How do FDMA, TDMA, and CDMA compare as multiple-access schemes, which mobile generations use them, and what replaces them in 4G/5G?
FDMA splits the spectrum by frequency, TDMA by time, CDMA by code. GSM (2G) combines FDMA+TDMA; UMTS (3G) uses CDMA; LTE/5G move to OFDMA-style scheduling over many orthogonal subcarriers, with 5G also relying heavily on spatial separation through beamforming.
| FDMA | TDMA | CDMA | |
|---|---|---|---|
| Separates users by | frequency band | time slot | spreading code |
| Resource a user holds | one exclusive frequency, whole call | one exclusive slot per frame | one code; shares all freq + all time |
| Capacity limit | hard (# of bands) | hard (# of slots) | soft (interference-limited) |
| Adding too many users | rejected — no band free | rejected — no slot free | everyone's noise floor rises gradually |
| Generation | building block of 1G/2G | with FDMA in 2G GSM | 3G UMTS (W-CDMA) |
Generation map:
| Generation | Main access idea |
|---|---|
| 1G analog | FDMA |
| 2G GSM | FDMA + TDMA |
| 3G UMTS | W-CDMA / CDMA |
| 4G LTE | OFDMA downlink; SC-FDMA uplink |
| 5G NR | OFDMA/OFDM plus SDMA through massive MIMO and beamforming |
The mental model for the three:
- FDMA = different rooms — each conversation in its own soundproof room (frequency).
- TDMA = one room, take turns — everyone in one room but speaking in scheduled slots (time).
- CDMA = one room, different languages — everyone talks at once in the same room; you tune your ear to one "language" (code) and the rest is background babble.
Key contrast: FDMA and TDMA enforce hard partitions — capacity is a fixed count, and the access scheme alone prevents interference. CDMA does not avoid interference; it manages it. Capacity is soft (interference-limited), which is why CDMA cells "breathe" — the usable cell radius shrinks as more users load the cell and raise the shared noise floor.
Generational arc: GSM (2G) = FDMA + TDMA + slow frequency hopping → UMTS (3G) = CDMA → LTE/5G (4G/5G) = OFDMA, a frequency-domain successor that schedules many narrow orthogonal subcarriers rather than spreading.
Go deeper:
FDMA vs TDMA vs CDMA — multiple access compared (YouTube) — the three "rooms / turns / languages" pictures animated side by side.
Channel access method (Wikipedia) — FDMA/TDMA/CDMA/SDMA compared in one place, including their hard- vs soft-capacity behaviour.- Orthogonal frequency-division multiple access (Wikipedia) — the OFDMA scheme that succeeds CDMA in 4G LTE and 5G NR.