KR20100048497A - Device and method for controlling hand-over in wireless communication system - Google Patents

Abstract

PURPOSE: A device and a method for controlling handover in a wireless communication system are provided to optimize handover efficiently by exchanging the information of neighboring cells. CONSTITUTION: A terminal is handed over to a target cell in a common service zone of both the target cell and a service cell. If RLF(Radio Link Failure) occurs in the common service zone after the handover operation, the terminal and the target cell detect the RLF. The serving cell is reconnected to the terminal by allowing the target cell or the terminal to detect and report the RLF to the serving cell.

Description

Handover method of mobile wireless communication system {DEVICE AND METHOD FOR CONTROLLING HAND-OVER IN WIRELESS COMMUNICATION SYSTEM}

The present invention relates to a handover method of a mobile wireless communication system, and more particularly, to a method for optimizing handover in case of link failure.

In general, many researches on hand-over optimization have been conducted for 2G / 3G wireless communication systems. Conventional handover methods include a handover using a local average technique, a time-based handover, and a handover based on the speed and location of a terminal. ), A handover according to a speed of a terminal, a handover using a fuzzy logic, or a method of multi-criteria inputs.

Look at the handover method as described above.

The first handover method (Local average technique) is used to eliminate the effect of fast fading without affecting the overall statistics of the received signal strength. Averaging is done by placing a timer with a shorter interval than the timer used in traditional handover algorithms. That is, when Ts is an existing timer, the timer Ts' used in the local-averaging technique is defined as in Equation 1 below.

Ts = M * Ts'

Here, the M value of Equation 1 is described in the following second handover technique.

The second handover method (Timer-based handover) is a drop timer defined by M, in order to solve the problem of local averaging due to the timer (short timer) of the first handover method (drop timer) Suggest. That is, in order to determine the handover to the target eNB to be handed over, the associated signal strength must be continuously smaller (or larger) than the setting (hystersis + threshold) value for M consecutive time slots. ) Triggers an event.

The third location-based handover method is an algorithm for improving a problem of conventional hand-over decision by using the speed and location of the UE. In this algorithm, two decision criteria are applied, the first of which is similar to the existing hand-over decision based on hysteresis. If this condition is satisfied, the proposed scheme calculates the window using the speed and position of the UE and checks the second conditional expression based on the calculated window.

The fourth speed-sensitive handover method adaptively adjusts a window (temporal window (time-to-trigger)) according to the speed of the UE. Although the number of sampling may be constant, the sampling period may be adjusted, or the sampling period may be constant, but the number of sampling may be adjusted. To this end, various speed measurement methods such as Level Crossing Rates (LCR), Zero Crossing Rates (ZCR), and Covariance Approximation Method (COV) are proposed.

The fifth handover method (Fuzzy logic based algorithm using multi-criteria inputs) determines the handover by using fuzzy logic that inputs various handover related metrics. Criteria used as inputs include multi-criteria inputs; received signal strength, ratio of the used capacity to the total capacity, relative direction of UEs to the serving eNB, speed of UEs, and the like. Fuzzy logic is performed by inputting various reference signals (multi-criteria measurement) as described above. A membership value is obtained as an output of the fuzzy logic, and when the membership value is smaller than a set threshold, handover is performed.

The above handover related technology performs optimization in consideration of characteristics (location, speed, etc.) of the UE connected to the current serving cell and information of the serving cell itself (RRM information, etc.). do. In this case, there is a limitation in handover optimization because information is not used in an adjacent cell (particularly, a handover target cell).

1 illustrates an example in which a UE is located in a coverage of serving cell and neighbor cell area and is handed over from a serving cell to a target cell.

At this time, immediately after the UE handovers from the serving cell to the target cell (ie, after establishing RRC_CONNECTION with the target cell), a link connection fails within a short time (radio link failure; hereinafter referred to as RLF). Is generated, it is recognized as a handover (mobility) related RLF. The RLF may be generated when there is a coverage hole that cannot be served by the target cell in a common area between the serving cell and the target cell. For example, if there is an obstacle (for example, a tall building) between the UE and the target cell in the common area, the UE can receive the signal of the serving cell before the handover, but the signal of the handed target cell cannot be received. It can be impossible. In this case, the handed-over UE generates an RLF that is disconnected from the target cell.

