JP5449602B2 - Wireless communication system and wireless terminal - Google Patents

Description

  The present invention relates to a radio communication system, a radio terminal, and a radio communication method to which inter-base station cooperative MIMO communication is applied.

  Conventionally, as one technique that can improve frequency utilization efficiency in a wireless communication system, a wireless signal transmission side and a reception side each use a plurality of antennas and transmit a plurality of signal sequences at the same frequency and the same time. MIMO (Multi-Input Multi-Output) communication is known.

  In order to exhibit the performance of MIMO communication, it is necessary to adaptively perform transmission control according to the state of the radio propagation path. For this reason, closed-loop control is introduced in a wireless communication system in which MIMO communication is introduced. Specifically, the reception side of the radio signal generates propagation path information corresponding to the characteristics of the wireless propagation path (hereinafter referred to as propagation path characteristics), and feeds back the propagation path information to the transmission side. In particular, in an FDD wireless communication system in which there is no correlation between propagation path characteristics in uplink and downlink, feedback of propagation path information is essential.

  In recent years, as a technology developed from MIMO communication, inter-base station cooperative MIMO communication (also referred to as “multi-cell cooperative transmission / reception”) in which a plurality of radio base stations communicate with one radio terminal at the same frequency and at the same time has attracted attention. (For example, refer to Patent Document 1). According to inter-base station cooperative MIMO communication, the number of antennas that can be used for MIMO communication can be increased by using the antennas of each of a plurality of radio base stations, and the transmission speed can be improved compared to conventional MIMO communication. And reception quality can be improved.

Special table 2008-523665 gazette

  However, when closed-loop control is performed in cooperative MIMO communication between base stations, feedback is performed from a wireless terminal to each of a plurality of wireless base stations, and the overhead associated with feedback is increased compared to conventional MIMO communication. . That is, by setting a feedback control channel between the radio terminal and each of the plurality of radio base stations, the amount of radio resource consumption in the uplink increases.

  Alternatively, a control channel for feedback is set only between one radio base station among a plurality of radio base stations, and feedback from the one radio base station to another radio base station using inter-base station communication By transferring, overhead associated with feedback can be reduced. However, when transferring feedback from one radio base station to another radio base station, the feedback may not be in time due to the effect of transfer delay, and inter-base station cooperative MIMO communication may not function normally.

  Therefore, an object of the present invention is to provide a wireless communication system, a wireless terminal, and a wireless communication method that can reduce overhead caused by feedback while normally functioning cooperative MIMO communication between base stations.

  In order to solve the above-described problems, the present invention has the following features. First, the first feature of the present invention is a radio terminal (radio terminal UE) and a first radio base station that transmits a first radio signal (radio signal RS1) to the radio terminal in response to feedback from the radio terminal. (Radio base station BS1) and a second radio signal (radio signal RS2) transmitted to the radio terminal at the same frequency and at the same time as the first radio signal in response to feedback from the radio terminal. A wireless communication system (wireless communication system 1) having a wireless base station (wireless base station BS2), wherein a time interval at which the wireless terminal performs feedback to the first wireless base station is determined by the wireless terminal. The gist is that it is longer than the time interval for performing feedback to the two radio base stations.

  A second feature of the present invention relates to the first feature of the present invention, wherein the first radio base station receives at least one of a phase and an amplitude of the first radio signal according to feedback from the radio terminal. The first transmission antenna weight (transmission antenna weight 1) used for control is updated, and the second radio base station receives at least one of the phase and the amplitude of the second radio signal according to feedback from the radio terminal. A second transmission antenna weight (transmission antenna weight 2) used for control is updated, and a time interval for the first radio base station to update the first transmission antenna weight is determined by the second radio base station in the second transmission. The gist is that it is longer than the time interval for updating the antenna weight.

  A third feature of the present invention relates to the first feature or the second feature of the present invention, wherein the wireless terminal transmits the first wireless base station during a period in which feedback to the first wireless base station is omitted. First propagation path information (propagation path information 1) according to propagation path characteristics from the wireless terminal to the wireless terminal, and second propagation path information (propagation information) according to propagation path characteristics from the second wireless base station to the wireless terminal. The gist is to perform feedback based on the route information 2) to the second radio base station.

  A fourth feature of the present invention relates to the third feature of the present invention, wherein the wireless terminal uses the first propagation path information as a reference during a period in which feedback to the first wireless base station is omitted. The gist is to normalize the second propagation path information and feed back the normalized second propagation path information to the second radio base station.

  A fifth feature of the present invention relates to the third feature of the present invention, wherein the wireless terminal is configured to receive the first propagation path information and the first information during a period in which feedback to the first wireless base station is omitted. 2 propagation path information is fed back to the second radio base station, and the second radio base station is fed back from the radio terminal based on the first propagation path information fed back from the radio terminal. The gist is to normalize propagation path information and transmit the second radio signal in accordance with the normalized second propagation path information.

