METHOD AND APPARATUS OF MONITORING PDCCH IN WIRELESS COMMUNICATION SYSTEM

Abstract

A method and an apparatus of monitoring a physical downlink control channel (PDCCH) in a wireless communication system, carried in a user equipment (UE), are provided. The method includes receiving a PDCCH map, and monitoring a set of PDCCH candidates based on the PDCCH map.

Description

TECHNICAL FIELD

The present invention relates to wireless communications, and more particularly, to a method and an apparatus of monitoring a physical downlink control channel (PDCCH) in a wireless communication system.

BACKGROUND ART

In wireless communication systems, one base station (BS) generally provides services to a plurality of user equipments (UEs). The BS schedules user data for the plurality of UEs, and transmits the user data together with control information containing scheduling information for the user data. In general, a channel for carrying the control information is referred to as a control channel, and a channel for carrying the user data is referred to as a data channel. The UE finds control information of the UE by searching for the control channel, and processes data of the UE by using the control information.

In order for the UE to receive user data assigned to the UE, control information for the user data on a control channel must be received. In a given bandwidth, a plurality of pieces of control information for a plurality of UEs are generally multiplexed within one transmission interval. That is, to provide a service to the plurality of UEs, the BS multiplexes the plurality of pieces of control information for the plurality of UEs and then transmits the control information through a plurality of control channels. The UE searches for control channel of the UE among the plurality of control channels.

Blind decoding is one of schemes for detecting specific control information from the plurality of pieces of multiplexed control information. The blind decoding attempts to recover a control channel by using several combinations of information in a state where a UE has no information required to recover the control channel. That is, in a state where the UE does not know whether control information transmitted from the BS is control information of the UE and the UE does not know in which portion the control information of the UE exists, the UE decodes all pieces of given control information until the control information of the UE is found. The UE can use information unique to each UE to detect the control information of the UE. For example, when the BS multiplexes control information of each UE, an identifier unique to each UE can be transmitted by being masked onto a cyclic redundancy check (CRC). The CRC is a code used for error detection. The UE de-masks unique identifier of the UE from the CRC of the received control information, and then can detect the control information of the UE by performing CRC checking.

Meanwhile, as a mobile communication system of a next generation (i.e., post-3rd generation), an international mobile telecommunication-advanced (IMT-A) system is standardized aiming at support of an Internet protocol (IP)-based seamless multimedia service in an international telecommunication union (ITU) by providing a high-speed transmission rate of 1 gigabits per second (Gbps) in downlink communication and 500 megabits per second (Mbps) in uplink communication. In a 3rd generation partnership project (3GPP), a 3GPP long term evolution-advanced (LTE-A) system is considered as a candidate technique for the IMT-A system. The LTE-A system is evolved to increase a completion level of the LTE system, and is expected to maintain backward compatibility with the LTE system. This is because the provisioning of compatibility between the LTE-A system and the LTE system is advantageous in terms of user convenience, and is also advantageous for a service provider since existing equipment can be reused.

In general, a wireless communication system is a single carrier system supporting a single carrier. The transmission rate is proportional to transmission bandwidth. Therefore, for supporting a high-speed transmission rate, transmission bandwidth shall be increased. However, except for some areas of the world, it is difficult to allocate frequencies of wide bandwidths. For effectively using fragmented small frequency bands, a spectrum aggregation (also referred to as bandwidth aggregation or carrier aggregation) technique is being developed. The spectrum aggregation technique is to obtain the same effect as if which a frequency band of a logically wide bandwidth may be used by aggregating a plurality of physically discontiguous frequency bands in a frequency domain. Through the spectrum aggregation technique, multiple carrier (multi-carrier) can be supported in the wireless communication system. The wireless communication system supporting multi-carrier is referred to as a multi-carrier system. The carrier may be also referred to as a radio frequency (RF), component carrier (CC), etc.

However, if a BS transmits a control channel and a UE monitors the control channel with a same manner used in a single carrier system in a multi-carrier system, the complexity of blind decoding is significantly increased.

Accordingly, there is a need for a method and an apparatus of effectively transmitting a control channel and monitoring a control channel in a multi-carrier system.

DISCLOSURE OF INVENTION

Technical Problem

The present invention provides a method and an apparatus of monitoring a physical downlink control channel (PDCCH) in a wireless communication system.

Technical Solution

In an aspect, a method of monitoring a physical downlink control channel (PDCCH) in a wireless communication system, carried in a user equipment (UE), is provided. The method includes receiving a PDCCH map from a base station (BS), and monitoring a set of PDCCH candidates based on the PDCCH map.

Preferably, the PDCCH map comprises a monitoring set field, the monitoring set field indicating N downlink component carriers (CCs) out of L downlink CCs (L≧N, where L and N each is a natural number), and a UE monitors the set of PDCCH candidates in each of the N downlink CC.

Preferably, the PDCCH map is received on a PDCCH.

Preferably, the PDCCH map is received through a constant downlink CC out of a plurality of downlink CCs.

Preferably, the PDCCH map is received through a downlink CC, the downlink CC is hopped among a plurality of downlink CCs in accordance with a hopping rule.

Preferably, the PDCCH map is received via radio resource control (RRC) signal.

Preferably, the PDCCH map comprises a control channel element (CCE) field, the CCE field indicating Y CCE aggregation levels out of X CCE aggregation levels (X≧Y, where X and Y each is a natural number), and a UE monitors the set of PDCCH candidates at each of the Y CCE aggregation levels.

Preferably, the PDCCH map comprises a monitoring set field and a CCE field.

In another aspect, a UE is provided. The UE includes a radio frequency (RF) unit transmitting and/or receiving a radio signal and a processor coupled with the RF unit and configured to receive a PDCCH map, and monitor a set of PDCCH candidates based on the PDCCH map.

In still another aspect, a method of transmitting a PDCCH in a wireless communication system, carried in a BS, is provided. The method includes transmitting a PDCCH map to a UE, and transmitting a PDCCH in accordance with the PDCCH map to the UE.

Advantageous Effects

A method and an apparatus of effectively monitoring a physical downlink control channel (PDCCH) are provided. Accordingly, overall system performance can be improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a wireless communication system.

FIG. 2 shows an example of a plurality of component carriers (CCs) used in a multi-carrier system.

FIG. 3 is a block diagram showing an example of a multi-carrier system.

FIG. 4 shows an example of a plurality of physical channels (PHYs).

FIG. 5 shows an example of a bandwidth used by a PHY.

