Patent Description:
In a UMTS (Universal Mobile Telecommunications System) network, the specifications of long term evolution (LTE) have been drafted for the purpose of further increasing high-speed data rates, providing lower delay and so on (see non-patent literature <NUM>). In LTE, as multiple-access schemes, a scheme that is based on OFDMA (Orthogonal Frequency Division Multiple Access) is used in downlink channels (downlink), and a scheme that is based on SC-FDMA (Single Carrier Frequency Division Multiple Access) is used in uplink channels (uplink). Also, successor systems of LTE (referred to as, for example, "LTE-advanced" or "LTE enhancement" (hereinafter referred to as "LTE-A")) are under study for the purpose of achieving further broadbandization and increased speed beyond LTE, and the specifications thereof have been drafted (Rel. <NUM>/<NUM>).

As duplex modes in LTE and LTE-A systems, there are frequency division duplex (FDD) to divide between the uplink (UL) and the downlink (DL) based on frequency, and time division duplex (TDD) to divide between the uplink and the downlink based on time (see <FIG>). In TDD, the same frequency region is applied to uplink and downlink communication, and signals are transmitted and received to and from one transmitting/receiving point by dividing the uplink and the downlink based on time.

Also, the system band of the LTE-A system (Rel. <NUM>/<NUM>) includes at least one component carrier (CC), where the system band of the LTE system constitutes one unit. Gathering a plurality of components carriers (cells) to achieve a wide band is referred to as "carrier aggregation" (CA).

Non-Patent Literature <NUM> describes DAI Design for LTE-A. DAI is needed for both FDD and TDD. ACK/NAK transmission on PUCCH is described.

Patent Literature <NUM> describes that the <NUM>-bit DAI originally defined for TDD may be used for FDD downlink data transmissions for more efficient ACK/NACK feedback via the PUCCH on the TDD CC.

In carrier aggregation (CA), which was introduced in Rel. <NUM>/<NUM>, the duplex mode to employ between a plurality of CCs (also referred to as "cells," "transmitting/receiving points," etc.) is limited to the same duplex mode (see <FIG>). On the other hand, future radio communication systems (for example, Rel. <NUM> and later versions) may anticipate CA to employ different duplex modes (TDD+FDD) between multiple CCs (see <FIG>).

<NUM>/<NUM> anticipates intra-base station CA (intra-eNB CA), which controls CA by using one scheduler between multiple CCs. In this case, the PUCCH signals (transmission acknowledgment signals (ACKs/NACKs), etc.) in response to DL data signals (PDSCH signals) transmitted in each CC are multiplexed to be aggregated in a specific CC (primary cell (PCell)) and transmitted.

When conventional feedback mechanism is used in CA in which different duplex modes (TDD+FDD) are employed between multiple CCs, there is a risk that transmission acknowledgment signals and so on cannot be transmitted adequately on the uplink.

The present invention has been made in view of the above, and it is therefore an object of the present invention to provide a user terminal, a base station and a radio communication method, whereby uplink transmission can be carried out adequately even when CA is executed by applying different duplex modes between multiple cells.

According to the present invention, it is possible to carry out uplink transmission adequately even when CA is executed by applying different duplex modes between multiple cells.

As noted earlier, in LTE and LTE-A systems, two duplex modesnamely, FDD and TDD -- have been provided (see above <FIG>). Also, from Rel. <NUM> onward, support for intra-base station CA (intra-eNB CA) has been provided. However, CA in Rel. <NUM>/<NUM> is limited to the same duplex mode (FDD+FDD intra-eNB CA or TDD+TDD intra-eNB CA) (see above <FIG>).

Meanwhile, the systems of Rel. <NUM> and later versions presume intra-base station CA (intra-eNB CA), which employs different duplex modes (TDD+FDD) between multiple CCs (see above <FIG>). Furthermore, the systems of Rel. <NUM> and later versions also presume employing inter-base station CA (inter-eNB CA) (see <FIG>). Note that inter-base station CA is supported regardless of the duplex mode, and it may be possible to introduce inter-base station CA, which accommodates different duplex modes (TDD+FDD).

In intra-base station CA (intra-eNB CA), scheduling is controlled using one scheduler between multiple cells (see <FIG>). That is, a user terminal has only to feed back uplink control signals (UCI) such as a transmission acknowledgment signal (ACK/NACK (hereinafter also referred to as "A/N")) and/or the like to a specific cell (PCell) alone.

Meanwhile, in inter-base station CA (inter-eNB CA), schedulers are provided separately for each of multiple cells, and scheduling is controlled on a per cell basis. Also, inter-eNB CA presumes that each base station is connected in such a manner that the delay is not negligible (non-ideal backhaul connection). Consequently, the user terminal has to feed back uplink control signals (UCI) to each cell (see <FIG>).

When CA is executed by applying different duplex modes between multiple CCs (cells) (TDD-FDD CA), the problem is how user terminals should send A/N feedback. For example, it may be possible that each cell employs conventional feedback mechanism on an as-is basis in TDD-FDD CA.

<FIG> shows a case where, in a cell employing FDD (hereinafter also referred to as an "FDD cell"), a user terminal feeds back A/N's in response to PDSCH signals with conventional timing. In this case, the user terminal feeds back the A/N's in UL subframes that come a predetermined number of subframes after (for example, <NUM> after) the DL subframes in which the PDSCH signals are allocated.

<FIG> shows a case where, in a cell employing TDD (hereinafter also referred to as a "TDD cell"), a user terminal feeds back A/N's in response to PDSCH signals with conventional timing. In this case, the user terminal feeds back the A/N's in UL subframes that are assigned in advance to the DL subframes in which the PDSCH signals are allocated.

In TDD up to the Rel. <NUM> system, the configuration ratio of UL and DL has had a plurality of patterns (DL/UL configurations <NUM> to <NUM>), and, in each DL/UL configuration, the DL subframes to be allocated to UL subframes are determined. For example, <FIG> shows the case of DL/UL configuration <NUM> (DL/UL Config. <NUM>), in which each DL subframe is allocated to (associated with) a predetermined UL subframe. In <FIG>, the number that is assigned to each DL subframe (including special subframes) shows the number of subframes from the corresponding UL subframe.

