Reference Plane / Episode 1
Watch the ACK/NACK pipeline. Stretch the link. See where it stalls.
Link scenario
Block error rate (BLER)
HARQ processes
UE-side process IDs, one TB each
Each cell is one HARQ process. A TB occupies it from first transmission until ACK or max retx.
UE perspective, last events first
Fraction of Shannon limit achieved
N · t_slot / RTT = 1.000
Pipe depth needed
4 processes
One slot = 1 ms. To keep the link full, processes must equal or exceed RTT in slots.
TBs transmitted
0
ACKs received
0
NACKs / retx
0
Stall slots
0
The UE sends a transport block (TB) on a free HARQ process. The block flies up for one one-way delay (τ). The gNB decodes, then sends an ACK if the CRC passes, or a NACK if it fails. The feedback flies back for another τ, so the UE learns the outcome 2τ after first transmission. While waiting, the UE can use other HARQ processes to keep sending. If N processes × slot time is shorter than the round-trip, the UE runs out of free processes and the link sits idle. That is the NTN HARQ stall.