JP5687100B2 - Mobile communication system and base station apparatus - Google Patents

Description

The present invention, OFDM: relates to a mobile communication system and a base station apparatus location using (Orthogonal Frequency Division Multiplexing Orthogonal Frequency Division Multiplexing) scheme, in particular, it relates to cooperative transmission and reception techniques among multiple base stations in the downlink.

In 3GPP LTE-Advanced, coordinated multi-point transmission and reception (CoMP) technology is being studied. As one of the CoMP technologies, there is a multi-base station cooperative MIMO transmission system that expands the channel capacity of a cell edge user by regarding the antennas of a plurality of base stations as a virtual large-scale array antenna.
A technique based on SFN (Single Frequency Network) is known as an implementation example of this multi-base station cooperative MIMO transmission system. According to this method, the same information is transmitted from different base stations using the same radio resource, and the same information that has arrived at the receiver in the guard interval is combined to increase the received power of the desired signal. be able to.

  An example of multi-base station cooperative MIMO transmission will be described with reference to FIG. In this figure, 1 is a master base station (Master BS), 2 is a slave base station (Slave BS) that performs transmission in cooperation with the master base station, and 3 is a mobile station (UE). As illustrated, each of the master base station 1, the slave base station 2, and the mobile station 3 includes a plurality of antennas. The base stations 1 and 2 are assumed to be synchronously controlled such that a shift in reception timing of signals transmitted from the base stations 1 and 2 in the mobile station 3 falls within the OFDM guard interval (GI).

When the received SINR at the mobile station is low, as shown in (a) of FIG. 5, single rank transmission is selected, and the same substream # 0 is precoded or SFN over the master base station 1 and the slave base station 2. Is transmitted toward the mobile station 3. Since the data transmitted from the master base station 1 and the data transmitted from the slave base station 2 reach the mobile station 3 within the same guard interval, they are combined and demodulated, so that the reception quality can be improved.
When the received SINR is high, 2-rank transmission is performed as shown in FIG. In this case, the master base station 1 transmits the first substream # 0 toward the mobile station 3, and the slave base station 2 transmits the second substream # 2 different from the first substream # 0 toward the mobile station 3. Substream # 1 is transmitted. In this way, when the received SINR is high, throughput can be improved by transmitting different substreams between a plurality of base stations and separating and detecting them at the mobile station.

  In the downlink of the 3GPP LTE standard (Rel.8), OFDMA (Orthogonal Frequency Division Multiple Access) is adopted. The downlink radio frame is composed of 10 subframes, and each subframe is composed of 2 time slots. One time slot is 0.5 [ms], one subframe is 1 [ms], and one radio frame is 10 [ms]. Multiple OFDM symbols are included per time slot. Option 1 includes 7 OFDM symbols in one time slot, Option 2 includes 6 OFDM symbols in one time slot, and Option 3 includes 3 OFDM symbols in one time slot. It is. Allocation of radio resources to one user is performed using a resource block (RB) of 1 [ms] (1 subframe) × 180 kHz (12 subcarriers) as a basic unit.

  In the downlink, a reference signal (RS), a primary (first) synchronization signal (PSS), a secondary (second) synchronization signal (SSS), and a PBCH (Physical Broadcast Channel: Broadcast channel), PCFICH (Physical Control Format Indicator Channel: control channel configuration indication channel), PDCCH (Physical Downlink Control Channel: downlink control channel), PHICH (Physical Hybrid-ARQ Indicator Channel: hybrid ARQ indication channel), PDSCH (Physical Downlink) Each physical channel includes Shared Channel (downlink shared channel) and PMCH (Physical Multicast Channel).

A reference signal (RS) is a signal transmitted to a mobile station with a known transmission power and phase, and is used for measurement of a radio transmission line state for synchronous detection, radio link control, scheduling, cell search, handover, and the like. . The arrangement of the reference signal will be described later.
The primary synchronization signal (PSS) and the secondary synchronization signal (SSS) are signals used for cell search for detecting a base station to which a mobile station should be connected. The primary synchronization signal is multiplexed in the last OFDM symbol of slots # 1 and # 11, and the bandwidth is 72 bands of the center band. The secondary synchronization signal is multiplexed on the second OFDM symbol from the tail of slots # 1 and # 11, and the bandwidth is 72 subcarriers in the center band.
PBCH is a channel for broadcasting system-specific and cell-specific control information, and is multiplexed on the first 4 OFDM symbols in slot # 2 of subframe # 1, and has a bandwidth of 72 subcarriers in the center band.
PCFICH indicates the number of OFDM symbols used for PDCCH that performs layer 1 / layer 2 control accompanying PDSCH and PUSCH (uplink shared channel).
PDCCH indicates format information such as allocation information, modulation method, coding rate, and the like by the PDSCH and PUSCH schedulers.
PHICH transmits ACK or NAK information of hybrid ARQ for PUSCH.
PCFICH, PHICH, and PDCCH are multiplexed into the first 1 to 3 OFDM symbols of each subframe.
PMCH is used for MBMS transmission across multiple cells. PMCH is not supported in LTE Rel.8. The PMCH is time-multiplexed with the subframe that transmits the PDSCH.

The cell-specific reference signal (CS-RS) is the reference signal described above, and is spread and arranged over the entire band. When the base station includes a plurality of transmission antennas, a CS-RS specific to each transmission antenna is transmitted for each transmission antenna. Thereby, channel information between each transmitting antenna of the base station and the receiving antenna of the mobile station can be acquired.
The CS-RSs are distributed throughout the resource block, and the positions of subcarriers to which the CS-RSs are mapped are totally shifted according to the cell ID, that is, different frequency shifts ( The entire subcarrier to be mapped is shifted in the frequency direction). In addition, the CS-RS is scrambled by a cell-specific spread signal.

The LTE-Rel.8 standard CS-RS mapping when the base station has a single transmission antenna will be described with reference to FIG. In the following, the case where the Extended Cyclic Prefix specification is applied as an OFDM guard interval will be exemplified.
6A shows that when the remainder modulo cell ID 6 is 0 (mod (Cell_ID, 6) = 0), FIG. 6B shows that the remainder modulo cell ID 6 is 1 (mod (Cell_ID , 6) = 1), (c) is a remainder modulo cell ID 6 (mod (Cell_ID, 6) = 2), and (d) is a remainder modulo cell ID 6. Is 3 (mod (Cell_ID, 6) = 3), (e) is when the remainder modulo cell ID 6 is 4 (mod (Cell_ID, 6) = 4), (f) is the cell ID It is a figure which shows the mapping state of CS-RS transmitted from the single transmission antenna (Antenna port # 0) in case the remainder modulo 6 is 5 (mod (Cell_ID, 6) = 5). In these drawings, the horizontal axis indicates time, and the vertical axis indicates frequency, and one resource block including one subframe (2 time slots) and 12 subcarriers is illustrated. The example shown in this figure is the case of option 2 described above, and one time slot is divided into six cells. That is, each square in the horizontal direction corresponds to one OFDM symbol. Each square in the vertical direction corresponds to one subcarrier. Each cell of 1 OFDM symbol × 1 subcarrier is called a resource element (RE).

In FIG. 6A to FIG. 6F, resource elements with left-down hatching indicate CS-RS transmitted from each base station.
As shown in the figure, the CS-RS is always mapped to the first OFDM symbol and the third OFDM symbol from the back in each time slot. The CS-RS is inserted every 6 subcarriers, and the position of the inserted subcarrier is shifted according to the cell ID.

  That is, the CS-RS from the base station of the cell in which the remainder modulo 6 of the cell ID is 0 (mod (Cell_ID, 6) = 0) is shown in each time slot as shown in FIG. The first subcarrier and the seventh subcarrier at the timing of the first OFDM symbol, and the fourth subcarrier and the tenth subcarrier at the timing of the fourth OFDM symbol of each time slot. It is mapped and transmitted. Also, as shown in (b), the CS-RS from the base station of the cell whose remainder modulo cell ID 6 is 1 (mod (Cell_ID, 6) = 1) is the first OFDM symbol. The second subcarrier and the eighth subcarrier are transmitted at the timing of, and the fifth subcarrier and the eleventh subcarrier are mapped and transmitted at the timing of the fourth OFDM symbol. .

