Handoff method for cellular digital mobile communication system and mobile station

Abstract: According to a handoff method for a cellular digital mobile communication system, a communication test of a new channel is performed by utilizing a free time of a time-divisionally multiplexed channel. A mobile station of this invention includes a communication circuit for performing communication by using a time-divisionally multiplexed channel, a switching circuit for connecting a new channel during a free time of the time-divisionally multiplexed channel, and a communication test circuit for performing a communication test when the switching circuit is connected to the new channel.

Description: The present invention relates to a handoff (busy channel switching) method for a cellular digital mobile communication system constituted by small radio zones (cellular system), to a mobile station in such a system, and to a cellular digital mobile communication system. A service area of the cellular system is divided into a large number of radio zones (cell sites), and identical frequencies are utilized again in remote cellsites to increase the number of mobile telephone sets per frequency. The number of potential users of mobile telephones is larger than the actual number of users covered by the existing systems. Therefore, strong demand has arisen for a large-capacity system. The size of the cell site may be reduced to increase the system capacity within the limited frequency band. For example, if the radius of the cell site is reduced to 1/10, frequency utilization efficiency can be increased to 100 times. When the size of the cell site is minimized, however, the frequency of communication channel switching, i.e., a handoff is increased since a mobile station is moved from one site to another during communication. A handoff sequence of an existing system is described in Bell System Technical Journal Vol. 58, No. 1, January 1979, P. 65. Upon designation of a new cell site channel, a mobile station interrupts communication via the present channel, and the channel is then switched to a new channel. In this state, the mobile station performs a communication test and restarts communication on the new channel. According to this method, communication is interrupted for 0.7 to 0.8 sec during the communication test. Since the communication is interrupted for 0.7 to 0.8 sec according to the conventional handoff method, communication quality is greatly degraded when the channels are frequently switched upon an increase in the number of cell sites. In addition, in the conventional handoff method, detection of movement of a mobile station between cell sites is concentrated on a base station and a radio channel control station. When the size of the cell site is minimized, the handoff operation must be frequently performed, and a control amount is locally increased, thereby requiring a large-capacity apparatus. WO-A-87 05458 relates to a method of operating a radio transmission or communication system including a central station and a plurality of individual remote stations. In this method, all the control signals are generated and transmitted by the central station, thus locally concentrating the control amount at the central station. Each control signal includes a first part constituting a first period of time of the control signal and including information identifying one or more intended receiving stations, and a second part constituting a second period of time of the control signal and including control information for the intended receiving station or stations. Each individual remote station is shiftable between the first operational mode in which it receives the control signal from the central station to which it is assigned, and a second operational mode in which it is not receiving the control signal. During the first period of time, each remote station is operated in the first operational mode in order to evaluate whether it is an intended receiving station. For the second period of time, each remote station which is not an intended receiving station is operated in the second operational mode for receiving a control signal from an alternative central station, whereas each remote station which is an intended receiving station is operated in the first operational mode for receiving the second part of the control signal. Thus, only those stations which are not intended receiving stations of the signals from the original central station, i.e. those remote stations which are not communicating with the original central station, receive a control signal from an alternative central station. EP-A-0 145 098 relates to a bidirectional radio communication method between base stations and mobile stations, and discloses a hand-off method for a cellular digital mobile communication system, wherein each base station transmits a signal representing presence/absence of a free communication channel, and a mobile station causes a level monitor to receive the signals transmitted from the base station and a base station adjacent thereto, to compare reception levels of the base stations having free communication channels to monitor a change in cell site. It is an object of the present invention to provide a handoff method substantially free from an interruption time and in which the amount of control is not locally concentrated, and a mobile station for realizing the method. This object is achieved by the handoff method and the mobile station according to the Claims. In the handoff method of a cellular digital mobile communication system according to the present invention, a free time of a time-divisionally multiplexed channel is utilized to perform a communication test of a new channel, as defined in