JP2013243706A - Wireless base station apparatus, mobile station device, wireless communication method, and wireless communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wireless base station apparatus, a mobile station device, a wireless communication method, and a wireless communication system, which can realize extremely high orthogonality between layers.SOLUTION: A wireless base station apparatus 200 includes: a plurality of transmission antennas; an orthogonalizing part for orthogonalizing a downlink reference signal disposed in a plurality of wireless resources in a time direction at the same frequency, using an orthogonal code between layers; and a transmission part for transmitting signals obtained by multiplexing the downlink reference signal and a transmission data, from the plurality of transmission antennas in multi-layer format.

Description

  The present invention relates to a radio base station apparatus, a mobile station apparatus, a radio communication method, and a radio communication system that transmit a downlink reference signal (reference signal).

  Successor to the wideband code division multiple access (W-CDMA) system, high speed downlink packet access (HSDPA) system, high speed uplink packet access (HSUPA) system, etc., that is, long term evolution (LTE) ) Was established in the WCDMA standardization body 3GPP (Release-8). As a radio access method in Release-8 LTE (hereinafter referred to as Rel8-LTE), an orthogonal frequency division multiplexing access (OFDMA) method is used for the downlink, and a single carrier frequency division multiplexing connection is used for the uplink. (SC-FDMA: Single-Carrier Frequency Division Multiple Access) is defined.

  The OFDMA scheme is a multicarrier transmission scheme in which a frequency band is divided into a plurality of narrow frequency bands (subcarriers) and data is transmitted on each subcarrier. It can be expected that high-speed transmission can be realized by increasing the frequency utilization efficiency by arranging subcarriers densely while being orthogonal to each other on the frequency axis.

  The SC-FDMA scheme is a single carrier transmission scheme in which a frequency band is divided for each terminal, and transmission is performed using different frequency bands among a plurality of terminals. In addition to being able to reduce interference between terminals easily and effectively, the variation in transmission power can be reduced, so this method is preferable from the viewpoint of reducing the power consumption of terminals and expanding the coverage.

  Further, Rel8-LTE defines a downlink reference signal configuration. The downlink reference signal includes 1) downlink CQI (Channel Quality Indicator) measurement for scheduling and adaptive control, and 2) channel estimation for downlink synchronous detection in a user terminal (hereinafter referred to as LTE terminal) that supports Rel8-LTE. 3) Used for estimation of downlink channel conditions for cell search and handover. In the downlink reference signal, a cell-specific reference signal, a reference signal common to a plurality of cells, and an individual reference signal for beam forming are defined.

  In Rel8-LTE, a wireless transmission method (MIMO: Multiple-Input Multiple-Output) is provided that improves communication quality by providing a plurality of antennas for each of a transmitter and a receiver (for example, non-patent literature). 1). A distinction is made between cases where the layers (data streams) to be transmitted at the same time belong to the same user (single user MIMO) and cases where they belong to different users (multiuser MIMO).

  Single-user MIMO can perform four-layer spatial multiplexing using a maximum of four transmission antennas in a base station. Each layer is transmitted from all the transmission antennas by using different transmission phase / amplitude control (precoding) instead of corresponding to the transmission antenna on a one-to-one basis. By precoding, each layer transmitted ideally simultaneously is received orthogonally (without interfering with each other) at the receiver side. For this reason, precoding vectors (weighting of transmitting antennas) are considered in consideration of fading fluctuations so that the layers (data streams) transmitted simultaneously do not interfere with each other and are received at a high SINR in the LTE terminal. ). Also, precoding enables beam forming that realizes directional transmission in which a desired wave is emphasized for a specific user terminal.

  Multi-user MIMO is realized by assigning the same resource block (RB) of a certain subframe to a plurality of user terminal layers. In the case of multi-user MIMO, the number of layers assigned to each user is limited to one.

3GPP TR 25.913 [1] T. Ihara et al., IEEE ICCS 2002

  By the way, a virtual antenna (Antenna virtualization) technique has been proposed in which a reference signal is precoded from each transmission antenna of a transmitter and transmitted to transmit with a smaller number of virtual antennas than the actual number of antennas (for example, Non-Patent Document 2), the configuration of the downlink reference signal in the case of forming a virtual antenna in the MIMO system has not been studied.

  The present invention has been made in view of the above points, and an object thereof is to provide a radio base station apparatus, a mobile station apparatus, a radio communication method, and a radio communication system that can realize extremely high orthogonality between layers. .

  The radio base station apparatus of the present invention includes a plurality of transmission antennas, an orthogonalization unit that orthogonalizes between layers using orthogonal codes for downlink reference signals arranged in a plurality of radio resources in the time direction at the same frequency, A transmission unit configured to transmit a signal obtained by multiplexing a downlink reference signal and transmission data from the plurality of transmission antennas in multiple layers.

  According to the present invention, very high orthogonality between layers can be realized.

Block diagram of transmission system of base station apparatus according to embodiment Block diagram of reception system of mobile station apparatus according to embodiment The figure which shows the state which allocated CRS prepared for all four antenna ports to the radio | wireless resource. Conceptual diagram showing how CRSs for all antenna ports are precoded to form one virtual antenna and transmitted to the entire cell / sector The figure which shows the state which allocated the radio | wireless resource of CRS reduced to 1 antenna port Conceptual diagram showing a state in which CRS of one antenna port is precoded to form one virtual antenna and transmitted to the entire cell / sector. The figure which shows the state which allocated the radio | wireless resource of CRS reduced to 2 antenna ports Conceptual diagram showing a state where CRS of two antenna ports is precoded to form one virtual antenna and transmitted to the entire cell / sector. Conceptual diagram when CRS reduction method is applied to distributed antenna system (A) A diagram showing a pattern in which CRS is assigned to a plurality of symbols in one subframe in a high-density CRS structure, (b) a pattern in which CRS is assigned only to the first symbol in one subframe in the CRS structure in FIG. Figure showing Diagram showing time division multiplex transmission of "normal structure" and "low density structure" The figure which shows the transmission interval of CQI-RS. Explanatory drawing about the multiplexing method for multiplexing CQI-RS to a sub-frame Explanatory drawing about the multiplexing method for multiplexing CQI-RS to a sub-frame The figure which shows DM-RS allocated similarly to the reference signal peculiar to a user prescribed | regulated by Rel8 LTE. The figure which illustrated the density of DM-RS optimal for data transmission with a single stream The figure which illustrated the density of DM-RS optimal for data transmission with a single stream The figure which shows DM-RS arrangement | positioning optimal for data transmission by multi-stream Conceptual diagram of orthogonalization between DM-RS streams by FDM Conceptual diagram of orthogonalization between DM-RS streams by CDM LTE-based system conceptual diagram

