JP2017139684A - Transmission weight control method in mu-mimo and selection method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a transmission weight control method in cooperative MU-MIMO in a plurality of base stations.SOLUTION: In a cooperative MU-MIMO system among a plurality of base stations, in which two base stations transmit signals to two mobile stations, a transmission weight control method, which multiplies a transmission weight by a transmission signal, includes the steps of: receiving, in a first base station, channel information from a first mobile station, as feedback information, and receiving, in a second base station, channel information from a second mobile station, as feedback information; generating in the two base stations respective transmission signals; generating, in the respective base stations, transmission weights on the basis of the received channel information; receiving, in the first base station, the transmission signal of the second base station, and receiving, in the second base station, the transmission signal of the first base station, through a transmission line between the base stations; multiplying, in the two base stations, the transmission weights by the transmission signals; and transmitting, from the two base stations, transmission signals obtained by multiplying the transmission weights.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a transmission weight control method in a multi-user MIMO (MU-MIMO) system, and more particularly to a transmission weight control method and a transmission weight control method selection method in multi-base station cooperative MU-MIMO.

  MIMO (Multiple-Input Multiple-Output) transmission, which performs communication by spatially multiplexing a plurality of data using a plurality of antennas for transmission and reception of wireless communication, is realized. MIMO transmission includes single user MIMO (SU-MIMO) for the purpose of increasing the peak data rate for one mobile station and MU-MIMO for the purpose of increasing the throughput for a plurality of mobile stations.

  MU-MIMO is mainly used as a downlink MU-MIMO in which one or a plurality of base stations simultaneously transmit signals to a plurality of mobile stations. As downlink MU-MIMO, a single base station MU-MIMO that transmits signals from one base station to multiple mobile stations, and a multi-base station cooperative MU-MIMO that transmits signals from multiple base stations to multiple mobile stations is assumed.

  In MU-MIMO, MIMO transmission is performed to a plurality of mobile stations using radio resources having the same frequency and the same time. For this reason, interference occurs between mobile stations. Since interference between mobile stations causes a decrease in signal transmission characteristics, it is important to suppress interference between mobile stations. As means for suppressing interference between mobile stations, transmission weight control (directivity control) on the base station side is necessary. In transmission weight control, a weight that reduces interference between mobile stations on the receiving side is preliminarily applied to the transmission signal. Such weights are set based on channel information between the base station and the mobile station. When the duplex method is FDD, channel information is measured at the mobile station and fed back as feedback information from the mobile station to the base station. On the other hand, in the case of TDD, channel information can be obtained by a base station measuring an uplink signal from a mobile station.

  One transmission weight control method is a block diagonalization method. In the block diagonalization method, a transmission weight is generated in advance such that a non-diagonal block of a composite channel obtained by multiplying a channel information matrix by a transmission weight is zero. Non-patent document 1 is an example of the block diagonalization method.

Quentin H. Spencer, et al., “Zero-Forcing Methods for Downlink Spatial Multiplexing in Multiuser MIMO Channels”, IEEE TRANSACTIONS ON SIGNAL PROCESSING, Vol. 52, No. 2, pp. 461-471, Feb. 2004.

  The multi-base station cooperative MU-MIMO is mainly intended to improve throughput for mobile stations in places where the SNR of the propagation path is low, such as near cell boundaries. A transmission weight control method using a block diagonalization method can be applied to suppress interference between mobile stations in multi-base station cooperative MU-MIMO. In the block diagonalization method, a transmission weight in which the non-diagonal block of the composite channel is 0 is generated. In order to generate such a transmission weight, it is necessary to share a part of the transmission weight between base stations that perform multi-base station cooperative MU-MIMO. That is, transmission weight information generated by one base station is transmitted to another base station and used by another base station. Transmission weight sharing between base stations is the same for other general weight generation algorithms such as Zero Forcing (ZF) -based transmission weight control and Minimum Mean Square Error (MMSE) -based transmission weight control.

  However, information transmission between base stations has a larger transmission delay than information transmission on a radio propagation path. For this reason, a large delay also occurs when transmission weight information is transmitted between base stations in multi-base station cooperative MU-MIMO. The transmission weight is generated based on the channel information. When a transmission weight generated in one base station is transmitted to another base station with a transmission delay time and a signal considering the transmission weight is transmitted, there is a possibility that the channel state greatly fluctuates. is there. When the channel state is fluctuating, the transmission weight control does not operate effectively and causes deterioration of characteristics.

  An object of the present invention is to propose a transmission weight control method that does not require transmission weight sharing between base stations in transmission weight control of multi-base station cooperative MU-MIMO.

  In the multi-base station cooperative MU-MIMO, there are methods other than the transmission weight control method according to the present invention, such as a block diagonalization method. These transmission weight control methods have different characteristics depending on the location of the destination mobile station as well as the complexity of the algorithm and the processing time. For this reason, adaptively selecting a transmission weight control method improves system throughput.

  Furthermore, the present invention provides a transmission weight control method for selecting one transmission weight control method from among a plurality of transmission weight control methods in accordance with the position of a mobile station in the transmission weight control of multiple base station cooperative MU-MIMO. The purpose is to propose a selection method.

を生成し、前記第２基地局が送信ウェイト

を生成するステップであって、H_xyは基地局yと移動局xとの間のチャネル情報であり、H⁺はチャネル情報の疑似逆行列であり、Iは単位行列であるステップと、基地局間伝送路を介して、前記第１基地局は、前記第２基地局で前記生成された送信信号を受信し、前記第２基地局は、前記第１基地局で前記生成された送信信号を受信するステップと、前記２つの基地局が、それぞれ、前記生成した送信信号と前記受信した送信信号に前記送信ウェイトを掛けるステップと、前記２つの基地局が、それぞれ前記送信ウェイトが掛けられた送信信号を送信するステップと、を有する。 The present invention relates to a multi-base station cooperative MU-MIMO system in which two base stations comprising a first base station and a second base station transmit signals to two mobile stations comprising a first mobile station and a second mobile station. In the transmission weight control method, the two base stations multiply transmission signals by transmission weights, the first base station receives channel information as feedback information from the first mobile station, and Two base stations receiving channel information as feedback information from the second mobile station, two base stations generating respective transmission signals, and based on the received channel information, 1 base station has transmission weight

And the second base station transmits a transmission weight

H _xy is channel information between the base station y and the mobile station x, H ⁺ is a pseudo inverse matrix of the channel information, I is a unit matrix, and the base station The first base station receives the transmission signal generated by the second base station via the inter-transmission path, and the second base station receives the transmission signal generated by the first base station. Receiving, the two base stations multiplying the generated transmission signal and the received transmission signal by the transmission weight, respectively, and the two base stations respectively transmitting the transmission weight multiplied by the transmission weight Transmitting a signal.

であり、前記第２基地局の送信ウェイトが、

であり、H_xyは基地局yと移動局xとの間のチャネル情報であり、H⁺はチャネル情報の疑似逆行列であり、Iは単位行列である送信ウェイト制御方法を選択するステップと、を有する。 Furthermore, the present invention provides a multi-base station cooperative MU- that transmits signals to two mobile stations consisting of a first mobile station and a second mobile station from two base stations consisting of a first base station and a second base station. In the MIMO system, the two base stations select one transmission weight control method from a plurality of transmission weight control methods, wherein the first base station is a first mobile station to the first mobile station. Measuring the distance; measuring the second distance to the second mobile station by the second base station; and if the first distance and the second distance are smaller than a predetermined threshold, Two base stations have a transmission weight of the first base station,

And the transmission weight of the second base station is

H _xy is channel information between the base station y and the mobile station x, H ⁺ is a pseudo inverse matrix of the channel information, and I is a transmission weight control method that is a unit matrix; and Have

  In transmission weight control of multi-base station cooperative MU-MIMO, it is not necessary to share transmission weights between base stations, so that it is possible to suppress characteristic degradation due to transmission delay between base stations.

  Higher throughput characteristics can be obtained by selecting an appropriate transmission weight control method according to the mobile station position.

It is a figure explaining a multiple base station cooperation MU-MIMO system. It is a figure explaining the transmission weight control by a block diagonalization method in a multi-base station cooperation MU-MIMO system. It is a figure explaining the transmission weight control by this invention in multiple base station cooperation MU-MIMO system. It is a figure which shows the flowchart of the transmission weight control method by this invention. It is a figure explaining the adaptive selection of the transmission weight control method in multiple base station cooperation MU-MIMO system.

  Hereinafter, a transmission weight control method in multi-base station cooperative MU-MIMO will be described with reference to the drawings. However, it should be understood that the invention is not limited to the drawings or the embodiments described below.

  A simple example of a multi-base station cooperative MU-MIMO system is shown in FIG. In FIG. 1, reference numeral 11 is a multi-base station cooperative MU-MIMO system. Here, it is assumed that two base stations perform MIMO communication with two mobile stations. Reference number 12 is one base station (BS # 1), 13 is BS # 1 (12) MIMO antenna # 1, 14 is BS # 1 (12) MIMO antenna # 2, and 15 is the other one The base station (BS # 2), 16 represents the MIMO antenna # 3 of BS # 2 (15), and 17 represents the MIMO antenna # 4 of BS # 2 (15). Reference numeral 18 is one mobile station (MS # 1), 19 is a MIMO antenna for MS # 1 (18), 20 is another mobile station (MS # 2), and 21 is MS # 2 (20 ) Represents a MIMO antenna.

  The multiple base station cooperative MU-MIMO system shown in FIG. 1 is one example, and the number of base stations, the number of mobile stations, the number of antennas, and the like can be variously set.

In FIG. 1, two base stations BS # 1 (12), BS # 2 (15) and two mobile stations MS # 1 (18), MS # 2 (20) each have two MIMO antennas. And BS # 1 (12) MIMO antenna # 1 (13) and # 2 (14) transmit signal s ⁽¹⁾ , BS # 2 (15) MIMO antenna # 3 (16), # The signal s ⁽²⁾ is transmitted from 4 (17). BS # 1 (12) signal from s ⁽¹⁾ is received by the MS # 1 (18) through the channel of the channel information H _11, it is received by the MS # 2 (20) through the channel of the channel information H ₂₁ The Further, BS # signal from ^{2 (15)} s (2) is received by the MS # 1 (18) through the channel of the channel information H _12, with MS # 2 (20) through the channel of the channel information H ₂₂ Received.

ここで、Hは伝搬路のチャネル行列で、

と表され、送信信号ベクトルs、雑音信号ベクトルnは、それぞれ、

と表される。 In the multi-base station cooperative MU-MIMO system shown in FIG. 1, when transmission weight control is not performed, the received signal vector y is expressed by the following equation.

Where H is the channel matrix of the propagation path,

And the transmission signal vector s and the noise signal vector n are respectively

It is expressed.

As shown in Equation (1), the signal received by MS # 1 (18) includes the inter-mobile station interference component H ₁₂ s ⁽² ), and the signal received by MS # 2 (20) Includes an inter-mobile station interference component H ₂₁ s ⁽¹⁾ . Such inter-mobile station interference components lower the SINR (Signal-to-Interference + Noise Ratio) of the received signal.

  In order to suppress inter-mobile station interference, the base station performs transmission weight control in which a transmission signal is transmitted in advance with a transmission weight. An example of a multi-base station cooperative MU-MIMO system to which transmission weight control by block diagonalization is applied is shown in FIG.