In conventional systems, the timing to feed back A/N's (DL HARQ timing) stays the same even when CA is employed. However, even when CA is applied to UL, A/N transmission using the PUCCH is determined to be carried out only in a specific cell (PCell).

Also, in conventional systems, a plurality of formats (PUCCH formats) are defined for the PUCCH transmission of uplink control signals such as transmission acknowledgment signals (A/N signals) and channel quality information (CQI). Now, PUCCH format 1b defined for A/N feedback will be described below.

When CA is not employed in an FDD cell (non-CA), the A/N's that are fed back from each user terminal in one subframe are one or two bits. In this case, the user terminals apply PUCCH format 1a/1b and feed back one or two A/N bits by using BPSK or QPSK (BPSK or QPSK modulation). In PUCCH format 1a/1b, the PUCCH resource to allocate an A/N is determined based on the place where downlink control information (DL DCI) is scheduled (PDCCH/EPDCCH resource index (CCE/ECCE index)) and a parameter that is reported through RRC signaling (RRC parameter) (see <FIG>). In this case, it is possible to encode and multiplex maximum thirty six A/N's per RB.

When CA (two CCs) is employed in an FDD cell, the A/N's that are fed back from each user terminal in one subframe require maximum four bits. In this case, the user terminals apply PUCCH format 1b with channel selection and transmit maximum four A/N bits. In PUCCH format 1b with channel selection (hereinafter also referred to simply as "channel selection"), a PUCCH resource candidate is determined from the place where DL DCI for the PCell is scheduled (CCE/ECCE index), and an RRC parameter. Also, another PUCCH resource candidate is determined from a TPC command (ARI) that is included in DL DCI for an SCell, and an RRC parameter (see <FIG>).

The ARI is an ACK/NACK resource indicator (A/N resource indicator) that was introduced in Rel. <NUM>, and is used to specify the PCell's PUCCH resource that is used to send A/N feedback for the PDSCH transmitted from the SCell when CA is employed. To be more specific, a plurality of PUCCH resource candidates are reported in advance to a user terminal through higher layers such as RRC, and one among these is specified by the ARI.

In channel selection, maximum four A/N bits are represented by using a plurality of PUCCH resource candidates and QPSK symbols. The user terminals select and feed back predetermined PUCCH resources/QPSK symbol points depending on each cell's A/N contents.

For example, assume a case here where, in PUCCH format 1b with channel selection, four PUCCH resource candidates are configured. In this case, the PUCCH resource for when channel selection is not executed (PUCCH format 1b) and the PUCCH resource following that PUCCH resource will be referred to as PUCCH resource candidates <NUM> and <NUM>, respectively. The PUCCH resource candidate <NUM> can be calculated by adding +<NUM> to the CCE/ECCE that is used to calculate the PUCCH resource candidate <NUM>. Also, from a set of four resource candidates that are configured in advance by RRC signaling, PUCCH resources that are specified dynamically by TPC commands (ARIs) contained in the SCell's DCI are PUCCH resource candidates <NUM> and <NUM> (see <FIG>).

In PUCCH format 1b with channel selection, a user terminal targets different PUCCH resources and/or different QPSK symbol points for mapping, depending on the A/N/DTX state (hereinafter also referred to as the "A/N state"). To be more specific, the user terminal controls A/N feedback based on a relationship table (mapping table), which defines the associations/relationships between A/N states, PUCCH resources and QPSK symbol points.

On the other hand, since A/N's for a plurality of DLs are allocated to one UL in a TDD cell, even when CA is not employed (non-CA), A/N feedback of more than two bits is required. Consequently, in TDD, it is possible to execute A/N bundling, which bundles and processes A/N's for a plurality of DL subframes as one A/N. In this case, feedback can be sent by using PUCCH format 1a/1b. Meanwhile, in TDD, even when CA is not employed, it is possible to configure the above-noted PUCCH format 1b with channel selection and PUCCH format <NUM>. When CA is employed, the above PUCCH format 1b with channel selection and PUCCH format <NUM> are employed. In PUCCH format <NUM>, a PUCCH resource candidate is determined from a TPC command (ARI) that is included in DL DCI for an SCell, and an RRC parameter.

In this way, existing systems provide different PUCCH mechanisms between FDD and TDD, and do not assume PUCCH transmission for when CA is carried out by applying different duplex modes between multiple cells (multiple CCs) (TDD-FDD CA). For example, in TDD-FDD CA, when ACKs/NACKs for multiple CCs (for example, two CCs) are gathered in one cell (CC) and transmitted, how a user should apply the PUCCH format and carry out A/N transmission is the problem.

In particular, in existing system, each A/N bit is defined to correspond to PUCCH resources/QPSK symbol points differently between FDD and TDD. Consequently, when existing feedback mechanism is used in TDD-FDD CA, there is a threat that troubles might occur.

So, the present inventors have come with the idea of, when channel selection is carried out in TDD-FDD CA by using tables which each define different contents between the FDD cell and the TDD cell, selecting between the tables based on the duplex mode of the cell where PUCCH transmission is carried out. Also, the present inventors have come up with idea of carrying out channel selection by using the TDD table, regardless of the cell where PUCCH transmission is carried out. Furthermore, the present inventors have come up with the idea of employing A/N bundling in the FDD cell and/or in the TDD cell in TDD-FDD CA channel selection.

Now, a first aspect, a second aspect, a third example and a fourth example will be described below in detail with reference to the accompanying drawings. Only the fourth example is covered by the claims. The first aspect, the second aspect and the third example are nevertheless useful for understanding the invention.

With the First aspect, when PUCCH format 1b with channel selection is employed in TDD-FDD CA, a predetermined relationship table is selected for use, based on the duplex mode of the cell (CC) to carry out PUCCH transmission. That is, regardless of which cell's downlink control information (DL DCI) is detected or which cell's PDSCH is received, a user terminal executes channel selection by using the mapping table that corresponds to the duplex mode of the cell (CC) to carry out PUCCH transmission.

