Mobile communication systems

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a mobile communication system, and more particularly, to a mobile communication system that is capable of switching the duplex modes of an uplink or downlink depending on the transmission volume communicated between a base station and a mobile station.

2. Description of the Related Art

Duplexing carried out in the current mobile communication system is generally classified into frequency division duplex (FDD) and time division duplex (TDD).

Fig. 1 illustrates a conventional cell structure in FDD communication, and Fig. 2 illustrates the basic concept of conventional FDD communication. As shown in Fig. 1, base stations 11₁, 11₂, and 11₃ are located in the respective cells 400, 500, and 600, and connected to mobile stations 21₁, 21₂, and 21₃, respectively, via radio channels. In FDD, different frequencies f₁ and f₂ are allocated to the uplink, which is directed from the mobile station to the base station, and the downlink, which is directed from the base station to the mobile station. The radio frequencies f₁ and f₂ occupy the associated links all the time, as shown in Fig. 2. In addition, the radio frequencies f₁ and f₂ are used equally in all the cells 400, 500, and 600.

Fig. 3 illustrates a conventional FDD mobile communication system. This FDD system includes a base station 11₁ and a mobile station 21₁. The base station 11₁ has a signal processing unit 80 connected to the backbone network (e.g., mobile communication network), a transmitter 81, a receiver 82, a band pass filter (BPF) 83 with a center frequency of f₁, a band pass filter (BPF) 84 with a center frequency of f_2, and antennas 85, 86 connected to the receiver 82 and the transmitter 81, respectively.

The mobile station 21₁ has an input unit 70 (e.g., microphone 70a or keyboard 70b), an output unit 71 (e.g., display 71a or speaker 71b), a signal processing unit 72, a transmitter 73, a receiver 74, a band pass filter (BPF) 75 with a center frequency of f₁, a band pass filter (BPF) 76 with a center frequency of f₂, and antennas 77, 78 connected to the transmitter 73 and the receiver 74, respectively.

In operation of the mobile station 21₁, digital signals supplied from the input unit 70 are converted to baseband signals by the signal processing unit 72. The baseband signals are converted to radio signals by the transmitter 73, which then pass through the band pass filter 75. Radio signal components of center frequency f₁ are extracted by the band pass filter 75, and the signals are transmitted from the antenna 77 to the base station 11₁ at a frequency f₁.

The radio signals transmitted from the mobile station 21₁ are received by the antenna 85 of the base station 11₁, and supplied to the receiver 82 via the band pass filter 83. The received radio signals are converted to baseband signals, and the baseband signals are then converted to digital signals by the signal processing unit 80. The digital signals are transmitted to the backbone network, such as a mobile communication network.

On the other hand, in the downlink, digital signals received by the base station 11₁ from the backbone network are converted to baseband signals by the signal processing unit 80, and then converted to radio signals by the transmitter 81. The band pass filter 84 extracts only those signal components with a center frequency of f₂, which are then transmitted to the mobile station 21₁.

The radio signals transmitted from the base station 11₁ are received by the antenna 78 of the mobile station 21₁, and supplied to the receiver 74 via the band pass filter 76. The radio signals are converted into digital signals through the receiver 74 and the signal processing unit 72, and supplied to the output unit 71.

In FDD, radio communication is carried out by allocating different frequencies f₁ and f₂ to the uplink (from the mobile station 21₁ to the base station 11₁) and the downlink (from the base station 11₁ to the mobile station 21₁.

Figs. 4-7 illustrate the concept of conventional TDD communication. In conventional TDD, the same radio frequency is used on the uplink and the downlink, unlike FDD. As illustrated in Fig. 4, base stations 12₁-12₁₄ are located in the respective cells 700-713. In this cell structure, a mobile station 22₁, which is under management of the base station 12₁, is connected to a mobile station 22₂, which is under management of the base station 12₂ by means of TDD communication. In TDD, three or more frequencies (f₁-f₃) are allocated alternately and repeatedly to the cells (#1-#3). This arrangement is required to guarantee a sufficient spatial distance between those cells that use the same radio frequency in order to preclude interference from the neighborhood cells. Each frequency allocated to one of the cells #1-#3 is used for the uplink and the downlink alternately each at an equal time interval. The time period allocated to the alternate links is called a time slot.

Fig. 6 illustrates a conventional TDD mobile communication system. This TDD system includes a base station 12₁ and a mobile station 22₁. The base station 12₁ has a signal processing unit 120 connected to the backbone network (e.g., mobile communication network), a transmitter 122, a receiver 123, a switch (SW) 124, a band pass filter (BPF) 125 with a center frequency of f₁, and an antenna 126 for transmitting and receiving radio signals.

The mobile station 22₁ has an input unit 110, such as a microphone 110a or a keyboard 110b, an output unit 111, such as a display 111a or a speaker 111b, a signal processing unit 112, a transmitter 114, a receiver 115, a switch (SW) 116, a band pass filter (BPF) 117 with a center frequency of f₁, and an antenna 118 for transmitting and receiving radio signals.

In operation of the mobile station 22₁ in the TDD system, digital signals supplied from the input unit 110 are converted to baseband signals by the signal processing unit 112. The baseband signals are converted to radio signals by the transmitter 114, which then pass through the switch (SW) 116 and the band pass filter (BPF) 117. Radio signal components of center frequency of f₁ are extracted, and are transmitted from the antenna 118 to the base station 12₁ at a frequency f₁.

The radio signals transmitted from the mobile station 22₁ are received by the antenna 126 of the base station 12₁, and supplied to the receiver 123 via the band pass filter (BPF) 125 and the switch (SW) 124. The received radio signals are converted to digital signals by the receiver 123 and the signal processing unit 120. The digital signals are then transmitted to the backbone network.

On the other hand, in the downlink, digital signals received at the base station 12₁ from the backbone network are converted to radio signals by the signal processing unit 120 and the transmitter 122. The radio signals pass through the switch (SW) 124 and the band pass filter (BPF) 125, in which only those signal components with a center frequency of f₁ are extracted. The radio signals of frequency f₁ are transmitted from the antenna 126 to the mobile station 22₁.

The radio signals transmitted from the base station 12₁ are received by the antenna 118 of the mobile station 22₁, supplied to the receiver 115 via the band pass filter (BPF) 117 and the switch (SW) 116. The radio signals are converted into digital signals through the receiver 115 and the signal processing unit 112, and supplied to the output unit 111.

The switching controllers 121 and 113 of the base station 12₁ and the mobile station 22₁, respectively, superpose control signals on the downlink communication signals so that the base station 12₁ and the mobile station 22₁ synchronize with each other. The switching controllers 121 and 113 switch the switches (SWs) 124 and 116, respectively, based on the control signal supplied from the signal processing unit 120 of the base station.

With this arrangement, during uplink communication, the signal processing unit 120 of the base station 12₁ stops supplying signals to the transmitter 122. Accordingly, the transmitter 122 of the base station 12₁ and the receiver 115 of the mobile station 22₁ suspend operations. On the other hand, during the downlink communication, the signal processing unit 112 of the mobile station 22₁ stops supplying signals to the transmitter 114. Accordingly, the transmitter 114 of the mobile station 22₁ and the receiver 123 of the mobile station 12₁ suspend operations.

In recent years, the volume of data transmitted in mobile radio communication has increased greatly. Unlike voice communication on telephone lines, data communication is likely to produce asymmetric situations in the volume of data between the uplink and the downlink. In particular, as browser phones, which are cellular phones allowing the users to access homepages uploaded in servers on the Internet or use electronic mail, have rapidly spread, digital contents is frequently downloaded from the servers set by providers. As a result, the data amount transmitted on the downlink is likely to be larger than that on the uplink.

In the conventional duplex method (either FDD or TDD), even if the data amount on the downlink is much larger than that on the uplink because of asymmetric transmission volume, an equal radio frequency or a time slot has to be allocated to the uplink and the downlink. For this reason, the radio resources, such as a time slot or a frequency band, are wasted. Besides, since the radio frequency band allocated to mobile radio communication is limited, the transmission capacity of the system is also restricted.

Accordingly, it is an important requirement to eliminate waste of radio resources so as to maximize deterioration of the transmission capacity of the system in the limited radio frequency band. From this viewpoint, the 3^rd Generation Partnership Project (3GPP), which is an organization that investigates technological standards of IMT-2000, has proposed asymmetric duplex as illustrated in Fig. 7.

In this method, a series of frames (F#1, F#2,... F#n), each consisting of a plurality of time slots (for example, four in the example of Fig. 7), are provided. The numbers of time slots allocated to the uplink and the downlink are set asymmetric in order to cope with asymmetric data volume on the uplink and the downlink.

However, this asymmetric duplexing is unsuitable to a large-capacity mobile communication system because data are transmitted in the same radio frequency band in both the uplink and the downlink, and therefore, a continuing wide range of frequency bands must be guaranteed. Unfortunately, the allocated radio frequency bands are overcrowded, and it is difficult to obtain a continuing wide range of frequency bands.

Another problem arises if TDD is applied to W-CDMA systems, and if the data volumes in the uplink and the downlink become asymmetric. In asymmetric TDD-type A-CDMA, as the volume of the communication data increases, high-speed processing is required in generating clock frequencies for the baseband signal and modulating the baseband signal to a radio signal. This causes the structures of both the mobile terminal and the base station to be complicated.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a mobile communication system that can realize an efficient use of the radio frequency resources without complicating the structure, even if the data volume is asymmetric in parallel transmissions on the uplink and the downlink.

To achieve the object, in one aspect of the invention, a mobile communication system includes a base station, and a mobile station configured to communicate with the base station using a radio frequency. This mobile communication system has a first communication mode, a second communication mode, a third communication mode, and a mode switcher that switches the first through third communication mode in accordance with data transfer volumes on the uplink and the downlink. In the first communication mode, a first radio frequency is used for an uplink and a second radio frequency is used for a downlink. In the second communication mode, the first radio frequency is used for the uplink, and the second radio frequency is shared by the uplink and the downlink. In the third communication mode, the second radio frequency is used for the downlink, and the first radio frequency is shared by the uplink and the downlink.

If the communication volumes on the uplink (from the mobile station to the base station) and the downlink (from the base station to the mobile station) are symmetric, the first mode is selected. If the communication volumes on the uplink and the downlink become asymmetric, the mode is switched to the second or the third communication mode, in which either the first or second radio frequency is used for bidirectional communication (e.g., for both the uplink and the downlink) in a duplex manner. With this mobile communication system, radio resources required for radio communication is allocated efficiently to both symmetric and asymmetric communication.

In another aspect of the invention, a mobile communication system that can easily cope with increase of the data transfer volume on the uplink is provided. This mobile communication system includes a base station and a mobile station configured to communicate with the base station, and it has a first communication mode, a second communication mode, and a mode switcher that switches the first and second communication modes in accordance with data transfer volumes on the uplink and the downlink. In the first communication mode, a first radio frequency is used for an uplink and a second radio frequency is used for a downlink. In the second communication mode, the first radio frequency is used for the uplink, and the second radio frequency is shared by the uplink and the downlink.

If the data transfer volume on the uplink increases, the communication mode is switched to the second mode, in which the up channel communication and the down channel communication are switched in a duplex manner at the same second frequency. Since the mode switching causes the downlink frequency (e.g., the second frequency) to be used in a duplex manner, the mode control operation is simplified, without causing the structures of the base station and the mobile station to be complicated.

In still another aspect of the invention, a mobile communication system that can easily cope with increase of the data transfer volume on the downlink is provided. This mobile communication system includes a base station and a mobile station configured to communicate with the base station using a radio frequency, and it has a first communication mode, in which a first radio frequency is used for an uplink and a second radio frequency is used for a downlink, and a second communication mode, in which the second radio frequency is used for the downlink and the first radio frequency is shared by the uplink and the downlink. The system also has a mode switcher configured to switch the first and second communication modes in accordance with data transfer volumes on the uplink and the downlink.

In yet another aspect of the invention, a base station configured to communicate with a mobile terminal in a prescribed geographical is provided. The base station communicates with the mobile terminal using at least two of a first communication mode in which a first radio frequency is used for an uplink and a second radio frequency is used for a downlink; a second communication mode in which the first radio frequency is used for the uplink, and the second radio frequency is shared by the uplink and the downlink; and a third communication mode in which the first radio frequency is shared by the uplink and the downlink and the second radio frequency is used for the downlink. The base station comprises a signal processing unit configured to produce a control signal for designating one of the communication modes, and a switching controller configured to receive the control signal and change the communication mode based on the control signal.

BRIRF DESCRIPTION OF THE DRAWINGS

Other objects, features, and advantages of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which <ul id="ul0001" list-style="none" compact="compact"><li>Fig. 1 illustrates a cell structure of the conventional FDD system;</li><li>Fig. 2 illustrates the concept of conventional FDD;</li><li>Fig. 3 illustrates the structure of a conventional FDD mobile communication system;</li><li>Fig. 4 illustrates a cell structure of the conventional TDD system;</li><li>Fig. 5 illustrates the concept of conventional TDD;</li><li>Fig. 6 illustrates the structure of a conventional TDD mobile communication system;</li><li>Fig. 7 further illustrates the concept of conventional TDD communication;</li><li>Fig. 8 illustrates an example of the cell structure of a mobile communication system according to an embodiment of the present invention;</li><li>Figs. 9A-9C illustrate three types of communication modes used in the mobile communication system according to an embodiment of the invention;</li><li>Fig. 10 illustrates the structure of the mobile communication system according to an embodiment of the invention;</li><li>Fig. 11 illustrates the operation of the mobile communication system in the FDD mode, in which the data volumes on the downlink and the uplink are almost equal;</li><li>Fig. 12 illustrates the operation of the mobile communication system in the FDD/TDD mixed mode A, in which the data volume on the downlink is greater than that on the uplink;</li><li>Fig. 13 illustrates the operation of the mobile communication system in the FDD/TDD mixed mode B, in which the data volume on the uplink is greater than that on the downlink; and</li><li>Fig. 14 illustrates an example of the base station used in the mobile communication system according to the embodiment;</li><li>Fig. 15 illustrates a modification of the base station;</li><li>Fig. 16 illustrates another modification of the base station; and</li><li>Fig. 17 illustrates another example of the cell structure of the mobile communication system according to the invention.</li></ul>

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The details of the preferred embodiment of the invention will now be described with reference to the attached drawings.

Fig. 8 illustrates an example of the cell structure of the mobile communication system according to an embodiment. The mobile communication system comprises base stations (BS1-BS3) 10₁-10₃ positioned in cells 100, 200, 300, respectively, and mobile stations (MS1-MS3) 201-20₃ connected to the associated base stations 10₁-10₃ via radio communication to achieve voice communication, non-telephone communication, and other types of communication.

In this system, radio frequency f₁ is used on the uplink from the mobile stations 20₁-20₃ to the base stations 10₁-10₃, and radio frequency f₂ is used on the downlink from the base stations 10₁-10₃ to the mobile stations 20₁-20₃. These frequencies f₁ and f₂ are equally used in all the cells.

Figs. 9A-9C illustrate three types of communication modes used in the mobile communication system in an embodiment. In this example, one of the three modes is selected depending on the communication volumes on the uplink and the downlink. These three modes include the FDD mode (Fig. 9A), the FDD/TDD mixed mode A (Fig. 9B), and the FDD/TDD mixed mode B (Fig. 9C)

If the data volume on the uplink (from the mobile station to the base station) is almost the same as that on the downlink (from the base station to the mobile station), the FDD mode is selected. In the FDD mode, for example, frequency f₁ is used exclusively for the uplink, and frequency f₂ is used exclusively for the downlink. This mode is similar to the conventional FDD technique.

If the data volume on the downlink becomes greater than that on the uplink, the mode is switched to the FDD/TDD mixed mode A, in which the uplink frequency f₁ is shared between the uplink and the downlink, while the downlink frequency f₂ is still used exclusively for the downlink. In other words, the downlink channel and the uplink share the uplink frequency f₁ in alternate time slots, as in the TDD technique. In the example shown in Fig. 9B, the time slots (intervals) for the uplink and the downlink communication are set equal, and the data capacity ratio of the uplink to the downlink becomes 1 to 3 (1:3).

If the data volume on the downlink becomes smaller than that on the uplink, the FDD/TDD mixed mode B is selected. In this mode, the uplink frequency f₁ is used exclusively for the uplink, and the downlink frequency f₂ is shared between the uplink and the downlink in alternate time slots, as in TDD. In this case, the communication ratio of the uplink to the downlink is 3 to 1 (3:1).

By switching these three modes depending on the situation, the system can deal with the asymmetric radio communication.

Fig. 10 illustrates the mobile communication system according to an embodiment, which includes a mobile station 20₁ and a base station 10₁. The mobile station 20₁ has an input unit 31, such as a microphone 31a or a keyboard 31b, an output unit 32, such as a display 32a or a speaker 32b, a signal processing unit 33, a switching controller 34, a serial to parallel converter (S/P) 35, and a parallel to serial converter (P/S) 36. The mobile station 20₁ also has two transmitters 37 and 38, two receivers 39 and 40, two switches (SW1 and SW2) 41 and 42, a first band pass filter (BPF) 43 with a center frequency of f₁, a second band pass filter (BPF) 44 with a center frequency of f₂, and two antennas 45 and 46. Each antenna is used for both transmitting and receiving radio signals.

The base station 10₁ has a signal processing unit 50 connected to the backbone network (e.g., mobile communication network), two transmitters 51 and 52, a parallel to serial converter (P/S) 54, a serial to parallel converter (S/P) 55, and two receivers 56 and 57. The base station 10₁ also has two switches (SW1 and SW2) 58 and 59, a first band pass filter (BPF) 60 with a center frequency of f₁, a second band pass filter (BPF) 61 with a center frequency of f_2, and antennas 62, 63, each antenna used for both transmitting and receiving radio signals.

In operation, a digital signal input from the input unit 31 of the mobile station 20₁ is converted to a baseband signal by the signal processing unit 33, and divided into two lines of baseband signals by the serial to parallel converter 35. These baseband signals are converted to radio signals by the first and second transmitters 37 and 38. The radio signals are supplied to the first and second band pass filters (BPS) 43 and 44 via the first and second switches (SW1 and SW2) 41 and 42, respectively. The first band pass filter 43 extracts the signal component of frequency f₁, and the second band pass filter 44 extracts the signal component of frequency f₂. These radio signals of frequencies f₁ and f₂ are to be transmitted to the base station 10₁ via the antennas 45 and 46, respectively.

The radio signals transmitted from the mobile station 20₁ at frequencies f₁ and f₂ are received by the associated antennas 62 and 63, and supplied to the first and second receivers 56 and 57, respectively, via the associated band pass filters 60 and 61, and the associated switches 58 and 59. The radio signals are converted to baseband signals by the receivers 56 and 57, and synthesized by the parallel to serial converter (P/S) 54. The composite baseband signal is converted to a digital signal by the signal processing unit 50, and transmitted to the backbone network.

On the downlink, the digital signal supplied from the backbone network is converted to a baseband signal by the signal processing unit 50. The baseband signal is divided into two lines of baseband signals by the serial to parallel converter (S/P) 55, and then converted to radio signals by the associated transmitters 51 and 52. These two radio signals pass through the associated switches (SW1 and SW2) 58 and 59, and the associated band pass filters (BPFs) 60 and 61. The first band pass filter 60 extracts only the signal component of frequency f₁, and the second band pass filter 61 extracts only the signal component of frequency f₂. These radio signals are transmitted to the mobile station 20₁ from the associated antennas 62 and 63.

The radio signals transmitted from the base station 10₁ at frequencies f₁ and f₂ are received by the associated antennas 45 and 46 of the mobile station 20₁, and supplied to the receivers 39 and 40 via the associated band pass filters 43 and 44, and the switches (SW1 and SW2) 41 and 42, respectively. The radio signals are then converted to baseband signals by the receivers 39 and 40. The two lines of baseband signals are synthesized by the parallel to serial converter (P/S) 36. The composite signal is converted to a digital signal by the signal processing unit 33, and output to the output unit 32.

The switching controller 53 of the base station 10₁ and the switching controller 34 of the mobile station 20₁ superpose control signals on the downlink to allow the base station 10₁ and the mobile station 20₁ to synchronize with each other. Under the synchronization, the switching controllers 53 and 34 switch the communication mode of the system based on the control signal supplied from the signal processing unit 50 of the base station 10₁. The switching controllers 53 and 34, and the signal processing unit 50 constitute a mode switcher.

In other words, based on the control signal from the signal processing unit 50, the switching controllers 53 and 34 regulate operations of the first switches (SW1) 58 and 41, the second switches (SW2) 59 and 42, the serial to parallel converters (S/P) 55 and 35, and the parallel to serial converters (P/S) 54 and 36 in three different manners illustrated in Figs. 11-13.

With the data volume on the uplink almost the same with that on the downlink, the mobile communication system operates in the FDD mode, as illustrated in Figs. 11 A and 11B. In the FDD mode, radio frequency f₁ is used exclusively for the uplink, and radio frequency f₂ is used exclusively for the downlink, as illustrated in Fig. 11A. The control signal generated by the signal processing unit 50 of the base station 10₁ contains commands for regulating the first switches (SW1) 58 and 41, the second switches (SW2) 59 and 42, the serial to parallel converters (S/P) 55 and 35, and the parallel to serial converters (P/S) 54 and 36, so that the mobile station 20₁ transmits at frequency f₁ and receives at frequency f₂, and that the base station receives at frequency f₁ and transmits at frequency f₂.

To be more precise, in the mobile station 20₁, the first switch (SW1) 41 is connected to the transmitter A (37), and the second switch (SW2) 42 is connected to the receiver B (40), as illustrated in Fig. 11B. No serial to parallel conversion is carried out by S/P converter 35, and all the signals from the signal processing unit 33 are supplied to the transmitter A (37). Similarly, no parallel to serial conversion is carried out by P/S converter 36, and all the signals received at the receiver B (40) are supplied to the signal processing unit 33.

On the other hand, in the base station 10₁, the first switch (SW1) is connected to the receiver A (56), and the second switch (SW2) is connected to the transmitter B (52). No serial to parallel conversion is carried out by S/P converter 55, and all the signals from the signal processing unit 50 are supplied to the transmitter B (52). Similarly, no parallel to serial conversion is carried out by P/S converter 54, and all the signals received at the receiver A (56) are supplied to the signal processing unit 50.

If the data volume on the downlink becomes greater than that on the uplink, the mode is changed to the FDD/TDD mixed mode A, as illustrated in Figs. 12A and 12B. In this mode, radio frequency f₁ on the uplink is shared by the up channel and the down channel in the time division duplex (TDD) manner, as illustrated in Fig. 12A. The control signal generated by the signal processing unit 50 of the base station 10₁ contains control commands for regulating the first switches (SW1) 58 and 41, the second switches (SW2) 59 and 42, the serial to parallel converters (S/P) 55 and 35, and the parallel to serial converters (P/S) 54 and 36_, so that the base station 10₁ transmits and the mobile station 20₁ receives at frequency f₂, and that the uplink frequency f₁ is used alternately by the base station 20₁ and the mobile station 10₁ to transfer data between them.

To be more precise, in the mobile station 10₁, the first switch (SW1) 41 is switched alternately between the transmitter A (37) and the receiver A (39), and the second switch (SW2) 42 is connected to the receiver B (40), as illustrated in Fig. 12B. No serial to parallel conversion is carried out by S/P converter 35, and all the signals from the signal processing unit 33 are supplied to the transmitter A (37). Parallel to serial conversion is carried out by P/S converter 36 so that the signal ratio of the receiver A (39) to the receiver B (40) becomes 1 to 2 (1:2).

In the base station 10₁, the first switch (SW1) 58 is switched alternately between the receiver A (56) and the transmitter A (51), and the second switch (SW2) 59 is connected to the transmitter B (52). Serial to parallel conversion is carried out by S/P converter 55 so that the signal ratio of the transmitter A (51) to transmitter B (52) becomes 1 to 2 (1:2). No parallel to serial conversion is carried out by P/S converter 54, and the all the signals from the receiver A (56) are supplied to the signal processing unit 50.

If the data volume on the uplink becomes greater than that on the downlink, the mode is changed to the FDD/TDD mixed mode B, as is illustrated in Figs. 13A and 13B. In this mode, the downlink frequency f2 is shared by the up channel and the down channel in the time division duplex (TDD) manner, as illustrated in Fig. 13A. The control signal generated by the signal processing unit 50 of the base station 10₁ contains control commands for regulating the first switches (SW1) 58 and 41, the second switches (SW2) 59 and 42, the serial to parallel converters (S/P) 55 and 35, and the parallel to serial converters (P/S) 54 and 36, so that the base station 10₁ transmits and the mobile station 20₁ receives at frequency f₁, and that the downlink frequency f₂ is used alternately by the base station 20₁ and the mobile station 10₁ to transfer data between them.

