In recent years, demands for larger capacity and higher speed of a wireless communication have been growing, and methods for improving an effective availability of finite frequency resources have been actively researched. As one of those methods, attention has been attracted to a technique using a space domain.
In a MIMO (multiple input multiple output) technology, a plurality of antenna elements are equipped in each of a transmitter and a receiver, and spatial multiplexing transmission is realized under a propagation environment that is low in correlativity of reception signals among antennas (refer to Non-patent Literature 1). In this case, the transmitter transmits different data series from a plurality of attached antennas by using a physical channel having the same time, the same frequency, and the same code for each antenna element. The receiver separates and receives the reception signals from a plurality of attached antennas on the basis of the different data series. In this way, a plurality of spatial multiplexing channels is used so that high speed can be achieved without using multilevel modulation. When the transmitter and the receiver are equipped with the same number of antennas under the environments where a large number of scatters exist between the transmitter and the receiver in a sufficient S/N (signal to noise ratio) condition, a communication capacity can be enlarged in proportion to the number of antennas.
Also, as another MIMO technology, a multiuser MIMO technology (multiuser MIMO, or MU-MIMO) has been known. The MU-MIMO technology has been already discussed in a next-generation wireless communication system standard. For example, in a draft of 3GPP LTE standard or IEEE 802.16m standard (hereinafter referred to as “16 m”), the standardization of a transmission system using the multiuser MIMO has been included (refer to Non-patent Literature 2 and Non-patent Literature 3). Hereinafter, as one example, a description will be given of an outline of the multiuser MIMO system in a downlink in the 16m.
FIG. 21 illustrates a frame format in a downlink.
In the figure, SFn (n=an integer of 0 to 7) denotes a subframe. In transmitting individual data of a terminal (or user) using an individual data area (blocks indicated by DL in the figure) in the downlink, a base station device allows control information such as terminal allocation information to be included in a signal to be transmitted from the base station device to a terminal device existing within a communication area. In the 16m, the base station device allows the control information to be included in areas allocated as A-MAP in FIG. 21.
FIG. 22 illustrates an example of main parameters included in the control information (individual control information) for a specific terminal device MS#n. Resource allocation information RA#n that is one of the parameters illustrated in FIG. 22 includes information related to a position, an allocation size, and distributed/continuous mapping of a transmission area of individual data of the terminal (or user) in the individual data area DL to be transmitted by using an OFDM symbol subsequent to A-MAP.
In MIMO mode information MEF illustrated in FIG. 22, transmission information of a spatial multiplexing mode or a temporal-spatial diversity transmission mode is transmitted. When the MIMO mode information MEF indicates the MU-MIMO mode, the MIMO mode information MEF further includes pilot sequence information PSI#n and the number of spatial streams Mt in the MU-MIMO as a whole. The MCS information notifies the terminal device MS#n of modulation multi-level value of the spatial stream and code rate information.
The MCRC#n that is terminal destination information illustrated in FIG. 22 is CRC information masked with terminal identification information CID (connection ID) allocated to the terminal MS#n by the base station device at the time of establishing a connection. With this information, the terminal device detects individual control information addressed to the own station together with error detection.
A description will be given of the operation of a conventional base station device 80 that performs the above-mentioned MU-MIMO transmission with reference to FIG. 23. FIG. 23 is a block diagram illustrating a configuration of the conventional base station device 80 and a conventional terminal device 90 (terminal device MS#n; n is a natural number). The base station device 80 illustrated in FIG. 23 notifies the individual terminal of the MU-MIMO allocation information through a downlink individual control channel allocated as an A-MAP, prior to the MU-MIMO transmission. As illustrated in FIG. 22, the MU-MIMO allocation information includes, as parameters necessary for a receiving process at the terminal device MS#n side, the number of spatial streams (MO, the code rate and modulation information MCS#n of the error correcting code performed on a spatial stream addressed to MS#n, pilot information (PSI#n) addressed to the MS#n, and resource allocation information RA#n addressed to the MS#n. In this case, n=1, . . . Mt. Also, it is assumed that one spatial stream is allocated to the terminal device MS#n.
A control information and data generator 84#n includes an individual pilot generator 85, a modulated data generator 86, a precoding weight multiplier 87, and an individual control information generator 88, and generates individual control information and data for the terminal device MS#n.
The individual control information generator 88 generates an individual control signal including the above-mentioned MU-MIMO allocation information. The modulated data generator 86 generates a modulated data signal #n addressed to the terminal device MS#n that performs the spatial multiplexing transmission on the basis of the code rate and modulation information MCS#n. The individual pilot generator 85 generates a pilot signal #n used for channel estimation on the basis of pilot information (PSI#n) addressed to the MS#n. The precoding weight multiplier multiplies the modulated data signal #n by the pilot signal #n with the use of a common precoding weight #n to generate spatial streams. The spatial multiplexing streams are generated by the number of spatial multiplexing streams (Mt) by the control information and data generator 84#n1, . . . #Mt.
An OFDM symbol configuration section 81 allocates an individual control signal to an A-MAP control information area on an OFDM symbol. Further, the spatial streams that are individual data addressed to Mt terminal devices are mapped to a source based on the resource allocation information RA#n by spatial multiplexing. IFFTs sections 82 performs OFDMA modulation on an output of the OFDM symbol configuration section, and add a cyclic prefiex (or guard interval) thereto. After frequency conversion, the outputs are transmitted from respective antennas 83.
