KR20000003011A - Hard hand-off using common frequency in cellular system - Google Patents

Abstract

PURPOSE: A hard handoff is provided to reduce the load of a system and to remove the unnecessary hand-off. CONSTITUTION: A hard handoff comprises the steps of: measuring the pilot strength can be sensed by a mobile terminal; returning to the pilot strength measuring step if the measured pilot strength is over the critical pilot value; handing down the presently connected frequency channel to the common frequency able to be served in all base stations if the measured pilot strength is not over the critical pilot value; and performing the soft handoff among the cells in the common frequency.

Description

Hard Handoff Method Using Common Frequency in Cellular System

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to cellular systems and, more particularly, to a method of performing hard handoff using a common frequency.

The cellular mobile communication system divides the entire service area into a plurality of wireless base station areas, constitutes cells, which are small service areas, and centrally controls these wireless base stations with a switching system to allow subscribers to move calls between cells. Allow you to continue. In a cellular system, a handoff function allows a call to continue when a mobile terminal leaves a cell of one base station and enters a new cell.

1 illustrates a schematic diagram of a cellular system using a conventional Code Division Multiple Access (CDMA) technique. As shown, a mobile station (MS) 40 receiving a mobile communication service and a base station transceiver subsystem (BTS) 30, 31 providing a service to the mobile terminal 40. And a base station controller (BSC) 20 for controlling the base stations 30 and 31 and a mobile switch that connects the base station controller to a public switched telephone network (PSTN) 11. (Mobile Switching Center: MSC) 10. The mobile switch 10 obtains information on the mobile terminal 40 from a home location register (HLR) 12 and a visitor location register (VLR) 13 to provide a service. do.

In the code division multiple access system configured as described above, a link directed from the base station toward the mobile terminal is called a forward link, and a link directed from the mobile terminal toward the base station is called a reverse link. All forward channels in a base station have the same Pseudorandom Noise (PN) sequence offset. Since the PN offset is transmitted through a pilot channel, which is one of the forward channels, it is called a pilot signal and becomes an identification signal for distinguishing several base stations.

Cellular and Personal Communication Service (PCS) systems using a code division multiple access scheme provide various types of handoffs to ensure call continuity. In particular, a hard handoff is called a method of guaranteeing continuity of a call by setting a new channel within a short time that is difficult for a caller to recognize after disconnecting an existing channel.

The cellular system adds a frequency assignment (FA) assigned to the base station when the increase in capacity is required due to an increase in users. In particular, in the case of downtown base stations, because of the large number of subscribers, a large number of frequency allocations are required, and in the case of an outer base station, small frequency allocations are required. In this case, when the mobile terminal moves to a neighbor base station that does not service the frequency to which the channel is currently connected, and also moves to a neighbor base station where there is no free traffic channel at the frequency to which the current channel is connected, hard handoff occurs.

The mobile terminal recognizes that it has entered the area of the adjacent base station by receiving the pilot signal of the base station through the pilot channel. Therefore, in order to support hard handoff, a separate pilot transmitter for transmitting an identify pilot signal of an adjacent base station is used.

A hard handoff method using a pilot signal is as described in US Patent No. 5,594,718, "METHOD AND APPARATUS FOR PROVIDING MOBILE UNIT ASSISTED HARD HANDOFF FROM A CDMA COMMUNICATION SYSTEM TO AN ALTERNATIVE ACCESS COMMUNICATION SYSTEM."

2 illustrates a pilot signal generator of a neighbor base station for hard handoff by the prior art. As shown, each base station has a digital modem portion (200) (210) (220) that converts a low frequency audio signal (AF) into an intermediate frequency (IF) signal. Transceiver portions 300, 310 and 320 for converting a frequency signal into a radio frequency (RF) signal and a power amplifier (PA) 400 for amplifying a high frequency signal transmitted to an antenna ( 410, 420 has a high frequency path portion. The digital modem parts 200, 210, transceivers # 1, 2 (300) 310, and power amplifiers 400, 410 are parts that are operated for actual service.

The pilot signal generator comprises a digital modem portion 220, a transceiver # 3 320 and a power amplifier 420 for generating a pilot signal, and generates a pilot signal of an adjacent base station. The mobile terminal moving in the direction of the neighboring base station senses that the pilot signal of the neighboring base station is strong while the pilot signal of the original base station is weak and requests a hard handoff.

In the conventional mobile communication system using the pilot generator as described above, the hard handoff procedure is as follows. The mobile terminal detects that the pilot signal of the neighboring base station is sufficiently calculated, determines the handoff decision parameter and the decision time point, and requests a handoff. The base station controller determines that there is no corresponding communication resource in the adjacent base station if the corresponding frequency of the base station detected by the mobile terminal is operated as a dummy pilot for hard handoff. The UE performs a handdown to a common frequency commonly used by the base stations currently talking to the mobile terminal and the additionally detected base station, that is, an Intracell FA Hard Handoff. The mobile terminal then connects the call using a common frequency. The inter-frequency hard handoff is then completed by performing an inter-cell soft handoff to the target base station.

