BACKGROUND

Technical Field

The present invention relates to a transmission apparatus, a reception apparatus, a transmission method, and a reception method.

Description of the Related Art

In recent years, it has become common to transmit not only audio data but also large-volume data, such as still image data and moving image data in addition to audio data in cellular mobile communication systems, in response to spread of multimedia information. Active studies associated with techniques for achieving a high transmission rate in a high-frequency radio band have been conducted to achieve large-volume data transmission.

When a high frequency radio band is utilized, however, attenuation increases as the transmission distance increases, although a higher transmission rate can be expected within a short range. Accordingly, the coverage area of a radio communication base station apparatus (hereinafter, abbreviated as “base station”) decreases when a mobile communication system using a high frequency radio band is actually put into operation. Thus, more base stations need to be installed in this case. The installation of base stations involves reasonable costs, however. For this reason, there has been a high demand for a technique that provides a communication service using a high-frequency radio band while limiting an increase in the number of base stations.

In order to meet such a demand, studies have been carried out on a relay technique in which a radio communication relay station apparatus (hereinafter, abbreviated as “relay station”) is installed between a base station and a radio communication mobile station apparatus (hereinafter, abbreviated as “mobile station”) to perform communication between the base station and mobile station via the relay station for the purpose of increasing the coverage area of each base station. The use of relay technique allows a mobile station not capable of directly communicating with a base station to communicate with the base station via a relay station.

It is required for an LTE-A (long-term evolution advanced) system for which the introduction of the relay technique described above has been studied, to maintain compatibility with LTE (long-term evolution) in terms of a smooth transition from and coexistence with LTE. For this reason, mutual compatibility with LTE is required for the relay technique as well.

FIG. 1illustrates example frames in which control signals and data are assigned in the LTE system and the LTE-A system.

In the LTE system, DL (downlink) control signals from a base station to a mobile station are transmitted through a DL control channel, such as PDCCH (physical downlink control channel). In LTE, DL grant (also referred to as “DL assignment”) indicating DL data assignment and UL (uplink) grant indicating UL data assignment are transmitted through PDCCH. A DL grant indicates that a resource in the subframe in which the DL grant is transmitted has been allocated to the mobile station. Meanwhile, in an FDD system, a UL grant indicates that a resource in the fourth subframe after the subframe in which the UL grant is transmitted has been allocated to the mobile station. In a TDD system, a UL grant indicates that the resource in a subframe transmitted after four or more subframes from the subframe in which the UL grant is transmitted has been allocated to the mobile station. In the TDD system, the subframe to be assigned to the mobile station, or the number of subframes before the assigned subframe in which the UL grant is transmitted is determined in accordance with the time-division pattern of the UL and DL (hereinafter referred to as “UL/DL configuration pattern”). Regardless of the UL/DL configuration pattern, the UL subframe is a subframe after at least four subframes from the subframe in which the UL grant is transmitted.

In the LTE-A system, relay stations, in addition to base stations, also transmit control signals to mobile stations in PDCCH regions in the top parts of subframes. With reference to a relay station, DL control signals have to be transmitted to a mobile station. Thus, the relay station switches the processing to reception processing after transmitting the control signals to the mobile station to prepare for receiving signals transmitted from the base station. The base station, however, transmits DL control signals to the relay station at the time the relay station transmits the DL control signals to the mobile station. The relay station therefore cannot receive the DL control signals transmitted from the base station. In order to avoid such inconvenience in LTE-A, studies have been carried out on providing a region for mapping downlink control signals for relay stations (i.e., relay PDCCH (R-PDCCH) region) in a data region as illustrated inFIG. 2in LTE-A. Similar to the PDCCH, mapping a DL grant and UL grant to the R-PDCCH is studied. In the R-PDCCH, as illustrated inFIG. 1, mapping a DL grant in the first slot and a UL grant in the second slot is studied (refer to Non-patent Literature 1). Mapping the DL grant only in the first slot reduces a delay in decoding the DL grant and allows relay stations to prepare for ACK/NACK transmission for DL data (transmitted in the fourth subframes following reception of DL grant in FDD). Each relay station finds the downlink control signals intended for the relay station by performing blind-decoding on downlink control signals transmitted using an R-PDCCH region from a base station within a resource region indicated using higher layer signaling from the base station (i.e., search space). As described above, the base station notifies the relay station of the search space corresponding to the R-PDCCH by higher layer signaling.

