Mobile communication system and method for establishing synchronization in mobile communications

[0002] The invention relates to a mobile communication system suitable for use, for example, in digital cellular phones and mobile communications of small-zone time division multiaccess, and a method for establishing synchronization in mobile communications.

[0003] In TDD (time division duplex)-TDMA (time division multiple access) mobile communications, such as PHS (personal handy-phone system), timing of transmission/receiving should be adjusted between a plurality of base stations installed within the service area to prevent the occurrence of interference.

[0004] Means, which has hitherto been used, for coping with this demand is such that the standard is set so as to be consistent with that of other device with which the base station is in synchronization, alternatively such that all the base stations are brought to synchronization with one standard station.

[0005] When the standard is set so as to be consistent with that of other device with which the base station is in synchronization, however, the management of these devices is likely to be influenced by other devices. Further, in areas where any standard device is absent, it is impossible to provide synchronization between base stations.

[0006] On the other hand, when all the base stations are brought to synchronization with one standard system, a lot of time is disadvantageously taken for achieving synchronization in the whole service area.

[0007] The invention has been made under these circumstances, and it is an object of the invention to provide a mobile communication system and a method for establishing synchronization in mobile communications that can ensure synchronization between a plurality of base stations and can enhance frequency reuse in time division transmission without suffering from increased interference.

[0008] According to the first feature of the invention, a mobile communication system comprises:

[0009] a plurality of first base stations each having a predetermined radio zone, for performing communication with one or more mobile stations;

[0010] a plurality of second base stations each having a predetermined radio zone, for performing communication with one or more mobile stations and, in addition, giving a synchronous standard to said plurality of first base stations; and

[0011] an absolute synchronous station for giving a synchronous standard to each of the plurality of second base stations.

[0012] In the mobile communication system, preferably, the plurality of second base stations each have a predetermined synchronous area and a plurality of the first base stations with the radio zones being in contact with or overlapped with each other are present within the synchronous area.

[0013] Preferably, the second base stations each have first timing control means for controlling the timing of a control channel signal transmitted to other second base station based on the timing of a control channel signal transmitted by the absolute synchronous station.

[0014] Preferably, the second base stations each have second timing control means for controlling the timing of a control channel signal transmitted to the mobile station based on the timing of a control channel signal transmitted by the absolute synchronous station.

[0015] Preferably, the second base stations each have second timing control means for controlling the timing of a control channel signal transmitted to the mobile station based on the timing of a control channel signal transmitted by the other second base stations.

[0016] Preferably, the control channel signal has a predetermined fixed code at a predetermined position of all bits, and the second base stations each delay the control channel signal to be transmitted so that the bit position of the predetermined fixed code contained in the control channel signal transmitted is consistent with the bit position of the predetermined fixed code contained in the received control channel signal.

[0017] According to the second feature of the invention, a method for establishing synchronization in mobile communication, comprises the steps of:

[0018] providing a mobile communication system comprising

[0019] a plurality of first base stations each having a predetermined radio zone, for performing communication with one or more mobile stations,

[0020] a plurality of second base stations each having a predetermined radio zone and each having a predetermined synchronous area, for performing communication with one or more mobile stations and, in addition, giving a synchronous standard to the plurality of first base stations with the radio zones being in contact with or overlapped with each other within the synchronous area, and

[0021] an absolute synchronous station for giving a synchronous standard to each of the plurality of second base stations; and

[0022] through each of the second base stations, controlling the timing of a control channel signal transmitted to the other second base stations based on the timing of a control channel signal transmitted by the absolute synchronous station.

[0023] In the method for establishing synchronization in mobile communication, preferably, the second base stations each control the timing of a control channel signal transmitted to the mobile station based on the timing of a control channel signal transmitted by the absolute synchronous station.

[0024] Preferably, the second base stations each control the timing of a control channel signal transmitted to the mobile station based on the timing of a control channel signal transmitted by the other second base stations.

[0025] Preferably, the control channel signal has a predetermined fixed code at a predetermined position of all bits, and the second base stations each delay the control channel signal to be transmitted so that the bit position of the predetermined fixed code contained in the control channel signal transmitted is consistent with the bit position of the predetermined fixed code contained in the received control channel signal.

