Mobile communication system capable of performing soft-handover between base station controllers connected to different mobile communication exchanges

[0002] This invention relates to a mobile communication system and, in particular, to a soft-handover technique between base station controllers of the mobile communication system.

[0003] Conventionally, a handover technique is used in a mobile communication system to prevent the communication from being interrupted when a mobile terminal is moved among cells. A soft-handover technique is known as one type of the handover technique and indispensable to a CDMA communication system.

[0004] In general, the soft-handover technique is applied to each mobile service switching center (or mobile communication exchange) of the mobile communication system. In other words, each mobile service switching center independently performs the soft-handover. Thus, a hard-handover technique is used when the mobile terminal moves from an service area covered by the mobile service switching center to another service area covered by another mobile service switching center. The hard-handover brings about problems, such as instantaneous disconnection of the communication, deterioration of speech quality of the communication, etc.

[0005] A conventional soft-handover technique for performing of the soft-handover between base stations connected to different mobile service switching centers is disclosed in Japanese Unexamined Patent Publication No. Hei 09-505948. The soft-handover technique is used in a mobile communication system which comprises mobile communication exchanges, base stations, and at least one mobile terminal.

[0006] In the mobile communication system, each of mobile communication exchanges can perform the soft-handover both between base stations connected to it and between base stations connected to it and to another mobile communication exchange. Therefore, throughout a whole cellular service area, a seamless soft-handover with a signal diversity synthesis can be achieved between the base stations connected to different mobile communication exchanges.

[0007] For example, it is assumed that the mobile terminal is located in a first cell covered by a first base station connected to a first mobile communication exchange and communicates with the other system user (or terminal) through the first base station and the first mobile communication exchange. The first mobile communication exchange receives user information signals sent from the mobile terminal through the first base station and performs the signal diversity synthesis of the received user information signals to form a synthesized user information signal and to send the other system user the synthesized user information. Under this situation, when the mobile terminal moves from the first cell to a second cell adjacent to the first cell and covered by a second base station connected to a second mobile communication exchange, connection between the mobile station and the second mobile communication exchange is established through the second base station. The second mobile communication exchange relays user information signals, which is sent from the mobile station and which is received by the second base station, to the first communication exchange. The first mobile communication exchange carries out signal diversity synthesis of the user information signals received by the first and the second base station to transmit a new synthesized user information signal to the other system user.

[0008] However, the conventional soft-handover technique can not applied to a larger mobile communication system, such as a mobile communication system having base station controllers between mobile communication exchanges and the base stations.

[0009] Moreover, the conventional soft-handover technique requires a line or channel via the mobile communication exchanges because the handover is performed between the mobile communication exchanges. It is therefore necessary to provide another device for synthesizing speech signal and data signal. This leads to the problem that the system is increased in scale.

[0010] Furthermore, each of the mobile communication exchanges must store cell ID analysis information for the whole system.

[0011] Another conventional soft-handover technique for performing of the soft-handover between base stations connected to different mobile communication exchanges is disclosed in Japanese Unexamined Patent Publication No. Hei 10-145834. The soft-handover technique is applied to a mobile communication system which comprises mobile communication exchanges, base stations, at least one mobile terminal, and lines each of which connects between two base stations adjacent to each other. The lines are logically independent of the mobile communication exchanges.

[0012] In the mobile communication system, traffic information signals are exchanged between two base stations adjacent to each other to carry out the soft-handover when the mobile station moves between two cells covered by the two base stations. The soft-handover can be carried out even if the base stations are connected to different mobile communication exchanges.

[0013] However, the leading base station, which is one of the two base stations and leads the soft-handover, is changed from one to the other with the movement of the mobile station, according to the soft-handover technique. This certainly brings necessity of a hard-handover when the soft-handover is carried out between the base stations connected to the different mobile communication exchanges. Consequently, the soft-handover can hardly decrease frequency of the hard-handover.

[0014] It is therefore an object of this invention to provide a mobile communication system capable of achieving soft-handover between base station controllers connected to different mobile communication exchanges.

[0015] It is another object of this invention to provide a mobile communication system capable of achieving soft-handover without increasing the scale of the system and without storing cell ID analysis information of the whole system in each of base station controllers.

[0016] It is still another object of this invention to provide a mobile communication system capable of decreasing frequency of hard-handover.