Given the introduction of various apparatuses as radio communication terminals in the future M2M (machine to machine) communication, for example, there is a concern for a shortage of resources in the mapping region for PDCCH (i.e., “PDCCH region”) due to an increase in the number of terminals. If PDCCH cannot be mapped due to such a resource shortage, the DL data cannot be assigned for the terminals. Thus, the resource region for mapping DL data (i.e., “PDSCH (physical downlink shared channel) region”) cannot be used even if there is an available region, which may cause a decrease in the system throughput. Studies have been carried out to solve such resource shortage through mapping control signals for terminals served by a base station also in a data region to which R-PDCCH is mapped. The resource region to which control signals for terminals served by the base station are mapped and which can be utilized as a data region at different timings is called an “enhanced PDCCH (E-PDCCH) region, “new-PDCCH (N-PDCCH) region” or “X-PDCCH region” or the like. As described above, in LTE-A, a relay technique is introduced and relay control signals are mapped to the data region. Since the relay control signal may be expanded and used as a control signal for a terminal, the resource region to which control signals for terminals served by the base station are mapped and which can be utilized as a data region at different timings is also called “R-PDCCH.” Mapping the control signals (i.e., E-PDCCH) to a data region in such a manner enables transmission power control for control signals transmitted to terminals near a cell edge or interference control for interference to another cell by control signals to be transmitted or for interference to the cell from another cell. In LTE-Advance, a high transmission rate is achieved using a wideband radio bandwidth, multiple-input multiple-output (MIMO) transmission technique and interference control technique.

PDCCH and R-PDCCH have four aggregation levels, i.e., levels 1, 2, 4, and 8 (for example, refer to Non-patent Literature (hereinafter, abbreviated as “NPL”) 1). Levels 1, 2, 4, and 8 respectively have six, six, two, and two “resource region candidates.” The term “resource region candidate” refers to a candidate region to which control signals are to be mapped. Each resource region candidate is composed of as many control channel elements (CCE) as corresponding aggregation levels. In addition, when a single terminal is set with one aggregation level, control signals are actually mapped to one of the multiple resource region candidates of the aggregation level.FIG. 2illustrates example search spaces corresponding to R-PDCCH. The ovals represent search spaces at various aggregation levels. The multiple resource region candidates in the search spaces at the different aggregation levels are arranged consecutively on VRBs (virtual resource blocks). The resource region candidates in the VRBs are mapped to PRBs (physical resource blocks) through higher layer signaling.

A search space corresponding to E-PDCCH is a resource region to which control signals transmitted from a base station to a terminal may be mapped. A search space corresponding to E-PDCCH is individually set for each terminal.

As described above, in the R-PDCCH region, a DL grant is mapped to the first slot and UL grant is mapped to the second slot. That is, the resource to which the DL grant is mapped is separated from the resource to which the UL grant is mapped in the time domain. In contrast, in E-PDCCH, as shown inFIG. 3, studies are also underway to separate the resource to which the DL grant is mapped from the resource to which the UL grant is mapped in the frequency domain (that is, subcarriers or PRB pair). Here, the term “PRB (physical resource block) pair” refers to a set of PRBs of the first slot and the second slot, whereas the term “PRB” refers to each of the PRBs of the first slot and the second slot.

For the design of E-PDCCH, part of the design of R-PDCCH may be used or a design completely different from the design of R-PDCCH may be used. Actually, studies are underway to make the design of E-PDCCH different from the design of R-PDCCH.

