Code Division Multiple Access lets many users share the same frequency at the same time. Each call is assigned a unique code. The receiver uses that code to pick out the right signal from the mix. Other signals sound like noise. CDMA was the basis for 3G networks and is still used in some systems.

CDMA was developed by Qualcomm in the 1980s. It was based on spread spectrum technology used in military communications. The signal is spread across a wide band of frequencies. The code is the key. Without the code, the signal is unintelligible. With it, the original data is recovered.

Key features

• Spread spectrum: signal spread over wide bandwidth
• Unique codes: each user has a distinct code
• Soft handoff: can connect to multiple cells
• Capacity: limited by interference, not hard limit

CDMA competed with GSM, which used time division. CDMA offered better capacity and call quality in some conditions. But GSM won globally. Later, CDMA was used in 3G standards like WCDMA and CDMA2000. The technique influenced LTE and 5G, which use OFDMA but share the same goal: many users, one spectrum.