Source: https://patents.google.com/patent/US8965294B2/en
Timestamp: 2020-04-01 22:06:30
Document Index: 416075345

Matched Legal Cases: ['Application No. 61', 'Application No. 61', 'Application No. 61', 'Application No. 61', 'Application No. 61', 'Application No. 61']

US8965294B2 - Method and apparatus for reducing inter-cell interference in radio communication system - Google Patents
Method and apparatus for reducing inter-cell interference in radio communication system Download PDF
US8965294B2
US8965294B2 US13/582,340 US201113582340A US8965294B2 US 8965294 B2 US8965294 B2 US 8965294B2 US 201113582340 A US201113582340 A US 201113582340A US 8965294 B2 US8965294 B2 US 8965294B2
US13/582,340
US20120329400A1 (en
2010-03-24 Priority to US31724110P priority Critical
2010-03-26 Priority to US31770410P priority
2010-04-05 Priority to US32077610P priority
2010-05-04 Priority to US33090110P priority
2010-08-25 Priority to US37668110P priority
2010-10-08 Priority to US39107710P priority
2011-03-24 Priority to PCT/KR2011/002029 priority patent/WO2011118993A2/en
2011-03-24 Priority to US13/582,340 priority patent/US8965294B2/en
2011-03-24 Application filed by LG Electronics Inc filed Critical LG Electronics Inc
2012-09-04 Assigned to LG ELECTRONICS INC. reassignment LG ELECTRONICS INC. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: KIM, BYOUNGHOON, KIM, HAKSEONG, KIM, KIJUN, SEO, HANBYUL, SEO, INKWON, LEE, DAEWON
2012-12-27 Publication of US20120329400A1 publication Critical patent/US20120329400A1/en
2015-02-24 Publication of US8965294B2 publication Critical patent/US8965294B2/en
238000004891 communication Methods 0 title abstract description 17
A method and apparatus for reducing inter-cell interference in a radio communication system are disclosed. A method for reducing inter-cell interference includes determining REs of a downlink subframe of a first cell overlapping with CRS transmission REs of a downlink subframe of a second cell, determining a portion of REs of the downlink subframe of the first cell overlapping with CRS transmission REs of the downlink subframe of the second cell as punctured REs, mapping one or more downlink channel to the downlink subframe of the first cell other than the punctured REs, and transmitting the one or more downlink channel mapped to the downlink subframe of the first cell to a UE.
This application is the National Phase of PCT/KR2011/002029 filed on Mar. 24, 2011, which claims priority under 35 U.S.C. 119(e) to U.S. Provisional Application No. 61/317,241 filed on Mar. 24, 2010, U.S. Provisional Application No. 61/317,704 filed on Mar. 26, 2010, U.S. Provisional Application No. 61/320,776 filed on Apr. 5, 2010, U.S. Provisional Application No. 61/330,901 filed on May 4, 2010, U.S. Provisional Application No. 61/376,681 filed on Aug. 25, 2010 and U.S. Provisional Application No. 61/391,077 filed on Oct. 8, 2010, all of which are hereby expressly incorporated by reference into the present application.
The present invention relates to a radio communication system, and more particularly, to a method and apparatus for reducing inter-cell interference in a radio communication system.
A terminal (or user equipment) 131 may directly receive services from (or be served by) the macro base station 110 (hereinafter referred to as a macro-terminal). And, alternatively, a terminal (or user equipment) 132 may directly receive services from (or be served by) the micro base station 121 (hereinafter referred to as a micro-terminal). In some cases, a terminal 132 existing within the coverage area of the micro base station 121 may receive services from the macro base station 110.
Depending upon whether or not the terminal (or user equipment) has limited access, the micro base station may be categorized into two different types, the first type being a CSG (Closed Subscriber Group) micro base station, and the second type being an OA (Open Access) or OSC (Open Subscriber Group) micro base station. More specifically, the CSG micro base station may serve (or transmit services to) only specific terminals that are authorized, and the OSG micro base station may serve (or transmit services to) all types of terminals without any particular access limitations.
In the above-described heterogeneous network, if a macro-terminal served by a macro base station is adjacent to a micro base station, interference may occur in a downlink signal received by the macro-terminal from the macro base station due to a strong downlink signal from the micro base station. In addition, a micro-terminal served by the micro base station may be subject to interference by a downlink signal from the macro base station. Alternatively, an uplink signal from the macro-terminal served by the macro base station may subject the micro base station adjacent to the corresponding macro-terminal to strong interference.
An object of the present invention devised to solve the problem lies on a method and apparatus for minimizing interference with respect to another base station when a base station transmits signals to a terminal.
The object of the present invention can be achieved by providing a method for reducing inter-cell interference, including determining, by a first cell, REs (Resource Elements) of a downlink subframe of the first cell overlapped with CRS (Cell-specific Reference Signal) transmission REs of a downlink subframe of a second cell, determining, by the first cell, a portion of REs of the downlink subframe of the first cell overlapped with CRS transmission REs of the downlink subframe of the second cell as punctured REs, mapping, by the first cell, one or more downlink channel to the downlink subframe of the first cell other than the punctured REs, and transmitting, by the first cell, the one or more downlink channel mapped to the downlink subframe of the first cell to a UE (User Equipment).
In another aspect of the present invention, provided herein is an apparatus for reducing inter-cell interference a reception module for receiving uplink signal from a UE (User Equipment), a transmission module for transmitting downlink signal to the UE, and a processor for controlling signal reception and transmission of a first cell via the reception module and transmission module, wherein the processor configured to determine REs (Resource Elements) of a downlink subframe of the first cell overlapped with CRS (Cell-specific Reference Signal) transmission REs of a downlink subframe of a second cell, determine a portion of REs of the downlink subframe of the first cell overlapped with CRS transmission REs of the downlink subframe of the second cell as punctured REs, map one or more downlink channel to the downlink subframe of the first cell other than the punctured REs, and transmit, via the transmission module, the one or more downlink channel mapped to the downlink subframe of the first cell to the UE.
The following description may be commonly applied to embodiments of the present invention.
The punctured REs may include REs positioned in at least one of control region and data region of the downlink subframe of the first cell among the REs of the downlink subframe of the first cell overlapped with CRS transmission REs of the downlink subframe of the second cell.
The punctured REs may include REs corresponding to a portion of CRS transmit antenna ports among the REs of the downlink subframe of the first cell overlapped with CRS transmission REs of the downlink subframe of the second cell.
The punctured REs may be determined separately for different downlink subframe of the first cell.
The punctured REs may further include REs of the downlink subframe of the first cell overlapped with PDCCH (Physical Downlink Control Channel) transmitting region of the downlink subframe of the second cell.
The method may further include transmitting information indicating position of the punctured REs to the UE.
A boundary of the downlink subframe of the first cell may be shifted by a predetermined number of OFDM symbols from a boundary of the downlink subframe of the second cell.
The downlink subframe of the second cell may be configured as a MBSFN (Multicast/Broadcast over Single Frequency Network) subframe. REs other than CRS transmission REs of the downlink subframe in the second cell may be configured as null REs.
The method may further include transmitting DMRS (Demodulation Reference Signal) in the downlink subframe of the first cell, wherein the DMRS are transmitted according to one of DMRS patterns not overlapped with the punctured REs, and wherein the DMRS patterns are DMRS pattern for normal subframe, DMRS pattern for DwPTS (Downlink Pilot Time Slot) with length of 11 or 12 OFDM symbols, and DMRS pattern for DwPTS with length of 9 or 10 OFDM symbols.
According to the present invention, when a base station transmits signals to a UE, a method and apparatus for minimizing interference with another base station can be provided.
The effects obtained from the present invention are not limited to the above-described effect and other effects that are not mentioned herein will be clearly understood to those skilled in the art from the following description of the present invention.