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
In one aspect of the present invention, three types of reference signals are included as downlink reference signals to which virtual antennaization is applied.

  The first uses a reference signal common to a plurality of cells (referred to as “CRS” (Common Reference Signal) in this specification). CRS can reuse a common reference signal that is common among cells defined in Rel8-LTE. CRS is used for demodulation of at least a shared data channel (PDSCH) when supporting LTE terminals in the same band. Further, a terminal (hereinafter referred to as LTE-A terminal) of LTE-Advanced (hereinafter referred to as LTE-A terminal), which is a radio access method succeeding Rel8-LTE, is used for paging channel (PCH), broadcast channel (BCH), It is used for demodulating the common control channel.

  Second, in the MIMO system, a reference signal (referred to as “CQI-RS” (Channel Quality Indicator-Reference Signal) in this specification) used for CSI (Channel State Information) measurement for each antenna is prepared. Since the above-mentioned CRS is a reference signal common to cells, when the virtual antenna is used, there is no reference signal for each antenna, and CSI (Channel State Information) measurement for each antenna cannot be performed. Therefore, a CQI-RS is prepared for each actual antenna.

  The third type is specific to the LTE-A terminal (user) under the base station, and a reference signal (“DM-RS” (in this specification) used for demodulation of the common data channel (PDSCH) in the LTE-A terminal. Prepared as Demodulation-Reference Signal). Reference signals that are orthogonal between streams are used.

  Hereinafter, three types of downlink reference signals CRS, CQI-RS, and DM-RS to which virtual antennaization is applied will be specifically described.

  First, in the 4-antenna MIMO system, consider a case in which a 4-antenna CRS defined by Rel8-LTE is allocated (multiplexed) to a radio resource.

  FIG. 3 shows a state where four CRSs prepared for each of the four antennas are multiplexed with radio resources. FIG. 3 shows a radio resource having a size of one resource block in the frequency axis direction and one subframe in the time axis direction. In Rel8-LTE, 100 resource blocks are allocated to a 20 MHz system band, and it is specified that one resource block is composed of 12 subcarriers. Further, it is specified that one subframe is a transmission time unit, data is transmitted by dividing one subframe into two time slots, and one time slot is composed of seven symbols.

  Since Rel8-LTE defines different CRSs for all of the four antennas, the pattern illustrated in FIG. 3 is obtained when the radio resources are assigned so that different CRSs are transmitted by the four antennas. However, FIG. 3 illustrates a state in which the DM-RS newly defined this time is also allocated to the radio resource, and the CRS is arranged so as not to overlap with the DM-RS. DM-RSs are transmitted from all four antennas without being made virtual antennas.

  Now, a case is assumed in which four CRSs multiplexed as shown in FIG. 3 are precoded, and transmission of each CRS is made into a virtual antenna (one). When the base station transmits the four antennas with antenna weights of 1, 1, −1, −1 and the like, transmission into one virtual antenna can be realized.

  FIG. 4 conceptually shows a state in which different CRSs are precoded, converted into one virtual antenna, and transmitted to the entire cell / sector. Each CRS precoded so as to be a virtual antenna is transmitted to the entire cell / sector. Moreover, DM-RS is transmitted with 4 antennas to the LTE-A terminal.

  Since four CRSs are made into one virtual antenna, the LTE terminal can demodulate the shared data channel using any received CRS, and the LTE-A terminal can perform common control using any received CRS. The channel can be demodulated.

  However, as shown in FIG. 3, if four CRSs corresponding to four antennas are multiplexed four times and DM-RSs are multiplexed four times, there is a problem that the overhead becomes very large. If the LTE terminal and the LTE-A terminal can receive one CRS, the shared data channel or the common control channel can be demodulated. There is no need to transmit the three CRSs.

  Therefore, in response to the number of antennas converted into virtual antennas in one cell / sector, the CRS does not transmit all CRSs corresponding to the actual number of antennas, but transmits only the number of antennas converted into virtual antennas. To. As a result, the number of CRSs allocated to radio resources can be reduced corresponding to the number of antennas that are made into virtual antennas, and overhead can be reduced.

  In the example shown in FIG. 5, as in the case of FIG. 4, the actual four antennas are converted into a single virtual antenna, so that one CRS is transmitted with the four antennas. DM-RSs are assigned to the same positions as in FIG. FIG. 6 shows a state in which one CRS is pre-coded from each antenna and transmitted corresponding to the number of virtual antennas (one), and then transmitted with one virtual antenna. Moreover, DM-RS is transmitted with 4 antennas to the LTE-A terminal.

  FIG. 7 is a diagram showing CRS radio resource allocation with the CRS type reduced to two antenna ports. DM-RSs are assigned to the same positions as in FIG. Two CRSs are assigned to radio resources corresponding to two antennas. As shown in FIG. 8, two different CRSs are precoded, converted into a single virtual antenna, and transmitted to the entire cell / sector.

  In this way, four antennas are virtually made into two antennas by making virtual antennas, and the number of CRSs is reduced to two corresponding to the number of antennas made into virtual antennas, thereby enabling LTE terminals adapted for two-antenna transmission. In other words, transmission diversity of two antennas can be applied to the shared data channel. In communication with the LTE-A terminal, two-antenna transmission diversity can be applied to the control channel.

