Communications Toolbox™

modem.pamdemod - Construct PAM demodulator object

Syntax

h = modem.pamdemod(property1, value1, ...) h = modem.pamdemod(PAMmod_object) h = modem.pamdemod(PAMmod_object, property1, value1, ...) h = modem.pamdemod

Description

The modem.pamdemod function creates a modulator 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.pamdemod(property1, value1, ...) constructs a PAM demodulator object h with properties as specified by the property/value pairs.

h = modem.pamdemod(PAMmod_object) constructs a PAM demodulator object h by reading the property values from the PAM modulator object, PAMmod_object. The properties that are unique to the PAM demodulator object are set to default values.

h = modem.pamdemod(PAMmod_object, property1, value1, ...) constructs a PAM demodulator object h by reading the property values from the PAM modulator object, PAMmod_object. Additional properties are specified using property/value pairs.

h = modem.pamdemod constructs a PAM demodulator object h with default properties. It constructs a demodulator object for BPAM demodulation, and is equivalent to:

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

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

An PAM demodulator object has the following properties. All the properties are writable unless explicitly noted otherwise.

Property	Description
`Type`	Type of modulation object. This is a fixed value, set to `'PAM Demodulator'`.
`M`	M-ary value.
`Constellation`	Ideal signal constellation. This property is not writable and is automatically computed based on `M`.
`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).
`OutputType`	Type of output to be computed by the PAM demodulator object. The choices are `'bit'` (bit/binary output), and `'integer'` (integer/symbol output).
`DecisionType`	Type of output values to be computed by the PAM demodulator object. The choices are `'hard decision'` (hard decision values), `'llr'` (log-likelihood ratio), and `'approximate llr'` (approximate log-likelihood ratio).
`NoiseVariance`	Noise variance of the received signal to be processed by the PAM demodulator object. This is used to compute only the LLR or approximate LLR. Hence, `NoiseVariance` is visible only when `DecisionType` is set to `'llr'` or `'approximate llr'`.

Methods

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

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

Examples

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