Demodulate QPSKmodulated data
PM, in Digital Baseband sublibrary of Modulation
The QPSK Demodulator Baseband block demodulates a signal that was modulated using the quaternary phase shift keying method. The input is a baseband representation of the modulated signal.
The input must be a complex signal. This block accepts a scalar or column vector input signal. For information about the data types each block port supports, see Supported Data Types.
HardDecision QPSK Demodulator Signal Diagram for Trivial Phase Offset (odd multiple of )
HardDecision QPSK Demodulator FloatingPoint Signal Diagram for Nontrivial Phase Offset
HardDecision QPSK Demodulator FixedPoint Signal Diagram for Nontrivial Phase Offset
The exact LLR and approximate LLR cases (softdecision) are described in Exact LLR Algorithm and Approximate LLR Algorithm in the Communications System Toolbox User's Guide.
The phase of the zeroth point of the signal constellation.
Determines how the block maps each integer to a pair of output bits.
Determines whether the output consists of integers or bits.
If the Output type parameter is set to Integer
and Constellation
ordering is set to Binary
, then
the block maps the point
exp(jθ + jπm/2)
to m, where θ is the Phase offset parameter and m is 0, 1, 2, or 3.
The reference page for the QPSK Modulator
Baseband block shows the signal constellations for the cases
when Constellation ordering is set to either Binary
or Gray
.
If the Output type is set to Bit
,
then the output contains pairs of binary values if Decision
type is set to Hard decision
. The most
significant bit (i.e. the leftmost bit in the vector), is the first
bit the block outputs.
If the Decision type is set to Loglikelihood
ratio
or Approximate loglikelihood ratio
,
then the output contains bitwise LLR or approximate LLR values, respectively.
Specifies the use of hard decision, LLR, or approximate LLR
during demodulation. This parameter appears when you select Bit
from
the Output type dropdown list. The output values
for Loglikelihood ratio and Approximate loglikelihood ratio decision
types are of the same data type as the input values. For integer output,
the block always performs Hard decision demodulation.
See Exact LLR Algorithm and Approximate LLR Algorithm in the Communications System Toolbox User's Guide for algorithm details.
This field appears when Approximate loglikelihood
ratio
or Loglikelihood ratio
is selected
for Decision type.
When set to Dialog
, the noise variance can
be specified in the Noise variance field. When
set 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.
Data Types Pane for HardDecision
For bit outputs, when Decision type is
set to Hard decision
, the output data type can
be set to 'Inherit via internal rule'
, 'Smallest
unsigned integer'
, double
, single
, int8
, uint8
, int16
, uint16
, int32
, uint32
,
or boolean
.
For integer outputs, the output data type can be set to 'Inherit
via internal rule'
, 'Smallest unsigned integer'
, double
, single
, int8
, uint8
, int16
, uint16
, int32
,
or uint32
.
When this parameter is set to 'Inherit via internal
rule'
(default setting), the block will inherit the output
data type from the input port. The output data type will be the same
as the input data type if the input is a floatingpoint type (single
or double
).
If the input data type is fixedpoint, the output data type will work
as if this parameter is set to 'Smallest unsigned integer'
.
When this parameter is set 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 of the model.
If ASIC/FPGA
is selected in the Hardware
Implementation pane, and Output type is Bit
,
the output data type is the ideal minimum onebit size, i.e., ufix(1)
.
For all other selections, it is an unsigned integer with the smallest
available word length large enough to fit one bit, usually corresponding
to the size of a char (e.g., uint8
).
If ASIC/FPGA
is selected in the Hardware
Implementation pane, and Output type is Integer
,
the output data type is the ideal minimum twobit size, i.e., ufix(2)
.
For all other selections, it is an unsigned integer with the smallest
available word length large enough to fit two bits, usually corresponding
to the size of a char (e.g., uint8
).
This parameter only applies when the input is fixedpoint and Phase offset is not an even multiple of $$\frac{\pi}{4}$$.
You can select Same word length as input
or Specify
word length
, in which case you define the word length using
an input field.
Data Types Pane for SoftDecision
For bit outputs, when Decision type is
set to Loglikelihood ratio
or Approximate
loglikelihood ratio
, the output data type is inherited
from the input (e.g., if the input is of data type double
,
the output is also of data type double
).
Port  Supported Data Types 

Input 

Var 

Output 

This block supports HDL code generation using HDL Coder™. HDL Coder provides additional configuration options that affect HDL implementation and synthesized logic. For more information on implementations, properties, and restrictions for HDL code generation, see QPSK Demodulator Baseband in the HDL Coder documentation.