WO2016067319A1

WO2016067319A1 - Communication system and method, base station, and user terminal

Info

Publication number: WO2016067319A1
Application number: PCT/JP2014/005470
Authority: WO
Inventors: Boonsarn Pitakdumrongkija; Naoto Ishii
Original assignee: Nec Corporation
Priority date: 2014-10-29
Filing date: 2014-10-29
Publication date: 2016-05-06
Also published as: JP6418334B2; JP2017538369A

Abstract

Disclosed is a method in a mobile communication system comprising a base station communicating with at least one user terminal via a radio link, wherein the base station is adapted to create a precoding matrix for applying to data transmitted to the user terminal by using a channel estimate determined from Uplink Reference Signal transmitted from the user terminal to the base station, the method comprising: the base station notifying to the user terminal the precoding matrix, and the user terminal estimating and reporting a Signal to Interference pluses Noise Ratio (SINR) for each data layer conditioned on the notified precoding matrix.

Description

COMMUNICATION SYSTEM AND METHOD, BASE STATION, AND USER TERMINAL

The present invention relates to a mobile communication system and method, a base station, and a user terminal.

Background

Recently, exploding growth of mobile data communication traffic due to proliferation of smart phones and other smart devices has accelerated mobile network operators to deploy or consider deploying various technologies to increase network capacity. One of key enabling technologies is MIMO (Multi-Inputs and Multi-Outputs) technology.

Specifically, MIMO technology has already been standardized in Third Generation Partnership Project (3GPP) Long-Term Evolution (LTE) and LTE-Advanced standards. MIMO technology comprises two sub-categories, namely, Single-User-MIMO (SU-MIMO) and Multi-User MIMO (MU-MIMO). SU-MIMO technology enables transmission of multiple layers of data on the same time and frequency resource between a base station and a single user terminal. On the other hand, MU-MIMO technology, while offering the same benefit as SU-MIMO, can additionally enable data transmission on the same time and frequency resource between a base station and multiple user terminals. Therefore, in order to maximize network capacity, mobile network operators worldwide are considering deployment of MU-MIMO technology.

MU-MIMO is the most effective when channels from a base station to all user terminals can be precisely known. For a downlink communication direction (from a base station to user terminals), which dominates most of mobile data communication traffic, this can be realized easily especially in Time-Division Duplexing (TDD) system comprising a base station configured to exploit channel reciprocity property between channels in the downlink direction and channels in an reverse (uplink) direction.

Fig. 1 illustrates a typical example of a system in a related art in which channel reciprocity between one base station and one user terminal is exploited by a base station.

Specifically, referring to Fig.1, a user terminal 20R transmits Uplink Reference Signal (Uplink RS, which is equivalent to Sounding Reference Signal (SRS) in LTE system) from M antennas 21R (operation S11).

A base station 10R receives from the user terminal 20R Uplink RS transmitted from each of the user terminal's antennas 21R (operation S12).

Then, the base station 10R estimates channel from every user terminal's antenna 21R to every base station's antenna 11R to obtain Uplink channel matrix H_U(operation S13). H_U is an N_TxM impulse response matrix, where N_T is the number of antennas 11R of the base station 10R and M is the number of antennas 21R of the user terminal 20R.

Finally, the base station 10R invokes channel reciprocity property to obtain Downlink channel matrix H_D by using the Uplink channel matrix H_U,
H_D = H^H _U,
where H_D is an MxN_T impulse response matrix and the superscript H denotes complex conjugate transpose or Hermitian transpose (operation S14).

Once the base station 10R obtains all Downlink channel matrices to all user terminals, the base station 10R can create a precoding matrix (hereinafter also referred to as "Base-station-created precoding matrix") for each user terminal. Data of one user terminal with the precoding matrix applied thereto is made not to interfere with data of other user terminals. There are several methods to create the precoding matrix.

Fig. 2 is a diagram for illustrating an example of Block-Diagonalization (BD) precoding method described in NPL1. Fig. 2 illustrates a case where the base station 10R creates two Base-station-created precoding matrices F₁ and F₂ for User terminals 1 and 2 (12-1 and 12-2), respectively, when respective Downlink channel matrices H₁ and H₂ to

User terminals

1 and 2 are known at the base station 10R.

To ensure that data of one user terminal do not interfere with those of the other user terminal, the base station 10R creates the Base-station-created precoding matrix for the user terminal (F₁, F₂) that is equivalent to a null-space matrix of the other user terminal's channel (V₂ ⁽ⁿ⁾, V₁ ⁽ⁿ⁾). That is,
F₁ = V₂ ⁽ⁿ⁾, and F₂ = V₁ ⁽ⁿ⁾,
where V_i ⁽ⁿ⁾ (i=1,2) each forms orthogonal basis for null space of H_i (channel matrix from base station to i-th user terminal).

