MAC CE-driven TA updates, TAGs, timeAlignmentTimer, and why UEs get de-synced
UEs at different distances from gNB have different propagation delays. Without TA correction, their uplink signals arrive at different times, causing inter-symbol interference (ISI). TA advances each UE's TX timing by 2 × propagation delay so all UL signals arrive aligned at the gNB.
Move the UE slider to change distance. Watch how TA adjusts TX timing to keep UL arrivals aligned at gNB.
RTT = 0.67 μs → Small TA. UE transmits almost on time.
RTT = 6.67 μs → Medium TA. UE transmits 6.67 μs early.
RTT = 33.3 μs → Large TA. UE transmits 33.3 μs early.
| Parameter | RAR (Initial) | MAC CE (Maintenance) |
|---|---|---|
| Field size | 12 bits | 6 bits |
| Type | Absolute value | Relative adjustment |
| Range | 0–3846 | −31 to +32 |
| Resolution | 32 × Tc = 16.28 ns | 16 × Tc = 8.14 ns |
| Max TA | ~31 μs (4.7 km) | ±260 ns per update |
After initial TA from RACH Msg2, the gNB continuously monitors UL timing and sends TA Command MAC CEs (1 byte: 4-bit TAG ID + 6-bit relative adjustment). These fine-tune the TA to track UE mobility.
Animated feedback loop: gNB measures UL timing drift, sends TA MAC CE, UE adjusts. Watch the TA value converge.
SCells at different physical locations need different TA values. TAGs allow the UE to maintain up to 4 independent TA values — one pTAG (PCell) and up to 3 sTAGs (SCell groups).
See how cells are grouped into TAGs with independent TA values. Toggle sTAGs to see the configuration change.
The timeAlignmentTimer tracks how long since the last TA update. When it expires, the UE considers itself not time-aligned — it flushes all HARQ buffers, releases PUCCH/SRS/CG, and must perform RACH to re-acquire TA.
Watch the timer count down. TA MAC CEs reset the timer. If it expires, UE de-syncs and must RACH.
UE is time-aligned. Can transmit on UL. TA updates restart the timer.
UE NOT time-aligned. All UL resources released. Must RACH to re-acquire TA.
New TA from RAR Msg2. Timer restarted. CG/SPS must be re-activated by gNB.
| Scenario | timeAlignTimer | TA Update Rate | TAG Config |
|---|---|---|---|
| Stationary IoT | ms10240 | Very rare | pTAG only |
| Pedestrian | ms2560 | Every ~200 ms | pTAG only |
| Urban vehicle | ms1280 | Every ~50 ms | pTAG + sTAG |
| High-speed train | ms500 | Every ~4 ms | pTAG + sTAG |
| NTN (satellite) | infinity | Continuous | pTAG + sTAG |
| Aspect | LTE | 5G NR |
|---|---|---|
| TA resolution (RAR) | 11 bits, 16×Ts | 12 bits, 32×Tc |
| TA resolution (CE) | 6 bits, 16×Ts | 6 bits, 16×Tc |
| Max TAGs | 4 | 4 |
| NTN support | None | Extended TA (Rel-17) |
UE adjusts TA from DL timing measurements without waiting for MAC CE. Near-zero TA maintenance latency. Critical for 500 km/h rail.
AI at gNB predicts TA from UE trajectory. Pre-sends correction before drift occurs. 60-80% fewer TA MAC CEs.
18-bit TA (vs 12-bit). UE ephemeris-aware TA prediction. Dual TA: common (cell-level) + UE-specific (fine).
Compare TA capabilities across generations.
| Dimension | 4G (LTE) | 5G NR | 6G (Target) |
|---|---|---|---|
| TA resolution | 11 bits | 12 bits | 18 bits |
| Max range | ~100 μs | ~31 μs | 26+ ms (NTN) |
| TAGs | 4 | 4 | 8+ |
| Updates | gNB-driven | gNB-driven | UE autonomous + AI |
| Update latency | 2-4 ms | 2-4 ms | < 1 ms |