How does GSM combine FDMA and TDMA, and what role does slow frequency hopping play?

GSM assigns each user both a specific frequency (FDMA) and a specific time slot (TDMA). Slow frequency hopping then switches the frequency between frames to combat fading and distribute interference.

The combination:

1. FDMA — the user is assigned a frequency channel (one of 124 in GSM-900, each 200 kHz)
2. TDMA — within that channel, the user gets one of 8 time slots (577 µs each)
3. Frequency hopping — the assigned slot stays the same, but the frequency changes between TDMA frames

Slow frequency hopping explained:

• "Slow" because the frequency changes once per frame (every 4.615 ms), not within a frame
• The hopping sequence is predefined and known to both the phone and the base station
• Each hop moves to a different frequency channel

Why hop?

• Frequency-selective fading — if one frequency happens to be in a deep fade at your location, it only affects one frame; the next frame will be on a different frequency that's likely fine
• Interference averaging — interference on any single frequency is spread across all users, rather than one user being permanently stuck on a noisy channel
• Quality degradations from fading and interference get distributed evenly across all users

This scheme is used exclusively in 2G. Starting with 3G (UMTS), the access method changed to CDMA, which handles frequency diversity differently (through wideband spreading).

Go deeper:

• Frequency-hopping spread spectrum (Wikipedia) — the hopping principle behind GSM's slow frequency hopping and how it averages out fading and interference.