JP2015119232A - Wireless communication system, wireless communication method, and base station device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress deterioration in a modulation encoding system caused by that a slave base station cannot transmit cooperative transmission data due to influences of a jitter in a transmission path between base stations and the like, to improve the throughput of the base station, in such a communication system that one base station becomes a master base station, and the other cooperative base stations become slave base stations, and the master base station transfers the cooperative transmission data to the slave base stations, among wireless communication systems utilizing a cooperative multipoint transmission/reception technology.SOLUTION: A slave base station determines whether or not transmission of cooperative transmission data is successful, and transmits the information to a master base station. The master base station determines a modulation encoding system on the basis of successful/unsuccessful information of cooperative transmission received from the slave base station and ACK/NACK (Negative ACK) information of a HARQ (Hybrid Automatic Request Repeat) received from a terminal.

Description

  The present invention relates to a radio communication system and method using a cooperative multipoint transmission / reception technique, and a base station apparatus.

1. Cooperative multipoint communication In recent years, technology development of the fourth generation mobile radio communication system (IMT-Advanced) has been active. As IMT-Advanced, there is LTE-Advanced (LTE-A), which is discussed in the standardization organization 3GPP. In the LTE-A scheme, by applying a coordinated multiple-point transmission / reception (CoMP) technology in which a plurality of base station apparatuses cooperate to communicate with one terminal at the same time, a cell boundary or Throughput at sector boundaries can be improved. Non-Patent Document 1 describes CoMP technology in LTE-A.

  FIG. 1 is a diagram illustrating an example of a Joint Transmission (JT) method, which is one of CoMP technologies. In the JT system, the terminal 10 is connected to one base station 1-1 and receives a signal transmitted from the base station 1-1 in a downlink control channel (Physical Downlink Control Channel: PDCCH) region. In addition, the terminal 10 receives signals transmitted in the downlink shared channel (PDSCH) area from the plurality of base stations 1-1 and 1-2. The plurality of base stations 1-1, 1-2 transmit different data A and B, or transmit the same data A, in the same resource block (Resource Block: RB). FIG. 1 shows an example in which different data A and B are transmitted.

  FIG. 2 is a diagram illustrating an example of a dynamic point selection (DPS) method which is one of CoMP technologies. In the DPS scheme, one base station having a good wireless environment is selected from the plurality of base stations 1-1 and 1-2, and the base station transmits data A to the terminal 10 using the RB included in the PDSCH region. However, other cooperating base stations do not transmit (mutate) radio waves in the RB. FIG. 2 illustrates an example in which the base station 1-1 transmits data A and the base station 1-2 performs muting. Also in this scheme, the terminal 10 is connected to one base station 1-1 and receives a signal transmitted from the base station in the PDCCH region.

  As a method for realizing the CoMP technology described above, there is a distributed control method in which an arbitrary base station serves as a master node to control a cooperative node. In this method, for example, the base station to which the terminal is connected is the control base station (master base station), the other cooperating base stations are subordinate base stations (slave base stations), and the master base station leads the cooperative operation. Necessary information such as scheduling information and cooperative transmission data is transferred to the slave base station, and the slave base station operates in accordance with instructions from the master base station. When transferring information necessary for cooperative operation from a master base station to a slave base station, an IP (Internet Protocol) line called an X2 interface defined by 3GPP, an S1 interface via a backhaul network, or between base stations A wired transmission line such as a dedicated line for realizing communication is used. Note that information necessary for cooperative operation may be transferred via a wireless transmission path instead of a wired transmission path.

2. Link adaptation
Link adaptation is a technique for increasing the transmission rate as much as possible while ensuring communication quality by selecting an appropriate modulation and coding scheme (MCS) according to the radio environment. Generally, when an MCS with high transmission efficiency is selected, the throughput increases. In LTE-A, CQI (Channel Quality Indicator) and PMI (Precoding Matrix Indicator) are fed back from the terminal to the base station. Since an error may occur between the MCS obtained from these information and the optimum MCS in a certain wireless environment, the purpose of link adaptation is to fill in the error. As a method of correcting the error, for example, a predetermined block error rate (NACK (Negative Acknowledge): Negative ACK) information used in a hybrid automatic request repeat (HARQ) information is used. There is a method of controlling the MCS so that it converges to Block Error Rate (BLER). In this method, if the ACK is received N times continuously, it is determined that the radio environment is good, and an MCS having higher transmission efficiency is selected. If the NACK is received M times continuously, the radio environment is determined to be bad. Thus, an MCS with lower transmission efficiency is selected.

3GPP TS 36.814 v9.0.0, “Evolved Universal Terrestrial Radio Access (E-UTRA); Fur- erances for E-UTRA physical layer aspects”, p15-19.

  When the CoMP technology introduced in the background art is applied to a wireless communication system, the master base station performs HARQ retransmission control and MCS management used in the slave base station, and the PDCCH is transmitted only from the master base station. Also, when the slave base station transmits data different from the master base station in the JT method, or when data is transmitted from the slave base station in the DPS method, the terminal transmits an ACK / NACK for the data transmitted from the slave base station. To the master base station. Here, when transferring information necessary for cooperative operation from the master base station to the slave base station, the transfer time may be longer than expected due to the increase in processing load of the master base station and jitter in the transmission path between base stations. There is. At this time, the slave base station may not be able to schedule the coordinated transmission data by the time instructed by the master base station, and the slave base station may not be able to transmit the coordinated transmission data. If the slave base station is transmitting data different from that of the master base station, the terminal cannot receive the desired data from the slave base station if the coordinated transmission data cannot be transmitted, so the NACK for the data transmitted from the slave base station To the master base station. In link adaptation, the radio environment between the slave base station and the terminal is estimated using HARQ ACK / NACK. For this reason, the master base station cannot distinguish whether the master base station has received NACK for data transmitted from the slave station due to degradation of the radio environment, or whether the slave base station has received NACK because the cooperative transmission data could not be transmitted. . Therefore, the master base station assumes that the radio environment between the slave base station and the terminal has deteriorated, and lowers the MCS for the slave base station more than necessary. For this reason, MCS control by link adaptation cannot be performed correctly, and an MCS having lower transmission efficiency than an MCS adapted to the original wireless environment is selected, resulting in a deterioration in throughput.

