Patent Publication Number: US-2022216952-A1

Title: Terminal, base station, receiving method, and transmitting method

Description:
TECHNICAL FIELD 
     The present disclosure relates to a terminal, a base station, a receiving method and a transmitting method. 
     BACKGROUND ART 
     In the standardization of 5G, New Radio access technology (NR) was discussed in 3GPP and the NR Release 15 (Rel. 15) specification was published. 
     CITATION LIST 
     Non-Patent Literature 
     
         
         NPL 1 
       
    
     3GPP, TR38.811 V15.0.0, “Study on New Radio (NR) to support non terrestrial networks (Release 15)”, 2018-06 
     SUMMARY OF INVENTION 
     However, there is a room to study about appropriate retransmission control processing in accordance with a radio propagation environment. 
     A non-limiting and exemplary embodiment facilitates providing a terminal, a base station, a receiving method and a transmitting method capable of realizing appropriate retransmission control processing in accordance with a radio propagation environment. 
     In an embodiment, the techniques disclosed here feature a terminal including: reception circuitry, which, in operation, receives control information on buffering of data relating to a retransmission request; and control circuitry which, in operation, controls the buffering based on the control information. 
     It should be noted that general or specific embodiments may be implemented as a system, an apparatus, a method, an integrated circuit, a computer program, a storage medium, or any selective combination thereof. 
     According to an embodiment of the present disclosure, it is possible to realize appropriate retransmission control processing in accordance with a radio propagation environment. 
     Additional benefits and advantages of the disclosed embodiments will become apparent from the specification and drawings. The benefits and/or advantages may be individually obtained by the various embodiments and features of the specification and drawings, which need not all be provided in order to obtain one or more of such benefits and/or advantages. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating exemplary operations of HARQ; 
         FIG. 2  is a block diagram illustrating an exemplary configuration of a part of a base station according to Embodiment 1; 
         FIG. 3  is a block diagram illustrating an exemplary configuration of a part of a terminal according to Embodiment 1; 
         FIG. 4  is a block diagram illustrating an exemplary configuration of the base station according to Embodiment 1; 
         FIG. 5  is a block diagram illustrating an exemplary configuration of the terminal according to Embodiment 1; 
         FIG. 6  is a diagram illustrating exemplary operations of a radio communication system according to Embodiment 1; 
         FIG. 7  is a diagram illustrating exemplary operations of the radio communication system according to Embodiment 1; 
         FIG. 8  is a diagram illustrating exemplary operations of the radio communication system according to Embodiment 2; 
         FIG. 9  is a diagram illustrating exemplary operations of the radio communication system according to Embodiment 2; 
         FIG. 10  is a diagram illustrating exemplary operations of the radio communication system according to Variation 1 of Embodiment 2; 
         FIG. 11  is a diagram illustrating exemplary operations of the radio communication system according to Variation 2 of Embodiment 2; 
         FIG. 12  is a diagram illustrating exemplary operations of the radio communication system according to Variation 2 of Embodiment 2; 
         FIG. 13  is a diagram illustrating exemplary operations of the radio communication system according to Embodiment 3; 
         FIG. 14  is a diagram illustrating an example of indication information of ACK/NACK according to Embodiment 3; and 
         FIG. 15  is a diagram illustrating another example of indication information of ACK/NACK according to Embodiment 3. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present disclosure will be described in detail below by referring to the drawings. 
     [Extension to Non-Terrestrial Network (NTN)] 
     NR is being studied for extension to Non-Terrestrial Networks (NTNs) such as a communication using a satellite and/or a High-altitude platform station (RAPS) (e.g., NPL 1). 
     In an NTN environment, a coverage area (e.g., one or more cells) of a satellite for a terrestrial terminal (also called as a UE (User Equipment)) or an aircraft terminal is formed by beams from the satellite. Also, a round trip time of radio wave propagation between the terminal and the satellite is determined by an altitude (e.g., up to about 36,000 km) of the satellite and/or an angle viewed from the terminal. 
     For example, the satellite forms a cell with a diameter of several hundreds of kilometers. The cell formed by the satellite is larger than a cell with a diameter of several kilometers formed by a terrestrial base station (also called as a gNB, for example) or the like. For example, NPL 1 describes that a propagation delay time between a satellite and a terminal, that is, a round trip propagation time (RTT: Round Trip Time) takes about 544 ms at most in arm NTN. 
     [HARQ] 
     In LTE or NR, for example, HARQ (hybrid automatic repeat request) is applied to retransmission control in data transmission. 
     In HARQ, a transmitting side performs channel coding (FEC: Forward Error Correction) such as turbo coding or LDPC (Low Density Parity Check) coding on data, for example, and transmits the channel coded data. When there is an error in received data in data decoding, a receiving side saves (in other words, also called as buffers, stores or holds) the received data (e.g., soft determination value) in a buffer. Note that a buffer is also called as, for example, an HARQ soft buffer or simply a soft buffer. In retransmitting the data, the receiving side combines (soft combines) the received data (in other words, retransmission data or data relating to a retransmission request) and the previously received data (in other words, saved data), and decodes the combined data. 
     As a result, in HARQ, the receiver side can decode the data using data with an improved received quality SNR (Signal to Noise Ratio)). Also, in HARQ, the transmission side can improve a coding gain by transmitting a parity bit (in other words, a different RV (Redundancy version)) different from that in the previous transmission. Further, in HARQ, a plurality of processes (also called as HARQ processes, for example) can be used to continuously transmit data in view of a propagation path delay or processing delays of the transmitting side and the receiving side (see, for example,  FIG. 1 ). In this case, the receiving side saves the received data in the buffer separately for each process ID (sometimes referred to as “PID”) which is identification information for identifying the process (or data). 
     Furthermore, for example, in LTE or NR, in data allocation, a base station indicates, to a terminal, information on HARQ including a process ID, an NDI (New Data Indicator), an RV and the like. The terminal performs data reception processing (e.g., soft combining processing) based on the information on HARQ indicated from the base station. 
     On the other hand, for example, an HARQ method for an NTN has not been fully discussed. Since the number of processes in an RTT is increased in an NTN, which is a cell environment with a larger propagation delay (or RTT) as compared with a terrestrial network, an amount of soft buffer (in other words, soft buffer size) included in the receiving side (for example, terminal) may be increased. The increase in the amount of soft buffer may increase, for example, the cost and chip size of the device on the receiving side (terminal). 
