JP2009278424A - Base station device and method for transmitting data - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress the occurrence of a delay at a time when data transmission to a terminal device is stopped for correcting synchronous displacement (synchronous error). <P>SOLUTION: Base station devices 2 and 3 have control sections 34 stopping the transmissions of data to the terminal device in a time zone which should transmit data to the terminal device, and synchronous error detecting sections 33 acquiring synchronous signals transmitted from other base station devices and detecting the synchronous errors (synchronous displacements) during the stop of the data transmissions, and the synchronous errors can be corrected on the basis of the acquired synchronous signals. The base station devices 2 and 3 have buffers 26 storing data stopping transmissions, and reopen the data transmissions to the terminal device and transmit data stored by the buffers 26 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 a mobile terminal 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.
Here, as a technique for synchronization between base stations, for example, there is one described in Patent Document 1.
JP 59-6642 A

In order to achieve synchronization between base stations, it is conceivable that each base station apparatus receives a GPS signal from a GPS satellite and each base station operates with a common synchronization signal as in Patent Document 1.
However, when synchronizing using a GPS signal, 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 becomes impossible to establish synchronization between base stations.

Therefore, it is conceivable to receive a synchronization signal transmitted by another adjacent base station, detect the transmission timing of the other adjacent base station, and synchronize at the transmission timing.
In this case, since synchronization can be achieved by wireless communication using the same frequency as that used for communication with the mobile terminal, a special receiving system for synchronization is not required unlike a GPS receiver for receiving GPS signals.
For this reason, it is possible to reduce the size and cost of the base station, and it is suitable as a small base station installed indoors.

Here, the WiMAX described above employs a communication method that realizes duplex communication by TDD (Time Division Duplex) that switches between transmission and reception at high speed for wireless communication with a mobile terminal.
Specifically, as shown in FIG. 12, in WiMAX, one basic frame is composed of a downlink subframe (base station signal transmission time) and an uplink subframe (base station signal reception time) arranged in the time direction. Has been placed. The downlink subframe has a preamble at the beginning.

FIG. 12 shows a state in which transmission timings and reception timings match and are synchronized among a plurality of base stations.
Such processing for synchronization between base stations is performed when the base station is activated, and normal communication with a mobile terminal is performed after synchronization between base stations is established.

  However, depending on the difference in the accuracy of the clock generators possessed by the base station, the synchronization may shift due to the passage of time or the like. In other words, even if synchronization with other base stations is established when the base station is started, if communication is subsequently performed with the mobile terminal (terminal device), synchronization deviation gradually occurs due to a difference in accuracy of the clock generator. .

For this reason, the present inventor has come up with the idea of stopping the transmission of data to the terminal device in the time zone in which data should be transmitted to the terminal device, and re-synchronizing during the suspension.
However, if the transmission of data to the terminal device is stopped, the data transmission is delayed correspondingly and the throughput may be reduced.

  Therefore, an object of the present invention is to suppress the occurrence of a delay when data transmission to a terminal device is stopped in order to correct a synchronization error (synchronization error).

  The present invention relates to a stop means for stopping data transmission to a terminal apparatus in a time zone in which data is to be transmitted to the terminal apparatus, and a synchronization transmitted from another base station apparatus while the data transmission is stopped by the stop means. A means for acquiring a signal, a correction means for correcting a synchronization error based on the acquired synchronization signal, a storage means for storing data stopped by the stop means, and after resuming data transmission to the terminal device And a transmission means for transmitting the data accumulated by the accumulation means to the terminal device.

  According to the present invention described above, in order to store the data whose transmission has been stopped even if transmission of data to the terminal apparatus is stopped in the time zone in which data should be transmitted to the terminal apparatus, After the transmission is resumed, the accumulated data can be transmitted to the terminal device. Therefore, even if transmission of data to the terminal device is stopped in a time zone in which data is to be transmitted to the terminal device, data loss does not occur when viewed from the terminal device, and delay associated with retransmission of missing data can be prevented.

The transmission means compares the transmission priority of the data stored in the storage means, the transmission being stopped by the stop means, and the data to be transmitted after resuming data transmission to the terminal device. It is preferable that data with high priority is transmitted with priority.
The transmission priority is preferably based on a data service class.

