JP2018160743A - Communication system, base station, and time synchronization method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a base station, a communication system, and a time synchronization method that can conduct time synchronization at the same accuracy level as an external time source even at a place a radio signal from the external time source with high time accuracy such as a GPS signal cannot be received.SOLUTION: A small cell base station 20 receives a synchronization signal generated on the basis of an SFN based on a reference time from a macro cell base station 10 time-synchronized on the basis of a GPS signal, synchronizes an SFN' of the small cell base station with the SFN of the macro cell base station 10 on the basis of the received synchronization signal, calculates and sets a value of Ntime to count up at a longer cycle than the SFN' on the basis of the reference time on the basis of time information (time stamp) obtained from an NTP server, and calculates current time on the basis of the value of Ntime, the value of the SFN', and the reference time.SELECTED DRAWING: Figure 8

Description

  The present invention relates to a communication system, a base station, and a time synchronization method.

  Conventionally, a base station that acquires absolute time information (positioning time) based on a radio signal (GPS signal) received from a GPS (Global Positioning System) satellite and performs time synchronization is known (see, for example, Patent Document 1). ).

  However, there is a problem that the time synchronization control using the radio signal (GPS signal) cannot be used in a base station installed in a place where a radio signal (GPS signal) from a GPS satellite cannot be received.

A communication system according to an aspect of the present invention includes a first base station that is time-synchronized based on first time information acquired from a first time information source, and a second base that is capable of wireless communication with the first base station. The first base station includes a first reference information storage unit that stores information of reference time information used for time synchronization, and the reference time information as a reference. A synchronization signal for transmitting to the second base station a synchronization signal generated based on a first time parameter of a radio frame that increases by one in a predetermined first time period from a predetermined initial value to a maximum value as a round unit. A second reference information storage unit that stores information of the reference time information; a synchronization signal reception unit that receives the synchronization signal from the first base station; The synchronization signal received from the first base station Accordingly, the first time parameter of the radio frame of the own station, which is incremented by 1 in a predetermined first time period, from the predetermined initial value to the maximum value as a round unit, is set to the first time of the radio frame of the first base station. Second time information with lower accuracy than the first time information acquired from the first time information source from a radio frame synchronization processing unit synchronized with a time parameter and a second time information source different from the first time information source A time information acquisition unit for acquiring the first time parameter of the local station synchronized with the first base station based on the reference time information, and increments by 1 at a predetermined second time period As a value when the second time information of the second time parameter set so as to be acquired, a value of the second time parameter is calculated and set from the second time information with reference to the reference time information. To do A meter calculation setting unit; and a time calculation unit that calculates time based on the reference time information, the value of the second time parameter, and the value of the first time parameter of the local station synchronized with the first base station. Prepare.
In the communication system, a second time period of the second time parameter may be larger than an error of the second time information (for example, a maximum error of the second time information during the second time period). Alternatively, it may be more than twice the error of the second time information.
Further, in the communication system, the parameter calculation setting unit of the second base station may determine the value of the second time parameter and the value of the first time parameter from the second time information with reference to the reference time information. And the calculated second time parameter may be corrected and set based on the calculated first time parameter value.
In the communication system, the parameter calculation setting unit of the second base station determines whether or not the calculated first time parameter value is greater than or equal to the half value of the predetermined maximum value. If the determination is negative, the next update timing of the second time parameter based on the calculated first time parameter is corrected and set by adding 1 to the calculated second time parameter, When the determination is affirmative, the calculated second time parameter may not be corrected.
In the communication system, the first time parameter is a system frame number standardized in the LTE / LTE-Advanced specification of the mobile communication system, and the synchronization signal is LTE / LTE-Advanced specification of the mobile communication system. And at least one of the second synchronization signal.
In the communication system, the first time information source is a GPS (Global Positioning System) satellite.
In the communication system, the second time information source may be an NTP (Network Time Protocol) server.
In the communication system, the second base station may be a small cell base station, and the first base station may be a macro cell base station.

In addition, a base station according to another aspect of the present invention is a base station of a mobile communication system, and a base station that is time-synchronized based on first time information acquired from a first time information source. A storage unit for storing information of reference time information used for time synchronization of the reference, and a predetermined first time period in a cycle from a predetermined initial value to a maximum value based on the reference time information in the reference base station The synchronization signal generated based on the first time parameter of the radio frame that increases by 1 is received from the reference base station, and the synchronization signal received from the reference base station Synchronize the first time parameter of the radio frame of the own station, which is incremented by 1 in a predetermined first time period, from the initial value to the maximum value in one cycle, to the first time parameter of the radio frame of the reference base station The time at which the second time information is acquired from the radio frame synchronization processing unit to be activated and the second time information different from the first time information source, and the second time information is less accurate than the first time information acquired from the first time information source. Each time the value of the first time parameter of the information acquisition unit and the own station synchronized with the reference base station on the basis of the reference time information makes a round, it is set to increase by 1 at a predetermined second time period. A parameter calculation setting unit that calculates and sets the value of the second time parameter from the second time information on the basis of the reference time information as a value when the second time information of the second time parameter is acquired And a time calculation unit that calculates time based on the reference time information, the value of the second time parameter, and the value of the first time parameter of the own station synchronized with the reference base station.
In the base station, the second time period of the second time parameter may be larger than an error of the second time information (for example, a maximum error of the second time information in the period of the second time period). Alternatively, it may be more than twice the error of the second time information.
In the base station, the parameter calculation setting unit calculates the value of the second time parameter and the value of the first time parameter from the second time information based on the reference time information, and calculates the value The calculated second time parameter may be corrected and set based on the value of the first time parameter.
In the base station, the parameter calculation setting unit determines whether the calculated first time parameter value is greater than or equal to a half value of the predetermined maximum value, and the determination is negative. In this case, the next update timing of the second time parameter (Ntime) based on the calculated first time parameter is corrected by adding 1 to the calculated second time parameter, and the determination is affirmative. In this case, the calculated second time parameter may not be corrected.
In the base station, the first time parameter may be a system frame number standardized in LTE / LTE-Advanced specifications of a mobile communication system, and the synchronization signal may be LTE / LTE of the mobile communication system. -It may be at least one of the first synchronization signal and the second synchronization signal standardized by the Advanced specification.
In the base station, the first time information source may be a GPS (Global Positioning System) satellite, and the second time information source may be an NTP (Network Time Protocol) server.
The base station may be a small cell base station, and the reference base station may be a macro cell base station.