In this case, since the RLF occurs after the UE completes the handover from the previous serving cell to the target cell during the handover, the UE does not know that the RLF has occurred even if the UE reconnects to the previous serving cell after the RLF. Is the point. That is, the detection of the RLF occurrence by the previous serving base station (source cell) as described above is not supported by the current 3gpp LTE spec. Accordingly, in the case of the handover (handover according to mobility-related RLF), only the information of the previous serving cell alone may not effectively detect and optimize the handover.

An embodiment of the present invention can solve an inter-cell problem, such as a hand-over to non-suitable cell, by using a framework of a self organizing network (SON). I suggest a way.

To this end, in the embodiment of the present invention, if the RLF occurs after performing the handover, the RLF report (RLF report) so that each device of the mobile wireless communication system can share the RLF situation. In this case, the RLF report may use a method of reporting to a previous serving cell of a target cell UE or server OAM / SON server. By using the RLF report, a previous serving cell (hereinafter referred to as a source eNB) may detect that a handover to a target cell (hereinafter referred to as a target eNB) is not legal, and thus generates a mobility-related RLF (RLF). The number of times can be reduced.

Therefore, in a mobile wireless communication system, if there is a problem between cells during handover (inter-cell hand-over problem), the handover is effectively optimized by detecting information of adjacent cells (particularly, target cells). )can do.

A handover processing method of a mobile wireless communication system according to an embodiment of the present invention comprises the steps of a UE handover to a target cell in a common service area of a serving cell and a target cell, and when the RLF occurs in the common service area after the handover. And detecting the RLF by the terminal and the target cell, and detecting the RLF by the target cell or the terminal and reporting the RLF to the serving cell to reconnect the serving cell with the terminal. do.

The RLF report process may include a process in which the target cell generates an RLF report during a handover time and a process in which the target cell transmits an RLF report to the serving cell as an X2 message. Generating an RLF report during a handover time and transmitting the generated RLF report to an OAM / SON server, and generating, by the OAM / SON server, an RAM message as an OAM message and transmitting the generated RLF message to the serving cell; Attempting to reconnect to the serving cell may be performed, and the terminal may transmit a RLF report to the reconnected serving cell.

As described above, when the RLF is generated after performing the handover, the RLF is reported so that each device of the mobile wireless communication systems can share the RLF situation. In this case, the RLF report may use a method of reporting to a previous serving cell of a target cell UE or server OAM / SON server. By using the RLF report, the previous serving cell (hereinafter referred to as the source eNB) can detect that the handover to the target cell (hereinafter referred to as the target eNB) is not legitimate, such RLF statistics for a certain period of time (RLF statistics) By collecting, the number of occurrences of mobility-related RLF may be reduced by lowering hand-over priority for the target eNB.

Therefore, in a mobile wireless communication system, if there is a problem between cells during handover (inter-cell hand-over problem), the handover is effectively optimized by detecting information of adjacent cells (particularly, target cells). )can do.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that, in the drawings, the same components are denoted by the same reference symbols as possible. Further, the detailed description of well-known functions and constructions that may obscure the gist of the present invention will be omitted.

In the following description, the term "serving base station" refers to a base station to which a terminal has been linked before handover. In an embodiment of the present invention, the terms serving cell and source eNB are also described. It will be used in the same sense as the serving base station. The term " target base station " refers to a base station to which a terminal is linked after handover. In an embodiment of the present invention, terms of a target cell and a target eNB also mean the same as the serving base station. Will be used in the following terms. In the embodiment of the present invention, the terms "base station", "cell" and "eNB" will be used interchangeably, and all of these terms will be used interchangeably.

An embodiment of the present invention proposes a handover technique for minimizing radio link failure (RLF) in a handover situation of a terminal by improving a handover method between base stations in a mobile wireless communication system. That is, an embodiment of the present invention uses an inter-cell problem such as a hand-over to non-suitable cell using a framework of a self organizing network (SON). Suggest ways to solve it.

