What is CDMA?
CDMA is used for mobile to mobile or cellular communication similar to GSM. CDMA is the short form of Code Division Multiple Access. It is more secured way of communication due to it spread spectrum property. In the spread spectrum modulation signal to be transmitted is modulated using spreading code or PN code, which will increase the bandwidth of the signal significantly. This makes signal power level below the noise power level, hence it makes impossible for intruders to intercept the signal and decode the signal. Only the receiver can decode the signal using correlation method provided the PN code is known at the receiver.
Various unique codes are used by mobile to differentiate them at Base station. These unique codes are called Walsh codes in CDMA. Base station to mobiles path is also differentiated by unique codes assigned. These codes are called PN codes.
Mobile to Base station is called reverse channel and Base station to mobile is called forward channel
QPSK modulation is used on forward channel and OQPSK modulation is used on reverse channel to modulate the information bits before RF frequency up conversion.
This page explains what is a CDMA technology is all about and provides useful links as mentioned below.
TD-SCDMA Physical layer CDMA versus GSM Cell Re-Selection what is CDMA WCDMA compressed mode CDMA tutorial TD-SCDMA Tutorial Multicarrier CDMA