Abstract:
A radio apparatus includes a first radio-frequency unit configured to receive a reception signal transmitted from a communication partner, and configured to transmit a transmission signal to the communication partner, a second radio-frequency unit configured to receive a reception signal transmitted from the communication partner, configured to transmit a transmission signal to the communication partner, a baseband processing unit configured to be supplied with the reception signal from each of the first and second radio-frequency units, and supply, to each of the first and second radio-frequency units, data to be transmitted to the communication partner, and the baseband processing unit configured to generate a reference signal supplied to each of the first radio-frequency unit and the second radio-frequency unit, and a digital signal supplying unit configured to supply a digital signal containing the reference signal from the baseband processing unit to each of the first and second units.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2005-283631, filed Sep. 29, 2005, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a radio apparatus having a digital interface and, more particularly, to a radio apparatus having a plurality of radio units.  
         [0004]     2. Description of the Related Art  
         [0005]     Conventionally, in a radio apparatus including an RF (Radio Frequency) unit (radio unit) and a baseband processing unit, an interface includes an analog signal line and a digital and/or analog control line. However, recently, along with the spread of CMOS (Complementary Metal-Oxide Semiconductor) RF-ICs (Integrated Circuit), an ADC (Analog/Digital Converter) or DAC (Digital/Analog Converter) can be incorporated in the RF-IC. Under-this situation, so-called DigRF for connecting the RF-IC to a digital IC via a digital interface is standardized (see, e.g., [searched on Aug. 4, 2005] in the Internet &lt;URL: http://146.101.169.51/DigRF %20Standard %20v112.pdf&gt; “DigRF BASEBAND/RF DIGITAL INTERFACE SPECIFICATION Logical, Electrical and Timing Characteristics EGPRS Version, Digital Interface Working Group, Rapporteur: Andrew Fogg, TTPCom, Version 1.12”, see the drawing on p. 6).  
         [0006]     On the other hand, recently, a scheme called MIMO (Multi-Input, Multi-Output) which uses a plurality of radio units and antennas to increase the transmission speed has been researched and developed, and put into practical use. In such radio apparatus, signals must be transmitted and received, or controlled for the respective RF units.  
         [0007]     However, since DigRF is originally a standard for GSM (Global System for Mobile communications), it is not premised on an arrangement having a plurality of RF units, such as MIMO which is examined to be applied to a next-generation wireless LAN and next-generation cellular phone. More specifically, in the MIMO-compatible radio apparatus, quartz oscillators for generating reference signals must be arranged in the respective RF units. However, since the characteristics of the respective oscillators vary, the characteristics of the signals to be demodulated and modulated by the radio units also vary, thus increasing an error rate.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     In accordance with a first aspect of the invention, there is provided a radio apparatus comprising: a first radio-frequency unit configured to receive a reception signal transmitted from a communication partner, and configured to transmit a transmission signal to the communication partner; a second radio-frequency unit configured to receive a reception signal transmitted from the communication partner, configured to transmit a transmission signal to the communication partner; a baseband processing unit configured to be supplied with the reception signal from each of the first radio-frequency unit and the second radio-frequency unit, and supply, to each of the first radio-frequency unit and the second radio-frequency unit, data to be transmitted to the communication partner, and the baseband processing unit configured to generate a reference signal supplied to each of the first radio-frequency unit and the second radio-frequency unit; and a digital signal supplying unit configured to supply a digital signal containing the reference signal from the baseband processing unit to each of the first radio-frequency unit and the second radio-frequency unit.  
         [0009]     In accordance with a second aspect of the invention, there is provided a radio apparatus comprising: a first radio-frequency unit configured to receive a reception signal transmitted from a communication partner, and configured to transmit a transmission signal to the communication partner; a second radio-frequency unit configured to receive a reception signal transmitted from the a communication partner, configured to transmit a transmission signal to the communication partner; a baseband processing unit configured to be supplied with the reception signal, and supply, to each of the first radio-frequency unit and the second radio-frequency unit, data to be transmitted to the communication partner, and the baseband processing unit configured to generate at least one of a switching control signal for switching a transmission operation and a reception operation in each of the first radio-frequency unit and the second radio-frequency unit, a power consumption control signal for changing power consumed by each of the first radio-frequency unit and the second radio-frequency unit, a transmission power control signal for changing transmission power in each of the first radio-frequency unit and the second radio-frequency unit, and a setting control signal for setting an oscillation frequency of each of the first radio-frequency unit and the second radio-frequency unit; and a digital signal supplying unit configured to transmit a digital signal containing the control signal from the baseband processing unit to each of the first radio-frequency unit and the second radio-frequency unit.  
