CDMA

CDMA, or code-division multiple access, is a spread-spectrum multiple-access technology that lets multiple users share the same radio channel by assigning each user a unique code sequence. In contrast to time-division or frequency-division schemes, users transmit simultaneously using the same frequency band, differentiated by their codes.

In direct-sequence CDMA (DS-CDMA), each user's data is multiplied by a pseudorandom code, or chip sequence, that spreads the signal over a wide bandwidth. The receiver uses the same code to despread and recover the data. Codes are drawn from a large set, providing multiplexing gain, robustness to narrowband interference, and a degree of privacy since the signal appears noise-like without the code.

A key feature of CDMA systems is the use of spreading and power control to manage interference and capacity. Because signals from many users occupy the same spectrum, accurate uplink power control is essential to limit near-far effects and allow successful decoding. Techniques such as soft handoff can provide seamless service when users move between cells.

CDMA has been implemented in several cellular standards. The IS-95 (cdmaOne) family defined 2G/early 3G networks. CDMA2000 represents a 3G evolution of those technologies. Wideband CDMA (W-CDMA) is a related approach used in UMTS; it shares the same underlying spread-spectrum concept but is a distinct standard. CDMA has also been applied in satellite communications and specialized wireless systems.

Advantages of CDMA include higher spectral efficiency in multipath environments, robustness to interference, and simplified frequency planning. Limitations include system complexity, reliance on effective power control, and the need for careful code management and network design to mitigate interference.