  The present invention has been made to solve the above-described problems, and in a wireless communication system using cooperative multipoint transmission / reception technology, grasps whether a cooperative base station has been able to transmit cooperative transmission data and performs appropriate modulation. And it aims at improving the throughput of a base station by selecting an encoding system.

  In order to solve the above problems, in the present invention, a control base station apparatus to which a terminal connects and a dependent base station apparatus that cooperates with the control base station apparatus, the control base station apparatus cooperates with the dependent base station apparatus. Transmitting transmission data, the subordinate base station apparatus determines whether the transmission of the coordinated transmission data to the terminal has succeeded or failed, and transmits the success or failure information of the coordinated transmission indicating the determination result to the control base station apparatus. The control base station apparatus determines the modulation and coding scheme based on the success or failure information of the coordinated transmission and the HARQ ACK or NACK information received from the terminal.

  ADVANTAGE OF THE INVENTION According to this invention, the throughput of a base station can be improved in the radio | wireless communications system using a cooperation multipoint transmission / reception technique.

It is a figure which shows the example of the Joint Transmission (JT) system which is one of the CoMP techniques. It is a figure which shows the example of the Dynamic Point Selection (DPS) system which is one of the CoMP techniques. It is a sequence diagram explaining the flow of exchanging information required in order to perform the data flow and link adaptation for CoMP in one Example of this invention. It is a figure explaining the format which notifies the information which succeeded / failed in the cooperative transmission in one Example of this invention. It is a flowchart explaining the process which determines MCS used for the data transmitted from the master base station in one Example of this invention. It is a mapping table of CQI to MCS used in LTE. It is a flowchart explaining the process which determines MCS used for the data transmitted from the slave base station in one Example of this invention. 4 is a mapping table of ACK / NACK information and success / failure information of cooperative transmission for data transmitted from a slave base station managed by a master base station in an embodiment of the present invention. It is a flowchart explaining the outer loop control used for the process which determines MCS used for the data transmitted from the slave base station in one Example of this invention. It is a block diagram of the base station in one Example of this invention. It is a block diagram of the baseband signal processing part in one Example of this invention. It is a flowchart explaining the outer loop control used for the process which determines MCS used for the data transmitted from the master base station and slave base station in one Example of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings using examples. Note that the same reference numerals are assigned to substantially the same parts, and description thereof will not be repeated. In addition, the wireless communication system will be described based on LTE and LTE-Advanced systems, but the wireless communication system is not limited thereto.

  In the following embodiments, when it is necessary for the sake of convenience, the description will be divided into a plurality of embodiments. However, unless otherwise specified, they are not irrelevant to each other. Some or all of the modifications, details, supplementary explanations, and the like exist.

  In addition, in the following examples, when referring to the number of elements (including the number, numerical value, quantity, range, etc.), unless otherwise specified, or in principle limited to a specific number in principle. The number is not limited to the specific number, and may be a specific number or more.

  Further, in the following embodiments, the constituent elements (including element steps) are not necessarily essential unless explicitly stated or considered to be clearly essential in principle. Needless to say.

  Example 1 will be described. Hereinafter, a case where the slave base station transmits data different from the master base station will be described as an example.

  Conventionally, whether the master base station has received a NACK for data transmitted from a slave station due to degradation of the radio environment or whether the slave base station has received a NACK due to the failure to transmit coordinated transmission data. Since it was not possible, MCS control by link adaptation could not be performed correctly. In other words, if it can be known that the NACK has been received because the coordinated transmission data could not be transmitted, the MCS control can be correctly performed.

  In the present embodiment, a method for selecting an appropriate MCS in a wireless communication system to which the CoMP technology is applied by reflecting information on whether or not the slave base station has been able to transmit the cooperative transmission data in the link adaptation is presented.

  A data flow and a flow for exchanging information necessary for link adaptation in a wireless communication system to which the CoMP technology is applied will be described with reference to FIGS.

  FIG. 3 is a sequence diagram showing a flow of data for performing CoMP and a flow of exchanging information necessary for performing link adaptation.

  FIG. 4 is a diagram illustrating a format for notifying information on successful / failed coordinated transmission.

  As shown in FIG. 3, the master base station notifies the slave base station of control information such as the transmission time of MCS, frequency resources, and coordinated transmission data (S101). Next, the master base station transfers data that the slave base station transmits to the terminal to the slave base station (S102). In the transfer of control information and transmission data, an X2 interface or the like is used. However, in this embodiment, the interface used for transfer is not limited to the X2 interface. When the data transmission time is reached, the master base station transmits transmission data scheduling information on the PDCCH and uplink transmission resource information for returning HARQ ACK / NACK for the transmission data to the terminal (S103). Data is transmitted from each of the master base station and the slave base station (S104-1, S104-2). When the transmission time instructed by the master base station is T, the slave base station regards the coordinated transmission data as scheduled at time T using the control information and the transmission data received from the master base station. Data can be sent to the terminal. On the other hand, when data cannot be received from the master base station, or when coordinated transmission data cannot be scheduled at time T due to delay in data reception, the coordinated transmission data cannot be transmitted.

  Returning to FIG. 3, the slave base station determines whether or not the cooperative transmission data transferred from the master base station has been transmitted (S105), and notifies the result to the master base station using 1 bit of 0/1 ( S106). 0 indicates that the coordinated transmission has failed, and 1 indicates that the coordinated transmission has succeeded. In the process S105, it is determined that the coordinated transmission data is transmitted in the process S104, and it is determined that the coordinated transmission is successful. In process S106, as shown in FIG. 4, the success / failure information of the coordinated transmission is notified in association with the transmitted frame number and subframe number. In order to notify the success / failure information of the coordinated transmission from the slave base station to the master base station, for example, an X2 interface is used. Here, regarding the notification method of the success / failure information of the coordinated transmission, information on one transmission data may be notified sequentially or information on a plurality of transmission data may be notified collectively.

