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LteSchedulerEnbDl::checkEligibility() fell through to 'return true' when no H-ARQ process was free. The UE was granted, the RLC built PDUs, and LteMacEnb::macPduMake() then deleted the MAC PDU without any statistic.
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Hi,
while working with Simu5G 1.2.2 on a DL scenario with a lot of interference, we lost packets at the application while the MAC reported 0 % loss. We traced it to
LteSchedulerEnbDl::checkEligibility(). The same code is still on master.If a UE has no free H-ARQ process,
firstAvailable()returnsHARQ_NONE. That case falls through toreturn true, so the UE still gets a grant. The RLC builds PDUs for it, andLteMacEnb::macPduMake()then finds no free process and deletes the MAC PDU (“no available process for this MAC PDU in TxHarqBuffer”). No statistic counts this. The UE’s RLC sees the sequence gaps and drops the affected SDUs.In our run the gNB deleted 214 MAC PDUs in the first 3 seconds. With this change it deleted none, and delivery of a 300 B periodic flow went from 58 % to 90 % (mean over 20 seeds).
The fix returns
falsein that case, so the UE simply waits a TTI. Retransmissions go throughschedulePerAcidRtx()and aren’t affected. Fingerprints of scenarios where a UE runs out of H-ARQ processes will probably change.LteMacUe::macPduMake()has a similar delete on the UL side; we haven’t looked into that one.Thanks!