The present invention relates to synchronization between a plurality of base stations and a mobile station in a wireless communication system using a wireless transmission method which requires synchronization between users in base station cells.
In a conventional wireless communication system constituted by a plurality of base stations, a technique referred to as handover is used (For example, see Patent Document 1). In a conventional handover method, a mobile station measures received powers of signals received from the plurality of base stations. On the basis of the measurement results, a first base station providing a maximum received-power to the mobile station and a second base station providing the second highest received-power to the mobile station are selected. A condition is used as a trigger which causes the mobile station to switch the base station. The condition is that a received-power difference between the first and second base stations is smaller than a preset threshold value and time at which the difference is smaller than the threshold value is equal to or longer than set time set in advance. As a result, the mobile station can maintain high communication quality by switching between the base stations with reference to the received-power difference. Furthermore, only when the time at which the received-power difference is smaller than the threshold value is equal to or longer than the set time, the mobile station switches between the base stations. Therefore, a system load caused by switching between the base stations can be reduced.
A conventional handover process will be described below with reference to the flow chart shown in
When the received-power difference between the first and second base stations is smaller than the threshold value set in advance (step S103), and it is determined that time at which the difference is smaller than the threshold value is equal to or longer than the set time set in advance (step S104), switching between the base stations is carried out (step S105).
On the other hand, a wireless transmission method which requires synchronization between users of base stations includes a method which performs synchronization by using Cyclic Prefixes to control a timing error between the users within a Cyclic Prefix so as to prevent interference between the users (For example, see Non-patent Document 1).
However, when a conventional handover method is applied to a wireless communication system using a wireless transmission method which requires synchronization between users in base station cells, delay occurs to obtain new synchronization in a non-serving base station serving as a handover destination. Furthermore, since a mobile station is synchronized with only a serving base station serving as a handover source, an error may be generated in measurement of reverse link receiving quality from the non-serving base station or forward link receiving quality in the non-serving base station.
Therefore, it is an object of the present invention to provide a wireless communication system and method using a wireless transmission method which requires synchronization between users in base station cells, wherein a mobile station can be synchronized with a switched base station the moment it is determined to switch base stations.
In order to solve the above problem, the present invention provides a wireless communication system constituted by a plurality of base stations and a mobile station, wherein each of the base stations includes: means for generating synchronization information to synchronize the mobile station with the base station from a forward pilot signal received from the mobile station; means for generating forward propagation quality information from the forward pilot signal; and means for generating a reverse control signal including the synchronization information and forward propagation quality information and transmitting, and the mobile station includes: means for generating and transmitting the forward pilot signal; means for receiving a signal including the reverse control signal from each of the base stations; means for extracting a forward propagation quality and the synchronization information with the base station, from the received reverse control signal; means for selecting an optimum base station as a new serving base station from the received forward propagation quality; and means for changing a transmission timing of the forward pilot signal to a transmission timing based on the synchronization information extracted from the reverse control signal of the selected base station and transmitting when the selected base station is different from the serving base station.
With the above configuration, the serving base station and the non-serving base station estimate reception timings of a forward link on the base of the forward pilot signal and calculate information used to cause the mobile station to be synchronized with the base stations to notify the mobile station of the information. The moment the mobile station determines to switches the base station, the mobile station can be synchronized with the destination base station on the basis of the notification information.
According to the present invention, the serving base station and the non-serving base station estimate reception timings of a forward link on the base of the forward pilot signal and calculate information used to cause the mobile station to be synchronized with the base stations to notify the mobile station of the information. The moment the mobile station determines to switches the base station, the mobile station can be synchronized with the destination base station on the basis of the notification information. Furthermore, the mobile station can transmits a forward link pilot signal used when the mobile station is synchronized with the serving base station by using the notification information. As a result, even in the non-serving base station, forward link receiving quality can be estimated.
A best mode of the present invention will be described below with reference to drawings.
1. A mobile station (MS: Mobile Station) transmits a forward pilot signal to a serving base station (SBS: Serving Base Station) serving as a handover source and a non-serving base station (NSBS: Non-Serving BS) except for the serving base station. The SBS and the NSBS estimate forward reception timings. The SBS and the NSBS generate transmission timings to the mobile station which the mobile station is synchronized with the SBS and the NSBS, as synchronization information. The SBS and the NSBS transmit reverse (downward) control signals including at least the synchronization information to the mobile station. The mobile station changes transmission timings of data and a forward pilot signal to a destination SBS to be synchronized with the destination SBS when SCs (Serving Cells) are switched. When the SCs are not switched, the mobile station changes the transmission timings of the data and the forward pilot signal to be synchronized with the SBS.
More specifically, the serving base station and the non-serving base station estimate a reception timing of a forward link on the basis of the forward pilot signal, calculate information used to cause the mobile station to be synchronized with the base stations to notify the mobile station of the information. On the basis of the notification information, an effect in that the mobile station can be synchronized with the destination base station the moment the mobile station determines to switch between the base stations can be obtained.
