Patent Application
20160381583
The present invention relates to a radio base station, a user terminal and a radio communications method in a next-generation mobile communications system.
In the UMTS (Universal Mobile Telecommunications System) network, long-term evolution (LTE) is under study for the purposes of further increasing high-speed data rates, providing lower delays and so on (see, for example, non-patent literature 1). In LTE, as multiple-access schemes, a scheme that is based on OFDMA (Orthogonal Frequency Division Multiple Access) is used for the downlink, and a scheme that is based on SC-FDMA (Single Carrier Frequency Division Multiple Access) is used for the uplink.
Successor systems of LTE—referred to as, for example, “LTE-advanced” or “LTE enhancement”—have been under study for the purpose of achieving further broadbandization and increased speed beyond LTE, and the specifications thereof have been drafted as LTE Rel. 10/11 (LTE-A). To cope with the growing number of subscribers and the growing traffic per user, in LTE and LTE-A, MIMO (Multiple-Input Multiple-Output) multiplexing technique is under study as a radio communication technique to achieve improved cell throughput and spectral efficiency by transmitting and receiving data with a plurality of antennas.
In LTE-A, MIMO multiplexing technique to use maximum eight antennas is stipulated. In MIMO multiplexing, a base station transmits transmitting-antenna-specific orthogonal reference signals (RSs) for measuring CSI (Channel State Information), and a user terminal measures each transmitting antenna's CSI. When the number of transmitting antennas increases, the number of reference signals for measuring CSI also increases, and therefore the resources for transmitting information symbols run short.
The present invention has been made in view of the above, and it is therefore an object of the present invention to provide a radio base station, a user terminal and a radio communication method which can reduce the overhead of reference signals for measuring CSI in high-order MIMO multiplexing technique.
The radio base station of the present invention is a radio base station that is used in a radio communications system of a frequency division duplexing (FDD) scheme, and that has a receiving section that receives reference signals for measuring time division duplex (TDD) channel state information, transmitted from a plurality of antennas provided in a user terminal, a measurement section that measures the channel state information, in a plurality of receiving antennas, by using the reference signals, a generation/selection section that generates an optimal precoding vector from the channel state information measured in each receiving antenna, or selects the optimal precoding vector from a set of precoding vectors that is defined in advance, and a transmission section that transmits a physical downlink shared channel, in MIMO multiplexing transmission, by using the precoding vector selected in the generation/selection section.
According to the present invention, it is possible to reduce the overhead of reference signals for measuring CSI in high-order MIMO multiplexing technique.
Now, an embodiment of the present invention will be described below in detail with reference to the accompanying drawings. As duplex modes in radio communication in LTE and LTE-A systems, there are frequency division duplex (FDD) to divide between the uplink (UL) and the downlink (DL) based on frequency, and time division duplex (TDD) to divide between the uplink and the downlink based on time.
An advantage of the FDD scheme is that, since timing synchronization between base stations is not necessary, radio resources can be allocated in the uplink or the downlink, independently, per cell, depending on traffic, in a cellular-based multi-cell environment. A disadvantage of the FDD scheme is that the uplink and the downlink require independent frequency bands—that is, paired bands.
An advantage of the TDD scheme is that paired bands are not necessary and the reciprocity of channels can be used. Consequently, the TDD scheme is effective in frequency bands where paired bands cannot be reserved. A disadvantage of the TDD scheme is that timing synchronization is required between cells in a cellular-based multi-cell environment.
A user terminal measures each transmitting antenna's CSI, selects the precoding vector that maximizes the received SNR from a set of precoding vectors (codebook) that is defined in advance, and reports this to the base station. Although there are methods of calculating optimal precoding vectors, apart from code book-based precoding, the precoding vector information to feed back to the base station increases. Consequently, in LTE and LTE-A, code book-based precoding is employed. Meanwhile, the base station needs to transmit antenna-specific CSI measurement reference signals in order to allow the user terminal to measure the receiving level from all transmitting antennas. For the transmitting-antenna-specific reference signals, cell-specific reference signals are defined for up to four transmitting antennas, and CSI-RSs are defined for five to eight transmitting antennas.