  Returning to FIG. 3 again, the terminal transmits ACK / NACK for the data transmitted from the master base station and the slave base station to the master base station (S107-1, S107-2). In FIG. 3, the processing S107 is shown to be performed after the processing S106. However, in this embodiment, the order may be changed. Further, the terminal reports CQI to the master base station (S108). In FIG. 3, the process S108 is shown to be performed after the process S107, but the order of this process does not depend on other processes, and may be performed before the process S101, for example. Then, the master base station performs link adaptation using the CQI reported from the terminal, the ACK / NACK information notified from the terminal, and the success / failure information of the coordinated transmission notified from the slave base station, and the master base station The MCS used for data to be transmitted and the MCS used for data transmitted from the slave base station are determined (S109).

  Hereinafter, the detailed operation of the process S109 will be described.

  The process of determining the MCS used for data transmitted from the master base station will be described using mathematical expressions and FIGS.

  FIG. 5 is a flowchart for explaining processing for determining MCS used for data transmitted from the master base station. The period for performing this process may be every subframe, every frame, or every few frames.

  FIG. 6 is a CQI-to-MCS mapping table used in LTE.

  First, it is determined whether CQI is received from the terminal (S201). If CQI is received, Efficiency is calculated from the received CQI index, and the value is set to Effi_curr (S202). The mapping from CQI index to Efficiency is performed using the mapping table shown in FIG. After calculating Effi_curr, Effi_ave (t), which is a value obtained by averaging Efficiency using Equation 1, is calculated (S203).

数１に示されるように、Ｅｆｆｉ＿ａｖｅ（ｔ）は前回処理Ｓ１０９を行ったときに算出したＥｆｆｉ＿ａｖｅ（ｔ−１）から求められる。そのため、Ｅｆｆｉ＿ａｖｅ（ｔ）の計算結果は、処理Ｓ１０９を行う周期毎、数１の計算が完了した後に基地局内部のメモリに保持しておいて次回の処理の際に参照できるようにしておく。また、端末が接続されて処理を開始した時点ではＥｆｆｉ＿ａｖｅ（ｔ）は計算されていないので、初期値を０などに設定しておく。ここで、αは平均化に用いられる忘却係数を表すコンフィグレーションパラメータであり、この値をいくつにするかは、例えば、オペレータがあらかじめ設定しておく。αの値としては、例えば、α＝０．９のようにオペレータが任意の値を設定する。処理Ｓ２０１にて端末からＣＱＩを受信していないと判断した場合、数２にて求められる、前回処理Ｓ１０９を行ったときに算出したＥｆｆｉ＿ａｖｅ（ｔ−１）を平均化した結果Ｅｆｆｉ＿ａｖｅ（ｔ）として以降の処理に用いる（Ｓ２０４）。

As shown in Equation 1, Effi_ave (t) is obtained from Effi_ave (t−1) calculated when the previous processing S109 was performed. Therefore, the calculation result of Effi_ave (t) is held in the memory inside the base station after the calculation of Formula 1 is completed for each cycle in which processing S109 is performed, so that it can be referred to in the next processing. Also, since Effi_ave (t) is not calculated at the time when the terminal is connected and starts processing, the initial value is set to 0 or the like. Here, α is a configuration parameter representing a forgetting factor used for averaging, and the number of this value is set in advance by an operator, for example. As the value of α, for example, the operator sets an arbitrary value such as α = 0.9. If it is determined in step S201 that CQI has not been received from the terminal, Effi_ave (t−1) calculated when performing the previous processing S109, which is obtained by Equation 2, is averaged as Effi_ave (t). This is used for subsequent processing (S204).

次に、端末からマスタ基地局から送信されたデータに対するＡＣＫ／ＮＡＣＫを少なくとも１つ以上通知を受けているかどうか判断する（Ｓ２０５）。ＡＣＫ／ＮＡＣＫの通知を受けている場合、数３を用いてＥｆｆｉｃｉｅｎｃｙを補正するためのオフセット値ｏｆｆｓｅｔ＿ｏｐｔ（ｔ）を算出する（Ｓ２０６）。

Next, it is determined whether at least one ACK / NACK notification for data transmitted from the master base station is received from the terminal (S205). If an ACK / NACK notification is received, an offset value offset_opt (t) for correcting Efficiency is calculated using Equation 3 (S206).

この補正はアウターループ制御として言及され、無線環境に合わせた適切なＭＣＳを選択させるために行われる。数３に示されるように、ｏｆｆｓｅｔ＿ｏｐｔ（ｔ）は前回処理Ｓ１０９を行ったときに算出したｏｆｆｓｅｔ＿ｏｐｔ（ｔ−１）から求められるので、その計算結果は数３の計算が完了した後に基地局内部のメモリに保持しておいて次回の処理の際に参照できるようにしておく。また、端末が接続されて処理を開始した時点ではｏｆｆｓｅｔ＿ｏｐｔ（ｔ）は計算されていないので、初期値を０などに設定しておく。数３で用いているｎｕｍ ｏｆ ＡＣＫは前回の処理から今回の処理を行うまでの間に端末から受信したＡＣＫの総数、ｎｕｍ ｏｆ ＮＡＣＫは前回の処理から今回の処理を行うまでの間に端末から受信したＮＡＣＫの総数を表す。ここで、ＢｌｅｒＳｔｅｐはＥｆｆｉｃｉｅｎｃｙを上げ下げするステップ幅を制御するコンフィグレーションパラメータ、ＴａｒｇｅｔＢｌｅｒはアウターループ制御で目標とするＢＬＥＲを定義するコンフィグレーションパラメータであり、これらの値は、例えば、ＢｌｅｒＳｔｅｐ＝０．１、ＴａｒｇｅｔＢｌｅｒ＝１０のようにオペレータが任意の値を設定する。数３が意味するところは、ＡＣＫを受信したら、無線環境が良いので現在のＭＣＳよりも１つ上のＭＣＳに遷移して伝送速度を上げるべきと判断してオフセット値を上げ、ＮＡＣＫを受信したら、無線環境が悪く現在のＭＣＳでは通信品質が確保できないので、１つ下のＭＣＳに遷移して伝送速度を落としてでも通信品質を確保すべきと判断してオフセット値を下げる。そして、ＡＣＫで上げるオフセット値の幅とＮＡＣＫで下げるオフセット値の幅をＴａｒｇｅｔＢｌｅｒ対１００−ＴａｒｇｅｔＢｌｅｒの関係にすることで、環境に合わせてＭＣＳが変動してもＢＬＥＲがＴａｒｇｅｔＢｌｅｒに収束させる、ということである。