2. All the base stations transmit reverse control signals to the mobile station.
The effect obtained by the operation is the easiest method as a transmission method of reverse control information since the base station independently transmits a reverse control signal. Furthermore, a base station control device such as an RNC (Radio Network Controller) is not necessary.
3. All the NSBSs directly transmit reverse control signals to the SBS.
An effect obtained by the operation is that the mobile station is required only to receive the reverse control signal from the SBS because the reverse control signals are collected and transmitted to the mobile station at once by the SBS. Furthermore, a base station control device such as an RNC is not necessary.
4. All the NSBSs transmit reverse control signals to the RNC, and the RNC transmits the reverse control signals of all the NSBSs to the SBS.
An effect obtained by the operation is that the reverse control signals of the NSBSs are transmitted to the SBS with a cable through the RNC.
5. All the NSBSs directly transmit the reverse control signals to the SBS.
An effect obtained by the operation is that the SBS is required to transmit only synchronization information to the MS as the reverse control signal because the SBS selects the base station. For this reason, an amount of information can be reduced in comparison with item 3.
6. All the NSBSs transmit reverse control signals to the RNC, and the RNC transmits the reverse control signals of all the NSBSs to the SBS.
An effect obtained by the operation is that the SBS is required to transmit only synchronization information to the MS as a reverse control signal. For this reason, an amount of information can be reduced in comparison with item 4.
7. All the NSBSs and the SBS transmit the reverse control signals to the RNC, and the RNC selects a new SBS on the basis of the reverse control signals of all the base stations.
An effect obtained by the operation is that the SBS is required to transmit only synchronization information to the MS as the reverse control signal because the RNC selects the base station. For this reason, an amount of information can be reduced in comparison with item 4.
A transmitting method of a forward pilot signal will be described below.
8. Forward pilot signals transmitted from the mobile station to the SBS and the NSBSs are the same forward pilot signals.
An effect obtained by the operation is that generation and transmission of the forward pilot signals are maximally easy because the same forward pilot signals are transmitted by all the base stations.
9. The mobile station changes a transmission timing of forward pilots to the NSBSs in accordance with the synchronization information of the SBS.
An effect obtained by the operation is that the forward pilot signals to the NSBSs do not interfere with the SBS because the forward pilot signals to the NSBSs are always synchronized with the SBS.
10. The mobile station changes a transmission timing of forward pilots to the NSBSs in accordance with the synchronization information of the NSBSs.
An effect obtained by the operation is that forward propagation quality obtained when the mobile station is synchronized with the NSBSs can be reliably estimated.
11. The base station is characterized by including a means for estimating forward propagation quality on the basis of the forward pilot signal and transmitting the forward propagation quality including the reverse control signal to the mobile station when the mobile station requires the forward propagation quality.
An effect obtained by the operation is that the mobile station can switch the base stations on the basis of the forward propagation quality.
A criterion for transmitting the reverse control signal will be described below. An effect obtained by the operation is that a transmission frequency of the reverse control signal is reduced by setting a criterion for transmitting the reverse control signal.
12. The SBS and the NSBS periodically transmit reverse control signals to the mobile station.
13. The SBS periodically transmits the reverse control signal of the SBS to the mobile station. The SBS and the NSBS transmit the reverse control signals to the mobile station when the forward propagation quality exceeds a threshold value P.
14. The SBS periodically transmits the reverse control signal of the SBS to the mobile station. The SBS transmits the reverse control signals of the SBS and the NSBS to the mobile station when the forward propagation quality in the NSBS exceeds the threshold value P.
15. The SBS periodically transmits the reverse control signal of the SBS to the mobile station. The mobile station estimates reverse propagation quality and transmits request information as the forward control signal to the SBS when the reverse propagation quality exceeds the threshold value P. The SBS and the NSBS transmit the reverse control signals to the mobile station when request information is received.
16. The SBS periodically transmits the reverse control signal of the SBS to the mobile station. The mobile station estimate the reverse propagation quality. When the reverse propagation quality corresponding to at least one base station exceeds a threshold value Q, the mobile station transmits request information which requests the reverse control signal of the base station corresponding to the reverse propagation quality exceeding the threshold value Q from the SBS to the SBS. The SBS receives the request information, and the SBS transmits reverse control signals corresponding to the base station corresponding to the request information to the mobile station at once.
A case in which transmission timings of forward pilot signals are not changed in units of NSBSs will be described below.
17. The SBS periodically transmits the reverse control signal of the SBS to the mobile station. When any one of the base station and the mobile station determines to switch SCs, an NSBS (new SBS) having the new switched SC transmits a reverse control signal of the new SBS to the mobile station.
18. The SBS periodically transmits the reverse control signal of the SBS to the mobile station. When any one of the base station and the mobile station determines to switch SCs, the SBS transmits a reverse control signal of the new SBS to the mobile station.
A case in which transmission timings of forward pilot signals are changed in units of NSBS will be described below.
19. The SBS and the NSBS periodically transmit the reverse control signals to the mobile station. When any one of the base station and the mobile station determines to switch SCS, a new SBS transmits a reverse control signal of the new SBS to the mobile station.
