FDMA; FDM in telephone hierarchy

FDMA

Frequency Division Multiple Access (FDMA) is a multiple-access method in which a shared medium (e.g. a satellite transponder or a cellular band) is divided into frequency sub-bands, and each user/station is assigned its own sub-band for the duration of its use. All users transmit at the same time on different frequencies. Examples: satellite SCPC/MCPC systems, 1G AMPS cellular (30 kHz channels). FDM is the multiplexing of signals at one location; FDMA is the same idea applied to users at different locations.

FDM in telephony hierarchy

Each voice channel (300–3400 Hz) is given a 4 kHz slot and frequency-translated by SSB-SC (LSB) modulation. Channels are built up in standard blocks (ITU-T/CCITT):

12 ch  -> Basic group       60-108 kHz
x5     -> Supergroup  (60)  312-552 kHz
x5     -> Mastergroup (300) 812-2044 kHz
x3     -> Supermaster (900) 8516-12388 kHz

1. Basic group: 12 voice channels modulate carriers 64, 68, ..., 108 kHz; LSBs selected; band 60–108 kHz (48 kHz).
2. Supergroup: 5 groups translated by carriers 420, 468, 516, 564, 612 kHz into 312–552 kHz; 60 channels.
3. Mastergroup: 5 supergroups, 300 channels, 812–2044 kHz (8 kHz guard bands between supergroups).
4. Supermastergroup: 3 mastergroups, 900 channels, 8516–12388 kHz.

In the AT&T (Bell) system a mastergroup is 10 supergroups (600 channels, 564–3084 kHz) and a jumbo group is 6 mastergroups (3600 channels).

Advantages of the hierarchy: standard modular equipment, filters at convenient frequencies, and whole blocks can be switched or routed together. Carriers come from one master oscillator, and pilot tones keep levels and frequencies correct.