What is FDMA/TDMA combined access, and what is frequency hopping?

FDMA/TDMA combines frequency and time division — each user gets a specific time slot on a specific frequency. Frequency hopping adds robustness by switching frequencies between time slots.

* Frequency hopping: the burst changes carrier every frame along a sequence both ends know. *

FDMA/TDMA (used in GSM):

• First, spectrum is divided into frequency channels (FDMA) — GSM-900 has 124 channels of 200 kHz each
• Each frequency channel is divided into 8 time slots (TDMA frame)
• A user gets one time slot on one frequency → they transmit in short bursts
• Between bursts, other users use the same frequency in their time slots

Frequency hopping:

• Instead of staying on one frequency, the device switches frequency after each TDMA frame according to a predefined hopping sequence
• The hopping pattern is known to both the device and the base station

Why frequency hop?

• Combats frequency-selective fading — if one frequency has a deep fade, the next hop will be on a different frequency that's likely fine
• Reduces interference — an interfering signal on one frequency only affects one hop, not the entire call
• Averages out channel quality across frequencies
• Technically more complex — requires fast frequency synthesizers

Tip: Bluetooth also uses frequency hopping (1600 hops/second across 79 channels) — it was actually inspired by a WWII patent for torpedo guidance!

Go deeper:

• Frequency-hopping spread spectrum (Wikipedia) — how hopping fights narrowband interference and fading, plus its use in GSM and Bluetooth.