     On the contrary, for example, when the number of processes in an RTT is decreased, an amount of soft buffer included in the receiving side (for example, terminal) can be decreased. However, when the number of processes in the RTT is decreased, the throughput per user (in other words, terminal) (also called as maximum user throughput, for example) may be decreased. 
     Further, for example, when HARQ (in other words, retransmission in the physical layer or the MAC (Medium Access Control)) is not applied, retransmission of a higher layer (e.g., the RLC (Radio Link Control) layer) is performed. However, in the retransmission of the higher layer, a combining gain is not obtained and a delay becomes larger as compared with HARQ. For example, in the retransmission of the higher layer, the user experience may be degraded due to delays in various network controls or a delay of user data. 
     Then, in an embodiment of the present disclosure, retransmission control in a case where a propagation delay between a terminal and a base station is larger compared with a terrestrial network environment, such as an NTN environment, will be described. According to an embodiment of the present disclosure, for example, appropriate retransmission control can be realized for each of different data (for example, data of different processes). 
     Embodiment 1 
     [Overview of Communication System] 
     A communication system according to the present embodiment includes, for example, base station  100  and terminal  200 . In the following description, as an example, base station  100  (corresponding to a transmitting apparatus) transmits data (also called as downlink data or a PDSCH (Physical Downlink Shared Channel), for example). Further, terminal  200  (corresponding to a receiving apparatus) receives data and feeds back a response signal for the data. Note that a response signal is also called as an ACK (Acknowledgement)/NACK (Negative acknowledgement) signal or ACK/NACK information, for example, 
       FIG. 2  is a block diagram illustrating an exemplary configuration of a part of base station  100  according to the embodiment of the present disclosure. In base station  100  illustrated in  FIG. 2 , controller  101  generates control information (e.g., buffer priority information) on buffering for retransmission control. Radio transmitter  103  transmits data and the control information. 
       FIG. 3  is a block diagram illustrating an exemplary configuration of a part of terminal  200  according to the embodiment of the present disclosure. In terminal  200  illustrated in  FIG. 3 , radio receiver  202  receives control information (e.g., buffer priority information) on buffering for retransmission control. Soft buffer controller  204  controls the buffering based on the control information. 
     [Configuration of Base Station] 
       FIG. 4  is a block diagram illustrating an exemplary configuration of base station  100  according to the present embodiment. Base station  100  includes controller  101 , coder and modulator  102 , radio transmitter  103 , antenna  104 , radio receiver  105 , demodulator and decoder  106  and ACK/NACK determiner  107 , for example. 
     Controller  101 , for example, generates control information (e.g., data allocation information) for terminal  200 , and outputs the generated control information to coder and modulator  102 . The control information may include allocation information of a time resource and a frequency resource (also called as time and frequency resource allocation information), information on a coding rate and a modulation scheme (also called as MCS (Modulation and Coding Scheme) information, for example), and information on HARQ (also called as HARQ information, for example), for example. 
     The HARQ information may include a process ID, an NDI and an RV, for example. The HARQ information may also include “buffer priority information (BPF: Buffer Priority Flag)” indicating a priority of buffering in a buffer (e.g., soft buffer  205  illustrated in  FIG. 5 ) included in terminal  200  between a plurality of data (e.g., data of different processes). Note that an example of the BPF will be described later. 
     Further, controller  101  controls retransmission of transmission data based on the determination result of ACK/NACK input from ACK/NACK determiner  107 . For example, controller  101  outputs a retransmission instruction to coder and modulator  102  in a case of a NACK, and outputs a data discard instruction to coder and modulator  102  in a case of an ACK. 
     Coder and modulator  102  codes (encodes) and modulates input transmission data (for example, data relating to new transmission (simply also called as new data) or the initial transmission data), and outputs the modulated signal to radio transmitter  103 . The data may be coded by error correction coding such as turbo code, LDPC code and polar code, for example. The data may be modulated by QPSK (Quadrature Phase Shift Keying), QAM (Quadrature Amplitude Modulation), or the like, for example. Coder and modulator  102  codes and modulates the control information input from controller  101  and outputs the modulated signal to radio transmitter  103 . 
     Further, coder and modulator  102  holds transmitted transmission data, for example, and performs processing similar to the above-described processing on the held data (in other words, retransmission data) when a retransmission instruction is output from controller  101  (in other words, there is an error in the transmission data). In addition, coder and modulator  102  discards the held data, for example, when a data discard instruction is output from controller  101  (in other words, there is no error in the transmission data). 
     Note that a retransmission unit of transmission data may be a transport block unit, a code block unit, a code block group unit or another data unit, for example. 
     For example, in LTE or 5G NR, transmission data corresponds to a PDSCH and data allocation information corresponds to DCI (Downlink Control Information) or a PDCCH (Physical Downlink Control Channel). 
     Radio transmitter  103  performs transmission processing such as D/A conversion, up-conversion and amplification, for example, on the signal input from coder and modulator  102 , and transmits the radio signal obtained by performing the transmission processing from antenna  104 . 
     Radio receiver  105  performs reception processing such as down-conversion and A/D conversion on a signal received via, antenna  104  from terminal  200 , and outputs the signal after performing the reception processing to demodulator and decoder  106 . The Signal from terminal  200  include a data signal (also called as uplink data or a PUSCH (Physical Uplink Shared Channel, for example) and an ACK/NACK signal, for example. 
     Demodulator and decoder  106  performs, for example, channel estimation, demodulation and decoding processing on the signal input from radio receiver  105 . Demodulator and decoder  106  outputs the data signal (in other words, received data) included in the received signal, and outputs the ACK/NACK signal included in the received signal to ACK/NACK determiner  107 . 
     ACK/NACK determiner  107  determines an ACK or HACK for the transmitted transmission data (e.g., transport block or the like) based on the ACK/NACK signal input from demodulator and decoder  106 . ACK/NACK determiner  107  outputs the determination result to controller  101 . 