  In FIG. 2, reference numeral 31 is a multi-base station cooperative MU-MIMO system to which transmission weight control is applied. Here, it is assumed that two base stations perform MIMO communication with two mobile stations. Reference number 32 is one base station (BS # 1), 33 is a transmitter, 34 is a weight controller, 35 is a MIMO antenna # 1 for BS # 1 (32), and 36 is a MIMO for BS # 1 (32) Antennas # 2 and 37 are another base station (BS # 2), 38 is a transmission unit, 39 is a weight control unit, 40 is a BS # 2 (37) MIMO antenna # 3 and 41 is BS # 2 This represents (37) MIMO antenna # 4. Reference numeral 42 is one mobile station (MS # 1), 43 is an antenna for MIMO of MS # 1 (42), 44 is another mobile station (MS # 2), 45 is MS # 2 (44 ) Represents a MIMO antenna.

  The multiple base station cooperative MU-MIMO system shown in FIG. 2 is one example, and the number of base stations, the number of mobile stations, the number of antennas, and the like can be variously set.

In FIG. 2, two base stations BS # 1 (32) and BS # 2 (37) and two mobile stations MS # 1 (42) and MS # 2 (44) each have two MIMO antennas. And The signal s ⁽¹⁾ generated by the transmission unit 33 of the BS # 1 (32) is subjected to transmission weight control by the weight control unit 34 and transmitted from the MIMO antennas # 1 (35) and # 2 (36). The signal s ⁽²⁾ generated by the transmission unit 38 of # 2 (37) is subjected to transmission weight control by the weight control unit 39 and transmitted from the MIMO antennas # 3 (40) and # 4 (41). Signal transmitted from the BS # 1 (32) is received by the MS # 1 (42) through the channel of the channel information H _11, is received by the MS # 2 (44) through the channel of the channel information H _21. The signal transmitted from the BS # 2 (37) is received by the MS # 1 (42) through the channel of the channel information H _12, is received by the MS # 2 (44) through the channel of the channel information H ₂₂ The

Interference between mobile stations is suppressed by performing transmission weight control. When the transmission wait control is ideally performed, BS # 1 (32) and the BS # 2 (37) signal component is received by the MS # 2 (44) through the respective channel of the channel information H ₂₁ and H ₂₂ from s ⁽¹⁾ can be set to 0. Also be a BS # 1 (32) and the BS # 2 (37) MS # 1 (42) signal component s ⁽²⁾ 0, which is received in through the channel of the channel information H ₁₁ and H ₁₂ from .

コンポジットチャネルHW_BDは、

と表され、非対角ブロックH₁W₂及びH₂W₁が0となるように送信ウェイトが決定される。 Transmission weight control by the block diagonalization method will be described below. A transmission weight W _BD is generated in advance so that the non-diagonal block of the composite channel HW _BD becomes zero.

Composite channel HW _BD

And the transmission weight is determined so that the off-diagonal blocks H ₁ W ₂ and H ₂ W ₁ are 0.

となり、移動局間干渉成分を除去することができる。 When transmitting a transmission signal with a transmission weight W _BD , the received signal vector is

Thus, the inter-mobile station interference component can be removed.

  However, if the accuracy of weight generation deteriorates, the inter-mobile station interference component cannot be completely removed, and the interference component remains. For example, when channel information measured at a mobile station is fed back, accurate weight generation cannot be performed when the channel estimation accuracy at the mobile station is low. Also, since the amount of information that can be fed back is limited, the quantization accuracy of the channel information also affects the weight generation accuracy. Further, when the weight control granularity on the frequency axis and the time axis is coarse, the weight cannot accurately follow frequency selective fading and channel time variation. Also, when channel information is fed back, if the feedback delay is large, the channel state fluctuates during weight control, so that the generated weight is not properly reflected and inter-mobile station interference occurs.

  Furthermore, multi-base station cooperative MU-MIMO is affected by a transmission delay when transmitting transmission weight information between base stations. In multi-base station cooperative MU-MIMO, when transmission weight control is performed by a method such as a block diagonalization method, it is necessary to share some transmission weights between base stations. A transmission weight generated in one base station is subjected to a transmission delay when transmitted to another base station. This delay is, for example, 10 ms, which is much larger than the above feedback delay. Such a delay greatly deteriorates the characteristics in transmission weight control. Note that the delay time in information transmission between base stations depends on a wireless communication system, a transmission path of information transmission between base stations, and the like.