  The time synchronization method between base stations of a mobile communication system according to another aspect of the present invention is based on the first time information acquired from the first time information source by the reference first base station. A synchronization signal generated based on a first time parameter of a radio frame that increases by one in a predetermined first time period (10 ms) with a predetermined first time period to a maximum value as a round unit with reference time information as a reference is transmitted. And the second base station subject to time synchronization receives the synchronization signal from the first base station, and based on the synchronization signal received from the first base station, a maximum from a predetermined initial value Synchronizing the first time parameter of the radio frame of the own station, which is incremented by 1 in a predetermined first time period up to a value, with the first time parameter of the radio frame of the first base station; First time information Acquiring second time information having a lower accuracy than the first time information acquired from the first time information source from a second time information source different from the first time information source, and the second base station The second time parameter of the second time parameter set so as to increase by 1 at a predetermined second time period each time the value of the first time parameter of the own station synchronized with the first base station makes a round with respect to the first base station. Calculating and setting the value of the second time parameter from the second time information on the basis of the reference time information as a value when acquiring the two time information; and the second base station, Calculating time based on reference time information, a value of the second time parameter, and a value of the first time parameter of the own station synchronized with the reference base station.

  According to the present invention, time synchronization can be performed with the same accuracy as the external time source even in a place where a radio signal from an external time source with high time accuracy such as a GPS signal cannot be received.

Explanatory drawing which shows an example of schematic structure of the mobile communication system provided with the macrocell base station and small cell base station which concern on embodiment of this invention. Explanatory drawing which shows an example of the format of the time-axis direction of the radio frame of a downlink signal. Explanatory drawing which shows an example of the mode of the transmission stop in the sub-frame by ABS employ | adopted with the inter-cell interference control technique (eICIC). Explanatory drawing which shows an example of the mode of interference when the time synchronization between base stations is incomplete. (A) And (b) is a figure explaining the radio | wireless frame synchronization in this embodiment. (A) And (b) is a figure which illustrates the traffic condition of the daytime and nighttime in an urban area, and the ABS pattern according to it, respectively. The figure explaining Ntime used by the time synchronization of this embodiment. The control block diagram which shows the principal part of the macrocell base station and small cell base station in the mobile communication system which concerns on this embodiment. The flowchart of the time synchronous process with the macrocell base station and small cell base station of the mobile communication system which concerns on this embodiment. The figure which shows an example of the time synchronizer in a macrocell base station. The figure which shows an example of the listening synchronizer in each base station. The figure which shows an example of the time synchronizer in a small cell base station. The figure explaining an example in the case of being influenced by the error of the time acquired from the NTP server in calculation of Ntime. The control flowchart which shows an example of the time synchronization and the time calculation in the small cell base station which concerns on other embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Here, the embodiment of the present invention will be described on the assumption that it is applied to LTE / LTE-Advanced, but the concept of the present invention can be applied to any system as long as the system uses a similar cell configuration and physical channel configuration. Applicable.

First, the overall configuration of a mobile communication system to which the present invention can be applied will be described.
FIG. 1 is an explanatory diagram showing a schematic configuration of a mobile communication system in which a macro cell base station and a small cell base station according to an embodiment of the present invention are arranged.

  In FIG. 1, a mobile communication system according to the present embodiment is a communication system compliant with LTE (Long Term Evolution) / LTE-Advanced standard specifications, and a macro cell base station 10 as a reference first base station, And a small cell base station 20 as a second base station to be synchronized with time, which is located in a cell (hereinafter referred to as “macro cell”) 10A that is a radio communication area of the macro cell base station 10. A cell (hereinafter referred to as “small cell” as appropriate) 20 </ b> A that is a radio communication area of the small cell base station 20 is included inside the macro cell 10 </ b> A of the macro cell base station 10. Since the macro cell 10A is mainly outdoors, it is also called an outdoor macro cell.

  In recent years, in large urban areas, communication traffic in indoor offices of medium- and high-rise buildings has increased rapidly, and means for efficiently carrying communication traffic in the height direction is required. Therefore, a three-dimensional space cell configuration in which the small cell base station 20 is expanded and installed three-dimensionally including the height direction in the building 40 such as a building located in the macro cell 10A is effective.

  In FIG. 1, a user terminal device (UE: User Equipment) 30 that is a first mobile station is a user terminal device (MUE) that is located in the macro cell 10 </ b> A of the macro cell base station 10 and connected to the macro cell base station 10. Yes, wireless communication for telephone or data communication is possible via the macrocell base station 10. Since this user terminal device 30 is located near the boundary between the macro cell 10A and the small cell 20A, the user terminal device 30 is likely to receive interference from the small cell 20A.

  In addition, the user terminal device (UE) 31 as the second mobile station is located in the outer edge of the small cell 20A of the small cell base station 20 installed in the building 40 and is connected to the small cell base station 20. The user terminal device (SUE) is in a state in which wireless communication for telephone or data communication is possible via the small cell base station 20. Since this user terminal device 31 is located near the boundary between the small cell 20A and the macro cell 10A, the user terminal device 31 is susceptible to interference from the macro cell 10A.

  When the user terminal devices 30 and 31 are located in the macro cell 10A or the small cell 20A, the user terminal devices 30 and 31 use a predetermined communication method and wireless communication resources between the macro cell base station and the small cell base station corresponding to the cell in which they are located Wireless communication. The user terminal devices 30 and 31 are configured using hardware such as a computer device having a CPU and a memory, an external communication interface unit for a core network, a wireless communication unit, and the like, and a base station is executed by executing a predetermined program Wireless communication with 10, 20, etc. can be performed.

  The macro cell base station 10 is a wide area base station that covers a macro cell that is a wide area having a radius of about several hundreds to several kilometers that is installed outdoors in a mobile communication network, and is “Macro e-Node B”. , Sometimes called “MeNB”. The macrocell base station 10 is connected to other base stations via a wired communication line, for example, and can communicate with a predetermined communication interface. The macrocell base station 10 is connected to a core network of a mobile communication network via a communication line such as a line termination device and a dedicated line, and has a predetermined communication interface with various nodes such as a server device on the core network. Communication is possible.