In the MIMO system shown in
Receiving antennas Rx1 to Rx4 in the user terminal, provided as a receiving section, receive the signals transmitted from one or more transmitting antennas via MIMO propagation paths. The signals received in each receiving antenna are separated into received signals that correspond to respective streams, via a channel estimation section and a signal demultiplexing section. The received signals pertaining to each stream are converted through a parallel-to-serial converter (P/S), providing decoded bits.
In the MIMO system, rank adaptation, which controls the number of transmission streams (rank) depending on the magnitude of the eigenvalues of the channel matrix generated based on the channel response between the transmitting and the receiving antennas.
The precoding vector selection section determines the channel response in each receiving antenna in the event precoding vectors are transmitted from a codebook, which is a set of precoding vectors that is defined in advance—that is, in the event the transmission signals are multiplexed by a precoding matrix—from the channel responses that are estimated using transmitting-antenna-specific reference signals included in the received signals in each receiving antenna. The precoding vector selection section measures the received signal power and the noise power from each receiving antenna's channel response, and calculates the desired signal power-to-noise power ratio (SNR). A precoding vector selection section averages the received SNRs between the receiving antennas, and finds the average received SNR for each precoding vector. Then, the precoding vector selection section selects the precoding vector that maximizes the average received SNR as an optimal precoding vector.
Although the maximum number of transmitting antennas in existing LTE-A systems is eight, CSI-RSs, which are different from cell-specific reference signals (CS-RSs) are defined for antenna ports 5 to 8, so that it is no longer necessary to multiplex reference signals for measuring CSI on all resource blocks. However, in resource blocks for a user that carries out MIMO multiplexing transmission with eight transmitting antennas, CSI-RSs for eight antennas need to be multiplexed. The problem then arises that, if, in the future, the number of transmitting antennas increases even more, the number of reference signals for measuring CSI also increases, and the resources for transmitting information symbols run short.
So, the present inventors have found out measuring CSI by using carrier frequency swapping in high-order MIMO multiplexing technique. By this means, it is possible to reduce the overhead of reference signals for measuring CSI in high-order MIMO multiplexing technique. Now, an embodiment of the present invention will be described below in detail
Now, existing CSI measurement and MIMO multiplexing transmission will be described using
Following this, CSI measurement and MIMO multiplexing transmission to use carrier frequencies according to an embodiment of the present invention will be described with reference to
First, a user terminal transmits TDD CSI-RSs or sounding reference signals, in the downlink carrier frequency (fDL), by using one or a plurality of FFT blocks in an uplink subframe. Presuming PDSCH transmission on the downlink, the base station measures channel response in the frequency domain, in a plurality of receiving antennas, by using the CSI-RSs. The CSI-RSs are transmitted in the downlink carrier frequency, so that the reciprocity of propagation channels can be used. The base station transmits an optimal precoding vector from the CSIs measured per receiving antenna, and transmits the downlink PDSCH using the selected precoding vector.
In the example shown in
In the example shown in
Note that it is also possible to multiplex uplink control information in the transmission period in the uplink frequency spectrum region where the CSI-RS is transmitted using the downlink carrier frequency (fDL). In FFT blocks apart from one FFT block at the top, uplink user information and control information, and downlink user information and control information are allocated to radio resources.
Next, an example of a CSI-RS multiplexing method using carrier frequency swapping in MIMO multiplexing technique will be described. Distributed FDMA is one such example, and orthogonal CDMA is another. In
In distributed FDMA, as shown in
In orthogonal CDMA, as shown in
Now, with the aid of
As shown in
The overhead of CSI-RS and CQI feedback in the conventional method and the proposed method will be shown in comparison. A structure will be assumed here where the antennas are used for both transmission and reception in common, and where the number of antennas in a base station is NBS and the number of antennas in a user terminal is NUE.
In single-user MIMO multiplexing, if NBS=NUE holds, the overhead of transmitting-antenna-specific orthogonal CSI-RSs does not vary between the proposed method and the conventional method. However, in the proposed method, CSI-RSs are transmitted on the uplink, so that the CQI overhead is reduced compared to the conventional method. Furthermore, since CSI is measured directly in the base station, it is possible to reduce the deterioration of the reliability of measurement due to the quantization of CQI feedback.
