Patent Publication Number: US-2002004396-A1

Title: Mobile communication system and method and mobile stations and base stations in the system

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention relates to a mobile communication system and method, and mobile stations and base stations in the system, in particular, in which the code division multiple access (CDMA) system having a handover function is used.  
       DESCRIPTION OF THE RELATED ART  
       [0002] At a mobile communication system used the CDMA system, when a mobile station executes handover from a base station, with which the mobile station is communicating, to an adjacent base station, the mobile station executes communication with plural base stations by using the same frequency at the same time, which is a feature of the CDMA system. This technology is called soft handover.  
       [0003] When the mobile station executes the soft handover, it is indispensable that plural base stations transmit the same content signal to the mobile station at the same timing. In order to execute this, each base station provides a receiver of the global positioning system (GPS), and when the plural base stations transmit the signal, the plural base stations synchronize the signal by using the receiver.  
       [0004] However, at the case that each base station provides the receiver of the GPS, the manufacturing cost of the base station increases and also the load for the mobile communication system increases compared with that the base station does not provide the receiver of the GPS. Therefore, it must be studied that the soft handover can be executed without using the GPS.  
       [0005] And when the mobile station executes the soft handover, the plural base stations must transmit the signal by synchronizing the signal among the plural base stations. In order to execute this, it is necessary that complex hardware and complex software are controlled. Therefore, this control must be improved.  
       SUMMARY OF THE INVENTION  
       [0006] It is therefore an object of the present invention to provide a mobile communication system and method, and mobile stations and base stations in the system, in which a technology being capable of simply executing soft handover with low cost is used.  
       [0007] According to a first aspect of the present invention for achieving the object mentioned above, there is provided a mobile communication system. The mobile communication system provides plural base stations and plural mobile stations and a mobile station requests handover from the wireless zone of a base station in which the mobile station exists to the wireless zone of another base station to which the handover is executed at the time when the mobile station moves from the wireless zone of the base station to the wireless zone of another base station. And the mobile station provides a first measuring means for measuring first phase difference between a signal that the mobile station is transmitting to the base station and a signal that was transmitted from the base station to the mobile station, and a first transmitting means for transmitting the first phase difference measured at the first measuring means to another base station when the mobile station requests the handover. And another base station provides a second measuring means for measuring second phase difference between a signal that another base station is transmitting to the mobile station and a signal that was transmitted from the mobile station to another base station when the handover is executing, an adjusting means for adjusting phase so that the phase of the signal transmitting from another base station to the mobile station is made to be equal to the phase of the signal transmitted from the base station to the mobile station by using information of the first phase difference and the second phase difference, and a second transmitting means for transmitting a signal, which the adjusting means adjusted, to the mobile station after the handover was executed. And another base station transmits a signal synchronized with the signal, which was transmitted from the base station to the mobile station, to the mobile station after the handover was executed, by that the second transmitting means transmits the signal adjusted at the adjusting means to the mobile station.  
       [0008] According to a second aspect of the present invention, there is provided a mobile station in a mobile communication system. The mobile station provides a measuring means for measuring first phase difference between a signal that the mobile station is transmitting to a base station from which handover is executed and a signal that was transmitted from the base station from which the handover is executed to the mobile station, and a transmitting means for transmitting the first phase difference measured at the measuring means to another base station to which the handover is executed when the mobile station requests the handover.  
       [0009] According to a third aspect of the present invention, there is provided a base station in a mobile communication system. The base station provides a measuring means for measuring second phase difference between a signal that the base station is transmitting to a mobile station executing handover and a signal that was transmitted from the mobile station executing the handover to the base station, an adjusting means for adjusting phase so that the phase of the signal transmitting from another base station to which the handover is executed to the mobile station is made to be equal to the phase of the signal transmitted from the base station to the mobile station by using information of first phase difference measured at the mobile station and the second phase difference, and a transmitting means for transmitting a signal, which the adjusting means adjusted, to the mobile station after the handover was executed.  
       [0010] According to a fourth aspect of the present invention, there is provided a mobile communication method in a mobile communication system in which plural base stations and plural mobile stations are provided. And at the case that a mobile station requests handover from the wireless zone of a base station in which the mobile station exists to the wireless zone of another base station to which the handover is executed when the mobile station moves, the mobile station provides the steps of, measuring first phase difference between a signal that the mobile station is transmitting to the base station and a signal that was transmitted from the base station to the mobile station, and transmitting the measured first phase difference to another base station when the mobile station requests the handover. And another base station provides the steps of; measuring second phase difference between a signal that another base station is transmitting to the mobile station and a signal that was transmitted from the mobile station to another base station when the mobile station is executing the handover, adjusting phase so that the phase of the signal transmitting from another base station to the mobile station is made to be equal to the phase of the signal transmitted from the base station to the mobile station by using information of the first phase difference and the second phase difference, and transmitting a signal, which the adjusting means adjusted, to the mobile station after the handover was executed. And another base station transmits a signal synchronized with the signal, which was transmitted from the base station to the mobile station, to the mobile station after the handover was executed, by that the second transmitting means transmits the signal adjusted at the adjusting means to the mobile station.  