  The transmission unit preferably transmits the data stored in the storage unit with priority over other data after resuming data transmission to the terminal device.

In response to a data retransmission request from the terminal apparatus, means for retransmitting data to the terminal apparatus, an accumulation mode in which the accumulation means accumulates data stopped by the stop means, and transmission stop by the stop means. It is preferable to further comprise mode switching means for switching between a missing mode in which the accumulated data is not accumulated by the accumulation means and is lost for the terminal device.
In this case, when it is more advantageous for the terminal device to make the data missing without accumulating the data whose transmission has been stopped, the mode can be switched to the missing mode.

  The switching means preferably switches the mode based on the frequency of stopping data transmission to the terminal device. Since the frequency of data transmission suspension has a significant effect on determining which of accumulation mode and loss mode is advantageous, switching the mode based on the frequency of data transmission suspension to the terminal device enables appropriate switching. Can be done.

  Another aspect of the present invention is a data transmission method in a base station apparatus for acquiring a synchronization signal transmitted from another base station apparatus in a time zone in which data should be transmitted to the terminal apparatus. The transmission of data to the device is stopped, the data that has been stopped is stored, and after the data transmission to the terminal device is resumed, the data stored by the storage means is transmitted to the terminal device. This is a data transmission method.

  According to the present invention, even if data transmission to a terminal device is stopped for correction of synchronization error (synchronization error), occurrence of delay can be suppressed.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 1 shows a mobile radio communication system using a TCP / IP network NW such as the Internet as a backbone 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.
Communication between the base station apparatuses 1, 2, 3 and the terminal apparatuses 101, 102, 103 in the wireless communication system is performed based on a protocol corresponding to layer 2 (data link layer) in the OSI reference model.

  In this wireless communication system, for example, a method compliant with “WiMAX” (mobile WiMAX) defined in IEEE 802.16 that supports an orthogonal frequency division multiple access (OFDMA) method is adopted to realize broadband wireless communication. Yes.

Each base station apparatus 1, 2, 3 can communicate with terminal apparatuses 101, 102, 103 in the area (cell) covered by each base station apparatus 1, 2, 3.
As shown in FIG. 11, in WiMAX, one basic frame is arranged such that a downlink subframe (signal transmission time of the base station apparatus) and an uplink subframe (signal reception time of the base station apparatus) are arranged in the time direction. It is a communication system that performs duplexing of transmission and reception by TDD (Time Division Duplexing).

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. 2, the plurality of base station apparatuses 1, 2 and 3 include at least one master base station apparatus (master BS) 1 and a plurality of slave base station apparatuses (slave BS) 2 and 3. It is.
The master base station apparatus 1 is a base station apparatus that does not need to detect and acquire the timing for synchronization between base stations from a received wave of a signal (synchronization signal; preamble) transmitted by another base station apparatus. 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 itself. Note that the master base station apparatus 1 may include a GPS receiver and determine a signal transmission timing using a GPS signal.

  Slave base station apparatuses 2 and 3 are base station apparatuses that detect and acquire timing for synchronization between base stations from a received wave of a signal (synchronization signal; preamble) transmitted by another base station apparatus.

  FIG. 3 shows the configuration of the slave base station apparatuses 2 and 3. In FIG. 3, the structure centering on the receiving part 10 and the transmission part 20 in the base station apparatus is shown. The reception unit 10 includes an amplifier 11 that amplifies the reception signal, an A / D conversion unit 12 that performs A / D conversion on the reception signal, and a demodulation unit (DEM) 13 that demodulates the reception signal converted into a digital signal. .

  The transmission unit 20 includes a buffer 20 for storing transmission packets obtained by blocking transmission data, a modulation unit (MOD) 21 that modulates transmission data, a D / A conversion unit 22 that performs D / A conversion on transmission signals, And the amplifier 23 which amplifies a transmission signal is provided.

  The modulator 21 performs OFDM (A) modulation on the transmission data stored in the buffer 20 and generates a downlink subframe (transmission frame). The downlink subframe (digital signal) generated by the modulation unit 21 is converted into an analog signal by the D / A conversion unit 22 and transmitted to the terminal device.

