CDMA

Code Division Multiple Access is a radio transmission method that allows multiple users to share the same frequency band simultaneously without interfering with each other. Each user's data is spread across the available bandwidth using a unique digital code, then recombined at the receiver using that same code. The receiver effectively filters out all transmissions that do not match its assigned code sequence, recovering only the intended message from what appears to be noise to anyone without the correct code.

The technology relies on pseudorandom spreading sequences, commonly called chips, which run at a much higher clock rate than the original data. A typical CDMA signal might use sequences with millions of chips per second, spreading a kilobit-per-second data stream across several megahertz of spectrum. The processing gain, the ratio of spread bandwidth to original data rate, determines how many simultaneous users the system can support while maintaining acceptable signal quality.

Deployment and variants

CDMA saw its widest adoption in mobile telephony, particularly in second and third generation cellular networks including IS-95 (cdmaOne) and CDMA2000. It competed with GSM, which used time division multiplexing instead. Early satellite and military communications also employed CDMA because its spread-spectrum nature provides inherent resistance to jamming and eavesdropping. Modern cellular networks have largely migrated to OFDMA and LTE variants, which offer better spectral efficiency at the cost of less robust performance under poor channel conditions.

The primary limitation of CDMA is the near-far problem: a strong signal close to the receiver can drown out weaker distant signals even with perfect code orthogonality. Practical systems require precise power control to keep all received signals at nearly equal strength. Additionally, as the number of active users increases, the cumulative effect of non-orthogonal code interference degrades performance faster than in time or frequency division systems.

The name reflects the core concept: the code itself becomes the division mechanism, allowing access independent of time slot or frequency channel. This contrasts with TDMA (time slots) and FDMA (frequency channels), where users are kept separate by scheduling or spectrum allocation rather than by code assignment.