  Further, the macro cell base station 10 includes a GPS (Global Positioning System) receiver, and includes GPS data including high-accuracy time data with an error of 1 [μs] from an atomic clock included in the GPS satellite 200 that is the first time source. The signal can be received.

  Unlike the macro cell base station in a wide area, the small cell base station 20 has a wireless communicable distance of about several meters to several hundred meters, and can be installed inside a building 40 such as a general home, a store, or an office. A small-capacity base station. Since the small cell base station 20 is provided so as to cover an area smaller than the area covered by the wide-area macro cell base station in the mobile communication network, the small cell base station 20 may be referred to as “Small e-Node B” or “Small eNB”. is there. The small cell base station 20 is also connected to the core network of the mobile communication network via a communication line such as a line terminating device and a broadband public communication line such as an ADSL (Asymmetric Digital Subscriber Line) line or an optical line. It is possible to communicate with various nodes such as the server apparatus via a predetermined communication interface.

  The small cell base station 20 installed in the building 40 may not be able to receive a GPS signal including highly accurate time data from the GPS satellite 200. Therefore, the small cell base station 20 includes an NTP (Network Time Protocol) server, which is a second time source, via a line termination device and a communication line such as an ADSL (Asymmetric Digital Subscriber Line) line or a broadband public communication line such as an optical line. Thus, it is possible to receive low-accuracy time data having an error of several ms or more.

  Each base station of the macro cell base station 10 and the small cell base station 20 is configured using hardware such as a computer device having a CPU, a memory, and the like, an external communication interface unit for a core network, a wireless communication unit, and the like. By executing this program, various processes for suppressing interference, which will be described later, are executed, or wireless communication is performed with the user terminal devices 30, 31 using a predetermined communication method and wireless communication resources. can do.

  Each of the base stations 10 and 20 is a base station capable of downlink radio communication using an OFDM (Orthogonal Frequency Division Multiplexing) scheme with a user terminal device that is a mobile station. The base stations 10 and 20 include, for example, an antenna, a radio signal path switching unit, a duplexer (DUP), a downlink radio reception unit, an OFDM (Orthogonal Frequency Division Multiplexing) demodulation unit, an uplink radio reception unit, an SC-FDMA ( (Single-Carrier Frequency-Division Multiple Access) Demodulator. Furthermore, each base station 10 and 20 includes an OFDM modulation unit, a downlink radio transmission unit, a control unit and the like.

  The SC-FDMA demodulator performs SC-FDMA demodulation processing on the received signal received by the uplink radio receiver, and passes the demodulated data to the controller. The OFDM modulation unit modulates the data of the downlink signal transmitted from the control unit to the user terminal device residing in the cell of the local station by the OFDM method so as to be transmitted with a predetermined power. For example, when the base station receives information on a transmission stop target subframe from the server apparatus, the OFDM modulation unit is controlled to stop downlink transmission only for a specific subframe in the radio frame. The downlink radio transmission unit transmits the transmission signal modulated by the OFDM modulation unit via the duplexer, the radio signal path switching unit, and the antenna.

  The control units of the base stations 10 and 20 are configured by computer devices, for example, and control and execute various processes by reading and executing predetermined programs. The control unit also functions as means for receiving, from the server device, ABS pattern information, which is information on a subframe to be transmitted, in cooperation with the external communication interface unit. Further, the control unit also functions as means for controlling to stop the downlink transmission in a specific transmission stop target subframe based on the transmission stop target subframe information (ABS pattern information) received from the server device. To do.

  The control unit of the small cell base station 20 among the base stations 10 and 20 performs frame synchronization that matches the period and phase of the radio frame with the macro cell base station 10 based on the synchronization signal received from the macro cell base station 10. It also functions as means for synchronizing the time based on time information (time stamp) from the NTP server. The synchronization process in the small cell base station 20 will be described later.

  In FIG. 1, one macro cell base station 10 and one small cell base station 20 are illustrated, but there may be a plurality of macro cell base stations 10 and small cell base stations 20. In addition, FIG. 1 illustrates a case where one user terminal device is located in each of the macro cell 10A and the small cell 20A, but there are a plurality of user terminal devices located in each cell. Also good.

  Further, in the mobile communication system of the present embodiment, in addition to the NTP server described above, a server device capable of communicating with each of the base stations 10 and 20 via a communication line may be provided. This server device is also called a SON (Self-Organizing Network) server or the like, and is configured using hardware such as a computer device having a CPU, a memory, and the like, and an external communication interface unit for a core network. The server device can communicate with the macro cell base station 10 and the small cell base station 20 via a predetermined communication line by executing a predetermined program.

  Each of the base stations 10 and 20 has an internal clock and has an inter-base station time synchronization function. The macrocell base station 10 has means for performing time synchronization processing (GPS synchronization method) based on GPS signals received from GPS (Global Positioning System) satellites. On the other hand, the small cell base station 20 that cannot receive a GPS signal from the GPS satellite 200 performs listening synchronization, which will be described later, that performs frame synchronization processing that matches the cycle and phase of the radio frame based on the synchronization signal received from the macrocell base station 10, and NTP. Means for performing time synchronization processing in combination with time information acquired using NTP or SNTP communication with the server.

Next, inter-cell interference control that can effectively apply the method of adjusting the downlink signal transmission timing of the macro cell base station and the small cell base station 20 synchronized in time with each other in the mobile communication system configured as described above will be described.
In FIG. 1, when the same frequency band is used in the macro cell base station 10 and the small cell base station 20, since interference occurs in the small cell base station 20, control for suppressing the interference is necessary. As this interference control method, LTE-Advanced standard eICIC (enhanced Inter-Cell Interference Coordination) technology is effective.

  FIG. 2 is an explanatory diagram illustrating an example of a format in a time axis direction of a downlink radio frame compliant with LTE / LTE-Advanced. As shown in FIG. 2, a radio frame 100 having a predetermined length (10 [ms] in the illustrated example) which is one unit of a downlink signal has a predetermined number (10 in the illustrated example) of a predetermined length (in the illustrated example). In the example, the subframe 110 is 1.0 [ms]). Each subframe 110 has a control channel region 110A and a data channel region 110B.

