# comm.PNSequence

Generate a pseudo-noise (PN) sequence

## Description

The `PNSequence` object generates a sequence of pseudorandom binary numbers using a linear-feedback shift register (LFSR). This object implements LFSR using a simple shift register generator (SSRG, or Fibonacci) configuration. Pseudo-noise sequences are typically used for pseudorandom scrambling and in direct-sequence spread-spectrum systems.

To generate a PN sequence:

1. Create the `comm.PNSequence` object and set its properties.

2. Call the object with arguments, as if it were a function.

To learn more about how System objects work, see What Are System Objects? (MATLAB).

## Creation

### Syntax

``pnSequence = comm.PNSequence``
``pnSequence = comm.PNSequence(Name,Value)``

### Description

````pnSequence = comm.PNSequence` creates a pseudo-noise (PN) sequence generator System object™. This object generates a sequence of pseudorandom binary numbers using a linear-feedback shift register (LFSR).```
````pnSequence = comm.PNSequence(Name,Value)` sets properties using one or more name-value pairs. Enclose each property name in quotes.```

## Properties

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Unless otherwise indicated, properties are nontunable, which means you cannot change their values after calling the object. Objects lock when you call them, and the `release` function unlocks them.

If a property is tunable, you can change its value at any time.

For more information on changing property values, see System Design in MATLAB Using System Objects (MATLAB).

Generator polynomial that determines the shift register's feedback connections, specified as one of these:

• A polynomial character vector that includes the number `1`.

• A binary row vector that represents the coefficients of the generator polynomial in order of descending power. The first and last entries must be `1`. The length of this vector is N + 1, where N is the degree of the generator polynomial.

• A vector containing the exponents of z for the nonzero terms of the polynomial in descending order of powers. The last entry must be `0`.

For more information, see Character Representation of Polynomials.

Example: `'z^8 + z^2 + 1'`, `[1 0 0 0 0 0 1 0 1]`, and `[8 2 0]` represent the same polynomial, p(z) = z8 + z2 + 1.

Data Types: `double` | `char`

Source of the initial conditions that determines the start of the PN sequence, specified as one of these:

• `'Property'` — Specify PN sequence generator initial conditions as a binary scalar or binary vector using the `InitialConditions` property.

• `'Input port'` — Specify PN sequence generator initial conditions by using an additional input argument, when calling the object. The object accepts a binary scalar or binary vector input. The length of the input must equal the degree of the generator polynomial that the `Polynomial` property specifies.

Data Types: `char`

Initial conditions of shift register when the simulation starts, specified as a binary scalar or binary vector.

If you set this property to a vector, each element of the vector corresponds to the initial value of the corresponding cell in the shift register. The length of the vector must equal the degree of the generator polynomial. If you set this property to a scalar, the initial conditions of all the cells of the shift register are the specified scalar value.

The scalar, or at least one element of the specified vector, must be nonzero for the object to generate a nonzero sequence.

#### Dependencies

This property is available when `InitialConditionsSource` is set to `'Property'`.

Data Types: `double`

Source of the mask that determines the shift of the PN sequence, specified as one of these:

• `'Property'` — Specify the mask as an integer scalar or binary vector using the `Mask` property.

• `'Input port'` — Specify the mask by using an additional input argument, when calling the object. The mask can only be specified as a binary vector, whose length must equal the degree of the generator polynomial that the `Polynomial` property specifies.

Data Types: `char`

Mask that determines how the PN sequence is shifted from its starting point, specified as an integer scalar or a binary vector.

When you set this property to an integer scalar, the value is the length of the shift. A scalar shift can be positive or negative. When the PN sequence has a period of N = 2n – 1, where n is the degree of the generator polynomial that the `Polynomial` property specifies, the object wraps shift values that are negative or greater than N.

When you set this property to a binary vector, its length must equal the degree of the generator polynomial that the `Polynomial` specifies. For more information, see Shifting PN Sequence Starting Point.

The mask vector can be calculated using the `shift2mask` function.

#### Dependencies

This property is available when `MaskSource` is set to `'Property'`.

Set this property to `true` to enable variable-size outputs by using an additional input argument when calling the object. The enabled input specifies the output size of the PN sequence. The input value must be less than or equal to the value of the `MaximumOutputSize` property.

When you set this property to `false`, the `SamplesPerFrame` property specifies the number of output samples.

Maximum output size, specified as a positive integer two-element row vector that denotes the maximum output size of the PN sequence. The second element of the vector must be `1`.