FIG. 6 shows an example of an asymmetric structure of downlink and uplink in a multi-carrier system.

FIG. 7 shows a structure of a radio frame.

FIG. 8 shows an example of a resource grid for one downlink slot.

FIG. 9 shows a structure of a radio frame and a subframe in a frequency division duplex (FDD) system.

FIG. 10 shows an example of an resource element group (REG) structure when a base station (BS) uses one or two transmit (Tx) antennas.

FIG. 11 shows an example of an REG structure when a BS uses four Tx antennas.

FIG. 12 shows an example of mapping of a physical control format indicator channel (PCFICH) to REGs.

FIG. 13 is a flow diagram showing an example of a method of transmitting data and receiving data performed by a user equipment (UE).

FIG. 14 is a flowchart showing an example of a method of configuring a physical downlink control channel (PDCCH).

FIG. 15 shows an example of a method of multiplexing a plurality of PDCCHs for a plurality of UEs, performed by a BS.

FIG. 16 shows an example of a method of monitoring a control channel, performed by a UE.

FIG. 17 shows an example of a PDCCH transmission method in a multi-carrier system.

FIG. 18 is a flow diagram showing a control channel transmission method and/or a control channel monitoring method according to an embodiment of the present invention.

FIG. 19 shows an example of transmitting a PDCCH by using a PDCCH map in a multi-carrier system.

FIG. 20 shows another example of transmitting a PDCCH by using a PDCCH map in a multi-carrier system.

FIG. 21 shows an example of semi-statically configured a PDCCH map.

FIG. 22 shows another example of semi-statically configured a PDCCH map.

FIG. 23 shows a control channel monitoring method performed by a UE in a multi-carrier system.

FIG. 24 is a block diagram showing wireless communication system to implement an embodiment of the present invention.

MODE FOR THE INVENTION

FIG. 1 is a block diagram showing a wireless communication system.

Referring to FIG. 1, a wireless communication system 10 includes at least one base station (BS) 11. The BSs 11 provide communication services to specific geographical regions (generally referred to as cells) 15a, 15b, and 15c. The cell can be divided into a plurality of regions (referred to as sectors). A user equipment (UE) 12 may be fixed or mobile, and may be referred to as another terminology, such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), a wireless device, a personal digital assistant (PDA), a wireless modem, a handheld device, etc. The BS 11 is generally a fixed station that communicates with the UE 12 and may be referred to as another terminology, such as an evolved node-B (eNB), a base transceiver system (BTS), an access point, etc.

Hereinafter, a downlink (DL) denotes communication from the BS to the UE, and an uplink (UL) denotes communication from the UE to the BS. In the DL, a transmitter may be a part of the BS, and a receiver may be a part of the UE. In the UL, the transmitter may be a part of the UE, and the receiver may be a part of the BS.

The wireless communication system supports multi-antenna. The transmitter may use a plurality of transmit (Tx) antennas, and the receiver may use a plurality of receive (Rx) antennas. The Tx antenna is a logical or physical antenna used to transmit one signal or one stream, and the Rx antenna is a logical or physical antenna used to receive one signal or one stream.

If the transmitter and the receiver use multi-antenna, the wireless communication system may be called as multiple input multiple output (MIMO) system.

FIG. 2 shows an example of a plurality of component carriers (CCs) used in a multi-carrier system.

Referring to FIG. 2, a multi-carrier system may use N CCs (CC #1, CC #2, . . . , CC #N). Although it is described herein that adjacent CCs are physically discontiguous in a frequency domain, this is for exemplary purposes only. Adjacent CCs may be physically contiguous in a frequency domain

Therefore, a frequency band of a logically wide bandwidth may be used in the multi-carrier system by aggregating a plurality of physically discontiguous and/or contiguous CCs in a frequency domain.

In downlink, a BS concurrently can transmit information to one UE through one or more CCs. In uplink, the UE can also transmit data to the BS through one or more CCs.

FIG. 3 is a block diagram showing an example of a multi-carrier system.

Referring to FIG. 3, each of a transmitter 100 and a receiver 200 uses N CCs (CC #1, CC #2, . . . , CC #N) in a multi-carrier system. A CC includes one or more physical channels (hereinafter, simply referred to as PHYs). A wireless channel is established between the transmitter 100 and the receiver 200.

The transmitter 100 includes a plurality of PHYs 110-1, . . . , 110-M, a multi-carrier multiplexer 120, and a plurality of Tx antennas 190-1, . . . , 190-Nt. The receiver 200 includes a multi-carrier demultiplexer 210, a plurality of PHYs 220-1, . . . , 220-L, and a plurality of Rx antennas 290-1, . . . , 290-Nr. The number M of PHYs of the transmitter 100 may be identical to or different from the number L of PHYs of the receiver 200. Although it is described herein that each of the transmitter 100 and the receiver 200 includes a plurality of antennas, this is for exemplary purposes only. The transmitter 100 and/or the receiver 200 includes a single antenna.

The transmitter 100 generates Tx signals from information based on the N CCs, and the Tx signals are transmitted on M PHYs 110-1, . . . , 110-M. The multi-carrier multiplexer 120 combines the Tx signals so that the Tx signals can be simultaneously transmitted on the M PHYs. The combined Tx signals are transmitted through the Nt Tx antennas 190-1, . . . , 190-Nt. The Tx radio signals are received through the Nr Rx antennas 290-1, . . . , 290-Nr of the receiver 200 through the wireless channel. The Rx signals are de-multiplexed by the multi-carrier demultiplexer 210 so that the Rx signals are separated into the L PHYs 220-1, . . . , 220-L. Each of the PHYs 220-1, . . . , 220-L recovers the information.

The multi-carrier system may include one or more carrier modules. The carrier module upconverts a baseband signal to a carrier frequency to be modulated onto a radio signal, or downconverts a radio signal to recover a baseband signal. The carrier frequency is also referred to as a center frequency. The multi-carrier system may use a plurality of carrier modules for each carrier frequency, or use a carrier module which can change a carrier frequency.

FIG. 4 shows an example of a plurality of PHYs. FIG. 4 shows an example of N CCs consisting of M PHYs (PHY #1, PHY #2, . . . , PHY #M).

Referring to FIG. 4, each of M PHYs has a specific bandwidth (BW). An PHY #m has a center frequency f_c,m and a bandwidth of N_IFFT,m×Δf_m (where m=1, . . . , M). Herein, N_IFFT,m denotes an inverse fast Fourier transform (IFFT) size of the PHY #m, and M_m denotes a subcarrier spacing of the PHY #m. The IFFT size and the subcarrier spacing may be different or identical for each PHY. Center frequencies of the respective PHYs may be arranged with a regular interval or an irregular interval.