Claim 1. The mobile station of the present invention is defined in Claim 2. The invention is described in detail in connection with the drawings in which Fig. 1 is a view showing allocation of cell sites of a cellular digital mobile communication system which employs the present invention; Figs. 2A, 2B and 2C are views showing frame formats of channels of each cell site according to an embodiment of a handoff method of the present invention; Fig. 3 is a view showing a handoff sequence using the frame formats shown in Figs. 2A to 2C; Fig. 4 is a view showing another handoff sequence using the frame formats shown in Figs. 2A to 2C; Fig. 5 is a block diagram of a mobile station for realizing the handoff sequences of Figs. 3 and 4; Figs. 6A, 6B, 7A, and 7B are views showing frame formats of channels according to another embodiment of the handoff method of the present invention; Fig. 8 is a flow chart showing a handoff sequence using the channel frame formats shown in Figs. 6A to 7B; Fig. 9 is a flow chart showing another handoff sequence using the channel frame formats shown in Figs. 6A to 7B; Fig. 10 is a block diagram showing a mobile station for realizing the handoff sequences of Figs. 8 and 9; Fig. 11 is a view showing a system employing still another embodiment of a handoff method of the present invention; Fig. 12 is a view showing a time division operation of the carrier frequency in the handoff method practiced by the system shown in Fig. 11; Fig. 13 is a view showing assignment of time-divisional carrier frequencies shown in Fig. 12 to cell sites; Fig. 14 is a view showing a handoff sequence executed according to assignment of the carrier frequencies shown in Figs. 12 and 13; Fig. 15 is a view showing another time division operation of carrier frequencies in the handoff method practiced by the system shown in Fig. 11; Fig. 16 is a view showing assignment of the time-divisional carrier frequencies (Fig. 15) to cell sites. Preferred embodiments of the present invention will be described with reference to the accompanying drawings. Fig. 1 shows allocation of base stations of a cellular digital mobile communication system which employs a handoff method, i.e., a method of switching communication channels (to be referred to as channels hereinafter). Referring to Fig. 1, a coverage area of the system constitutes a grid consisting of radio zones, i.e., cell sites of a large number of base stations 1. Fig. 2A shows a format assigned to a base station A, Fig. 2B shows a format assigned to base stations B, D, and F, and Fig. 2C shows a format assigned to base stations C, E and G. In the channel format of the base station 1, a frame having a length of 8T (to be referred to as an 8T frame hereinafter) constituted by time slots each having a length of 2T is repeated twice, and a frame having a length of 4T (to be referred to as a 4T frame hereinafter) of time slots each having a length of T is repeated eight times. When the frame format is assigned to the base station 1 as described above, while the 8T frame is repeated twice, the adjacent base stations repeat the 4T frame. Therefore, all channel switching operations of the base stations can be completed within the 6T free time which is available during repetition of the 8T frame twice. Assume that a new channel B3 is designated for a mobile station which is moving by using a channel A1 at present. In this case, the mobile station switches the present channel to the channel B3, performs a communication test with the base station B, and then returns to the channel A1 when the first half of the continuous 2T time slots of the channel A1 is completed. When the communication test is not completed within the time slot, the communication test can be performed using a plurality of time slots. With this arrangement, the mobile station can continuously perform communication via the channel A1 and at the same time the communication test of the newly designated channel B3. The communication test for other channels than the channel B3 can be performed in the same manner as described above. Fig. 3 is a view showing control procedures of a first handoff sequence using the frame formats shown in Figs. 2A to 2C. When a mobile station connected to the base station A via the channel A1 enters into a cell site of base station B, this entrance is detected by an exchange. The exchange then selects a free channel B3 of the base station B as a new channel. This detection technique is well known to those skilled in the art. The exchange designates the channel number B3 of the base station B to the mobile station via the base station A by using a signal in the channel A1. The exchange renders the base station B to transmit a test signal via the channel B3. The mobile station receives the channel B3 designated by utilizing the free time of the channel. The mobile station detects the test signal and sends back an acknowledge signal via the channel B3. The exchange which receives the acknowledge signal via the base station B designates switching to the mobile station by using the signal in the channel A1. Upon designation of switching, the mobile station switches the channel from A1 to B3. In this manner, when the communication test is performed by utilizing the free time of the channel, the interruption time of communication can be shortened. Fig. 4 is a view showing control procedures of a second handoff sequence using the channel frame formats shown in Figs. 2A to 2C. When a mobile station connected to the base station A via the channel A1 enters into a cell site of the base station B, this mobile station detects entrance into the cell site of the base station B. This detection can be performed such that a control channel commonly and time-divisionally