JP2010034819A - Base station device and method of synchronization of carrier frequencies between base stations - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To synchronize carrier frequencies between base stations while using a clock generated by a built-in clock generator as a reference signal which decides the carrier frequency of a transmission signal. <P>SOLUTION: A base station device is configured so that a radio communication of an OFDM signal is performed with a terminal device in time-divisional duplex operation. In the base station device, the accuracy of the OFDM signal carrier frequency is affected by the accuracy of the clock frequency generated by the built-in clock generator 18. The base station device receives an OFDM signal transmitted from an other base station while transmission to a terminal device is halted, estimates the carrier frequency offset of the OFDM signal, and corrects the carrier frequency of the OFDM signal to be transmitted to the terminal device. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a base station apparatus and a data transmission method.

  In a wireless communication system in which mobile terminals can communicate, such as WiMAX (Worldwide Interoperability for Microwave Access), a large number of base stations are installed in various places. A mobile terminal in an area (cell) covered by each base station can communicate with a base station covering the area.

When the mobile terminal moves, the base station with which the mobile terminal communicates is changed. However, when the base station is changed, the mobile terminal simultaneously changes from two base stations (serving base station and target base station). Will receive the signal.
For this reason, in order to smoothly move a mobile terminal between base stations, it is necessary to ensure synchronization between base stations with the same transmission timing between adjacent base stations.

When synchronization between base stations is established, when a mobile terminal moves between base stations, the mobile terminal can simultaneously receive signals from two base stations, and can smoothly move between base stations (handover).
Here, as a technique for obtaining timing synchronization between base stations, for example, there is one described in Patent Document 1.

  Patent Document 1 discloses a technology in which each base station receives a GPS signal from a GPS satellite, and each base station synchronizes timing based on the GPS signal.

JP 59-6642 A

Now, WiMAX employs an OFDM (Orthogonal Frequency Division Multiplexing) system. In the OFDM signal, since subcarriers are densely arranged and the subcarrier interval is small, if the carrier frequency error is large between the signal transmission side and the signal reception side, the OFDM demodulation characteristics deteriorate. For this reason, the carrier frequency error is required to be small.
Therefore, it is essential to synchronize carrier frequencies between communication apparatuses that are generally assumed to communicate, such as between a base station and a mobile terminal. Such carrier frequency synchronization is achieved by the receiving side detecting a carrier frequency error from the received signal and correcting the carrier frequency error of the received signal. In this case, the detection and correction of the carrier frequency error of the reception signal is performed by an AFC (automatic frequency control) circuit provided in the reception circuit.

  On the other hand, the present inventors have obtained the knowledge that carrier frequency synchronization is also required between base stations when the mobile terminal is moved between base stations (handover) on the premise of the OFDM scheme. However, carrier frequency synchronization based on the knowledge of the present inventors means that each base station matches the carrier frequency of the signal transmitted to the mobile terminal in its own area between the base stations, and the receiving unit of the base station or terminal This is different from carrier frequency synchronization in which a carrier frequency error is detected in order to demodulate a signal of a communication partner and a carrier frequency error of a received signal is corrected.

In order for each base station as the signal transmission side to match the carrier frequency of the transmission signal, each base station needs to operate with a common reference signal (clock).
However, since the accuracy of the clock generator (crystal oscillator) built in each base station varies, each base station uses a clock generated by the built-in clock generator of each base station as a reference signal, and each base station has a predetermined carrier frequency. When trying to transmit a signal, the carrier frequency inevitably differs between base stations due to the difference in clock frequency accuracy.
Therefore, it is generally considered that the clock generated by the built-in clock generator is not suitable as a reference signal for matching the carrier frequency of the transmission signal between the base stations.

  Here, when each base station can receive a GPS signal from a GPS satellite as in Patent Document 1, each base station uses the clock signal included in the GPS signal as a carrier frequency reference signal, The carrier frequency of the transmission signal can be matched. Since the GPS signal is a signal that can be commonly used by each base station, it is suitable as a reference signal for matching the carrier frequency of the transmission signal between the base stations.

  However, when a GPS signal is used, each base station needs to have a GPS receiver, resulting in an increase in size and cost. In addition, in the case of a base station installed in an environment that cannot receive GPS signals such as indoors, it is impossible to receive GPS signals.

