JP4514938B2 - Mobile communication system and base station apparatus - Google Patents

Description

[0001]
[Technical field to which the invention belongs]
The present invention relates to a mobile communication system and a base station apparatus that have a large number of base stations and communicate with mobile stations at transmission timings and reception timings in which the base stations are synchronized, and more particularly to time division multiple access (TDMA) systems. The present invention relates to a mobile communication system and a base station apparatus that are suitable when applied to a mobile communication system to establish frame synchronization between base stations.
[0002]
[Prior art]
In a mobile communication system such as a portable wireless device, a communication line using radio waves is set between a wireless base station and a mobile station, and communication is performed by transmitting and receiving voice, data, etc. wirelessly. Since then, digital cellular phone systems based on TDMA and CDMA (Code Division Multiple Access) have been widely used. The base station and mobile station of this TDMA system operate so as to receive a reception slot assigned to the own station, transmit in the transmission slot assigned to the own station, and repeat transmission / reception intermittently. . In addition, CDMA base stations and mobile stations transmit and receive digital signals modulated by spreading codes assigned to the CDMA system. In these digital mobile communication systems, base stations and mobile stations must synchronize and transmit and receive at the same timing.
[0003]
In the TDMA scheme, in order to synchronize the base station and the mobile station, the mobile station that has received the signal from the base station transmits / receives its own station based on the synchronization signal (synchronization word) included in the signal from the base station. Synchronize the frames and start transmission from the mobile station. In addition, in order to absorb the frame timing shift caused by the distance between the base station and the mobile station, a guard time is provided at the beginning of each reception slot so that reception is not affected even if a transmission slot enters. .
[0004]
In such a mobile communication system, at the time of start-up of a base station, it has high-accuracy time information (for example, a high-accuracy oscillator is provided or time information from a GPS satellite is received). It has been proposed to synchronize the frame timing between base stations by forming a hierarchical synchronization group by synchronizing the frame timing.
[0005]
[Problems to be solved by the invention]
However, in such a mobile communication system, since synchronization with other base stations is taken when the power is turned on, the frame timing shifts as time passes, and the transmission slot and the reception slot interfere with each other. Normal reception is not possible.
[0006]
In particular, in the TDMA-TDD system that performs time-division bidirectional transmission, an uplink signal to the base station and a downlink signal to the mobile station are mixed at the same frequency. The signal and the downstream signal collide.
[0007]
Specifically, in the PHS system (Personal Handyphone System) that employs the TDMA-TDD system, as shown in FIG. 10, four reception slots and four transmission slots are arranged in one TDD / TDMA frame. The transmission / reception slots are repeated with a millisecond cycle. At this time, if the transmission / reception timing (frame timing) between the base station 1 and the base station 2 is shifted, the transmission slot and the reception slot interfere with each other between the base stations. That is, while the mobile station transmits an uplink signal to the base station 1 (RX4), the base station 2 transmits a downlink signal to the mobile station (TX1), and the uplink signal (RX4). And the downstream signal (TX1) collide with each other. In many cases, the transmission output of the base station is larger than that of the mobile station, and the base station 1 cannot receive the upstream signal in the reception slot RX4.
[0008]
Various methods have been proposed for synchronizing frame timing between base stations in a mobile communication system. For example, there is a method in which each base station autonomously establishes synchronization in accordance with a base station having the strongest electric field strength among surrounding base stations. However, in this method, a base station is generated that deviates greatly from the frame timing of the reference station due to an infinite chain of synchronization. In addition, there are a plurality of base stations serving as a reference for synchronization, a plurality of synchronization groups are formed, and frame timing is shifted between the synchronization groups, so that interference occurs between the synchronization groups.
[0009]
In addition, since each base station is connected by a wired line, there is a method of establishing synchronization by distributing a reference signal via this wired line. However, since a standard interface such as ISDN cannot distribute a reference signal, and a special interface is required to distribute the reference signal, it is not suitable for a large-scale public network. In addition, it is difficult to synchronize base stations across a plurality of exchanges in consideration of the effect of transmission delay caused by the exchanges.
[0010]
Further, since each base station is controlled by a center facility that comprehensively controls the communication network, there is a method of correcting the frame timing according to an instruction from the center facility. However, this method requires an instruction from the center facility by the operator, resulting in an operation cost. In addition, complicated processing is required, for example, the synchronization instruction must be given again when adding base stations.
[0011]
An object of the present invention is to provide a mobile communication system that easily and reliably maintains transmission timing and reception timing in which synchronization is established among a plurality of base stations.