According to a UE or a cell, each PHY may use a bandwidth narrower than a maximum bandwidth. For example, if it is assumed that each PHY has a maximum bandwidth of 20 mega Hertz (MHz), and M is 5, then a full bandwidth of up to 100 MHz can be supported.

FIG. 5 shows an example of a bandwidth used by a PHY.

Referring to FIG. 5, if it is assumed that a maximum bandwidth of the PHY is 20 MHz, the PHY can use a bandwidth (e.g., 10 MHz, 5 MHz, 2.5 MHz, or 1.25 MHz) narrower than the maximum bandwidth. Regardless of a bandwidth size used by the PHY in downlink, a synchronization channel (SCH) may exist in each PHY. The SCH is a channel for cell search. The cell search is a procedure by which a UE acquires time synchronization and frequency synchronization with a cell and detects a cell identifier (ID) of the cell. If the SCH is located in all downlink PHYs, all UEs can be synchronized with the cell. In addition, if a plurality of downlink PHYs are allocated to the UE, cell search may be performed for each PHY or may be performed only for a specific PHY.

As such, a UE or a BS can transmit and/or receive information based on one or more PHYs in the multi-carrier system. The number of PHYs used by the UE may be different from or equal to the number of PHYs used by the BS. In general, the BS can use M PHYs, and the UE can use L PHYs (M≧L, where M and L are natural numbers). Herein, L may differ depending on a type of the UE.

The multi carrier system can have several types of uplink and downlink configurations. In a frequency division duplex (FDD) system or a time division duplex (TDD) system, a structure of downlink and uplink may be an asymmetric structure in which an uplink bandwidth and a downlink bandwidth are different from each other. Alternatively, the structure of downlink and uplink may be configured in which an uplink bandwidth and a downlink bandwidth are identical to each other. In this case, the structure of downlink and uplink may be configured to a symmetric structure in which the same number of PHYs exist in both uplink and downlink transmissions or an asymmetric structure in which the number of PHY differs between uplink and downlink transmissions.

FIG. 6 shows an example of an asymmetric structure of downlink and uplink in a multi-carrier system. A transmission time interval (TTI) is a scheduling unit for information transmission. In each of the FDD system and the TDD system, a structure of downlink and uplink is an asymmetric structure. If the structure of downlink and uplink is an asymmetric structure, a specific link may have a higher information throughput. Therefore, system can be optimized flexibly.

Hereinafter, for convenience of explanation, it is assumed that a CC includes one PHY.

All transmission/reception methods used in a single carrier system using can also be applied to each CC of the transmitter and the receiver in a multi-carrier system. In addition, it is desirable for the multi-carrier system to maintain backward compatibility with the single carrier system which is legacy system of the multi-carrier system. This is because the provisioning of compatibility between the multi-carrier system and the single carrier system is advantageous in terms of user convenience, and is also advantageous for a service provider since existing equipment can be reused.

Now, a single carrier system will be described.

FIG. 7 shows a structure of a radio frame.

Referring to FIG. 7, the radio frame consists of 10 subframes. One subframe consists of two slots. Slots included in the radio frame are numbered with slot numbers #0 to #19. A time required to transmit one subframe is defined as a TTI. For example, one radio frame may have a length of 10 milliseconds (ms), one subframe may have a length of 1 ms, and one slot may have a length of 0.5 ms.

The structure of the radio frame is for exemplary purposes only, and thus the number of subframes included in the radio frame or the number of slots included in the subframe may change variously.

FIG. 8 shows an example of a resource grid for one downlink slot.

Referring to FIG. 8, the downlink slot includes a plurality of orthogonal frequency division multiplexing (OFDM) symbols in a time domain and NDL resource blocks (RBs) in a frequency domain. The OFDM symbol is for expressing one symbol period, and may be referred to as an orthogonal frequency division multiple access (OFDMA) symbol or a single carrier-frequency division multiple access (SC-FDMA) symbol according to a multiple access scheme. The number NDL of resource blocks included in the downlink slot depends on a downlink transmission bandwidth configured in a cell. One RB includes a plurality of subcarriers in the frequency domain.

Each element on the resource grid is referred to as a resource element (RE). Although it is described herein that one RB includes 7×12 resource elements consisting of 7 OFDM symbols in the time domain and 12 subcarriers in the frequency domain for example, the number of OFDM symbols and the number of subcarriers in the RB are not limited thereto. Thus, the number of OFDM symbols and the number of subcarriers may change variously depending on a cyclic prefix (CP) length, a subcarrier spacing, etc. For example, when using a normal CP, the number of OFDM symbols is 7, and when using an extended CP, the number of OFDM symbols is 6.

The resource grid for one downlink slot of FIG. 8 can be applied to a resource grid for an uplink slot.

FIG. 9 shows a structure of a radio frame and a subframe in a FDD system.

Referring to FIG. 9, the radio frame includes 10 subframes, and each subframe includes two consecutive slots. When using a normal CP, the subframe includes 14 OFDM symbols. When using an extended CP, the subframe includes 12 OFDM symbols. A SCH is transmitted in every radio frame. The SCH includes a primary (P)-SCH and a secondary (S)-SCH. The P-SCH is transmitted through a last OFDM symbol of a 1st slot of a subframe 0 and a subframe 5 in a radio frame. When using the normal CP, the P-SCH is an OFDM symbol 6 in the subframe, and when using the extended CP, the P-SCH is an OFDM symbol 5 in the subframe. The S-SCH is transmitted through an OFDM symbol located immediately before an OFDM symbol on which the P-SCH is transmitted.

A maximum of three OFDM symbols (i.e., OFDM symbols 0, 1, and 2) located in a front portion of a 1st slot in every subframe correspond to a control region. The remaining OFDM symbols correspond to a data region. A physical downlink shared channel (PDSCH) can be assigned to the data region. Downlink data is transmitted on PDSCH.

Cntrol channels such as a physical control format indicator channel (PCFICH), a physical HARQ (hybrid automatic repeat request) indicator channel (PHICH), a physical downlink control channel (PDCCH) etc., can be assigned to the control region.

Resource element groups (REGs) are used for defining the mapping of a control channel to resource elements.

