Communications Blockset

PN Sequence Generator

Generate pseudonoise sequence

Library

Comm Sources

Description

The PN Sequence Generator block generates a sequence of pseudorandom binary numbers. A pseudonoise sequence can be used in a pseudorandom scrambler and descrambler. It can also be used in a direct-sequence spread-spectrum system.

Below is a schematic of the pseudorandom sequence generator. All adders perform addition modulo 2.

All M registers in the generator update their values at each time step. The state of each switch is defined by the generator polynomial, which is a polynomial in z^-1 having binary coefficients. The constant term of the generator polynomial must be 1. You can specify the Generator polynomial parameter using either of these formats:

• A vector of coefficients of the polynomial, starting with the constant term and proceeding in order of increasing powers of z^-1. The first and last elements must be 1.
• A vector containing the exponents of z (not z^-1) for the nonzero terms of the polynomial. The first element must be zero.

For example, p = [1 0 0 0 0 0 1 0 1] and p = [0 -6 -8] represent the same polynomial p(z) = 1 + z^-6 + z^-8.

It is very important that the Initial states parameter have a suitable value. The initial state of at least one of the registers must be nonzero in order to generate a nonzero sequence. The length of the Initial states vector must equal the order of the generator polynomial, and its elements must be binary numbers. If the Generator polynomial parameter is a vector that lists the coefficients in order, then the order of the generator polynomial is one less than the vector length.

Attributes of Output Signal

If the Frame-based outputs box is checked, then the output signal is a frame-based column vector whose length is the Samples per frame parameter. Otherwise, the output signal is a one-dimensional scalar.

Dialog Box

Generator polynomial

A polynomial that determines the shift register's feedback connections.

Initial states

A vector of initial states of the shift registers.

Sample time

The period of each element of the output signal.

Frame-based outputs

Determines whether the output is frame-based or sample-based.

Samples per frame

The number of samples in a frame-based output signal. This field is active only if Frame-based outputs is checked.

See Also

Scrambler

PM Modulator Passband

Poisson Int Generator