[0017] Other object of this invention will become clear as the description proceeds.

[0018] On describing the gist of this invention, it is possible to understand that a mobile communication system includes a plurality of base stations having cells for being connected to a mobile terminal by radio and a plurality of base station controllers connected between the base stations and a mobile exchange.

[0019] According to the gist of this invention, the mobile communication system comprises control signal lines connected among the base station controllers for translating control signals to perform soft-handover and to put concerning base station controllers into a soft-handover state regardless of the mobile exchange when the mobile terminal moves between cells. Connection lines is connected among the base station controllers for keeping communication between the mobile terminal and another terminal in the soft-handover state. Each of the base station controllers keep transmitting/receiving communication signals to/from the mobile exchange regardless location of the mobile terminal, when the communication starts through it.

[0020] In the mobile communication system, the base stations broadcast predetermine radio waves over the cells, respectively. The mobile terminal detects the predetermined radio waves to produce a field strength report signal representative of the predetermined radio waves. Each of the base station controllers comprises a soft-handover control section to perform the soft-handover on the basis of the field strength report signal. A conversion section converts the communication signals between a first type for the base stations and a second type for the mobile exchange. A switching section is connected to the soft-handover control section, the conversion section, predetermined base stations of the base stations, the predetermined connections lines of the base stations, and predetermined control signal lines of the control signal lines for switching paths among them.

[0021] According to another gist of this invention, a method of performing a soft-handover in a mobile communication system comprises the steps of receiving at a first base station controller a field strength report signal transmitted by a mobile terminal which communicates with another terminal through the first base station controller. A step of establishing establishes a connection on a connection line connected between the first base station controller and a second base station controller which is connected to a base station indicated by the field strength report signal.

[0022] According to still another gist of this invention, a mobile communication system includes first and second base stations having first and second cells adjacent to each other, respectively. The first and the second base stations are connected to a mobile terminal by radio. First and second base station controllers is connected to the first and the second base stations, respectively. The first and the second base station controllers connect the first and the second base stations with a first mobile exchange. The mobile terminal transmits/receives communication signals to/from another terminal through the first base station, the first base station controller, and the first mobile exchange when the mobile terminal locates in the first cell. The mobile communication system comprises a control signal line connected between the first base station controller and the second base station controller to perform soft-handover between the first base station controller and the second base station controller without respect to the first mobile exchange and to put the first base and the second base station controllers into a soft-handover state when the mobile terminal moves from the first cell to the second cell. A connection line is connected between the first base station controller and the second base station controller to translate the communication signals between the first base station controller and the second base station controller in the soft-handover state and to keep the first base station signal controller transmitting/receiving the communication signals to/from the first mobile exchange.

[0023] In the mobile communication system, the first and the second base stations periodically broadcast predetermined radio waves. The mobile terminal detects electric field strength of the predetermined radio waves to produce a field strength report signal representative of the electric field strength. Each of the first base station controllers comprises a soft-handover control section to perform the soft-handover on the basis of the field strength report signal. A conversion section converts the communication signals between a first type for the first and the second base stations and a second type for the first mobile exchange. A switching section is connected to the soft-handover control section, the conversion section, the first or the second base station, the connections line, and the control signal line to switch paths among them.

[0024] According to further still another gist of this invention, a mobile communication system includes a plurality of mobile terminals, a plurality of base stations each of which has a cell and is connected to each of the mobile terminals through radio communication wave, a plurality of base station controllers each of which is connected to predetermined base stations of the base stations through communication lines to carry out transmission and reception of control signals, speech signals, and data signals to and from the predetermined base stations through the communication lines, and a mobile communication exchange which is connected to each of the base station controllers through a communication line and which controls exchanging operation with the base station controllers, the system performing soft-handover between the base station controllers when the mobile terminal is moved between two cells. The mobile communication system comprises a connection line as a communication line and a control signal line for transmission of control signals both of which connect the base station controllers.

[0025] In the mobile communication system, each of the base station controllers comprising a soft-handover control section which establishes the connection line with reference to a received electric field strength at said mobile terminal. A QCELP/PCM conversion section synthesizes QCELP signals of the speech signals and the data signals transmitted/received to and from the mobile terminal via the base station to produce a synthesized signal and converting the synthesized signal into a PCM signal. A switching section carries out path-connection between the communication line connected to the base station, the soft-handover control section, the QCELP/PCM conversion section, the connection line, and the control signal line. The soft-handover control section is connected to a soft-handover control section in another base station controller through the switching section and the control signal line.

