`	Ideal signal constellation. This property is not writable and is automatically computed based on the `M` and `PhaseOffset` properties.
`SymbolOrder`	Type of mapping employed for mapping symbols to ideal constellation points. The choices are `'binary'` (binary mapping), `'gray'` (Gray mapping), and `'user-defined'` (custom mapping). Default is `'binary'`.
`SymbolMapping`	Symbol mapping values corresponding to ideal constellation points. This property is writable only when `SymbolOrder` is set to `'user-defined'`. Each element of the symbol mapping vector contains the symbol mapped to the corresponding element of the constellation vector. Thus, the first element of the symbol mapping vector contains the symbol mapped to the first element of the constellation vector, the second element contains the symbol mapped to the second element of the constellation vector, and so on.
`OutputType`	Type of output to be computed by the PSK demodulator object. The choices are `'bit'` (bit/binary output), and `'integer'` (integer/symbol output). Default is `'integer'`.
`DecisionType`	Type of output values to be computed by the PSK demodulator object. The choices are `'hard decision'` (hard-decision values), `'llr'` (log-likelihood ratio), and `'approximate llr'` (approximate log-likelihood ratio). Default is `'hard decision'`.
`NoiseVariance`	Noise variance of the channel or equalized signal to be processed by the PSK demodulator object. The noise variance is used to compute LLR or Approximate LLR, hence `NoiseVariance` is visible only when `DecisionType` is set to `'llr'` or `'approximate llr'`. If the `NoiseVariance` value is very small (i.e., SNR is very high), LLR computations may yield `Inf` or `-Inf` because the LLR algorithm would involve computing exponentials of very large or very small numbers using finite precision arithmetic. In such cases, use approximate LLR, as its algorithm does not involve computing exponentials.

Methods

A PSK demodulator object has the following four functions for inspection, management, and simulation:

See Using Modem Objects for details and examples of their use.

Algorithms

See Exact LLR Algorithm and Approximate LLR Algorithm.

Examples

% Construct a demodulator object for QPSK demodulation.
h = modem.pskdemod(4)
 
% Construct an object to compute log-likelihood ratio of
% a baseband signal using 16-PSK modulation. The
% constellation has Gray mapping and is shifted by -pi/16
% radians. The estimated noise variance of input
% signal is 1.2.
h = modem.pskdemod('M', 16, 'PhaseOffset', -pi/16, ...
    'SymbolOrder', 'Gray', 'OutputType', 'Bit', ...
    'DecisionType', 'LLR', 'NoiseVariance', 1.2)
 
% Construct a demodulator object from an existing
% modulator object for PSK modulation in order to
% compute approximate log-likelihood ratio for
% a baseband signal whose estimated noise variance is 0.81.
modObj = modem.pskmod('M', 8, 'InputType', 'Bit')
demodObj = modem.pskdemod(modObj, 'DecisionType', ...
           'Approximate LLR', 'NoiseVariance', 0.81)