From 5d4e17f8ca59a2b330e0a2ad0f14a136d43e7f1a Mon Sep 17 00:00:00 2001 From: Robert Schmidt Date: Wed, 3 Dec 2025 16:16:42 +0100 Subject: [PATCH] doc: nFAPI: add Troubleshoot section and explain ra_responseWindow --- doc/nfapi.md | 23 +++++++++++++++++++++++ 1 file changed, 23 insertions(+) diff --git a/doc/nfapi.md b/doc/nfapi.md index 6198307220..5f2d8a35da 100644 --- a/doc/nfapi.md +++ b/doc/nfapi.md @@ -285,3 +285,26 @@ done over virtual time, i.e., frames/slots as executed by the 5G Systems. For instance, these numbers might be slightly higher or slower in RFsim than in wall-clock time, depending if the system advances faster or slower than wall-clock time. + +## Troubleshoot + +When operating using the FAPI split, the PNF needs to give the VNF extra time +to schedule the next slot. Especially since the current nFAPI split still +relies on slot indications, extra time due to transport delays need to be +accounted for. Currently, this delay is set conservatively, meaning that it +should work for most systems, but can create problems during random access: + + [NR_MAC] exceeded RA window: preamble at 411.19 now 413.0 (diff 21), ra_ResponseWindow 5/20 slots + [NR_MAC] sfn: 413.0 UE RA-RNTI 010b TC-RNTI 5d82: exceeded RA window, cannot schedule Msg2 + +means that the VNF received a preamble (411.19), but the current slot to be +scheduled (413.0) is beyond the random access response window (20 slots). In +this case, try one of the following: + +- If the radio allows, reduce the L1 TX advance `RUs.[0].sl_ahead` by some + slots, but note that this could make the system less stable. +- Change the code to reduce `sl_ahead` inside function `handle_nr_slot_ind()`, + to reduce the FAPI scheduling slot time budget. +- Non-standard: You can manually increase the response window by setting + `gNBs.[0].servingCellConfigCommon.[0].ra_ResponseWindow` to, e.g., 6. Note + that the maximum allowed response window is 10ms.