[0026] FIG. 1 is a block diagram of a mobile communication system according to an embodiment of the present invention;

[0027] FIG. 2 is a signal sequence chart for describing soft-handover processing in the mobile communication system of FIG. 1;

[0028] FIG. 3 is a signal sequence chart for describing soft-handover processing in the mobile communication system of FIG. 1;

[0029] FIG. 4 is a signal sequence chart for describing soft-handover processing in the mobile communication system of FIG. 1; and

[0030] FIG. 5 is a block diagram of a mobile communication system according to another embodiment of the present invention.

[0031] FIG. 6 is a block diagram of a mobile communication system according to still another embodiment of the present invention.

[0032] FIG. 7 is a block diagram of a mobile communication system according to further still another embodiment of the present invention.

[0033] Referring to FIG. 1 through 4, the description will proceed to a mobile communication system according to one embodiment of this invention.

[0034] FIG. 1 is a block diagram of a mobile communication system according to an embodiment of the present invention.

[0035] Referring to FIG. 1, the mobile communication system comprises a plurality of mobile terminals 10aand 10b. Base stations 20ato 20dhave cells 22ato 22d, respectively. Each of the base stations 20ato 20dcan be connected to each of the mobile terminals 10aand 10bthrough a radio communication wave when each of the mobile terminals 10aand 10blocates in its cell. The cell 22ais adjacent to the cell 22band these cells have a common area. The cell 22bis also adjacent to the cell 22cand these cells have another common area. Moreover, the cell 22cis adjacent to the cell 22dand these cells has sill another common area.

[0036] A base station controller 40ais connected to the base stations 20aand 20bthrough communication lines 30aand 30b, respectively. The communication lines 30aand 30bare used for transmission and reception of control signals, speech signals, and data signals between the base station controller 40aand the base stations 20aand 20b, respectively.

[0037] Similarly, a base station controller 40bis connected to the base stations 20cand 20dthrough communication lines 30cand 30d, respectively. The communication lines 30cand 30dis used for transmission and reception of control signals, speech signals, and data signals between the base station controller 40band the base stations 20cand 20d, respectively.

[0038] A mobile communication exchange 60 is connected to the base station controllers 40aand 40bthrough communication lines 50aand 50b, respectively. The mobile communication exchange 60 controls the exchange between the base station controllers 40aand 40b, between the base station controller 40aor 40band another mobile communication exchange (not shown), and between the base station controller 40aor 40band another network (not shown).

[0039] The base station controller 40aand the base station controller 40bare connected to each other via connection lines 70aand 70bas communication lines and via a control signal line 80 through which the control signals are transmitted and received.

[0040] The base station controller 40acomprises a soft-handover control section 41a, a QCELP/PCM (Qualcomm Code-Excited Linear Prediction/Pulse Code Modulation) conversion section 42a, and a switching section 43awhich is connected to both the soft-handover control section 41aand the QCELP/PCM conversion section 42a. The switching section 43ais connected to the communication lines 30aand 30b, the connection lines 70aand 70b, and the control signal line 80.

[0041] The switching section 43aswitches paths among the communication lines 30aand 30b, the connection lines 70aand 70b, the control signal line 80, the soft-handover control section 41a, and QCELP/PCM conversion selection 42a. For example, the switching section 43amay selectively connect the communication line 30ato the QCELP/PCM conversion section 42aor the connection line 70a. The switching section 43amay selectively connect the communication line 30bto the QCELP/PCM conversion section 42aor the connection line 70b. Moreover, the switching section 43amay connect the connection lines 70aand 70bto the QCELP/PCM conversion section 42a. In addition, the switching section 43amay connect the soft-handover control section 41ato the control signal line 80 if it carry out as the occasion demands. In FIG. 1, the switching section 43aconnects the communication lines 30aand 30band the connection lines 70aand 70bto the QCELP/PCM conversion section 42aand connects the soft-handover control section 41ato the control signal line 80.

[0042] Similarly, the base station controller 40bcomprises a soft-handover control section 41b, a QCELP/PCM conversion section 42b, and a switching section 43bwhich is connected to both the soft-handover control section 41band the QCELP/PCM conversion section 42b. The switching section 43bis connected to the communication lines 30cand 30d, the connection lines 70aand 70b, and the control signal line 80.

