JP2013059025A - Wireless communication system and control method of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wireless communication system capable of avoiding a communication interruption and disconnection, even when a moving speed of a wireless communication terminal is high or a wireless communication terminal is located in an area end of a base station and residence time in the service area is short.SOLUTION: A plurality of geographically neighboring base stations A, B, and C allocate a wireless resource of the same frequency domain and the same time domain for a wireless communication terminal 7, moving at a high speed. The wireless communication terminal 7, moving at a high speed, communicates with respective base stations A, B, and C using an allocated wireless resource.

Description

  The present invention relates to a wireless communication system that realizes stable communication of a wireless communication terminal and a control method of the wireless communication system.

  Conventionally, when a wireless communication terminal in communication moves, the wireless communication terminal continues communication while successively performing wireless connection procedures (handovers) to a base station in a better wireless state. In the case of FIG. 6, when the wireless communication terminal 7 communicating with the base station A at the point 1 moves to the point 2, the wireless communication terminal 7 performs a wireless connection procedure with the base station B. Release and move to base station B. Further, when the wireless communication terminal 7 moves to the point 3, the wireless communication terminal 7 performs a wireless connection procedure with the base station C, and after completing the connection, releases the connection with the base station B and moves to the base station C.

  A procedure when the wireless communication terminal 7 moves from point 1 to point 3 will be described with reference to FIG. At the point 1, when the wireless communication terminal 7 is communicating with the base station A (FIG. 7 (1)), when the wireless communication terminal 7 moves to the point 2, the base station A The information of the wireless communication terminal 7 (service being provided, security information, etc.) is transferred to the base station B via the route, and preparations for the wireless communication terminal 7 to connect to the base station B are made (FIG. 7 (2)). The wireless communication terminal 7 performs a wireless connection procedure with the base station B and enters a communication state with the base station B after completion (FIG. 7 (3)). When the wireless connection procedure between the base station B and the wireless communication terminal 7 is completed, the mobility management device 1 releases the connection between the wireless communication terminal 7 and the base station A.

  Further, when the wireless communication terminal 7 moves to the point 3, the base station B transfers the information of the wireless communication terminal 7 to the base station C via the movement management device 1, and the wireless communication terminal 7 is transferred to the base station C. Prepare for connection (FIG. 7 (4)). The wireless communication terminal 7 performs a wireless connection procedure with the base station C, and enters a communication state with the base station C after completion (FIG. 7 (5)). When the wireless connection procedure between the base station C and the wireless communication terminal 7 is completed, the mobility management device 1 releases the connection between the wireless communication terminal 7 and the base station B.

JP 2009-010989 A

  However, in the conventional connection procedure, when the wireless communication terminal moves, it is necessary to perform the wireless connection procedure with the destination base station prior to the communication with the destination base station. When the mobile station moves at a high speed, there is a problem that the time required for connection with the destination base station does not keep up with the movement of the wireless communication terminal and the connection is cut off.

  In addition, when the wireless communication terminal moves so as to graze the edge of the area of the base station, the service time is shortened, the wireless connection procedure becomes complicated, and as a result, the time during which communication is interrupted becomes longer and the service quality is improved. descend.

  The present invention has been made in view of such a problem, and an object of the present invention is to stabilize the wireless communication terminal even when the moving speed of the wireless communication terminal is high or when the service area time is short. An object of the present invention is to provide a radio communication system and a control method for the radio communication system for realizing the communication.

  In order to achieve the above object, a wireless communication system of the present invention is a wireless communication system including a base station and a wireless communication terminal. A base station in communication with a fast moving wireless communication terminal is an adjacent base station. On the other hand, radio resources in the same frequency domain and the same time domain are allocated for the radio communication terminal, and the radio communication terminal communicates with each base station using the radio resource.

  The radio communication system of the present invention preferably includes an uplink signal aggregation device that aggregates uplink signals from the respective base stations that communicate with the radio communication terminal using the allocated radio resources.

  It is preferable that the uplink signal aggregation device decodes the uplink signal at each base station, selects the data that has been successfully decoded, and sets it as uplink data.

  Alternatively, the uplink signal aggregating device preferably uses the uplink signal received by combining the uplink signal received at each base station by the uplink signal aggregating device and decoding it.

