Patent Publication Number: US-2018041889-A1

Title: Wireless communication system

Description:
This application is a continuation application of International Application PCT/JP2015/063883 filed on May 14, 2015 and designated the U.S., the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     The embodiments discussed herein are related to a wireless communication system that performs device-to-device (D2D) communication, and a wireless communication device used in the wireless communication system. 
     BACKGROUND 
     The Third Generation Partnership Project (3GPP) has been considering the standardization of a mobile communication scheme. As an example, a high-speed wireless communication scheme such as long-term evolution (LTE) has been standardized by the 3GPP. In 3GPP Standards (Release 12), as one example of a new wireless communication scheme, the standardization of D2D communication is being promoted. D2D communication is one example of an extended specification of LTE, and may be referred to as LTE device-to-device proximity services. 
     In D2D communication, a terminal device can perform direct communication with another terminal device without going through a base station. Therefore, communication with a small delay is expected when D2D communication is used. In addition, D2D communication can be performed in an area that is difficult for radio waves from a base station to reach (or an area in which no base stations exist), and therefore D2D communication can contribute to the extension of network coverage. Further, also in a situation in which a base station is unavailable (for example, when a major earthquake has occurred), D2D communication can be performed, and therefore D2D communication can contribute to the securement of communications in disasters. A communication link established between terminal devices for D2D communication may be referred to as a D2D link. 
     When a terminal device fails to perform direct communication with a base station, D2D communication can be used such that another terminal device that supports D2D communication operates as a relay device between the terminal device and the base station. Namely, the network coverage is substantially extended by the UE-to-network relay. In addition, in D2D communication, data (such as emergency information) to be transmitted from a source terminal to a destination terminal can be relayed without going through a base station. Stated another way, D2D communication of 2 hops or more is implemented by the UE-to-UE relay. 
     Note that related technologies are described in, for example, WO2014/050557. 
     In a case in which data is relayed by using D2D communication, a plurality of terminal devices that can operate as a relay station may exist between a transmission source and a destination. In this case, one of the plurality of terminal devices operates as a relay station. 
     However, D2D communication is a new technology, and a relay scheme using D2D communication has not yet been sufficiently considered. As an example, a method for selecting a terminal device that will operate as a relay station from among a plurality of terminal devices has not yet been determined. Therefore, when a terminal device that is not preferable as a relay station is selected as the relay station, the quality of communication between a transmission source and a destination may deteriorate. This problem does not arise only in D2D communication described in the 3GPP Standards (Release 12), but may also arise in a wireless communication system that can perform direct communication between terminal devices. 
     SUMMARY 
     According to an aspect of the invention, a wireless communication system includes: a destination device; a source device configured to support device-to-device (D2D) communication and transmit data to the destination device; and a plurality of terminal devices respectively configured to support the D2D communication. One or more terminal devices among the plurality of terminal devices respectively transmit a discovery signal including parameter information to the source device when the one or more terminal devices are respectively configured to be selectable as a relay station for communication with the source device by the destination device. The source device performs a measurement of the discovery signal that is respectively received from the one or more terminal devices, and selects a terminal device that will relay data to be transmitted from the source device to the destination device from among the one or more terminal devices in accordance with the measurement. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates an example of a wireless communication system. 
         FIGS. 2A-2C  illustrate an example of messages used in a procedure for selecting a relay station. 
         FIG. 3  illustrates an example of a parameter table. 
         FIG. 4  illustrates an example of a terminal device. 
         FIG. 5  illustrates an example of a hardware configuration of a terminal device. 
         FIG. 6  illustrates an example of a base station. 
         FIG. 7  illustrates an example of a sequence for selecting a relay station according to a first embodiment. 
         FIG. 8  is a flowchart illustrating an example of the operation of a terminal device that selects a relay station. 
         FIG. 9  is a flowchart illustrating an example of the operation of a terminal device that may be selected as a relay station. 
         FIG. 10  is a diagram explaining the grouping of terminal devices. 
         FIG. 11  illustrates another example of a sequence for selecting a relay station according to the first embodiment. 
         FIG. 12  illustrates an example of a sequence for selecting a relay station according to a second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  illustrates an example of a wireless communication system according to embodiments of the invention. In this example, the wireless communication system includes a plurality of terminal devices (device-to-device user equipment (DUE))  1 - 5  and a base station  6 . 
     The base station  6  is an evolved Node B (eNB) in this example. The eNB is a base station used in LTE. Accordingly, the base station  6  manages and controls cellular communication according to LTE. Namely, the base station  6  can receive and process a data signal and a control signal in cellular communication that are transmitted from a terminal device. The base station  6  can also transmit a data signal and a control signal in cellular communication to a terminal device. 
     The base station  6  periodically transmits a broadcast signal. The broadcast signal is received by all of the terminal devices in a cell. In the example illustrated in  FIG. 1 , each of the terminal devices  1 - 3  and  5  receives the broadcast signal. A terminal device that has received the broadcast signal can determine that the terminal device itself exists within a cell of the base station  6 . The terminal device that has received the broadcast signal may transmit a response signal to the base station  6 . In this case, the base station  6  can manage terminal devices that exist within the cell. 
     Each of the terminal devices  1 - 5  supports cellular communication and D2D communication. Namely, each of the terminal devices  1 - 5  can transmit data to another terminal device via the base station  6 , and can receive data from another terminal device via the base station  6 . In addition, each of the terminal devices  1 - 5  can perform direct communication with another terminal device via a D2D link rather than via the base station  6 . Data carried in cellular communication or D2D communication is not particularly limited, but includes audio data, image data, video data, text data, and the like. In the description below, a terminal device that supports D2D communication may be referred to as a “DUE”. 
     Each of the terminal devices broadcasts a discovery signal in D2D communication periodically for example. The discovery signal is used to report the existence of a terminal device that generates the discovery signal to another terminal device. Accordingly, the discovery signal carries a message including identification information of a source terminal device of the discovery signal. As an example, a discovery signal transmitted from a DUE  4  carries a message including “source ID: DUE  4 ”. A discovery sequence is based on, for example, a physical random access channel (PRACH), a sounding reference signal (SRS), and/or a primary synchronization signal (PSS)/secondary synchronization signal (SSS). In addition, the message of the discovery signal is carried by using, for example, a physical uplink shared channel (PUSCH). 
     The discovery signal transmitted from DUE  4  is received by a terminal device that is located near DUE  4 . In this example, each of DUE  1  to DUE  3  receives the discovery signal transmitted from DUE  4 . In this case, DUE  4  can receive a discovery signal that is transmitted from each of DUE  1  to DUE  3 . 
     In the wireless communication system having the configuration above, when DUE  4  performs cellular communication, DUE  4  performs, for example, a cell search in order to detect abase station. However, in the example illustrated in  FIG. 1 , DUE  4  does not exist within the cell of the base station  6 , and therefore DUE  4  fails to detect a base station. In this case, DUE  4  accesses the base station  6  by using D2D communication. 
     DUE  4  broadcasts a relay station discovery signal with a specified transmission power. The transmission power of the relay station discovery signal may be different for each of the terminal devices in a case in which transmission power information is included in the discovery message, as illustrated in the example of  FIG. 2A . The relay station discovery signal carries a message including identification information of a source terminal device of the relay station discovery signal, similarly to a normal discovery signal. The discovery message carried by the relay station discovery signal includes, for example, a source ID, transmission power information, and data type information, as illustrated in  FIG. 2A . The source ID identifies a transmission source of the relay station discovery signal. The transmission power information indicates the transmission power of the relay station discovery signal. The data type information indicates the type of data that is carried in D2D communication after a D2D link is established. As an example, the data type may indicate whether emergency communication or non-emergency communication. The emergency communication includes, for example, a call for calling the police or a fire station. The data type is specified by a user of a terminal device. The discovery message may include a destination ID. The destination ID identifies a device that is a destination of data. 
     In this example, the relay station discovery signal transmitted from DUE  4  is received by DUE  1  to DUE  3 . Each of DUE  1  to DUE  3  calculates a priority value. The priority value indicates the priority of operating as a relay station, which will be described later in detail. DUE  1  to DUE  3  may calculate the priority value only when data type information included in the received discovery message indicates a specified type (for example, emergency communication). 
     Each of DUE  1  to DUE  3  transmits the calculated priority value to DUE  4 . At this time, each of DUE  1  to DUE  3  transmits the calculated priority value to DUE  4  by using the response message illustrated in  FIG. 2B . 
     When the relay station discovery signal transmitted from DUE  4  is received by multiple terminal devices, priority values are reported from the multiple terminal devices to DUE  4 . In this case, resources for D2D communication may be congested due to a procedure for carrying the priority values. Accordingly, each of the terminal devices that have received the relay station discovery signal (in  FIG. 1 , DUE  1  to DUE  3 ) transmits a calculated priority value to DUE  4 , only when the calculated priority value is greater than a specified threshold. By employing this method, only a preferable number of terminal devices transmit priority values by appropriately specifying a threshold. 
     DUE  4  determines a relay station DUE that will operate as a relay station in accordance with the priority values received from one or more terminal devices. At this time, DUE  4  selects a terminal device that has generated the largest priority value as a relay station DUE. In the example illustrated in  FIG. 1 , DUE  1  is selected from DUE  1  to DUE  3 . In this case, a D2D link is established between DUE  4  and DUE  1 . In addition, DUE  1  is configured so as to transfer, to the base station  6 , data received from DUE  4  via the D2D link. By doing this, D2D communication is performed between DUE  4  and DUE  1 , and cellular communication is performed between DUE  1  and the base station  6 . 
     The priority value is calculated according to at least one of the parameters below. 
     (1) Reference signal received power (RSRP) on a transmission-source side
 
(2) RSRP on a destination side
 
(3) Residual amount of a battery
 
(4) The number of adjacent DUEs
 
     (5) Interference 
     The RSRP on the transmission-source side indicates the received power of a signal transmitted from a transmission source of the relay station discovery signal. The transmission power of the relay station discovery signal is specified in advance, as described above. Alternatively, the transmission power of the relay station discovery signal is reported by using the relay station discovery signal, as illustrated in  FIG. 2A . Accordingly, the relay station discovery signal is used as a reference signal in the measurement of the RSRP. In the example illustrated in  FIG. 1 , each of DUE  1  to DUE  3  measures the received power of the relay station discovery signal transmitted from DUE  4 . 
