GB2281177A

GB2281177A - Seamless handoff method

Info

Publication number: GB2281177A
Application number: GB9414209A
Authority: GB
Inventors: Masayoshi Inoue
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 1993-07-14
Filing date: 1994-07-14
Publication date: 1995-02-22
Also published as: JPH0730949A; JP2590692B2; GB9414209D0; AU6746694A; CA2128137A1

Abstract

In a TDMA cellular radio system a first time slot is used when the mobile subscriber is in a first cell and a second time slot is used when the mobile subscriber enters a second cell. During the hand-off both time slots are used respectively for up and down communications with the mobile switching centre. In this way an instantaneous break in a communication in progress is avoided.

Description

TDMA MOBILE COMMUNICATION WITH SELECTION OF TWO TIME SLOTS DURING MOVEMENT OF A MOBILE STATION BETWEEN TWO RADIO ZONES BACKGROUND OF THE INVENTION: This invention relates to channel switching in a time division multiple access (TDMA) mobile communication network and, more particularly, to a channel switching method, a channel switching system, and a mobile station of the system.

When related to this invention, the mobile communication network comprises a plurality of base stations individually covering a plurality of radio zones or cells, a mobile station movable in the radio zones and capable of communicating with the base stations through a plurality of radio time slots of a time division multiple access frame, and a mobile exchange station connected to the base stations and to exchange offices of a fixed network.

The channel exchange station will herein be referred to briefly as an exchange station, The channel switching is between a first and a second time slot of the radio time slots during movement of the mobile station from a first zone to a second zone of the radio zones and during current communication between the mobile station and the exchange station through a first station covering the first zone among the base stations with a communication quality kept at a predetermined value between the current communication and fresh communication of the mobile station with a second station covering the second zone among the base stations.

Usually, a plurality of mobile stations are used in the mobile communication network. Each mobile station may be a portable telephone set, an automobile communication set, or the like. Between such mobile stations and the base stations, several frequencies of a radio frequency band are reused in the manner know in the art. Merely for simplicity of the description, attention will be directed in the following to only one of the mobile stations.

One of typical examples of the mobile communication network is specified in a standard, when transliterated in accordance with ISO 3602, published by "Denpa Sisutemu Xahatu Sent" (Electromagnetic Wave System Development Center) under the title of "Daini' Sedai Kodoresu Denwa Sisutemu Hyozyun Kikiku" (Standards for Second-generation Cordless Telephone Systems), RCR STD-28, First Edition. To briefly describe the standard under the situations mentioned above, it will be presumed that the communication quality becomes objectionable between the mobile station and the first station. In this event, the mobile station establishes a link channel to the second station and transmits a call through the link channel. The second station acknowledges the call.Responsive to this acknowledgement, the mobile station stops to continue the current communication. It is consequently inevitable that an instantaneous break or interruption occurs in communication between establishment of the link channel and beginning of the fresh communication.

Another conventional mobile communication network is disclosed in Japanese Patent Prepublication (A) No. 34,044 of 1990 by Kamiya-Tosiaki. According to this network, each mobile station comprises first and second transmitter/receivers for the first and the second stations* As a consequence, the mobile station becomes bulky, heavy-weighted, and expensive. This is particularly objectionable when the mobile station is a portable telephone set. No exchange station is described.

Still another conventional mobile communcation network is revealed in Japanese Patent Prepublication (A) No. 41,529 of 1989 by Maruyama-Yasuo and another. A plurality of radio frequencies are assigned to a service area of the mobile communication network. In a time division multiple access frame, first and second time slots are fixedly allotted to the first and the second stations. One of the radio frequencies is used at the first and the second stations in commun. The exchange station takes part in channel switching to transmit a channel switch timing signal to the mobile station through the first and the second stations. When a common radio frequency is not available at the first and the second stations, the isstantenous break is unavoidable.Furthermore, it is technically difficult to generate the channel switch timing signal with propagation delay time exactly taken into consideration. This ivevitably results in a shift between propagation times from the exchange station to mobile station through the first and the second stations and in another cause of the instantaneous break.

Yet another conventional mobile communication network is described by Eiji Kito in United States Patent Application No. 898,710 filed June 15, 1992, for assignment to the instal assignee. Although the exchange station is not so called, this prior patent application will herein be incorporated by reference in order to simplify the description and the drawing figures.