  A sixth feature of the present invention relates to any one of the third to fifth features of the present invention, wherein the first propagation path information includes each transmission antenna of the first radio base station and each reception of the radio terminal. It is information indicating propagation path characteristics between the antennas, and the second propagation path information is information indicating propagation path characteristics between each transmission antenna of the second radio base station and each reception antenna of the radio terminal. It is a summary.

  A seventh feature of the present invention relates to any one of the third to fifth features of the present invention, wherein the first propagation path information is used for controlling at least one of a phase or an amplitude of the first radio signal. The first transmission antenna weight (transmission antenna weight 1) or an index indicating the first transmission antenna weight, and the second propagation path information is used for controlling at least one of the phase and the amplitude of the second radio signal. The second transmission antenna weight (transmission antenna weight 2) or an index indicating the second transmission antenna weight.

  An eighth feature of the present invention relates to any one of the first to seventh features of the present invention, wherein the first radio base station is located farther from the radio terminal than the second radio base station. The gist is that it is a radio base station.

  A ninth feature of the present invention relates to any one of the first to seventh features of the present invention, wherein the first radio base station is a radio base station having higher processing performance than the second radio base station. This is the gist.

  A tenth feature of the present invention relates to any one of the first to seventh features of the present invention, wherein the first radio base station is a radio base station whose processing performance is lower than that of the second radio base station. This is the gist.

  An eleventh feature of the present invention relates to any one of the first to seventh features of the present invention, wherein the first radio base station propagates to the radio terminal more than the second radio base station. The gist is that the wireless base station has a small path fluctuation.

  A twelfth feature of the present invention relates to any one of the first to seventh features of the present invention, wherein the first radio base station propagates to the radio terminal more than the second radio base station. The gist is that the wireless base station has a large road loss.

  The thirteenth feature of the present invention is that the first radio signal (radio signal RS1) is received from the first radio base station (radio base station BS1), and the second radio signal is transmitted at the same frequency and at the same time as the first radio signal. A radio terminal (radio terminal UE) that receives a radio signal (radio signal RS2) from a second radio base station (radio base station BS2), and provides feedback to the first radio base station and the second radio base station A transmission unit (transmission unit 122) for transmission, and the time interval at which the transmission unit performs feedback to the first radio base station is greater than the time interval at which the transmission unit performs feedback to the second radio base station. The gist is also long.

  A fourteenth feature of the present invention is a first radio base station (radio base station BS1) that transmits a first radio signal (radio signal RS1) to the radio terminal in response to feedback from a radio terminal (radio terminal UE). In response to feedback from the radio terminal, a second radio base station (radio base station) transmits a second radio signal (radio signal RS2) to the radio terminal at the same frequency and at the same time as the first radio signal. Instructing the radio terminal to set a time interval for performing feedback to the first radio base station to be longer than a time interval for performing feedback to the second radio base station. The gist is to do.

  A fifteenth feature of the present invention is that a first radio base station (radio base station BS1) transmits a first radio signal to the radio terminal in response to feedback from a radio terminal (radio terminal UE); In response to feedback from the terminal, the second radio base station (radio base station BS2) transmits the second radio signal to the radio terminal at the same frequency and at the same time as the first radio signal. The time interval at which the radio terminal performs feedback to the first radio base station is longer than the time interval at which the radio terminal performs feedback to the second radio base station.

  According to the features of the present invention, it is possible to provide a radio communication system, a radio terminal, and a radio communication method that can reduce overhead caused by feedback while normally functioning cooperative MIMO communication between base stations.

It is a schematic block diagram of the radio | wireless communications system which concerns on 1st Embodiment and 2nd Embodiment of this invention. It is a block diagram which shows the structure of the radio | wireless terminal which concerns on 1st Embodiment of this invention. It is a block diagram which shows the structure of the radio base station (1st radio base station) which concerns on 1st Embodiment and 2nd Embodiment of this invention. It is a block diagram which shows the structure of the wireless base station (2nd wireless base station) which concerns on 1st Embodiment of this invention. It is a figure for demonstrating the normalization process performed by the normalization part which concerns on 1st Embodiment of this invention (the 1). It is a figure for demonstrating the normalization process performed by the normalization part which concerns on 1st Embodiment of this invention (the 2). 3 is a flowchart showing an operation of the radio communication system according to the first embodiment of the present invention. It is a block diagram which shows the structure of the radio | wireless terminal which concerns on 2nd Embodiment of this invention. It is a block diagram which shows the structure of the wireless base station (2nd wireless base station) which concerns on 2nd Embodiment of this invention. It is a flowchart which shows operation | movement of the radio | wireless communications system which concerns on 2nd Embodiment of this invention.