     The RSRP on the destination side indicates the received power of a signal transmitted from a destination device. The “destination” is, for example, the base station  6 . In this case, each of the terminal devices measures, for example, the received power of a broadcast signal transmitted from the base station  6 . When the transmission power of the broadcast signal is specified in advance or known, the broadcast signal is used as a reference signal in the measurement of the RSRP. In the example illustrated in  FIG. 1 , each of DUE  1  to DUE  3  measures the received power of the broadcast signal transmitted from the base station  6 . 
     The residual amount of the battery indicates the residual amount of a battery implemented in a terminal device. In the example illustrated in  FIG. 1 , each of DUE  1  to DUE  3  monitors the residual amount of its own battery. 
     The number of adjacent DUEs indicates the number of DUEs that can be accessed by 1 hop. When each of the DUEs periodically broadcasts a discovery signal, each of the DUEs can detect adjacent DUEs. The number of transmission sources of the detected discovery signals corresponds to the number of adjacent DUEs. Assume, for example, that, in the example illustrated in  FIG. 1 , each of DUE  1  to DUE  4  broadcasts a discovery signal. Also assume that DUE  1  receives the discovery signal from each of DUE  2  to DUE  4 . In this case, the “number of adjacent DUEs” of DUE  1  is 3. The DUE is a wireless communication node that supports D2D communication, and therefore the “number of adjacent DUEs” may be referred to as the “number of adjacent D2D nodes”. 
     The interference is not particularly limited, but in this example, the interference is calculated according to the received power and the predicted interference power of a target signal (or according to a ratio of the received power and the predicted interference power of the target signal). As an example, in  FIG. 1 , assume that DUE  1  receives a target signal from DUE  4 . In this case, DUE  1  first calculates the path loss PL(4,1) between DUE  4  and DUE  1  according to the formula below. 
         PL (4,1)= P 0− Pr ( d )
 
     P0 indicates the transmission power of the relay station discovery signal. The transmission power of the relay station discovery signal is reported by the relay station discovery message, as illustrated in  FIG. 2A . Pr(d) indicates the received power of the relay station discovery signal at DUE  1 . Further, assume that DUE  1  measures in advance a path loss between DUE  1  and an adjacent DUE. In the example illustrated in  FIG. 1 , DUE  1  measures in advance the path loss PL(2,1) between DUE  2  and DUE  1  and the path loss PL(3,1) between DUE  3  and DUE  1 . Assume that each of the DUEs can measure or estimate a path loss between an adjacent DUE and the DUE itself by using a periodically broadcast discovery signal. 
     When the path loss PL(4,1) is small, the received power of the target signal at DUE  1  is large. In this case, interference with the target signal is likely to be small. When the path loss PL(4,1) is large, the received power of the target signal at DUE  1  is small. In this case, interference with the target signal is likely to be large. When a path loss (PL(2,1), PL(3,1)) between DUE  1  and an adjacent DUE is small, predicted interference power is large in DUE  1 . In this case, interference with the target signal is likely to be large. When a path loss between DUE  1  and an adjacent DUE is large, predicted interference power is small in DUE  1 . In this case, interference with the target signal is likely to be small. Accordingly, DUE  1  can calculate interference, for example, based on a ratio of a path loss PL between DUE  4  and DUE  1  and a path loss between an adjacent DUE and DUE  1 . At this time, interference may be calculated according to a ratio of the path loss PL between DUE  4  and DUE  1  and a minimum value of path losses between adjacent DUEs and DUE  1 . 
     Each of the DUEs may include a parameter table that stores the parameters described above. An example of the parameter table is illustrated in  FIG. 3 .  FIG. 3  illustrates an example of a parameter table provided in DUE  1 . In the example illustrated in  FIG. 3 , DUE  2 , DUE  3 , and DUE  4  have been detected as an adjacent DUE. The number of adjacent DUEs is 3 in this example. When the number of adjacent DUEs is calculated by using the parameter table, a transmission source (DUE  4 ) of the relay station discovery signal may be excluded from the detected DUEs (DUE  2  to DUE  4 ). The path loss is used to calculate interference, as described above. The parameter table is updated, for example, periodically. 
     Upon receipt of the relay station discovery signal, the terminal device calculates the priority value, as described above. The priority value P is calculated, for example, according to the formula below. 
         P=w 1* RSRP ( S )+ w 2* RSRP ( D )+ w 3* BTT−w 4* NUM _ DUE−w 5*INTER 
     Each of w1-w5 indicates a weight. Each of w1-w5 is zero or a positive value. RSRP(S) indicates the received power of a reference signal received from a source terminal device. RSRP(D) indicates the received power of a reference signal received from a destination device. BTT indicates the residual amount of a battery. NUM_DUE indicates the number of adjacent DUEs. INTER indicates interference. 
