Method and apparatus for splitting data in multi-connectivity

Abstract

A method for multi-connectivity between a plurality of base stations (10, 10', 10a, 10b, 20, 20', 20a, 20b) and user equipment (30, 30', 30", 30a, 30b, 100) through a plurality of channels, the method comprising: estimating a first round trip time (RTT) at the user equipment (30, 30', 30", 30a, 30b, 100), the first RTT being an expected time required to transmit data and receive an acknowledgment response (ACK) through a first channel; estimating a second RTT at the user equipment (30, 30', 30", 30a, 30b, 100), wherein the second RTT is an expected time required to transmit data and receive an ACK through a second channel; and determining at the user equipment (30, 30', 30", 30a, 30b, 100) a size of uplink data to be transmitted through the first channel based on the first RTT and the second RTT, where estimating the first RTT has the following: obtaining re-transmission parameters corresponding to the first channel; and obtaining the first RTT based on the re-transmission parameters, where: the retransmission parameters have a retransmission period c1 and a maximum retransmission number N1, and obtaining the first RTT obtaining an RTT S1 which is expressed in an equation below: RTTS 1 = ( 1 − BLER 1 ) ∗ ∑ n = 0 N 1 − 1 BLER 1 n ( p 1 + n ∗ c 1 ) , where BLER1 and p1 are a block error rate and a signal movement time in the first channel, respectively.

Description

BACKGROUND

[0001] The inventive concept relates to wireless communication and more particularly to a method and apparatus for splitting data in multi-connectivity.

[0002] In wireless communication between a user equipment and a base station, various techniques can be used to transmit a larger amount of data at a higher speed. For example, multi-connectivity refers to a process in which a user equipment communicates with two or more base stations. In multi-connectivity, data can be transmitted and / or received over a plurality of channels between a user equipment and two or more base stations, and thus, data throughput can be increased and communication quality can be prevented from degrading due to a poor-quality channel. To increase the efficiency of multi-connectivity, it is desirable to efficiently distribute data transmission among a plurality of channels.

[0003] WO 2017 / 140361 A1 relates to uplink selection for a wireless network based on the weighting of the network cells and the specific weighting of the wireless links.

[0004] US 2011 / 0113299 A1 relates to signaling in wireless communication systems, particularly in OFDMA and CDMA.

[0005] US 2015 / 0189551 A1 relates generally to communication systems and, in particular, to techniques for dynamically sharing carriers between different radio access technologies (RATs).

[0006] US 2017 / 0188248 A1 relates to methods, a first network device and a second network device, in particular to the evaluation of a connection between a first network node and a user terminal via a second cell. SUMMARY

[0007] The inventive concept provides a method and apparatus for efficiently splitting data in multi-connectivity.

[0008] Aspects of the inventive concept are set out in the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Embodiments of the inventive concept will be more clearly understood from the following detailed description taken together with the accompanying drawings, in which: Fig. 1 is a diagram illustrating multi-connectivity according to an exemplary embodiment; Fig. 2 is a diagram showing a structure of a wireless protocol in the multi-connectivity of the Fig. 1 according to an exemplary embodiment; Fig. 3 is a diagram illustrating a structure of a wireless protocol in a user equipment according to an exemplary embodiment; Fig. 4 is a flowchart illustrating a method for multi-connectivity according to an exemplary embodiment; Fig. 5 is a flowchart illustrating a method for multi-connectivity according to an exemplary embodiment; Fig. 6 is a flowchart illustrating operation S200 of the Fig. 4 according to an exemplary embodiment; Fig. 7A and Fig. 7B are flowcharts used in operation S240 of the Fig. 6 according to exemplary embodiments; Fig. 8 is a flowchart illustrating operation S600 of the Fig. 4 according to an exemplary embodiment; Fig. 9A and Fig. 9B flowcharts are which operation S640 of the Fig. 8 according to exemplary embodiments; Fig. 10 is a block diagram illustrating user equipment according to an exemplary embodiment; Fig. 11A and Fig. 11B are diagrams illustrating a method for multi-connectivity with respect to time flow according to example embodiments; Fig. 12 is a diagram illustrating multi-connectivity according to an exemplary embodiment; and Fig. 13A and Fig. 13B are diagrams illustrating a method for multi-connectivity with respect to time flow according to example embodiments. DETAILED DESCRIPTION

[0010] Fig. 1 is a diagram illustrating multi-connectivity according to an exemplary embodiment. In detail, Fig. 1 is a diagram illustrating a plurality of wireless communication systems comprising a first wireless communication system RAT1 and a second wireless communication system RAT2, each of which comprises user equipment (UE) 30 and a plurality of base stations including a first base station 10 and a second base station 20.

[0011] In one non-limiting embodiment, each of the first and second wireless communication systems RAT1 and RAT2 may be a fifth generation (5G) system, a new radio 5G (5G NR) system, a long-term evolution (LTE) system, a code division multiple access (CDMA) system, a global system for mobile communications (GSM) system, a wireless local area network (WLAN) system, or any other wireless communication system. Herein, a wireless communication system may be referred to as a radio access technology (RAT).

[0012] The first and second base stations 10 and 20 can communicate with the UE 30 based on multi-connectivity. For example, as shown in Fig. 1, the UE 30 and the first base station 10 may establish a first channel CH1 therebetween according to the first wireless communication system RAT1 and may communicate with each other via the first channel CH1. The UE 30 and the second base station 20 may establish a second channel CH2 therebetween according to the second wireless communication system RAT2 and may communicate with each other via the second channel CH2. In some embodiments, the first wireless communication system RAT1 may be the same as the second wireless communication system RAT2. In some other embodiments, the first wireless communication system RAT1 may be different from the second wireless communication system RAT2.Hereinafter, in exemplary embodiments, an example in which the first wireless communication system RAT1 is a 5G NR system (i.e., the first base station 10 is a 5G NR base station) and the second wireless communication system RAT2 is an LTE system (i.e., the second base station 20 is an LTE base station) will be mainly described. However, it should be understood that exemplary embodiments are not limited thereto.

[0013] A base station (e.g., the first base station 10 and / or the second base station 20) may refer to a fixed station and may communicate with the UE 30 and / or another base station to exchange data and control information. For example, a base station may be referred to as a Node B, an Evolved Node B (eNB), a Next Generation Node B (gNB), a sector, a location, a Base Transceiver System (BTS), an Access Point (AP), a Relay Node, a Remote Radio Head (RRH), a Radio Unit (RU), or a Small Cell.Herein, a base station or a cell may be construed as a comprehensive meaning representing a function performed or a specific area represented by a Base Station Controller (BSC) in CDMA, a Node B in WCDMA, an iNB in LTE or a gNB or a sector (a location) in 5G NR, and may cover various coverage areas such as a megacell, a microcell, a picocell, a femtocell, a relay node, an RRH, an RU and a small cell communication area.

