KR0173906B1 - Soft Handover Method by Distinguishing Indirect and Direct Wave Relationships of Base Stations in Microcell Environments - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a soft handover method in which a soft handover is performed according to an indirect wave and a direct wave relationship in consideration of a sudden decrease in signal strength due to a corner effect, which is an electromagnetic characteristic of a microcell, in a mobile communication system. When approaching, handover to base station 1 or base station 3 which is indirect wave relationship in advance to prevent deterioration of call quality of received signal due to corner effect when mobile station moves from intersection to base station which is indirect wave relationship, and compulsory termination rate of handover call There is an effect that can reduce.

Description

Soft Handover Method by Distinguishing Indirect and Direct Wave Relationships of Base Stations in Microcell Environments

1 is an explanatory diagram for a soft handover signal level of a direct wave relationship in a conventional base station.

2 is an explanatory diagram of a soft handover signal level of an indirect wave relationship in a conventional base station.

3a and 3b are flowcharts of a soft handover method in a conventional mobile communication system (IS-95).

4 is a cell diagram of a Manhattan road model, which is a downtown model in a typical microcell environment.

5 is a configuration diagram of a mobile communication system to which the present invention is applied.

6A and 6B show an overall flowchart of a soft handover method according to the present invention when the mobile station is not in a handover state.

7A and 7B are flowcharts of a soft handover method according to the present invention when the mobile station is in a handover state.

* Explanation of symbols for main parts of the drawings

501: switching center 502: base station

503: home location register 504: voice mailbox system

505: fax mail system

The present invention relates to a soft handover method for performing a soft handover according to an indirect wave and a direct wave in consideration of a sudden decrease in signal strength due to a corner effect, which is an electromagnetic wave characteristic of a microcell, in a mobile communication system.

In order to increase user capacity, the mobile communication environment changes to a microcell whose cell radius is much smaller than a conventional microcell, with the signal strength of the currently connected base station as the mobile station turns to a new base station at the intersection near the cell boundary. A sharply decreasing corner effect occurs.

This results in a very poor call quality in the vicinity of the corner, and the forced termination of the handover call may occur when the mobile station gathers traffic to the base station to which the handover is to be made and the channel allocation for the handover is not made quickly after the corner. There is this.

Accordingly, there is a need for a method in which a mobile station can quickly perform handover to a new base station after cornering in order to minimize this disadvantage.

1 is an explanatory diagram of a soft handover signal level of a direct wave relationship in a conventional base station. As the mobile station moves from base station 1 to base station 2, the strength of the received signal of the base station 1 decreases, and the strength of the received signal of the base station 2 Gradually increasing.

The parameters for measuring pilot strength used to determine handover timing are as follows.

T-ADD is the value of the base station pilot strength (hereinafter, referred to as T-ADD) that neighboring base stations must satisfy in order to enter the candidate set, and T-DROP is the lowest signal level value (hereinafter, T) for the base station to remain in the active set. T-COMP (T-COMPL) is a pilot of a base station belonging to an active set necessary for the base station of the candidate set to enter the active set when the base station of the candidate set has a direct wave relationship with the base station of the active set. The difference between the pilot strength of the base station belonging to the strength and the candidate set (hereinafter referred to as T-COMP), and the T-TDROP is a channel drop type required for the base stations having a direct wave relationship to leave the active set. -Called TDROP)

2 is an explanatory diagram of a soft handover signal level of indirect wave relation in a conventional base station. As the mobile station moves from base station 1 to base station 2, the strength of the received signal of the base station 1 in the soft handover area drops sharply. The intensity of the received signal of 2 shows a sharp increase.

The parameters for measuring pilot strength used to determine handover timing are as follows.

T-COMPN is the difference between the pilot strength of the base station belonging to the active set necessary to enter the active set and the pilot strength of the base station belonging to the candidate set when the base station of the candidate set is an indirect wave relation to the base station of the active set (hereinafter, T). T-TDROPN is the channel drop timer value required by the base station to leave the active set when the mobile station is in soft handover with base stations with indirect wave relations (hereinafter referred to as T-TDROPN). Indicates.

3A and 3B are flowcharts of a soft handover method in a conventional mobile communication system (IS-95).

