Patent Document

TECHNICAL FIELD 
     The present invention relates to a radio relay apparatus, a radio transmitting apparatus and a radio relay method. 
     BACKGROUND ART 
     In recent years, studies are being carried out on a mobile communication system in which a radio communication mobile station apparatus (hereinafter simply referred to as “mobile station”) has a relay function (repeater function) that relays a signal directed to another mobile station (e.g., see Patent Literature 1). 
     In this mobile communication system, for example, the mobile station having the relay function relays a signal directed to the other mobile station transmitted from a radio communication base station apparatus (hereinafter simply referred to as “base station”). This allows even a mobile station that has difficulty in directly communicating with the base station to communicate with the base station via the mobile station having the relay function, and thereby increases system capacity. 
     CITATION LIST 
     Patent Literature 
     PLT 1 
     Japanese Patent Application Laid-Open No. 2009-159457 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in the above mobile communication system, the mobile station having the relay function needs to consume power of the mobile station itself to relay a signal directed to the other mobile station. To be more specific, when relaying a signal directed to the other mobile station, the mobile station having the relay function consumes power for operating a reception RF circuit and demodulation circuit in a receiving circuit and consumes power for operating a modulation circuit and transmission RF circuit in a transmitting circuit. 
     Such power consumption involved in relay processing causes the charge capacity of a battery provided for the mobile station having the relay function to decrease. Therefore, the more relay processing the mobile station having the relay function performs on a signal directed to the other mobile station, the less the charge capacity of the battery becomes, resulting in a problem that the mobile station having the relay function itself can no longer transmit/receive signals. 
     On the other hand, when a certain mobile station does not perform relay processing for other mobile stations, there may be mobile stations that cannot perform communication at all, with the result that it is not possible to increase the system capacity of the mobile communication system. 
     It is an object of the present invention to provide a radio relay apparatus, a radio transmitting apparatus and a radio relay method that reduce power consumption required for relay processing in a mobile station having a relay function, and can thereby increase system capacity. 
     Solution to Problem 
     A radio relay apparatus according to the present invention is a radio relay apparatus that relays communication between a radio transmitting apparatus and a radio receiving apparatus, and adopts a configuration including: a demodulation section that demodulates a signal transmitted from the radio transmitting apparatus; a decoding section that decodes an amplitude bit among a plurality of bits constituting each symbol of the demodulated signal and obtains a signal directed to the radio relay apparatus, and a modulation section that modulates a phase bit among the plurality of bits constituting each symbol of the demodulated signal and generates a relay signal directed to the radio receiving apparatus. 
     A radio transmitting apparatus according to the present invention is a radio transmitting apparatus that transmits a signal to a radio relay apparatus that relays communication between the radio transmitting apparatus and a radio receiving apparatus, and the radio receiving apparatus, and adopts a configuration including: an allocation section that allocates data directed to the radio relay apparatus at a bit position corresponding to an amplitude bit among a plurality of bits constituting each symbol of the signal and allocates data directed to the radio receiving apparatus at a bit position corresponding to a phase bit among the plurality of bits constituting each symbol of the signal; and a modulation section that modulates the signal including the amplitude bit and the phase bit. 
     A radio relay method according to the present invention is a radio relay method in a radio relay apparatus that relays communication between a radio transmitting apparatus and a radio receiving apparatus, including: a demodulating step of demodulating a signal transmitted from the radio transmitting apparatus; a decoding step of decoding an amplitude bit among a plurality of bits constituting each symbol of the demodulated signal and obtaining a signal directed to the radio relay apparatus; and a modulating step of modulating a phase bit among the plurality of bits constituting each symbol of the demodulated signal and generating a relay signal directed to the radio receiving apparatus. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to reduce power consumption required for relay processing in a mobile station having a relay function and thereby increase a system capacity. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a communication system according to each embodiment of the present invention; 
         FIG. 2  is a block diagram illustrating a configuration of a source node according to Embodiment 1 of the present invention; 
         FIG. 3  is a block diagram illustrating a relay node according to Embodiment 1 of the present invention; 
         FIG. 4  is a block diagram illustrating a configuration of a destination node according to Embodiment 1 of the present invention; 
         FIG. 5A  is a diagram illustrating bits constituting each symbol of a signal transmitted from the source node according to Embodiment 1 of the present invention; 
         FIG. 5B  is a diagram illustrating a constellation of the signal transmitted from the source node according to Embodiment 1 of the present invention; 
         FIG. 5C  is a diagram illustrating relay processing in the communication system according to Embodiment 1 of the present invention; 
         FIG. 6  is a diagram illustrating a specific example of relay processing in the communication system according to Embodiment 1 of the present invention; 
         FIG. 7  is a diagram illustrating bits constituting each symbol of a signal transmitted from a source node according to Embodiment 2 of the present invention; 
         FIG. 8  is a diagram illustrating relay processing in a communication system according to Embodiment 2 of the present invention; 
         FIG. 9  is a diagram illustrating bits constituting each symbol of a signal transmitted from a source node according to Embodiment 3 of the present invention; 
         FIG. 10  is a diagram illustrating relay processing in a communication system according to Embodiment 3 of the present invention; 
         FIG. 11  is a block diagram illustrating a configuration of a source node according to Embodiment 4 of the present invention; 
         FIG. 12  is a block diagram illustrating a configuration of a relay node according to Embodiment 4 of the present invention; 
         FIG. 13  is a diagram illustrating bits constituting each symbol of a signal transmitted from the source node according to Embodiment 4 of the present invention; 
         FIG. 14  is a block diagram illustrating another configuration of the source node according to Embodiment 4 of the present invention; 
         FIG. 15  is a diagram illustrating a general polar modulation circuit; 
         FIG. 16  is a block diagram illustrating a configuration of a polar modulation circuit in the transmission RF section of the relay node according to Embodiment 4 of the present invention; 
         FIG. 17A  is a diagram illustrating a constellation of a signal transmitted from a source node according to a variation of the present invention; 
         FIG. 17B  is a diagram illustrating symbols transmitted from the source node according to the variation of the present invention; 
         FIG. 18A  is a diagram illustrating a constellation of a signal transmitted from the source node according to the variation of the present invention; 
         FIG. 18B  is a diagram illustrating symbols transmitted from the source node according to the variation of the present invention; 
         FIG. 19A  is a diagram illustrating a constellation of a signal transmitted from the source node according to the variation of the present invention; and 
         FIG. 19B  is a diagram illustrating the number of amplitude bits and the number of phase bits in each modulation scheme according to the variation of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     As shown in  FIG. 1 , the following description will describe a communication system including a radio transmitting apparatus (hereinafter, referred to as “source node” such as the aforementioned base station), a radio relay apparatus (hereinafter, referred to as “relay node” such as the aforementioned mobile station having a relay function), and a radio receiving apparatus (hereinafter, referred to as “destination node” such as the aforementioned other mobile station). 
     In this communication system, the source node transmits data directed to the relay node and data directed to the destination node as shown in  FIG. 1 . On the other hand, the relay node receives the signal transmitted from the source node and relays the data directed to the destination node out of the received signal to the destination node. 
     Embodiment 1 
       FIG. 2  shows a configuration of source node  100  according to the present embodiment. 
     In source node  100  shown in  FIG. 2 , reception RF section  102  receives a signal from a destination node (which will be described later) or a relay node (which will be described later) via antenna  101 . Reception RF section  102  performs reception processing such as down-conversion, A/D conversion on the received signal and outputs the signal after the reception processing to destination node CQI detection section  103  and relay node CQI detection section  104 . This signal includes channel quality information (here, CQI (Channel Quality Indicator)) generated at the destination node and relay node respectively. 