FIG. 9 is a conceptual diagram when the CRS reduction technique is applied to a distributed antenna system.
In the distributed antenna system, a plurality of remote antenna units (base stations BS) geographically distributed in one area are connected via a communication cable to form one cell. The distributed base station (BS) has one (or a plurality of) antennas, but the central apparatus (Central eNB) performs antenna transmission / reception processing from the plurality of distributed base stations (BS) in an integrated manner. For example, MIMO transmission that improves diversity effect and transmission speed using multiple antennas is realized using multiple distributed base stations (BS).

  In the distributed antenna system shown in FIG. 9, one cell is covered by four transmitters (BS), and each transmitter (BS) has one transmission antenna. The central device (Central eNB) centrally manages antenna transmission / reception processing from a plurality of distributed base stations (BS) to realize MIMO transmission. In this case, as shown in FIG. 9, if the same CRS is precoded from a plurality of distributed base stations (BS) and transmitted to the entire cell, different CRS is transmitted from each distributed base station (BS). , Overhead can be reduced. In FIG. 9, different DM-RSs are transmitted for each antenna (for each distributed base station (BS)) and transmitted to the LTE-A terminal.

  Thus, not only when one base station is provided with a plurality of antennas, but also in a distributed antenna system, transmission of CRS reduced corresponding to the number of antennas made into virtual antennas similarly reduces overhead. The effect to do.

FIGS. 10A and 10B are diagrams showing a CRS structure.
FIG. 10A shows a CRS structure up to two antennas supported by the LTE terminal. In the CRS structure of FIG. 9, two different CRSs are alternately allocated to four subcarriers equally in one resource block in the frequency axis direction within one symbol. Further, four symbols are assigned to one CRS. It is desirable that the symbol intervals to which the CRS is assigned be substantially equal within the subframe. The CRS structure in FIG. 10A is referred to as a “normal structure”. Such a “normal structure” CRS is continuously assigned to each resource block in the frequency axis direction.

  The base station apparatus pre-codes two different CRSs (normal structure) corresponding to two antennas shown in FIG. 10 (a), and actually uses the four transmitting antennas to form virtual antennas for the entire cell / sector. Multiplex and transmit with one antenna.

  FIG. 10B shows a CRS structure with up to two antennas, but shows a pattern in which CRS is assigned only to the first symbol of one subframe in the CRS structure of FIG. The density of CRS is lower than that of the normal structure. The low density CRS structure shown in FIG. 10B is referred to as a “low density structure”. Even in the case of two-antenna transmission, overhead can be greatly reduced by transmitting a CRS having a “low density structure”.

  As shown in FIG. 11, the “normal structure” and the “low density structure” may be time-division multiplexed and transmitted from the radio base station apparatus. In this case, transmission can be performed by switching between the normal structure and the low density structure in units of subframes. The LTE terminal can receive the “normal structure” CRS and use it for demodulation of the shared data channel, but cannot support the “low density structure” CRS. Therefore, the LTE terminal is signaled so as to recognize the transmission section transmitting the “low density structure” CRS as a section of an MBSFN (Multimedia Broadcast Multicast Service Single Frequency Network) subframe. In the MBSFN scheme, all adjacent base stations use the same scrambling code and transmit the same radio signal in synchronization with the same MBMS. Since the LTE terminal does not capture the MBSFN subframe, the demodulation operation can be continued only with the “normal structure” CRS. On the other hand, the LTE-A terminal supports both “normal structure” and “low density structure”.

  In this way, by transmitting the “normal structure” and “low density structure” in time division multiplexing, LTE terminals that support the “normal structure” receive the “normal structure” CRS and demodulate the shared data channel. it can. The LTE-A terminal can demodulate the common / individual control channel by receiving both the “normal structure” and the “low density structure”. Further, a terminal that has received a CRS having a “normal structure” and / or a “low density structure” can perform RSRP (Reference Signal Received Power) measurement for handover based on the received CRS.

  The ratio between the “normal structure” and the “low density structure” may be dynamically switched according to the system environment. For example, when the number of LTE terminals supporting the “normal structure” decreases, the ratio of the “normal structure” CRS is reduced, and when the LTE terminal is completely removed, the “normal structure” CRS is not transmitted. Anyway. That is, it is possible to construct a radio access system in which CRS structures with different densities are configured to be time-division multiplexed and controlled to an appropriate density according to the situation.

Next, CQI-RS will be specifically described.
As described above, when virtual antennaization is applied to CRS, CRS cannot be used for channel estimation for each antenna when the actual number of antennas is 4, 8, or more. In the distributed antenna system, since channel estimation for each distributed base station is required, a reference signal for each antenna is required.

  Therefore, separately from the CRS applied to the virtual antenna, channel estimation for each antenna can be performed using CQI-RS defined for each antenna and each cell.

  Since CQI-RS is for channel estimation, low density is sufficient. In LTE, a sounding reference signal is defined as a reference signal for channel estimation in the uplink. Since this is a reference signal for measuring channel quality in the same way as an uplink sounding reference signal, transmission is performed at a density (transmission interval) similar to that of an uplink sounding reference signal.

  Specifically, as shown in FIG. 12A, transmission is performed at intervals of 2 ms (2 TTI) as the most dense sending method. Further, similarly to the sounding reference signal, it is configured to transmit once every 5 ms and once every 10 ms.

Next, a multiplexing method for multiplexing CQI-RSs into subframes will be described with reference to FIGS.
Since LTE-A is determined to support up to 8 antennas, it is necessary to transmit 8 different CQI-RSs corresponding to at least 8 antennas.