<FIG> shows a case where an A/N in response to the DL signal from the TDD cell and an A/N in response to the DL signal from the FDD cell are transmitted by using a PUCCH resource of the TDD cell, by employing channel selection. That is, based on the state of the A/N for the TDD cell and the state of the A/N for the FDD cell, the user terminal selects a predetermined PUCCH resource and a QPSK symbol point from the mapping table and carries out PUCCH transmission from the TDD cell.

In this case, the user terminal selects the channel selection relationship table for the TDD cell to carry out PUCCH transmission. Note that, for the channel selection relationship table for the TDD cell, the TDD cell mapping table that is defined in Rel. <NUM> (see <FIG>) can be used. Note that mapping table to be associated with the TDD cell is not limited to that illustrated in <FIG>.

<FIG> shows a case where an A/N in response to the DL signal from the TDD cell and an A/N in response to the DL signal from the FDD cell are transmitted by using a PUCCH resource of the FDD cell by employing channel selection. That is, based on the state of the A/N for the TDD cell and the state of the A/N for the FDD cell, the user terminal selects a predetermined PUCCH resource and a QPSK symbol point from the mapping table and carries out PUCCH transmission from the FDD cell.

In this case, the user terminal selects the channel selection relationship table for the FDD cell to carry out PUCCH transmission. Note that, for the channel selection relationship table for the FDD cell, the TDD cell mapping table defined in Rel. <NUM> (see <FIG>) can be used. Note that the mapping table to be associated with the FDD cell is not limited to that of <FIG>.

In this way, with the first aspect, the user terminal performs channel selection by using a relationship table that corresponds to the duplex mode of the cell where PUCCH transmission is carried out. By this means, when the user terminal fails to receive DL DCI in the CC where the PUCCH is not transmitted (detection failure), it is still possible to apply the feedback mechanism which does not employ CA (non-CA) on an as-is basis (fallback).

For example, when the user terminal detects only a DL DCI assignment in the TDD cell (fails to detect the FDD cell's DCI), the user terminal allocates an A/N in response to this TDD cell's DL DCI, to a PUCCH resource of the TDD cell (see <FIG>). On the other hand, when the user terminal detects only a DL DCI assignment in the FDD cell (fails to detect the TDD cell's DCI), the user terminal allocates an A/N in response to this FDD cell's DL DCI, to a PUCCH resource of the FDD cell (see <FIG>).

In this case, seen from the NW (for example, base stations), it is possible to judge that all the user terminals that carry out PUCCH transmission in the same CC are transmitting A/N's in accordance with the same mapping table. Consequently, as long as mapping decoding algorithms are implemented on a per CC basis, it is possible to simplify the circuit structure in the base stations.

Also, by employing the first aspect, it is possible to support enhancement to inter-base station CA (inter-eNB CA). As described above, when non-ideal backhaul connection is used in inter-eNB CA, the user terminal may carry out PUCCH transmission to each base station. In inter-eNB CA, the first aspect can be applied on an as-is basis, when channel selection is carried out in one or in each base station.

<FIG> shows a case where, in TDD-FDD CA (inter-eNB CA), A/N's in response to the DL signal (PDSCH signal) of each cell are allocated to a PUCCH resource of each cell.

<FIG> shows a case where, in the TDD cell, A/N's in response to a plurality of DL subframes are allocated to a PUCCH resource of the TDD cell by using channel selection. With the first aspect, the user terminal selects the channel selection relationship table for the TDD cell for PUCCH transmission in the TDD cell. Consequently, even in the channel selection in inter-eNB CA shown in <FIG>, the user terminal selects the relationship table for the TDD cell for PUCCH transmission in the TDD cell, and therefore A/N feedback can be executed adequately.

<FIG> shows a case where a plurality of FDD cells are engaged in intra-eNB CA, and where these multiple FDD cells and TDD cells are engaged in inter-eNB CA. In this case, the user terminal gathers the A/N's of the individual FDD cells in one FDD cell (for example, the PCell), and carries out A/N transmission. That is, the user terminal employs channel selection and carries out PUCCH transmission from one FDD cell. With the first aspect, the user terminal selects the channel selection relationship table for the FDD cell for PUCCH transmission in the FDD cell. Consequently, even in <FIG>, the user terminal selects the relationship table for the FDD cell for PUCCH transmission in the FDD cell, and therefore can execute A/N feedback adequately. Note that the TDD cell is the same as in above <FIG>.

As described above, PUCCH format 1b with channel selection supports up to <NUM>-CC CA. In the FDD cell, A/N transmission in response to a DL signal is fed back in a UL subframe that comes a predetermined period (<NUM>) after the DL subframe in which that DL signal is transmitted. Consequently, in FDD, A/N's never exceed maximum four bits in the event of two CCs.

On the other hand, in TDD, A/N's for a plurality of DL subframes are transmitted in one UL in each CC, so that cases might occur where more than four bits are involved in the event of two CCs. For example, in TDD, if CA is executed in UL/DL configuration <NUM>, the A/N's to feed back in one UL become sixteen bits (four subframes × two CWs × two CCs) (see <FIG>). TDD in existing systems provides for, when more than four bits are involved, employing A/N spatial bundling and making A/N's for two CWs a one-bit A/N.

By employing spatial bundling of A/N's, the A/N's to feed back in one UL subframe in <FIG> become maximum eight bits (=<NUM>/<NUM>). PUCCH format 1b with channel selection in TDD provides for A/N feedback of up to eight bits (four bits in FDD).

As noted earlier, although a plurality of DL/UL configurations are defined in TDD (DL/UL configurations <NUM> to <NUM>), DL/UL configuration <NUM> alone is designed so that A/N's in response to DL subframes of over four subframes are concentrated as one (see <FIG>). Consequently, in existing systems, TDD DL/UL configuration <NUM> does not support channel selection.

Now, in TDD-FDD CA, if channel selection for the FDD cell is employed as shown in <FIG>, in the TDD cell, it is not possible to provide DL assignments for multiple subframes (A/N transmission in response to multiple DL subframes). This is because existing systems provide for channel selection for FDD-FDD CA (two CCs), and provide support for only maximum four bits.