  Next, a first embodiment, a second embodiment, and other embodiments of the present invention will be described with reference to the drawings. In the description of the drawings in the following embodiments, the same or similar parts are denoted by the same or similar reference numerals.

[First Embodiment]
In the first embodiment, (1) configuration of the radio communication system, (2) normalization processing, (3) base station selection processing, (4) operation of the radio communication system, and (5) effects of the first embodiment. explain.

(1) Configuration of radio communication system
First, (1.1) overall schematic configuration, (1.2) configuration of radio terminal UE, (1.3) configuration of radio base station BS1, and (1.4) configuration of radio base station BS2 will be described.

(1.1) Overall schematic configuration
FIG. 1 is a schematic configuration diagram of a wireless communication system 1 in which inter-base station cooperative MIMO communication (multi-cell cooperative transmission / reception) is introduced.

  The wireless communication system 1 has a configuration based on LTE-Advanced, which is positioned as a fourth generation (4G) mobile phone system, for example. The wireless communication system 1 employs an FDD (Frequency Division Duplex) system as a duplex system. In the following, cooperative MIMO communication between base stations mainly in the downlink will be described.

  As shown in FIG. 1, the radio communication system 1 includes a radio base station BS1 (first radio base station), a radio base station BS2 (second radio base station), a radio terminal UE, and a control device 11. The radio terminal UE is located in an overlapping portion between the cell C1 formed by the radio base station BS1 and the cell C2 formed by the radio base station BS2.

  The radio base station BS1 and the radio base station BS2 may be macro cell base stations or femto cell base stations. A femtocell base station is a small radio base station installed mainly indoors.

  The radio base station BS1 and the radio base station BS2 are connected to each other via a backhaul network 10 that is a wired communication network. The control device 11 is provided in the backhaul network 10 and controls the radio base station BS1 and the radio base station BS2 via the backhaul network 10. However, the radio base station BS1 and the radio base station BS2 can directly perform communication between base stations without going through the control device 11.

  Closed loop control is introduced in the inter-base station cooperative MIMO communication in the wireless communication system 1.

  The radio base station BS1 transmits a radio signal RS1 (first radio signal) to the radio terminal UE in response to feedback from the radio terminal UE. In response to feedback from the radio terminal UE, the radio base station BS2 transmits the radio signal RS2 (second radio signal) to the radio terminal UE at the same frequency and the same time as the radio signal RS1.

  The radio base station BS1 performs transmission signal weighting processing (referred to as “precoding”) according to feedback from the radio terminal UE, and transmits a radio signal RS1 including the weighted transmission signal. By the weighting process, the phase and amplitude of the transmission signal (radio signal RS1) are controlled for each transmission antenna of the radio base station BS1.

  Similarly, the radio base station BS2 performs transmission signal weighting processing according to feedback from the radio terminal UE, and transmits a radio signal RS2 corresponding to the weighted transmission signal. By the weighting process, the phase and amplitude of the transmission signal (radio signal RS2) are controlled for each transmission antenna of the radio base station BS2.

  Here, the transmission signal sequence (also referred to as “stream”) included in the radio signal RS1 transmitted by the radio base station BS1 is the same as the transmission signal sequence included in the radio signal RS2 transmitted by the radio base station BS2. It may be different.

  When the transmission signal sequence included in the radio signal RS1 and the transmission signal sequence included in the radio signal RS2 are the same (during single stream transmission), the reception quality is improved due to the diversity effect. As shown in FIG. 1, in the case where the radio terminal UE is located at the cell edge of the radio base station BS1 and the radio base station BS2, it is preferable to improve the reception quality by single stream transmission.

  When the transmission signal sequence included in the radio signal RS1 and the transmission signal sequence included in the radio signal RS2 are different (during multi-stream transmission), the transmission speed is improved. In the case where the radio base station BS1 or the radio base station BS2 is, for example, a femtocell base station and the radio terminal UE is located near the radio base station BS1 and the radio base station BS2, the transmission rate is increased by multi-stream transmission. It is preferable to improve.

  The time interval at which the radio terminal UE performs feedback to the radio base station BS1 (hereinafter, feedback interval 1) is longer than the time interval at which the radio terminal UE performs feedback to the radio base station BS2 (hereinafter, feedback interval 2). The feedback interval 1 is an integral multiple of the feedback interval 2, for example.

  In the present embodiment, as an example, it is assumed that the radio terminal UE performs feedback to the radio base station BS2 in each frame, and performs feedback to the radio base station BS1 once in a plurality of frames.

  In this way, the radio terminal UE appropriately omits feedback to the radio base station BS1. In the following, a case where feedback to the radio base station BS1 is appropriately omitted will be described, but a radio base station where feedback is appropriately omitted can be selected. A radio base station selection method for which feedback is appropriately omitted will be described later.