FIG. 10 shows an example of an REG structure when a BS uses one or two Tx antennas. FIG. 11 shows an example of an REG structure when a BS uses four Tx antennas. In FIGS. 10 and 11, it is assumed that a maximum of three OFDM symbols (i.e., OFDM symbols 0, 1, and 2) located in a front portion of a 1st slot in a subframe are control regions.

Referring to FIGS. 10 and 11, Rp indicates a resource element which is used to transmit a reference signal (hereinafter referred to as an ‘RS’) through antenna p (pε{0, 1, 2, 3}). The RS may be also referred to as a pilot. One REG is composed of four adjacent resource elements in the frequency domain other than resource elements which are used for RS transmission. In the OFDM symbol 0 in the subframe, two REGs exist within one resource block in the frequency domain. It is to be noted that the above REG structures are only illustrative and the number of resource elements included in the REG may change in various ways.

The PHICH carries an HARQ acknowledgement (ACK)/not-acknowledgement (NACK) for uplink data.

The PCFICH carries information about the number of OFDM symbols used for transmission of PDCCHs in a subframe. Although the control region includes three OFDM symbols herein, this is for exemplary purposes only. According to an amount of control information, the PDCCH is transmitted through the OFDM symbol 0, or the OFDM symbols 0 and 1, or the OFDM symbols 0 to 2. The number of OFDM symbols used for PDCCH transmission may change in every subframe. The PCFICH is transmitted through a 1st OFDM symbol (i.e., the OFDM symbol 0) in every subframe. The PCFICH can be transmitted through a single antenna or can be transmitted through a multi-antenna using a transmit diversity scheme. When a subframe is received, the UE evaluates control information transmitted through the PCFICH, and then receives control information transmitted through the PDCCH.

The control information transmitted through the PCFICH is referred to as a control format indicator (CFI). For example, the CFI may have a value of 1, 2, or 3. The CFI value may represent the number of OFDM symbols used for PDCCH transmission in a subframe. That is, if the CIF value is 2, the number of OFDM symbols used for PDCCH transmission in a subframe is 2. This is for exemplary purposes only, and thus information indicated by the CFI may be defined differently according to a downlink transmission bandwidth. For example, if the downlink transmission bandwidth is less than a specific threshold value, the CFI values of 1, 2, and 3 may indicate that the number of OFDM symbols used for PDCCH transmission in the subframe is 2, 3, and 4, respectively.

The following table shows an example of a CFI and a 32-bit CFI codeword which generates by performing channel coding to the CFI.

TABLE 1
           CFI codeword
CFI        <b0, b1, . . ., b31>
1          <0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0,
           1, 1, 0, 1, 1, 0, 1, 1, 0, 1>
2          <1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1,
           0, 1, 1, 0, 1, 1, 0, 1, 1, 0>
3          <1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1,
           1, 0, 1, 1, 0, 1, 1, 0, 1, 1>
4          <0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
(Reserved) 0, 0, 0, 0, 0, 0, 0, 0, 0, 0>

The 32-bit CFI codeword can be mapped to a 16 modulated symbols using a quadrature phase shift keying (QPSK) scheme. In this case, 16 resource elements (or subcarriers) are used in PCFICH transmission. That is, 4 REGs are used in PCFICH transmission.

FIG. 12 shows an example of mapping of a PCFICH to REGs.

Referring to FIG. 12, the PCFICH is mapped to 4 REGs, and the respective REGs to which the PCFICH are mapped are spaced apart from one another. An REG to which the PCFICH is mapped may vary according to the number of resource blocks in a frequency domain. In order to avoid inter-cell interference of the PCFICH, the REGs to which the PCFICH is mapped may be shifted in a frequency domain according to a cell ID.

Now, a PDCCH will be described.

A control region consists of a set of control channel elements (CCEs). The CCEs are indexed 0 to N(CCE)-1, where N(CCE) is the total number of CCEs constituting the set of CCEs in a downlink subframe. The CCE corresponds to a plurality of REGs. For example, one CCE may correspond to 9 REGs. A PDCCH is transmitted on an aggregation of one or several consecutive CCEs. A PDCCH format and the possible number of bits of the PDCCH are determined according to the number of CCEs constituting the CCE aggregation. Hereinafter, the number of CCEs constituting the CCE aggregation used for PDCCH transmission is referred to as a CCE aggregation level. In addition, the CCE aggregation level is a CCE unit for searching for the PDCCH. A size of the CCE aggregation level is defined by the number of contiguous CCEs. For example, the CCE aggregation level may be an element of {1, 2, 4, 8}.

The following table shows an example of the PDCCH format, the number of REGs and the number of PDCCH bits.

	TABLE 2
	PDCCH	CCE	Number	Number
	format	aggregation level	of REGs	of PDCCH bits
	0	1	9	72
	1	2	18	144
	2	4	36	288
	3	8	72	576

Control information transmitted on the PDCCH is referred to as downlink control information (DCI). The DCI transports uplink scheduling information, downlink scheduling information, or an uplink power control command, etc. The downlink scheduling information is also referred to as a downlink grant, and the uplink scheduling information is also referred to as an uplink grant.

FIG. 13 is a flow diagram showing an example of a method of transmitting data and receiving data performed by a UE.

Referring to FIG. 13, a BS transmits an uplink grant to a UE at step S11. The UE transmits uplink data to the BS based on the uplink grant at step S12. The uplink grant may be transmitted on a PDCCH, and the uplink data may be transmitted on a physical uplink shared channel (PUSCH). A relationship between a subframe in which a PDCCH is transmitted and a subframe in which a PUSCH is transmitted may be previously set between the BS and the UE. For example, if the PDCCH is transmitted in an nth subframe in a FDD system, the PUSCH may be transmitted in an (n+4)th subframe.

The BS transmits an downlink grant to the UE at step S13. The UE receives downlink data from the BS based on the downlink grant at step S14. The downlink grant may be transmitted on a PDCCH, and the downlink data may be transmitted on a PDSCH. For example, the PDCCH and the PDSCH are transmitted in the same subframe.

As described above, a UE shall receive DCI on PDCCH to receive downlink data from a BS or transmit uplink data to a BS.

DCI may use a different DCI format in accordance with usage. For example, a DCI format for an uplink grant and a DCI format for a downlink grant is different each other. A size and usage of DCI may differ according to a DCI format.

The following table shows an example of the DCI format.