     Destination node CQI detection section  103  detects a CQI generated at the destination node (hereinafter, referred to as “destination node CQI”) from the signal inputted from reception RF section  102 . On the other hand, relay node CQI detection section  104  detects a CQI generated at the relay node (hereinafter, referred to as “relay node CQI”) from the signal inputted from reception RF section  102 . Destination node CQI detection section  103  and relay node CQI detection section  104  then output the destination node CQI and the relay node CQI to scheduler  105 . 
     As the channel quality information, not only CQIs but also reception intensity of a pilot signal, reception SNR, reception SINR, reception CIR, reception CINR, variance or standard deviation of a reception SINR or variance or standard deviation of reception CINR may be used. 
     Scheduler  105  performs scheduling on resources (time resources, frequency resources, space resource, multipath or the like) to be allocated to a transmission signal transmitted from the source node using the destination node CQI and relay node CQI inputted from destination node CQI detection section  103  and relay node CQI detection section  104  respectively. Scheduler  105  then outputs the scheduling result to MCS (Modulation and channel Coding Scheme) determining section  106 . 
     MCS determining section  106  determines a modulation scheme of a transmission signal including a coding rate of data directed to the destination node, a coding rate of data directed to the relay node and data directed to the destination node and data directed to the relay node, based on the scheduling result inputted from scheduler  105 . Here, MCS determining section  106  determines the modulation scheme of the transmission signal, and thereby allocates the data directed to the destination node and the data directed to the relay node to an amplitude bit (amplitude information) which is a bit whose value varies when the amplitude of each symbol of the transmission signal changes, and a phase bit (phase information) which is a bit whose value varies when the phase of each symbol of the transmission signal changes among a plurality of bits constituting each symbol of the transmission signal. For example, MCS determining section  106  may select one of a predetermined plurality of constellation patterns to determine a modulation scheme of the transmission signal. Alternatively, MCS determining section  106  may also adaptively determine an arbitrary modulation scheme based on the CQIs inputted via scheduler  105 . MCS determining section  106  then outputs MCS information including the determined coding rate and modulation scheme to coding sections  107  and  108 , rate matching sections  109  and  110 , bit allocation section  111  and modulation section  112 . 
     Coding section  107  performs error correcting coding processing on the data directed to the destination node based on the coding rate indicated in the MCS information inputted from MCS determining section  106 , and outputs the coded signal to rate matching section  109 . 
     Coding section  108  performs error correcting coding processing on the data directed to the relay node based on the coding rate indicated in the MCS information inputted from MCS determining section  106 , and outputs the coded signal to rate matching section  110 . 
     Instead of coding section  107  and coding section  108 , one encoder may perform error correcting coding processing on the data directed to the destination node and data directed to the relay node collectively. That is, source node  100  may multiplex the data directed to the destination node and data directed to the relay node, and perform error correcting coding processing collectively. Generally, there is a characteristic; the longer the unit (data length) in which error correcting decoding is performed, the less is the likelihood of errors. In this case, it is possible to improve the error correcting decoding performance at the relay node, and improve the error rate characteristic more than when performing error correcting coding processing on the data directed to the destination node and data directed to the relay node individually. 
     Rate matching section  109  performs rate matching processing on the data directed to the destination node inputted from coding section  107  based on the coding rate indicated in the MCS information inputted from MCS determining section  106 . Rate matching section  109  then outputs the signal after the rate matching processing (data directed to the destination node) to bit allocation section  111 . 
     Rate matching section  110  performs rate matching processing on the data directed to the relay node inputted from coding section  108 , based on the coding rate indicated in the MCS information inputted from MCS determining section  106 . Rate matching section  110  then outputs the signal after the rate matching processing (data directed to the relay node) to bit allocation section  111 . 
     Bit allocation section  111  performs bit allocation processing on the data directed to the destination node inputted from rate matching section  109  and the data directed to the relay node inputted from rate matching section  110  based on the modulation scheme indicated in the MCS information inputted from MCS determining section  106 . To be more specific, bit allocation section  111  allocates the data directed to the relay node at a bit position corresponding to the amplitude bit among a plurality of bits constituting each symbol of the transmission signal modulated by modulation section  112 . Furthermore, bit allocation section  111  allocates the data directed to the destination node at a bit position corresponding to the phase bit among the plurality of bits constituting each symbol of the transmission signal modulated by modulation section  112 . Bit allocation section  111  then outputs a bit sequence in which the data directed to the relay node and data directed to the destination node are allocated to modulation section  112 . 
     Modulation section  112  modulates the bit sequence inputted from bit allocation section  111 , based on the modulation scheme indicated in the MCS information inputted from MCS determining section  106 . That is, modulation section  112  modulates the signal (bit sequence) including the amplitude bit to which the data directed to the relay node is allocated and the phase bit to which the data directed to the destination node is allocated. That is, modulation section  112  modulates the bit sequence according to the modulation scheme using the amplitude and phase. Modulation section  112  then outputs the modulated signal to transmission RF section  113 . 
     Transmission RF section  113  performs transmission processing such as D/A conversion, amplification and up-conversion on the signal inputted from modulation section  112 , and transmits the signal after the transmission processing from antenna  101 . 
     Next, the configuration of relay node  200  according to the present embodiment is shown in  FIG. 3 . 
     In relay node  200  shown in  FIG. 3 , reception RF section  202  receives a signal transmitted from source node  100  via antenna  201 . Reception RF section  202  then performs reception processing such as down-conversion, A/D conversion on the received signal and outputs the signal after the reception processing to demodulation section  203 . The signal transmitted from source node  100  includes data directed to relay node  200  and data directed to the destination node. 
     Demodulation section  203  demodulates the signal inputted from reception RF section  202  and outputs the demodulated signal (bit sequence) to amplitude bit extraction section  204  and phase bit extraction section  206 . 
     Amplitude bit extraction section  204  extracts the data directed to relay node  200  from the signal (bit sequence) inputted from demodulation section  203 . To be more specific, amplitude bit extraction section  204  extracts the amplitude bit among a plurality of bits constituting each symbol of the signal transmitted from source node  100 . Amplitude bit extraction section  204  then outputs the extracted amplitude bit (that is, data directed to relay node  200 ) to decoding section  205 . 
     Decoding section  205  performs error correcting decoding processing on the data directed to relay node  200  inputted from amplitude bit extraction section  204 , and outputs the decoding result as received data to a higher layer in relay node  200 . That is, decoding section  205  decodes the amplitude bit among a plurality of bits constituting each symbol of the signal demodulated by demodulation section  203  to obtain data directed to the relay node. 
     Phase bit extraction section  206  extracts data directed to the destination node from the signal (bit sequence) inputted from demodulation section  203 . To be more specific, phase bit extraction section  206  extracts the phase bit among a plurality of bits constituting each symbol of the signal transmitted from source node  100 . Phase bit extraction section  206  outputs the extracted phase bit (that is, data directed to the destination node) to relay processing section  207 . 
     Relay processing section  207  performs relay processing such as processing of forming the data directed to the destination node inputted from phase bit extraction section  206  into a signal format directed to the destination node which is the relay transmission destination. Relay processing section  207  then outputs the signal after the relay processing to modulation section  208 . 
