# Reply LS on Issues Related to Support of IMS Voice over NB-IoT NTN Connected to EPC

## Document Information

- **Source:** RAN1
- **To:** SA2
- **CC:** RAN2, SA4, CT1, SA3, SA1
- **Release:** Release 20
- **Work Item:** FS_5GSAT_Ph4_ARC
- **Meeting:** RAN1 Meeting #122bis (Prague, CZ, 13-17 October 2025)
- **Document Number:** R1-2508096
- **Response to:** R1-2506707

## Overall Description

This LS provides RAN1's response to SA2's question regarding the probability of consecutive packet losses in IMS voice over NB-IoT NTN scenarios.

### SA2 Question Addressed

**Question 2 (To RAN1):** Can RAN1 provide any data regarding the probability that such number of consecutive packets (e.g., 16 or 64) can be lost or erroneously decompressed? How often such event can occur?

### RAN1 Response

#### Scenario 1: Line-of-Sight Guaranteed

Under the following conditions:
- Line-of-sight maintained for the duration of the call
- No change in large scale parameters (e.g., fixed shadowing)
- Using NTN TDL-C channel model (previously used by RAN1 and SA4 evaluations)

**Finding:** The probability of having 16 or 64 packets consecutively lost is **negligible** under typical operating conditions (e.g., up to 10% packet error rate).

#### Scenario 2: Line-of-Sight Not Guaranteed

When line-of-sight cannot be guaranteed for the duration of the call:
- Intermittent blockage may occur in practice (e.g., according to the land mobile satellite channel model in TR 38.811)
- In this case, the probability of having 16 or 64 packets consecutively lost **may not be negligible**

**Note:** RAN1 has **not reached consensus** on whether to consider this case.

## Actions

**ACTION:** RAN1 asks SA2 to take the above information into account.