FIG. 3 is an explanatory diagram illustrating an example of a state in which subframe transmission is stopped using inter-cell interference control technology (eICIC) in a state where time synchronization between base stations is performed.
As shown in FIG. 3, in eICIC, for example, data in some subframes (subframes # 1 to # 3 and # 6 to # 8 in the illustrated example) in a radio frame transmitted from the macrocell base station 10 are used. Stop sending Such a subframe is called ABS in LTE. The small cell base station 20 uses the same number of subframes (subframes # 1 to # 3 and # 6 to # 8 in the illustrated example) as the subframes designated as ABSs in the macrocell base station 10, and uses the user terminals. By transmitting data to the device 30, interference of the data channel from the macro cell base station 10 in the user terminal device 30 connected to the small cell base station 20 can be reduced.

  Further, for example, as shown in FIG. 3, by similarly setting the ABS in a part of subframes of the small cell base station 20 (subframes # 0, # 4, # 5, and # 9 in the illustrated example). The macrocell base station 10 uses the same number of subframes (# 0, # 4, # 5, and # 9 subframes in the illustrated example) as the subframes designated as ABSs in the small cell base station 20. By transmitting data to the terminal device, it is possible to reduce data channel interference from the small cell base station 20 in the user terminal device connected to the macro cell base station 10.

  FIG. 4 is an explanatory diagram illustrating an example of a state of interference when time synchronization of each base station is incomplete. In the inter-cell interference control technology (eICIC), since interference is controlled on the time axis, the time lag when the downlink signals transmitted from the antennas of the base stations 10 and 20 reach the user terminal device 30 is It is necessary to make it below the allowable range (for example, 1 [μs] or less). If the time lag when the downlink signals transmitted from the base stations 10 and 20 reach the user terminal device 30 exceeds an allowable range, the ABS is set in the macro cell base station 10 as shown in FIG. 4, for example. The rear end portion 111 of the subframe 110a (# 1) including the transmission data immediately before the subframe 110a (# 2), and the front end portion of the subframe 110b (# 2) including the transmission data of the small cell base station 20 112 interfere with each other. That is, the downlink signal transmitted from the macro cell base station 10 when the front end portion 112 of the subframe 110b (# 2) of the downlink signal transmitted from the small cell base station 20 is received by the user terminal device 30. The rear end portion 111 of the subframe 110a (# 1) reaches the user terminal device 30 and interferes therewith. Therefore, it is necessary to improve the accuracy of time synchronization of the base stations 10 and 20.

  As a time synchronization method for a plurality of base stations, there is a GPS synchronization method for performing time synchronization processing based on GPS signals received from GPS satellites at each base station. In the GPS synchronization method, time synchronization processing can be performed using a GPS signal including time data having an error of 1 [μs] or less from a GPS satellite, and highly accurate time synchronization can be performed. The macrocell base station 10 that can receive GPS signals can perform time synchronization processing based on GPS signals received from GPS satellites. However, the small cell base station 20 installed in the building 40 may not be able to receive a GPS signal from the GPS satellite 200 and cannot be time synchronized by the GPS synchronization method.

  In addition, time synchronization processing may be performed using PTP (Precision Time Protocol) defined by the IEEE (The Institute of Electrical and Electronic Engineers) 1588 standard. However, in this case, it is necessary to construct a network of IEEE (The Institute of Electrical and Electronic Engineers) 1588 standard. Furthermore, it is necessary to use a highly accurate and expensive 1588 standard clock source without error, and it is expensive to construct a communication system.

  On the other hand, by using NTP or SNTP communication with an NTP server, a network can be constructed at a low cost, but the error is several ms and high-precision time synchronization cannot be performed.

  Therefore, in this embodiment, a base station that cannot use the GPS synchronization method receives a synchronization signal of a radio frame transmitted by a reference base station that is time-synchronized by a highly accurate time synchronization method such as the GPS synchronization method. Time stamp information acquired from a time synchronization system other than GPS, such as NTP, a time parameter for managing a radio frame provided in an LTE standard base station, and a parameter obtained by extending the parameter Is used to achieve highly accurate time synchronization.

  In the small cell base station 20 of the present embodiment, listening synchronization is performed to perform frame synchronization processing for matching the period and phase of a radio frame based on the synchronization signal received from the macro cell base station 10. As a result, even if the small cell base station 20 does not perform highly accurate time synchronization, the period and phase of the radio frame 100 can be accurately determined between the macro cell base station 10 and the small cell base station 20 (for example, (With an accuracy of 1 [μs] or less).

FIGS. 5A and 5B are diagrams for explaining radio frame synchronization in the present embodiment.
In the listening synchronization method, the small cell base station 20 receives synchronization signals (PSS, SSS) periodically transmitted from the macro cell base station 10. For example, the small cell base station 20 measures the time from the start timing of the radio frame 100 until the synchronization signal is received. When the transmission timing of the radio frame 100 of the small cell base station 20 is synchronized with the macro cell base station 10, the measured time becomes the specified time ΔK. However, as shown in FIG. 5A, when the start timing of the radio frame 100 of the small cell base station 20 is not synchronized with the macro cell base station 10, the measured time is ΔK + ΔS. The small cell base station 20 calculates a time ΔS that deviates from the specified time ΔK, and starts the start timing of the next radio frame 100 with a delay of ΔS, as shown in FIG. Thereby, even if the small cell base station 20 does not acquire the time from the time source, the period and phase of the radio frame 100 can be accurately determined between the macro cell base station 10 and the small cell base station 20 (for example, 1 [ μs] can be synchronized (with an accuracy of

  In the inter-cell interference suppression technology using the ABS described above, the assignment of ABS between the macro cell base station 10 and the small cell base station 20 depends on the traffic demand, so the ABS pattern is changed according to the traffic situation. Is preferred.