     Modulation section  208  modulates the signal (that is, data directed to the destination node) inputted from relay processing section  207 . That is, modulation section  208  modulates the phase bit among a plurality of bits constituting each symbol of the signal demodulated by demodulation section  203  to generate data directed to the destination node (relay signal). At this time, modulation section  208  modulates the signal according to a modulation scheme with a smaller amplitude variation than the modulation scheme of the signal transmitted from source node  100  (that is, modulation scheme using the amplitude and phase). For example, modulation section  208  modulates the signal according to a modulation scheme using only the phase. Modulation section  208  then outputs the modulated signal to transmission RF section  209 . 
     Transmission RF section  209  performs transmission processing such as D/A conversion, amplification and up-conversion on the signal inputted from modulation section  208  and transmits the signal after the transmission processing from antenna  201  to the destination node. 
     Next, the configuration of destination node  300  according to the present embodiment is shown in  FIG. 4 . 
     In destination node  300  shown in  FIG. 4 , reception RF section  302  receives a signal (relay signal) transmitted from relay node  200  via antenna  301 . Reception RF section  302  performs reception processing such as down-conversion, A/D conversion on the received signal and outputs the signal after the reception processing to demodulation section  303 . 
     Demodulation section  303  demodulates the signal inputted from reception RF section  302 , and outputs the demodulated signal (bit sequence) to phase bit extraction section  304 . 
     Phase bit extraction section  304  extracts data directed to destination node  300  from the signal (bit sequence) inputted from demodulation section  303 . To be more specific, phase bit extraction section  304  extracts the phase bit which is a bit constituting each symbol of the signal transmitted from relay node  200  (that is, data directed to destination node  300 ). Phase bit extraction section  304  then outputs the data directed to extracted destination node  300  to decoding section  305 . 
     Decoding section  305  performs error correcting decoding processing on the data directed to destination node  300  inputted from phase bit extraction section  304 , and outputs the decoding result as received data to a higher layer in destination node  300 . Error correcting decoding processing is necessary when error correcting coding processing is in progress in source node  100  or relay node  200  and error correcting decoding processing is omitted when error correcting coding processing is not in progress in source node  100  or relay node  200 . 
     Furthermore, relay node  200  ( FIG. 3 ) and destination node  300  ( FIG. 4 ) according to the present embodiment estimate a channel between relay node  200  and source node  100 , and a channel between destination node  300  and source node  100  using pilot signals transmitted from source node  100  ( FIG. 2 ). Relay node  200  and destination node  300  then generate CQIs (aforementioned relay node CQI and destination node CQI) using the channel estimate values which are the estimation results. Destination node  300  then transmits the CQI information indicating the generated destination node CQI to source node  100  or relay node  200 . Furthermore, relay node  200  transmits the generated relay node CQI and the destination node CQI received from destination node  300  to source node  100 . 
     Next, operation of the communication system according to the present embodiment will be described in detail. 
     In the following description, source node  100  ( FIG. 2 ) transmits a signal made up of three bits per symbol as shown in  FIG. 5A . Here, as shown in  FIG. 5A , a high-order one bit of the three bits constituting one symbol is an amplitude bit and low-order two bits are phase bits. 
     That is, the constellation of a transmission signal transmitted from source node  100  is as shown in  FIG. 5B .  FIG. 5B  shows the constellation called “8QAM” (which may be referred to as “8PASK (8 Phase Amplitude Shift Keying”). As shown in  FIG. 5B , when the high-order one bit (amplitude bit) of the three bits constituting one symbol is ‘1’, the signal is modulated on an outer circle (ring) and when the high-order one bit (amplitude bit) is ‘0’, the signal is modulated on an inner circle (ring). That is, in 8PSK shown in  FIG. 5B , the value of the high-order one bit (amplitude bit) varies as the amplitude of the transmission signal (symbol) changes. That is, the amplitude of the transmission signal is determined only by the value of the amplitude bit independently of the values of the phase bits. 
     Furthermore, as shown in  FIG. 5B , when the low-order two bits (phase bits) of the three bits constituting one symbol are ‘01’, ‘11’, ‘10’ and ‘00’, the signal is modulated in the first to fourth quadrants respectively. That is, in 8PSK shown in  FIG. 5B , values of the low-order two bits (phase bits) vary as the phase of the transmission signal (symbol) changes. That is, the phase of the transmission signal is determined only by the values of the phase bits independently of the value of the amplitude bit. For example, as shown in  FIG. 5B , in the case of symbols ‘001’ and ‘101’, both phase bits (low-order two bits) are ‘01’ and therefore the signal is modulated in the first quadrant irrespective of the value of the amplitude bit. 
     As shown in  FIG. 5A  and  FIG. 5B , bit allocation section  111  of source node  100  allocates data r directed to relay node  200  at a bit position corresponding to the high-order one bit (amplitude bit) of three bits constituting each symbol of the transmission signal and allocates data d directed to destination node  300  at hit positions corresponding to the low-order two bits (phase bits). Modulation section  112  then modulates the bit sequence in which data r directed to relay node  200  and data d directed to destination node  300  are allocated as shown in  FIG. 5A  based on the constellation shown in  FIG. 5B . 
     As shown in  FIG. 5C , source node  100  transmits the transmission signal made up of the symbols in the configuration shown in  FIG. 5A  to relay node  200 . 
     Demodulation section  203  of relay node  200  demodulates the transmission signal transmitted from source node  100 . Amplitude bit extraction section  204  extracts the high-order one bit (r) which is the amplitude bit of the three bits constituting each symbol shown in  FIG. 5A , and decoding section  205  decodes the amplitude bit. Relay node  200  thereby obtains data r directed to relay node  200 . 
     On the other hand, phase bit extraction section  206  of relay node  200  extracts the low-order two bits (d, d) which are phase bits of the three bits constituting each symbol shown in  FIG. 5A . Modulation section  208  of relay node  200  modulates the phase bits (data directed to destination node  300 ). 
     At this time, modulation section  208  modulates the phase bits (low-order two bits) shown in  FIG. 5A  using QPSK expressed by four phases. In other words, modulation section  208  modulates the phase bits using QPSK (modulation scheme using only phases) which is a modulation scheme with a smaller amplitude variation than 8PASK (modulation scheme using amplitude and phases) which is a modulation scheme for the transmission signal transmitted from source node  100 . To be more specific, modulation section  208  modulates only the phase bits (low-order two bits) using QPSK expressed by four phases irrespective of the value of the amplitude bit of the transmission signal transmitted from source node  100 . As a result, the relay signal, which is relayed, becomes a QPSK signal expressed by two bits per symbol, that is, by four phases. 
     Thus, relay node  200  can relay to destination node  300 , the low-order two bits (phase bits) of the three bits constituting each symbol of the transmission signal transmitted from source node  100  just as they are. In other words, relay node  200  does not relay the high-order one bit which is the amplitude bit among the three bits constituting each symbol of the transmission signal transmitted from source node  100 . 
     That is, as shown in  FIG. 5C , relay node  200  relays a relay signal only including the phase bits (d, d) (data directed to destination node  300 ) to destination node  300 . As shown in  FIG. 5C , destination node  300  receives the phase bits (d, d) from relay node  200 , decodes the received phase bits and thereby obtains data directed to destination node  300 . 
     Here, to be more specific, as shown in  FIG. 6 , a case will be described where data directed to relay node  200  are r(1), r(2), . . . , r(n) and data directed to destination node  300  are d(1), d(2), . . . , d(2n). 