  In addition, when the host network connected to each base station is a communication line capable of supplying a clock such as ISDN, each base station acquires the clock from ISDN and uses the clock as a reference signal. It is possible to match the carrier frequency of the transmission signal between the base stations.

  However, in a communication system in which the Internet is assumed as an upper network, such as WiMAX, a clock cannot be acquired from the upper network.

  Therefore, the present invention synchronizes carrier frequencies between base stations while using a clock generated by a built-in clock generator, which is generally considered inappropriate, as a reference signal for determining the carrier frequency of a transmission signal. With the goal.

  The present invention is configured to perform wireless communication of an OFDM signal by time division duplex with a terminal device, and includes a built-in clock generator for generating an operation clock, and a clock generated by the built-in clock generator A base station apparatus in which the accuracy of the carrier frequency of the OFDM signal is affected by the accuracy of the frequency, and means for receiving an OFDM signal transmitted from another base station apparatus while transmission to the terminal apparatus is stopped, to the terminal apparatus Based on the OFDM signal received during the transmission stop, estimation means for obtaining an estimated value of the carrier frequency offset of the OFDM signal, and correcting the carrier frequency of the OFDM signal transmitted to the terminal device based on the estimated value And a frequency correcting means.

According to the present invention, in the base station apparatus, the accuracy of the carrier frequency of the OFDM signal is affected by the accuracy of the clock frequency generated by the built-in clock generator. However, the base station apparatus receives an OFDM signal transmitted from another base station apparatus, and estimates the difference (carrier frequency offset) between the carrier frequency of the base station apparatus and the carrier frequency of the other base station apparatus To do.
And the said base station apparatus correct | amends the carrier frequency of the OFDM signal transmitted to a terminal device based on the estimated value. Therefore, the carrier frequency of the transmission signal of the base station apparatus is synchronized with the carrier frequency of the transmission signal of another base station apparatus.

  Moreover, in the present invention, the base station device performs communication with the terminal device by time division duplex. Therefore, normally, the other base station apparatus is also in the reception state in the time zone in which the base station apparatus is in the reception state, and the other base station apparatus also transmits in the time period in which the base station apparatus is in the transmission state. It becomes a state time zone. For this reason, the base station apparatus cannot receive an OFDM signal from another base station apparatus.

  However, in the present invention, since the base station apparatus stops transmission to the terminal apparatus and receives an OFDM signal transmitted from another base station apparatus while transmission to the terminal apparatus is stopped, the base station apparatus does not use time division duplexing. However, it is possible to receive OFDM signals transmitted from other base station apparatuses.

 In the present invention, OFDM naturally includes OFDMA (Orthogonal Frequency Division Multiple Access) which is an extension of OFDM.

  The estimation means obtains an estimated value of the communication timing offset of the OFDM signal based on the OFDM signal received while transmission to the terminal apparatus is stopped, and determines the carrier frequency of the OFDM signal based on the estimated value of the communication timing offset. It is preferably configured to determine an estimate of the offset.

  Further, the estimating means determines the first estimated value of the communication timing offset obtained at the first transmission stop time and the communication timing obtained at the second transmission stop time that is different from the first transmission stop time. A phase rotation amount calculating means for calculating a phase rotation amount of the OFDM signal between the first transmission stop time and the second transmission stop time based on the difference between the second estimated value of the offset, and the phase rotation amount And a clock error calculating means for calculating the error of the clock frequency based on the calculated error of the clock frequency and obtaining the estimated value of the carrier frequency offset based on the calculated error of the clock frequency.

  Preferably, the base station apparatus further includes means for correcting a communication frame timing based on the estimated value of the communication timing offset.

  The OFDM signal received from another base station apparatus while transmission to the terminal apparatus is stopped is preferably a preamble signal transmitted from the other base station apparatus to the terminal apparatus.

  The frequency correction means preferably corrects the carrier frequency of the received OFDM signal based on the estimated value of the carrier frequency offset.

  Further, it is preferable to periodically stop transmission to the terminal device. In this case, the period for stopping transmission to the terminal device may be constant or may vary.

