CDMA (Code division multiple access) is a channel access method utilized by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne and CDMA2000 (which are often referred to as simply "CDMA"), that use CDMA as their underlying channel access methods.
CDMA is a spread spectrum multiple access technique. In CDMA a locally generated code runs at a much higher rate than the data to be transmitted. Data for transmission is simply logically XOR (exclusive OR) added with the faster code.
Each user in a CDMA system uses a different code to modulate their signal. Choosing the codes used to modulate the signal is very important in the performance of CDMA systems. The best performance will occur when there is good separation between the signal of a desired user and the signals of other users.
- Universal frequency reuse
The frequency spectrum is a limited resource. Therefore, wireless telephony, like radio, must reuse frequency assignments.
For example, two radio stations might transmit at 91.3 FM. There is no interference as long as the radio stations are far enough apart.
- Fast and accurate power control
Power control is a CDMA feature that enables mobiles to adjust the power at which they transmit. This ensures that the base station receives all signals at the appropriate power.
The CDMA network independently controls the power at which each mobile transmits.
Both forward and reverse links use power control techniques.
- Rake receiver
CDMA's rake receiver is multiple receivers in one. The rake receiver identifies the three strongest multi-path signals and combines them to produce one very strong signal.
The rake receiver therefore uses multipath to reduce the power the transmitter must send.
Both the mobile and the BTS use rake receivers.
- Different types of handoff
Handoff is the process of transferring a call from one cell to another. This is necessary to continue the call as the phone travels.
CDMA is unique in how it handles handoff.
Types of CDMA handoff
CDMA has three primary types of handoff: