The LTE Timing Advance

One nice thing about GSM ‘in the old days’ was, that there was a timing advance (TA) value that indicated to the mobile how much earlier it had to start transmitting relative to the network to account for the limited speed of radio waves. In my early ‘mobile days’, the TA was an invaluable help to get an idea of how far cell sites are apart from each other in the city and in the countryside, and if a cell site I stood in front of was actually the one my mobile communicated with or if it was from a different network operator. Unfortunately, the timing advance went away with UMTS, and when it came back with LTE, it was not shown in the ‘engineering menu’ of mobile devices any more. A pity. Also, I thought that the timing advance value was deeply buried in MAC messages, and hence difficult to get to with professional trace tools. Recently, however, I found out that at least the later assumption was not quite true.

In LTE and in 5G NR as well, the timing advance is given to the UE during the random access procedure. In LTE, the value is given in steps of 78,12 meters. A value of 4, for example, means that the cell site is around 4*78,12= 312 meters away. Unfortunately, none of the mobile devices I’m aware of shows the value in an engineering menu. But professional (expensive) traces tools can fortunately show the value.

When the device is not stationary, the value can then be adapted by the network on the MAC layer by giving increments or decrements to this initial value. The field has 6 bits (64 values), and a value of 31 (i.e. the middle point of the range) means not to change the current timing advance value. For more details have a look here.

And while we are at it, there’s a timing advance value in 5G as well. The distance per step of the TA value given in the random access response message depends on the sub-carrier spacing (SCS). Typically, and SCS of 15 kHz is used below 3.5 GHz, and in this case, the distance per step is the same as in LTE: 78,12 meters. In the 3.5 GHz band (NR band n78), an SCS of 30 kHz is typically used, and here, each timing advance step is half the value, i.e. 39,06 meters. Have a look here for the details.