       [0011] According to the present invention, when handover is executed, another base station, to which the handover is executed, obtains the timing of a signal transmitted from a base station, from which the handover is executed, when another base station received an uplink signal from the mobile station, and another base station adjusts the timing with which another base station transmits a signal. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012] The objects and features of the present invention will become more apparent from the consideration of the following detailed description taken in conjunction with the accompanying drawings in which:  
     [0013]FIG. 1 is a block diagram showing a structure of an embodiment of a mobile communication system of the present invention;  
     [0014]FIG. 2 is a block diagram showing an internal structure of an upper rank apparatus in the mobile communication system at the embodiment of the present invention;  
     [0015]FIG. 3 is a block diagram showing an internal structure of a base station in the mobile communication system at the embodiment of the present invention;  
     [0016]FIG. 4 is a block diagram showing an internal structure of a mobile station in the mobile communication system at the embodiment of the present invention;  
     [0017]FIG. 5 is a diagram showing a structure of a signal transmitting and receiving between the mobile station and the base stations at the embodiment of the mobile communication system of the present invention;  
     [0018]FIG. 6 is a diagram showing a state before the mobile station executes soft handover from the wireless zone of the base station to the wireless zone of another base station at the embodiment of the mobile communication system of the present invention;  
     [0019]FIG. 7 is a timing chart showing the phase difference of transmitting and receiving signals between the mobile station and the base stations at the embodiment of the radio communication system of the present invention; and  
     [0020]FIG. 8 is a diagram showing a state that the mobile station is executing soft handover when the mobile station is moving from the wireless zone of the base station to the wireless zone of another base station at the embodiment of the mobile communication system of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0021] Referring now to the drawings, an embodiment of the present invention is explained in detail. FIG. 1 is a block diagram showing a structure of an embodiment of a mobile communication system of the present invention. As shown in FIG. 1, base stations  12  and  13  are connected to an upper rank apparatus  11  through asynchronous transfer mode (ATM) lines. And the upper rank apparatus  11  is also connected to switching equipment  15  for mobile communication through ATM lines. And a mobile station  14  communicates with the base station  12  or the base station  13  at the CDMA system. In this, the number of the base stations is not limited to two, and also the number of the mobile stations is not limited to one. And actually, a large number of base stations and a large number of mobile stations are provided in the mobile communication system.  
     [0022]FIG. 2 is a block diagram showing an internal structure of the upper rank apparatus  11  in the mobile communication system at the embodiment of the present invention. As shown in FIG. 2, a state, in which packet data attached the frame number (FN)  0  to  2  are being transmitted to the both base stations  12  and  13  at the same timing, is shown at the time of the soft handover. In FIG. 2, a processor  54  executes a coding process for signals of the upper rank apparatus  11 , and transmits the same content signals to the base stations  12  and  13  at the same timing. In this, the signals are voice signals and image signals.  
     [0023]FIG. 3 is a block diagram showing an internal structure of the base station  12  in the mobile communication system at the embodiment of the present invention. As shown in FIG. 3, the base station  12  provides an ATM switch (ATM SW)  703  being an interface that receives signals outputted from the upper rank apparatus  11  through ATM lines and outputs signals from the mobile station  14  through the ATM lines to the upper rank apparatus  11 , a reference timing generator  701  that generates transmitting/receiving timing of signals at the base station  12  itself, plural signal processors  704  that apply various processes to received signals and transmitting signals at the generated timing, an amplifier  710  that amplifies the power of the transmitting signals and the received signals, an antenna  711  that transmits the amplified signals and receives the signals transmitted from the mobile station  14 , and a controller  702  that controls a timing adjuster  705  in the signal processor  704  so that both a base station from which handover is executed and a base station to which the handover is executed process signals at the same timing based on the phase difference measured at a phase difference measuring section  708  in the signal processor  704 .  
