JP5921590B2 - Inter-base station handover method in wireless communication system - Google Patents

Description

  The present invention relates to a handover method between base stations in a wireless communication system, and more particularly to a handover method between an outdoor small IP base station and a base station compliant with 3GPP specifications.

  With the spread of wireless communication terminals such as mobile phones and smartphones, wireless communication networks are spread nationwide, and users of wireless communication terminals can use these wireless communication terminals anytime, anywhere. I was able to receive it. In addition, services provided by wireless communication systems have been diversified and sophisticated year by year, and users have been able to enjoy services using various applications.

  For example, a user can connect to a wireless communication network without delay even in a place where there is a large amount of wireless communication traffic such as a downtown area, and a wireless communication terminal can also be used in a place away from a remote place such as a mountain forest. can do. In addition, various services are provided by wireless communication systems, from voice calls and short message transmission to real-time video communication and transmission of large data files.

  Wireless communication providers that provide these services through wireless communication systems are making various efforts to further improve the services. For example, by increasing the number of installed base stations and improving communication efficiency as a wireless communication system, it is possible to cope with an increase in the number of local users and traffic volume and to provide a higher-speed data communication service. As techniques for improving communication efficiency, methods such as MIMO communication using a plurality of transmission / reception antennas and band coupling using a plurality of communication bands have been developed and put into practical use.

  In addition, construction of a wireless communication system is being promoted so that base stations can be installed throughout the country so that wireless communication terminals can be used anywhere in Japan. In this case, the base stations are arranged so that more efficient system operation can be performed in consideration of the population and traffic volume. In an area where the population is small, the distance between the base stations is set to, for example, a few ten kilometers so that the coverage area (referred to as “cell”) of the base stations becomes large. On the contrary, in an area where the population density is high and the traffic volume is large, the distance between base stations is set to several tens of meters, and a small area cell is arranged.

  Also, in dead zones where radio waves are difficult to reach, such as between underground malls and high-rise buildings, relay stations can be used to relay radio waves to dead zones, or distributed base stations connected to base stations via wires. By installing in the dead zone, wireless communication in the dead zone is possible. In addition, when radio waves are difficult to reach indoors such as a general home, a femto base station which is a small base station can be installed in the home. The coverage area of this femto base station is about several tens of meters.

  The femto base station is connected to an access network for wireless communication via an Internet line. A signal of a wireless communication terminal wirelessly connected to the femto base station is input / output to / from the wireless access network through a plurality of nodes in the Internet line. For this reason, there is a possibility that the signal transmission time of the wireless communication terminal varies greatly depending on the congestion state of communication traffic in each node in the Internet line. Such a change in transmission time makes it difficult to synchronize the timing between the handover source base station and the handover destination base station when the wireless communication terminal is handed over.

  Assuming that a wireless network control device and a base station in a wireless access network are connected via the Internet, Patent Document 1 discloses a method for dealing with fluctuations in transmission path delay caused by an Internet line.

International Publication No. 2003/086003

  When some transmission paths become very long, such as when a satellite access line is included in a wireless communication system, transmission delay of the transmission path becomes a problem. In such a wireless communication system, when a wireless communication terminal performs handover, it is difficult to synchronize timing between the handover source base station and the handover destination base station due to this transmission delay. As a result, handover fails in the radio communication terminal. For this reason, there is a need for a mechanism that enables handover without greatly changing the specifications of existing networks, base stations, and wireless communication terminals in a wireless communication system in which a large transmission path delay is assumed.

  An object of the present invention is to provide a method for performing handover between base stations in a wireless communication system in which a transmission path is long and a very large transmission path delay is assumed.

  The present invention is a method in which a radio communication terminal performs a handover from a first base station to a second base station, wherein a radio network controller that manages the first base station uses the second base station Including transmitting a first channel synchronization to a plurality of base stations adjacent to the first base station, the plurality of adjacent base stations responding to the reception of the first channel synchronization, and the wireless network Transmitting the second channel synchronization to the control device, and the radio network control device from the transmission time of the first channel synchronization and the reception time of the second channel synchronization, Calculating a respective signal transmission time between a plurality of adjacent base stations, and the radio network controller comprising the plurality of adjacent base stations and the calculated signal. A step of recording a relationship with a transmission time; and when the wireless network control device receives a handover request from the wireless communication terminal to the second base station via the first base station, Determining whether the relationship between the second base station and the signal transmission time is recorded; and when the relationship between the second base station and the signal transmission time is recorded, the wireless network control device Transmitting a downlink data frame to the second base station simultaneously with transmitting a radio channel switching instruction to the terminal.