と表せる。 The sharing of transmission weights between base stations in the block diagonalization method will be described below. The composite channel HW _BD is obtained from equation (6):

It can be expressed.

となる。従って、送信ウェイトW₂₁、W₂₂は、チャネル情報H₁₁、H₁₂を用いて生成されることになる。図２に示されるように、チャネル情報H₁₁、H₁₂は、MS#1（42）で測定される。同様に、送信ウェイトW₁₁、W₁₂は、チャネル情報H₂₁、H₂₂を用いて生成され、チャネル情報H₂₁、H₂₂は、MS#2（44）で測定される。 In the block diagonalization method, the transmission weight is determined so that the non-diagonalized component becomes zero. That is,

It becomes. Accordingly, the transmission weights W ₂₁ and W ₂₂ are generated using the channel information H ₁₁ and H ₁₂ . As shown in FIG. 2, channel information H ₁₁ and H ₁₂ are measured by MS # 1 (42). Similarly, the transmission weights W _11, W ₁₂ is generated by using the channel information H _21, H _22, the channel information H _21, H ₂₂ is measured by the MS # 2 (44).

Channel information H ₁₁ and H ₁₂ measured by MS # 1 (42) is transmitted to BS # 1 (32) via feedback channel 46. In BS # 1 (32), transmission weights W ₂₁ and W ₂₂ are generated based on channel information H ₁₁ and H ₁₂ . Similarly, channel information H ₂₁ and H ₂₂ measured by MS # 2 (44) are transmitted to BS # 2 (37) via feedback channel 47. In BS # 2 (37), transmission weights W ₁₁ and W ₁₂ are generated based on channel information H ₂₁ and H ₂₂ .

となる。BS#1（32）において、s⁽¹⁾、W₂₁は、BS#1（32）で生成され、s⁽²⁾、W₁₁は、BS#2（37）から伝送される。即ち、送信信号s⁽²⁾と送信ウェイトW₁₁を両基地局で共有する。一方、BS#2（37）において、s⁽²⁾、W₁₂は、BS#2（37）で生成され、s⁽¹⁾、W₂₂は、BS#1（32）から伝送される。即ち、送信信号s⁽¹⁾と送信ウェイトW₂₂を両基地局で共有する。これらの情報は、図２において、基地局間伝送路48を通して伝送される。 The weight-controlled transmission signal transmitted from each base station is

It becomes. BS # 1 in ^{(32), s (1)} , W 21 is generated in BS # 1 (32), s (2), W 11 is transmitted from the BS # 2 (37). That is, sharing the transmission signal s ⁽²⁾ and the transmission weight W ₁₁ at both base stations. On the other hand, in BS # 2 (37), s (2), W 12 is BS # is produced by ^{2 (37), s (1} ), W 22 is transmitted from the BS # 1 (32). That is, the transmission signal s ⁽¹⁾ and the transmission weight W ₂₂ are shared by both base stations. These pieces of information are transmitted through the inter-base station transmission path 48 in FIG.

  As described above, in the transmission weight control method, when transmission weight information is shared between base stations, the transmission weight control characteristic is deteriorated with a large transmission delay in the inter-base station transmission path 48. For this reason, when applying transmission weight control to multi-base station cooperative MU-MIMO, transmission weights that do not require sharing of transmission weight information between base stations so as not to be affected by transmission delay in information transmission between base stations A control method is proposed.

コンポジットチャネルHW_CANは、式(11)から、

となり、非対角項が0となる。受信信号ベクトルyは、

となり、移動局間干渉を0とすることができる。 In the proposed method, the transmission weight is set as follows.

The composite channel HW _CAN can be calculated from equation (11):

And the off-diagonal term is 0. The received signal vector y is

Thus, interference between mobile stations can be reduced to zero.

