Multiplex communications – Wide area network – Packet switching
Patent
1991-04-24
1993-11-02
Olms, Douglas W.
Multiplex communications
Wide area network
Packet switching
3701001, 375114, H04J 316
Patent
active
052589803
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a radio channel switching control method in a digital mobile communication system employing TDMA techniques.
BACKGROUND ART
In a mobile communication system like land mobile radiotelephone a zone which permits a mobile station to communicate with one base station is limited by the radio wave transmitting power used; therefore, in the case of implementing a wide service area, a number of zones Z1, Z2, ... including base stations B1, B2, ..., respectively, are provided, for example, as shown in FIG. 1, in such a manner as to permit communication from any zone via its base station and an exchange 20. Adjacent zones use signals of different frequencies to prevent interference between signals of different zones. In such a mobile communication system, when a certain mobile station 10 moves from the zone Z1 to the adjoining one Z2 while maintaining communication, the radio channel frequency used must be switched from F1 to F2 and during this hand off the communication is temporarily interrupted; this is a well-known problem in the art.
In recent years there has been under study the introduction of a digital mobile communication system employing TDMA (Time Division Multiple Access) techniques. In the digital mobile communication system utilizing the TDMA techniques each mobile station holds communication in each zone, using a designated time slot of a frequency specified by the base station in that zone. A communication channel specified in terms of frequency and time slot will hereinafter be referred to as a radio channel, but in the case where it is not intended to specify any particular time slot in the following description, the channel will be also called a frequency channel. Also in this instance, when the mobile station moves from a certain zone to an adjacent one, the radio channel must be switched accordingly. It is easily predictable that the switching of the radio channel will cause a momentary interruption of communication also in such a well-known digital mobile communication system which utilizes the TDMA techniques.
FIG. 2 is a timing chart for explaining the radio channel hand off procedure in the digital mobile communication system employing the TDMA techniques and the momentary interruption of communication which results from the channel hand off. Now, let it be assumed that the mobile station 10 moves from the zone Z1 to the zone Z2 in FIG. 1. Row A in FIG. 2 shows a signal of a frequency F1 transmitted from the base station B1. The transmitted signal is a recurrence of a TDMA frame FR which is composed of a predetermined number of, for instance, three time slots S1, S2 and S3, and for communication with the mobile station 10 in the zone, one time slot, the first time slot S1 in this example, is used. Row B in FIG. 2 shows a signal of a frequency F2 being transmitted from the base station B2 of the zone Z2 into which the mobile station 10 is to enter. Assume that the mobile station 10 uses an idle third time slot S3 of the transmitted signal from the base station B2 in the zone Z2. Row C shows a signal received by the mobile station 10 and row D the frequency of the received signal. Row E shows a signal transmitted from the mobile station 10 and row F a signal received by the base station B2 in the zone Z2.
When the reception at the frequency F1 gets worse as the mobile station 10 approaches the zone Z2, the base station B1 specifies the radio channel to be used, i.e. the frequency F2 and the time slot S3, through a signal A1 of the first slot S1 of the frame FR1 shown in row A of FIG. 2. Upon reception of such a radio channel specify signal, the mobile station 10 switches the receiving frequency to F2 as depicted in row D of FIG. 2, receives a signal of the frequency F2 from the base station B2, shown in row B of FIG. 2, and then establishes synchronization with a signal B1 of the third slot S3 of the frame FR1. This ensures correct reception of signals of the third slot S3 in the frame FR2 and subsequent frames FR3, FR4, ... of the signal of
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Adachi Fumiyuki
Fujii Toshinori
Hata Masaharu
Maebara Akihiro
Onoe Seizo
Hom Shick
Nippon Telegraph and Telephone Corporation
Olms Douglas W.
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