In order to enter the handover with the neighboring base station, the mobile station determines whether the pilot strength of the neighboring base station of the mobile station is greater than the T-ADD (301), and if the pilot strength of the neighboring base station is less than the T-ADD, the mobile station transitions to the initial state, If the pilot strength of the T-ADD is greater than T-ADD, it is determined whether the pilot strength of the base station to which the mobile station is currently connected from the base station's pilot strength is greater than the T-COMP (302). If the pilot strength of the base station is less than T-COMP, the transition to the initial state is performed. If the pilot strength of the base station to which the mobile station is currently connected at the pilot strength of the base station is greater than the T-COMP, the mobile station is assigned to this new base station and soft hand. Perform over and terminate (303).

On the other hand, when the mobile station attempts to disconnect from the accessing base station, it is determined whether the pilot strength of the accessing base station is smaller than the T-DROP (304). If the pilot strength of the accessing base station is larger than the T-DROP, the mobile station transitions to the initial state. If the pilot strength of the accessing base station is smaller than the T-DROP, it is determined whether the pilot strength of the accessing base station is smaller than the T-DROP for the time T-TDROP (305). If the state smaller than -DROP is not maintained for the time of T-TDROP, the state transitions to the initial state, and the soft handover is terminated if the pilot strength of the accessing base station is smaller than T-DROP for the time of T-TDROP. 306).

4 is a cell diagram of a Manhattan road model, which is a city model in a general microcell environment, in which base station 2 has a direct wave relationship with respect to base station 0, and base stations 1 and 3 have an indirect wave relationship.

When the mobile station is connected to the base station 0 and proceeds toward the intersection, the corner of the intersection turns to the base station 1 or the base station 3, which are indirect wave relations, and the strength of the signal received from the base station 0 to the mobile station is drastically reduced by the corner effect. The call quality is very poor. Therefore, soft handover with base station 1 or base station 3 to which the mobile station is directed should be made quickly.

However, in the soft handover method of the conventional mobile communication system (IS-95), the mobile station requires a handover only when the pilot strength of the base station to which the mobile station passes after fully turning the corner is required to handover, until the call is assigned until the actual handover channel is allocated. If the quality remains intact and traffic is concentrated on either base station 1 or base station 3 and the handover call needs to wait for channel allocation, the probability of the call being terminated is forced because the received signal strength decreases faster than the direct wave due to the corner effect. There was a problem in that it was larger than when attempting to hand over to the base station 2 which is a direct wave relationship.

Accordingly, the present invention devised to solve the above problems, when the mobile station approaches the intersection, when the mobile station moves to the base station in the indirect wave relationship by performing a handover to the base station 1 or base station 3 in advance. It is an object of the present invention to provide a soft handover method that prevents a drop in call quality due to a corner effect and reduces the forced termination rate of a handover call.

One embodiment of the present invention for achieving the above object is a soft handover method applied to a microcell environment of a mobile communication system when the mobile station is not in a handover state, wherein the pilot strength of the neighboring base station is determined by the neighboring base station. A first step of determining whether the pilot strength of the neighboring base station is a direct wave or an indirect wave relationship if the pilot strength of the neighboring base station is greater than the value of the base station pilot strength that must be satisfied to enter the candidate set; As a result of the determination in the first step, if the direct wave relationship is determined by subtracting the pilot strength of the accessing base station from the pilot strength of the neighboring base station, the pilot strength of the base station belonging to the active set having a direct wave relationship and the pilot strength of the base station belonging to the candidate set are determined. A second step of performing a soft handover if it is greater than the difference value T-COMPL; And as a result of the determination in the first step, in the case of indirect wave relation, the pilot strength of the base station belonging to the active set in which the pilot strength of the neighboring base station is subtracted from the pilot strength of the neighboring base station is mutually indirect wave relation and the pilot strength of the base station belonging to the candidate set If greater than the difference value T-COMPN, characterized in that it comprises a third step of performing a soft handover.