To this end, in the embodiment of the present invention, if the RLF occurs after performing the handover, the RLF report (RLF report) so that each device of the mobile wireless communication system can share the RLF situation. In this case, the RLF report may use a method of reporting to a previous serving cell of a target cell UE or server OAM / SON server. By using the RLF report, a previous serving cell (hereinafter referred to as a source eNB) may detect that a handover to a target cell (hereinafter referred to as a target eNB) is not legal, and thus generates a mobility-related RLF (RLF). The number of times can be reduced.

Therefore, when there is a problem between cells during handover in the mobile wireless communication system (inter-cell hand-over problem), the handover state is effectively detected by exchanging information of adjacent cells (especially target cells), Over can be optimized.

If the RLF is generated shortly after the handover is performed from the serving base station to the target base station in the mobile wireless communication system, the previous serving base station cannot know the RLF situation occurring at the target base station. A process in which such a situation occurs will be described with reference to FIGS. 2 and 3.

FIG. 2 is a diagram for describing an example in which an RLF is generated within a short time after a handover is performed in a mobile wireless communication system.

FIG. 2 illustrates an example in which there is a coverage hole of Cell_B when the UE is located in a common area of Cell_A (which may be a serving base station) and Cell_B (here, it may be a target base station). It is shown. For example, the UE is located in a common area (cell edge region, regions r1, r2, and r3 in FIG. 2) of Cell_A and Cell_B, and there is a high building between the common area and the Cell_B, and immediately adjacent shades of the building. Assume a case where a UE in a shade region is located. The UE is located in the r1 region and linked to Cell_A to move to the r2 region to perform a handover. In this state, it is assumed that the UE is moved to the r3 region. In this case, the UE may receive a Cell_A signal, but the Cell_B signal may be a kind of Cell_B shadow hole. As a result, the UE generates an RLF in which a link with Cell_B is disconnected. When the RLF is generated, the UE scans a signal of a neighbor cell to connect with the base station again. Since the signal of Cell_B cannot be received in the region where the UE is currently located (for example, the r3 region), the UE attempts to reconnect to Cell_A, and as a result, is connected to Cell_A (RRC_connected).

Here, since the RLF occurs immediately after the UE completes the handover from Cell_A to Cell_B, even if the UE reconnects after the RLF, Cell_A does not know that the RLF has occurred. When handing over from Cell_B to Cell_A by a normal handover procedure, handover of the UE in Cell_A using UE history information included in the RRC context of the UE A ping-pong hand-over problem can be detected. Therefore, when the handover occurs as described above, it is preferable that the UE, the target base station or the server controlling the handover inform the serving base station of the handover occurrence state.

3 is a flowchart illustrating a procedure of processing handover related messages and events to be solved in an embodiment of the present invention.

Referring to FIG. 3, a procedure of handing over a UE from a source eNB to a target eNB follows a general handover procedure. Looking at the handover procedure, first, the UE is connected to Cell_A, which is the source eNB, and the UE measures the signal of the source eNB and reports the result to the source eNB (eg, A3.entry.A.B event). When the source eNB determines the handover, the source eNB transmits a handover request signal to the target eNB. The target eNB then sends a hand-over accept message to the source eNB, and the source eNB commands the handover to the UE. The UE then disconnects from the source eNB and connects to the target eNB (detach from Cell_A and RRC_connectd to Cell_B), the source eNB delivers packets to the target eNB (deliver buffered and in transit packets to target eNB). Buffer packets transmitted from the source eNB (buffer packets from source eNB). In this state, the UE confirms a handover with the target eNB.