This correction is referred to as outer loop control, and is performed in order to select an appropriate MCS suitable for the wireless environment. As shown in Equation 3, offset_opt (t) is obtained from the offset_opt (t−1) calculated when the previous processing S109 was performed, and thus the calculation result is obtained after the calculation of Equation 3 is completed. It is stored in the memory so that it can be referred to in the next processing. Further, since offset_opt (t) is not calculated at the time when the terminal is connected and starts processing, the initial value is set to 0 or the like. The num of ACK used in Equation 3 is the total number of ACKs received from the terminal from the previous process to the current process, and the num of NACK is received from the terminal from the previous process to the current process. Represents the total number of NACKs received. Here, BlerStep is a configuration parameter that controls the step width for increasing / decreasing Efficiency, and TargetBler is a configuration parameter that defines a target BLER in outer loop control. These values are, for example, BlerStep = 0.1, The operator sets an arbitrary value such as TargetBler = 10. The meaning of Equation 3 is that if ACK is received, the wireless environment is good, so it is determined that the transmission rate should be increased by shifting to MCS one higher than the current MCS, and if NACK is received Since the wireless environment is poor and communication quality cannot be ensured with the current MCS, it is determined that the communication quality should be ensured even if the transmission speed is lowered by shifting to the next lower MCS, and the offset value is lowered. And, by making the width of the offset value raised by ACK and the width of the offset value lowered by NACK a relationship of TargetBler vs. 100-TargetBler, BLER converges to TargetBler even if MCS changes according to the environment. is there.

  Returning to FIG. 5, if it is determined in step S205 that the terminal has never received an ACK / NACK notification, offset_opt (t−1) calculated when the previous process S109 is performed, which is obtained in equation (4). ) Is used for subsequent processing as offset_opt (t) (S207).

アウターループ制御によりｏｆｆｓｅｔ＿ｏｐｔ（ｔ）を算出した後、数５を用いて、Ｅｆｆｉ＿ａｖｅ（ｔ）にｏｆｆｓｅｔ＿ｏｐｔ（ｔ）による補正を加えた、適切なＥｆｆｉｃｉｅｎｃｙを表すＥｆｆｉ＿ｏｐｔを算出する（Ｓ２０８）。

After calculating offset_opt (t) by outer loop control, Effi_opt representing appropriate Efficiency is calculated by adding correction by offset_opt (t) to Effi_ave (t) using Equation 5 (S208).

そして、Ｅｆｆｉ＿ｏｐｔの値が高すぎたり低すぎたりすることでＭＣＳが上限あるいは下限に張り付くことがないよう、Ｅｆｆｉ＿ｏｐｔの上限値、下限値に制限を設ける（Ｓ２０９）。処理Ｓ２０８で得られたＥｆｆｉ＿ｏｐｔの値がコンフィグレーションパラメータＥｆｆｉＴｈＭｉｎを下回っていた場合、ｏｆｆｓｅｔ＿ｏｐｔ（ｔ）を

Then, the upper limit value and lower limit value of Effi_opt are limited so that the MCS does not stick to the upper limit or lower limit due to the value of Effi_opt being too high or too low (S209). If the value of Effi_opt obtained in step S208 is lower than the configuration parameter EffiThMin, offset_opt (t) is set.

に再計算し、その後、Ｅｆｆｉ＿ｏｐｔを

Then recalculate Effi_opt

に従って値をＥｆｆｉＴｈＭｉｎにする。一方、Ｅｆｆｉ＿ｏｐｔの値がコンフィグレーションパラメータＥｆｆｉＴｈＭａｘ以上であった場合、ｏｆｆｓｅｔ＿ｏｐｔ（ｔ）を

To EffiThMin. On the other hand, if the value of Effi_opt is greater than or equal to the configuration parameter EffiThMax, offset_opt (t) is set to

に再計算し、その後、Ｅｆｆｉ＿ｏｐｔを

Then recalculate Effi_opt

に従って値をＥｆｆｉＴｈＭａｘにする。処理Ｓ２０９によってｏｆｆｓｅｔ＿ｏｐｔ（ｔ）を再計算した場合、その計算結果を数６または数８の計算が完了した後に基地局内部のメモリに保持する。ここで、ＥｆｆｉＴｈＭｉｎの値は基地局が取りうる最低のＭＣＳに対応するＥｆｆｉｃｉｅｎｃｙに設定するのが望ましく、例えば、ＥｆｆｉＴｈＭｉｎ＝０．０１のように設定する。ＥｆｆｉＴｈＭａｘについても同様で、この値は基地局が取りうる最大のＭＣＳに対応するＥｆｆｉｃｉｅｎｃｙに設定するのが望ましく、例えば、ＥｆｆｉＴｈＭａｘ＝５．８のように設定する。

To EffiThMax. When offset_opt (t) is recalculated in step S209, the calculation result is held in the memory inside the base station after the calculation of Equation 6 or Equation 8 is completed. Here, it is desirable to set the value of EffiThMin to Efficiency corresponding to the lowest MCS that the base station can take, for example, EffiThMin = 0.01. The same applies to EffiThMax, and this value is desirably set to Efficiency corresponding to the maximum MCS that the base station can take, for example, EffiThMax = 5.8.

  Finally, MCS is determined from Effi_opt using the mapping table shown in FIG. 6 (S210). As a specific determination method, for example, there is a method of selecting an MCS having the closest Efficiency from Effi_opt. In this example, if Effi_opt = 2.6, Efficiency = 2.7305 is the closest. MCS is determined to be 64QAM-466 / 1024.

  By performing the above processing, it is possible to appropriately select the MCS used for data transmitted from the master base station.

  The process of determining the MCS used for the data transmitted from the slave base station will be described using equations, FIGS. 3, 4, 5, 7, 7, and 9.

  FIG. 7 is a flowchart for explaining the process of determining the MCS used for data transmitted from the slave base station, and FIG. 8 shows the cooperation with the ACK / NACK information for the data transmitted from the slave base station managed by the master base station. FIG. 9 is a flowchart for explaining outer loop control used for processing for determining MCS used for data transmitted from a slave base station.