  Moreover, it is desirable that each CQI-RS be orthogonal to each antenna, and it is desirable that each CQI-RS can be extended to a plurality of cells. Then, since eight antenna orthogonalization is required per cell and it is necessary to support orthogonalization between cells, it is expected that the number of orthogonal reference signals increases for CQI-RS. Therefore, CQI-RS is time-division multiplexed with other symbols so as not to overlap other symbols to which signals other than CQI-RS are assigned.

  FIG. 13A shows an example in which CQI-RS is time-division multiplexed with other symbols and CQI-RS itself is frequency division multiplexed (FDM). In order to support 8 antennas, signals composed of 8 CQI-RSs (numerals 1 to 8) and orthogonal to each other are used.

  In LTE, a control channel is assigned to the first three symbols of each subframe. FIG. 13 (a) shows a case where one CRS is used in correspondence with virtualizing a plurality of antennas as one by using virtual antennas. The number (type) of CRS increases or decreases according to the number of antennas to be virtualized. Further, as described above, CRSs are equally arranged in radio resources. The DM-RS can be assigned to the same position because a user-specific reference signal is defined in LTE, for example. The symbol to which CQI-RS is assigned is not particularly limited as long as it does not overlap with other symbols, but the last symbol of the subframe can be used. By using the last symbol of the subframe for CQI-RS transmission, it is possible to minimize the influence on the already defined control channel, CRS and DM-RS defined this time.

  FIG. 13B shows CQI-RS multiplexing (TDM in symbol units + FDM in subcarrier units) shown in FIG. 13A, and other CQI-RSs in subcarriers to which each CQI-RS is assigned. An example of code division multiplexing is shown. Thus, by combining three types of multiplexing (TDM + FDM + CDM) with CQI-RS, it becomes possible to efficiently transmit increasing CQI-RS.

  FIG. 14 shows an example in which CQI-RS is assigned to a plurality of symbols and symbol-multiplexed. Eight CQI-RSs corresponding to eight antennas are assigned to the last symbol of the subframe, and eight CQI-RSs are assigned to other symbols to which CRS and DM-RS are not assigned.

  Further, a hybrid type in which eight CQI-RSs multiplexed into two symbols as shown in FIG. 14 are further code division multiplexed as shown in FIG. The symbol to be code division multiplexed may be two symbols or one symbol.

Next, DM-RS will be specifically described.
As described above, when virtual antennaization is applied to CRS, CRS cannot be used for demodulation for each antenna when the actual number of antennas is 4, 8, or more. Further, since a distributed antenna system requires demodulation for each distributed base station, a reference signal for each antenna is required. In addition, since LTE-A needs to support multi-stream, it is necessary to determine the density of DM-RS in consideration of multi-stream. In addition, when extending to multi-stream, it is necessary to ensure orthogonality between streams.

  Therefore, apart from the CRS applied to the virtual antenna, the demodulation for each antenna can be performed using DM-RS defined for each antenna and each cell.

  Since the reference signal used for demodulating the common data channel is defined in Rel8 LTE as in DM-RS, the basic structure of DM-RS is the same as the user-specific reference signal defined in Rel8 LTE. .

  FIG. 15 shows DM-RSs assigned in the same manner as user-specific reference signals defined in Rel8 LTE. The first 3 symbols of one subframe are allocated to the control channel, and the remaining areas do not overlap with the CRS, so that there are 12 symbols in total in the 4th, 7th, 10th, and 13th symbols. DM-RS is frequency-multiplexed for each symbol in the resource element.

  First, the DM-RS density optimum for data transmission with a single stream will be described. Also in this case, the density of the DM-RS is determined in consideration of the case where the multi-stream is extended.

  FIG. 16 (a) shows an example in which DM-RSs are multiplexed at a density of 16 resource elements per resource block (1 subframe). FIG. 16B shows an example in which DM-RSs are multiplexed at a density of 12 resource elements per 1 resource block (1 subframe). FIG. 17 shows an example in which DM-RSs are multiplexed at a density of 8 resource elements per 1 resource block (1 subframe).

  In FIGS. 16 (a), (b), and 17, the DM-RS density is different, but the symbol positions to which DM-RSs are assigned are the same in both cases, and the fourth symbol, the seventh symbol, and the tenth symbol. Eyes and 13th symbol. It is also common with user-specific reference signals defined in Rel8 LTE. Also, the arrangement of DM-RSs assigned to each symbol in the symbols is arranged so as to be evenly distributed in the frequency direction. Also, as shown in FIGS. 16B and 17, it is also desirable from the viewpoint of equalization to arrange symbols so that mapping positions in the frequency direction do not overlap between symbols.

  As described above, for a single stream, a DM-RS allocated to a DM-RS is shared (shared with a user-specific reference signal defined in Rel8 LTE) and arranged in one resource block (one subframe). It is desirable to be able to optimize the density by allowing the density of the to be changed.

Next, DM-RS density optimum for data transmission in multi-stream will be described.
FIGS. 18A and 18B show DM-RS arrangements that are optimal for data transmission in multi-streams. FIG. 18A is an example in which DM-RSs of the first stream # 1 and the second stream # 2 are multiplexed at a density of 24 resource elements per resource block (one subframe). FIG. 18B shows an example in which DM-RSs of the first stream # 1 and the second stream # 2 are multiplexed at a density of 16 resource elements per resource block (one subframe). . Even if the densities are different, the DM-RS of the first stream # 1 and the DM-RS of the second stream # 2 are assigned to a common symbol. The symbol positions are the fourth symbol, the seventh symbol, the tenth symbol, and the thirteenth symbol. It is also common with user-specific reference signals defined in Rel8 LTE. Moreover, the arrangement | positioning within the symbol of each DM-RS of the different stream allocated to the same symbol is arrange | positioned so that it may disperse | distribute equally in a frequency direction.

  As described above, for multi-streams, DM-RSs allocated to DM-RSs are made common (common to user-specific reference signals defined in Rel8 LTE) and arranged in one resource block (one subframe). It is desirable that the density be optimized according to the number of transmission data streams so that the density can be optimized.

