The core idea that led to today’s system was the cellular concept.
The cellular concept : multiple lower-power base stations that service mobile users within their coverage area and handoff users to neighboring base stations as users move. Together base stations tessellate the system coverage area.
The TDMA used by real cellular systems (like AT&T’s) is actually a combination of FDMA/TDMA.
Base station breaks up its total frequency band into smaller subbands.
Base station also divides time into slots and frames.
Each user is now assigned a frequency and a time slot in the frame.
Hybrid FDMA/TDMA (Cont’d) Time … … User 1 User 2 User 10 User 11 User 12 User 20 … … User 31 User 32 User 40 User 21 User 22 User 30 Assume a base station divides its frequency band into 4 subbands and time into 10 slots per frame. … … … … Frame Frequency Subband 1 Frequency Subband 2 Frequency Subband 3 Frequency Subband 4 … … User 1 User 2 User 10 User 11 User 12 User 20 … … User 31 User 32 User 40 User 21 User 22 User 30
There maybe other users who do this at the same time using the same frequency.
If they do, the signals will interfere with each other and the base station will not receive anything.
This indicates a failure (aka collision), when this happens, each user will backoff for some random amount of time and try again. Since they backoff for a random amount of time, chances are they won’t retry at the same time.