SUMMARY OF THE INVENTION: In view of the foregoing, it is an object of the present invention to provide a channel switching method for use in a time division multiple access mobile communication network of the type described, by which method it is possible to get rid of an instantaneous break of communication in progress while a mobile station moves from a radio zone to another radio zone covered by two base stations of the network.

It is another object of this invention to provide a channel switching method which is of the type described and in which the mobile station is compact, light-weighted, and. not expensive.

It is still another object of this invention to provide a channel switching method which is of the type described and in which a channel switch timing signal need not be generated in exact consideration of a propagation delay from a mobile exchange station of the network to the mobile station along signal paths through the two base stations.

It is a different object of this invention to provide a channel switching system for use in carrying out the channel switching method of the dype described.

It is a further different object of this invention to provide a mobile station for use in the channel switching system of the type described.

Other objects of this invention will become clear as the description proceeds.

In accordance with an aspect of this invention, there is provided a channel switching method which is for switching, in a mobile communication network comprising a plurality of base stations individually covering a plurality of radio zones, a mobile station movable in the radio zones and capable of communicating with the base stations through a plurality of radio time slots of a time division multiple access frame, and an exchange station connected to the base stations, between a first and a second time slot of the radio time slots during movement of the mobile station from a first zone to a second zone of the radio zones and during current communication between the mobile station and the exchange station through a first station covering the first zone among the base stations with a communication quality kept at a predetermined value between the current communication and fresh communication of the mobile station with a second station covering the second zone among the base stations and which comprises the steps of: (A) simultaneously using first the first and the second time slots with one of the first and the second time slots ueed in the current communication; and (B) subsequently using the other of the first and the second time slots alone.

In accordance with a different aspect of this invention, there is provided a channel switching system which is for switching, in a mobile communication network comprising a plurality of base stations individually covering a plurality of radio zones, a mobile station movable in the radio zones and capable of communicating with the base stations through a plurality of radio time slots of a time division multiple access frame, and an exchange station connected to the base stations, between a first and a second time slot of the radio time slots during movement of the mobile station from a first zone to a second zone of the radio zones and during current communication between the mobile station and the exchange station through a first station covering the first zone among the base stations with a communication quality kept at a predetermined value between the current communication and fresh communication of the mobile station with a second station covering the second zone among the base stations and which comprises in the mobile station: (A) up selector and transmitter means for transmitting an up communication signal to the exchange station selectively in one or both of the first and the second time slots; (B) down selector and receiver means for receiving a dom communication signal from the exchange station in the first and the second time slots as first and second reception signals and for selecting one of the first and the second reception signals as a received signal; and (C) mobile control means for making the up selector and transmitter means select the above-mentioned one or the both of first and second time slots and making the down selector and receiver means select the above-muntioned one of first and second reception signals * In accordance with a further different aspect of this invention, there is provided a mobile station which is for use in a mobile communication network comprising a plurality of base stations individually covering a plurality of radio zones and an exchange station connected to the base stations and which is movable in the radio zones and capable of communicating with the base sat ions through a plurality of radio time slots of a time division multiple access frame, is for use in a channel switching system for switching between a first and a second time slot of the radio time slots during movement from a first zone to a second zone of the radio zones and during current communication between the mobile station and the exchange station through a first station covering the first zone among the base stations with a communication quality kept i a predetermined value between the current communication and fresh communication of the mobile station with a second station covering the second zone among the base stations, and comprises: (A) up selector and transmitter means for transmitting an up communication signal to the exchange station selectively in one or both of the first and the second time slots; (B) down selector and receiver means for receiving a down communication signal from the exchange station in the first and the second time slots as first and second reception signals and for selecting one of the first and the second reception signals as a received signal; and (C) mobile control means for making the up selector and transmitter means select the above-mentioned one or the both of first and second time slots and making the down selector and receiver means select the above-mentioned one of first and second reception signals.