  Furthermore, in the case of multi-stream, DM-RSs are transmitted orthogonally between streams. As an orthogonalization technique between DM-RS streams, FDM, CDM, and combinations thereof can be used.

  FIGS. 19A and 19B show the concept of orthogonalization between DM-RS streams by FDM. FIGS. 19A and 19B are examples in which the DM-RS multi-stream (# 1, # 2) transmission shown in FIG. 18B is orthogonalized by FDM. FIG. 19A shows the DM-RS structure of the first multi-stream # 1, and the resource element indicated by “x” indicates that the signal of the first stream is not transmitted. FIG. 19B shows the DM-RS structure of the second multi-stream # 2, and the resource element indicated by “x” indicates that the signal of the second stream is not transmitted.

  The first stream # 1 and the second stream # 2 are assigned to the common symbols (fourth symbol, seventh symbol, tenth symbol, thirteenth symbol), and the DM- of the first stream # 1 in each common symbol. In the subcarrier that transmits the RS, the DM-RS is not transmitted in the second stream # 2.

  Thereby, when transmitting a downlink reference signal (DM-RS) in multi-stream transmission, signal transmission is not performed in the other stream in the same subcarrier of the same symbol transmitting DM-RS in one stream. Therefore, interference between streams does not occur, and very high orthogonality can be realized.

  FIGS. 20A and 20B show the concept of orthogonalization between DM-RS streams by CDM. FIG. 20A shows the arrangement of the DM-RS of the first stream # 1 and a two-dimensional orthogonal code for encoding the DM-RS. The DM-RSs of the first stream # 1 shown in FIG. 20A are equally arranged at a density of 16 resource elements in one resource block (one subframe). A two-dimensional Walsh code can be used as a two-dimensional orthogonal code used for encoding the DM-RS of the first stream # 1. The two-dimensional Walsh code shown in FIG. 20A is a 4 × 4 Walsh code in accordance with the DM-RS structure, and all the coefficients are set to “+1” as shown in FIG. That is, it means that the DM-RS of the first stream # 1 multiplied by the two-dimensional Walsh code shown in FIG.

  On the other hand, FIG. 20B shows the arrangement of the DM-RS of the second stream # 2 and a two-dimensional orthogonal code for encoding the DM-RS. The DM-RS of the second stream # 2 illustrated in FIG. 20B has the same density as the first stream # 1, and is arranged in the same resource element as the first stream # 1. A two-dimensional Walsh code having the same size as the first stream # 1 is used as a two-dimensional orthogonal code used for encoding the DM-RS of the second stream # 2, but the time axis direction and the frequency axis direction as shown in FIG. In the configuration, “+1” and “−1” are alternately set. That is, when the first stream # 1 and the second stream # 2 encoded using the two-dimensional Walsh code that is the orthogonal code shown in FIGS. 20A and 20B are added in the time axis direction or the frequency axis direction. Since the other stream signal disappears, no interference occurs between the streams, and a very high orthogonality can be realized.

  In this way, by performing code division multiplexing between a plurality of streams, DM-RSs of a plurality of streams can be placed on the same resource element (same subcarrier of the same symbol) in a radio resource, so that the DM of each stream -The density of RS can be increased. As a result, it is possible to follow fading fluctuations in the time axis direction and channel fluctuations in the frequency axis direction at high speed, and always achieve high reception quality.

  20A and 20B show an example in which a two-dimensional Walsh code is used as the two-dimensional orthogonal code, but other two-dimensional orthogonal codes can be equally applied.

  Next, a radio communication method using the downlink reference signals (CRS, CQI-RS, DM-RS) and an embodiment of a radio base station apparatus and a radio terminal to which such a radio communication method is applied will be described. Hereinafter, a radio access system targeting LTE and LTE-A will be described as an example, but application to other systems is not limited.

  FIG. 1 is a functional block diagram of a base station apparatus, which mainly shows a transmission function configuration of a baseband processing unit. FIG. 2 is a functional block diagram of the mobile station, and mainly shows the reception function configuration of the baseband processing unit. Before describing the functions of the base station apparatus and the mobile station, a mobile communication system having the mobile station and the base station apparatus will be described with reference to FIG.

The mobile communication system 1000 is based on the LTE system, and a radio communication method using CRS, CQI-RS, and DM-RS as downlink reference signals is applied. The mobile communication system 1000 includes a base station device 200 and a plurality of mobile stations 100 (100 ₁ , 100 ₂ , 100 ₃ ,... 100n, n is an integer of n> 0) communicating with the base station device 200. . Base station apparatus 200 is connected to an upper station, for example, access gateway apparatus 300, and access gateway apparatus 300 is connected to core network 400. The mobile station 100n communicates with the base station apparatus 200 in the cell 50 by LTE. The access gateway device 300 may be called MME / SGW (Mobility Management Entity / Serving Gateway).

Since each mobile station (100 ₁ , 100 ₂ , 100 ₃ ,... 100n) has the same configuration, function, and state, the following description will be given as the mobile station 100n unless otherwise specified. For convenience of explanation, the mobile station communicates with the base station apparatus wirelessly, but more generally, a user apparatus (UE: User Equipment) including both a mobile terminal and a fixed terminal may be used.

  In the radio communication system 1000, OFDMA (Orthogonal Frequency Division Multiple Access) is applied for the downlink and SC-FDMA (Single Carrier Frequency Division Multiple Access) is applied for the uplink as the radio access scheme. As described above, OFDMA is a multi-carrier transmission scheme that performs communication by dividing a frequency band into a plurality of narrow frequency bands (subcarriers) and mapping data to each subcarrier. SC-FDMA is a single carrier transmission scheme that reduces interference between terminals by dividing a system band into bands each consisting of one or continuous resource blocks for each terminal, and a plurality of terminals using different bands. .