However, the prior art performed as described above requires a separate hardware called a pilot generator. This can be especially fatal for small base stations. Therefore, there is a need for a hard handoff method that does not use additional hardware.

SUMMARY OF THE INVENTION The present invention has been made to solve the problems of the prior art operating as described above, and includes a hard handoff between intra-cell frequencies to a common frequency through setting and optimizing appropriate parameters for handdown in a cellular mobile communication system. To provide a method for performing inter-cell hard handoff as a result.

Other objects and advantages of the present invention will become more apparent from the following detailed description of the invention and the accompanying drawings.

1 is a block diagram of a cellular system using a conventional code division multiple access technique.

2 illustrates a pilot signal generator of a neighbor base station for hard handoff by the prior art.

3 is a flowchart illustrating an overall hard handoff method according to the present invention.

4 is a flowchart illustrating a detailed hard handoff method according to the present invention.

<Explanation of symbols for main parts of the drawings>

10: mobile switching center (MSC)

11: Public Switched Telephone Network (PSTN)

12: Home Location Registration System (HLR)

13: Visitor Location Registration System (VLR)

20: base station controller (BSC)

30,31: base station (BTS)

40: mobile terminal (MS)

200,210,220: Digital modem part

300,310,320: transceiver part

400,410,420: power amplifier

A preferred embodiment of the hard handoff method using a common frequency in a cellular system according to the present invention, which is designed to achieve the above object, is to connect a subscriber signal to a plurality of base stations and distinguish each base station by a pilot signal. A base station controller comprising a mobile terminal, at least one service area having the unique pilot signal and providing a mobile communication service with a mobile terminal, and a base station controller connecting a plurality of base stations to an exchange and providing a handoff to the mobile terminal. ,

Periodically measuring pilot strength that can be detected by the mobile terminal;

If the measured pilot strength is greater than a pilot threshold, returning to measuring the pilot strength;

If the measured pilot strength is not greater than a pilot threshold, handing down a frequency channel to which a current call is connected to a common frequency that can be serviced by all base stations; And

Performing inter-cell soft handoff within a common frequency.

In an embodiment of the present invention, the step of measuring pilot strength may include: periodically sending a pilot measurement request command to a mobile station by a base station controller;

Preferably, the mobile terminal receiving the pilot measurement request command includes sending a pilot strength measurement message.

The process of measuring the pilot strength, the mobile terminal preferably comprises the step of periodically sending a power measurement report message,

The hard handoff method includes measuring a load on a system;

Preferably, the method further includes setting a pilot threshold in consideration of the measured system load.

In the process of measuring the load of the system, it is preferable to measure the load of the system based on the number of calls currently in the system,

In the process of setting the pilot threshold, it is preferable to set the pilot threshold low if the load of the system is low, and set the pilot threshold high if the load of the system is high,

The step of handing down to the common frequency includes: driving a handdown timer if the measured pilot strength is not greater than a pilot threshold;

It is preferable to further include the step of handing down the frequency channel to the common frequency if the measured pilot strength continues to be below the threshold until the handdown timer normally terminates.

The handing down to the common frequency may include comparing the RTD slope with an RTD threshold if the measured pilot strength is not greater than a pilot threshold;

Preferably, if the RTD slope is above the RTD threshold, the method further includes handing down the frequency channel to a common frequency.

The handing down process may include measuring load on the system;

Preferably, the method further includes handing down the frequency channel to a common frequency with the least load on the system.

Another preferred embodiment of the hard handoff method using a common frequency in a cellular system according to the present invention is a mobile terminal for connecting a subscriber signal to a plurality of base stations and identifying each base station by a pilot signal, and the unique pilot signal. A base station comprising at least one service area and a base station for providing a mobile communication service with a mobile terminal, and a base station controller for connecting a plurality of base stations to a switch and providing a handoff to the mobile terminal,

Periodically measuring pilot strength that can be detected by the mobile terminal;

Measuring system load;

Calculating a pilot threshold, a handdown timer, and an RTD slope using the measured system load;

Comparing the measured pilot strength with the calculated pilot threshold and returning to measuring the pilot strength when the measured pilot strength is large;

Driving the handdown timer if the measured pilot strength is less than the pilot threshold;

Resetting a handdown timer and returning to measuring the pilot strength if the timer count is not greater than the handdown timer;

Determining if the RTD slope is greater than the RTD threshold if the timer count is greater than the handdown timer;

Resetting a handdown timer and returning to measuring the pilot strength if the RTD slope is less than the RTD threshold;

Searching for a common frequency to hand down if the RTD slope is greater than or equal to the RTD threshold;

Performing handdown to a common frequency when a common frequency to be handed down is found; And

Within a common frequency, performing the inter-cell soft handoff.