Example: `[10 1]` gives a `10`-by-`1` maximum sized output signal.

#### Dependencies

This property is available when `VariableSizeOutput` is set to `true`.

Number of samples output per frame by the PN sequence object, specified as a positive integer. If you set this property to a value of M, then the object outputs M samples of a PN sequence that has a period of N = 2n – 1, where n represents the degree of the generator polynomial that the `Polynomial` specifies.

If you set the `BitPackedOutput` property to `false`, the samples are bits from the PN sequence. If you set the `BitPackedOutput` property to `true`, then the output corresponds to `SamplesPerFrame` groups of bit-packed samples.

Set this property to `true` to enable PN sequence generator reset by using an additional input argument when calling the object. This input resets the states of the PN sequence generator to the initial conditions specified in the `InitialConditions` property. For more information, see Resetting a Signal.

#### Dependencies

This property is available when `InitialConditionsSource` is set to `'Property'`.

Option to output bit-packed words, specified as `false` or `true`. Set this property to `true` to enable bit-packed outputs. The first bit from the left in the bit-packed word is considered the most significant bit.

When `BitPackedOutput` is `true`, the object outputs a column vector of length M, which contains integer representations of bit words of length P. M is the number of samples per frame specified in the `SamplesPerFrame` property. P is the size of the bit-packed words specified in the `NumPackedBits` property.

Number of bits packed into each output data word, specified as an integer in the range [1, 32].

#### Dependencies

This property is available when `BitPackedOutput` is set to `true`.

Set this property to `true` to obtain signed, bit-packed, output words. In this case, a `1` in the most significant bit (sign bit) indicates a negative value. The property indicates negative numbers in a two's complement format.

#### Dependencies

This property is available when `BitPackedOutput` is set to `true`.

Output data type, specified as one of these:

• When `BitPackedOutput` is set to `false`, `OutputDataType` can be `'double'`, `'logical'`, or ```'Smallest unsigned integer'```.

• When `BitPackedOutput` is set to `true`, `OutputDataType` can be `'double'` or `'Smallest integer'`.

### Note

You must have a Fixed-Point Designer™ user license to use this property in ```'Smallest unsigned integer'``` or `'Smallest integer'` mode.

#### Dependencies

The valid settings for `OutputDataType` depends on the setting of `BitPackedOutput`.

## Usage

### Syntax

``outSequence = pnSequence()``
``outSequence = pnSequence(initCond)``
``outSequence = pnSequence(maskVec)``
``outSequence = pnSequence(outputSize)``
``outSequence = pnSequence(reset)``

### Description

example

````outSequence = pnSequence()` outputs a frame of the PN sequence. Specify the frame length with the `SamplesPerFrame` property. The PN sequence has a period of N = 2n – 1, where n is the degree of the generator polynomial that you specify in the `Polynomial` property.You can combine optional input arguments when you set their enabling properties. Optional inputs must be listed in the same order as the order of the enabling properties.```
````outSequence = pnSequence(initCond)` provides an additional input with values specifying the initial conditions of the linear-feedback shift register.This syntax applies when you set the `InitialConditionsSource` property of the object to `'Input port'`.```
````outSequence = pnSequence(maskVec)` provides an additional input specifying the mask vector that determines how the PN sequence is shifted from its starting point.This syntax applies when you set the `MaskSource` property of the object to `'Input port'`.```
````outSequence = pnSequence(outputSize)` provides an additional input specifying the output size of the PN sequence.This syntax applies when you set the `VariableSizeOutput` property of the object to `true`.```
````outSequence = pnSequence(reset)` provides an additional input indicating whether to reset the PN sequence generator.This syntax applies when you set `InitialConditionsSource` to `'Property'` and `ResetInputPort` to `true`.```

### Input Arguments

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Initial conditions of the shift register when the simulation starts, specified as a binary scalar or binary vector. When you set `initCond` to a binary vector, the length of the vector must equal the degree of the `Polynomial` property. The scalar, or at least one element of `initCond`, must be nonzero for the object to generate a nonzero sequence.

Example: `outSequence = pnSequence([1 1 0])` corresponds to possible initial register states for a PN sequence generator specified by a generator polynomial of degree 3.

Data Types: `double`

Mask that determines how the PN sequence is shifted from its starting point, specified as a binary vector. The length of the vector must equal the degree of the `Polynomial` property.

Output size of the PN sequence, specified as a scalar or two-element row vector. When you set `outputSize` to a two-element row vector, the second element must be equal to `1`.

When you specify `reset` as a scalar, the object resets to the initial conditions specified in the `InitialConditions` property and generates a new output frame.

When you specify `reset` as a column vector, the length of the vector must equal the number of samples per frame specified in `SamplesPerFrame`. A column vector input allows multiple resets within an output frame. A non-zero value at the ith element of the vector will cause a reset at the ith element of the generated PN sequence.

### Output Arguments

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PN sequence generated by the object, returned as a column vector.

## Object Functions

To use an object function, specify the System object as the first input argument. For example, to release system resources of a System object named `obj`, use this syntax:

`release(obj)`

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 `step` Run System object algorithm `release` Release resources and allow changes to System object property values and input characteristics `reset` Reset internal states of System object

## Examples

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Generate a 14-sample frame of a maximal length PN sequence given generator polynomial, ${x}^{3}+{x}^{2}+1$.

Generate PN sequence data by using the `comm.PNSequence` object. The sequence repeats itself as it contains 14 samples while the maximal sequence length is only 7 samples (${2}^{3}-1$).

