KR101373381B1 - Apparatus and method for Decreasing Hand over interruption Time In a Communication System - Google Patents

Abstract

The present invention provides a method of reducing handover delay time by a terminal in a communication system. When an error occurs in a packet initially received from a target base station which performs the handover immediately after performing the handover, an error occurs in the packet. Transmitting a signal informing to the target base station, and receiving the packet from the target base station within a number of retransmissions limited to a predetermined value by the target base station. The modulation and coding scheme (MCS) level determined by considering the number of retransmissions by the target base station is applied.

Intra system handover interruption time, HARQ retransmission limit, MCS level, additional frequency assignment HARQ operation time.

Description

Apparatus and method for Decreasing Hand over interruption Time In a Communication System}

1 shows a handover in an intra system according to the prior art.

FIG. 2 is a diagram illustrating a method of pre-allocating resources allocated in units of time as frequency resources according to a second embodiment of the present invention.

3 is a block diagram of a base station according to a preferred embodiment of the present invention.

4 is a view showing the position between the terminal and the base station according to an embodiment of the present invention.

5 is a flowchart illustrating an operation between a terminal and a target base station of FIG. 4 according to an exemplary embodiment of the present invention.

6 is a diagram showing a performance result according to an embodiment of the present invention.

7 is a diagram illustrating an average throughput of receiving a packet by a terminal performing a handover according to an embodiment of the present invention.

The present invention relates to a method and apparatus for reducing handover delay in a communication system, and more particularly, to a method and apparatus for reducing packet transmission delay in performing an intra-system handover.

Various wireless communication services that provide voice, data, audio, video and real-time video are evolving rapidly. Next generation wireless communication systems will provide various services through a device in a ubiquitous manner.

At this time, a mobile station receiving delay-sensitive real-time services such as voice over IP and video streaming performs intra system handover without interrupting the service. To do this, the packet transmission delay fluctuation caused by the handover must be less than a certain time.

1 illustrates a handover in an intra system according to the prior art.

Referring to FIG. 1, a serving base station 100 and a target base station 110 exist in the same system, and the terminal 105 is in the state where the serving base station 100 uses a real-time service. At 110, a handover is attempted.

In step 115, the serving base station 100 transmits the last Nth packet for the real-time service to the terminal 105 before handover. In step 120, the terminal 105 disconnects from the serving base station 100 and is handed over to the target base station 110. When the handover with the terminal 105 is completed, the target base station 110 transmits the N + 1 th packet, which is the first packet for the corresponding service, to the terminal in step 125.

That is, as in steps 115 through 125, the N + 1 th packet transmitted by the target base station 110 for the first time from the time when the terminal 105 receives the N th packet transmitted by the serving base station 100 lastly. The delay time until receiving is the handover interruption time.

In order to guarantee the quality of service (QoS) of the real-time service, the handover interruption time is required to be less than a specific time, and the handover interruption time is 50ms even to stably obtain Robust Transmission Control Protocol (TCP) performance. It should be In addition, in order to guarantee QoS of a highly interactive service such as real-time collaboration and real time gambling, an interruption time of handover of a terminal should be 20 ms or less.

As a result, the interruption time generated when the UE performs the handover must satisfy the delay constraint for each service to satisfy the QoS of the corresponding service.

In the physical layer, a hybrid automatic repeat request (HARQ) technique that performs fast retransmission to reduce link adaptation error and improve throughput performance is used. First, a Modulation and Coding Scheme (MCS) level is selected within a range satisfying an arbitrary frame error rate (FER) (e.g., 10%) using CQI (Channel Quality Information) information indicating a current channel state. In addition, when a NACK is generated for the transmitted packet, the same packet or additional redundant information is transmitted by the maximum number of HARQ retransmissions that can occur at a predetermined maximum in the system. UE performs soft combining to obtain a signal to interference and noise ratio (SINR) by using the first transmitted packet and the retransmitted packets from the base station, and thus the decoding success probability of the corresponding packet. Increase

In the HARQ scheme based on chase combining, the base station repeatedly transmits the same packet as the packet initially transmitted to the terminal. The terminal obtains a gain of SINR by adding the energy of the packets transmitted every time, thereby increasing the probability of decoding by using a soft combined packet.