  Since the base station apparatus communicates with the terminal apparatus by TDD (Time Division Duplex), a switch (SW) 31 for switching the connection with the antenna 30 between the receiving unit 10 side and the transmitting unit 20 side is provided. I have. That is, at the timing of the transmission frame (downlink subframe), the switch 31 is switched to the transmission unit 20 side, and at the timing of the reception frame (uplink subframe), the switch 31 is switched to the reception unit 10 side.

  The operation clocks of the A / D converter 12 and the D / A converter 22 are supplied from a reference signal generator 40. The reference signal generator 40 includes a clock generator such as a crystal resonator and generates an operation clock having a predetermined frequency. Needless to say, the operation clock also serves as an operation clock for other digital circuits in the base station apparatus such as the frame timing counter 40.

  Here, the accuracy of the operation clock of the D / A converter 22 affects the accuracy of the time length of the transmission frame (downlink subframe). Therefore, if the accuracy of the reference signal generator is different for each base station apparatus, an error occurs in the operation clock between the base station apparatuses, and the time length of the generated transmission frame is slightly different for each base station apparatus. Become.

  Switching between transmission and reception is performed according to the counter value in the frame timing counter 32. That is, the time length of the transmission frame, the time length of the reception frame, and the time interval between the frames are determined in advance by the standard, and when the counter value matches a predetermined transmission / reception switching timing, Switching between transmission and reception is performed.

  When a synchronization shift with another base station apparatus occurs, the synchronization shift can be corrected by correcting the counter value of the frame timing counter 32. For example, when it is detected that there is n synchronization values (synchronization errors), the transmission / reception switching timing can be matched with other base station apparatuses by shifting the counter value of the counter 32 in the correct direction by n. it can.

  The synchronization error (synchronization error) is detected by the synchronization error detector 33. As shown in FIG. 4, the synchronization error detector 33 detects a synchronization signal (preamble) from the received signal (received wave), and detects a synchronization error (timing offset). The detected synchronization error is given to the frame timing counter 32, and the synchronization error (synchronization error) is corrected.

  In order to detect a synchronization error using a signal transmitted from another base station apparatus, the reception unit 10 includes a changeover switch 14 for switching the reception signal to the demodulation unit 13 side or the synchronization error detection unit 33 side. . The change-over switch 14 applies the received signal to the demodulator 13 during the communication mode in which the signal from the terminal device can be received, and detects the synchronization error in the synchronized mode in which the communication mode is stopped (paused). To part 33.

The transmission unit 20 also has a changeover switch 24. The changeover switch 24 applies a transmission signal to the D / A converter 22 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. 22 is not to be given.
Furthermore, the transmission unit 20 has a switch 25. This switch 25 is for switching whether or not to provide a buffer (transmission data (packet) storage unit) 26 that stores transmission data (packets) to the modulation unit 25 to form a transmission frame.

  Switching of the switches 14, 24, and 25 of the receiving unit 10 and the transmitting unit 20 is performed by the cycle control unit 34. The cycle control unit 34 is for controlling the cycle (synchronization timing) for suspending the communication mode. The cycle control unit 34 performs processing according to the flowchart of FIG. The process of FIG. 5 will be described later.

  In addition, the cycle control unit 34 has a function of changing the cycle for suspending the communication mode. That is, the cycle control unit 34 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 34 performs 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 the synchronization mode is frequently executed by shortening the cycle for suspending the communication mode, etc., in a situation where synchronization deviation (synchronization error) is likely to increase. In this way, the synchronization error deviation is prevented from becoming large, and in the situation where the synchronization deviation (synchronization error) does not occur so much, the frequency of executing the synchronization mode is decreased by increasing the period of stopping (pausing) the communication mode. It is.

  In the present embodiment, the cycle control unit 34 changes the cycle based on the past synchronization error. For this reason, as shown in FIG. 3, the base station apparatus includes a synchronization error history storage unit 35 that stores a past history of synchronization errors.