[0014] The UE 30 may be a wireless communication device and may be fixed or mobile. Likewise, the UE 30 may refer to various devices that communicate with a base station to transmit or receive data and / or control information. For example, the UE 30 may be referred to as data terminal equipment, a mobile station (MS), a user data terminal (UT), a subscriber station (SS), a wireless device, or a handheld device. Likewise, the UE 30 may support multi-connectivity, and thus, as in Fig. 1, the UE 30 may be connected to two or more base stations, for example, the first and second base stations 10 and 20. In particular, as shown in Fig. 1, a UE 30 connected to two base stations, for example, the first and second base stations 10 and 20, may be referred to as dual connectivity. Hereinafter, exemplary embodiments will primarily describe dual connectivity, but it should be understood that exemplary embodiments apply to multi-connectivity, where the UE 30 communicates with three or more base stations.

[0015] A wireless communication network between the first and second base stations 10 and 20 and the UE 30 can share available network resources and can therefore support a plurality of users. For example, information can be transmitted over the wireless communication network by using various multiple access schemes such as CDMA, Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Orthogonal Frequency Division Multiple Access (OFDMA), Single Carrier Frequency Division Multiple Access (SC-FDMA), OFDM-FDMA, OFDM-TDMA, and OFDM-CDMA. The first base station 10 can communicate with the second base station 20 via an interface IF. In some embodiments, the first base station 10 can access the second base station 20 via an X2 interface. In some other embodiments, as described below with reference to Fig. 12, the first base station 10 can access the second base station 20 via a core network.

[0016] In multi-connectivity, data may be divided and transmitted, and divided portions (e.g., first and second data) of the data may be transmitted via the first channel CH1 and the second channel CH2, respectively. For example, on a downlink, the first and second base stations 10 and 20 may respectively transmit the first and second data obtained by dividing data to be transmitted to the UE 30 to the UE 30 via the first and second channels CH1 and CH2. Similarly, on an uplink, the UE 30 may divide data to be transmitted into the first and second data and may respectively transmit the first and second data to the first and second base stations 10 and 20 via the first and second channels CH1 and CH2.Hereinafter, according to exemplary embodiments, as described below with reference to the drawings, the UE 30 and the first and second base stations 10 and 20 can efficiently divide data based on a state of each of the first and second channels CH1 and CH2. Accordingly, a delay due to data reordering can be reduced, and an improved quality of experience (QoE) can be provided to a user of the UE 30.

[0017] Fig. 2 is a diagram showing a structure of a wireless protocol in multi-connectivity of the Fig. 1 according to one embodiment.

[0018] As above with reference to Fig. 1, the UE 30' can access a first base station 10' and a second base station 20'. In Fig. 2, it can be assumed that the first base station 10' is a base station (e.g., gNB) of a 5G NR system, and the base station 20' is a base station (e.g., eNB) of an LTE system. As in Fig. 2, the UE 30' (or a Packet Data Convergence Protocol (PDCB) of the UE 30') may support a wireless protocol of the first wireless communication system RAT1 (e.g., the 5G NR system) and a wireless protocol of the second wireless communication system RAT2 (e.g., the LTE system). Hereinafter, Fig. 2 with reference to Fig. 1 are described.

[0019] Referring to Fig. 2, in each of the first base station 10' and the UE 30', the wireless protocol of the first wireless communication system RAT1 may include physical (PHY) layers 11 and 31a, media access controllers (MACS) 12 and 32a, radio link controllers (RLCs) 13 and 33a, and PDCPs 14 and 34. Each of the PHY layers 11 and 31a, the MACs 12 and 32a, the RLCs 13 and 33a, and the PDCPs 14 and 34 may perform unique functions thereof based on a prescription by the first wireless communication system RAT1. For example, the PHY layers 11 and 31a may encode and modulate data of the MACs 12 and 32a, generate an OFDM symbol to transmit the generated OFDM symbol to the first channel CH1, demodulate and decode an OFDM symbol transmitted over the first channel CH1, and transmit data to the MACs 12 and 32a.MACs 12 and 32a may perform functions including Hybrid Automatic Repeat Request (HARQ) retransmission, RLCs 13 and 33 may perform functions including Automatic Repeat Request (ARQ), and PDCPs 14 and 34 may perform functions including reordering.

[0020] In each of the second base station 20' and the UE 30', the wireless protocol of the second wireless communication system RAT2 may include PHY layers 21 and 31b, MACs 22 and 32b, RLCs 23 and 33b, and PDCPs 24 and 34. Each of the PHY layers 21 and 31b, the MACs 22 and 32b, the RLCs 23 and 33b, and the PDCPs 24 and 34 may perform unique functions thereof based on a prescription by the second wireless communication system RAT2. For example, the PHY layers 21 and 31b may encode and modulate data of the MACs 22 and 32b, generate an OFDM symbol to transmit the generated OFDM symbol to the second channel CH2, demodulate and decode an OFDM symbol transmitted over the second channel CH2, and transmit data to the MACs 22 and 32b.MACs 22 and 32b may perform functions including Hybrid Automatic Repeat Request (HARQ) retransmission, RLCs 13 and 33 may perform functions including ARQ, and PDCPs 14 and 34 may perform functions including reordering.

[0021] The PDCP 34 may include a split bearer in dual connectivity. The split bearer may distribute a data packet to a plurality of different RLC entities in a PDCP to transmit data over a plurality of channels, and may thus designate a data radio bearer (DRB) to improve a data transmission rate. For example, in the uplink, the PDCP 34 may process data packets (or PDCP service data units (SDUs)) into PDCP protocol data units (PDUs) and transmit the PDCP PDUs to two RLC entities 33a and 33b. Here, if the PDCP PDUs transmitted to the two RLC entities 33a and 33b are not properly distributed, namely, if a size of split data is not properly determined, a delay may occur due to reordering in the PDCP 34.If the delay increases due to reordering, a delay in an upper layer (e.g., an application layer (e.g., 35 of the . Fig. 3)) of the PDCP 34, causing degradation in the user QoE of the UE 30'. Likewise, a problem similar to the one described above may occur in the downlink.

[0022] Fig. 3 is a diagram illustrating a structure of a wireless protocol in UE 30 according to an exemplary embodiment. In detail, Fig. 3 a structure of a wireless protocol in the uplink. Hereinafter, Fig. 3 with reference to Fig. 1 and when describing the Fig. 3, the same or similar description to those given above with reference to Fig. 2 are omitted.