  The master base station determines the success / failure information of the cooperative transmission notified from the slave base station in accordance with the format shown in FIG. 4 in step S106 in FIG. 3, and the slave notified from the terminal in step S107-2 in FIG. The ACK / NACK information for the data transmitted from the base station is associated with the frame number and the subframe number. For example, the master base station receives an ACK for data transmitted from the slave base station at frame number N and subframe number # 3 from the terminal, and the result of coordinated transmission from the slave base station at frame number N and subframe number # 3 is 1, that is, when receiving the notification that the coordinated transmission is successful, the master base station obtains information that the data transmitted from the slave base station in frame number N and subframe number # 3 is ACK and the coordinated transmission is successful. This information is managed using the table shown in FIG. 8 using, for example, a memory in the base station, and used for processing for determining the MCS used for data transmitted from the slave base station. By this association, it is possible to determine whether the NACK information for the data transmitted from the slave base station notified from the terminal is due to the failure of coordinated transmission or due to degradation of the radio environment. it can.

Returning to FIG. 7, processing for determining the MCS used for data transmitted from the slave base station will be described. The period for performing the processing shown in FIG. 7 needs to be at least equal to or greater than the period in which the success / failure information of cooperative transmission is returned from the slave base station. Processing S301 to processing S304 performs the same processing as processing S201 to processing S204 shown in the flowchart of FIG. In step S305, it is determined whether or not at least one ACK / NACK notification for data transmitted from the slave base station is received from the terminal. If an ACK / NACK notification has been received, offset_opt (t) is calculated using the flowchart shown in FIG. 9 (S306). If the ACK / NACK notification is not received, the same processing as the processing S207 shown in FIG. 5 is performed. (S307)
The procedure for calculating offset_opt (t) will be described with reference to the flowchart of FIG. First, a variable offset_tmp used to temporarily hold an offset value for outer loop control is initialized to 0 (S401). Next, it is confirmed whether information to be used for calculation of outer loop control remains in the mapping table of FIG. 8 (S402). If the information remains, the head information of the mapping table in FIG. 8 is referred to, and it is confirmed whether the cooperative transmission result is successful: 1 in the information (S403). If the result is “1”, the offset value offset_tmp for correcting Efficiency is calculated using Equation 10 (S404).

協調送信に成功していることの通知をスレーブ基地局から受けている場合は、ＡＣＫ／ＮＡＣＫ情報が信用できるものと判断し、通常のアウターループ制御に従ってｏｆｆｓｅｔ＿ｔｍｐを計算すればよい。ここで、数１０で用いているｎｕｍ ｏｆ ＡＣＫは図８のマッピングテーブルの先頭の情報がＡＣＫになっていれば値が１となり、ＮＡＣＫになっていれば値が０となる。ｎｕｍ ｏｆ ＮＡＣＫについても同様で、図８のマッピングテーブルの先頭の情報がＡＣＫになっていれば値が０となり、ＮＡＣＫになっていれば値が１となる。一方、処理Ｓ４０３において協調送信結果が失敗：０となっている場合、数１１を用いてＥｆｆｉｃｉｅｎｃｙを補正するためのオフセット値ｏｆｆｓｅｔ＿ｔｍｐを計算する（Ｓ４０５）。

If the slave base station receives a notification that the coordinated transmission is successful, it is determined that the ACK / NACK information can be trusted, and offset_tmp may be calculated according to normal outer loop control. Here, the value of num of ACK used in Equation 10 is 1 if the top information in the mapping table of FIG. 8 is ACK, and 0 if it is NACK. The same applies to num of NACK. The value is 0 if the top information of the mapping table in FIG. 8 is ACK, and the value is 1 if the information is NACK. On the other hand, when the cooperative transmission result is failure: 0 in process S403, the offset value offset_tmp for correcting Efficiency is calculated using Equation 11 (S405).

数１０との違いは、ＡＣＫ／ＮＡＣＫにしたがってＥｆｆｉｃｉｅｎｃｙの補正値を計算する式に係数βがかかっている点である。βは端末から通知されるスレーブ基地局から送信されたデータに対するＮＡＣＫ情報の信頼度を決定するコンフィグレーションパラメータであり、０から１の範囲をとりうる。β＝１であるということは、スレーブ基地局が協調送信に失敗したとしても、端末から通知されるＮＡＣＫ情報が信頼できるものとみなすことを意味し、β＝０であるということは、スレーブ基地局が協調送信に失敗している場合は端末から通知されるＮＡＣＫ情報が信頼できないとみなし、ＮＡＣＫである場合にアウターループ制御によるＥｆｆｉｃｉｅｎｃｙの補正を一切行わないことを意味する。つまり、β＝０に設定することにより、無線環境の劣化以外の要因でＭＣＳを必要以上に下げることがなくなり、環境に合わせて適切なＭＣＳを選択できるようになる。

The difference from Equation 10 is that a coefficient β is applied to the equation for calculating the correction value of Efficiency according to ACK / NACK. β is a configuration parameter that determines the reliability of the NACK information for the data transmitted from the slave base station notified from the terminal, and can take a range from 0 to 1. β = 1 means that even if the slave base station has failed in coordinated transmission, the NACK information notified from the terminal is regarded as reliable, and β = 0 means that the slave base station If the station has failed in the coordinated transmission, the NACK information notified from the terminal is regarded as unreliable, and if it is NACK, it means that the correction of efficiency by the outer loop control is not performed at all. That is, by setting β = 0, the MCS is not lowered more than necessary due to factors other than deterioration of the radio environment, and an appropriate MCS can be selected according to the environment.

  Returning to FIG. 9, after calculating offset_tmp, the top information in the mapping table of FIG. 8 used for the calculation of the outer loop control this time is deleted (S406).

  After the processing of S403 to S406 is repeated and all information is deleted from the mapping table of FIG. 8, if it is determined that there is no information in processing S402, offset_opt (t -1) and offset_tmp are used to calculate offset_opt (t) (S407).