Here, a communication channel in the LTE system will be described.
For the downlink, a reference signal for transmitting CRS, CQI-RS, and DM-RS, which are newly defined reference signals this time, a physical downlink shared channel (PDSCH) shared by each mobile station 100n, and a physical A downlink control channel (downlink L1 / L2 control channel) is used. CRS, CQI-RS, and DM-RS are transmitted by the reference signal by applying the above-described multiplexing method. A user data signal is transmitted through the physical downlink shared channel. With the physical downlink control channel, DM-RS sequence information, scheduling information, user ID that communicates using the physical downlink shared channel, and information on the transport format of the user data, that is, downlink scheduling information, and physical The user ID that performs communication using the uplink shared channel and the information on the transport format of the user data, that is, the uplink scheduling grant, etc. are notified. Specifically, the DM-RS sequence information is defined as an index from the stream 1 to the stream 8 when the DM-RS is defined. When single stream transmission is applied, which index is used, the PDCCH or The mobile station is notified by higher layer signaling. When multi-stream transmission is applied, a control signal notifies which index is used by other users multiplexed in the same resource block.

  In the downlink, broadcast channels such as Physical-Broadcast Channel (P-BCH) and Dynamic Broadcast Channel (D-BCH) are transmitted. The information transmitted by the P-BCH is a Master Information Block (MIB), and the information transmitted by the D-BCH is a System Information Block (SIB). The D-BCH is mapped to the PDSCH and transmitted from the base station apparatus 200 to the mobile station 100n.

  For the uplink, a physical uplink shared channel (PUSCH) shared by each mobile station 100 and a physical uplink control channel (PUCCH) that is an uplink control channel are used. . User data is transmitted through the physical uplink shared channel. The physical uplink control channel transmits precoding information for downlink MIMO transmission, delivery confirmation information for the downlink shared channel, downlink radio quality information (CQI: Channel Quality Indicator), and the like.

  In the uplink, a physical random access channel (PRACH) for initial connection or the like is defined. The mobile station 100 transmits a random access preamble on the PRACH.

Next, a base station apparatus 200 according to an embodiment of the present invention will be described with reference to FIG.
The base station apparatus 200 according to the present embodiment includes a plurality of transmission antennas # 1 to #N. By precoding and transmitting a CRS from each transmission antenna, the number of virtual antennas is smaller than the actual number of antennas. Can be sent. Here, for convenience of explanation, the actual number of antennas is assumed to be eight.

  The base station apparatus 200 generates downlink control information and CRS for each virtual antenna, downlink transmission data and DM-RS for each stream, CQI-RS for each transmission antenna, and for each transmission antenna. Are transmitted by downlink channel multiplexing.

  The base station apparatus 200 includes a CRS sequence generation unit 11 that generates a CRS corresponding to the number of virtual antennas, a downlink control information generation unit 12 that generates downlink control information, and a CRS and downlink control information generation generated by the CRS sequence generation unit 11. A multiplexing unit 13 that multiplexes the downlink control information generated by the unit 12 on radio resources (time resources and frequency resources) is provided.

  The CRS sequence generation unit 11 generates CRSs # 1 to # 8 that correspond to the actual antennas # 1 to # 8 on a one-to-one basis when not creating a virtual antenna. In addition, when a virtual antenna is formed, a CRS corresponding to the number of virtual antennas is generated. In this example, when the number of virtual antennas is “1”, CRS # 1 and # 2 for two antennas are transmitted by the virtual antenna, but if the number of CRSs corresponding to the number of virtual antennas is set to 1: 1, The overhead can be reduced to the maximum.

  As described above, the CRS sequence generation unit 11 can dynamically change the number of CRSs generated corresponding to the number of virtual antennas (minimum value is 0). The number of virtual antennas can be notified to the CRS sequence generation unit 11 from the upper layer.

  Further, the CRS sequence generation unit 11 has a CRS structure of “normal structure” (for example, FIG. 10A) and “low density structure” (for example, FIG. 10B) in accordance with an instruction from an upper layer. Switch to Then, in the multiplexing unit 13, the “normal structure” and the “low density structure” are time-division multiplexed and transmitted (for example, FIG. 11). The LTE-A terminal can receive and demodulate both the “normal structure” and the “low density structure”, but the LTE terminal cannot support the “low density structure”. In the LTE terminal, control information (for example, MBSFN subframe information) for recognizing that the “low density structure” is an unnecessary subframe is signaled from the downlink control information generation unit 12 to the LTE terminal.

  The downlink control information generation unit 12 generates downlink control information mainly transmitted on the PDCCH. The downlink control information includes PDSCH and PUSCH scheduling information indicating the subcarrier position allocated by the scheduler, the modulation method, the channel coding rate, format information such as precoding information, the DM-RS sequence information, and “low When the “density structure” is time-division multiplexed, it can include control information for recognizing that the “low density structure” is a subframe that is not required to be captured.

  The precoding information is used for precoding information used for precoding CRS for virtual antenna conversion and for simultaneously receiving streams (layers) transmitted at the same time orthogonally on the receiver side. Distinguished from precoding information. When virtual antennaization is applied, these two types of precoding information are included in downlink control information.

  The precoding unit 14 assigns a weight for virtual antenna formation to each transmission antenna, and transmits a signal in which CRS and downlink control information are multiplexed. The number of virtual antennas is adjusted by the weighting for virtual antennas that the precoding unit 14 gives to each transmission antenna. When CRS transmission is performed using the CRS structure shown in FIGS. 10A and 10B, two CRSs are precoded from each transmission antenna and transmitted.

  As a result, when it is assumed that the number of virtual antennas is “1”, two CRSs that are twice the number of virtual antennas are transmitted even in the case of an 8-transmit antenna configuration. Thus, the overhead can be reduced as compared with the case where four CRSs are multiplexed and transmitted corresponding to all four transmission antennas, and the overhead reduction effect is further enhanced in the case of eight transmission antennas.

  Even when the number of virtual antennas is assumed to be “1”, by transmitting CRS for two antennas, an LTE terminal that supports two antennas can transmit diversity effect by transmitting two antennas for the control channel. Is obtained.

