JP2009010752A - Mobile communication system, base station apparatus, and method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To generate a suitable precoding vector in accordance with performance of user equipment in a downlink multiuser MIMO (multi input multi output) type mobile communication system. <P>SOLUTION: A mobile communication system includes: a precoding means which duplicates each of a plurality of transmission streams into a plurality of sequences and weights each of the duplicated sequences using the precoding vector; a plurality of combination means each for combining the plurality of sequences weighted by different precoding vectors and applies the combined sequence to any one of a plurality of transmission antennas; and a means which calculates a product of a channel matrix to first user equipment and the conjugate transposed matrix thereof to determine a specific vector corresponding to a maximum specific value of the product matrix. A first precoding vector applied to a transmission stream addressed to the first user equipment for a certain period of time is represented by the specific vector. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to the technical field of mobile communication, and in particular, to a mobile communication system, a base station apparatus, a user apparatus, and a method used in such a system that use a multi-user MIMO (Multi Input Multi Output) system in the downlink. Related.

  The multi-input multi-output (MIMO) system is a multi-antenna system communication that uses a plurality of antennas for communication to increase the speed and / or quality of transmission signals. When communication is performed with a plurality of users in the downlink using the MIMO scheme, it is referred to as a “downlink multi-user MIMO” scheme. A precoding scheme is used in the downlink multi-user MIMO scheme. The precoding method is a technique for transmitting a signal to a communication partner with a beam having a controlled directivity by duplicating a stream of a transmission signal and combining and transmitting each of the duplicated streams with appropriate weights. is there. The weights used in the precoding scheme are called transmission weights or precoding vectors. In the next generation mobile communication system (for example, long term evolution (LTE)), the OFDM scheme is used for the downlink, and a fairly wide band is prepared as a system band.

FIG. 1 schematically shows how precoding is performed. The two transmission streams 1 and 2 are respectively duplicated into two sequences by the copy unit, and each sequence is multiplied by a precoding vector, combined and transmitted. The precoding vector is adaptively controlled to be a more appropriate value based on feedback from the reception side (user apparatus). The precoding is described in Non-Patent Document 1, for example.
3GPP R1-070236, "Precoding for E-UTRA downlink MIMO", LG Electronics, Samsung and NTT-DoCoMo

  When a beam whose directivity is controlled is used for each user apparatus, interference with other users may be increased depending on the user environment (particularly the position). For example, a user and another user are communicating at the same place at the same time. In such a case, interference needs to be appropriately suppressed, but a user device having a strong interference suppression function is generally expensive, and expects a strong interference suppression function to be exhibited for all user devices. It is not possible. Therefore, it is desirable that some measures should be taken on the transmission side so as to reduce interference for user devices that do not have such high performance. However, it seems that the beam control considering the performance of the user equipment has not been sufficiently studied so far.

  An object of the present invention is to generate an appropriate precoding vector according to the performance of a user apparatus in a downlink multi-user MIMO mobile communication system.

  In the present invention, a downlink multi-user MIMO mobile communication system is used. The system includes a precoding unit that duplicates each of a plurality of transmission streams into a plurality of sequences, and weights each duplicated sequence with a precoding vector, and a plurality of synthesis units, each of which is different. A plurality of combining means for combining a plurality of sequences weighted by a precoding vector and providing the combined sequence to any one of the plurality of transmission antennas; a channel matrix for the first user apparatus; and its conjugate transpose matrix; And a means for obtaining an eigenvector corresponding to the maximum eigenvalue of the matrix of the product. The first precoding vector applied to the transmission stream addressed to the first user apparatus during a certain period is expressed by the eigenvector.

  According to the present invention, it is possible to generate an appropriate precoding vector according to the performance of a user apparatus in a downlink multi-user MIMO mobile communication system.

  Hereinafter, the present invention will be described in several embodiments, but the division of each embodiment is not essential to the present invention, and two or more embodiments may be used as necessary.

  In order to simplify the explanation, the base station apparatus transmits four transmission streams to four users from four antennas, but the number of transmission streams, the number of users, the number of antennas, and other values are merely examples. Instead, any suitable numerical value may be used.

  FIG. 2 shows a part of a base station apparatus according to an embodiment of the present invention. This base station apparatus is used in a downlink multi-user MIMO mobile communication system. In FIG. 2, precoding units 21-1 to 21-4, duplication unit 22, transmission weight generation units 23-1 to 23-4, transmission weight multiplication units 24-1 to 24, combining units 25-1 to 25, and transmission / reception antenna 27-are shown. 1-4 are shown.