utilized by a plurality of base stations is monitored by using the channel free time and mean values of reception levels of the control channel transmitted from the respective base stations are compared. The mobile station which detects crossing between the cell sites requests a channel to the base station B. In response to this, the base station B designates the channel number B3 to the mobile station. The request and the designation of the channel described above are performed by utilizing the channel free time, and therefore communication is not interfered. The mobile station then transmits a test signal by utilizing the free time of the channel A1 via the newly designated channel B3. When the base station B detects this test signal, it sends back an acknowledge signal via the channel B3. When the mobile station receives the acknowledge signal from the base station B, the mobile station designates switching to the exchange via the base station A by using a signal in the channel A1. The mobile station then switches the channel from A1 to B3. The exchange which receives a signal for designating switching switches a communication line from the base station A to the base station B. Fig. 5 is a block diagram showing a mobile station for realizing the handoff sequences of Figs. 3 and 4. Reference numeral 10 denotes a switching circuit; 20, a communication circuit; and 30, a communication test circuit. An RF signal received by an antenna 100 is converted into an IF signal by a mixer 11 and demodulated by a modulator/demodulator 40. The demodulated signal is input to a time division multiplexer 21. The time division multiplexer 21 extracts a channel signal of present communication from outputs from the modulator/demodulator 40, and the extracted signal is sent to an encoder 22. The encoder 22 extracts a control signal which is then supplied to a control circuit 13. The encoder 22 converts an output from the time division multiplexer 21 into a speech signal and sends it to a telephone set 23. A speech signal as an output from the telephone set 23 is converted into a digital signal by the encoder 22, and the digital signal is input to the time division multiplexer 21. A control signal as an output from the control circuit 13 constituted by a microprocessor is input to the time division multiplexer 21. These signals are converted into a time division multiplexed signal by the time division multiplexer 21. The multiplexed signal is modulated by the modulator/demodulator 40. The modulated signal is converted into an RF signal by the mixer 11. The RF signal is transmitted from the antenna 100. Time-division multiplexing and modulation/demodulation techniques are not limited to any specific ones. Upon detection of the signal for designating the new channel number, when the present channel time slot is completed, the control circuit 13 changes a frequency of an oscillator 12 to tune the mixer 11 to a frequency of the new channel thereby to cause the communication test circuit 30 to connect with the new channel through the time division multiplexer 21. When the communication test circuit 30 detects a test signal via the new channel, it sends back an acknowledge signal. Alternatively, the communication test circuit 30 transmits a test signal via the new channel and receives an acknowledge signal. When the time slots of the new channel are completed, the control circuit 13 changes the frequency of the oscillator 12 to tune to the frequency of the present channel and connects the communication circuit 20 to the present channel via the time division multiplexer 21 again. With the above operation, the communication test can be performed without adversely affecting communication. Therefore, the interruption time of communication during the handoff sequence can be shortened. Since a conventional mobile telephone system employs analog modulation, an SCPC (Single Channel Per Carrier) method is generally employed in which one channel is assigned to one carrier frequency. In the SCPC method, since the mobile station is always kept in the transmission or reception state during communication, two receiver/transmitters are required to perform a communication test while communication is kept continued. However, when digital modulation is performed, N channels can be assigned to one carrier frequency by time division multiplexing. In this case, a time required for setting each mobile station in the transmission and reception states can be 1/N of the total communication time. The mobile station is kept in the standby state during the remaining time. According to the present invention, this free time is utilized to perform a communication test of a new channel by one receiver/transmitter without interfering the present communication. In general, in time division multiplexing, the time slots of the channel have equal intervals. When a new channel which overlaps the present channel along the time axis is designated, the time slots of the present channel always overlap those of the new channel. Therefore, a communication test cannot be performed even if a free time is utilized. According to the present invention, the time slots of the channels have unequal intervals. Even if any channel is newly assigned, a free time is available which does not involve overlapping between the time slots of the present and new channels. With this time slot assignment, a communication test can be performed without adversely affecting communication, and the interruption time can be almost eliminated. Figs. 6A, 6B, 7A, and 7B show channel frame formats according to another embodiment of a handoff method of the present invention. Figs. 6A and 6B show frame formats of a downward channel from a base station A to a