  Similarly, the CS-RS from the base station of the cell in which the remainder modulo cell ID 6 is 2 (mod (Cell_ID, 6) = 2) is the first OFDM as shown in (c). The third and ninth subcarriers at the symbol timing and the fifth and twelfth subcarriers at the timing of the fourth OFDM symbol are transmitted on the fifth and twelfth subcarriers. The CS-RS from the base station of the cell with 3 (mod (Cell_ID, 6) = 3) is the fourth and tenth subcarriers at the timing of the first OFDM symbol, as shown in (d). , And cells transmitted in the first and seventh subcarriers at the timing of the fourth OFDM symbol, and the remainder modulo 6 of the cell ID is 4 (mod (Cell_ID, 6) = 4) As shown in (e), the CS-RS from the base station of the first OFDM symbol is the tag of the first OFDM symbol. Are transmitted on the 5th and 11th subcarriers in the timing and the 2nd and 8th subcarriers at the timing of the 4th OFDM symbol, and the remainder modulo 6 of the cell ID is 5 ( The CS-RS from the base station of the cell with mod (Cell_ID, 6) = 5) is, as shown in (f), the sixth subcarrier and the twelfth subcarrier at the timing of the first OFDM symbol. It is transmitted on the third subcarrier and the ninth subcarrier at the timing of the carrier and the fourth OFDM symbol.

CS-RSs are transmitted using the same subcarrier at the same timing from the base stations of the cells with the same number of remainders modulo cell ID 6, but CS-RS is transmitted to each cell. Since it is scrambled using the associated code, it can be demodulated.
FIG. 6 shows the case of option 2, and as described above, the fourth OFDM symbol from the beginning is the third OFDM symbol from the back. Since the slot contains 7 OFDM symbols, the fifth OFDM symbol from the beginning becomes the third OFDM symbol from the back, and CS-RS uses the timing of the first OFDM symbol and the fifth OFDM symbol from the beginning. It will be inserted at the timing.
In this way, CS-RSs are mapped and transmitted from each base station to subcarriers determined according to the cell ID of the cell at a predetermined timing.

When the base station is provided with a plurality of transmission antennas, the CS-RS specific to the transmission antenna is mapped on the frequency axis and the time axis so that CS-RSs in the same cell do not interfere with each other, that is, The signals are transmitted orthogonally on the frequency axis and the time axis and transmitted from each transmitting antenna. As in the case described above, CS-RS is subjected to different frequency shift and scrambling depending on the cell ID.
7A to 7F, LTE Rel. 8 when the base station is provided with two antennas, a first antenna (Antenna port # 0) and a second antenna (Antenna port # 1). The standard CS-RS mapping will be described. In the following, an example is given of the case where the Extended Cyclic Prefix specification is applied as an OFDM guard interval.

  FIG. 7A is a diagram showing an arrangement of CS-RSs transmitted from two antennas provided in a base station of a cell in which the remainder modulo cell ID 6 is 0 (mod (Cell_ID, 6) = 0). (A) shows the arrangement of CS-RS transmitted from the first antenna, and (b) shows the arrangement of CS-RS transmitted from the second antenna. Here, the hatching at the lower left is a resource element for transmitting CS-RS from the first antenna, the hatching at the lower right is a resource element for transmitting CS-RS from the second antenna, and Null is It indicates that no signal is transmitted from the antenna.

As shown in (a) of FIG. 7A, the CS-RS from the first antenna is the first subcarrier and the seventh subcarrier at the timing of the first OFDM symbol of each time slot, and each It is transmitted after being mapped to the fourth subcarrier and the tenth subcarrier at the timing of the fourth OFDM symbol of the time slot. Also, as shown in (b), the CS-RS from the second antenna is the fourth and tenth subcarriers and the fourth in the timing of the first OFDM symbol of each time slot. The first OFDM symbol is mapped to the first and seventh subcarriers of the OFDM symbol.
As shown in the figure, for the timing and subcarrier at which the CS-RS is transmitted from the first antenna, the second antenna is in a null state in which no signal is transmitted, and the CS-RS is transmitted from the second antenna. With respect to the timing and subcarrier at which RS is transmitted, the first antenna is in a null state in which no signal is transmitted. Thus, the CS-RSs in the same cell are mapped so as not to interfere with each other.

FIG. 7B shows an arrangement of CS-RSs transmitted from two antennas provided in a base station of a cell whose remainder modulo cell ID 6 is 1 (mod (Cell_ID, 6) = 1). FIG. 5A shows a CS-RS transmitted from a first antenna, and FIG. 5B shows a CS-RS mapping transmitted from a second antenna.
As shown in FIG. 7B (a), the CS-RS from the first antenna includes the second subcarrier, the eighth subcarrier, and the fourth subcarrier at the timing of the first OFDM symbol. The CS-RS is transmitted on the fifth and eleventh subcarriers at the timing of the OFDM symbol, and the CS-RS from the second antenna is the fifth subcarrier and the eleventh subcarrier at the timing of the first OFDM symbol. It is transmitted on the second subcarrier and the eighth subcarrier at the timing of the carrier and the fourth OFDM symbol. Then, at the timing when the CS-RS is transmitted from one antenna and the subcarrier, the other antenna is in a null state in which no signal is transmitted.
In this way, when the remainder modulo 6 of the cell ID is 1, the subcarrier frequency is shifted by 1 as compared with the case where the remainder modulo 6 shown in FIG. 7A is 0. , The CS-RSs from the respective antennas are mapped and transmitted so as not to interfere with each other.

  FIG. 7C shows an arrangement of CS-RSs transmitted from two antennas provided in a base station of a cell whose remainder modulo cell ID 6 is 2 (mod (Cell_ID, 6) = 2). FIG. 5A shows a CS-RS transmitted from a first antenna, and FIG. 5B shows an arrangement of CS-RS transmitted from a second antenna. As shown in this figure, from the two antennas provided in the base station of the cell whose remainder is 2, subcarriers whose subcarrier frequency is shifted by 1 with respect to the case where the remainder shown in FIG. And CS-RSs are transmitted so as not to interfere with each other.

Similarly, from the two antennas provided in the base station of the cell whose modulus modulo 6 is 3 (mod (Cell_ID, 6) = 3), (a) and (b) in FIG. From the two antennas provided in the base station of the cell in which the CS-RS arranged as shown in FIG. 5 is transmitted and the remainder modulo the cell ID 6 is 4 (mod (Cell_ID, 6) = 4) CS-RS arranged as shown in (a) and (b) of FIG. 7E is transmitted, and a cell whose modulus modulo 6 is 5 (mod (Cell_ID, 6) = 5) CS-RSs arranged as shown in (a) and (b) of FIG. 7F are transmitted from the two antennas provided in the base station.
Thus, when two antennas are provided in the base station, CS-RSs corresponding to the antennas are transmitted from each antenna so as not to interfere with each other.

The physical layer specifications of 3GPP LTE Rel.8 are described in Non-Patent Documents 1 to 3. 3GPP LTE-Advanced is described in Non-Patent Documents 4 and 5.
In addition, as a technique for obtaining a highly accurate channel estimation value even in a low reception SINR environment, the channel response of a subcarrier including a reference signal (Reference Signal) is converted to the time domain by IDFT, and interference noise components are removed. A time domain channel estimation method that obtains a highly accurate channel estimation result is known (Non-patent Document 6).