[0026] The invention will be explained in more detail in conjunction with the appended drawings, wherein:

[0027] FIG. 1 is a diagram showing the whole construction of the mobile communication system according to one preferred embodiment of the invention;

[0028] FIG. 2 is a detailed block diagram showing one example of the construction of an absolute synchronous station in the preferred embodiment of the invention;

[0029] FIG. 3 is a detailed block diagram showing one example of the construction of a master base station in the preferred embodiment of the invention;

[0030] FIG. 4 is a detailed block diagram showing one example of the construction of a slave base station in the preferred embodiment of the invention;

[0031] FIG. 5 is a flow chart showing one example of the flow of processing in the absolute synchronous station in the preferred embodiment of the invention;

[0032] FIG. 6 is a flow chart showing one example of the flow of processing in the master base station in the preferred embodiment of the invention; and

[0033] FIGS. 7A to 7D are diagrams showing the state of a control channel signal in the preferred embodiment of the invention.

[0034] The preferred embodiments of the invention will be explained in FIGS. 1 to 7.

[0035] FIG. 1 is a diagram showing the whole construction of the mobile communication system according to one preferred embodiment of the invention. In this case, the mobile communication system of the invention is applied to microcellular mobile communication systems, such as PHS's.

[0036] In FIG. 1, numeral 3 designates an absolute synchronous station that serves as a standard of the synchronization of synchronous areas 4₋₁, 4₋₂, 4₋₃. . . (hereinafter often referred to simply as “synchronous area 4”).

[0037] A master base station 1 and a plurality of slave base stations 2, 2 . . . are present in each of the synchronous areas 4₋₁, 4₋₂, 4₋₃. . . .

[0038] This master base station 1 is not used for call processing services and communication services but for establishment of synchronization of the slave base stations 2, 2^{. . .}On the other hand, the slave base station 2 is used in actual communication services.

[0039] The absolute synchronous station 3 is for synchronization in the preferred embodiment of the invention, and is not used in communication with mobile stations (not shown). FIG. 2 is a detailed block diagram showing one example of the construction of an absolute synchronous station 3.

[0040] As shown in FIG. 2, the absolute synchronous station 3 has, for example, four directional antennas 31₋₁, to 31₋₄. The directional antennas 31₋₁, to 31₋₄are installed so that the main lobe faces a desired master base station 1.

[0041] The directional antennas 31₋₁to 31₋₄each predetermine a master base station 1 as the corresponding transmission destination to ensure the synchronous area 4 and, at the same time, prevents propagation delay attributable to multipath.

[0042] According to this preferred embodiment, the directional antennas 31₋₁to 31₋₄each, for example, have a gain of not less than 10 dBi, and transmit control channel signals with frequencies in commercial use at a power of 200 mW to the master base station 1.

[0043] Numeral 32 designates an antenna switch comprising a gate circuit. This antenna switch 32 functions to direct a transmission control channel signal to any one of the antennas 31₋₁to 31₋₄through a single transmission signal S_S3.

[0044] Numeral 33 designates a radio controller. The radio controller 33 selects and controls the directional antenna 31₋₁to 31₋₁for the antenna switch 32 based on a time notification signal T_imsent from a slot counter 34 for counting the number of slots of TDMA.

[0045] FIG. 3 is a detailed block diagram showing one example of the construction of the master base station 1. Numerals 11₋₁and 11₋₂each designate a transmitting directional antenna, and numerals 15₋₁and 15₋₂each a receiving directional antenna. The details of these transmitting directional antennas and receiving directional antennas are the same as those of the directional antennas 31₋₁to 31₋₄.

[0046] Numerals 17₋₁to 17₋₄each designate nondirectional antennas for use in communication with PS (personal station of PHS) . Examples thereof include sleeve antennas. The antennas 17₋₁to 17₋₄are also used for radio synchronization with the slave base station 2.