  According to the present invention, it is possible to synchronize carrier frequencies between base stations while using a clock generated by a built-in clock generator as a reference signal for determining a carrier frequency of a transmission signal.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 1 shows a mobile radio communication system having a TCP / IP network NW such as the Internet as an upper network.
This communication system includes a plurality of base station apparatuses (BSs) 1, 2, and 3 that perform wireless communication with mobile terminals (MSs) 101, 102, and 103 that are terminal apparatuses. A plurality (thousands) of base stations 1, 2, and 3 are connected to an ASN-GW (Access Service Network Gateway) 105 serving as an access control device. Further, the ASN-GW 105 is connected to an upper network NW such as the Internet via an HA (Home Agent) 106.

  Therefore, packets (downlink data) transmitted from the servers 107 and 108 on the upper network NW such as the Internet to the terminal are transmitted to the terminal devices 101, 102, and 103 via the base station devices 1, 2, and 3. Will be sent.

  In this wireless communication system, for example, a “WiMAX” (mobile WiMAX) system compliant with IEEE802.16 that supports an orthogonal frequency division multiple access (OFDMA) system is employed to realize broadband wireless communication.

Each base station device 1, 2 and 3 can communicate with terminal devices (mobile terminals) 101, 102 and 103 in the area (cell) covered by each base station device 1, 2 and 3. is there.
As shown in FIG. 2, in WiMAX, one basic frame is composed of a downlink subframe (base station apparatus signal transmission time) and an upstream subframe (base station apparatus signal reception time) arranged side by side in the time direction. Therefore, the communication system performs duplexing of transmission and reception by TDD (Time Division Duplex).

The length of one basic frame is 5 msec. The downlink subframe is a time zone in which the base station apparatuses 1, 2, and 3 transmit signals to the terminal apparatuses 101, 102, and 103 in its own area, and the uplink subframe is transmitted by the base station apparatuses 1, 2, and 3 This is a time zone for receiving signals from the terminal devices 101, 102, and 103 in its own area.
The downlink subframe includes a preamble that is a known signal at the head.

As shown in FIG. 3, the plurality of base station apparatuses 1, 2, and 3 in the wireless communication system include at least one master base station apparatus (master BS) 1 and a plurality of slave base station apparatuses (slave BS) 2. , 3 are included.
In this wireless communication system, processing for obtaining frame timing synchronization and carrier frequency synchronization is performed between the base station apparatuses 1, 2, and 3. The master base station apparatus 1 is a reference station for frame timing and carrier frequency, and the slave base station apparatuses 2 and 3 are directly connected to the master base station apparatus 1 or indirectly through other slave base station apparatuses. Frame timing synchronization and carrier frequency synchronization are taken.

  The frame timing synchronization is such that communication frames of the base station devices 1, 2, and 3 are transmitted at the same timing. That is, as shown in FIG. 2, in the time zone in which a certain base station device (first base station) is transmitting to the terminal device (down subframe time zone) by frame timing synchronization, other base station devices (Second base station) also transmits to the terminal device, and in a time zone (time zone of the uplink subframe) in which a certain base station device (first base station) receives from the terminal device, another base station device The communication timings of the base station apparatuses 1, 2, and 3 can be matched so that the (second base station) also receives from the terminal apparatus.

  Since the frame timing synchronization is established between the base station apparatuses, the terminal apparatus smoothly communicates with each base station apparatus even when the terminal apparatus communicates with a plurality of base station apparatuses at the time of handover or the like. Communication can be performed.

The carrier frequency synchronization is to match the carrier frequency of the signal (OFDM (A) signal) transmitted from each base station apparatus 1, 2, 3 to the terminal apparatus between the base station apparatuses.
Since the carrier frequency synchronization is established between the base station apparatuses, the terminal apparatus can smoothly communicate with each base station apparatus even when the terminal apparatus communicates with a plurality of base station apparatuses at the time of handover or the like. Can communicate with.

Here, each terminal apparatus detects an error in the carrier frequency of the OFDM signal received from the base station apparatus, and corrects the carrier frequency error in the received OFDM signal (the difference in carrier frequency between the transmission side and the reception side). (Automatic frequency control) function.
Therefore, each terminal apparatus can perform OFDM demodulation after correcting the error even if there is an error in the carrier frequency of the OFDM signal received from the base station apparatus.

  However, when the terminal device is in a state of communicating with a plurality of base station devices at the time of handover or the like, if the carrier frequency synchronization is not established between the base stations, the terminal device can use the AFC function even if the carrier frequency is not synchronized. It is very difficult to correct the frequency error.