Another embodiment of the present invention for achieving the above object is a soft handover method applied to the microcell calla mirror of the mobile communication system when the mobile station is in the handover state, the two base stations involved in the handover A first step of determining whether each other is indirect wave relationship; A second step of maintaining the handover state if the two base stations are indirectly related to each other as a result of the determination of the first step; As a result of the determination in the first step, if the two base stations are in a direct wave relationship and the pilot strengths of the neighboring base stations are greater than the value of the base station pilot strengths (T-ADD) that the neighboring base stations must satisfy to enter the candidate set, the pilots of the neighboring base stations A third step of determining whether the strength is a direct wave or an indirect wave relationship with the pilot strength of the currently connected base station; As a result of the determination in the third step, in the case of indirect wave relation, if the value of the pilot strength of the neighboring base station is subtracted from the pilot strength of the neighboring base station is greater than the difference value T-COMPN of the pilot strength of the base stations belonging to the active set having the indirect wave relation A fourth step of performing soft handover; And as a result of the determination in the third step, if the direct wave relationship is determined by subtracting the pilot strength of the accessing base station from the pilot strength of the neighboring base stations, the pilot strength of the base station belonging to the active set having a direct wave relationship and the pilot strength of the base station belonging to the candidate set If greater than the difference value (T-COMPL) is characterized in that it comprises a fifth step of performing a soft handover.

5 is a configuration diagram of a mobile communication system to which the present invention is applied, where 501 is an exchange station, 502 is a base station, 503 is a home location register, 504 is a voice mail system, and 505 is a fax mail system.

The switching center 501 is composed of a control subsystem (CS), an interconnection subsystem (IS), and a switching subsystem (SS).

The CS is responsible for the overall operation and maintenance of the system, and the IS is located at the center of the system to connect the SSs or the SSs and CSs, translate numbers, and connect space switches. The main function of SS is the call processing function, which exists in one function, but in the application stage, SS-S (Subscriber), SS-T (Trunk), SS-M (Mobile), SS-7 (No.7), SS-A ( ARS) and many other combinations of these are possible.

In addition, the switching center 501 includes a control station (BSC) for controlling the base station 502 and a base station management (BSM) for managing functions, and includes a voice mailbox system 504 and a fax mail system. 505 is connected.

A home location register (HLR) 503 is connected to the switching center 501 and is responsible for a database management system (DBMS) function that manages various types of information of mobile subscribers. Do this.

The base station 502 is composed of a Digital Channel Unit (DCU), a Backhaul Inteface Unit (BIU), a Timing / Frequency Control Unit (TCU), an FR Unit (RFU), a GPS (Golbal Position System), and various functions for mobile subscribers. In charge of.

Here, it is assumed that the mobile station can be connected simultaneously with up to two base stations.

6A and 6B are an overall flowchart of the soft handover method according to the present invention when the mobile station is not in the handover state.

First, to enter a handover with a neighboring base station when the mobile station is not in the handover state, it is determined whether the pilot strength of the neighboring base station is greater than the T-ADD (601), if the pilot strength of the neighboring base station is less than the T-ADD. When the initial state transitions and the pilot strengths of the neighboring base stations are greater than the T-ADD, it is determined whether the pilot strengths of the neighboring base stations are the direct wave relation or the indirect wave relation (602).

As a result of the determination, if the pilot strengths of the two base stations are in a direct wave relationship, it is determined whether the pilot strength of the neighboring base station is less than the T-COMPL in the pilot strength of the neighboring base stations (603). If the pilot strength is less than T-COMPL, the state transitions to the initial state. If the pilot strength of the neighboring base station is less than T-COMPL, the soft handover is performed (605). do.

As a result of the determination, if the pilot strengths of the two base stations are in a direct wave relationship, it is determined whether the pilot strength of the neighboring base station from the pilot strength of the neighboring base stations is greater than T-COMPN (604). If the value of pilot strength subtracted is less than T-COMPN, the state transitions to the initial state, and if the value of pilot intensity decremented of the accessing base station is greater than T-COMPN in pilot strength of neighboring base stations, after soft handover is performed (605). do.

On the other hand, when the mobile station attempts to disconnect the currently connected base station, first, it is determined whether the pilot strength of the accessing base station is smaller than the T-DROP (606). Transition to the state, the timer of the base station being connected is operated less than the T-DROP timer to determine whether the pilot strength of the base station being connected is less than the T-DROP is maintained for T_TDROP time (607) If the state of pilot strength less than T-DROP is not maintained for T_TDROP time, it transitions to the initial state. If the state of pilot base station of less than T-DROP is maintained for T-TDROP time, it is disconnected and handover is performed. End (608).

At this time, by setting the value of the T-COMPL higher than the value of the T-COMPN it is possible to perform a handover to the base station in the indirect wave relationship before reaching the intersection.

7A and 7B are flowcharts of the soft handover method according to the present invention when the mobile station is in the handover state.

