Originally published on 4GNation, December 2015 — republished here as part of the Telco & Beyond Engineering Archive.
This is one of the simplest concepts in LTE, but also one of the most foundational — nothing in the network works without the transmitter and receiver agreeing on a shared sense of time. I used a wristwatch analogy back in 2015 to explain it to colleagues, and it still holds up as the easiest mental model for anyone new to the topic.
In any communication system, one of the most important requirements is that the transmitter and receiver operate at the same tempo — technically speaking, in synchronized mode.
Put simply: the transmitter and receiver each have their own clock, and they need to synchronize those clocks before communication can begin.
What kind of clock does LTE use? Like an analog wristwatch, the LTE clock has two “arms.” One ticks every 10ms, the other every 1ms. And just like a wristwatch, each arm’s ticks are numbered within a fixed range.
- The arm ticking every 10ms has numbers between 0 and 1023 — this is the System Frame Number (SFN).
- The arm ticking every 1ms has numbers between 0 and 9 — this is the subframe number.
Before the transmitter (eNodeB) and receiver (UE) can start communicating, they have to set both clock arms to the same number. This synchronization happens during cell search and timing sync. In short, they synchronize tempo — the exact moment each arm ticks — through cell search and timing sync, and the UE gets its SFN sync from the MIB, which carries the SFN value directly.