  In addition, the base station apparatus 200 includes a CRI-RS sequence generation unit 15 that generates a CQI-RS for CSI measurement for each transmission antenna, and an antenna between antennas that orthogonalizes the CRI-RS generated for each transmission antenna. And an orthogonalizing unit 16. Since the inter-antenna orthogonalizing unit 16 generates CQI-RSs for each antenna corresponding to the eight transmitting antennas from the CRI-RS sequence generating unit 15, multiplexing is performed for orthogonalizing the eight antennas.

  For example, each CQI-RS is allocated to different subcarriers in the same resource block in the last one symbol of each subframe (for example, FDM shown in FIG. 13A). Also, when CRI-RS increases to support inter-cell orthogonalization, each CQI-RS is assigned to different subcarriers in the same resource block in the last one symbol of each subframe, and each CQI -The resource element to which the RS is assigned is code division multiplexed (for example, CDM shown in FIG. 13B). Also, each CQI-RS is allocated to different subcarriers in the same resource block in a plurality of symbols in the same subframe (for example, FDM shown in FIG. 14).

  In this way, as a downlink reference signal, a CQI-RS is generated and transmitted for each actual transmission antenna separately from the CRS that is made into a virtual antenna, so that it is made into a virtual antenna in the LTE terminal and the LTE-A terminal. Even when signals are transmitted, CSI measurement for each antenna is possible, and channel quality can be measured.

  In addition, it is possible to cope with an increase in the number of CQI-RSs by preparing various multiplexing methods in consideration of orthogonalization between antennas and orthogonalization between cells.

  Also, the base station apparatus 200 includes a DM-RS sequence generation unit 18 that generates a DM-RS for each data stream, and an inter-stream orthogonalization that performs orthogonalization between streams when generating a multi-stream DM-RS. Part 19.

  The DM-RS sequence generation unit 18 generates a DM-RS for user-specific PDSCH demodulation, and the DM-RS density per resource block (subframe) is optimized for the DM-RS. For this reason, the DM-RS density per resource block (subframe) can be changed to several density patterns (for example, FIGS. 16A, 16B, and 17). Common symbols (for example, 4th symbol, 7th symbol, 10th symbol, 13th symbol) are used as symbols for multiplexing the RS.

  In addition, the DM-RS sequence generation unit 18 multiplexes DM-RSs for different streams into common symbols, but assigns them to different subcarriers (for example, FIGS. 18A and 18B).

  The inter-stream orthogonalization unit 19 adds multiplexing for orthogonalizing the streams to the DM-RS for which the DM-RS density is optimized by the DM-RS sequence generation unit 18.

  As shown in FIGS. 19A and 19B, the first stream # 1 and the second stream # 2 are common symbols (fourth symbol, seventh symbol, tenth symbol, thirteenth symbol). While the DM-RS is allocated, the sub-carrier that transmits the DM-RS of the first stream # 1 in each common symbol is configured not to transmit the DM-RS in the second stream # 2.

  Thereby, when transmitting DM-RS in multi-stream transmission, signal transmission is not performed in the other stream in the same subcarrier of the same symbol transmitting DM-RS in one stream. Interference is not generated, and extremely high orthogonality can be realized.

  The other is, as shown in FIGS. 20 (a) and 20 (b), the DM-RS of the first stream # 1 is multiplied by a two-dimensional Walsh code whose coefficients are all set to “+1”. The 2-stream # 2 DM-RS is encoded by multiplying two-dimensional Walsh codes in which “+1” and “−1” are alternately set in the time axis direction and the frequency axis direction.

  Note that the DM-RS of the second stream # 2 has the same density as the first stream # 1, and is arranged in the same resource element as the first stream # 1. The two-dimensional Walsh code can be signaled to the mobile station by being included in the DM-RS sequence information. Alternatively, it may be set as known information in advance in the mobile station.

  As described above, the first stream # 1 and the second stream # 2 encoded using the two-dimensional Walsh code, which is the orthogonal code shown in FIGS. 20A and 20B, are arranged in the time axis direction or the frequency axis direction. When added, the signal of the other stream disappears, so that no interference occurs between the streams, and very high orthogonality can be realized.

  The base station apparatus 200 includes a downlink transmission data generating unit 22 that generates downlink transmission data for the mobile station, and a downlink transmission data encoding / modulating unit 23 that encodes and modulates the downlink transmission data. The downlink transmission data encoding / modulating unit 23 modulates and outputs the data by error correction encoding and a predetermined data modulation method. The downlink transmission data generation unit 22 and the downlink transmission data encoding / modulation unit 23 are provided for each stream.

  The multiplexing unit 21 multiplexes downlink transmission data and DM-RS for each stream and outputs the multiplexed data to the precoding unit 24. The precoding unit 24 performs weighting for each antenna so that the streams (layers) transmitted at the same time are orthogonally received on the receiver side (normal precoding for MIMO transmission).

  In the downlink channel multiplexing unit 25 of the base station apparatus 200, a reference signal for transmitting a CRS that is made into a virtual antenna, a CQI-RS generated for each transmission antenna, and a DM-RS generated for each stream, and downlink control A PDCCH that transmits information, a PDSCH that transmits downlink transmission data, and other necessary downlink channels are multiplexed. The channel-multiplexed signal is subjected to inverse fast Fourier transform by the inverse fast Fourier transform unit 26 to be converted into a signal in the time domain, and a CP prefixing unit 27 is provided with a cyclic prefix serving as a guard interval for preventing intersymbol interference. Thereafter, it is amplified by the transmission amplifier 28 and transmitted.

  The transmission process as described above is performed for each transmission antenna. However, as described above, downlink control information and CRS are generated in units of virtual antennas, and downlink transmission data and DM-RS are generated in units of streams.