     Weights w1-w5 are determined according to a network policy. As an example, when communication quality is important, w1, w2, and w5 are increased. In order to reduce the concentration of a load on a specified terminal device, w4 may be increased. 
     The priority value is calculated according to at least one of the parameters described above. Accordingly, as an example, when the priority vale is calculated according to only the RSRP, w1 and w2 are 1, and w3-w5 are zero. 
     Each of the terminal devices that receives the relay station discovery signal compares the calculated priority value with a specified threshold. When the priority value is greater than the threshold, the terminal device reports the priority value to a source terminal device. The source terminal device determines a relay station DUE that will operate as a relay station in accordance with the priority values received from one or more terminal devices. At this time, DUE  4  may select a terminal device that has generated the largest priority value as a relay station DUE that will operate as a relay station. 
     The threshold is determined in advance according to, for example, simulation or measurement. The threshold is used to determine whether a DUE can operate as a relay station. As an example, when the threshold is high, only a DUE that can provide a high performance as a relay station transmits a priority value to the source terminal device. In this case, a DUE that will operate as a relay station is selected from one or more DUEs that can provide a high performance as the relay station. Accordingly, communication with a good quality is provided. However, when the threshold is excessively high, candidates for the DUE that will operate as the relay station may fail to be found. On the other hand, when the threshold is excessively low, priority values may be reported from a large number of DUEs to the source terminal device. In this case, D2D communication may be congested due to a procedure for carrying the priority values. Accordingly, the threshold is determined in such a way that the number of priority values received by the source terminal device is close to a specified number (for example, 5). 
     In an area in which a density at which DUEs exist is high, the threshold may be increased in comparison with an area in which the density at which DUEs exist is low. In this case, the threshold may be dynamically specified for each of the DUEs by a base station. 
       FIG. 4  illustrates an example of the terminal device. A terminal device  10  corresponds to each of DUE  1  to DUE  4  in the example illustrated in  FIG. 1 . The terminal device  10  supports cellular communication and D2D communication. The terminal device  10  may have other functions that are not illustrated in  FIG. 4 . 
     The terminal device  10  includes a traffic processor  11 , a channel encoder  12 , an IFFT circuit  13 , a CP adder  14 , an RF transmitter  15 , an RF receiver  16 , a channel demodulator  17 , and an RSRP calculator  18  in order to support cellular communication, as illustrated in  FIG. 4 . 
     The traffic processor  11  generates traffic to be transmitted in cellular communication. The channel encoder  12  encodes the traffic output from the traffic processor  11 . The IFFT circuit  13  performs inverse Fast Fourier Transform on an output signal of the channel encoder  12  so as to generate a time domain signal. The CP adder  14  adds a cyclic prefix (CP) to the time domain signal output from the IFFT circuit  13 . The RF transmitter  15  transmits a cellular signal via an antenna. The cellular signal is received by a base station. 
     The RF receiver  16  receives a cellular signal transmitted from the base station. The channel demodulator  17  demodulates the received cellular signal. When the received cellular signal includes a D2D resource allocation instruction, the channel demodulator  17  extracts the D2D resource allocation instruction from the received cellular signal, and guides the D2D resource allocation instruction to the D2D scheduler  21  described later. The RSRP calculator  18  calculates the received power of a reference signal (for example, a broadcast signal) that is transmitted from the base station. The received power obtained by the RSRP calculator  18  may be used as an RSRP on a destination side when a priority value is calculated. 
     The terminal device  10  includes a D2D scheduler  21 , a D2D data generator  22 , a discovery signal generator  23 , an RF transmitter  24 , an RF receiver  25 , a data signal demodulator  26 , a discovery signal detector  27 , an RSRP calculator  28 , an interference calculator  29 , a priority value calculator  30 , a threshold decision unit  31 , and a selector  32  in order to support D2D communication. 
     The D2D scheduler  21  can determine a resource used for D2D communication from among resources provided by the wireless communication system or prepared resources. As an example, when a frequency to be used for D2D communication is determined by the D2D scheduler  21 , the terminal device  10  performs D2D communication at the frequency. The D2D scheduler  21  can control D2D communication of the terminal device  10  in accordance with the resource allocation instruction received from the base station. As an example, when the frequency of D2D communication is specified by the resource allocation instruction, the D2D scheduler  21  controls the D2D data generator  22  and/or the RF transmitter  24  in such a way that a D2D signal is transmitted at the specified frequency. In addition, the D2D scheduler  21  may control the RF receiver  25  and/or the data signal demodulator  26  in such a way that a D2D signal is received at the specified frequency. 
     The D2D data generator  22  generates transmission data in D2D communication according to the control of the D2D scheduler  21 . The D2D data generator  22  can also generate a response message including the priority value obtained by the priority value calculator  30 . The discovery signal generator  23  generates a discovery signal/relay station discovery signal. The discovery signal/relay station discovery signal carries the identification information of the terminal device itself. The discovery signal/relay station discovery signal is transmitted by using, for example, PUSCH. The RF transmitter  24  transmits a D2D signal (including a D2D data signal, a discovery signal, and a relay discovery signal) via an antenna. 
