How does the eNodeB handle handovers in the LTE network?

Intra-LTE handovers can be further classified in to two category of S1 and X2 based on the signalling route it takes to complete the handover procedure. A X2 link is a network interface between eNodeBs and it is always the preferred choice of path for handover, on the other hand S1 link is a network interface between eNodeB and MME so it adds an extra latency to the signalling hence always becomes the second choice. One more advantage of X2 handover is possibility of data forwarding between cells during handover is in progress, this results into seamless experience during handover with zero packet loss.

eNB evaluates the measurement reports &triggers an event if the conditions for HO are met.Event A3 or A5, HO request is sent to the target node,tgt node receives the ho request and respond back with the ack .eNBs coordinate the allocation of resources in the target cell.The tgt eNB configures the necessary parameters for the UE in its cell.The serving eNB sends a HO Command msg to the UE, instructing it to prepare for HO.HO Prep message is sent to the tgt eNB.The serving eNB sends a HR msg to the target eNB, indicating that the HO process is about to start.UE switches its connection to the target eNB.data forwarding path is activated,target eNB sends a HO Command Ack MSG to the serving eNodeB, confirming that the HO successfully executed.

The handovers are mostly triggered based on the measurement events reported by UE. There may be very limited scenarios where handovers are triggered without any measurement event, for example unexpected reestablishment triggered on neighbour cell and in this case neighbour cell can directly request for handover of UE from the source cell. The measurement events are configured by eNodeBs during initial attach process and can be reconfigured based on conditional changes, for example UE might be moving towards inter frequency LTE neighbour or it might be moving towards inter RAT neighbour in both cases measurement events has to be updated with correct report configuration.

Here are another additional challenges related to handovers in LTE networks: Handover decision inaccuracies,Interference and signal degradation,Load balancing issues in this case uneven distribution of traffic across cells can lead to overloaded cells and congestion, making it challenging for handovers to occur smoothly and efficiently , and Handover complexity in heterogeneous networks when LTE networks often consist of a mix of different cell types, such as macrocells, small cells, and Wi-Fi hotspots. Coordinating handovers between these heterogeneous cells can introduce additional complexity and potential challenges, such as compatibility issues and coordination overhead.

The major and foremost challenge of handover is the successful handoff of UE to the target cell which means UE is successfully attached to the target cell or not. Once the UE receives the reconfiguration message with mobility information from source cell, it immediately triggers the RACH procedure on target cell and if the radio conditions are poor there are high chances of RACH failure on the target cell, which leads to radio link failure of UE and signalling and data loss. These RLFs are generally categorized as too early or too late handover, and as name suggest it means the handover was triggered either too early before radio conditions of target cell are getting better or too late after radio conditions of target cell is too strong.

Best handover solution is the network planning and the understanding of the cell coverage. Measurement events can be optimized according to the cell coverage, on top of that cell individual offset can be optimized based on too early or too late handover events. In LTE, Self Organizing Networks aka SON plays a key role in optimizing the handover solution. It collects the performance indicators along with event data to optimize and suggest the correct set of report configuration for the cells.

The handovers are very important feature to make sure seamless mobility with great user experience. It can also be used as load balancing tool for the event of overload on the source cell. During initial days of LTE when IMS was not widely available, the voice calls were always redirected to 3G or 2G network and Inter RAT handovers had been preferred choice of these redirections.