FIG. 1 shows a heterogeneous network wireless communication system;
FIG. 2 shows the structure of a downlink radio frame;
FIG. 3 shows a resource grid in a downlink slot;
FIG. 4 shows the structure of a downlink subframe;
FIG. 5 shows the structure of an uplink subframe;
FIG. 6 shows the configuration of a radio communication system having multiple antennas;
FIG. 7 shows patterns of CRSs and DRSs defined in the existing 3GPP LTE system;
FIG. 8 shows the structure of an uplink subframe including an SRS symbol;
FIG. 9 shows an example of implementing transmission and reception functions of an FDD-mode RN;
FIG. 10 shows transmission to a UE from an RN and downlink transmission to the RN from a base station;
FIG. 11 shows a CRS transmission pattern of a first base station according to the present invention;
FIG. 12 shows a CRS transmission pattern of a second base station according to the present invention;
FIG. 13 shows punctured REs in a downlink subframe according to the present invention;
FIG. 14 shows exemplary REs punctured in a downlink frame according to the present invention;
FIG. 15 shows subframes of first and second base stations having shifted boundaries according to the present invention;
FIG. 16 shows exemplary REs punctured in a downlink subframe according to the present invention;
FIG. 17 shows exemplary REs punctured in a downlink subframe according to the present invention;
FIG. 18 shows exemplary REs punctured in a downlink subframe according to the present invention;
FIG. 19 shows exemplary REs punctured in a downlink subframe according to the present invention;
FIG. 20 shows a DMRS pattern for a normal subframe;
FIG. 21 shows a DMRS pattern for a DwPTS;
FIG. 22 shows exemplary REs punctured in a downlink subframe according to the present invention;
FIG. 23 shows exemplary REs punctured in a downlink subframe according to the present invention;
FIG. 24 shows exemplary subframe shifts according to the present invention;
FIG. 25 shows exemplary subframe shifts according to the present invention;
FIG. 26 shows exemplary REs punctured in a downlink subframe according to the present invention;
FIG. 27 is a diagram explaining an operation for reducing inter-cell interference according to an exemplary embodiment of the present invention;
FIG. 28 is a flowchart showing a process for reducing inter-cell interference according to an exemplary embodiment of the present invention; and
FIG. 29 is a diagram showing a base station (eNB) device according to an exemplary embodiment of the present invention;
FIG. 2( a) is a diagram showing the structure of the type 1 radio frame. A downlink radio frame includes 10 subframes, and one subframe includes two slots in time domain. A time required for transmitting one subframe is defined in a Transmission Time Interval (TTI). For example, one subframe may have a length of 1 ms and one slot may have a length of 0.5 ms. One slot may include a plurality of OFDM symbols in time domain and include a plurality of Resource Blocks (RBs) in frequency domain. Since the 3GPP LTE system uses OFDMA in downlink, the OFDM symbol indicates one symbol duration. The OFDM symbol may be called a SC-FDMA symbol or a symbol duration. A RB is a resource allocation unit and includes a plurality of contiguous subcarriers in one slot.
FIG. 2( b) is a diagram showing the structure of the type 2 radio frame. The type 2 radio frame includes two half frames, each of which includes five subframes, a downlink pilot time slot (DwPTS), a guard period (GP), and an uplink pilot time slot (UpPTS). One of these subframes includes two slots. The DwPTS is used for initial cell search, synchronization and channel estimation at a user equipment. The UpPTS is used for channel estimation and uplink transmission synchronization of the user equipment. The guard period is to remove interference occurring in an uplink due to multi-path delay of a downlink signal between the uplink and a downlink. Meanwhile, one subframe includes two slots regardless of a type of the radio frame.
As shown in FIG. 6( a), if the number of transmission antennas is increased to NT and the number of reception antennas is increased to NR, a theoretical channel transmission capacity is increased in proportion to the number of antennas, unlike the case where a plurality of antennas is used in only a transmitter or a receiver. Accordingly, it is possible to improve a transfer rate and to remarkably improve frequency efficiency. As the channel transmission capacity is increased, the transfer rate may be theoretically increased by a product of a maximum transfer rate R0 upon using a single antenna and a rate increase ratio Ri.
Consider that the NT actually transmitted signals x1, x2, . . . , xN T are configured by applying a weight matrix W to the information vector ŝ with the adjusted transmit powers. The weight matrix W serves to appropriately distribute the transmitted information to each antenna according to a transport channel state, etc., x1, x2, . . . , xN T may be expressed by using the vector x as follows.
x = [ x 1 x 2 ⋮ x i ⋮ x N T ] = [ w 11 w 12 … w 1 ⁢ ⁢ N T w 21 w 22 … w 2 ⁢ N T ⋮ ⋱ w i ⁢ ⁢ 1 w i ⁢ ⁢ 2 … w i ⁢ ⁢ N T ⋮ ⋱ w N T ⁢ 1 w N T ⁢ 2 … w ⁢ N T ⁢ N T ] ⁡ [ s ^ 1 s ^ 2 ⋮ s ^ j ⋮ s ^ N T ] = W ⁢ s ^ = WPs Equation ⁢ ⁢ 5
FIG. 6( b) is a diagram showing channels from the NT transmission antennas to the reception antenna i. The channels may be combined and expressed in the form of a vector and a matrix. In FIG. 6( b), the channels from the NT transmission antennas to the reception antenna i may be expressed as follows.
H = [ h 1 T h 2 T ⋮ h i T ⋮ h N R T ] = [ h 11 h 12 … h 1 ⁢ ⁢ N T h 21 h 22 … h 2 ⁢ N T ⋮ ⋱ h i ⁢ ⁢ 1 h i ⁢ ⁢ 2 … h i ⁢ ⁢ N T ⋮ ⋱ h N R ⁢ 1 h N R ⁢ 2 … h ⁢ N R ⁢ N T ] Equation ⁢ ⁢ 8
y = [ y 1 y 2 ⋮ y i ⋮ y N R ] = [ h 11 h 12 … h 1 ⁢ ⁢ N T h 21 h 22 … h 2 ⁢ N T ⋮ ⋱ h i ⁢ ⁢ 1 h i ⁢ ⁢ 2 … h i ⁢ ⁢ N T ⋮ ⋱ h N R ⁢ 1 h N R ⁢ 2 … h ⁢ N R ⁢ N T ] ⁡ [ x 1 x 2 ⋮ x j ⋮ x N T ] + [ n 1 n 2 ⋮ n i ⋮ n N R ] = Hx + n Equation ⁢ ⁢ 10
FIG. 7 is a diagram showing a pattern of CRSs and DRSs mapped on a downlink RB defined in the existing 3GPP LTE system (e.g., Release-8). The downlink RB as a mapping unit of the RSs may be expressed in units of one subframe on a time domain×12 subcarriers on a frequency domain. That is, on the time axis, one RB has a length of 14 OFDM symbols in case of the normal CP (FIG. 7( a)) and has a length of 12 OFDM symbols in case of the extended CP (FIG. 7( b)).
k = 6 ⁢ ⁢ m + ( v + v shift ) ⁢ ⁢ mod ⁢ ⁢ ⁢ 6 ⁢ ⁢ l = { 0 , N symb DL - 3 if ⁢ ⁢ p ∈ { 0 , 1 } 1 if ⁢ ⁢ p ∈ { 2 , 3 } ⁢ ⁢ m = 0 , ⁣ 1 , … ⁢ , 2 · N RB DL - 1 ⁢ ⁢ m ′ = m + N RB max , DL - N RB DL ⁢ ⁢ v = { ⁢ 0 ⁢ if ⁢ ⁢ p = 0 ⁢ ⁢ and ⁢ ⁢ l = 0 ⁢ 3 ⁢ if ⁢ ⁢ p = 0 ⁢ ⁢ and ⁢ ⁢ l ≠ 0 ⁢ 3 ⁢ if ⁢ ⁢ p = 1 ⁢ ⁢ and ⁢ ⁢ l = 0 ⁢ 0 ⁢ if ⁢ ⁢ p = 1 ⁢ ⁢ and ⁢ ⁢ l ≠ 0 ⁢ 3 ⁢ ( n s ⁢ ⁢ mod ⁢ ⁢ 2 ) ⁢ if ⁢ ⁢ p = 2 ⁢ 3 + 3 ⁢ ( n s ⁢ ⁢ mod ⁢ ⁢ 2 ) ⁢ if ⁢ ⁢ p = 3 ⁢ ⁢ v shift = N ID cell ⁢ ⁢ mod ⁢ ⁢ 6 Equation ⁢ ⁢ 12
In Equation 12, k denotes a subcarrier index, l denotes a symbol index, and p denotes an antenna port index. Nsymb DL denotes the number of OFDM symbols of one downlink slot, NRB DL denotes the number of RBs allocated to the downlink, ns denotes a slot index, and NID cell denotes a cell ID. mod indicates a modulo operation. The location of the RS in the frequency domain depends on a value Vshift. Since the value Vshif depends on the cell ID, the location of the RS has a frequency shift value which varies according to the cell.