3GPP TS36.211 V8.6.0, "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation", March 2009. 3GPP TS36.212 V8.6.0, "Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding", March 2009. 3GPP TS36.213 V8.6.0, "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures", March 2009. 3GPP TR36.913 V9.0.0, "Requirements for further advancements for Evolved Universal Terrestrial Radio Access (E-UTRA) (LTE-Advanced)", Dec. 2009. 3GPP TR36.814 V9.0.0, "Evolved Universal Terrestrial Radio Access (E-UTRA); Further advancements for E-UTRA physical layer aspects", Dec. 2009. Hidenori Kayama, Katsuhiko Hiramatsu, Koichi Honma, “Examination of high-accuracy channel estimation method for broadband OFDM communication using sinc function replica” IEICE Technical Report RCS2007-70, Aug. 2008.

Multiple base station coordinated transmission / reception technology (CoMP) was excluded from standardization items in LTE-Advanced Rel.10, but is expected to be included in standardization study items in Rel.11, which will be studied in 2011.
In order to achieve smooth system migration when implementing multi-base station cooperative MIMO transmission with LTE-Advanced Rel.11, as with LTE-Advanced Rel.10, backward compatibility with LTE Rel.8 (Backward Compatibility) ) Is considered essential.
In order for LTE standard (Rel.8) terminals and LTE-Advanced CoMP compatible terminals to coexist, the cell-specific reference signal (CS-RS), which is a downlink channel estimation signal, has the same configuration as LTE Rel.8. Must be realized in
However, when the resource element mapping rule for the downlink physical shared channel (PDSCH) and cell-specific reference signal (CS-RS) as in 3GPP LTE Rel.8 specifications is applied in the multi-base station cooperative MIMO transmission scheme, the cell-specific reference signal (CS-RS) insertion position differs between multiple base stations, so CS-RS and PDSCH from cooperating cells (adjacent cells) interfere with each other, and multiple signal separation in multi-base station cooperative MIMO transmission (cooperative MIMO transmission) -There is a problem that synthesis cannot be realized accurately.

With reference to FIG. 8, a case will be described in which two base stations that perform cooperative MIMO transmission apply LTE Rel.8 standard resource element mapping. Here, the remainder modulo 6 of the cell ID of the master base station (Master BS) is 0 (mod (Cell_ID, 6) = 0), and the cell ID 6 of the slave base station (Slave BS) is modulo. The remainder is 1 (mod (Cell_ID, 6) = 1).
FIG. 8A is a diagram illustrating an example of mapping of information transmitted on the downlink from the master base station, and FIG. 8B is a diagram illustrating an example of mapping of information transmitted on the downlink from the slave base station. It is.

  (A) of FIG. 8 shows the master base station first antenna (Master BS Antenna Port # 0) when the remainder modulo 6 of the cell ID of the master base station is 0 (mod (Cell_ID, 6) = 0). ) And the second antenna (Master BS Antenna Port # 1), the mapping of data transmitted is shown, which is the same as that shown in FIG. 7A. FIG. 8B shows the slave base station first antenna (Slave BS Antenna Port #) when the remainder modulo 6 of the cell ID of the slave base station is 1 (mod (Cell_ID, 6) = 1). 0) and the mapping of data transmitted from the second antenna (Slave BS Antenna Port # 1), which is the same as that shown in FIG. 7B.

  However, FIG. 8 also illustrates mapping of data other than CS-RS and null. The subcarriers other than CS-RS or null at the timing of the first OFDM symbol of each time slot are PDCCH (downlink control channel) as shown in the figure. Also, the second and third OFDM symbols in the first time slot (slot # 0) are CFI (Control Format Indicator) transmitted via PCFICH (Control Channel Configuration Indication Channel) multiplexed on the first OFDM symbol. Depending on the value (1-3), the corresponding OFDM symbol is used as the PDCCH. Resource elements that are not used by CS-RS, null, and other physical channels are PDSCHs (downlink shared channels).

The position of the CS-RS transmitted from the first and second antennas of the master base station shown in (a) of FIG. 8, and the CS-- transmitted from the first and second antennas of the slave base station shown in (b). When the RS positions are compared, it can be seen that the CS-RS and the PDSCH (or PDCCH) of the master base station and the slave base station are mapped to the same resource element, causing interference.
For example, in the first OFDM symbol of the first time slot (slot # 0) in the first subcarrier, the CS-RS is transmitted from the first antenna of the master base station, and the second antenna of the master base station is Although it is in a null state, PDCCH is transmitted from the first and second antennas of the slave base station. In addition, the first OFDM symbol of slot # 0 in the second subcarrier is transmitted by the CS-RS from the first antenna of the slave base station, and the second antenna of the slave base station is set to the null state. PDCCH is transmitted from the first and second antennas of the base station.
The first OFDM symbol of the second time slot (slot # 1) in the first subcarrier is transmitted by the CS-RS from the first antenna of the master base station, and the second antenna of the master base station is Although it is in a null state, PDSCH is transmitted from the first and second antennas of the slave base station. Further, the first OFDM symbol of slot # 1 in the second subcarrier is transmitted by the CS-RS from the first antenna of the slave base station, and the second antenna of the slave base station is set to the null state. PDSCH is transmitted from the first and second antennas of the base station.

In this way, when the LTE Rel.8 standard resource element mapping is applied, the cell IDs are different between the cooperative cells, so the subcarrier frequency into which the CS-RS is inserted is different. PDSCH or PDCCH will interfere.
For this reason, in particular, when performing single rank coordinated MIMO transmission shown in FIG. 5A, at the timing when PDSCH interferes with CS-RS of an adjacent coordinated cell, a normal signal detection method, for example, maximum There is a problem that SFBC decoding based on Maximum Ratio Combining (MRC) cannot be applied.

Therefore, the present invention provides a mobile communication system and a base station device that can reduce the influence of interference due to a cell-specific reference signal (CS-RS) from a cell that performs coordinated transmission when multiple base station coordinated MIMO transmission is applied. It is intended to provide a device.

  In order to solve the above problems, a mobile communication system of the present invention is a mobile communication system in which a plurality of base stations and a mobile station communicate by OFDM, and a frequency band common to the plurality of base stations in downlink transmission. Is used, the transmission timing of all base stations that interfere with each other is controlled so that the shift in reception timing including the multipath of the signal from each base station in the mobile station falls within the OFDM guard interval. The cell-specific reference signal is distributed on the time axis and the frequency axis, and the position of the subcarrier on which the cell-specific reference signal is allocated is determined according to the cell ID. When the slave base station that performs MIMO transmission in cooperation with the master base station arranges data symbols with the same mapping rule as the master base station, The data symbol arranged in the same resource element as the resource element in which the cell-specific reference signal is inserted is transmitted using the same resource element as the resource element in which the cell-specific reference signal of the master base station is transmitted. And the mobile station transmits a cell-specific reference transmitted from the master base station and the slave base station based on a channel estimation value and a transmission pattern of a known cell-specific reference signal transmitted from the master base station and the slave base station. An interference component due to a signal is removed, and the data symbol is decoded.

  The base station apparatus of the present invention is a mobile communication system in which a plurality of base stations and mobile stations communicate by OFDM, and when using a common frequency band in a plurality of base stations in downlink transmission, The transmission timing of all base stations that cause interference is controlled so that the shift in reception timing including the multipath of signals from each base station in the mobile station falls within the OFDM guard interval. Are distributed on the time axis and the frequency axis, and the position of the subcarrier on which the cell-specific reference signal is arranged is determined according to the cell ID. When the own base station performs MIMO transmission in cooperation with the master base station, when data symbols are arranged with the same mapping rule as the master base station The data symbol arranged in the same resource element as the resource element into which the cell-specific reference signal is inserted in the own base station is transmitted using the same resource element as the resource element in which the cell-specific reference signal of the master base station is transmitted It has a means to do.

According to the mobile communication system and a base station apparatus location of the present invention, upon application of a plurality of base stations coordinated MIMO transmission, the cell-specific reference signal of the cooperative cell (CS-RS) according to such a downlink shared channel (PDSCH) It becomes possible to reduce the influence of interference.