[0043] The switching section 43bswitches paths among the communication lines 30cand 30d, the connection lines 70aand 70b, the control signal line 80, the soft-handover control section 41b, and QCELP/PCM conversion selection 42b. For example, the switching section 43bmay selectively connect the communication line 30cto the QCELP/PCM conversion section 42bor the connection line 70b. The switching section 43dmay selectively connect the communication line 30cto the QCELP/PCM conversion section 42bor the connection line 70b. Furthermore, the switching section 43bmay connect the connection lines 70aand 70bto the QCELP/PCM conversion section 42b. In addition, the switching section 43bmay connect the soft-handover control section 41bto the control signal line 80 if it carry out as the occasion demands. In FIG. 1, the switching section 43bconnects the communication lines 30cand 30dto the connection lines 70aand 70b, respectively, and connects the soft-handover control section 41bto the control signal line 80.

[0044] Hereinbelow, the description will be made about the soft-handover processing in the mobile communication system having the above-mentioned structure.

[0045] At first, during communication with one of the base stations 20a-20d, the mobile terminal 10acontinuously watches predetermined radio waves, such as pilot signals, transmitted from all of the base stations 20a-20d. When the mobile terminal 10areceives the stronger radio wave than that sent from the base station communicating with it, field strength report information representative of the electric field strength measured at the mobile terminal 10ais reported through the base station communicating with it to the base station controller 40aor 40bconnected to the base station communicating with it.

[0046] Next, the mobile terminal 10areceives a soft-handover execution request signal from the base station controller 40aor 40b. In this event, the mobile terminal 10atransmits a soft-handover execution notice signal through the communicating base station to the base station controller 40aor 40b.

[0047] Then, the base station controller 40aor 40bperforms soft-handover with respect to a cell ID in the soft-handover execution notice signal. The communication is kept by two of the base stations 20a-20d.

[0048] In order to disconnect the communication, the mobile terminal 10atransmits a disconnection request signal through the communicating base station(s) to the base station controller 40aor 40b. The base station controller 40aor 40breceives the disconnection request signal. Then, the base station controller 40aor 40btransmits a disconnection response signal through the communicating base station(s) to the mobile terminal 10a. The communication comes to an end at the time when the disconnection response signal is received at the mobile terminal 10a. When the disconnection request signal is transmitted from the mobile communication exchange 60 to the mobile terminal 10athrough the base station controller 40aor 40band the communicating base station(s), the mobile terminal 10awhich has received the disconnection request signal transmits the disconnection response signal to bring the communication to an end.

[0049] During the communication, the communicating base station(s) transmits/receives control signals and speech or data signals in a radio zone between the communicating base station(s) and the mobile terminal 10aand to/from the communication line 30a, 30b, 30c, or 30dbetween the communicating base station(s) and the base station controller 40aor 40b.

[0050] Each of the base station controllers 40aand 40bestablishes a radio channel by the use of a channel establishment request signal which is transmitted to the base station(s) 20a-20dand releases the radio channel by the use of a channel release request signal. In the base station controller 40a, communication signals transmitted from the communicating base stations are supplied to the QCELP/PCM conversion section 42athrough the switching section(s) 43a(and 43b). The QCELP/PCM conversion section 42asynthesizes the speech signals or the data signals supplied thereto to produce a synthesized signal which is transmitted to the mobile communication exchange 60 connected thereto.

[0051] Thereafter, the communication signal transmitted from the mobile communication exchange 60 is supplied to the QCELP/PCM conversion section 42a. The QCELP/PCM conversion section 42adistributes the speech signal or the data signal which is transmitted through the switching section(s) 43a(and 43b)to the communicating base stations.

[0052] The soft-handover control section 41astores cell ID information including cell IDs assigned to the base stations 20aand 20btogether with related cell IDs assigned to neighboring base stations which are neighboring to each of the base station 20aand 20b. It is unnecessary that the neighboring base stations are connected to the base station controller 40a. Similarly the soft-handover control section 41bstores cell ID information including cell IDs assigned to the base stations 20cand 20dtogether with related cell IDs assigned to neighboring base stations which are neighboring to each of the base station 20cand 20d. It is unnecessary that the neighboring base stations are connected to the base station controller 40b.