     [Configuration of Terminal] 
       FIG. 5  is a block diagram illustrating an exemplary configuration of terminal  200  according to the present embodiment. Terminal  200  includes antenna  201 , radio receiver  202 , demodulator and decoder  203 , soft buffer controller  204 , soft buffer  205 , ACK/NACK generator  206 , coder and modulator  207  and radio transmitter  208 , for example. 
     Radio receiver  202  performs reception processing such as down-conversion and A/D conversion on a signal received via antenna  201  from base station  100 , and outputs the received signal (including data and control information, for example) after performing the reception processing to demodulator and decoder  203 . 
     Demodulator and decoder  203  demodulates and error correction decodes the received signal input from radio receiver  202 , and outputs the decoded signal to soft buffer controller  204 . 
     For example, demodulator and decoder  203  may demodulate and error correction decode the received data included in the received signal based on a modulation scheme and a coding rate indicated in the control information (e.g., data allocation information) included in the received signal. Demodulator and decoder  203  outputs the decoded signal to soft buffer controller  204 . 
     Further, demodulator and decoder  203  determines whether the received data is new data or retransmission data based on an NDI included in the control information, for example. When the received data is the new data, demodulator and decoder  203 , for example, error correction decodes the received data and performs CRC (Cyclic Redundancy Check) determination. When the received data is the retransmission data and data received by the previous reception is saved in soft buffer  205 , demodulator and decoder  203  combines the saved data and the data received this time, and then error correction decodes the combined to data. On the other hand, when data received by the previous reception is not saved in soft buffer  205 , demodulator and decoder  203  error correction decodes the data received this time without performing combining processing, and performs CRC determination. Furthermore, demodulator and decoder  203  outputs the result of the CRC determination to soft butter controller  204 . 
     When the CRC determination result input from demodulator and decoder  203  is CRC OK (in other words, there is no error), soft buffer controller  204  instructs soft buffer  205  to discard (in other words, Flush) received data saved in the past by a corresponding process ID. 
     When the CRC determination result is CRC NG (in other words, there is an error), soft buffer controller  204  controls buffering of the received data in soft buffer  205  based on buffer priority information (BPF) included in the signal input from demodulator and decoder  203 . For example, soft buffer controller  204  controls save of the received data in soft buffer  205  and discard of the received data based on the BPF. In a case of saving the received data, soft buffer controller  204  instructs soft buffer  205  to save the received data. Note that an example of operations of soft buffer controller  204  will be described later. 
     Soft buffer controller  204  outputs the CRC determination result input from demodulator and decoder  203  to ACK/NACK generator  206 . When the CRC determination result is CRC OK, soft buffer controller  204  outputs the corresponding received data. 
     Soft buffer  205  is a buffer for saving data (e.g., soft determination value) received in the past by terminal  200 . The saved data is used, for example, for HARQ combining Soft buffer  205  saves the received data for each process of HARQ, for example. 
     Note that in terminal  200 , soft buffer  205  does not need to include a buffer corresponding to the number of processes corresponding to an RTT. In other words, a buffer size of soft buffer  205  may be smaller than the buffer size corresponding to the number of processes corresponding to the RTT. 
     ACK/NAM generator  206  generates an ACK/NACK signal based on the CRC determination result input from soft buffer controller  204 . ACK/NACK generator  206  generates an ACK in a case of CRC OK (in a case where there is no error) and generates a NACK in a case of CRC NG (in a case where there is an error), for example. ACK/NACK generator  206  outputs the generated ACK/NACK signal to coder and modulator  207 . 
     Note that when a plurality of transport blocks or code blocks are transmitted, ACK/NACK generator  206  may generate an ACK/NACK signal for each transport block or each code block, and generate an ACK/NACK code block including the plurality of ACK/NACK signals. 
     Coder and modulator  207  error correction codes and modulates transmission data (in other words, uplink data or a PUSCH), and outputs the modulated signal to radio transmitter  208 . Further, coder and modulator  207  error correction codes and modulates the ACK/NAM signal input from ACK/NACK generator  206 , and outputs the modulated signal to radio transmitter  208 . 
     Radio transmitter  208  performs transmission processing such as D/A conversion, up-conversion and amplification, for example, on the signal input from coder and modulator  207 , and transmits the radio signal obtained by performing the transmission processing from antenna  201 . 
     [Exemplary Operations of Base Station  100  and Terminal  200 ] 
     Next, exemplary operations of base station  100  and terminal  200  will be described. 
     When there is an error in received data (for example, in a case of CRC NG), terminal  200  (in other words, receiving side) saves the received data in soft buffer  205  and waits for retransmission of the corresponding data. On the other hand, when there is no error in received data (for example, in a case of CRC OK), terminal  200  does not need to save the received data in soft buffer  205 . In other words, terminal  200  uses soft buffer  205  when there is an error in the received data (e.g., received packet). 
     Also, for example, a less propagation path variation is assumed in an NTN environment (e.g., an environment for an aircraft or seacraft) compared with a terrestrial network environment. Thus, a lower error rate (also called as BLER (Block Error Rate) or packet error rate, for example) is more likely to be realized in the NTN environment compared with the terrestrial network environment. 
     Therefore, it is assumed that a case where terminal  200  uses soft buffer  205  (in other words, a case where received data is saved (or combined)) is less in the NTN environment compared with the terrestrial network environment. 
     For example, terminal  200  may use, on average, a buffer corresponding to the number of processes corresponding to an RTT multiplied by a BUR in soft buffer  205 . As an example, when the BLER is 1 and the number of processes corresponding to the MT is 512 (e.g., RTT=512 ms when a TTI (Transmission Time interval) is 1 ms), terminal  200  may use a buffer corresponding to about 6 processes on average. 
     Thus, in an NTN environment, HARQ can be realized with soft buffer  205  having a size smaller than the soft buffer size corresponding to the number of processes corresponding to the RTT. 
     Therefore, in the present embodiment, a buffer size of soft buffer  205  may be smaller than, for example, the buffer size corresponding to the number of processes corresponding to the RTT. 
     However, it is desirable to transmit higher-priority data (data having a higher priority) such as a control packet or important user data with a low delay by a combining gain of HARQ so as not to impair the user experience. In other words, retransmission of the MAC layer or a higher layer in which a combining gain of HARQ is not obtained, for example, may be applied to lower-priority data (data having a lower priority) transmission. 
