Communications Toolbox™

modem.genqammod - Construct General QAM modulator object

Syntax

h = modem.genqammod(property1, value1, ...) h = modem.genqammod(GENQAMdemod_object) h = modem.genqammod(GENQAMdemod_object, property1, value1, ...) h = modem.genqammod

Description

The modem.genqammod function creates a modulator object that you can use with the modulate method to modulate a signal. To learn more about the process for modulating a signal, see Using Modem Objects.

h = modem.genqammod(property1, value1, ...) constructs a General QAM modulator object h with properties as specified by the property/value pairs.

h = modem.genqammod(GENQAMdemod_object) constructs a General QAM modulator object h by reading the property values from the General QAM demodulator object, GENQAMdemod_object. The properties that are unique to the General QAM modulator object are set to default values.

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

h = modem.genqammod constructs a General QAM modulator object h with default properties. It constructs a modulator object for 16-QAM modulation, and is equivalent to:

h = modem.genqammod('Constellation', [-3+j*3, -3+j*1, ...
      -3-j*1, -3-j*3, -1+j*3, -1+j*1, -1-j*1, -1-j*3, ...
      1+j*3, 1+j*1, 1-j*1, 1-j*3, 3+j*3, 3+j*1, 3-j*1, ...
      3-j*3], 'InputType', 'integer')

Modem Modulation Method

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

The syntax is y = modulate(h, x), where h is the handle to a modulator object and x is a signal. This syntax outputs the baseband signal y.

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

When mapping input bits to symbols, the first bit is interpreted as the most significant bit.

For h.inputtype = ‘bit' (i.e., x represents binary input), nBits consecutive elements in each channel or column represent a symbol, where nBits = log₂(h.M). The number of elements in each channel must be an integer multiple of nBits, and elements of x must be 0 or 1. For an input x of size , an output y of size is computed.

For h.inputtype = ‘integer' (i.e., x represents symbol input), elements of x must be in the range [0, h.M-1]. For an input x of size , an output y of size is computed.

See Using Modem Objects for usage examples.

Properties

Using Modem Objects

A General QAM modulator 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 `'General QAM Modulator'`.
`M`	M-ary value. This property is not writable, and is automatically computed based on `Constellation`.
`Constellation`	Signal constellation..
`InputType`	Type of input to be processed by the General QAM modulator object. The choices are `'bit'` (bit/binary output), and `'integer'` (integer/symbol output).

Methods

A General QAM modulator object has the following functions for inspection, management, and simulation:

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

Examples

% Construct a General QAM modulator object with an
% equidistant 3-point constellation on the unit circle.
M = 3;
h = modem.genqammod('Constellation', exp(j*2*pi*[0:M-1]/M))
 
% Construct a General QAM object to modulate binary data 
% using a two-tiered constellation.
h = modem.genqammod('Constellation', [exp(j*2*pi*[0:3]/4) ...
         2*exp(j*(2*pi*[0:3]/4+pi/4))], 'InputType', 'Bit')
plot(h.Constellation, '*');grid on;axis('equal',[-2 2 -2 2]);
 
% Construct a modulator object from an existing
% demodulator object for general QAM demodulation in order
% to compute approximate log-likelihood ratio for a baseband 
% signal whose estimated noise variance is 0.81.
demodObj = modem.genqamdemod('Constellation', [-1 1 2*j -2*j], ...
  'OutputType', 'Bit')
modObj = modem.genqammod(demodObj)