To be more precise, in the mobile station 10₁, the first switch (SW1) 41 is connected to the transmitter A (37), and the second switch (SW2) is switched alternately between the transmitter B (38) and the receiver B (40), as illustrated in Fig. 13B. Serial to parallel conversion is carried out by S/P converter 35 so that the signal ratio of the transmitter A (37) to transmitter B (38) becomes 2 to 1 (2:1). No parallel to serial conversion is carried out by P/S converter 36, and the all the signals from the receiver B (40) are supplied to the signal processing unit 33.

In the base station 10₁, the first switch (SW1) 58 is connected to the receiver A (56), and the second switch (SW2) 59 is switched alternately between the receiver B (57) and the transmitter B (52). No serial to parallel conversion is carried out by S/P converter 55, and all the signals from the signal processing unit 50 are supplied to the transmitter B (52). Parallel to serial conversion is carried out by P/S converter 54 so that the signal ratio of the receiver A (39) to the receiver B (40) becomes 2 to 1 (2:1).

In this manner, the mobile communication system according to the embodiment changes the communication mode depending on the data volumes on the uplink and the downlink. With the substantially equal data volumes between the uplink and the downlink, radio frequencies f₁ and f₂ are used exclusively for the uplink and the downlink, respectively. With the uplink data volume increased over the downlink data volume, the downlink frequency f₂ is shared between the uplink and the downlink in the TDD manner. With the downlink data volume increased over the uplink data volume, then the uplink frequency f₁ is shared between the uplink and the downlink in the TDD manner, so that the up channel communication and the down channel communication are carried out alternately. With this arrangement, the radio resources are appropriately allocated to the links in accordance with the data volumes on the uplink and the downlink. As a result, the radio resources are utilized efficiently.

The mode switcher, which comprises the signal processing unit 50 of the base station 10₁ and the switching controllers 53 and 34, also controls the time slot allocation ratio between the uplink and the downlink. In the example shown in Fig. 9B, the time slots of the uplink frequency f₁ are allocated to the uplink and the downlink alternately at the same ratio. However, this allocation rate may be adjusted in accordance with the data transfer volumes in the uplink and downlink. For example, the time slots may be allocated to the uplink and the downlink at a ratio of 1 to 2, so that the up channel uses frequency f₁ one of every three time slots. In this case, the signal processing unit 50 generates a control command that regulates the switchers, the transmitters, the receivers, the P/S converters, and the S/P converters so as to operate at an appropriate timing and ratio.

Although the explanation has been made using an example in which the three communication modes are used, only two of them may be used. For example, the FDD mode and the FDD/TDD mixed mode A are used and the radio communication may be switched between these two modes if the downlink generally has a heavy traffic. Alternatively, the FDD mode and the FDD/TDD mixed mode B are used if the uplink traffic is generally heavy.

Next, the mode switching technique will be explained in more detail below. The mode change described above may be implemented in cooperation with any one or any combination of the means described below.

(1) Remote Monitoring Controller

Fig. 14 illustrates a modification of the base station 10₁, which carries out mode switching. The signal processing unit 50 includes a remote monitoring controller (RMC) 50a and a control signal generator (CSG) 50b. The remote monitoring controller 50a receives via the backbone network a monitoring control signal from, for example, a monitoring station that monitors the data transfer volumes on the uplink and the downlink. The remote monitoring controller 50a selects an appropriate mode from the FDD mode, FDD/TDD mixed mode A, and FDD/TDD mixed mode B illustrated in Figs. 11-13, based on the received monitoring signal.

The selected mode is supplied to the control signal generator 50b. The control signal generator 50b generates a control signal based on the selected mode. The control signal is supplied to the switching controller 53, which then switches the first and second switches (SW1 and SW2) 58 and 59, and controls the operations of the P/S converter 54 and the S/P converter 55, in an appropriate manner. This control signal is supplied to the switching controller 34 of the mobile station 20₁ on the downlink, and it causes the switching controller 34 to control the connections of the switches (SW1 and SW2) 41 and 42 and the operations of the S/P converter 35 and the P/S converter 36.