FIG. 6 is a diagram illustrating daytime and nighttime traffic situations in an urban area and an ABS pattern corresponding to the traffic situation. FIG. 6A illustrates daytime situations, and FIG. b) illustrates the night situation.
As shown in FIG. 6A, in the urban area during the day, many user terminal devices (MUE) 31 exist in the small cell 20A of the small cell base station 20. Therefore, during daytime, data communication with the user terminal devices (MUE) 30 and 31 is efficiently performed by reducing the allocation of ABS of the small cell base station 20 and increasing the data communication of the small cell base station 20. be able to. On the other hand, as shown in FIG. 6 (b), in the urban area at night, many user terminal devices (MUEs) move to the suburbs. Therefore, the user terminals residing in the small cell 20A of the small cell base station 20 The number of devices (MUE) 31 is reduced. Therefore, at night, data communication with the user terminal device (MUE) 31 can be efficiently performed by increasing the allocation of ABS of the small cell base station 20 and reducing the data communication of the small cell base station 20. .

  Even when the ABS pattern is switched, if the ABS pattern change timing is not accurately synchronized, the above-described data channel interference occurs. The switching of the ABS pattern is performed, for example, by sending an ABS pattern or switching time (day, hour, minute, second) from the server device to each base station, and at each base station, at the time sent from the server device. The pattern is switched at the timing. As described above, since the ABS pattern is switched based on the time information sent from the server device, in the ABS pattern switching time synchronization, the ABS pattern change is performed based on the highly accurate time at each base station. There is a need to do. As described above, the macrocell base station 10 installed outdoors can acquire a highly accurate time with an error of 1 [μs] or less from a GPS satellite, and can perform highly accurate time management. In the small cell base station 20 that cannot receive a GPS signal, a highly accurate time is acquired by using the PTP (Precision Time Protocol) defined by the above-mentioned IEEE (The Institute of Electrical and Electronic Engineers) 1588 standard. However, as described above, there is a problem that the cost is high.

  LTE defines a system frame number (SFN) that identifies consecutive radio frames 100 from each other. The transmission interval of the radio frame 100 is 10 [ms], and is synchronized with the macrocell base station 10 and is always transmitted at a constant interval. Therefore, it is possible to obtain a highly accurate current time by calculating the current time using the SFN of the radio frame synchronized with the macro cell base station synchronized with the time with high accuracy. However, since the value of SFN is defined as 0 (minimum value) to 1023 (maximum value), in the time calculation using SFN, the time is only in the range of 10.24 seconds (10 [ms] × 1024). It cannot be calculated.

  Therefore, in the present embodiment, a new time parameter (hereinafter referred to as “Ntime”) that counts up with a period longer than that of the SFN is introduced, and the time information (time) acquired from the SFN as the first time parameter and the NTP server. By using Ntime, which is a second time parameter set based on the (stamp), it is possible to calculate the current time with high accuracy in a wider time range (for example, 48 hours), and perform time synchronization of ABS pattern switching with high accuracy. I did it. This will be specifically described below.

FIG. 7 is a diagram for explaining Ntime used in time synchronization according to the present embodiment.
As shown in FIG. 7, Ntime is a parameter that is counted up when the SFN is reset, and the Ntime countup interval is 10.24 seconds (10240 [ms]). In FIG. 7, the setting is reset at Ntime = 16874 (48 hours), but such setting may be appropriately set by the system.

The time calculation by Ntime and SFN uses a reference time which is the time when SFN and Ntime = 0.
The SFN at a time (d: h: m: s) at which a predetermined time has elapsed from the reference time (D: H: M: S) can be expressed by the following (Formula 1).
SFN = {f (d, h, m, s) mod 1024} div 10 (Equation 1)

Ntime can be expressed by the following (formula 2).
Ntime = {f (d, h, m, s)} div 10240 (Expression 2)

The above f (d, h, m, s) is an elapsed time (unit: ms) from the reference time (D: H: M: S) to an arbitrary time (d: h: m: s). (Formula 3).
f (d, h, m, s) = 10 ³ × {(s−S) + (m−M) × 60 + (h−H) × 60 ² + (d−D) × 24 × 60 ² } (Formula 3)

For example, when the reference time is 0 day 0 hour 0 minute 0 second and the time after a predetermined period from the reference time (D: H: M: S) is 0 day 12:30:30, the SFN at this time, Ntime can be obtained by the following calculations 1 to 3.
Calculation 1: f (0, 12, 30, 0) = 10 ³ × (30 × 60 + 12 × 60 ² )
= 4500000 [ms]
Calculation 2: Ntime = 45000 div 10240 = 439
Calculation 3: SFN = (4500000 mod 10240)
= 4640 div 10
= 464

  Therefore, 0 day 12:30:30 can be expressed as Ntime = 439 and SFN = 464 based on the reference time.

FIG. 8 is a control block diagram showing main parts of the macro cell base station 10 and the small cell base station 20 in the mobile communication system according to the present embodiment.
The macro cell base station 10 includes a GPS signal receiving unit 11 that receives a GPS signal from the GPS satellite 200, and a reference information storage unit that stores information on reference times (D, H, M, S) shared with the small cell base station 20. 12 and an SFN calculator 13. The SFN calculation unit 13 includes a radio frame control unit 13a that controls the start timing of the radio frame 100, and an SFN count unit 13b that counts SFN. In addition, the macro cell base station 10 includes a downlink signal transmission unit 14 that transmits a downlink signal including a synchronization signal such as PSS and SSS and SFN information to the small cell base station 20.

  The small cell base station 20 includes a downlink signal receiving unit 21 that receives a downlink signal including a synchronization signal (PSS, SSS) and SFN information transmitted from the macro cell base station 10, and the SFN ( Hereinafter, in order to distinguish from the SFN of the macrocell base station, it is expressed as “SFN ′”), and a synchronization processing unit 22 that synchronizes with the SFN of the macrocell base station 10 is provided. The synchronization processing unit 22 adjusts the start timing of the radio frame 100 to synchronize with the radio frame start timing of the macro cell base station 10 and the SFN sent from the macro cell base station 10 to SFN ′ And an SFN ′ setting unit 22b for setting. Similarly to the macro cell base station 10, the small cell base station 20 also includes a reference information storage unit 23 that stores information on the shared reference time.

  Further, the small cell base station 20 calculates a time stamp acquisition unit 24 that acquires a time stamp as time information from the NTP server 201, and calculates Ntime as a second time parameter based on the acquired time stamp and the reference time. An Ntime calculation unit 25 that is a parameter calculation setting unit to be set, and a time calculation unit 26 that is a time calculation unit that calculates the current time based on SFN ′, the reference time, and Ntime.