     [0024] The signal processor  704  provides a transmitting and receiving section  709  that modulates the signals from the upper rank apparatus  11  and demodulates the signals transmitted from the mobile station  14 , the phase difference measuring section  708  being a second measuring means that measures the phase difference between signals that are transmitted to a mobile station executing the handover and signals transmitted from the mobile station, a spreading section  706  that spreads signals to be transmitted, a de-spreading section  707  that de-spreads received signals, and the timing adjuster  705  that adjusts the timing so that both a base station from which handover is executed and a base station to which the handover is executed process signals at the same timing. In this, a second transmitting means is defined as that signals are modulated at the transmitting and receiving section  709  and the signals are amplified at the amplifier  710  and the amplified signals are transmitted to the mobile station  14  through the antenna  711 .  
     [0025] At the explanation mentioned above, only the base station  12  was explained, however, the base station  13  has also the same structure that the base station  12  has.  
     [0026]FIG. 4 is a block diagram showing an internal structure of the mobile station  14  in the mobile communication system at the embodiment of the present invention. As shown in FIG. 4, the mobile station  14  provides a reference timing generator  801  that generates timing for processing transmitting and receiving signals at the mobile station  14 , a phase difference measuring section  805  being a first measuring means that measures the phase difference between signals transmitting to the base stations  12  and  13  and signals transmitted from the base stations  12  and  13 , a spreading section  803  that spreads signals to be transmitted, a controller  802  that controls the signal spreading timing at the spreading section  803  based on the phase difference measured at the phase difference measuring section  805 , a de-spreading section  804  that de-spreads the received signals corresponding to the generated timing, a transmitting and receiving section  806  that modulates the signals to be transmitted and demodulates the received signals, an amplifier  807  that amplifies the signals transmitted from the base stations  12  and  13  and the signals to be transmitted to the base stations  12  and  13 , and an antenna  808  that transmits the amplified signals to the base stations  12  and  13  and receives the signals transmitted from the base stations  12  and  13 . In this, a first transmitting means is defined as that signals are modulated at the transmitting and receiving section  806  and the signals are amplified at the amplifier  807  and the amplified signals are transmitted to the base stations  12  and  13  through the antenna  808 .  
     [0027]FIG. 5 is a diagram showing a structure of a signal transmitting and receiving between the mobile station  14  and the base stations  12  and  13  at the embodiment of the mobile communication system of the present invention. And this structure of the signal shows an ATM cell, that is, packet data. This signal is generated by that a signal transmitted from another mobile station through the switching equipment  15  is processed being such as coding processed at the upper rank apparatus  11 . And the signal transmitting/receiving between the mobile station  14  and the base stations  12  and  13  is made to be packet data every 10 ms, and a FN is attached to each packet data. The FN is counted, for example, by the repetition of 0 to 127, and one radio frame is composed of FN=0 to FN=the counted number.  
     [0028] In FIG. 5, one radio frame consists of FN  0  to FN  126 , and the interval among the packet data is 10 ms, however, this is an example and one radio frame is not limited to this example.  
     [0029] Next, referring to drawings, the operation of the mobile communication system of the present invention is explained. FIG. 6 is a diagram showing a state before the mobile station  14  executes soft handover from the wireless zone of the base station  12  to the wireless zone of the base station  13  at the embodiment of the mobile communication system of the present invention. In this, a signal having the structure shown in FIG. 5 is transmitting from the base station  12  to the mobile station  14  by that, for example, the frequency is f 1 , and the spreading code is c 1 , and a signal having the structure shown in FIG. 5 is transmitting from the mobile station  14  to the base station  12  by that, for example, the frequency is f 2 , and the spreading code is c 2 .  
     [0030] In this, a signal at downlink communication from the base station  12  to the mobile station  14  is a signal that a signal outputted from the upper rank apparatus  11  through an ATM line is spread and radio modulated at the base station  12 . That is, the signal spread and radio modulated at the base station  12  is transmitted to the mobile station  14 .  
     [0031] Actually, a signal from the upper rank apparatus  11  is inputted to the ATM SW  703  in the base station  12 . And the signal is inputted to the spreading section  706  from the ATM SW  703 , and the spreading section  706  spreads the signal based on an instruction from the controller  702  through the timing adjuster  705 , and outputs the spread signal to the transmitting and receiving section  709 .  
     [0032] The transmitting and receiving section  709  modulates the inputted signal and outputs the modulated signal to the amplifier  710 . The amplifier  710  amplifies the power of the modulated signal and transmits the amplified signal to the mobile station  14  through the antenna  711  by that the frequency is f 1  and the spreading code is c 1 . In this, the signal outputted from the spreading section  706  is also inputted to the phase difference measuring section  708 , however the inputted signal is discarded at the phase difference measuring section  708  at this time.  