The synchronization error history storage unit 35 can store synchronization errors for the past predetermined period (one or more synchronization errors in the past).
The cycle control unit 34 calculates information (statistics) indicating the past tendency of the synchronization error based on the history information of the synchronization error, and the synchronization mode is executed according to the size of the information (statistic). Change the cycle (frequency). That is, if the past synchronization error is large, the cycle is shortened (increased frequency), and if the past synchronization error is small, the cycle is lengthened (lower frequency).

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

  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. 2, 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.

  FIG. 6 shows a synchronization deviation (timing offset) between the master base station apparatus (master BS) 1 and the slave base station apparatus (slave BS) 2. This synchronization shift occurs while the slave base station device 2 communicates with the terminal device in its own area, and the reason why the synchronization shift occurs is 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 device) at the time of startup, A reception wave (source reception wave) of a signal (preamble; known signal; synchronization signal) transmitted by the source base station apparatus is detected, and timing (signal transmission timing) for synchronization between base stations is acquired.
Note that processing for synchronization between base stations performed when the base station apparatus is activated is referred to as initial synchronization processing. The initial synchronization process is performed at the time of activation as described above, and more specifically, is performed after the base station apparatus is activated and before communication with the terminal apparatus is started. The specific contents of the initial synchronization process are almost the same as those in a “synchronization mode in which communication is stopped” described later.

Therefore, after the initial synchronization processing, the communication (communication mode) performed by the slave base station device with the terminal device is the transmission timing and reception of the source base station device (other base station device) as shown in FIG. The timing (communication timing) coincides with the timing.
However, if the accuracy of the clock generator of the slave base station device 2 is not sufficient or the clock accuracy varies among the base station devices, a synchronization shift occurs over time. That is, when the base station device is communicating with the terminal device, a transmission / reception timing (communication timing) of another base station device and a shift (synchronization shift) gradually occur.

That is, since an error in the clock frequency of the clock generator included in the base station apparatus exists between the base station apparatuses, one transmission frame (downlink subframe) generated based on the clock frequency (reference signal) The time length (for example, 5 msec in the standard) is slightly different between the base station apparatuses. Even if the error of the time length of one frame is slight, if the transmission of the frame to the terminal device is repeated, the error accumulates and there is a possibility that a relatively large synchronization shift (for example, about 1 μsec) may occur.
As described above, even when the communication timing between the base station apparatuses is aligned in the initial synchronization process, the synchronization deviation gradually increases during the communication with the terminal apparatus as shown in FIG.

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. 5 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 illustrated in FIG. 5, the base station devices 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 are in the synchronization mode, synchronization processing (synchronization processing in which communication is stopped) is performed. During this synchronization process, communication with the terminal device (downlink subframe transmission) is stopped (paused), and a signal is originally received even during the downlink subframe time.

In the synchronization process (synchronization process in which communication is suspended), first, a signal from another base station apparatus 2 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. For this reason, the base station apparatus 1 detects the timing of the preamble at the head of the downlink subframe DL transmitted by the other base station apparatus 2.
Note that the synchronization signal may be a midamble, a pilot signal, or the like.

Base station apparatuses 2 and 3 have a function of scanning (receiving signals) received waves from other base station apparatuses adjacent to the base station apparatuses 2 and 3 in order to detect preamble timing.
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 base station apparatuses 2 and 3 detect the preamble timing and the like using these known preamble patterns.

そして、図４（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.

Then, as shown in FIG. 4B, 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 base station apparatuses 2 and 3 correct the synchronization error by setting the acquired preamble timing t to be the transmission start timing of the downlink subframe. That is, the difference between the transmission timings of the devices 2 and 3 and the detected preamble timing t is detected as a synchronization shift (timing offset; synchronization error). Then, by correcting the transmission timing (frame timing) of the own devices 2 and 3 based on the detected synchronization shift, the transmission timing of the own devices 2 and 3 is changed to the transmission timing of the other base station devices 1 and 2. Can match. In other words, synchronization can be reestablished by shifting the transmission timing (frame timing) of the own apparatus in the correct direction by the amount of the detected synchronization error with reference to the timing of the detected synchronization signal.