TABLE 3
DCI
format Objectives
0      Scheduling of PUSCH
1      Scheduling of one PDSCH codeword
1A     Compact scheduling of one PDSCH codeword
1B     Closed-loop single-rank transmission
1C     Paging, RACH response and dynamic BCCH
1D     MU-MIMO
2      Scheduling of closed-loop rank-adapted spatial
       multiplexing mode
2A     Scheduling of open-loop rank-adapted spatial
       multiplexing mode
3      TPC commands for PUCCH and PUSCH with 2 bit
       power adjustments
3A     TPC commands for PUCCH and PUSCH with single
       bit power adjustments

Referring to above table, a DCI format 0 is used for PUSCH scheduling. The DCI format 0 is used for an uplink grant.

A DCI format 1 is used for scheduling of one PDSCH codeword. A DCI format 1A is used for compact scheduling of one PDSCH codeword. A DCI format 1B is used for compact scheduling of one PDSCH codeword in a closed-loop rank 1 transmission mode. A DCI format 1C is used for paging, random access channel (RACH) response, and dynamic broadcast control channel (BCCH). A DCI format 1D is used for PDSCH scheduling in a multi-user (MU)-MIMO mode. A DCI format 2 is used for PDSCH scheduling in a closed-loop rank-adapted spatial multiplexing mode. A DCI format 2A is used for PDSCH scheduling in an open-loop rank-adapted spatial multiplexing mode. Each of from DCI format 1 to DCI format 2A is used for a downlink grant. However, the DCI format may differ according to a usage of DCI or transmission mode of a BS.

DCI formats 3 and 3A are used for transmission of a transmission power control (TPC) command for a physical uplink control channel (PUCCH) and a PUSCH. The DCI formats 3 and 3A is used for an uplink power control command.

Each DCI format consists of a plurality of information fields. The type of information fields constituting a DCI format, the size of each of the information fields, etc. may differ according to the DCI format. For example, a downlink grant (or an uplink grant) includes resource allocation field indicating radio resource. The downlink grant (or the uplink grant) may include further a modulation and coding scheme (MCS) field indicating modulation scheme and channel coding sheme. In addition, the downlink grant (or the uplink grant) may include further various information fields.

FIG. 14 is a flowchart showing an example of a method of configuring a PDCCH.

Referring to FIG. 14, in step S21, a BS generates information bit stream in accordance with a DCI format. In step S22, the BS attaches a cyclic redundancy check (CRC) for error detection to the information bit stream. The information bit stream may be used to calculate the CRC. The CRC is parity bits, and the CRC may be attached in front of the information bit stream or at the back of the information bit stream.

The CRC is masked with an identifier (referred to as a radio network temporary identifier (RNTI)) according to an owner or usage of the DCI. The masking may be scrambling of the CRC with the identifier. The masking may be an modulo 2 operation or exclusive or (XOR) operation between the CRC and the identifier.

If the DCI is for a specific UE, a unique identifier (e.g., cell-RNTI (C-RNTI)) of the UE may be masked onto the CRC. The C-RNTI may be also referred to as a UE ID. The CRC can be masked with different RNTI except C-RNTI, such as paging-RNTI (P-RNTI) for a paging message, system information-RNTI (SI-RNTI) for system information, random access-RNTI (RA-RNTI) for indicating random access response which is response of random access preamble transmitted by a UE, etc.

In step S23, the BS generates coded bit stream by performing channel coding to the information bit stream attached the CRC. The channel coding scheme is not limited. For example, convolution coding scheme can be used. The number of PDCCH bits may differ in accordance with channel coding rate.

In step of S24, the BS generates rate matched bit stream by performing rate matching to the coded bit stream. In step of S25, the BS generates modulated symbols by modulating the rate matched bit stream. In step of S26, the BS maps the modulated symbols to resource elements.

As described above, a method of configuring one PDCCH is explained. However, a plurality of control channels may be transmitted in a subframe. That is, a plurality of PDCCHs for several UEs can be transmitted by being multiplexed in one subframe. Generation of information bit stream, CRC attachment, channel coding, and rate matching, etc. are performed independently for each PDCCH. The aforementioned process of configuring the PDCCH of FIG. 14 can be performed independently for each PDCCH.

FIG. 15 shows an example of a method of multiplexing a plurality of PDCCHs for a plurality of UEs, performed by a BS.

Referring to FIG. 15, a CCE set constituting a control region in a subframe is constructed of a plurality of CCEs indexed from 0 to N(CCE)−1. That is, the number of CCEs is N(CCE). A PDCCH for a UE #1 is transmitted on a CCE aggregation with a CCE index 0 at a CCE aggregation level of 1. A PDCCH for a UE #2 is transmitted on a CCE aggregation with a CCE index 1 at a CCE aggregation level of 1. A PDCCH for a UE #3 is transmitted on a CCE aggregation with CCE indices 2 and 3 at a CCE aggregation level of 2. A PDCCH for a UE #4 is transmitted on a CCE aggregation with CCE indices 4, 5, 6, and 7 at a CCE aggregation level of 4. A PDCCH for a UE #5 is transmitted on a CCE aggregation with CCE indices 8 and 9 at a CCE aggregation level of 2.

The CCEs are mapped to resource elements (REs) according to a CCE-to-RE mapping rule. In this case, a PDCCH of each UE is mapped to the REs by being interleaved in the control region in the subframe. Locations of the REs to be mapped may change according to the number of OFDM symbols used for transmission of the PDCCHs in the subframe, the number of PHICH groups, the number of Tx antennas, and the frequency shifts.

The BS does not provide the UE with information indicating where a PDCCH of the UE is located in the subframe. In general, in a state where the UE does not know a location of the PDCCH of the UE in the subframe, the UE finds the PDCCH of the UE by monitoring a set of PDCCH candidates in every subframe. Monitoring implies that the UE attempts decoding each of the PDCCH candidates according to all the monitored DCI formats. This is referred to as blind decoding or blind detection. If no CRC error is detected if the UE performs CRC checking after de-masking C-RNTI from a PDCCH candidate, it is regarded that the PDCCH candidate is detected by the UE as the PDCCH of the UE.

In addition, the UE does not know at which CCE aggregation level the PDCCH of the UE is transmitted. Therefore, the UE needs to attempt decoding on the set of PDCCH candidates for each of all possible CCE aggregation levels.

FIG. 16 shows an example of a method of monitoring a control channel, performed by a UE.