In multi-user (MU) MIMO multiplexing, if NBS>NUE holds, according to the proposed method, each user terminal has only to transmit NUE or an equivalent number of orthogonal CSI-RSs, so that, compared to the conventional method, the overhead of orthogonal CSI-RSs per user terminal can be reduced significantly. Also, in comparison to the conventional method, the proposed method is the same as SU-MIMO in that the overhead of CQI feedback can be reduced.
According to the conventional method, calibration to correct the deviation of phase or amplitude in the RF transmission section and receiving section circuitry in base stations is not necessary. The proposed method, on the other hand, requires this calibration.
Furthermore, the proposed method is different from the conventional method in that the number of resource elements that can be used in the main link in subframes decreases. By this means, with the proposed method, the number of reference signals that miss insertion can be made slightly less.
As described above, according to the proposed method of transmitting CSI-RSs by using carrier frequency swapping, the overhead of CSI-RS and CQI feedback can be reduced compared to the conventional method.
(Structure of Radio Communications System)
Now, a structure of a radio communications system according to the present embodiment will be described below. In this radio communications system, the above-described TDD CSI-RS transmission method to use carrier frequency swapping is employed.
The radio base stations 10 are radio base stations that have predetermined coverages. Note that a radio base station 10 may be a macro base station (also referred to as “eNodeB,” “macro base station,” “central node,” “transmission point,” “transmitting/receiving point,” etc.) to have a relatively wide coverage, or may be a small base station (also referred to as “small base station,” “pico base station,” “femto base station,” “HeNB” (Home eNodeB), “RRH” (Remote Radio Head), “micro base station,” “transmission point,” “transmitting/receiving point,” etc.) to have a local coverage.
The user terminals 20 are terminals to support various communication schemes such as LTE, LTE-A and so on, and may include both mobile communication terminals and stationary communication terminals. A user terminal 20 can communicate with other user terminals 20 via the radio base stations 10.
Note that the higher station apparatus 30 may be, for example, an access gateway apparatus, a radio network controller (RNC), a mobility management entity (MME) and so on, but is by no means limited to these.
Also, in the radio communications system 1, a downlink shared channel (PDSCH: Physical Downlink Shared Channel), which is used by each user terminal 20 on a shared basis, downlink control channels (PDCCH (Physical Downlink Control Channel), EPDCCH (Enhanced Physical Downlink Control Channel), etc.), a broadcast channel (PBCH) and so on are used as downlink channels. User data, higher layer control information and predetermined SIBs (System Information Blocks) are communicated by the PDSCH. Downlink control information (DCI) is communicated by the PDCCH and the EPDCCH.
In the radio communications system 1, an uplink shared channel (PUSCH: Physical Uplink Shared Channel), which is used by each user terminal 20 on a shared basis, and an uplink control channel (PDCCH: Physical Uplink Control Channel) and so on are used as uplink channels. User data and higher layer control information are communicated by the PUSCH.
User data to be transmitted from the radio base station 10 to a user terminal 20 on the downlink is input from the higher station apparatus 30, into the baseband signal processing section 104, via the interface section 106.
In the baseband signal processing section 104, a PDCP layer process, division and coupling of user data, RLC (Radio Link control) layer transmission processes such as an RLC retransmission control transmission process, MAC (Medium Access Control) retransmission control, including, for example, an HARQ transmission process, scheduling, transport format selection, channel coding, an inverse fast Fourier transform (IFFT) process and a pre-coding process are performed, and the result is forwarded to each transmitting/receiving section 103. Furthermore, downlink control signals are also subjected to transmission processes such as channel coding and an inverse fast Fourier transform, and are forwarded to each transmitting/receiving section 103.
Each transmitting/receiving section 103 converts a downlink signal, pre-coded and output from the baseband signal processing section 104 per antenna, into a radio frequency band. The amplifying sections 102 amplify the radio frequency signals having been subjected to frequency conversion, and transmit the signals through the transmitting/receiving antennas 101.
