Descrambler

Descramble input signal

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Libraries:
Communications Toolbox / Sequence Operations

Description

The Descrambler block applies multiplicative descrambling to input data. It performs the inverse operation of the Scrambler block used in the transmitter.

This schematic shows the multiplicative descrambler operation. The adders and subtracter operate modulo N, where N is the value specified by the Calculation base parameter.

At each time step, the input causes the contents of the registers to shift sequentially. Using the Scramble polynomial parameter, you specify the on or off state for each switch in the descrambler. To make the Descrambler block reverse the operation of the Scrambler block, use the same parameter settings in both blocks. If there is no signal delay between the scrambler and the descrambler, then the Initial states in the two blocks must be the same.

To achieve repeatable initial descrambler conditions, you can use one of these optional input ports:

• Select the Reset on nonzero input via port parameter and reset the scrambler with Rst.

• Set the Initial states source parameter to Input port and provide the initial states with ISt.

This block can accept input sequences that vary in length during simulation. For more information about sequences that vary in length, see Variable-Size Signal Basics (Simulink).

Note

To apply additive descrambling to input data, you can use the PN Sequence Generator block and the Logical Operator (Simulink) block configured as an XOR logical operator. For an example, see Additive Scrambling of Input Data in Simulink.

Examples

Scrambler-Descrambler Block Pair Use

The slexScramblerDescrambler model illustrates use of a scrambler-descrambler block pair.

The Scrambler and Descrambler block pair are initialized to the same values by setting block parameters to variables assigned using the InitFcn Callback. To view or change the initialization values, go to File > Model Properties > Callbacks > InitFcn. When you run the model, it displays the percentage of zeros at the input to the scrambler, output from the scrambler and output from the descrambler. For a binary signal, the scrambler outputs zeros or ones with equal probability. For perfect data recovery, the bit error rate is zero.

Extended Examples

QPSK Transmitter and Receiver

Model a QPSK transmitter and receiver in Simulink^®.

• Open Model

Ports

Input

In — Input data signal

vector

Input data signal, specified as an N_S-by-1 vector. N_S represents the number of samples in the input signal. The input values must be integers from 0 to Calculation base – 1.

Data Types: double | Boolean | int8 | int16 | int32 | uint8 | uint16 | uint32

Rst — Reset scrambler

scalar

Reset scrambler, specified as a scalar. The scrambler is reset if a nonzero input is applied to the port.

Dependencies

To enable this port, set Initial states source to Dialog Parameter and select Reset on nonzero input via port.

ISt — Initial states

scalar

Initial states of the descrambler registers when the simulation starts, specified as a nonnegative integer vector. The length of ISt must equal the order of the Scramble polynomial parameter. The vector element values must be integers from 0 to Calculation base – 1.

Dependencies

To enable this port, set Initial states source to Input port.

Output

Out — Output descrambled data

vector

Output descrambled data, returned as an N_S-by-1 vector. N_S equals the number of samples in the input signal.

Data Types: double

Parameters

To edit block parameters interactively, use the Property Inspector. From the Simulink Toolstrip, on the Simulation tab, in the Prepare gallery, select Property Inspector.

Calculation base — Calculation base

4 (default) | nonnegative integer

Calculation base used in the descrambler for modulo operations, specified as a nonnegative integer. The input and output of this block are integers from 0 to Calculation base – 1.

Scramble polynomial — Polynomial that defines connections in descrambler

'1 + x^-1 + x^-2 + x^-4' (default) | character vector | integer vector | binary vector

Polynomial that defines the connections in the descrambler, specified as a character vector, integer vector, or binary vector. The Scramble polynomial parameter defines if each switch in the descrambler is on or off. Specify the polynomial as:

• A character vector, such as '1 + x^-6 + x^-8'. For more details on specifying polynomials in this way, see Representation of Polynomials in Communications Toolbox.

• An integer vector, such as [0 -6 -8], listing the descrambler coefficients in order of descending powers of x^-1, where p(x^-1) = 1 + p₁x^-1 + p₂x^-2 + ...

• A binary vector, such as [1 0 0 0 0 0 1 0 1], listing the powers of x that appear in the polynomial that has a coefficient of 1. In this case, the order of the descramble polynomial is one less than the binary vector length.

Example: '1 + x^-6 + x^-8', [0 -6 -8], and [1 0 0 0 0 0 1 0 1] all represent this polynomial:

p(x^-1) = 1 + x^-6 + x^-8

Initial states source — Set the source for descrambler initial states

Dialog Parameter (default) | Input port

• Dialog Parameter – Specify descrambler initial states by using the Initial states parameter.

• Input port – Specify descrambler initial states by using the ISt port.

Initial states — Initial states of descrambler registers

[0 1 2 3] (default) | nonnegative integer vector

Initial states of descrambler registers when the simulation starts, specified as a nonnegative integer vector. The length of Initial states must equal the order of the Scramble polynomial parameter. The vector element values must be integers from 0 to Calculation base – 1.

Dependencies

This parameter is available when Initial states source is set to Dialog Parameter.

Reset on nonzero input via port — Reset descrambler via input port

off (default) | on

Select this parameter to reset the Descrambler block via input port Rst.

Dependencies

This parameter is available when Initial states source is set to Dialog Parameter.

Block Characteristics

Data Types	Boolean | double | integer
Multidimensional Signals	no
Variable-Size Signals	no

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

Version History

Introduced before R2006a