[0047] Numeral 12 designates an antenna switch for switching between the directional antennas 15₋₁and 15₋₂, and numeral 14 an antenna switch for switching between the directional antennas 11₋₁and 11₋₂. The antenna switches 12 and 14 each comprise a gate circuit or the like.

[0048] Thus, the master base station 1 performs switching between the two receiving directional antennas 15₋₁, 15₋₂and switching between the two transmitting directional antennas 11₋₁, 11₋₂respectively through the antenna switch 12 and the antenna switch 14.

[0049] This permits a main path and a bypass to be provided in the synchronous circuit between the master base stations 1. That is, duplexing is provided to improve the reliability of the synchronization.

[0050] The master synchronous controller 13 comprises a synchronization generator and a software (both not shown). The timing of the generation of the synchronization is variable by the software. By virtue of this constitution, adjustment can be made to the target timing of the absolute synchronous station 3 and of the mater base station 1.

[0051] The slave synchronous controller 18 is a part of the controller used in service, and is synchronized with the master synchronous controller 13. Further, this slave synchronous controller 18 performs diversity control of a radio transmitter 16 to control the gain of the radio transmitter 16.

[0052] The synchronous area 4 for each of the master base stations 1 is wider in terms of cell than the cell of each of the slave base stations 2, 2 . . . , and, for example, has a radius of about 3.5 km.

[0053] FIG. 4 is a detailed block diagram showing one example of the construction of the slave base station 2. Numerals 21₋₁to 21₋₄each designate a nondirectional antenna of which the details are the same as those of the nondirectional antennas 17₋₁to 17₋₄. These nondirectional antennas 21₋₁to 21₋₄perform space diversity.

[0054] Numeral 22 designates a radio receiver for a receiving circuit for each of the nondirectional antennas 21₋₁to 21₋₄. The radio receiver 22 detects an analog radio signal and converts the analog radio signal to a base band signal. Further, the radio receiver 22 determines a receive level used in a demodulator 23.

[0055] The demodulator 23 synthesizes a baseband signal, from each of the nondirectional antennas 21₋₁to 21₋₄, received from the radio receiver 22. Thus, demodulation is performed. In the demodulation, the demodulator 23 adds weights for respective nondirectional antennas 21₋₁to 21₋₄depending upon a reference level from the radio receiver 22, to thereby obtain gains. Numeral 24 designates a UW decision section that detects UW (unique word) showing the head of an information field from received data and, based on the position of the head of UW, calculates a deviation of the timing from the master base station 1 as the origin of synchronization.

[0056] The information on the deviation in timing calculated in the UW decision section 24 is input into a synchronous controller 25. Based on the information on the deviation in timing, the timing of operation is forcibly adjusted to the timing of the master base station 1 as the origin of synchronization.

[0057] FIG. 5 is a flow chart showing one example of the flow of processing in the absolute synchronous station 3. In this preferred embodiment, the start-up of the whole system is carried out first in the absolute synchronous station 3, and radio synchronization is performed from a master base station 1 designated by the directional antenna.

[0058] Upon the start-up, the absolute synchronous station 3 initiates the transmission of a control channel signal (step Sa1) The directional antennas 31₋₁to 31₋₄are directed to separate respective master base stations 1. Therefore, in the directional antennas 31₋₁to 31₋₄, whether or not the control channel signal has been transmitted for 100 msec is successively judged (step Sa2). After 100 msec has elapsed, the transmission antenna (directional antennas 31₋₁to 31₋₄) is switched (step Sa3).

[0059] In the switching of the directional antennas 31₋₁to 31₋₄, a slot counter 34 informs the radio controller 33 of the spacing of 100 msec to control the antenna switch 32. Thus, the control channel signal is successively transmitted from the selected directional antennas 31₋₁to 31₋₄.

[0060] That is, for each of the directional antennas 31₋₁, to 31₋₄, the control channel signal is transmitted at spacings of 400 msec.

[0061] FIG. 6 is a flow chart showing one example of the flow of processing in the master base station 1.

[0062] The master base station 1 has a directional antenna 15₋₁and a directional antenna 15₋₂.