  When a handover trigger occurs in a base station to which the wireless communication terminal is connected, a wireless channel set by the wireless communication terminal is transmitted by transmitting a downlink data frame to the handover destination base station simultaneously with a wireless channel switching instruction to the wireless communication terminal. The handover process can be completed within the switching timer. As a result, the handover process is executed even when the transmission line delay is large.

It is a figure for demonstrating the hand-over in a radio | wireless communications system. FIG. 10 is a sequence diagram illustrating a general handover procedure in a wireless communication network system. It is a figure which shows the structure of the radio | wireless communication network system containing an outdoor small IP base station. It is a figure for demonstrating the hand-over in the radio | wireless communication network system containing an outdoor small IP base station. It is a sequence diagram at the time of applying a general handover procedure to a wireless communication network system including an outdoor small IP base station. It is a sequence diagram explaining the handover procedure by this invention.

  Hereinafter, a handover method in a wireless communication system will be described with reference to the drawings. However, it should be understood that the invention is not limited to the drawings or the embodiments described below.

  First, a handover method in an existing wireless communication network system compliant with 3GPP specifications will be described. FIG. 1 is a diagram for explaining handover in an existing wireless communication network system. Here, the handover assumes a hard handover in which the connection is switched from one base station to another base station.

  In FIG. 1, reference numeral 10 is an existing radio communication network system, 11 is a radio access network, 12 is a radio network controller (RNC) connected to a handover source base station, and 13 is an RNC connected to a handover destination base station. , 14 is an interface (Iur) connecting RNC (12) and RNC (13), 15 is a handover source base station (NodeB 1), 16 is a handover destination base station (NodeB 2), 17 is an RNC and a base station 18 is a wireless communication terminal, 19 is a signal transmitted from NodeB 1 (15) to the wireless communication terminal 18, and 20 is a signal transmitted from NodeB 2 (16) to the wireless communication terminal 18. is there. FIG. 1 shows a state in which the radio communication terminal 18 is handed over from the NodeB 1 (15) cell to the NodeB 2 (16) cell.

  In FIG. 1, NodeB 1 (15) and NodeB 2 (16) are connected to separate RNCs (RNC (12) and RNC (13), respectively), and handover occurs between the RNCs. 15) and NodeB 2 (16) may be connected to the same RNC and handed over within the RNC. Also, the RNCs connected to NodeB 1 (15) and NodeB 2 (16) may be connected to separate exchanges.

  Next, a handover procedure in the hard handover model shown in FIG. 1 will be described using FIG. FIG. 2 is a diagram illustrating a general handover sequence. It is assumed that the wireless communication terminal (UE) 18 is connected to NodeB 1 (15) and is handed over to NodeB 2 (16). A flow of control signals and data signals between the RNC (12) managing the NodeB 1 (15) and the NodeB 2 (16) and the UE (18) of the handover destination base station is shown. Between RNC (12) and NodeB 2 (16), there is RNC (13) that manages NodeB 2 (16), and the signal passes through RNC (13) but is assumed to be transparent. I will omit it. The UE (18) measures the signal strength of the reference signal transmitted by the currently connected NodeB 1 (15) and also measures the signal strength of the reference signals from the adjacent base stations including NodeB 2 (16). ing. When the UE (18) moves, the signal strength of the reference signal from NodeB 1 (15) weakens and the signal strength of the reference signal from NodeB 2 (16) rises, and a handover request to NodeB 2 (16) Occurs.

  The UE (18) reports the signal strength measurement result of the reference signal to NodeB 1 (15) and RNC (12), and the signal strength of the reference signal from NodeB 2 (16) is exceeded. Will occur. In response to the handover trigger, the RNC (12) transmits a radio channel switching instruction for switching the radio channel to the handover destination to the UE (18) in S101. In the UE (18) that has received the radio channel switching instruction, the radio channel switching timer 50 is set based on that time. When the downlink data frame from the handover destination base station NodeB 2 (16) is received in the radio channel switching timer 50, the UE (18) determines that the handover has succeeded, and the NodeB 1 (15) to which it is currently connected Disconnect communication with.

  In the RNC (12) in which the handover trigger occurs, the transmission delay between the UE (18) and the NodeB 2 (16) notified as the handover destination base station is measured. This transmission delay measurement enables the signal from NodeB 1 (15) and the signal from NodeB 2 (16) to be received continuously without interruption when UE (18) hands over. This is a necessary procedure. In S102, the RNC (12) transmits channel synchronization to the NodeB 2 (16). A transmission time (tc1) in the RNC (12) is added to this channel synchronization. The NodeB 2 (16) that has received the channel synchronization returns the channel synchronization to the RNC (12) in S103. The channel synchronization returned includes the transmission time (tc3) in NodeB 2 (16), the transmission time (tc1) in RNC (12) added to the channel synchronization in S102, and the channel synchronization in S102 in NodeB 2 ( The time (tc2) received in 16) is added. The RNC (12) that has received the channel synchronization of S103 uses the reception time (tc4) and the times tc1, tc2, and tc3 added to the channel synchronization to establish the relationship between the RNC (12) and the NodeB 2 (16). Calculate the transmission delay between. However, the transmission delay can be calculated using only the transmission time (tc1) and the reception time (tc4).

  The RNC (12) that has completed the transmission delay measurement with the handover destination base station transmits the downlink data frame to the NodeB 2 (16) in S104. The transmission time is t1. This downlink data frame is transmitted from the NodeB 2 (16) serving as a new connection destination base station of the UE (18) to the UE (18) at the time of handover.

  A connection frame number (CFN: Connection Frame Number) indicating a frame number of the data frame is assigned to the downlink data frame. In NodeB 2 (16), in order to minimize the delay from the reception of the downlink data frame from the RNC (12) to the transmission to the UE (18) and the buffering time causing the delay, the downlink data frame is appropriately timed. It is required to be received at. For this reason, the RNC (12) assigns the CFN advanced by the transmission delay to the downlink data frame to be transmitted to the current NodeB 2 (16) and transfers it. For example, when the timing when RNC (12) tries to transmit to NodeB 2 (16) corresponds to CFN 10 of the downlink data frame and the transmission delay corresponds to 5 data frames, CFN 15 is assigned to the downlink data frame. Forward. Then, the Node B 2 (16) can receive the downlink data frame to which the CFN 15 is added immediately before the transmission timing of the CFN 15 downlink data frame to the UE (18). This makes it possible to minimize the delay in NodeB 2 (16) until the downlink data frame is sent to the UE (18) and the buffering time causing the delay. That is, the CFN of the downlink data frame in S104 is obtained by adding the number of data frames corresponding to the transmission delay to the CFN at the transmission time t1.

  In NodeB 2 (16), a reception window 51 corresponding to the CFN of the downlink data frame of S104 is set, and if the downlink data frame of the corresponding CFN is transferred from the RNC (12) in this reception window 51, It is determined that the data can be received without any problem.

  In Node B 2 (16) that has received the downlink data frame of the desired CFN within the reception window 51, signal processing for transmitting downlink data to the UE (18) is performed, and in S105, this downlink data frame and radio channel switching are performed. Send a completion instruction (time t2). The UE (18) determines that the handover has succeeded when receiving the radio channel switching completion from the NodeB 2 (16) in the radio channel switching timer 50 set after receiving the radio channel switching instruction. The radio channel connection with NodeB 1 (15) which is the destination base station is disconnected.

  Currently, wireless communication systems are being built nationwide, and wireless communication networks are in place. On the other hand, base stations have not been installed so far in areas scattered in mountainous areas, and residents in this area may not be able to use wireless communication terminals. This is because the area's population is small, and even if a base station is installed, the frequency of use of wireless communication terminals is low, the base station installation and system operation are not profitable, cable installation is difficult, and the base station is wired. This is because it is difficult to connect to a wireless communication network.

  However, there has been a demand from residents in the area where a base station has not been installed so far and a wireless communication terminal could not be used to use the wireless communication terminal. In addition, one of the goals of system construction is to enable wireless communication operators to use wireless communication terminals at any location in the country. For this reason, the installation of base stations is being promoted in the above areas.

