1 definition of switching
Handover refers to the process in which a mobile station transfers from one base station or channel to another during a call. By switching, communication continuity and call quality can be effectively guaranteed.
2 Switch classification
In the WCDMA system, according to the handover mode, the handover can be mainly divided into the following three types: hard handover, soft handover, and soft handover.
Hard handover refers to channel switching between different base station coverage cells. The mobile subscriber 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 establish a connection with two or more cells at the same time to communicate. During the handover process, the terminal does not have to disconnect the original base station before establishing a connection with the new base station, and there is no communication interruption. This switching occurs in channel switching between base station coverage cells of the same carrier frequency.
For softer handover, refers to handovers that occur 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, handover execution; first, the terminal obtains the parameter that needs to be measured by receiving the measurement control information sent by the network; then, the terminal sends the measurement report information to The network; then, the network makes a determination of the handover; in the execution phase, the terminal responds according to the signaling procedure.
4 soft switching features
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. The signal can be received by multiple BSs as long as the BS knows the relevant connection parameters, such as the code used by the UE. After entering the SHO, these user-specified information and other parameters are sent to the new BS.
In soft handoff, signals are received and processed by different BSs. The detected bit sequence is transmitted to the current SRNC, and then the SRNC receives the data packet from a selected combination of all relevant BSs.
Since the signal can be received and synthesized by multiple BSs, the UE's transmit power can be reduced, thereby reducing interference.
Similarly, each active set channel in the downlink represents an independent slave-to-UE link. The data from the UTRAN is copied by the SRNC and eventually transferred to all relevant BSs. Since the BS uses different scrambling codes, the UE can recognize different signals according to different scrambling codes.
4.2 Power Control
In the case where 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 the UTRAN, which is received by a plurality of different BSs. however. During the transmission of TPC commands over the air, errors may occur for some reason. Therefore, some downlink wireless links may increase power, while other downlinks may reduce power. This produces a so-called power drift. When the SRNC detects a power drift, the downlink power balancing process is initiated through the Iub interface signaling.
These TPC commands are set to TPCest = 1 (increased power) or TPCest = 0 (power reduced). Therefore, the current downlink power P(k-1) changes as the new transmission power P(k) as the updated downlink power changes.
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 the power to the UE, the UE takes corresponding measures to perform. But for some unreliable power control commands, the UE will block it.
4.3 Soft Handoff Algorithm
Several common terms in soft handoff algorithms:
â— Activation Set: A collection of cells that establish a connection with a mobile station. The result of the soft handoff is represented by the increase and decrease of the cells in the active set.
â— Monitoring set: The cell of the monitoring set is the neighboring cell of the active set cell, and is sent to the UE by the RNC through the measurement control. The pilot Ec/Io of these cells is not yet strong enough to be added to the active set.
• Detection Set: The cell that is not in the active set and the monitoring set, but the UE is able to detect its presence.
In addition to the active set, there are monitoring sets and detection sets in the UE. For a monitoring set, it can control up to 32 internal frequency cells, including cells within the active set. These cells periodically check the "trigger conditions". If the trigger condition is met, the UE will create a report and then send it to the UTRAN, which means that whether or not 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, which include measurement identification and target cell information. Since the measurement identification is set in the measurement control information, it can be used to identify which condition can trigger the measurement report, and then the UTRAN selects an appropriate operation. Possible operations include adding wireless links, removing wireless links, and replacing wireless links that include adding and removing links.
Each operation has its own fixed trigger condition. The trigger condition contains several parameters, such as hysteresis time parameters and trigger time parameters. These parameters are sent to the UE before the connection and can be updated during the connection, such as updates made to suit the propagation environment. The measurement is a cyclic event that 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 schematic of the WCDMA soft handoff algorithm based on Ec/Io.
(1) Event 1A: In the figure, the pilot signal strength of cell 2 is gradually increased, and when the strength Ec/Io reaches the sum of the activation set increase threshold T_ADD is the strongest 1-cell pilot strength of the active set, and maintains a segment Time T. At this time, in the case where the active set is not full, the cell 2 is added to the active set.
(2) Event 1C: Observe the signal strength of the cell 3 and the cell 1. The signal strength of cell 3 is gradually increased, the signal strength of cell 1 is gradually weakened, 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 pilot Ec/Io and the replacement hysteresis threshold T_Replace of the weakest 1 cell (the weakest cell in the active set). And maintain a period of time T. In the state where the number of active sets is full, cell 3 is replaced by cell 1 and is added to the active set.
(3) Event 1B: In the next case, the signal strength of the cell 3 is gradually weakened. When the pilot signal strength of the cell 3 is weak enough that the sum of the active set erasure threshold T_Delete is the strongest 2-cell pilot strength of the active set, and 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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