[0012]
[Means for Solving the Problems]
  A first invention has a plurality of base stations, and a mobile station selectively connects a line with one or more base stations, performs communication by transmitting and receiving at a predetermined timing, and the base station Among them, a base station that generates high-accuracy transmission timing and reception timing is set as a reference station at the upper level of the hierarchy, and other base stations select a higher-level base station and set transmission timing and reception timing. In a mobile communication system that determines and hierarchically synchronizes with the reference station as a center, the base station performs a synchronization monitoring process for correcting a deviation in transmission timing and reception timing with a base station of a synchronization partner at a predetermined time interval. lineIn addition, the synchronization monitoring process is performed in order from the base station in the higher hierarchy by dividing into a plurality of time zones according to the base station hierarchy.
[0014]
  The second invention isIn the first invention, the synchronization monitoring process is executed at a time determined by the base station in a time zone in which the synchronization monitoring process of its own hierarchy is executed.
[0015]
  According to a third invention, in the first or second invention,The synchronization monitoring process includes a process of searching for an appropriate base station as a synchronization partner among neighboring base stations.
[0016]
  In a fourth aspect based on the first aspect, the plurality of base stations are connected to an exchange through a communication line,The base station corrects the difference between its own transmission timing and reception timing based on the clock signal sent through the communication line.In addition, the transmission timing and reception timing deviations with the synchronization partner base station are corrected at predetermined time intervals.It is characterized by doing.
[0017]
  A fifth invention is the fourth invention,The communication line is a digital communication line, and the clock signal is a clock signal used for digital transmission on the communication line.
[0018]
  A sixth invention is the first to fifth inventions,The mobile communication system communicates with a TDMA system, and the base station determines frame timing as transmission timing and reception timing.
[0019]
  The seventh inventionHaving a plurality of base stations, a mobile station selectively connects to one or more base stations and performs communication by transmitting and receiving at a predetermined timing. The base station that generates the transmission timing and the reception timing is set as a reference station at the upper level of the hierarchy, the other base stations select the base station of the hierarchy higher than the own station, determine the transmission timing and the reception timing, In a base station apparatus of a mobile communication system that hierarchically synchronizes with a reference station as a center, the base station apparatus corrects a deviation in transmission timing and reception timing with a synchronization partner base station at a predetermined time interval. The lineThe synchronization monitoring process is divided into a plurality of time zones according to the base station hierarchy, and is executed in order from the base station in the higher hierarchy..
[0021]
  In an eighth aspect based on the seventh aspect,The synchronization monitoring process is executed at a time determined by the base station apparatus in a time zone in which the synchronization monitoring process of its own hierarchy is executed.
[0022]
  A ninth invention is the seventh or eighth invention, whereinThe synchronization monitoring process includes a process of searching for an appropriate base station as a synchronization partner among neighboring base stations.
[0023]
  In a tenth aspect based on the seventh aspect, the plurality of base stations are connected to a communication line.The base station corrects the difference between its own transmission timing and reception timing based on the clock signal sent through the communication line.In addition, the transmission timing and reception timing deviations with the synchronization partner base station are corrected at predetermined time intervals.It is characterized by doing.
[0024]
  In an eleventh aspect based on the tenth aspect,The communication line is a digital communication line, and the clock signal is a clock signal used for digital transmission on the communication line.
[0025]
  In a twelfth invention according to the seventh to tenth inventions,The base station apparatus communicates with a TDMA system, and the base station determines frame timing as transmission timing and reception timing.
[0026]
Operation and effect of the invention
  In a mobile communication system in which a base station synchronizes hierarchically with a reference station as a center, the base station performs synchronization for correcting a shift in transmission timing and reception timing with a synchronization-target base station at predetermined time intervals. Since the monitoring process is performed, high-precision synchronization can be maintained.In addition, since the synchronization monitoring process is executed in order from the base station higher in the hierarchy, accurate synchronization can be maintained.
[0027]
  The seventh inventionIn a base station apparatus of a mobile communication system in which the base station is hierarchically synchronized with the reference station as the center, the base station apparatus corrects a deviation in transmission timing and reception timing with the synchronization partner base station at a predetermined time interval. Since the synchronization monitoring process is performed, highly accurate synchronization can be maintained.In addition, since the synchronization monitoring process is executed in order from the base station higher in the hierarchy, accurate synchronization can be maintained.
[0029]
  In the second invention and the eighth invention,Since the synchronization monitoring process is executed at the time determined by each base station in the time zone in which the synchronization monitoring process of the own hierarchy is executed, the synchronization monitoring process is not performed simultaneously in a plurality of base stations.
[0030]
  In the third and ninth inventions,The synchronization monitoring process includes a process of searching for a suitable base station as a synchronization partner among neighboring base stations. Therefore, even when a base station higher in hierarchy than the current synchronization partner is installed in the vicinity, it automatically The (high accuracy) base station can be used early.