[0023] Referring to Fig. 3, the wireless protocol may include a first PHY layer 31a', a first MAC 32a', and a first RLC 33' for the first wireless communication system RAT1, and may include a second PHY layer 31b', a second MAC 32b', and a second RLC 33b' for the second wireless communication system RAT2, and a PDCP 34' may support the first wireless communication system RAT1 and the second wireless communication system RAT2. Likewise, an application 35 corresponding to an upper layer of the PDCP 34' may provide the PDCP 34' with a data packet to be transmitted through the uplink.

[0024] The PDCP 34' can, as described above with reference to Fig. 2, may determine data (e.g., size of first data and second data) to be transmitted through the first channel CH1 and the second channel CH2 in multi-connectivity, and may accordingly divide the data packet provided by the application 35 and may respectively provide portions of divided data to the first RLC (or a first RLC entity) 33a' and the second RLC (or a second RLC entity) 33b'. For example, according to the NR PDCP standard (3GPP TS 38.323 V15.2.0, 2018-06) recently published, a means for minimizing a PDCP reordering delay caused by a data division operation performed by the UE 30" is defined as a capability of the UE 30".

[0025] In some embodiments, the PDCP 34' may be provided with a plurality of lower layer indicators and may distribute data to the first RLC 33a' and the second RLC 33b' based on the plurality of indicators. For example, as shown in Fig. 3, the PDCP 34' may be provided with first indicators IND1 from the first PHY layer 31a', the first MAC 32a', and the first RLC 33a', and may be provided with second indicators IND2 from the second PHY layer 31b', the second MAC 32b', and the second RLC 33b'. The PDCP 34' may detect states of the first channel CH1 and the second channel CH2 based on the first indicators IND1 and the second indicators IND2, and may distribute data to the first RLC 33a' and the second RLC 33b' based on the states of the first channel CH1 and the second channel CH2. An exemplary operation of the PDCP 34' is described below with reference to Fig. 4 are described.

[0026] Fig. 4 is a flowchart illustrating a method for multi-connectivity according to an exemplary embodiment. For example, the method of Fig. 4 by the PDCP 34' of the Fig. 3 and as described below with reference to Fig. 5, it can be triggered by various factors. Hereinafter, Fig. 4 with reference to the Fig. 1 and Fig. 3 are described.

[0027] Referring to Fig. 4, in operation S200, an operation for estimating round-trip times can be performed. A round-trip time (RTT) can be defined as a time required for a transmitting side to transmit data to a receiving side and to receive a response (e.g., an acknowledgement response (ACK)) for the transmitted data from the receiving side. A paper "Stability of end-to-end algorithms for joint routing and rate control" (F. Kelly and T. Voice, ACM SIGCOMM CCR, 35, 2005) has proposed a method for maximizing network efficiency in a transmission control protocol (TCP), and the proposed method can be expressed as the following equation (1). A transmitting side can set a congestion window "cwnd" of a corresponding path by Δw p the following equation (1): Δwp=maxi∈P(wiRTTi2)Σi∈P(wiRTTi)2

[0028] In equation (1), P can denote a total set of paths formed by a single host, w i can denote a current transmission window of an i-th path and RTT i may denote a round-trip time of the i-th path. In some embodiments, in multi-connectivity, data splitting may be performed based on a change in a transmission window provided in Equation (1). In TCP, a round-trip time may be determined based on traffic of a network, and thus, the transmitting side may measure, as the round-trip time, a difference between a time when data is transmitted and a time when ACK is received. On the other hand, in multi-connectivity, a round-trip time may depend on a state of a channel and may be estimated as described below.

[0029] To apply equation (1) to multi-connectivity, which is Fig. 1, a first round trip time RTT1 and a second round trip time RTT2 corresponding to the first channel CH1 and the second channel CH2, respectively, can be estimated. For example, as described above with reference to Fig. 3, the PDCP 34' of the UE 30" may estimate the first round trip time RTT1 based on the first indicators ID1 provided by lower layers and may estimate the second round trip time RTT2 based on the second indicators ID2 provided by lower layers. An example of operation S200 is described below with reference to Fig. 6 are described.

[0030] In operation S400, an operation for acquiring channel bandwidths may be performed. As in Fig. 1, in multi-connectivity, where the UE 30 communicates with the first and second base stations 10 and 20, the UE 30 and a base station (e.g., the first base station 10 or the second base station 20) can be considered as a one-hop network, and w i of equation (1) can be expressed as a multiplication of a channel bandwidth and a round trip time as in the following equation (2): wi=BWi×RTTi

[0031] Therefore, in equation (1) w i by “BW i ×RTT i", and in operation S400, an operation for obtaining channel bandwidths (i.e., a first channel bandwidth BW1 of the first channel CH1 and a second channel bandwidth BW2 of the second channel CH2) may be performed. For example, the first PHY layer 31a' and the second PHY layer 31b' may respectively measure the first channel bandwidth BW1 and the second channel bandwidth BW2, the PDCP 34' may obtain the first channel bandwidth BW1 based on an indicator provided by the first PHY layer 31a' among the first indicators IND1, and may obtain the second channel bandwidth BW2 based on an indicator provided by the second PHY layer 31b' among the second indicators IND.

[0032] In operation S600, an operation for determining sizes of pieces of divided data (e.g., first and second data) may be performed. When the first channel bandwidth BW1 and the second channel bandwidth BW2 correspond to channel bandwidths of the first channel CH1 and the second channel CH2, respectively, equation (1) may be expressed as the following equation (3) in multi-connectivity, which is expressed in Fig. 1 is illustrated, can be expressed as follows: Δw=max(BW1RTT1,BW2RTT2)(BW1+BW2)2

[0033] Furthermore, in multi-connectivity based on M (where M is an integer greater than 1) number of channels, equation (1) can be expressed as the following equation (4): Δw=max1≤i≤MBWiRTTi(∑i=1MBWi)2

[0034] The first round trip time RTT1 and the second round trip time RTT2 may be estimated in operation S200, and the first channel bandwidth BW1 and the second channel bandwidth BW2 may be obtained in operation S400. Accordingly, in operation S600, the PDCP 34' may calculate a variation (i.e., Δw) of a data size based on equation (3) and may reflect the variation Δw in dividing the data packet to be transmitted. An example of operation S600 is described below with reference to Fig. 8 are described.

[0035] Fig. 5 is a flowchart illustrating a method for multi-connectivity according to an exemplary embodiment. In detail, performing the method for multi-connectivity described above with reference to Fig. 4, in operation S100 of the Fig. 5 can be triggered, and after operation S100 is carried out, operation S200 of the Fig. 4. Likewise, after operation S600 of the Fig. 4 is carried out, operation S100 of the Fig. 5. As in Fig. 5, operation S100 may include operations S120, S140, and S160. In some embodiments, when at least one of operations S120, S140, and S160 of operation S100 is performed, the method may proceed to operation S200. In some other embodiments, operation S100 may include only some of operations S120, S140, and S160. Hereinafter, Fig. 5 with reference to Fig. 3 are described.