ｏｆｆｓｅｔ＿ｏｐｔ（ｔ）を算出した後の処理は、図７の処理Ｓ３０８、Ｓ３０９、及びＳ３１０が行われ、これらは図５の処理Ｓ２０８、Ｓ２０９、及びＳ２１０にそれぞれ対応している。上記の処理により、スレーブ基地局から送信されるデータに用いるＭＣＳが決まる。

Processing after calculating offset_opt (t) is performed in steps S308, S309, and S310 in FIG. 7, which correspond to steps S208, S209, and S210 in FIG. 5, respectively. The MCS used for data transmitted from the slave base station is determined by the above processing.

  Whether the NACK was received for the data transmitted from the slave station due to the deterioration of the radio environment or the slave base station was unable to transmit the coordinated transmission data by performing the above processing. The MCS used for the data transmitted from the slave base station can be appropriately selected after being distinguished, and the throughput can be improved.

  FIG. 10 shows a configuration example of the base station.

  In FIG. 10, the base station includes an antenna 1001, an RF (Radio Frequency) unit 1002, a baseband signal processing unit 1003, a CPU (Central Processing Unit) unit 1004, a network interface (NW I / F) unit 1007, , And a memory 1008. The CPU unit 1004 includes a scheduler unit 1005 and a link adaptation unit 1006 that performs processing related to MCS selection in the embodiment. Note that the scheduler unit 1005 and the link adaptation unit 1006 can be realized by hardware or software. When implemented by software, the CPU unit 1004 reads out and executes each program to implement these functions.

  A network interface (NW I / F) unit 1007 has an interface with the network network, and between the master base station and the slave base station, the cooperative transmission data of the embodiment, the control information necessary for the cooperative transmission, and the success / Send and receive failure information. The CPU unit 1004 controls the entire base station. The scheduler unit 1005 is built in the CPU unit 1004 and determines transmission timing, transmission beam, transmission power, and frequency resource allocation. The scheduler unit 1005 of the master base station extracts cooperative transmission data to be transferred to the slave base station and generates control information such as frequency resources. The scheduler unit 1005 of the slave base station determines the success / failure of the coordinated transmission, which is the processing of the above-described embodiment, and generates a transmission message that notifies the information. The link adaptation unit 1006 is built in the CPU unit 1004 and performs selection of MCS, which is the processing of this embodiment described above. The memory 1008 accumulates the CQI-to-MCS mapping table shown in FIG. 6, the control information used for link adaptation calculation of the embodiment, the control information necessary for transmission / reception, and the downstream signal transmitted from the network. The master base station memory 1008 stores mapping information (FIG. 8) of ACK / NACK information and cooperative transmission success / failure information for data transmitted from the slave base station. The slave base station memory 1008 stores coordinated transmission data transferred from the master base station and control information necessary for transmission. The baseband signal processing unit 1003 performs baseband signal processing. The RF unit 1002 performs conversion processing between the analog transmission / reception signal and the baseband signal. In this embodiment, the scheduler unit 1005 and the link adaptation unit 1006 are built in the CPU unit 1004. However, the scheduler unit 1005 and the link adaptation unit 1006 may be built in a DSP (Digital Signal Processor) or the like.

  FIG. 11 shows a configuration example of the baseband signal processing unit of the base station.

  In FIG. 11, the transmission unit of the baseband signal processing unit 1003 includes a channel encoder unit 2001, a modulation unit 2002, a MIMO (Multiple Input Multiple Output) encoder unit 2003, a power control unit 2004, a resource block mapper unit 2005, , An IFFT (Inverse FFT) unit 2006 and a CPI (Cyclic Prefix Insulator) unit 2007.

  Note that the above-described configuration can be realized by hardware or software. When realized by software, the CPU unit 1004 reads out and executes each program, thereby realizing each function of the above-described configuration.

  Channel encoder section 2001 performs error correction coding on transmission data of a plurality of users from user i to user k according to the coding rate determined by scheduler section 1005 based on the information determined by link adaptation section 1006. Do. Modulation section 2002 performs modulation processing according to the modulation scheme determined by scheduler section 1005 based on the information determined by link adaptation section 1006. The MIMO encoder unit 2003 performs a conversion process to MIMO. The power control unit 2004 adjusts transmission power. The resource block mapper unit 2005 performs mapping to resources allocated for each user according to the frequency resource allocation determined by the scheduler unit 1005. IFFT section 2006 performs conversion processing from a frequency domain signal to a time domain signal. The CPI unit 2007 adds a CP. Hereinafter, transmission signal processing when the slave base station transmits a transmission signal by CoMP will be described.

  Returning to FIG. 10, first, the NW I / F unit 1007 receives the coordinated transmission data signal and the control information transmitted from the master base station. The memory 1008 connected to the CPU unit 1004 temporarily stores the received signal. The scheduler unit 1005 built in the CPU 1004 determines the transmission beam, modulation and coding scheme, transmission power, and frequency resource allocation for the data signal using the control information transmitted from the master base station. The data signal is processed into a transmission signal according to the determination.

  In FIG. 11, for user transmission data stored in a memory 1008 connected to the CPU unit 1004, the channel encoder unit 2001 encodes the scheduler unit 1005 based on the control information transmitted from the master base station. Error correction coding is performed according to the rate. Next, modulation section 2002 converts the error correction coded data into a modulated signal according to the modulation scheme determined by scheduler section 1005 based on the control information transmitted from the master base station. The modulation signal is a signal having a constellation on the IQ signal plane such as QPSK, 16QAM, and 64QAM. Thereafter, MIMO encoder section 2003 performs MIMO signal processing on the modulated signal and distributes the signal to each antenna. The power control unit 2004 adjusts the power of the input signal. The signal whose power is controlled by the power control unit 2004 is input to the resource block mapper unit 2005. The resource block mapper unit 2005 maps each user's signal to a resource allocated for each user according to the frequency resource allocation determined by the scheduler 1005. Resource mapping is performed for each antenna. The IFFT (Inverse FFT) unit 2006 converts the frequency domain information for each antenna into a time domain signal. With respect to the obtained time domain signal, a CPI (Cyclic Prefix Inserter) unit 2007 adds a CP and sends a baseband transmission signal to the RF unit 1002 of FIG. The RF unit 1002 converts a baseband signal into an RF signal and emits a transmission signal from the antenna 1001.