BRIEF DESCRIPTION OF THE DRAWING: Fig. 1 is a block diagram of a mobile station for use in a channel switching system according to anembodiment of the instant invention; Fig. 2 is a block diagram of an exchange station for use in the channel switching system mentioned in conjunction with Fig. 1; Fig. 3 shows a time chart for use Aq describe: operation of the channel switching system mentioned in connection with Figs. 1 and 2; Figs. 4 (a) and (b) show in blocks the channel switching system mentioned in relation to Fig. 3 in various states of operation; and Fig. 5 shows time slots for use in describing operation of the channel switching system mentioned in conjunction with Figs. 3 and 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT: Referring to Fig. 1, the description will begin with a mobile station for use in a channel switching system according to a preferred embodiment of the present invention. The channel switching system is for use in a time division multiple access (TIBIA) mobile communication network.

Consequently, the mobile communication network will first be described in outline. In the manner depicted in the drawing figures of the prior patent application referred to hereinabove, the mobile communication network comprises a plurality of base stations or cell sites which individually cover a plurality of radio zones or cells in a service area of the network. A mobile station 11 (Fig. 1 herein) is movable in the radio zones and capable of communicating with the base stations through a plurality of radio time slots of a time division multiple access frame. The base stations are connected to a fixed network which is typically a public dommu-ication network. Although not so named in the prior patent application, a mobile exchange station is interposed between the base station and the fixed network.

The mobile exchange station is herein referred to briefly as an exchange station. As will later be described, the fixed network comprises a plurality of exchange offices connected to the exchange stationd and to a great number of subscriber substations through the exchange offices.

The channel switching system is for switching channels during movement df the mobile station 11 from a first zone to a second zone of the radio zones and during current communication between the mobile station 11 and the exchange station through a first station covering the first zone among the base stations with a communication quality kept at or above a predetermined value between the current communication and fresh communication of the mobile station 11 with the exchange station through a second station of the bastations that covers the second zone.

The fresh communication is a continuation of the current communication and is differently named because paths of communication is different from the current communication.

Digital signals are bidirectionally transmitted in the mobile communication network. Throughout the instant specification, the signals will be called an up signal when transmitted from the mobile station to at least one of the base stations and thence to the exchange station. In a like manner, the signals will be referred to as a down signal when transmitted from the exchange station to one or two base stations and therefrom to the mobile station 11.

In Fig. 1, the mobile station 11 comprises a transmitter (TX) 13 and a receiver (RX) 15 which may be those used in a telephone handset. Consequently, the transmitter 13 may be a microphone, The receiver 15 may be a loudspeaker. Alternatively, the transmitter 13 and the receiver 15 may be parts of a terminal set and are for transmitting and receiving data signals. Merely for brevity of the description, a voice signal alone will be taken into consideration as a communication signal.

From the transmitter 13, an analog up voice signal is delivered to a transmitter circuit (TX CKT) 17 and is converted into a digital up voice signal. The receiver 15 is supplied with an analog down voice signal from a receiver circuit (RX CKT) 19 for converting a digital down voice signal supplied thereto into the analog down voice signal. The digital up voice signal is sent to a transmission- or up selector (TX SEL) 21. The digital down voice signal is supplied to the receiver circuit 19 from a reception or down selector (RX SEL) 23. The transmission and the reception selectors 21 and 23 are cross connected to mobile br-frst,and second call controllers (CONT) 25 and 27 which are parts of a microprocessor, The digital up and down voise signals will briefly be called up and down voice signals.

In the manner which will become clear as the description proceeds, the first and the second call controllers 25 and 27 produce first and second up call control signals. Controlled by the first and the second up call control signals depicted by thin lines, the transmitter circuit 17 sends the up voice signal to the first and the second call controllers 25 and 27 and further to a time division multiplexer (MUX) 29 in the manner illustrated by thick lines.

It will be presumed that the mobile station 11 is currently either standstill or moving in the first zone covered by the first station and should transmit the up voice signal to the exchange station through the first station and that the first call controller 25 is put in operation as an active controller with the second call controller 27 kept in a standby state before channel switching becomes necessary, In any event, such an active call controller 25 or 27 specifies one of a plurality of radio time slots, such as four time slots, in a time division multiple access frame and specifies a radio frequency of which a carrier signal is modulated by a digital up voice signal into an up radio modulated signal.