         [0010]     In accordance with a third aspect of the invention, there is provided a radio apparatus comprising: a first radio-frequency unit configured to receive a reception signal transmitted from a communication partner; a second radio-frequency unit configured to receive a reception signal transmitted from the communication partner; a baseband processing unit configured to be supplied with the reception signal, and supply, to each of the first radio-frequency unit and the second radio-frequency unit, data to be transmitted to the communication partner, and configured to supply power to the baseband processing unit and each of the first radio-frequency unit and the second radio-frequency unit; and a transmitting unit configured to transmit, from the baseband processing unit to each of the first radio-frequency unit and the second radio-frequency unit, power to be supplied to each of the first radio-frequency unit and the second radio-frequency unit.  
         [0011]     In accordance with a fourth aspect of the invention, there is provided a radio apparatus comprising: a first radio-frequency unit configured to receive a reception signal transmitted from a communication partner; a second radio-frequency unit configured to receive a reception signal transmitted from the communication partner; a baseband processing unit configured to be supplied with the reception signal, and supply, to each of the first radio-frequency unit and the second radio-frequency unit, data to be transmitted to the communication partner, and configured to supply, to each of the first radio-frequency unit and the second radio-frequency unit, a power supply voltage output from an external battery; and a supplying unit configured to supply the power supply voltage from the baseband processing unit to each of the first radio-frequency unit and the second radio-frequency unit.  
         [0012]     In accordance with a fifth aspect of the invention, there is provided a radio apparatus comprising: a first radio-frequency unit configured to receive a reception signal transmitted from a communication partner, and a transmission signal transmitter which transmits a transmission signal to the communication partner; a second radio-frequency unit configured to receive a reception signal transmitted from the communication partner, and a transmission signal transmitter which transmits a transmission signal to the communication partner; a baseband processing unit configured to be supplied with the reception signal, and supply, to each of the first radio-frequency unit and the second radio-frequency unit, data to be transmitted to the communication partner, and configured to generate an oscillation frequency signal for setting a transmission frequency or a reception frequency in each of the first radio-frequency unit and the second radio-frequency unit; and a supplying unit configured to supply the oscillation frequency signal from the baseband processing unit to each of the first radio-frequency unit and the second radio-frequency unit. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0013]      FIG. 1  is a block diagram of a radio apparatus according to the first embodiment of the present invention;  
         [0014]      FIG. 2  is a block diagram of a radio apparatus according to the second embodiment of the present invention;  
         [0015]      FIG. 3  is a timing chart showing a CLOCK signal, DATA signal, and STROBE signal which are transmitted via a digital interface shown in  FIG. 2 ;  
         [0016]      FIG. 4  is a block diagram of a radio apparatus according to the modification of the second embodiment of the present invention;  
         [0017]      FIG. 5  is a block diagram of a radio apparatus according to the third embodiment of the present invention;  
         [0018]      FIG. 6  is a view showing a transmission period of a control signal in the radio apparatus shown in  FIG. 5 ;  
         [0019]      FIG. 7  is a block diagram of a radio apparatus according to the modification of the third embodiment of the present invention;  
         [0020]      FIG. 8  is a block diagram of a radio apparatus according to the fourth embodiment of the present invention;  
         [0021]      FIG. 9  is a block diagram showing a detailed power feeding scheme in the radio apparatus shown in  FIG. 8 ; and  
         [0022]      FIG. 10  is a block diagram of a radio apparatus according to the fifth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     A radio apparatus according to each embodiment of the present invention will be described below with reference to the accompanying drawing.  
         [0024]     The radio apparatus according to the embodiment of the present invention can prevent degradation of its characteristics, with a simpler arrangement.  
       First Embodiment  
       [0025]     A radio apparatus according to the first embodiment of the present invention will be described with reference to  FIG. 1 .  FIG. 1  is a block diagram when a baseband processing unit has a reference signal source, and a reference signal from the reference signal source is distributed to radio units via digital interfaces.  