mobile station and an upward channel from the mobile station to the base station A. Figs. 7A and 7B show frame formats of a downward channel from a base station B to a mobile station and an upward channel from the mobile station to the base station B. The downward and upward channels of each base station have a predetermined frequency difference. An 8T frame constituted by eight time slots each having a length of T is repeated to constitute each channel. In addition, a time difference between the downward and upward channels is one time slot. Therefore, after the mobile station receives information in a time slot of the downward channel, it transmits information in a time slot of the upward channel. Assume that a mobile station connected to the base station A via a channel A1 performs a communication test with the base station B during communication. After the mobile station receives information via the upward channel A1, it changes a frequency to that of the base station B and waits for a test signal via a new downward channel. When the mobile station detects a test signal via a new downward channel, it sends back an acknowledge signal via the new upward channel, immediately changes the channel to the channel of the base station A, and starts communication. Frequency switching generally requires a given period of time. Accordingly, when the mobile station transmits the signal via the upward channel A1 and then changes the frequency, the signal cannot be received via the downward channel B3 but by the channel B4. If newly assigned channels are B4, B5, and B6, the mobile station detects a test signal via a downward channel and sends back an acknowledge signal via an upward channel. The mobile station then switches the frequency to that of the base station A. In this case, information in the next time slot of the channel A1 can be received, and therefore no interruption caused by the communication test occurs. If newly assigned channels are B7, B8, B1, B2, and B3, the mobile station detects a test signal via a downward channel and sends back an acknowledge signal via an upward channel. The mobile station then switches the frequency to that of the channel of the base station A. In this case, the mobile station cannot receive information in the next time slot of the channel A1. Therefore, one time slot of the channel A1 is lost and interruption occurs. However, such interruption has almost no effect on the communication quality and is negligible. In the above frame formats, a new downward channel can be detected within one frame upon frequency switching. However, even if a new downward channel is not detected upon a lapse of one frame, the present channel is forcibly restored, and a loss of the time slots can be reduced to one, thereby preventing degradation of communication quality. In addition, if a loss of a plurality of time slots assures allowable communication quality, a new downward channel can be observed for a plurality of frames. Two handoff sequences according to the channel frame formats shown in Figs. 6A to 7B will be described with reference to flow charts of Figs. 8 and 9, respectively. In the handoff sequence of Fig. 8, a mobile station is connected to the base station A and repeats downward channel reception (step 81) via the channel A1 and transmission via the upward channel (step 82) of the channel A1 of the frame formats of Figs. 6A, 6B, 7A, and 7B. When this mobile station enters into a cell site of the base station B, the exchange detects this entrance and selects a free channel, e.g., B3 of the base station B as a new channel. The exchange designates the new channel to the mobile station by using a signal in the channel A1 via the base station A. The exchange causes the base station B to transmit a test signal via the channel B3. In step 83, upon designation of the new channel, the mobile station tunes to the carrier of the downward channel B3 at the end of the time slots of the channel A1 in step 85. In step 86, a timer is set at one frame period. The mobile station then waits for a test signal via the downward channel B3. When the mobile station detects the test signal in step 87, the mobile station transmits an acknowledge signal via the new upward channel B3 in step 89. The mobile station returns to the channel A1 and continues communication. If the test signal is not detected upon a lapse of a one-frame period in step 88, the flow returns to step 81. In this case, no operation is performed, and the mobile station continues communication via the channel A1. When the exchange receives the acknowledge signal via the base station B, the exchange designates switching (handoff) to the mobile station by using the signal in the channel A1. The exchange itself switches the channel from A1 to B3. In response to the designation of the handoff (step 84), the mobile station switches the channel from A1 to B3 (step 90). In this manner, when the communication test is performed while communication continues, an interruption time during the handoff sequence can be shortened. In the handoff sequence shown in Fig. 9, the mobile station communicates with the base station via a channel and at the same time receives a common downward control channel time-divisionally utilized by a plurality of base stations by using a channel free time (step 91). The mean values of the reception levels of the downward control channel of the base stations are compared to detect whether the mobile station enters into another cell site (step 92). For example, when a mobile station connected to the base station A via the channel A1 enters into a cell site of the base station B, the mobile station which has detected the entrance in the cell site in step 95 transmits a signal for requesting