20. The SBS periodically transmits the reverse control signals of the SBS and the NSBS to the mobile station. When any one of the base station and the mobile station determines to switch SCs, the SBS transmits the reverse control signal of the new SBS to the mobile station.
21. The mobile station determines to switch SCs and transmits a request of a reverse control signal as a forward control signal.
A method of improving the accuracy of synchronization information will be described below.
22. The mobile station increases at least one of a transmission frequency of a forward pilot signal, the number of repetitions of transmission, and a transmission power in NSBSs ranked in the top X of propagation quality. Increments are increased in a descending order of propagation quality.
An effect obtained by the operation is that synchronization accuracy to a base station which is probably switched to an SBS is improved.
23. The mobile station increases at least one of a transmission frequency of a forward pilot signal, the number of repetitions of transmission, and a transmission power in an NSBS the propagation quality of which exceeds a threshold value Y. Increments are increased in a descending order of propagation quality.
An effect obtained by the operation is that synchronization accuracy to a base station which is probably switched to an SBS is improved.
24. The mobile station increases at least one of a transmission frequency of a forward pilot signal, the number of repetitions of transmission, and a transmission power in NSBSs ranked in the bottom Z of propagation quality. Increments are increased in an ascending order of propagation quality.
An effect obtained by the operation is that synchronization accuracy to all the base stations is increased.
25. The mobile station increases at least one of a transmission frequency of a forward pilot signal, the number of repetitions of transmission, and a transmission power in an NSBS the propagation quality of which is equal to or less than a threshold value W. Increments are increased in an ascending order of propagation quality.
An effect obtained by the operation is that synchronization accuracy to all the base stations is increased.
A wireless communication system to which the present invention is applied is constituted by a plurality of base stations and at least one mobile station. In the following embodiment, to make it easy to understand a correspondence between the base stations and the mobile station, an explanation will be made on the assumption that first to Nth base stations and first to Mth mobile stations are used. For descriptive convenience, each of the first to Nth base stations must serve as any one of a serving as a serving base station (for a mobile station, the base station which communicates with the mobile station) or non-serving base station (for the mobile station, the base station which does not communicate with the mobile station) for each mobile station. However, the same first to Nth base stations do not always serve as serving base stations or non-serving base stations for all the mobile stations. It is assumed that a signal between an ath base station and an ith mobile station in a wireless communication system having N base stations is expressed as an ((i*N)+a)th signal to uniquely specify a signal between a specific base station and a specific mobile station. Reference symbol * is a symbol which means multiplication. The mobile stations, the base stations, and a base station control device realize functions (will be described below) by control programs stored in respectively memories.
In the first embodiment, a mobile station selects a base station, and a criterion for selecting a base station is forward propagation quality. Reverse (Downward) control signals to the moving station are transmitted by all the base stations, the transmission timing is determined by a periodic trigger. Information of the reverse control signal transmitted to the mobile station includes synchronization information of the base stations and forward propagation quality.
A periodic trigger generated by the base station in a predetermined cycle is monitored. When the trigger is detected (step S201), a reception timing in an ath base station is estimated on the basis of a pilot signal transmitted by an ith (i is a natural number which is equal to or less than M) mobile station, and ((i*N)+a)th synchronization information to synchronize the ith mobile station with the ath base station is generated (step S202). Forward propagation quality in the ath base station is estimated, ((i*N)+a)th forward propagation quality information is generated, the ((i*N)+a)th synchronization information and the ((i*N)+a)th forward propagation quality information are generated as an ((i*N)+a)th reverse control signal, and the ((i*N)+a)th reverse control signal is transmitted to the ith mobile station (step S203).
When the trigger is not detected, the processes are ended.
The ith mobile station receives ((i*N)+1)th to ((i*N)+N)th reverse control signals from the first to Nth base stations. First, ((i*N)+1)th to ((i*N)+N)th pieces of synchronization information and ((i*N)+1)th to ((i*N)+N)th pieces of propagation quality information are separated from the ((i*N)+1)th to ((i*N)+N)th reverse control signals, respectively (step S301).
An ith forward data signal and an ith forward pilot signal are generated, and the ith forward data signal and the ith forward pilot signal are generated as ith forward transmission signals (step S302).
((i*N)+j)th (j is a natural number which is equal to or less than N) forward propagation quality information corresponding to a serving base station, ((i*N)+1)th to ((j*N)+(j−1))th pieces of forward propagation quality information corresponding to non-serving base stations, and ((i*N)+(j+1))th to ((i*N)+N)th pieces of forward propagation quality information are compared with each other. When any one of the ((i*N)+1)th to ((i*N)+(j−1))th pieces of forward propagation quality information and the ((i*N)+(j+1))th to ((i*N)+N)th pieces of forward propagation quality information exceeds the ((i*N)+j)th forward propagation quality information (step S304), on the basis of the maximum ((i*N)+s)th synchronization information, corresponding to an sth base station, in the ((i*N)+1)th to ((i*N)+(j−1))th pieces of forward propagation quality information and the ((i*N)+(j+1))th to ((i*N)+N)th pieces of forward propagation quality information, the ith forward transmission signal is transmitted such that a transmission timing of the ith forward transmission signal is changed to synchronize the ith mobile station with the sth base station (step S305). When the ((i*N)+j)th forward propagation quality information is equal to or higher than the ((i*N)+1)th to ((i*N)+(j−1))th pieces of forward propagation quality information and the ((i*N)+(j+1))th to ((i*N)+N)th pieces of forward propagation quality information (step S304), the ith forward transmission signal is transmitted such that the transmission timing of the ith forward transmission signal is changed on the basis of ((i*N)+j)th synchronization information to synchronize the ith mobile station with a jth base station (step S306).