  As one method for installing a base station in the above area, a small base station having a configuration equivalent to that of a femto base station is installed outdoors, and the small base station is wirelessly communicated via a satellite access line. There is a way to connect to the network. An example of such a configuration is shown in FIG. Here, the small base station is called an outdoor small IP base station.

  In FIG. 3, reference numeral 30 is a wireless communication network system including an outdoor small IP base station, 31 is an existing wireless communication network system based on 3GPP specifications, 33 is a wireless access network, and 12 is a wireless network for managing the base stations. Control device (RNC: Radio Network Controller), 14 is an interface (Iur) for connecting nodes in the radio access network 33, 15 is a base station (NodeB), 17 is an interface (Iub) for connecting RNC 12 and the base station 15 , 18 represent wireless communication terminals connected to any of the base stations 15. The wireless communication terminal 18 can perform seamless communication even during movement by performing handover within a cell managed by each base station.

  In FIG. 3, reference numeral 32 is a wireless communication network system using an outdoor small IP base station, 34 is a packet data gateway (PDG: Packet Data Gateway) that connects a wireless access network 33 and an external node, and 35 is satellite access. A ground station for a line, 36 is a communication satellite, 37 is an outdoor small IP base station (IP_BS), and 38 is a wireless communication terminal connected to the outdoor small IP base station 37. The PDG 34 is a gateway that transfers a signal from the ground station 35 to the radio access network 33 and transmits a signal to the ground station 35 from the radio access network 33. One or two ground stations 35 are installed throughout Japan.

  The small outdoor IP base station 37 has an RNC function in addition to the base station function. The apparatus configuration can be equivalent to that of the femto base station, but covers a relatively wide range outdoors than the femto base station. The femto base station is connected to the wireless access network 33 by wire, whereas the outdoor small IP base station 37 is connected to the wireless access network 33 via a satellite access line. The small outdoor IP base station 37 has jurisdiction over one cell, and the radio communication terminal 38 in the cell is connected to the radio access network 33 via a satellite access line.

  The cell of the outdoor small IP base station 37 is generally formed independently, and therefore, the radio communication terminal 38 connected to the outdoor small IP base station 37 does not perform handover to the adjacent cell. However, in consideration of the convenience of the user of the radio communication terminal 38, it is desirable to be able to perform handover to an adjacent cell.

  In a wireless communication network system including a satellite access line, it is difficult to perform handover between an outdoor small IP base station and a general base station. A signal delay in a transmission path between RNCs or RNC-base stations in a general radio access network is several milliseconds to several tens of milliseconds. On the other hand, when a satellite access line is used for a part of the transmission line, the transmission line delay becomes several hundred msec to several thousand msec. In the 3GPP specifications, handover with a base station having such a long transmission line delay is not assumed. Therefore, a function for dealing with such a long transmission line delay is also installed in a wireless communication terminal. It has not been.

  A case is assumed in which a handover is performed between an outdoor small IP base station and a general base station. FIG. 4 is a diagram for explaining handover in a wireless communication network system including an outdoor small IP base station. In FIG. 4, reference numeral 40 is a radio communication network system including an outdoor small IP base station, 33 is a radio access network, 16 is a base station NodeB 2, 34 is a packet data gateway PDG connecting the radio access network 33 and an external node, 35 is a ground station for satellite communication, 36 is a communication satellite, 37 is an outdoor small IP base station (IP_BS), 38 is a wireless communication terminal UE connected to the outdoor small IP base station 37, 41 is a UE from the outdoor small IP base station 37 to UE A signal transmitted to (38) and 42 represent a signal transmitted from NodeB 2 (16) to UE (38). In FIG. 4, it is assumed that UE (38) performs handover from outdoor small IP base station 37 to NodeB 2 (16).

  FIG. 5 is a diagram for explaining a handover procedure for the handover model shown in FIG. The UE (38) is currently connected to the outdoor small IP base station IP_BS (37), and is handed over to the NodeB 2 (16) as it moves. The UE (38) measures the signal strength of the reference signal transmitted by the currently connected IP_BS (37), and measures the signal strength of the reference signal from a plurality of adjacent base stations including NodeB 2 (16). Yes. As the UE (38) moves, the signal strength of the reference signal from the IP_BS (37) weakens, and when the signal strength of the reference signal from the NodeB 2 (16) exceeds, the handover request to the NodeB 2 (16) Arise.

