How does CDMA (Code Division Multiple Access) work, and how is it different from FDMA and TDMA?

In CDMA, all users transmit on the same frequency at the same time, but each user's signal is encoded with a unique orthogonal code — the receiver uses the code to filter out the desired signal from the mix.

How it works:

• Every user gets a unique spreading code (a mathematical sequence)
• The user's data is multiplied by this code, spreading it across a wide frequency band
• All users transmit simultaneously on the same frequency band
• The receiver knows the sender's code and uses it to extract the desired signal
• Signals from other users appear as background noise — they can be filtered out

Visualized: On a frequency-vs-time diagram, all users occupy the entire frequency band for the entire time — they're separated only in the code dimension.

Advantages:

• Graceful degradation — adding more users increases noise slightly but doesn't cause hard cutoffs
• No frequency planning needed — all cells can use the same frequency (frequency reuse factor = 1)
• Soft handover possible — phone can talk to two base stations simultaneously

Disadvantages:

• Near-far problem — a nearby user's signal can overwhelm a distant user's signal → requires precise power control
• More complex signal processing

Used in: 3G (UMTS/WCDMA) is built on CDMA. It was a fundamental paradigm shift from GSM's FDMA/TDMA approach.

Go deeper:

• What is CDMA? (Electronics Notes) — the "many languages in one room" intuition plus how the matching code decodes one user despite the others.
• Code-division multiple access (Wikipedia) — spreading codes, correlation receivers and the worked example of recovering one user from the combined signal.