Under the circumstances being in consideration, it will additionally be presumed that the up call control signal indicates a first transmission time slot T1 (later illustrated) of the radio time slots for transmission of the up voice signal to the first station and a first reception time slot R1 (later depicted) for reception of the down voice signal from the first station.

The time division multiplexer 29 places the up voice signal in the first transmission time slot and delivers this signal to a mobile radio transmitter (TX) 31. Produced by the transmitter 31 in the first transmission time slot, an up radio eignal is fed through a duplexer 33 to an antenna 35 and transmitted to the first station in the first transmission time slot.

In the manner which will presently be described, the first station transmits a down radio signal in the first reception time slot to the mobile station 11. Received by the antenna 35 and supplied with the down radio signal through the duplexer 33, a mobile radio receiver (RX) 37 demodulates the down radio signal into the down voice signal for delivery to the time division multiplexer 29 in the first reception time slot. The multiplexer 29 sends the down voice signal to the first and the second call controllers 25 and 27 in the first reception time slot.

One or both of the first and the second call controllers 25 and 27 is or are active. In any event, the first and the second call controllers 25 and 27 produce first and second down call control signals. in other words, the first and the second call controllers 25 and 27 select one thereof as a selected call controller for delivering a digital down voice signal in one of the radio time slots to the receiver circuit 23. In the manner presumed above. the first call controller 25 is active and sends the down voice signal to the receiver circuit 27 as described before.

Turning to Fig. 2, the exchange station is indicated at 41. Fig. 1 will additionally be referred to. The first and the second stations and other base stations of the mobile communication network are M in number and are depicted with a common legend "BASE STA" at 43(1), 43(2), ..., and 43 (M). For convenience of legends, the first and the second stations 43(1) and 43(2) will alternatively be labelled BASE A and BASE B. Furthermore, the base stations 43(1) through 43(M) will collectively be denoted by a single reference numeral 43 without use of the suffixes enclosed with parentheses.Similarly, the exchange offices~Df the fixed network are N in number and are designated at 45(1), 45(2), ..., and 45(N) with a common legend "EX OFF" and will either collectively or individually be denoted by a sole reference numeral 45 with omission of the suffixes.

Receiving the up radio signal from the mobile station 11 in the first transmission time slot, the first station 43(1) delivers a first interstation up signal to the exchange station 41. Others of the base stations 43 likewise send second through M-th up interstation signals to the exchange station 41 when supplied with up radio signals through pertinent ones of the radio time slots.

In Fig. 2, the exchange station 41 comprises a base station interface unit (STA INT) 47 connected to the base stations 43 to receive the up interstation signals. An exchange office interface unit (OFF INT) 49 is connected to the exchange officies 45 and therefrom to subscriber substations (not shown) of the fixed network. Sent to the first station 43(1) in the first transmission time slot, the up radio signal and consequently the first up interstation signal is accompanied by an up destination signal indicative of an up destination address, such as a telephone directory number, of one of the subscriber substations to which the up voice signal shouled be delivered through a relevant one of the exchange offices 45.

Supplied from the base station interface unit 47 in the first transmission time slot with the first up interstation signal, an exchange call controller (CONT) 51 delivers an up interface control signal to the exchange office interface unit 49 and an up switch control signal to a time division switch (SW) 53. Sent from the station interface unit 47 in the first transmission time slot, the up voice signal is delivered to the office interface unit 49 by the time division switch 53 under control by the up switch control signal. Controlled by the up interface control signal, the office interface unit 49 sends the up voice signal to the relevant one of the exchange offices 45 and thence in the known manner to the above-mentioned one of the subscriber substations.

It will be assumed that a call originates at one of the subsciber substations as an originating call. Through a pertinent one of the exchange offices 45, the originating call is delivered to the exchange station 41 and is supplied to the exchange office interface unit 49 together with a down destination signal indicative of a station number of the mobile station 11.

Responsive to the originating call delivered through the office interface unit 49, the exchange call controller 53 sends a down interface control signal to the base station interface unit 47 and a down switch control signal to the time division switch 53. In the manner described before, this call is delivered to the first station 43(1) and is transmitted thence to the mobile station 11 in the first reception time slot.