     [0033] That is, only when the mobile station  14  requests handover, the phase difference measuring section  708  measures the phase difference. At this case, the phase difference is measured by the same method that the mobile station  14  executes the measurement of the phase difference. Measuring the phase difference is explained later.  
     [0034] At the mobile station  14 , the signal transmitted from the base station  12  is received at the antenna  808  and the received signal is inputted to the amplifier  807 . The amplifier  807  amplifies the power of the received signal and the amplified signal is outputted to the transmitting and receiving section  806 . The transmitting and receiving section  806  demodulates the inputted signal and outputs the demodulated signal to the de-spreading section  804  and the phase difference measuring section  805 . The de-spreading section  804  de-spreads the inputted signal at the timing generated at the reference signal generator  801  and outputs the de-spread signal to the controller  802 . The phase difference measuring section  805  stores temporarily the inputted signal in a memory (not shown) in the phase difference measuring section  805 .  
     [0035] After this, when the mobile station  14  desires to transmit a signal to the base station  12 , a signal generated at a signal generator (not shown) is outputted to the spreading section  803 . The spreading section  803  spreads the inputted signal at the timing generated at the reference timing generator  801  and outputs the spread signal to the transmitting and receiving section  806  and the phase difference measuring section  805 .  
     [0036] The transmitting and receiving section  806  modulates the inputted signal and outputs the modulated signal to the amplifier  807 . The amplifier  807  amplifies the inputted signal and transmits the amplified signal to the base station  12  thorough the antenna  808  by that the frequency is f 2  and the spreading code is c 2 . The phase difference measuring section  805  measures the phase difference between the signal storing in the memory (signal received from the base station  12 ) and the signal transmitting to the base station  12 , and outputs the measured result to the controller  802 . The controller  802  stores the measured result in a memory (not shown).  
     [0037] The base station  12  receives the signal through the antenna  711 . The received signal is outputted to the amplifier  710 . The amplifier  710  amplifies the power of the inputted signal and outputs the amplified signal to the transmitting and receiving section  709 . The transmitting and receiving section  709  modulates the inputted signal and outputs the modulated signal to the de-spreading section  707 . The de-spreading section  707  de-spreads the inputted signal at the timing generated at the reference timing generator  701  and outputs the de-spread signal to the upper rank apparatus  11  through the ATM SW  703 . The signal outputted from the transmitting and receiving section  709  is also inputted to the phase difference measuring section  708 , however, the inputted signal is discarded at the phase difference measuring section  708 .  
     [0038] Next, the phase difference of the transmitting and receiving signals is explained. FIG. 7 is a timing chart showing the phase difference of transmitting and receiving signals between the mobile station  14  and the base stations  12  and  13  at the embodiment of the radio communication system of the present invention.  
     [0039] As shown in FIG. 7( a ) and ( b ), there is phase difference of, for example, two frames, that is, 20 ms between a signal transmitted from the base station  12  to the mobile station  14  and a signal transmitting from the mobile station  14  to the base station  12 . And as shown in FIG. 7( c ) and ( d ), there is phase difference of, for example, one frame, that is, 10 ms between a signal transmitted from the mobile station  14  to the base station  13  and a signal transmitting from the base station  13  to the mobile station  14 . When it is assumed that the phase difference occurs as mentioned above, as shown in FIG. 7( a ) and ( d ), there is the phase difference of 30 ms between the signal transmitted from the base station  12  to the mobile station  14  and the signal transmitting from the base station  13  to the mobile station  14 . This occurs because the signals transmitting from the base stations  12  and  13  are not synchronized.  
     [0040] And at FIG. 7( b ) and ( c ), the timing of the transmitting signal from the mobile station  14  is shown, therefore the timing is the same.  
     [0041] As mentioned above, the phase difference information, which was measured at the phase difference measuring section  805  in the mobile station  14  and outputted to the controller  802  and memorized in the controller  802 , is 20 ms, and this phase difference information is transmitted to the base station  13  to which the handover is executed. This transmission of the information is explained later in more detail. And when the mobile station  14  is executing the handover from the base station  12  to the base station  13 , the base station  13  can obtain the phase difference of, for example, 10 ms between the uplink signal and the downlink signal between the base station  13  and the mobile station  14 , by transmitting and receiving signals between the base station  13  and the mobile station  14 .  
     [0042] At the embodiment of the mobile communication system of the present invention, a signal, which the base station  13  to which the handover is executed transmits to the mobile station  14 , is synchronized with a signal, which the base station  12  from which the handover is executed transmits to the base station  14 .  