In another technique, an incremental redundancy-based HARQ technique, when a NACK occurs due to a decoding failure of a previously transmitted packet, the base station sends additional redundant information as a packet to be retransmitted to the terminal. The terminal adjusts a coding rate and SINR until the decoding of the corresponding data is successful using the redundant information. In the first packet transmission, transmission is performed at the highest coding rate, and additional redundant bits are transmitted whenever retransmission is needed.

)은 하기 <수학식 1>과 같이 표현된다.The handover interruption time according to the prior art

) Is expressed by Equation 1 below.

는 서빙 기지국과 타겟 기지국 간에 패킷을 포워딩(forwarding)하는 시간이고, 상기

은 레이어1 파라미터들을 타겟 기지국으로부터 받아서 동기화(synchronize)를 수행하고 PDU들을 주고 받을 수 있는데 (is ready to transmit) 걸리는 시간이다. 상기

시간 동안 단말은 기지국으로부터 CQI정보를 전송할 수 있는 채널(CQICH)을 할당 받는다. 상기

은 타겟 기지국이 단말로부터 수신한CQI정보를 이용하여 해당 단말을 스케쥴링하는데 걸리는 시간이다. 상기

는 단말이 타겟 기지국으로의 핸드오버 직후에 전송되는 패킷의 HARQ 오퍼레이션 시간이다. Here,

Is a time for forwarding a packet between the serving base station and the target base station,

Is the time taken to receive layer 1 parameters from the target base station to perform synchronization and to send and receive PDUs. remind

During the time, the UE is allocated a channel (CQICH) capable of transmitting CQI information from the base station. remind

Is the time taken by the target base station to schedule the terminal using the CQI information received from the terminal. remind

Is a HARQ operation time of a packet transmitted immediately after a handover to a target base station.

That is, the handover interruption time, after the L1 switching (switching) is finished, the UE transmits the CQI information to the target base station through the CQICH. Thereafter, the target base station takes a scheduling operation of selecting a terminal to transmit data in a current transmission time interval (TTI) based on the CQI information transmitted by the terminals, and then takes a packet to transmit a packet using HARQ to the selected terminal. It's time.

In the handover formed in the cell edge region, HARQ processing time is long because channel quality is not good. Therefore, if the HARQ processing time of the packet transmitted from the target base station is extended immediately after the handover is performed, the handover interruption time becomes long, and thus a delay constraint for each service is not satisfied.

Accordingly, an object of the present invention, which is designed to solve the problems of the prior art operating as described above, is to provide a QoS improvement by satisfying a delay constraint for each service by reducing an interruption time occurring when performing a handover. And to provide an apparatus.

Another object of the present invention is to provide a method and apparatus for limiting the number of HARQ retransmissions for packet transmission and satisfying a delay constraint for each service by allocating an additional frequency in advance in an environment exceeding the number of HARQ retransmissions. It is.
In a communication system according to an embodiment of the present invention, a method for reducing a handover delay time by a terminal may include an error in the packet if an error occurs in a packet initially received from a target base station which performed the handover immediately after performing the handover. Transmitting a signal indicating that a signal has occurred to the target base station, and receiving the packet from the target base station within a number of retransmissions limited to a predetermined value by the target base station, and receiving within the number of retransmissions. The packet is characterized in that the modulation and coding scheme (MCS) level determined by considering the number of retransmissions by the target base station is applied.
In a communication system according to an embodiment of the present invention, a terminal for reducing a handover delay time recognizes that an error occurs in a packet initially received from a transceiver and a target base station that performs the handover immediately after performing the handover. In this case, the transmission and reception unit is controlled to transmit a signal indicating that an error has occurred in the packet to the target base station, and the transmission and reception is performed so as to receive the packet from the target base station within a number of retransmissions limited to a predetermined value by the target base station. And a control unit for controlling a unit, wherein a packet received within the number of retransmissions is applied with a modulation and coding scheme (MCS) level determined by the target base station in consideration of the number of retransmissions.