In the mobile communication system of this invention, it is a figure which shows an example of the signal transmitted from a master base station and a slave base station. In the mobile communication system of this invention, it is a figure which shows the other example of the signal transmitted from a master base station and a slave base station. It is a block diagram which shows an example of the structure by the side of the base station in the mobile communication system of this invention. It is a block diagram which shows an example of a structure of the mobile station in the mobile communication system of this invention. It is a figure for demonstrating an example of multiple base station cooperation MIMO transmission, (a) is a mode of single rank transmission, (b) is a figure which shows the mode of 2 rank transmission. It is a figure for demonstrating the mapping of LTE-Rel.8 standard CS-RS in case a base station has a single transmission antenna, (a)-(f) respectively modulo cell ID 6 It is a figure which shows the case where the remainder to perform is 0-5. It is a figure for demonstrating the mapping of LTE Rel.8 standard CS-RS when a base station has two antennas, (a) when the remainder modulo cell ID 6 is 0 It is a figure which shows the mapping of CS-RS transmitted from a 1st antenna, and the mapping of CS-RS transmitted from a (b) 2nd antenna. (A) mapping of CS-RS transmitted from the first antenna when the remainder modulo 6 of the cell ID is 1, and (b) CS-RS transmitted from the second antenna It is a figure which shows mapping. (A) mapping of CS-RS transmitted from the first antenna when the remainder modulo 6 of the cell ID is 2, and (b) CS-RS transmitted from the second antenna It is a figure which shows mapping. (A) mapping of CS-RS transmitted from the first antenna when the remainder modulo 6 of the cell ID is 3, and (b) CS-RS transmitted from the second antenna It is a figure which shows mapping. (A) mapping of CS-RS transmitted from the first antenna when the remainder modulo 6 of the cell ID is 4, and (b) CS-RS transmitted from the second antenna It is a figure which shows mapping. (A) mapping of CS-RS transmitted from the first antenna, and (b) CS-RS transmitted from the second antenna when the cell modulus modulo 6 is 5. It is a figure which shows mapping. It is a figure for demonstrating the case where 2 base stations which perform cooperative MIMO transmission apply the resource element mapping of LTE Rel.8 standard, (a) is an example of the mapping of the information transmitted by a downlink from a master base station. (B) is a figure which shows an example of the mapping of the information transmitted by a downlink from a slave base station.

First, a description will be given of the basic concept of definitive to the mobile communication system and a base station apparatus location of the present invention.
Similar to LTE Rel.8, the mobile communication system of the present invention is a downlink of a radio access system using OFDM or OFDMA in which cell-specific reference signals (CS-RS) are distributed on the time axis and the frequency axis. Intended for links. A plurality of base stations (master base station and slave base station) are synchronized with high accuracy using GPS or the like, and when using a common frequency band in a plurality of base stations in downlink transmission, It is assumed that transmission timings of all base stations that cause interference are controlled so that a shift in reception timing including a multipath of signals from each base station in the mobile station falls within the OFDM guard interval.

The CS-RS can perform different scrambling for each cell on the base station side. However, the scrambling pattern is uniquely determined by the cell ID.
In addition, the subcarrier frequency at which the CS-RS is arranged on the base station side can be changed for each cell. However, the subcarrier frequency in which CS-RS is arranged is uniquely determined by the cell ID.
The cell ID is set so that the master base station and the slave base station (usually the base station of an adjacent cell) that performs transmission in cooperation with the master base station are mapped to different subcarrier frequencies. Shall. That is, different cell IDs are set for adjacent cells.
In the mobile station, the CS-RS transmission pattern is known not only for the master base station but also for the slave base station. This can be realized by acquiring the cell ID information of the slave base station.
Further, it is assumed that a highly accurate channel estimation value is obtained even in a low reception SINR environment by applying a channel estimation method having an interference noise suppression effect such as a time domain channel estimation method on the mobile station side.

  Under such a premise, in the mobile communication system of the present invention, when coordinated transmission is performed in the slave base station, data symbols are arranged with the same mapping rule as that of the master base station. A data symbol arranged in a resource element (RE) in which RS is inserted is moved and mapped to a resource element in which CS-RS of the master base station is inserted. Thereby, the CS-RS transmitted from the master base station and the CS-RS transmitted from the slave base station become interference components for the data symbols transmitted using the same resource element. In the mobile station, the master base station and the slave base station using the channel estimation value and the CS-RS of a known coordinated cell, that is, the known CS-RS transmission pattern transmitted from the master base station and the slave base station. By generating a replica of the interference component due to CS-RS transmitted from the base station and subtracting the generated replica from the received signal, the interference component due to CS-RS transmitted from the master base station and slave base station is removed. I am doing so. The received signal from which the interference component has been removed is used to demodulate the signal by applying an algorithm such as MMSE (Minimum Mean Square Error) method, and a soft decision output is obtained and transmitted. Play the data symbol.

Thus, according to the mobile communication system and a base station apparatus location of the present invention, in a system in which the cell-specific reference signal for each cell ID (CS-RS) subcarrier frequencies that are disposed is shifted, a plurality of base stations coordinated MIMO When implementing the transmission scheme, transmit data symbols to be transmitted at the resource element position where the CS-RS of the slave base station is arranged at the resource element position where the CS-RS of the corresponding master base station is arranged, The receiver (mobile station) removes the CS-RS from the slave base station, which is an interference component, using the known channel estimation value and the CS-RS transmission pattern transmitted from the slave base station. The received data symbols are demodulated.
This makes it possible to reduce the influence of interference due to CS-RS when performing signal transmission by multi-base station cooperative MIMO transmission in a system in which the frequency at which CS-RS is arranged for each cell ID shifts. .

This will be described using a specific example.
Similar to the multiple base station cooperative MIMO transmission system shown in FIG. 5, each base station and mobile station are equipped with two antennas, and perform single rank transmission when communication quality is low (low reception SINR). When the quality is high (high reception SINR), two rank transmission is performed. Here, in the case of low reception SINR, description will be made assuming that SFBC (space frequency block coding) is used for precoding performed on the base station side. SFBC is defined by 3GPP. In the SFBC scheme, consecutive data symbols c _{i, j} and c _{i, j + 1} from the first antenna port are arranged and transmitted on adjacent subcarriers, and −c _{i, j + 1} ^* from the second antenna port ^. And ci _{, j} ^* (* is a complex conjugate) are arranged on the same adjacent subcarrier and transmitted.

As described above, when LTE Rel.8 standard resource element mapping is applied, since the cell ID is different between the cooperative cells, the subcarrier frequency into which the CS-RS is inserted is different. And PDSCH or PDCCH interfere with each other.
With reference to FIG. 1, a method employed in the present invention to solve the above-described problem will be described.
FIG. 1 is a diagram illustrating an example of signals transmitted from both base stations of a master base station and a slave base station that performs coordinated MIMO transmission of rank 1 with the master base station in the mobile communication system of the present invention. (A) is a signal transmitted from the first antenna of the master base station, (b) is a signal transmitted from the second antenna of the master base station, and (c) is a signal transmitted from the first antenna of the slave base station. , (D) shows a signal transmitted from the second antenna of the slave base station. Here, in order to avoid complexity, only signals for 6 subcarriers and 3 OFDM symbols indicated by k ₁ to k ₆ are shown. Each base station is assumed to have two transmission antennas. A case where both base stations transmit data precoded by the SFBC method will be described.

