In CDMA user signal is multiplied by pseudo random sequence. This sequence must be known by the transmitter and also by the receiver to be able to realize synchronisation and despreading. To be used in real systems the sequence should be able to be constructed from a finite number of randomly pre-selected parameters. On the other hand the PN sequence should look like noise
A PN sequence has three following properties:
· The number of ‘1’s and the number of ‘0’s in a PN sequence are only different by one.
· Run lengths of zeroes or ones are the same as in a coin flipping experiment. Half of the run lengths are unity, one-quarter are of length two, one-eighth are of length three and a fraction 1/2n of all runs are of length n.
· If the sequence is shifted by any non-zero number of elements, the resulting sequence will have an equal number of agreements and disagreements with the original sequence.
A deterministic generated sequence that nearly satisfies these three requirements, within extremely small difference, will be a pseudorandom sequence.
A PN sequence can be generated by a linear shift register. Fig 3.2 shows a linear shift register sequence generator.
In each clock cycle the register shifts all its contents to the right. The sequence an can be written as
an = c1 an-1 + c2 an-1 + ... + cr an-r
where c1 to cr are the connection variables (0 for no connection and 1 for connection ).