Consequently, with the second aspect, the relationship table for TDD is used when channel selection is employed in TDD-FDD CA. That is, in TDD-FDD CA, regardless of which cell's downlink control information (DL DCI) is detected, which cell's PDSCH is received, or which cell's PUCCH is transmitted, the user terminal uses the mechanism of PUCCH format 1b with channel selection for TDD. <FIG> shows a case where the channel selection relationship table for TDD is used for PUCCH transmission from the FDD cell.

Given that TDD-FDD CA is based on the premise that at least one TDD cell is engaged in CA, although there is a high possibility that more than four bits of A/N's are produced, it is still possible to use channel selection adequately in TDD-FDD CA, by applying the second aspect.

Note that, with the second aspect, when there are more than four bits of A/N's, bundling may be executed in the spatial direction (spatial bundling) in the FDD cell and/or in the TDD cell, as in TDD CA. By this means, even when employing spatial multiplexing (or MIMO) results in an increased number of A/N bits, adequate feedback is still possible. Also, in TDD, when a large number of DL subframes correspond to A/N's, it is possible to execute channel selection by using a mapping table which defines the relationships between A/N states and PUCCH resources/QPSK symbol points (constellation)/code sequences (RM code input bits).

When TDD-FDD CA is employed, a user terminal can use A/N bundling in the subframe direction in the TDD cell. For example, the user terminal employs PUCCH format 1b with channel selection by using the TDD cell's A/N's, which are bundled in one bit (or two bits) by A/N bundling, and the FDD cell's A/N's (one bit or two bits). <FIG> shows a case where A/N's (one bit or two bits) to correspond to the TDD cell, bundled by A/N bundling in the subframe direction, and A/N's (one bit or two bits) to correspond to the FDD cell are transmitted in PUCCH transmission from the FDD cell, by using channel selection.

In this way, by making A/N's maximum four bits by employing A/N bundling, regardless of whether the PUCCH-transmitting cell is in FDD or in TDD, the user terminal can execute channel selection by using the mapping table for either the TDD cell or the FDD cell.

Also, since channel selection is executed after A/N bundling applied in the subframe direction, with the third example, channel selection can be applied even to TDD DL/UL configuration <NUM>, which is not supported in existing systems. To be more specific, the user terminal can bundle A/N's for maximum nine DL subframes, and multiplex and transmit the bundling result with the FDD cell's A/N's by way of channel selection (see <FIG>).

Furthermore, with the third example, A/N bundling is executed in the subframe direction so as to provide one bit, bundling in the spatial direction is not necessary. In other words, when A/N bundling is employed in the subframe direction and yet bundling is not executed in the spatial direction, the maximum number of A/N bits can be made four bits. In this way, by not executing bundling in the spatial direction, HARQ for when spatial multiplexing (or MIMO) is employed can be executed per spatial multiplexing (or MIMO) stream, so that, by executing adaptive HARQ in a minute manner, it is possible to heighten the effect of improving throughput.

Note that, although a case has been shown with reference to <FIG> where A/N bundling is applied to the TDD cell, this is by no means limiting, and it is equally possible to apply bundling to the FDD cell as well. For example, as shown in <FIG>, it is possible to bundle A/N's for a plurality of DL subframes in the FDD cell, and multiplex and transmit these bundled A/N's with A/N's that are bundled in the TDD cell, by way of channel selection. Note that, although <FIG> shows a case where A/N's for the same number of DL subframes (for example, four subframes) are bundled in the FDD cell and in the TDD cell, the number of A/N's to bundle may vary in each cell.

In this way, by applying A/N bundling to the FDD cell, it is possible to allocate A/N's for more DL subframes to one UL subframe and feed them back in one PUCCH. AS a result, it is possible to improve the spectral efficiency of UL resources.

As described above, in the TDD cell, DL subframes are allocated over multiple subframes, and A/N's for a plurality of DL subframes are fed back in one UL subframe. At this time, if the user terminal fails to detect a DL assignment (PDCCH signal) in a DL subframe in the middle amongst these multiple DL subframes, the user terminal cannot send adequate A/N feedback.

For example, assume a case where DL signals are transmitted to the user terminal in four consecutive subframes (SFs #<NUM> to #<NUM>). In this case, if the user terminal fails to detect the DL assignment (PDCCH signal) of SF #<NUM>, the user terminal judges that DL signals are transmitted in three subframes SFs #<NUM>, #<NUM> and #<NUM>. Consequently, if the user terminal executes A/N bundling in the subframe direction, the user terminal feeds back an ACK if these three subframes (SFs #<NUM>, #<NUM> and #<NUM>) are OK (ACK). In this way, if a detection failure occurs on the user terminal side, DL HARQ cannot be executed properly.

In order to solve this problem, TDD has heretofore provided support for the two-bit DAI in downlink control information (DCI). The DAI functions as a counter, and its value increases by one per DL assignment. That is, when the user terminal fails to detect a DL assignment in the middle, the DAI count value skips one count, so that the failed detection is brought to attention.

For example, when DL signals are transmitted to the user terminal in four consecutive subframe (SFs #<NUM> to #<NUM>), the DCIs of SFs #<NUM> to #<NUM> include DAI=<NUM> to <NUM>, respectively. When the user terminal fails to detect the DL assignment (PDCCH signal) in SF #<NUM>, this results in the state in which DAI=<NUM> cannot be acquired and is missing in the user terminal, so that the user terminal can judge that a detection failure has occurred with the DL assignment of SF #<NUM> (see <FIG>). As a result of this, the user terminal can recognize that the A/N for SF #<NUM> which comes in second is wrong.

