Communications Toolbox™

modem.qammod - Construct QAM modulator object

Syntax

h = modem.qammod(M) h = modem.qammod(M, phaseoffset) h = modem.qammod(property1, value1, ...) h = modem.qammod(QAMdemod_object) h = modem.qammod(QAMdemod_object, property1, value1, ...) h = modem.qammod

Description

The modem.qammod 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.qammod(M) constructs a QAM modulator object h for M-ary modulation.

h = modem.qammod(M, phaseoffset) constructs a QAM modulator object h whose constellation has a phase offset of phaseoffset radians.

h = modem.qammod(property1, value1, ...) constructs a QAM modulator object h with properties as specified by the property/value pairs. See the following section on properties.

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

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

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

h = modem.qammod('M', 16, 'PhaseOffset', 0, 'SymbolOrder',...
    'binary', '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

The following table describes the properties of the QAM modulator object.

Property	Description
`Type`	Type of modulation object. This property is a fixed value, set to `'QAM Modulator'`.
`M`	M-ary value. Default is `16`.
`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. The first element of this vector corresponds to the top-leftmost point of the constellation, with subsequent elements running down column-wise, from left to right. The last element corresponds to the bottom-rightmost point. Note that when the alphabet size is 4, this top-down mapping in binary mode effectively creates a gray-mapped constellation.
`InputType`	Type of input to be processed by the QAM modulator object. The choices are `'bit'` (bit/binary output), and `'integer'` (integer/symbol output). Default is `'integer'`.

Methods

A 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 modulator object for 32-QAM modulation.
h = modem.qammod(32)
 
% Construct an object to modulate binary data using 64-QAM
% modulation. The constellation has Gray mapping.
h = modem.qammod('M', 64, 'SymbolOrder', 'Gray', ...
                 'InputType', 'Bit')
 
% Construct a modulator object from an existing demodulator
% object for QAM demodulation in order to modulate binary
% inputs.
demodObj = modem.qamdemod('M', 8) 
modObj = modem.qammod(demodObj, 'InputType', 'Bit')