In this manner, according to the embodiment, when the base station determined to switch the base stations, the mobile station can be synchronized with the destination base station.
In a second embodiment, a serving base station selects a base station, and a criterion for selecting a base station is forward propagation quality. A reverse control signal to a mobile station is transmitted by the serving base station. When the transmission timing is a timing when the serving base station determines to switch base stations, information of a reverse control signal transmitted to the mobile station is synchronization information of a destination base station. When the transmission timing is a timing when a periodic trigger is detected, information of the reverse control signal transmitted to the mobile station is synchronization information of the serving base station.
A trigger generating unit 1107 periodically generates a trigger signal. A base station reverse control signal transmitting unit 1108 of its own base station transmits the base station control signal of its own base station to the mobile station as a base station reverse control transmission signal of its own base station when the trigger signal is received. A base station control signal separating unit 1109 of its own base station separates forward propagation quality information and synchronization information from the base station control signal of its own base station and outputs the forward propagation quality information and the synchronization information. A non-serving base station control signal receiving unit 1110 separates and outputs control signals of all the non-serving base stations in signals received from all the non-serving base stations. A forward propagation quality information extracting unit 1111 separates and outputs forward propagation quality information in the control signal of the non-serving base station. A base station selecting unit 1112 selects an optimum base station on the basis of the pieces of forward propagation quality information of all the base stations and generates and outputs information representing the base station as optimum base station selection information. A synchronous information extracting unit 1113 separates and outputs synchronization information in a control signal of a non-serving base station. When the base station indicated by the optimum base station selection information is different from the serving base station, a reverse control signal generating unit 1114 generates an outputs synchronization information corresponding to the base station indicated by the optimum base station selection information as a reverse control signal. A multiplexing unit 1115 multiplexes the reverse control signal and the optimum base station selection information and transmits a multiplexed signal as the reverse transmission signal. An antenna is used for both transmission and reception, and may also be configured to be switchably connected to a transmission signal and a reception signal.
A trigger signal periodically generated by the SBS is monitored by the SBS. When the SBS detects the trigger signal, the control operation shifts to step S405. When the SBS does not detect the trigger signal, the control operation shifts to step S406 (step S404). The SBS transmits control information of the own base station to the mobile station as a base station reverse control signal of its own base station (step S405). A control signal is received from the NSBS, and the forward propagation quality information and the synchronization information are separated and output (step S406). On the basis of pieces of forward propagation quality information of all the base stations, an optimum base station is selected, and information indicating an optimum base station is generated as optimum base station selection information (step S407). When the base station indicated by the optimum base station selection information is different from the SBS, the control operation shifts to step S409. When the base station indicated by the optimum base station selection information is the same as the SBS, the processes are ended (step S408). The base station connected to the mobile station is switched to the base station indicated by the optimum base station selection information (step S409). The synchronization information corresponding to the base station indicated by the optimum base station selection information is generated as a reverse control signal. A signal obtained by multiplexing the reverse control signal and the optimum base station selection information is transmitted to the mobile station as a reverse transmission signal (S410).
The processes in the first to Nth base stations will be further described below such that the processes are divided into processes in the first to (j−1)th base stations serving as non-serving base stations which are handover destinations and the (j+1)th to Nth base stations (j is a natural number which is N or less except for j) and processes in the jth base station serving as a serving base station which is a handover source.
In a sth (s is a natural number which is N or less) base station serving as a non-serving base station, a reception timing in the sth base station is estimated on the basis of a pilot signal transmitted by the ith mobile station, and the ((i*N)+s)th synchronization information to synchronize the ith mobile station with the sth base station is generated. Forward propagation quality in the sth base station is estimated to generate ((i*N)+s)th forward propagation quality information, the ((i*N)+s)th synchronization information and the ((i*N)+s)th forward propagation quality information are generated as an ((i*N)+s)th reverse control signal and transmitted to the jth base station.
In the jth base station serving as a serving base station, the ((i*N)+1)th to ((i*N)+(j−1))th reverse control signals and the ((i*N)+(j+1))th to ((i*N)+N)th reverse control signals respectively transmitted by the first to (j−1)th base stations and the (j+1)th to Nth base stations are received, and the ((i*N)+1)th to ((i*N)+(j−1))th pieces of synchronization information, the ((i*N)+(j+1))th to ((i*N)+N)th pieces of synchronization information, the ((i*N)+1)th to ((i*N)+(j−1))th pieces of forward propagation quality information, and the ((i*N)+(j+1))th to ((i*N)+N)th pieces of forward propagation quality information are separately output.