  The radio terminal UE has propagation path information 1 (first information) corresponding to the propagation path characteristics from the radio base station BS1 to the radio terminal UE in a period in which feedback to the radio base station BS1 is omitted (hereinafter referred to as feedback omission period). Feedback based on propagation path information) and propagation path information 2 (second propagation path information) according to propagation path characteristics from the radio base station BS2 to the radio terminal UE is performed to the radio base station BS2. The propagation path characteristic means parameters such as an attenuation amount, a phase rotation amount, and a delay amount that are received when a wireless signal passes through the wireless propagation path.

(1.2) Configuration of radio terminal UE
FIG. 2 is a block diagram showing a configuration of the radio terminal UE.

  In the following description, portions related to the present invention will be mainly described. Therefore, it should be noted that the radio terminal UE may have a configuration (such as a power supply unit) that is not illustrated or omitted in the description.

  As illustrated in FIG. 2, the radio terminal UE includes a plurality of antennas 111, a transmission / reception unit 120, a control unit 130, and a storage unit 140. In the example of FIG. 2, the radio terminal UE has a plurality of antennas 111, but it is not always necessary to have a plurality of antennas 111, and only one antenna 111 may be provided.

  The transceiver 120 is configured using, for example, a radio frequency (RF) circuit, a baseband (BB) circuit, or the like. The control unit 130 is configured using, for example, a CPU, and controls various functions provided in the radio terminal UE. The storage unit 140 is configured using a memory, for example, and stores various types of information used for controlling the radio terminal UE and the like.

  The transmission / reception unit 120 includes a reception unit 121 and a transmission unit 122. The control unit 130 includes a propagation path information generation unit 131 and a normalization unit 132.

  The receiving unit 121 receives the radio signal RS1 from the radio base station BS1 and also receives the radio signal RS2 from the radio base station BS2. Note that, at the time of single stream transmission or the like, the reception unit 121 preferably receives the radio signal RS1 and the radio signal RS2 in the same phase.

  The propagation path information generation unit 131 uses a reference signal (pilot signal) included in the radio signal RS1 received by the reception unit 121 and the like between each transmission antenna of the radio base station BS1 and each reception antenna of the radio terminal UE. The process (what is called channel estimation) which calculates the propagation path estimated value (henceforth, propagation path estimated value 1) which shows a propagation path characteristic is performed.

  The propagation path information generation unit 131 uses a reference signal (pilot signal) included in the radio signal RS2 received by the reception unit 121, between each transmission antenna of the radio base station BS2 and each reception antenna of the radio terminal UE. A process of calculating a propagation path estimation value (hereinafter, propagation path estimation value 2) indicating the propagation path characteristics is performed.

  The propagation path information generation unit 131 generates propagation path information 1 that is information to be fed back to the radio base station BS1 based on the propagation path estimated value 1. The transmission path information 1 is not limited to the case where the propagation path estimation value 1 is used as the propagation path information 1. The transmission path information 1 is used for weighting processing (that is, control of at least one of the phase and the amplitude of the radio signal RS1) in the radio base station BS1. It may be an antenna weight 1 or an index indicating the transmission antenna weight 1 (referred to as “PMI (Pre-coding Matrix Index)”).

  The propagation path information generation unit 131 generates propagation path information 2 that is information to be fed back to the radio base station BS2 based on the propagation path estimation value 2. The propagation path information 2 is not limited to the case where the propagation path estimated value 2 is used as the propagation path information 2, and the transmission path information 2 is used for weighting processing (that is, control of at least one of the phase and the amplitude of the radio signal RS 2) in the radio base station BS 2. It may be an antenna weight 2 or an index indicating the transmission antenna weight 2.

  The normalizing unit 132 normalizes the propagation path information 2 based on the propagation path information 1 in the feedback omission period. Details of the normalization process will be described later.

  The transmission unit 122 transmits feedback to the radio base station BS1 at a feedback interval 1 and transmits feedback to the radio base station BS2 at a feedback interval 2. Further, the transmission unit 122 feeds back the propagation path information 2 normalized by the normalization unit 132 to the radio base station BS2 during the feedback omission period.

(1.3) Configuration of radio base station BS1
FIG. 3 is a block diagram showing a configuration of the radio base station BS1.

  As illustrated in FIG. 3, the radio base station BS1 includes a plurality of antennas 211, a transmission / reception unit 220, a control unit 230, a storage unit 240, and a wired communication unit 250. In the example of FIG. 3, the radio base station BS <b> 1 has a plurality of antennas 211, but it is not always necessary to have a plurality of antennas 211, and only one antenna 211 may be provided.

  The transmission / reception unit 220 is configured using, for example, an RF circuit or a BB circuit. The control unit 230 is configured using a CPU, for example, and controls various functions provided in the radio base station BS1. The storage unit 240 is configured using, for example, a memory, and stores various types of information used for controlling the radio base station BS1. The wired communication unit 250 communicates with the radio base station BS2 and the control device 11 via the backhaul network 10.