k = ( k ′ ) ⁢ ⁢ mod ⁢ ⁢ N sc RB + N sc RB · n PRB ⁢ ⁢ k ′ = { 4 ⁢ ⁢ m ′ + v shift if ⁢ ⁢ l ∈ { 2 , 3 } 4 ⁢ ⁢ m ′ + ( 2 + v shift ) ⁢ ⁢ mod ⁢ ⁢ 4 if ⁢ ⁢ l ∈ { 5 , 6 } ⁢ ⁢ l = { 3 l ′ = 0 6 l ′ = 1 2 l ′ = 2 5 l ′ = 3 ⁢ ⁢ l ′ = { 0 , 1 if ⁢ ⁢ n s ⁢ ⁢ mod ⁢ ⁢ 2 = 0 2 , 3 if ⁢ ⁢ n s ⁢ ⁢ mod ⁢ ⁢ 2 = 1 ⁢ ⁢ m ′ = 0 , 1 , … ⁢ , 3 ⁢ ⁢ N RB PDSCH - 1 ⁢ ⁢ v shift = N ID cell ⁢ ⁢ mod ⁢ ⁢ 3 Equation ⁢ ⁢ 13 k = ( k ′ ) ⁢ ⁢ mod ⁢ ⁢ N sc RB + N sc RB · n PRB ⁢ ⁢ k ′ = { 3 ⁢ m ′ + v shift if ⁢ ⁢ l = 4 3 ⁢ m ′ + ( 2 + v shift ) ⁢ ⁢ mod ⁢ ⁢ 3 if ⁢ ⁢ l = 1 ⁢ ⁢ l = { 4 l ′ ∈ { 0 , 2 } 1 l ′ = 1 ⁢ ⁢ l ′ = { 0 if ⁢ ⁢ n s ⁢ ⁢ mod ⁢ ⁢ 2 = 0 1 , 2 if ⁢ ⁢ n s ⁢ ⁢ mod ⁢ ⁢ 2 = 1 ⁢ ⁢ m ′ = 0 , 1 , … ⁢ , 4 ⁢ ⁢ N RB PDSCH - 1 ⁢ ⁢ v shift = N ID cell ⁢ ⁢ mod ⁢ ⁢ 3 Equation ⁢ ⁢ 14
A RN forwards data transmitted or received between the eNodeB and the UE, two different links (backhaul link and access link) are applied to the respective carrier frequency bands having different attributes; The eNodeB may include a donor cell. The RN is wirelessly connected to a radio access network through the donor cell.
Operation for Reducing Inter-Cell Interference
Referring back to FIG. 1, if the terminal 132 is a macro-terminal (served by the macro base station 110), a signal generated from the micro base station 122 may create interference in a downlink signal to the terminal 132 from the macro base station 110. For example, if the micro base station 122 is a CSG cell (namely, a cell that is accessible only by authorized terminals) and if the terminal 132 does not belong to a CSG, then the terminal 132 is not served by the micro base station 122 and should transmit and receive signals to and from the macro base station 110 even though the terminal 132 is located within a coverage range of the micro base station 122. As a result, the terminal 132 may be subject to strong interference from the micro base station 122 upon receiving the downlink signal.
In this case, RSs (e.g., CRSs), which are used for, for example, measurement of a downlink channel state in the terminal 132, may be subject to strong interference and thus strength of a downlink reception signal, for example, a Signal-to-Interference plus Noise Ratio (SINR) may be lowered to a prescribed level or less. Then, the terminal 132 recognizes a current state as Radio Link Failure (RLF) and may perform an operation for connection re-establishment to another base station. That is, if data of the base station 122 is transmitted to REs for RS transmission transmitted from the macro base station 110 to the terminal 132, strong interference may occur in the RSs received by the terminal 132. To reduce such ICI, it is possible not to interfere with RSs of a cell which is subject to interference, in downlink transmission of a cell producing interference.
According to the present invention, when a base station, which subjects a specific UE to strong interference, transmits a downlink signal (PDCCH and/or PDSCH), all or some of RE locations to which RSs (e.g., CRSs) measured by the specific terminal are transmitted may be punctured so as not to interfere with the corresponding RSs. For example, referring back to FIG. 1, upon transmitting a PDCCH and/or a PDSCH, the micro base station 122 may operate to transmit no signals in REs overlapping with REs where CRSs of the macro base station 110 are transmitted.
In the present invention, RE locations for CRS transmission between cells interfering with each other, that is, between cells which are targets of Inter-Cell Interference Coordination (ICIC) may be set not to overlap. For instance, proper frequency offset (V-shift) may be applied so that a CRS transmission RE of the macro base station 110 does not overlap with a CRS transmission RE of the base station 122. Namely, a CRS transmission RE of one base station may be set not to overlap with a CRS transmission RE of another base station.
In addition, even when RSs (e.g., CRSs) of the micro base station 122 are transmitted to data transmission REs transmitted from the macro base station 110 to the terminal 132, ICI may occur. Since the RSs are generally transmitted at a high transmission power (namely, in a power boosted state), the RSs may create strong interference in data transmission of other cells. To reduce such ICI, in downlink transmission of a cell which is subject to interference, data may not be transmitted in RS transmission locations of a cell which creates interference. More specifically, since a terminal (especially, a legacy terminal) served by a cell which creates interference expects that a serving cell thereof (i.e., the cell which creates interference) always transmits CRSs, the serving cell cannot help transmitting the CRSs even though CRSs of a cell create interference in data transmission of another cell. Accordingly, non-transmission of data in a CRS transmission RE of a cell which creates interference by a cell which is subject to interference may be considered.
According to the present invention, when a first base station which is subject to interference from a second base station transmits a PDCCH and/or a PDSCH to a specific terminal, all or part of RE locations to which RSs (e.g., CRSs) of the second base station are transmitted are punctured so that the specific terminal may not be subject to interference by the RSs of the second base station upon transmitting control signals and/or data. For example, referring back to FIG. 1, when the macro base station 110 transmits a PDSCH and/or a PDSCH, no signals may be transmitted in REs overlapping with REs where CRSs of the micro base station 122 are transmitted.
In all exemplary embodiments of the present invention described hereinbelow, a punctured RE location may refer to an RE location which is subject to interference by another base station or an RE location which subjects a terminal associated with another base station to interference.
In addition, a base station which creates interference is referred to as an interfering cell, a terminal which is subject to interference is referred to as a victim UE, and a base station serving a terminal which is subject to interference is referred to as a victim cell. In two cells which are targets of ICIC, an interfering cell may be referred to as a coordinated cell.
According to the first embodiment, in two base stations interfering with each other, one base station may puncture all REs overlapping with CRS transmission REs of the other base station. In other words, an interfering cell in downlink transmission thereof may puncture all REs overlapping with all CRS transmission REs of a victim cell, and/or the victim cell in downlink transmission thereof may puncture all REs overlapping with all CRS transmission REs of the interfering cell. The former serves to reduce ICI in CRS measurement of a victim UE and the latter serves to reduce ICI in data demodulation of the victim UE. Puncturing all REs overlapping with CRS transmission REs of another cell indicates that a cell sets REs corresponding to all antenna ports 0 to 3 (i.e., 4 CRS antenna ports) to which CRSs of another cell are transmitted to null REs and transmits signals. In other words, REs overlapping with CRS transmission REs of another cell are muted in a downlink subframe of a cell.
FIGS. 11 to 13 are diagrams explaining examples of puncturing REs according to the first embodiment of the present invention.
FIG. 11 illustrates a CRS transmission pattern (locations of REs to which CRSs are transmitted in one RB) of a first base station. It is assumed that the first base station performs transmission of 2 antenna ports for example. That is, the first base station transmits CRSs for antenna ports 0 and 1.
FIG. 12 illustrates a CRS transmission pattern of a second base station. It is assumed that the second base station performs transmission of 4 antenna ports for example. That is, the second base station transmits CRSs for antenna ports 0 to 3.
The CRS transmission patterns of FIGS. 11 and 12 show patterns shifted by an offset (V-shift) of one subcarrier.
FIG. 13 shows puncturing REs overlapping with all CRS ports of the second base station while the first base station performs downlink transmission according to an exemplary embodiment of the present invention. It is assumed in FIG. 13 that a PDCCH has a length of 2 OFDM symbols.
In a description of FIGS. 11 to 13 and exemplary embodiments of the present invention which will be described hereinbelow, the first base station may be an interfering cell and the second base station may be a victim cell, or the first base station may be a victim cell and the second base station may be an interfering cell. The former serves to prevent CRSs of the victim cell from interfering due to data transmitted by the interfering cell, and the latter serves to eliminate an effect of CRSs transmitted by the interfering cell on data transmission of the victim cell. Thus, in two base stations interfering with each other, the two methods for puncturing REs overlapping with the CRS pattern of a counterpart base station may be separately applied or may be simultaneously applied.