となる。BS#1（32）において、送信信号s⁽¹⁾はその送信部33で生成され、チャネル情報H₁₁、H₁₂はフィードバックチャネル46を通じてMS#1（42）から伝送されるため、送信信号s⁽²⁾のみがBS#2（37）から通知される。同様に、BS#2（37）において、送信信号s⁽²⁾はその送信部38で生成され、チャネル情報H₂₁、H₂₂はフィードバックチャネル47を通じてMS#2（44）から伝送されるため、送信信号s⁽¹⁾のみがBS#1（32）から通知される。従って、送信ウェイト情報を基地局間で共有する必要はない。図３に示すように、基地局間伝送路48を通して、BS#1（32）からBS#2（37）に送信信号s⁽¹⁾が伝送され、BS#2（37）からBS#1（32）に送信信号s⁽²⁾が伝送される。 The transmission weight controlled signal transmitted from each base station is

It becomes. BS # in 1 (32), for transmitting signal s ⁽¹⁾ is generated at the transmitter 33, the channel information H _11, H ₁₂ is transmitted from the MS # 1 (42) through the feedback channel 46, the transmission signal s Only ⁽²⁾ is notified from BS # 2 (37). Similarly, in BS # 2 (37), the transmission signal s ⁽²⁾ is generated by the transmission unit 38, and the channel information H ₂₁ and H ₂₂ are transmitted from the MS # 2 (44) through the feedback channel 47. Only the transmission signal s ⁽¹⁾ is notified from BS # 1 (32). Therefore, there is no need to share transmission weight information between base stations. As shown in FIG. 3, the transmission signal s ⁽¹⁾ is transmitted from BS # 1 (32) to BS # 2 (37) through the transmission path 48 between base stations, and BS # 1 (32 ⁾ is transmitted from BS # 2 (37) to BS # 1 ( The transmission signal s ⁽²⁾ is transmitted to 32).

FIG. 4 shows a flowchart of processing on the base station side of the present invention. In S51, channel channel information transmitted from the mobile station through the feedback channel is received. This feedback information is transmitted from a mobile station located in the base station. That is, in the base station BS # 1 (32), feedback information is transmitted from the mobile station MS # 1 (42) in the cell. MS # in 1 (42), can be measured and BS # 1 (32) the channel information H ₁₂ for receiving the signal transmitted from the channel information H ₁₁ and BS # 2 (37) for receiving the signal transmitted from , H ₁₁ and H ₁₂ are fed back. In the base station BS # 2 (37), feedback information is transmitted from the mobile station MS # 2 (44) in the cell. MS # in 2 (44), can be measured and BS # 2 channel information H ₂₁ for receiving the signal transmitted from the channel information H ₂₂ and BS # 1 for receiving the signal transmitted (32) from (37) feeds back H ₂₁ and H _22.

In S52, the transmission weight shown in Equation (11) is created using the channel information fed back. In S53, the transmission signal is shared between base stations. That is, BS # 1 (32) transmits transmission signal s ⁽¹⁾ to BS # 2 (37) and receives transmission signal s ⁽²⁾ from BS # 2 (37). In S54, the transmission signal is multiplied by a transmission weight to generate a transmission signal with transmission weight control transmitted from the base station. This is shown by equation (14). In S55, a transmission weight controlled signal is transmitted.

The number of MIMO antennas will be described below. In the system shown in FIG. 3, the number of MIMO antennas for each base station and mobile station is set to two. That is, it is assumed that 2 × 2 MIMO communication is performed in one base station and one mobile station. However, the number of antennas is not limited to this, and can be other numbers. In this case, the number of antennas may be different between the transmission side and the reception side. At this time, the channel information is not a square matrix. The inverse matrix H ⁻¹ of the channel matrix cannot be generated with the transmission weights shown in Equation (11).

When the channel information is not a square matrix, the inverse matrix can be obtained as a pseudo inverse matrix (or generalized inverse matrix) H ⁺ . In this case, the transmission weight is indicated as follows.

  Further, although the present invention has been described as being applied to multi-base station cooperative MU-MIMO, it can also be applied to a single base station MU-MIMO. For example, one base station has MIMO antennas # 1 to # 4, antennas # 1 and # 2 transmit signals for MS # 1, and antennas # 3 and # 4 transmit signals for MS # 2. Even in such a case, the transmission weight represented by the equation (11) or (15) can be similarly applied.