Supplied from the down voice signal of the originating call and controlled by the down switch control signal, the time division switch 53 delivers the down voice signal to the base station interface unit 47. Controlled by the down interface control signal, the station interface unit 47 sends the down voice signal to the first station 43(1) as a down interstation signal in the first reception time slot. The first station 43(1) transmits the down radio signal to the mobile station 11 in the first reception time slot.

Referring afresh to Figs. 3, 4 (a), 4 (b), and 5 and again to Figs. 1 and 2, operation will be described with regard to the channel switching systen being illustrated. In Fig. 3, the transmitter 13 and the receiver 15 of the mobile station 11 are indicated by TX/RX 13/15. The mobile station 11 is labelled MOBILE. The exchange station 41 is indicated by a legend "EX STA". The fixed network and the pertinent or the relevand one of the exchange offices 45 are collectively indicated with a legend "OFF" at 45. Similar legends and designations are used in Figs. 4 (a) and (b), wherein only the first and the second stations 43(1) and 43(2) are illustrated as representatives of the base stations 43.In Fig. 5, the first transmission and reception time slots T1 and R1 are depicted, First through ninth states of operation are indicated by (1) to (9) in Figs. 3 through 5. Such time slots are used in transmission and reception of not only the voice signals but also various control signals as will shortly be described.

While the mobile station 11 is in the first zone to proceed with the current communication, the channel switching system is in the first state (1). In this state, the transmitter 13 and the receiver 15 transmit and receive the up and the down voice signals to and from the exchange office 45 through the first station 43(1) or BASE A in the manner indicated by thick lines labelled UP/DOWN in Fig. 3 at (1). Signal paths are indicated by thick lines with arrowheads in Fig. 4 at (1). The first transmis zion and reception time slots T1 and R1 are used as so labelled in Fig. 5 at (1).

In the mobile station 11, the first or active call controller 25 always measures an R1 reception level of the down radio signal received in the first reception time slot R1 and informs the second call controller 27 of the R1 reception level.

In the meantime, the second call controller 37 measures reception levels of other electromagnetic waves received in the second through the fourth reception time slots R2, R3, and R4 as R2, R3, and R4 reception levels and compares these reception levels with the R1 reception level delivered from the first call controller 25.

It is now assumed that the mobile station 11 moves from the first zone of the first station 43(1) to the second zone of the second station 43(2) and that the second call controller 27 finds that the R3 reception level is superior to the R1 reception level. The channel switching system is put in the second state (2). That is, the second call controller 27 is rendered active from the standby state. The first call controller 25 is kept active. Rendered active, the second call controller 27 indicates use of the third transmission and reception time slots T3 and R3 to the transmitter circuit 21, to the first call controller 25, to the time division multiplexer 29, and to the radio transmitter 31. It is unnecessary that the radio frequency should be kept identical with the radio frequency used in the state (1).

Link channels are established between the mobile station 11 and the second station 43(2) or BASE B in the manner depicted in Fig. 3 at (2) by a dashed line labelled EST. The link channels are illustrated in Fig. 4 at (2) by dashed lines. In Fig. 5 at (2), the third transmission and reception time slots are indicated with labels T3 and R3 enclosed with parentheses to show the fact that these time slots are not yet in actual use in fresh communication between the mobile station 11 and the second station 43(2) as a continuation of the current communication.

If the R2 and/or the R4 reception levels are equally superior to the R1 reception level, it is possible to establish the link channels to the second station 43(2) in whichever of such other time slots. If none of the other time slots are idle at the second station 43(2), this fact is transmitted from the second station 43(2) to the mobile station 11. If the first transmission and reception time slots are idle at the second station 43(2) and furthermore if at least one of the other time slots is idle at the first station 43(1), the first and the second call controllers 25 and 27 preliminarily switches the first transmission and reception time slots to such at least one of the radio time slots. It is thereby possible to establish the link channels for the fresh communication.