According to the second embodiment, in two base stations interfering with each other, one base station may puncture a part of REs overlapping with CRS transmission REs of the other base station. In other words, an interfering cell in downlink transmission thereof may puncture a part of REs overlapping with CRS transmission REs of a victim cell, and/or the victim cell in downlink transmission thereof may puncture a part of REs overlapping with CRS transmission REs of the interfering cell. The former serves to reduce ICI in CRS measurement of the victim UE and the latter serves to reduce ICI in data demodulation of the victim UE. Puncturing a part of REs overlapping with all CRS transmission REs of another cell indicates that a cell sets REs corresponding to one or more antenna ports of antenna ports 0 to 3 to which CRSs of another cell are transmitted to null REs and transmits signals. One or more antenna ports may be one of the antenna ports 0 to 3 or may be set to a combination of two or more antenna ports such as the antenna ports 0 and 2.
FIG. 14 is a diagram illustrating an example of puncturing REs according to the second exemplary embodiment of the present invention.
Here, a CRS transmission pattern of a first base station assumes transmission of 2 antenna ports as in FIG. 11 and a CRS transmission pattern of a second base station assumes transmission of 4 antenna ports as in FIG. 12.
FIG. 14 shows puncturing REs overlapping with REs corresponding to an antenna port 0 among CRS transmission REs of the second base station while the first base station performs downlink transmission according to an exemplary embodiment of the present invention. It is assumed in FIG. 14 that a PDCCH has a length of 2 OFDM symbols.
In a description of FIG. 14, the first base station may be an interfering cell and the second base station may be a victim cell, or the first base station may be a victim cell and the second base station may be an interfering cell. The former serves to prevent CRSs of the victim cell from interfering due to data transmitted by the interfering cell, and the latter serves to eliminate an effect of CRSs transmitted by the interfering cell on data transmission of the victim cell. Especially, in the former case, puncturing a part of REs overlapping CRS transmission REs of another cell in downlink transmission of a cell is suitable for the case where a UE served by another cell uses only CRSs of a part of CRS antenna ports (e.g., only CRSs of the antenna port 0) upon determining RLF. Thus, in two base stations interfering with each other, the two methods for puncturing a part of REs overlapping with the CRS pattern of a counterpart base station may be separately applied or may be simultaneously applied.
While the above first and second embodiments have described puncturing REs in both a PDCCH area and a PDSCH area of a first base station when the first base station in downlink transmission thereof punctures all or some REs overlapping with the CRS pattern of a second base station in the two base stations interfering with each other, the present invention is not limited thereto. That is, in the downlink transmission of the first base station, all or some REs overlapping with the CRS pattern of the second base station may be punctured in either the PDCCH area or the PDSCH area. Alternatively, when REs overlapping with REs of some antenna ports of the CRS pattern of the second base station are punctured in downlink transmission of the first base station, the partial antenna ports may be differently set in the PDCCH area and the PDSCH area. For example, REs overlapping REs of antenna ports 0 and 1 of the CRS pattern of the second base station may be punctured in the PDCCH area of the first base station, and REs overlapping REs of the antenna port 0 of the CRS pattern of the second base station may be punctured in the PDSCH area of the first base station.
The above first and second embodiments have described the methods for puncturing REs in a base station. Hereinafter, a method for a base station to inform a UE served thereby of an RE puncturing pattern is described.
In two base stations interfering with each other, if one base station performs an operation of puncturing REs overlapping with a CRS pattern of the other base station, the corresponding base station may inform UEs connected thereto of an RE puncturing pattern through an upper layer signal or a physical layer signal. Upon receiving the signal, the UEs are able to determine the locations where RE puncturing is applied and incorporate the puncturing locations in channel decoding, thereby reducing capability degradation due to RE puncturing. For example, in the PDCCH, more PDCCH resources may be used (by applying CCE aggregation of a higher level) to reduce capability degradation, and in the PDSCH, a Modulation and Coding Scheme (MCS) may be appropriately controlled in consideration of RE puncturing.
To control a proper MCS, rate matching may be used as another method (i.e., a null RE transmission scheme) in which, in two base stations interfering with each other, one base station transmits REs overlapping a CRS pattern of another base station as null REs. Then, a base station configures a transmission signal using rate matching and UEs perform decoding according to the rate matching. In some cases, the respective UEs should decode different null RE transmission schemes. To remove uncertainty due to such cases, the base station may suitably apply RE puncturing and rate matching using UE capability information (e.g., release information) of a corresponding UE.
If an RE puncturing pattern signaling scheme and a rate matching application scheme according to RE puncturing are mixed, the base station should inform the UE which scheme is used to transmit null REs, so that the UE may perform decoding suitable for each transmission scheme.
Although the above first to third embodiments have been described under the assumption that synchronization has been established such that subframe boundaries (or timings) of two base stations interfering with each other coincide with each other, the present invention is not limited thereto. Hereinafter, exemplary embodiments applied to the case where the subframe boundaries of two base stations interfering with each other do not coincide with each other will be described.
The fourth embodiment relates to a method for reducing ICI when a subframe boundary of one of two base stations interfering with each other is shifted by a prescribed number of OFDM symbols from a subframe boundary of the other base station. For example, in two base stations interfering with each other, when any base station punctures REs overlapping a CRS pattern of the other base station in PDCCH and PDSCH areas, locations of the overlapping REs may be determined in consideration of the shifted number of OFDM symbols of a subframe boundary of one base station from a subframe boundary of the other base station. In this case, a base station performing RE puncturing informs UEs connected thereto to what extent (e.g., the number of OFDM symbols) a subframe boundary is shifted from the other base station so that the UEs may identify the locations of punctured REs. Such a shift in a subframe boundary may be used to adjust OFDM symbols where CRSs are concentrated so as to be transmitted at different times.
In FIG. 15, a subframe boundary of a first base station (eNB1) is shifted by one OFDM symbol from a subframe boundary of a second base station (eNB2). Then, when the first base station transmits a PDCCH and/or a PDSCH, the locations of REs overlapping with a CRS pattern of the second base station, that is, the locations of punctured REs are also shifted by one OFDM symbol.
FIGS. 16 and 17 illustrate punctured REs in downlink transmission of a first base station, that is, REs overlapping with a CRS pattern of a second base station when a subframe boundary of the first base station is shifted by one OFDM symbol from a subframe boundary of the second base station as shown in FIG. 15. In FIGS. 16 and 17, it is assumed that CRS patterns of the first and second base stations are the same as those shown in FIGS. 11 and 12 and embodiments to which a shift in the subframe boundaries of the first and second base stations is applied are shown. FIG. 16 shows an example of puncturing all REs overlapping with the CRS pattern of the second base station in downlink transmission of the first base station. FIG. 17 shows an example of partially puncturing REs overlapping with the CRS pattern of the second base station in downlink transmission of the first base station. Namely, the examples of FIGS. 16 and 17 show the embodiments to which a shift in subframe boundaries is applied in the examples of FIGS. 13 and 14.
When subframe boundaries of two base stations do not coincide with each other as shown in FIGS. 16 and 17, that is, when any one of the subframe boundaries is shifted, a subframe boundary of one base station is within one subframe of the other base station. In FIG. 16 for example, in view of a first base station, OFDM symbols 12 and 13 are within the same subframe, but in view of a second base station, they are within different subframes. Namely, the OFDM symbols 12 and 13 in FIG. 16 correspond to an OFDM symbol 13 of one subframe and an OFDM symbol 0 of a subsequent subframe, respectively, in view of the second base station. In other words, the OFDM symbol 13 of the first base station overlaps with the OFDM symbol 0 of the second base station.
Important control information such as a PCFICH, a PHICH, and a PDCCH is conveyed in the first k OFDM symbols (e.g., OFDM symbols 0, 1, and 2) of a downlink subframe. If such important control information is subject to interference by other signals, a problem may occur in that a UE cannot receive any downlink transmission signals. Accordingly, in order to avoid interference with important downlink control information when subframe boundaries of two base stations do not coincide with each other, a first base station may not transmit a PDSCH and/or a CRS in the last k OFDM symbols of a first base station corresponding to the first k OFDM symbols of the second base station, in addition to puncturing REs overlapping with a CRS pattern of the second base station. For example, if a subframe boundary of one base station is shifted by one OFDM symbol from a subframe boundary of the other base station as shown in FIG. 15, the first base station may transmit a null RE instead of transmitting a PDSCH and/or a CRS in an OFDM symbol 13. Generally, if a subframe boundary of the first base station is shifted by k OFDM symbols, transmission may not be performed in the last k OFDM symbols in each subframe to prevent interference with a control channel of the second base station.