a new channel to the base station B via the control channel. In response to this, the base station B designates the channel number B3 to the mobile station via the downward control channel and transmits a test signal via the channel B3. Exchange of signals between the mobile station and the base station B in steps 96 and 97 is performed by utilizing a free time of the channel. The mobile station which is designated with the new channel B3 in step 98 tunes the frequency to the carrier of the channel B3 (step 102) upon completion of the time slots of the channel A1. The timer is set at one frame period (step 103). The mobile station then waits for a test signal via the channel B3. When the mobile station detects the test signal in step 104, the mobile station transmits an acknowledge signal via the channel B3 in step 101. The flow returns to step 96. The mobile station then continues communication via the channel A1. However, if the test signal is not detected upon a lapse of a one-frame period in step 105, the flow returns to step 91. No operation is performed, and the mobile station continues communication via the channel A1. When the base station B receives the acknowledge signal, the base station B transmits a switching signal to the exchange and the mobile station. The exchange and the mobile station which receive the switching signal in step 99 switch the channel from A1 to B3 in step 106. Fig. 10 is a block diagram of a mobile station for realizing the handoff sequence shown in Fig. 9. Reference numeral 80 denotes a communication circuit; 30, a communication test circuit; and 90, a switching circuit. An RF signal received by an antenna 100 is converted into an IF signal by a mixer 40. The IF signal is demodulated by a modulator/demodulator 50. The demodulated signal is input to a time division multiplexer 60. The time division multiplexer 60 extracts a speech signal from the output from the modulator/demodulator 50. The speech signal is sent to a switch 91. The time division multiplexer 60 extracts a control signal from the output from the modulator/demodulator 50 and sends it to a control circuit 92. The switch 91 is controlled by the control circuit 92 to connect the time division multiplexer 60 to an encoder 81 or the communication test circuit 30. The time division multiplexer 60 is normally connected to the encoder 81, and the encoder 81 converts an output from the time division multiplexer 60 into a speech signal. The speech signal is sent to a telephone set 82. A speech signal as an output from the telephone set 82 is converted into a digital signal by the encoder 81. This digital signal is input to the time division multiplexer 60. A control signal as an output from the control circuit 92 is input to the time division multiplexer 60. These input signals are multiplexed by the time division multiplexer 60, and the multiplexed signal is modulated by the modulator/demodulator 50. The modulated signal is converted into an RF signal by the mixer 40. The RF signal is then transmitted from the antenna 100. Time-division multiplexing and modulation/demodulation techniques are not limited to any specific ones, and a detailed description will be omitted. Upon detection of the signal for designating a new channel number, when the time slots of the present channel are completed, the control circuit 92 changes a frequency of an oscillator 70 and tunes to the frequency of the new channel. The switch 91 connects the time division multiplexer 60 to the communication test circuit 30, and a timer 93 is set at one frame period. When a one-frame period has elapsed, the timer 93 outputs a detection signal to the control circuit 92. When the communication test circuit 30 detects a test signal via the new channel, it sends back or transmits an acknowledge signal to signal the end of communication test to the control circuit 92. When the communication test is completed, or a one-frame period has elapsed, the control circuit 92 changes the frequency of the oscillator 70 and tunes it to the frequency of the present channel. The time division multiplexer 60 is connected again to the encoder 81, and communication continues. With the above operations, the communication test can be performed without adversely affecting communication. Therefore, a communication interruption time during the handoff sequence can be shortened. According to the above embodiment as has been described above, the communication test of the new communication channel can be performed while communication continues. Therefore, a new channel can be switched during communication without degrading communication quality. Fig. 11 is a view showing a cellular digital mobile communication system employing a handoff method according to still another embodiment of the present invention. Referring to Fig. 11, the cellular digital mobile communication system is constituted by an exchange 200, base stations 220, and a mobile station 230. The base station 220 includes at least one TDMA radio transmitter/receiver, and the mobile station 230 includes one TDMA radio transmitter/receiver. The mobile station 230 located within the cell site of a given base station 220 (to be referred to as a BS-A hereinafter) sets a communication channel with the BS-A and is wired via the exchange 200. When the mobile station 230 is moved from the cell site of the BS-A to a cell site of an adjacent base station (e.g., BS-B), the mobile station 230 must detect that a new channel can be set with the BS-B. The mobile station 230 switches the channel with the BS-A into that with the BS-B and continues communication. Figs. 12 and 13 show a first example of time division of carrier frequencies and spatial allocation of the time-divisionally carrier