On the other hand, as for uplink signals, radio frequency signals that are received in the transmitting/receiving antennas 101 are each amplified in the amplifying sections 102, converted into the baseband signal through frequency conversion in each transmitting/receiving section 103, and input into the baseband signal processing section 104.
Each transmitting/receiving section 103 receives the TDD CSI-RSs that are transmitted from a plurality of antennas provide in the user terminal 20. Each transmitting/receiving section 103 transmits the downlink PDSCH in MIMO multiplexing transmission by using a selected precoding vector. The transmitting/receiving sections 103 apply MIMO multiplexing to transmission streams, the number of which is determined in a channel estimation section to be described later, and transmits PDSCHs in MIMO multiplexing transmission.
In the baseband signal processing section 104, the user data that is included in the input uplink signals is subjected to an FFT process, an IDFT process, error correction decoding, a MAC retransmission control receiving process, and RLC layer and PDCP layer receiving processes, and the result is forwarded to the higher station apparatus 30 via the interface section 106. The call processing section 105 performs call processing such as setting up and releasing communication channels, manages the state of the radio base station 10 and manages the radio resources.
The interface section 106 transmits and receives signals to and from neighboring radio base stations (backhaul signaling) via an inter-base station interface (for example, optical fiber, the X2 interface, etc.). Alternatively, the interface section 106 transmits and receives signals to and from the higher station apparatus 30 via a predetermined interface.
The control section 301 controls the scheduling of downlink user data that is transmitted in the PDSCH, downlink control information that is communicated in one or both of the PDCCH and the enhanced PDCCH (EPDCCH), downlink reference signals and so on. Also, the control section 301 controls the scheduling of RA preambles communicated in the PRACH, uplink data that is communicated in the PUSCH, uplink control information that is communicated in the PUCCH or the PUSCH, and uplink reference signals (allocation control). Information about the allocation control of uplink signals (uplink control signals, uplink user data, etc.) is reported to the user terminal 20 by using a downlink control signal (DCI).
The control section 301 controls the allocation of radio resources to downlink signals and uplink signals based on command information from the higher station apparatus 30, feedback information from each user terminal 20, and so on. That is, the control section 301 functions as a scheduler.
The downlink control signal generating section 302 generates downlink control signals (which may be both PDCCH signals and EPDCCH signals, or may be one of these) that are determined to be allocated by the control section 301. To be more specific, the downlink control signal generating section 302 generates a DL assignment, which reports downlink signal allocation information, and a UL grant, which reports uplink signal allocation information, based on commands from the control section 301.
The downlink data signal generating section 303 generates downlink data signals (PDSCH signals) that are determined to be allocated to resources by the control section 301. The data signals that are generated in the data signal generating section 303 are subjected to a channel coding process and a modulation process, based on channel coding rates and modulation schemes that are determined based on CSI from each user terminal 20 and so on.
The mapping section 304 controls the allocation of the downlink control signals generated in the downlink control signal generating section 302 and the downlink data signals generated in the downlink data signal generating section 303 to radio resources based on commands from the control section 301.
The demapping section 305 demaps an uplink signal transmitted from the user terminal 20 and separates the uplink signal. The channel estimation section 306 estimates channel states from the reference signals included in the received signals separated in the demapping section 305, and outputs the estimated channel states to the uplink control signal decoding section 307 and the uplink data signal decoding section 308. That is, the channel estimation section 306 functions as a measurement section that measures CSI by using the TDD CSI-RSs that are received. Also, the channel estimation section 306 calculates an optimal number of transmission streams from the CSIs measured per receiving antenna.
The uplink control signal decoding section 307 decodes the feedback signal (delivery acknowledgement signals and/or the like) transmitted from the user terminal in the uplink control channel (PRACH, PUCCH, etc.), and outputs the result to the control section 301. The uplink data signal decoding section 308 decodes the uplink data signal transmitted from the user terminal through the uplink shared channel (PUSCH), and outputs the result to the decision section 309. The decision section 309 makes a retransmission control decision (A/N decisions) based on the decoding result in the uplink data signal decoding section 308, and outputs result to the control section 301.