In this state, the target eNB requests a path switch request from the server, and the server switches a path and transmits an end marker to the source eNB. Wherein the endmarker then attaches the endmarker to the last packet sent to the target eNB. In this case, the target eNB transmits the buffered packets transmitted from the source eNB to the UE, and when the end marker is confirmed, the target eNB starts to transmit its received packets to the UE. Upon receiving the path switch request, the server responds to the path switch request, whereby the target eNB sends a UE release release message to the source eNB, whereby the source eNB sends UE related information. Release (UE-related context / resource release after recognition as hand-over success). The source eNB then forwards the buffering packet to the target eNB, which continues when end marker data is verified. Thereafter, the source eNB releases the UE related information, and the UE performs a communication function with the target eNB.

In the above state, if the UE moves to a coverage hole in which communication with the target base station is impossible (which may be an r3 region in FIG. 2), the UE and the target eNB detect an RLF generation. . In this case, in the embodiment of the present invention, the UE does not perform an operation of scanning with neighboring base stations and then reconnecting with the scanned base station (which may be a source eNB). That is, in the embodiment of the present invention, when the RLF situation occurs in step 311, the UE or target eNB that detects it in step 313 detects the RLF, and in step 315 the UE, target eNB or SON server sends the RLF to the source eNB. report. The source eNB then knows the RLF situation of the UE and can quickly reconnect with the UE. At this time, the source eNB (source cell) may collect the information on the RLF generation, and may perform a hand-over optimization algorithm based on a statistical value (RLF performance statistic).

As described above, the embodiment of the present invention proposes three RLF report methods as shown in Table 1 below to inform the source eNB of the RLF generation related information that triggers the related handover.

RLF detection RLF report path Target eNB  Target eNB-> source eNB via X2 interface UE  UE-> source eNB via RRC signaling Target eNB  Target eNB-> OAM / SON server-> source eNB via OAM / SON interface

After the source eNB collects information on the RLF generation by the method as shown in Table 1, the handover optimization algorithm is performed based on the RLF performance statistic. For example, if a statistic in which an RLF occurs immediately after a handover to a specific target eNB is higher than a certain threshold, the hand-over priority to the target target eNB is changed. You can lower or prohibit it.

The RLF report method according to an embodiment of the present invention will be described in detail. Here, the RLF report method may correspond to steps 311 to 315 of FIG. 3.

First, a first report method (RLF report from target eNB to source eNB via X2 interface) according to an embodiment of the present invention will be described. 4 shows a procedure for the RNB to detect the RLF and report it to the source eNB.

Referring to FIG. 4, when the UE completes the handover from the source eNB to the target eNB while performing the procedure of FIG. 3, the UE wirelessly links to the target eNB to perform communication. In this case, when the RLF is generated in the ecology in which the handover is performed as described above, the target eNB detects the RLF with the corresponding UE in step 411, and dwell time from the time of handover to the time of occurrence of the RLF. And detect the source eNB information (examine, detect). Thereafter, the target eNB transmits the RLF reload to the source eNB through the X2 interface message in step 413. In this case, the RLF report may include information such as a UE dwell time, a hand-over timestamp, the last measurement report, and the like.

Secondly, a second report method (RLF report from UE to source eNB) according to an embodiment of the present invention will be described. FIG. 5 illustrates a procedure where a UE detects an RLF and the UE reconnects to the source eNB to report the RLF report.

Referring to FIG. 5, when the UE completes the handover from the source eNB to the target eNB while performing the procedure as shown in FIG. 3, the UE wirelessly links to the target eNB to perform communication. In this case, when the RLF is generated in the ecology in which the handover is performed, the UE detects this in step 611, reconnects with the source eNB in step 513, and then generates an RLF report in step 515. To the source eNB.

That is, when the UE is RLF connected to the target eNB in the state where the handover is completed from the source eNB to the target eNB, the UE attempts to reconnect (RRC_re_establishment) with the target eNB connected at the time of the RLF. At this time, when the UE is located in the coverage hole of the target eNB, reconnection with the target eNB (RRC_re-establishment) fails. Then, the UE scans the cell and re-connects with another base station, and the UE notifies the newly connected eNB of the occurrence of the RLF. In this case, if the connected eNB is the source eNB that was the previous serving base station, a hand-over problem may be detected, and the detected RLF report is transmitted to the source eNB.