  That is, when carrier frequency synchronization is not established between base stations, the error of the carrier frequency for one base station device is different from the error of the carrier frequency for another base station device as seen from a certain terminal device. Therefore, when communication is performed simultaneously with the plurality of base station apparatuses, the carrier frequency error cannot be corrected.

Now, since the master base station apparatus 1 is a reference station for frame timing and carrier frequency, it is necessary to acquire a signal for achieving frame timing synchronization or carrier frequency synchronization between base stations from other base station apparatuses. Absent.
For example, the master base station apparatus 1 can be configured as a self-running master base station apparatus that determines its own signal transmission timing based on a clock generated by its own built-in clock generator (crystal oscillator). Note that the master base station apparatus 1 may include a GPS receiver and determine a signal transmission timing using a GPS signal.

  On the other hand, the slave base station devices 2 and 3 send signals for frame timing synchronization or carrier frequency synchronization between base stations to other base station devices (master base station devices or other slave base station devices). )

FIG. 4 shows the configuration of the slave base station apparatuses 2 and 3.
The base station apparatuses 2 and 3 each receive an amplifier 11 for amplifying the received signal, a quadrature demodulator 12 for performing a quadrature demodulation (orthogonal detection) process on the received signal output from the amplifier 11, and a quadrature demodulation The A / D converter 13 performs A / D conversion on the received signal output from the device 12. The received signal converted into the digital signal is given to a DSP (digital signal processor) 20.

  Also, the base station apparatuses 2 and 3 perform orthogonal modulation processing on the transmission signal output from the D / A converter 15 and the D / A converter 15 for D / A conversion of the digital transmission signal for signal transmission. A quadrature modulator 16 to be performed and an amplifier 17 for amplifying a transmission signal output from the quadrature modulator 16 are provided.

The operation clocks of the quadrature demodulator 12, the A / D converter 13, the D / A converter 15, and the quadrature modulator 16 are provided from a built-in clock generator (reference signal generator) 18. . The built-in clock generator 18 includes a crystal resonator and generates an operation clock having a predetermined frequency. Note that the clock of the clock generator 18 is given to the A / D converter 13 and the like via the multipliers 19a and 19b.
The operation clock of the built-in clock generator 18 is also supplied to the DSP 20 and becomes an operation clock in the DSP 20.

  Here, the accuracy of the operation clock given to the D / A converter 15 affects the accuracy of the time length of the transmission frame (downlink subframe). Therefore, if the accuracy of the clock generator 18 is different for each base station device, the time length of the generated transmission frame is slightly different for each base station device. When the frame transmission is repeated, the difference in the frame time length is accumulated, and the frame timing between the base station devices is shifted (communication frame timing offset) (see FIG. 5).

The DSP (signal processing unit) 20 performs signal processing on the reception signal and / or transmission signal.
The main functions of the DSP 20 are a function as an OFDM demodulator for a received signal, a function as an OFDM modulator for a transmitted signal, a function for switching between transmission and reception (transmission frame and reception frame), a frame timing synchronization function between base stations, And a carrier frequency synchronization function between base station apparatuses. In FIG. 4, the blocks shown in the DSP 20 indicate these functions.

The carrier frequency correction unit 21 in FIG. 4 corrects the carrier frequency of the received signal. A carrier frequency correction unit 22 that corrects the carrier frequency of the transmission signal is also provided.
The carrier frequency correction units 21 and 22 correct the carrier frequency of the reception signal and / or the transmission signal based on the carrier frequency offset estimated by the estimation unit 23.

The output of the carrier frequency correction unit 21 of the received signal is given to the demodulation unit (DEM) 25 via the changeover switch 24. The demodulation unit 25 performs demodulation (OFDM demodulation) processing on the received signal that has been subjected to carrier frequency correction.
The change-over switch 24 applies the received signal to the demodulator 25 during the communication mode in which the signal from the terminal apparatus can be received, and the received signal is estimated in the synchronous mode in which the communication mode is stopped (paused). It is for giving to.
Note that the switch 24 is switched by the synchronization control unit 26. The communication mode and the synchronization mode will be described later.