frequencies, respectively. Referring to Figs. 12 and 13, the carrier frequencies to be used are given as f₁, f₂, f₃,... f n , and twelve channels are time-divisionally multiplexed so as to synchronize with the carrier frequencies f₁, f₂, f₃,... f n . Channel groups T₁ to T₁₂ are constituted by channels at identical timings. The channel groups T₁ to T₁₂ are assigned to the base stations such that the channel groups are not used in the adjacent base stations. In this case, the number of cell sites in a group having an identical frequency is 12. Since the channel group T₁ is assigned to the BS-A, the mobile station 230 located in the cell site of the BS-A performs transmission or reception at the timing T₁ of each of the carrier frequencies f₁ to f n . With the this arrangement, when the mobile station 230 which uses the channel of the channel group T₁ is moved to the cell site of an adjacent base station (one of BS-B to BS-G), the channel groups which may be newly assigned to this mobile station 230 are T₄, T₆, T₇, T₉, T₁₀, and T₁₁. Since these channel groups have differences of one or more time slots from the channel group T₁ which has been used, respectively, a channel group newly assigned to the free time can be designated while communication continues via the channel group T₁, and therefore a communication can be performed via the new channel group. Fig. 14 is a view showing a handoff sequence executed according to carrier frequency allocation shown in Figs. 12 and 13. With reference to Fig. 14, assume that a mobile station using a channel of the frequency f₂ of the channel group T₁ (to be referred to as a channel (T₁,f₂) hereinafter) of the BS-A is moved to a cell site of the BS-B. When this mobile station is moved in the cell site of the BS-B, the exchange detects this movement and selects a free channel, e.g., (T₆,f₁) as a new channel of the cell site of the BS-B, and designates this to the mobile station by using a signal in the channel (T₁,f₂). This detection by the exchange is well known to those skilled in the art, and a detailed description thereof will be omitted. The exchange transmits a test signal to the BS-B via the channel (T₆,f₁). The mobile station designated with the new channel (T₆,f₁) waits for a test signal transmitted via the channel (T₆,f₁) upon completion of the time slots of the channel (T₁,f₂). When the mobile station detects the test signal, it transmits or sends back an acknowledge signal via the channel (T₆,f₁), returns to the channel (T₁,f₂), and continues communication. A communication test is performed through two frames in Fig. 14. However, a communication test may be completed within one frame or may continue for three or more frames. When the exchange receives the acknowledge signal via the channel (T₆,f₁) of the BS-B, the exchange designates switching to the mobile station by using a signal in the channel (T₁,f₂). The exchange itself switches the communication line from the BS-A to the BS-B. Upon designation of a handoff sequence, the mobile station switches the channel from (T₁,f₂) to (T₆,f₁). In this manner, when the communication test is performed while communication continues, an interruption time during the handoff sequence can be shortened. Figs. 15 and 16 are views showing a second example for time division of carrier frequencies and spatial allocation of the frequency-divided carrier frequencies. Referring to Figs. 15 and 16, carrier frequencies to be used are given as f₁₁, f₁₂, f₁₃, f₂₁, f₂₂, f₂₃,... f k ℓ, (k: an integer satisfying 1 ≦ k ≦ n;ℓ = 1, 2, 3),... f n1 , f n2 , and f n3 , and the channels are time-divisionally multiplexed to synchronize with the above carrier frequencies, thereby constituting channel groups T₁, T₂,... T₁₂. The same time slots as the channel T i (i: an integer satisfying 1 ≦ i ≦ 12) are given as S i and R i . For example, S₂ represents that the carrier frequencies f₁₂, f₂₂,... f k2 ,... f n2 are time-divisionally multiplexed at the same time slots as in the channel T₂. In this manner, the channels at identical timings constitute channel groups T₁ to T₁₂, S₁ to S₁₂, and R₁ to R₁₂. The channel groups T₁ to T₁₂, S₁ to S₁₂, and R₁ to R₁₂ are assigned to the cell sites, as shown in Fig. 15. In this case, clusters each having a size of 12 are combined to constitute a cluster having a size of 36. Therefore, the number of cell sites at an identical frequency is 36. Similarly, it is possible to increase the number of cell sites at an identical frequency while the degree of multiplexing of channels is kept at 12. According to this embodiment as described above, a plurality of channels are time-divisionally multiplexed so as to synchronize with the carrier frequencies, the channels at identical timings constitute channel groups, the channel groups are assigned to the cell sites such that the channel group is used as a unit, and the channel groups continuous on the time axis are not used by the adjacent cell sites. With this arrangement, a difference of one or more channels occurs between the present channel and the new channel of the cell site adjacent to the present channel. Therefore, a communication test of the new channel can be performed while communication continues via the present channel. Therefore, an interruption time during the handoff sequence can be eliminated. According to this embodiment as has been described above, there is provided a handoff method in a cellular digital mobile communication system, wherein transmission timings of the TDMA radio transmitters/receivers are equal to each other within a given base station but are different by one or more time slots between adjacent base stations, so that an interruption time during the handoff sequence can be substantially eliminated