     The RF receiver  25  receives a D2D signal (including a D2D data signal, a discovery signal, and a relay discovery signal) that is transmitted from another terminal device. The data signal demodulator  26  demodulates the received D2D data signal so as to recover D2D data. 
     The discovery signal detector  27  detects the discovery signal/relay station discovery signal from the D2D signal transmitted from the other terminal device. At this time, the discovery signal detector  27  obtains the identification information of a terminal device that is a transmission source of the discovery signal/relay station discovery signal. In addition, the discovery signal detector  27  can extract transmission power information and data type information from the relay station discovery signal. 
     The RSRP calculator  28  calculates the received power of a reference signal (for example, the discovery signal/relay station discovery signal) that is transmitted from another terminal device. The received power obtained by the RSRP calculator  28  may be used to calculate the priority value. The interference calculator  29  calculates interference that a target signal may be subject to in accordance with the received power of the discovery signal/relay station discovery signal. The interference is calculated according to a path loss between terminal devices, as described above. 
     The priority value calculator  30  calculates the priority value by using at least one of parameters (1) to (5) described above, when the terminal device  10  receives the relay station discovery signal. The RSRP is calculated by the RSRP calculator  18  or  28 . The residual amount of a battery is obtained by monitoring a battery that is not illustrated in  FIG. 4 . The number of adjacent DUEs is obtained by counting the number of terminal devices for which a discovery signal is detected by the discovery signal detector  27 . The interference is calculated by the interference calculator  29 . 
     The threshold decision unit  31  decides whether the priority value calculated by the priority value calculator  30  will be transmitted to a source terminal device. Namely, the threshold decision unit  31  decides that the priority value will be transmitted to the source terminal device when the calculated priority value is greater than the threshold, and the threshold decision unit  31  decides that the priority value will not be transmitted to the source terminal device when the calculated priority value is smaller than or equal to the threshold. When it is decided that the priority value will be transmitted to the source terminal device, the priority value calculated by the priority value calculator  30  is transmitted to the source terminal device by using the D2D data generator  22 . 
     The selector  32  selects a DUE that will operate as a relay station (hereinafter referred to as a relay station DUE) in accordance with the priority values received from one or more other terminal devices. As an example, a DUE that has generated the largest priority value is selected as the relay station DUE. Note that the terminal device  10  has a function of establishing a D2D link with the relay station DUE. At this time, in the relay station DUE, a communication circuit is configured in such a way that data received from the terminal device  10  via the D2D link is forwarded to a destination device (in the example illustrated in  FIG. 1 , the base station  6 ). 
       FIG. 5  illustrates an example of a hardware configuration of the terminal device. As illustrated in  FIG. 5 , the terminal device  10  includes a processor  10   a , a memory  10   b , a transceiver circuit  10   c , and a battery  10   d . The terminal device  10  may include other hardware elements. 
     The processor  10   a  implements the functions of the terminal device  10  by executing a given program. As an example, the functions of the traffic processor  11 , the channel encoder  12 , the IFFT circuit  13 , the CP adder  14 , the channel demodulator  17 , the RSRP calculator  18 , the D2D scheduler  21 , the D2D data generator  22 , the discovery signal generator  23 , the data signal demodulator  26 , the discovery signal detector  27 , the RSRP calculator  28 , the interference calculator  29 , the priority value calculator  30 , the threshold decision unit  31 , and the selector  32  that are illustrated in  FIG. 4  may be implemented by the processor  10   a.    
     The memory  10   b  stores a program executed by the processor  10   a . The memory  10   b  also stores the parameter table illustrated in  FIG. 3 . The memory  10   b  includes a work area of the processor  10   a . The transceiver circuit  10   c  corresponds to the RF transmitter  15 , the RF receiver  16 , the RF transmitter  24 , and the RF receiver  25  that are illustrated in  FIG. 4 . The battery  10   d  supplies power to the processor  10   a , the memory  10   b , and the transceiver circuit  10   c . The residual amount of the battery  10   d  is periodically monitored by the processor  10   a  (the priority value calculator  30 ). 
       FIG. 6  illustrates an example of the base station. The base station  6  includes an RF receiver  41 , a CP remover  42 , an FFT circuit  43 , a channel separator  44 , a data signal demodulator  45 , a channel decoder  46 , a control signal demodulator  47 , a channel decoder  48 , a D2D scheduler  49 , a data signal generator  50 , a DUE selector  51 , a control signal generator  52 , an IFFT circuit  53 , a CP adder  54 , and an RF transmitter  55 , as illustrated in  FIG. 6 . The base station  6  may include other functions. 
     The RF receiver  41  receives a cellular signal transmitted from the terminal device  10 . The CP remover  42  removes a cyclic prefix from the received cellular signal. The FFT circuit  43  performs Fast Fourier Transform on the received signal so as to generate a frequency domain signal. The channel separator  44  separates the received signal into a data signal and a control signal in a frequency domain. 
     The data signal demodulator  45  demodulates the received data signal so as to recover data. The channel decoder  46  decodes the recovered data. The control signal demodulator  47  demodulates the received control signal. The channel decoder  48  decodes the demodulated control signal so as to recover control information. 