  Since the precoding units 21-1 to 21-4 have the same configuration and function, 21-1 will be described as a representative. The precoding unit 25-1 includes a duplication unit 22 and transmission weight multiplication units 24-1 to 24-4. The duplication unit 22 receives the first transmission stream s1 (t) and duplicates them into four streams. Each duplicated stream is provided to transmission weight multipliers 24-1 to 24-1, respectively. Each of the transmission weight multiplication units 24-1 to 24-1 weights the stream supplied from the duplication unit 22 with a precoding vector, and provides them to the synthesis units 25-1 to 25-4.

  The transmission weight generation units 23-1 to 23-4 respectively prepare precoding vectors used in the precoding units 21-1 to 21-4.

  The synthesizers 25-1 to 25-4 are prepared in association with the transmitting and receiving antennas 27-1 to 27-4, respectively. One combining unit receives a plurality of streams weighted by the precoding units 21-1 to 21-4 and combines them.

  FIG. 3 shows a part of a user equipment according to an embodiment of the present invention. FIG. 3 shows transmission / reception antennas 31-1 and 31, a reference signal receiving unit 33, a channel estimation unit 35, and a precoding vector estimation unit 37. In order to simplify the illustration, only two transmission / reception antennas are illustrated, but any number of transmission / reception antennas equal to or less than the number of transmission antennas of the base station apparatus may be used.

  The reference signal receiving unit 33 extracts a reference signal (RS) from reception signals received by each of the plurality of transmission / reception antennas, and converts the reference signal (RS) into a format that can be processed in baseband. The reference signal is a known signal from the base station apparatus and user apparatus before the start of communication, and may be referred to as a pilot signal, a training signal, a reference signal, or the like.

The channel estimation unit 35 estimates the channel state of the radio link from the reception state of the reference signal received by each transmission / reception antenna, and obtains the channel matrix H _m (m represents the m-th user and is an integer between 1 and N) N is the total number of users communicating at the same time.) Each matrix element of the channel matrix H _m represents a channel state between a certain transmission antenna of the base station apparatus and a certain reception antenna of the user apparatus. Therefore, the number of matrix elements is generally (N _Tx × N _Rx ). (N _Tx represents the number of transmission antennas of the base station apparatus, and N _Rx represents the number of reception antennas of the user apparatus.

Precoding vector estimating unit 37, the channel matrix H _m, determine the most suitable precoding vector to the own device. The precoding vector is obtained as follows. First, a product H _m ^H H _{m of} a channel matrix H _m and a conjugate transpose matrix H _m ^H of the channel matrix is calculated. Here, the superscript “H” represents conjugate transpose. The matrix H _m ^H H _m is a non-negative Hermitian matrix having a dimension of (N _Tx × N _Tx ). Next, this matrix H _m ^H H _m is eigenvalue decomposition as:.

H _m ^H H _m = U _m Λ _m U _m ^H
Here, Λ _m is a diagonal matrix having eigenvalues λ _{m, i} (i = 1,..., N _Tx ) of H _m ^H H _m as matrix elements. U _m is a unitary matrix for diagonalization, and the eigenvector e _{m, i} (i = 1, ..., N) corresponding to the eigenvalue λ _{m, i} (i = 1, ..., N _Tx ) _Tx ).

_{U m = [e m, 1} e m, 2 ··· e m, NTx].
Since H _m ^H H _m is a non-negative Hermitian matrix, all eigenvalues are positive real numbers. For convenience of explanation, it is assumed that λ _{m, 1} is the maximum among the eigenvalues λ _{m, i} (i = 1,..., N _Tx ).

Precoding vector estimating unit 37, the maximum eigenvalue or identifying a first eigenvalue lambda _{m, 1,} identifying a first eigenvector e _{m, 1} corresponding to the first eigenvalue lambda _{m, 1.} This first eigenvector is an optimal precoding vector for the m-th user apparatus.

  Note that the precoding vector estimation unit 37 may be provided in the user apparatus as illustrated, or may be provided in the base station apparatus.

The operation will be described next. The base station apparatus of FIG. 2, the optimum first eigenvector to each of a plurality of (N number of) user devices each _{e 1,1, e 2,1, ...,} e m, 1, ..., e Notification of _{N, 1} is received (N is the total number of users, N = 4 in FIG. 2). Alternatively, the first eigenvector of each user may be calculated from the channel matrix H _m (m = 1,..., N) notified from each user apparatus. These first eigenvectors are given to the transmission vector generation units 23-1 to 23-4 and set as transmission weights or precoding vectors. For example, when the third stream s ₃ (t) is transmitted to the third user apparatus, a precoding vector corresponding to the eigenvector e _3,1 is provided from the transmission weight generation unit 23-3, and the precoding vector Is multiplied by the third stream s ₃ (t). The third stream s ₃ (t) is transmitted from the four transmission / reception antennas 27-1 to 27-4 to the third user apparatus. If one transmission stream corresponds to one user, the mth transmission stream s _m (t) is weighted with the vector em _{, 1} .