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According to another embodiment of the present invention, a method for reducing a handover delay time by a base station in a communication system,

Limiting the number of times of retransmission of the packet to a predetermined value when receiving a signal indicating that an error has occurred in the first packet received immediately after the handover occurs;

Determining a Modulation and Coding Scheme (MCS) level of the packet in consideration of the limited number of retransmissions.

According to another aspect of the present invention, a method for reducing a handover delay time in a communication system,

Selecting terminals to transmit data in the current transmission interval based on channel quality information received from the terminals;

And retransmitting packets transmitted through the terminal immediately after completion of the handover among the selected terminals based on a predetermined number of retransmissions.

Apparatus according to another embodiment of the present invention, in the base station for reducing the handover delay time in the communication system,

A scheduling unit for selecting terminals to transmit data in a current transmission interval based on channel quality information received from the terminals;

And a retransmitter configured to retransmit packets transmitted through the terminal immediately after the handover completion among the selected terminals based on a predetermined number of retransmissions.

The operation principle of the preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. Like reference numerals refer to the same elements as shown in the drawings, even though they may be shown on different drawings, and detailed descriptions of related well-known functions or configurations are not required in the following description of the present invention. If it is determined that it can be blurred, the detailed description thereof will be omitted. The following terms are defined in consideration of the functions of the present invention, and may be changed according to the intentions or customs of the user, the operator, and the like. Therefore, the definition should be based on the contents throughout this specification.

Hereinafter, the present invention limits the number of HARQ retransmission of the packet received from the target base station immediately after the handover, in order to reduce the handover interruption time occurring in the intra system, thereby satisfying the delay constraints for each service and Suggest a device.

)은 상기 기지국간 데이터 포워딩 지연 시간(

)보다 더 길다. 또한, 상기 스케쥴링 지연 시간(

)은 단말이 타겟 기지국으로 CQI 정보를 전송하는 시간과, 타겟 기지국이 현재 TTI에서 데이터를 전송할 단말을 선택하는 시간으로 나뉠 수 있다. 이때, 상기 CQI 정보를 전송하는 시간은 대략 3 TTI가 소요된다. 그러나, 본 발명은 핸드오버를 수행할 단말에게 가장 높은 스케쥴링 우선순위를 부여하여, 데이터를 전송할 단말들을 선택하는 데 걸리는 시간은 없다고 본다. 결국, 상기 핸드오버 인터럽션 타임은 L1 스위칭 시간과 핸드오버 직후에 전송되는 패킷의 HARQ 오퍼레이션 시간에 의해서 결정된다. As shown in Equation 1, the L1 switching delay time among the configuration times of the handover interruption time

) Is the data forwarding delay time between the base stations

Longer than) In addition, the scheduling delay time (

) May be divided into time at which the terminal transmits CQI information to the target base station and time at which the target base station selects a terminal to transmit data in the current TTI. At this time, it takes approximately 3 TTI to transmit the CQI information. However, the present invention considers that it takes no time to select terminals to transmit data by giving the highest scheduling priority to the terminal to perform the handover. As a result, the handover interruption time is determined by the L1 switching time and the HARQ operation time of the packet transmitted immediately after the handover.

Hereinafter, in the first embodiment of the present invention, to reduce the HARQ operation time, the number of HARQ retransmissions is limited, and the target base station assigns scheduling priority to the terminal to perform handover. In particular, when terminals to perform handover are provided with a real-time service, a priority scheduling priority is given. Next, when the scheduling of the terminals using the real-time service is finished, among the terminals performing the handover, the scheduling of the terminal receiving the TCP service is performed. Thereafter, after the terminal performs the handover, the number of HARQ retransmissions of the packet transmitted to the terminal immediately after the connection with the target base station is limited.

If more retransmissions are required than the number of retransmissions defined in the present invention, the second embodiment of the present invention additionally allocates frequency resources at this point. Because, even after the connection with the target base station, the terminal is still in the handover area and the channel state is not good, HARQ processing time may be long. Therefore, by additionally allocating frequency resources in advance in the current TTI, the terminal can successfully decode the packet within a limited number of HARQ interlaces.

First, according to the first embodiment of the present invention, a technique for determining the number of HARQ interlaces for each service is represented by Equation 2 below.