If the transmission timing is matched with the transmission timing of another base station, the reception timing is also matched naturally. That is, the frame synchronization is established with other base stations.
In this way, the communication mode in which communication is performed with the terminal device is stopped, and synchronization is performed using a synchronization signal from another base station device, so even if there is no control channel for synchronization, Can be synchronized.

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

FIG. 7A shows an offset of the clock frequency of the slave base station device 2 with respect to the clock frequency of the master base station device 1. As shown in FIG. 7A, the clock frequency of the slave base station device 2 has an offset larger than the clock frequency of the master base station device 1 as time elapses. The cause of this change in offset is a change in environmental temperature.
The clock frequency offset does not change abruptly with the passage of time, but gradually changes with time as shown in FIG. Accordingly, as described above, the cycle control unit 34 can perform an appropriate cycle change by changing the cycle based on the past tendency of the synchronization error.

  FIG. 7B shows a change in the synchronization shift (timing offset) in the slave base station apparatus 2 when the synchronization mode is executed every 5 minutes. When the synchronization mode is executed at the timing indicated by the arrow in FIG. 7B, the timing offset becomes almost zero, but the timing offset gradually increases between the synchronization mode and the synchronization mode (during the communication mode).

As shown in FIG. 8, since the base station apparatuses 2 and 3 in the synchronous mode perform reception in a time zone that should be a transmission frame (downlink subframe), a packet that should have been transmitted in that time zone There is a risk that packet loss will not occur.
Specifically, when the synchronization mode is set, the control unit 34 controls the changeover switch 24 so that the modulated signal (transmission frame; transmission data) is not given to the D / A conversion unit 22. 21 is disconnected from the D / A converter 22. For this reason, when the modulation unit 21 generates a transmission frame in synchronization with each transmission frame timing, the transmission frame is not transmitted at the timing of the synchronous mode.
In this case, packet loss occurs for one or a plurality of packets corresponding to frames that have not been transmitted.

Therefore, in the present embodiment, the control unit 34 controls the switch 25 so that data (packets) is not given from the buffer (transmission buffer) 26 to the modulation unit 21 during the synchronous mode.
Therefore, during the synchronous mode, the data (packets) in the buffer 26 is stopped in transmission and is not stored for transmission but remains stored in the buffer 26. As a result, no transmission frame is generated during the synchronous mode.

As described above, in the present embodiment, even when the data is to be transmitted to the mobile terminal MS, the data (packets) are stored and stopped in the buffer 26 even if the data is in the reception state for the synchronous mode. (Packet) is not used for transmission frame generation by the modulator 21 during the synchronous mode.
When the communication mode is restored from the synchronous mode and data transmission to the mobile terminal MS is resumed, the data (packets) stored in the buffer 26 are transmitted to the mobile terminal MS.

  Here, let us consider a case where the switch 25 is not provided in FIG. 3 and the data (packet) in the buffer 26 is given to the modulator 21 even in the synchronous mode. Here, as shown in FIG. 9A, packets are transmitted in the order of packet numbers. Further, as shown in FIG. 9A, it is assumed that the transmission timing of the transmission frame corresponding to the packet whose packet number is “5” and the synchronous mode are set.

In this case, the transmission frame corresponding to the packet with the packet number “5” is not transmitted because of the synchronous mode. However, from the viewpoint of the buffer 26, the packet with the packet number “5” is considered to have already been transmitted. Therefore, after the communication mode is resumed, the packet with the packet number “6” becomes the transmission target.
As a result, in the base station apparatus (BS), even if the packets with the bucket numbers 1 to 8 are to be transmitted, the packet with the packet number “5” is not actually transmitted, and therefore received by the mobile terminal MS. I can't.

  In this case, between the base station apparatus (BS) and the mobile terminal (MS), the lost packet (data) is retransmitted by ARQ (Automatic Repeat reQuest). The ARQ includes a GBN (Go Back N) type ARQ and an SR (Selective Repeat) type ARQ. The GBN type is a method of retransmitting data back to the Nth data (packet) that could not be received by the mobile terminal MS that is the receiving side. The SR type is the type that the mobile terminal MS that is the receiving side is the SR type. This is a method for requesting retransmission of only the Nth data (packet) that could not be received.

