Types Of Handover

Types of Handover(s) Before we start discussing the handovers in detail we would like to list all of them for convenience of the reader 1. Softer Handover 2. Soft Handover 3. Intra-frequency hard handover 4. Inter-frequency hard handover 5. SRNS Relocation 6. Combined Hard handover and SRNS Relocation 7. Inter-RAT hard handover Softer Handover
Fig 1: Softer HandoverStrictly speaking softer handover is not really a handover. In this case the UE combines more than one radio link to improve the reception quality. On the other hand the Node B combines the data from more than one cell to obtain good quality data from the UE. [1] Specifies the maximum number of Radio Links that a UE can simultaneously support as 8. In practice this would be limited to 4 as it is verydifficult to make the receiver with 8 fingers. Generally speaking when RRC connection is established, it would always be established on one cell. The network initiates Intra-Frequency measurements to check if there are any other cells the UE can connect simultaneously to improve the quality of the data being transferred between the RNC and the UE. If a suitable cell is found then Active Set Updateprocedure is initiated. Using this Active Set Update message, the network adds or deletes more than one radio link to the UE. The only requirement is that from the start till the end of this Active Set Update procedure, one Radio Link should remain common. Soft Handover
Fig 2: Soft Handover Soft Handover is the same as softer handover but in this case the cells belong to more than one node B. Inthis case the combining is done in the RNC. It is possible to simultaneously have soft and softer handovers.
Fig 3: Soft Handover with Iur connection A more complicated soft handover would include a cell that belongs to a Node B in different RNC. In this case an Iur connection is established with the drift RNC (RNC 2) and the data would be transferred to the Serving RNC (RNC 1) via Iurconnection. In a typical UMTS system, the UE is in soft/softer handover around 50% of the time. One of the very important requirements for the soft/softer handover is that the frames from different cells should be within 50ms of each other or this would not work. The last thing one needs to remember is that the soft/softer handover is initiated from the RNC and the core network is not involved in thisprocedure. Hard Handover Hard handover occurs when the radio links for UE change and there are no radio links that are common before the procedure is initiated and after the procedure is completed. There are two types of hard handover. First is Intra-frequency hard handover and the second is Inter-frequency hard handover. Intra-frequency hard handover will not occur for the FDD system. It would happenin TDD system. In this case the code spreading/scrambling code for UE will change but the frequency remains the same. Inter-frequency hard handover generally occurs when hierarchical cells are present. In this case the frequency at which the UE is working changes. This happens when the current cell is congested, etc. Have a look at the Inter-Frequency Measurement primer for more information. Hardhandover procedure can be initiated by the network or by the UE. Generally it would be initiated by the network using one of the Radio Bearer Control messages. In case of UE initiated, it would happen if the UE performs a Cell Update procedure and that Cell Update reaches the RNC on a different frequency. The Core Network is not involved in this procedure. SRNS Relocation SRNS Relocation procedureis not strictly speaking a handover procedure but it can be used in combination with the handover procedure. A simple SRNS Relocation can be explained with the help of figures present in [9].
Fig 4: Data flow before SRNS Relocation procedure ([9], Fig 37)

Fig 5: Data flow after SRNS Relocation procedure ([9], Fig 38)The UE is active on a cell that belongs to a different RNC (than the one...