     Therefore, in the present embodiment, buffer priority information (BPF) indicating a priority of buffering of data in soft buffer  205  is indicated from base station  100  to terminal  200 . In other words, the BPF indicates a priority of use of soft buffer  205  between a plurality of data or a priority of combining processing with HARQ. 
     For example, in data transmission allocation, base station  100  generates a BPF for allocated data for terminal  200 , and indicates the BPF to terminal  200 . Based on the BPF terminal  200  controls save of received data in soft buffer  205  (in other words, data combining processing). 
     Hereinafter, an example of operations of data save in soft buffer  205  according to the present embodiment will be described. 
     For example, a BPF may be 1-bit information (e.g., 0 or 1). For example, BPF=1 indicates higher-priority data for save in soft buffer  205  and BPF; =0 indicates lower-priority data for save in soft buffer  205 . 
     For example, the higher-priority data is data including a control message of a higher layer such as Radio Resource Control (RRC) or data for higher-priority Logical Channel. The lower-priority data may be data involving a stricter delay requirement such as data for a streaming service such as voice data or video data, for example. This is because the need for save in soft buffer  205  is low since retransmission may not be performed for the data involving the stricter delay requirement. Note that the higher-priority data and the lower-priority data are not limited thereto, but may be other types of data. 
     For example, when there is an error in data decoding of lower-priority data (e.g., BPF=0), terminal  200  saves the received data in soft buffer  205  in a case where there is a free space in soft buffer  205 , and discards the received data without saying the received data in soft buffer  205  in a case where there is no free space in soft buffer  205 . 
     Further, for example, when there is an error in data decoding of higher-priority data (e.g., BPF=1), terminal  200  preferentially saves the received data in soft buffer  205 . 
     For example, when soft buffer  205  is all used for other data (in other words, there is no free space), terminal  200  discards saved data corresponding to lower-priority data among the data saved in soft buffer  205  and saves (in other words, overwrites save) the higher-priority data. In other words, when there is no free space in soft buffer  205 , terminal  200  overwrites the data having the lower priority than the received data with the received data. 
     On the other hand, for example, when soft buffer  205  is all used for other data and there is no lower-priority data in the data saved in soft buffer  205 , terminal  200  discards the received data. 
       FIGS. 6 and 7  illustrate examples of the save operation in soft buffer  205 . 
     In the examples illustrated in  FIGS. 6 and 7 , soft buffer  205  has a buffer corresponding to three processes. In other words, as illustrated in  FIGS. 6 and 7 , it is assumed that the number of processes corresponding data that can be saved in soft buffer  205  is less than the number of processes corresponding to an RTT Note that the buffer size of soft buffer  205  is not limited to the buffer size corresponding to three processes, but may be any other size. 
     As illustrated in  FIGS. 6 and 7 , the decoding result for the data (e.g., PDSCH) with process ID=1 (PID 1 ) is an error (e. g., CRC NG). Therefore, terminal  200  saves the received data with PID 1  in soft buffer  205 . At this time, terminal  200  saves the process ID (in other words, label) corresponding to the received data together with the received data in soft buffer  205 . Further, as illustrated in  FIGS. 6 and 7 , since BPF=1 is indicated with respect to the data with PID=1, terminal  200  sets (in other words, labels) “higher priority” to the saved data with PID=1, for example. 
     As illustrated in  FIGS. 6 and 7 , the decoding results for the data with process ID=2 (PID 2 ) and the data with process ID=10 (PID 10 ) are errors (e.g., CRC NG). Therefore, terminal  200  saves the received data with PID 2  and the received data with PID 10  in soft buffer  205 . Further, as illustrated in  FIGS. 6 and 7 . BPF=0 is indicated for each of the data with PID=2 and the data with PID=10. Therefore, terminal  200  sets (in other words, labels) “lower priority” to each of the saved data with PID=2 and the saved data with PID=10. 
     As illustrated in  FIGS. 6 and 7 , at the time when the received data with PID 10  is saved, soft buffer  205  is all in a used state (in other words, a state where there is no free space). 
       FIG. 6  illustrates exemplary operations in a case where the BPF of the data with PID=41 transmitted from base station  100  to terminal  200  is 1 (the priority is higher) after the received data with PID 10  is saved. 
     As illustrated in  FIG. 6 , the decoding result for the data with PID 41  and BPF=1 is an error (e.g., CRC NG). In this case, terminal  200  discards any data of the lower-priority data (data with BPF=0 or data labeled with the lower priority label) among the data saved in soft buffer  205 , and saves the data with PID 41  in soft buffer  205 . For example, terminal  200  may overwrite the data with the older process ID (data with PID 2  in  FIG. 6 ) among the lower-priority data saved in soft buffer  205  with the high-priority data (data with PID 41  in  FIG. 6 ), and save the high-priority data. 
     On the other hand,  FIG. 7  illustrates exemplary operations in a case where the BPF of the data with ND=41 transmitted from base station  100  to terminal  200  is 0 (the priority is lower) after the received data with PID 10  is saved. 
     As illustrated in  FIG. 7 , the decoding result for the data with PID 41  and BPF=0 is an error (e.g., CRC NG). In this case, there is no data having a lower priority than the data with PID 41  among the data saved in soft buffer  205 . Therefore, terminal  200  discards the data with PID 41  without saving it. In other words, terminal  200  does not discard the data saved in soft buffer  205 . 
     The examples of the data save operation in soft buffer  205  have been described above. 
     As described above, although the number of processes in an KIT is increased in an NTN environment as compared with a terrestrial network environment, the number of multipaths is less and a severe propagation path variation is unlikely to occur, so that a lower error rate is easily realized. Therefore, terminal  200  can use, for example, soft buffer  205  having a size smaller than the soft buffer size corresponding to the number of processes corresponding to the RTT Thus, even when the number of processes in the RTT is increased as in the NTN environment, the size of soft buffer  205  included in terminal  200  can be suppressed. Therefore, according to the present embodiment, for example, the chip size or cost of terminal  200  can be decreased. 
     Further, in the present embodiment, a priority (e.g., BPF) for buffering in soft buffer  205  (in other words, HARQ combining, is configured for transmission data. In other words, information (e.g., BPF) on buffering for retransmission control is indicated from base station  100  to terminal  200 . 
