codes in CDMA

Walsh Code :

In CDMA the traffic channels are separated by uinique “Walsh” code. All such codes are orthogonal to each other. The individual subscriber can start communication using one of these codes. These codes are traffic channel codes and are used for orthogonal spreading of the information in the entire bandwidth. Orthogonality provides nearly perfect isolation between the multiple signals transmitted by the base station.

The basic concept behind creation of the code is as follows:

(a)        Repeat the function right

(b)        Repeat the function below

(c)                Invert function (diagonally)

0  -----  0          0 --------           0          0          0          0

             0         1                     0          1          0          1

                                                0          0          1          1

                                                0          1          1          0

Long code :

the long pseudo random noise (PN) sequence is based on 2⁴² characteristic polynomial. With this long code the data in the forward direction (Base to Mobile) is scrabled. The PN codes are generated using linear shift registers. The long code is unique for the subscribers and is known as users address mask.

Short Code :

The short pseudo random noise (PN) sequence is based on 2¹⁵ characteristic polynomial. This short code differentiates the cells & the sectors in a cell. It also consists of codes for I & Q channel feeding the modulator.