[0053] The electric field strength report measured at the mobile terminal 10aand transmitted therefrom is analyzed at the base station controllers 40aor 40b. When the cell at which the mobile terminal 10ais currently positioned is the cell of the base station under the neighboring base station controller, the soft-handover between the base station controllers is started.

[0054] The switching sections 43aand 43bcarry out connection of the connection lines 70aand 70b.

[0055] Then, the electric field strength report signal transmitted from the mobile terminal 10ais received by the base station controller 40a. At the same time, the base stations 20a-20dare selectively supplied through the communication lines 30a-30dwith the channel establishment/channel release request signal.

[0056] The base station controller 40aas an origination of the soft-handover performs, through the control signal line 80 for the control signal to/from the base station controller 40b, transmission of a request signal for starting the soft-handover between the base station controllers, transmission of a request signal for the cell ID analysis, transmission of a request signal for channel establishment for a relevant base station, transmission of a request signal for ending the soft-handover between the base station controllers, reception of a start response signal, reception of a cell ID analysis response signal, reception of a channel establishment response signal, and reception of end response signal from the base station controller 40b.

[0057] On the other hand, the base station controller 40bas a destination of the soft-handover performs, through the control signal line 80 for the control signal to/from the base station controller 40a, reception of the start request signal of the soft-handover, reception of the cell ID analysis request signal, reception of the channel establishment request signal of the relevant base station, reception of the end request signal of the soft-handover between the base station controllers, transmission of the start response signal to the base station controller 40aas the origination of the soft-handover, transmission of the cell ID analysis response signal, transmission of the channel establishment response signal, and transmission of the end response signal to the base station controller 40a.

[0058] Thereafter, the mobile communication exchange 60 performs mobile communication exchange of the speech signal and the data signal to/from the base station controllers 40aand 40b, or connection to the base station controllers 40aand 40band to other exchange networks.

[0059] Now, description will be made in detail about the above-mentioned soft-handover in conjunction with the case where the mobile terminal 10amoves from the cell 22ato the cell 22b.

[0060] FIG. 2 is a view for explaining the soft-handover process performed at the mobile communication system shown in FIG. 1 in case where the mobile terminal 10ais moving from the cell 22ato the cell 22b.

[0061] When the mobile terminal 10aperforms communication within the cell 22a, the communication is carried out by using of the base station 20a, the communication line 30a, the switching section 43a, the QCELP/PCM conversion section 42a, and the mobile communication exchange 60.

[0062] When the mobile terminal 10amoves toward the cell 22b, the radio wave (or the pilot signal) transmitted from each of the base stations 20band 20cand received the mobile terminal 10abecomes strong. In other words, the mobile terminal 10areceives the radio wave transmitted from each of the base stations 20band 20cwith high level of electric field strength. The electric field strength is measured and a report of the measured electric field strength is reported to the base station 20aby radio (step S1). The report signal includes the cell IDs of the base stations 20band 20cexcept for the cell ID of the base station 20a.

[0063] The report signal reported to the base station 20ais also reported through the communication line 30ato the base station controller 40a(step S2).

[0064] Then, the base station controller 40aanalyzes the report signal and detects the cell IDs. One of the cell IDs is coincident with the cell ID of the base station 20band another one is coincident with the cell ID of the base station 20ccontrolled by the base station controller 40bconnected to the base station controller 40athrough the control signal line 80.

[0065] Next, the soft-handover control section 41awithin the base station controller 40ais started and transmits through the control signal line 80 to the base station controller 40bthe request signal for starting the soft-handover between the base station controllers (step S3). Hereafter, transmission and reception of the control signals between the base station controller 40aand the base station controller 40bare all carried out through the control signal line 80.

[0066] When the base station controller 40breceives the request signal for starting the soft-handover between the base station controllers, the soft-handover control section 41bin the base station controller 40bis started and linking of the control signals to be transmitted/received is carried out.

[0067] Thereafter, the base station controller 40btransmits through the control signal line 80 to the base station controller 40athe response signal to the request signal for starting the soft-handover between the base station controllers (step S4).

[0068] When the base station controller 40areceives the response signal to the request signal for starting the soft-handover between the base station controllers, it transmits the cell ID analysis request signal to the base station controller 40b(step S5).