[0031]
  4th invention and 10th invention are:Since the base station corrects the deviation of the transmission timing and reception timing of its own station based on the clock signal sent by the connected communication line, it maintains synchronization based on the clock signal from the same exchange High precision synchronization can be maintained.
[0032]
  The fifth invention and the eleventh invention are:The communication line is a digital communication line (for example, ISDN line), and the clock signal used for digital transmission on the communication line (for example, ISDN reference clock signal) is used as the clock signal. High precision synchronization can be maintained without the need for a special device (for example, an interface device).
[0033]
  The sixth invention and the twelfth invention areThe base station apparatus communicates by the TDMA method, and the base station determines the frame timing as the transmission timing and the reception timing, so that the transmission slot and the reception slot do not collide.
[0034]
DETAILED DESCRIPTION OF THE INVENTION
Next, embodiments of the present invention will be described with reference to the drawings.
[0035]
FIG. 1 is a configuration diagram of an entire PHS system to which the present invention is applied.
[0036]
A PHS terminal (PS) 1, which is a mobile station of the PHS system, establishes a wireless line with the base station apparatus (CS) 2 and performs voice and data communication. Since this PHS method uses a multiple access method (multi-channel access method), the terminal 1 and the base station apparatus 2 exchange information necessary for setting a call channel in the control channel ( Negotiate), set the call channel, and start communication on the call channel.
[0037]
The base station apparatus 2 is connected to a subscriber exchange 3, and the subscriber exchange 3 is further connected to an ISDN network 5 via a relay exchange 4. Further, the base station apparatus 2 is connected to the center facility 6 via the exchanges 3 and 4, and the center facility 6 performs overall control of starting, stopping, setting change, and the like of the base station apparatus 2.
[0038]
Further, a clock signal (corresponding to 160 kbit / s) included in the ISDN line is sent from the subscriber exchange 3 to the base station apparatus 2. Therefore, the exchange 3 acts as a highly accurate frequency source for the base station apparatus 2.
[0039]
FIG. 2 is a block diagram of the base station apparatus 2 according to the embodiment of this invention.
[0040]
The base station apparatus 2 is connected to the exchange 3 via an ISDN line. The signal input from the exchange 3 to the base station apparatus 2 is level-converted by an interface unit (DSU) 11, encoded by an encoding / decoding unit 12, modulated and multiplied by a baseband unit 13 and a radio unit 14, Amplification or the like is performed to obtain a high frequency signal, which is transmitted from the antenna 15 as a downstream signal to the PHS terminal 1.
[0041]
On the other hand, the upstream signal received from the PHS terminal 1 received by the antenna 15 is subjected to processing such as frequency conversion in the radio unit 14 and sent to the baseband unit 13 for baseband processing such as detection (demodulation). The The encoder / decoder 12 decodes the baseband-processed signal and converts it into an analog signal. The decoded signal is converted to a level that can be input to the exchange 3 by the DSU 11 and sent to the exchange 3.
[0042]
The CPU 17 controls each unit of the base station device 2. The clock unit 16 is configured to generate a clock signal and to control the oscillation frequency by an action from the outside (CPU 17). The CPU 17 instructs the radio unit 14 on the transmission / reception switching timing based on this clock signal. The storage unit 18 stores a control program for the CPU 17 to control the base station apparatus 2 and data necessary for the CPU 17 to operate. The control unit of the base station apparatus 2 is configured by the clock unit 16, the CPU 17, the storage unit 18, and the like.
[0043]
The signal branched from the DSU 11 is introduced into the clock extraction unit 19, which extracts the ISDN clock signal sent through the ISDN line. Since this ISDN clock signal is generated by the exchange 3 with a frequency source with higher accuracy than the clock unit 16 provided in the base station apparatus 2, an accurate timing signal can be generated.
[0044]
The ISDN clock signal extracted by the clock extraction unit 19 is sent to the CPU 17, and the deviation from the timing signal currently generated by the clock unit 16 is monitored, so that the deviation of the clock unit 16 falls within a certain range. The frame synchronization timing signal is adjusted by correcting the oscillation frequency. Specifically, the base clock signal of 160 kbit / s is converted into a predetermined frequency by frequency division and used.
[0045]
Some base station apparatuses 2 include a GPS receiver (not shown) and receive signals from GPS satellites via a GPS receiving antenna. The GPS satellite has an atomic clock and transmits accurate time information. In the base station device 2 including the GPS receiver, the clock extraction unit 19 extracts time information from the signal from the GPS satellite, and generates a frame synchronization timing signal according to the extracted time information. Since a timing signal without any deviation can be generated in about 1 microsecond), it becomes a master station serving as a reference for synchronization of surrounding base stations.
[0046]
The GPS receiver and the GPS receiving antenna constitute a GPS unit and are configured to be detachable from the base station apparatus 2. For this reason, it is possible to easily add a master station by changing a station not equipped with GPS to a station equipped with GPS.