CITATION LIST

Non-Patent Literature

NPL 13GPP TS 36.216 V10.1.0 Physical layer for relaying operation

BRIEF SUMMARY

Technical Problem

When the resource to which a DL grant is mapped is separated from the resource to which a UL grant is mapped in the frequency domain (that is, subcarriers or RB pair), one PRB pair may be designated as a minimum unit (that is, a CCE) when resources are allocated to E-PDCCH. However, when a PRB pair made up of two slots is designated as a CCE, the resource amount of CCE increases. For this reason, a reception SINR of E-PDCCH increases and the possibility of receiving quality becoming excessively high, which results in an increased possibility of resources being wasted. Therefore, a “divided resource region” obtained by dividing one PRB pair may be used as a CCE of E-PDCCH.

However, when the division number per PRB pair increases, the resource amount of CCE for E-PDCCH (that is, the number of resource elements (REs) forming one CCE) decreases. Moreover, when the aggregation level of E-PDCCH is assumed to be 1, 2, 4 or 8 as in the cases of PDCCH and R-PDCCH, the number of terminals that can be supported decreases. That is, the receiving quality of terminals that can be supported is determined by the receiving quality of highest aggregation level 8. When the resource amount of CCE for E-PDCCH is small, the receiving quality of E-PDCCH degrades, and therefore the number of terminals that satisfy the desired receiving quality decreases.

Furthermore, even when the division number per PRB pair is fixed, the number of REs forming a CCE varies from one subframe to another. The following is the description of factors that cause the number of REs forming a CCE to vary from one subframe to another even when the division number per PRB pair is fixed. In LTE and LTE-A, one PRB has 12 subcarriers in the frequency direction and has a width of 0.5 msec in the time direction as shown inFIG. 4. A unit of two PRBs combined in the time direction is called a “PRB pair.” That is, a PRB pair has 12 subcarriers in the frequency direction and has a width of 1 msec in the time direction. However, when a PRB pair represents a block of 12 subcarriers in the frequency domain, the PRB pair may be simply called “RB.” In addition, a unit defined by one subcarrier and one OFDM symbol is a resource element (RE). The items described about PRBs here also apply to VRBs. The term “RB” is used to generically call a PRB and VRB.

[1] CP Length of OFDM Symbol:

The number of OFDM symbols per PRB varies depending on a CP (cyclic prefix) length of OFDM symbol. Therefore, the number of REs forming a CCE varies depending on the CP (cyclic prefix) length even if the division number per PRB pair is fixed.

To be more specific, a normal downlink subframe includes 14 OFDM symbols in the case of a normal CP and includes 12 OFDM symbols in the case of an extended CP. Furthermore, a DwPTS region of a special subframe shown inFIG. 5(that is, region used for DL transmission) includes three, nine, ten, eleven or twelve OFDM symbols in the case of a normal CP and three, eight, nine or ten OFDM symbols in the case of an extended CP.

[2] Number of REs Used for Reference Signal (RS):

The number of REs to which reference signals are mapped in one PRB varies from one subframe to another. Therefore, the number of REs forming a CCE varies depending on the number of REs to which reference signals are mapped in one PRB even when the division number per PRB pair is fixed.

(1) CRS:

CRS is transmitted in all RBs. Although CRS is also transmitted in a data region in subframes other than MBSFN subframes, CRS is transmitted using only two initial OFDM symbols in MBSFN subframes.

(2) DMRS (12 REs, 24 REs or 16 REs):

The use of DMRS is dynamically indicated from a base station to a terminal using downlink assignment control information (DL assignment). The number of DMRSs to be set can be made to vary from one user to another. DMRS is transmitted in a data region and the value to be set may vary from one RB to another.

(3) CSI-RS (2 REs or More):

CSI-RS is transmitted in all RBs. A subframe to be transmitted is determined by a previously set period. CSI-RS has a muting function of not transmitting data in order to receive a CSI-RS of another cell. Once the CSI-RS muting is set, the number of REs usable as a data region or E-PDCCH region further decreases.

(4) PRS (Positioning Reference Signals):

PRS (positioning reference signals) is an RS used for position measurement. In such a setting that REs set for this PRS is not used for the E-PDCCH region, the number of REs available for E-PDCCH further decreases.

[3] Number of OFDM Symbols Forming PDCCH Region:

The number of OFDM symbols used for PDCCH is variable from one to four. Therefore, in such a setting that the PDCCH region is not used for E-PDCCH, the number of OFDM symbols available for E-PDCCH decreases as the number of OFDM symbols of the PDCCH region increases. That is, the number of REs forming a CCE varies depending on the number of OFDM symbols forming the PDCCH region even if the division number per PRB pair is fixed.