[0036] In operation S120, an operation for receiving a data packet may be performed. For example, when the data packet is received from the application 35 corresponding to the upper layer, the PDCP 34' may perform the process of Fig. 4. The data packet received by the application 35 may be data to be transmitted by the application 35 via wireless communication and may be referred to as a PDCP SDU, and in some embodiments, the data packet may include a header and a payload.

[0037] In operation S140, an operation for receiving ACK may be performed. For example, when the ACK corresponding to an RLC PDU in an RLC Acknowledgement (AM) mode is received, the PDCP 34' may perform the process of Fig. 4. In some embodiments, when a predetermined number of ACKs are received, the PDCP 34' may initiate the process of Fig. 4. Likewise, in some embodiments, as described below with reference to equation (8), the PDCP 34' may initiate the method of Fig. 4 based on a predetermined period or other factor, and in this case the number of ACKs received can be used.

[0038] In operation S160, an operation for receiving updated retransmission parameters may be performed. As described below with reference to Fig. 6, round trip times based on a re-transmission in operation S200 of the Fig. 4. A wireless communication system may prescribe values of retransmission parameters; some wireless communication systems (e.g., the 5G NR system) may prescribe that the values of the retransmission parameters vary, and a base station may update the retransmission parameters based on a channel condition. If the values of the retransmission parameters vary, the estimated round-trip times may vary, and accordingly, when updated retransmission parameters are received from the base station, the PDCP 34' may perform the Fig. 4 trigger.

[0039] Fig. 6 is a flowchart illustrating operation S200 of the Fig. 4 according to an exemplary embodiment. As described above with reference to Fig. 4, an operation for estimating round trip times can be performed in operation S200' of the Fig. 6. In detail, a round trip time corresponding to one channel can be set in operation S200' of the Fig. 6 can be estimated, and operation S200' of the Fig. 6 can be performed consecutively or in parallel a plurality of times based on a plurality of channels. As in Fig. 6 may include operation S200', operation S220 and operation S240. Hereinafter, Fig. 6 with reference to the Fig. 1 and Fig. 3 and an example estimating the first round trip time RTT1 corresponding to the first channel CH1 will be described based on the assumption that the UE 30 of the Fig. 1 the UE 30" of the Fig. 3 is.

[0040] In operation S220, an operation for obtaining retransmission parameters may be performed. For example, the PDCP 34' may be provided with the first indicators IND1 comprising first retransmission parameters provided by the first base station 10. In some embodiments, the PDCP 34' may reflect HARQ retransmission when estimating the first round trip time RTT1, and for example, the first retransmission parameters may include a retransmission period c1 and a maximum retransmission number N1. In some other embodiments, the PDCP 34' may reflect RLC retransmission when estimating the first round trip time RTT1, and for example, the first retransmission parameters may include a maximum RLC retransmission number R1.

[0041] In operation S240, an operation for calculating a round-trip time may be performed. For example, the PDCP 34' may calculate the first round-trip time RTT1, which is used to calculate the variation Δw of the data size in equation (3), based on the retransmission parameters obtained in operation S220. In some embodiments, the PDCP 34' may calculate a round-trip time including a HARQ retransmission provided by the first MAC 32'. In some embodiments, the PDCP 34' may calculate a round-trip time further including an RLC retransmission provided by the first RLC 33a'. Examples of operation S240 are described below with reference to Fig. 7A and Fig. 7B are described.

[0042] The Fig. 7A and Fig. 7B are flowcharts showing operation S240 of the Fig. 6 according to exemplary embodiments.

[0043] An operation for calculating the first round trip time RTT1 as described above with reference to Fig. 6 can be described in operation S240a of the Fig. 7A and S240b of the Fig. 7B. Repeated descriptions will be omitted when describing the embodiments of the Fig. 7A and Fig. 7B can be omitted. The Fig. 7A and Fig. 7B are made with reference to the Fig. 1 and Fig. 3, and an example estimating the first round trip time RTT1 corresponding to the first channel CH1, based on the assumption that the UE 30 of the Fig. 1 the UE 30" of the Fig. 3 will be described.

[0044] Referring to Fig. 7A may include operation S240a, operation S242a, and operation S244a, and an operation for calculating a round trip time including HARQ retransmission may be performed in operation S242a. For example, the PDCP 34' may calculate a first round trip time RTT S1 which has a HARQ re-transmission, by using a block error rate BLER as in the following equation 5: RTTS1=(1−BLER1)×∑n=0N1−1BLER1n(p1+n×c1)

[0045] In Equation 5, a first block error rate BLER1 may denote a block error rate measured in the first channel CH1. In some embodiments, the PDCP 34' may obtain the first block error rate BLER1 from an indicator provided by the first PHY layer 31a' below the first indicator IND1.

[0046] A first propagation delay p1 may represent a propagation delay occurring in the first channel CH1. In some embodiments, the PDCP 34' may obtain the first propagation delay p1 from the first MAC 32a'. For example, the first base station 10 may allocate a dedicated random access preamble for the UE 30", and if the UE 30" does not have a wireless resource for first accessing the base station 10 or transmitting a signal to the first base station 10, the UE 30" may perform a random access procedure (RACH) based on the random access preamble. The first base station 10 may measure a transmission time of the UE 30" by using a random access preamble (or a sounding reference signal (SRS)), calculate a correction timing value, and inform the UE 30" of the calculated correction timing value.The correction timing value (i.e., a timing advance value) provided by the first base station 10 to the UE 30" may be referred to as a timing advance command (TAC), and the TAC may be processed in a MAC layer. Accordingly, the first MAC 32a' of the UE 30" may generate the first propagation delay p1 based on the TAC and may provide the PDCP 34' with the first propagation delay p1 as one of the first indicators IND1.

[0047] In some embodiments, the first propagation delay p1 may be used in calculating the first round trip time RTT S1 which has a HARQ retransmission can be omitted. For example, in equation (5), the first propagation delay p1 can have a small value compared to “n1×c1”, and thus the first round trip time RTT S1which has HARQ re-transmission can be calculated as in the following equation (6). In this case, an operation for obtaining the first propagation delay p1 as a first re-transmission parameter from the first MAC 32a' in operation S220 of the Fig. 6 are omitted: RTTS1=(1−BLER1)×∑n=0N1−1BLER1n(n×c1)

[0048] In some embodiments, the PDCP 34' may determine the first round trip time RTT S1 as the first propagation delay p1 in an RLC non-acknowledgement (UM) mode. For example, if the first block error rate BLER1 is maintained approximately at zero, the RLC UM may be set, and the PDCP 34' may determine the first round trip time RTT S1 as the first propagation delay p1. Likewise, in some other embodiments, the PDCP 34' may determine the variation Δw at each first propagation delay p1 (i.e., first round trip time RTT S1 ) reflect.