     As a result, terminal  200  can preferentially buffer higher-priority data in soft buffer  205  and perform HARQ combining even when there is no free space in soft buffer  205 . Therefore, according to the present embodiment, since an HARQ combining gain is preferentially obtained with respect to higher-priority data, it is possible to improve the reception success probability of the higher-priority data and to improve the user experience. 
     Thus, according to the present embodiment, base station  100  and terminal  200  can appropriately perform retransmission control in accordance with a priority of buffering for each of data of different processes. Therefore, for example, even when a size of soft buffer  205  is smaller than the number of processes corresponding to an RTT length, base station  100  can continuously perform data transmission without decreasing the number of processes in the RTT. Thus, according to the present embodiment, it is possible to suppress a decrease in the throughput per user (e.g., maximum user throughput). 
     Therefore, according to the present embodiment, it is possible to realize appropriate retransmission control processing in accordance with a radio propagation environment. 
     Note that the data (e.g., data with PID 2  of  FIG. 6 ) that is overwritten with other data in soft buffer  205  and the data (e.g., data with PID 41  of  FIG. 7 ) that is not saved in soft buffer  205  are discarded by terminal  200 . In other words, HARQ combining is not applied to these data. In this case, normal retransmission (ARQ) may be applied to these data. In ARQ, for example, retransmission based on an ACK/NACK signal is performed, and combining of signals is not performed at the receiving side (e.g., terminal  200 ). Alternatively, for example, retransmission of a higher layer e.g., the RLC layer) may be applied to these data. As described above, the retransmission of the higher layer involves a larger delay as compared with HARQ. For example, in a case where a lower priority (e.g., BPF=0) is configured for data for which a low delay is not required, degradation of the user experience can be suppressed, for example, even when the normal retransmission or the retransmission of the higher layer in which combining is not presupposed instead of HARQ is applied to the data. 
     Variation of Embodiment 1 
     In Embodiment 1, the BPF indicating the priority of save in soft buffer  205  regardless whether new transmission or retransmission has been described. 
     On the other hand, in the variation of Embodiment 1, a case where a value of an NDI, that is, the content (or meaning) indicated by the BPF differs depending on new transmission and retransmission will be described. 
     For example, at the time of new transmission, that is, when the NDI value is toggled (for example, when the value is different from the previous value for the same process), the meaning indicated by the BPF and the operations of terminal  200  are similar to those in Embodiment 1. 
     On the other hand, at the time of retransmission, that is, when the NDI value is not toggled (for example, when the value is the same as the previous value for the same process), the BPF indicates a decoding method of the corresponding data. 
     In other words, the BPF includes information indicating a priority of buffering of new data in soft buffer  205  and information indicating whether or not to perform combining processing on retransmission data, for example. 
     For example, in a case of BPF=0 at the time of data retransmission, terminal  200  performs error correction decoding without performing soft combining. On the other hand, in a case of BPF=1 at the time of data retransmission, terminal  200  soft combines data saved in the past and retransmission data, and then performs error correction decoding. 
     Further, when there is an error (e.g., CRC NG) after error correction decoding, terminal  200  saves the errorneous data as lower-priority data in soft buffer  205  in a case of BPF=0 and saves the erroneous data as higher-priority data in soft buffer  205  in a case of BPF=1. 
     For example, base station  100  may indicate BPF=0 for retransmission data when lower-priority data transmitted in the past may have been overwritten with higher-priority data in terminal  200 . With this indication, base station  100  can instruct terminal  200  to perform decoding without performing combining. In other words, for example, terminal  200  can determine whether or not to perform combining without preforming processing of determining (or confirming) whether or not the data for the retransmission data is saved in soft buffer  205 . 
     Therefore, according to the variation of Embodiment 1, it is possible to further simplify the processing of terminal  200 . 
     Embodiment 2 
     In the present embodiment, operations of the radio communication system in the operation of reusing the same process ID for different data transmission in an RTT will be described. 
     Note that since a base station and a terminal according to the present embodiment have the same basic configuration as base station  100  and terminal  200  according to Embodiment 1, respectively, they will be described with reference to  FIGS. 4 and 5 . 
     The number of indication bits for process IDs can be decreased by reusing the process ID. Fax example, when an RTT is 512 ms and a TTI length is 1 ins, there are 512 processes in the RTT, and the number of bits for indication of process IDs is 9. In the present embodiment, for example, by configuring (in other words, limiting) the number of process IDs to be indicated to 64, the number of indication bits for the process IDs can be decreased to 6. 
     However, since the same process ID is reused for different data in the RTT, a receiving side (for example, terminal  200 ) may combine received data with different data (in other words, data whose process ID is the same as a process ID of the received data) saved in soft buffer  205 . 
     In the present embodiment, a method of avoiding combining of different data having the same process ID by indicating information on use of the soft buffer with a BPF indicated from base station  100  to terminal  200  will be described. In other words, the present embodiment differs from Embodiment 1 in the content indicated by the BPF. 
     For example, base station  100  (e.g., controller  101 ) uses the process IDs in order for data transmission to terminal  200  (see, for example,  FIG. 5 ) in the RTT, and uses (reuses) the used process ID(s) when all the process IDs are used. As a result, the same process ID may be allocated to different data. 
     When allocating the used process ID to the data, base station  100  (e.g., controller  101 ) configures BPF=1 for higher-priority data and configures BPF=0 for lower-priority data. 
     Note that when allocating the unused process ID to the data, base station  100  may, for example, configure BPF=1 for higher-priority data and configure BPF=0 for low-priority data in the same manner as in Embodiment 1. 
     At the time of retransmission, base station  100  configures BPF=0 when higher-priority different data has been transmitted using the same process ID and configures BPF=1 when higher-priority different data is not transmitted using the same process ID. 
     Base station  100  transmits data (e.g., new data or retransmission data) and a BPF for the data to terminal  200 . 
     When the new data is received (in other words, an NDI value is toggled), terminal  200  performs error correction decoding. When the decoding result of the new data is an error (e.g., CRC NG), in a case of BPF=1, terminal  200  discards saved data having the same process ID as a process ID of the received data among the data saved in soft buffer  205 , and saves the received data in soil buffer  205 . When there is no saved data having the same process ID as the process ID of the received data in soft buffer  205 , terminal  200  saves the received data in soft buffer  205 . 