  The DSP 20 also includes a modulation unit (MOD) 27 that performs modulation (OFDM modulation) processing on the transmission signal. The carrier frequency of the signal generated by the modulation unit 27 is determined by the quadrature modulator 16 based on the clock frequency of the clock generator 18. Further, since the carrier frequency error in the quadrature modulator 16 is the same as that in the quadrature demodulator 12, the carrier frequency error estimated by the estimation unit 23 from the received signal is directly inverted by the carrier frequency correction unit 22 as described later. If shifted, the carrier frequency of the base station transmission signal is accurately matched.

The transmission signal output from the modulation unit 27 is given to the carrier frequency correction unit 22 via the changeover switch 28.
The changeover switch 28 provides a transmission signal to the D / A converter 15 during a communication mode in which a signal can be transmitted to the terminal device, and transmits the transmission signal to a D / A converter in the synchronous mode in which the communication mode is suspended. 15 is not to be given.
The switching of the switch 28 is also performed by the synchronization control unit 26.

The estimation unit 23 detects a preamble that is a synchronization signal from a received signal, a communication frame timing offset with another base station device, a carrier frequency offset with another base station device, Is estimated.
For this reason, the estimation unit 23 includes a preamble detection unit 23a that detects a preamble included in the received signal, and a clock error estimation unit 23b that estimates a clock error between another base station device and the own device. ing.

In this embodiment, the preamble at the head of the downlink subframe DL transmitted by another base station apparatus 2 is used as a synchronization signal for synchronization between base stations. Therefore, the detection unit 23a detects the timing of the preamble at the head of the downlink subframe DL transmitted by another base station apparatus 2.
Note that the synchronization signal may be a midamble, a pilot signal, or the like.

  The base station apparatuses 2 and 3 have a preamble pattern that may be used by other base station apparatuses 1 and 2 in a memory as a known pattern. The detection unit 23a of the base station apparatuses 2 and 3 detects the preamble timing and the like using these known preamble patterns.

そして、図６（ｂ）に示すように、受信波Ｘ（ｔ）と既知プリアンブルパターンＰ（ｎ）の相関値がピークをとった位置を、プリアンブルのタイミングｔとして検出することができる。 Here, since the preamble is a known signal, the signal waveform of the preamble is also known. If the received signal after sampling is X (t) and the signal in the discrete time domain of the preamble is P (n) (n = 0,..., N−1), the received wave X shown in FIG. For (t), the sliding correlation of P (n) is taken in the time direction based on the following equation.

As shown in FIG. 6B, the position where the correlation value between the received wave X (t) and the known preamble pattern P (n) takes a peak can be detected as the preamble timing t.

  The detection unit 23a detects the difference between the transmission timings of the devices 2 and 3 and the detected preamble timing t as an estimated value of the communication timing offset (synchronization timing error). The communication timing offset (communication frame timing offset) is provided to the storage unit 29 and accumulated in the storage unit 29 each time it is detected.

The communication frame timing offset detected by the detection unit 23 a is given to the frame timing control unit 30. The frame timing control unit (TDD control unit) 30 performs control for switching between transmission and reception.
Receiving the communication frame timing offset, the frame timing control unit 30 shifts the transmission timing (transmission subframe timing) of the own apparatus in the correct direction by the detected communication frame timing offset. Thereby, it is possible to synchronize the frame timing between the base station apparatuses by matching the transmission timing of the own apparatus with the transmission timing of the other base station apparatus.

If the transmission timing is matched with the transmission timing of another base station apparatus, the reception timing is also matched naturally. That is, the frame timing is synchronized with other base station apparatuses.
Further, in the present embodiment, the communication mode for communicating with the terminal device is stopped, and the synchronization is performed using the synchronization signal (preamble) transmitted from the other base station device to the terminal device. Synchronization can be achieved even if there is no control channel for achieving the above.

  Based on the communication frame timing offset detected by the preamble detector 23a, the clock error estimator 23b and the clock frequency of the built-in clock generator 18 of its own device on the receiving side and other base station devices on the transmitting side The difference (clock frequency error) from the clock frequency of the internal clock generator 18 is estimated.

  The clock error estimator 23b has a communication frame timing offset t1 detected in the previous synchronization mode and a communication frame timing offset t2 detected in the current synchronization mode under the situation where the synchronization mode is periodically executed. To estimate the clock error. The previous timing offset t1 can be acquired from the storage unit 29.