Also, when DL signals are transmitted to the user terminal in three subframes (SFs #<NUM>, #<NUM> and #<NUM>), the DCIs of SFs #<NUM>, #<NUM> and #<NUM> include DAI=<NUM> to <NUM>, respectively. If the user terminal fails to detect the DL assignment (PDCCH signal) in SF #<NUM>, this results in the state in which DAI=<NUM> cannot be acquired and is missing in the user terminal, so that the user terminal can judge that a detection failure has occurred with the DL assignment of SF #<NUM> or #<NUM> (see <FIG>). As a result of this, the user terminal can recognize that the first A/N (for SF #<NUM> or #<NUM>) is wrong.

In this way, in TDD where A/N bundling in the subframe direction may be employed, DAI is supported. That is, conventionally, support for the DAI has been provided in TDD, and in TDD DL/UL configurations <NUM> to <NUM>, which feed back A/N's in response to multiple DLs in one UL.

Note that, among the TDD DL/UL configurations, in DL/UL configuration <NUM>, which has a low DL subframe ratio (a high UL subframe ratio), the DAI is not supported because A/N's for multiple DLs are not fed back in one UL subframe. Also, FDD has no motive to provide support for the DAI, and so the DAI is not supported in FDD either.

Consequently, the present inventors have found out that, when, in TDD-FDD CA, A/N bundling and so on are applied to A/N's for a plurality of consecutive DL subframes in FDD, it is not possible to utilize the DAI, and therefore there is threat that the performance of DL HARQ might deteriorate.

So, the present inventors have found out supporting the DAI in FDD. For example, when A/N bundling is applied to the FDD cell in TDD-FDD CA, a DAI of predetermined bits (for example, two bits) is configured in the FDD cell's DL DCI. By this means, even in the FDD cell, it is possible to maintain the DL HARQ performance for when A/N bundling is employed, as in TDD.

Also, according to the fourth example, instead of configuring a DAI of predetermined bits (for example, two bits) in the FDD cell's DL DCI, it is possible to realize a predetermined function by limiting the DL assignments of the FDD cell to the same subframes as those of the TDD cell's DL assignments. That is, the DAI that is contained in the TDD cell's DL DCI is used as a DAI for both the FDD cell and the TDD cell. For example, assume a case here where a user terminal sends A/N's for the TDD cell that are bundled in A/N bundling and A/N's for the FDD cell that are bundled in A/N bundling in PUCCH transmission from one cell, by using channel selection (see <FIG>).

For example, when DL assignments are provided in predetermined DL subframes (SFs #<NUM>, #<NUM> and #<NUM>) in the TDD cell, DL assignments are provided in SFs #<NUM>, #<NUM> and #<NUM> in the FDD cell as well, as in the TDD cell. Consequently, the DAI of the TDD cell functions as a counter for both the TDD cell and the FDD cell. That is, the DAI included in the DCI in each DL subframe of the TDD cell is used for the FDD cell as well.

By this means, it is possible to adequately execute A/N bundling in the FDD cell, as in the TDD cell, and, given that it is not necessary to add a two-bit DAI to the DL DCI of the FDD cell, it is possible to reduce the increase of DCI overhead.

Note that, although cases have been described with the above embodiment where the feedback timing for when CA is not employed is used as the HARQ timing in response to the allocation of DL signals (PDSCH signals) of both the FDD cell and the TDD cell, the present embodiment is by no means limited to this. For example, it is possible to make the DL HARQ timing in the TDD cell the same as the DL HARQ timing in FDD, in intra-eNB CA (see <FIG>). In this case, the A/N in response to the PDSCH signal that is transmitted in a DL subframe of the TDD cell can be fed back in a UL subframe of the FDD cell that comes a predetermined period (for example, <NUM>) after the subframe in which the PDSCH signal is transmitted. By this means, it is possible to reduce the feedback delay in TDD DL HARQ to <NUM>. Also, since it is possible to reduce the number of transmission acknowledgment signals to feed back in one UL subframe and distribute these signals over a plurality of subframes, even when a base station fails to detect a transmission acknowledgment signal, it is possible to reduce the impact this has on DL HARQ.

Meanwhile, in the case illustrated in <FIG>, in timings (TDD cell's UL subframes) where both the FDD cell and the TDD cell are directed to UL, in which CC the A/N's should be multiplexed and PUCCH transmission should be carried out is the problem. Also, when channel selection is used in PUCCH transmission, how to execute control (how to determine the type of the table to select in channel selection) is the problem. In this case, it is possible to select the cell to carry out PUCCH transmission by using one of the examples shown with the above embodiment. For example, referring to <FIG>, in a subframe where the FDD cell and the TDD cell are both directed to UL, cases might occur including the case where PUCCH transmission is carried out only in one cell (the FDD cell or the TDD cell) regardless of the configuration of the primary cell, the case where PUCCH transmission is carried out in the PCell or in the SCell, and the case where PUCCH transmission is carried out in the cell that carries out A/N transmission in this subframes. Furthermore, in each PUCCH transmission, it is possible to employ channel selection by using one of the mechanisms shown with the above embodiment.

Now, an example of the operation of user terminals according to the present embodiment will be described below.

First, a user terminal connects with the TDD cell or the FDD cell. Following this, TDD-FDD CA is configured from the NW (for example, the connecting base station) to the user terminal. At this time, the TDD cell's DL/UL configuration is reported to the user terminal via system information (SIB <NUM>) or via higher layer signaling such as RRC and so on. Also, through higher layer signaling such as RRC, the number of CCs and the use of PUCCH format 1b with channel selection are configured. In addition, the PUCCH resource and other parameters are reported at the same time.

Following this, the NW schedules the allocation of the PDSCH in the PCell and the SCell by means of the PDCCH/EPDCCH. The user terminal decodes the PDCCH/EPDCCH, decodes the PDSCHs of the PCell and the SCell, and makes decisions regarding retransmission control (ACKs/NACKs). Then, the user terminal sends feedback by using the PUCCH transmission method in which A/N's that are provided in accordance with the retransmission control decisions are configured (the above first aspect to the fourth example).

Now, an example of a radio communication system according to the present embodiment will be described in detail below.