Referring to FIG. 16, a CCE set constituting a control region in a subframe is constructed of a plurality of CCEs indexed from 0 to N(CCE)−1. That is, the number of CCEs is N(CCE). There are four types of a CCE aggregation level L, that is, {1, 2, 4, 8}. A set of PDCCH candidates monitored by the UE is differently defined according to the CCE aggregation level. For example, if the CCE aggregation level is 1, the PDCCH candidates correspond to all CCEs constituting the CCE set. If the CCE aggregation level is 2, the PDCCH candidates correspond to a CCE aggregation with CCE indices 0 and 1, a CCE aggregation with CCE indices 2 and 3, and so on. If the CCE aggregation level is 4, the PDCCH candidates correspond to a CCE aggregation with CCE indices 0 to 3, a CCE aggregation with CCE indices 4 to 7, and so on. If the CCE aggregation level is 8, the PDCCH candidates correspond to a CCE aggregation with CCE indices 0 to 7, and so on.

A frame structure of a single carrier system, a PDCCH transmission and monitoring method, etc., have been described above. For optimization of a multi-carrier system, a multi-antenna scheme or a control channel shall be designed by considering a frequency channel property for each CC. Therefore, it is important to properly use a system parameter and an optimal transmission/reception scheme for each CC. In addition, the same frame structure as a legacy system may be used in one CC of the multi-carrier system. In this case, the control channel shall be properly modified to operate both a UE for the legacy system and a UE for the multi-carrier system. Hereinafter, the UE for the legacy system is referred to as a long term evolution (LTE) UE, and the UE for the multi-carrier system is referred to as an LTE-advanced (LTE-A) UE.

FIG. 17 shows an example of a PDCCH transmission method in a multi-carrier system.

Referring to FIG. 17, the multi-carrier system uses a plurality of CCs, i.e., CC #1, CC #2, . . . , CC #L. A PDCCH for a UE #1 is transmitted for each CC in every subframe. The UE #1 has to attempt blind decoding to find the PDCCH of the UE #1 for each CC in every subframe.

Therefore, if L downlink CCs are used in the multi-carrier system, the LTE-A UE has to receive the PDCCH with a reception complexity which is L times higher than that of the LTE UE. This causes a problem of great power consumption in the LTE-A. To solve this problem, there is a need for an effective control channel transmission method and control channel monitoring method in the multi-carrier system, whereby a reception complexity of the control channel can be minimized according to a scheduling condition or a channel condition.

FIG. 18 is a flow diagram showing a control channel transmission method and/or a control channel monitoring method according to an embodiment of the present invention.

Referring to FIG. 18, a BS transmits a PDCCH map to a UE (step S110). The UE monitors a set of PDCCH candidates based on the PDCCH map (step S120).

To decrease a blind decoding complexity of the UE, the PDCCH map includes information regarding a PDCCH transmitted by the BS to the UE. The PDCCH map may include a monitoring set field and/or a CCE field. The PDCCH map may be configured differently in accordance with each DCI format

First, the monitoring set field is described.

If a UE uses L downlink CCs, a BS can transmit a PDCCH simultaneously through N downlink CCs out of the L downlink CCs (L≧N, where L and N are natural numbers). In this case, when the BS informs the UE of the N downlink CCs on which the PDCCH is transmitted, the UE performs blind decoding only in the N downlink CCs. Accordingly, a blind decoding complexity of the UE can be decreased. That is, the monitoring set field indicates the N downlink CCs out of the L downlink CCs. The BS transmits the PDCCH to the UE only through the N downlink CCs. The UE monitors the PDCCH only in the N downlink CCs. That is, the UE monitors a set of PDCCH candidates in each of the N downlink CCs. The monitoring set field may be configured differently in accordance with each DCI format.

FIG. 19 shows an example of transmitting a PDCCH by using a PDCCH map in a multi-carrier system.

Referring to FIG. 19, the PDCCH map includes a monitoring set field. The PDCCH map is transmitted only through a CC #1. The PDCCH map is dynamically transmitted in every subframe. If the PDCCH map is dynamically transmitted, flexibility of scheduling can be increased. The PDCCH map transmission method of FIG. 19 is for exemplary purposes only, and the PDCCH map transmission method of the present invention is not limited thereto.

Hereinafter, a radio resource for transmitting a PDCCH map will be described. The radio resource used for PDCCH map transmission may be constructed by combining a time resource, a frequency resource, and/or a code resource. The radio resource by which the PDCCH map is transmitted may be determined according to a rule predetermined between a BS and a UE. Alternatively, the PDCCH map may be transmitted in a PDCCH format. That is, in a state where the UE does not know a location of the PDCCH map in a subframe, the UE can attempt blind decoding to find a PDCCH map in every subframe. For example, the PDCCH map may be generated in such a manner that an information bit stream based on the PDCCH map format is generated and then an identifier (ID) of the UE, which is an owner of the PDCCH map, is masked to a CRC. The PDCCH map format may include a monitoring set field and/or a CCE field as an information field.

Even if the PDCCH map is transmitted in the PDCCH format, it is preferable that the UE knows a downlink CC on which the PDCCH map is transmitted. This is because a purpose of transmitting the PDCCH map is to reduce the blind decoding complexity, and this purpose is not achieved if the UE does not know the downlink CC on which the PDCCH map is transmitted.

Hereinafter, a CC on which a PDCCH map is transmitted will be described.

A BS can transmit the PDCCH map to a UE through a constant downlink CC. Alternatively, the downlink CC on which the PDCCH map is transmitted may change over time. For example, the downlink CC on which the PDCCH map is transmitted may change with a specific period according to a channel condition. Alternatively, the downlink CC on which the PDCCH map is transmitted may change in a specific pattern according to a hopping rule predetermined between the BS and the UE. In this case, the PDCCH map can be transmitted by being distributed over a plurality of downlink CCs. In another method, the BS may configure a downlink CC on which the PDCCH map is transmitted semi-statically through higher layer signaling such as a radio resource control (RRC) signaling. In this case, the downlink CC on which the PDCCH map is transmitted is semi-statically modified.

The downlink CC on which the PDCCH map is transmitted may be determined according to the UE. A plurality of downlink CCs to be allocated may differ according to the UE. The UE can use a downlink CC of a lowest frequency band among the plurality of allocated downlink CCs in transmission of the PDCCH map. This is because a low frequency band has a high reliability.

If the PDCCH is not transmitted in all downlink CCs, the BS may transmit the PDCCH map to the UE to report a presence or absence of the PDCCH. Alternatively, the BS may not transmit the PDCCH map to the UE, which can be regarded as error occurrence.