  For example, when the carrier frequency is 2.6 [GHz], T1 is detected as a timing offset in the previous synchronization mode (synchronization timing = t1) as shown in FIG. Shall be made. The corrected timing offset is 0 [msec]. In the current synchronization mode (synchronization timing = t2) after T = 10 seconds, the timing offset is detected again, and the timing offset is T2 = 0.1 [msec].

At this time, a timing offset of 0.1 [msec] generated during 10 seconds is an accumulated value of an error between the clock cycle of the synchronization source base station and the clock cycle of the synchronization destination base station.
That is, the following equation holds between the timing offset and the clock period.
Synchronization source base station clock cycle: Synchronization source base station clock cycle = T: (T + T2) = 10: (10 + 0.0001)

And since the clock frequency is the reciprocal of the clock period,
(Synchronization source base station clock frequency-synchronization destination base station clock frequency)
= Synchronization source base station clock frequency × T2 / (T + T2)
≒ Synchronization source base station clock frequency x 0.00001
It becomes.

  Therefore, in this case, there is an error of 0.00001 = 10 [ppm] between the clock frequency of the other base station apparatus on the transmission side and the clock frequency of the base station apparatus on the reception side. The clock error estimation unit 23b estimates the clock frequency error as described above.

Since the carrier frequency and the timing offset are shifted in the same manner, the carrier frequency is also shifted by 10 [ppm], that is, a shift of 2.6 [GHz] × 1 × 10 ⁻⁵ = 26 [kHz]. . Thus, the clock error estimation unit 23b can also estimate the carrier frequency error (carrier frequency offset) from the clock frequency error.

The carrier frequency error estimated by the clock error estimation unit 23b is given to the carrier frequency correction units 21 and 22. In the present embodiment, not only the carrier frequency of the reception signal but also the carrier frequency of the transmission signal can be corrected as in a normal AFC (automatic frequency control) function.
That is, the estimated value of the carrier frequency error of the OFDM signal transmitted from another base station device is also given to the carrier frequency correction unit 22 on the transmission side, and in this carrier frequency correction unit 22, the transmission signal to the terminal device is transmitted. The carrier frequency is corrected. As a result, even if there is a clock frequency error, the carrier frequency of the transmission signal is almost the same between the own apparatus and the other base station apparatus.

In the present embodiment, the normal AFC function is used to estimate the communication frame timing offset required for frame timing synchronization instead of estimating the carrier frequency error of the received signal. Since this is used to estimate the carrier frequency error, it is advantageous in configuration.
Note that an estimated value of the carrier frequency error of the OFDM signal transmitted from another base station apparatus may be obtained using a normal AFC function, and the estimated value may be given to the carrier frequency correction unit 22 on the transmission side.
Further, in this embodiment, in order to simplify the description, a direct conversion transceiver configuration that receives and generates a direct radio frequency (RF Radio Frequency) signal using an analog quadrature modem is used. Instead, a superheterodyne transceiver that receives and generates an IF intermediate frequency signal may be used. Alternatively, direct conversion can be used for transmission, and superheterodyne can be used for reception, or vice versa. Further, the quadrature modulator / demodulator may be realized by a digital circuit, and the IF frequency may be directly sampled by A / D and generated by D / A.

Returning to FIG. 4, as described above, the synchronization control unit 26 controls the cycle (synchronization timing) for suspending the communication mode, and executes the synchronization mode.
The synchronous mode is executed as follows.

  First, the slave base station devices 2 and 3 select one base station device as a source base station device among other base station devices (master base station device or other slave base station devices) at the time of startup, A received wave (source received wave) of a signal (preamble; known signal; synchronization signal) transmitted by the source base station apparatus is detected, and frame timing synchronization and carrier frequency synchronization are established between the base station apparatuses.

  A process for synchronization between base stations performed when the base station apparatus is activated is referred to as an initial synchronization mode. The initial synchronization mode is executed at the time of activation as described above, and more specifically, is performed after the base station apparatus is activated until the communication with the terminal apparatus is started.

After the execution of the initial synchronization mode, the base station apparatus can communicate with the terminal apparatus in its own area.
However, since the clock accuracy varies among the base station apparatuses, the frame timing and the carrier frequency vary between the base station apparatuses over time.