  The procedure after the handover trigger is generated in the IP_BS (37) in response to the handover request from the UE (38) is the same as the procedure described with reference to FIG.

  In the transmission section between the outdoor small IP base station IP_BS (37) and the radio access network 33, a satellite access line using the communication satellite 36 exists. Since such a satellite access line is a very long transmission line, the transmission delay during signal transmission also increases. For this reason, as shown in FIG. 5, the time until the channel synchronization procedure in S102 and S103 is completed becomes very long. As a result, when the downlink data frame is transmitted to NodeB 2 (16) at time t1 immediately after the channel synchronization procedure is completed, the downlink data frame and radio channel switching completed transmitted from NodeB 2 (16) to UE (38) (S105 ) Is not received in the radio channel switching timer 50 set in the UE (38), and the handover is determined to be unsuccessful in the UE (38).

  The present invention addresses the problem that, as described above, it takes time to measure the transmission delay of a transmission path including a satellite access line, and handover fails in a wireless communication terminal. In the present invention, transmission delay measurement performed when a handover trigger occurs is recorded in advance, and the measurement result is recorded, and when handover occurs, the value is used to perform handover processing. To do.

  FIG. 6 shows a handover procedure for a handover according to the present invention. In the outdoor small IP base station IP_BS (37), transmission delay measurement with NodeB 2 (16), which is one of the handover destination base station candidates, is performed in advance. The transmission delay measurement is the same as the procedure described with reference to FIG. That is, in S201, IP_BS (37) transmits channel synchronization to NodeB 2 (16). At this time, the transmission time tc1 in the IP_BS (37) is added to the channel synchronization. The NodeB 2 (16) that has received the channel synchronization returns the channel synchronization to the IP_BS (37) in S202. In this channel synchronization, a transmission time tc3, a transmission time tc1 of channel synchronization of S201, and a reception time tc2 are added. In the IP_BS (37) that receives the channel synchronization, the transmission delay is calculated from the reception times tc4, tc1, tc2, and tc3, and is stored in the memory in the IP_BS (37) in association with the NodeB 2 (16). It is assumed that such transmission delay measurement is performed in advance for all candidates of the handover switching destination base station and the value is held.

  Assume that there is a handover request from the UE (38), and a handover trigger occurs in S203. IP_BS (37) checks whether the measurement result of transmission delay exists in the memory for the handover destination base station notified from UE (38). If there is a measurement result of the transmission delay, a handover procedure for the base station is executed. If there is no measurement result of the transmission delay, the measurement is performed to prepare for the next handover request.

  When there is a transmission delay measurement result of the notified handover destination base station (NodeB 2 (16) in FIG. 6), IP_BS (37) transmits a radio channel switching instruction to UE (38) in S204. . The UE (38) that has received the radio channel switching instruction sets the radio channel switching timer 50.

  Further, the IP_BS (37) transmits a downlink data frame to the NodeB 2 (16) in S104 simultaneously with the transmission of the radio channel switching instruction (time t1). The CFN of the downlink data frame is obtained by adding the transmission delay to NodeB 2 (16) measured and recorded in advance to the transmission time t1. When the downlink data frame is received in the reception window 51 at NodeB 2 (16), the downlink data frame and radio channel switching completion are transmitted to the UE (38) at S105. When the completion of the radio channel switching is received within the radio channel switching timer 50 set in the UE (38), it is determined that the handover is successful.

  Thus, by measuring and measuring the transmission delay to the handover destination base station in advance, the transmission delay measurement after the occurrence of the handover trigger can be omitted. In particular, when a satellite access line exists between a small outdoor IP base station and a radio access network, the transmission delay becomes long, and the handover fails, the transmission delay measurement time after the handover trigger can be omitted. The handover can be completed within the radio channel switching timer set by the radio communication terminal.

  Here, a satellite access line is assumed as a part of the transmission path, but it can also be applied to other lines in which transmission delay increases. It can also be applied to a general wireless communication network. Even in this case, by using the recorded measurement value of the transmission delay, it is not necessary to measure the transmission delay each time a handover trigger occurs.