電力の正規化を以下のように行う。

このように、各アンテナの送信電力が１となるように、アンテナ毎のノルムで正規化を行うことで、電力の増加を抑えることができる。 Also, when the transmission weight according to the proposed method is applied, the power of the dediagonalization term of the weight matrix increases. For this reason, the power increase is suppressed by normalizing the power for each antenna. Since normalization is performed for each antenna, there is no need to share information between base stations. For simplicity, the transmission weight of a 2 × 2 matrix is expressed as follows:

Power normalization is performed as follows.

In this manner, normalization is performed with the norm for each antenna so that the transmission power of each antenna becomes 1, so that an increase in power can be suppressed.

  In the transmission weight control according to the proposed method, good characteristics (that is, high throughput) tend to be obtained when the mobile station is located near the base station. The closer the mobile station is to the base station, that is, the further away from the adjacent base station that performs cooperative communication, the smaller the interference from the adjacent base station and the smaller the off-diagonal term component in the transmission weight. As a result, the ratio of the desired signal component is increased and the throughput improvement effect is increased.

  FIG. 5 shows simple examples of throughput characteristics when the transmission weight control by the proposed method is applied and when the transmission weight control by the block diagonalization method is applied. It is assumed that multiple base station cooperative MU-MIMO is performed between two base stations BS # 1 and BS # 2. It is assumed that mobile station MS # 1 is in the cell of base station BS # 1 and moves in the direction of base station BS # 2 from a position near base station BS # 1. The horizontal axis of the graph of FIG. 5 is the position of the mobile station MS # 1, and shows from the vicinity of the base station BS # 1 to the position of the cell boundary between the base stations BS # 1 and BS # 2. The mobile station MS # 2 is assumed to be in the cell of the base station BS # 2 and fixed. The vertical axis represents the total throughput of the two mobile stations MS # 1 and MS # 2. When the transmission weight control according to the proposed method is applied, the throughput fluctuation due to the position of the mobile station becomes large. In particular, high throughput is obtained near the base station. Conversely, the throughput decreases near the cell boundary.

  The characteristics shown in FIG. 5 differ depending on the environment in which there are two base stations that perform multi-base station cooperative MU-MIMO, such as cell size and radio signal propagation characteristics. If such characteristics are known in advance, it is possible to adaptively select a transmission weight control method according to the mobile station position. In FIG. 5, when the mobile station is located outside the position A (ie, near the cell boundary), the transmission weight control by the block diagonalization method is selected to obtain a higher throughput than the transmission weight control by the proposed method. It is done. On the other hand, when the mobile station is inside position A (that is, a position close to the base station), higher throughput can be obtained by applying the transmission weight control method according to the proposed method.

  The base station can obtain higher throughput by determining whether the mobile station is inside or outside the position A and switching the transmission weight control method according to the determination result. Further, when three or more transmission weight control methods are applied, two or more switching positions can be considered.

A system that performs multi-base station cooperative MU-MIMO acquires throughput characteristics by a plurality of transmission weight control methods including a proposed method and a block diagonalization method for each base station using means such as simulation or actual measurement. Based on the obtained throughput characteristics, an optimum transmission weight control method according to the mobile station position and its switching position are determined. The determined switching position and transmission weight control method are stored in the form of a table or the like. Table 1 shows an example of a selection table of transmission weight control methods according to the distance of the mobile station from the base station.

  In Table 1, the transmission weight control method is selected according to the distance between MS # 1 and BS # 1 and the distance between MS # 2 and BS # 2. The applicable transmission weight control methods are assumed to be three types A, B, and C. This transmission weight control method includes a proposed method, a block diagonalization method, a ZF-based transmission weight control method, an MMSE-based transmission weight control method, and the like. As described above, since the proposed method has a property that a high throughput is obtained when the mobile station is in the vicinity of the base station, it can be selected as the method A in Table 1.