[0049] It should be understood that the second round trip time RTT S2 , which has a HARQ re-transmission and corresponds to the second channel CH2, is similarly calculated based on equation (5) and / or equation (6).

[0050] In operation S244a, an operation for calculating a round trip time including an RLC retransmission may be performed. For example, the PDCP 34' may calculate a first round trip time RTT T1 which has an RLC re-transmission, as in the following equation (7): RTTT1=RTTS1×∑r=0R1(BLER1N1)r

[0051] In the embodiment of the Fig. 7A, the PDCP 34' can be the first round trip time RTT T1 , which is calculated based on equation (7), as the data variation Δw of equation (3). It should also be understood that the second round trip time RTT S2, which has a HARQ re-transmission and corresponds to the second channel CH2, is calculated similarly based on equation (7).

[0052] Referring to Fig. 7B may include operation S240b, operation S242b, operation S243b, and operation S244b. Compared with operation S240a of Fig. 7A can perform operation S240b of the Fig. 7B further comprise operation S243b. Similar to operation S242a of Fig. 7A, an operation for calculating a round trip time including a HARQ retransmission may be performed in operation S242b. Accordingly, a first round trip time RTT S1 and a second round trip time RTT S2 , each of which has a HARQ re-transmission, are calculated.

[0053] In operation S243b, an operation for comparing a block error rate BLER with a predefined first threshold value THR1 may be performed. For example, in a process of calculating the first round trip time RTT S1 a first block error rate BLER1 is compared with the predetermined first limit value THR1, and as in Fig. 7B, if the first block error rate BLER1 is less than the first threshold value THR1, operation S240b may end. On the other hand, if the first block error rate BLER1 is equal to or greater than the first threshold value THR1, operation S244b may be performed, and an operation for calculating a first round trip time RTT T1 , which has an RLC re-transmission, can be performed in operation S244b.

[0054] As a result, in the embodiment of the Fig. 7B, if the first block error rate BLER1 is less than the first threshold THR1, the first round trip time RTTS1 which has a HARQ retransmission, the equation (5) or (6) can be determined as a final first round trip time RTT1. On the other hand, if the first block error rate BLER1 is equal to or greater than the first threshold value THR1, the first round trip time RTT T1 which has an RLC retransmission can be determined as the final first round trip time RTT1. In a state where a block error rate is low, a possibility that an RLC retransmission occurs may be low, and thus, in the embodiment of the Fig. 7B Operation S244b (for example, calculation based on equation (7)) may be omitted.

[0055] Fig. 8 is a flowchart showing operation S600 of the Fig. 4 according to an exemplary embodiment. As described above with reference to Fig. 4, an operation for determining sizes of pieces of divided data (for example, first data and second data) may be performed in operation S600' of the Fig. 8. As in Fig. 8 may include operation S600', operation S620 and operation S640. Hereinafter, Fig. 8 with reference to the Fig. 3 and Fig. 4 are described.

[0056] In operation S620, an operation for calculating a variation of a data size may be performed. For example, the PDCP 34' may calculate a variation Δw of a data size as in equation (3) based on the round-trip times obtained by operation S200 and the channel bandwidths obtained by operation S400. In some embodiments, the variation Δw may be applied identically to a plurality of channels.

[0057] In some embodiments, if a variation in data size is not calculated every time ACK occurs, a variation Δw1 in data size corresponding to the first channel CH may be calculated as in the following equation (8): Δw1=max(BW1RTT1,BW2RTT2)(BW1+BW2)2×NACK1

[0058] In equation (8), N ACK1 denotes the number of ACKs received through the first channel CH1 from the first base station 10. For example, the PDCP 34' can calculate the variation Δw1 of the data size in the RLC AM based on equation (8). Similarly, a variation Δw2 of the data size corresponding to the second channel CH2 can be calculated by using N ACK2 which denotes the number of ACKs received from the second base station 20 via the second channel CH2.

[0059] In multi-connectivity, based on a number M of channels, a variation Δw1 of a data size corresponding to an i-th channel CHi can be calculated as in the following equation (9): Δwi=max1≤i≤MBWiRTTi(Σi=1MBWi)2×NACKi

[0060] In operation S640, an operation for changing data sizes may be performed. For example, the PDCP 34' may add the variation Δw calculated by operation S620 to data sizes set in a plurality of channels to change the data sizes. In some embodiments, when the variation Δw is applied identically to the plurality of channels, the same variation Δw may be added to the data sizes set in the plurality of channels. For example, retransmission may rarely occur in a channel having sufficient quality, and thus, even if the variation Δw has a negative value, a large amount of data may be transmitted through the channel having sufficient quality (or channel condition) compared to a channel having lower quality.Likewise, retransmission may occur frequently in the channel having poor quality, and therefore, even if the variation Δw has a positive value, a small amount of data may be transmitted through the channel having lower quality compared to the channel having sufficient quality.

[0061] The Fig. 9A and Fig. 9B are flowcharts showing operation S640 of the Fig. 8 according to exemplary embodiments. In detail, operation S640a of the Fig. 9A and Operation S640b of the Fig. 9B, respectively, comprise operations S644a and S644b of blocking the transmission of data through a channel having a bad channel condition. As described below with reference to Fig. 8, an operation for changing data sizes in operation S640a of the Fig. 9A and Operation S640b of the Fig. 9B. Hereinafter, repeated descriptions will be omitted when describing the Fig. 9A and Fig. 9B can be omitted, and the Fig. 9A and Fig. 9B are described with reference to Fig. 3 are described.

[0062] Referring to Fig. 9A, operation S640a may include operations S641a to S646a, and an initialization operation may be performed in operation S641a. For example, as in Fig. 9A, a variable i, which represents an index of a channel, is set to 1. In the embodiment of Fig. 9A, the size of data parts or data chunks to be transmitted through a number M of channels can be determined, and accordingly, the variable i can have a value "1 to M".

[0063] In operation S642a, an operation for comparing a block error rate BLER iof an i-th channel with a predetermined second threshold value THR2. As in Fig. 9A, if the block error rate BLER i of the i-th channel is greater than the second threshold value THR2, operation S644a may be performed, and an operation for determining a data size as zero may be performed in operation S644a. On the other hand, if the block error rate BLER i of the i-th channel is equal to or less than the second threshold value THR2, operation S643a may be performed, and an operation for reflecting a variation in data size in a data distribution may be performed in operation S643a. As a result, when a block error rate BLER of a channel is high, the PDCP 34' may block transmission of data through a corresponding channel.

