Detailed analysis of LTE system switching technology (1)
I. Switching Technology of TD-LTE System
1. Overview of TD-LTE Switching
As a TD-SCDMA System of TD-LTE evolution technology, the fast hardware switching method can be used to realize the switching between different frequency bands and between systems, so as to achieve better regional coverage and seamless switching, it is also compatible with the existing 3GPP and non-3GPP. Soft switchover is not used because of the high complexity of the device and the high timing difficulty, which will bring high processing capability requirements. The design of the core network has also changed accordingly, adding the System Architecture Evolution (SAE) and 3GPP modules, and implementing compatibility between the LTE System and the 3GPP and non-3GPP system switching.
The switchover process is divided into four steps: Measurement, reporting, decision, and execution. The received power, bit rate, and link distance can be used as measurement standards for theoretical estimation and corresponding processing. TD-LTE system switching is UE-assisted hard switching, he and FDD-LTE hard switching the biggest difference is: In the TD-LTE, the pilot signal is transmitted in a special time slot, in the FDD-LTE system, the pilot channel occupies a whole frame length, so the measurement standards based on the pilot channel is not so accurate for the TD-LTE. Therefore, for the measurement of TD-LTE, we also need to combine the channel quality, UE position and the intensity of the pilot signal.
2. Switch Type
In the connection mode, switching within the E-UTRAN is the switching of the terminal-assisted network control. The switchover is divided into three parts: preparation, execution, and completion. ENB includes the following types of switches:
A. Switching Based on Wireless Quality
The reason for this type of switching is that the UE's measurement report shows that there are neighboring communities with better channel quality than the current service community.
B. switchover based on wireless access technology coverage
This type of switching occurs when the UE loses the current wireless access technology (RAT) coverage to connect to other rats. For example, if an UE is far away from a city area and thus the TD-LTE coverage is lost, the network will switch to the quality RAT detected by UE, such as the general mobile communication system (UMTS) or Global Mobile Communication System (GSM ).
C. Load-Based Switching
This type of switchover is used to balance the load conditions between different rats belonging to the same operator when a given cell is overloaded. For example, if a TD-LTE area is very crowded, some users need to move to the adjacent TD-LTE area or adjacent UMTS area.
3. Switching Process
A. Switch preparation: the source eNB configures the UE measurement report based on roaming restrictions, and the UE sends a report based on the predefined measurement rules. The source eNB determines whether the UE needs to be switched based on the report and RRM information. When switching is required, the source eNB sends a switching request to the target eNB. The target eNB executes the acceptance Control Based on the received QoS information and returns the ACK.
B. switch execution: The Source eNB sends a switch command to the UE. After receiving the switch, the UE switches and synchronizes the switch to the target eNB. The network responds to the synchronization. After the UE successfully accesses the target eNB, send a switch confirmation message to the target eNB.
C. Switching completed: MME sends a user-side update request to the S-GW, and the user-side switches the downstream path to the target side; the target eNB notifies the source eNB to release the original resources. The switchover is complete.
4. Switch decision criteria
Switching measurement plays an important role in switching algorithms. UE measurement reports play a key role in eNB switching decision-making. The switching measurement and decision criteria defined in the LTE standard are as follows:
A. reference signal receiving power (RSRP): refers to the electromagnetic interference (RE) that carries the exclusive reference signal of the residential area in the measurement frequency band to be considered) the linear mean of power contribution (in W.
B. Switching Delay Difference (HOM): The RSRP difference between the current service zone and the adjacent zone. The value can be set based on the communication environment. The value determines the switching delay.
C. Trigger duration (TTT): During this period, the decision can be switched only when the HOM condition is continuously met. TTT can effectively prevent the occurrence of the "ping-pong effect" during the switchover.
Next we will introduceSwitching Algorithm:
UE monitors the RSRP of all tested cells after passing through the filter and sends the measurement report to the eNB of the service area. When the current condition is continuously met within the given TTT, eNB will switch the UE. UE sets the TTT parameter based on its speed. RSRPT is the reference signal receiving power of the target community, while RSRPS is the reference signal receiving power of the service community.
RSRPT> RSRPS + HOM
Describes an important example of this switching algorithm:
After receiving the measurement report, The eNB of the current service starts to use the internal program of the network to switch the UE to the new target cell. Assume that the target community has enough resources for the UE to be switched over. The preparation time is modeled as a constant protocol delay, expressed as P in the figure. After the preparation is complete, the Service Community sends a switch Command Message to the UE downstream.