QPSK Demodulator Baseband
Demodulate QPSKmodulated data
Library
PM, in Digital Baseband sublibrary of Modulation
Description
The QPSK Demodulator Baseband block demodulates a signal that was modulated using the quadrature 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.
Algorithm
HardDecision QPSK Demodulator Signal Diagram for Trivial Phase Offset (odd multiple of π/4)
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.
Dialog Box
 Phase offset (rad)
The phase of the zeroth point of the signal constellation.
 Constellation ordering
Determines how the block maps each integer to a pair of output bits.
 Output type
Determines whether the output consists of integers or bits.
If the Output type parameter is set to
Integer
and Constellation ordering is set toBinary
, then the block maps the pointexp(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
orGray
.If the Output type is set to
Bit
, then the output contains pairs of binary values if Decision type is set toHard 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
orApproximate loglikelihood ratio
, then the output contains bitwise LLR or approximate LLR values, respectively. Decision type
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 for algorithm details.
 Noise variance source
This field appears when
Approximate loglikelihood ratio
orLoglikelihood ratio
is selected for Decision type.When set to
Dialog
, the noise variance can be specified in the Noise variance field. When set toPort
, a port appears on the block through which the noise variance can be input. Noise variance
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
toInf
if Noise variance is very highNaN
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
 Output
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
, orboolean
.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
, oruint32
.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
ordouble
). 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 isBit
, 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 isInteger
, 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
). Derotate factor
This parameter only applies when the input is fixedpoint and Phase offset is not an even multiple of π/4.
You can select
Same word length as input
orSpecify 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
).
Examples
Supported Data Types
Port  Supported Data Types 

Input 

Var 

Output 