  Since a general handover is often performed in the same RNC or between adjacent RNCs, the transmission path in the radio access network is short. On the other hand, when the wireless communication system includes a satellite access line, the transmission path in the wireless access network may be very long. This is due to the fact that only one or two ground stations with satellite access lines are installed in Japan. Regardless of where the communication partner of the wireless communication terminal connected to the outdoor small IP base station is in the country, the signal between the two parties passes through one of the ground stations. For this reason, the transmission path of the wireless communication system including the outdoor small IP base station is subject to a long delay due to the satellite access line and also a fluctuation in the transmission delay depending on the congestion situation in the node in the wireless access network. Is expected. When the fluctuation of the transmission delay is large, in FIG. 6, the downlink data frame having the frame number CFN transmitted from the IP_BS (37) is received outside the range of the reception window 51 waiting at the frame number CFN in NodeB 2 (16). There is a possibility that. In such a case, the downlink data frame is discarded and the handover fails.

  One method for dealing with such fluctuations in the transmission line delay is to always perform transmission delay measurement using channel synchronization and update the transmission delay record to the latest value. When a handover trigger occurs, a value close to the actual transmission delay can be used by using the latest measured value as the transmission delay value. As another method, in order to suppress a transmission delay measurement error, an average value obtained by averaging the latest measurement results of a plurality of times can be used. In addition, the congestion state of the nodes of the radio access network is expected to fluctuate in the time zone, so that if the time zone has regularity, the measured value can be held for each time zone. Alternatively, it is also possible to calculate a predicted value of transmission delay using a specific estimation formula or the like.

  In the example of FIG. 4, it is assumed that the UE (38) performs handover from the cell of the outdoor small IP base station IP_BS (37) to the cell of the standard base station NodeB 2 (16). On the other hand, it is also possible to hand over from the cell of the standard base station NodeB 2 (16) to the cell of the outdoor small IP base station IP_BS (37). In this case, the RNC (13) connected to the NodeB 2 (16) measures the transmission delay with respect to the IP_BS (37).

  Here, it is assumed that the outdoor small IP base station incorporates the RNC function and the base station function in one apparatus. However, other configurations are possible. For example, the RNC function and the base station function can be stored in separate devices, and only the base station function can be installed outdoors. In this case, one RNC function may manage a plurality of base station functions.

11, 33 Radio access network 12, 13 Radio network controller (RNC)
15, 16 Base station (NodeB)
18, 38 Wireless communication terminal 35 Ground station 36 Communication satellite 37 Outdoor small IP base station (IP_BS)

Claims (4)

A method in which a radio communication terminal performs handover from a first base station to a second base station,
A radio network controller that manages the first base station transmits first channel synchronization to a plurality of base stations adjacent to the first base station including the second base station;
The plurality of adjacent base stations responding to reception of the first channel synchronization and transmitting second channel synchronization to the radio network controller;
Each of the radio network control devices transmits a signal between the radio network control device and the plurality of adjacent base stations from the transmission time of the first channel synchronization and the reception time of the second channel synchronization. Calculating a time;
The radio network controller recording a relationship between the plurality of adjacent base stations and the calculated signal transmission time;
When the radio network controller receives a handover request from the radio communication terminal to the second base station via the first base station, the relationship between the second base station and the signal transmission time is Determining whether it is recorded;
When the relationship between the second base station and the signal transmission time is recorded, the radio network control device transmits a radio channel switching instruction to the radio communication terminal and simultaneously transmits a downlink data frame to the second base station. A step of sending
A handover method comprising:   The handover method according to claim 1, wherein the radio network controller is connected to a radio access network via a satellite access line.   The frame number of the downlink data frame transmitted to the second base station is a frame number obtained by adding the number of frames corresponding to the signal transmission time to the second base station to the frame number of the current downlink data frame. The handover method according to claim 1 or 2, wherein:   The wireless network control device calculates and records a signal transmission time between the plurality of adjacent base stations, the wireless network control device transmits a wireless channel switching instruction to the wireless communication terminal. The handover method according to any one of claims 1 to 3, which is completed before the step of transmitting a downlink data frame to the second base station is performed.

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