     The D2D scheduler  49  generates a resource allocation instruction of D2D communication by using the control information recovered by the channel decoder  48 . The data signal generator  50  generates a data signal to be transmitted to the terminal device  10 . The DUE selector  51  specifies one or more candidates for a terminal device that will operate as a relay station from among terminal devices that exist within a cell. The DUE selector  51  outputs identification information for identifying the specified terminal device. The control signal generator  52  generates a control signal that controls the terminal device  10 . The resource allocation instruction generated by the D2D scheduler  49  is transmitted to the terminal device  10  by the data signal generator  50  or the control signal generator  52 . The identification information indicating the terminal device selected by the DUE selector  51  is transmitted to the terminal device  10  by the control signal generator  52 . 
     The IFFT circuit  53  performs inverse Fast Fourier Transform on the control signal and the data signal so as to generate a time domain signal. The CP adder  54  adds a cyclic prefix to the time domain signal output from the IFFT circuit  53 . The RF transmitter  55  transmits a cellular signal via an antenna. 
     First Embodiment 
       FIG. 7  illustrates an example of a sequence for selecting a relay station according to a first embodiment. In this example, assume that DUE  4  accesses the base station  6 . Namely, DUE  4  is a source terminal device, and the base station  6  is a destination device. However, DUE  4  is located outside the cell of the base station  6 , as illustrated in  FIG. 1 , and thus fails to directly access the base station  6 . Stated another way, DUE  4  fails to detect the base station  6  when a cell search is performed. Accordingly, DUE  4  selects a DUE that will operate as a relay station between DUE  4  and the base station  6 . 
     DUE  4  broadcasts a relay station discovery signal. The relay station discovery signal carries a discovery message including “source ID: DUE  4 ” “transmission power” and “data type: emergency”. The relay station discovery signal is received by each of DUE  1  to DUE  3 . The relay station discovery signal may include a destination ID that specifies a “base station” in order to indicate the UE-to-network relay. When the relay station discovery signal does not include the destination ID, each of the DUEs may be configured to recognize that the relay station discovery signal is requesting the UE-to-network relay as a default. In these cases, each of the DUEs that have received the relay station discovery recognizes the nearest base station as a destination. 
     When each of DUE  1  to DUE  3  detects that a data type reported by the discovery message is “emergency”, each of DUE  1  to DUE  3  starts a process for calculating a priority value. The priority value is calculated by using at least one of an RSRP on a transmission-source side, an RSRP on a destination side, the residual amount of a battery, the number of adjacent DUEs, and interference, as described above. It is preferable that the priority value be calculated by using at least the RSRP on the transmission-source side and the RSRP on the destination side. 
     Each of DUE  1  to DUE  3  compares the calculated priority value with a specified threshold. When the calculated priority value is greater than the threshold, the DUE transmits the priority value to DUE  4 . In the example illustrated in  FIG. 7 , the priority values calculated by DUE  1  and DUE  2  are respectively greater than the threshold, and the priority value calculated by DUE  3  is smaller than the threshold. Accordingly, each of DUE  1  and DUE  2  transmits the priority value calculated by the local device to DUE  4 . Stated another way, priority value 1 is reported from DUE  1  to DUE  4 , and priority value 2 is reported from DUE  2  to DUE  4 . 
     DUE  4  selects a DUE that will operate as a relay station in accordance with the priority value received from each of one or more DUEs. In this example, DUE  4  receives priority value 1 from DUE  1 , and receives priority value 2 from DUE  2 . Assume that priority value 1 is greater than priority value 2. In this case, DUE  4  selects DUE  1  as a relay station DUE. 
     DUE  4  establishes a D2D link with DUE  1 . At this time, a communication circuit is configured in such a way that DUE  1  forwards, to the base station  6 , data received from DUE  4  via the D2D link. By doing this, data to be transmitted from DUE  4  to the base station  6  is relayed by DUE  1 , and is forwarded to the base station  6 . At this time, D2D communication is performed between DUE  4  and DUE  1  via the D2D link, and cellular communication is performed between DUE  1  and the base station  6 . 
     As described above, according to the first embodiment, when a terminal device fails to perform direct communication with a destination device, a relay station DUE that will operate as a relay station is selected from a plurality of DUEs. At this time, the relay station DUE is selected in consideration of the communication environment and the operation state of each of the DUEs, and therefore the quality of communication between the terminal device and the destination device is good. The relay station DUE is selected according to priority values generated by the respective DUEs. At this time, only a priority value that is greater than a specified threshold is transmitted from a corresponding DUE to the terminal device. Accordingly, traffic for reporting the priority values is reduced. 
       FIG. 8  is a flowchart illustrating the processing of a terminal device that selects a relay station. The processing of this flowchart is performed, for example, by DUE  4  illustrated in FIG.  1  or  FIG. 7 . In addition, the processing of this flowchart is performed by the processor  10   a  of the terminal device that selects the relay station. Further, the processing of this flowchart is performed, for example, when the terminal device fails to detect a base station in a cell search. 
