10.1.6 Radio Link Failure

36.3003GPPEvolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)Overall descriptionRelease 17Stage 2TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

Two phases govern the behaviour associated to radio link failure as shown on Figure 10.1.6-1:

– First phase:

– started upon radio problem detection;

– leads to radio link failure detection;

– no UE-based mobility;

– based on timer or other (e.g. counting) criteria (T₁).

– Second Phase:

– started upon radio link failure detection or handover failure;

– leads to RRC_IDLE;

– UE-based mobility;

– Timer based (T₂).

Figure 10.1.6-1: Radio Link Failure

Table 10.1.6-1 below describes how mobility is handled with respect to radio link failure:

Table 10.1.6-1: Mobility and Radio Link Failure

Cases	First Phase	Second Phase	T2 expired
UE returns to the same cell	Continue as if no radio problems occurred	Activity is resumed by means of explicit signalling between UE and eNB	Go via RRC_IDLE
UE selects a different cell from the same eNB	N/A	Activity is resumed by means of explicit signalling between UE and eNB	Go via RRC_IDLE
UE selects a cell of a prepared eNB (NOTE)	N/A	Activity is resumed by means of explicit signalling between UE and eNB	Go via RRC_IDLE
UE selects a cell of a different eNB that is not prepared (NOTE)	N/A	Go via RRC_IDLE	Go via RRC_IDLE
NOTE: a prepared eNB is an eNB which has admitted the UE during an earlier executed HO preparation phase, or obtains the UE context during the Second Phase.

For a NB-IoT UE that only uses Control Plane CIoT EPS/5GS optimisations, as defined in TS 24.301 [20] and does not support RRC Connection re-establishment for the control plane as defined in TS 36.331 [16], at the end of the first phase, the UE enters RRC_IDLE (there is no second phase).In the Second Phase, in order to resume activity and avoid going via RRC_IDLE when the UE returns to the same cell or when the UE selects a different cell from the same eNB, or when the UE selects a cell from a different eNB, the following procedure applies:

– The UE stays in RRC_CONNECTED;

– The UE accesses the cell through the random access procedure;

– Except for a NB-IoT UE using only Control Plane CIoT EPS/5GS optimisations, the UE identifier used in the random access procedure for contention resolution (i.e. C‑RNTI of the UE in the cell where the RLF occurred + physical layer identity of that cell + short MAC-I based on the keys of that cell) is used by the selected eNB to authenticate the UE and check whether it has a context stored for that UE:

– If the eNB finds a context that matches the identity of the UE, or obtains this context from the previously serving eNB, it indicates to the UE that its connection can be resumed;

– If the context is not found, RRC connection is released and UE initiates procedure to establish new RRC connection. In this case UE is required to go via RRC_IDLE.

– For a NB-IoT UE using only Control Plane CIoT EPS/5GS optimisations, the UE identifier used in the random access procedure for contention resolution (i.e. S-TMSI (for EPS) or truncated 5G-S-TMSI (for 5GS) of the UE at the time where the RLF occurred + UL NAS MAC + UL NAS COUNT) is used by the selected (ng-)eNB to request the MME/AMF to authenticate the UE’s re-establishment request and provide the UE context:

– If the authentication of the UE is successful and a context is provided, it indicates to the UE that its connection can be resumed;

– If no context is provided, the RRC connection is released and UE initiates procedure to establish new RRC connection. In this case UE is required to go via RRC_IDLE.

The radio link failure procedure applies also for RNs, with the exception that the RN is limited to select a cell from its DeNB cell list. Upon detecting radio link failure, the RN discards any current RN subframe configuration (for communication with its DeNB), enabling the RN to perform normal contention-based RACH as part of the re-establishment. Upon successful re-establishment, an RN subframe configuration can be configured again using the RN reconfiguration procedure.

For DC, PCell supports above phases. In addition, the first phase of the radio link failure procedure is supported for PSCell. However, upon detecting RLF on the PSCell, the re-establishment procedure is not triggered at the end of the first phase. Instead, UE shall inform the radio link failure of PSCell to the MeNB.

NOTE: If the recovery attempt in the second phase fails, the details of the RN behaviour in RRC_IDLE to recover an RRC connection are up to the RN implementation.

In case of DAPS handover, the UE continues the detection of radio link failure at the source cell until the successful completion of the random access procedure to the target cell. If RLF is declared in the source cell, the UE:

– stays in RRC_CONNECTED;

– stops any data transmission or reception via the source link and releases the source link, but maintains the source RRC configuration;

– if handover failure is declared at the target cell after source cell RLF was declared,

– selects a suitable cell and initiates RRC re-establishment;

– enters RRC_IDLE if a suitable cell was not found within a certain time after handover failure was declared.

In case of CHO, after RLF is declared in the source cell, the UE:

– stays in RRC_CONNECTED;

– selects a suitable cell and if the selected cell is a CHO candidate and if network configured the UE to try CHO at the selected CHO candidate cell after RLF, then the UE attempts CHO execution, otherwise re-establishment is performed;

– enters RRC_IDLE if a suitable cell was not found within a certain time after RLF was declared.