     In a case of BPF=0, when the decoding result of the new data is an error, there is no saved data having the same process ID as a process ID of the received data in soft buffer  205 , and there is a free space in soft buffer  205 , terminal  200  saves the received data in soft buffer  205 . On the other hand, when there is saved data having the same process ID as the process ID of the received data with BPF=0 in soft buffer  205  or there is no free space in soft buffer  205 , terminal  200  discards the received data without saving it. 
     Thus, in the present embodiment, the BPF indicates the priority of the buffering in soft buffer  205  between different data associated with the same process ID related to an HARQ process (in other words, retransmission control process). 
       FIGS. 8 and 9  illustrate examples of the save operation in soft buffer  205  according to the present embodiment. 
     In the examples illustrated in  FIGS. 8 and 9 , soft buffer  205  has a buffer corresponding to three processes. In other words, as illustrated in  FIGS. 8 and 9 , it is assumed that the number of processes corresponding data that can be saved in soft buffer  205  is less than the number of processes corresponding to are RTT Note that the buffer size of soft buffer  205  is not limited to the buffer size corresponding to three processes, but may be any other size. 
     In  FIGS. 8 and 9 , as an example, the same process ID=10 (PID 10 ) is reused for different data in the RTT. For example, in  FIGS. 8 and 9 , the decoding result of the first data with PID 10  in terminal  200  is an error (e.g., CRC NG), and the data is saved in soft buffer  205 . In  FIGS. 8 and 9 . BPF=0 is indicated to terminal  200  when the first data with PID 10  is transmitted. 
     In  FIG. 8 , BPF=1 (priority: high) is indicated to terminal  200  when the second data with PID 10  is transmitted, and in  FIG. 9 , BPF  0  (priority: low) is indicated to terminal  200  when the second data with PID 10  is transmitted. 
     For example, as illustrated in  FIG. 8 , for BPF′=1 for the second data with PID 10 , terminal  200  discards the first data with PID 10  saved in soft buffer  205  and saves (overwrites save) the second data with PID 10 . In other words, the second data with PID 10  and BPF=1 is saved in soft buffer  205  (in other words, HARQ combined) preferentially over the first data with PID 10  and BPF=0. 
     On the other hand, for example, as illustrated in  FIG. 9 , for BPF=0 for the second data with PID 10 , terminal  200  discards the second data with PID 10  without discarding the first data with PID 10  saved in soft buffer  205 . 
     When the retransmission data is received (in other words, the NDI is not toggled), and with respect to the retransmission data with BPF=1, data having the same process ID as a process ID of the retransmission data is saved in soft buffer  205 , terminal  200  combines the saved data and the retransmission data, and error correction decodes the combined data. On the other hand, when with respect to the retransmission data with BPF=1, data having the same process ID as the process ID of the retransmission data is not saved in soft buffer  205 , terminal  200  error correction decodes the retransmission data without performing combining. Terminal  200  saves the received data in soft buffer  205  when the decoding to result of the retransmission data is an error (e.g., CRC NG). 
     On the other hand, when the retransmission data is received, with respect to the retransmission data with BPF=0, terminal  200  error correction decodes the retransmission data without performing combining regardless of whether or not data having the same process ID as the process ID of the retransmission data is saved in soft buffer  205 . Terminal  200  does not save the received data in soft buffer  205  even when the decoding result is an error (e.g., CRC NG). 
     The examples of the data save operation in soft buffer  205  have been described above. 
     As described above, according to the present embodiment, when the data having the same process ID as the process ID of the received data is saved in soft buffer  205 , with respect to the received data with BPF=1, terminal  200  discards the past data (in other words, saved data), and newly saves (overwrites save) the received data in soft buffer  205 . On the other hand, when the data having the same process ID as the process ID of the received data is saved in soft buffer  205 , with respect to the received data with BPF=0, terminal  200  discards the received data and holds the past data (in other words, saved data). 
     As a result, in the present embodiment, even when the process ID is reused, terminal  200  can preferentially save (in other words, HARQ combine) any of the different data using the same process ID in soft buffers  205 . In other words, in the present embodiment, terminal  200  can prevent different data using the same process ID from being combined at the time of retransmission. 
     Further, according to the present embodiment, it is possible to improve the efficiency of HARQ while decreasing the number of indication bits for the process IDs (in other words, control overhead). Therefore, according to the present embodiment, it is possible to realize appropriate retransmission control processing in accordance with a radio propagation environment. 
     Variation 1 of Embodiment 2 
     In Variation 1 of Embodiment 2, a case where a period (also called as a segment, for example) in which a process ID is valid is configured will be described. 
     In Variation 1 of Embodiment 2, different data associated with the same process ID is transmitted in different segments of a plurality of segments. In other words, data having the same process ID in different segments are treated as separate dat. 
       FIG. 10  is a diagram illustrating exemplary operations according to Variation 1 of Embodiment 2. 
     An RTT interval is divided into a plurality of segments (in  FIG. 8 , four segments of seg 0  to seg 3 ). For example, as illustrated in  FIG. 10 , seg 0  to seg 3  are repeatedly configured with the RTT as a cycle. 
     For example, terminal  200  saves data having the same process ID of different segments as different data in soft buffer  205 . In other words, terminal  200  saves the received data in soft buffer  205  based on a combination of the segment and the process ID. 
     For example, when the decoded result of the data with the process ID=0 (PID 0 ) is an error (CRC NG) in seg 0  illustrated in  FIG. 10 , terminal  200  saves the received data with PID 0  in seg 0  in soft buffer  205 , and assumes that retransmission of the data with PID 0  is performed during seg 0  period of the next RTT interval that is the same segment of the current RTT. 
     Also, when terminal  200  receives data having the same process ID (e.g., data with PID 1  illustrated in  FIG. 10 ) in different segments (e.g., seg 0  and seg 1  illustrated in  FIG. 10 ), terminal  200  performs reception processing on these data as different data. For example, as illustrated in  FIG. 10 , when the decoding results of the data with PID 1  in seg 0  and the data with PID 1  in seal are CRC NG, terminal  200  separately saves each of these received data (for example, data with PID 1  in seg 0  and data with PIM in seg 1 ) in soft buffer  205 . In other words, terminal  200  manages soft buffer  205  based on the segment and the process ID. 