The channel switching system is now put in the third state (3). It is presumed that the link channels are established in the third transmission and reception time slots. In this event, the second call controller 27 makes the transmitter circuit 21 deliver the up voice signal not only to the first call controller 25 and thence eventually to the exchange office 45 but also to the second call controller 27 and thence up to the exchange station 41. This is illustrated in Fig. 3 at (3) by a line labelled UP to the second station BASE B. This is furthermore depicted in Fig. 4 at (3) with addition of arrowheads to thin lines and in Fig. 5 at (3) with the parentheses removed from the label T3. In addition, the second call controller 27 delivers a channel switch request signal to the second station 43(2) as depicted in Fig. 3 at (3) by a line labelled CH SW REQ. Aothough the up voice signal is sent through the second station 43(2) to the exchange station 41 in the third transmission time slot, the time division switch 53 delivers this up voice signal nowhere in response to the up switch control signal produced by the exchange call controller 51 in response to the channel switch request signal supplied through the second station 43(2) and through the base station interface unit 47.

The channel switching system proceeds to the fourth state (4). Receiving the channel switch request signal, the exchange call controller 51 makes the time division switch 53 send the down voice signal not only in the first reception time slot but also in the third reception time slot. Responsive to the down voice signal in the first and the third reception time slots, both the first and the second station 43(1) and 43(2) transmit the down radio signals to the mobile station 11 including the down voice signal. Furthermore, the exchange call controller 51 delivers a channel switch acknowledge signal to the station interface unit 47 in the third reception time slot. In response, the second station 43(2) makes the down radio signal include the channel switch acknowledge signal.The second call controller 27 therefore does not yet make the receiver circuit 19 deal with the down voice signal of the third reception time slot. Only the first call controller 25 makes the receiver circuit 19 process the down voice signal of the first reception time slot. This is illustrated in Fig. 3 at (4) by lines labelled DOWN and labelled CH SW ACK and in Fig. 4 at (4) by thin lines with arrowheads.

The channel switching system further proceeds to the fifth state (5). Receiving the channel switch acknowledge signal in the third reception time slot, the second call controller 27 makes the receiver circuit 19 respond to the down voice signal supplied thereto through the second call controller 27 in place of the down voice signal delivered in the first reception time slot through the first call controller 25. The up voice signal is delivered to the first transmission time slot still through the first call controller 25. The second call controller 27 sends an up channel switch request signal in the third transmission time slot towards the exchange station 41.

In Fig. 3 at (5), the line DOWN extends to the transmitter/ receiver 13/15 and makes the receiver 15 produce a received signal.

The line UP is extended to the first station BASE A and thence to the exchange office 45. The up channel switch request signal is indicated by lines labellep UP 5W REQ. In Fig. 4 at (5), the down voice signal is illustrated by a thin line through the exchange station 41 to the first station 43(1). In Fig. 5 at (5), the first reception time slot is depicted by enclosing the label R1 with parentheses.

The channel switching system is now put in the sixth state (6). Receiving the up channel switch request signal, the exchange call controller 51 makes the time division switch 53 substitute the third transmission time slot for the first transmission time slot. The up and the down voice signals are now transmitted and received between the mobile station 11 and the exchange office 45 through the second station 43(2). The exchange call controller 51 furthermore produces an up channel switch completion signal for supply in the third reception time slot towards the mobile station 11. In Fig. 3 at (6), the up and the down voice signals are illustrated by lines labelled UP/DOWN. The up channel switch completion signal is indicated by a line labelled UP SW COMPL.

In Fig. 4 at (6), delivery of the up channel switch completion signal to the mobile station 11 is indicated by a dashed line.

The channel switching system is put in the seventh state (7). Receiving the up channel switch completion signal in the third reception time slot, the second call controller 27 transfers this up channel switch completion signal to the first call controller 25. In response, the first call controller 25 produces a channel release request signal.in the first transmission time slot for delivery towards the first station 43(1) and thence towards the exchange station 41. This is depicted in Fig. 3 at (7) by a line labelled REL REQ. In r'ig, 4 at (7), this is indicated by another dashed line drawn from the mobile station 11 to the first station 43(1).

The channel switching system proceeds to the eighth state (8). Receiving the channel release request signal, the exchange call controller 51 makes the time division switch 53 stop use of the first reception time slot and produces a release completion signal for supply in the first reception time slot towards the mobile station 11. In Fig. 3 at (8), this is depicted by a line labelled REL COMPL. In Fig. 4 at (8), this is indicated by changing a thin line into a dashed line from the first station 43(1) to the mobile station 11.