  In the radio communication system of the present invention, it is preferable that a base station in communication with the high-speed moving radio communication terminal is a master base station, and the master base station processes a control channel signal.

  The radio communication system of the present invention further includes an uplink signal aggregating device that aggregates uplink control channel signals from the respective base stations communicating with the radio communication terminal using the allocated radio resources, and the master base The station preferably processes the signal of the uplink control channel aggregated by the uplink signal aggregation device.

  Further, the present invention provides a wireless communication system including a base station and a wireless communication terminal, a cooperative operation area that secures radio resources for performing communication with the wireless communication terminal, and the outside of the cooperative operation area When a plurality of adjacent base stations are in the coordinated operation area, they have the same frequency domain and the same time domain among the radio resources. Radio resources are allocated to the radio communication terminals, the radio communication terminals communicate with each of the plurality of base stations using radio resources in the same frequency domain and the same time domain, and each of the plurality of base stations If the notification from the higher-level device causes the buffer area to be out of the range, the radio resources reserved for the radio communication terminal are Assigned to the radio communication terminal.

Further, the present invention provides a control method of a radio communication system comprising a base station and a radio communication terminal, wherein a base station in communication with a high-speed moving radio communication terminal transmits the radio communication to an adjacent base station. Allocate radio resources in the same frequency domain and time domain for terminals,
The wireless communication terminal communicates with each base station using the wireless resource.

  The present invention provides a radio communication system that realizes stable communication of a radio communication terminal because each base station secures the same radio resource for a high-speed mobile radio communication terminal.

It is a figure which shows the structure of the radio | wireless communications system which concerns on the 1st Embodiment of this invention. It is a figure explaining the positional relationship of a radio | wireless communication terminal and an adjacent base station. It is a figure explaining the allocation state of the radio | wireless resource of each base station. It is a figure which shows the procedure at the time of the movement of the radio | wireless communication terminal which concerns on embodiment of this invention. It is a figure explaining the usage example of the control channel in each base station, and a communication channel. It is a figure which shows a state when the radio | wireless communication terminal is moving, implementing the radio | wireless connection procedure (handover) with a base station one after another. It is a figure which shows the conventional procedure at the time of movement of a radio | wireless communication terminal. It is a figure explaining the allocation state of the radio | wireless resource of each base station in detail. It is a figure which shows the area formation image of the radio | wireless resource for high-speed mobile terminals formed along track | orbits, such as a railroad. It is a figure which shows the area formation image of the radio | wireless resource for high-speed mobile terminals in the radio | wireless communications system which concerns on 2nd Embodiment. It is a figure which shows how the radio | wireless resource is used on the geographical plane. It is a figure which shows the structure of the radio | wireless communications system which concerns on the 2nd Embodiment of this invention. It is a flowchart explaining the control of a cooperation operation | movement base station, and the procedure of operation | movement. It is a flowchart explaining the control of a buffer operation | movement base station, and the procedure of operation | movement.

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a configuration of a wireless communication system according to the first embodiment of the present invention. Wireless communication system according to a first embodiment of the present invention, the back plurality of base stations _{2 1,} 2 2, and · · · _{2 N,} a plurality of base stations _{2 1,} 2 2, the · · · _{2 N} A mobile management device 1 and an upstream signal aggregation device 6 connected via a hall line (wired).

  Each base station 2 performs wireless communication with a wireless communication terminal located in the area of the local station, for example, by an OFDMA (Orthogonal Frequency Division Multiple Access) method. Note that examples of the wireless communication terminal include a mobile phone and a portable information terminal (for example, a smartphone) with a built-in communication function. The base station 2 includes a radio control unit 3, a radio resource management unit 4, and a radio unit 5. The radio resource management unit 4 selects and assigns radio resources to radio communication terminals.

  The mobility management device 1 manages the movement of the wireless communication terminal. The uplink signal aggregation device 6 aggregates uplink signals from the respective base stations.

  Like the adjacent base stations A, B, and C shown in FIG. 2, base stations that are close to the radio communication terminal 7 geographically or wirelessly control the radio communication terminal 7 that is moving at high speed in cooperation. Signals are transmitted and received through channels and communication channels. Adjacent base stations A, B, and C are linked through a backhaul line (wired). Radio conditions are collected at each adjacent base station and aggregated to the master base station. The master base station determines a radio resource to be allocated to the radio communication terminal 7 and notifies the adjacent base station. The master base station does not necessarily need to be in a state where wireless connection with the wireless communication terminal 7 is possible. As the wireless communication terminal 7 moves, the master base station determines the next master base station and makes it changeable.