[0064] In operation S645a, an operation can be performed to compare the variable i with M. As in Fig. As illustrated in Fig. 9A, when the variable i does not agree with M, namely, when the variable i is less than M, the variable i may increase by 1 in operation S646a, and operation S642a may be performed again. On the other hand, when the variable i agrees with M, namely, when data sizes corresponding to the M channels are all determined, operation S640 may end.

[0065] Referring to Fig. 9B may include operation S640b, operations S641b to S646b. Operations similar to the operations described in some operations under operations S641a and S643a to S646a of operation S640a of Fig. 9A can be performed in some operations under operations S641b and S643b to S646b of the Fig. 9B.

[0066] In operation S642b, an operation for comparing a predetermined threshold value THR3 with a rate NACK % of a negative non-acknowledgement response (NACK) occurring in the i-th channel may be performed. As in Fig. As illustrated in Figure 9B, if the NACK rate % of NACKs occurring in the i-th channel is higher than the threshold value THR3, operation S644b may be performed, and if the NACK rate % of NACKs occurring in the i-th channel is equal to or lower than the threshold value THR3, operation S643b may be performed. As a result, if a NACK rate NACK % of the channel is high, the PDCP 34' may block transmission of data over a corresponding channel.

[0067] Fig. 10 is a block diagram illustrating a UE 100 according to an exemplary embodiment. As described above with reference to Fig. 1, the UE 100 of the Fig. 10 support multi-connectivity and can perform wireless communication with two or more base stations. As in Fig. 10, the UE 100 may include an antenna 110, a transceiver 120, a data processor 130, a memory 140, and a main processor 150. The elements of the UE 100 are independently Fig. 10, but in some embodiments, two or more elements may be implemented as a single unit (e.g., a semiconductor chip).

[0068] Antenna 110 may receive a radio frequency (RF) signal from a base station or may transmit an RF signal to the base station. In some embodiments, antenna 110 may be configured as an antenna array including a plurality of antennas and may support multiple-input-multiple-output (MIMO) and beamforming.

[0069] The transceiver 120 may process a signal between the antenna 110 and the data processor 130. For example, the transceiver 120 may include a duplexer, a switch, a filter, a multiplexer, and an amplifier. Likewise, the transceiver 120 may process an RF signal received via the antenna 110 to generate a receive signal RX and may provide the receive signal RX to the data processor 130. Likewise, the transceiver 120 may process a transmit signal TX provided by the data processor 130 to generate an RF signal and may provide the generated RF signal to the antenna 110. In some embodiments, the transceiver 120 may be referred to as a radio frequency integrated circuit (RFIC).

[0070] The data processor 130 may process a data packet PKT received from the main processor 150 to generate the transmission signal TX, process the reception signal RX received from the transceiver 120 to generate the data packet PKT, and provide the generated data packet PKT to the main processor 150. The data processor 130 may perform an operation corresponding to at least one layer in a wireless protocol structure. For example, the data processor 130 may be referred to as a communication protocol and may perform functions of the first and second PHY layers 31a' and 31b', the first and second MACs 32a' and 32b', the first and second RLCs 33a' and 33b', and the PDCP 34' of the Fig. 3. In some embodiments, the data processor 130 may include hardware that includes a logic block designed based on a logic combination, a processing unit that includes software and at least one core (or at least one processor) for executing the software, and a combination of the hardware and the processing unit. For example, the data processor 130 may include hardware blocks and / or software blocks that respectively correspond to the first and second PHY layers 31a' and 31b', the first and second MACs 32a' and 32b', the first and second RLCs 33a' and 33b', and the PDCP 34' of the Fig. 3. The method according to exemplary embodiments described above with reference to the drawings and at least one operation included in the method may be performed by the data processor 130. In some embodiments, a base station (e.g., 10 and / or 20 of the Fig. 1) a structure similar to that of the UE 100 of the Fig. 10, and a data processor included in the base station may perform a method for dual connectivity and at least one operation included in the method.

[0071] Memory 140 may store data required for a process of processing a signal and / or data by the data processor. In some embodiments, memory 140 may store software (i.e., a series of instructions) executed by data processor 130.

[0072] The main processor 150 may include at least one core (or one processor). Likewise, the main processor 150 may transmit the data packet PKT to be transmitted via wireless communication to the data processor 130 and may receive data transmitted from the base station based on the data packet PKT provided by the data processor 130. The main processor 150 may control an operation of the UE 100 and may generate the data packet PKT or perform an operation based on a received data packet PKT.

[0073] The Fig. 11A and Fig. 11B are diagrams illustrating a method for multi-connectivity with respect to a time flow according to exemplary embodiments. In detail, the Fig. 11A and Fig. 11B are diagrams illustrating examples of a method for multi-connectivity in the downlink. In some embodiments, one of the base stations connected to the UE may perform data splitting for multi-connectivity, and examples in which first base stations 10a and 10b, which are base stations (e.g., gNBs) of a 5G NR system, perform data splitting for multi-connectivity are described below with reference to FIGS. Fig. 11A and Fig. 11B. However, it should be understood that exemplary embodiments are not limited thereto. Hereinafter, Fig. 11A and Fig. 11B with reference to Fig. 1 and it can be assumed that the first base station 10, the second base station 20 and the UE 30 of the Fig. 1 of the first base station 10a, the second base station 20a and the UE 30a of the Fig. 11A and the first base station 10b, the second base station 20b and the UE 30b of the Fig. 11B. Repeated descriptions are avoided when describing the Fig. 11A and Fig. 11B can be omitted.

[0074] Referring to Fig. 11A, a second round trip time RTT2 corresponding to the second channel CH2 may be estimated by the second base station 20a establishing the second channel CH2 with the UE 30a, and the estimated second round trip time RTT2 may be provided by the second base station 20a to the first base station 10a.

[0075] In operation S11a, the first base station 10a may estimate a first round trip time RTT1. The first base station 10a may establish the first channel CH1 with the UE 30a, and thus, as described above with reference to the drawings, the first base station 10a may estimate the first round trip time RTT1 in a manner similar to a manner of estimating the first round trip time RTT1 by the UE 30a. Likewise, in operation S12a, the first base station 10a may obtain a first channel bandwidth BW1.

[0076] In operation S13a, the second base station 20a may estimate the second round trip time RTT2. The second base station 20a may establish the second channel CH2 with the UE 30a, and thus, as described above with reference to the drawings, the second base station 20a may estimate the second round trip time RTT2 in a manner similar to the manner of estimating the second round trip time RTT2 by the UE 30a. Likewise, in operation S14a, the second base station 20a may obtain a second channel bandwidth BW2.

