RLF stands for “Radio Link Failure” in LTE, which occurs when the radio link between the user equipment (UE) and the eNodeB (base station) is lost. This can happen due to various reasons such as interference, coverage issues, handover failures, or equipment malfunction.
When an RLF occurs, the UE sends an RLF indication message to the eNodeB, which triggers a series of actions to recover the lost connection. The eNodeB can try to re-establish the connection by retransmitting the lost packets, adjusting the transmission parameters, or performing a handover to a neighboring cell with better coverage.
If these recovery attempts fail, the UE is instructed to perform a cell reselection to find a better cell to connect to. This is done by measuring the signal strength of neighboring cells and selecting the one with the strongest signal.
RLF are implemented at both UE and eNB side
- UE Detected Radio Link Failures (RLF)
- eNodeB Detected Radio Link Failures (RLF)
We will discuss mostly UE detected Radio Link Failure because 3GPP does not specify eNB radio link failures
UE Detected Radio Link Failures (RLF)
- Physical Layer problem detected (T310 Expiry)
- RLF due to Maximum UL RLC Retransmissions Exceeded
- RLF due to failure in Handover
- RLF due to RACH problems and issues.
Physical Layer problem detected (T310 Expiry)
When Physical Layer problem detected, UE will get to know from N310 consecutive “out-of-sync” from the Physical Layer to start the timer T310.
Once T310 expires Radio Link Failure is considered and triggered by UE.
If N311 consecutive “in-sync” indication comes from physical layer then UE can resume the RRC connection as before. And will not go to RRC re-establishment process.
Note: N311: This constant defines the maximum number of retransmissions allowed for a RRC Connection Release message. The value of this constant is typically set to 1.
After the RLF occurred, RRC re-establishment process is attempted.
Once RRC re-establishment process completed and UE not able to gain the normal ongoing RRC connection, then UE will go back to Idle and freshly start cell reselection and tracking area update (TAU).
RLF due to Maximum UL RLC Retransmissions Exceeded
First of all, need to know what is UL RLC retransmission, UL RLC Retransmissions refer to the retransmission of uplink (UL) RLC protocol data units (PDUs) from the UE to the eNodeB. When the eNodeB receives a PDU from the UE, it sends an acknowledgment (ACK) or negative acknowledgment (NACK) to indicate whether the PDU was successfully received or not. If the eNodeB sends a NACK, the UE needs to retransmit the PDU.
UL RLC Retransmissions can occur due to various reasons such as channel interference, transmission errors, congestion in the network, or UE movement. The number of UL RLC Retransmissions can impact the overall system performance, as it may result in increased latency and reduced throughput.
In this case RLF will get triggered by UE when if maximum number of UL retransmission is reached.
Once RLF triggered the it will go for RRC connection re-establishment.
Generally, in RRC connection reconfiguration message, the value for maximum retransmission used to me set i.e it can go max up to 16 transmission.
RLF due to failure in Handover
During a handover, the RRC (Radio Resource Control) protocol manages the transfer of radio resources from the source eNodeB to the target eNodeB. If the transfer is unsuccessful, the UE may experience a Radio Link Failure (RLF) and lose its connection to the network.
In the case of an RLF due to handover, the UE may not receive the necessary handover parameters from the target eNodeB or may not be able to establish a connection with the target eNodeB, resulting in a loss of radio link. This can occur due to various reasons such as weak signal strength, interference, or network congestion.
When an RLF occurs due to handover, the UE will trigger a re-establishment procedure to re-establish a connection with the network. The re-establishment procedure involves sending a request to the network to allocate a new radio resource and establish a new connection.
To prevent RLF due to handover, network operators can take measures such as optimizing the handover parameters, improving network coverage, and deploying advanced techniques such as interference management and load balancing.
In this scenario of RLF UE will get the handover command message in RRC connection re-configuration and once UE gets this command, it trigger the T304 Timer.
While T304 is running UE must successes in PRACH procedure. If nor so, Handover failure occurs.
After the RLF declaration UE will try for RRC connection re-establishment.
RLF due to RACH problems and issues.
If the UE experiences difficulty accessing the RACH or if the RACH is congested, it may lead to RLF. This can occur due to various reasons such as interference, insufficient RACH resources, or collisions with other UEs.
When the UE experiences an RLF due to RACH problems and issues, it may try to re-establish the connection with the network by attempting to access the RACH again. If the RACH is still congested or the UE continues to experience difficulty accessing the RACH, it may trigger a re-establishment procedure to establish a new connection with the network.
To know more on RACH you can refer