More specifically, assuming that the number of antennas 11R of the base station 10R is N_T, the number of antennas (21R-i) (i=1, 2) of i-th user terminal (20R-i) is N_ri(=M), with N_r=N_r1+N_r2(=2M), and H_i is a Downlink channel matrix (N_rixN_T impulse response matrix with a rank L_i), H_i is decomposed using SVD(singular value decomposition),
H_i = U_iD_i [V_i ^(s) V_i ⁽ⁿ⁾]^H,
where U_i(i=1,2) is an (N_r - N_ri)x(N_r - N_ri) Unitrary matrix,
D_i (i=1,2) is an (N_r - N_ri) x N_T matrix having L_i (=rank of H_i) positive singular values and zeros in diagonal elements and having zeros in off diagonal elements,
V_i ^(s) (i=1,2) is an N_TxL_i matrix having, as column vectors, N_T-orthonormal vectors corresponding to L_i positive singular values, and
V_i ⁽ⁿ⁾ (i=1,2) is an N_Tx(N_T -L_i) matrix having, as column vectors, N_T-orthonormal vectors corresponding to zero singular values.

In the base station 10R, each modulated codeword (codeword from a coder not shown is modulated by a modulator not shown) is mapped onto one or more layers. The number of layers is less than or equal to the number of transmit antenna ports. Each layer is mapped by precoding-matrix onto one or more transmit antenna ports associated with physical transmission antennas. In the base station 10R, each of adders 13-1 to 13-N_T connected to each of N_T antennas adds associated mapped layers for

user terminals

1 and 2.

When one user terminal (20R-1 / 20R-2) receives signals transmitted from the base station 10R, the one user terminal does not experience interference from the other user terminal (20R-2 / 20R-2).

Therefore, the user terminal can adjust a receiving matrix G to only extract multiple layers of data intended for the user terminal itself by for example, using a Zero-Forcing (ZF) or Minimum Mean-Squared Error (MMSE) criterion.

Note: The following gives an outline about a Zero-Forcing (ZF) receiver. Assuming that the received signal (vector) r observed at the user terminal is modeled as:
y= Hx+ v, where x is a transmit vector, His achannel matrix from the base station to the user terminal and v is a noise vector (additive white Gaussian noise (AWGN)), when channel state information (CSI) is perfect, the ZF estimate of the transmitted vector can be expressed as:
y^~ = G(Hx + v) = x + Gv,
where G = [H^H H]^-1 H^H is the ZF receiver. The superscript -1 denotes inverse of a matrix.
H⁺ = [H^H H]^-1 H^H is a pseudo inverse matrix (left inverse of H, that is, H⁺ H = I, where I is an Identity Matrix).

Although interference between user terminals (Inter-user interference: IUI) can be handled by the Base-station-created precoding matrices, the base station still needs to select appropriate Modulation and Coding Scheme (MCS) for each data layer in order to maximize each user terminal's throughput, and thus network capacity.

The selection of MCS by the base station is performed as follows.

The base station first acquires received channel quality observed by each user terminal conditioned on the Base-station-created precoding matrix. The received channel quality reflects a power of a desired signal with respect to a power of interference and noise experienced at the user terminal. The interference in this case refers to an undesired signal that may be generated due to imperfect nulling of Inter-user interference by the base station that is serving the user terminal or transmission of signals from neighboring base stations that are serving different user terminals.

Then, the received channel quality is used to determine the highest MCS that satisfies a predefined data transmission error rate. The received channel quality metric most commonly used is Signal to Interference pluses Noise Ratio (SINR), defined as the desired signal power divided by the total power of power of interference and noise.

Fig. 3 depicts an example of MCS-SINR mapping table that depicts an example of how SINR is mapped to MCS. Fig.3 is taken from NPL2's Section 7.2.3 Channel Quality Indicator (CQI) definition.

Referring to Fig.3, in "Modulation", there are QPSK (Quadrature Phase Shift Keying) and 16QAM (Quadrature Amplitude Modulation) and 64QAM.
"Code rate" is k/n, for every k bits of useful information, while the coder generates totally n bits of data, of which n-k are redundant.
"Spectral efficiency (usage) (C) (bit/s/Hz)" is a net bit-rate (bit/s) (useful information rate excluding error-correcting codes) or maximum throughput, divided by a bandwidth in Hertz of a communication channel.
Regarding "SINR", from the well known Shannon's channel capacity equation, "Spectral efficiency (C)" is given as C= log₂(1+SINR). Accordingly, SINR(dB) is given as log₁₀(2^C-1).

Therefore, in conclusion, the base station needs to obtain SINR observed by the user terminal conditioned on the Base-station-created precoding matrix in order to select an appropriate MCS for each data layer and maximize channel (network) capacity.