The channel switching system is now put eventually in the ninth state (9). Receiving the release completion signal in the first reception time slot, the first call controller 25 is put in the standby state. The ilnk channels are used between the mobile station 11 and the exchange station 41 through the second station 43(2) instead of through the first station 43(1).

This is illustrated in Fig. 3 at (9) by dashed lines labelled CH REL. In Fig. 4 at (9), no arrowheads are attached to lines between the first station 43(1) and the exchange station 41.

In Fig. 5 at (9), indickbd are only the third transmission and reception time slots.

Reviewing Figs. 1 through 5, it is now understood that the mobile station 11 is capable of switching channels between two of the base sations 43 with no instantaneous interruption or break in the communication in progress between the mobile station 11 and the exchange offices 45. The mobile station 11 is compact. No problem arises from a difference in propagation delays in paths through the two base stations* Furthermore, use of time slots in each time division multiple access frame is flexible. This makes it possible to flexibly assign the radio frequencies to the radio zones and to design the radio zones in the service area of the mobile communication network, Incidentally, it is appreciated that the first and the second call controllers 25 and 27 take part in dealing with various control signals, such as origination of a call, interruption of the call, switching of the channels, and so forth. Above all, the controllers 25 and 27 serve to indicate the transmission and the reception time slots, While this invention has thus far beeen described in specific conjunction with a single preferred embodiment thereof, it will now be readily possible for one skilled in the art to carry this invention into effect in various other manners.For example, it is possible to apply this invention to channel switching between two or more mobile stations, such as 11, rather than between a mobile station 11 and an exchange office 45, and to channel switching at the exchange station 41 rather than at the mobile station 11. When the channels should be switched at the exchange station 41, the exchange call controller 5 is implemented by a microprocessor. In addition, reception levels of an electromagnetic wave from the mobile station 11 are compared at two or more of the base stations 43 with the exchange call controller 51 used to provide a result of comparison between the reception levels and to deal with the control signals exemplified above.

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features.

The appended abstract as filed herewith is included in the specification by reference.

Claims

WHAT IS CLAIMED IS:

1. A channel switching method n a mobile communication network comprising a plurality of base stations individually covering a plurality of radio zones, a mobile station movable in said radio zones and capable of communicating with said base stations through a plurality o radio time slots of a time division multiple access frame, and an exchange station connected to said base stations between a first and a second time slot of said radio time slots during movement of said mobile station from a first zone to a second zone of said radio zones and during current communication between said mobile station and said exchange station through a first station covering said first zone among said base stations with a communication quality kept at a predetermined value between said current communication and fresh communication of said mobile station with a second station covering said second zone among said base stations, said method comprising the steps of: simultaneously using first said first and said second time slots with one of said first and said second time slots used in said current communication; and subsequently using the other of said first and said second time slot alone.

2. A channel switching method as claimed in Claim 1, wherein said first time slot is used in said current communication with said second time slot used for said fresh communication on simultaneously using said first and said second time slots if said second time slot is idle for said fresh communication.

3. A channel switching method as claimed in Claim 1, said first time swot being used in said current communication, wherein said second time slot is used, on simultaneously using said first and said second time slots, in said current communication with said first time slot used for said fresh communication if said second time slot is busy at said second station on simultaneously using said first and said second time slots.

4. A channel switching method as claimed in Claim 1, said first time slot being used in said current communication with said second time slot used for said fresh communication, wherein said first and said second time slots are used on switching from said current communication to said fresh communication in transmitting an up communication signal from said mobile station to said exchange station and in receiving a down communication signal at said mobile station from said exchange station, said other of first and second time slots alone being used in transmitting said up communication signal from said.mobile station to said exchange station and also in receiving said down communication signal at said mobile station from said exchange station.

5. A channel switching method as claimed in Claim 4, wherein said simultaneously using step comprises the steps of: simultaneously establishing during said movement said second time slot in addition to said first time slot; first transmitting said up communication signal in said second time slot; and subsequently transmitting said up communication signal in said first time slot.