  When the coordinated transmission data can be transmitted according to the transmission signal processing described above, the scheduler unit 1005 determines that the coordinated transmission is successful, and generates a transmission message of the coordinated transmission success information according to the format of FIG. On the other hand, when the above transmission signal processing cannot be performed by the transmission time transmitted from the master base station, the scheduler unit 1005 determines that the coordinated transmission has failed, and transmits the transmission message of the coordinated transmission failure information according to the format of FIG. Is generated. Thereafter, the NW I / F unit 1007 transmits a message to the master base station.

  In this embodiment, the link adaptation method in the case where the slave base station transmits data different from the master base station in the wireless communication system to which the JT method, which is one of the CoMP technologies, is described. Even when the same data as the base station is transmitted, the link adaptation method of this embodiment can be applied. In this case, the MCS used for data transmitted from the slave base station is the same as the MCS used for data transmitted from the master base station, and the MCS is determined by the master base station. Since the data transmitted from the two base stations are the same, the terminal notifies the master base station of ACK / NACK for one data. Even in this case, since it is possible to distinguish whether the NACK information notified from the terminal is due to the failure of the coordinated transmission or due to the deterioration of the radio environment, according to the flowchart of FIG. 7 described in the present embodiment. What is necessary is just to determine MCS.

  Also, the link adaptation method of this embodiment can be applied to a wireless communication system to which the DPS method, which is one of CoMP technologies, is applied. In this case, the MCS determined by the flowchart of FIG. 5 may be applied when data is transmitted from the master base station, and the MCS determined by the flowchart of FIG. 7 may be applied when data is transmitted from the slave base station.

  Next, Example 2 will be described.

  In the first embodiment, the link adaptation method in the wireless communication system to which the JT method, which is one of the CoMP technologies, is described. In the second embodiment, a link adaptation method in a wireless communication system to which the JT method, which is one of CoMP technologies, is extended, and a method that takes into account the probability that cooperative transmission data could not be transmitted from a slave base station will be described. Hereinafter, a case where the slave base station transmits the same data as the master base station will be described as an example.

  When the slave base station transmits the same data as the master base station, even if the slave base station cannot transmit the coordinated transmission data, the terminal receives data from the master base station, so the coordinated transmission data cannot be transmitted. However, NACK is not always notified from the terminal. Therefore, it is considered that the merit of distinguishing NACK information is low compared to the case where the slave base station transmits data different from the master base station. However, when viewed from the terminal, since the place that originally receives data from both the master base station and the slave base station receives data only from the master base station, the reception CINR (Carrier to Interference plus Noise Ratio) decreases, and NACK It becomes easy to become.

  Based on the above characteristics, if the probability that the slave base station cannot transmit the coordinated transmission data is low, it is considered that there is no problem in normal handling even if NACK is received because the coordinated transmission data could not be transmitted, If the probability that the slave base station cannot transmit the coordinated transmission data is high, it is considered that the reliability of the NACK information should be lowered as in the first embodiment. Based on these ideas, a method for selecting an MCS in the present embodiment will be described below.

  The method for selecting the MCS in the present embodiment is almost the same as that in the first embodiment, but the method is different only in the process of the flowchart for calculating offset_opt (t) shown in FIG. Hereinafter, a flowchart for calculating offset_opt (t) in this embodiment will be described with reference to FIG.

  FIG. 12 is a flowchart illustrating the outer loop control used in the process of determining the MCS used for data transmitted from the master base station and the slave base station in the present embodiment. The processing from S501 to S507 is the same as the processing from S401 to S407 in the flowchart shown in FIG. 9, but the condition for calculating offset_tmp with the reliability of the NACK information lowered by the number 11 in processing S505. As a result, it is added in the processing S503 that the cooperative transmission result is failure: 0, and in the processing S508, the failure rate of the cooperative transmission exceeds the configuration parameter TargetCompErr. As a method of calculating the failure rate of coordinated transmission, the success / failure information of coordinated transmission notified from the slave base station is stored in the past N times in the memory, and the ratio of the number of information failed in the coordinated transmission contained therein is failed. There is a way to define rate. For example, if N = 50 and the number of failures is 5, the failure rate is 5/50 × 100 = 10%. As the value of the configuration parameter TargetCompErr, the operator sets an arbitrary value such as 5, for example. Even if the coordinated transmission result is failed in process S503: 0, if the failure rate of coordinated transmission is equal to or less than TargetCompErr in process S508, offset_tmp is calculated according to Equation 10 in process S504.

  By performing the above processing, if the probability that the slave base station cannot transmit the coordinated transmission data is low, even if the slave base station has received NACK from the terminal because the coordinated transmission data could not be transmitted, The MCS is selected by the conventional calculation method because it is determined that the influence on the link adaptation is small. On the other hand, if there is a high probability that the slave base station cannot transmit the coordinated transmission data, if the slave base station has received NACK from the terminal because the coordinated transmission data could not be transmitted, the reliability of the NACK information is lowered. MCS is selected. As a result, an appropriate MCS is selected in accordance with the degree of successful transmission of the coordinated transmission data, and the throughput is improved.

  In this embodiment, the case where the slave base station transmits the same data as that of the master base station in the wireless communication system to which the JT method, which is one of the CoMP technologies, is described as an example. In the system, even when the slave base station transmits data different from that of the master base station, the link adaptation method of the present embodiment can be applied to the wireless communication system to which the DPS method is applied.

  The point of the present invention is a mechanism for exchanging information indicating whether or not cooperative transmission data has been correctly transmitted using CoMP between base stations and selecting an appropriate modulation and coding scheme based on the information. Is to have. The operation of the scheduler that determines whether or not the coordinated transmission data has been correctly transmitted can be said to be within the scope of the present invention.

DESCRIPTION OF SYMBOLS 1001 ... Antenna, 1002 ... RF part, 1003 ... Baseband part, 1004 ... CPU part, 1005 ... Scheduler part, 1006 ... Link adaptation part, 1007 ... NW / IF part, 1008 ... Memory part, 2001 ... Channel encoder part, 2002 ... Modulation section, 2003 ... MIMO encoder section, 2004 ... Power control section, 2005 ... Resource block mapper section, 2006 ... IFFT section, 2007 ... CPI section.