In the following description of the present invention, if it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted. Terms to be described later are terms defined in consideration of functions in the present invention, and may be changed according to intentions or customs of users or operators. Therefore, the definition should be made based on the contents throughout the specification.

The present invention consists of a process of measuring the pilot signal strength of a base station in a mobile terminal and a process of performing a handdown to a common frequency.

The mobile terminal reports the strength of all pilot signals it can detect to the base station controller through a pilot strength measurement message (PSMM) and a power measurement report message (PMRM).

When the strength of the pilot signal measured by the mobile terminal is less than or equal to a certain threshold, the frequency channel of the mobile terminal is handed down to a common frequency. Parameters required for handdown include a pilot strength value according to the system load, a handdown timer, and a round trip delay (RTD) slope.

The system load is measured using the number of active calls of the system. This is determined by the number of active calls currently assigned at the main processor of the base station controller. The present invention applies different thresholds of pilot strength depending on the amount of system load. In other words, when the load of the system is high, the threshold is set high to reduce the handdown frequency, and the load is set low when the load is low. This is because less load on the system can handle more handoffs.

If the handdown is performed by applying the threshold constantly without considering the system load, the service coverage is reduced. This is because the service area of the forward link is determined by Ec / It received from the terminal, and Ec / It depends on the system load. Therefore, if the same threshold is applied to a system with different loads, it may cause a decrease in the performance of the system.

The handdown timer is to prevent the handoff from being performed when the strength of the pilot momentarily decreases due to a change in the propagation environment. In addition, the RTD slope is for estimating the direction of the mobile terminal in order to solve the problem that the handdown timer cannot solve.

When a mobile terminal connecting a call with a base station to which multiple frequencies are allocated moves to a base station where a frequency allocation offset is generated, a hard handoff between frequencies is performed by the following procedure. Here, a base station to which a mobile terminal connects a call is called a serving base station, and a base station to be moved is called a neighbor base station.

3 is a flowchart illustrating an overall hard handoff method according to the present invention. As shown, the process of measuring and reporting the pilot strength in the mobile terminal (510), the process of measuring the system load (520), the process of calculating the handdown parameter using the system load (530), the measured pilot The strength is compared with the threshold, and if the measured pilot strength is not less than the threshold, the process returns to step 510 (540), if the measured pilot strength is less than the threshold, performing handdown (550) and the inter-cell soft hand A process 560 of performing an off is included.

Hereinafter, with reference to the accompanying drawings will be described in detail the operating principle of the preferred embodiment of the present invention.

In step 510, the mobile station measures the pilot strength of the frequency allocation that can be detected and reports it to the base station controller through a pilot strength measurement message (PSMM) or a power measurement report message (PMRM). The pilot strength measurement message is generated when a change occurs in a set of pilot strengths that can be detected by the current mobile terminal, that is, an active set. In the present invention, the base station controller periodically sends a pilot measurement request order (PMRO) to the mobile terminal in order to periodically receive the pilot strength measurement message in addition to the existing pilot strength measurement message generation condition. The power measurement report message is periodically reported from the mobile terminal to the base station controller.

In step 520, to measure the load on the system, the number of calls currently in a call is calculated. Since the base station controller is currently monitoring the status of the traffic channel assigned to the mobile terminal, it measures the number of active calls and uses it as a handoff parameter.

In step 530, the threshold of pilot strength is calculated using the measured system load. In other words, the threshold value is set high when the system load is high, and the threshold value is set low when the load is light. It also calculates handdown timers and RTD slope values.

In step 540, the strength of the pilot measured in step 510 is compared with the threshold calculated in step 530, and when the threshold is less than or equal to the threshold, the handdown timer is driven. If the pilot strength continues to be below the threshold until the timer ends normally, the system calculates the RTD slope. If the slope does not decrease, the mobile terminal assumes that it is moving away from the serving base station and determines the handdown.

Once the handdown is determined, in step 550, the channel is allocated to the common frequency with the least load with reference to the system load measured in step 520.

In step 560, intercell soft handoff is performed within a common frequency to complete the intercell frequency hard handoff.

4 is a flowchart illustrating a detailed hard handoff method according to the present invention.

As shown, first, it is checked whether the neighboring base station has a different frequency allocation from the serving base station. (602) Otherwise, since there is no need to perform a hard handoff using a common frequency, the process ends.

The pilot strength that can be detected by the mobile station is periodically measured through a power measurement report message and a pilot strength measurement message. (606) When the pilot strength is measured at the base station controller, the system load is measured (608), and various handoffs are performed. Calculate the parameters: pilot threshold and handdown timer and RTD slope.