Third, a third report method (report from Target eNB to OAM / SON server and from OAM / SON interface to source eNB) according to an embodiment of the present invention will be described. FIG. 6 illustrates a procedure in which an RLF is detected by the target eNB and delivered to the OAM / SON server, and the OAM / SON server analyzes the report and reports it to the source eNB.

Referring to FIG. 6, when the UE completes the handover from the source eNB to the target eNB while performing the procedure of FIG. 3, the UE wirelessly links to the target eNB to perform communication. In this case, when the RLF is generated in the ecology in which the handover is performed as described above, the target eNB detects the RLF with the corresponding UE in step 611, and the time from the handover (over-in in) time to the RLF occurrence time (dwell time) And detect the source eNB information (examine, detect). Thereafter, the target eNB transmits an RLF report to the OAM / SON server in step 613, where the RLF report includes information such as UE dwell time, hand-over timestamp, and the last measurement report. Then, the OAM / SON server analyzes the RLF information in step 615, and the OAM / SON server transmits the RLF generation and related information to the source eNB through the OAM / SON interface in step 617.

As described above, in the embodiment of the present invention, when the RLF is generated after performing the handover, the RLF generation of the UE and the target cell is reported to the previous serving cell (RLF report). In this case, the RLF reporting method may include a method in which the target eNB detects an RLF and reports it to a source eNB, a method in which a UE detects an RLF and a UE reconnects to the source eNB to report the RLF report, or a target eNB reports an RLF. Detect and forward it to the OAM / SON server, and the OAM / SON server may use one of the methods of analyzing and analyzing the RLF report to the source eNB. Then, after the source eNB (source cell) collects information on the occurrence of the RLF, the handover optimization algorithm is performed based on the RLF performance result (RLF performance statistic). That is, when the statistic in which the RLF occurs immediately after a handover to a specific target eNB is higher than a predetermined threshold, the hand-over priority to the corresponding target eNB is lowered. Or forbidden.

The embodiments of the present invention disclosed in the present specification and drawings are merely illustrative of specific embodiments of the present invention and are not intended to limit the scope of the present invention in order to facilitate understanding of the present invention. It will be apparent to those skilled in the art that other modifications based on the technical idea of the present invention can be carried out in addition to the embodiments disclosed herein.

1 is a diagram illustrating an example in which a UE is located in a coverage of serving cell and neighbor cell region and is handed over from a serving cell to a target cell;

2 is a diagram illustrating an example in which an RLF is generated when a terminal is located in a shadow area of a target cell when the terminal is located in a common area of a serving cell and a target cell.

3 is a flowchart illustrating a procedure of processing handover related messages and events to be solved in an embodiment of the present invention.

4 is a diagram illustrating a procedure by which a target eNB detects an RLF and reports it to a source eNB;

FIG. 5 illustrates a procedure in which a UE detects an RLF and the UE reconnects to the source eNB to report the RLF report.

FIG. 6 is a diagram illustrating a procedure in which an RLF is detected by a target eNB and delivered to an OAM / SON server, and the OAM / SON server analyzes the report and reports it to a source eNB.

Claims (4)

In the handover processing method of a mobile wireless communication system, Performing a handover by the terminal to the target cell in a common service area of the serving cell and the target cell; Detecting, by the UE and the target cell, the RLF when the RLF occurs in the common service area after the handover; The RLF detection The handover processing method characterized in that the target cell or the terminal detects the RLF and the RLF report to the serving cell and the serving cell reconnects with the terminal. The method of claim 1, wherein the RLF report process, Generating, by the target cell, an RLF report during a handover time; And the target cell transmits an RLF report to the serving cell as an X2 message. The method of claim 1, wherein the RLF report process, Generating, by the target cell, an RLF report during a handover time and transmitting the generated RLF report to an OAM / SON server; And generating, by the OAM / SON server, the RLF message as an OAM message and transmitting the generated RLF message to the serving cell. The method of claim 1, wherein the RLF report process, Attempting to reconnect the terminal to the serving cell; And transmitting the RLF report to the serving cell to which the terminal is reconnected.

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