Understanding CDMA Technology: Code Division Multiple Access

CDMA, or Code Division Multiple Access, is a technology used for mobile (cellular) communication, similar to GSM.

It’s considered a more secure method of communication thanks to its spread spectrum property. In spread spectrum modulation, the signal to be transmitted is modulated using a spreading code, often called a PN code (Pseudo-Noise code). This significantly increases the signal’s bandwidth.

This bandwidth increase effectively lowers the signal power level to below the ambient noise level. As a result, it becomes extremely difficult for unauthorized parties to intercept and decode the signal.

Only the intended receiver can decode the signal, and this is accomplished using a correlation method, assuming the receiver knows the PN code used for spreading. Different mobile devices use unique codes to allow the base station to differentiate between them. These unique codes in CDMA are called Walsh codes. The path from the base station to the mobile is also differentiated by unique codes, called PN codes.

The path from the mobile to the base station is called the reverse channel, while the path from the base station to the mobile is the forward channel.

QPSK (Quadrature Phase-Shift Keying) modulation is typically used on the forward channel, and OQPSK (Offset Quadrature Phase-Shift Keying) modulation is used on the reverse channel. This is done to modulate the information bits before the RF frequency up-conversion.