Communications Blockset

Sequence Generators

You can use blocks in the Sequence Generators sublibrary of the Comms Sources library to generate sequences for spreading or synchronization in a communication system. You can open the Sequence Generators sublibrary by double-clicking its icon in the main Communications Blockset library (commlib), or by typing

• commseqgen2p1
  

at the MATLAB prompt.

Blocks in the Sequence Generators sublibrary generate

• Pseudorandom sequences
• Orthogonal codes
• Synchronization codes

Pseudorandom Sequences

The following table lists the blocks that generate pseudorandom or pseudonoise (PN) sequences. The applications of these sequences range from multiple-access spread spectrum communication systems to ranging, sychronization, and data scrambling.

Sequence	Block
Gold sequences	Gold Sequence Generator
Kasami sequences	Kasami Sequence Generator
PN sequences	PN Sequence Generator

All three blocks use shift registers to generate pseudorandom sequences. The following is a schematic diagram of a typical shift register.

All r registers in the generator update their values at each time step according to the value of the incoming arrow to the shift register. The adders perform addition modulo 2. The shift register can be described by a binary polynomial in z, g_rz^r + g_r-1z^r-1 + ... + g₀. The coefficient g_i is 1 if there is a connection from the ith shift register to the adder, and 0 otherwise.

The Kasami Sequence Generator block and the PN Sequence Generator block use this polynomial description for their Generator polynomial parameter, while the Gold Sequence Generator block uses it for the Preferred polynomial [1] and Preferred polynomial [2] parameters.

The lower half of the preceding diagram shows how the output sequence can be shifted by a positive integer d, by delaying the output for d units of time. This is accomplished by a single connection along the dth arrow in the lower half of the diagram.

See Example: Pseudorandom Sequences for an example that uses these blocks.

Synchronization Codes

The Barker Code Generator block generates Barker codes to perform synchronization. Barker codes are subsets of PN sequences. They are short codes, with a length at most 13, which are low correlation sidelobes. A correlation sidelobe is the correlation of a codeword with a time-shifted version of itself.

Orthogonal Codes

Orthogonal codes are used in systems in which the receiver is perfectly synchronized with the transmitter. For such systems, the despreading operation is ideal when orthogonal codes are used for the spreading. For example, they are used in the forward link of the IS-95 system, in which the base station transmits a pilot signal to help the receiver gain synchronization.

Code	Block
Hadamard codes	Hadamard Code Generator
OVSF codes	OVSF Code Generator
Walsh codes	Walsh Code Generator

See Example: Orthogonal Sequences for an example that uses these blocks.

Random Noise Generators

Sequence Generator Examples