[0047]
FIG. 3 is a configuration diagram of an identification code (CSID) according to the embodiment of the present invention.
[0048]
The base station apparatus 2 transmits its own identification code (CSID) at regular time intervals (100 millisecond intervals) in the control channel. This CSID includes a 3-bit synchronization control code in addition to a 9-bit operator identification code, a general call area number, and an additional ID. This synchronization control code represents the position of each base station in the hierarchy of the synchronization level, and defines whether it is a base station that is a synchronization partner as viewed from other base stations.
[0049]
FIG. 4 is a table showing the contents of the synchronization control code according to the embodiment of the present invention.
[0050]
The synchronization control code is composed of a 3-bit signal, and in this embodiment, five levels of synchronization are set. The synchronization master station is set to the highest synchronization level (synchronization level 1) and is represented by a synchronization control code of “100”.
[0051]
“Synchronization level 2” is a station whose frame timing is synchronized based on a signal from a master station at synchronization level 1, and a base station whose priority is a synchronization level 1 from other base stations. Is represented by a synchronous control code of “101”.
[0052]
“Synchronization level 3” is a station that synchronizes the frame timing of its own station based on a signal from a base station of synchronization level 2, and the priority that is based on other base stations is that of synchronization levels 1 and 2 It is next to the base station and is represented by a synchronization control code of “110”.
[0053]
“Synchronization level 4” is a station whose frame timing is synchronized based on a signal from a base station of synchronization level 3, and has the lowest priority as a reference from other base stations. This is represented by the synchronization control code.
[0054]
The “asynchronous station” is a synchronization level of a base station that is not a synchronization master station and is not synchronized with another base station, and is represented by a synchronization control code of “000”. A base station that has synchronized frame timing based on a signal from a base station at synchronization level 4 is also set to a synchronization level of “asynchronous station”.
[0055]
This synchronization level has the highest priority as a reference for synchronization as viewed from other stations because “sync level 1” has the highest frame timing accuracy. In addition, since “sync level 4” has the lowest frame timing accuracy, the priority as a reference for synchronization is the lowest when viewed from other stations. Further, the “asynchronous station” is provided at the lower level of the synchronization level 4 and the frame timing error of the asynchronous station is estimated to be large, so that it is not used as a reference for synchronization by other base stations.
[0056]
In the present embodiment, the synchronization control code is represented by 3 bits, but the number of bits of the synchronization control code may be increased to set a higher-level synchronization level. In this case, it is desirable to set the hierarchy so that the synchronization is lost due to the delay and the line cannot be connected.
[0057]
FIG. 5 is a flowchart showing an operation at power-on of the base station apparatus 2 according to the embodiment of the present invention.
[0058]
When the base station apparatus 2 is newly started or restarted by a reset signal or the like, the constants are initialized when the power is turned on, and the respective circuits (baseband unit 13, radio unit 14, etc.) are initialized. (S101). The CPU 17 of the control unit detects whether the base station device 2 is a master station (has a GPS receiver) (S102), and if it is a master station, extracts a clock signal from a signal from a GPS satellite. Thus, the frame timing of the own station is synchronized with this time information (S103). Thereafter, the synchronization level of the local station is set to “1” (S104), and transmission on the control channel is started (S109). By using a synchronization control code included in the identification code (CSID) transmitted on this control channel, the base station is notified to the other base station that it is a master station.
[0059]
On the other hand, if the base station apparatus 2 is not a master station (S102), it receives a control channel and receives CSIDs from surrounding base stations (S105). This CSID includes a synchronization control code. Receiving the identification code (CSID) from the surrounding base station, the base station apparatus 2 selects a synchronization partner (S106). The operations in steps S105 to S106 will be described in detail later with reference to FIG.
[0060]
Then, in synchronization with the synchronization signal (synchronization word) of the partner station, a synchronization clock of the station is generated, and the frame timing of the station is synchronized with the frame timing of the partner station (S107).
[0061]
Thereafter, 1 is added to the synchronization level of the partner station that is the reference for frame synchronization, and the synchronization level of the own station is set as a reference and set to a lower level of the synchronization level of the base station (S108). Then, transmission of the CSID on the control channel is started (S109), and the synchronization level of the own station is notified to other base stations.
[0062]
Also, the center facility 6 is notified by a wired line that the synchronization level of the own station has been determined (S110). At this time, instead of notifying the center facility 6 unconditionally after synchronization as in the present embodiment, the center facility 6 may be notified only when the station becomes an asynchronous station. With this configuration, the center facility 6 can grasp only the base station in which an abnormality has occurred.
[0063]
FIG. 6 is a diagram illustrating a hierarchy of base stations synchronized according to an embodiment of the present invention.