     As shown in  FIG. 5A , of the three bits constituting each symbol, the high-order one bit is an amplitude bit and the low-order two bits are phase bits. Thus, as shown in  FIG. 6 , source node  100  allocates data r(1) directed to relay node  200  to the high-order one bit of the three bits constituting symbol #1 and allocates data d(1) and d(2) directed to destination node  300  to the low-order two bits. Similarly, as shown in  FIG. 6 , source node  100  allocates data r(2) directed to relay node  200  to the high-order one bit of the three bits constituting symbol #2 and allocates data d(3) and d(4) directed to destination node  300  to the low-order two bits. The same applies to symbols #3 to #n. 
     That is, as shown in  FIG. 6 , of 3n transmission bits transmitted with symbols #1 to #n from source node  100 , n bits are data directed to relay node  200  and  2   n  bits are data directed to destination node  300 . 
     Thus, source node  100  modulates the transmission signal using 8PASK which is a modulation scheme using amplitude and phase, and transmits the signal to relay node  200 . Upon receiving the transmission signal modulated using 8PASK, relay node  200  modulates only the phase bits included in each symbol of the transmission signal according to a modulation scheme having a smaller amplitude variation than in 8PASK (here QPSK using only phase), and relays the signal to destination node  300 . 
     Here, examples of operation of the radio transmission amplifier in the transmission circuit (transmission RF section  209  shown in  FIG. 3 ) include “linear operation” in which the output voltage varies depending on a supply voltage of the radio transmission amplifier, and “limiter operation” in which the output voltage is constant (saturation state) irrespective of the supply voltage of the radio transmission amplifier. 
     Furthermore, in the transmission circuit, power consumption of the radio transmission amplifier does not depend on the output voltage of the radio transmission amplifier, but only depends on the supply voltage. By contrast, transmission power of the signal transmitted from the transmitting antenna depends on the output voltage of the radio transmission amplifier. Thus, when the output voltage (average output voltage) of the radio transmission amplifier is kept constant (when the transmission power of the signal transmitted from the transmitting antenna is kept constant), the difference between the energy corresponding to the supply voltage of the radio transmission amplifier (that is, power consumption of the radio transmission amplifier) and energy corresponding to the actual instantaneous output voltage (that is, transmission power of the signal) becomes useless energy (power). 
     For example, in the linear operation, the radio transmission amplifier causes the output voltage to vary centered on the set average output voltage. That is, in the linear operation, the radio transmission amplifier needs to output a higher output voltage than the average output voltage. Thus, the difference between the energy equivalent to the supply voltage corresponding to a maximum output voltage to be outputted to keep the average output voltage constant (that is, power consumption of the radio transmission amplifier) and the energy equivalent to an instantaneous output voltage at each time (that is, transmission power of the transmission signal) becomes useless. Furthermore, in the linear operation, even when the average output voltage is identical, the greater the amplitude variation of the output voltage, the greater the maximum output voltage, and therefore useless energy (useless power) increases further. 
     By contrast, in the limiter operation in which the output voltage is constant irrespective of the supply voltage of the radio transmission amplifier (without amplitude variation), the radio transmission amplifier can continue to supply the same output voltage as the set average output voltage. For this reason, in the limiter operation, there is substantially no difference between the energy equivalent to the supply voltage corresponding to the maximum output voltage to be outputted to keep the average output voltage constant (that is, power consumption of the radio transmission amplifier) and the energy equivalent to the instantaneous output voltage at each time (that is, transmission power of the transmission signal) and useless energy (useless power) is extremely small. 
     That is, the limiter operation has higher power efficiency of the radio transmission amplifier than the linear operation, and can transmit a signal with less power consumption than the linear operation. Furthermore, of the linear operation, linear operation having a smaller amplitude variation has higher power efficiency of the radio transmission amplifier than linear operation having a greater amplitude vibration and can transmit a signal with less power consumption. 
     However, transmission through the linear operation (hereinafter, referred to as “linear transmission”) can change the amplitude of the output voltage, and can thereby transmit both amplitude information (amplitude bit) and phase information (phase bits) in the transmission signal. On the other hand, transmission through the limiter operation (hereinafter, referred to as “limiter transmission”) has constant amplitude of the output voltage, and can transmit only phase information (phase bits) in the transmission signal. 
     Therefore, in the aforementioned modulation scheme using both phase and amplitude, it is necessary to perform linear transmission not limiter transmission. By contrast, in the modulation scheme using only phase, it is possible to use both linear transmission and limiter transmission. 
     Here, relay node  200  uses QPSK (modulation scheme using only phase) which is a modulation scheme with smaller amplitude variation than 8PASK (modulation scheme using amplitude and phase) which is a modulation scheme for a transmission signal from source node  100  during relay transmission. Thus, when a relay signal is linear-transmitted using QPSK during relay transmission, the amplitude variation of the radio transmission amplifier becomes smaller than the modulation scheme (8PASK) for the transmission signal from source node  100 , and it is possible to improve power efficiency of the radio transmission amplifier. Furthermore, when a relay signal is limiter-transmitted using a modulation scheme having a constant amplitude characteristic such as GMSK (Gaussian filtered Minimum Shift Keying) or FSK (Frequency Shift Keying) during relay transmission, relay node  200  can further improve the power efficiency of the radio transmission amplifier compared to linear transmission. 
     Thus, relay node  200  only relays a signal having phase bits (phase information) as the relay signal (data directed to destination node  300 ). This allows relay node  200  to change the modulation scheme in relay transmission to a modulation scheme with a smaller amplitude variation than the modulation scheme of a transmission signal from source node  100 . It is thereby possible to improve power efficiency of the radio transmission amplifier (transmission RF section  209 ) of relay node  200 . That is, relay node  200  can suppress useless power consumption produced in relay processing. Moreover, relay node  200  transmits a relay signal modulated according to a modulation scheme using only phase through limiter operation, and can thereby suppress an increase of useless power consumption produced in relay processing compared to that during transmission through linear operation. 
     Therefore, the present embodiment reduces power consumption required for relay processing in a mobile station having a relay function (relay node  200  in the present embodiment), and can thereby suppress a decrease in charge capacity of the battery when performing relay processing on the signal directed to another mobile station. Thus, the present embodiment relays a signal of the other mobile station while transmitting/receiving a signal of the mobile station having the relay function (relay node  200 ), and can thereby increase system capacity in the entire mobile communication system. That is, the present embodiment reduces power consumption required for relay processing in the mobile station having the relay function, and can thereby increase system capacity. 
     Furthermore, in the present embodiment, source node  100  allocates data directed to relay node  200  to the amplitude bit and allocates data directed to destination node  300  to the phase bits. Here, as shown in  FIG. 5B , a determination axis for an amplitude bit is located between symbols in each quadrant. For example, in the first quadrant shown in  FIG. 5B , a determination axis for determining two symbols is located between ‘001’ and ‘101’. By contrast, a determination axis for phase bits is located on a boundary between quadrants. That is, 8PASK has a feature that amplitude bits are more error-prone than phase bits. However, the distance from source node  100  to relay node  200  is assumed to be shorter than the distance from source node  100  to destination node  300 . That is, data directed to relay node  200  is less error-prone than data directed to destination node  300  when the communication distance is taken into consideration. Thus, even when source node  100  allocates data directed to relay node  200  to the amplitude bit (bit which is more error-prone than phase bit), there is a higher possibility that relay node  200  may be able to reliably receive data directed to relay node  200  allocated to the amplitude bit. 