On the basis of the pilot signal transmitted by the ith mobile station, a reception timing in the jth base station is estimated, and ((i*N)+j)th synchronization information to synchronize the ith mobile station with the jth base station is generated. Forward propagation quality in the jth base station is estimated, ((i*N)+j)th forward propagation quality information is generated, and the ((i*N)+j)th synchronization information and the ((i*N)+j)th forward propagation quality information are generated as a ((i*N)+j)th reverse control signal.
The jth forward propagation quality information corresponding to the serving base station is compared with the ((i*N)+1)th to ((i*N)+(j−1))th pieces of forward propagation quality information and the ((i*N)+(j+1))th to ((i*N)+N)th pieces of forward propagation quality information corresponding to the non-serving base stations, respectively. When the ((i*N)+s)th forward propagation quality information which exceeds the jth forward propagation quality information is detected, base stations are switched, and information indicating the destination base station is generated.
Information indicating the destination base station and the first to Nth control signals are transmitted to the ith mobile station.
A forward data signal generating unit 2105 generates and outputs a forward data signal. A forward pilot signal generating unit 2106 generates and outputs a forward pilot signal. A forward transmission signal generating unit 2107 generates and outputs the forward data signal and the forward pilot signal as forward generating signals. A transmission timing setting unit 2104 transmits the forward generating signals such that a transmission timing is changed on the basis of the new base station selection information to synchronize the mobile station with a newly selected base station. An antenna is used for both transmission and reception, and may also be configured to be switchably connected to a transmission signal and a reception signal.
The ith mobile station receives information indicating a destination base station and the ((i*N)+1)th to ((i*N)+N)th reverse control signals from the jth base station. First, the ((i*N)+1)th to ((i*N)+N)th pieces of synchronization information and the ((i*N)+1)th to ((i*N)+N)th pieces of propagation quality information are separated from the ((i*N)+1)th to ((i*N)+N)th reverse control signals, respectively (step S501).
An ith forward data signal and an ith forward pilot signal are generated as ith forward transmission signals (step S502).
When the information indicating the destination base station changes the base station into the sth (s is a natural number which is N or less except for j) base station (step S503), the ith forward transmission signal is transmitted such that a transmission timing of the ith forward transmission signal is changed on the basis of the ((i*N)+s)th synchronization information to synchronize the ith mobile station with the sth base station (step S504). When the information indicating the destination base station indicates the same base station as the jth base station serving as a serving base station (step S503), the transmission timing of the ith forward transmission signal is changed on the basis of the ((i*N)+j)th synchronization information to synchronize the ith mobile station with the jth base station (step S505).
In this manner, according to the embodiment, when the mobile station determined to switch the base stations, the mobile station can be synchronized with the destination base station.
In a third embodiment, a mobile station selects a base station, and a criterion for selecting a base station is forward propagation quality. When a reverse control signal to a mobile station is transmitted by a base station having forward propagation quality exceeding a threshold value, a transmission timing of the reverse transmission signal is a timing when the forward propagation quality exceeds the threshold value, and information of the reverse transmission signal transmitted to the mobile station includes pieces of synchronization information of the base stations and forward propagation quality information. When the reverse transmission signal to the mobile station is transmitted by a serving base station (SBS), a transmission timing of the reverse transmission signal is a timing when a periodic trigger is detected, and information of the reverse transmission signal transmitted to the mobile station includes synchronization information of the serving base station and the forward propagation quality information.
A trigger generating unit 1206 periodically generates a trigger signal. A base station control signal transmitting unit 1207 of its own base station transmits the base station control signal of its own base station to the mobile station as a base station reverse control signal of its own base station when the trigger signal is received. An antenna is used for both transmission and reception, and may also be configured to be switchably connected to a transmission signal and a reception signal.
The processes in the first to Nth base stations will be further described below such that the processes are divided into processes in the first to (j−1)th base stations serving as non-serving base stations which are handover destinations and the (j+1)th to Nth base stations (j is a natural number which is N or less) and processes in the jth base station serving as a serving base station which is a handover source.
In a sth (s is a natural number which is N or less except for j) base station serving as a non-serving base station, a reception timing in the sth base station is estimated on the basis of a pilot signal transmitted by the ith mobile station, and the ((i*N)+s)th synchronization information to synchronize the ith mobile station with the sth base station is generated. Forward propagation quality in the sth base station is estimated to generate ((i*N)+s)th forward propagation quality information, the ((i*N)+s)th synchronization information and the ((i*N)+s)th forward propagation quality information are generated as an ((i*N)+s)th reverse control signal and transmitted to the jth base station.
In the jth base station serving as a serving base station, on the basis of the pilot signal transmitted by the ith mobile station, the reception timing in the jth base station is estimated, and the ((i*N)+j)th synchronization information to synchronize the ith mobile station with the jth base station is generated. Forward propagation quality in the jth base station is estimated, and ((i*N)+j)th forward propagation quality information is generated. The ((i*N)+j)th synchronization information and the ((i*N)+j)th forward propagation quality information are generated as ((i*N)+j)th reverse control signal.