[0064]
The base stations shown in the figure are indicated by circles (black circles, white circles). A base station (CS1) indicated by a black circle in the center of the figure is a master station, and is a base station that is the center of a synchronous group of the illustrated base stations.
[0065]
CS1 is a master station and the synchronization level is 1. CS2 and CS3 generate their own frame timing in synchronization with the CS1 frame timing, and the synchronization level of CS2 and CS3 is set to “2” which is one rank lower than the synchronization level of CS1. Since CS4 synchronizes with the frame timing of CS3 and CS5 generates the frame timing of its own station in synchronization with the frame timing of CS2, the synchronization level of CS4 and CS5 is one rank lower than the synchronization level of CS2 and CS3. Set to “3”.
[0066]
The master station is appropriately installed so that the order of the surrounding base stations is low (the synchronization level is large) and the base station serving as a reference for synchronization does not exist in the surroundings. For this reason, the center facility 6 monitors the synchronization level of the base station, and it is not necessary to arrange many master stations. Therefore, the system is flexible and the system can be configured at low cost.
[0067]
When a plurality of base stations are hierarchized in this way, a plurality of synchronization groups are formed when base stations are arranged in a wide area, but the base station serving as a reference is time information from a GPS satellite. Therefore, the error between the synchronization groups is small, and even when different synchronization groups are in contact with each other, the frame timing does not exceed the guard time.
[0068]
In the present embodiment, the base station apparatus 2 maintains synchronization with other base station apparatuses 2 by the clock signal sent from the subscriber exchange 3 to the base station apparatus 2 on the ISDN line. However, the frequency from the exchange (frequency source) 3 may also fluctuate, and an accurate clock signal is generated by the influence of jitter and noise generated by the line state, path length, and equipment installed in the path. There is a possibility of not reaching the device 2. Therefore, it is necessary to perform a synchronization monitoring process that periodically detects a frame timing shift of the base station apparatus 2 and corrects this.
[0069]
FIG. 7 is a flowchart showing the base station synchronization monitoring processing according to the embodiment of the present invention.
[0070]
This synchronization monitoring process is executed at predetermined time intervals (see FIG. 9).
[0071]
When the base station apparatus 2 reaches a predetermined time, it receives a signal from the synchronization partner base station via the control channel, and based on the signal from the partner station, the difference between the frame timing of the partner station and the frame timing of its own station. Is detected. If this deviation is within the allowable range (S111), the operation is resumed without changing the frame timing of the own station (without performing the processing of S112 to S118).
[0072]
On the other hand, when a deviation exceeding the allowable range is detected between the frame timing of the partner station and the frame timing of the own station (S111), a correction process of the frame timing of the own station is executed (S112 to S118).
[0073]
Specifically, it receives the signal of the base station of the synchronization partner at the frame timing of its own station, and if it can be received normally, it judges that there is no frame timing deviation from the synchronization partner station and receives it normally. If not, it is determined that the frame timing with the synchronization partner station is shifted. Further, even when the signal can be normally received, processing (S112 to S118) for changing the frame timing of the own station may be performed in order to synchronize more accurately.
[0074]
First, the base station apparatus 2 receives a control channel and receives an identification code (CSID) from a surrounding base station (S112). This CSID includes a synchronization control code. The base station device 2 that has received the CSID from the surrounding base station selects a synchronization partner (S113). The operation of S112 to S113 will be described in detail later with reference to FIG.
[0075]
Then, the synchronization clock of the own station is generated so as to synchronize with the synchronization signal included in the signal from the other station, and the frame timing of the own station is synchronized with the frame timing of the other station (S114). This synchronization between base stations synchronizes the frame timing of the own station based on a synchronization signal (synchronization word) included in the signal from the partner station.
[0076]
Thereafter, 1 is added to the synchronization level of the partner station that is the reference for frame synchronization, and the synchronization level of the own station is set to be lower than the synchronization level of the base station that is the reference (S115). Then, the transmission of the CSID on the control channel is resumed (S116), and the synchronization level of the own station is notified to other base stations.
[0077]
If the synchronization level of the own station has been changed (S117), this is notified to the center facility 6 via a wired line (S118). At this time, instead of notifying the center facility 6 when the synchronization level is changed as in the present embodiment, the center facility 6 may be notified unconditionally after synchronization. Moreover, you may comprise so that the center installation 6 may be notified only when it becomes an asynchronous station. If comprised in this way, only the base station in which abnormality occurred in the center facility 6 can be grasped.
[0078]
In addition to the predetermined time interval, the synchronization monitoring process can be executed when an abnormality occurs in the clock signal of the communication line (wired line) or when interference is detected in the wireless line.