FIG. 6andFIG. 7illustrate the number of REs of the first slot and the second slot when resources of the fourth and subsequent OFDM symbols of the PRB pair are used for E-PDCCH.FIG. 6andFIG. 7illustrate an example where CSI-RS is mapped to the second slot in particular.FIG. 6andFIG. 7together form one table:FIG. 6showing the first half of the table andFIG. 7showing the second half of the table.

As described above, when the number of REs located in a PRB pair and available for E-PDCCH fluctuates considerably, receiving quality of a control signal is more likely to degrade.

An object of the present invention is to provide a transmitting apparatus, a receiving apparatus, a transmission method and a reception method that are capable of improving receiving quality of a control signal.

Solution to Problem

A transmitting apparatus according to an aspect of the present invention includes: a calculation section that calculates a division number of each one of physical channel resource blocks based on: a first number of resource elements in a corresponding one of the physical channel resource blocks to which resource elements an assignment control signal is capable of being mapped; a second number of resource elements to which a signal other than the assignment control signal is mapped; and a reference value which is a number of resource elements that satisfy receiving quality of the assignment control signal in a receiving apparatus; a control section that sets a resource region candidate including at least one control channel element obtained by dividing each one of the physical channel resource blocks into the division number and that determines, based on an aggregation level, a search space made up of a plurality of the resource region candidates set in each one of the physical channel resource blocks; and a transmitting section that transmits, to the receiving apparatus, the assignment control signal mapped in one of the plurality of resource region candidates forming the search space.

A receiving apparatus according to an aspect of the present invention includes: a calculation section that calculates a division number of each one of physical channel resource blocks based on: a first number of resource elements in a corresponding one of the physical channel resource blocks to which resource elements an assignment control signal is capable of being mapped; a second number of resource elements to which a signal other than the assignment control signal is mapped; and a reference value which is a number of resource elements that satisfy receiving quality of the assignment control signal in a receiving apparatus; an identification section that sets a resource region candidate including at least one control channel element obtained by dividing each one of the physical channel resource blocks into the division number and that identifies, based on an aggregation level, a search space made up of a plurality of the resource region candidates set in each one of the physical channel resource blocks; and a receiving section that receives the assignment control signal mapped in one of the plurality of resource region candidates forming the identified search space.

A transmission method according to an aspect of the present invention includes: calculating a division number of each one of physical channel resource blocks based on: a first number of resource elements in a corresponding one of the physical channel resource blocks to which resource elements an assignment control signal is capable of being mapped; a second number of resource elements to which a signal other than the assignment control signal is mapped; and a reference value which is the number of resource elements that satisfy receiving quality of the assignment control signal in a receiving apparatus; setting a resource region candidate including at least one control channel element obtained by dividing each one of the physical channel resource blocks into the division number; determining, based on an aggregation level, a search space made up of a plurality of the resource region candidates set in each one of the physical channel resource blocks; and transmitting, to the receiving apparatus, the assignment control signal mapped in one of the plurality of resource region candidates forming the search space.

A reception method according to an aspect of the present invention includes: calculating a division number of each one of physical channel resource blocks based on: a first number of resource elements in a corresponding one of the physical channel resource blocks to which resource elements an assignment control signal is capable of being mapped; a second number of resource elements to which a signal other than the assignment control signal is mapped; and a reference value which is the number of resource elements that satisfy receiving quality of the assignment control signal; setting a resource region candidate including at least one control channel element obtained by dividing each one of the physical channel resource blocks into the division number; identifying, based on an aggregation level, a search space made up of a plurality of the resource region candidates set in each one of the physical channel resource blocks; and receiving the assignment control signal mapped in one of the plurality of resource region candidates forming the identified search space.

Advantageous Effects of Invention

According to the present invention, it is possible to provide a transmitting apparatus, a receiving apparatus, a transmission method and a reception method that are capable of improving receiving quality of a control signal.