<FIG> is a schematic structure diagram of the radio communication system according to the present embodiment. Note that the radio communication system shown in <FIG> is a system to incorporate, for example, the LTE system or SUPER <NUM>. This radio communication system can adopt carrier aggregation (CA) to group a plurality of fundamental frequency blocks (component carriers) into one, where the system bandwidth of the LTE system constitutes one unit. Also, this radio communication system may be referred to as "IMT-advanced," or may be referred to as "<NUM>," "FRA (Future Radio Access)," etc..

The radio communication system <NUM> shown in <FIG> includes a radio base station <NUM> that forms a macro cell C1, and radio base stations 12a and 12b that form small cells C2, which are placed inside the macro cell C1 and which are narrower than the macro cell C1. Also, user terminals <NUM> are placed in the macro cell C1 and in each small cell C2. The user terminals <NUM> can connect with both the radio base station <NUM> and the radio base stations <NUM> (dual connectivity). Also, intra-base station CA (intra-eNB CA) or inter-base station CA (inter-eNB CA) is applied between the radio base station <NUM> and the radio base stations <NUM>. Furthermore, it is possible that one of the radio base station <NUM> and the radio base stations <NUM> employs FDD and the other one employs TDD.

Between the user terminals <NUM> and the radio base station <NUM>, communication is carried out using a carrier of a relatively low frequency band (for example, <NUM>) and a narrow bandwidth (referred to as, for example, "existing carrier," "legacy carrier" and so on). Meanwhile, between the user terminals <NUM> and the radio base stations <NUM>, a carrier of a relatively high frequency band (for example, <NUM>. <NUM> and so on) and a wide bandwidth may be used, or the same carrier as that used in the radio base station <NUM> may be used. A new carrier type (NCT) may be used as the carrier type between the user terminals <NUM> and the radio base stations <NUM>. Between the radio base station <NUM> and the radio base stations <NUM> (or between the radio base stations <NUM>), wire connection (optical fiber, X2 interface and so on) or wireless connection is established.

The radio base station <NUM> and the radio base stations <NUM> are each connected with a higher station apparatus <NUM>, and are connected with a core network <NUM> via the higher station apparatus <NUM>. Note that the higher station apparatus <NUM> may be, for example, an access gateway apparatus, a radio network controller (RNC), a mobility management entity (MME) and so on, but is by no means limited to these. Also, each radio base station <NUM> may be connected with the higher station apparatus via the radio base station <NUM>.

Note that the radio base station <NUM> is a radio base station having a relatively wide coverage, and may be referred to as an "eNodeB," a "macro base station," a "transmitting/receiving point" and so on. Also, the radio base stations <NUM> are radio base stations having local coverages, and may be referred to as "small base stations," "pico base stations," "femto base stations," "home eNodeBs," "micro base stations," "transmitting/receiving points" and so on. Hereinafter the radio base stations <NUM> and <NUM> will be collectively referred to as "radio base station <NUM>," unless specified otherwise. The user terminals <NUM> are terminals to support various communication schemes such as LTE, LTE-A and so on, and may be both mobile communication terminals and stationary communication terminals.

In this radio communication system, as radio access schemes, OFDMA (Orthogonal Frequency Division Multiple Access) is applied to the downlink, and SC-FDMA (Single-Carrier Frequency Division Multiple Access) is applied to the uplink. OFDMA is a multi-carrier transmission scheme to perform communication by dividing a frequency band into a plurality of narrow frequency bands (subcarriers) and mapping data to each subcarrier. SC-FDMA is a single-carrier transmission scheme to mitigate interference between terminals by dividing the system band into bands formed with one or continuous resource blocks per terminal, and allowing a plurality of terminals to use mutually different bands.

Now, communication channels used in the radio communication system shown in <FIG> will be described. Downlink communication channels include a PDSCH (Physical Downlink Shared CHannel), which is used by each user terminal <NUM> on a shared basis, and downlink L1/L2 control channels (PDCCH, PCFICH, PHICH and enhanced PDCCH). User data and higher control information are transmitted by the PDSCH. Scheduling information for the PDSCH and the PUSCH and so on are transmitted by the PDCCH (Physical Downlink Control CHannel). The number of OFDM symbols to use in the PDCCH is transmitted by the PCFICH (Physical Control Format Indicator Channel). HARQ ACKs/NACKs for the PUSCH are transmitted by the PHICH (Physical Hybrid-ARQ Indicator CHannel). Also, the scheduling information for the PDSCH and the PUSCH and so on may be transmitted by the enhanced PDCCH (EPDCCH) as well. This EPDCCH is frequency-division-multiplexed with the PDSCH (downlink shared data channel).

Uplink communication channels include a PUSCH (Physical Uplink Shared CHannel), which is used by each user terminal <NUM> on a shared basis as an uplink data channel, and a PUCCH (Physical Uplink Control CHannel), which is an uplink control channel. User data and higher control information are transmitted by this PUSCH. Also, by means of the PUCCH, downlink radio quality information (CQI: Channel Quality Indicator), ACKs/NACKs and so on are transmitted.

<FIG> is a diagram to show an overall structure of a radio base station <NUM> (which may be either a radio base station <NUM> or <NUM>) according to the present embodiment. The radio base station <NUM> has a plurality of transmitting/receiving antennas <NUM> for MIMO transmission, amplifying sections <NUM>, transmitting/receiving sections <NUM>, a baseband signal processing section <NUM>, a call processing section <NUM> and a transmission path interface <NUM>.

User data to be transmitted from the radio base station <NUM> to the user terminals <NUM> on the downlink is input from the higher station apparatus <NUM>, into the baseband signal processing section <NUM>, via the transmission path interface <NUM>.

In the baseband signal processing section <NUM>, a PDCP layer process, division and coupling of user data, RLC (Radio Link Control) layer transmission processes such as an RLC retransmission control transmission process, MAC (Medium Access Control) retransmission control, including, for example, an HARQ transmission process, scheduling, transport format selection, channel coding, an inverse fast Fourier transform (IFFT) process and a precoding process are performed, and the result is transferred to each transmitting/receiving section <NUM>. Furthermore, downlink control channel signals are also subjected to transmission processes such as channel coding and an inverse fast Fourier transform, and are transferred to each transmitting/receiving section <NUM>.