[0079]
Specifically, in this embodiment, since the frame timing of the base station is corrected based on the clock signal of the ISDN line, if an abnormality occurs in the clock signal of the ISDN line, the transmission / reception frame timing is abnormal. It will occur. For example, when the clock signal from the ISDN line is interrupted or when the frequency of the clock signal from the ISDN line changes significantly, it is determined that an abnormality has occurred in the clock signal and the synchronization monitoring process is executed. Thus, the frame timing of the own station is synchronized (matched) with the frame timing of surrounding base stations.
[0080]
Also, if there is a discrepancy between the frame timing of the local station and the frame timing of surrounding stations, the slot interferes and receives interference from other stations. For example, RSSI is good When the FER is bad, it is determined that the frame timing of the own station is shifted, and the synchronization monitoring process is executed to synchronize (match) the frame timing of the own station with the frame timing of the surrounding base stations.
[0081]
FIG. 8 is a flowchart showing details of an operation of selecting a partner base station that synchronizes the frame timing of the own station in the base station according to the embodiment of the present invention, and includes steps S105 to S106 in FIG. Details of operations from reception of base stations around steps S112 to S113 to selection of a synchronization partner station are shown.
[0082]
The CPU 17 of the control unit of the base station sets a 100 millisecond timer as a time for receiving the control channel, and starts measuring 100 milliseconds (S121). Thereafter, the control channel is continuously received (S123) until the timer expires and 100 milliseconds elapse (S122). In the PHS system, each base station transmits a CSID on the control channel at intervals of 100 milliseconds. Therefore, if the control channel is received continuously for 100 milliseconds, all the base stations that can be received by the base station are received. A CSID can be received.
[0083]
If the base station apparatus 2 can receive another base station in any time slot within 100 milliseconds (S124), the CSID of the received base station, the synchronization control code, the received electric field strength (RSSI, Bit error rate and the like) are stored (S125). On the other hand, if another base station cannot receive (S124), the control channel continues to be received (S122 to S125) until a predetermined time (100 milliseconds) elapses after the timer expires (S122 to S125). Search for nearby base stations.
[0084]
When the continuous reception of the control channel for a predetermined time (100 milliseconds) is completed, a synchronization partner base station is selected from the received base stations. The CPU 17 of the control unit selects a base station having the lowest synchronization level (higher hierarchy and higher synchronization priority) from the data of neighboring base stations stored in the storage unit 18 (S126). If a plurality of base stations having the same synchronization level have been received at this time (S127), a base station having the maximum reception level is selected from the base stations having the same synchronization level (S128).
[0085]
Here, referring to FIG. 6, at this time, CS5 was synchronized with CS4. However, both base stations of CS2 and CS4 could be received by the synchronization monitoring process. Since CS2 and CS4 have higher synchronization levels (CS2 is synchronization level 2 and CS4 is synchronization level 3), CS2 that is a higher base station is selected and synchronized with the frame timing of CS2. Then, the frame synchronization clock signal of the own station is generated.
[0086]
CS4 was synchronized with CS2, but CS2 and CS3 having the same level could be received in the synchronization monitoring process. Since CS2 and CS3 are closer to each other than CS2 and the received electric field strength is stronger, CS4 selects CS3 and resynchronizes with the frame timing of CS3.
[0087]
FIG. 9 is a diagram illustrating the timing of the base station synchronization monitoring processing according to the embodiment of this invention.
[0088]
In the present embodiment, since a plurality of base stations are hierarchized to form a synchronization group, it is necessary to correct the synchronization shift in order from the higher base station in the hierarchy.
[0089]
First, the synchronization shift of the secondary station at the synchronization level 2 which is higher in the hierarchy and forms the frame timing of the own station in synchronization with other base stations is corrected. The base station at the synchronization level 2 executes the synchronization monitoring process described with reference to FIG. 7 between 2 o'clock and 2:55 to correct the synchronization shift.
[0090]
Next, with respect to a synchronization level 3 base station (tertiary station) located in a hierarchy below synchronization level 2, the synchronization shift of the synchronization level 2 station in which the frame timing of the local station is synchronized is corrected. The synchronization level 2 base station executes the synchronization monitoring process described with reference to FIG. 7 between 3 o'clock and 3:55 to correct the synchronization shift from the synchronization level 3 base station.
[0091]
Next, for the synchronization level 4 base station (quaternary station) located in the hierarchy below the synchronization level 3, the synchronization deviation of the synchronization level 3 station where the frame timing of the own station is synchronized is corrected. The synchronization level 2 base station executes the synchronization monitoring process described with reference to FIG. 7 between 4 o'clock and 4:55 to correct the synchronization shift from the synchronization level 4 base station.
[0092]
Next, the asynchronous station located at the lowest synchronization level executes the synchronization monitoring process described in FIG. 7 between 5 o'clock and 5:55 to search for a synchronizable base station and with other base stations. Correct the sync gap.