  The transmission / reception unit 220 includes a reception unit 221 and a transmission unit 222. The control unit 230 includes a weight control unit 231.

  When the propagation path information 1 is fed back from the radio terminal UE, the reception unit 221 receives the fed back propagation path information 1.

  The weight control unit 231 controls the transmission antenna weight 1 used for transmission of the radio signal RS1. When the reception unit 221 receives the propagation path information 1, the weight control unit 231 updates the transmission antenna weight 1 according to the propagation path information 1 and notifies the transmission unit 222 of the updated transmission antenna weight 1.

  During the period in which the receiving unit 221 does not receive the propagation path information 1, that is, the feedback skip period, the weight control unit 231 notifies the transmitting unit 222 of the transmitting antenna weight 1 that has been used without updating the transmitting antenna weight 1. To do.

  The transmission unit 222 weights (precodes) the transmission signal using the transmission antenna weight 1 notified from the weight control unit 231 and transmits the radio signal RS1 including the weighted transmission signal.

(1.4) Configuration of radio base station BS2
FIG. 4 is a block diagram showing a configuration of the radio base station BS2.

  As illustrated in FIG. 4, the radio base station BS2 includes a plurality of antennas 311, a transmission / reception unit 320, a control unit 330, a storage unit 340, and a wired communication unit 350. In the example of FIG. 4, the radio base station BS <b> 2 has a plurality of antennas 211, but it is not always necessary to have a plurality of antennas 211, and only one antenna 211 may be provided.

  The transmission / reception unit 320 is configured using, for example, an RF circuit or a BB circuit. The control unit 330 is configured using, for example, a CPU, and controls various functions provided in the radio base station BS2. The storage unit 340 is configured using, for example, a memory, and stores various types of information used for controlling the radio base station BS2. The wired communication unit 350 communicates with the radio base station BS1 and the control device 11 via the backhaul network 10.

  The transmission / reception unit 320 includes a reception unit 321 and a transmission unit 322. The control unit 330 includes a weight control unit 331.

  The receiving unit 321 receives the propagation path information 2 fed back from the radio terminal UE. The propagation path information 2 received by the receiving unit 321 may be normalized or not normalized.

  The weight control unit 331 controls the transmission antenna weight 2 used for transmitting the radio signal RS2. The weight control unit 331 updates the transmission antenna weight 2 according to the propagation path information 2 received by the reception unit 321, and notifies the transmission unit 322 of the updated transmission antenna weight 2.

  The transmission unit 322 weights (precodes) the transmission signal using the transmission antenna weight 2 notified from the weight control unit 331, and transmits the radio signal RS2 including the weighted transmission signal.

(2) Normalization processing
Next, normalization processing executed by the normalization unit 132 will be described with reference to FIGS. 5 and 6. First, the case where the propagation path information 1 is the propagation path estimation value 1 and the propagation path information 2 is the propagation path estimation value 2 will be described.

  The propagation path estimation value has a value for each antenna (for each combination of transmission and reception antennas), but if the relative relationship is maintained, even if the value itself is changed, it is processed appropriately on the receiving side (signal separation, etc.) it can. For example, for channel estimation value 1 (a, b) and channel estimation value 2 (c, d) shown in FIG. 5A, as shown in FIG. 5B, (axe, bxe, cxe) , dxe), the phase relationship between the propagation channel estimation value 1 and the propagation channel estimation value 2 is maintained even if all the propagation channel estimation values are multiplied by the constant e, and the same phase relationship is maintained. Therefore, there is no particular problem.

  Therefore, as shown in FIG. 6 (a), the propagation path estimated value 1 (a) and the propagation path estimated value 2 (b, c, d) that require feedback are shown in FIG. 6 (b). Thus, the channel estimation value 2 is divided by the channel estimation value 1. Specifically, the propagation path estimation value 1 (a / a) = (1) and the propagation path estimation value 2 (b / a, c / a, d / a) are normalized by division to obtain propagation path estimation. The value 1 can be virtually always fixed to (1), and feedback to the radio base station BS1 can be made unnecessary.

  In this example, normalization is performed by dividing the channel estimation value 2 by the channel estimation value 1, but any other calculation method can be used as long as the relative relationship between the channel estimation value 1 and the channel estimation value 2 is maintained. For example, normalization may be performed by subtracting the propagation path estimated value 1 from the propagation path estimated value 2.

  When the propagation path information 1 is the transmission antenna weight 1 or its index and the propagation path information 2 is the transmission antenna weight 2 or its index, in addition to the above processing, for example, a candidate for the transmission antenna weight 2 ( The transmission antenna weight 2 corresponding to the normalized propagation path estimated value 2 is selected from the “codebook”), and the selected transmission antenna weight 2 or its index is fed back to the radio base station BS2. .