FIG. 20 shows another example of transmitting a PDCCH by using a PDCCH map in a multi-carrier system.

Referring to FIG. 20, the multi-carrier system uses three downlink CCs, i.e., CC #1, CC #2, and CC #3. The PDCCH map includes a monitoring set field. A PDCCH map of a UE #1 is transmitted through the CC #1 in every subframe. In a subframe n, a monitoring set field for the UE #1 indicates the CC #1, the CC #2, and the CC #3. Therefore, the PDCCHs for the UE #1 are transmitted through each of the CC #1, the CC #2, and the CC #3. In a subframe n+1, the monitoring set field for the UE #1 indicates the CC #2. Therefore, the PDCCH for the UE #1 is transmitted only through the CC #2. In a subframe n+k, the monitoring set field for the UE #1 indicates the CC #1 and the CC #3. Therefore, the PDCCHs for the UE #1 are transmitted through each of the CC #1 and the CC #3.

The monitoring set field may use a bitmap to indicate a downlink CC on which a PDCCH is transmitted. A plurality of downlink CCs correspond to respective bits of the monitoring set field, and the downlink CC on which the PDCCH is transmitted may be expressed by ‘1’. For example, in case of FIG. 20, the monitoring set field may have a size of 3 bits. In the subframe n, the monitoring set field for the UE #1 may be 111. In the subframe n+1, the monitoring set field for the UE #1 may be 010. In the subframe n+k, the monitoring set field for the UE #1 may be 101.

In FIG. 19 and FIG. 20, the PDCCH map is dynamically transmitted in every subframe. In this case, the downlink CC on which the PDCCH is transmitted changes dynamically in every subframe.

The BS may configure the PDCCH map semi-statically via higher layer signaling such as RRC. In this case, the downlink CC on which the PDCCH is transmitted changes semi-statically.

FIG. 21 shows an example of semi-statically configured a PDCCH map.

Referring to FIG. 21, a multi-carrier system uses three downlink CCs, i.e., CC #1, CC #2, and CC #3. The PDCCH map includes a monitoring set field. It is assumed that a monitoring field set for the UE #1 indicates the CC #1 and the CC #3 and does not indicate the CC #2. The PDCCH map is semi-statically configured, and thus the PDCCH for the UE #1 is transmitted only through the CC #1 and the CC #3 from a subframe n to a subframe n+k. In FIG. 21, a PDSCH corresponding to the PDCCH is transmitted only through a downlink CC on which the PDCCH is transmitted. That is, if the PDCCH is transmitted through the CC #1, the PDSCH corresponding to the PDCCH is transmitted only through the CC #1. In this case, the PDSCH for the UE #1 cannot be transmitted through the CC #2. Therefore, the UE #1 may not receive any information in the CC #2. If the BS intends to transmit small-sized downlink data to the UE #1 through the PDSCH, there is no problem even if the CC #2 is not used for transmission of downlink data for the UE #1.

If a radio resource scheduling scheme is a semi-persistent scheduling (SPS) scheme, the UE can read downlink data on the PDSCH without having to receive the PDCCH. Therefore, the BS can transmit the PDSCH based on the SPS scheme through the CC #2.

FIG. 22 shows another example of semi-statically configured a PDCCH map. In FIG. 22, similarly to FIG. 21, a multi-carrier system uses three downlink CCs, i.e., CC #1, CC #2, and CC #3. It is assumed that a monitoring field set for the UE #1 is included in the PDCCH map, and indicates the CC #1 and the CC #3 and does not indicate the CC #2.

Referring to FIG. 22, a downlink CC on which a PDSCH is transmitted may be equal to or different from a downlink CC on which a PDCCH for scheduling the PDSCH is transmitted. Therefore, a PDCCH for the UE #1 is transmitted only through the CC #1 and the CC #3 indicated by the monitoring set field, whereas the PDSCH may be transmitted through all downlink CCs, i.e., the CC #1 to the CC #3. In a subframe n+1, a PDCCH for the CC #2 is transmitted through the CC #1. In this case, according to a CC indicator or a predetermined rule, the UE #1 can know for which downlink CC the PDCCH is used.

If L CCs can be constructed for one LTE-A UE, only A CCs out of the L CCs can be used (L≧N, where L and N are natural numbers). An active CC set is a CC set having the A CCs out of the L CCs as its elements. The aforementioned monitoring set field can be used in the active CC set defined with the A CCs.

Next, the CCE field is described.

A UE has to attempt blind decoding for each CCE aggregation level. Therefore, a blind decoding complexity can be significantly decreased if the CCE aggregation level to be monitored by the UE is limited. If a wireless communication system uses X CCE aggregation levels, a BS can transmit a PDCCH to the UE by using Y CCE aggregation levels (X≧Y, where X and Y are natural numbers). In this case, when the BS reports information regarding the Y CCE aggregation levels to the UE, the UE performs blind decoding only for the Y CCE aggregation levels. Accordingly, the blind decoding complexity of the UE can be decreased. That is, the CCE field indicates the Y CCE aggregation levels out of the X CCE aggregation levels. The UE can monitor a PDCCH only for each of the Y CCE aggregation levels indicated by the CCE field. The Y CCE aggregation levels indicated by the CCE field will be referred hereinafter as a subset level.

For example, if the wireless communication system uses 4 CCE aggregation levels, such as, {1, 2, 4, 8}, the subset level can be indicated according to a CCE field value as described in the following table.

TABLE 4
CCE field Subset level
0         {1, 2, 4, 8}
1         {1, 2, 4}
2         {2, 4, 8}
3         {4, 8}

Table 4 is for exemplary purposes only, and thus the subset level can be variously configured according to the CCE field value. The CCE field may indicate a different subset level for each a DCI format or a DCI format group. Alternatively, the CCE field may indicate a subset level only for a specific DCI format by using the CCE field. In addition, the CCE field may indicate a different subset level for each a downlink CC or a downlink CC group.

The CCE field may be included in a PDCCH map. Therefore, the aforementioned description on the PDCCH map can be equally applied to the CCE field. The CCE field can be transmitted dynamically or semi-statically.

As such, the CCE aggregation level may be explicitly limited to the subset level by using the CCE field. Alternatively, the CCE aggregation level may be implicitly limited to the subset level. For example, all PDCCHs in a subframe may be specified to use the same CCE aggregation level. If the UE finds one PDCCH from a plurality of PDCCHs at one CCE aggregation level, the UE monitors the remaining PDCCHs also at the CCE aggregation level.