while communication continues. According to the present invention each mobile station can compare the reception levels of the downward signals of the base stations to monitor a change in cell site, so that monitoring control can be shared by the base and mobile stations. For this purpose, the level monitor is arranged in the mobile station to receive the downward signals from other base stations excluding a given station while the mobile station communicates with the given station. The downward signal may be received by the level monitor via a communication channel, but preferably via a control channel which normally transmits the downward channel. When the control channel is time-divisionally multiplexed by each base station throughout the cell sites, the reception frequency of the monitor need not be changed to simplify the apparatus arrangement. The base station transmits a downward signal including the flag representing the presence/absence of a free communication channel, and the mobile station compares the reception levels of the downward signals of the base stations having free communication channels. Therefore, a base station having no free communication channel cannot be selected as a designation station. In this manner, the mobile station monitors a change in cell site during communication. When the change in cell site is detected, the mobile station transmits a switching request signal and is switched to the designated base station having a free communication channel. Therefore, local concentration of the control amount during the handoff sequence can be prevented. According to the present invention as has been described above, there is provided a handoff method and a mobile station in a cellular digital mobile communication system substantially free from a local increase in control amount caused by an increase in the frequency of occurrence of channel switching.

Claims: A handoff method for a cellular digital mobile communication system including a number of base stations (1; 220; 120) and a plurality of mobile stations (230; 130), each of said mobile stations communicating with its serving base station using a time-divisionally multiplexed communication channel, wherein a mobile station's serving base station is changed during a communication, characterized by the steps of : tuning a mobile station to a frequency of a new communication channel of a new serving base station during a free time of the TDMA communication channel, in which said mobile station does not send an information signal to its current serving base station and it does not receive an information signal from said current serving base station; sending a communication test signal from said mobile station to a new serving base station while said mobile station is tuning to a frequency of said new communication channel; and operating said mobile station to receive a communication test signal from said new serving base station while said mobile station is tuning to a frequency of said new communication channel. A mobile station in a cellular digital mobile communication system including a number of base stations (1; 220; 120) and a plurality of mobile stations (230; 130), each of said mobile stations communicating with its serving base station using a time-divisionally multiplexed communication channel, wherein a mobile station's serving base station is changed during a communication, characterized in that the mobile station comprises: means (10; 90) for tuning said mobile station to a frequency of a new communication channel of a new serving base station during a free time of the TDMA communication channel, in which said mobile station does not send an information signal to its current serving base station and it does not receive an information signal from said current base station; communication test means (30) for sending a communication test signal to a new serving base station while said mobile station is tuning to a frequency of said new communication channel; and means (30) for receiving a communication test signal from said new serving base station while said mobile station is tuning to a frequency of said new communication channel. A mobile station according to claim 2, wherein said tuning means (10; 90) tuner said mobile station to the frequency of said new communication channel when time slots of the time-divisionally multiplexed commuunication channel are completed and switches from said new communication channel to the time-divisionally multiplexed communication channel when the communication test of the new communication channel is completed or a predetermined period of time has elapsed. A cellular digital mobile communication system comprising a number of base stations (1;220;120) and a plurality of mobile stations (230;130), each of said mobile stations communicating with its serving base station using a time-divisionally multiplexed communication channel, wherein a mobile station's serving base station is changed during a communication, characterized in that the cellular digital mobile communication system comprises a plurality of mobile stations (230;130) according to claims 2 or 3. A cellular digital mobile communication system according to claim 4, wherein each base station (1; 220; 120) comprises at least one time-divisional multiplex access radio transmitter/receiver, and the transmission/reception timings of the time-divisional multiplex access radio transmitter/receiver are identical within a given base station but are differentiated in adjacent base stations by one or more time slots.

Inventors: FURUYA YUKITSUNA, KANAI TOSHIHITO, FURUYA, YUKITSUNA, KANAI, TOSHIHITO
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1988
Main classification: H04Q7
IPC codes: H04W36