     [0043]FIG. 8 is a diagram showing a state that the mobile station  14  is executing soft handover when the mobile station  14  is moving from the wireless zone of the base station  12  to the wireless zone of the base station  13  at the embodiment of the mobile communication system of the present invention. As shown in FIG. 8, when the mobile station  14  is moving from the wireless zone of the base station  12  to the wireless zone of the base station  13 , the mobile station  14  transmits an uplink communication signal by, for example, that the frequency is f 2  and the spreading code is c 2  to the base station  13 .  
     [0044] At this time, the signal transmitted from the mobile station  14  to the base station  13  has the phase difference information at the mobile station  14 . Therefore, the base station  13  can obtain the information that the phase difference of, for example, 20 ms exists between the signal transmitted from the base station  12  to the mobile station  14  and the signal transmitting from the mobile station  14  to the base station  12  as shown in FIG. 7( a ) and ( b ).  
     [0045] And as mentioned above, the base station  13  can obtain the phase difference between the uplink signal and the downlink signal between the base station  13  and the mobile station  14 , by transmitting and receiving signals between the base station  13  and the mobile station  14 . Actually, when the mobile station  14  requests the handover of the base station  13 , at the base station  13 , the signal (transmitted from the mobile station  14  to the base station  13 ) inputted to the phase difference measuring section  708  outputted from the transmitting and receiving section  709  is stored in the memory (not shown) in the phase difference measuring section  708 . After this, when the base station  13  transmits a signal to the mobile station  14 , at the base station  13 , a signal outputted from the spreading section  706  is inputted to the phase difference measuring section  708 , and the phase difference measuring section  708  measures the phase difference between the signal outputted from the spreading section  706  and the signal storing in the memory.  
     [0046] After this, the base station  13  adjusts the timing of the signal transmitting from the base station  13  to the mobile station  14  shown in FIG. 7( d ) to FIG. 7( d ′) by using an adjusting means such as a buffer memory (not shown) in the controller  702  for the mobile station  14 . With this, the timing of the signal transmitting from the base station  13  to the mobile station  14  shown in FIG. 7 ( d ′) is synchronized with the timing of the signal transmitted from the base station  12  to the mobile station  14  shown in FIG. 7( a ), and the base station  13  transmits the signal to the mobile station  14  by that the frequency is f 1  and the spreading code is c 3 . Actually, the signal shown in FIG. 7 ( d ′) is synchronized with the signal shown in FIG. 7( a ) in a state that the signal shown in FIG. 7 ( d ′) was delayed by one radio frame.  
     [0047] And the upper rank apparatus  11  is controlling both the base stations  12  and  13 , therefore, the upper rank apparatus  11  can transmit a signal, which is transmitted to the mobile station  14 , to the base stations  12  and  13  at the same timing. Therefore, the timing of the downlink signal from the base station  13  to the mobile station  14  by transmitting at the frequency f 1  and the spreading code c 3  becomes the same timing of the downlink signal from the base station  12  to the mobile station  14  by transmitting at the frequency f 1  and the spreading code c 1 , and the soft handover can be realized.  
     [0048] In this, at the case that the amount of phase difference at the base station  13  becomes large and exceeds the capacity of the buffer memory, the base station  13  informs the upper rank apparatus  11  of this. And the upper rank apparatus  11  adjusts the transmitting timing of signals by using this as a trigger.  
     [0049] As mentioned above, according to the embodiment of the mobile communication system of the present invention, it is not necessary that the receiver of the GPS system is provided in each of the base stations  12  and  13 , therefore, the base stations  12  and  13  can be small sized. And the base stations  12  and  13  transmit signals to the mobile station  14  without synchronizing with the upper rank apparatus  11 . Therefore, at a case that a mobile communication system is a system in which signals are transmitted between a base station and a mobile station by synchronizing with an upper rank apparatus, when the upper rank apparatus has some trouble, there will be some problem in the radio communication system. However, the mobile communication system of the present invention does not have this problem and the reliability of the radio communication system is improved.  
     [0050] At the embodiment of the present invention, the case, in which signals are synchronized in a frame unit, is explained, however, it is possible that the signals are synchronized in a chip rate level, in this case, the synchronization can be adjusted in a finer level.  
     [0051] As mentioned above, according to the present invention, the phase difference of signals transmitting and receiving between a mobile station and base stations is measured, and the signals whose phase difference does not exist are transmitted from a base station, to which the handover is executed, to the mobile station. Therefore, low cost and easy handover can be executed without synchronizing signals between base stations.  
     [0052] While the present invention has been described with reference to the particular illustrative embodiment, it is not to be restricted by that embodiment but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiment without departing from the scope and spirit of the present invention.