[0069] When the base station controller 40breceives the cell ID analysis request signal, it analyzes the cell ID. Then, the cell ID of the base station 20cas the result of the analysis is transmitted to the base station controller 40atogether with the cell ID analysis response signal (step S6).

[0070] When the base station controller 40areceives the cell ID analysis response signal, it transmits the channel establishment request signal to the base station 20b(step S7).

[0071] The base station controller 40bis also supplied with the channel establishment request signal (step S8).

[0072] When the base station controller 40breceives the channel establishment request signal, it supplies the base station 20cwith the channel establishment request signal (step S9).

[0073] Next, the switching section 43bin the base station controller 40bcarries out the path connection between the connection line 70a, which is formed between this base station controller and the base station controller 40a, and the communication line 30c. Then, the channel establishment response signal is transmitted to the base station controller 40a(step S10).

[0074] When the base station controller 40areceives the channel establishment response signal, the switching section 43ain the base station controller 40acarries out the path establishment between the connection line 70aand the QCELP/PCM conversion section 42a.

[0075] Thereafter, the base station controller 40atransmits through the communication line 30ato the base station 20athe soft-handover execution request signal carrying the cell ID of each of the base stations 20band 20cwhich became available in communication (step S11).

[0076] Thereafter, the soft-handover execution request signal transmitted to the base station 20ais sent from the base station 20ato the mobile terminal 10aby radio (step S12).

[0077] When the mobile terminal 10areceives the soft-handover execution request signal, it transmits a soft-handover execution notification signal to the base station 20a(step S13).

[0078] After the soft-handover execution notification signal is transmitted from the mobile terminal 10a, the mobile terminal 10aperforms the soft-handover to the cell ID designated by the soft-handover execution request signal. At this time instant, the mobile terminal 10ais put into a soft-handover state communication lines through the base station 20b—the communication line 30band through the base station 20c—the communication line 30c—the switching section 43c—the connection line 70a.

[0079] When the base station 20areceives the soft-handover execution notification signal, it transmits the soft-handover execution notification signal through the communication line 30ato the base station controller 40a(step S14).

[0080] Thereafter, the base station controller 40asends a channel release request signal through the communication line 30 to the base station 20ain order to release the channel of the base station 20awhich is no longer used (step S15).

[0081] Then, the switching section 43ain the base station 20areleases the path between the communication line 30aand the QCELP/PCM conversion section 42a. Thus, in the base station 20a, the channel to which the release request signal is addressed is released.

[0082] Hereinbelow, description will be made about the processing where the mobile terminal 10ais moved from the cell 22bto the cell 22d.

[0083] FIG. 3 is a view for describing the soft-handover processing in the mobile communication system shown in FIG. 1 in case where the mobile terminal 10ais moved from the cell 22btoward the cell 22d.

[0084] When the mobile terminal 10amoves into the cell 22c, it receives the radio wave transmitted from each of the base stations 20band 20cwith high level of electric field strength and the electric field strength is measured. However, since the cell IDs added to the report signal of the measured electric field strength are not changed from the current cell IDs, the communication is continued without reporting the cell IDs.

[0085] When the mobile terminal 10amoves into the cell 22dand leaves form the cell 22c, it receives the radio wave transmitted from the base station 20dwith high level of electric field strength. The electric field strength is measured and the measured electric field strength is reported to the base stations 20band 20cby radio (step S101).

[0086] The report signal of the electric field strength reported to the base station 20bis also reported through the communication line 30bto the base station controller 40awhile the report signal of the electric field strength reported to the base station 20cis also reported through the communication line 30cand the control signal line 80 to the base station controller 40a(step S102).

[0087] Then, the base station controller 40aanalyzes the report signal to detect that the cell ID is not coincident with the cell ID of the base station controller 40abut is coincident with the cell ID of the base station on the base station controller 40b.

[0088] Then, the base station controller 40atransmits the cell ID analysis request signal to the base station controller 40b(step S103).

[0089] When the base station controller 40breceives the cell ID analysis request signal, it analyzes the cell ID. Then, the cell ID of the base station 20das the result of the analysis is transmitted to the base station controller 40atogether with the cell ID analysis response signal (step S104).

[0090] When the base station controller 40areceives the cell ID analysis response signal, it transmits the channel establishment request signal to the base station 40b(step S105).

[0091] When the base station controller 40breceives the channel establishment request signal, it supplies the base station 20dwith the channel establishment request signal (step S107).