The ((i*N)+1)th to ((i*N)+(j−1))th reverse control signals and the ((i*N)+(j+1))th to ((i*N)+N)th reverse control signals respectively transmitted by the first to (j−1)th base stations and the (j+1)th to Nth base stations are received, and the ((i*N)+1)th to ((i*N)+(j−1))th pieces of synchronization information, the ((i*N)+(j+1))th to ((i*N)+N)th pieces of synchronization information, the ((i*N)+1)th to ((i*N)+(j−1))th pieces of forward propagation quality information, and the ((i*N)+(j+1))th to ((i*N)+N)th pieces of forward propagation quality information are separately output.
The non-serving base stations are sequentially selected one by one. When forward propagation quality indicated by the ((i*N)+s)th forward propagation quality information corresponding to the sth base station selected in the previous step exceeds a threshold value U, the ((i*N)+s)th reverse control signal corresponding to the sth base station is determined to be transmitted to the ith mobile station.
Even though the forward propagation quality is equal to or less than the threshold value U, when a trigger periodically generated is detected, the ((i*N)+s)th reverse control signal corresponding to the sth base station is determined to be transmitted to the ith mobile station.
When it is checked that all the non-serving base stations are selected, a reverse control signal corresponding to a determined non-serving base station is transmitted to the ith mobile station.
A transmission timing setting unit 2205 changes a transmission timing of the forward data signal on the basis of new base station selection information to synchronize the mobile station with a base station corresponding to the new base station selection information. Transmission timings of the forward pilot signals transmitted to the base stations corresponding to the first to Kth pieces of synchronization information are changed to synchronize the mobile station with the base stations. With respect to base stations except for the base station indicated by the new base station selection information, transmission powers of forward pilot signals transmitted to base stations ranked in the top X (X is a natural number which is K or less) of forward propagation quality are set to be high in proportion to the forward propagation properties. The mobile station transmits the forward data signal to the base station corresponding to the new base station selection information and transmits the forward pilot signals to the base stations corresponding to the first to Kth pieces of synchronization information, respectively. An antenna is used for both transmission and reception, and may also be configured to be switchably connected to a transmission signal and a reception signal.
On the basis of synchronization information corresponding to a newly selected base station, a transmission timing of the forward data signal is changed to synchronize the mobile station with the newly selected base station. On the basis of the first to Kth pieces of synchronization information, transmission timings of the first to Kth forward pilot signals are changed to synchronize the mobile station with the K base stations. In these pilot signals, transmission powers of forward pilot signals transmitted to base stations ranked in the top X (X is a natural number which is K or less) of forward propagation quality are set to be high in proportion to the forward propagation properties (step S705).
On the basis of synchronization information corresponding to a serving base station, a transmission timing of the forward data signal is changed to synchronize the mobile station with the serving base station. On the basis of the first to Kth pieces of synchronization information, transmission timings of the first to Kth forward pilot signals are changed to synchronize the mobile station with the K base stations. In these pilot signals, transmission powers of forward pilot signals transmitted to base stations ranked in the top X of forward propagation quality are set to be high in proportion to the forward propagation properties, the control operation shifts to step S707 (step S706). Forward transmission signals (the forward data signal and the first to Kth forward pilot signals) are transmitted (step S707).
The processes in the first to Mth mobile stations will be further described below by using the processes in the ith mobile station as an example. It is assumed that the first to Mth mobile stations perform the same processes.
((i*N)+1)th to ((i*N)+N)th forward pilot signals and the ith forward data signal are generated. In step ST801, the ith mobile station receives the ((i*N)+1)th to ((i*N)+N)th reverse control signals from the jth base station and separates the ((i*N)+1)th to ((i*N)+N)th pieces of synchronization information and the ((i*N)+1)th to ((i*N)+N)th pieces of propagation quality information from the ((i*N)+1)th to ((i*N)+N)th reverse control signals, respectively.
The ((i*N)+j)th forward propagation quality information corresponding to the serving base station, the ((i*N)+1)th to ((i*N)+(j−1))th pieces of forward propagation quality information corresponding to the non-serving base station, and the ((i*N)+(j+1))th to ((i*N)+N)th pieces of forward propagation quality information are compared with each other. When any one of the ((i*N)+1)th to ((i*N)+(j−1))th pieces of forward propagation quality information and the ((i*N)+(j+1))th to ((i*N)+N)th pieces of forward propagation quality information exceeds the ((i*N)+j)th forward propagation quality information, on the basis of the ((i*N)+s)th synchronization information corresponding to the sth base station having a maximum one of the ((i*N)+1)th to ((i*N)+(j−1))th pieces of forward propagation quality information and the ((i*N)+(j+1))th to ((i*N)+N)th pieces of forward propagation quality information, a transmission timing of the ith forward data signal is changed to synchronize the ith mobile station with the sth base station. When the ((i*N)+j)th forward propagation quality information is equal to or larger than the ((i*N)+1)th to ((i*N)+(j−1))th pieces of forward propagation quality information and the ((i*N)++1))th to ((i*N)+N)th pieces of forward propagation quality information, on the basis of the ((i*N)+j)th synchronization information, a transmission timing of the ith forward data signal is changed to synchronize the ith mobile station with the jth base station.
