CDMA Codes


PN Long Code
The long code gets its name from the fact that it takes about 41.4 days for the code to repeat.
Information about the long code is broadcast to the mobile station by the Sync Channel to help the mobile lock onto the base station and helps provide separation from other base stations. The long code is used to scramble the interleaved signal to provide additional security against interception and interference. An additional advantage of the long code is that it allows the transmitter to use less power, maintaining control over the ambient RF environment and increasing the overall capacity of the cell.
PN Short Code
One of the codes used in conjunction with the Walsh Code is the PN (pseudorandom noise)
code. The pseudorandom noise code, more commonly referred to as the PN code, is used to
provide the base station with a unique identification that the mobile station uses to identify the serving base station. The PN code can be further modified with a time offset which allows additional PN codes to be generated. The time offsets used for the PN code is based on orthogonal coding in which the spread signal is split and sent to a quadrature spreader whose output is offset by ninety degrees. The resulting offset outputs are combined and transmitted to the mobile.
Walsh Function
The user signal (or control channel) is multiplied by the Walsh code. The Walsh code provides each user or channel with a unique identifier and, in DS spreading, spreads the frame across the entire 1.23 MHz bandwidth.