The generation/selection section 310 generates an optimal precoding vector from the CSI measured in each receiving antenna. Also, the generation/selection section 310 selects an optimal precoding vector from the codebook based on the CSI that is measured in each receiving antenna.
As for downlink data, radio frequency signals that are received in the plurality of transmitting/receiving antennas 201 are each amplified in the amplifying sections 202, and subjected to frequency conversion and converted into the baseband signal in the transmitting/receiving sections 203. This baseband signal is subjected to an FFT process, error correction decoding, a retransmission control receiving process and so on in the baseband signal processing section 204. In this downlink data, downlink user data is forwarded to the application section 205. The application section 205 performs processes related to higher layers above the physical layer and the MAC layer, and so on. Furthermore, in the downlink data, broadcast information is also forwarded to the application section 205.
Meanwhile, uplink user data is input from the application section 205 to the baseband signal processing section 204. In the baseband signal processing section 204, a retransmission control (H-ARQ (Hybrid ARQ)) transmission process, channel coding, precoding, a DFT process, an IFFT process and so on are performed, and the result is forwarded to each transmitting/receiving section 203. The transmitting/receiving section 203 convert the baseband signal output from the baseband signal processing section 204 into a radio frequency band. After that, the amplifying sections 202 amplify the radio frequency signal having been subjected to frequency conversion, and transmit the resulting signal from the transmitting/receiving antennas 201.
The transmitting/receiving sections 203 transmit TDD CSI-RSs in the downlink carrier frequency by using, for example, one or a plurality of FFT blocks in a subframe.
The control section 401 controls the generation of uplink control signals (A/N signals and so on) and uplink data signals based on downlink control signals (PDCCH signals) transmitted from the radio base stations, retransmission control decisions with respect to the PDSCH signals received, and so on. The downlink control signals received from the radio base station are output from the downlink control signal decoding section 408, and the retransmission control decisions are output from the decision section 410.
The uplink control signal generating section 402 generates uplink control signals (feedback signals such as delivery acknowledgement signals, channel state information (CSI) and so on) based on commands from the control section 401. The uplink data signal generating section 403 generates uplink data signals based on commands from the control section 401. Note that, when a UL grant is contained in a downlink control signal reported from the radio base station, the control section 401 commands the uplink data signal generating section 403 to generate an uplink data signal.
The mapping section 405 controls the allocation of the uplink control signals (delivery acknowledgment signals and so on) and the uplink data signals to radio resources (PUCCH, PUSCH, etc.) based on commands from the control section 401.
The demapping section 406 demaps the downlink signals transmitted from the radio base station 10 and separates the downlink signals. The channel estimation section 407 estimates channel states from the reference signals included in the received signals separated in the demapping section 406, and outputs the estimated channel states to the downlink control signal decoding section 408 and the downlink data signal decoding section 409.
The downlink control signal decoding section 408 decodes the downlink control signal (PDCCH signal) transmitted in the downlink control channel (PDCCH), and outputs the scheduling information (information regarding the allocation to uplink resources) to the control section 401. Also, when information related to the cell to feed back delivery acknowledgement signals or information as to whether or not to apply RF tuning is included in a downlink control signal, these pieces of information are also output to the control section 401.
The downlink data signal decoding section 409 decodes the downlink data signals transmitted in the downlink shared channel (PDSCH), and outputs the results to the decision section 410. The decision section 410 makes a retransmission control decision (A/N decision) based on the decoding result in the downlink data signal decoding section 409, and outputs the result to the control section 401.
The present invention is by no means limited to the above embodiment and can be implemented with various changes. The sizes and shapes illustrated in the accompanying drawings in relationship to the above embodiment are by no means limiting, and may be changed as appropriate within the scope of optimizing the effects of the present invention. Besides, implementations with various appropriate changes may be possible without departing from the scope of the object of the present invention.
This application is based on Japanese Patent Application No. 2014-038647, filed on Feb. 28, 2014, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-038647 | Feb 2014 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2015/055904 | 2/27/2015 | WO | 00 |