Therefore, the slave base station devices 2 and 3 pause (stop) communication (transmission signal; downlink subframe) with the terminal device at a predetermined timing, and stop the synchronization mode (communication was paused). Synchronized mode).
FIG. 8 shows a synchronization mode in which the base station apparatuses 2 and 3 receive signals from other base station apparatuses (master base station apparatus or slave base station apparatus) from the (normal) communication mode in which communication with the terminal apparatus is performed. The flowchart for switching to is shown.

As shown in FIG. 8, the base station apparatuses 2 and 3 determine whether or not it is a synchronization timing that should be a synchronization mode (step S1). The synchronization timing is set, for example, as a cycle (every predetermined time or every predetermined number of frames) in which the synchronization mode is set. When setting a period by time, it can be set as about 5 minutes, for example.
When it is determined that it is time to shift to the synchronization mode in the normal communication mode in which communication is performed with the terminal device (step S2), the base station devices 2 and 3 are in the synchronization mode (step S3). Migrate to When the synchronization mode ends, the normal communication mode is resumed (step S4).
The base station devices 2 and 3 communicate with the terminal device, but can eliminate the synchronization loss by executing the synchronization mode periodically or whenever necessary. it can.

  When the base station devices 2 and 3 enter the synchronous mode, communication with the terminal device (downlink subframe transmission) is stopped (paused), and a state in which signals are received even during the time when the base station devices are originally in the downlink subframe. It becomes.

  In the synchronous mode, a signal (OFDM signal) transmitted from another base station device 2 to the terminal device is received. In this embodiment, the preamble at the head of the downlink subframe DL transmitted by another base station apparatus 2 is used as a synchronization signal for synchronization between base stations, and frame timing synchronization and carrier frequency synchronization are achieved.

  When the above synchronization mode ends, the base station devices 2 and 3 return from the synchronization mode to the normal communication mode, and can communicate with the terminal device.

  In addition, the synchronization control unit 26 has a function of changing a cycle for pausing the communication mode. That is, the cycle control unit 26 can set the cycle for suspending the communication mode to, for example, 5 minutes in some cases and 6 minutes in other cases. That is, the cycle control unit 26 can perform adaptive control of a cycle (synchronization timing) for suspending the communication mode.

  The adaptive control of the cycle for stopping the communication mode (synchronization timing interval) means that in the situation where the synchronization shift (timing offset or carrier frequency offset) tends to increase, the cycle for pausing the communication mode is shortened and the synchronization is frequently performed. The mode is executed so that the synchronization deviation does not become large, and in the situation where the synchronization deviation does not occur so much, the cycle of stopping (pausing) the communication mode is lengthened and the frequency of executing the synchronization mode is lowered. is there.

  In the present embodiment, the synchronization control unit 26 changes the cycle based on the past synchronization shift (timing offset).

The storage unit 20 can store synchronization deviation history information (a past one or a plurality of timing offsets) for a predetermined period in the past.
The synchronization control unit 26 calculates information (statistics) indicating the past tendency of synchronization deviation based on the history information of synchronization deviation, and the synchronization mode is executed according to the size of the information (statistic). Change the cycle (frequency). That is, if the past synchronization deviation is large, the cycle is shortened (increased frequency), and if the past synchronization deviation is small, the cycle is lengthened (frequency is lowered).

  Note that the information (statistic) indicating the past tendency of synchronization deviation may be an average of past synchronization deviations, or may be a variance value, standard deviation, or mean square value of past synchronization deviations. .

  Note that the change of the period (interval) for entering the synchronization mode may be based on other information that affects the synchronization shift. For example, since the environmental temperature affects the accuracy of the clock frequency, the base station apparatus may be provided with a temperature sensor to acquire temperature information, and the period (interval) of the synchronization mode may be changed based on the temperature information. . Specifically, if the temperature change detected by the temperature sensor is large, the synchronous mode cycle (interval) is reduced, and if the temperature change is small, the synchronous mode cycle (interval) is increased. Can do.