FIG. 4 schematically shows how a beam is transmitted to each user apparatus with a transmission weight related to the maximum eigenvalue. Each user apparatus receives a stream precoded with an eigenvector corresponding to its maximum eigenvalue. Therefore, the precoding vectors e _1,1 , e _2,1 ,..., Em _{, 1} ,..., E _{N, 1} used in the base station apparatus are generally non-orthogonal to each other. However, if other-user interference is appropriately suppressed, reception quality (SNR) is theoretically maximized by reception diversity by maximum ratio combining (MRC).

  One method of reducing interference with other users is to perform a process for suppressing interference in each user apparatus. For example, by performing linear minimum mean square error (LMMSE) filtering in each user apparatus, interference from other users can be considerably suppressed.

  As described above, when other-user interference cannot be ignored, it must be appropriately suppressed. However, a user device having a strong interference suppression function is generally expensive and has strong interference with all user devices. The suppression function cannot be expected. Therefore, it is desirable that some measures should be taken on the transmission side so as to reduce interference for user devices that do not have such high performance.

  FIG. 5 shows a part of a base station apparatus according to a second embodiment of the present invention. The illustrated portion relates to the transmission weight generation units 23-1 to 23-4 in FIG. It is a figure which shows a part of base station apparatus in case orthogonal transmission weight is used. FIG. 5 shows transmission weight generating units 51-1 to 51-4, an orthogonalizing unit 52, and a vector selecting unit 53.

The transmission weight generation units 51-1 to 51-4 output the first eigenvector em _{, 1} (m = 1,..., N) for each user apparatus based on the feedback signal from each user apparatus. When such a first eigenvector is calculated in each user apparatus and fed back from each user apparatus, the transmission weight generation unit may output them as they are.

The orthogonalizing unit 52 derives four sets of orthogonal systems from four generally non-orthogonal vectors from the transmission weight generating units 51-1 to 51-4. Specifically, the orthogonalization unit fixes one vector e _{1,1 out} of a set of four vectors {e _1,1 , e _2,1 , e _3,1 , e _4,1 }, By deriving three orthogonal vectors, one orthogonal system or orthonormal system {f _{1, k} } is derived. {f _{1, k} } (k = 1,..., 4) represents a set including three vectors orthogonal to e _1,1 and e _1,1 . Similarly, the orthogonalization unit 52 also derives one orthogonal system or orthonormal system {f _{2, k} } by fixing another vector e _2,1 and deriving three vectors orthogonal thereto. To do. Further, the orthogonalization unit 52 fixes the vector e _3,1 to derive the orthogonal system {f _{3, k} }, and fixes the vector e _4,1 to derive the orthogonal system {f _{4, k} }. The orthogonalization may be performed by any appropriate method known in the art. As an example, in this embodiment, the Gram-Schmidt (GS) orthogonalization method is used.

The vector selection unit 53 selects each of the vectors {f _{1, k} }, {f _{2, k} }, {f _{3, k} }, {f _{4, k} } forming the orthogonal system given from the orthogonalization unit 52. A transmission weight (precoding vector) actually applied to the transmission stream is selected. For example, when optimally transmitting the first stream s ₁ (t), the vector selection unit 53 selects the orthogonal system {f _{1, k} } and outputs it as precoding vectors w1,..., W4. . When optimally transmitting the second stream s ₂ (t), the vector selection unit 53 selects the orthogonal system {f _{2, k} } and outputs it as precoding vectors w1,..., W4. Similarly, the vector selection unit 53 uses the orthogonal system {f _{3, k} } when optimally transmitting the third stream s ₃ (t), and uses the orthogonal system when optimally transmitting the fourth stream s ₄ (t). Select {f _{4, k} }.

  The selection in the vector selection unit 53 may be made based on various determination criteria. For example, when the importance of each transmission stream is different, a precoding vector may be selected based on the importance. Alternatively, when focusing on one user apparatus, the precoding vector may be switched over time so that a precoding vector suitable for the user apparatus is selected at a constant period.

FIG. 6 shows how the orthogonal beams are switched between the first period and the second period. For simplicity of illustration, it is assumed that the first transmission stream s1 (t) is transmitted from the base station apparatus eNB to the first user apparatus UE1, and the second transmission stream s2 (t) is transmitted to the second user apparatus UE2. During the first period, each transmission stream is transmitted from the base station apparatus with a precoding vector {f _{1, k} } that is optimal for the first user apparatus. The beams 1 and 2 formed by the precoding vector {f _{1, k} } are orthogonal to each other, and the beam 1 is an optimum beam for the first user apparatus UE1. The first user apparatus UE1 can receive the first transmission stream s1 (t) transmitted by the beam 1 with little interference. Therefore, even if the first user apparatus cannot exhibit a powerful interference cancellation function, high reception quality can be obtained by performing reception diversity by maximum ratio combining. This is because the beam 2 of another user is orthogonal to the beam 1. The reception quality at the second user apparatus UE2 is not optimum, but this state may be maintained as long as it is equal to or higher than the minimum reception quality acceptable for the second user apparatus.

