KR100540573B1 - Handover method of mobile communication system - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a handover method of a mobile communication system. Conventionally, in order to search for a reverse signal from a mobile station entering a target base station, a search window having a fixed size according to a preset calculation method or a target base station is used. Since the size of the search window is set in consideration of all possible propagation delay times, there is a problem in that the efficiency of reverse signal search is lowered and the performance of handover is reduced.

Accordingly, the present invention divides the reverse signal search section of the mobile station, which is set in consideration of all propagation delays according to the cell area of the base station, into a plurality of unit search sections, and has the highest unit search probability (Pdf). By first setting the path by searching the section, it provides a handover method of a mobile communication system that can not only save the resources of the system but also reduce the search time of the reverse signal.

Description

Handover method of mobile communication system {HANDOVER METHOD OF MOBILE COMMUNICATION SYSTEM}             

1 is a flowchart of a handover method of a conventional mobile communication system;

2 is a multi-cell model diagram of a mobile communication system showing the position of a mobile station for each service area during handover;

3 is a flowchart of a handover method of a mobile communication system according to the present invention;

4 is a flowchart illustrating a routing probability distribution calculation method according to an embodiment of a handover method of a mobile communication system according to the present invention;

5 is a flowchart illustrating a routing probability distribution calculation method according to another embodiment of a handover method of a mobile communication system according to the present invention;

6 is a routing probability distribution diagram between a mobile station and a target base station.

*** Explanation of symbols for the main parts of the drawing ***

 1: target base station service area 3: serving base station service area

 5: neighboring base station service area 10: mobile station

The present invention relates to a handover method of a mobile communication system. More particularly, the present invention relates to a handover method of a mobile communication system capable of improving handover performance by shortening a search time of a reverse signal from a mobile station during handover.

Standards of wireless digital communication systems include 3GPP-oriented asynchronous W-CDMA systems centered on Europe and Japan and 3GPP2 synchronous CDMA2000 systems centered on the United States.

In the CDMA system, when a handover event occurs in which a mobile station moves from a serving base station with which a mobile station is currently communicating to a cell area of an adjacent target base station, the target base station searches for an uplink Tx from the mobile station through a path searcher. To establish a route with the mobile station.

Here, the target base station is referred to as a propagation delay (TP) between the mobile station and the target base station based on the MR (Measurement Report) of the mobile station and the round trip time (RTT) between the serving base station and the mobile station. ) And sets the size W of the search window of the path searcher according to the calculated propagation delay time.

The search window is a reverse signal search section configured to search for propagation delay time required for receiving a reverse signal from the mobile station to the base station after transmitting a signal from the base station to the base station. Typically, the size of the search window is 2 of the maximum TP. It is set to 2TP.

In the synchronous system, the transmission timing (TX timing) between the cells of each base station is synchronized according to the reference time from the GPS (Global Positioning System), so that the size of the search window is calculated by calculating the propagation delay time (TP) during handover. In the asynchronous system, since the transmission timings of the base stations are different and the transmission time differences between the base stations are not known, the search window is set in consideration of all propagation delay times (TP) that may occur in the base station.

1 is a flowchart of a conventional handover method. As shown in FIG. 1, when a handover event occurs in which the mobile station moves from the serving base station with which the mobile station is currently communicating to the cell area of the adjacent target base station (S1), the target base station starts a multi-path search. . The target base station sets the size (W) of the search window so that all the reverse signals of the mobile stations that can be received are searched in consideration of all possible propagation delay times (TP) in the service area (S3). In particular, in the case of an asynchronous system, since the target base station cannot calculate the propagation delay time TP of the mobile station, the size of the search window is set in consideration of all possible propagation delay times based on the cell radius. Accordingly, when the cell radius of the target base station is 1 Km, the size W of the search window is 25.6 chips, and when the 5 Km is 5 Km, the size W of the search window is 256 chips.

The target base station searches for the reverse signal from the mobile station through the search window (S5), and determines whether or not the synchronization with the searched reverse signal (S7). When the target base station acquires synchronization with the mobile station and becomes in an in-sync satus, the target base station searches for a path for the corresponding channel and transmits and receives a signal with the mobile station (S9).