The pilot strength measured in the process 606 is compared with the calculated pilot threshold, and if the measured pilot strength is larger, the process returns to the process 606. (610) If the measured pilot strength is smaller, the handdown timer is set. If the timer count is not greater than the handdown timer end value, the handdown timer is reset and the process returns to step 606. If the timer count is greater than the handdown timer end value, the RTD slope is equal to the RTD threshold. If the RTD slope is less than the RTD threshold, the handdown timer is reset and the process returns to step 606.

If the slope of the RTD is greater than or equal to the RTD threshold, a common frequency to be handed down is searched for (618). At this time, the common frequency with a low load is found in consideration of the load for each frequency. If a frequency to be handed down is found, a handoff direction message (HDM) is sent to the mobile terminal to indicate a handoff. (620) When the mobile terminal completes a handdown to a common frequency through a handoff procedure, Inter-cell soft handoff is performed within the common frequency to complete the inter-cell hard handoff (622).

The present invention can be variously modified and can take various forms and only the specific embodiments thereof are described in the detailed description of the invention. It is to be understood, however, that the present invention is not limited to the specific forms mentioned in the detailed description of the invention, but rather includes all modifications, equivalents, and substitutions within the spirit and scope of the invention as defined by the appended claims. It should be understood to do.

In the present invention operating as described above, the effects obtained by the representative ones of the disclosed inventions will be briefly described as follows.

According to the present invention, the reliability of hard handoff can be improved by using handoff parameters such as pilot threshold, handdown timer, and RTD slope according to system load.

That is, the pilot threshold is differently applied according to the load of the system to adjust the number of handoff occurrences, reduce the load on the system, and use the handdown timer to eliminate unnecessary handoffs due to the propagation environment. In addition, by using the RTD slope to estimate the direction of the mobile terminal, it is possible to determine the handoff more accurately.

Claims (7)

A mobile terminal which connects a subscriber signal to a plurality of base stations and distinguishes each base station by a pilot signal, at least one service area having the unique pilot signal, and provides a mobile terminal and a mobile communication service; A base station controller for connecting a base station to an exchange and providing a handoff to a mobile terminal, Periodically measuring pilot strength that can be detected by the mobile terminal; If the measured pilot strength is greater than a pilot threshold, returning to measuring the pilot strength; If the measured pilot strength is not greater than a pilot threshold, handing down a frequency channel to which a current call is connected to a common frequency that can be serviced by all base stations; And A method of hard handoff using a common frequency in a cellular system, comprising performing intercell soft handoff within a common frequency. The method of claim 1, wherein the measuring of the pilot strength comprises: The base station controller periodically sending a pilot measurement request command to the mobile terminal; And sending a pilot strength measurement message to a mobile terminal that has received a pilot measurement request command. The method of claim 1, wherein the measuring of the pilot strength comprises: And periodically sending a power measurement report message by the mobile terminal. The method of claim 1, wherein the handing down to the common frequency comprises: Driving a handdown timer if the measured pilot strength is not greater than a pilot threshold; And handing down the frequency channel to the common frequency if the measured pilot strength continues to be below the threshold until the handdown timer normally terminates. The method of claim 1, wherein the handing down to the common frequency comprises: Comparing the RTD slope with an RTD threshold if the measured pilot strength is not greater than a pilot threshold; And handing down the frequency channel to a common frequency if the RTD slope is above the RTD threshold. The method of claim 1, wherein the handdown process comprises: Measuring the load of the system; And handing down the frequency channel to the common frequency with the least load on the system. A mobile terminal which connects a subscriber signal to a plurality of base stations and distinguishes each base station by a pilot signal, at least one service area having the unique pilot signal, and provides a mobile terminal and a mobile communication service; A base station controller for connecting a base station to an exchange and providing a handoff to a mobile terminal, Periodically measuring pilot strength that can be detected by the mobile terminal; Measuring system load; Calculating a pilot threshold, a handdown timer, and an RTD slope using the measured system load; Comparing the measured pilot strength with the calculated pilot threshold and returning to measuring the pilot strength when the measured pilot strength is large; Driving the handdown timer if the measured pilot strength is less than the pilot threshold; Resetting a handdown timer and returning to measuring the pilot strength if the timer count is not greater than the handdown timer; Determining if the RTD slope is greater than the RTD threshold if the timer count is greater than the handdown timer; Resetting a handdown timer and returning to measuring the pilot strength if the RTD slope is less than the RTD threshold; Searching for a common frequency to hand down if the RTD slope is greater than or equal to the RTD threshold; Performing handdown to a common frequency when a common frequency to be handed down is found; And Performing the inter-cell soft handoff within the common frequency; and using the common frequency in a cellular system.