In this case, because in a precoded MIMO propagation channel the channel estimation can be performed with the use of the pilot signal precoded by the same precoding weight as that of the data signal, the MIMO mode information requires no precoding information.
Also, the MIMO propagation channel in the terminal device MS#n can be estimated with the use of signals orthogonal to each other between the spatial multiplexing streams using frequency division as the respective pilot signals.
On the other hand, the terminal device MS#n performs the following terminal receiving process. First, the terminal device MS#n detects the MU-MIMO allocation information addressed to the own terminal device from a downlink individual control signal received by a downlink control information detector 92 through antennas 91. Then, the terminal device MS#n extracts data in an area where the resource is allocated to the MU-MIMO transmission from data in which OFDMA demodulation not shown has been performed.
Then, an MIMO separator 93 performs the channel estimation of the MIMO propagation channel with the use of the pilot signal precoded by the number of spatial multiplexing streams (Mt). Further, the MIMO separator 93 generates a reception weight based on MMSE criteria on the basis of the result of the channel estimation of the MIMO propagation channel and pilot information (PSI) addressed to the own terminal device, and separates a stream addressed to the own terminal device from the data in the resource allocated area which has been spatially multiplexed. Then, after separation of the stream addressed to the own terminal device, the terminal device MS#n demodulates and decodes the stream with the use of the MCS information by a demodulator/decoder 94.
In this case, the resource allocation information RA#n addressed to the MS#n which is a parameter required for the receiving process at the terminal device MS#n side includes distributed/continuous mapping information, position (start, end) information, and allocation size information.
In the 16m, the resources are placed on the basis of a physical resource unit (PRU) including a given OFDM symbol and subcarrier. A given number of pilot signals are arranged within the PRU.
FIG. 24 illustrates an example of a physical resource unit (PRU) configuration at the time of transmitting two streams. The PRU illustrated in FIG. 24 includes 6 OFDM symbols and 18 subcarriers. The PRU includes 12 pilot symbols (blocks indicated by 1 or 2 in the figure) and 96 data symbols.
Also, there are two kinds of resource allocation methods which are a continuous mapping (Continuous Resource Unit (CRU) or localized Resource Unit), and a distributed mapping (Distributed Resource Unit (DRU)). The continuous mapping continuously allocates a resource to the terminal device with the subcarriers whose reception quality is relatively high, on the basis of a reception quality status from the terminal device. This is a resource allocation method particularly suitable for a case in which a travel speed of the terminal is low, and a temporal change in the reception quality is gentle. On the other hand, the distributed mapping allocates the resources distributed on the subcarriers to the terminal to easily obtain a frequency diversity effect. This is a resource allocation method particularly suitable for a case in which the travel speed of the terminal is high, and the temporal change in the reception quality is severe.
<Resource Allocation Method: Continuous Mapping>
Subsequently, a description will be given of the continuous mapping that is the resource allocation method with reference to FIG. 25.
The individual data of the user (individual data or user individual data) which is transmitted to the terminal device, individually, is allocated to the physical resource unit PRU with a logical resource unit (LRU) as a unit. In this example, the LRU includes data as much as the number of data symbols except for the pilot symbols included in the PRU, and is allocated to a data symbol placed portion in the physical resource PRU in a given order. Also, the LRU is allocated to the continuous subcarriers with one PRU as a unit (hereinafter called “miniband unit”) or n plural PRUs as an assembled unit (hereinafter called “subband unit”). FIG. 25 illustrates an example of the resource continuous mapping using the subband of n=4. As illustrated in FIG. 25, in the individual data of the user, LRU#1 to LRU #4 are allocated to PRU#1 to PRU#4, respectively.
<Resource Allocation Method: Distributed Mapping>
Subsequently, a description will be given of the distributed mapping that is the resource allocation method with reference to FIG. 26.
The individual data of the user which is transmitted to the terminal device, individually, is allocated to the physical resource unit PRU with the logical resource unit LRU as a unit. In this example, the LRU includes data as much as the number of data symbols except for the pilot symbols included in the PRU. A subcarrier, interleaver (or tone permutation) distributes a plurality of LRU data into a plurality of PRU in conformity to a given rule.
As illustrated in FIG. 26, when a transmission diversity manner such as an SFBC (space-frequency block coding) is applied in the subcarrier leaver, in order to ensure continuity between two subcarriers, the distributed mapping is performed with the two subcarriers as one unit (two-subcarrier based interleaver or two-tone based permutation).
The SFBC is disclosed in Non-patent Literature 6.
Also, when the maximum likelihood estimation (MLD) reception that obtains a high reception quality at the time of receiving the MU-MIMO is applicable in the terminal device, “modulation information on the spatial streams addressed to another user” which are spatially multiplexed at the same time is further included in the individual control information.
FIG. 27 illustrates an example of bit allocation (per one user) of the modulation information on another user as disclosed in Non-patent literature 5. Referring to FIG. 27, another user is informed of any modulation format of QPSK, 16QAM, and 64QAM (constellation information at the time of modulation) by use of 2 bits.