On the basis of the ((i*N)+1)th to ((i*N)+N)th pieces of synchronization information, transmission timings of the ((i*N)+1)th to ((i*N)+N)th forward pilot signals are changed to synchronize the mobile station with the first to Nth base stations. Furthermore, at least one of a transmission frequency, the number of repetitions of transmission, and transmission powers of forward pilot signals transmitted to base stations ranked in the top X of ((i*N)+1)th to ((i*N)+N)th pieces of forward propagation quality information is set to increase in an ascending order of the forward propagation qualities, and the ((i*N)+1)th to ((i*N)+N)th forward pilot signals and the ith forward data signal are output as an ith forward transmission signal.
In this manner, according to the embodiment, when the mobile station determined to switch the base stations, the mobile station can be synchronized with the destination base station.
In a fourth embodiment, a mobile station selects a base station, and a criterion for selecting a base station is reverse propagation quality. When a reverse control signal to a mobile station is transmitted by a serving base station (SBS). A transmission timing of the reverse transmission signal is a timing when request information is received, information of the reverse control signal transmitted to the mobile station is synchronization information corresponding to the base station which makes the request. When the transmission timing is a timing when a periodic trigger is detected, the information of the reverse control signal transmitted to the mobile station is synchronization information of the serving base station.
A trigger generating unit 1308 periodically generates and outputs a trigger signal. When the trigger signal is received, a base station reverse control signal transmitting unit 1309 of its own base station transmits the base station control signal of its own base station to the mobile station as a base station reverse control transmission signal of its own base station. A base station control signal separating unit 1311 of its own base station separates synchronization information from the base station control signal of the own base station and outputs the synchronization information. A request information separating unit 1310 separates request information from the forward link signal transmitted by the ith mobile station and outputs the request information. A non-serving base station control signal receiving unit 1312 separates and outputs a control signal for each of the non-serving base stations in signals received from all the non-serving base stations. A synchronous information extracting unit 1313 separates and outputs synchronization information in the control signal for the non-serving base station. A reverse control signal generating unit 1314 transmits synchronization information corresponding to the base station indicated by the request information to the ith mobile station as a reverse control signal. An antenna is used for both transmission and reception, and may also be configured to be switchably connected to a transmission signal and a reception signal.
On the basis of the forward pilot signal, a reception timing is estimated, and information to synchronize the mobile station with the base station is output as synchronization information. The synchronization information is output as a control signal for the own base station (step S801). It is checked whether the own base station is a base station (SBS) which is being connected to the mobile station. When the own base station is the SBS, the control operation shifts to step S804. When the base station is a base station (NSBS) which is not being connected to the mobile station, the control operation shifts to step S803 (step S802).
Control information of the own base station is transmitted to the SBS (step S803). A trigger signal periodically generated by the SBS is monitored by the SBS. When the SBS detects the trigger signal, the control operation shifts to step S805. When the SBS does not detect the trigger signal, the control operation shifts to step S806 (step S804). The control information for the own base station is transmitted to the mobile station as a base station reverse control signal of the own base station (step S805). A control signal is received from the NSBS, synchronization information is extracted and output. (S806) When request information is received from the mobile station, the control operation shifts to step S808. When no request information is received, the processes are ended (step S807). A control signal for the base station corresponding to the received request information is transmitted to the mobile station as a reverse control signal (step S808).
In this case, the processes in the processes in the first to Nth base stations will be further described by using processes in an ath (a is an arbitrary natural number which is N or less) base station as an example.
When request information which transmits a reverse control signal is received from an ith (i is a natural number which is M or less), on the basis of a pilot signal transmitted by the ith mobile station, a reception timing in the ath base station is estimated, and ((i*N)+a)th synchronization information to synchronize the ith mobile station with the ath base station is generated as an ((i*N)+a)th reverse control signal. A ((i*N)+a)th reverse pilot signal transmitted by the ath base station to the ith mobile station is generated by the ath base station.
Thereafter, the ((i*N)+a)th reverse control signal and the ((i*N)+a)th reverse pilot signal are transmitted to the ith mobile station.
Even though the request information to transmit the reverse control signal is not received, a trigger periodically generated by the base station in a predetermined cycle is monitored. When the trigger is detected, the same processes as described above are performed, the ((i*N)+a)th reverse control signal and the ((i*N)+a)th reverse pilot signal are transmitted to the ith mobile station.
When the trigger is not detected, the processes in the ath base station are ended.
A nonperiodic reverse control signal separating unit 2306 separates the reverse control signal transmitted by the SBS on the basis of the request information from the reception signal received by the mobile station, and outputs the reverse control signal as a nonperiodic reverse control signal. A nonperiodic synchronous information extracting unit 2307 extracts nonperiodic synchronization information from the nonperiodic reverse control signal and outputs the nonperiodic synchronization information.