In the example shown in FIG. 1, the cell ID of the cell to which the master base station belongs is 0 (the remainder modulo 6 of the cell ID is 0), and the cell ID of the cell to which the slave base station belongs is 1 (cell ID 6 The modulo residue is 1).
As shown in FIG. 1A, as in FIG. 7A, at the timing of the first OFDM symbol of the _first subcarrier k ₁ , the first antenna of the master base station receives its CS-RS (r ₀ ^{( M)} (k ₁ )), and the second antenna is placed in a null state.
Then, according to the SFBC scheme, at the timing of the first OFDM symbol of subcarrier k ₂ and subcarrier k ₃ , the first antenna of the master base station transmits data symbols c _1,1 and c _1,2 , and the second antenna Data symbols-c _1,2 ^* and c _1,1 ^* are transmitted. The first antenna of the master base station is set to the null state in the first OFDM symbol of the _fourth subcarrier k ₄ , and the second antenna transmits its CS-RS (r ₁ ^(M) (k ₄ )). At the timing of the first OFDM symbols of subcarriers k ₅ and k ₆ , the first antenna of the master base station transmits data symbols c _2,1 and c _2,2 and the second antenna receives data symbols −c _2,2 ^*. And c _2,1 ^* are transmitted.

Since the slave base station has a remainder modulo 6 of the cell ID, as shown in FIG. 7B, the first antenna of the slave base station at the timing of the first OFDM symbol of the _second subcarrier k ₂ . Transmits its CS-RS (r ₀ ^(S) (k ₂ )), and the second antenna is set to the null state. Also, at the timing of the first OFDM symbol of the fifth subcarrier k ₅ , the first antenna of the slave base station is set to the null state, and the second antenna transmits its CS-RS (r ₁ ^(S) (k ₅ )). To do. This mapping is required to maintain compatibility with LTE Rel.8.

For MIMO coordinated transmission, the slave base station needs to transmit the same data symbol at the same frequency and at the same timing as the master base station. Since the timings are defined, the same data symbols as the master base station cannot be transmitted at those timings.
For example, the timing of the first OFDM symbol of subcarrier k _{2 is} such that the first antenna and the second antenna of the master base station transmit data symbols c _1,1 and −c _1,2 ^* , respectively, and cooperate with multiple base stations. In order to execute SFBC, the same data symbols c _1,1 and -c _1,2 ^* must be transmitted from the first antenna and the second antenna of the slave base station, respectively, but CS-RS is transmitted. These data symbols cannot be transmitted because they are resource elements to be null or null resource elements. Also, the timing of the first OFDM symbol of subcarrier k ₅ is such that the first antenna and the second antenna of the master base station transmit data symbols c _2,1 and −c _2,2 ^* , respectively. Although it is necessary to transmit the same data symbol from the first antenna and the second antenna, the data symbol is transmitted because it is a resource element for transmitting CS-RS or a resource element to be in a null state. I can't. As shown in the figure, in other resource elements, the slave base station can transmit the same data symbols as the master base station.

Therefore, in the present invention, on the slave base station side, the data element to be transmitted in the resource element at the CS-RS insertion position and null position of the slave base station, the resource element in which the CS-RS of the master base station is transmitted To send to.
That is, the slave base station transmits the first OFDM of subcarrier k ₂ in which the first antenna of the slave base station transmits its CS-RS, r ₀ ^(S) (k ₂ ), and the second antenna is in the null state. Data carrier c _1,1 and -c _1,2 ^* to be transmitted at symbol timing, the first antenna of the master base station transmits its CS-RS, and the second antenna is set to the null state. Transmission is performed from the first antenna and the second antenna at the timing of _one first OFDM symbol. Also, the first antenna of the slave base station should be null, and the second antenna should be transmitted at the timing of the first OFDM symbol of subcarrier k ₅ that transmits r ₁ ^(S) (k ₅ ) that is the CS-RS. Data symbols c _2,1 and -c _2,2 ^* are transmitted, with the first antenna of the master base station being in the null state and the second antenna transmitting its CS-RS, r ₁ ^(M) (k ₄ ). Transmission is performed from the first antenna and the second antenna at the timing of the first OFDM symbol of subcarrier k ₄ .

  As described above, in the present invention, when performing multi-base station cooperative MIMO transmission of a single rank, the slave base station is arranged in the same resource element as the resource element in which CS-RS is inserted or in the null state. The data symbol to be transmitted is moved to the resource element in which the CS-RS of the master base station is inserted and transmitted. In other resource elements, the same data symbol is transmitted between the master base station and the slave base station.

The signal transmitted from the first antenna of the master base station shown in FIG. 1A reaches the receiving antenna of the mobile station MS via a propagation path having a channel response value of h _i0 ^(M) (k). The signal transmitted from the second antenna of the master base station shown in (b) reaches the receiving antenna of the mobile station MS through the propagation path of h _i1 ^(M) (k), and the slave base shown in (c). The signal transmitted from the first antenna of the station reaches the receiving antenna of the mobile station MS via a propagation path having a channel response value of h _i0 ^(S) (k), and the signal of the slave base station shown in (d) The signal transmitted from the two antennas reaches the receiving antenna of the mobile station MS via a propagation path having a channel response value of h _i1 ^(S) (k).

  In the mobile station MS, signals transmitted from the first and second antennas of the master base station that perform coordinated transmission and the first and second antennas of the slave base station are received. Using CS-RS transmission pattern information, a CS-RS interference component replica transmitted from the slave base station is generated and removed from the received signal. Then, using the received signal from which the interference signal is removed, an algorithm such as MMSE is applied to demodulate the signal, and a soft decision output is obtained to decode the transmission signal.

ここで、ｘ_i（ｋ_j）は移動局の第i+1本目の受信アンテナにおけるサブキャリアｋ_jの受信信号、ｈ_i,0 ^(M)（ｋ_j）はマスター基地局の第１アンテナから移動局の第i+1本目の受信アンテナまでのサブキャリアｋ_jの周波数におけるチャネル応答値、ｈ_i,1 ^(M)（ｋ_j）はマスター基地局の第２アンテナから移動局の第i+1本目の受信アンテナまでのサブキャリアｋ_jの周波数におけるチャネル応答値、ｈ_i,0 ^(S)（ｋ_j）はスレーブ基地局の第１アンテナから移動局の第i+1本目の受信アンテナまでのサブキャリアｋ_jの周波数におけるチャネル応答値、ｈ_i,1 ^(S)（ｋ_j）はスレーブ基地局の第２アンテナから移動局の第i+1本目の受信アンテナまでのサブキャリアｋ_jの周波数におけるチャネル応答値、ｎ_i（ｋ_j）は移動局の第i+1本目の受信アンテナにおけるサブキャリアｋ_jの周波数における受信機雑音である。 A description will be given of demodulation of a received signal in a square surrounded by a broken line in FIG. 1, that is, at the timing of the first OFDM symbol of subcarriers k _{1 to} k ₆ .
The number of receive antennas at the mobile station MS and N _r present. In the mobile station when signals within the squares enclosed by broken lines in FIG. 1 are transmitted from the first and second antennas of the master base station and the first and second antennas of the slave base station. The received signal is expressed as follows.
The received signal vector at the timing of the first OFDM symbol of the _first subcarrier k ₁ is expressed by the following equation (1).

Here, x _i (k _j ) is the received signal of subcarrier k _{j at} the ⁽ i + 1) th receiving antenna of the mobile station, and h _{i, 0} ^(M) (k _j ) is from the first antenna of the master base station. The channel response value, h _{i, 1} ^(M) (k _j ) at the frequency of the subcarrier k _j up to the (i + 1) th receiving antenna of the mobile station is the i + th of the mobile station from the second antenna of the master base station. The channel response value at the frequency of subcarrier k _j up to the first receiving antenna, h _{i, 0} ^(S) (k _j ) is from the first antenna of the slave base station to the i + 1 th receiving antenna of the mobile station. Channel response value at the frequency of the subcarrier k _j , h _{i, 1} ^(S) (k _j ) is the subcarrier k _j from the second antenna of the slave base station to the i + 1 th receiving antenna of the mobile station The channel response value in frequency, n _i (k _j ), is applied to the i + 1 th receiving antenna of the mobile station. It is a receiver noise in the frequency of the subcarrier _kj in this.