는 1번의 HARQ를 수행하는데 걸리는 시간이고, 상기

는 각 서비스 타입별 딜레이 바운드(delay bound)를 만족하기 위한 핸드오버 인터럽션 타임 요청 시간이다.

가 1보다 작을 경우에는 1로 설정하여 HARQ 오퍼레이션을 수행한다.here,

Is the time taken to perform one HARQ, and

Is a handover interruption time request time for satisfying a delay bound for each service type.

If is less than 1, it is set to 1 to perform HARQ operation.

That is, the number of HARQ retransmissions occurring in the packet transmitted immediately after the handover is determined to satisfy the delay constraint for each service, and the MCS level is determined so that the packet can be successfully decoded within the predetermined number of retransmissions. The MCS level determination algorithm is used for a packet transmitted immediately after performing a handover. In addition, it is assumed that the channel state is constant during HARQ operation due to the low moving speed of the terminal. The MCS level determination algorithm is applied to a HARQ technique based on chase combining.

을 나타내는 식이다. The MCS level determination algorithm determines an MCS level for real time services using Equation 3 below, and determines an MCS level for a TCP service using Equation 4 below. Equation 3 below is the determined MCS level of the real-time service

Is an expression representing.

는 단말이 핸드오버 직후, 실시간 서비스를 제공하는 타겟 기지국으로부터 처음 수신한 패킷 데이터에 대해서 미리 정해진 HARQ 재전송을 포함한 총 전송횟수이다. 상기

는 SINR 이

값을 가질 때 MCS레벨(i) 에서의 에러발생 확률을 나타내고, q는 상기

의 지시자로,

는 q번째 재전송에서의 결합된 SINR 값이다. here,

Is a total number of transmissions including a predetermined HARQ retransmission for packet data initially received from a target base station providing a real-time service immediately after handover. remind

SINR is

Error value at the MCS level (i) when the value has

As an indicator of,

Is the combined SINR value in the q th retransmission.

가 <수학식 3>와 같이 계산되어 미리 정해지는 임계값

를 만족할 경우, 상기 실시간 서비스의 MCS레벨은

로 결정된다. 이때, 상기 결정되는MCS 레벨이 다수일 경우, 처리량을 최대화할 수 있는 최대 MCS 레벨값으로 결정한다. remind

Is a predetermined threshold value calculated as in Equation 3

If satisfied, the MCS level of the real-time service is

. At this time, if the determined MCS level is a plurality, it is determined as the maximum MCS level value that can maximize the throughput.

Equation 4 is an expression representing the determined MCS level of the TCP service.

을 나타내는 식이다.Equation 4 below is the determined MCS level of the TCP service.

Is an expression representing.

는 단말이 핸드오버 직후, TCP서비스를 제공하는 타겟 기지국으로부터 처음 수신한 패킷 데이터에 대해서 미리 정해진 HARQ 재전송을 포함한 총 전송 횟수이다.상기

는 SINR 이

값을 가질 때 MCS레벨(i)에서 에러발생 확률을 나타내고, q는 상기

의 지시자로,

는 q번째 재전송에서의 결합된 SINR 값이다. Here,

Is a total number of transmissions including a predetermined HARQ retransmission for the packet data first received from the target base station providing the TCP service immediately after the handover.

SINR is

Error value at the MCS level (i) when the

As an indicator of

Is the combined SINR value in the q th retransmission.

가 <수학식 4>와 같이 계산되어 미리 정해지는 임계값

을 만족할 경우, 상기 TCP 서비스의 MCS레벨은

로 결정된다. 이때, 상기 결정되는MCS 레벨이 다수일 경우, 처리량을 최대화 할 수 있는 최대 MCS 레벨 값으로 결정한다. remind

Is a predetermined threshold value calculated as

If the following is satisfied, the MCS level of the TCP service is

. At this time, if the determined MCS level is a plurality, it is determined as the maximum MCS level value that can maximize the throughput.