  FIG. 9 is a flowchart of time synchronization processing in the macro cell base station 10 and the small cell base station 20 of the mobile communication system according to the present embodiment. FIG. 10 is an example of the time synchronization unit 501 in the macro cell base station 10. FIG.

  As shown in FIG. 9, a reference time (D, H, M, S) shared by each base station for performing time synchronization from the server to each base station 10, 20 is transmitted from the server, and each base station Stores the received reference time in the reference information storage unit 23 (S101, S201).

  The time synchronization unit 501 of the macrocell base station 10 shown in FIG. 10 receives a GPS signal from the GPS satellite 200 by the GPS signal reception unit 11, acquires time information with an error of 1 [μs] or less, and corrects the internal clock ( S102 of FIG. 9). Thereby, the macro cell base station 10 is time-synchronized with an error of 1 [μs] or less with respect to an accurate time.

  Next, the radio frame control unit 13a of the SFN calculation unit 13 adjusts the start timing so that the start timing of the radio frame 100 is synchronized with the reference time stored in the reference information storage unit 12 (FIG. 9). Part of S103). Specifically, based on the time information of the acquired GPS signal, the wireless frame is started at the timing when the reference time is reached.

  As described above, the time information included in the GPS signal acquired by the macrocell base station 10 has an error of 1 [μs] or less. Accordingly, based on the time information included in the GPS signal, the time lag of the adjusted start timing of the radio frame 100 with respect to the reference time is suppressed to 1 [μs] or less.

  The SFN count unit 13b counts up the SFN based on the start timing of the radio frame 100, and resets the SFN when SFN = 1023 (part of S103 in FIG. 9). Since the interval between the radio frames 100 is 10 [ms], the SFN count unit 13b functions like a clock that keeps time at an interval of 10 [ms]. Further, since the count-up is performed based on the start timing of the radio frame adjusted using the time information included in the GPS signal, the time lag of the SFN count-up is suppressed to 1 [μs] or less.

  The SFN counting unit 13b determines the radio frame start timing at the reference time based on the reference time and the GPS signal, and starts counting the SFN.

FIG. 11 is a diagram illustrating an example of the listening synchronization unit 502 in each base station.
Based on the start timing of the radio frame 100 adjusted by the radio frame control unit 13a, the downlink signal transmission unit 14 of the macro cell base station 10 transmits a downlink signal including a synchronization signal such as PSS and SSS to the small cell base station. 20 (S104 in FIG. 9). This downlink signal includes the value of SFN acquired from the SFN count unit 13b.

  The downlink signal reception unit 21 of the small cell base station 20 receives the synchronization signals (PSS, SSS) and the value of SFN transmitted from the macro cell base station 10. The radio frame control unit 22a of the synchronization processing unit 22 adjusts the start timing of the radio frame 100 by the method described with reference to FIG. 5 and performs radio frame transmission between the small cell base station 20 and the macro cell base station 10. 100 start timings are synchronized. Further, the SFN ′ setting unit 22b of the synchronization processing unit 22 sets the value of SFN transmitted from the macrocell base station 10 to the value of SFN ′. As a result, the SFN ′ of the small cell base station 20 and the SFN of the macro cell base station 10 are synchronized (S203 in FIG. 9). As a result, the small cell base station 20 also becomes SFN ′ based on the reference time, and the time lag of the timing at which SFN ′ is counted up is suppressed to 1 [μs] or less.

FIG. 12 is a diagram illustrating an example of the time synchronization unit 503 in the small cell base station.
The Ntime calculation unit 25 of the small cell base station 20 is based on the time stamp including an error of several ms unit acquired by the time stamp acquisition unit 24 from the NTP server 201 and the reference time stored in the reference information storage unit 23. Ntime is calculated and set (S204 in FIG. 9). In calculating Ntime, first, the elapsed time f (d, h, m, s) from the reference time to the time of the time stamp is calculated in milliseconds using the above equation (3). Next, Ntime is calculated using the calculated elapsed time f (d, h, m, s) and the previous equation (2).

  As described with reference to FIG. 7, the Ntime is a time parameter that is counted up when the SFN is reset. The time width is 10.24 seconds (1024 [ms]), which is an NTP server. It is 10 times or more the time stamp error (several ms) obtained from 201. Here, the error of the time stamp is, for example, the maximum error of the time stamp in the period of the second time period (10240 ms) that is the Ntime update period. Therefore, the calculated Ntime can absorb the error of the time stamp acquired from the NTP server 201, and the value of Ntime is hardly affected by the error of the time stamp acquired from the NTP server 201. . In addition, if the time width of Ntime is at least twice the time error acquired by the time stamp acquisition unit 24, the error can be absorbed well.

  Next, the Ntime calculation unit 25 sets the calculated Ntime value as the current Ntime value, and updates Ntime each time SFN ′ is reset.

The time calculation unit 26 calculates time based on SFN ′, Ntime, and the reference time. Specifically, first, an elapsed time (unit: ms) from the reference time is calculated from the following equation 4.
f (d, h, m, s) = 10240 × Ntime + SFN ′ × 10 (Expression 4)

  Next, the elapsed time f (d, h, m, s) calculated at the reference time (D, H, M, S) is added. Thereby, the current time can be calculated.

  The calculation of the current time is calculated from SFN ′ whose time lag with respect to the reference time is 1 [μs] or less and Ntime in which the error of the time stamp acquired from the NTP server 201 is absorbed. The time of the following error can be calculated. Therefore, by performing time management based on this calculated time, even the small cell base station 20 that cannot receive a GPS signal can perform time synchronization with an error of 1 [μs] or less with respect to the accurate time.

  As described above, each of the macro cell base station 10 and the small cell base station 20 can perform time management with high accuracy, so that time synchronization of ABS pattern switching can be performed accurately, and data at the time of ABS pattern switching is obtained. The occurrence of channel interference can be suppressed.

  In the time synchronization processing of the present embodiment, depending on the timing of acquiring a time stamp from the NTP server 201, the calculation of Ntime may be affected by the error in the time acquired from the NTP server 201.