Also, the baseband signal processing section <NUM> reports, to the user terminals <NUM>, control information for allowing communication in the cell, through higher layer signaling (RRC signaling, broadcast signal and so on). The information for allowing communication in the cell includes, for example, the uplink or downlink system bandwidth, feedback resource information and so on. Each transmitting/receiving section <NUM> converts baseband signals that are pre-coded and output from the baseband signal processing section <NUM> on a per antenna basis, into a radio frequency band. The amplifying sections <NUM> amplify the radio frequency signals having been subjected to frequency conversion, and transmit the signals through the transmitting/receiving antennas <NUM>.

On the other hand, as for data to be transmitted from the user terminals <NUM> to the radio base station <NUM> on the uplink, radio frequency signals that are received in the transmitting/receiving antennas <NUM> are each amplified in the amplifying sections <NUM>, converted into baseband signals through frequency conversion in each transmitting/receiving section <NUM>, and input in the baseband signal processing section <NUM>.

In the baseband signal processing section <NUM>, the user data that is included in the input baseband signals is subjected to an FFT process, an IDFT process, error correction decoding, a MAC retransmission control receiving process, and RLC layer and PDCP layer receiving processes, and the result is transferred to the higher station apparatus <NUM> via the transmission path interface <NUM>. The call processing section <NUM> performs call processing such as setting up and releasing communication channels, manages the state of the radio base stations <NUM> and manages the radio resources.

<FIG> is a diagram to show a principle functional structure of the baseband signal processing section <NUM> provided in the radio base station <NUM> according to the present embodiment. As shown in <FIG>, the baseband signal processing section <NUM> provided in the radio base station <NUM> is comprised at least of a control section <NUM>, a downlink control signal generating section <NUM>, a downlink data signal generating section <NUM>, a mapping section <NUM>, a demapping section <NUM>, a channel estimation section <NUM>, an uplink control signal decoding section <NUM>, an uplink data signal decoding section <NUM> and a decision section <NUM>.

The control section <NUM> controls the scheduling of downlink user data that is transmitted in the PDSCH, downlink control information that is transmitted in the PDCCH and/or the enhanced PDCCH (EPDCCH), downlink reference signals and so on. Also, the control section <NUM> controls the scheduling of uplink data that is transmitted in the PUSCH, uplink control information that is transmitted in the PUCCH or the PUSCH, and uplink reference signals (allocation control). Information about the allocation control of uplink signals (uplink control signals and uplink user data) is reported to user terminals by using a downlink control signal (DCI).

To be more specific, the control section <NUM> controls the allocation of radio resources with respect to downlink signals and uplink signals, based on command information from the higher station apparatus <NUM>, feedback information from each user terminal <NUM> and so on. That is, the control section <NUM> functions as a scheduler. Also, in inter-eNB CA, the control section <NUM> is provided for each of multiple CCs separately, and, in intra-eNB CA, the control section <NUM> is provided to be shared by multiple CCs.

The downlink control signal generating section <NUM> generates downlink control signals (PDCCH signal and/or EPDCCH signal) determined to be allocated by the control section <NUM>. To be more specific, based on commands from the control section <NUM>, the downlink control signal generating section <NUM> generates a DL assignment to report downlink signal allocation information, and a UL grant to report uplink signal allocation information.

For example, according to the above fourth example, the downlink control signal generating section <NUM> generates downlink control information with DAI included therein. The downlink control signal generating section <NUM> can generate a DAI to apply to the PCell and the SCell on a shared basis, included in the SCell's downlink control information. In this case, the control section <NUM> can make a DL assignment in the TDD cell and a DL assignment in the FDD cell for the user terminal the same (provide DL assignments in the same subframe in the TDD cell and the FDD cell) (see above <FIG>).

The downlink data signal generating section <NUM> generates downlink data signals (PDSCH signals). The data signals that are generated in the data signal generating section <NUM> are subjected to a coding process and a modulation process, based on coding rates and modulation schemes that are determined based on CSI from each user terminal <NUM> and so on.

Based on commands from the control section <NUM>, the mapping section <NUM> controls the allocation of the downlink control signals generated in the downlink control signal generating section <NUM> and the downlink data signals generated in the downlink data signal generating section <NUM>, to radio resources.

The demapping section <NUM> demaps the uplink signals transmitted from the user terminals and separates the uplink signals. The channel estimation section <NUM> estimates the channel states from the reference signals included in the received signals separated in the demapping section <NUM>, and outputs the estimated channel states to the uplink control signal decoding section <NUM> and the uplink data signal decoding section <NUM>.

The uplink control signal decoding section <NUM> decodes the feedback signals (transmission acknowledgment signals, etc.) transmitted from the user terminals through an uplink control channel (PUCCH), and outputs the results to the control section <NUM>. The uplink data signal decoding section <NUM> decodes the uplink data signals transmitted from the user terminals through an uplink shared channel (PUSCH), and outputs the results to the decision section <NUM>. The decision section <NUM> makes retransmission control decisions (ACK/NACK) based on the decoding results in the uplink data signal decoding section <NUM>, and outputs the results to the control section <NUM>.

<FIG> is a diagram to show an overall structure of a user terminal <NUM> according to the present embodiment. A user terminal <NUM> has a plurality of transmitting/receiving antennas <NUM> for MIMO transmission, amplifying sections <NUM>, transmitting/receiving sections (receiving sections) <NUM>, a baseband signal processing section <NUM> and an application section <NUM>.

As for downlink data, radio frequency signals that are received in the plurality of transmitting/receiving antennas <NUM> are each amplified in the amplifying sections <NUM>, and subjected to frequency conversion and converted into the baseband signal in the transmitting/receiving sections <NUM>. This baseband signal is subjected to receiving processes such as an FFT process, error correction decoding and retransmission control, in the baseband signal processing section <NUM>. In this downlink data, downlink user data is transferred to the application section <NUM>. The application section <NUM> performs processes related to higher layers above the physical layer and the MAC layer. Also, in the downlink data, broadcast information is also transferred to the application section <NUM>.