The received signal vector in the second subcarrier k ₂ is expressed by the following equation 2.

The received signal vector in the third subcarrier k ₃ is expressed by the following equation (3).

The received signal vector in the fourth subcarrier k ₄ is expressed by the following equation 4.

The received signal vector in the _fifth subcarrier k ₅ is expressed by the following equation (5).

The received signal vector in the _sixth subcarrier k ₆ is expressed by the following equation (6).

When the above-mentioned formulas _{1 to 3} relating to the subcarriers k _{1 to} k 3 are put together, the following formula 7 is obtained.

Here, r ₀ ^(M) (k ₁ ) which is CS-RS transmitted from the first antenna of the master base station and r ₀ ^(S) which is CS-RS transmitted from the first antenna of the slave base station. All of (k ₂ ) are known. That is, the mobile station acquires cell ID information at the time of cell search, and can know the pattern of the cell-specific reference signal transmitted from the base station of each cell based on the cell ID. Based on the estimated channel response value and a known CS-RS transmission pattern, a replica of the interference component by CS-RS of the adjacent cell is created and removed from the received signal. be able to.
Therefore, the equation 7 becomes the following equation 8.

X ₀ (k ₁ ) −h _0,0 ^(M) (k ₁ ) r ₀ ^(M) (k ₁ ) on the left-hand side in Equation 8 is obtained from the received signal x ₀ (k ₁ ) and is estimated channel response h Product of _0,0 ^(M) (k ₁ ) and CS-RS r ₀ ^(M) (k ₁ ), that is, interference component replica h _0,0 ^(M) (k ₁ ) r ₀ ^(M) ( It represents a received signal from which k ₁ ) has been removed. Interference components are similarly removed from other received signals related to subcarrier k ₁ and received signals related to subcarrier k ₂ .

のMMSE（Minimum Mean Square Error：最小平均２乗誤差）法に基づく軟判定出力ｃ^〜 ₁は、次式で求めることができる。

ここで、σ²は協調セルからの干渉成分を除去した後の雑音電力である。

Of MMSE: soft-output c ^~ ₁ based on (Minimum Mean Square Error minimum mean square error) method, can be obtained by the following expression.

Here, σ ² is the noise power after removing the interference component from the cooperative cell.

Similarly, the equations _{4 to 6} relating to the subcarriers k _{4 to} k ₆ can be summarized as the following equation 11.

Then, similarly to the above, the interference component due to CS-RS can be removed, and Equation 11 becomes Equation 12.

のMMSEに基づく軟判定出力ｃ^〜 ₂は、次式で求めることができる。

数１０及び数１４で得られた軟判定出力に基づいてデータシンボルを復号する。

The soft decision outputs c ^to ₂ based on the MMSE can be obtained by the following equation.

Data symbols are decoded based on the soft decision outputs obtained in Equations (10) and (14).

In the example described above, the remainder modulo 6 of the cell ID of the cell to which the master base station belongs is 0, and the remainder 1 is modulo 6 of the cell ID of the cell to which the slave base station belongs. This is the case where the amount of deviation between the subcarrier for transmitting CS and the subcarrier for transmitting CS-RS from the slave base station is 1.
Next, a CS-RS is transmitted from the master base station with a remainder modulo 6 of the cell ID of the cell of the master base station and 2 modulo 6 of the cell ID of the cell of the slave base station. An example in which the frequency difference between the subcarrier to which the CS-RS is transmitted from the slave base station and the subcarrier to which the CS-RS is transmitted is two subcarrier frequencies will be described with reference to FIG.
2A is a signal transmitted from the first antenna of the master base station, FIG. 2B is a signal transmitted from the second antenna of the master base station, and FIG. 2C is transmitted from the first antenna of the slave base station. (D) shows a signal transmitted from the second antenna of the slave base station.
Signals transmitted from the first and second antennas of the master base station shown in FIGS. 2A and 2B are the same as those shown in FIGS. 1A and 1B.

Since the slave base station has a remainder modulo 6 of the cell ID, from the first antenna and the second antenna of the slave base station, according to the mapping shown in FIG. 7C (a) and (b). A signal is transmitted.
That is, as shown in FIG. 2C, the first antenna of the slave base station has its CS-RS (r ₀ ^(S) (k ₃ )) at the timing of the first OFDM symbol of the third subcarrier k ₃ . sends, it is null state at the timing of the 1OFDM symbol of the sixth sub-carrier k _6. The second antenna of the slave base station, as shown in FIG. 2 (d), is the null state in the third second 1OFDM symbol subcarrier k ₃ timing, the 1OFDM symbol of the sixth sub-carrier k ₆ The CS-RS (r ₁ ^(S) (k ₆ )) is transmitted at the timing shown in FIG.

As shown in FIGS. 2A and 2B, the timing of the first OFDM symbol of the third subcarrier k ₃ is determined by transmitting data symbols c _1,2 from the first antenna of the master base station. The data symbol c _1,1 ^* is transmitted from the second antenna of the base station, and the same data symbol c _1,2 is transmitted from the first antenna and the second antenna of the slave base station that perform cooperative MIMO transmission with the master base station. And c _1,1 ^* , but the first antenna of the slave base station transmits its CS-RS and the second antenna must be in the null state, so the data symbols c _1,2 and c _1,1 ^* cannot be sent. Therefore, in the present invention, the first base station of the master base station transmits its CS-RS and the second antenna is set to the first OFDM symbol of the first subcarrier k ₁ in which the second antenna is in the null state. Data symbol c _1,2 is transmitted from one antenna, and data symbol c _1,1 ^* is transmitted from the second antenna.

Similarly, the timing of the first OFDM symbol of the sixth subcarrier k ₆ is such that data symbols c _2,2 are transmitted from the first antenna of the master base station, as shown in (a) and (b) of FIG. This is the timing at which the data symbol c _2,1 ^* is transmitted from the second antenna, and it is necessary to transmit the same data symbol c _2,2 and c _2,1 ^* from the first antenna and the second antenna of the slave base station, respectively. However, since the first antenna of the slave base station is set to the null state and the second antenna must transmit its CS-RS (r ₁ ^(S) (k ₆ )), the data symbol c _2,2 And c _2,1 ^* cannot be transmitted. Therefore, in the present invention, the first OFDM of the fourth subcarrier k ₄ in which the first antenna of the master base station is set to the null state and the second antenna transmits the CS-RS (r ₁ ^(M) (k ₄ )). At the symbol timing, the data symbol c _2,2 is transmitted from the first antenna of the slave base station, and the data symbol c _2,1 ^* is transmitted from the second antenna.

Thus, the difference between the remainder modulo 6 of the cell ID of the cell to which the master base station belongs and the remainder modulo 6 of the cell ID of the cell to which the slave base station belongs is also shown in FIG. As in the case shown, data symbols that cannot be transmitted because each antenna of the slave base station hits a resource element specified by LTE Rel. The antenna is transmitted to the resource element that transmits the CS-RS.
Similarly, when the difference between the remainder modulo 6 of the cell ID of the cell to which the master base station belongs and the remainder modulo 6 of the cell ID of the cell to which the slave base station belongs is other than 1 and 2, Data symbols to be transmitted for coordinated MIMO to resource elements whose base station antenna is CS-RS transmitted or null state Resource elements whose master base station antenna is CS-RS transmitted or null state Send.

  As in FIG. 1, the signals transmitted from the first and second antennas of the master base station reach the receiving antenna of the mobile station via the respective propagation paths, and the first and second antennas of the slave base station. The signal transmitted from the second antenna also reaches the receiving antenna of the mobile station via each propagation path.

A description will be given of demodulation of a received signal in a square surrounded by a broken line in FIG. 2, that is, at the timing of the first OFDM symbol of subcarriers k _{1 to} k ₆ .
The received signal vector at the timing of the first OFDM symbol of the first subcarrier k ₁ is expressed by the following equation (15).