[0093]
During this synchronization monitoring process, since the frame timing changes, it is not possible to provide a communication service via the base station. If a large number of base stations perform synchronization monitoring at the same time, there may be areas where communication is not possible (blank areas) within the service area where the base stations are installed, or traffic concentrates on nearby base stations that can communicate. May end up. In order to prevent the base stations having the same synchronization level from starting the synchronization monitoring process at the same time in the predetermined time zone (2 to 2:55 for synchronization level 2 stations) Among them, the time zone for starting the synchronization monitoring process is shifted.
[0094]
Specifically, a method for determining the time to start the synchronization monitoring process by the CSID (sum of each digit, the last digit, etc.), which is a number unique to the base station device, or a random number at the base station device There is a method in which the time for starting the synchronization monitoring process is determined by the generated random number value.
[0095]
Since the master station receives the signal from the GPS satellite and corrects the frame synchronization timing, there is no need to periodically correct the synchronization shift, but a predetermined time interval (one day / once) The frame timing can be corrected by receiving a signal from a GPS satellite. In this case, the master station needs to perform the synchronization monitoring process before the time zone when the synchronization monitoring process of the secondary station is executed.
[0096]
Furthermore, when the synchronization level is lowered by this synchronization monitoring process, the synchronization monitoring process is performed again in the time period of the synchronization monitoring process assigned at the changed synchronization level.
[0097]
Specifically, the secondary station synchronized with the master station performs synchronization monitoring processing from 2 o'clock to 2:55, but if the synchronization partner station cannot be detected, the control channel is set to 100 milliseconds. Receive continuously. When another station (master station) of the same level cannot be detected by receiving this control channel, a secondary station that can be received by continuously receiving this control channel is searched for and synchronized with this secondary station. Therefore, the base station synchronized with the secondary station becomes a tertiary station. However, since the synchronization monitoring process of the secondary station is performed from 2 o'clock to 2:55, the newly synchronized secondary station has already performed the synchronization monitoring process and corrected to an accurate frame timing. I don't know if it is. Therefore, the base station synchronized with the secondary station is the time when the synchronization monitoring process of the secondary station of the synchronization partner is completed, that is, the time zone in which the tertiary station performs the synchronization monitoring process (from 3:00 to 3:55). During the period, the synchronization monitoring process is performed again to synchronize again with the secondary station whose frame timing deviation is corrected.
[0098]
In this way, the base station that is not the master station sets the master station at the upper level of the hierarchy centering on the master station having a high-accuracy clock source, and the other base stations transmit synchronization control codes transmitted by each base station. By selecting a base station in a higher hierarchy than its own station, frame timing is determined, and each base station autonomously forms a hierarchy and synchronizes around the master station, so that high accuracy (master station The transmission / reception timings can be synchronized with each other at about ± 1 μsec.
[0099]
Furthermore, since the master stations are arranged in a distributed manner, even if one master station fails, the base station synchronized with this master station only needs to synchronize with other surrounding base stations. The operation of the communication system does not stop, and the tolerance to failure is strong.
[0100]
Thereafter, since the frame timing of the own station is corrected based on the clock signal of the ISDN line to which the base station apparatus is connected, the frame timing once synchronized with the master station as a center does not shift.
[0101]
In addition, once a day, synchronization monitoring processing is performed to correct the frame timing shift with the base station that became the synchronization partner, so correction is maintained before the effect of the shift increases, and accurate synchronization is maintained. can do.
[0102]
Further, since this synchronization monitoring process is executed in order from the upper base station in the hierarchy, the frame timing shift is corrected in order from the base station located in the upper hierarchy, and therefore, based on a more accurate base station. Frame timing can be synchronized. In other words, if the synchronization monitoring process is executed regardless of the hierarchy, a frame timing shift is detected in the upper base station that is the reference after the lower base station corrects the synchronization relationship with the upper base station. When the upper base station corrects the frame timing, the frame timing of the lower base station becomes erroneous including this deviation.
[0103]
In addition, since the time for starting the synchronization monitoring process is determined by the number (CSID) unique to the base station apparatus, the synchronization monitoring process is simultaneously performed in a number of base stations in the time zone in which the base stations in the same layer perform the synchronization monitoring process. Will not stop the communication service.
[0104]
Furthermore, since the synchronization monitoring process searches for a base station apparatus having a higher synchronization level than the current synchronization partner, it is possible to generate frame timing with high accuracy in synchronization with a base station with higher accuracy than the current one.
[0105]
Furthermore, since the frame timing is synchronized by selecting a base station having a strong signal strength among other base stations in the same hierarchy, the synchronization can be surely performed.
[0106]
In addition, when the synchronization level of the own station is determined by synchronizing with other base stations, the center facility is notified by a wired line that the synchronization level of the own station has been determined. It is possible to grasp the status within.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an entire PHS system to which the present invention is applied.