6. A channel switching system for switching, in a mobile communication network comprising a plurality of base stations individually covering a plurality of radio zones, a mobile station movable in said radio zones and capable of communicationg with said base stations through a plurality of radio time slots a time division multiple access frame, and an exchange station connected to said base stations, between a first and a second time slot of said radio time slots during movement of said mobile station from a first zone to a second zone of said radio zones and during current communication between said mobile station and said exchange station through a first station covering said first zone among said base stations with a communication quality kept at a predetermined value between said current communication and fresh communication of said mobile station with a second station covering said second zone among said base stations, said system comprising in said mobile station:: up selector and transmitter means for transmitting an up communication signal to said exchange station selectively in one or both of said first and said second time slots; down selector and receiver means for receiving a down communication signal from said exchange station in said first and said second time slots as first and second reception signals and for selecting one of said first and said second reception signals as a received signal; and mobile control means for making said up selector and transmitter means select said one or said both of first and second time slots and making said down selector and receiver means select said one of first and second reception signals.

7. A channel switching system as claimed in Claim 6, said one of first and second time slots being used in said current communication, wherein said mobile control means makes said up selector and transmitter means select said both of first and second time slots and again said one of first and second time slots while said mobile station is moving from said first zone to said second zone and when said mobile station is ih said second zone, respectively, the last-mentioned one of first and second time slots being different from the first-mentioned one of first and second time slots.

8. A channel switching system as claimed in Claim 6, wherein said mobile control means makes said up selector and transmitter means select said one of first and second time slots during said current communication and during said fresh communication and said both of first and second time slots during switch from said current communication to said fresh communication and makes said down selector and receiver means select said first reception signal during said current communication and said second reception signal during switch from said current communication to said fresh communication and during said fresh communication.

9. A channel switching system as claimed in Claim 8, wherein said mobile control means makes said up selector and transmitter means select said first time slot during said current communication, said second time slot during said switch in an earlier state, and again said first time slot during said switch in a later state.

10. A channel switching system as claimed in Claim 6, wherein: (Claim 10 continued) said up selector and transmitter means transmits an up control signal to said exchange station in addition to said up communication signal: said exchange station comprising exchange station control means responsive to said up control signal for producing a down control signal for controlling said mobile control means, 11.A mobile station for use in a mobile communication network comprising a plurality of base stations individually covering a plurality of radio zones and an exchange station connected to said base stations, said mobile station being movable in said radio zones and capable of communicating with said base stations through a plurality of radio time slots of a time division multiple access frame, being for use in a channel switching system for switching between a first and a second time slot of said radio time slots during movement from a first zone to a second zone of said radio zones and during current-communication between said mobile station and said exchange station through a first station covering said first zone among said base stations with a communication quality kept at a predetermined value between said current communication and fresh communication of said mobile station with a second station covering said second zone among said base stations, and comprising: : up selector and transmitter means for transmitting an up communication signal to said exchange station selectively in one or both of said first and said second time slots; down selector and receiver means for receiving a down communication signal from said exchange station in said first (Claim 11 continued) and said second time slots as first and second reception signals and for selecting one of said first and said second reception signals as a received signal; and mobile control means for making said up selector and transmitter means select said one or said both of first and second time slots and making said down selector and receiver means select said one of first and second reception signals.

12. A mobile station as claimed in Claim 11, said one of first and second time slots being used in said current communication, wherein said mobile control means makes said up selector and transmitter means select said both of first and second time slots and again said one of first and second time slots while said mobile station is moving from said first zone to said second zone and when said mobile station is in said second zone, respectively, the last-mentioned one of first and second time slots being different from the first-mentioned one of first and second time slots.

13. A mobile station as claimed in Claim 11, wherein said mobile control means makes said up selector-and transmitter means select said one of first and second time slots during said current communication and during said fresh communication and said both of first and second time slots during switch from said current communication to said fresh communication and makes said down selector and receiver means select said first reception signal during said current communication and said second reception signal during switch from said current communication to said fresh communication and during said fresh communication 14, A mobile station as claimed in Claim 13, wherein said mobile control means makes said up selector and transmitter means select said first time slot during said current communication, said second time slot during said switch in at an earlier state, and again said first time slot during said switch at a later state.

15. A mobile station as claimed in Claim 11, wherein: said up selector and transmitter means transmits an up control signal to said exchange station in addition to said up communication signal; said exchange station comprising exchange station control means responsive to said up control signal for producing a down control signal for controlling said mobile control means

16. A channel switching method or a channel switching system substantially as herein described with reference to the accompanying drawings.