KR20100053000A - Method and system for forwarding data upon handover in wireless communication system - Google Patents

Abstract

PURPOSE: A data forwarding method upon hand over in wireless telecommunication system and a system thereof are provided to reduce waiting time for re-ordering of forwarded data by preventing generation of forwarding delay caused by answer delay from a terminal upon the hand over. CONSTITUTION: A higher layer of a source eNB senses a hand over event to a target eNB(910). The higher layer transmits set response waiting time to PDCP(Packet Data Convergence Protocol) layer(911). If the response waiting time is expired, the PDCP layer forwards a PDCP PDU(Protocol Data Unit) 2 to the target eNB(912). The higher layer forwards the PDCP SDU 3 to the target eNB after processing of a downlink packet(915,916).

Description

Method for forwarding data when handing over in wireless communication system and system for performing the same {Method and System for forwarding data upon handover in wireless communication system}

The present invention relates to a data forwarding method for handover in a wireless communication system and a system for performing the same. In particular, the source base station targets data waiting for a response when the source base station waits for a response while the source base station transmits data to the terminal and waits for a response. A method of forwarding to a base station and a system for performing the same.

UMTS (Universal Mobile Telecommunication Service) system is based on the European mobile communication system Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS), and Wideband Code Division Multiple Access, Third generation asynchronous mobile communication system using WCDMA).

The 3rd Generation Partnership Project (3GPP), which is in charge of UMTS standardization, is discussing Long Term Evolution (LTE) as the next generation mobile communication system of the UMTS system. LTE is a technology that implements high-speed packet-based communication with a transmission speed of up to 300 Mbps, and aims to commercialize it in 2010. To this end, various schemes are discussed. For example, a scheme of reducing the number of nodes located on a communication path by simplifying a network structure, or approaching wireless protocols as close as possible to a wireless channel are under discussion.

In addition, the operation of the handover (Handover) in the LTE system is roughly as follows. The upper layer of the source ENB (Evolved Node B) transmits a handover event progress command to a user equipment (UE) to hand over the UE to the target ENB, and operates an RLC (Radio Link Control) layer to disconnect the UE. Stop and forward the downlink PDCP Service Data Unit (SDU) transmitted through the Packet Data Convergence Protocol (PDCP) layer to the target ENB.

In particular, the PDCP layer should also forward the PDCP Protocol Data Unit (PDU) waiting for a response from the RLC layer in RLC-AM (Acknowledge Mode) to the target ENB. However, since the RLC layer stops operating when a progress command of a handover event is transmitted to the UE, the source ENB does not receive the response even though the UE has successfully received the PDCP PDU, that is, an ACK to the source ENB. Cannot be delivered to the PDCP Layer. Therefore, the PDCP layer waits until the response wait timer expires and transmits the PDCP PDU to the target ENB. Therefore, the delay time is inevitable.

Also, since the PDCP PDU is a packet that must be transmitted before the PDCP SDU, the target ENB has a long waiting time for rearranging the forwarded packets when the forwarding delay occurs due to the response waiting timer. There is this. In addition, as the waiting time increases, the target ENB increases the PDCP SDU storing and waiting for rearrangement, thereby increasing the rearrangement complexity.

An object of the present invention is to propose a method and apparatus for forwarding data waiting for a response from a terminal during handover in a wireless communication system.

In the wireless communication system of the present invention for solving the above problems, the data forwarding method at the time of handover is performed by the source base station transmitting data to the terminal and waiting for a response, when a handover occurs while waiting for the response. Setting a response waiting time of data waiting for a response by the source base station (hereinafter, referred to as “response waiting data”); and when the set response waiting time elapses, the source base station forwards the response waiting data to a target base station. It is characterized by including. The setting of the response waiting time may further include receiving, by the source base station, received signal strength (RSS) information from the terminal and setting the response waiting time in proportion to the received signal strength. Characterized in that.

According to the data forwarding method and apparatus during handover in the wireless communication system of the present invention, the forwarding delay is prevented due to the response delay from the terminal during the handover, thereby reducing the waiting time for rearranging the forwarded data and sequential data. Forwarding can be executed to reduce the rearrangement complexity.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this case, it should be noted that like elements are denoted by like reference numerals as much as possible. Further, the detailed description of well-known functions and constructions that may obscure the gist of the present invention will be omitted.