     Thus, when indicating the process ID to terminal  200 , base station  100  only needs to indicate the process ID in the segment, so that indication bits for the process IDs can be decreased. For example, when an RTT is 512 ms, a TTI length is 1 ms, and four segments each of which has a segment length of 128 ms are configured, the maximum number of processes in the segment is 128. Therefore, for example, for the process IDs corresponding to the RTT length, the number of indication bits is 9, whereas in Variation 1 of Embodiment 2, the number of indication bits for the process IDs is 7. Thus, the number of indication bits can be decreased. 
     Further, the process ID may be reused in the segment by combining Variation 1 and Embodiment 2. For example, in the above example (when the segment length is 128 ms and the maximum number of processes is 128), 16 process IDs may be indicated, for example. In this case, the number of indication bits for the process IDs is 4. Thus, the number of indication bits can be further decreased. When the process ID in the segment is reused, for example, as described in Embodiment 2, a BPF can be used to avoid combining data differing from each other in terminal  200 , thereby enabling HARQ combining of higher-priority data. 
     Here, it is assumed that data transmitted in a certain segment is retransmitted in the same segment after the RTT has elapsed. However, base station  100  may not be able to perform transmission in the same segment due to other processing with a higher priority or data transmission and reception for another user. In this case, base station  100  may transmit retransmission data together with BPF=0 in the next segment without waiting for the same segment in the next RTT interval for transmission, for example. In this case, since the BPF is 0, terminal  200  performs decoding processing without soft combining the data saved for the segment (seg 1 ) and the received data. Terminal  200  does not save the received data in soft buffer  205  even when the decoding result of the received data is an error. In this manner, when data transmitted in a certain segment of a plurality of segments is retransmitted in a different segment, a BPF for the data retransmitted in the different segment indicates that combining processing (in other words, save in soft buffer  205 ) is not performed on the data. 
     For example, in  FIG. 10 , data with PID 1  in seg 0  of a certain RTT is originally retransmitted in seg 0  after the elapse of the RTT. On the contrary, when transmission of the data with PID 1  in seg 0  is delayed, base station  100  illustrated in  FIG. 10  transmits, in seg 1 , the data with PID 1  in seg 0  together with BPF=0, for example. As illustrated in  FIG. 10 , base station  100  transmits, in seg 1 , retransmission data for the data with PID 1  in seg 1  without a BPF. In this manner, in  FIG. 10 , the different data associated with PID 1  are transmitted in seg 1 . 
     In this case, as illustrated in  FIG. 10 , terminal  200  combines the retransmission data with PID 1  transmitted without the BPF seg 1  and saved data with PID 1  seg 1  saved in soft buffer  205 . On the other hand, as illustrated in  FIG. 10 , terminal  200  performs reception processing. on the data with PID 1  transmitted together with BPF=0 in seal without performing combining (in other words, saving the data). 
     Thus, even when data are across segments, base station  100  can avoid combining different data and retransmit the data, so that even when data cannot be transmitted in the same segment, a delay can be decreased. For example, it is possible to further prevent generation of a delay equal to or more than 1 RTT. 
     Note that segment lengths in an RTT may all be the same, or at least a part of the segment lengths may be different (may be shorter or longer). 
     Variation 2 of Embodiment 2 
     In Variation 2 of Embodiment 2, a case where a process ID and a slot number are associated (in other words, tied or linked) with each other will be described. 
     In synchronous HARQ, a transmission timing of each process is predetermined. For example, after certain data is transmitted, corresponding retransmission data is transmitted after the RTT has elapsed. As described above, in synchronous HARQ, since the data transmission timing is predetermined, there may be no indication of process IDs. On the other hand, in synchronous HAW), since the transmission timing is determined, flexibility of scheduling (e.g., data allocation to each terminal) is decreased. Further, for example, although indicating the process ID can improve the flexibility of scheduling, an amount of control data for the indication of the process Ms is increased. 
     Therefore, in Variation 2 of Embodiment 2, a process ID is indicated based on a slot number and a relative value from the slot number (in other words, an offset value to the slot number; also called as a process ID relative value). This cam improve the flexibility of scheduling while decreasing the amount of control data for the indication of the process ID. 
     For example, let the slot number be “n,” let the relative value (offset value) to be indicated be “p,” and let the number of processes corresponding to the RTT (in other words, the value obtained by dividing the RTT by the TTI) be “nRIT” In this case, the process ID (PID) is calculated by the following Equation (1): 
         PID =( n+p +nRTT)mod nRTT  (1)
 
     Here, p may be represented by 4 bits, for example, −8, −7, . . . , +7. In this case, base station  100  indicates a 4-bit p to terminal  200 . Terminal  200  calculates the process II) intended by base station  100  according to Equation (1), Note that the number of bits for p is not limited to 4, but may be another number. 
       FIG. 11  is a diagram illustrating exemplary operations according to Variation 2 of Embodiment 2. 
     For example, as illustrated in  FIG. 11 , when data with PID 1  and with p=0 configured transmitted in slot 1  is retransmitted in slot 3  with a timing delayed by two slots from the time when the RTT elapses (in other words, next slot 1 ), base station  100  configures p=−2. Thus, the value of (n+p) in Equation (1) at the time of new transmission (n=1, p=0) is the value of (n+p) at the time of retransmission (n=3, p=−2) is ‘1.’ As a result, the PIDs are the same with each other. As illustrated in  FIG. 11 , when the data with PID 1  is retransmitted in slot 3 , BPF=1 is configured. 
     Thus, the value of p (offSet value) may be determined based on a difference between a slot number corresponding to the time of new transmission of data and a slot number corresponding to the time of retransmission of the data. A BPF for the retransmission data indicates that combining processing is performed. Thus, base station  100  can indicate, to terminal  200 , the same process ID (e.g., PID 1 ) at a timing different from the predetermined transmission timing. 
     In transmitting higher-priority data, base station  100  may configure a process ID to which lower-propriety data saved in soft buffer  205  is allocated to a process ID of the higher-priority data and configure BPF=1 in order to overwrite the lower-priority data. 