  FIG. 3 is a diagram for explaining a radio resource allocation state of each base station. In FIG. 3, resources filled in with black are radio resources for high-speed mobile terminals, and blank resources are resources that are allocated locally by the base stations A and B. As shown in FIG. 3, adjacent base stations A and B use part of the radio resources of each base station as radio resources for high-speed mobile terminals. Base stations A and B dynamically allocate the same frequency domain and the same time domain to a certain terminal from radio resources for high-speed mobile terminals. However, the resource to be allocated does not necessarily have to be a continuous area. Further, in order to avoid interference, radio resources for high-speed mobile terminals are secured in common in the area separately from resources allocated locally, and are dynamically allocated to the high-speed mobile terminal group from among them. The area may be set along the track of a high-speed rail or along the road of a highway. A resource to be allocated locally can be freely allocated to a normal wireless communication terminal accommodated in the base station.

  FIG. 4 shows a procedure when the wireless communication terminal according to the embodiment of the present invention moves. The uplink signal aggregating apparatus 6 that selects or combines the uplink signals may be inside the master base station or outside the base station.

  At point 1, the wireless communication terminal 7 detects the high-speed movement and notifies the base station A in the state of communication with the base station A (FIG. 4 (1)), or the high-speed movement of the wireless communication terminal 7 The base station A detects and shifts the wireless communication terminal 7 to the control state of the high-speed mobile terminal. The base station A becomes a master base station, selects adjacent base stations B and C, and starts cooperation with the base stations B and C (FIG. 4 (2)).

  The base station A selects an allocation resource based on the information of the base stations A, B, and C (radio state, resource availability information, etc.) and notifies the base stations B and C (FIG. 4 (3)). Information of base stations A, B, and C is exchanged via a backhaul line. The wireless communication terminal 7 is in communication with the base stations A, B, and C ((4) in FIG. 4). The base stations A, B, and C transmit radio resource (communication channel) allocation information to the radio communication terminal 7 using the same radio resource (control channel).

  The radio communication terminal 7 selects or synthesizes a signal including allocation information from the base stations A, B, and C, decodes it, and extracts the allocation information. According to the allocation information, the radio communication terminal 7 receives a downlink communication channel signal using a reception radio resource or transmits an uplink communication channel signal using a transmission radio resource. The wireless communication terminal 7 selects or combines the signals from the base stations A, B, and C received on the downlink communication channel and decodes them. Each of the base stations A, B, and C decodes the signal from the wireless communication terminal 7 received through the uplink communication channel and transmits it to the uplink signal aggregation device 6. The uplink signal aggregation device 6 selects the data that has been successfully decoded as uplink data. Alternatively, the signals received by the base stations A, B, and C are combined by the uplink signal aggregation device 6, and the decoded data is set as uplink data. Control channel signals such as a radio resource request for uplink data transmission and downlink radio quality information are selected or combined by the uplink signal aggregating apparatus 6 and processed by the master base station.

  The wireless communication terminal 7 moves to point 2 and continues communication with the wireless communication terminal 7 at the base stations A, B, and C by the same operation as in FIG. 4 (4) (FIG. 4 (5)). The wireless communication terminal 7 moves to the point 3 and continues communication with the wireless communication terminal 7 at the base stations A, B, and C by the same operation as in FIG. 4 (4) (FIG. 4 (6)).

  In the adjacent base station, the same signal is transmitted and received using the same radio resource. Therefore, in the area formed by the adjacent base station, the wireless communication terminal can receive any base station from any base station. You may send it. For example, as illustrated in FIG. 5, as the wireless communication terminal moves, the wireless communication terminal receives a control channel from the base station A, receives a downlink communication channel from the base station B, and transfers an uplink communication channel to the base station C. Communication is established even if it is transmitted.

  As described above, the wireless communication system according to the first embodiment of the present invention does not require a wireless connection procedure even if the base station communicating with the wireless communication terminal changes within the area formed by the adjacent base stations. Therefore, even when the moving speed of the wireless communication terminal is fast and the wireless communication terminal grazes the area edge of the base station (when the time in the area is short), there is no communication interruption and the connection is not disconnected. Therefore, there is no deterioration in service quality due to the wireless connection procedure.