FIGS. 18 and 19 illustrate puncturing operations in the last OFDM symbol of a subframe of a first base station by the shifted number of OFDM symbols of a subframe boundary in the operations of FIGS. 16 and 17, respectively. In FIGS. 18 and 19, it is assumed that a subframe boundary is shifted by one OFDM symbol, and examples of puncturing the last OFDM symbol are shown in a downlink subframe of the first base station.
The first base station may transmit a signal informing a UE of information about a CRS pattern of the second base station and information about to what extent a subframe boundary is shifted. The UE then recognizes locations of punctured REs through information received from the first base station and performs channel decoding.
Upon application of the above-described first to fifth embodiments, in two base stations interfering with each other, a first base station may not puncture REs to which a PCFICH and a PHICH thereof are transmitted when puncturing REs overlapping with CRSs and/or important control information of a second base station during downlink transmission thereof. That is, REs to which the PCFICH and the PHICH of the first base station are transmitted may be transmitted without puncturing even though they overlap with the CRSs and/or important downlink control information of the second base station. This is because waste of resources generated is very significant when the PCFICH and the PHICH are not transmitted or when the UE fails to decode the PCFICH and the PHICH, whereas resources occupied by the PCFICH and the PHICH are not great. However, the REs to which the PCFICH and the PHICH of the first base station are transmitted are not excluded from being punctured in the same way as in REs applied to the PDCCH.
In an LTE-A system, DMRS based data demodulation is considered as described previously. A DMRS for two or more layers is defined according to the configuration of an extended antenna supported in the LTE-A system. A DMRS pattern may be designed as shown in FIGS. 20 and 21. FIG. 20 illustrates REs for DMRS transmission in a normal subframe of a normal CP. FIG. 21 illustrates REs for DMRS transmission which are designed for use in a DwPTS in a special subframe in the type 2 radio frame (FIG. 2( b)) applied to TDD. A length of the DwPTS in the special subframe of a TDD type frame structure is different from that in a normal subframe. For example, the DwPTS may have a length of 3, 9, 10, 11, or 12 OFDM symbols in case of a normal CP. Since no data is transmitted when a length of the DwPTS is 3 OFDM symbols, DMRSs are not needed. When a length of the DwPTS is 9, 10, 11, or 12 OFDM symbols, since a UpPTS interval for uplink transmission is present in some symbols starting from the last symbol in the special subframe, REs for DMRS transmission are present in symbols except for the symbols for the UpPTS interval. FIG. 21( a) illustrates a DMRS pattern when a length of the DwPTS is 11 or 12 OFDM symbols. FIG. 21( b) illustrates a DMRS pattern when a length of the DwPTS is 9 or 10 OFDM symbols.
Meanwhile, in two base stations interfering with each other, channel estimation through the DMRSs may be problematic if the last few OFDM symbols of a downlink subframe of any base station are punctured. This is because the DMRSs are present in the last two OFDM symbols of the downlink subframe in case of a normal subframe as shown in FIG. 20.
To solve this problem, according to the present invention, a DMRS pattern designed for use in a DwPTS present in a special subframe in a TDD radio frame structure, for example, the DMRS pattern of FIG. 21 may be used when some symbols are sequentially punctured from the last OFDM symbol of a downlink subframe. For instance, if a base station does not puncture the last symbol (e.g., as in the first to fourth embodiments), the DMRS pattern designed for the normal subframe may be used as shown in FIG. 20. Meanwhile, if a base station punctures the last few OFDM symbols (e.g., as in the fifth embodiment), the DMRS pattern designed for the DwPTS may be used as shown in FIG. 21. The DMRS pattern of FIG. 21( a) may be used to puncture the last one or two OFDM symbols, and the DMRS pattern of FIG. 21( b) may be used to puncture the last three or four OFDM symbols.
A base station may explicitly signal a DMRS pattern to be used in a UE through an upper layer signal or a physical layer signal or may cause the UE to implicitly identify a DMRS pattern to be used in a downlink subframe based on the number of punctured OFDM symbols of the last part of the corresponding downlink subframe. For example, when the last one or two OFDM symbols of a downlink subframe are punctured, the UE may define the DMRS pattern (FIG. 21( a)) designed for a DwPTS having a length of 11 or 12 OFDM symbols.
The seventh embodiment relates to a method, in downlink transmission of one of two base stations interfering with each other, for setting different RE puncturing patterns in every downlink subframes upon puncturing REs overlapping with a CRS pattern of the other base station.
For example, in the above-described first to sixth embodiments, the first base station may apply different RE puncturing patterns according to the configuration of an MBSFN subframe of the second base station. The MBSFN subframe is essentially used for a Multimedia Broadcast and Multicast Service (MBMS) which simultaneously transmits the same signal in the same cell. Accordingly, an RS transmission scheme defined in the MBSFN subframe differs from a unicast scheme in which different data is transmitted to each cell.
Specifically, for a subframe set by the second base station as a normal subframe, the first base station may puncture REs overlapping with CRSs present in both a PDCCH area and a PDSCH area of a downlink subframe of the second base station. FIGS. 13, 14, 16, 17, 18, and 19 show embodiments of such an operation.
Meanwhile, for a subframe set by the second base station as an MBSFN subframe, the first base station may puncture REs overlapping with CRSs present in the PDCCH area (i.e., OFDM symbols 0 and 1) of the MBSFN subframe of the second base station, and may not puncture REs overlapping CRSs present in the PDSCH area (i.e., OFDM symbols 2 to 13) of the MBSFN subframe of the second base station. FIG. 22 shows an embodiment of such an operation.
FIG. 22 illustrates an example of puncturing REs in a first base station in the case where a downlink subframe of a second base station is set as the MBSFN subframe, when a subframe boundary of one of two cells interfering with each other is shifted by one OFDM symbol from a subframe boundary of the other cell as shown in FIG. 16. In this way, if the number of REs punctured in the first base station is reduced according to the configuration of the MBSFN subframe of the second base station, a data rate of a UE served by the first base station can be increased without affecting CRS measurement in a UE served by the second base station (i.e., in a UE which is subject to interference from the first base station).
For such an operation, the first and second base stations may exchange MBSFN subframe configuration information, and each of the base stations may inform a corresponding UE of MBSFN subframe configuration information of the other base station (or information about a puncturing pattern to be applied to a corresponding subframe) through an upper layer signal or a physical layer signal. The UE may recognize a proper puncturing pattern to be applied to a corresponding subframe through the received information. For example, the UE may distinguish between a normal subframe and an MBSFN subframe through subframe index information and the MBSFN subframe configuration information so that it may apply a proper RE puncturing pattern according to a corresponding subframe.
In the eighth embodiment, an RE puncturing operation for reducing ICI is described in detail when a subframe boundary of one first base station is shifted by one or more OFDM symbols from a subframe boundary of the other base station in two base stations interfering with each other.
FIG. 23 illustrates an example of puncturing REs in a first base station in the case where a downlink subframe of a second base station is set as an MBSFN subframe when a subframe boundary of one base station is shifted by 2 OFDM symbols from a subframe boundary of the other base station. The example of FIG. 23 has an advantage of not performing RE puncturing in a control area of a downlink subframe of the first base station. That is, PDCCH areas of respective downlink subframes of the two base stations interfering with each other may be prevented from overlapping with each other. Since a PDCCH area in an LTE system may have a maximum length of 4 OFDM symbols, the PDCCH areas of the two base stations do not overlap if a subframe boundary of one of two base stations interfering with each other is shifted by 4 or more OFDM symbols from a subframe boundary of the other base station.
If a subframe boundary of one of two base stations is shifted by a plurality of OFDM symbols from a subframe boundary of the other base station, OFDM symbols to which CRSs of the two base stations are transmitted may be shifted to non-overlapping locations when considering this case. In the LTE system, CRS antenna ports 0 and 1 are located in OFDM symbol indexes 0, 4, 7, and 11 and CRS antenna ports 2 and 3 are located in OFDM symbol indexes 1 and 8. Hence, a subframe shift may be applied as shown in FIG. 24. FIG. 24 shows exemplary subframe shifts when each of a first base station (eNB1) and a second base station (eNB2) transmits CRSs (CRS ports 0, 1, 2, and 3) over four antenna ports. As shown in FIG. 24, a downlink subframe boundary of one of two base stations may be shifted by 2, 5, 9, or 12 OFDM symbols from a downlink subframe boundary of the other base station so that OFDM symbols for transmitting CRSs in PDCCH areas of downlink subframes of two base stations interfering with each other do not overlap.