  Further, since the synchronization accuracy is also affected by the number of stages from the master base station apparatus 1, the period of the synchronization mode may be changed according to the number of stages from the master base station apparatus 1. Here, the number of stages from the master base station apparatus 1 is the slave base station apparatus 2 having the master base station apparatus 1 as a source base station, as shown in FIG. 3, where the master base station apparatus 1 is the first stage. Becomes the second stage, and the slave base station apparatus 3 having the second-stage base station apparatus 2 as the source base station becomes the third stage. Since the base station apparatus having a larger number of stages from the master base station apparatus 1 has a lower synchronization accuracy, the period of the synchronization mode can be reduced, and the base station apparatus having a smaller number of stages can increase the period of the synchronization mode.

  Note that the number of stages from the master base station apparatus 1 may be set in advance in each base station apparatus, or in the synchronous mode, obtain the number of stages of other base station apparatuses (source base station apparatuses) A value obtained by adding 1 to the number of stages may be the number of stages of the own device. In order to obtain the number of stages of other base station devices (source base station devices), for example, in the case of WiMAX, since a plurality of types are defined as preamble patterns, this can be used. Specifically, if a predetermined preamble pattern is assigned to each stage in advance, the base station apparatus that performs the synchronization process grasps the number of stages of other base station apparatuses (source base station apparatuses) by identifying the preamble pattern. can do.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the meanings described above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is a figure which shows the mobile radio | wireless communications system which uses internet NW as a high-order network. It is a figure which shows the state of a WiMAX frame when the synchronization between base stations is taken. It is a figure which shows the master base station apparatus and slave base station apparatus in a radio | wireless communications system. It is a functional block diagram of a base station apparatus. It is a figure which shows the flame | frame which a timing offset produced. It is explanatory drawing for detecting the timing of a preamble. It is explanatory drawing which shows the timing offset amount in the last and this time synchronous mode. It is a flowchart which shows switching of normal communication mode and synchronous mode.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Base station apparatus 2 Base station apparatus 3 Base station apparatus 18 Clock generator 20 DSP
DESCRIPTION OF SYMBOLS 21 Carrier frequency correction part 22 Carrier frequency correction part 23 Estimation part 23a Preamble detection part 23b Clock error estimation part 24 Changeover switch 25 Demodulation part 26 Synchronization control part 27 Modulation part 28 Changeover switch 29 Storage part

Claims (7)

It is configured to perform OFDM signal wireless communication with a terminal device by time division duplex, and includes a built-in clock generator that generates an operation clock, and according to the accuracy of the clock frequency generated by the built-in clock generator. A base station apparatus in which the accuracy of the carrier frequency of the OFDM signal is affected,
Means for receiving an OFDM signal transmitted from another base station device while transmission to the terminal device is stopped;
Based on the OFDM signal received while transmission to the terminal device is stopped, estimating means for obtaining an estimated value of the carrier frequency offset of the OFDM signal;
Frequency correcting means for correcting the carrier frequency of the OFDM signal transmitted to the terminal device based on the estimated value;
A base station apparatus comprising:   The estimation means obtains an estimated value of the communication timing offset of the OFDM signal based on the OFDM signal received while transmission to the terminal apparatus is stopped, and determines the carrier frequency of the OFDM signal based on the estimated value of the communication timing offset. The base station apparatus according to claim 1, wherein the base station apparatus is configured to obtain an estimated value of an offset. The estimation means includes
A first estimated value of the communication timing offset obtained at the first transmission stop time, and a second estimated value of the communication timing offset obtained at the second transmission stop time that is a time different from the first transmission stop time; Phase rotation amount calculating means for calculating the phase rotation amount of the OFDM signal between the first transmission stop time and the second transmission stop time based on the difference between
Clock error calculating means for calculating an error of the clock frequency based on the phase rotation amount;
And obtaining an estimated value of the carrier frequency offset based on the calculated error of the clock frequency.
The base station apparatus according to claim 2, wherein   The base station apparatus according to claim 2, further comprising means for correcting a communication frame timing based on the estimated value of the communication timing offset.   The OFDM signal received from another base station apparatus while transmission to the terminal apparatus is stopped is a preamble signal transmitted from the other base station apparatus to the terminal apparatus. Base station equipment.   The base station apparatus according to claim 1, wherein the frequency correction unit corrects a carrier frequency of the received OFDM signal based on the estimated value of the carrier frequency offset.   The base station apparatus according to claim 1, wherein transmission to the terminal apparatus is periodically stopped.

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