System Frame Number (SFN) and Subframe Number

SFN and Subframe

In any communication, one of the most important requirement would be that the transmitter and receiver operate at the same tempo, more technically speaking that the transmitter and receiver should operates in synchronized mode.

Speaking in laymen's terminology, the transmitter and receiver has it's own clock and they have to synchronize the clock before the communication starts.

What kind of clock they have in LTE ? Like our analog wrist clock, LTE clock has two arms. One arm ticks every 10 ms and the other arm ticks every 1 ms. Again as in the wrist clock, each of ticks has specific numbers and the numbers has a certain range.

In LTE, the arm ticking in 10 ms interval has numbers between 0 and 1023 and these numbers are called SFN (System Frame Number) and the other arm ticking in 1 ms interval has numbers between 0 and 9, and this number is called subframe number.

Before the transmitter (eNode B) and the receiver (UE) in LTE start communicating each of other they have to set these two clock arms to be the same number and this synchronization happens during cell search and timing sync process. In short, they synchronize the tempo (exact time the arm ticks) by cell search and timing sync and UE get SFN sync from MIB which carries SFN number in it.