Alternatively, if the number of antenna ports of the second base station is limited to 2 or less, a subframe shift shown in FIG. 25 may be performed so that OFDM symbols for transmitting CRSs of two base stations interfering with each other do not overlap. This may be usefully applied when the second base station is an HeNB. FIG. 25 illustrates exemplary subframe shifts when a first base station (eNB1) transmits CRSs (CRS ports 0, 1, 2, and 3) over 4 antenna ports and a second base station (eNB2) transmits CRSs (CRS ports 0 and 1) over two antenna ports. As shown in FIG. 25, a downlink subframe boundary of one of two base stations may be shifted by 2, 3, 5, 6, 9, 12 or 13 OFDM symbols from a downlink subframe boundary of the other base station so that OFDM symbols for transmitting CRSs in PDCCH areas of downlink subframes of two base stations interfering with each other do not overlap.
The above-described fifth embodiment (FIGS. 18 and 19) is a method for preventing interference with a control channel of a second base station by puncturing the last k OFDM symbols of a subframe in a first base station when a subframe boundary of the first base station is shifted by k (where k≧1) OFDM symbols. In the ninth embodiment, a method is described for partially puncturing OFDM symbols in the middle of a downlink subframe of one base station when a subframe boundary of one of two base stations interfering with each other is shifted with respect to a subframe boundary of the other base station.
As in a part of the embodiments of FIGS. 24 and 25, if a subframe boundary of one of two base stations interfering with each other is shifted by a large number of OFDM symbols (e.g., 3 or more OFDM symbols) from a subframe boundary of the other base station, too many OFDM symbols are not transmitted when a base station, a subframe boundary of which is shifted, punctures the shifted OFDM symbols sequentially from the last symbol of a downlink subframe, thereby greatly lowering transmission capabilities. Accordingly, the present embodiment proposes puncturing OFDM symbols, in a downlink subframe of a first base station, overlapping with control channel transmission OFDM symbols of a second base station.
For example, if a subframe boundary of one of two base stations interfering with each other is shifted by 6 OFDM symbols from a subframe boundary of the other base station as shown in FIG. 26, OFDM symbols (8, 9, and 10 in FIG. 26) of a downlink subframe of a first base station overlapping with control channel transmission OFDM symbols (OFDM symbols 0, 1, and 2 of a subframe of a second base station) of the second base station may be punctured.
Although FIG. 26 shows non-puncturing REs overlapping with CRS transmission REs of the second base station in the downlink subframe of the first base station, the present embodiment is not limited thereto. Namely, in the embodiment of FIG. 16, REs overlapping with CRS transmission REs of the second base station in the downlink subframe of the first base station may be additionally punctured.
Alternatively, if a subframe boundary of one of two base stations interfering with each other is shifted by one half of a subframe (e.g., by 13 OFDM symbols) from a subframe boundary of the other base station, punctured OFDM symbols of the first base station overlapping with the control channel transmission OFDM symbols of the second base station may be present in the front part of the subframe.
In this way, in order to perform an operation for puncturing a part of OFDM symbols of the middle of one subframe, a base station may inform a UE of the number of shifted and/or punctured OFDM symbols in a subframe through an upper layer signal or a physical layer signal.
In all of the above-described embodiments of the present invention, punctured REs (i.e., REs overlapping with CRS transmission REs of the second base station and/or OFDM symbols overlapping with the control channel transmission OFDM symbols of the second base station) in the downlink subframe of the first base station are purely exemplary and the present invention includes puncturing only a part of corresponding REs. For example, REs may partially punctured in either a PDCCH area or a PDSCH area, or only REs corresponding to some CRS antenna ports may be punctured. Moreover different RE puncturing patterns may be applied to each subframe (e.g., according to MBSFN subframe configuration). Thus, one or more methods for applying RE puncturing may be simultaneously or independently applied.
When an RE puncturing pattern is applied according to all the above-described embodiments, it is necessary to determine a DMRS pattern to be used in a corresponding subframe. For example, when OFDM symbols 8, 9, and 10 in one subframe are punctured as shown in FIG. 26, a DMRS pattern for a DwPTS having a length of 9 or 10 OFDM symbols shown in FIG. 21( b) may be used. Here, a base station and a UE may determine whether a DMRS pattern is influenced by an RE puncturing pattern of a corresponding subframe in order of a DMRS pattern for a normal subframe, a DMRS pattern for a DwPTS having a length of 11 or 12 OFDM symbols, and a DMRS pattern for a DwPTS having a length of 9 or 10 OFDM symbols. Then the base station and the UE may search for a DMRS pattern which is not influenced by an RE puncturing pattern (i.e., a DMRS pattern in which punctured REs do not coincide with DMRS transmission REs) and specify a counterpart operation to use a corresponding DMRS pattern.
Hereinafter, a method is described for signaling information informing UEs belonging to a specific base station of the locations of punctured or rate-matched REs when the specific base station performs puncturing or rate matching for coordination with another base station. REs which are punctured or rate-matched will be collectively referred to as punctured REs. Punctured REs mean REs which are subjected to interference caused by another base station. Information about the locations of punctured REs may include presence/absence of punctured REs in each subframe, a time and/or frequency offset between a punctured RE pattern and a reference pattern, the number of transmission antennas of a base station related to punctured REs, and the like. The information about the locations of punctured REs (or information about the locations of REs subjected to interference caused by another base station) is described in detail hereinbelow.
(1) Presence/Absence of Punctured REs in Each Subframe
Whether punctured REs are present in a corresponding subframe may be signaled in predetermined units (e.g., in units of a radio frame (10 ms) or a subframe (1 ms)). This may be signaled by a bit map method or a method for defining an index according to a specific pattern and indicating a corresponding index, through an upper layer signal or a physical layer signal.
(2) Time/Frequency Offset Between a Punctured RE Pattern and a Reference Pattern
To indicate a punctured RE pattern, a reference pattern for puncturing REs is defined and a time/frequency offset from the reference pattern is indicated. If a CRS pattern used in each cell is determined through the time/frequency offset, the reference pattern may be defined as a pattern which is the same as a CRS pattern used in a corresponding base station or may be defined as an arbitrary RE puncturing pattern. Thus, since an RE puncturing pattern can be signaled to a UE by indicating to what extent a time/frequency offset is generated compared with the reference pattern, signaling overhead can be reduced compared with indicating the RE puncturing pattern itself.
(3) Number of Transmission Antennas of Other Cells
In two cells interfering with each other, locations of REs punctured in a first cell may be determined by locations of CRS transmission REs of a second cell. The number of CRS transmission REs may vary according to the number of transmission antennas used by a corresponding cell. Accordingly, a cell (i.e., the first cell) which informs a UE of information about punctured REs may signal the number of transmission antennas used by a cell (i.e., the second cell) related to the punctured REs to the UE.
(4) Use/Non-Use of MBSFN Mode
As is described in conjunction with FIGS. 22 and 23, upon determining punctured REs in a downlink subframe of a first cell in two cells interfering with each other, if a downlink subframe of a second cell is set to an MBSFN subframe (i.e., if only CRSs of a PDCCH area in a second cell are transmitted), UE capabilities can be improved by reducing the number of punctured REs in the downlink subframe of the first cell. For this operation, a base station performing RE puncturing may signal whether a subframe of another base station is an MBSFN subframe or a normal subframe with respect to a subframe to which RE puncturing is applied.
(5) Symbol Level Puncturing
Although RE puncturing may be performed in units of REs, it may also be performed in units of OFDM symbols as in the above-described fifth embodiment. Accordingly, upon puncturing all OFDM symbols, a base station may signal information indicting punctured symbols to a UE.
(6) Time Domain in which RE Puncturing is Performed
Signaling may be used to indicate whether RE puncturing is used in a specific time domain of downlink transmission. For example, performing RE puncturing in either a PDCCH area or a PDSCH area may be signaled. Alternatively, performing RE puncturing in either a first slot or a second slot may be signaled. Further, performing RE puncturing in one of N (N=14 in a normal CP) OFDM symbols or a plurality of OFDM symbols may be signaled. Through such signaling, whether RE puncturing is applied to various units of time domains (PDCCH/PDSCH, slots, OFDM symbols) may be informed.
