TECHNICAL FIELD

The present invention relates to a reference signal allocation method and a radio communication base station apparatus.

BACKGROUND ART

3GPP-LTE adopts OFDMA (Orthogonal Frequency Division Multiple Access) as a down link communication method. With 3GPP-LTE, a radio communication base station apparatus (hereinafter “base station”) transmits RSs (reference signals) using predetermined communication resources, and radio communication terminal apparatuses (hereinafter “terminals”) perform channel estimation using received reference signals to demodulate data (see Non-Patent literature 1.) In addition, terminals use the reference signals to perform adaptive MCS (modulation and channel coding scheme) control, PMI (precoding matrix indicator) control in MIMO (multiple-input multiple-output) transmission, or received quality measurement for adaptive scheduling. Then, terminals feed obtained PMIs and received quality information (CQI: channel quality indicator) back to a base station.

In addition, when a base station has a plurality of antennas, the base station enables diversity transmission. For example, a base station transmits a plurality of data streams from a plurality of antennas (MIMO transmission) to allow high-speed transmission. In order to receive diversity-transmitted signals as described above with no error, terminals need to know the channel states from a group of antennas used for transmission in a base station, to the terminals. Therefore, RSs need to be transmitted without interfering with each other, from all antennas provided in a base station. To realize this, 3GPP-LTE adopts a method of transmitting RSs from respective antennas in a base station, using timings and carrier frequencies varying in the time domain and the frequency domain.

FIG. 1shows a configuration of a base station having four antennas (4Tx base station) anticipated with 3GPP-LTE, andFIG. 2shows an RS transmission method in a 4Tx base station (see Non-Patent Literature 2.) Here, inFIG. 2, the vertical axis (frequency domain) is indicated by a unit of subcarriers and the horizontal axis (time domain) is indicated by a unit of OFDM symbols. In addition, R0, R1, R2and R3indicate RSs transmitted from antennas0,1,2and3(the first, second, third and fourth antennas), respectively. Moreover, inFIG. 2, one block unit enclosed by a heavy-line frame (six subcarriers in the frequency domain and fourteen OFDM symbols in the time domain) is referred to as a resource block (RB.) Although one RB is composed of twelve subcarriers in 3GPP-LTE, the number of subcarriers constituting one RB is six here for ease of explanation. In addition, each unit of one subcarrier with one OFDM symbol constituting one RB is referred to as a resource element (RE.) As seen fromFIG. 2, in order to minimize RS transmission overhead, a 4Tx base station reduces transmission frequencies of RSs (R2and R3) from antenna2(third antenna) and antenna3(fourth antenna.)

Here, RSs shown inFIG. 2are common to all terminals in the cell covered by a base station, and referred to as cell-specific reference signals. In addition, a base station may additionally transmit RSs multiplied by a specific weight on a per terminal basis (UE-specific reference signals) for beamforming transmission.

As described above, with 3GPP-LTE, the maximum number of antennas in a base station is four, and terminals supporting 3GPP-LTE perform data demodulation and downlink signal quality measurement, using RSs (R0to R3shown inFIG. 2) transmitted from a base station having maximum four antennas (4Tx base station.)

By contrast with this, LTE-advanced, which is improved 3GPP-LTE, is studying a base station having maximum eight antennas (8Tx base station.) Here, LTE-advanced needs to provide a base station complying with 3GPP-LTE in order to allow communication of terminals supporting only base stations (4Tx base stations) in 3GPP-LTE. In other words, LTE-advanced is required to accommodate both terminals supporting only a 4Tx base station (hereinafter “LTE terminals”) and terminals supporting also an 8Tx base station (hereinafter “LTE+ terminals” or “LTE-advanced terminals”).

CITATION LIST

Non-Patent Literature

[NPL 1] 3GPP TS 36.213 V8.2.0(ftp://ftp.3gpp.org/specs/2008-03/Re1-8/36_series/36213-820.zip)[NPL 2] 3GPP TS 36.211 V8.2.0(ftp://ftp.3gpp.org/specs/2008-03/Re1-8/36_series/36211-820.zip)

SUMMARY OF INVENTION

Technical Problem

With LTE-advanced, a base station needs to transmit RSs for eight antennas in order to allow LTE+ terminals to receive diversity-transmitted signals with no error. For example, as shown inFIG. 3, it may be possible to allocate R0to R7corresponding to eight antennas, to all RBs. By this means, LTE+ terminals are able to receive signals with no error. Moreover, terminals can obtain the CQI and PMI for each antenna, on a per subframe basis, so that it is possible to improve throughput by MIMO transmission.

However, LTE terminals only know the positions of allocating RSs (R0to R3) shown inFIG. 2. That is, LTE terminals do not know the presence of RSs used only in LTE+ terminals, that is, R4to R7shown inFIG. 3. Therefore, when RSs (R4to R7) used only in LTE+ terminals are allocated to REs, LTE terminals recognizes the RSs as data signals and receives them. As described above, when LTE terminals and LTE+ terminals exist together, the LTE terminals may not correctly receive signals. This causes error rate performances and throughput of LTE terminals to deteriorate.

It is therefore an object of the present invention to provide a reference signal allocation method and a radio communication base station apparatus that makes it possible to prevent the throughput of LTE terminals from deteriorating even if LTE terminals and LTE+ terminals exist together.

Solution to Problem

The reference signal allocation method according to the present invention includes: allocating first reference signals to all resource blocks in one frame, the first reference signals being used in first radio communication terminal apparatuses supporting a radio communication base station apparatus having N antennas and also used in second radio communication terminal apparatuses supporting a radio communication base station apparatus having more antennas than the N antennas; and allocating second reference signals used only in the second radio communication terminal apparatuses to part of the resource blocks in the one frame.

The radio communication base station apparatus according to the present invention that transmits first reference signals and second reference signals, the first reference signals being used in first radio communication terminals supporting another radio communication base station apparatus having N antennas and also used in second radio communication terminal apparatuses supporting the radio communication base station apparatus having more antennas than the N antennas, and the second reference signals being used only in the second radio communication terminal apparatuses adopts a configuration to include: a setting section that sets resource blocks to allocate the second reference signals on a per subframe basis, based on allocation patterns of the second reference signals; and an allocation section that allocates the first reference signals to all the resource blocks in one frame and allocates the second reference signals to part of the resource blocks set in the one frame.

Advantageous Effects of Invention

According to the present invention, it is possible to prevent throughput of LTE terminals from deteriorating even if LTE terminals and LTE+ terminals exist together.