)보다 더 많은 재전송이 필요할 경우, 현재 전송된 CQI 정보를 이용하여 각 서비스 별로 정해진 HARQ 인터레이스 횟수를 만족하는 MCS 레벨을 결정할 수 없는 경우로 해석할 수 있다. However, due to poor channel conditions, the number of HARQ interlaces limited for each service (

If more retransmission is needed, it may be interpreted as a case in which the MCS level that satisfies the number of HARQ interlaces determined for each service cannot be determined using the currently transmitted CQI information.

Therefore, in the second embodiment of the present invention, after obtaining the SINR gain through the allocation of additional frequency resources at this point, the MCS level is determined again.

FIG. 2 is a diagram illustrating a method of pre-allocating resources allocated in units of time as frequency resources according to a second embodiment of the present invention.

Referring to FIG. 2, by further allocating a frequency resource in advance in the current TTI 205, a frequency twice that of the frequency allocated in the TTI 200 corresponding to the current TTI 205 is allocated in the existing technology.

By allocating the frequency resource in advance as shown in FIG. 2, the packet transmission between the terminal and the target base station can be successfully decoded within a predetermined number of retransmissions.

 Orthogonal Frequency Division Multiplex (OFDM) -based systems operate in a diversity mode during packet transmission in order to minimize inter-cell interference in adjacent cell edge regions. In this case, an additional gain of SINR can be obtained by additionally allocating a sub-channel in frequency units to a terminal to perform handover. That is, in the case of using the diversity mode when transmitting a packet of the UE existing in the handover region, the MCS level to be applied and the number of additional subchannels to be additionally allocated to each service are represented by the following Equations 5 and 6 By using> respectively.

, 는 실시간 서비스와 TCP 서비스 각각에 대해서 추가적으로 할당된 자원 수로 즉, 호핑할 시퀀스 수로 <수학식 3> 또는 <수학식 4>의 SINR의 이득인

보다

배로 통해서 SINR 이득(

)이 증가된다.here,

Is the number of additional resources allocated for each of the real-time service and the TCP service, that is, the number of sequences to be hopped.

see

SINR gain through ship (

) Is increased.

3 is a block diagram of a base station according to an embodiment of the present invention.

Referring to FIG. 3, the base station 300 includes a controller 302, a radio frequency (RF) processor 304, a CQI information processor 306, a scheduling performer 308, and a movement speed estimator 310. ), And an MCS level and subchannel allocation determiner 312 and a HARQ performer 314.

The control unit 302 is responsible for the overall control of the configuration of the base station 300, and in particular, the RF processing unit 304, the CQI information processing unit 306, the scheduling unit 308, and the moving speed estimate By controlling the government 310, the MCS level and the subchannel allocation determination unit 312 and the HARQ execution unit 314, the HARQ performance is controlled for the terminal to perform handover while moving at a low movement speed.

The RF processor 304 receives a plurality of pieces of information including CQI information transmitted from terminals through an air interface and transmits the plurality of pieces of information to the controller 302.

The CQI information processing unit 306 receives the CQI information transmitted by the corresponding terminal through the control unit 302 and recognizes or detects a channel state of the current TTI.

The scheduling unit 308 acquires channel state information and use service information of the terminals through the CQI information processing unit 306 or the control unit 302 and gives priority to selection authority, thereby transmitting data in the current TTI. Select them.

The movement speed estimator 310 estimates the movement speed of the terminal based on the signal strength value detected by the controller 302 for the base station currently being serviced by the terminal.

The MCS level and subchannel allocation determiner 312 recognizes the moving speeds of the terminals performing the handover through the control unit 302, and for each terminal performing the handover while moving at a lower moving speed, the corresponding terminal. The number of HARQ retransmissions determined based on the service type is determined using Equation 2, and based on the CQI information, the MCS level and the allocation unit using Equations 5 to 6 are assigned. Determine the number of channels.

The HARQ performing unit 314 may HARQ the packet of each terminal based on the MCS level determined through the control unit 302 or the MCS level and the subchannel allocation determining unit 312, the allocated subchannel and the predetermined number of HARQ retransmissions. Perform the action.

4 is a view showing the position between the terminal and the base station according to an embodiment of the present invention.

Referring to FIG. 4, the terminal 400 has a low mobility of 3 km / h or less and is currently located between the cell 1 410 and the cell 2 420. The terminal 400 attempts to perform a handover to the target base station 425 located in the neighboring cell while receiving a service from the serving base station 415 located in the cell 1 410.

