KR20040104145A - 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 and sets the size of the search window that is set in consideration of all propagation delays according to the cell area of the base station by a predetermined number and preferentially searches for a section having the highest probability of setting the handover path (Pdf). The present invention provides a handover method of a mobile communication system that not only saves resources of the system but also shortens the search time of the reverse signal by setting up a reverse path by searching for a reverse signal.

Description

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

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.

A search window is a mobile station code search section set to search for a propagation delay time required for a reverse signal from a mobile station to be received by a base station after transmitting a signal from a base station to a terminal. 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 a mobile station moves from a serving base station with which the mobile station is currently communicating to a cell area of an adjacent target base station (S1), the target base station receives in consideration of all possible propagation delay times (TP). The size (W) of the search window is set to search for all possible reverse codes of the mobile station (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. Dividing the mobile station code search section into a plurality of unit search sections during handover;

The target base station searches for the backward signal from the mobile station through the search window (S5) to obtain synchronization with the mobile station (S7). When in sync with the mobile station (In-Sync satus), the target base station transmits and receives a signal with the mobile station by searching for a multipath for the corresponding channel (S9).

On the other hand, when the size of the search window is too small, the reception rate of the reverse signal decreases, and when the size of the search window is too large, it takes a lot of time to search for the mobile station 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 probability of the reverse path between the target base station and the mobile station is high, thereby reverse path from the mobile station. An object of the present invention is to provide a handover method of a mobile communication system that can improve handover performance by shortening a search time.

1 is a flowchart of a handover method of a conventional asynchronous system,

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

3 is a flowchart of a handover method of an asynchronous 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

A handover method of a mobile communication system according to the present invention for achieving the above object comprises: dividing a mobile station code 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; Searching for a reverse code of a mobile station that matches the mobile station code according to the search result to determine whether to synchronize a reverse path according to the reverse code; And communicating with the mobile station through a multipath search according to the determination result.

The dividing of the mobile station code search section into a plurality of unit search sections may include: a mobile station code search section used for multipath searching of an entire mobile station code search section set for searching for a reverse code of a mobile station in the service area. It may be a step of dividing by the size of.

The dividing of the mobile station code search section into a plurality of unit search sections may include: a unit having the next priority, rather than a time taken to determine whether the reverse path is synchronized with the unit search section in which the reverse code is searched; It is also possible to set the maximum value set such that the search time of the reverse code for the search section is the same or smaller.

The setting of the priority for the unit search section may include: storing a mobile station code search section in which a backward path is detected during a previous handover; Calculating a probability distribution of routing for each unit search section based on the stored mobile station code search section in which the stored backward path is detected; The method may further include assigning a priority to a section having the high routing probability.

Alternatively, setting the priority for the unit search section may include calculating a distance d1 between the serving base station and the mobile station based on the RTT value provided by the 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; Dividing the calculated probability distribution of d2 at predetermined intervals; It is preferable to include a step of giving priority to the mobile station code search section corresponding to the section of the 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 reverse path according to the searched reverse code is synchronized, is performed simultaneously with the search process of the reverse code for the unit search section having the next priority of the unit search section in which the reverse code is searched Is preferable,

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 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 be identical, and thus the mobile station 10 and the base station move to the corresponding service areas 1, 3, and 5 of each base station Node B1, Node B2, and Node B3. A mobile station 10 receives and synchronizes a backward signal from the mobile station.

Accordingly, each base station sets a mobile station code search section in order to search for a reverse code included in an uplink signal of the mobile station 10 entering the corresponding service areas 1, 3, and 5. At this time, each base station first sets the entire mobile station code 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). The mobile station code search section is divided into a plurality of sections, and the reverse code is searched for each section.

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

The path searcher of the target base station Node B2 determines the priority of each divided 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). That is, since the reverse direction signal of the mobile station 10 is searched and priority is given to a section in which the path setting is likely to succeed, the unit search section (size of the search window) is divided into W / n when the entire search window is divided into n. Each unit search section is reduced, and ranks from 1 to n are given in order of highest priority.

The path searcher of the target base station Node B2 has a search window of size W / n based on the priority, and the reverse code included in the reverse signal of the mobile station 10 from the first priority search section W (1) first. Search for (S20).

The target base station Node B2 checks whether the reverse code of the searched mobile station 10 matches or not and determines whether to synchronize with the reverse signal Uplink (S22).

