M6 — Mobility: CHO + DAPS handover

[josephnef]

Part of the 5G-NR RAN EPIC #254. Phase B, the payoff capability.

Goal: a moving UE's schedule migrates cell-to-cell with no scan / no re-association / no clock re-acquisition — make-before-break, riding the network-wide synced clock.

5G mapping

CHO (Conditional Handover — pre-staged A3-style condition) and DAPS (Dual Active Protocol Stack — dual-cell active slots + uplink duplication during the crossing).

Deliverables (tools/ran-controller/ + src/cell/)

• measurement aggregation — each DU tags UE-uplink RxAtrib RSSI/SNR/EVM (from M0's per-UE attribution, filtered — never raw RSSI) and reports to the CU over F1.
• CHO — a pre-staged A3-style condition (offset margin + time-to-trigger hysteresis) evaluated at the DU/CU.
• make-before-break slot migration — the UE's grant appears in cell B's beacon and disappears from A; shared clock ⇒ no re-sync; FastRetune for the channel; security context pre-staged from ap_wpa2.cpp (802.11r-style).
• DAPS soft handover — same-channel first (from the review). With one radio a UE cannot listen to A while tuned to B, so different-channel DAPS is sequential duplication (deaf during retune), not simultaneous dual connectivity. First demo = same-channel dual-cell duplication + sequence dedup over Xn/F1. The different-channel variant must budget retune time+variance, RF settling, missed beacons/ACKs, and a deadline to return to the serving channel.
• shared sequence handling + duplicate suppression for HARQ and DAPS, and state cleanup during handover.

Reuses

M5 CU + F1/Xn, M4 synced TSF (the enabler — TSF continuity across cells), FastRetune, ap_wpa2.cpp security context, M0 per-UE attribution.

Risks (from EPIC)

• A3/CHO tuning — raw RSSI ping-pongs at the edge; hysteresis + time-to-trigger is the classic pitfall (drive off filtered quality).
• Single-radio deaf-time — different-channel DAPS is sequential; ~ms FastRetune bounds it.

Dependencies

M5 (#254 tracks).

Test — tests/ran_handover.sh

Move a UE A→B: the grant migrates after the time-to-trigger window with TSF continuity, no re-association, vs the ~100 ms roam "ghost bar." Kill one path mid-crossing → the duplicated UL still arrives via B (DAPS).