[0092] Next, the switching section 43bin the base station controller 40bcarries out the path establishment for the connection line 70b, which is formed between this base station controller and the base station controller 40a, and the communication line 30d. Then, the channel establishment response signal is transmitted to the base station controller 40a(step S107).

[0093] When the base station controller 40areceives the channel establishment response signal, the switching sections 43ain the base station controller 40acarries out path establishment for the connection line 70band the QCELP/PCM conversion section 42a.

[0094] Thereafter, the base station controller 40atransmits through the communication channel 30band the control signal line 80 to the base stations 20band 20c, respectively, the soft-handover execution request signal carrying the cell ID of the base station 20dwhich has become available in communication (step S108).

[0095] The soft-handover execution request signal transmitted to each of the base stations 20band 20cis sent from the base stations 20band 20cto the mobile terminal 10aby radio (step S109).

[0096] When the mobile terminal 10areceives the soft-hand-over execution request signal, it transmits a soft-hand-over execution notice signal to each of the base stations 20band 20c(step S110).

[0097] After the soft-handover execution notice signal is transmitted from the mobile terminal 10a, the mobile terminal 10aperforms the soft-handover upon the cell ID designated by the soft-handover execution request signal. At this time instant, the mobile terminal 10ais put into a soft-handover state having a communication line through the base station 20d—the communication line 30d—the switching section 43b—the connection line 70bto the base station controller 40a.

[0098] When the base stations 20band 20dreceive the soft-handover execution notice signal, this notice signal is transmitted to the base station controller 40afrom the base station 20bthrough the communication line 30band from the base station 20dthrough the communication line 30b, the soft-handover control section 41b, and the control signal line 80 (step S111).

[0099] Thereafter, the base station controller 40asends the channel release request signal through the communication line 30bto the base station 20bin order to release the channel of the base station 20bwhich is no longer used (step S112).

[0100] Then, the switching section 43ain the base station controller 40areleases the paths between the QCELP/PCM conversion section 42aand the communication line 30band through the QCELP/PCM conversion section 42aand the connection line 70a. Thus, in the base station 20b, the channel to which the release request signal is addressed is released.

[0101] Thereafter, the base station controller 40asends a channel release request signal to the base station controller 40b(step S113).

[0102] When the base station controller 40breceives the channel release request signal, it transmits the channel release request signal through the communication line 30cto the base station 20c.

[0103] Then, the switching section 43bin the base station controller 40breleases the path between the connection line 70aand the communication line 30c. Accordingly, in the base station 20c, the channel to which the channel release request signal is addressed is released. The base station controller 40btransmits the channel release response signal to the base station controller 40a(step S115).

[0104] As described above, when the QCELP/PCM conversion section 42ain the base station controller 40ais used and if the base station under the control of the base station controller 40ais no longer used, the communication can be carried out without switching the QCELP/PCM conversion section 42a.

[0105] Hereinbelow, description will be made about the processing where the mobile terminal 10afinishes the communication in the cell 22d.

[0106] FIG. 4 is a view for describing the processing in the mobile communication system shown in FIG. 1 in case where the mobile terminal 10afinishes the communication in the cell 22d.

[0107] When the mobile terminal 10afinishes the communication in the cell 22d, it at first transmits the disconnection request signal to the base station 20d(step S201).

[0108] When the base station 20dreceives the disconnection request signal, it transmits the disconnection request signal to the base station controller 40a(step S202).

[0109] When the base station controller 40areceives the disconnection request signal, it transmits the disconnection request signal to the mobile communication exchange 60 (step S203). Simultaneously, a disconnection response signal is transmitted through the control signal line 80, the soft-handover control section 41b, and the communication line 30dfrom the base station controller 40ato the base station 20d(step S204).

[0110] The disconnection response signal transmitted to the base station 20dis transmitted to the mobile terminal 10aby radio (step S205). At this stage, the mobile terminal 10afinishes the communication and transmission/reception for the communication.

[0111] Next, the switching section 43ain the base station controller 40areleases the path between the QCELP/PCM conversion section 42aand the connection line 70b. The channel release request signal is transmitted from the base station controller 40ato the base station controller 40b(step S207).

[0112] When the base station controller 40breceives the channel release request signal, it transmits the channel release request signal to the base station 20d(step S208). The switching section 43bin the base station controller 40breleases the path between the connection line 70band the communication line 30d.