The received signal vector in the second subcarrier k ₂ is expressed by the following equation (16).

The received signal vector in the third subcarrier k ₃ is expressed by the following equation (17).

The received signal vector in the fourth subcarrier k ₄ is expressed by the following equation (18).

The received signal vector in the fifth subcarrier k ₅ is expressed by the following equation 19.

The received signal vector in the sixth subcarrier k ₆ is expressed by the following equation (20).

Summarizing Formulas 15 to 17 indicating the received signal vectors of the first subcarrier k ₁ to the third subcarrier k ₃ , the following Formula 21 is obtained.

As in the previous case, r ₀ ^(M) (k ₁ ), which is a CS-RS transmitted from the first antenna of the master base station, and r, which is a CS-RS transmitted from the first antenna of the slave base station. ₀ ^(S) (k ₃ ) are all known, and based on the estimated channel response value and the known CS-RS transmission pattern, a replica of the interference component by the CS-RS of the adjacent cell is created and the received signal Is removed from the above equation, the equation 21 becomes the following equation 22.

のMMSEに基づく軟判定出力ｃ^〜 ₁は、次の数２４により求めることができる。

The soft decision outputs c ^to ₁ based on MMSE can be obtained by the following equation (24).

Summarizing Expressions 18 to 20 indicating the received signal vectors of the fourth subcarrier k ₄ to the sixth subcarrier k ₆ , the following Expression 25 is obtained.

Also for this equation, since the CS-RS transmitted from the master base station and the slave base station is known, the interference component due to CS-RS can be removed in the same manner as described above, and Equation 25 becomes Equation 26.

のMMSEに基づく軟判定出力ｃ^〜 ₂は、次の数２８で求めることができる。

このようにして得られた軟判定出力を用いて受信信号を復号することができる。

The soft decision outputs c ^to ₂ based on the MMSE can be obtained by the following equation (28).

The received signal can be decoded using the soft decision output obtained in this way.

Next, the configuration of the base station side device and the configuration of the mobile station in the mobile communication system of the present invention will be described. In addition, although the case where there are two base stations is shown here, the same configuration can be made when there are three or more base stations.
FIG. 3 is a block diagram showing an example of the configuration on the base station side in the mobile communication system of the present invention. Note that the mobile communication system of the present invention basically assumes a system in which a plurality of mobile stations can communicate with one base station, but FIG. 3 shows elements necessary for communication with one mobile station. Only shows. Since the present invention relates to downlink transmission (base station → mobile station), blocks related to uplink transmission (mobile station → base station) are omitted.

  In FIG. 3, 10 is a first base station, 30 is a second base station, and 50 is a base station controller. One of the first base station 10 and the second base station 30 is a master base station, and the other is a slave base station. The first base station 10 and the second base station 30 are connected to each other via a network and are connected to the base station control device 50. The first base station 10 and the second base station 30 have the same configuration. Here, it is assumed that each base station 10 and 30 is provided with a plurality of Nt transmission / reception antennas, but the number of transmission / reception antennas provided in each base station does not have to be the same, and an arbitrary number It can be. In the figure, in the first base station 10 and the second base station 30, blocks indicated by broken lines are blocks used in the case of multiple rank transmission.

In the first base station 10, user data to be transmitted is input to the rank adaptation unit 12 via the buffer 11. The rank adaptation unit 12 divides the user data from the buffer 11 into a number of substreams corresponding to the number of ranks specified by the scheduler 17 and inputs the data to a channel encoder 13 provided for the substreams. Do. The data error-correction-encoded by the channel encoder 13 is interleaved by the interleaver 14, then converted to complex symbols by the I / Q mapping unit 15, multiplied by the precoding matrix by the precoder 16, and adapted to the transmission antenna. Input to the provided multiplexers 18-1 to 18 -N _t . The multiplexer 18-1 to 18-N _t, respectively cell-specific reference signal (CS-RS) item and the control signal are multiplexed, into parallel signals by the serial to parallel converter (S / P) 19-1~19-N t after being converted, the inverse Fourier transform in the inverse fast Fourier transform (IFFT) unit 20-1 to 20-N _t, is converted into a serial signal by the parallel-serial converter (P / S) 21-1~21-N t The The output signal of each serializer 21 - 1 to 21-N _t, after the cyclic prefix corresponding to the guard interval is added by the CP (Cyclic Prefix) adding section 22-1 to 22-N _t, not shown is frequency-converted to a carrier frequency in the mixer, is transmitted to power amplification by not shown power amplifier from the antenna 23-1 to 23-N _t.

Most of the MIMO / OFDM based systems apply rank adaptation. For example, in 2 × 2 open-loop MIMO standardized by LTE, transmission diversity using SBC (space-frequency block coding) corresponding to two modes of single rank transmission and two rank transmission, and SDM (space Two modes called division multiplexing are adaptively switched.
The rank adaptation unit 12 performs SFBC encoding on user data when the scheduler 17 is instructed to transmit single rank, and divides user data into two substreams when instructed to transmit two ranks. In order to perform spatial multiplexing, the data is input to the channel encoder 13 and the channel encoder indicated by the broken line for error correction coding.

The second base station 30 also includes a buffer 31, a rank adaptation unit 32, a channel encoder 33, an interleaver 34, an I / Q mapping unit 35, a precoder 36, a scheduler 37, multiplexers 38-1 to 38-N _t , and serial-parallel conversion. instrument (S / P) 39-1~39-N t, the inverse fast Fourier transform (IFFT) unit 40-1 to 40-N _t, parallel to serial converter (P / S) 41-1~41-N t, CP addition units 42-1 to 42-N _t and antennas 42-1 to 42-N _t are provided, and are configured in the same manner as the first base station 10.

The base station control device 50 controls synchronization and coordinated scheduling between a plurality of base stations. The base station control device 50 may be arranged on a network to which the plurality of base stations are connected, or may be arranged inside any one of the base stations.
In LTE, each mobile station can transmit a measurement report to a base station in the area. Therefore, for example, the mobile station transmits a measurement report including the cell ID and RSPP (Reference Signal Received Power: radio quality) of the own cell and the cell ID and RSPP of the neighboring cell to the own cell base station. Transmits the received measurement report to the base station controller 50. Thereby, the base station control apparatus 50 can acquire the existence of the cell edge mobile station, the information of the own cell base station to which the cell edge mobile station belongs and the neighboring cell base stations, and the information of the reception quality. Based on the information obtained in this way, the base station control device 50 determines whether or not to perform multi-base station cooperative MIMO transmission, and whether to perform single rank transmission shown in FIG. The scheduler 17 of the neighboring cell base station (slave base station) that determines whether to perform 2-rank transmission shown in (b) and performs MIMO transmission in cooperation with the own cell base station (master base station) and the master base station. And 37 are controlled. For example, when the received SINR is low, the master base station 10 and the slave base station 30 transmit the same substream Stream # 0 to perform the single rank multiple base station cooperative MIMO transmission shown in FIG. The scheduler 17 and 37 are instructed to perform SFBC precoding. Further, when the received SINR is high, the master base station 10 transmits the substream Stream # 0 and the slave base station 30 performs the two-rank multi-base station cooperative MIMO transmission shown in (b) of FIG. Control to transmit substream Stream # 1.

  When performing single rank multiple base station cooperative MIMO transmission, the scheduler 37 of the slave base station 30 is described in (c), (d) of FIG. 1 or (c), (d) of FIG. Execute the process. That is, when data symbols are arranged according to the same mapping rule as that of master base station 10, CS-RS of the master base station inserts data symbols arranged in the resource element in which CS-RS is inserted in slave base station 30. To move to the resource element to be processed.

FIG. 4 is a block diagram showing an example of the configuration of the mobile station 60 in the mobile communication system of the present invention. Since the present invention relates to downlink transmission (base station → mobile station), description of blocks related to uplink transmission (mobile station → base station) is omitted.
As shown in the figure, the mobile station 60 has a plurality of N _r reception antennas 61-1 to 61-N _r . Signals received by the receiving antennas 61-1 to 61-N _r are input to the signal receiving unit 62-1 to 62-N _r provided corresponding to each. Each signal receiving unit 62-1 to 62-N _r are the same structure.