Meanwhile, uplink user data is input from the application section <NUM> to the baseband signal processing section <NUM>. In the baseband signal processing section <NUM>, a retransmission control (H-ARQ (Hybrid ARQ)) transmission process, channel coding, precoding, a DFT process, an IFFT process and so on are performed, and the result is transferred to each transmitting/receiving section <NUM>. The baseband signal that is output from the baseband signal processing section <NUM> is converted into a radio frequency band in the transmitting/receiving sections <NUM>. After that, the amplifying sections <NUM> amplify the radio frequency signal having been subjected to frequency conversion, and transmit the resulting signal from the transmitting/receiving antennas <NUM>.

<FIG> is a diagram to show a principle functional structure of the baseband signal processing section <NUM> provided in the user terminal <NUM>. As shown in <FIG>, the baseband signal processing section <NUM> provided in the user terminal <NUM> is comprised at least of a control section <NUM> (feedback control section), an uplink control signal generating section <NUM>, an uplink data signal generating section <NUM>, a mapping section <NUM>, a demapping section <NUM>, a channel estimation section <NUM>, a downlink control signal decoding section <NUM>, a downlink data signal decoding section <NUM> and a decision section <NUM>.

The control section <NUM> controls the generation of uplink control signals (feedback signals) and uplink data signals based on downlink control signals (PDCCH signals) transmitted from the radio base stations, retransmission control decisions with respect to the PDSCH signals received, and so on. The downlink control signals are output from the downlink control signal decoding section <NUM>, and the retransmission control decisions are output from the decision section <NUM>.

Also, the control section <NUM> also functions as a feedback control section that controls the feedback of transmission acknowledgment signals (ACKs/NACKs) in response to PDSCH signals. To be more specific, in a communication system in which CA is employed, the control section <NUM> controls the selection of the cell (or CC) to feed back transmission acknowledgment signals, the PUCCH resource to allocate the transmission acknowledgment signals, and so on. For example, based on downlink control signals transmitted from the radio base stations, the control section <NUM> determines the cell to feed back transmission acknowledgment signals, the PUCCH resource to use and so on, and indicate these to the mapping section <NUM>.

For example, assume a case where, in TDD-FDD CA (intra-eNB CA), A/N's in response to DL signals of both cells are transmitted by using channel selection, with reference to a table in which the states of these A/N's are associated at least with PUCCH resources and QPSK symbol points. Note that the table defines different contents for the FDD cell and the TDD cell.

In this case, regardless of the cell where downlink control information is detected or the cell where downlink shared data is received, the control section <NUM> can use the table to correspond to the duplex mode of a predetermined cell to transmit A/N's (the above first aspect). Alternatively, regardless of which cell's downlink control information is detected, which cell's downlink shared data is received or which cell's transmission acknowledgment signal is allocated, the control section <NUM> can use the table that corresponds to the TDD cell (the above second aspect). Alternatively, transmission acknowledgment signals that correspond to a plurality of DL subframes of the TDD cell, respectively, can be made predetermined bits or less in the control section <NUM> by employing A/N bundling, and multiplexed with the FDD cell's transmission acknowledgment signals (the above third example).

Alternatively, the control section <NUM> can apply A/N bundling to A/N's corresponding to a plurality of DL subframes of the FDD cell (the above fourth example). At this time, based on the DAI included in the downlink control information, the control section <NUM> applies A/N bundling to the transmission acknowledgment signals corresponding to a plurality of DL subframes of the FDD cell, respectively. Furthermore, it is possible to use the DAI included in the TDD cell's downlink control information, for A/N bundling for both the TDD cell and the FDD cell.

The uplink control signal generating section <NUM> generates uplink control signals (feedback signals such as transmission acknowledgment signals, channel state information (CSI), and so on) based on commands from the control section <NUM>. Also, the uplink data signal generating section <NUM> generates uplink data signals based on commands from the control section <NUM>. Note that, when a UL grant is included in a downlink control signal reported from the radio base stations, the control section <NUM> commands the uplink data signal generating section <NUM> to generate an uplink data signal.

The mapping section <NUM> (allocation section) controls the allocation of uplink control signals (transmission acknowledgment signals, etc.) and uplink data signals to radio resources (PUCCH and PUSCH) based on commands from the control section <NUM>. For example, depending on the CC (cell) to send feedback (PUCCH transmission), the mapping section <NUM> allocates the transmission acknowledgment signals to the PUCCH of that CC.

The demapping section <NUM> demaps a downlink signal transmitted from the radio base station <NUM> and separates the downlink signal. The channel estimation section <NUM> estimates the channel state from the reference signals included in the received signal separated in the demapping section <NUM>, and outputs the estimated channel state to the downlink control signal decoding section <NUM> and the downlink data signal decoding section <NUM>.

The downlink control signal decoding section <NUM> decodes the downlink control signal (PDCCH signal) transmitted in the downlink control channel (PDCCH), and outputs the scheduling information (information regarding the allocation to uplink resources) to the control section <NUM>.

The downlink data signal decoding section <NUM> decodes the downlink data signal transmitted in the downlink shared channel (PDSCH), and outputs the result to the decision section <NUM>. The decision section <NUM> makes a retransmission control decision (ACK/NACK) based on the decoding result in the downlink data signal decoding section <NUM>, and also outputs the result to the control section <NUM>.

Claim 1:
A terminal (<NUM>) comprising:
a receiving section (<NUM>) adapted to:
receive, as a downlink assignment, a first downlink control information, DCI, including a first two-bit downlink assignment index, DAI, when a first duplex mode of a cell, where a downlink shared channel is scheduled, is a frequency division duplex, FDD, mode, and
receive, as a downlink assignment, a second DCI including a second two-bit DAI when a second duplex mode of the cell, where the downlink shared channel is scheduled, is a time division duplex, TDD, mode; and
a control section (<NUM>) adapted to control, based on a value of the received first or second DAI, transmission of a transmission acknowledgement signal for the downlink shared channel, wherein
the control section (<NUM>) is adapted to apply ACK/NACK bundling to the transmission acknowledgement signal.