On the other hand, when the size of the search window is too small, the reception rate of the reverse signal is decreased. When the size of the search window is too large, the mobile station spends a lot of time searching for the reverse signal, thereby degrading handover performance.

However, in the current mobile communication system, the size of the search window is set by calculating the propagation delay time according to a predetermined calculation formula. In particular, in the case of an asynchronous system, the size of the search window considering the propagation delay time possible in the target base station is set. . That is, since the size of the search window is simply set in a fixed manner, there is a problem in that efficiency is low.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems. In the case of handover, the search window is divided into a plurality of sections to search for a reverse signal from a place where the routing probability between the target base station and the mobile station has a high probability, thereby multiplying at the target base station. It is an object of the present invention to provide a handover method of a mobile communication system that can improve handover performance by shortening a path search time.

A handover method of a mobile communication system according to the present invention for achieving the above object comprises the steps of: dividing a reverse signal search section into a plurality of unit search sections; Setting priorities for the divided unit search sections; Searching for the unit search section according to the set priority; And determining whether to synchronize according to the search result.

In this case, the unit search section is most preferably set to W / n (where W = size of the entire backward signal search section, n = integer).

It is most preferable that the size of the unit search section is set to the largest value such that the relationship between the time Tps for path search of one section and the time Tsd for determining backward synchronization is Tps > Tsd.

The setting of the priority for the unit search section may include: storing the unit search section in which a path is detected; Calculating a probability distribution of route setting for each of the stored unit search sections; The method may further include assigning a priority to a section having the high routing probability.

The setting of the priority for the unit search section may include calculating a distance d1 between the serving base station and the mobile station based on a round trip time (RTT) value provided by a serving base station; Calculating a probability distribution of a distance d2 between a target base station and the mobile station based on the calculated d1 value; It is preferable to include the step of giving priority to the unit search interval having a high probability distribution of d2.

Here, the calculating of the probability distribution of the distance d2 between the target base station and the mobile station may be a step of calculating the probability distribution of d2 through Equation 3 based on the calculated d1 value. Do.

<Equation 3>

ξ: shadowing coefficient

On the other hand, the step of determining whether the searched reverse signal is synchronized, it is preferable to be performed simultaneously with the search process of the reverse signal for the unit search section having the next priority of the unit search section in which the reverse signal is searched.

Hereinafter, with reference to the accompanying drawings will be described in detail with respect to preferred embodiments of the present invention.

FIG. 2 is a multi-cell model diagram of a mobile communication system showing the position of the mobile station 10 with respect to each service area 1, 3, 5 at the time of handover.

The plurality of base stations Node B1, Node B2, and Node B3 randomly set the PN roll of PN (Pseudo Noise) code of each of the base stations Node B1, Node B2, and Node B3, and then select the area (1, 3) of each base station. , 5) separately transmit and receive signals by synchronizing with each mobile station 10 belonging to each of the base stations (Node B1, Node B2, Node B3).

In order for the mobile station 10 and the base station to transmit and receive signals, the synchronization of channel codes repeated in 256 chip periods must match, so that each base station Node B1, Node B2, Node B3 moves to the corresponding service area (1, 3, 5). A mobile station 10 receives a reverse signal from the mobile station to obtain synchronization.

Accordingly, each base station establishes a reverse signal search section in order to search for the uplink signal of the mobile station 10 entering the corresponding service areas 1, 3, and 5. At this time, each base station first sets a reverse signal search section (size of search window, W) in consideration of all propagation delays that may occur according to the cell size of the corresponding service area (1, 3, 5), and then sets the set mobile station. The code search section is divided into a plurality of unit search sections, and a reverse signal is searched for each unit search section.

3 is a flowchart of a handover method of a mobile communication system according to the present invention. As shown in FIG. 3, when a handover event occurs (S12), the target base station Node B2 sets a reverse signal search section (size of the search window W) set according to the cell size of the target base station service area 2. Is divided into a plurality of unit search sections (W / n) (S14).