     Furthermore, in the present embodiment, destination node  300  extracts only phase bits of the signal relayed from relay node  200 . Here, the circuit that extracts only phase bits can perform delay detection on a differentially coded/modulated signal. Thus, the circuit that extracts only phase bits (the circuit that can perform delay detection) has a simpler circuit configuration than a circuit that performs coherent detection. That is, the circuit that extracts only phase bits has a simpler circuit configuration than a circuit that extracts both of the amplitude bit and phase bit. It is thereby possible to prevent the circuit configuration of destination node  300  from increasing. For example, when a mobile station is provided with a relay function, even if the circuit scale increases by an addition of the relay processing related circuit (that is, the circuit corresponding to relay node  200 ), it is possible to prevent the circuit scale of the entire mobile station from increasing by simplifying the reception circuit provided in the mobile station (that is, the circuit corresponding to destination node  300 ). 
     Embodiment 2 
     The present embodiment will describe a case where a source node (radio transmitting apparatus) further transmits broadcast data which is data directed to a plurality of receiving nodes. 
     Hereinafter, the present embodiment will be described more specifically. Bit allocation section  111  of source node  100  according to the present embodiment ( FIG. 2 ) further performs bit allocation processing on broadcast data inputted from an input section (not shown) in addition to data directed to destination node  300 , and data directed to relay node  200  as in the case of Embodiment 1. Here, the broadcast data is transmitted to a plurality of receiving nodes (hereinafter, referred to as “broadcast data receiving nodes”) including destination data. 
     To be more specific, bit allocation section  111  allocates data directed to a relay node at a bit position corresponding to an amplitude bit among a plurality of bits constituting each symbol of a transmission signal as in the case of Embodiment 1. On the other hand, bit allocation section  111  allocates data directed to the relay node and broadcast data at bit positions corresponding to phase bits among a plurality of bits constituting each symbol of the transmission signal. 
     For example, as shown in  FIG. 7 , source node  100  transmits a signal made up of three bits per symbol as in the case of Embodiment 1. Furthermore, as shown in  FIG. 7 , of the three bits constituting one symbol, a high-order one bit is an amplitude bit and low-order two bits are phase bits. 
     Thus, bit allocation section  111  allocates data r directed to relay node  200  at a bit position corresponding to a high-order one bit (amplitude bit) of the 3 bits constituting each symbol of the transmission signal as shown in  FIG. 7 . Furthermore, bit allocation section  111  allocates broadcast data b and data d directed to destination node  300  at bit positions corresponding to the low-order two bits (phase bits) of the three bits constituting each symbol of the transmission signal in descending order of bits. As shown in  FIG. 7 , modulation section  112  modulates a bit sequence in which data r directed to relay node  200 , broadcast data b and data d directed to destination node  300  are allocated based on the constellation shown in  FIG. 5B . 
     Source node  100  then transmits the transmission signal in which data (r, b, d) are allocated to the three bits making up one symbol to relay node  200  in descending order of bits as shown in  FIG. 8 . 
     Next, amplitude bit detection section  204  ( FIG. 3 ) of relay node  200  according to the present embodiment extracts the high-order one bit (r) which is the amplitude bit among a plurality of bits constituting each symbol of the transmission signal as in the case of Embodiment 1. Decoding section  205  then decodes the amplitude bit and thereby obtains data r directed to relay node  200 . 
     On the other hand, phase bit extraction section  206  extracts the phase bits (broadcast data b and data d directed to destination node  300  shown in  FIG. 7 ) among the plurality of bits constituting each symbol of the transmission signal. That is, relay node  200  relays only the phase bits (b, d) of the transmission signal transmitted from source node  100  as shown in  FIG. 8 . Thus, relay node  200  transmits data d to destination node  300 , and at the same time transmits data b to nodes other than destination node  300  (receivers in  FIG. 8 ). 
     At this time, modulation section  208  of relay node  200  modulates the phase bits (broadcast data b and data d directed to destination node  300 ) according to a modulation scheme (e.g., QPSK (modulation scheme using only phase)) having a smaller amplitude variation than the modulation scheme (e.g., 8PASK (modulation scheme using amplitude and phase)) of the transmission signal transmitted from source node  100  as in the case of Embodiment 1. 
     Next, destination node  300  according to the present embodiment ( FIG. 4 ) receives broadcast data b and data d directed to destination node  300  constituting each symbol of the relay signal transmitted from relay node  200  as shown in  FIG. 8 . 
     On the other hand, the receivers (broadcast data receiving nodes) shown in  FIG. 8  extract broadcast data b of bits constituting each symbol of the relay signal transmitted from relay node  200 , and also receive only broadcast data b by discarding (shown by X in  FIG. 8 ) data d directed to destination node  300 . 
     Thus, source node  100  modulates the transmission signal according to a modulation scheme (e.g., 8PASK) using amplitude and phase, and transmits the transmission signal to relay node  200  as in the case of Embodiment 1. However, source node  100  allocates broadcast data directed to a plurality of broadcast data receiving nodes and individual data directed to the destination node to the phase bits among a plurality of bits constituting each symbol of the transmission signal. Thus, relay node  200  relays only phase bits included in each symbol of the transmission signal from source node  100 , and can thereby relay not only data directed to destination node  300  but also broadcast data to a plurality of broadcast data receiving nodes as in the case of Embodiment 1. 
     At this time, in the same way as in Embodiment 1, relay node  200  modulates the phase bits included in each symbol of the transmission signal from source node  100  using a modulation scheme (e.g., QPSK) with a smaller amplitude variation than the modulation scheme (e.g., 8PASK) of the transmission signal, and relays the phase bits. Thus, according to the present embodiment, relay node  200  can suppress an increase of useless power consumption in not only relay processing of data directed to destination node  300  but also relay processing of broadcast data. That is, relay node  200  can reduce power consumption required for relay processing on a plurality of broadcast data receiving nodes. 
     Thus, the present embodiment reduces power consumption required for relay processing in a mobile station having a relay function (relay node), and can thereby increase system capacity as in the case of Embodiment 1. Furthermore, according to the present embodiment, the mobile station having the relay function can also suppress power consumption required when relaying broadcast data to a plurality of broadcast data receiving nodes. 
     In the present embodiment, the allocation positions and the number of bits of broadcast data and data directed to the destination node in the phase bits among a plurality of bits constituting each symbol of the transmission signal transmitted from source node  100  are not limited to the case shown in  FIG. 7 . That is, the allocation positions and the number of bits of broadcast data and data directed to the destination node in the phase bits may be changed arbitrarily. For example, when there is no data directed to the destination node transmitted from source node  100 , source node  100  may allocate only broadcast data in the phase bits and transmit the broadcast data. Alternatively, source node  100  may adaptively change the allocation positions and the number of bits of broadcast data and data directed to the destination node in the phase bits according to CQIs received from the destination node and broadcast data receiving node respectively. This allows the destination node and broadcast data receiving node to receive data more reliably. 
     Embodiment 3 
     The present embodiment will describe a case where a source node (radio transmitting apparatus) transmits data directed to a relay node (radio relay apparatus) using not only an amplitude bit but also some of phase bits among a plurality of bits constituting each symbol of a transmission signal. 
     Hereinafter, the present embodiment will be described more specifically. Bit allocation section  111  ( FIG. 2 ) of source node  100  according to the present embodiment allocates data directed to relay node  200  at a bit position corresponding to an amplitude bit and bit positions corresponding to some of phase bits among a plurality of bits constituting each symbol of a transmission signal. On the other hand, bit allocation section  111  allocates data directed to relay node  200  at bit positions corresponding to bits other than the above-described some bits (bits to which data directed to relay node  200  is allocated) among the plurality of bits constituting each symbol of the transmission signal. 