To perform a handover to another base station when the mobile station is in the soft handover state, first, it is determined whether two base stations involved in the handover are indirect or direct wave relations with each other (701). In case of a wave relationship, the handover state is maintained. If the two base stations are in a direct wave relationship, it is determined whether the pilot strength of the neighboring base station is greater than the T-ADD (702). In operation 703, if the pilot strength of the neighboring base station is greater than T-ADD, it is determined whether the pilot strength of the neighboring base station and the pilot strength of the currently connected base station are in the direct wave relationship or the indirect wave relationship.

As a result of the determination, if the pilot strengths of the two base stations are indirect wave relations, it is determined whether the pilot strength of the neighboring base station from the pilot strength of the neighboring base stations is greater than T-COMPN (704). If the value of the pilot strength subtracted is less than the T-COMPN, the state transitions to the initial state. If the value of the pilot strength of the neighboring base station is less than the T-COMPN, the soft handover is performed (706). do.

As a result of the determination, if the pilot strengths of the two base stations are in a direct wave relationship, it is determined whether the pilot strength of the neighboring base station from the pilot strength of the neighboring base stations is greater than T-COMPL (705). If the pilot strength is less than T-COMPL, the state transitions to the initial state. If the pilot strength of the neighboring base station is less than the T-COMPL, the soft handover is performed (706). do.

At this time, by setting the value of the T-COMPL higher than the value of the T-COMPN it is possible to perform a handover to the base station in the indirect wave relationship before reaching the intersection.

On the other hand, when the mobile stations are handing over base stations having indirect wave relations with each other, in order to disconnect the base station in order to quickly stop the handover after the intersection, first, the two base stations involved in the handover are connected to each other. It is determined whether the direct wave relationship or the indirect wave relationship (707).

As a result of determination, if the two base stations are indirect wave relation, it is determined whether the pilot strength of the accessing base station is smaller than the T-DROP (708). If the pilot strength of the accessing base station is larger than the T-DROP, the transition to the initial state is performed. If the pilot strength is less than the T-DROP, the timer is operated to determine whether the pilot strength of the accessing base station is less than the T-DROP for the duration of the T-TDROPN period (709), and the pilot strength of the accessing base station is T-DROP. If the smaller state is not maintained during the T-TDROPN period, the state transitions to the initial state. If the state of the pilot of the accessing base station is smaller than the T-DROP period is maintained for the T-TDROPN period, the connection is terminated and the handover is terminated (712). ).

As a result of determination, if the two base stations are in a direct wave relationship, it is determined whether the pilot strength of the accessing base station is smaller than the T-DROP (710). If the pilot strength of the accessing base station is larger than the T-DROP, the transition to the initial state is performed. If the pilot strength is less than the T-DROP, the timer is operated to determine whether the pilot strength of the accessing base station is less than the T-DROP for the duration of the T-TDROP (711), and the pilot strength of the accessing base station is T-DROP. If the smaller state is not maintained during the T-TDROP period, the state transitions to the initial state, and if the pilot strength of the connected device station is smaller than the T-DROP period, the connection is terminated and the handover is terminated. 712).

At this time, the T-TDROPN is made shorter than the T-DROP so that the soft handover state between the base stations of the indirect wave relationship can be quickly terminated after the mobile station crosses the intersection.

In the present invention as described above, when a mobile station approaches an intersection, the mobile station moves to a base station having an indirect wave relationship by performing a handover to base station 1 or base station 3 having an indirect wave relationship in advance. There is an effect that can prevent the deterioration of quality and reduce the forced termination rate of the handover call.

Claims (7)