Also, the terms and words used in the present specification and claims should not be construed to be limited to ordinary or dictionary meanings, and the inventor is not limited to the concept of terms in order to describe his invention in the best way. It should be construed in accordance with the meaning and concept consistent with the technical idea of the present invention based on the principle that it can be properly defined.

In addition, the terms of the embodiment of the present invention will be in accordance with the 3GPP LTE system standard.

1 is a diagram illustrating a schematic structure of a mobile communication system according to an embodiment of the present invention.

Referring to FIG. 1, in a wireless communication system according to an exemplary embodiment of the present invention, an Evolved Radio Access Network (E-RAN) 110 and 112 may be an Evolved Node B (ENB) 120. , 122, 124, 126, and 128 and an Evolved Packet Core (EPC) 130, 132.

The UE (User Equipment) 101 connects to the Internet Protocol (IP) network 114 by the E-RANs 110 and 112. The ENBs 120, 122, 124, 126, and 128 are nodes corresponding to the existing Node Bs and are connected to the user UE 101 by a radio channel. In order to realize a transmission rate of up to 300 Mbps, a wireless communication system uses Orthogonal Frequency Division Multiplexing (OFDM) as a wireless access technology in a 20 MHz bandwidth. In addition, an adaptive modulation & coding (AMC) scheme that determines a modulation scheme and a channel coding rate according to the channel state is applied. Meanwhile, many next generation mobile communication systems including LTE use HARQ (Hybrid Automatic Retransmission Request) as an error correction technique. HARQ is a technique of increasing reception success rate by soft combining with retransmitted data without discarding previously received data.

In more detail, the HARQ receiver determines whether an error exists in the received packet, and then transmits a HARQ positive acknowledgment signal or a HARQ negative acknowledgment signal to the transmitter. The transmitter performs retransmission of the HARQ packet or transmission of a new HARQ packet according to the above-described acknowledgment signals. The HARQ receiver soft-combines the retransmitted packet with the previously received packet to reduce the probability of error occurrence.

In addition, when a handover from the source ENB 120 to the target ENB 122 occurs according to the movement of the UE 101, the source ENB 120 transmits the data waiting for a response to the UE 101 for response response time. After the timer is elapsed, the packet data is forwarded to the target ENB 122 and newly received packet data from the EPC 130 is immediately forwarded to the target ENB 122.

Hereinafter, the E-RAN 110, 112 including the ENBs 120, 122, 124, 126, 128 and the EPCs 130, 132, the base station 200, the UE 101, the mobile terminal 100, and the like. It will be called.

 Next, a hierarchical structure of a radio protocol of a radio communication system according to an embodiment of the present invention will be described. 2 is a diagram illustrating a hierarchy of protocols in a control plane in a wireless communication system according to an exemplary embodiment of the present invention.

2, a radio protocol according to an embodiment of the present invention is a Radio Resource Control (RCC) 201, a Packet Data Convergence Protocol (PDCP) 202, a Radio Link Control (RLC) 203 (203). And a Medium Access Control 204 (MAC) layer, and further includes a physical layer (PHY Layer, PhysicalLayer) 205.

The RRC layer 201 performs measurement, radio bearer setup, and RRC connection control, and the PDCP layer 202 is responsible for IP header compression / restore. The RLC layer 203 reconfigures a PDCP PDU (Packet Data Unit, hereinafter referred to as a PDU of the protocol) to an appropriate size to perform an Automatic Repeat Request (ARQ) operation.

The MAC 204 is connected with several RLC layer devices configured in one terminal. The MAC 204 performs multiplexing of multiple RLC PDUs output from the RLC layer devices into MAC PDUs, and demultiplexes the RLC PDUs from the MAC PDUs. In addition, the physical layer 205 performs channel coding and modulation on higher layer data, forms an OFDM symbol, and transmits it to a wireless channel, or demodulates, channel decodes, and transmits the OFDM symbol received through the wireless channel to a higher layer.

In particular, in the present invention, the PDCP layer 202 performs sequence numbering, header compression, and ciphering to transmit a packet over a radio section. Such an operation is defined in 3GPP standard document TS 36.323.

3 is a diagram illustrating a general processing procedure of downlink data in the PDCP layer.