The mobile communication system, such as LTE, has some mechanism in which a user terminal reports received SINR observed by the user terminal to the base station. NPL2's Section 7.2 UE procedure for reporting Channel State Information (CSI) describes such mechanism in details.

Fig. 4 is a simplified sequence chart illustrating the CSI reporting operations disclosed in NPL2. Here, the system according to the example includes one base station and two user terminals only for the sake of simplicity.

Both user terminals 1 and 2 (20R-1, and 20R-2) first receive Downlink Reference Signal (Downlink RS, which is equivalent to Channel State Information Reference Signal (CSI-RS) in LTE system) broadcasted from each of the base station's antennas to every user terminals (operation S21).

Then, the user terminals 1 and 2 (20R-1 and 20R-2) estimate, respectively Down link channel matrices H₁and H₂ from the base station's antennas to the

user terminals

1 and 2, based on the received Downlink RS, respectively (operations S22-1 and 2).

The base station 10R transmits a request for reporting channel quality information to the respective user terminals 1 and 2 (20R-1 and 20R-2) (operations S23-1 and 2).

After that, the user terminals 1 and 2 (20R-1 and 20R-2) create respectively precoding matrices F_{user1-created} and F_{user2-created} (hereinafter also termed as User-created precoding matrix) by using the estimated channel in order to maximize received SINR (operations S24-1 and 2).

Next, the user terminals 1 and 2 (20R-1 and 20R-2) estimate received SINR for each data layer conditioned on the User-created precoding matrices F_{user1-created} and F_{user2-created}, respectively (operations S25-1 and 2).

Finally, the user terminal 1 (20R-1) reports both the User-created precoding matrix F_{user1-created}, and the received SINR conditioned on F_{user1-created} to the base station 10R (operation S26-1) and the user terminal 2 (20R-2) reports both the User-created precoding matrix F_{user2-created} and the received SINR conditioned on the User-created precoding matrix F_{user2-created} to the base station 10R (operation S26-2).

More specifically, in LTE, the user terminal performs the process of creating the User-created precoding matrix by selecting one precoding matrix from a predefined set of precoding matrices (candidates: called codebook) known to both the user terminal and the base station. The user terminal reports the User-created precoding matrix and the corresponding SINR to the base station using Precoding Matrix Indicator (PMI) index and Channel Quality Indicator (CQI) index, respectively. Higher the CQI index (from 0 to 15) reported by the user terminal to the base station, the base station uses higher modulation scheme (from QPSK to 64QAM) and higher code rate to achieve higher efficiency.

In PTL1, there is disclosed a method in an eNodeB in a MIMO system, wherein the method comprises: receiving signals from a user apparatus in a space division multiplex group to estimate an uplink channel characteristic according to the received signal, determining reciprocity property calibration information between the uplink channel characteristic and a downlink channel characteristic; determining a downlink precoding matrix using zero-forcing; and transmitting a downlink signal to the user apparatus in a space division multiplex group based on the determined downlink precoding matrix. The user apparatus judges whether the measured downlink vector channel procession satisfies predetermined conditions. When conditions are satisfied, the user apparatus transmits the information relevant to the estimated downlink vector channel procession to eNodeB, in order to carry out channel reciprocity calibration.
In PTL2, there is disclosed a MIMO system in which a base stations includes extraction means to extract a matrix element for stream closest to a channel matrix indicating a channel state in channel transmission path among the precoding matrix corresponding to the PMI received from a mobile station equipment, and generation means to generate precoding weight based on the matrix element extracted by the extraction means. User equipment measures channel variation amount using a received signal from receiving antennas thereof. The user equipment, based on the measured channel variation amount, chooses PMI according to phase and amplitude control amount (precoding weight (precoding matrix)) that maximizes the received SINR of transmit data from each of transmitting antennas of the base station eNodeB and feeds back the selected PMI as channel information to the base station eNodeB via uplink. The base station eNodeB responds to PMI fed back from the user equipment UE and transmits data to which precoding has been applied to the user equipment.

Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. JP2012-531132A Japanese Unexamined Patent Application Publication No. JP2011-151540A

Q. H. Spencer, A. L. Swindlehurst, and M. Haardt, "Zero-forcing methods for downlink spatial multiplexing in multiuser MIMO channels," IEEE Transactions on Signal Processing, vol. 52, no. 2, Feb. 2004. 3GPP TS 36.213 V11.7.0, "Technical specification: Physical layer procedures (Release 11)," 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA), Jun. 2014.

The following describes analysis of the related arts.

The mechanism of the mobile communication system in the related art, such as NPL2 fails to provide the base station with sufficient information in order to select appropriate MCS for the Base-station-created precoding matrix. This is because the received SINR reported by the user terminal in the related art is conditioned on the User-created precoding matrix instead of the Base-station-created precoding matrix. These two kinds of precoding matrices can be very different from each other in MU-MIMO operation.