  Alternatively, as a method for selecting an optimum PMI from a list (code book) of an index (herein referred to as “PMI”) of transmit antenna weight 2, the following method may be employed. Specifically, based on the propagation path estimation value 1 and the propagation path estimation value 2, the reception SNR when the PMI in the list is used is calculated for each PMI, and the maximum reception among the calculated reception SNRs. The PMI corresponding to the SNR is selected as the optimum PMI. Such a method is effective when the PMI in the list is small. When employing this method, the normalization unit 132 maximizes the reception SNR based on the propagation path estimated value 1 and the propagation path estimated value 2 under the assumption that the radio base station BS1 performs fixed transmission. The PMI is selected (searched) as a PMI to be fed back to the radio base station BS2.

  In the above example, the radio base station BS1 has one transmission antenna, but the same processing can be applied to the case where there are a plurality of transmission antennas.

(3) Base station selection processing
Next, a base station selection process that is a process of selecting a radio base station (radio base station BS1) from which feedback is appropriately omitted will be described. The base station selection process may be executed by any of the radio terminal UE, the radio base station BS1, the radio base station BS2, and the control device 11.

  As the base station selection process, for example, any of the following selection methods 1 to 5 can be used. However, it is not limited to the case where any one of the selection methods 1 to 5 is used in a fixed manner, but the selection methods 1 to 5 are appropriately switched depending on the situation of the radio terminal UE, the radio base station BS1, and the radio base station BS2. May be.

(Selection method 1)
In the selection method 1, a radio base station located far from the radio terminal UE is selected as the radio base station BS1, and a radio base station located close to the radio terminal UE is selected as the radio base station BS2. Thereby, the radio terminal UE always performs feedback to the radio base station BS2 located close to the radio terminal UE, and appropriately omits feedback to the radio base station BS1 located far from the radio terminal UE. The transmission power for feedback can be reduced, and the amount of interference can be reduced. The selection method 1 is preferable when the battery level of the radio terminal UE is low. Note that, in the selection method 1, information on the distance between the radio terminal UE and the radio base station BS1 and the distance between the radio terminal UE and the radio base station BS2 are required, but the information is provided in the radio terminal UE. Distance information can be obtained using GPS or the like.

(Selection method 2)
In the selection method 2, a radio base station with high processing performance (specifically, a macro cell base station) is selected as the radio base station BS1, and a radio base station with low processing performance (specifically, a femtocell base station) Is selected as the radio base station BS2. Usually, since the femtocell base station exists near the radio terminal UE, the radio terminal UE always performs feedback to the radio base station BS2 (femtocell base station) located nearby from the radio terminal UE. Feedback to the radio base station BS1 (macrocell base station) located far from the UE is omitted as appropriate. For example, when the radio terminal UE is not provided with GPS or the like, the selection method 2 is effective.

(Selection method 3)
In the selection method 3, a radio base station with low processing performance (specifically, a femtocell base station) is selected as the radio base station BS1, and a radio base station with high processing performance (specifically, a macrocell base station) is selected. Is selected as the radio base station BS2. Selection method 3 can be achieved because a radio base station with low processing performance performs constant transmission with fixed weight and a radio base station with high processing performance performs multi-antenna multi-stream transmission to obtain a high transmission rate. Is effective when a high transmission rate is required or when the battery of the radio terminal UE is high.

(Selection method 4)
In the selection method 4, a radio base station having a small propagation path fluctuation with the radio terminal UE is selected as the radio base station BS1, and a radio base station having a large propagation path fluctuation with the radio terminal UE is selected as the radio base station BS2. Choose as. This is because the radio base station BS1 performs constant transmission with a fixed weight, and therefore it is preferable that the propagation path fluctuation is small. Selection method 4 is effective when there is a large difference in propagation path fluctuation with each radio base station.

(Selection method 5)
In the selection method 5, a radio base station having a large propagation path loss with the radio terminal UE is selected as the radio base station BS1, and a radio base station having a small propagation path loss with the radio terminal UE is selected as the radio base station BS2. Choose as. The propagation path loss is calculated as the difference between the transmission power at the radio base station and the reception power at the radio terminal UE, and reflects the distance between the terminal and the base station. For this reason, the radio terminal UE can always perform feedback to the radio base station BS2 located close to the radio terminal UE and appropriately omit feedback to the radio base station BS1 located far from the radio terminal UE. Since the propagation path loss is used for other purposes such as transmission power control and can be easily obtained, the selection method 5 can effectively utilize the existing system configuration.

(4) Operation of the wireless communication system
Next, the operation of the wireless communication system 1 according to the first embodiment will be described with reference to FIG.