         [0026]     A plurality of antennas  101 , RF units  102 , and digital interfaces  111 , and a baseband processing unit  112  are provided. Each RF unit  102  includes a radio unit  103 , synthesizer (VCO: Voltage-Controlled Oscillator)  109 , and PLL circuit  110 . The radio unit  103  serves as an RF-IC, and incorporates a receiver (RX)  104 , a transmitter (TX)  105 , a reception ADC  106 , a transmission DAC  107 , and a serial-to-parallel converter (S/P)  108 . The baseband processing unit  112  serves as a baseband IC, and incorporates a serial-to-parallel converter  113 , a temperature compensation quartz oscillator (TCXO)  114  serving as the reference signal source, and a digital signal processing unit  115 . Note that in each of the following embodiments, a MIMO radio apparatus having three RF units  102  is exemplified. In this arrangement, the receiver  104  receives a plurality of different signals transmitted from a communication partner at the same frequency. The baseband processing unit  112  demultiplexes the signal received by each receiver, and multiplexes the demultiplexed signals, thereby reconstructing one piece of information. On the other hand, each transmitter  105  transmits different signals at the same frequency via the antenna  101 . The synthesizer  109  is an oscillator which oscillates at a predetermined oscillation frequency. The oscillation frequency changes depending on an applied voltage.  
         [0027]     The TCXO  114  generates a reference signal corresponding to a reference frequency used in the PLL circuit  110 .  
         [0028]     The PLL circuit  110  generally includes a phase detector (not shown), loop filter (not shown), and frequency divider (not shown). The PLL circuit  110  compares the oscillation frequency of the synthesizer  109  with the frequency of a reference frequency generator (TCXO  114  in the first embodiment). The PLL circuit  110  then controls to pass the output from the phase detector through the loop filter, feed back the output to the synthesizer  109 , and accurately match the phases of the reference frequency and the output.  
         [0029]     The serial-to-parallel converter  108  in each radio unit  103  is connected to the serial-to-parallel converter  113  in each baseband processing unit  112  via each digital interface  111 .  
         [0030]     As the characteristic feature of the first embodiment, the reference signal generated by the TCXO  114  is transmitted to each RF unit  102  via the digital interface  111 . More specifically, this feature is effective for the radio apparatus including the plurality of RF units  102  as shown in  FIG. 1 .  
         [0031]     The reference signal transmitted from the TCXO  114  via the digital interface  111  is used as a PLL reference signal for the synthesizers  109  incorporated in the respective RF units  102 . In this arrangement, all the synthesizers  109  can share a common reference signal, and the oscillation frequencies of the synthesizers  109  can match each other.  
         [0032]     Since the remaining components are generally used, a detailed description will be omitted, and the operations of the remaining components will be briefly described.  
         [0033]     (Reception) The antenna  101  receives a signal, the receiver  104  receives this signal, and the ADC  106  converts the received signal into a digital signal. The serial-to-parallel converter  108  converts this digital signal into a serial signal, and this serial signal is transmitted to the serial-to-parallel converter  113  in the baseband processing unit  112  via the digital interface  111 . The serial-to-parallel converter  113  reconverts this serial signal into the digital signal, and the digital signal processing unit  115  demodulates this digital signal.  
         [0034]     (Transmission) The digital signal processing unit  115  generates a transmission signal, and the serial-to-parallel converter  113  converts this transmission signal into a serial signal. This serial signal is transmitted to the serial-to-parallel converter  108  in the RF unit  102  via the digital interface  111 . The serial-to-parallel converter  108  converts this serial signal into a digital signal, and the DAC  107  converts this digital signal into an analog signal. The transmitter  105  transmits this analog signal as a transmission signal from the antenna  101 .  
         [0035]     As described above, in the radio apparatus according to the first embodiment, all the synthesizers  109  can share the common reference signal generated by the TCXO  114 , and the oscillation frequencies of the synthesizers  109  can match each other. In this arrangement, the quartz oscillator need not be arranged in each RF unit, and the structure of the RF unit becomes simple to prevent the characteristics of the radio apparatus from degrading. The embodiment of the present invention is applied to the MIMO radio apparatus in which each receiver receives the plurality of different signals at the same frequency, thereby implementing the radio apparatus having higher performance.  
       Second Embodiment  
       [0036]     A radio apparatus according to the second embodiment of the present invention will be described with reference to  FIGS. 2 and 3 . As the characteristic feature of the second embodiment, a reference signal generated by a TCXO  114  in the first embodiment is transmitted as a clock of a digital interface  201 .  FIG. 2  is a block diagram when the signal from the TCXO is used as the reference clock of the digital interface, and the reference signal is clock-extracted from the transmitted serial bus signal on a radio unit side as the reference of a synthesizer. The same reference numerals described above denote the same components as those in the second embodiment, and a description thereof will be omitted.  