     For example, as shown in  FIG. 9 , source node  100  transmits a signal made up of three bits per symbol as in the case of Embodiment 1. Furthermore, as shown in  FIG. 9 , of the three bits constituting one symbol, a high-order one bit is an amplitude bit and low-order two bits are phase bits. 
     As shown in  FIG. 9 , bit allocation section  111  allocates data r directed to relay node  200  at bit positions corresponding to high-order two bits (amplitude bit and one of phase bits) among the three bits constituting each symbol of the transmission signal. Furthermore, as shown in  FIG. 9 , bit allocation section  111  allocates data d directed to destination node  300  at a bit position corresponding to the low-order one bit (bit other than the one bit to which the data directed to relay node  200  is allocated among the phase bits) among the three bits constituting each symbol of the transmission signal. That is, source node  100  allocates two bits (one amplitude bit and one phase bit) among the three bits constituting each symbol of the transmission signal to the data directed to relay node  200  and allocates the remaining one bit (one phase bit) to the data directed to destination node  300 . Modulation section  112  modulates a bit sequence in which data (r, r) directed to relay node  200  and data d directed to destination node  300  are allocated as shown in  FIG. 9 , based on the constellation shown in  FIG. 5B . 
     Source node  100  then transmits the transmission signal with data (r, r, d) allocated to the three bits constituting one symbol, as shown in  FIG. 10 , to relay node  200 . 
     Next, amplitude bit extraction section  204  ( FIG. 3 ) of relay node  200  according to the present embodiment extracts the high-order one bit (r) which is the amplitude bit among the plurality of bits constituting each symbol of the transmission signal. Furthermore, phase bit extraction section  206  extracts the phase bits (r and d shown in  FIG. 9 ) among the plurality of bits constituting each symbol of the transmission signal. 
     Relay node  200  then decodes the amplitude bit (r shown in  FIG. 9 ) and one of the phase bits (r shown in  FIG. 9 ) among the plurality of extracted bits, and thereby obtains data directed to relay node  200 . Furthermore, relay node  200  relays the bit (d) other than the above-described one bit (r) of the phase bits among the plurality of extracted bits. That is, relay node  200  relays only one bit (d) of the phase bits, that is, the bit other than the data (r) directed to relay node  200  of the phase bits, of the transmission signal transmitted from source node  100  as shown in  FIG. 10 . 
     At this time, modulation section  208  of relay node  200  modulates one bit (data d directed to destination node  300 ) of the phase bits using a modulation scheme (e.g., BPSK (modulation scheme using only phase)) with a smaller amplitude variation than the modulation scheme (e.g., 8PASK (modulation scheme using amplitude and phase)) of the transmission signal transmitted from source node  100  as in the case of Embodiment 1. That is, the number of bits (one bit) constituting one symbol generated in modulation section  208  in the present embodiment is smaller compared to the constellation (QPSK: two bits per symbol) during the relay of relay node  200  in Embodiment 1. For example, modulation section  208  modulates one of the phase bits (data d directed to destination node  300 ) using BPSK. 
     Next, destination node  300  ( FIG. 4 ) according to the present embodiment receives data d directed to destination node  300  constituting each symbol of the relay signal transmitted from relay node  200  as shown in  FIG. 10 . 
     Thus, source node  100  allocates data directed to relay node  200  not only to the amplitude bit but also to one of the phase bits among the plurality of bits constituting each symbol of the transmission signal. That is, source node  100  can allocate data directed to relay node  200  to any one bit among the plurality of bits constituting each symbol of the transmission signal. This allows source node  100  to adaptively change the proportion of the number of bits of the data directed to the relay node and data directed to the destination node among the plurality of bits constituting each symbol of the transmission signal. 
     However, as shown in  FIG. 9 , data d directed to destination node  300  (that is, relayed signal) is allocated to only the phase bits as in the case of Embodiment 1 (e.g.,  FIG. 5A ). Therefore, relay node  200  may modulate only one phase bit (only d shown in  FIG. 9 ) among the plurality of bits constituting each symbol of the transmission signal from source node  100  using a modulation scheme (e.g., BPSK) with a smaller amplitude variation than the modulation scheme (e.g., 8PASK) of the transmission signal, and relay the modulated phase bit as in the case of Embodiment 1. 
     That is, even when the proportion of the number of bits among the plurality of bits constituting each symbol of the transmission signal is adaptively changed, source node  100  may not have to allocate data directed to destination node  300  to the amplitude bit. Thus, relay node  200  never loses the effect of improving power consumption efficiency of the aforementioned radio transmission amplifier (transmission RF section  209  shown in  FIG. 3 ). 
     Thus, according to the present embodiment, source node  100  allocates data directed to relay node  200  to the amplitude bit and one of the phase bits among a plurality of bits constituting each symbol of the transmission signal. However, source node  100  allocates data directed to destination node  300  to only the phase bits among the plurality of bits constituting each symbol of the transmission signal as in the case of Embodiment 1. This makes it possible to transmit more data directed to relay node  200  without reducing power efficiency during relay transmission in relay node  200 . Therefore, the present embodiment can reduce power consumption required for relay processing in a mobile station (relay node) having a relay function, and thereby increase system capacity as in the case of Embodiment 1. Furthermore, the present embodiment can adaptively change data allocation (bit allocation in one symbol) of data directed to relay node  200  and data directed to destination node  300 . 
     Embodiment 4 
     The present embodiment will describe a case where a signal indicating control information (control channel) to control operation of relay processing at a relay node is transmitted as the aforementioned data directed to the relay node (radio relay apparatus). 
     Hereinafter, the present embodiment will be described more specifically.  FIG. 11  shows a configuration of source node  400  according to the present embodiment. In  FIG. 11 , the same components as those in Embodiment 1 ( FIG. 2 ) will be assigned the same reference numerals and descriptions thereof will be omitted. 
     In source node  400  shown in  FIG. 11 , a destination node CQI is inputted to scheduler  105  from destination node CQI detection section  103 . Scheduler  105  performs scheduling on resources allocated to a transmission signal transmitted from source node  400  using the destination node CQI. MCS determining section  106  then determines a coding rate and modulation scheme of data directed to the destination node based on the scheduling result inputted from scheduler  105 . Furthermore, MCS determining section  106  outputs MCS information including the determined coding rate and modulation scheme to coding section  107 , modulation section  112  and relay node directed control information generation section  401 . 
     Relay node directed control information generation section  401  generates control information including the MCS information or the like inputted from MCS determining section  106 . Relay node directed control information generation section  401  outputs the generated control information to amplitude bit generation section  403 . 
     Phase bit generation section  402  performs control so that data directed to the destination node inputted from coding section  107  is allocated at bit positions corresponding to phase bits among a plurality of bits constituting each symbol of the transmission signal transmitted from source node  400 . That is, phase bit generation section  402  generates phase bits including data directed to the destination node. 
     Amplitude bit generation section  403  performs control so that control information inputted from relay node directed control information generation section  401  is allocated at a bit position corresponding to the amplitude bit among a plurality of bits constituting each symbol of the transmission signal transmitted from source node  400 . That is, amplitude bit generation section  403  generates the amplitude bit including control information. 
     Source node  400  may also use bit allocation section  111  described in Embodiment 1 instead of phase bit generation section  402  and amplitude bit generation section  403 . That is, bit allocation section  111  provided in source node  400  may allocate data directed to the destination node at a bit position corresponding to the phase bit and allocate control information at a bit position corresponding to the amplitude bit. 
