Demodulate QAMmodulated data
AM, in Digital Baseband sublibrary of Modulation
The General QAM Demodulator Baseband block demodulates a signal that was modulated using quadrature amplitude modulation. The input is a baseband representation of the modulated signal.
The input must be a discretetime complex signal. The Signal constellation parameter defines the constellation by listing its points in a lengthM vector of complex numbers. The block maps the mth point in the Signal constellation vector to the integer m1.
This block accepts a scalar or column vector input signal. For information about the data types each block port supports, see the Supported Data Types table on this page.
A real or complex vector that lists the constellation points.
Determines whether the block produces integers or binary representations of integers.
If you set this parameter to
Integer
, the block
produces integers.
If you set this parameter to
Bit
, the block produces
a group of K bits, called a binary
word, for each symbol, when
Decision type is set to
Hard decision
. If
Decision type is set to
Loglikelihood ratio
or
Approximate loglikelihood
ratio
, the block outputs bitwise LLR and
approximate LLR, respectively.
This field appears when
Bit
is selected in the
pulldown list Output
type.
Specifies the use of hard decision, LLR, or approximate LLR during demodulation. See Exact LLR Algorithm and Approximate LLR Algorithm in the Communications Toolbox User's Guide for algorithm details.
This field appears when you set
Approximate loglikelihood
ratio
or Loglikelihood
ratio
for Decision
type.
When you set this parameter to
Dialog
, you can then specify
the noise variance in the Noise
variance field. When you set this
option to Port
, a port appears
on the block through which the noise variance can
be input.
This parameter appears when the
Noise variance source is set
to Dialog
and specifies the
noise variance in the input signal. This parameter
is tunable in normal mode, Accelerator mode and
Rapid Accelerator mode.
If you use the Simulink^{®} Coder™ rapid simulation (RSIM) target to build an RSIM executable, then you can tune the parameter without recompiling the model. This is useful for Monte Carlo simulations in which you run the simulation multiple times (perhaps on multiple computers) with different amounts of noise.
The LLR algorithm involves computing exponentials of very large or very small numbers using finite precision arithmetic and would yield:
Inf
to
Inf
if Noise
variance is very high
NaN
if Noise
variance and signal power are both very
small
In such cases, use approximate LLR, as its algorithm does not involve computing exponentials.
FixedPoint Signal Flow Diagram for Hard Decision Mode
In the figure above, M represents the size of the Signal constellation .
The general QAM Demodulator Baseband block
supports fixedpoint operations for computing Hard
Decision (Output type set to
Bit
and
Decision type is set to
Hard decision
) and
Approximate LLR (Output type
is set to Bit
and
Decision type is set to
Approximate LogLikelihood
ratio
) output values. The input
values must have fixedpoint data type for
fixedpoint operations.
FixedPoint operations are NOT yet supported for Exact LLR output values.
FixedPoint Signal Flow Diagram for Approximate LLR Mode
In the figure above, M represents the size of the Signal constellation.
FixedPoint Signal Flow Diagram for Approximate LLR Mode: Noise Variance Operation Modes
If Noise variance is
set to Dialog
, the
block performs the operations shown inside the
dotted line once during initialization. The block
also performs these operations if the
Noise variance value changes
during simulation.
The block supports the following Output options:
When you set the parameter to
Inherit via internal rule
(default setting), the block inherits the output
data type from the input port. The output data
type is the same as the input data type if the
input is of type single
or
double
.
For integer outputs, you can set this
block's output to Inherit via internal
rule
(default setting),
Smallest unsigned integer
,
int8
, uint8
,
int16
,
uint16
,
int32
,
uint32
,
single
, and
double
.
For bit outputs, when you set
Decision type to
Hard decision
, you can set the
output to Inherit via internal
rule
, Smallest unsigned
integer
, int8
,
uint8
,
int16
,
uint16
,
int32
,
uint32
,
boolean
,
single
, or
double
.
When you set Decision
type to Hard
decision
or Approximate
loglikelihood ratio
and the input is a
floating point data type, then the output inherits
its data type from the input. For example, if the
input is of data type double
,
the output is also of data type
double
. When you set
Decision type to
Hard decision
or
Approximate loglikelihood
ratio
, and the input is a fixedpoint
signal, the Output parameter,
located in the FixedPoint algorithm parameters
region of the DataType tab, specifies the output
data type.
When you set the parameter to
Smallest unsigned integer
, the
output data type is selected based on the settings
used in the Hardware
Implementation pane of the
Configuration Parameters dialog box. If you select
ASIC/FPGA
in the
Hardware Implementation pane,
the output data type is the ideal minimum size,
i.e., ufix(1)
for bit outputs,
and $$ufix\left(\lceil {\mathrm{log}}_{2}M\rceil \right)$$ for integer outputs. For all
other choices, the Output
data type is an unsigned integer with the smallest
available word length large enough to fit the
ideal minimum size, usually corresponding to the
size of a char (e.g.,
uint8
).
Use this parameter to specify the rounding method to be used when the result of a fixedpoint calculation does not map exactly to a number representable by the data type and scaling storing the result.
For more information, see Rounding Modes (DSP System Toolbox) or Rounding Mode: Simplest (FixedPoint Designer).
Use this parameter to specify the method to be used if the magnitude of a fixedpoint calculation result does not fit into the range of the data type and scaling that stores the result:
Saturate represents positive overflows as the largest positive number in the range being used, and negative overflows as the largest negative number in the range being used.
Wrap uses modulo arithmetic to cast an overflow back into the representable range of the data type. See Modulo Arithmetic (FixedPoint Designer) for more information.
For more information, see the Saturate on integer overflow parameter subsection of Specify FixedPoint Attributes for Blocks (DSP System Toolbox).
Use this parameter to define the data type of the Signal constellation parameter.
When you select Same word
length as input
the word length of
the Signal constellation
parameter matches that of the input to the block.
The fraction length is computed to provide the
best precision for given signal constellation
values.
When you select Specify word
length
, the Word
Length field appears, and you may enter
a value for the word length. The fraction length
is computed to provide the best precision for
given signal constellation values.
Use this parameter to specify the data type for Accumulator 1:
When you select Inherit via
internal rule
, the block
automatically calculates the output word and
fraction lengths. For more information, see the
Inherit via Internal Rule (DSP System Toolbox) subsection of
the DSP System
Toolbox™ User's Guide.
When you select Binary point
scaling
, you can enter the word
length and the fraction length of
Accumulator 1, in
bits.
Use this parameter to specify the data type for Product input.
When you select Same as
accumulator 1
, the Product
Input characteristics match those of
Accumulator 1.
When you select Binary point
scaling
you can enter the word
length and the fraction length of
Product input, in
bits.
Use this parameter to select the data type for Product output.
When you select Inherit via
internal rule
, the block
automatically calculates the output signal type.
For more information, see the Inherit via Internal
Rule
(DSP System Toolbox) subsection of the
DSP System
Toolbox User's Guide.
When you select Binary point
scaling
enter the word length and
the fraction length for Product
output, in bits.
Use this parameter to specify the data type for Accumulator 2:
When you select Inherit via
internal rule
, the block
automatically calculates the accumulator data
type. The internal rule calculates the ideal,
fullprecision word length and fraction length as
follows:
WL_{ideal accumulator 2} = WL_{input to accumulator 2}
FL_{ideal accumulator 2} = FL _{input to accumulator 2}
After the fullprecision result is calculated, your particular hardware may still affect the final word and fraction lengths set by the internal rule. For more information, see The Effect of the Hardware Implementation Pane on the Internal Rule (DSP System Toolbox) subsection of the DSP System Toolbox User's Guide.
The internal rule always sets the sign of
datatype to Unsigned
.
When you select Binary point
scaling
, you are able to enter the
word length and the fraction length of
Accumulator 2, in
bits.
The settings for the following fixedpoint
parameters only apply when you set
Decision type to
Approximate loglikelihood
ratio
.
When you select Inherit via
internal rule
, the block
automatically calculates the accumulator data
type. The internal rule first calculates ideal,
fullprecision word length and fraction length as
follows:
WL_{ideal accumulator 3} = WL_{input to accumulator 3} + 1
FL _{ideal accumulator 3} = FL _{input to accumulator 3}.
After the fullprecision result is calculated, your particular hardware may still affect the final word and fraction lengths set by the internal rule. For more information, see The Effect of the Hardware Implementation Pane on the Internal Rule (DSP System Toolbox) subsection of the DSP System Toolbox User's Guide.
The internal rule always sets the sign of
datatype to Signed
.
When you select Same as
accumulator 3
, the Noise
scaling input characteristics match
those of Accumulator
3.
When you select Binary point
scaling
you are able to enter the
word length and the fraction length of
Noise scaling input, in
bits.
This field appears when Noise variance source is set to Dialog.
When you select Same word
length as input
the word length of
the Inverse noise variance
parameter matches that of the input to the block.
The fraction length is computed to provide the
best precision for a given inverse noise variance
value.
When you select Specify word
length
, the Word
Length field appears, and you may enter
a value for the word length. The fraction length
is computed to provide the best precision for a
given inverse noise variance value.
When you select Inherit via
internal rule
, the Output
data type is automatically set for you.
If you set the Noise variance
source parameter to
Dialog
, the output is a
result of product operation as shown in the Noise
Variance Operation Modes Signal Flow Diagram FixedPoint Signal Flow Diagram for Approximate LLR Mode: Noise Variance Operation Modes. In this case, it follows the internal rule for
Product data types specified in the Inherit via
Internal Rule (DSP System Toolbox) subsection of the
DSP System
Toolbox User's Guide.
If the Noise variance
source parameter is set to
Port
, the output is a
result of division operation as shown in the
signal flow diagram. In this case, the internal
rule calculates the ideal, fullprecision word
length and fraction length as follows:
WL _{output} = max(WL _{Noise scaling input}, WL_{ Noise variance})
FL _{output} = FL _{Noise scaling input (dividend)}– FL _{Noise variance (divisor)} .
After the fullprecision result is calculated, your particular hardware may still affect the final word and fraction lengths set by the internal rule. For more information, see The Effect of the Hardware Implementation Pane on the Internal Rule (DSP System Toolbox) subsection of the DSP System Toolbox User's Guide.
The internal rule for
Output always sets the sign
of datatype to Signed
.
For additional information about the parameters pertaining to fixedpoint applications, see Specify FixedPoint Attributes for Blocks (DSP System Toolbox).
Port  Supported Data Types 

Input 

Var 

Output 