FIG. 2 is a block diagram of a base station apparatus according to an embodiment of the present invention.
FIG. 3 is a configuration diagram of a CSID according to the embodiment of this invention.
FIG. 4 is a table showing contents of synchronization control bits according to the embodiment of the present invention.
FIG. 5 is a flowchart showing an operation at power-on of a base station according to the embodiment of the present invention.
FIG. 6 is a diagram illustrating a hierarchy of base stations synchronized according to an embodiment of the present invention.
FIG. 7 is a flowchart showing base station synchronization monitoring processing according to the embodiment of the present invention;
FIG. 8 is a flowchart showing details of an operation of selecting a synchronization partner in the base station according to the embodiment of the present invention.
FIG. 9 is a diagram illustrating timing of base station synchronization monitoring processing according to the embodiment of this invention.
FIG. 10 is a diagram illustrating transmission / reception timing of a conventional base station apparatus.
[Explanation of symbols]
1 PHS terminal (PS)
2 Base station equipment (CS)
3 subscriber exchange
4 Relay exchange
5 ISDN network
6 Center facilities
11 Interface unit (DSU)
12 Encoding / Decoding Unit (CODEC)
13 Baseband (B / B)
14 Radio section (TRX)
15 Antenna
16 Clock part (CLK)
17 CPU
18 Storage unit (RAM / ROM)
19 Clock extractor

Claims (12)

Having a plurality of base stations, a mobile station selectively connects a line with one or more base stations, communicates by transmitting and receiving at a predetermined timing,
Among the base stations, a base station that generates high-accuracy transmission timing and reception timing is set as a reference station at the top of the hierarchy,
In the mobile communication system in which other base stations select a higher-tier base station to determine transmission timing and reception timing, and hierarchically synchronize around the reference station,
The base station, have rows synchronization monitoring process for correcting the deviation of transmission timing and reception timing of the synchronization counterpart base station at predetermined time intervals,
The synchronization monitoring process is divided into a plurality of time zones according to a hierarchy of the base station, and is executed in order from the base station in the higher hierarchy .   The mobile communication system according to claim 1, wherein the synchronization monitoring process is executed at a time determined by the base station in a time zone in which the synchronization monitoring process of its own hierarchy is executed.   The mobile communication system according to claim 1, wherein the synchronization monitoring process includes a process of searching for an appropriate base station as a synchronization partner among neighboring base stations.   The plurality of base stations are connected to an exchange through a communication line,
  The base station corrects a shift in transmission timing and reception timing of the local station based on a clock signal transmitted through the communication line, and further transmits a transmission timing with a synchronization partner base station at a predetermined time interval, and The mobile communication system according to claim 1, wherein a shift in reception timing is corrected.   5. The mobile communication system according to claim 4, wherein the communication line is a digital communication line, and the clock signal is a clock signal used for digital transmission on the communication line.   The mobile communication system communicates with a TDMA system,
  The mobile communication system according to claim 1, wherein the base station determines a frame timing as a transmission timing and a reception timing.   Having a plurality of base stations, a mobile station selectively connects a line with one or more of the base stations, communicates by transmitting and receiving at a predetermined timing,
  Among the base stations, a base station that generates high-accuracy transmission timing and reception timing is set as a reference station at the top of the hierarchy,
  In the base station apparatus of the mobile communication system, the other base station selects a base station in a higher hierarchy than the own station, determines transmission timing and reception timing, and hierarchically synchronizes around the reference station.
  The base station device performs synchronization monitoring processing for correcting a shift in transmission timing and reception timing with a synchronization partner base station at a predetermined time interval,
  The base station apparatus, wherein the synchronization monitoring process is divided into a plurality of time zones according to a hierarchy of the base station, and is executed in order from the base station of a higher hierarchy.   The base station apparatus according to claim 7, wherein the synchronization monitoring process is executed at times determined by the base station apparatus in a time zone in which the synchronization monitoring process of its own hierarchy is executed.   The base station apparatus according to claim 7, wherein the synchronization monitoring process includes a process of searching for an appropriate base station as a synchronization partner among neighboring base stations.   The plurality of base stations are connected to a communication line,
  The base station corrects a shift in transmission timing and reception timing of the local station based on a clock signal transmitted through the communication line, and further transmits a transmission timing with a synchronization partner base station at a predetermined time interval, and The base station apparatus according to claim 7, wherein a shift in reception timing is corrected.   The base station apparatus according to claim 10, wherein the communication line is a digital communication line, and the clock signal is a clock signal used for digital transmission on the communication line.   The base station apparatus according to any one of claims 7 to 10, wherein the base station apparatus performs communication by a TDMA system, and the base station determines a frame timing as a transmission timing and a reception timing. Station equipment.

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