     Next,  FIG. 12  shows the configuration of relay node  500  according to the present embodiment. In  FIG. 12 , the same components as those in Embodiment 1 ( FIG. 3 ) will be assigned the same reference numerals and descriptions thereof will be omitted. 
     In relay node  500  shown in  FIG. 12 , control information receiving section  501  controls operation of the relay processing in relay node  500  based on a signal inputted from amplitude bit extraction section  204 , that is, the control information directed to relay node  500  transmitted from source node  400 . 
     In addition to the aforementioned MCS information, the control information includes, for example, the demodulation scheme in demodulation section  203 , relay scheme (e.g., instruction information instructing any one of decode and forward type and amplitude and forward type) in relay processing section  207 , modulation scheme (e.g., the number of phases or constellation candidates) in modulation section  208 , operating mode in transmission RF section  209 , system control information for controlling the entire system of relay node  500  and control information for a higher layer. Furthermore, the control information may also include information indicating a control channel relating to data directed to relay node  500  or information indicating a control channel relating to data directed to the destination node. In this case, control information receiving section  501  controls demodulation section  203 , relay processing section  207 , modulation section  208 , transmission RF section  209 , that is, relay processing of data directed to destination node  300  according to the instruction from source node  400  indicated in the control information directed to relay node  500 . Furthermore, control information receiving section  501  controls relay node system control section  502  and higher layer according to the instruction from source node  400  indicated in the control information directed to relay node  500 . 
     In the present embodiment, relay node  500  extracts control information directed to relay node  500  (control information including information relating to relay processing) from the amplitude bit among a plurality of bits constituting each symbol of the transmission signal transmitted from source node  400 , and extracts data directed to destination node  300  from the phase bits. Relay node  500  relays data directed to destination node  300  based on an instruction from source node  400  indicated in the control information. 
     Thus, according to the present embodiment, relay node  500  reduces power consumption required for relay processing of data directed to destination node  300 , and can thereby increase system capacity as in the case of Embodiment 1. Furthermore, the present embodiment allocates control information directed to relay node  500  to the amplitude bit among a plurality of bits constituting each symbol of the transmission signal transmitted from source node  400 . Thus, the present embodiment controls the relay operation in relay node  500  symbol by symbol, and can thereby control operation of the relay processing more meticulously than Embodiment 1. 
     In the present embodiment, the control information directed to the relay node may include information indicating an ON/OFF setting (ON/OFF setting information) of the relay function in relay node  500 . For example, as shown in  FIG. 13 , of the three bits (b0, b1, b2) constituting each symbol of the transmission signal, suppose amplitude bit b0 is ON/OFF setting information and phase bits b1 and b2 are data. Here, as shown in  FIG. 13 , when the ON/OFF setting information is ‘0’, suppose phase bits b1 and b2 are data directed to the relay node (that is, the relay function of relay node  500 : OFF). On the other hand, when the ON/OFF setting information is ‘1’, suppose phase bits b1 and b2 are data directed to the destination node (that is, the relay function of relay node  500 : ON). Therefore, when ON/OFF setting information b0 out of the three bits (b0, b1, b2) constituting each symbol of the transmission signal from source node  400  is ‘0’, relay node  500  receives phase bits b1 and b2 as data directed to relay node  500 . On the other hand, when ON/OFF setting information b0 out of the three bits (b0, b1, b2) constituting each symbol of the transmission signal from source node  400  is ‘1’, relay node  500  relays phase bits b1 and b2 as data directed to destination node  300 . This allows source node  400  to adaptively change data to be transmitted according to the situation of data directed to relay node  500  and data directed to destination node  300 . 
     Furthermore, the present embodiment may allocate control information directed to the relay node inputted from a higher layer in source node  400   a  shown in  FIG. 14  to the amplitude bit among a plurality of bits constituting each symbol of the transmission signal. For example, relay node directed control information generation section  401   a  of source node  400   a  shown in  FIG. 14  may generate one or both of MCS information inputted from MCS determining section  106  and control information inputted from the higher layer, as control information directed to the relay node. 
     Furthermore, by combining the present embodiment and Embodiment 2, information relating to the ratio between broadcast data and data directed to the destination node within phase bits and a configuration thereof (e.g., the number of bits and allocation position) may be included as control information directed to the relay node. 
     Furthermore, by combining the present embodiment and Embodiment 3, the control information directed to the relay node may be allocated not only to the amplitude bit but also to some of the phase bits, among a plurality of bits constituting each symbol of the transmission signal. 
     Embodiment 5 
     The present embodiment will describe a case where a polar modulation circuit is used as a circuit constituting a transmission RF section of a relay node (radio relay apparatus). 
     The following description will describe a case where the operating mode in transmission RF section  209  ( FIG. 3 ) of relay node  200  according to the present embodiment is a limiter operation. 
     Here,  FIG. 15  shows general polar modulation circuit  10 . Here, for example, a signal modulated in modulation section  208  of relay node  200  is expressed using an I (in-phase) component and Q (quadrature) component on an orthogonal coordinate plane as shown in  FIG. 15 . 
     In polar modulation circuit  10  shown in  FIG. 15 , polar coordinate conversion section  11  performs polar coordinate conversion on the modulated signal (I component, Q component) to obtain amplitude information γ and phase information θ. Polar coordinate conversion section  11  then outputs amplitude information γ to the terminal of an input voltage of saturation amplifier  12 , and outputs phase information θ to the terminal of an input signal of saturation amplifier  12 . Saturation amplifier  12  then amplifies phase information θ inputted to the terminal of the input signal based on amplitude information γ inputted to the terminal of the input voltage. 
     Here, as described above, in relay node  200 , a signal (data directed to destination node  300 ) to be relayed out of the transmission signal transmitted from source node  100  is allocated to only the phase bit (that is, phase information θ shown in  FIG. 15 ). Therefore, only phase information θ (data directed to the destination node) is inputted to transmission RF section  209  from modulation section  208 . 
     Furthermore, in the present embodiment, since the operating mode in transmission RF section  209  is limiter operation, the amplitude of the signal to be relayed (data directed to destination node  300 ) has a constant and maximum value. Therefore, amplitude information γ=MAX which indicates a constant and maximum amplitude value is inputted as an input voltage of the saturation amplifier to the polar modulation circuit constituting transmission RF section  209 . 
     Thus, polar modulation circuit  20  constituting transmission RF section  209  ( FIG. 3 ) of relay node  200  in the present embodiment is as shown in  FIG. 16 . That is, polar modulation circuit  20  is provided with only saturation amplifier  21 . Furthermore, the input voltage of saturation amplifier  21  is a constant value (γ=MAX). Thus, only data directed to the destination node, that is, only phase information θ is inputted to saturation amplifier  21 . 
     That is, when compared with general polar modulation circuit  10  shown in  FIG. 15 , since a phase bit (phase information θ) is directly inputted from modulation section  208 , polar modulation circuit  20  constituting transmission RF section  209  according to the present embodiment shown in  FIG. 16  eliminates the necessity for the polar coordinate conversion section. Furthermore, performing limiter transmission causes the input voltage of the saturation amplifier to become a constant value (and maximum value), and thereby eliminates the necessity for the terminal for the input voltage of saturation amplifier  21 . That is, polar modulation circuit  20  according to the present embodiment shown in  FIG. 16  is made up of only saturation amplifier  21  provided with only the terminal of the input signal to which phase information θ (data directed to destination node  300 ) is inputted. 
     Thus, by using a polar modulation circuit as the circuit of the radio transmission amplifier (transmission RF section  209 ) in relay node  200 , it is possible to simplify the circuit configuration of the radio transmission amplifier compared to a case where a general polar modulation circuit is used. That is, the polar modulation circuit that performs modulation using amplitude information and phase information is suitably applicable to a transmission RF section of a relay node that relays a relay signal (data directed to the destination node) using only phase (phase information). 
     Thus, the present embodiment can further suppress power consumption required for relay processing in relay node  200 . That is, relay node  200  can further reduce power consumption required for relay processing of data directed to destination node  300  and increase system capacity. 
     The embodiments of the present invention have been described so far. 
     A case has been described in the above embodiments where the source node uses 8PASK as a modulation scheme. However, the modulation scheme used by the source node is not limited to 8PASK, but the modulation scheme may be any modulation scheme as long as the modulation scheme is capable of expressing an amplitude bit whose value varies as the amplitude of a symbol changes and phase bits whose values vary as the phase of a symbol changes in a constellation. 
     For example, a case will be described where the source node uses 4QAM as the modulation scheme. As shown in  FIG. 17A , in a constellation of 4QAM, the number of bits constituting one symbol is 2, the high-order one bit is an amplitude bit and the low-order one bit is a phase bit. That is, as shown is  FIG. 17A , of the two bits constituting one symbol, when the high-order one bit (amplitude bit) is ‘1’, the signal is modulated on the outside circle (ring) and when the high-order one bit (amplitude bit) is ‘0’, the signal is modulated on the inside circle (ring). Furthermore, as shown in  FIG. 17A , of the two bits constituting one symbol, when the high-order one bit (amplitude bit) is ‘1’, a 90° offset of phase is added and when the high-order one bit (amplitude bit) is ‘0’, a offset 90° of phase is not added. In this case, in the source node, of two bits constituting each symbol, data r(n) directed to the relay node is allocated to the high-order one bit (amplitude bit) and data d(n) directed to the destination node is allocated to the low-order one bit (phase bit) as shown in  FIG. 17B . The relay node then extracts the high-order one bit (amplitude bit) out of the two bits constituting the symbol shown in  FIG. 17A  to obtain data directed to the relay node (r(n) shown in  FIG. 17B ) and extracts the low-order one bit (phase bit) to obtain data directed to the destination node (d(n) shown in  FIG. 17B ). The relay node then modulates data directed to the destination node corresponding to the low-order one bit (phase bit) using BPSK which is a modulation scheme using only phase, and relays the data. This makes it possible to reduce power consumption required for relay processing in the relay node as in the case of the above embodiment. 
     Furthermore, a case will be described by way of example where the source node uses 4PASK as the modulation scheme. As shown in  FIG. 18A , in the constellation of 4PASK, the number of bits constituting one symbol is 2, the low-order one bit is an amplitude bit and the high-order one bit is a phase bit. That is, as shown in  FIG. 18A , of the two bits constituting one symbol, when the low-order one bit (amplitude bit) is ‘1’, the signal is modulated by a symbol with greater amplitude (‘11’ or ‘01’). On the other hand, as shown in  FIG. 18A , of the two bits constituting one symbol, when the low-order one bit (amplitude bit) is ‘0’, the signal is modulated by a symbol with smaller amplitude (‘10’ or ‘00’). In this case, as shown in  FIG. 18B , the source node allocates data r(n) directed to the relay node to the low-order one bit (amplitude bit) out of the two bits constituting each symbol, and allocates data d(n) directed to the destination node to the high-order one bit (phase bit). The relay node then extracts the low-order one bit (amplitude bit) out of the two bits constituting the symbol shown in  FIG. 18A  to obtain the data directed to the relay node (r(n) shown in  FIG. 18B ) and extracts the high-order one bit (phase bit) to obtain the data directed to the destination node (d(n) shown in  FIG. 18B ). The relay node then modulates the data directed to the destination node corresponding to the high-order one bit (phase bit) using BPSK which is the modulation scheme using only phase, and relays the modulated data. This makes it possible to reduce power consumption required for relay processing in the relay node as in the case of the above embodiment. 
     Furthermore, as shown in  FIG. 19A , for example, the source node may also use 2 (k+1)  QAM (2 11  QAM in  FIG. 19A ) shown in  FIG. 19B  as the modulation scheme in which the number of amplitude bits is k (k=3 in  FIG. 19A ) and the number of phase bits is 1 (1=8 in  FIG. 19A ). That is, the source node allocates k bits of a plurality of bits ((k+1) bits) constituting one symbol to the data directed to the relay node, and allocates 1 bit to the data directed to the destination node. For example, as shown in  FIG. 19B , in 16(=2 (2+2) )QAM, the source node allocates two bits of a plurality of bits (4 bits) constituting one symbol to the data directed to the relay node, and allocates two bits to the data directed to the destination node. The same applies to other modulation schemes shown in  FIG. 19B . 
     A case has been described in the above embodiments where the relay node and the destination node feed back CQIs to the source node. Here, it is also possible to use as CQIs, the number of amplitude bits, the number of phase bits, a ratio between the number of amplitude bits and the number of phase bits, or a predetermined combination of the number of amplitude bits and the number of phase bits. For example, when the ratio between the number of amplitude bits and the number of phase bits is fed back instead of the aforementioned CQIs, the source node determines a modulation scheme or the like (that is, MCS information) based on the ratio between the number of amplitude bits and the number of phase bits. 
     Furthermore, a case has been described in the above embodiments by way of example where the present invention is configured by hardware, but the present invention may also be implemented by software. 
     Each function block employed in the description of each of the aforementioned embodiments may typically be implemented as an LSI constituted by an integrated circuit. These may be individual chips or partially or totally contained on a single chip. “LSI” is adopted here but this may also be referred to as “IC,” “system LSI,” “super LSI,” or “ultra LSI” depending on differing extents of integration. 
     Further, the method of circuit integration is not limited to LSI&#39;s, and implementation using dedicated circuitry or general purpose processors is also possible. After LSI manufacture, utilization of a programmable FPGA (Field Programmable Gate Array) or a reconfigurable processor where connections and settings of circuit cells within an LSI can be reconfigured is also possible. 
     Further, if integrated circuit technology comes out to replace LSI&#39;s as a result of the advancement of semiconductor technology or a derivative other technology, it is naturally also possible to carry out function block integration using this technology. Application of biotechnology is also possible. 
     The disclosure of Japanese Patent Application No. 2009-284345, filed on Dec. 15, 2009, including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     INDUSTRIAL APPLICABILITY 
     The present invention is applicable to a mobile communication system or the like. 
     REFERENCE SIGNS LIST 
     
         
           100 ,  400 ,  400   a  source node 
           200 ,  500  relay node 
           300  destination node 
           101 ,  201 ,  301  antenna 
           102 ,  202 ,  302  reception RF section 
           103  destination node CQI detection section 
           104  relay node CQI detection section 
           105  scheduler 
           106  MCS determining section 
           107 ,  108  coding section 
           109 ,  110  rate matching section 
           111  bit allocation section 
           112 ,  208  modulation section 
           113 ,  209  transmission RF section 
           203 ,  303  demodulation section 
           204  amplitude bit extraction section 
           205 ,  305  decoding section 
           206 ,  304  phase bit extraction section 
           207  relay processing section

Technology Category: 4