  When the inter-base station cooperative MIMO communication is started, in step S11, the radio base station BS1 from which feedback is appropriately omitted is selected by the process described in the above (3) base station selection process. When an apparatus other than the radio terminal UE (radio base station BS1, radio base station BS2, control apparatus 11) selects the radio base station BS1 for which feedback is appropriately omitted, the apparatus propagates to the radio base station BS1. The radio terminal UE is instructed to appropriately omit the feedback of the route information 1.

  When it is time to perform feedback to the radio base station BS1 (step S12; YES), in step S13, the transmission unit 122 of the radio terminal UE feeds back the propagation path information 1 to the radio base station BS1, and the propagation path information 2 To the radio base station BS2. The receiving unit 221 of the radio base station BS1 receives the propagation path information 1. The receiving unit 321 of the radio base station BS2 receives the propagation path information 2.

  In step S14, the transmitter 222 of the radio base station BS1 transmits the radio signal RS1 to the radio terminal UE using the transmission antenna weight 1 updated by the weight controller 231 according to the propagation path information 1 received by the receiver 221. To do. The transmission unit 322 of the radio base station BS2 transmits the radio signal RS2 to the radio terminal UE using the transmission antenna weight 2 updated by the weight control unit 331 according to the propagation path information 2 received by the reception unit 321.

  On the other hand, when it is time to omit the feedback to the radio base station BS1 (step S12; NO), in step S15, the normalization unit 132 of the radio terminal UE performs the process described in the above (2) normalization process. The propagation path information 2 is normalized with reference to the propagation path information 1.

  In step S16, the transmission unit 122 of the radio terminal UE omits feedback of the propagation path information 1 to the radio base station BS1, and feeds back the propagation path information 2 normalized by the normalization unit 132 to the radio base station BS2. To do. The receiving unit 321 of the radio base station BS2 receives the normalized propagation path information 2. The weight control unit 331 of the radio base station BS2 notifies the transmission unit 322 of the transmission antenna weight 2 updated according to the propagation path information 2 received by the reception unit 321.

  In step S17, the transmission unit 322 of the radio base station BS2 weights the transmission signal using the updated transmission antenna weight 2 notified from the weight control unit 331, and transmits the radio signal RS2 including the weighted transmission signal. To do. Further, the transmission unit 222 of the radio base station BS1 weights the transmission signal using the transmission antenna weight 1 that has not been updated, and transmits the radio signal RS1 including the weighted transmission signal.

(5) Effects of the first embodiment
The radio terminal UE appropriately omits feedback to the radio base station BS1. Thereby, the overhead accompanying feedback can be reduced. That is, in the feedback omission period, the radio terminal UE only needs to set a control channel for feedback with the radio base station BS2, so that the radio resource consumption in the uplink is reduced.

  In the first embodiment, the radio terminal UE normalizes the propagation path information 2 on the basis of the propagation path information 1 and feeds back the normalized propagation path information 2 to the radio base station BS2 in the feedback omission period. Thereby, since the radio base station BS2 transmits the radio signal RS2 according to the propagation path information 2 in which the relative relationship with the propagation path information 1 is maintained, the inter-base station cooperative MIMO communication is normally performed even during the feedback omitted period. Can function.

[Second Embodiment]
In the first embodiment, the radio terminal UE normalizes the propagation path information. However, in the second embodiment, the radio base station BS2 normalizes the propagation path information. Hereinafter, (1) the configuration of the wireless communication system, (2) the operation of the wireless communication system, and (3) the effects of the second embodiment will be described. However, differences from the first embodiment will be described.

(1) Configuration of radio communication system
FIG. 8 is a block diagram illustrating a configuration of the radio terminal UE according to the second embodiment. FIG. 9 is a block diagram showing a configuration of the radio base station BS2 according to the second embodiment.

  As illustrated in FIG. 8, the radio terminal UE does not include the normalization unit 132 described in the first embodiment. On the other hand, as shown in FIG. 9, the radio base station BS2 has a normalization unit 332 having the same function as the normalization unit 132 described in the first embodiment. About another structure, it is the same as that of 1st Embodiment.

(2) Operation of wireless communication system
Next, the operation of the wireless communication system 1 according to the second embodiment will be described with reference to FIG. However, since the process of step S21-S24 is the same as that of 1st Embodiment, the process of step S25-S27 is demonstrated.

  In step S25, the propagation path information generation unit 131 of the radio terminal UE generates propagation path information 1 and propagation path information 2. Then, the transmitter 122 of the radio terminal UE omits the feedback of the propagation path information 1 to the radio base station BS1, and transmits the propagation path information 1 and the propagation path information 2 generated by the propagation path information generation section 131 to the radio base station. Feedback to station BS2. The receiving unit 321 of the radio base station BS2 receives the fed back propagation path information 1 and propagation path information 2.