The path searcher of the target base station Node B2 gives priority to each unit search section in the order of the highest probability of the path setting according to the probability distribution (pdf) of the path setting (S16). Therefore, when the entire backward signal search section is divided into n units, the unit search section is reduced in size to W / n, and each unit search section is given a rank from 1 to n in order of high priority.

The path searcher of the target base station Node B2 starts the reverse signal search of the mobile station 10 from the unit search section W (1) having the first priority according to the priority (S20).

When the reverse signal of the mobile station 10 is found, the target base station Node B2 determines whether or not the searched uplink signal Uplink is synchronized (S22).

If the synchronization of the reverse signal is not obtained, the path searcher of the target base station Node B2 sequentially searches each unit search section according to the priority (S24). Here, the target base station allocates a finger to the unit search section W (n-1) where the reverse signal is searched to determine whether the reverse signal is synchronized, thereby determining the reverse signal search and the reverse signal synchronization for each section. Let's proceed at the same time. That is, steps S20 and S22 may be simultaneously performed for each unit search section.

When the synchronization of the uplink signal is acquired in a predetermined section, the target base station Node B2 stops searching for the uplink signal for handover, starts a normal multipath search for the unit search section in which the synchronization is obtained, and starts the mobile station 10. Communication with the server (S26).

On the other hand, the number of divisions (n) of all reverse signal search sections is preferably set to the largest value such that the relationship between the time Tps for searching paths of one section and the time Tsd for determining backward synchronization is Tps≥Tsd. It is also possible to set the number of divisions n = W / w with reference to the size w of a normal reverse signal search section.

As described above, the present invention divides the reverse signal search section set according to the cell size into n and searches the section having the highest setting probability of the path first.

Here, the probability distribution pdf of the routing can be calculated according to the flowchart of FIG. 4 or 5. 4 is a flowchart of a routing probability distribution (pdf) calculation method according to an embodiment of a handover method of a mobile communication system of the present invention, and FIG. 5 is a flowchart of a handover method of a mobile communication system of the present invention. According to the routing probability distribution (pdf) calculation method.

4 is a flowchart illustrating a method of calculating a distance between the mobile station 10 and the target base station Node B2 when there is no information on the service area cell of each base station.

When the handover occurs, the target base station Node B2 calculates a distance d1 between the serving base station Node B1 and the mobile station 10 based on the RTT value provided by the serving base station Node B1 (S30). The mobile station 10 provides an MR for measuring and reporting the received radio wave strength of the forward signal from the serving base station Node B1, and based on the MR provided by the mobile station 10, the serving base station Node B1 is a target base station. Node B2) may provide an RTT value.

Based on the calculated d1 value, the distance d2 between the target base station Node B2 and the mobile station 10 is calculated (S32), and d2 may be calculated using the following equation.

First, the reception power of the base station signal received by the mobile station 10 may be represented by the transmission power and the path loss as shown in Equation 1 below.

<Equation 1>

Here, PRX is the magnitude of the signal received by the mobile station 10, PTX is the magnitude of the signal transmitted from the base station, and ξ is the shadowing coefficient.

At the time of handover, the moment when the cell of the target base station service area 3 is added, the forward reception power of the cell of the serving base station service area 1 received from the mobile station 10 and the cell of the target base station service area 3 are received. It is time to show the difference within 3dB. Therefore, the following <Equation 2> can be established.

<Formula 2>

Here, assuming that the transmission power PTX of the two base stations are the same, the distance d2 between the mobile station 10 and the target base station Node B2 may be calculated by the following Equation 3.

<Equation 3>

Accordingly, if d1 calculated in step S30 is substituted into <Equation 3>, the distance of d2 is calculated and a probability distribution (pdf) of d2 is calculated (S34). Here, the section having a high probability distribution of d2 is a section in which the mobile station 10 is likely to exist from the target base station Node B2, which means that the section has the highest possible setting of the handover path.