Referring to FIG. 3, the PDCP layer 303 compresses a sequence number and a header into a PDCP SDU (Service Data Unit, 301, hereinafter, a packet output from a specific protocol layer device) received at a higher layer. In addition, PDCP PDU 302 is created by attaching PDCP Header containing Ciphering related information and delivered to lower layer RLC Layer.

At this time, the transmission operation of the PDCP PDU 302 is classified according to the operation mode of the RLC layer. In more detail, the operation mode of the RLC layer includes an RLC acknowledge mode (RLC-AM) for receiving a response (ACK / NACK) and an RLC unacknowledge mode (RLC-UM) for not receiving a response (ACK / NACK). Separated by. In the case of RLC-AM, the RLC layer informs the PDCP layer that the PDCP PDU 302 is normally transmitted in the physical layer, that is, an ACK to the PDCP layer. At the time when the Discard Timer expires, the PDCP-PDU 302 is discarded and the next PDCP SDU 301 is processed and transmitted to the RLC layer.

Meanwhile, according to the current 3GPP standard document, the response wait time (Discard Timer) is determined at the time when the radio bearer is configured in the upper layer (for example, RRC layer, etc.) and the value is generally determined based on the QoS level. . In other words, the current standard document defines the response waiting time in the upper layer, but this is limited to the general radio bearer setting process, and does not define processing for PDUs waiting for a response from the RLC layer existing in the handover process. have

4 is a flowchart illustrating a general method of forwarding data during handover processing in a base station operating in RLC-AM.

Referring to FIG. 4, in step 401, the source base station recognizes that a handover event should be executed to the target base station. In step 402, the source base station determines whether there is data waiting for a response from the RLC layer.

If it is determined in step 402 that there is no data waiting for a response, that is, if there is no PDCP PDU processed by the PDCP layer and transmitted to the RLC layer, the PDCP SDU is forwarded to the target base station. In addition, if it is determined in step 402 that there is a PDCP PDU waiting for a response, the base station first forwards the PDCP SDU to the target base station in step 404.

If the response wait time has expired, that is, if no ACK is received from the RLC layer before the response wait time has elapsed, the PDCP PDU waiting for the response is forwarded to the target base station in step 405.

5 is a flowchart illustrating transmission and reception of signals between a base station and a terminal when a handover occurs in a wireless communication system.

Referring to FIG. 5, in step 501, the Evolved Packet Core (EPC) transmits downlink packet 1 to the source ENB (Evolved Node B). In addition, the upper layer of the source ENB processes the downlink packet 1 and then transmits the PDCP SDU 1 to the PDCP layer in step 502. Subsequently, the PDCP layer generates a PDCP PDU 1 by attaching a PDCP header including a sequence number, header compression, and ciphering related information to the received PDCP SDU 1, and delivers it to the lower layer RLC layer in step 503. At this time, in step 504, a response wait time proceeds, and if the response ACK is not received from the RLC layer until the response wait time expires, the PDCP layer discards PDCP PDU 1 and transmits the next PDCP SDU to the RLC layer. Done. In step 505, PDCP PDU 1 transmitted to the RLC layer is transmitted to the UE through a wireless channel, and in step 506, the RLC layer transmits an acknowledgment (ACK) signal to the PDCP layer before the response waiting time expires.

Subsequently, the EPC transmits downlink packet 2 to the source ENB in step 507, processes downlink packet 2 in the upper layer of the source ENB, and then transmits PDCP SDU 2 to the PDCP layer in step 508. In addition, the PDCP layer processes the received PDCP SDU 2 to generate PDCP PDU 2, and delivers it to the lower layer RLC layer in step 509. In step 510, the upper layer of the source ENB (eg, the RRC layer) detects a handover event to the target ENB, and in step 511, the PDCP SDU transmitted from the EPC is forwarded to the target ENB to the PDCP layer. . In addition, the upper layer of the source ENB transmits an operation stop command to the RLC layer in step 512. On the other hand, the response waiting time of the PDCP PDU 2 transmitted in step 509 is in progress (step 513).

Even if the response wait time of PDCP PDU 2 is in progress, in step 514 the EPC sends downlink packet 3 to the source ENB, and the upper layer of the source ENB processes downlink packet 3 and then in step 515 PDCP SDU 3 Forwards to the target ENB.