Here, the path searcher of the target base station (Node B2) sequentially searches for the reverse code of the W (n) section in the order of high probability (S24), while the unit search section W (n- previously searched for the reverse code). By assigning a finger to 1) to determine whether the reverse path is synchronized, the determination of whether the reverse code and the reverse path are synchronized for each section is performed at the same time.

When the synchronization of the reverse path is obtained in a predetermined section, the target base station Node B2 stops searching the reverse code for handover, and starts a normal multipath search for the unit search section in which the reverse path is obtained. Perform communication with 10) (S26).

On the other hand, it is preferable that the number of divisions n of all the search windows W is set to the largest value such that the relationship between the time Tps for searching a route 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 conventional multipath search window.

As described above, the present invention divides the size of the search window set according to the cell size into n, and preferentially searches for a section having the highest probability of setting the handover path.

Here, the probability distribution pdf of the handover routing may 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 the handover method of the asynchronous mobile communication system of the present invention, and FIG. 5 is another embodiment of the handover method of the asynchronous mobile communication system of the present invention. It is a flowchart of the routing probability distribution (pdf) calculation method which concerns on an example.

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 entire d2 is divided at predetermined intervals (S36), and the path with the mobile station 10 is searched first by giving priority to the section having the high probability distribution of d2 among the divided sections (S38).

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 section of the search window in which the reverse path was detected during handover, that is, the mobile station code search section, 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).

Subsequently, when the handover event occurs again, the size of the entire search window is divided at predetermined intervals (S46), and among the divided sections, the mobile station 10 and the mobile station 10 are preferentially given to the section of the search window where the reverse path was most frequently detected during handover. Search the path of (S48).

For example, if the search window is divided into four, 150 handovers occur in the first section, 800 times in the second section, 30 times in the third section, and 20 times in the fourth section. 10) When searching for the route, the reverse route search proceeds in the order of the second section, the first section, the third section, and the fourth section, and the size of the search window 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 drawing, since the entire search window W is divided into four sections, the size of the search window is set to W / 4, and since the routing probability of the first section is calculated as the highest, the priority is given to the first section. The mobile station 10 searches for an uplink signal. When an uplink signal is acquired from the mobile station 10, a multipath search for the corresponding channel is performed to transmit and receive a signal with the mobile station 10.

As described above, when the handover is performed in the mobile communication system, the size of the search window is set by dividing the size of the search window, which is set in consideration of all propagation delays, according to the cell area of the base station by n and the probability of setting the handover path is increased. The highest section is searched first, and the probability distribution is calculated by using the probability distribution (Pdf) of the distance d2 between the mobile station and the target base station (Node B2) or by collecting a path set during actual handover. It shows how to calculate the distribution (Pdf). As a result, the size of the search window is reduced, which not only saves resources of the system but also preferentially searches for a high probability of setting the handover path, 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 present invention as described above, when performing the handover in the mobile communication system, the size of the search window that is set in consideration of all propagation delays according to the cell area of the base station is divided into n pieces and set the size of the handover path. Since the section having the highest setting probability is searched first, not only the resource of the system can be saved but also the search time of the reverse signal can be shortened.

Claims (7)

Dividing the mobile station code 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; Searching for a reverse code of a mobile station that matches the mobile station code according to the search result to determine whether to synchronize a reverse path according to the reverse code; And communicating with the mobile station through a multipath search according to the determination result. The method of claim 1, Dividing the mobile station code search section into a plurality of unit search sections, And dividing the entire mobile station code search section set to search for the reverse code of the mobile station in the service area by the unit of the size of the mobile station code search section used in the multipath search. Over way. The method of claim 1, Dividing the mobile station code search section into a plurality of unit search sections, A maximum value set such that the reverse code search time for the next unit search section having the next priority is equal to or smaller than the time taken to determine whether the reverse path is synchronized with the search unit for which the reverse code has been searched; Handover method of a mobile communication system, characterized in that the step of setting to The method of claim 1, Setting the priority for the unit search section, Storing a mobile station code search section in which a backward path is detected in a previous handover; Calculating a probability distribution of routing for each unit search section based on the stored mobile station code search section in which the stored backward path is detected; 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 the RTT value provided by the 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; Dividing the calculated probability distribution of d2 at predetermined intervals; And assigning a priority to a mobile station code search section corresponding to a section having a high probability distribution of d2. The method of claim 5, wherein 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 The method according to claim 1 or 3, The determining of the synchronization of the reverse path according to the searched reverse code, And the reverse code is searched simultaneously with the search process of the reverse code for the unit search section having the next priority of the search unit.