```pnSequence = comm.PNSequence('Polynomial',[3 2 0], ... 'SamplesPerFrame',14,'InitialConditions',[0 0 1]); x1 = pnSequence(); [x1(1:7) x1(8:14)]```
```ans = 7×2 1 1 0 0 0 0 1 1 1 1 1 1 0 0 ```

Create another maximal length sequence based on the generator polynomial, ${x}^{4}+x+1$. As it is a fourth order polynomial, the sequence repeats itself after 15 samples (${2}^{4}-1$).

```pnSequence2 = comm.PNSequence('Polynomial','x^4+x+1', ... 'InitialConditions',[0 0 0 1],'SamplesPerFrame',30); x2 = pnSequence2(); [x2(1:15) x2(16:30)]```
```ans = 15×2 1 1 0 0 0 0 0 0 1 1 0 0 0 0 1 1 1 1 0 0 ⋮ ```

The `comm.PNSequence` system object implements a linear-feedback shift register (LFSR) using a simple shift register generator (SSRG, or Fibonacci configuration). This configuration differs from the modular shift register generator (MSRG, or Galois configuration) by a phase difference, that can be determined empirically from the system object.

This phase difference can be specified as the `Mask` parameter for the `comm.PNSequence` system object to generate the equivalent MSRG configuration output. The block diagram represents the implementation of a 5-bit LFSR in the Galois (MSRG) configuration.

Load the file `GaloisLFSR`. The file contains the following variables that define the properties and output PN sequence of the 5-bit Galois LFSR:

• `polyVec`: Generator polynomial

• `polySize`: Degree of the generator polynomial

• `initStates`: Initial conditions of the shift register

• `maskVar`: Mask to shift the PN sequence

• `pn_msrg`: Output PN sequence of maximal length, from the 5-bit Galois LFSR

`load GaloisLFSR`

Generate PN sequence data by using the `comm.PNSequence` object with the same set of properties used to implement the 5-bit Galois LFSR. Compare this PN sequence with the output of the 5-bit Galois LFSR. The two sequences differ by a phase shift.

```pnSequence = comm.PNSequence('Polynomial',polyVec,'InitialConditions',initStates,... 'Mask',maskVar,'SamplesPerFrame',2^polySize-1); pn = pnSequence(); isequal(pn,pn_msrg)```
```ans = logical 0 ```

Compute the phase shift between the two configurations. Set the value of the `Mask` property based on this phase shift.

```for i = 1:length(pn) exp_pn = [pn(i:end);pn(1:(i-1))]; if isequal(exp_pn,pn_msrg) break end end maskVar = i-1;```

Generate PN sequence data by using the `comm.PNSequence` system object with the modifed `Mask` property value. Compare this sequence with the output of the 5-bit Galois LFSR. The two sequences are now equal.

```pnSequence_mod = comm.PNSequence('Polynomial',polyVec,'InitialConditions',initStates,... 'Mask',maskVar,'SamplesPerFrame',2^polySize-1); pn_mod = pnSequence_mod(); isequal(pn_mod,pn_msrg)```
```ans = logical 1 ```

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## References

[1] Proakis, John G. Digital Communications. 3rd Ed. New York. McGraw Hill, 1995.

[2] Lee, J. S., and L. E. Miller. CDMA Systems Engineering Handbook. Boston and London. Artech House, 1998.

[3] Golomb, S.W. Shift Register Sequences. Laguna Hills. Aegean Park Press, 1967.

## See Also

### Blocks

#### Bridging Wireless Communications Design and Testing with MATLAB

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