If the PDCP layer does not receive a response to the PDCP PDU 2 from the RLC layer before the response wait time expires, the PDCP layer forwards the PDCP PDU 2 to the target ENB without discarding it in step 516. Therefore, the delay time according to the response waiting time of PDCP PDU 2 occurs from the perspective of the target ENB, and since PDCP SDU 3 is received first and then PDCP PDU 2 is forwarded, it needs to be rearranged to process data.

6 is a flowchart illustrating a method for forwarding in a base station during handover according to an embodiment of the present invention.

Referring to FIG. 6, in step 601, a Handover Triggering Event of the UE occurs, and the source ENB checks whether the PDCP layer operates as an RLC AM in step 602. If it is determined in step 602 that the operation mode is RLC UM, the source ENB forwards only the PDCP SDU to the target ENB in step 603 and discards the PDCP PDU regardless of the response wait time DT.

In addition, when it is determined in operation 602 that the operation mode is RLC AM, in step 604, the response waiting time is set according to the result of the response waiting time calculation function F (x), and when the response waiting time in which the PDCP PDU is set in step 605 expires, Forward to the target ENB. After the PDCP PDU is transmitted, the PDCP SDU is forwarded to the target ENB in step 606. In this case, F (x) can be implemented in various ways using various factors. For example, the signal received from the source eNB by the target UE (Received Signal Strength, RSS), PDCP window size, UE mobility speed, etc. are possible. The present invention proposes a method using RSS and a method using PDCP window size, which will be described later with reference to FIGS. 7 and 8.

7 is a diagram illustrating a relationship between a value of a response wait time and a received signal strength according to an embodiment of the present invention.

Referring to FIG. 7, as an embodiment for the source ENB to set the response wait time DT, the received signal strength (= RSS) received by the target UE at the source eNB is F (x) as shown in Equation 1 below. As a value between 0 and DT _MAX (Maximum Response Wait Time) Use as input parameter x.

Equation 1

i) F (x) = 0 (when RSS <RSS_TH_min = minimum threshold of Received Signal Strength from source eNB)

RSS_TH_max = maximum threshold of Received Signal Strength from source eNB)ii) F (x) = DT _MAX (when RSS

RSS_TH_max = maximum threshold of Received Signal Strength from source eNB)

RSS < RSS_TH_max)iii) F (x) = a * RSS ^b (when RSS_TH_min

RSS <RSS_TH_max)

Here, a is a proportional constant, and b is a fading coefficient taking a value between 0 and 1, and can be changed according to the position where the source ENB is installed and the expected position of the UE.

When the response waiting time DT is set using RSS as described above, if the RSS of the source ENB is lower than RSS_threshold, the PDCP layer of the source ENB sets the response waiting time to zero. Therefore, when a handover event occurs, the PDCP PDU is forwarded to the target ENB. Thus, forwarding delays due to response delays can be prevented, and data rearrangement complexity at the target ENB can be reduced.

8 is a diagram illustrating a relationship between a value of a response wait time and a PDCP data size according to another embodiment of the present invention.

Referring to FIG. 8, if the PDCP layer processes a plurality of PDCP PDUs in parallel, it is assumed that the PDCP layer in the RLC AM knows the data size M of the PDCP PDU currently being processed. As shown in Equation 2 below, when the data size M is larger than the data size M_th of the maximum PDCP PDU which is a reference value, the DT of the packets existing in the window being processed is set to 0, and when the data size M_th is smaller than the current DT, Keep it.

Equation 2

F (x) = 0 (M> M_th)

F (x) = Preset DT value (M <M_th)

If the size M of the PDCP PDU being processed in the window of the PDCP layer is larger than the maximum MCP of the maximum PDCP PDU, the RLC AM is unlikely to receive all the ACKs for the PDCP-PDU. Therefore, PDCP PDUs that do not receive an ACK need to be forwarded to the target ENB. In this case, if the PDCP PDU is immediately forwarded by setting DT to 0 as shown in FIG. 8, the forwarding delay due to the response delay can be prevented and the data rearrangement complexity in the target ENB can be reduced.

9 is a flowchart illustrating transmission and reception of signals between a base station and a terminal when a handover occurs in a wireless communication system according to an exemplary embodiment of the present invention.

Referring to FIG. 9, since the process from step 901 to step 909 is performed by the same process as described in FIG. 5, a detailed description thereof will be omitted.

