fix(lorawan): NbTrans retransmissions skipped after LinkADRReq nb_trans change

[hallard]

Summary

Fixes a LoRaWAN spec 4.3.1.1 compliance failure where the first unconfirmed uplink after a LinkADRReq changes nb_trans is sent only once instead of nb_trans times, and FCnt is incremented after that single TX.

Root cause

get_QOS_level() has a state-updating side-effect: it writes _prev_qos_level = nb_trans each time it is called. get_QOS_level() is only called from post_process_tx_no_reception(), which is only entered when no downlink is received.

Typical scenario when NS sends a LinkADRReq:

1. The downlink carrying the LinkADRReq is received → process_reception() handles it → post_process_tx_no_reception() is never called → _prev_qos_level stays at its old value.
2. nb_trans is now updated to the new value.
3. Next application frame is sent → no downlink → post_process_tx_no_reception():

   prev_QOS_level = get_prev_QOS_level()  // old value, e.g. 1
   QOS_level      = get_QOS_level()       // new value, e.g. 3 — side-effect: _prev_qos_level = 3
   if (QOS_level > 1 && (prev_QOS_level == QOS_level))  // 1 == 3 → FALSE
   

   → retransmissions skipped → FCnt++ after a single TX. Spec violation.
4. Every subsequent frame: prev_QOS_level == QOS_level → retransmits correctly.

Fix

• LoRaWANStack.cpp: Remove the prev_QOS_level == QOS_level guard. The _qos_cnt counter — reset to 1 in handle_tx() for every new application frame — already correctly bounds the number of retransmissions regardless of nb_trans transitions.

• LoRaMac.cpp: Reset ul_nb_rep_counter to 0 at the start of each new frame send (LoRaMac::send). Without this reset, the counter accumulates across all frames (it is only reset on join/disconnect), so nb_retries in the TX confirmation grows unboundedly and the commented-out MBED_ASSERT(ul_nb_rep_counter <= nb_trans) in on_radio_tx_done would fire on the second application message.

Test plan

• Set nb_trans = 1 (default): device sends 1 TX per frame, FCnt increments correctly.
• Set nb_trans = 3 via LinkADRReq in a downlink: verify the first data frame after the change is sent 3 times with the same FCnt, then FCnt increments.
• Verify nb_retries in the TX confirmation equals the number of physical TXs for that frame only.
• LoRaWAN certification ADR/Redundancy test (NbTrans > 1).

🤖 Generated with Claude Code