For both mobiles and base station in direct sequence CDMA must be synchronized. In the IS-95A system, synchronization is based on the Global Positioning Satellite system time.
The CDMA system time is the time reference used by all IS-95 / IS-2000 and IS-856 systems. It is synchronous to UTC time (Universal Coordinated Time) except for leap seconds and uses the same time as the GPS time (Global Positioning System).
- How Does CDMA Achieve Synchronization for Efficient searching?
- Use GPS Satellite System
- Base Stations Use GPS Time via Satellite Receivers as a Common Time Reference
- GPS Clock Drives the Long Code Generator
Each CDMA base station incorporates a GPS receiver to provide exact system timing information for the cell. The base station then sends this information to each mobile via a special channel. In this manner, all radios in the system can maintain near perfect synchronization.
Most designs also include atomic clocks to provide a backup timing reference in the event that an insufficient number of GPS satellite can be received.
These are capable of maintaining synchronization for up to several hours. The GPS clock used for CDMA system time is then used to drive the long code pseudo-random sequence generator.
The GPS system time is recorded in Layer 3 CDMA messages and is called the CDMA system time.