[0077] In operation S15a, the second base station 20a may provide the second round trip time RTT2 and the second channel bandwidth BW2 to the first base station 10a. For example, as described above with reference to Fig. 1, the second base station 20a provides the second round trip time RTT2 and the second channel bandwidth BW2 for the first base station 10a via the interface IF.

[0078] In operation S16a, the first base station 10a may calculate data sizes. For example, the first base station 10a may collect information about the first and second channels CH1 and CH2 in operations S11a to S15a and may calculate a variation Δw of a data size as in equation (3) based on the collected information. The first base station 10a may reflect the variation Δw of the data size when calculating a size of first data to be transmitted from the first base station 10a to the UE 30a and a size of second data to be transmitted from the second base station 20a to the UE 30a.

[0079] In operation S17a, the first base station 10a may transmit the first data to the UE 30a over the first channel CH1 based on the calculated size of the first data. Similarly, in operation S18a, the first base station 10a may provide the second data to the second base station 20a. In operation S19a, the second base station 20a may transmit the second data provided by the first base station 10a to the UE 30a over the second channel CH2.

[0080] Referring to Fig. 11B, a second base station 20b establishing the second channel CH2 with UE 30b may provide a first base station 10b with information about the second channel CH2, and the first base station 10b may estimate a second round trip time RTT2 based on the information about the second channel CH2.

[0081] In operation S11b, the first base station 10b may estimate a first round trip time RTT1, and in operation S12b, the first base station 10b may obtain a first channel bandwidth BW1.

[0082] In operation S13b, the second base station 20b may provide second channel information to the first base station 10b. For example, as described above with reference to the drawings, the second base station 20b may provide the first base station 10b with information (e.g., second retransmission parameters corresponding to the second channel CH2) used to estimate the second round trip time RTT2 and information (e.g., second channel information having a second channel bandwidth BW2) used to calculate a variation Δw of the data size. In some embodiments, the second base station 20b may provide the first base station 10b with a propagation delay (i.e., second propagation delay p2) corresponding to the second channel CH2.

[0083] In operation S14b, the first base station 10b may estimate the second round trip time RTT2. For example, the first base station 10b may estimate the second round trip time RTT2 by using Equation (5), Equation (6), and / or Equation (7) based on the second channel information provided in operation S13b.

[0084] In operation S15b, the first base station 10b may calculate data sizes. In operation S16b, the first base station 10b may transmit first data to the UE 30b over the first channel CH1 based on a calculated size of the first data. Likewise, in operation S17b, the first base station 10b may provide second data to the second base station 20b. In operation S18b, the second base station 20b may transmit the second data provided by the first base station 10b to the UE 30b over the second channel CH2.

[0085] Fig. 12 is a diagram illustrating multi-connectivity according to an exemplary embodiment. In detail, Fig. 12 shows a structure having cores (e.g., a first core and a second core) 70 and 80 connected to base stations (e.g., a first base station and a second base station) 40 and 50. Hereinafter, when describing Fig. 12 a description which Fig. 1 overlapped.

[0086] Referring to Fig. 12, the UE 60 may access the first base station 40 via a first channel CH1 and may access the second base station 50 via a second channel CH2. The first base station 40 may access the first core 70, and the second base station 50 may access the second core 80. For example, the first base station 40 may be a base station of the 5G NR system, and the second base station 50 may be a base station of the LTE system. In this case, the second core 80 may be referred to as an evolved packet core (EPC).

[0087] The first core 70 and the second core 80 may access a common Internet Protocol (IP) anchor 90, and the common IP anchor 90 may be a network entity and may perform a function for routing data transmitted from a data network to a UE 60. In some embodiments, the common IP anchor 90 may perform data splitting in multi-connectivity, and relevant embodiments are described below with reference to the Fig. 13A and Fig. 13B.

[0088] The Fig. 13A and Fig. 13B are diagrams illustrating a method for multi-connectivity with respect to time flow according to exemplary embodiments. In detail, the Fig. 13A and Fig. 13B illustrates examples of a method for multi-connectivity in the downlink. In some embodiments, common IP anchors 90a and 90b may perform data splitting for multi-connectivity, and examples in which data is distributed to first base stations 40a and 40b of the 5G NR system and second base stations 50a and 50b of the LTE system are described below with reference to FIG. Fig. 13A and Fig. 13B. However, it should be understood that exemplary embodiments are not limited thereto.

[0089] Referring to Fig. 13A, the shared IP anchor 90a may partition data based on round-trip times and channel widths provided by the first base station 40a and the second base station 50a. In operation S21a, the first base station 40a may provide the shared IP anchor 90a with a first round-trip time RTT1 and a first channel bandwidth BW1, each of which corresponds to a first channel CH1. In operation S22a, the second base station 50a may provide the shared IP anchor 90a with a second round-trip time RTT2 and a second channel bandwidth BW2, each of which corresponds to a second channel CH2. For example, the first base station 40a and the second base station 50a may each calculate the first round-trip time RTT1 and the second round-trip time RTT2 based on equation (5), equation (6), and / or equation (7).

[0090] In operation S23a, the common IP anchor 90a may calculate sizes of pieces of divided data based on the round-trip times and the channel bandwidths provided by the first base station 40a and the second base station 50a. For example, the common IP anchor 90a may calculate a variation Δw of a data size based on equation (3) and may reflect the variation Δw of the data size when calculating a size of first data to be transmitted over the first channel CH1 and a size of second data to be transmitted through the second channel CH2. Subsequently, in operation S24a, the common IP anchor 90a may provide the first data to the first base station 40a, and in operation S25a, the common IP anchor 90a may provide the second data to the second base station 50a.

[0091] Referring to Fig.13B, the shared IP anchor 90b may divide data based on channel information provided by each of the first base station 40b and the second base station 50b. In operation S21b, the first base station 40b may provide the shared IP anchor 90b with first channel information corresponding to a first channel CH1. In operation S22b, the second base station 50b may provide the shared IP anchor 90b with second channel information corresponding to a second channel CH2. For example, the first channel information may include first retransmission parameters and a first channel bandwidth, and the second channel information may include second retransmission parameters and a second channel bandwidth.

[0092] In operation S23b, the shared IP anchor 90b may estimate round-trip times. For example, the shared IP anchor 90b may estimate the first round-trip time RTT1 and the second round-trip time RTT2 by using Equation (5), Equation (6), and / or Equation (7) based on the first channel information and the second channel information. In operation S24b, the shared IP anchor 90b may calculate sizes of pieces of split data. For example, the shared IP anchor 90b may calculate a variation Δw of the data size based on Equation (3) and may reflect the variation Δw of the data size when calculating a size of first data to be transmitted over the first channel CH1 and a size of second data to be transmitted over the second channel CH2.In operation S25b, the common IP anchor 90b may provide the first data to the first base station 40b, and in operation S26b, the common IP anchor 90b may provide the second data to the second base station 50b.

