Terminology » Timing Advance in 5G NTN : Difference between TAcommon vs TAspecific

Timing Advance in 5G NTN : Difference between TAcommon vs TAspecific

5g ntn 5g ntn satellite communication

Introduction : This is a core timing concept in 5G Non-Terrestrial Networks (NTN), particularly important for maintaining uplink synchronization despite the large and varying propagation delays caused by satellite distances. Let us understand TAcommon and TAspecific in the context of 5G NTN and summarize differences between them.

Timing Advance (TA) in 5G

In 5G (both terrestrial and NTN), Timing Advance (TA) is used to ensure that uplink signals from different UEs arrive at the base station (gNB) (or satellite in NTN) in time alignment — so they don’t overlap and cause interference.

However, in NTN, the propagation delay is much larger (tens to hundreds of milliseconds) and varies significantly depending on user location, satellite position, and beam geometry.

To handle this, 3GPP introduced two timing components in NTN. They are TAcommon and TAspecific.

TAcommon (Common Timing Advance)

  • TAcommon is the common or reference timing advance value applied to all UEs within a satellite beam to compensate for the major (average) propagation delay between the satellite and that beam’s coverage area.
  • Used as a baseline timing adjustment for all UEs in that beam.
  • Purpose : To pre-compensate the large common delay (e.g. 5 to 10 ms in LEO, 120 ms in GEO) that affects all UEs roughly equally.

TAspecific (Specific Timing Advance)

  • TAspecific is an additional fine tuning timing advance value applied per UE to compensate the differential propagation delay between that UE’s actual location and the beam’s reference point.
  • Corrects the small individual differences in propagation delay between UEs located at different positions within the same beam footprint.
  • Determined through timing measurements (e.g., PRACH timing).
  • Purpose : To ensure precise symbol level alignment among UEs in the same beam, after the common delay has been compensated.

=> The total timing advance (TA) applied by a UE is the sum of both components.

TAtotal=TAcommon+TAspecificTAtotal = TAcommon + TAspecific

Summary: TAcommon is a broadcast, baseline value for everyone in the cell. TAspecific is a UE calculated, individual correction.