How Long Code Generation and Scrambling in CDMA
All base stations digital transmissions are referenced to a common CDMA system-wide time scale that uses the GPS time scale, which is traceable to and synchronous with Universal Coordianted Time (UTC). GPS and UTC difer by an integer number of seconds, specifically the number of leap second corrections added to UTC since January 6, 1980.
The start of CDMA System Time is January 6, 1980 00:00:00:00 UTC, which coincides with the start of GPS time.
System Time keeps track of leap second corrections to UTC but does not use these corrections for physical adjustments to System Time Clocks.
The initial state of the Long Code is that state in which the output of the long code generator is the first 1 output following 41 consecutive zeros. This state of the register returns after 41.45 days.
- All CDMA Base Stations and Mobiles Must be Synchronized
- Base Stations Use GPS Time via Satellite Receivers as a Common Time Reference
- GPS Clock Drives the Long Code Generator.
Long Code Scrambling
- User’s Long Code Mask is Applied to the Long Code
- Masked Long Code is Decimated Down to 19.2 kbps
- Decimated Long Code is XOR’ed with Voice Data Bits
- Scrambles the Data to Provide Voice Security
In the forward link the long code is used to scramble the voice data to provide some measure of security. However, the long code is not used to spread the signal bandwidth in the forward channel. To accomplish the scrambling without increasing the data rate, the long code is decimated down to a lower rate after the user’s unique long code mask is applied.
The decimation is accomplished by essentially using every sixty-fourth bit out of the long code data stream. A sixty four times decimation reduces the data rate of the long code from 1.2288 Mbps to 19.2 kbps. At this point the long code data rate matches that of the encoded voice data it is exclusive OR’ed with.