     In S 1 , the terminal device broadcasts a relay station discovery signal. In S 2  and S 3 , the terminal device awaits a response message that corresponds to the relay station discovery signal. Upon receipt of a response message from one or more DUEs, the processing of the terminal device moves on to S 4 . When the terminal device does not receive the response message within a specified time period after the relay station discovery signal is transmitted, it is determined that the terminal device is isolated. In this case, the processing of the terminal device is terminated. 
     In S 4 , the terminal device obtains a priority value from the received response message. Namely, the terminal device obtains a priority value generated by each of the DUEs. In S 5 , the terminal device selects a relay station DUE according to the obtained priority values. The terminal device transmits, to the relay station DUE, a message indicating an instruction to operate as a relay station, and establishes a D2D link with the relay station DUE. 
       FIG. 9  is a flowchart illustrating the processing of a DUE that may be selected as the relay station. The processing of this flowchart is performed, for example, by DUE  1  to DUE  3  illustrated in  FIG. 1  or  FIG. 7 . In addition, the processing of this flowchart is performed by the processor  10   a  of the DUE. 
     In S 11 , the DUE receives a relay station discovery signal from a source terminal device. In S 12 , the DUE refers to a data type in a discovery message, and decides whether the DUE will respond to the relay station discovery signal. As an example, when the data type is “emergency”, the DUE decides that the DUE will respond to the relay station discovery signal. 
     In S 13 , the DUE calculates the received power of the relay station discovery signal. The received power is stored as an RSRP on a transmission-source side in a parameter table. In S 14 , the DUE obtains other selection parameters (such as an RSRP on a destination side, the residual amount of a battery, the number of adjacent DUEs, and/or interference). As an example, the selection parameters are measured in advance, and are stored in the parameter table illustrated in  FIG. 3 . The DUE may measure each of the selection parameters upon receipt of the relay station discovery signal. 
     In S 15 , the DUE calculates a priority value according to the selection parameters (including the RSRP on the transmission-source side). In S 16 , the DUE compares the priority value obtained in S 15  with a threshold. When the priority value is greater than the threshold, the DUE generates a response message including the priority value, and transmits the response message to the source terminal device in S 17 . When the priority value is smaller than or equal to the threshold, the DUE does not respond to the relay station discovery signal. 
     In the example described above, the destination device is a base station device, but the invention is not limited to this form. Namely, the destination device may be a terminal device (herein, a DUE). As an example, in  FIG. 1 , assume that DUE  4  desires to access DUE  5 , but that a wireless signal transmitted from DUE  4  does not reach DUE  5 . According to the first embodiment, also in this case, a DUE that will operate as a relay station is selected from DUE  1  to DUE  3 . 
     When the destination device is a DUE, the source terminal device may broadcast a discovery message including a destination ID by using a relay station discovery signal. In this case, a DUE that has received the relay station discovery signal can specify a destination device, and therefore the DUE can obtain the RSRP on the destination side. Stated another way, the DUE that has received the relay station discovery signal can calculate a priority value by using a method similar to the method in a case in which the destination is abase station. Therefore, according to the first embodiment, even when the destination device is a DUE, a DUE that preferably operates as a relay station is selected, and a relay operation is provided by the selected DUE. 
     The terminal devices that may operate as a relay station may be grouped. In the example illustrated in  FIG. 10 , DUE  7   a  and DUE  7   b  belong to a first group, DUE  7   b  and DUE  7   d  belong to a second group, and DUE  7   c  and DUE  7   f  belong to a third group. DUE  7   a  and DUE  7   e  that belong to the first group calculate priority values, and transmit the priority values to a source terminal device (DUE  4 ), only when DUE  7   a  and DUE  7   e  receive a relay station discovery signal that specifies a data type (for example, a report to the police) that corresponds to the first group. DUE  7   b  and DUE  7   d  that belong to the second group calculate priority values, and transmit the priority values to the source terminal device (DUE  4 ), only when DUE  7   b  and DUE  7   d  receive a relay station discovery signal that specifies a data type (for example, a report to the fire station) that corresponds to the second group. In this case, a threshold for deciding whether the priority value will be transmitted may be different for each group. 
     In the example described above, a source terminal device selects a relay station DUE according to priority values calculated by respective DUEs, but the invention is not limited to this method. As an example, a base station may select the relay station DUE according the priority values calculated by the respective DUEs. 
       FIG. 11  illustrates an example of a sequence in which abase station selects a relay station. A procedure in which a source terminal device broadcasts a relay station discovery signal and a procedure in which a DUE that has received the relay station discovery signal calculates a priority value are substantially the same in  FIG. 7  and  FIG. 11 . 
     In the example illustrated in  FIG. 11 , DUE  1  and DUE  2  respectively transmit priority value 1 and priority value 2 to the base station  6 . The base station  6  selects a DUE that will operate as a relay station in accordance with the received priority values. In this example, DUE  1  is selected as the relay station DUE. Then, the base station  6  transmits, to DUE  1 , a message indicating that DUE  1  has been selected as the relay station DUE, and DUE  1  forwards this message to the source terminal device (in this example, DUE  4 ). Then, similarly to the example illustrated in  FIG. 7 , DUE  1  relays communication between DUE  4  and the base station  6 . 