Specifically, while the Base-station-created precoding matrix takes into account channels to all user terminals in order to prevent data from one user terminal from interfering with other user terminals, the User-created precoding matrix only takes into account channels to one user terminal itself. Therefore, it is impossible in MU-MIMO to create a precoding matrix to effectively avoid Inter-UE (User Equipment) interference only from the User-created precoding matrix.

The present invention has been accomplished in consideration of the above mentioned problem, and an objective thereof is to provide a method and system for enabling a base station to estimate received SINR observed by a user terminal that is conditioned on a Base-station-created precoding matrix.

In accordance with one aspect of the present invention, there is provided a method in a mobile communication system comprising a base station communicating with at least one user terminal via a radio link, wherein the base station is adapted to create a precoding matrix for applying to data transmitted to the user terminal (Base station-created precoding matrix) by using a channel estimate determined from Uplink Reference Signal (Uplink RS) transmitted from the user terminal to the base station, the method comprising:
the base station notifying to the user terminal the Base station-created precoding matrix; and
the user terminal estimating a Signal to Interference pluses Noise Ratio (SINR) for each data layer conditioned on the notified precoding matrix to report the estimated SINR to the base station.

In accordance with another aspect of the present invention, there is provided a user terminal for a mobile communication system, comprising:
a transmission unit to transmit Uplink Reference Signal (Uplink RS) to the base station;
a reception unit to receive a precoding matrix notified by the base station, the precoding matrix being created by the base station by using a channel estimate determined from the Uplink RS, for applying to data transmitted to the user terminal; and
a channel quality report creation unit to estimate a Signal to Interference pluses Noise Ratio (SINR) for each data layer conditioned on the precoding matrix notified by the base station to report the estimated SINR to the base station.

In accordance with yet another aspect of the present invention, there is provided a base station comprising:
a generation unit to generate a precoding matrix for applying to data transmitted to the user terminal by using a channel estimate determined from Uplink Reference Signal (Uplink RS) transmitted from the user terminal to the base station; and
a channel quality report request unit to notify to the user terminal the precoding matrix and to request the user terminal for a report on Signal to Interference pluses Noise Ratio (SINR) conditioned on the precoding matrix notified to the user terminal by the base station.

In accordance with a further aspect of the present invention, there is provided a system comprising a base station and a user terminal respectively as set forth in the afore-mentioned aspects. In accordance with a further aspect of the present invention, there is provided a non-transitory computer readable storage medium that stores a program to cause a base station to execute the processing as set forth in the afore-mentioned aspects. There is also provided a non-transitory computer readable storage medium that stores a program to cause a user terminal
to execute the processing as set forth in the afore-mentioned aspects.

Advantageous Effect of Invention

According to the present invention, the information of the received SINR observed by the user terminal conditioned on the Base-station-created precoding matrix can be made available at the base station. Therefore, the base station can further use that information to select appropriate MCS for the Base-station-created precoding matrix and to maximize both user terminal's throughput and network capacity.

Fig. 1 is a diagram illustrating a base station estimating downlink channel matrix by exploiting uplink-downlink channel reciprocity property in a related art. Fig. 2 is a diagram illustrating a base station creating a precoding matrix for each user terminal for MU-MIMO operation in a related art. Fig. 3 illustrates in a table format mapping between MCS and SINR in a related art. Fig. 4 is a sequence diagram illustrating operations of user terminals creating a channel quality report in a related art. Fig. 5 is a diagram illustrating an example of mobile communication system according to an exemplary embodiment of the present invention. Fig. 6 is a diagram illustrating an example of a base station according to an exemplary embodiment of the present invention. Fig. 7 is a diagram illustrating a user terminal according to an exemplary embodiment of the present invention. Fig. 8 is a diagram illustrating an example of operations of the overall system according to an exemplary embodiment of the present invention. Fig. 9 is a flow chart illustrating an example of operations of a base station according to an exemplary embodiment of the present invention. Fig. 10 is a flow chart illustrating operations of a user terminal according to an exemplary embodiment of the present invention.

The following describes exemplary embodiments of the present invention with reference to the drawings. For illustrating modes of the present invention, the embodiments are constructed by assuming the application in the TDD LTE system.

First, a mobile communication system and devices, which are used in common for describing the present invention, will be described with reference to Figs. 5 to 7.

Fig. 5 illustrates an example of a mobile communication system according to the exemplary embodiments. Referring to Fig.5, the system comprises a base station (10) with multiple antennas (101), and a user terminal (20) with multiple antennas (201). The user terminal (20) is located in a base station radio coverage (30) and can communicate with the base station (10) in both uplink and downlink directions. Specifically, the user terminal (20) can transmit Uplink RS (equivalent to SRS in LTE system) and SINR report to the base station (10) via uplink channel. The base station (10) can transmit Downlink RS (equivalent to CSI-RS in LTE system) and control signal and data to the user terminal (20) via downlink channel.