  In addition, the wireless communication system according to the first embodiment of the present invention can follow an area formed by adjacent base stations by changing the master base station as the wireless communication terminal moves, and can move at high speed. There is no interruption of communication to the terminal, and the connection is not cut off. In addition, since the area is formed by the adjacent base station, the cover area is relatively enlarged, and there can be enough time for the procedure for changing the master base station.

  Further, the wireless communication terminal can improve the reception quality by combining the downlink signals from the adjacent base stations, and the uplink signal aggregation device can combine the uplink signals from the adjacent base stations, Reception quality can be improved.

  Moreover, since the radio | wireless communications system which concerns on the 1st Embodiment of this invention is the same as the radio | wireless resource and transmission / reception signal which are used for communication with the radio | wireless communication terminal which moves at high speed, it is based on interference with an adjacent base station. Area reduction does not occur, and the transmission output using radio resources for high-speed mobile terminals is made larger than the transmission output using radio resources for normal radio communication terminals to expand the area. It can also be overlapped.

  Next, a radio communication system according to the second embodiment of the present invention will be described.

  In the radio communication system according to the first embodiment, radio communication between a terminal and a base station is performed using radio resources for a high-speed mobile terminal secured in common by a plurality of base stations as shown in FIG. I do. As shown in FIG. 8, a part of the radio resources of the entire system is reserved for high-speed mobile terminals (fixed allocation), and necessary radio resources are allocated to the high-speed mobile terminals from the radio resources for the high-speed mobile terminals. The radio resources for normal terminals are allocated to normal terminals accommodated locally, and can be used repeatedly between cells.

  However, in the radio communication system according to the first embodiment, the base station needs to secure a fixed radio resource for the high-speed mobile terminal for communicating with the high-speed mobile terminal. Radio resources used for communication with normal terminals accommodated in a station are reduced. FIG. 9 is a diagram showing an area formation image of radio resources for a high-speed mobile terminal formed along a track such as a railroad. Along the trajectory 8, a plurality of base stations cooperate with each other (using the same radio resource for high-speed mobile terminals) and communicate with the high-speed mobile terminals 9. And a buffer area 11 for allowing radio resources for high-speed mobile terminals to be allocated to normal terminals in the normal operation area 12 are arranged over a wide area along the trajectory 8. Wireless resources for high-speed mobile terminals are secured in the base station. These reserved radio resources cannot be used for communication with the normal terminal even when the high-speed mobile terminal does not need to communicate.

  In the radio communication system according to the second embodiment of the present invention, a plurality of geographically adjacent base stations communicate with a high-speed mobile terminal in order to realize stable communication with the high-speed mobile terminal. Secure radio resources, and allocate the same (same frequency domain, same time domain) radio resources to the high-speed mobile terminals among the secured radio resources for the high-speed mobile terminals, and the high-speed mobile terminals use the same radio resources When each base station becomes capable of assigning radio resources for the high-speed mobile terminal as the high-speed mobile terminal moves, the radio resources for the high-speed mobile terminal are By dynamically controlling radio resources for high-speed mobile terminals so as to be allocated to the radio communication terminals, radio resources are optimized and radio utilization efficiency is improved.

  FIG. 10 is a diagram showing an area formation image of radio resources for a high-speed mobile terminal in the radio communication system according to the second embodiment.

  An area in which geographically adjacent base stations cooperate to communicate with the high-speed mobile terminal 13 using the same radio resource is formed by the cooperative operation cell group 9 of FIG. An area in which good communication with the high-speed mobile terminal 13 can be performed is formed by the cooperative operation cell group 9, but the radio wave used by the radio resources for the high-speed mobile terminal reaches outside the area. The arrival area is shown as a radiation area 10 in FIG. The buffer area 11 is provided between the radiation area 10 and the normal operation area 12 so that the same radio resources as those reserved for high-speed mobile terminals can be allocated to the normal terminals in the normal operation area 12. Is provided. The normal operation area 12 is an area in which the base station allocates all radio resources to the normal terminal and communicates with the normal terminal. In the radio communication system according to the second embodiment, the cooperative operation cell group 9 and the buffer area 11 are controlled to move following the movement of the high-speed mobile terminal 13.