  FIG. 9 shows GBN type ARQ. That is, “Go Back N Ack” is periodically transmitted from the mobile terminal MS on the receiving side to the base station apparatus. In FIG. 9A, since the packet number that could not be received by the mobile terminal MS is “5”, “Go Back N Ack” with N = 5 is transmitted to the base station apparatus. Then, the base station apparatus transmits again from the packet with the packet number 5.

  Although the ARQ can compensate for packet errors (packet loss) due to execution of the synchronous mode, a delay time occurs. In the case of GBN type ARQ, this delay time is longer than the execution time of one synchronization mode (time for one frame). As the delay time becomes longer, the TCP throughput characteristics deteriorate.

On the other hand, in the present embodiment, packets (data) are sent from the buffer 26 during the communication mode. However, during the synchronous mode, transmission is stopped and stored so that packets (data) are not sent from the buffer 26. The data accumulated in the buffer 26 is transmitted after the communication mode is resumed. For this reason, as shown in FIG. 9B, even if the communication mode is stopped and the synchronous mode is executed, packet loss does not occur when viewed from the mobile terminal MS on the receiving side.
Therefore, in “Go Back N Ack” periodically transmitted from the mobile terminal MS, a retransmission unnecessary instruction is given, and no delay due to retransmission occurs as shown in FIG. 9A. The delay that occurs in the case of FIG. 9B is at most the execution time of the synchronous mode.

  As described above, in the present embodiment, the transmission buffer 26 is provided, and during the synchronous mode, temporarily stores data (packets) that should be transmitted if the time zone is the communication mode but is stopped. In addition, since the data of the buffer 26 is transmitted when the communication mode is resumed, it is possible to suppress the occurrence of delay due to the retransmission by ARQ.

In this embodiment, when the synchronization mode ends and the communication mode resumes, it should be transmitted during the synchronization mode rather than the packet (data) that should have been originally transmitted in the transmission frame after the communication mode is resumed. A packet (data) corresponding to the transmitted frame is preferentially transmitted.
Specifically, it is assumed that the packet number that should have been transmitted during the synchronous mode is “5” and the packet number that should be transmitted when the communication mode is resumed is “6”. In this case, when the communication mode is resumed, the packet with the number “5” stored in the buffer 26 is first transmitted, and then the packet with the number “6” is transmitted.

  As described above, after resuming the communication mode, the packet stored in the buffer 26 is not transmitted preferentially, but the packet (data) that is supposed to be transmitted in the transmission frame after the resumption of the communication mode. The priority of the packet (data) corresponding to the transmission frame that should have been transmitted during the synchronous mode may be compared, and the packet (data) having a higher priority may be transmitted with priority.

  In this case, the configuration shown in FIG. 10 can be adopted for the buffer 26 of FIG. The buffer shown in FIG. 10 includes a first buffer 26a that stores packets (data) that are transmission targets during the synchronous mode and a second buffer 26b that stores packets (data) that are transmission targets during the communication mode. I have. During the communication mode, the priority comparison unit 26c compares the priorities of the packets stored in the buffers 26a and 26b, and sends out a packet having a high priority.

Here, the priority comparison unit 26c determines that the priority is high for data (packets) having higher real-time characteristics. Therefore, the priority comparison unit 26c compares and determines which packet should be prioritized based on, for example, the service class of the packet (data).
The priority comparison unit 26c performs identification by DSCP (Diff Service Code Point) in order to identify the service class of the packet.
Specifically, if the target packet is an IPv4 packet, the priority comparison unit 26c refers to the DSCP field (6 bits) included in the TOS (Type Of Service) field (8 bits) of the header, A service class (traffic type) can be identified.
If the target packet is IPv6, the service class (traffic type) can be identified by referring to the DSCP field (6 bits) included in the Traffic class field (8 bits) of the header.