In the illustrated example, the precoding vector is switched from {f _{1, k} } to {f _{2, k} } in a second period different from the first period. As a result, the beam 2 optimum for the second user apparatus UE2 is formed, and the beam 1 to the first user apparatus UE1 is formed so as to be orthogonal to the beam 2. Therefore, the second user apparatus UE2 can receive the second transmission stream s2 (t) transmitted by the beam 2 with little interference. In the present embodiment, the state of the first period and the state of the second period are switched alternately. In the illustrated example, there are two sets of orthogonal systems. However, more sets of orthogonal systems may be prepared for a larger number of user apparatuses, and these sets may be sequentially switched. By switching and using each of the plurality of orthogonal systems in order, each user apparatus can receive a transmission stream with high quality over at least a certain period.

The system conceptual diagram of a downlink multiuser MIMO system is shown. The partial functional block diagram of a base station apparatus is shown. 2 shows a partial functional block diagram of a user device. It is a figure which shows a mode that a beam is transmitted to each user apparatus with the transmission weight relevant to the largest eigenvalue. It is a figure which shows a part of base station apparatus in case orthogonal transmission weight is used. It is a figure which shows a mode that an orthogonal beam is switched.

Explanation of symbols

21-1-4 Precoding unit 22 Duplicating unit 23-1-4 Transmission weight generation unit 24-1-4 Transmission weight multiplication unit 25-1-4 Combining unit 27-1-4 Transmission / reception antenna 31-1, 2 Transmission / reception antenna 33 Reference signal receiver 35 Channel estimator 37 Precoding vector estimator 51-1-4 Transmission weight generator 52 Orthogonalizer 53 Vector selector

Claims (9)

A downlink multi-user MIMO mobile communication system,
Precoding means for replicating each of a plurality of transmission streams into a plurality of sequences, and weighting each replicated sequence with a precoding vector;
A plurality of combining means, wherein each of the combining means combines a plurality of sequences weighted with different precoding vectors, and gives a combined sequence to any one of the plurality of transmission antennas;
Means for calculating a product of a channel matrix for the first user equipment and its conjugate transpose matrix and determining an eigenvector corresponding to a maximum eigenvalue of the matrix of the product;
And a first precoding vector applied to a transmission stream addressed to the first user apparatus during a certain period is expressed by the eigenvector. A base station apparatus used in a downlink multi-user MIMO mobile communication system,
Multiple transmit antennas,
Precoding means for replicating each of a plurality of transmission streams into a plurality of sequences, and weighting each replicated sequence with a precoding vector;
A plurality of combining means, wherein each of the combining means combines a plurality of sequences weighted with different precoding vectors, and gives a combined sequence to any one of the plurality of transmission antennas;
And the first precoding vector applied to the transmission stream destined for the first user apparatus during a certain period is the maximum eigenvalue of the product of the channel matrix for the first user apparatus and its conjugate transpose matrix A base station apparatus represented by an eigenvector corresponding to.   A second precoding vector applied to a transmission stream addressed to a second user device different from the first user device during the certain period is the first precoding for the first user device. The base station apparatus according to claim 2, which is expressed by a vector orthogonal to the vector.   The first precoding vector applied to the transmission stream addressed to the first user apparatus during another period different from the certain period is expressed by a vector orthogonal to the eigenvector related to the first user apparatus. The base station apparatus according to claim 3.   The base station apparatus according to claim 3 or 4, wherein the orthogonal vector is derived from the eigenvector by a Gramschmitt orthonormalization method.   The base station apparatus according to claim 2, wherein information representing the eigenvector is notified from the first user apparatus.   The base station apparatus according to claim 2, wherein information representing a channel matrix for the first user apparatus is notified from the first user apparatus.   The base station apparatus according to claim 7, further comprising means for calculating a product of a channel matrix for the first user apparatus and its conjugate transpose matrix and obtaining an eigenvector corresponding to a maximum eigenvalue of the matrix of the product. A method used in a downlink multi-user MIMO mobile communication system,
A precoding step of replicating each of a plurality of transmission streams into a plurality of sequences, weighting each replicated sequence with a precoding vector;
Combining a plurality of sequences weighted with different precoding vectors and providing the combined sequence to any one of the plurality of transmission antennas;
Transmitting each sequence from each transmit antenna to the user equipment;
And the first precoding vector applied to the transmission stream destined for the first user apparatus during a certain period is the maximum eigenvalue of the product of the channel matrix for the first user apparatus and its conjugate transpose matrix A method represented by an eigenvector corresponding to.

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