Note that in Fig.5, the system is provided with a single user terminal only for the sake of simplifying explanation. In fact, the present invention can obviously be applied to a system with multiple user terminals.

Fig. 6 illustrates an example of an arrangement of the base station (10). Referring to Fig.6, a plurality of base station antennas (101) are used for both receiving uplink signals and transmitting downlink signals from and to the user terminal (20), respectively.

Uplink / Downlink Multiplexer (102) multiplexes reception of Uplink signals and transmission of Downlink signals in time.

Uplink RS / SINR report Demultiplexer (103) demultiplexes the reception of Uplink signals to reception of Uplink RS and reception of SINR report. Demultiplexed Uplink RS and SINR report are supplied to Channel reciprocity-based downlink channel matrix Estimator (104) and MCS selector (106), respectively.

Downlink RS / SINR report request / Data Multiplexer (108) multiplexes transmission of downlink signals, transmission of Downlink RS, transmission of SINR report request, and transmission of data into Downlink signals.

Channel reciprocity-based downlink channel matrix Estimator (104), upon reception of the Uplink RS from Uplink RS / SINR report Demultiplexer (103), estimates a downlink channel matrix from the base station to each user terminal by exploiting uplink-downlink channel reciprocity property. The estimation process of the downlink channel matrix is similar to that in the related art described with reference to Fig. 1.

Base-station-created precoding matrix Generator (105) obtains the estimated downlink channel matrix from the Channel reciprocity-based downlink channel matrix Estimator (104) and creates a Base-station-created precoding matrix for each user terminal. The creation process of the Base-station-created precoding matrix is similar to that in the related art described with reference to Fig. 2.

The Base-station-created precoding matrix is then applied to modulated data to be transmitted to the user terminal. The modulated data is obtained by coding (by a coder not shown) and modulating (by a modulator not shown) each layer of multiple-layer data stored in Data Buffer (107) with MCS selected by MCS Selector (106).

The modulated data having the precoding matrix applied thereto is mapped to Base station antennas (101) and transmitted through Downlink RS / SINR report request / Data Multiplexer (108) to the user terminal (20).

Downlink RS / SINR report request / Data Multiplexer (108) is also in charge of multiplexing transmissions of Downlink RS generated from Downlink RS Generator (109) and SINR report request generated from SINR report request Generator (110) besides the transmission of modulated data having the precoding matrix applied thereto.

The Downlink RS is necessary for the user terminal (20) in estimating downlink channel matrix for later creating SINR report. The SINR report request contains Base-station-created precoding matrix notification and request content that instructs the user terminal (20) to use the notified precoding matrix for creating the SINR report.

In order to provide the MCS Selector (106) with necessary information for selecting appropriate MCS for each data layer of the user terminal, Uplink RS / SINR report Demultiplexer (103) receives SINR report conditioned on the Base-station-created precoding matrix from the user terminal (20) and forwards that information to MCS Selector (106). Therefore, the MCS Selector (106) can use the information to select the highest MCS for each data layer that satisfies a predefined data transmission error rate. The selection can be done by using the SINR-MCS mapping table similar to the table of the related art described with reference to Fig. 3.

Fig. 7 illustrates an example of an arrangement of the user terminal (20). Referring to Fig.7, a plurality of user terminal antennas (201) are used for both receiving downlink signals and transmitting uplink signals from and to the base station (10), respectively.

Uplink / Downlink Multiplexer (202) multiplexes reception of Downlink signals and transmission of Uplink signals in time.

Downlink RS / SINR report request / Data Demultiplexer (203) demultiplexes the reception of Downlink signals to reception of Downlink RS, reception of SINR report request, and reception of Data.

Uplink RS / SINR report Multiplexer (207) multiplexes transmission of Uplink RS, and transmission of SINR report to Uplink signals.

Data reception Processor (204), upon reception of the modulated data to which the Base-station-created precoding matrix has been applied and transmitted from the base station (10), performs data reception processes that include demodulation and decoding.

Downlink RS-based downlink channel matrix Estimator (205), upon reception of the Downlink RS transmitted from the base station (10), estimates downlink channel matrix from the base station (10) to the user terminal (20) itself.

SINR report Generator (206) obtains the estimated downlink channel matrix and the SINR report request transmitted by the base station (10). Then, the SINR report Generator (206) creates SINR report conditioned on the notified Base-station-created precoding matrix as instructed by the base station (10). The created SINR report is then transmitted to the base station (10) through Uplink RS / SINR report Multiplexer (207).

Uplink RS Generator (208) generates Uplink RS to be transmitted to the base station (10) through Uplink RS / SINR report Multiplexer (207). The Uplink RS is necessary for the base station (10) in estimating downlink channel matrix by exploiting uplink-downlink channel reciprocity property for later creating Base-station-created precoding matrix.