A periodic reverse control signal separating unit 2308 separates a reverse control signal periodically transmitted by the SBS from the reception signal received by the mobile station and outputs the reverse control signal as a periodic reverse control signal. A periodic synchronous signal extracting unit 2309 extracts synchronization information from the periodic reverse control signal and outputs the synchronization information as periodic synchronization information.
A forward data signal generating unit 2311 generates and outputs a forward data signal. A forward pilot signal generating unit 2312 generates and outputs a forward pilot signal. A forward signal generating unit 2313 generates and outputs the forward data signal and the forward pilot signal as forward signals.
When the base station indicated by the optimum base station selection information is different from the serving base station, a transmission timing setting unit 2310 sets a transmission timing on the basis of the nonperiodic synchronization information corresponding to the base station indicated by the optimum base station selection information to synchronize the mobile station with the base station indicated by the optimum base station selection information. When the base station indicated by the optimum base station selection information is the same as the serving base station, a transmission timing is set by using the latest information of the nonperiodic synchronization information corresponding to the base station indicated by the optimum base station selection information and the periodic synchronization information to synchronize the mobile station with the serving base station, and a forward transmission signal is transmitted. An antenna is used for both transmission and reception, and may also be configured to be switchably connected to a transmission signal and a reception signal.
Request information representing that all pieces of reverse control information corresponding to the base station the reverse propagation quality of which exceeds the threshold value are requested from the SBS is transmitted from the mobile station to the SBS (step S905). A reverse control signal corresponding to the request information is received, and synchronization information is separated and output (step S906). On the basis of the reverse propagation quality information, an optimum base station is selected. Information representing the optimum base station is generated as optimum base station selection information (step S907). When the base station represented by the optimum base station selection information is different from the SBS, the control operation shifts to step S909. When the base station represented by the optimum base station selection information is the same as the SBS, the control operation shifts to step S910 (step S908). A transmission timing of a forward transmission signal is changed to synchronize a base station connected to the mobile station with a newly selected base station, and the control operation shifts to step S911 (step S909). The transmission timing of the forward transmission signal is changed to synchronize the base station connected to the mobile station with the serving base station (step S910). The forward signals (forward data signal and forward pilot signal) are transmitted (step S911).
The processes in the first to Mth mobile stations will be further described below by using the processes in the ith mobile station as an example. It is assumed that the first to Mth mobile stations perform the same processes.
First, the ith mobile station receives ((i*N)+1)th to ((i*N)+N)th reverse control signals and ((i*N)+1)th to ((i*N)+N)th reverse pilot signals from the first to Nth base stations and separates ((i*N)+1)th to ((i*N)+N)th pieces of synchronization information from the ((i*N)+1)th to ((i*N)+N)th reverse control signals, respectively.
Next, an ith forward data signal and an ith forward pilot signal are generated as ith forward transmission signals.
On the basis of the ((i*N)+1)th to ((i*N)+N)th reverse pilot signals, propagation qualities of reverse links corresponding to the first to Nth base stations are estimated, and ((i*N)+1)th to ((i*N)+N)th pieces of reverse propagation quality information are generated.
The pieces of reverse propagation quality information are compared with a threshold value (D), and request information is generated and transmitted to the base station the reverse propagation quality information of which exceeds the threshold value (D) such that the reverse control signal is transmitted to the ith mobile station.
Then, ((i*N)+j)th (j is a natural number which is N or less) reverse propagation quality information corresponding to the serving base station, ((i*N)+1)th to ((i*N)+(j−1))th pieces of reverse propagation quality information corresponding to the non-serving base station, and ((i*N)+(j+1))th to ((i*N)+N)th pieces of reverse propagation quality information are compared with each other, respectively. When any one of the ((i*N)+1)th to ((i*N)+(j−1))th pieces of reverse propagation quality information and the ((i*N)+(j+1))th to ((i*N)+N)th pieces of reverse propagation quality information exceeds the ((i*N)+j)th reverse propagation quality information, on the basis of the ((i*N)+s)th synchronization information corresponding to an sth (s is a natural number which is N or less except for j) base station having a maximum one of the ((i*N)+1)th to ((i*N)+(j−1))th pieces of reverse propagation quality information and the ((i*N)+(j+1))th to ((i*N)+N)th pieces of reverse propagation quality information, transmission is performed such that a transmission timing of the ith forward transmission signal is changed to synchronize the ith mobile station with the sth base station. When the ((i*N)+j)th forward propagation quality information is equal to or larger than the ((i*N)+1)th to ((i*N)+(j−1))th pieces of forward propagation quality information and the ((i*N)+(j+1))th to ((i*N)+N)th pieces of forward propagation quality information, on the basis of the ((i*N)+j)th synchronization information, transmission is performed such that the transmission timing of the ith forward transmission signal is changed to synchronize the ith mobile station with the jth base station.
In this manner, according to the embodiment, when the mobile station determines to switch the base stations, the mobile station can be synchronized with a destination base station.
Number | Date | Country | Kind |
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2005-241913 | Aug 2005 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2006/316369 | 8/22/2006 | WO | 00 | 2/4/2008 |