The above-described signaling information may be signaled independently or by combination.
If RE puncturing and rate matching are mixed as described in the third embodiment, information which can distinguish between RE puncturing and rate matching may be signaled together with signaling information indicating whether punctured REs are present. Then a UE is able to recognize a null RE transmission scheme.
Hereinafter, a scenario to which the above-described RE puncturing (or RE muting) is applied is described. Application of the RE puncturing method may be adaptively determined according to whether there is a base station which subjects another base station to interference and/or according to capabilities of a UE which is subject to interference (e.g., whether a UE is a legacy UE supporting only 3GPP LTE release-8 or release-9 or an advanced UE supporting 3GPP LTE-A). More specifically, the following operational scenario may be considered. Hereinbelow, it is assumed that a downlink signal from a macro base station (MeNB) to a macro UE (MUE) is subject to interference by a downlink signal from a micro base station (HeNB) to a micro UE (HUE). That is, it is assumed that the HeNB is an interfering cell, the MeNB is a victim cell, and the MUE is a victim UE. However, the present invention is not limited thereto and the same principle of the present invention may be applied even when interference occurs between two arbitrary base stations.
(1) Case where Both MUE and HUE are Legacy UEs
In this case, the HeNB should transmit null REs in CRS locations of the MeNB to prevent RLF of the MUE (i.e., to reduce interference). However, since the HUE cannot recognize transmission of the null REs, an operation for puncturing specific data (PDSCH) REs is performed by the HeNB.
(2) Case where MUE is Legacy UE and HUE is Advanced UE
In this case, the HeNB also should transmit null REs in CRS locations of the MeNB to prevent RLF of the MUE. Meanwhile, since the HUE can be aware of the locations of the null REs, the HeNB may inform an HUE of the locations of the null REs and may perform rate matching in which data REs are not mapped to corresponding locations. Alternatively, the HeNB may inform the HUE of the locations of the null REs and the HUE may perform an operation in which corresponding REs are not used for data decoding.
(3) Case where MUE is Advanced UE
In this case, the MUE may operate so as to perform radio link monitoring only with respect to resources having weak interference which are not transmitted by the HeNB in order to prevent unnecessary RLF. Since the HeNB is permitted to transmit data (PDSCH) in the locations of CRSs of the MeNB, an RE muting operation is not performed.
(4) Case where MUE is not Present in Adjacent Area of HeNB
In this case, since the MUE is not subject to interference from the HeNB, the HeNB may not perform the RE muting operation similarly to Case (3).
To perform an operation for reducing ICI according to the above-described scenario, the MeNB may transmit a signal informing the HeNB whether there is an MUE adjacent to the corresponding HeNB and, if an MUE adjacent to the HeNB is present, may transmit a signal indicating capabilities of the corresponding MUE, (e.g., whether the MUE is a legacy UE or an advanced UE). As a more direct method, the MeNB may transmit a signal informing the HeNB whether the corresponding HeNB is to perform RE muting. Whether to transmit this signal may be determined by the MeNB by judging to what extent a specific MUE is adjacent to the HeNB. For example, if a reception power level of an adjacent cell measured for the corresponding HeNB by the specific MUE is very high, the MeNB may transmit a signal for the ICI reduction operation to the HeNB.
If the advanced MUE has radio link monitoring capabilities with respect to only resources having weak interference, such resource-specific monitoring may be controlled by the MeNB to be performed only when the MUE is near the HeNB. Alternatively, the MUE may voluntarily perform the resource-specific monitoring. For example, the MUE generally performs radio link monitoring with respect to all resources but, if RLF occurs, the MUE may perform radio link monitoring only with respect to specific resources (the specific resources may be resources designated by a base station or may be resources determined as having weak interference through an interference power measured by the UE). If RLF does not occur in the resource-specific monitoring, the MUE may recognize that resource coordination is performed between an MeNB connected thereto and another eNB which creates strong interference therewith.
A description has been made hereinabove of an operation in which one base station (HeNB) does not transmit the PCFICH/PHICH/PDCCH and/or PDSCH in REs overlapping with CRS transmission REs of another base station (MeNB) and an operation of a UE (MUE) according thereto, in the case where ICI is very severe (e.g., when both the HeNB and the MeNB are present and the MUE is adjacent to the HeNB). These operations may be voluntarily performed by a UE without an additional signal (e.g., a signal for a CRS transmission pattern of an adjacent cell) from a base station. For example, if a UE (MUE) detects a strong CRS signal from an adjacent cell (HeNB), the MUE may decode corresponding channels using only REs except for REs which are subject to interference by CRSs of the corresponding adjacent cell (HeNB), in receiving and decoding the PCFICH/PHICH/PDCCH and/or PDSCH transmitted thereto from a serving cell (MeNB). Alternatively, if the interference strength of CRSs of an adjacent cell (HeNB) is above a given level as compared with the signal strength of a serving cell (MeNB), a UE (MUE) may be set to perform a decoding operation using REs except for REs which are subject to interference by CRSs of an adjacent cell (HeNB).
In the above-described various embodiments of the present invention, it should be noted that a UE performs decoding of a channel received from a serving cell by excluding only REs corresponding to CRSs of an adjacent cell even when any UE served by the serving cell is subject to severe interference by CRSs and data of an adjacent cell. Namely, a part corresponding to data of the adjacent cell is included in decoding of a UE. Such an operation by the UE may be performed such that a serving cell punctures REs overlapping with CRS transmission REs of an adjacent cell and signals the punctured REs to the UE, or the UE excludes REs overlapping with the CRS transmission REs of an adjacent cell producing severe interference without additional signaling. An operation for reducing ICI according to an exemplary embodiment of the present invention will now be described in detail with reference to FIG. 27.
In FIG. 27, it is assumed that a first cell (cell 1) is an interfering cell and a second cell (cell 2) is a victim cell. In other words, it is assumed that a UE served by the second cell is subject to strong interference by a signal from the first cell. As illustrated in FIG. 27, CRS transmission REs of one of the two cells are shifted by one subcarrier from CRS transmission REs of the other cell. For clarity of description, only OFDM symbols 0 and 1 of one subframe are shown and the other OFDM symbols of one subframe may transmit data from each cell and a UE may receive the symbols as in the above-described embodiments.
As shown in FIG. 27, a variety of control channels (PCFICH/PHICH/PDCCH) may be transmitted throughout the first two OFDM symbols (0 and 1) of one subframe. A UE served by the second cell (cell 2) may measure interference from the first cell (cell 1) in a subframe n. As denoted by oblique lines in the subframe n shown in FIG. 27, the first cell may transmit control channel signals thereof in the OFDM symbols 0 and 1. In terms of the UE, both CRSs (R0, R1, R2, and R3), and signals (indicated by oblique lines) except for the CRSs are determined as creating strong interference. To solve such severe interference, the first cell may perform an operation in which all signals except for the CRSs are not transmitted in a subframe n+1. In any downlink subframe, if only CRSs are transmitted and all signals except for the CRSs are not transmitted, such a subframe may be referred to as an Almost Blank Subframe (ABS). In this way, if the first cell sets the subframe n+1 as an ABS and transmits the ABS, the UE may expect an ABS transmission operation of the first cell and may receive a channel in the subframe n+1 from the second cell. Specifically, in the subframe n+1, the UE may perform channel decoding using REs except for CRS transmission REs (subcarrier locations 0, 3, 6, and 9 of OFDM symbols 0 and 1) under the assumption that the first cell creates interference only due to CRS transmission. In other words, REs (e.g., a subcarrier 2 of the OFDM symbol 0) which are subject to interference from signals except for the CRSs of the first cell may be used for channel decoding by the UE even if strong interference is detected in a part of subframes, such as the subframe n.
FIG. 28 is a flowchart illustrating a process for reducing ICI according to an exemplary embodiment of the present invention. In describing the process, it is assumed that there are two cells (i.e., a first cell and a second cell) interfering with each other. The following description may be applied to the case where the first cell is an interfering cell and the second cell is a victim cell, or the case where the first cell is a victim cell and the second cell is an interfering cell.
In step S2810, the first cell (eNB1) may determine REs overlapping with CRS transmission REs of a downlink subframe of the second cell (eNB2) among REs of a downlink subframe thereof. A CRS pattern of any cell may be determined by factors such as the number of transmission antennas of a corresponding cell, a type of a downlink subframe (whether a downlink subframe is a normal subframe or an MBSFN subframe), a shift (time shift) in a subframe boundary, a frequency shift (V-shift) of a CRS pattern, etc.
