Handover technology is an important part of WCDMA system, and soft handover is the most widely used method of this system. While increasing the system capacity, it reduces the call drop rate during the handover process and eliminates the "ping-pong effect" and "corner effect". Can effectively guarantee the stability, reliability and continuity of the user's conversation. Correctly understanding and understanding the switching technology plays a very important role in the optimization of daily networks.

1 Definition of handover
Handover refers to the process of a mobile station transferring from one base station or channel to another during a call. By switching, you can effectively ensure the continuity of communication and call quality.

2 Switch category
In the WCDMA system, according to the handover method, the handover can be divided into the following three types: hard handover, soft handover, and softer handover.
Hard handover refers to channel switching between cells covered by different base stations. Mobile users must disconnect from the original base station before establishing a connection with the new base station.
Soft handover means that when a mobile user moves from one cell to another, it can simultaneously establish connections with two or more cells to communicate. During the handover process, the terminal need not disconnect the connection with the original base station before establishing a connection with the new base station, and there is no phenomenon of communication interruption. This handover occurs when the base station with the same carrier frequency switches channels between the cells covered by the base station.
For softer handover, it refers to a handover that occurs between different sectors of the same cell.

3 Switching process
The handover process is generally divided into the following steps: measurement control, measurement report, handover decision, and handover execution; first, the terminal obtains the parameters that need to be measured by receiving measurement control information sent by the network; The network; then, the network makes a handover judgment; in the execution phase, the terminal responds according to the signaling process.

4 Features of soft handover
4.1 Uplink and Downlink
In the uplink direction, each UE utilizes its unique physical channel, such as a channel (or spread spectrum) code combined with a unique scrambling code sequence. As long as the BS knows the relevant connection parameters, such as the code used by the UE, the signal can be received by multiple BSs. After entering SHO, the information and other parameters specified by these users are sent to the new BS.
In soft handover, signals are received and processed by different BSs. The detected bit sequence is transmitted to the current SRNC, and then the SRNC receives data packets from all the relevant BSs in a selective combination.
Since the signal can be received and synthesized by multiple BSs, the transmit power of the UE can be reduced, thereby reducing interference.
Similarly, in the downlink, each active set channel represents an independent link from the BS to the UE. The data from UTRAN is copied by SRNC and finally transferred to all relevant BSs. Because the BS uses different scrambling codes, the UE can recognize different signals according to different scrambling codes.
4.2 Power control
When multiple SHO links are active on the downlink, power control must be performed on each signal radio link. There is only one TPC command transmitted by the UE to UTRAN, but the command is received by multiple different BSs. however. During the transmission of TPC commands over the air, errors may occur due to some reasons. Therefore, some downlink radio links may increase power, while others may reduce power. This creates what is known as power drift. When the SRNC detects that there is power drift, it initiates the downlink power balancing process through Iub interface signaling.
These TPC commands are set to TPCest = 1 (increase power) or TPCest = 0 (decrease power). Therefore, the current downlink power P (k-1) changes as the updated downlink power changes as the new transmit power P (k).
PTPC (k) represents the kth power adjustment in the inner loop power control process, and Pbal (k) represents a correction value obtained according to the downlink power control process in order to balance the power of the wireless link to a common reference power. Pref represents the downlink reference power, Pp-CPICH represents the transmission power of the primary common pilot channel (CPICH), and r is the adjustment coefficient, which is between 0 and 1. Pinit is the code domain power of the last time slot in the last adjustment period. By adopting this algorithm, the problem of power drift due to the detection error of the downlink TPC command can be solved.
There is a power control loop between each BS in the active set and the UE. The BS relies on different downlink TPC commands to control the corresponding UE. As long as the BS issues an instruction to reduce or increase power to the UE, the UE will take corresponding measures to execute. But for some unreliable power control commands, the UE will block it.
4.3 Soft handoff algorithm
Several commonly used terms in the soft handover algorithm:
● Active set: refers to the set of cells that establish a connection with a mobile station. The execution result of soft handover is represented by the increase and decrease of cells in the active set.
● Monitoring set: The cell of the monitoring set is the neighboring cell of the active set cell, and the RNC sends it to the UE through measurement control. The pilot Ec / Io of these cells is not strong enough to join the active set.
● Detection set: Cells that are not in the active set and monitoring set, but the UE can detect its presence.
In addition to the active set, there are a monitoring set and a detection set in the UE. For the monitoring set, it can control up to 32 internal frequency cells, including the cells in the active set. These cells regularly check for "trigger conditions". If the trigger condition is met, the UE will create a report and send it to UTRAN, which means that whether the cell joins the active set can be determined in the UE. This switching method is called mobile assisted switching. It should be noted that this report is only evaluated by UTRAN and does not consider other measurement operations. It contains measurement identification and target cell information. Since the measurement identification is set in the measurement control information, it can be used to identify the situation that can trigger the measurement report, and then UTRAN selects a suitable operation. Possible operations are: adding wireless links, removing wireless links, and replacing wireless links that include adding and removing links.
Each operation has its fixed trigger conditions. The trigger condition includes several parameters, such as: lag time parameter and trigger time parameter. These parameters are sent to the UE before connection and can be updated during the connection process, such as updates made to suit the propagation environment. The measurement is a cyclic event and is handled by the physical layer of the UE. The radio resource management (RRC) of the UE receives these measurements and decides whether to add or remove cells from the active set.
Figure 1 shows the principle diagram of WCDMA soft handover algorithm based on Ec / Io.

(1) Event 1A: In the figure, the pilot signal strength of cell 2 gradually increases. When the strength Ec / Io reaches the sum of the active set increase threshold T_ADD and the strongest pilot cell 1 pilot strength, and maintains a period Time T. At this time, if the active set is not full, cell 2 is added to the active set.
(2) Event 1C: Observe the signal strength of cell 3 and cell 1. The signal strength of cell 3 gradually increases, the signal strength of cell 1 gradually decreases, and the signal of cell 3 begins to exceed the signal strength of cell 1. If the pilot signal strength of cell 3 is the best pilot signal in the monitoring set, then its value reaches the sum of the weakest cell 1 (the weakest cell in the active set) pilot Ec / Io and the replacement lag threshold T_Replace And maintain T for a period of time. In the state where the number of active sets is full, cell 3 replaces cell 1 and is added to the active set.
(3) Event 1B: In the next case, the signal strength of cell 3 gradually weakens. When the strength of the pilot signal of cell 3 is so weak that the sum of the active set deletion threshold T_Delete is the strongest pilot strength of the 2 cell in the active set, and it is maintained for a period of time T. As the weakest signal in the active set, cell 3 will move out of the active set.

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