In the following, based on the system and devices described with reference to Figs. 5 to7, the detailed operations of the exemplary embodiment of the present invention will be described. Moreover, for clarity of explanation, it is assumed hereinafter that the base station (10) has 4 antennas and the user terminal (20) has 2 antennas, though not limited thereto.

<System operations>
Fig. 8 illustrates operations for the overall system comprising both the base station (10) and the user terminal (20).

Referring to Fig.8, at a beginning, the user terminal (20) transmits Uplink RS to the base station (10) (operation S1101).

[Corrected under Rule 26, 19.12.2014]
The base station (10) uses the Uplink RS to estimate a

downlink channel matrix (H) by exploiting uplink-downlink channel reciprocity (operation S1102). Note that operation S1101 and S1102 are similar to those in the related art described with reference to Fig. 1.

[Corrected under Rule 26, 19.12.2014]
Next, the base station (10) creates a

Base-station-created precoding matrix

(operation S1103). The base station (10) can use the method in the related art described in Fig. 2 to create the precoding matrix.

[Corrected under Rule 26, 19.12.2014]
Next, the base station (10) transmits the Downlink RS to the user terminal (20) (operation S1104). The user terminal (20) then uses the Downlink RS to estimate the

downlink channel matrix (H) (operation S1105). Note that the estimation of channel matrix by the Downlink RS is similar to that in the related art described with reference to Fig. 4.

[Corrected under Rule 26, 19.12.2014]
Then, the base station (10) transmits to the user terminal (20) the Base-station-created precoding matrix

notification and the request for SINR report (operation S1106).

[Corrected under Rule 26, 19.12.2014]
Upon reception of the notification and the request, the user terminal (20) uses the estimated downlink channel matrix (H) and the notified Base-station-created precoding matrix

to estimate the received SINR for each data layer (operation S1107). More specific details of operation S1107 will be described in later description of the user terminal operation.

[Corrected under Rule 26, 19.12.2014]
After the SINR report conditioned on the Base-station-created precoding matrix

created, the user terminal (20) transmits the SINR report to the base station (operation S1108).

Finally, the base station (10) uses the received SINR report for selecting MCS (operation S1109). The selection of MCS can be based on the MCS-SINR mapping table similar to the one used in the related art described with reference to Fig. 3.

<Base station operations>
Fig. 9 illustrates operations of the base station (10). For the sake of simplicity of explanation, it is assumed that the base station (10) has already created the Base-station-created precoding matrix. Therefore, referring to Fig.9, in the first operation S1201, the base station (10) transmits the Base-station-created precoding matrix notification and the request for SINR report to the user terminal.

Then, the base station (10) regularly checks whether the SINR report conditioned on the Base-station-created precoding matrix has been received (operation S1202).

Upon reception of the SINR report, the base station finally uses that information to select MCS (operation S1203).

In LTE system, the notification of Base-station-created precoding matrix and the request for SINR report can be transmitted by using Radio Resource Control (RRC) signaling. Specifically, already existing messages such as RRC Connection Reconfiguration message can be used to carry Information Element (IE) that describes the Base-station-created precoding matrix to the user terminal (20), when the base station (10) wants to notify the precoding matrix and request the SINR report.

<User terminal operations>
Fig. 10 illustrates operations of the user terminal (20). For the sake of simplicity of explanation, it is assumed that the user terminal (20) has already estimated the downlink channel matrix (H) by using the Downlink RS. Therefore, referring to Fig.10, in the first operation S1301, the user terminal (20) regularly checks whether the notification and the request from the base station have been received.

[Corrected under Rule 26, 19.12.2014]
Upon reception of the Base-station-created precoding matrix

notification and the request for SINR report, the user terminal (20) uses the estimated downlink channel matrix (H) and the notified Base-station-created precoding matrix

to estimate the received SINR for each data layer (operation S1302). Specifically, the user terminal (20) estimates the SINR for each data layer denoted as

, wherein

is a data layer index and

, by using the following mathematical expression.

[Corrected under Rule 26, 19.12.2014]

, for

(Eq. 1)

[Corrected under Rule 26, 19.12.2014]
where

is Average transmit power per data layer, which is already known by the user terminal (20) as a relative power with respect to Downlink RS transmit power.

[Corrected under Rule 26, 19.12.2014]
G is a

receiving matrix at the user terminal based on the
Base-station-created precoding matrix (F_Base-created). For example, when the ZF receiver is used, G is given as follows:

[Corrected under Rule 26, 19.12.2014]

, (Eq. 2)

[Corrected under Rule 26, 19.12.2014]

indicates

element of matrix X, and

R_I+N is an Interference pluses Noise covariance matrix reflecting an amount of interference and noise observed at the user terminal's antennas, which is well known to be derived by using the downlink channel matrix (H) and the Downlink RS.