When the probability distribution of d2 is calculated, the route with the mobile station 10 is searched first by giving the section having the high probability distribution of d2 among the unit search sections (S36).

5 is a flowchart of a method of calculating a routing probability distribution (pdf) through sample collection according to actual handover. When a handover event occurs from the serving base station Node B1 to the target base station Node B2 (S40), the backward signal search section in which the reverse path was detected during handover is stored (S42). When handover occurs, a sample is continuously collected, and when sufficient samples are collected, a probability distribution of route setting is calculated based on the collected route information (S44).

Thereafter, when the handover event occurs again, the reverse direction signal of the mobile station 10 is first searched for the unit search section in which the path was most frequently detected during the handover of the unit search section (S46).

For example, if the unit is divided into four unit search sections, 150 handover paths occur in the first section, 800 times in the second section, 30 times in the third section, and 20 times in the fourth section. When searching for the route with the mobile station 10, the reverse signal search proceeds in the order of the second section, the first section, the third section, and the fourth section, and the unit search section is set to W / 4. .

FIG. 6 illustrates a routing probability distribution diagram calculated according to the probability distribution calculation method of FIGS. 4 to 5. In the example shown in the figure, since the entire backward signal search section W is divided into four unit search sections, the size of the unit search section is set to W / 4, and the routing probability of the first section is calculated to be the highest. The mobile station 10 first searches for an uplink signal for the first section. When an uplink signal is acquired from the mobile station 10, a path for the corresponding channel is searched to transmit and receive a signal with the mobile station 10.

As described above, in the present invention, when searching for a multipath in a handover in a mobile communication system, the reverse signal search section set in consideration of all propagation delays according to the cell area of the base station is divided into n pieces and the path is set. The unit search section with the highest probability is searched first, and the probability distribution is calculated using a probability distribution (Pdf) of the distance d2 between the mobile station and the target base station (Node B2) or a path set during actual handover. We propose a method to calculate the probability distribution (Pdf) by collecting. Accordingly, the size of the search section where the search is performed is reduced, thereby not only saving the resources of the system, but also searching the place where the path setting probability is high, thereby reducing the search time of the reverse signal of the mobile station.

As described above, the handover method in the mobile communication system according to the present invention is not limited to the above-described embodiment, and various modifications can be carried out within a range obvious to those skilled in the art.

According to the handover method of the mobile communication system of the present invention as described above, the reverse signal search interval set in consideration of all propagation delays according to the cell area of the base station is divided into n pieces and the path setting probability is highest. Since the unit search section is searched first, the system resource can be saved and the search time of the reverse signal can be shortened.

Claims (7)

Dividing the backward signal search section into a plurality of unit search sections each having a largest value such that the relationship between the time Tps for path search of one section and the time Tsd for determining backward synchronization is Tps≥Tsd; Setting priorities for the divided unit search sections; Searching for the unit search section according to the set priority; Determining whether the reverse signal is synchronized when the reverse signal is found in the unit search section, and searching for a reverse signal in a unit search section having a next priority of the unit search section in which the reverse signal is searched. Handover method of a mobile communication system, characterized in that. The method of claim 1, A handover method of a mobile communication system, wherein the size of the unit search section is set to W / n, where W = size of the entire reverse signal search section and n = integer. delete The method of claim 1, Setting the priority for the unit search section, Storing a mobile station code search section in which a path is detected; Calculating a probability distribution of route setting for the stored unit search section; And assigning a priority to a section having a high routing probability. The method of claim 1, Setting the priority for the unit search section, Calculating a distance d1 between the serving base station and the mobile station based on a round trip time (RTT) value provided by a serving base station; Calculating a probability distribution of a distance d2 between a target base station and the mobile station based on the calculated d1 value; And assigning a priority to a unit search section having a high probability distribution of d2. The method of claim 5, Computing the probability distribution of the distance (d2) between the target base station and the mobile station, And a probability distribution of the d2 is calculated through the following Equation 3 based on the calculated d1 value. <Equation 3>

ξ: shadowing coefficient delete

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