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.
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