[0063] Accordingly, any one of the directional antennas 15₋₁and 15₋₂waits for a control channel signal from the absolute synchronous station 3 or the other master base stations 1, and monitoring is carried out on whether or not UW can be detected (step Sb1).

[0064] Further, monitoring is carried out on whether or not the wait time in the directional antenna exceeded one sec (step Sb2). As soon as one sec has elapsed, the receiving antenna is switched (step Sb3) to wait for the control channel signal.

[0065] When UW could be detected in step Sb1, the timing of the radio synchronization is varied based on the detected UW (step Sb4).

[0066] FIGS. 7A to 7D are diagrams showing the state of the control channel signal. Specifically, FIG. 7A shows a control channel signal in any one of the master base stations 1, FIG. 7B a received control channel signal, FIG. 7C a control channel signal after timing correction, and FIG. 7D a deviation in timing attributable to distance.

[0067] More specifically, timing deviation time A is determined based on the signal shown in FIG. 7A and the signal FIG. 7B. Therefore, as shown in FIG. 7C, synchronous timing is forcibly shifted by time A.

[0068] The deviation in timing attributable to distance also occurs. In this case, since the distance between the absolute synchronous station 3 and each of the master synchronous stations 1 is known, timing deviation time B can be predetermined. Therefore, as shown in FIG. 7D, also for time B, the timing may be shifted by time B.

[0069] Thus, the master base station 1 of which the synchronization has been established, after the correction of synchronization, transmits a control channel signal to the other master base stations 1 at spacings of 100 msec alternately from the directional antenna 11₋₁and the directional antenna 11₋₂(step Sb5)

[0070] Further, thereafter, the master base stations 1 are synchronized one by one, and synchronization among the master base stations 1 in the whole service area is carried out.

[0071] The slave base stations 2 wait from the start-up of this preferred embodiment for the completion of the synchronization of all the master base stations 1, and, upon the completion of the synchronization, execute radio synchronization (step Sb6).

[0072] In this case, the master base station 1 is detected which exhibits the highest receive level, followed by radio synchronization. The procedure for the correction of synchronization timing is the same as described above.

[0073] As described above, according to the preferred embodiment of the invention, in a mobile communication system comprising: a plurality of first base stations each having a predetermined radio zone, for performing communication with one or more mobile stations; a plurality of second base stations each having a predetermined radio zone and each having a predetermined synchronous area, for performing communication with one or more mobile stations and, in addition, giving a synchronous standard to the plurality of first base stations with the radio zones being in contact with or overlapped with each other within the synchronous area; and an absolute synchronous station for giving a synchronous standard to each of the plurality of second base stations, the second base stations each control the timing of a control channel signal transmitted to other second base station based on the timing of a control channel signal transmitted by the absolute synchronous station. Further, the second base stations each control the timing of a control channel signal transmitted to the mobile station based on the timing of a control channel signal transmitted by the absolute synchronous station. The second base stations each control the timing of a control channel signal transmitted to the mobile station based on the timing of a control channel signal transmitted by the other second base station. The control channel signal has a predetermined fixed code at a predetermined position of all bits, and the second base stations each delay the control channel signal to be transmitted so that the bit position of the predetermined fixed code contained in the control channel signal transmitted is consistent with the bit position of the predetermined fixed code contained in the received control channel signal. Therefore, it is possible realize a mobile communication system and a method for establishing synchronization in mobile communications that can ensure synchronization between a plurality of base stations and can enhance frequency reuse in time division transmission without increasing the occurrence of interference.

[0074] That is, a selected base station covering the whole area is first subjected to radio synchronization, followed by simultaneous synchronization of the other base stations used in service. This can shorten the time taken for the synchronization of the whole area to be established.

[0075] Further, only the wait time for synchronization of the master base station is required, and the operational base stations can be simultaneously synchronized. This can realize simplification of algorithm.

[0076] Furthermore, radio synchronization by the device within the self-system is possible. Therefore, there is no need to rely upon other systems.

[0077] The invention has been described in detail with particular reference to preferred embodiments, but it will be understood that variations and modifications can be effected within the scope of the invention as set forth in the appended claims.