In step 910, a higher layer (eg, an RRC layer) of the source ENB detects a handover event to the target ENB, and in step 911, a response waiting time is set and transmitted to the PDCP layer using the response waiting time calculation method described above. In addition, if the set response waiting time expires, the PDCP layer forwards to the target ENB without discarding PDCP PDU 2 in step 912.

Subsequently, in step 913, the PDCP SDU transmitted from the EPC is forwarded to the target ENB to the PDCP layer. In addition, the upper layer of the source ENB transmits an operation stop command to the RLC layer in step 914.

Subsequently, in step 915, the EPC transmits downlink packet 3 to the source ENB, and the upper layer of the source ENB processes downlink packet 3, and then forwards PDCP SDU 3 to the target ENB in step 515. Therefore, the forwarding delay of PDCP PDU 2 does not occur from the target ENB, and since PDCP SDU 3 is forwarded after PDCP PDU 2 is received first, delay and complexity due to rearrangement are also reduced.

On the other hand, embodiments of the present invention disclosed in the specification and drawings are merely presented specific examples to easily explain the technical contents of the present invention and help the understanding of the present invention, and are not intended to limit the scope of the present invention. It will be apparent to those skilled in the art that other modifications based on the technical idea of the present invention can be carried out in addition to the embodiments disclosed herein.

1 illustrates a schematic structure of a mobile communication system according to an embodiment of the present invention.

2 is a diagram illustrating a hierarchy of protocols in a control plane in a wireless communication system according to an embodiment of the present invention.

3 is a diagram illustrating general processing of downlink data in a PDCP layer.

4 is a flow chart illustrating a general method of forwarding data during handover processing in a base station operating with RLC-AM.

5 is a flowchart illustrating transmission and reception of signals between a base station and a terminal when a handover occurs in a wireless communication system.

6 is a flowchart illustrating a method of forwarding at a base station during handover according to an embodiment of the present invention.

7 is a diagram showing a relationship between a value of a response wait time and a received signal strength according to an embodiment of the present invention.

8 illustrates a relationship between a value of a response wait time and a PDCP data size according to another embodiment of the present invention.

9 is a flowchart illustrating transmission and reception of signals between a base station and a terminal when a handover occurs in a wireless communication system according to an embodiment of the present invention.

Claims (12)

A data forwarding method in handover in a wireless communication system, The source base station transmitting data to the terminal and waiting for a response; Setting a response waiting time of data waiting for a response by the source base station (hereinafter, 'response waiting data') when a handover occurs while waiting for the response; And forwarding, by the source base station, the response waiting data to a target base station when the set response waiting time elapses. The method of claim 1, wherein setting the response waiting time Receiving, by the source base station, received signal strength (RSS) information from the terminal; And setting the response waiting time in proportion to the received signal strength. The method of claim 2, wherein setting the response waiting time When the received signal strength is less than the minimum signal strength, setting the response waiting time to zero. The method of claim 2, wherein setting the response waiting time If the received signal strength exceeds the maximum signal strength, maintaining a predetermined response waiting time; data forwarding method in the handover in a wireless communication system further comprising. The method of claim 1, wherein setting the response waiting time And setting the response waiting time to 0 when the size of the response waiting data is greater than or equal to a data size limit. The method of claim 5, wherein setting the response waiting time If the size of the response waiting data is less than the data size limit, maintaining a predetermined response waiting time; data forwarding method in the handover in a wireless communication system further comprising. If a handover occurs after transmitting data to the terminal and before receiving a response, the response waiting time of the data waiting for a response (hereinafter referred to as 'response waiting data') is set. A source base station for forwarding standby data to a target base station; And a target base station receiving the response waiting data from the source base station and transmitting the received response data to the terminal. 8. The method of claim 7, wherein the source base station is And receiving the received signal strength information from a terminal and setting the response waiting time in proportion to the received signal strength. The method of claim 8, wherein the source base station is And when the received signal strength is less than the minimum signal strength, set the response waiting time to zero. The method of claim 8, wherein the source base station is And when the received signal strength exceeds a maximum signal strength, maintaining a preset response waiting time. 8. The method of claim 7, wherein the source base station is And setting the response waiting time to zero when the size of the response waiting data is equal to or larger than a data size limit. The method of claim 11, wherein the source base station is And when the size of the response waiting data is less than the data size limit, maintaining the response waiting time.

Images