[Corrected under Rule 26, 19.12.2014]
Note: Assuming that a received vector y(k) observed at the user terminal k is modeled as y(k)=HFs(k)+v(k), where H is a channel matrix from a base station to the user terminal k, F is a precoding matrix, s(k) is a symbol vector, and v(k) is a noise vector (interference pluses noise vector), the Eq.1 is based on the following well known formula:

,
where G is a receiving matrix, R_I+N is an Interference pluses Noise covariance matrix: R_I+N=E{v(k)v(k))^H} , and a symbol power E{s²(k)} is normalize to E{s²(k)}=1.

[Corrected under Rule 26, 19.12.2014]
After the SINR for each data layer conditioned on the Base-station-created precoding matrix

is created, the user terminal (20) finally transmits the SINRs to the base station (10) (operation S1303).

In LTE system, the SINR report can be transmitted to the base station (20) by using physical layer procedures for reporting Channel State Information (CSI). Specifically, the user terminal first quantizes the SINR for each data layer into the format of Channel Quality Indicator (CQI) index. Then, the user terminal transmits the CQI indices with either Physical Uplink Control Channel (PUCCH) or Physical Uplink Shared Channel (PUSCH).

<Advantageous effects>
According to the above described exemplary embodiment, the information of the received SINR observed by the user terminal conditioned on the Base-station-created precoding matrix can be made available at the base station. Therefore, the base station can further use that information to select appropriate MCS for the Base-station-created precoding matrix and to maximize both user terminal's throughput and network capacity.

<Generalization and variation>
Although all the descriptions on the exemplary embodiments of the present invention have been done by assuming the application in the TDD LTE system, the presented embodiments are not limited to only such application. For example, in the FDD (Frequency Division Duplex) LTE system that uses a pair of carrier frequencies close together for uplink and downlink communication, the uplink-downlink channel reciprocity property still holds, and thus the present invention can also be applied.

Variation of the embodiments are as follows, but not limited thereto.

One variation is a method to notify the user terminal with the Base-station-created precoding matrix and request for the SINR report. Instead of the base station transmitting both the notification and the request in a single message, the base station can separate them into different messages for transmitting at different time slots. This is beneficial when the frequencies required for transmitting precoding matrix notification and SINR report request are different.

For example, when the created Base-station-created precoding matrix is changing at slower rate than the received SINR for each data layer, the notification message can be transmitted at longer period than the request message. In this case, the user terminal can just use the latest notified Base-station-created precoding matrix for estimating the SINR when it receives only the request message.

In LTE system, in the case that the notification of the Base-station-created precoding matrix and the request for SINR report are transmitted separately, RRC signaling and uplink Downlink Control Indicator (DCI) format can be used to carry the notification and the request, respectively.

In the above PTL1-PTL2, there is not disclosed any arrangement in which a base station communicates with at least one user terminal via a radio link, wherein the base station is adapted to create a precoding matrix for applying to data transmitted to the user terminal (Base station-created precoding matrix) by using a channel estimate determined from Uplink Reference Signal (Uplink RS) transmitted from the user terminal to the base station, the base station notifies to the user terminal the Base station-created precoding matrix; and the user terminal estimating and reporting a Signal to Interference pluses Noise Ratio (SINR) for each data layer conditioned on the notified Base station-created precoding matrix.

Each disclosure of the above listed Patent Literatures and Non Patent Literatures is incorporated by reference into the present document. Modifications and adjustments of embodiments and examples are possible within bounds of the entire disclosure (including the scope and range of each of the claims) of the present invention, and also based on fundamental technological concepts thereof. Furthermore, a wide variety of combinations and selections of various disclosed elements is possible within the scope of the claims of the present invention. That is, the present invention clearly includes every type of transformation and modification that a person skilled in the art can realize according to technological concepts and the entire disclosure including the scope of the claims.

10, 10R base station
11R antennas
12-1 User terminal 1's Base-station-created precoding matrix
12-2 User terminal 2's Base-station-created precoding matrix
20, 20R user terminal
21R antennas
22- 1, 22-2 Receiving matrix
30 Base station radio coverage
101 Base station antennas
102 Uplink/Downlink Multiplexer
103 Uplink RS/CSI report Demultiplexer
104 Channel reciprocity-based downlink channel matrix Estimator
105 Base-station-created precoding matrix Generator
106 MCS Selector
107 Data Buffer
108 Downlink RS /SINR report request/ Data Multiplexer
109 Downlink RS Generator
110 SINR report request Generator
201 User terminal's antennas
202 Uplink/Downlink Multiplexer
203 Downlink RS/SINR report request/Data Demultiplexer
204 Data reception Processor
205 Downlink RS-based downlink channel matrix Estimator
206 SINRI report Generator
207 Uplink RS/CSI report Multiplexer
208 Uplink RS Generator