FIG. 13 is a diagram for explaining an example when the calculation of Ntime is affected by the error of the time acquired from the NTP server 201. Ntime shown in the figure is Ntime corresponding to an accurate time.
For example, as shown in FIG. 13, when the timing at which the time information (time stamp) is acquired from the NTP server 201 is immediately after the Ntime update timing (SFN ′ reset timing), the acquired time is delayed with respect to the accurate time. In this case, the value of Ntime calculated at the acquired time must be n + 1 if it is originally, but becomes n. As a result, when the calculated Ntime value is set, the time calculated by the time calculation unit 26 may be delayed by 10 seconds or more with respect to the accurate time.

  Therefore, as illustrated in FIG. 14, the Ntime calculation unit 25 determines Ntime and SFN (hereinafter, SFN calculated by the Ntime calculation unit 25 as “SFN”) based on the time acquired from the NTP server. And Ntime may be corrected and set based on the calculated value of SFN ″.

FIG. 14 is a control flow diagram illustrating an example of time synchronization and time calculation in a small cell base station according to another embodiment.
As shown in FIG. 14, the Ntime calculating unit 25 calculates Ntime and SFN ″ based on the time stamp acquired from the NTP server (S1). When the calculated SFN ″ value is not less than 0 and less than 512 (YES in S2), the calculated Ntime value is set as it is (S4).

  On the other hand, when the calculated SFN ″ is 512 or more (NO in S2), the calculated Ntime value may be one smaller than the original Ntime value. However, depending on the error and time stamp acquisition timing, the calculated Ntime value may be the original Ntime value. Therefore, when the calculated SFN ″ is 512 or more (NO in S2), 1 is added to the calculated Ntime at the timing when the calculated SFN ″ is reset (Ntime is updated based on SFN ″). The value is set to Ntime (S3).

  The time stamp acquired from the NTP server does not advance with respect to the correct time. Accordingly, the correct Ntime value at the timing when the calculated SFN ″ is reset (Ntime is updated based on SFN ″) is a value obtained by adding 1 to the calculated Ntime. Therefore, by setting a value obtained by adding 1 to the calculated Ntime, a correct Ntime can be set, and an accurate time calculation can be performed.

  In the example of FIG. 14 described above, the threshold value that is set by adding 1 to the calculated Ntime is set to the half value of SFN (512). However, this threshold value may be appropriately determined based on the error of the time to be acquired. The correct Ntime can be set by setting a value that goes back 10 times or more from the update timing as a threshold value. For example, in the present embodiment, since the error in the time to acquire is several ms, the threshold value is set to 923, and when SFN ″ is 923 or more, the value of Ntime + 1 is set at the Ntime update timing based on the calculated SFN ″. May be set.

  Note that the processing steps and the components of the communication system, the base station, and the user terminal device described in this specification can be implemented by various means. For example, these steps and components may be implemented in hardware, firmware, software, or a combination thereof. For example, the processing in each base station of the present embodiment is realized by a predetermined program being read and executed in hardware described later, or a predetermined program pre-installed in hardware described later is executed. Is done.

  For hardware implementation, means such as a processing unit used to realize the above steps and components in an entity (for example, various wireless communication devices, Node B, terminal, hard disk drive device, or optical disk drive device) One or more application specific IC (ASIC), digital signal processor (DSP), digital signal processor (DSPD), programmable logic device (PLD), field programmable gate array (FPGA), processor , A controller, a microcontroller, a microprocessor, an electronic device, other electronic units designed to perform the functions described herein, a computer, or a combination thereof.

  Also, for firmware and / or software implementation, means such as processing units used to implement the above components may be programs (eg, procedures, functions, modules, instructions) that perform the functions described herein. , Etc.). In general, any computer / processor readable medium that specifically embodies firmware and / or software code is means such as a processing unit used to implement the steps and components described herein. May be used to implement For example, the firmware and / or software code may be stored in a memory, for example, in a control device, and executed by a computer or processor. The memory may be implemented inside the computer or processor, or may be implemented outside the processor. The firmware and / or software code may be, for example, random access memory (RAM), read only memory (ROM), nonvolatile random access memory (NVRAM), programmable read only memory (PROM), electrically erasable PROM (EEPROM) ), FLASH memory, floppy disk, compact disk (CD), digital versatile disk (DVD), magnetic or optical data storage, etc. Good. The code may be executed by one or more computers or processors, and may cause the computers or processors to perform the functional aspects described herein.

  Also, descriptions of embodiments disclosed herein are provided to enable any person skilled in the art to make or use the present disclosure. Various modifications to the present disclosure will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other variations without departing from the spirit or scope of the disclosure. The present disclosure is therefore not limited to the examples and designs described herein, but should be accorded the widest scope consistent with the principles and novel features disclosed herein.

10 Macrocell base station (standard base station)
10A Macrocell 11 GPS signal receiver 12 Reference information storage unit 13 SFN calculator 13a Radio frame controller 13b SFN count unit 14 Downlink signal transmitter 20 Small cell base station 20A Small cell 21 Downlink signal receiver (synchronization signal receiver) )
22 Synchronization processing unit (synchronization processing unit)
22a Radio frame control unit 22b SFN 'setting unit 23 Reference information storage unit 24 Time stamp acquisition unit (time information acquisition unit)
25 Ntime calculation unit (parameter calculation setting unit)
26 Time calculation unit 30, 31 User terminal device 40 Building 100 Radio frame 110 Subframe 110a Subframe 110A Control channel area 110B Data channel area 200 GPS satellite (first time source)
201 NTP server (second time source)

JP 2010-28228 A

Claims (19)