     Second Embodiment 
     In the first embodiment, when a source terminal device broadcasts a relay station discovery signal, a procedure for selecting a DUE that will operate as a relay station is started. In a second embodiment, a DUE that desires to operate as a relay station broadcasts a transmission source discovery signal. Whether a DUE desires to operate as a relay station is specified, for example, by a user of the DUE. A DUE that is specified by the base station may broadcast the transmission source discovery signal. The transmission source discovery signal may be broadcast periodically. 
     The transmission source discovery signal includes the “source ID” and the “selection parameter”, as illustrated in  FIG. 2C . The source ID identifies a transmission source of the transmission source discovery signal. The selection parameter includes an RSRP on a destination side, the residual amount of a battery, and the number of adjacent DUEs. In this example, the destination device is the base station  6  illustrated in  FIG. 1 . In this case, the RSRP on the destination side indicates the received power of a reference signal (such as a broadcast signal) that is transmitted from the base station  6 . The residual amount of the battery and the number of adjacent DUEs are the same in the first embodiment and the second embodiment. It is preferable that the transmission power of the transmission source discovery signal be specified in advance. As an example, the transmission power of the transmission source discovery signal may be the same as a normal discovery signal or the relay station discovery signal described above. 
     The source terminal device performs a cell search when communication is performed with another terminal device via the base station  6 . When the cell search has failed, the source terminal device selects a DUE that will operate as a relay station by using a transmission source discovery signal received from each of one or more DUEs. At this time, the source terminal device calculates a corresponding priority value according to the received power of the transmission source discovery signal and the selection parameter carried by the transmission source discovery signal. The source terminal device selects a DUE that will operate as a relay station in accordance with the priority value calculated for each of the one or more DUEs. 
       FIG. 12  illustrates an example of a sequence for selecting a relay station according to the second embodiment. In this example, DUE  4  accesses the base station  6 , similarly to the sequence illustrated in  FIG. 7 . Namely, DUE  4  is a source terminal device, and the base station  6  is a destination device. Also assume that DUE  4  fails to detect the base station  6  when a cell search is performed. 
     From among DUE  1  to DUE  4 , DUE  1  and DUE  2  periodically broadcast a transmission source discovery signal. Namely, DUE  1  and DUE  2  are configured to be selectable as a relay station by a user. Alternatively, DUE  1  and DUE  2  are specified to be DUEs that may be selected as the relay station by the base station  6 . 
     The transmission source discovery signal carries a discovery message including the selection parameter, as illustrated in  FIG. 2C . The selection parameter includes an RSRP on a destination side, the residual amount of a battery, and the number of adjacent DUEs, as described above. As an example, the selection parameter collected by DUE  1  includes the received power of a broadcast signal transmitted from the base station  6 , the residual amount of a battery of DUE  1 , and the number of D2D nodes that are adjacent to DUE  1 . It is assumed that the selection parameter of each of the DUEs is stored in the parameter table illustrated in  FIG. 3 , as described above. 
     DUE  4  awaits a transmission source discovery signal that is broadcast by another DUE. In this example, DUE  4  receives transmission source discovery signal  1  including selection parameter 1 from DUE  1 , and receives transmission source discovery signal  2  including selection parameter 2 from DUE  2 . Then, DUE  4  calculates respective priority values for DUE  1  and DUE  2 . 
     Specifically, DUE  4  calculates the received power of transmission source discovery signal  1  transmitted from DUE  1 . Here, the loss of a path from DUE  1  to DUE  4  is almost the same as the loss of a path from DUE  4  to DUE  1 . Accordingly, the received power of transmission source discovery signal  1  at DUE  4  is equivalent to an RSRP at DUE  1  at the time when a reference signal is transmitted from DUE  4  to DUE  1 . Namely, the received power of transmission source discovery signal  1  at DUE  4  is equivalent to an RSRP on a transmission-source side at DUE  1  at the time when data is transmitted from DUE  4  via DUE  1  to the base station  6 . 
     DUE  4  calculates the priority value of DUE  1  by using the received power of transmission source discovery signal  1  transmitted from DUE  1  (namely, an RSRP on a transmission-source side), and an RSRP on a destination side, the residual amount of a battery, and the number of adjacent DUEs that are included in selection parameter 1 received from DUE  1 . Similarly, DUE  4  calculates the priority value of DUE  2  by using the received power of transmission source discovery signal  2  transmitted from DUE  2 , and selection parameter 2. 
     A method for selecting a DUE that will operate as a relay station according to a priority value and a method for establishing a link by using the selected DUE are substantially the same in the first embodiment and the second embodiment. Accordingly, similarly to the example illustrated in  FIG. 7 , also in the sequence illustrated in  FIG. 12 , a D2D link is established between DUE  4  and DUE  1 . In addition, a communication circuit is configured in such a way that DUE  1  forwards, to the base station  6 , data received from DUE  4  via the D2D link. By doing this, data to be transmitted from DUE  4  to the base station  6  is relayed by DUE  1 , and is forwarded to the base station  6 . 
     All examples and conditional language provided herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.