5 is a flowchart illustrating an operation between a terminal and a target base station of FIG. 4 according to an exemplary embodiment of the present invention.

In step 430, when the terminal 400 performs L1 switching through synchronization with the target base station 425, a CQI channel for CQI transmission is allocated.

In step 435, the terminal 400 measures the current channel state using the allocated CQI transmission channel and transmits the current channel state to the target base station 420 as CQI information.

In step 440, the target base station 420 selects terminals to be transmitted in the current TTI through a scheduling scheme used in the system. Here, the terminal 400 determines the scheduling priority by the service provided by the terminal 400. In the present invention, the terminal using the real-time service is given the highest priority when performing the handover. Thereafter, priority is given to a terminal that performs a handover among users who receive the TCP service.

In step 445, the target base station 425 receives CQI information and the type of service used by the terminal 400 from the terminal 400 performing the selected handover. Here, the target base station 425 can know the type of service used by the terminal through packet forwarding using backhaul communication with the existing serving base station. Thereafter, the target base station 425 calculates Equation 2 according to the type of service. Then, the target base station 425 using the CQI information using the <Equation 3> to <Equation 4> to <Equation 5> to <Equation 6>, HARQ operation by a predetermined number of HARQ retransmission To proceed, the MCS level and the number of subchannels allocated by the current TTI are determined.

In step 450, the target base station 425 stores the packet to be transmitted to the sub-channels allocated to the determined MCS level and transmits it to the terminal 400. In step 455, the terminal 400 decodes the HARQ processing, that is, the first packet using the HARQ scheme based on chase combining with the first packet. If decoding of the first packet fails, the terminal 400 transmits a NACK message to the target base station 420.

In step 465, the target base station retransmits the first packet transmitted in step 450 to the terminal 400 based on chase combining.

In step 470, if the terminal 400 successfully decodes a packet obtained through soft combining of the first packet and the retransmitted packet, in step 475, the terminal 400 sends an ACK message to the target base station 425. Send. Here, the case where one HARQ process is set for the first packet data transmitted by the terminal 400 after the handover is completed is described as an example. However, various HARQ processes may be set according to a channel environment or a service type of the corresponding terminal. .

6 is a diagram illustrating a performance result according to an exemplary embodiment of the present invention.

Here, CQI information in the current TTI can be used, and the total including the HARQ retransmission performed by the MCS level determined by each scheme for packets transmitted immediately after each handover occurs under the assumption that the CQI feedback delay is zero. The number of transmissions, the number of subchannels allocated to the packet, and the throughput for each packet are shown.

Referring to FIG. 6, the number of HARQ retransmissions that are actually performed by obtaining a high SINR gain by a subchannel additionally allocated by the proposed scheme is small. Thus, it can be seen that a relatively high throughput is obtained compared to other techniques.

7 is a diagram illustrating an average throughput of receiving a packet by a terminal performing a handover according to an exemplary embodiment of the present invention.

Referring to FIG. 7, it is an average throughput received by a UE performing a handover during a simulation time according to each simulation performed under the assumption that the CQI information in the current TTI, that is, the CQI feedback delay is zero. That is, the scheme according to the present invention reduces the handover interruption time by allocating an additional subchannel to the packet transmitted immediately after performing the handover. However, this additional allocation may cause another terminal not to be allocated a subchannel. That is, it is understood that there is almost no difference when compared with the throughput of the terminal that is not allocated the subchannel at the time when the packet is transmitted from the target base station immediately after every handover. In other words, even if the resource is not allocated at the time when the packet is transmitted immediately after the handover occurs, the number of retransmissions actually generated by the proposed scheme is small. It will not be done.

While the present invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments, but is capable of various modifications within the scope of the invention. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be determined by the scope of the appended claims, and equivalents thereof.

The present invention has an effect of reducing the handover interruption time by reducing the HARQ operation time generated in the packet transmitted immediately after the handover to the target base station. In addition, the present invention can satisfy the delay constraint of each service by allocating the MCS level and the number of subchannels in consideration of the HARQ interlace number determined for each service as well as the CQI information of the packet transmitted from the target base station after performing the handover. Can be.