In each signal receiving unit 62-1 to 62-N _r, the signal received by the corresponding receiving antennas 61-1 to 61-N _r is amplified by the low noise amplifier LNA81-1~81-N _r, The down converters D / Cs 82-1 to 82 -N _r multiply the carrier frequency signals to downconvert them to baseband, and input the signals to A / D converters 83-1 to 83 -N _r . The received signal converted into digital data by the A / D converters 83-1 to 83 -N _r is subjected to CP removal by a cyclic prefix (CP) removal unit (not shown), and then a serial-parallel converter (S / P ) 84-1 to 84-N _r and the FFT timing detector 63.
Deserializer 84-1~84-N _r is the received signal from which CP is removed from the receiving antenna corresponding converted into parallel signals, a fast Fourier transform provided corresponding (FFT) section 85 and outputs it to the -1~85-N _r.

The FFT timing detector 63 detects the OFDM symbol reception timing based on the primary synchronization signal (PSS) and the secondary synchronization signal (SSS) included in the reception signal. The FFT units 85-1 to 85 -N _r convert each input signal into a signal for each subcarrier based on the reception timing detected by the FFT timing detection unit 63.
The output signals of the FFT unit 85-1~85-N _r is the master base station and slave base station is input to the CS-RS interference canceller 86-1~86-N _r provided corresponding to each Are input to a channel estimation unit 64 that performs channel estimation.

The cell ID information of the master base station and the slave base station is acquired at the time of timing detection by the FFT timing detection unit 63, and the mobile station receives the CS-RS transmitted from each antenna of the master base station and the slave base station. The pattern is known. Channel estimation unit 64 from the received signal for each receive antenna for each subcarrier output from the FFT unit 85-1~85-N _r, the CS-RS of contained therein a master base station and slave base station It detects and acquires an estimated value of channel information indicating the state of the propagation path between each antenna of the master base station and slave base station and each receiving antenna of its own mobile station.

The CS-RS replica generation unit 65 generates a master base station and a slave base station based on the channel estimation value output from the channel estimation unit 64 and the CS-RS pattern transmitted from each antenna of the master base station and the slave base station. A replica of the received signal of CS-RS from each antenna is generated and supplied to the corresponding CS-RS interference cancellers 86-1 to 86 -N _r .
The CS-RS interference cancellers 86-1 to 86 -N _r subtract the corresponding CS-RS received signal replica generated by the CS-RS replica generator 65 from each received signal. As a result, the CS-RS signal transmitted from each antenna of the master base station and slave base station to the same resource element as the data symbol and causing interference with the data symbol is removed. This processing corresponds to the subtraction on the matrix component on the left side in the equations 8, 12, 22 and 26.

The output signal from the CS-RS interference canceller 86-1~86-N _r, the signal detection and LLR (Log Likelihood Ratio: log likelihood ratio) is input to the generating unit 68, the channel estimation unit of the master base station 66.
The channel estimation unit 66 of the master base station calculates a channel estimation value between each transmission antenna of the master base station and each reception antenna of the mobile station. The downlink layer 1 (L1) / layer 2L2) control information decoding unit 67 demodulates the PDCCH using the channel estimation value from the channel estimation unit 66 and supplies the PDCCH to the signal detection and LLR generation unit 68.
Based on the PDCCH demodulated by the downlink L1 / L2 control information decoding unit 67, the signal detection and LLR generation unit 68 demodulates the PDSCH signal of the subcarrier corresponding to the own station based on the MMSE method.

The signal demodulated by the signal detection and LLR generation unit 68 is converted into a serial signal by a parallel / serial converter (P / S) 69 and then subjected to error correction decoding processing by a channel decoder 70 and output.
When the number of ranks is 2 or more, as shown by a broken line in the figure, a signal of a sequence corresponding to the number of ranks is output from the signal detection and LLR generation unit 68, and parallel-serial conversion corresponding to each sequence is performed. After being converted into a serial signal by a device (P / S) 69, error correction decoding processing is performed by a channel decoder 70 corresponding to each sequence, and the output from the channel decoder 70 of each sequence is converted into a parallel-serial converter ( (P / S) 71 is converted into a serial signal and output.

1: Master base station, 2: Slave base station, 3: Mobile station, 10, 30: Base station, 11, 31: Buffer, 12, 32: Rank adaptation unit, 13, 33: Channel encoder, 14, 34: Inter Lever, 15,35: I / Q Mappinngu part, 16 and 36: pre-coder, 17, 37: _{scheduler, 18-1~18-N t, 38-1~38-} N t: multiplexer, 19-1～19- N _t , 39-1 to 39-N _t : serial-parallel converter, 20-1 to 20-N _t , 40-1 to 40-N _t : inverse fast Fourier transform unit, 21-1 to 21-N _t , 41-1 to 41-N _t: parallel to serial _{converter, 22-1~22-N t, 42-1~42-} N t: CP adding section, 23-1~23-N _t, 43-1~43 -N _t: antenna, 50: base station controller, 60: mobile station, 6 : Mobile station, 61-1 to 61-N _r: antenna, 62-1 to 62-N _r: signal receiving section, 63: FFT timing detector, 64: channel estimation unit, 65: CS-RS replica generating unit, 66: channel estimation unit, 67: downlink layer 1 / layer 2 control information decoding unit, 68: signal detection and LLR generation unit, 69: parallel-serial converter, 70: channel decoder, 71: parallel-serial converter, 81 −1 to 81-N _r : low noise amplifier, 82-1 to 82-N _r : down converter, 83-1 to 83-N _r : A / D converter, 84-1 to 84-N _r : serial parallel converter, 85-1~85-N _r: fast Fourier transform _{section, 86-1~86-N r: CS-} RS interference removal unit

Claims (2)

A mobile communication system in which a plurality of base stations and mobile stations communicate by OFDM,
When a common frequency band is used by a plurality of base stations in downlink transmission, the transmission timing of all the base stations that interfere with each other is the reception timing including the multipath of the signal from each base station in the mobile station. The deviation is controlled to be within the OFDM guard interval,
Cell-specific reference signals are distributed and arranged on the time axis and the frequency axis, and the position of the subcarrier on which the cell-specific reference signal is arranged is determined according to the cell ID,
The slave base station that performs MIMO transmission in cooperation with the master base station is the same as the resource element into which the cell-specific reference signal is inserted in the slave base station when the data symbols are arranged with the same mapping rule as the master base station. The data symbol arranged in the resource element is transmitted using the same resource element as the resource element to which the cell-specific reference signal of the master base station is transmitted,
The mobile station uses the channel estimation value and the interference caused by the cell-specific reference signal transmitted from the master base station and the slave base station based on the transmission pattern of the known cell-specific reference signal transmitted from the master base station and the slave base station. A mobile communication system, wherein a component is removed and the data symbol is decoded. A mobile communication system in which a plurality of base stations and mobile stations communicate by OFDM, and transmission timings of all the base stations that interfere with each other when a plurality of base stations use a common frequency band in downlink transmission However, the reception timing shift including the multipath of the signal from each base station in the mobile station is controlled so as to be within the OFDM guard interval, and the cell-specific reference signal is dispersed on the time axis and the frequency axis. A base station apparatus in a mobile communication system in which a position of a subcarrier in which a cell-specific reference signal is arranged is determined according to a cell ID,
When the own base station performs MIMO transmission in cooperation with the master base station, a resource element into which a cell-specific reference signal is inserted in the own base station when data symbols are arranged with the same mapping rule as the master base station A base station apparatus comprising: means for transmitting data symbols arranged in the same resource element using the same resource element as the resource element to which the cell-specific reference signal of the master base station is transmitted.

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