At present, 3GPP (3rd Generation Partnership Project) has been examining LTE Advanced (hereinafter referred to as “LTE-A”) for a further increase in communication speed. SC-FDMA (Single Carrier Frequency Division Multiple Access) system with excellent PAPR (Peak to Average Power Ratio) characteristics is adopted in uplink in LTE. In LTE uplink, spatial multiplexing using MIMO (Multiple Input Multiple Output) can be used. In particular, MU-MIMO (Multi-User MIMO) that performs spatial multiplexing in which multiple mobile station apparatuses use the same frequency and time resource can be used.
To reduce degradation of characteristics in MIMO communication caused by noise, interference, or the like, it is desirable to highly accurately calculate the channel (may also be referred to as “propagation path”) of each antenna port (defined for every physical antenna or combination of two or more physical antennas) in transmission and reception. In LTE, orthogonality in frequency domain is realized by code multiplexing implemented by allocating, to each antenna port, a sequence generated by applying a cyclic shift in time domain to an uplink demodulation reference signal (UL DMRS: Uplink Demodulation Reference Signal) generated on the basis of a Zadoff-Chu sequence. Further, since there are two sequences of UL DMRS (that is, two SC-FDMA symbols) in one subframe, LTE-A applies a Walsh sequence in units of SC-FDMA symbols, thereby realizing orthogonality in time domain (Non Patent Literature 1). This spread sequence applied in time domain is referred to as an orthogonal cover code or an OCC (Orthogonal Cover Code).
A condition in which orthogonality is established by a cyclic shift in UL DMRS is limited to the case in which the allocated frequency regions are completely the same. Therefore, when MU-MIMO is applied to mobile station apparatuses to which different frequency regions are allocated, orthogonality between the mobile station apparatuses can be realized by allocating orthogonal OCCs, such as [+1 +1] and [−1 −1], to the mobile station apparatuses, respectively. Further, as in Non Patent Literature 2, it has been proposed to use IFDM (Interleaved Frequency Domain Multiplexing) that arranges DMRS in a comb-shape on the frequency axis. Orthogonality of multiplexed UL DMRS can be realized by changing the offset of a subcarrier to be allocated.