  In Table 1, the distance is set every 10 m, but is not limited to this. The cell size (distance from the base station to the cell boundary), the measurement accuracy of the mobile station position, the type of transmission weight control method that can be selected, the amount of memory that can be recorded as a table, and the like can be set.

Further, instead of the distance between the mobile station and the base station, the level difference of the signal power from the base station received by the mobile station can also be used. For example, the signal power level difference is MS # 1 (BS # 1 to signal power level−BS # 2 to signal power level), and MS # 2 (BS # 2 to signal power level−BS # 1 to signal power). Level). Table 2 shows an example of a selection table of transmission weight control methods according to the signal power level difference.

  In Table 2, the larger the signal power level difference between base stations, the closer to the base station. A cell boundary can be set at a position where the signal power level difference is 0 dB. As in Table 1, the range and interval of the signal power level can be arbitrarily set.

  The tables shown in Tables 1 and 2 are an example, and the tables can be created by other methods.

Claims (7)

In the multi-base station cooperative MU-MIMO system in which two base stations composed of a first base station and a second base station transmit signals to two mobile stations composed of a first mobile station and a second mobile station, the 2 A transmission weight control method in which two base stations multiply transmission signals by their transmission weights,
The first base station receives channel information as feedback information from the first mobile station, and the second base station receives channel information as feedback information from the second mobile station;
The two base stations generating respective transmission signals;
Based on the received channel information, the first base station transmits a transmission weight.

And the second base station transmits a transmission weight

H _xy is channel information between the base station y and the mobile station x, H ⁺ is a pseudo inverse matrix of the channel information, I is a unit matrix, and
The first base station receives the generated transmission signal at the second base station via the inter-base station transmission path, and the second base station transmits the generated transmission at the first base station. Receiving a signal; and
Each of the two base stations multiplying the generated transmission signal and the received transmission signal by the transmission weight;
The two base stations each transmitting a transmission signal multiplied by the transmission weight;
A transmission weight control method comprising: When the first base station and the second base station, and the first mobile station and the second mobile station have the same number of MIMO antennas, the transmission weight generated by the first base station is:

The transmission weight generated by the second base station is

The transmission weight control method according to claim 1, wherein H ^-1 is an inverse matrix of channel information.   The transmission weight generated by the first base station and the second base station is a norm so that the transmission power of signals transmitted from the respective MIMO antennas of the first base station and the second base station is 1. The transmission weight control method according to claim 1, further comprising a normalizing step. In the multi-base station cooperative MU-MIMO system in which two base stations composed of a first base station and a second base station transmit signals to two mobile stations composed of a first mobile station and a second mobile station, the 2 A method in which one base station selects one transmission weight control method from among a plurality of transmission weight control methods,
The first base station measuring a first distance to the first mobile station;
The second base station measuring a second distance to the second mobile station;
When the first distance and the second distance are smaller than a predetermined threshold, the two base stations have a transmission weight of the first base station,

And the transmission weight of the second base station is

H _xy is channel information between the base station y and the mobile station x, H ⁺ is a pseudo inverse matrix of the channel information, and I is a transmission weight control method that is a unit matrix; and
A method of selecting a transmission weight control method.   When at least one of the first distance and the second distance is greater than the predetermined threshold, the two base stations select a transmission weight control method different from the selected transmission weight control method; The method for selecting a transmission weight control method according to claim 4, further comprising:   The method of selecting a transmission weight control method according to claim 5, wherein the different transmission weight control methods include a block diagonalization method, a ZF-based transmission weight control method, and an MMSE-based transmission weight control method. By comparing the first level difference received by the first base station from the first mobile station and the second level difference received by the second base station from the second mobile station with a predetermined second threshold value, Selecting one transmission weight control method from the plurality of transmission weight control methods;
The first level difference is a difference between a signal reception level from the first base station received by the first mobile station and a signal reception level from the second base station, and the second level difference is: 5. The method of selecting a transmission weight control method according to claim 4, wherein the transmission weight control method is a difference between a signal reception level from the second base station and a signal reception level from the first base station received by the second mobile station.

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