         [0037]     In the second embodiment, the operation clock of the digital interface  201  serves as the reference signal of the TCXO  114 . More specifically, as shown in  FIG. 3 , the reference signal of the TCXO  114  is used as CLOCK when serial transmission is performed via three interfaces, i.e., CLOCK, DATA, and STROBE interfaces.  
         [0038]     In this arrangement, since CLOCK of the serial transmission data is used as a PLL reference signal in a radio unit  103 , local oscillation outputs of the respective RF units  102  can match each other.  
       Modification of Second Embodiment  
       [0039]     A modification of the second embodiment will be described with reference to  FIG. 4 . Similar to  FIG. 2 ,  FIG. 4  is a block diagram when the signal from the TCXO is used as the reference clock of the digital interface, and the reference signal is clock-extracted from the transmitted serial bus signal on a radio unit side as the reference of a synthesizer.  
         [0040]     In this modification, each radio unit  103  additionally includes a clock extraction unit  402 . Each RF unit  102  is connected to a baseband processing unit  112  via a digital interface  401  using one signal line.  
         [0041]     Only a DATA signal is transmitted via the digital interface  401  serving as one serial signal line.  
         [0042]     The clock extraction unit  402  receives the DATA signal, and clock extraction is done from this DATA signal. Since the clock-extracted signal is the same as the reference signal of the TCXO  114 , this signal can be used as the reference signal for the synthesizers. As a result, similar to the above embodiments, local oscillation outputs of the respective radio units  103  can match each other.  
         [0043]     In the description of the second embodiment, the operation clock of a digital interface  201  is completely the same as the reference signal from the TCXO  114 . However, the clock signal and the reference signal may have the same phase. That is, for example, the same effect can be obtained even when an integral multiple (using a frequency multiplier) or integral fraction (using a frequency divider) of the frequency of the reference signal from the TCXO  114  serves as the operation clock of the digital interface  201  or  401 .  
         [0044]     As described above, in the second embodiment, all the synthesizers  109  can also share a common reference signal generated by the TCXO  114 , and the oscillation frequencies of the synthesizers  109  can match each other. In this arrangement, a quartz oscillator need not be arranged in each RF unit, and the structure of the RF unit becomes simple to prevent the characteristics of the radio apparatus from degrading. Additionally, since a common signal from TCXO is used as the reference signal of the digital interface, the structure becomes very simple.  
       Third Embodiment  
       [0045]     A radio apparatus according to the third embodiment of the present invention will be described with reference to  FIGS. 5 and 6 .  FIG. 5  is a block diagram when a radio unit control signal is transmitted via a digital interface, this control signal includes at least a battery saving signal, transmission/reception switching signal, transmission power control signal, and synthesizer frequency setting signal from a baseband processing unit, and an RSSI signal is transmitted from the radio unit. In the third embodiment, control signals for controlling an RF unit  102  and baseband processing unit  112  are transmitted via a digital interface  111  described in the first embodiment. In the radio apparatus in the third embodiment, each RF unit  102  includes a radio unit  103 , synthesizer (VCO: Voltage-Controlled Oscillator)  109 , and PLL circuit  110 , similar to that in the above-described embodiments. Additionally, in the third embodiment, the radio apparatus includes an RSSI detection unit  502  and control unit  503 . The synthesizer  109  receives the control signal (a synthesizer frequency setting signal to be described later) from the control unit  503 , and oscillates at a frequency designated in accordance with this control signal.  
         [0046]     The RSSI detection unit  502  detects the strength information of the signal received by each RF unit  102 , and outputs this strength information as an RSSI (Received Signal Strength Indicator) signal.  
         [0047]     The control unit  503  generates various control signals, e.g., a battery saving signal, transmission/reception switching signal, transmission power control signal, and synthesizer frequency setting signal, and outputs the generated control signals to a digital signal processing unit  115  or serial-to-parallel converter  113 . These control signals are then output to each RF unit  102  via the digital interface  111 .  
         [0048]     Examples of the control signals transmitted from the baseband processing unit  112  to the RF unit  102  are as follows.  