(2) Time Management Controller

Fig. 15 illustrates another modification of the base station 10₁. The switching controller 53 has a time schedule table (TST) 53a. In this modification, the switching controller 53 functions also as a time management controller. For example, the monitoring station takes statistics of the time-varying data volumes on the uplink and the downlink, and makes up a time schedule of a day or a week for mode change in advance, based on the statistics result. The time schedule is incorporated in the switching controller 53. The switching controller 53 controls the connections of the switches (SW1 and SW2) 58 and 59 and the operations of the P/S converter 54 and the s/P converter 55. Although not illustrated in Fig. 15, a corresponding time schedule (i.e., a time schedule table) is also incorporated in the switching controller 34 of the mobile station 20₁. Thus, the communication mode of the mobile communication system is automatically switched in accordance with the prescribed time schedule.

(3) Communication Observation Controller

Fig. 16 illustrates still another modification of the base station 10₁. The signal processing unit 50 has a control signal generator (CSG) 50b and a communication observation controller 50c. The communication observation controller 50c includes a communication volume monitoring (CVM) unit 50d for measuring the communication volumes on the uplink and the downlink, and a comparator 50e for comparing the measurements of the uplink and the downlink. The comparison result is supplied to the control signal generator (CSG) 50b, which generates a control signal for causing the switching controller 53 to appropriately switch the connection states of the switches (SW1 and SW2) 58 and 59 and the operations of the P/S converter 54 and the S/P converter 55. Although not illustrated in Fig. 16, the control signal is supplied to the switching controller 34 of the mobile station 20₁, and the switches 41 and 42, the S/P converter 35, and the P/S converter 36 are appropriately controlled. With this arrangement, mode control centralizes in each of the base stations 10₁-10₃ (Fig. 8), using the measuring results obtained at the base station.

The mode switch operations may be carried out independently among the base stations 10₁-10₃, or alternatively, all of or some neighborhood base stations may change the communication mode in synchronization with each other in order to prevent interference in carrier propagation. In the latter case, the transmitters 51, 52 and the receivers 56, 57 of the base station 10₁, and the receivers 39, 40 and the transmitters 37, 38 of the mobile station 20₁ synchronize with each other. In addition, if the FDD/TDD mixed mode A or B is selected, the up channel communication and the down channel communication may be switched using the same frequency after the base stations of all of or some neighborhood cells are synchronized with each other.

The mode switcher of the mobile communication system is not limited to the above-described examples. For example, a system manager that monitors and manages the mobile communication system may manually switch the communication mode of the system.

Fig. 17 illustrates another example of the cell structure. In this frame structure, adjacent cells use different combinations of radio frequencies. For example, cell 100 uses frequencies f₁ and f₂, the adjacent cell 200 uses frequencies f₂ and f₃, and another adjacent cell 300 uses frequencies f₂ are f₄. This arrangement can effectively prevent interference, which may occur in the cell structure shown in Fig. 8, in which the same frequencies f₁ and f₂ are used in all the cells. With the cell structure of Fig. 8, if the downlink data volume becomes greater than the uplink data volume, and therefore, uplink frequency f₁ is used in the TDD manner, then, the base station may receive radio frequency f₁ from the adjacent cell, which causes undesirable interference. By allocating different frequencies to the adjacent cells, as illustrated in Fig. 17, signal interference can be prevented effectively in the mobile communication system.

Preferably, the switching timing of the switches 41, 42, 58, 59 of the mobile station 10₁ and the base station 20₁, and the signal ratio defined by the serial to parallel converters 35, 55 and the parallel to serial converters 36, 54 are adjusted more finely. This arrangement allows the system to cover possible contingencies in the asymmetric data communication between the uplink and the downlink, and consequently, the resource utilization efficiency can be further improved.

Basically, the mobile communication system described in the embodiment operates in the FDD manner, and uses the TDD method as required depending on the data transfer volumes on the uplink and the downlink. Consequently, the mobile communication system does not require the clock rate and the modulation rate to be extremely increased, unlike in the conventional asymmetric TDD system, and the system structure, including the signal processing units and the transmitters/receivers of the base station and the mobile station, can be simplified.

Although the invention has been described using specific examples, the present invention is not limited to those examples, but includes many substitutions and modifications without departing from the scope of the invention. For example, although in the embodiment three communication modes are used, two of these modes (for example, the FDD mode and the FDD/TDD mixed mode A, or the FDD mode and the FDD/TDD mixed mode B) may be used. In this case, the mode switcher switches the system operation between the selected two modes.

This patent application is based on and claims the benefit of the earlier filing date of Japanese patent application No. 2001-146852 filed May 16, 2001, the entire contents of which are hereby incorporated by reference.