Claims (15)

A method for reducing handover delay time by a terminal in a communication system, Immediately after the handover, when an error occurs in a packet initially received from the target base station that performed the handover, transmitting a signal indicating that an error has occurred in the packet to the target base station; Receiving the packet from the target base station within a number of retransmissions limited to a predetermined value by the target base station; And a packet received within the number of retransmissions is subjected to a Modulation and Coding Scheme (MCS) level determined by the target base station in consideration of the number of retransmissions. The method of claim 1, wherein the number of retransmissions, The method for reducing the handover delay time of the terminal, characterized in that determined in consideration of the characteristics of the service used by the terminal. What is claimed is: 1. A method of reducing a handover delay time by a base station in a communication system, Limiting the number of times of retransmission of the packet to a predetermined value when receiving a signal indicating that an error has occurred in the first packet received immediately after the handover occurs; Determining a Modulation and Coding Scheme (MCS) level of the packet in consideration of the limited number of retransmissions. The method of claim 3, wherein the number of retransmissions, The method for reducing the handover delay time of the base station, characterized in that determined in consideration of the characteristics of the service used by the terminal. The method of claim 3, wherein the determining of the MCS level comprises: Receiving channel quality information from the terminal and additionally allocating subchannels when the channel quality information is less than a threshold value; And determining the MCS level in consideration of the subchannels. A method for reducing handover delay time of a base station in a communication system, Selecting terminals to transmit data in the current transmission interval based on channel quality information received from the terminals; And performing retransmission of packets transmitted through the terminal immediately after completion of the handover among the selected terminals based on a predetermined number of retransmissions. The method of claim 6, wherein the selecting process comprises: A method for reducing handover delay time of a base station, wherein the service information used by the terminal is considered. The method according to claim 6, Estimating a moving speed of a corresponding terminal based on a signal strength value detected by a serving base station of each of the selected terminals; Considering the estimated movement speed, a Modulation and Coding Scheme (MCS) level for determining a predetermined number of retransmissions based on a service type of a corresponding terminal and a subchannel to be allocated to the terminal which has performed handover among the selected terminals. The method for reducing the handover delay time of the base station further comprising the step of determining the number. 9. The method of claim 8, And retransmitting a packet of a corresponding terminal based on the determined MCS level, an allocated subchannel, and a predetermined number of retransmissions. A base station for reducing handover delay time in a communication system, A scheduling unit for selecting terminals to transmit data in a current transmission interval based on channel quality information received from the terminals; And a retransmitter configured to retransmit packets transmitted through the terminal immediately after completion of the handover among the selected terminals based on a predetermined number of retransmissions. The method of claim 10, wherein the scheduling unit, The base station, characterized in that for selecting the terminals to transmit data in the current transmission interval in consideration of the service information that the corresponding terminal of the selected terminals. The method of claim 10, A moving speed estimator for estimating a moving speed of the corresponding terminal based on values of signal strengths detected by the serving base stations of the selected terminals; The MCS (modulation and coding scheme) level for determining the number of retransmissions determined based on the service type of the corresponding terminal and the subchannel to be allocated to the terminal which performed the handover among the selected terminals in consideration of the estimated movement speed The base station further comprises a MCS level and subchannel allocation determining unit for determining the number. 13. The method of claim 12, And a retransmitter configured to retransmit the packet of the terminal based on the determined MCS level, the allocated subchannel, and a predetermined number of retransmissions. A terminal for reducing a handover delay time in a communication system, A transceiver unit, Immediately after performing the handover, if it is recognized that an error occurs in the first packet received from the target base station which performed the handover, the transceiver is controlled to transmit a signal indicating that an error has occurred in the packet. And a control unit controlling the transceiver to receive the packet from the target base station within a number of retransmissions limited by the target base station to a predetermined value. And a packet received within the number of retransmissions is applied with a modulation and coding scheme (MCS) level determined by the target base station in consideration of the number of retransmissions. 15. The method of claim 14, The number of retransmissions, Terminal determined according to the characteristics of the service used by the terminal.

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