Handover topic is one of the most important topics in mobile networks in general.
The meaning of handover is the transfer of communication, whether voice or data between two mobile stations, so that if there is a voice call or data transfer session and the mobile moves between different cells, the call or data transfer is not separated because the first wireless stations serving eNB transfers the service to the second target eNB wireless station.
A large percentage of call drop problems occur due to handover problems between stations. In LTE networks the type of handover used is hard handover, but in 3G soft/softer handover is used.
Here is a question, why does handover happen in the first place? There is more than one reason:
The first is: coverage based and this occurs while the mobile moves away from serving cell coverage towards the target cell in which case the value of the receiving power decreases from the serving cell and the receiving power increases from the target cell.
The second is: Priority based, and it occurs in the event of a desire to make a handover on a particular station, even if there is no problem with coverage and it occurs in the event that there is more than one frequency band on the stations.
The third is: load-based, and this is in case there is a load on a particular station, then a handover is done on another nearby station with less load.
The fourth is: service-based, where priority is given to a specific service, such as voice, to make a handover over a specific station type, such as 3G. Handover types can be classified into more than one type in LTE networks.
The first classification method from the point of view of the stations, it can be intra eNB in the event of a handover between two sectors in the same station,
Or inter eNB if it occurs between two stations and in this case it can be X2 Based handover if there is a x2 link between the stations
Or via an MME called S1 based handover because it is on the s1 link between the eNB and the MME.
In the latter type, there is more than one sub-type depending on whether the two stations are served by the same MME or SGW or not.
Handover can be classified from the point of view of station frequencies. There is an Intra-frequency Handover in case the serving cell and target cell operate at the same frequency.
Or inter-frequency handover in case the frequency is different, and in both cases the two stations are from the fourth generation LTE stations.
In the absence of LTE network coverage, the handover is called Inter-RAT handover, meaning handover between different types of mobile generations.
The Events of LTE Handover:
Inter-frequency handover and inter-RAT handover can be coverage-based, load-based or service-based handover.
A3/A4/A5 event can be used to trigger coverage-based inter-frequency handover. eNodeB can configure which event is used.
B1/B2 event can be used to trigger coverage-based inter-RAT handover. eNodeB can configure which event is used
A1 leaving condition：Ms+ Hys < Thresh
LTE Handover Fault Diagnosis
A2 leaving condition：Ms– Hys > Thresh
A3 leaving condition：Mn+ Ofn+ Ocn+ Hys < Ms + Ofs+ Ocs+ Off
A4 leaving condition：Mn+ Ofn+ Ocn+ Hys < Thresh
A5 leaving condition：Ms- Hys > Thresh1 or Mn+ Ofn+ Ocn+ Hys < Thresh2
B1 leaving condition：Mn+ Ofn+ Hys < Thresh
B2 leaving condition：Ms- Hys > Thresh1 or Mn+ Ofn+ Ocn+ Hys < Thresh2
The variables in the formula are defined as follows:
Ms is the measurement result of the serving cell, not taking into account any offsets.
Mn is the measurement result of the neighbouring cell, not taking into account any offsets.
Ofn is the frequency specific offset of the frequency of the neighbour cell.
Ocn is the cell specific offset of the neighbour, and set to zero if not configured for the neighbour cell.
Ofs is the frequency specific offset of the serving frequency.
Ocs is the cell specific offset of the serving, and is set to zero if not configured for the serving cell.
Hys is the hysteresis parameter for the events.
Thresh1 and Thresh2 is the threshold parameter for the events
Analysis of Intra-frequency Handovers Parameters:
Step 1: UE measures source cell and target cells. If the trigger condition is fulfilled, UE LTE Handover Fault Diagnosis reports measurement results.
Step 2 and step 3: After receiving measurement result, eNodeB makes decision to trigger
handover.
Source eNodeB sends Handover Request message to target eNodeB, including target cell ID and some parameters for handover preparation. After receiving handover request, target eNodeB tries access control and prepares resource for handover, then sends Handover Request Acknowledge to source eNodeB.
Step 4: Source eNodeB sends RRC Connection Reconfiguration including handover
Parameters for handover.
Step 5: Source eNodeB sends SN Status Transfer message to target eNodeB.
Step 6: UE triggers non-contention based random access in target cell.
Step 7: After successful random access in target cell, UE sends RRC Connection Reconfiguration Complete message to target eNodeB.
Step 8 and step 9: Target eNodeB sends Path Switch message to MME. MME informs S-GW to switch S1 connection to target eNodeB. After successful switch, MME sends Path Switch Acknowledge message to eNodeB.
Step 10: Target eNodeB sends UE Context Release message to source eNodeB to release the resource of the UE
Thank you.