[0113] After the path is released, the base station controller 40btransmits the channel release response signal to the base station controller 40a(step S209).

[0114] Thereafter, when all the communication resources are released, the base station controller 40atransmits the end request signal for ending the soft-handover between the base station controllers to the base station controller 40b(step S210).

[0115] When the base station controller 40breceives the end request signal, the base station controller 40bfinishes the linking of the control signals upon the transmission/reception already performed. The end response signal is transmitted to the base station controller 40a(step S211). Thus, the soft-handover processing between the base station controllers is brought to an end.

[0116] The above-described soft-handover processing comprising a series of steps may be written in a recording medium, such as an ROM, which is provided in each of the soft-handover control sections 41aand 41b.

[0117] As mentioned above, the mobile communication system has the connection lines and the control signal line which are arranged between a plurality of base station controllers. In the soft-handover control section arranged within each base station controller, the connection line is established with reference to the received electric field strength in the mobile terminal. The switching section in the base station controller performs a path-connection among the communication line, the soft-handover control section, the QCELP/PCM conversion section, the connection line, and the control signal line. The soft-handover control section is connected to another soft-handover control section in the other base station controller through the switching section and the control signal line, and establishes the connection line to the base station controller connected to the base station which has transmitted the radio wave received at the mobile terminal with high levels electric field strength.

[0118] As described above, the soft-handover processing is carried out by the signal transmission among the base station controllers. Therefore, it is not necessary for each of the base station controllers to have the cell ID analysis information of the whole system. The soft-handover is realized by having the analysis information of only the base station controller itself and the neighboring base station. In addition, by providing the control signal line, the connections lines between the base station controllers, it is not necessary to provide the mobile communication exchange with new lines or to additionally provide a device for synthesis of speech/data signals between the base station controllers through the mobile communication exchange.

[0119] FIG. 5 is a view showing a communication system according to another embodiment of the present invention. The structure illustrated in the figure includes a single soft-handover origination (41b) and two soft-handover destinations (41a, 41c).

[0120] The embodiment illustrated in FIG. 1 has a structure including a single soft-handover origination and a single soft-handover destination. In case where a single soft-handover origination and two soft-handover destinations are included as shown in FIG. 5, similar processing is carried out.

[0121] It is assume that the mobile terminal 10astart on communicating with another terminal (or a system user) through the base station controller. When the QCELP/PCM conversion section 42ais used and if the base station under the control of the base station controller 40ais no longer used, the communication can be carried out without switching the QCELP/PCM conversion section 42a. This is true even if the mobile terminal moves into a cell 22eand/or 22f. This is because the communication is kept through a base station(s) 20eor (or and) 20f, a switching section 43c, a connection line(s) 70cor (or and) 70d, the switching section 43b, the connection line(s) 70aor (or and) 70b.

[0122] The present invention having the above-described structure has the following effects.

[0123] The connection line is established between the base station controllers and a single QCELP/PCM conversion section is used without being switched. Therefore, the soft-handover between the base station controllers may be carried out without interruption.

[0124] The synthesizing process is performed on the ordinary operation in the base station controller without changing the operation of the mobile communication exchange. Therefore, it is not necessary to provide a new device for performing the combining process.

[0125] The cell ID is analyzed at a particular base station controller which controls the current position of the mobile terminal. Therefore, it is not necessary for each of the base station controllers to have the cell ID analysis information of the whole system. The soft-handover processing can be carried out by holding only the cell ID analysis information which is controlled by the base station controller itself and the analysis information of the neighboring base station in order to transmit/receive the cell ID analysis request signals.

[0126] While this invention has thus for been described in conjunction with the preferred embodiments thereof, it will readily be possible for those skilled in the art to put this invention into practice in various other manners. For example, the base station controller 40b(of FIG. 1 or 5) may be connected to another mobile communication exchange different from the mobile communication exchange 60 as shown in FIG. 6. Similarly, the base station controller 40c(of FIG. 5) may be connected to still another mobile communication exchange different form both of the mobile communication exchange 60 and the other mobile communication exchange connected to the base station controller 40bas shown in FIG. 7. Moreover, the mobile communication system may have four or more base station controllers. In this case, each of the base station controllers is connected to other base station controllers each of which cover a cell adjacent to a cell covered by it.