         [0049]     (1) Battery saving signal: This signal is a signal for shifting the power supply of each RF unit  102  to a battery saving mode during, e.g., a standby period in order to reduce power consumption. If circumstances require, only some of the plurality of RF units  102  may be activated, and the remaining RF units may be shifted to the battery saving mode. Generally, this signal is a variable signal capable of changing the power consumption of the radio apparatus.  
         [0050]     (2) Transmission/reception switching signal: In a packet communication system such as a wireless LAN, transmission and reception are not simultaneously carried out. In this case, a switching signal for switching between transmission and reception is transmitted to each RF unit  102 .  
         [0051]     (3) Transmission power control signal: The baseband processing unit  112  determines to change transmission power in accordance with a base station (access point) and propagation environment, and this control is implemented by the digital signal processing unit  115 .  
         [0052]     (4) Synthesizer frequency setting signal: This signal serves as an oscillation frequency setting signal for determining the oscillation frequency of a synthesizer in each RF unit  102 , and is distributed from the baseband processing unit  112  to each of the RF units  102 . In MIMO, the synthesizers in the respective RF units  102  use the same oscillation frequency. In the radio apparatus such as multichannel radio apparatus which performs transmission and reception by using a plurality of carrier frequencies, the oscillation frequencies of the synthesizers in the respective RF units  102  are set to different values.  
         [0053]     Examples of the control signals transmitted from the RF unit  102  to the baseband processing unit  112  are as follows.  
         [0054]     (1) RSSI signal indicating the strength of the received signal: The signal indicates a numerical value representing the strength of the signal received by each RF unit  102 . For example, in MIMO, based on these numerical values, the baseband processing unit  112  performs calculation for demultiplexing the signal received by each receiver, as a MIMO reception process.  
         [0055]     Conventionally, in the radio apparatus having the digital interface, the control signal is transmitted via a physically different signal line. Hence, the number of lines between the radio unit and the baseband processing unit increases, thus posing a problem especially in the radio apparatus having the plurality of radio units. However, in the third embodiment, since one digital interface  111  has the transmission functions of signal and control lines, the corresponding signals can be transmitted and received via the digital interface  111 , thus reducing the number of lines as a great merit. Especially, in the radio apparatus according to the third embodiment, the baseband processing unit  112  and the RF unit  102  are simply connected to each other when these units are arranged at physically separate locations. Hence, this is very advantageous in terms of space and cost.  
         [0056]     For example, as a scheme for superposing the control signal, the following schemes are available. One is a scheme for inserting the control signal between transmission and reception signals passing through the digital interface  111 , and another is a scheme for arranging buffers in the serial-to-parallel converter  108  and serial-to-parallel converter  113 , holding the signal in the buffer, transmitting the digital signal at a rate higher than a signal sampling rate, and inserting the control signal during an interval between the transmission processes. That is, as shown in  FIG. 6 , a control signal transmission period is set between transmission and reception periods.  
       Modification of Third Embodiment  
       [0057]     In this modification, control signals are collected and exchanged via a physically different signal line. A radio apparatus according to this modification will be described with reference to  FIG. 7 .  
         [0058]     In this modification, each RF unit  102  includes two serial-to-parallel converters  703  and  704  which are connected to a serial-to-parallel converter  113  via respective digital interfaces  701  and  702 . Transmission and reception data are transmitted between the serial-to-parallel converter  703  and the serial-to-parallel converter  113  via the digital interface  701 , and control data is transmitted between the serial-to-parallel converter  704  and the serial-to-parallel converter  113  via the digital interface  702 . Note that the example in the third embodiment may be combined with the modification of the third embodiment.  
         [0059]     More specifically, in the radio apparatus such as MIMO having the plurality of RF units, since the RF units  102  are controlled centered on the baseband processing unit  112 , the radio apparatus becomes very effective.  
         [0060]     In the third embodiment, all synthesizers  109  can share a common reference signal generated by a TCXO  114 , and the oscillation frequencies of the synthesizers  109  can match each other. In this arrangement, a quartz oscillator need not be arranged in each RF unit. Accordingly, the structure of the RF unit becomes simple to prevent degradation of the characteristics of the radio apparatus, e.g., an increase in error rate.  
       Fourth Embodiment  
       [0061]     A radio apparatus according to the fourth embodiment of the present invention will be described with reference to  FIG. 8 .  FIG. 8  is a block diagram when power is supplied from a baseband processing unit. The radio apparatus in the fourth embodiment has a digital interface  801  which includes a DC (power supply) line for an RF unit  102  in addition to a digital interface  111  in the first embodiment to distribute a signal. The digital interface  801  via which each RF unit  102  and a baseband processing unit  112  are connected is one connection line similar to the digital interface  111 , physically. In the fourth embodiment, a TCXO  114  also supplies, to each RF unit  102 , a common signal shared by all synthesizers  109 .  
         [0062]     The radio apparatus in the fourth embodiment has a power supply  802  in the baseband processing unit  112  which supplies power to each radio unit via the digital interface  801 . In this arrangement, another power supply line need not be arranged in addition to a signal line to the radio unit  103 , thus reducing the number of lines as a great merit, similar to the third embodiment. More specifically, the baseband processing unit  112  and the RF unit  102  are simply connected to each other when these units are arranged at physically separate locations. Hence, this is very advantageous in terms of space and cost.  
         [0063]     A detailed DC feeding scheme will be described next with reference to  FIG. 9 .  
         [0064]     A regulator  901  for the digital signal processing unit and a regulator  902  for the RF units are prepared in the baseband processing unit  112 . Each of these regulators is connected to an external power supply, e.g., a battery  903  to apply a constant voltage to each unit. The regulator  901  for the digital signal processing unit supplies power to the digital circuit of the baseband processing unit. On the other hand, the regulator  901  for the RF units generates a voltage to be supplied to the RF units  102 . The regulator  902  for the RF units supplies a power supply voltage to the RF units  102  via the respective digital interfaces  801 .  
         [0065]     With this operation, the voltage need not be directly and individually supplied from the battery to each RF unit  102 , thus largely reducing the space occupied by the lines. Also, the sources of the lines to the RF unit  102  can be centralized at the baseband processing unit  112 , and the lines need not be complicated. Note that the DC power may be supplied to each RF unit  102  via the line which is dedicated to power supply, and physically different from the signal line. Alternatively, the DC power may be supplied by superimposing on the digital interface  801 .  
       Fifth Embodiment  
       [0066]     A radio apparatus according to the fifth embodiment of the present invention will be described with reference to  FIG. 10 .  FIG. 10  is a block diagram when a synthesizer is included in a baseband processing unit to supply a local signal to each radio unit.  
         [0067]     In the radio apparatus according to the fifth embodiment, a synthesizer  1002  is included in a baseband processing unit  112 , and a signal from the synthesizer  1002  is distributed to radio units  103  via respective signal lines  1001 . The synthesizer  1002  includes a synthesizer  109 , PLL circuit  110 , and TCXO  114  shown in  FIG. 1 . In the first embodiment of the present invention, a reference signal from the TCXO is distributed to the radio units  103 . However, the fifth embodiment is different from the first embodiment in that the signal is supplied from the synthesizer to each radio unit.  
         [0068]     In the above-described first embodiment, the TCXO is included in the baseband processing unit  112 , and the signal from the TCXO is transmitted to each RF unit  102 . The transmitted signal is used as the reference signal of the synthesizers  109  in the RF units  102  to fully match the frequencies of the synthesizers.  
         [0069]     However, in the first embodiment, since the RF units  102  have the respective synthesizers  109 , the transient responses of the synthesizers  109  may be different from each other in the respective radio units. Generally, the synthesizer is very weak against disturbance, and an oscillation frequency sometimes transiently varies depending on a sneak transmission signal. In such case, although the oscillation frequencies can match each other in a steady state, for example, transmission and reception characteristics are sometimes degraded in MIMO, when the frequencies of the respective radio units slightly vary at, e.g., the heads of packets.  
         [0070]     When the synthesizer  1002  is included in the baseband processing unit  112  as in the fifth embodiment, even when the oscillation frequency of the synthesizer  1002  transiently varies depending on, e.g., the disturbance, the frequency distributed to the respective radio units  103  transiently varies at the same degree. Hence, the transmission and reception characteristics are not degraded even when the synthesizer is used in, e.g., MIMO, as a great merit.  
         [0071]     Note that the embodiments of the present invention are not limited to such precise embodiments, and various modifications may be effected without departing from the spirit or scope of the embodiments of the invention. For example, a MIMO radio apparatus is used in the above embodiments. However, the embodiments of the present invention can be applied to a diversity receiver.  
         [0072]     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.