FIG. 23 shows a control channel monitoring method performed by a UE in a multi-carrier system.

Referring to FIG. 23, three PDCCHs, i.e., PDCCH #1, PDCCH #2, and PDCCH #3, for a UE #1 are allocated to logically contiguous CCEs on a CCE aggregation.

As such, a BS can transmit a plurality of PDCCHs for one UE through one downlink CC. The plurality of PDCCHs can have independent CCE aggregation levels. The plurality of PDCCHs can be allocated to logically contiguous CCEs. If the UE detects the PDCCH #2, a CCE index (4 to 7) of a CCE aggregation on which the PDCCH #2 is transmitted can be used to facilitate detection of another PDCCH from CCEs located in a front or rear portion in the CCE aggregation.

FIG. 24 is a block diagram showing wireless communication system to implement an embodiment of the present invention. A BS 50 may include a processor 51, a memory 52 and a radio frequency (RF) unit 53. The processor 51 may be configured to implement proposed functions, procedures and/or methods described in this description. Layers of the radio interface protocol may be implemented in the processor 51. The memory 52 is operatively coupled with the processor 51 and stores a variety of information to operate the processor 51. The RF unit 53 is operatively coupled with the processor 11, and transmits and/or receives a radio signal. A UE 60 may include a processor 61, a memory 62 and a RF unit 63. The processor 61 may be configured to implement proposed functions, procedures and/or methods described in this description. The memory 62 is operatively coupled with the processor 61 and stores a variety of information to operate the processor 61. The RF unit 63 is operatively coupled with the processor 61, and transmits and/or receives a radio signal.

The processors 51, 61 may include application-specific integrated circuit (ASIC), other chipset, logic circuit, data processing device and/or converter which converts a baseband signal into a radio signal and vice versa. The memories 52, 62 may include read-only memory (ROM), random access memory (RAM), flash memory, memory card, storage medium and/or other storage device. The RF units 53, 63 include one or more antennas which transmit and/or receive a radio signal. When the embodiments are implemented in software, the techniques described herein can be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The modules can be stored in memories 52, 62 and executed by processors 51, 61. The memories 52, 62 can be implemented within the processors 51, 61 or external to the processors 51, 61 in which case those can be communicatively coupled to the processors 51, 61 via various means as is known in the art.

Accordingly, a reception complexity of a PDCCH in the UE can be decreased in a multi-carrier system. A method and an apparatus of effectively monitoring a PDCCH are provided. As a result, power consumption of the UE can be decreased. Therefore, overall system performance can be improved.

In view of the exemplary systems described herein, methodologies that may be implemented in accordance with the disclosed subject matter have been described with reference to several flow diagrams. While for purposed of simplicity, the methodologies are shown and described as a series of steps or blocks, it is to be understood and appreciated that the claimed subject matter is not limited by the order of the steps or blocks, as some steps may occur in different orders or concurrently with other steps from what is depicted and described herein. Moreover, one skilled in the art would understand that the steps illustrated in the flow diagram are not exclusive and other steps may be included or one or more of the steps in the example flow diagram may be deleted without affecting the scope and spirit of the present disclosure.

What has been described above includes examples of the various aspects. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the various aspects, but one of ordinary skill in the art may recognize that many further combinations and permutations are possible. Accordingly, the subject specification is intended to embrace all such alternations, modifications and variations that fall within the spirit and scope of the appended claims.

Claims

1-10. (canceled)

11. A method of monitoring a physical downlink control channel (PDCCH) in a wireless communication system, carried in a user equipment (UE), the method comprising:

receiving, from a base station (BS), a PDCCH map comprising a monitoring set field indicating a monitoring set comprising N downlink (DL) component carriers (CCs) among L DL CCs, where L≧N, L and N each is a natural number; and

monitoring a set of PDCCH candidates at each of the N DL CCs.

12. The method of claim 11, wherein the step of monitoring a set of PDCCH candidates includes:

decoding each PDCCH candidate; and

if a PDCCH candidate is successfully decoded, acquiring control information on the successfully decoded PDCCH candidate.

13. The method of claim 12, wherein the PDCCH candidate is successfully decoded if no CRC error is detected after de-masking the CRC of the PDCCH candidate with a UE's identifier.

14. The method of claim 13, wherein the PDCCH map further comprises a CCE field, the CCE field indicating the Y CCE aggregation levels among X CCE aggregation levels, where X≧Y, X and Y each is a natural number.

15. The method of claim 14, wherein the UE monitors the set of PDCCH candidates at each of Y control channel element (CCE) aggregation levels.

16. The method of claim 11, wherein the PDCCH map is received on a PDCCH.

17. The method of claim 11, wherein the PDCCH map is received through a specific DL CC among the L DL CCs.

18. The method of claim 11, wherein the PDCCH map is received through a DL CC, the DL CC being changed among the L DL CCs in accordance with a predefined rule.

19. The method of claim 11, wherein the PDCCH map is received via a radio resource control (RRC) message.

20. A UE comprising:

radio frequency (RF) unit transmitting and receiving a radio signal; and

a processor coupled with the RF unit and configured to receive, from a base station (BS), a PDCCH map comprising a monitoring set field indicating a monitoring set comprising N downlink (DL) component carriers (CCs) among L DL CCs, where L≧N, L and N each is a natural number, and to monitor a set of PDCCH candidates at each of the N DL CCs.

21. The UE of claim 20, wherein the processor is further configured to;

decode each PDCCH candidate; and

if a PDCCH candidate is successfully decoded, acquire control information on the successfully decoded PDCCH candidate.

22. The UE of claim 21, wherein the PDCCH candidate is successfully decoded if no CRC error is detected after de-masking the CRC of the PDCCH candidate with the UE's identifier.

23. The UE of claim 22, wherein the PDCCH map further comprises a CCE field, the CCE field indicating the Y CCE aggregation levels among X CCE aggregation levels, where X≧Y, X and Y each is a natural number.

24. The UE of claim 23, wherein the processor monitors the set of PDCCH candidates at each of the Y control channel element (CCE) aggregation levels.

25. The UE of claim 20, wherein the PDCCH map is received on a PDCCH.

26. The UE of claim 20, wherein the PDCCH map is received through a specific DL CC among the L DL CCs.

27. The UE of claim 20, wherein the PDCCH map is received through a DL CC, the DL CC being changed among the L DL CCs in accordance with a predefined rule.

28. The UE of claim 20, wherein the PDCCH map is received via a radio resource control (RRC) message.