  For example, in Mobile WiMAX, five classes are defined as service classes (also referred to as “QoS (Quality of Service) classes” in WiMAX). Specifically, the five classes include UGS (Unsolicited Grant Service), rtPS (Real-Time Polling Service), ErtPS (Extended Real-Time Poling Service), nrtPS (Non-Real-Time Polling Service), and BE (Best -Effort Service).

UGS is a transmission right assignment service and is a class applied to real-time applications such as VoIP (Voice over IP) that requires a fixed data transmission rate in real time.
rtPS is a class applied to applications such as music or video streaming.
ErtPS is a class applied to applications such as VoIP with silence suppression.
nrtPS is a class applied to applications such as FTP (File Transfer Protocol).
BE does not guarantee transmission such as transmission rate and transmission delay, and is a class applied to data communication, web browsing, and the like.

  When the service class is defined as described above, the service class to which the packet belongs can be determined by the priority comparison unit 26c referring to the DSCP field of the packet as described above.

In the present embodiment, a high priority is given to the service class according to the high real-time property. That is, the highest priority is given to UGS, and the priority becomes lower in the order of rtPS, ErtPS, nrtPS, and BE.
By preferentially transmitting those that require real-time performance in this way, it is possible to prevent the real-time performance from being impaired.
That is, in the case of simple data transfer (BE) where real-time performance is not so much required, there is no problem even if some delay occurs after transmission is delayed. On the other hand, in the case of data such as VoIP that requires real-time characteristics, the occurrence of delay causes an abnormality such as voice interruption, so that the occurrence of the abnormality can be prevented by preferential transmission.

The control unit 34 can be configured to perform switching control so that packets (data) to be transmitted are not necessarily stored in the buffer 26 but are not stored in the buffer during the synchronous mode.
In this case, during the synchronous mode, there are two modes of data (packet) handling, the accumulation mode and the loss mode, and the control unit 24 selects either the accumulation mode or the loss mode.

  Here, the accumulation mode is a mode in which transmission is stopped due to the synchronous mode and data (packets) is accumulated in the buffer 26. The missing mode is a mode in which data that has been stopped due to the synchronous mode is not accumulated in the buffer 26 and is made data (packets) that are missing from the viewpoint of the mobile terminal MS.

  The control unit 34 according to the present embodiment switches between the accumulation mode and the loss mode according to the period (frequency) of the synchronous mode. Specifically, the accumulation mode is set when the period of the synchronous mode is shorter than the predetermined threshold (the frequency is high), and the loss mode is set when the period is longer than the predetermined threshold (the frequency is low). .

  Here, since the size of the transmission buffer 26 is finite, a buffer overflow may occur. That is, when the data to be stored in the buffer 26 increases, the data cannot be stored in the buffer 26, resulting in buffer overflow loss. When the buffer overflow loss occurs, the base station apparatus itself loses transmission data (packets), and therefore, retransmission is performed by the upper protocol (retransmission at the TCP layer). That is, data is retransmitted from the servers 107 and 108 on the upper network NW. Retransmission with this higher-level protocol causes a relatively large delay, and is therefore desired to be avoided as much as possible.

  However, in this embodiment, since all data (packets) whose transmission has been stopped due to the synchronous mode are accumulated in the buffer 26, the buffer overflow is more likely to occur during the synchronous mode than during the normal communication mode. Become. That is, a delay due to buffer overflow is more likely to occur during the synchronous mode than during the communication mode.

However, if data (packets) whose transmission has been stopped is not stored in the buffer 26 in the synchronous mode, a delay associated with ARQ occurs.
In particular, when the frequency of the synchronous mode is high, if the data is not stored in the buffer 26, a delay due to ARQ frequently occurs, and the delay amount as a whole increases.
Therefore, in the present embodiment, the control unit 34 sets the accumulation mode when the period of the synchronous mode is shorter than the predetermined threshold (the frequency is high), and prevents frequent occurrence of delay due to ARQ. To do.

