In an uplink for LTE-Advanced, which is a developed version of 3GPP LTE (3rd Generation Partnership Project Long Term Evolution), support for transmission by a terminal using multiple antennas (SU-MIMO: single user-multiple input multiple output) is being studied. The SU-MIMO is a technique in which one terminal transmits data signal of the same frequency at the same time from multiple antenna ports and spatial-multiplexes data signals using a virtual communication path (stream) in space.
In order to improve communication performance by SU-MIMO, a base station must know the status of a propagation path (channel) from each antenna of a terminal using a sounding reference signal, one of the reference signals (hereinafter referred to as “sounding reference signal (SRS)”). Thus, the terminal must transmit an SRS from each antenna. “Sounding” refers to estimation of the quality of a propagation path between a terminal and a base station.
The SRS in the LTE is time-multiplexed with data, i.e. the symbol at the tail end of a subframe and transmitted to estimate the channel quality of an uplink data channel.
In LTE, because a terminal has one transmission antenna, the terminal transmits one SRS from the antenna. On the other hand, in LTE-Advanced, because a terminal has N antennas, overhead for SRS transmission may increase to N times, causing shortage of domains for SRS.
In order to increase the number of SRSs that can be transmitted in a cell every unit time, i.e. enhance the SRS capacity, a known method uses a code resource for reference signals for data demodulation (hereinafter referred to as “demodulation reference signal (DMRS)”) (see NPL 1).
NPL 1 discloses a base station that uses scheduling information for uplink (also referred to as an UL grant) to instruct a terminal to transmit SRS. An SRS for which transmission is prompted by scheduling information transmitted from a base station is called “scheduled SRS”. FIG. 1 shows information reported by an UL grant that gives an instruction on transmission of a data signal in LTE. The UL grant contains predetermined numbers of bits, each indicating a piece of information such as a frequency hopping flag (FH flag) that indicates the presence or absence of frequency hopping between slots and the amount of cyclic shift for DMRS (hereinafter referred to as “amount of CS (cyclic shift for DMRS)”).
Upon receiving the instruction on SRS transmission contained in an UL grant from a base station, a terminal transmits an SRS using the single carrier-frequency division multiple access (SC-FDMA) symbol in the physical uplink shared channel (PUSCH) in which a DMRS is to be assigned. A DMRS is transmitted using the symbol at the center in each slot of one subframe, as shown in FIG. 2, the subframe being consisting of two slots and being a scheduling unit of data signal. A DMRS and an SRS are signals using a cyclic shift sequence having the same sequence (ZC (Zadoff-Chu) sequence) cyclic-shifted. Hence, an SRS can be code-multiplexed in the same band as a DMRS using the amount of CS that is different from that for DMRS.
In the LTE specification, an SRS is transmitted using only the symbol at the tail end of one subframe. NPL 1 discloses transmission of SRS using the symbol at the center of a slot (position of the symbol for DMRS) in addition to the symbol at the tail end of one subframe, thus enhancing the SRS capacity.
Here, the FH flag contained in the UL grant for DMRS can also be applied to the scheduled SRS. That is, in the case where the Fit flag indicates that FH will be performed (“with FH”), there is an advantage that the channel quality can be measured for a plurality of bands because SRSs can be transmitted in two bands of slots 1 and 2. On the other hand, in the case where the FH flag indicates that FH will not be performed (“without FH”), there is an advantage that SRSs can be equalized in the same band of slot 1 and slot 2 and an influence of interference can be reduced (randomized). Furthermore, multiplication of the Walsh sequence with SRSs in the same band of slot 1 and slot 2 has another advantage in that an SRS can be multiplexed with DMRS in a different bandwidth.