  In step S26, the normalization unit 332 of the radio base station BS2 normalizes the propagation path information 2 based on the propagation path information 1 by the same process as that described in (2) normalization process in the first embodiment. To do. The weight control unit 331 of the radio base station BS2 notifies the transmission unit 322 of the transmission antenna weight 2 updated according to the propagation path information 2 normalized by the normalization unit 332.

  In step S27, the transmission unit 322 of the radio base station BS2 weights the transmission signal using the updated transmission antenna weight 2 notified from the weight control unit 331, and transmits the radio signal RS2 including the weighted transmission signal. To do. Also, the transmitter 222 of the radio base station BS1 weights the transmission signal using the transmission antenna weight 1 that has not been updated, and transmits the radio signal RS1 including the weighted transmission signal at a constant rate.

(3) Effects of the second embodiment
In the second embodiment, the radio terminal UE omits feedback to the radio base station BS1 during the feedback omission period, and feeds back the propagation path information 1 and the propagation path information 2 to the radio base station BS2, and the radio base station BS2 The propagation path information 2 fed back from the radio terminal UE is normalized with reference to the propagation path information 1 fed back from the radio terminal UE, and the transmission antenna weight 2 is updated according to the normalized propagation path information 2.

  Accordingly, since the radio terminal UE only needs to set a control channel for feedback with the radio base station BS2 during the feedback omission period, radio resource consumption in the uplink is reduced. Further, since the radio base station BS2 transmits the radio signal RS2 according to the propagation path information 2 in which the relative relationship with the propagation path information 1 is maintained, the inter-base station cooperative MIMO communication is normally performed even during the feedback omitted period. Can function.

[Other Embodiments]
As mentioned above, although this invention was described by embodiment, it should not be understood that the description and drawing which form a part of this indication limit this invention. From this disclosure, various alternative embodiments, examples and operational techniques will be apparent to those skilled in the art.

  In the above-described embodiment, the feedback channel information is a channel estimation value, a transmission antenna weight, or an index thereof. However, the present invention is not limited to this, and a future channel is calculated based on the channel estimation value. It may be a predicted channel value indicating the characteristic.

  In the second embodiment described above, since the propagation path information 1 and the propagation path information 2 are fed back to the radio base station BS2, the propagation path information 1 and the propagation path information 2 are transferred from the radio base station BS2 to the radio base station BS1. It is also possible to switch the radio base station from which the feedback is appropriately omitted after the transfer from the radio base station BS1 to the radio base station BS2.

  Thus, it should be understood that the present invention includes various embodiments and the like not described herein. Therefore, the present invention is limited only by the invention specifying matters in the scope of claims reasonable from this disclosure.

  1: wireless communication system, UE: wireless terminal, BS1, BS2: wireless base station, 10: backhaul network, 11: control device, 111: antenna, 120: transmission / reception unit, 121: reception unit, 122: transmission unit, 130 : Control unit, 131: propagation path information generation unit, 132: normalization unit, 140: storage unit, 211: antenna, 220: transmission / reception unit, 221: reception unit, 222: transmission unit, 230: control unit, 231: weight Control unit, 240: storage unit, 250: wired communication unit, 311: antenna, 320: transmission / reception unit, 321: reception unit, 322: transmission unit, 330: control unit, 331: weight control unit, 332: normalization unit, 340: Storage unit, 350: Wired communication unit.

Claims (2)

A radio terminal, a first radio base station that transmits a first radio signal to the radio terminal, and a second radio base that transmits a second radio signal to the radio terminal using the same radio resource as the radio resource of the first radio signal A wireless communication system having a station,
The wireless terminal is
A control unit for setting a first time interval for transmitting first feedback information for the first radio signal and a second time interval for transmitting second feedback information for the second radio signal;
The first feedback information is transmitted to the first radio base station at the first time interval set by the control unit, and the second feedback information is transmitted to the first radio base station at the second time interval. A first transmission unit for transmitting,
The controller sets the first time interval to be longer than the second time interval;
The first radio base station is
A second transmitter for transmitting the second feedback information received from the wireless terminal to a backhaul network;
The second radio base station is
A wireless communication system comprising: an acquisition unit configured to acquire the second feedback information via the backhaul network. A wireless terminal,
The first radio signal transmitted from the first radio base station for the radio terminal and the first radio signal transmitted from the second radio base station for the radio terminal by the same radio resource as the radio resource of the first radio signal. A receiver that receives two radio signals;
A controller configured to set a first time interval for transmitting first feedback information for the first radio signal and a second time interval for transmitting second feedback information for the second radio signal;
A transmitting unit that transmits the first feedback information at the first time interval set by the control unit and transmits the second feedback information at the second time interval to the first radio base station. and, the possess,
Wherein the control unit to set, as the first time interval is longer than the second time interval,
A wireless terminal characterized by that.

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