In a soft handover method applied to a microcell environment of a mobile communication system when the mobile station is not in a handover state, a value of the base station pilot strength that a pilot strength of a neighboring base station must satisfy in order for the neighboring base station to enter a candidate set ( T-ADD), a first step (601, 602) of determining whether the pilot strength of the neighboring base station is a direct wave or indirect wave relationship with the pilot strength of the base station currently connected; As a result of the determination of the first step (601, 602), if the direct wave relationship, the pilot strength of the neighboring base station is subtracted from the pilot strength of the neighboring base station belonging to the pilot strength and the candidate set of the base station belonging to the active set having a mutual direct wave relationship A second step (603, 605) of performing soft handover if greater than the difference value T-COMPL of the pilot gays of the base station; And as a result of the determination of the first steps 601 and 602, if the indirect wave relation is obtained, the pilot strength of the neighboring base station is subtracted from the pilot strength of the neighboring base station. And a third step (604, 605) of performing a soft handover if it is larger than the difference value T-COMPN of the pilot strength of the base station to which the base station belongs. 2. The method of claim 1, wherein when the mobile station attempts to disconnect from the currently connected base station, the states in which the pilot strength of the accessing base station is smaller than the lowest signal level value (T-DROP) for the base station to remain in the active set are mutually different. And a fourth step (606 to 608) for terminating the handover if the base station of the direct wave relationship is maintained for the time of the channel drop timer value (T-TDROP) necessary for exiting the active set. Way. The base station according to claim 1 or 2, wherein a difference (T-COMPL) between a pilot strength of a base station belonging to an active set having a direct wave relationship and a pilot strength of a base station belonging to a candidate set is determined by a base station belonging to an active point having an indirect wave relationship. Soft handover, characterized in that it is set higher than the difference between the pilot strength and the pilot strength of the base station belonging to the candidate set (T-COMPN) before the mobile station reaches the crossroads and performs handover to the base station in indirect wave relationship. Way. A soft handover method applied to a microcell environment of a mobile communication system when a mobile station is in a handover state, the first step of determining whether two base stations involved in the handover are indirect or direct wave relationships with each other 701; A second step of continuing to maintain the handover status when the two base stations are indirectly related to each other as a result of the determination of the first step (701); As a result of the determination in the first step 701, if the two base stations are in a direct wave relationship and the pilot strengths of the neighboring base stations are greater than the value of the base station pilot strengths (T-ADD) that the neighboring base stations must satisfy in order to enter the candidate set, A third step (702, 703) of determining whether the pilot strength of the base station is in a direct wave or indirect wave relationship with the pilot strength of the currently connected base station; As a result of the determination of the third step (702, 703), if the indirect wave relation, the pilot strength of the neighboring base station is subtracted from the pilot strength of the neighboring base station, the pilot strength of the base station belonging to the active set of the indirect wave relationship and belong to the candidate set A fourth step (704, 706) of performing a soft handover if it is larger than the difference value T-COMPN of the base station; And as a result of the determination in the third step 702 or 703, if the direct wave relation is determined by subtracting the pilot strength of the accessing base station from the pilot strength of the neighboring base stations, the pilot strength and candidate set of the base station belonging to the active set having a mutual direct wave relationship And a fifth step (705, 706) of performing a soft handover if it is larger than the difference value T-COMPL of the pilot strength of the base station to which the base station belongs. 5. The base station according to claim 4, wherein the mobile station is involved in the handover to disconnect the base station so as to quickly stop the handover after the intersection when the mobile stations are performing handovers between base stations having indirect wave relations with each other. A sixth step 707 of determining whether the direct wave relationship is the indirect wave relationship with each other; As a result of the determination of the sixth step 707, when the indirect wave relation is established, the base stations in which the pilot strength of the accessing base station is smaller than the lowest signal level value (T-DROP) for the base station to remain in the active set are in the indirect wave relationship. A seventh step (708, 709, 712) of terminating the handover if it is maintained for the time of the channel drop timer value (T-TDROPN) necessary for exiting; And as a result of the determination in the sixth step 707, when the direct wave relationship is established, the base stations in which the pilot strength of the accessing base station is smaller than the lowest signal level value (T-DROP) for the base station to remain in the active set are active. And an eighth step (710 to 712) of terminating the handover if the channel drop timer value (T-TDROP) required for exiting the set is maintained. The method according to claim 4 or 5, wherein the difference (T-COMPL) between the pilot strength of the base station belonging to the active set having a direct wave relationship and the pilot strength of the base station belonging to the candidate set is determined by the base station belonging to the active set having an indirect wave relationship. Soft handover, characterized in that it is set higher than the difference between the pilot strength and the pilot strength of the base station belonging to the candidate set (T-COMPN) before the mobile station reaches the crossroads and performs handover to the base station in indirect wave relationship. Way. 6. The channel drop timer value (T-TDROPN) required by the base stations in indirect wave relations to leave the active set is higher than the channel drop timer value (T-TDROP) required in order for the base stations in direct wave relations to leave the active set. Soft handover method, characterized in that the small value set.