A communication system comprising: a first base station that is time-synchronized based on first time information acquired from a first time information source; and a second base station that can wirelessly communicate with the first base station. There,
The first base station is
A first reference information storage unit for storing information of reference time information used for time synchronization;
A synchronization signal generated based on a first time parameter of a radio frame that increases by one in a predetermined first time period from a predetermined initial value to a maximum value on a basis of the reference time information as a cyclic unit, A synchronization signal transmitter for transmitting to the base station of
The second base station is
A second reference information storage unit for storing information of the reference time information;
A synchronization signal receiver for receiving the synchronization signal from the first base station;
Based on the synchronization signal received from the first base station, a first time parameter of the radio frame of the own station that increases by 1 in a predetermined first time period from a predetermined initial value to a maximum value as a cyclic unit, A radio frame synchronization processing unit for synchronizing with a first time parameter of a radio frame of the first base station;
A time information acquisition unit for acquiring second time information having a lower accuracy than the first time information acquired from the first time information source from a second time information source different from the first time information source;
A second time set so that the value of the first time parameter of the local station synchronized with the first base station is incremented by 1 at a predetermined second time period with reference to the reference time information. A parameter calculation setting unit configured to calculate and set the value of the second time parameter from the second time information based on the reference time information as a value when the second time information of the parameter is acquired;
And a time calculation unit for calculating a time based on the reference time information, the value of the second time parameter, and the value of the first time parameter of the own station synchronized with the first base station. Communications system. The communication system of claim 1.
The communication system, wherein a second time period of the second time parameter is larger than an error of the second time information. The communication system of claim 2,
2. The communication system according to claim 1, wherein a second time period of the second time parameter is at least twice an error of the second time information. The communication system according to any one of claims 1 to 3,
The parameter calculation setting unit of the second base station is
Calculating the value of the second time parameter and the value of the first time parameter from the second time information with reference to the reference time information;
A communication system, wherein the calculated second time parameter is corrected and set based on the calculated first time parameter value. The communication system of claim 4,
The parameter calculation setting unit of the second base station is
Determining whether the calculated value of the first time parameter is greater than or equal to a half value of the predetermined maximum value;
If the determination is negative, the next update time of the second time parameter based on the calculated first time parameter is corrected and set by adding 1 to the calculated second time parameter,
If the determination is affirmative, the calculated second time parameter is not corrected. The communication system according to any one of claims 1 to 5,
The first time parameter is a system frame number standardized in the LTE / LTE-Advanced specification of the mobile communication system,
The communication signal characterized in that the synchronization signal is at least one of a second synchronization signal that is standardized by LTE / LTE-Advanced specifications of a mobile communication system. The communication system according to any one of claims 1 to 6,
The communication system characterized in that the first time information source is a GPS (Global Positioning System) satellite. The communication system according to any one of claims 1 to 7,
The communication system characterized in that the second time information source is an NTP (Network Time Protocol) server. The communication system according to any one of claims 1 to 8,
The second base station is a small cell base station;
The communication system, wherein the first base station is a macrocell base station. A base station for a mobile communication system,
A reference information storage unit that stores information of reference time information used for time synchronization with a reference base station that is time-synchronized based on the first time information acquired from the first time information source;
The synchronization generated by the reference base station based on the first time parameter of the radio frame that increases by one in a predetermined first time period from the predetermined initial value to the maximum value as a cyclic unit with reference to the reference time information A synchronization signal receiver for receiving a signal from the reference base station;
Based on the synchronization signal received from the reference base station, the first time parameter of the radio frame of the own station that increases by 1 in a predetermined first time period from a predetermined initial value to a maximum value as a round unit, A radio frame synchronization processing unit for synchronizing with a first time parameter of a radio frame of a reference base station;
A time information acquisition unit for acquiring second time information having a lower accuracy than the first time information acquired from the first time information source from a second time information source different from the first time information source;
A second time parameter that is set so as to increase by one at a predetermined second time period every time the value of the first time parameter of the own station synchronized with the reference base station makes a round with respect to the reference time information A parameter calculation setting unit that calculates and sets the value of the second time parameter from the second time information based on the reference time information as a value when the second time information is acquired;
A time calculation unit for calculating time based on the reference time information, the value of the second time parameter, and the value of the first time parameter of the own station synchronized with the reference base station;
A base station comprising: The base station of claim 10,
The base station, wherein a second time period of the second time parameter is larger than an error of the second time information. The base station of claim 11,
The base station according to claim 2, wherein a second time period of the second time parameter is at least twice an error of the second time information. In the base station according to any one of claims 10 to 12,
The parameter calculation setting unit
Calculating the value of the second time parameter and the value of the first time parameter from the second time information with reference to the reference time information;
The base station, wherein the calculated second time parameter is corrected and set based on the calculated first time parameter value. The base station of claim 13,
The parameter calculation setting unit
Determining whether the calculated value of the first time parameter is greater than or equal to a half value of the predetermined maximum value;
If the determination is negative, the next update time of the second time parameter based on the calculated first time parameter is corrected and set by adding 1 to the calculated second time parameter,
When the determination is affirmative, the calculated second time parameter is not corrected. In the base station according to any one of claims 10 to 14,
The first time parameter is a system frame number standardized in the LTE / LTE-Advanced specification of the mobile communication system,
The base station characterized in that the synchronization signal is at least one of a second synchronization signal that is standardized in an LTE / LTE-Advanced specification of a mobile communication system. In the base station according to any one of claims 10 to 15,
The base station characterized in that the first time information source is a GPS (Global Positioning System) satellite. In the base station according to any one of claims 10 to 16,
The base station, wherein the second time information source is an NTP (Network Time Protocol) server. In the base station according to any one of claims 10 to 17,
The base station is a small cell base station,
The base station, wherein the reference base station is a macrocell base station. In a time synchronization method between base stations of a mobile communication system,
The reference first base station synchronizes the time based on the first time information acquired from the first time information source, and the predetermined first value to the maximum value based on the reference time information is set as a predetermined cycle. Transmitting a synchronization signal generated based on a first time parameter of a radio frame that is incremented by one in one hour period;
A second base station subject to time synchronization receives the synchronization signal from the first base station, and makes a round from a predetermined initial value to a maximum value based on the synchronization signal received from the first base station. Synchronizing the first time parameter of the radio frame of the own station, which increases by one at a predetermined first time period as a unit, with the first time parameter of the radio frame of the first base station;
Obtaining second time information having a lower accuracy than the first time information obtained from the first time information source from a second time information source different from the first time information source;
Each time the second base station makes a round of the value of the first time parameter of the local station synchronized with the first base station with reference to the reference time information, the second base station increases by one at a predetermined second time period. As the value when the second time information of the second time parameter set as described above is acquired, the value of the second time parameter is calculated from the second time information with reference to the reference time information and set. And
The second base station calculates a time based on the reference time information, the value of the second time parameter, and the value of the first time parameter of the local station synchronized with the reference base station; A time synchronization method comprising:

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