In step S2820, a portion of the REs determined in step S2810 may be determined as REs to be punctured. The REs determined in step S2810 correspond to REs overlapping with the CRS transmission REs of the downlink subframe of the second cell among the REs of the downlink subframe of the first cell. The present invention does not exclude the case where all of the REs determined in step S2820 are determined as REs to be punctured. However, the efficiency of interference coordination can be raised without greatly lowering the efficiency of data transmission by determining only necessary REs among REs related to interference as REs to be punctured.
The partial REs determined as REs to be punctured among the REs determined in step S2810 may be REs present in a control area and/or a data area of the downlink subframe of the first cell. In addition, the partial REs determined as REs to be punctured among the REs determined in step S2810 may be REs corresponding to a part of CRS transmission antenna ports of the second cell.
In step S2820, the punctured REs may be separately determined according to downlink subframes of the first cell. Namely, different RE puncturing patterns may be applied to each subframe. In addition to the REs determined in step S2810, the REs to be punctured in step 2820 may further include REs overlapping with a PDCCH transmission area of the downlink subframe of the second cell among REs of the downlink subframe of the first cell. Furthermore, the first cell may transmit information indicating the RE puncturing pattern to a served UE.
In step S2830, the first cell may map a PDCCH, a PDSCH, etc. to REs except for REs punctured in a downlink subframe thereof. In steps S2840, the first cell may transmit the PDCCH, PDSCH, etc. mapped to the downlink subframe to a UE.
Details described in the above embodiments of the present invention may be independently applied or two or more embodiments may be simultaneously applied to the ICI reduction process described in conjunction with FIG. 28. A repeated description is omitted for clarity.
FIG. 29 is a diagram illustrating a configuration of an eNB device 2910 according to an exemplary embodiment of the present invention.
Referring to FIG. 29, the eNB device 2910 may include a reception module 2911, a transmission module 2912, a processor 2913, a memory 2914, and a plurality of antennas 2915. The plurality of antennas 2915 supports MIMO transmission and reception. The reception module 2911 may receive signals, data, and information in downlink from a UE. The transmission module 2912 may transmit signals, data, and information in downlink to the UE. The processor 2913 may control the overall operation of the eNB device 2910.
The eNB device 2910 according to an exemplary embodiment of the present invention may be configured to reduce ICI. In describing the eNB device according to the exemplary embodiment of the present invention, it is assumed that there are two cells (i.e., a first cell and a second cell) interfering with each other. The following description may be applied to the case where the first cell is an interfering cell and the second cell is a victim cell, or the case where the first cell is a victim cell and the second cell is an interfering cell. The processor 2913 of the eNB device 2910 may control signal transmission and reception of the first cell through the transmission module 2913 and the reception module 2911. The processor 2913 may be configured to determine REs overlapping with CRS transmission REs of a downlink subframe of the second cell in a downlink subframe of the first cell. The processor 2913 may be configured to determine a part of REs overlapping with CRS transmission REs of the downlink subframe of the second cell in the downlink subframe of the first cell as REs to be punctured. The processor 2913 may be configured to map one or more downlink channels to the downlink subframe of the first cell except for the punctured REs and to transmit one or more downlink channels mapped to the downlink subframe of the first cell to the UE through the transmission module 2912.
While the processor 2913 of the eNB device 2910 determines the REs overlapping with the CRS transmission REs of the downlink subframe of the second cell among REs of the downlink subframe the first cell, a CRS pattern may be determined by factors such as the number of transmission antennas of a corresponding cell, a type of a downlink subframe (whether a downlink subframe is a normal subframe or an MBSFN subframe), a shift (time shift) in a subframe boundary, a frequency shift (V-shift) of a CRS pattern, etc. In addition, the punctured REs determined by the processor 2913 may be REs present in a control area and/or a data area of the downlink subframe of the first cell or REs corresponding to a part of CRS transmission antenna ports of the second cell among the REs present in a control area and/or a data area of the downlink subframe of the first cell. Furthermore, the punctured REs determined by the processor 2913 may further include REs overlapping with a PDCCH transmission area of the downlink subframe of the second cell among REs of the downlink subframe of the first cell. Moreover, the processor 2913 may be configured to separately determine the punctured REs according to downlink subframes of the first cell. The processor 2913 may be configured to transmit information indicating an RE puncturing pattern to the UE.
The processor 2913 of the eNB device 290 performs an operation processing function for information received by the eNB device 2910 and information to be transmitted to an external device. The memory 2914 stores the processed information for a given time and may be replaced with a constituent element such as a buffer (not shown).
Descriptions of the above embodiments of the present invention may be independently applied or two or more embodiments may be simultaneously applied to the configuration of the eNB device. A repeated description is omitted for clarity.
A description of the eNB device 2910 of FIG. 29 may be identically applied to a relay device as a downlink transmission subject or an uplink reception subject.
1. A method for reducing inter-cell interference, the method comprising:
transmitting, from a first cell to a user equipment, information on a Cell-specific Reference Signal (CRS) of a second cell,
wherein the information on the CRS of the second cell includes information explicitly indicating a number of a CRS antenna port of the second cell, and
wherein the information on the CRS of the second cell is used by the user equipment to reduce the inter-cell interference from the CRS of the second cell.
2. The method according to claim 1, wherein the information on the CRS of the second cell further includes a subframe configuration of the second cell indicating subframes containing the CRS in a data region.
3. The method according to claim 2, wherein the subframe configuration includes Multicast/Broadcast over Single Frequency Network (MBSFN) subframe configuration of the second cell.
4. The method according to claim 1, wherein the information on the CRS of the second cell is provided from the first cell via higher layer signaling.
5. The method according to claim 1, wherein CRS transmission Resource Elements (REs) of the second cell are determined by the user equipment based on the information on the CRS of the second cell.
6. The method according to claim 1, wherein at least a portion of REs of a subframe of the first cell overlapped with the CRS transmission REs of the second cell is not used for a downlink channel of the first cell.
7. The method according to claim 6, wherein the at least a portion of REs of the subframe of the first cell is configured as a null RE.
8. The method according to claim 1, wherein the first cell is a serving cell and the second cell is a neighbor cell.
9. A method for reducing inter-cell interference, the method comprising:
receiving, from a first cell to a user equipment, information on a Cell-specific Reference Signal (CRS) of a second cell,
10. A base station of a first cell for reducing inter-cell interference, the base station comprising:
a reception module configured to receive an uplink signal from a user equipment;
a transmission module configured to transmit a downlink signal to the user equipment; and
a processor configured to control the transmission module to transmit, from the first cell to the user equipment, information on a Cell-specific Reference Signal (CRS) of a second cell,
11. A user equipment for reducing inter-cell interference, the user equipment comprising:
a reception module configured to receive a downlink signal from a base station of a first cell;
a transmission module configured to transmit an uplink signal to the base station of the first cell; and
a processor configured to control the reception module to receive, from the first cell to the user equipment, information on a Cell-specific Reference Signal (CRS) of a second cell,
US13/582,340 2010-03-24 2011-03-24 Method and apparatus for reducing inter-cell interference in radio communication system Active 2031-08-04 US8965294B2 (en)
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PCT/KR2011/002029 WO2011118993A2 (en) 2010-03-24 2011-03-24 Method and apparatus for reducing inter-cell interference in radio communication system
US13/582,340 US8965294B2 (en) 2010-03-24 2011-03-24 Method and apparatus for reducing inter-cell interference in radio communication system
PCT/KR2011/002029 A-371-Of-International WO2011118993A2 (en) 2010-03-24 2011-03-24 Method and apparatus for reducing inter-cell interference in radio communication system
US14/597,831 Continuation US9667396B2 (en) 2010-03-24 2015-01-15 Method and apparatus for reducing inter-cell interference in radio communication system
US20120329400A1 US20120329400A1 (en) 2012-12-27
US8965294B2 true US8965294B2 (en) 2015-02-24
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US13/582,340 Active 2031-08-04 US8965294B2 (en) 2010-03-24 2011-03-24 Method and apparatus for reducing inter-cell interference in radio communication system
US14/597,831 Active 2031-06-01 US9667396B2 (en) 2010-03-24 2015-01-15 Method and apparatus for reducing inter-cell interference in radio communication system
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US16/242,280 Pending US20190140802A1 (en) 2010-03-24 2019-01-08 Method and apparatus for reducing inter-cell interference in radio communication system
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