Claims

A method in a mobile communication system comprising a base station communicating with at least one user terminal via a radio link, wherein the base station is adapted to create a precoding matrix for applying to data transmitted to the user terminal (Base station-created precoding matrix) by using a channel estimate determined from Uplink Reference Signal (Uplink RS) transmitted from the user terminal to the base station, the method comprising:
the base station notifying to the user terminal the Base station-created precoding matrix; and
the user terminal estimating a Signal to Interference pluses Noise Ratio (SINR) for each data layer conditioned on the notified precoding matrix to report the estimated SINR to the base station.
The method according to claim 1 further comprising:
in estimating the SINR, the user terminal first estimating a channel from the base station to the user terminal by using Downlink Reference Signal (Downlink RS) transmitted from the base station to the user terminal; and
the user terminal using the estimated channel and the notified precoding matrix to estimate the SINR for each data layer conditioned on the notified precoding matrix.
A user terminal for a mobile communication system, comprising:
a transmission unit to transmit Uplink Reference Signal (Uplink RS) to a base station;
a reception unit to receive a precoding matrix notified by the base station, the precoding matrix being created by the base station to apply to data to be transmitted to the user terminal by using a channel estimate determined from the Uplink RS; and
a channel quality report creation unit to estimate a Signal to Interference pluses Noise Ratio (SINR) for each data layer conditioned on the precoding matrix notified by the base station and to report the estimated SINR to the base station.
The user terminal according to claim 3, wherein
the channel quality report creation unit estimates a channel from the base station to the user terminal by using Downlink Reference Signal (Downlink RS) transmitted from the base station to the user terminal and, using the estimated channel and the precoding matrix notified by the base station, and estimates the SINR for each data layer conditioned on the precoding matrix notified by the base station.
A base station comprising:
a generation unit to generate a precoding matrix for applying to data to be transmitted to the user terminal by using a channel estimate determined from Uplink Reference Signal (Uplink RS) transmitted from the user terminal to the base station; and
a channel quality report request unit to notify to the user terminal the precoding matrix and to request the user terminal for a report on Signal to Interference pluses Noise Ratio (SINR) conditioned on the precoding matrix notified to the user terminal by the base station.
A base station comprising:
a transmitter configured to transmit a first signal including first information indicating a precoding matrix; and
a receiver configured to receive a second signal including a second information indicating a Signal to Interference pluses Noise Ratio (SINR), the SINR being estimated conditioned on the precoding matrix indicated by the first information; and
a determiner configured to determine a modulation and coding scheme (MCS) based on the received SINR,
wherein the transmitter transmits a third signal that is precoded with the precoding matrix indicated by the first information, the third signal being coded and modulated based on the determined MCS.
The base station according to the claim 6, wherein the SINR is estimated for each data layer.
A user terminal comprising:
a receiver configured to receive a first signal including first information indicating a precoding matrix; and
an estimator configured to estimate a Signal to Interference pluses Noise Ratio (SINR) conditioned on the precoding matrix indicated by the first information; and
a transmitter configured to transmit a second signal including second information indicating the estimated SINR,
wherein the receiver receives a third signal that is precoded with the precoding matrix indicated by the first information, the third signal being coded and modulated based on a modulation and coding scheme (MCS) determined by a base station based on the estimated SINR.
The user terminal according to the claim 8, wherein the SINR is estimated for each data layer.
A communication system comprising:
a user terminal; and
a base station, wherein
the base station includes:
a generation unit to generate a precoding matrix for applying to a signal to be transmitted to the user terminal;
a channel quality report request unit to notify to the user terminal the precoding matrix and to request the user terminal for a report on Signal to Interference pluses Noise Ratio (SINR) conditioned on the precoding matrix notified to the user terminal by the base station;
a reception unit to receive the SINR estimated and reported by the user terminal;
a selector to select a modulation and coding scheme (MCS) based on the received SINR;
a precoder to apply the precoding matrix to a signal coded and modulated based on the selected MCS; and
a transmission unit to transmit the precoded signal to the user terminal, and wherein
the user terminal includes:
a reception unit to receive the precoding matrix notified by the base station; and
a channel quality report creation unit to estimate a SINR conditioned on the precoding matrix notified by the base station and to report to the base station the estimated SINR, wherein the reception unit receives a signal having the precoding matrix applied thereto from the base station.
The communication system according to claim 10, wherein
the generation unit included in the base station generates the precoding matrix by using a channel estimate determined from Uplink Reference Signal (Uplink RS) transmitted from the user terminal and received by a plurality of antennas of the base station, and
the estimator included in the user terminal estimates a channel from the base station to the user terminal by using Downlink Reference Signal (Downlink RS) transmitted from the base station and received by a plurality of antennas of the user terminal, the estimator using the estimated channel and the precoding matrix notified by the base station, estimating the SINR for each data layer conditioned on the precoding matrix notified by the base station.