Claims (15)

A wireless communication system using cooperative multipoint transmission / reception technology,
A control base station device to which the terminal is connected;
A dependent base station device cooperating with the control base station device,
The control base station device transmits cooperative transmission data to the dependent base station device,
The dependent base station device determines whether the transmission of the coordinated transmission data to the terminal has succeeded or failed, and transmits the success or failure information of the coordinated transmission indicating the determination result to the control base station device,
The control base station apparatus determines a modulation and coding scheme based on the success or failure information of the coordinated transmission and HARQ ACK or NACK information received from the terminal. The wireless communication system according to claim 1,
The control base station apparatus determines whether or not the NACK information is due to a failure in coordinated transmission by associating the success or failure information of the coordinated transmission with the ACK or NACK information, and A radio communication system characterized by determining a modulation and coding scheme. The wireless communication system according to claim 2,
If the control base station apparatus determines that the NACK information is due to a failure in coordinated transmission, the NACK information has failed in coordinated transmission with the correction amount of the modulation and coding scheme based on the NACK information. A wireless communication system, characterized in that it is smaller than the correction amount when it is determined not to be caused by, or the NACK information is not used for calculation of a modulation and coding scheme. A wireless communication system according to claim 3,
The control base station apparatus estimates a failure rate of the coordinated transmission based on the success / failure information of the coordinated transmission, and NACK resulting from the failure of the coordinated transmission only when the failure rate is a certain value or more. A radio communication system characterized in that a correction amount of a modulation and coding scheme based on information is reduced or the NACK information is not used for calculation of a modulation and coding scheme. The wireless communication system according to claim 1,
The control base station apparatus transmits data to the terminal and the coordinated transmission data transmitted from the dependent base station apparatus to the terminal are different from each other,
The control base station apparatus receives the ACK or NACK information for the coordinated transmission data from the terminal,
The control base station apparatus determines the modulation and coding scheme used for the coordinated transmission data based on the success or failure information of the coordinated transmission and the ACK or NACK information for the coordinated transmission data. Wireless communication system. A wireless communication method in a wireless communication system that includes a control base station device to which a terminal is connected and a dependent base station device that cooperates with the control base station device and uses cooperative multipoint transmission / reception technology,
The control base station device transmits cooperative transmission data to the dependent base station device,
The dependent base station device determines whether the transmission of the coordinated transmission data to the terminal has succeeded or failed, and transmits the success or failure information of the coordinated transmission indicating the determination result to the control base station device,
The control base station apparatus determines a modulation and coding scheme based on success or failure information of the coordinated transmission and HARQ ACK or NACK information received from the terminal. The wireless communication method according to claim 6, comprising:
The control base station apparatus determines whether or not the NACK information is due to a failure in coordinated transmission by associating the success or failure information of the coordinated transmission with the ACK or NACK information, and performs modulation. A wireless communication method characterized by determining an encoding method. The wireless communication method according to claim 7, comprising:
If the control base station apparatus determines that the NACK information is due to a failure in coordinated transmission, the NACK information has failed in coordinated transmission with the correction amount of the modulation and coding scheme based on the NACK information. A wireless communication method characterized in that it is smaller than the correction amount when it is determined that it is not caused by the NACK information, or the NACK information is not used for calculation of the modulation and coding scheme. The wireless communication method according to claim 8, comprising:
The control base station apparatus estimates a failure rate of the coordinated transmission based on the success / failure information of the coordinated transmission, and NACK resulting from the failure of the coordinated transmission only when the failure rate is a certain value or more. A radio communication method characterized by reducing a correction amount of a modulation and coding scheme based on information or not using the NACK information for calculation of a modulation and coding scheme. The wireless communication method according to claim 6, comprising:
The control base station apparatus transmits data to the terminal and the coordinated transmission data transmitted from the dependent base station apparatus to the terminal are different from each other,
The control base station apparatus receives the ACK or NACK information for the coordinated transmission data from the terminal,
The control base station apparatus determines the modulation and coding scheme used for the coordinated transmission data based on the success or failure information of the coordinated transmission and the ACK or NACK information for the coordinated transmission data. Wireless communication method. A base station apparatus in a wireless communication system using cooperative multipoint transmission / reception technology,
A transmitting unit that transmits cooperative transmission data to another base station device that cooperates with the base station device to which the terminal is connected;
A receiving unit that receives the success or failure information of the coordinated transmission indicating the determination result of whether the transmission of the coordinated transmission data to the terminal has succeeded or failed, from the other base station device;
A base station apparatus comprising: a processor unit that determines a modulation and coding scheme based on success or failure information of the coordinated transmission and HARQ ACK or NACK information received from the terminal. The base station apparatus according to claim 11, wherein
The processor unit determines whether the NACK information is due to a failure in coordinated transmission by associating the success or failure information of the coordinated transmission with the ACK or NACK information. A base station apparatus characterized by determining a conversion method. The base station apparatus according to claim 12, wherein
When the processor unit determines that the NACK information is due to a failure in cooperative transmission, the processor unit determines a correction amount of the modulation and coding scheme based on the NACK information because the NACK information has failed in cooperative transmission. A base station apparatus characterized in that it is smaller than a correction amount when it is determined that it is not, or the NACK information is not used for calculation of a modulation and coding scheme. The base station apparatus according to claim 13, wherein
The processor unit estimates the failure rate of the coordinated transmission based on the success / failure information of the coordinated transmission, and only by the NACK information resulting from the failure of the coordinated transmission only when the failure rate is a certain value A base station apparatus that reduces a correction amount of a modulation and coding scheme or does not use the NACK information for calculation of a modulation and coding scheme. A base station apparatus in a wireless communication system using cooperative multipoint transmission / reception technology,
In cooperation with other base station devices to which the terminal is connected,
A receiving unit for receiving cooperative transmission data from the other base station device;
A processor unit for determining whether the transmission of the coordinated transmission data to the terminal has succeeded or failed;
A base station apparatus comprising: a transmission unit configured to transmit success or failure information of cooperative transmission indicating the determination result to the other base station apparatus.

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