  For the area formed by the cooperative operation cell group 9 and the buffer area 11, when the moving direction of the high-speed mobile terminal 13 can be predicted to some extent from the past movement route, etc., according to the probability that the high-speed mobile terminal 13 moves in that direction. Adjust the size of the area. That is, an area is formed that is wide in a direction where the probability of movement is high and narrow in a low direction.

  FIG. 11 is a diagram illustrating how radio resources are used on a geographical plane. From all the radio resources, a radio resource group A is secured for high-speed mobile terminals. The radio resource group A can be allocated to a normal terminal when the base station is in the normal operation area, but is not allowed to be allocated to the normal terminal when the base station is in the buffer area. The remaining radio resource group B can be allocated to the normal terminal regardless of whether the base station is in the buffer area or the normal operation area. The ratio of the radio resource group A and the radio resource group B is optimally controlled as necessary.

FIG. 12 is a diagram showing a configuration of a wireless communication system according to the second embodiment of the present invention. The wireless communication system according to the second embodiment includes a cooperative operation base station group 14, a buffer operation base station group 17, a cooperative operation base station group 14, a buffer operation base station group 17, and a backhaul line (wired). It consists of a high-speed mobile terminal area control device 19 connected via Cooperating base station group 14 has a plurality of cooperating base station _{15 1,} 15 2, made · · · 15 _N, cooperating base station _{15 1,} 15 2, · · · 15 _N are coordinated operation cell group 9 Configure. Further, the cooperative operation base station 15 ₁ to a master base station 16. Cushioning movement base station group 17 has a plurality of buffer operation the base station _{18 1,} 18 2, made · · · 18 _N, cushioning movement the base station _{18 1,} 18 2, · · · 18 _N of the buffer area cell group (radiation 20).

  Either the master base station 16 or the high-speed mobile terminal area control device 19 may have a function of determining radio resources for a high-speed mobile terminal. The high-speed mobile terminal area control device 19 uses a high-speed mobile terminal area control device 19 based on information about base stations around the high-speed mobile terminal obtained from the master base station 16 (for example, received signal strength of the signal from the high-speed mobile terminal received by the base station). A buffer area cell group 20 in radio resources for mobile terminals is formed, and the cooperative operation cell group 9 is controlled as necessary.

  Tracking of the buffer area cell group 20 with respect to terminal movement is realized by the high-speed mobile terminal area control device 19. The follow-up of the cooperative operation cell group 9 with respect to terminal movement is realized autonomously by the cooperative operation base station group 14 or by the high-speed mobile terminal area control device 19 based on a report from the master base station 16.

  The cooperative operation base station group 14 performs a link operation with the high-speed mobile terminal area control device 19 in addition to the function of realizing communication with the high-speed mobile terminal by the cooperative operation described in the first embodiment. The link operation refers to provision of information necessary for determining radio resources for high-speed mobile terminals, reporting of radio resource information for high-speed mobile terminals determined by the master base station 16, and When the cooperative operation cell group 9 is determined, this refers to the control of the cooperative operation cell group 9 based on the report of the information and the instruction of the high-speed mobile terminal area control device 19.

  The buffer operation base station group 17 forms a buffer area cell group 20 in accordance with an instruction from the high-speed mobile terminal area control device 19.

  FIG. 13 is a flowchart for explaining control and operation procedures of the cooperative operation base station. The master base station 16 or the high-speed mobile terminal area control device 19 secures radio resources for high-speed mobile terminals based on the information about the peripheral base stations of the high-speed mobile terminals, and forms the cooperative operation base station 9 Group 14 is determined (S101). The master base station 16 controls the cooperative operation base station 15 to dynamically allocate radio resources from radio resources for high-speed mobile terminals to high-speed mobile terminals (S102). The cooperative operation base station group 14 performs radio transmission / reception in cooperation with one high-speed mobile terminal using the allocated radio resource (S103). The cooperative operation base station group 14 allocates the remaining radio resources for normal terminals to the normal terminals and performs radio transmission / reception with the normal terminals (S104).