A mobile station 100 according to an embodiment of the present invention will be described with reference to FIG.
The reception processing system of the mobile station 100 receives a signal including a downlink reference signal configured by CRS, CQI-RS, and DM-RS as described above. After the CRS, CQI-RS, and DM-RS are separated from the received signal, the CRS is used for channel estimation of the shared / dedicated control channel in units of virtual antennas, and the CQI-RS is used to measure the channel quality for each actual transmission antenna. DM-RS is used for channel estimation in units of streams.

  In the reception processing system, the received signal is input to the CP removing unit 31 to remove the cyclic prefix. The fast Fourier transform unit 32 performs fast Fourier transform on the received signal from which CP has been removed, and converts a time-series signal component into a sequence of frequency components. The downlink channel separation unit 33 performs subcarrier demapping on the received signal, and transmits a reference signal for transmitting an RS sequence signal, a control channel (for example, PHICH, PDCCH) for transmitting downlink control information, and transmission data. A shared channel (for example, PDSCH) that is being transmitted is separated.

  Of the separated received reference signal symbols, the CRS is input to the CRS channel estimator 36. The PDCCH (or PDSCH) is input to the downlink control information demodulation / decoding unit 37.

  The CRS channel estimation unit 36 performs channel estimation on the PDCCH (or PDSCH) transmitted from the virtual antenna based on the received CRS information. The downlink control information demodulation / decoding unit 37 demodulates and decodes the downlink control information based on the CRS information. The DM-RS sequence information for each stream transmitted by the PDCCH is passed to the DM-RS channel estimation unit 38 for channel estimation of the corresponding stream.

  In addition, CQI-RSs of received reference signal symbols are input to CQI / PMI estimation units 34 of the corresponding antennas (or cells), respectively. The CQI / PMI estimation unit 34 measures the CSI for each antenna using the CQI-RS corresponding to each antenna, estimates the channel quality according to the CSI measurement result, and outputs the CQI measurement value to the feedback information generation unit 35. Output.

  As described above, the mobile station 100 can perform CQI measurement for each actual transmission antenna even when data is transmitted with a smaller number of virtual antennas than the actual number of transmission antennas due to the virtual antenna. Can be returned to the base station apparatus 200 as feedback information.

  In addition, DM-RSs of received symbols of reference signals are input to corresponding DM-RS channel estimation units 38, respectively. The PDSCH is input to the downlink transmission data demodulation / decoding unit 39. The DM-RS channel estimation unit 38 acquires the DM-RS of the corresponding stream using the DM-RS sequence information obtained by decoding the PDCCH (or PDSCH), and uses the DM-RS for the channel for the stream. presume. The downlink transmission data demodulation / decoding unit 39 demodulates and decodes the downlink transmission data based on the channel estimation.

  As described above, in the mobile station 100, even when data is transmitted with a virtual antenna number smaller than the actual number of transmission antennas due to virtual antennaization, DM-RS is acquired for each stream and PDSCH demodulation is performed. It becomes possible.

  The present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention.

  The present invention is applicable to a wireless communication system.

11 CRS sequence generator 12 Downlink control information generator 13 Multiplexer 14 Precoding unit (for virtual antenna)
15 CQI-RS sequence generation unit 16 Inter-antenna orthogonal unit 18 DM-RS sequence generation unit 19 Inter-stream orthogonal unit 21 Multiplexing unit 22 Downlink transmission data generation unit 23 Downlink transmission data encoding / modulation unit 24 Precoding unit 25 Downlink channel multiplexing unit 33 Downlink channel separation unit 34 CQI / PMI estimation unit 35 Feedback information generation unit 36 CRS channel estimation unit 37 Downlink control information demodulation / decoding unit 38 DM-RS channel estimation unit 39 Downlink transmission data demodulation / decoding unit

Claims (10)

Multiple transmit antennas,
An orthogonalization unit that orthogonalizes between layers using orthogonal codes for downlink reference signals arranged in a plurality of radio resources in the time direction at the same frequency;
A transmission unit for transmitting a signal obtained by multiplexing the downlink reference signal and transmission data from the plurality of transmission antennas in a multilayer;
A radio base station apparatus comprising:   The radio base station apparatus according to claim 1, wherein the downlink reference signal sequence information and the orthogonal code information are signaled to a mobile station apparatus.   The radio base station apparatus according to claim 1 or 2, wherein the downlink reference signal is defined for each antenna.   The radio base station apparatus according to any one of claims 1 to 3, wherein the density of the downlink reference signal is the same between layers. A receiving unit that is arranged in a plurality of radio resources in the time direction at the same frequency and receives a signal obtained by multiplexing downlink reference signals and transmission data that are orthogonalized between layers using orthogonal codes;
A data demodulator that demodulates the transmission data using the downlink reference signal;
A mobile station apparatus comprising:   The mobile station apparatus according to claim 5, wherein the downlink reference signal sequence information and the orthogonal code information are signaled from a radio base station apparatus.   The mobile station apparatus according to claim 5 or 6, wherein the downlink reference signal is defined for each antenna.   The mobile station apparatus according to any one of claims 5 to 7, wherein the density of the downlink reference signal is the same between layers. In the radio base station apparatus, a step of orthogonalizing between downlink layers using orthogonal codes for downlink reference signals arranged in a plurality of radio resources in the time direction at the same frequency, and multiplexing the downlink reference signal and transmission data A signal transmitted in multiple layers from the plurality of transmission antennas,
A mobile station apparatus, comprising: a step of receiving the signal; and a step of demodulating the transmission data using the downlink reference signal of the signal. An orthogonalization unit that performs orthogonalization between layers using orthogonal codes for downlink reference signals arranged in a plurality of radio resources in the time direction at the same frequency, and a signal obtained by multiplexing the downlink reference signal and transmission data A radio base station apparatus having a transmission unit that transmits in multiple layers from a plurality of transmission antennas;
A mobile station apparatus comprising: a reception unit that receives the signal; and a data demodulation unit that demodulates the transmission data using the downlink reference signal of the signal;
A wireless communication system comprising:

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