Claims

[1] A method for multi-connectivity between a plurality of base stations (10, 10', 10a, 10b, 20, 20', 20a, 20b) and user equipment (30, 30', 30", 30a, 30b, 100) through a plurality of channels, the method comprising: estimating a first round trip time (RTT) at the user equipment (30, 30', 30", 30a, 30b, 100), the first RTT being an expected time required to transmit data and receive an acknowledgment response (ACK) through a first channel; estimating a second RTT at the user equipment (30, 30', 30", 30a, 30b, 100), wherein the second RTT is an expected time required to transmit data and receive an ACK through a second channel; and determining at the user equipment (30, 30', 30", 30a, 30b, 100) a size of uplink data to be transmitted through the first channel based on the first RTT and the second RTT, where estimating the first RTT has the following: obtaining re-transmission parameters corresponding to the first channel; and obtaining the first RTT based on the re-transmission parameters, where: the retransmission parameters have a retransmission period c1 and a maximum retransmission number N1, and obtaining the first RTT obtaining an RTT S1 which is expressed in an equation below: RTTS1=(1−BLER1)∗∑n=0N1−1BLER1n(p1+n∗c1), where BLER1 and p1 are a block error rate and a signal movement time in the first channel, respectively. [2] The method according to claim 1, wherein the retransmission parameters further comprise a maximum retransmission number R1 of a radio link control (RLC) unit, and obtaining the first RTT obtaining an RTT T1than the first RTT, which is expressed in an equation below: RTTT1=RTTS1∗∑r=0R1(BLER1N1)r. [3] The method of claim 1, wherein obtaining the first RTT comprises determining the RTT S1 than the first RTT in response to the BLER1 being less than a first threshold. [4] The method of claim 1, wherein determining the size of the uplink data comprises determining the size of the uplink data as zero in response to the BLER1 being greater than a second threshold. [5] The method according to claim 1, wherein obtaining the retransmission parameters comprises obtaining updated retransmission parameters corresponding to the first channel, and obtaining the first RTT comprises obtaining the first RTT based on the updated retransmission parameters. [6] The method of claim 1, wherein the method is performed in a Packet Data Convergence Protocol (PDCP) layer. [7] The method of claim 1, wherein determining the size of the uplink data comprises determining the size of the uplink data as zero in response to a negative non-acknowledgement (NACK) rate by the first channel being greater than a third threshold. [8] The method of claim 1, further comprising determining a size of the uplink data to be transmitted through the second channel based on the first RTT and the second RTT. [9] The method of claim 1, wherein the user equipment (30, 30', 30", 30a, 30b, 100) communicates with a first base station (10, 10', 10a, 10b) through the first channel and a second base station (20, 20', 20a, 20b) through the second channel, and the first base station (10, 10', 10a, 10b) and the second base station (20, 20', 20a, 20b) communicate with the user equipment (30, 30', 30", 30a, 30b, 100) based on the same radio access technology. [10] The method of claim 1, wherein the user equipment (30, 30', 30", 30a, 30b, 100) communicates with a first base station (10, 10', 10a, 10b) through the first channel and a second base station (20, 20', 20a, 20b) through the second channel, and the first base station (10, 10', 10a, 10b) and the second base station (20, 20', 20a, 20b) communicate with the user equipment (30, 30', 30", 30a, 30b, 100) based on different radio access technologies. [11] A method for multi-connectivity between a plurality of base stations (10, 10', 10a, 10b, 20, 20', 20a, 20b) and a user equipment (30, 30', 30", 30a, 30b, 100) through a plurality of channels, the method comprising: estimating a first round trip time (RTT) at a first base station (10, 10', 10a, 10b), the first RTT being an expected time required to transmit data and receive an acknowledgment response (ACK) through a first channel; obtaining a second RTT at the first base station (10, 10', 10a, 10b), wherein the second RTT is an expected time required to transmit data and receive an ACK through a second channel; and determining at the first base station (10, 10', 10a, 10b) a size of downlink data to be transmitted from the first base station (10, 10', 10a, 10b) to the user equipment (30, 30', 30", 30a, 30b, 100) based on the first RTT and the second RTT, where estimating the first RTT has the following: obtaining re-transmission parameters corresponding to the first channel; and obtaining the first RTT based on the re-transmission parameters, where: the retransmission parameters have a retransmission period c1 and a maximum retransmission number N1, and obtaining the first RTT obtaining an RTT S1 which is expressed in an equation below: RTTS1=(1−BLER1)∗∑n=0N1−1BLER1n(p1+n∗c1), where BLER1 and p1 are a block error rate and a signal movement time in the first channel, respectively. [12] The method of claim 11, wherein obtaining the second RTT comprises receiving the second RTT estimated by a second base station (20, 20', 20a, 20b) from the second base station (20, 20', 20a, 20b). [13] The method of claim 11, wherein obtaining the second RTT comprises: receiving channel information about the second channel from a second base station (20, 20', 20a, 20b); and an estimation of the second RTT based on the channel information. [14] A method for multi-connectivity between a plurality of base stations (10, 10', 10a, 10b, 20, 20', 20a, 20b) and user equipment (30, 30', 30", 30a, 30b, 100) through a plurality of channels, the method comprising: estimating a first round trip time (RTT) at the user equipment (30, 30', 30", 30a, 30b, 100), the first RTT being an expected time required to transmit data and receive an acknowledgment response (ACK) through a first channel; estimating a second round trip time (RTT) at the user equipment (30, 30', 30", 30a, 30b, 100), the second RTT being an expected time required to transmit data and receive an acknowledgment response (ACK) through a second channel; obtaining channel bandwidths (BW1, BW2) of the first and second channels; and determining, at the user equipment (30, 30', 30", 30a, 30b, 100), sizes of uplink data to be transmitted over the first and second channels based on the first and second RTTs and the channel bandwidths (BW1, BW2) of the first and second channels, where estimating the first RTT has the following: obtaining re-transmission parameters corresponding to the first channel; and obtaining the first RTT based on the re-transmission parameters, where: the retransmission parameters have a retransmission period c1 and a maximum retransmission number N1, and obtaining the first RTT obtaining an RTT S1 which is expressed in an equation below: RTTS1=(1−BLER1)∗∑n=0N1−1BLER1n(p1+n∗c1), where BLER1 and p1 are a block error rate and a signal movement time in the first channel, respectively.

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