On the other hand, when the frequency of entering the synchronous mode is low, a delay due to ARQ occurs only occasionally without accumulation in the buffer 26, and the amount of delay due to ARQ is not so large. Instead, the amount of delay can be reduced as a whole by suppressing the possibility of a large delay due to buffer overflow.
Therefore, in the present embodiment, the control unit 24 sets the loss mode when the period of the synchronization mode is longer than the predetermined threshold (the frequency is low), and reduces the possibility of buffer overflow during the synchronization mode. Let

FIG. 11 shows the relationship between the synchronization mode interval (frequency) and the delay time. As shown in FIG. 11, the delay time caused by ARQ (when the buffer is not accumulated) decreases as the interval of the synchronization mode increases. The delay time due to buffer overflow (when the buffer is accumulated) is also smaller as the synchronization mode interval is larger, but has a gentler slope than the delay time caused by ARQ.
Therefore, the synchronization mode interval corresponding to the intersection of both delay time characteristics is used as a threshold value, and when the synchronization mode interval is smaller (higher frequency) than this threshold value, data is stored in the buffer 26 in the accumulation mode. Thus, an increase in delay time due to ARQ can be avoided. That is, in this case, the generated delay time is a delay time due to buffer overflow.

  On the other hand, when the interval of the synchronization mode is larger than the threshold (the frequency is low), the data is lost without being stored in the buffer 26 by the loss mode, and retransmitted to the mobile terminal MS by ARQ. In this case, a delay due to ARQ occurs, but a large delay due to buffer overflow can be avoided.

  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 the internet NW as a backbone network. 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 explanatory drawing for detecting the timing of a preamble. It is a flowchart which shows switching of normal communication mode and synchronous mode. It is a figure which shows a synchronization shift | offset | difference (timing offset produced) flame | frame. FIG. 15A is a graph showing a clock frequency offset, and FIG. 15B is a graph showing a timing offset. It is a figure which shows a mode that a packet loss arises in synchronous mode. (A) is a figure which shows that the packet loss which generate | occur | produced in synchronous mode is resent by ARQ, (b) is a figure explaining that packet loss does not arise in synchronous mode. It is a block diagram of a buffer having a function of comparing and sending out packet priorities. It is a characteristic view of the interval and delay time of a synchronous mode. It is a figure which shows the state of a WiMAX frame when the synchronization between base stations is taken.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Base station apparatus 2 Base station apparatus 3 Base station apparatus 21 Modulation part 24 Switch 25 Switch 26 Buffer 32 Frame timing counter 33 Synchronization error detection part 34 Period control part 35 Synchronization error history storage part

Claims (7)

Stop means for stopping transmission of data to the terminal device in a time zone in which data should be transmitted to the terminal device;
Means for acquiring a synchronization signal transmitted from another base station apparatus during data transmission suspension by the suspension means;
Correction means for correcting the synchronization error based on the acquired synchronization signal;
Accumulating means for accumulating data whose transmission has been stopped by the stopping means;
A transmission means for transmitting the data stored by the storage means to the terminal device after resuming data transmission to the terminal device;
A base station apparatus comprising:   The transmission means compares the transmission priority of the data stored in the storage means, the transmission being stopped by the stop means, and the data to be transmitted after resuming data transmission to the terminal device. The base station apparatus according to claim 1, wherein data having high priority is transmitted with priority.   The base station apparatus according to claim 2, wherein the transmission priority is based on a data service class.   The base station apparatus according to claim 1, wherein the transmission unit transmits the data stored in the storage unit with priority over other data after resuming data transmission to the terminal device. Means for retransmitting data to the terminal device in response to a data retransmission request from the terminal device;
The accumulation mode in which the accumulation means accumulates the data whose transmission has been stopped by the stopping means, and the deficiency that the data whose transmission has been stopped by the stopping means is not accumulated by the accumulation means and is lost to the terminal device. Mode switching means for switching between modes;
The base station apparatus according to any one of claims 1 to 4, further comprising:   The base station apparatus according to claim 5, wherein the switching unit switches the mode based on a frequency of stopping data transmission to the terminal apparatus. A data transmission method in a base station apparatus,
In order to acquire a synchronization signal transmitted from another base station device in a time zone in which data should be transmitted to the terminal device, stop transmission of data to the terminal device,
Accumulate data that has been suspended,
After resuming data transmission to the terminal device, the data stored by the storage means is transmitted to the terminal device;
A data transmission method characterized by the above.

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