  FIG. 14 is a flowchart for explaining control and operation procedures of the buffer operation base station. The high-speed mobile terminal area control device 19 controls the buffer operation base station group 17 so that a buffer area is formed following the cooperative operation cell group 9 (S201). The buffering operation base station 18 prohibits the allocation of radio resources for high-speed mobile terminals to its own terminal in accordance with instructions from the high-speed mobile terminal area control device 19 (S202). The buffer operation base station group 17 allocates the remaining radio resources for normal terminals to the normal terminals, and performs radio transmission / reception with the normal terminals (S203).

  As described above, the radio communication system according to the second embodiment of the present invention uses a plurality of identical radio resources selected from radio resources reserved for high-speed mobile terminals in cooperation with geographically adjacent base stations. The base station is used to communicate with the high-speed mobile terminal, and when the radio resource can be allocated as the high-speed mobile terminal moves, the radio resource for the high-speed mobile terminal is transferred to another radio communication. Since the allocated radio resources for the high-speed mobile terminal are dynamically controlled by allocating to the terminals, the minimum necessary radio resources can be secured as the radio resources for the high-speed mobile terminals and the radio utilization efficiency can be improved. Can do.

  In addition, the setting of radio resources for high-speed mobile terminals needs to be performed by the operator for each base station, and the complexity of operation and the increase in man-hours are problematic, but the radio according to the second embodiment of the present invention In the communication system, since the setting of radio resources for high-speed mobile terminals is automatically performed, there is no operation complexity and man-hours are reduced.

DESCRIPTION OF SYMBOLS 1 Mobility management apparatus 2 Base station 3 Radio | wireless control part 4 Radio | wireless resource management part 5 Radio | wireless part 6 Uplink signal aggregation apparatus 7 Radio | wireless communication terminal 8 Orbit 9 Cooperative operation cell group 10 Radiation area 11 Buffer area 12 Normal operation area 13 High-speed mobile terminal 14 Cooperative operation base station group 15 Cooperative operation base station 16 Master base station 17 Buffer operation base station group 18 Buffer operation base station 19 High-speed mobile terminal area controller 20 Buffer area cell group

Claims (8)

In a wireless communication system composed of a base station and a wireless communication terminal,
A base station in communication with a high-speed moving radio communication terminal allocates radio resources in the same frequency domain and the same time domain for the radio communication terminal to an adjacent base station,
The said radio | wireless communication terminal is a radio | wireless communications system which communicates with each base station using the said radio | wireless resource.   The radio communication system according to claim 1, further comprising an uplink signal aggregation device that aggregates uplink signals from the base stations that communicate with the radio communication terminal using the allocated radio resource.   The radio communication system according to claim 2, wherein the uplink signal aggregation device decodes the uplink signal at each base station, and selects uplink data from data that has been successfully decoded.   The radio communication system according to claim 2, wherein the uplink signal aggregation device uses uplink data received by combining the uplink signal received at each base station with the uplink signal aggregation device and decoding the data. The base station in communication with the high-speed mobile radio communication terminal is a master base station,
The radio communication system according to claim 1, wherein the master base station processes a control channel signal. An uplink signal aggregating device for aggregating uplink control channel signals from the respective base stations communicating with the radio communication terminal with the allocated radio resources;
6. The radio communication system according to claim 5, wherein the master base station processes uplink control channel signals aggregated by the uplink signal aggregation device. In a wireless communication system composed of a base station and a wireless communication terminal,
A high-order apparatus that forms a cooperative operation area that secures radio resources for communication with the wireless communication terminal, and a buffer area that prohibits assignment of the radio resource outside the cooperative operation area;
When a plurality of adjacent base stations are within the cooperative operation area, among the radio resources, radio resources in the same frequency domain and the same time domain are allocated to the radio communication terminal,
The radio communication terminal communicates with each of the plurality of base stations using radio resources in the same frequency domain and the same time domain,
When each of the plurality of base stations is out of the buffer area by the notification from the host device, the wireless communication assigns the radio resources reserved for the radio communication terminal to other radio communication terminals. system. In a control method of a wireless communication system composed of a base station and a wireless communication terminal,
A base station in communication with a high-speed moving radio communication terminal allocates radio resources in the same frequency domain and the same time domain for the radio communication terminal to an adjacent base station,
A control method of a radio communication system, wherein the radio communication terminal communicates with each base station using the radio resource.

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