3GPP (3rd Generation Partnership Project) has standardized LTE (Long Term Evolution) and LTE-Advanced, and is currently proceeding with the standardization for further enhancement in order to achieve high-speed and high-capacity communication between a radio communication base station apparatus (hereinafter abbreviated as “base station”) and a radio communication terminal apparatus (hereinafter abbreviated as “terminal”).
LTE and LTE-Advanced adopt OFDMA (Orthogonal Frequency Division Multiple Access) as a downlink communication scheme and SC-FDMA (Single Carrier Frequency Division Multiple Access) as an uplink communication scheme.
Channels and signals used on an uplink include, for example, PUSCH (Physical Uplink Shared Channel) and PUCCH (Physical Uplink Control Channel). PUSCH is a channel for transmitting a data signal. PUCCH is a channel for transmitting control information such as ACK/NACK or CSI (Channel State Information).
ACK/NACK is information indicating an error detection result of downlink data of each transport block (TB) transmitted from a base station and is 1-bit information indicating either ACK (no error) or NACK (error).
CSI is information indicating a measurement result of downlink channel quality. CSI includes CQI (Channel Quality Indicator), PMI (Precoding Matrix Indicator) and RI (Rank Indicator).
Here, LTE-Advanced Release 10 (hereinafter described as “Rel. 10”) supports carrier aggregation (CA) that operates a plurality of unit carriers (component carriers, hereinafter called “CCs”) based on the unit of a frequency bandwidth of a maximum of 20 MHz (that is, maximum frequency bandwidth in LTE) bundled together in order to support wideband transmission (e.g., see NPLs 1, 2 and 3). In CA, one PCC (Primary Component Carrier) and one or more SCCs (Secondary Component Carriers) are configured. The 3GPP specification may describe CC as a cell, PCC as a PCell (Primary Cell), and SCC as an SCell (Secondary Cell). When a plurality of CCs are used on an uplink, PUCCH is transmitted by only PCC so as not to increase PAPR (Peak to Average Power Ratio).
In Rel. 10, it is possible to use different PUCCH transmission formats according to the number of downlink CCs (hereinafter, simply described as “the number of CCs”) supported by a terminal, that is, the number of ACK/NACKs fed back by the terminal. More specifically, when the number of CCs is 1, the terminal uses format 1a/1b to feed back ACK/NACK to a base station. When the number of CCs is 2, the terminal uses channel selection using format 1b to feed back ACK/NACK to the base station. On the other hand, when the number of ACK/NACKs is 3 or more, the terminal uses format 3 to feed back ACK/NACKs to the base station.
Rel. 10 defines two types of CSI feedback method: periodic CSI and aperiodic CSI. With aperiodic CSI, the terminal feeds back CSI only once at timing instructed by the base station. With periodic CSI, the terminal feeds back CSI to the base station using format 2 at reporting cycles (e.g., 5 ms, 10 ms) configured for each terminal.
Format 1a/1b is a transmission format in which a channel is divided into Ncs×Noc(1) portions within one RB with Ncs cyclic shifts and Noc(1) OCC sequences. Note that signal points of BPSK are used for format 1a and signal points of QPSK are used for format 1b. Format 2 is a transmission format in which a channel is divided by Ncs cyclic shifts within one RB. Format 3 is a transmission format in which a channel is divided into Noc(3) portions within one RB. Note that Ncs=12, Noc(1)=3, and Noc(3)=4 or 5.
The transmission capacity of format 1a is 1 bit. The transmission capacity of format 1b is 2 bits. The transmission capacity of channel selection using format 1b is 4 bits. The transmission capacity of format 2 is 13 bits. The transmission capacity of format 3 is 21 bits.
Resources (transmission capacity) for the terminal to transmit ACK/NACK and periodic CSI are reserved. The resources reserved for periodic CSI are indicated from the base station to the terminal by RRC signaling semi-statically.
In Rel. 10, when ACK/NACK and periodic CSI are assigned to the same subframe, ACK/NACK is given priority over periodic CSI and periodic CSI is dropped (not transmitted). However, if periodic CSI is dropped frequently, the measurement accuracy of downlink channel quality deteriorates.
Thus, in Release 11 (hereinafter described as “Rel. 11”), which is LTE-Advanced, studies are being carried out on the possibility of supporting multiplexed transmission of ACK/NACK and periodic CSI when transmission subframes of ACK/NACK and periodic CSI overlap (e.g., NPL 4).
NPL 4 describes that format 3 is used when ACK/NACK and periodic CSI (hereinafter described as “CSI”) are simultaneously transmitted, and a PUCCH transmission format of Rel. 10 is used otherwise, that is, when ACK/NACK or CSI is singly transmitted. That is, ACK/NACK is transmitted in format 1b when the number of CCs is 1 or 2 and transmitted in format 3 when the number of CCs is 3 or more. CSI is transmitted in format 2.