There Are Three Codes in CDMA PN Long Code,PN Short Code & Walsh Code.
PN Long Code
The Long Code is a PN sequence that is 242 – 1 bits (chips) long. The long code is used to encrypt user information. Both the base station and the mobile unit have knowledge of this sequence at any given instant in time based on a specified private 42 bit “long code mask” that is exchanged.
PN Short Codes
The Short Code is a PN sequence that is 215 bits (chips) in length. This code is used for final spreading of the signal and is transmitted as a reference known as the “Pilot Sequence” by the base station. The same short code is used by all base stations. Base stations are differentiated from one another by offsetting their transmission of this code in (absolute) time. This time offset is known as a “PN Offset”. Mobile units initially search (in time) until they synchronize with a pilot code transmitted by a base station. The base station then conveys timing information to the mobile.
CDMA defines a group of 64 orthogonal sequences, each 64 bits long, known as Walsh Codes. These sequences are also referred to as Walsh Functions. Walsh codes are used to identify users on the forward link. For this reason they are loosely referred to as CDMA channels. Walsh codes are used for 64ary modulation as “symbols” on the reverse link.