     For example, as illustrated in  FIG. 12 , base station  100  configures p=0 and BPF=0 in slot 0 , and transmits data with PID 0 . Also, as illustrated in  FIG. 12 , base station  100  configures p=−5 and BPF=1 in slot 5  (n=5) in the same RTT, and transmits data with PID 0 . In other words, in slot 5  illustrated in  FIG. 12 , the same process ID as the process ID of the data transmitted by configuring p=0 and BPF:=0 is indicated to terminal  200 , In this case, with respect to indicated PID 0 , terminal  200  discards data (in other words, data with BPF=0) saved in soft buffer  205 , and saves the newly received data (in other words, data with BPF=1). 
     Thus, the value of p (offset value) is determined based on a difference between a slot number (slot 0  in  FIG. 12 ) corresponding to transmission of first data saved in soft buffer  205  of terminal  200  and a slot number corresponding to transmission of second data corresponding to the same PID as a PID (PID 0  in  FIG. 12 ) associated with the first data (slot 5  in  FIG. 12 ). A BPF for the second data indicates that buffering of the second data in soft buffer  205  is performed. 
     As described above, according to Variation 2 of Embodiment 2, it is possible to improve the flexibility of scheduling while decreasing the number of indication bits for the process IDs. Further, in Variation 2 of Embodiment 2, by combining the indication of the relative value p and the indication of the BPF, soft buffer  205  is preferentially used for the higher-priority data over the lower-priority data, so that the reliability of the higher-priority data can be improved and the delay can be shortened. 
     Note that the slot number is a number of a slot configured by base station  100  or the system, and may be a number repeated for each system frame. In this case, the value of n in Equation (1) may be calculated by the following Equation (2): 
         n =Slot number+System frame number*(Number of slots per system frame)  (2)
 
     The slot number may be a number repeated fir each subframe. In this case, the value of n in Equation (1) may be calculated by the following Equation (3): 
         n =Slot number+System frame number*Subframe number*(Number of slots per subframe)  (3)
 
     Note that although the slot has been described in Variation 2 of Embodiment 2, the time resource unit is not limited to the slot, but may be another time resource unit (e.g., frame, subframe, mini slot or the like). 
     Embodiment 3 
     In the present embodiment, a case where the terminal collectively indicates (feeds back) ACK/NACK signals for a plurality of transmission data (e.g., transport blocks) to the base station will be described. 
     For example, a periodicity of the indication of the ACK/NACK signals may be indicated from the base station to the terminal in advance. At an indication timing of ACK/NACK signals, for example, the terminal collectively transmits ACK/NACK signals for data received after the previous indication timing. 
       FIG. 13  illustrates exemplary operations of the radio communication system according to the present embodiment. 
     In  FIG. 13 , as an example, a fixed period (e.g., RTT) is divided into a plurality of segments (e.g., five segments in this example) (seg 0  to seg 4 ), and ACK/NACK signals are indicated once for each segment. For example, the terminal may hold ACK/NACK signals generated in a segment and performs transmission processing on the ACK/NACK signals at a timing of the break of the segment (in other words, border). 
     Further, terminal  200  may change an indication method for ACK/NACK signals based on, for example, a traffic condition, a channel condition, or an ACK/NACK For example, indication methods for ACK/NACK signals include the following. 
     Indication Method 1 
     As described above, in an NTN, a lower error rate (e.g., BLER) can be realized because a propagation path variation is smaller than that in a terrestrial network. 
     Therefore, in indication method 1, the terminal indicates a process ID of data whose ACK/NACK signal is a NACK (in other words, process ID of data whose decoding result is an error) among data received in a fixed period (e.g., segment). In other words, in indication method 1, the terminal does not indicate a process ID of data whose ACK/NACK signal is an ACK, among the data received in the fixed period. 
     For example, a case where ACK/NACK signals are collectively transmitted every 64 ms in indication method 1 will be described. Here, let the number of processes corresponding to NACKs (in other words, the number of data having an error in the decoding result) be “nError.” In this case, the terminal indicates information of nError×6 (=log2(64))+6 bits by considering indication of the number of errors and indication of the process ID(s) of the data having an error. For example, for the number of processes of data having an error “nError”=1, the number of indication bits is 12, which is less than 64 (bits). 
     Indication Method 2 
     In indication method 2, a bitmap represents one ACK/NACK signal for each process&#39;s data. For example, when collectively transmitting ACK/NACK signals every 64 ms, the terminal indicates information of up to 64 bits. 
     The indication methods for ACK/NACK signals have been described above. 
     Here, for example, when ACK/NACK signals are collectively transmitted every 64 ms and the number of processes of data having an error is 10 or more (nError&gt;=10), the number of indication bits is 66 or more in indication method 1. Therefore, the number of indication bits in indication method 2 (e.g., 64) is smaller than that in indication method 1. In other words, when ACK/NACKs are collectively transmitted every 64 ms and the number of processes of data having an error is less than 10 (nError&lt;10), the number of indication bits is less than 64 (that is, the number of indication bits in indication method 2) in indication method 1. 
     Therefore, in the present embodiment, the terminal switches between indication method 1 (in other words, ACK/NACK indication by a process ID corresponding to a NACK) and indication method 2 (in other words, ACK/NACK indication by a bitmap) depending on a situation. 
     Hereinafter, ACK/NACK indication switching methods 1 and 2 will be described. 
     Switching Method 1 
     In switching method 1, the base station configures an indication method for ACK/NACK signals for each terminal. 
     For example, an indication method for ACK/NACK signals is indicated from the base station to the terminal. For example, the base station may configure a configuration of an ACK/NACK indication method for each terminal, and indicate the configuration by a higher layer RRC reconfiguration message), or may configure a configuration of an ACK/NACK indication method for each cell, and indicate the configuration by system information. The terminal switches between indication method 1 and indication method 2 based on the indication from the base station. 
     For example, the base station may switch between indication method 1 and indication method 2 based on an occurrence frequency of data communication or a condition of a propagation path variation. For example, the base station may configure indication method 1 when the occurrence frequency of the data notification is low (for example, when the occurrence frequency is less than a threshold), and may configure indication method 2 when the occurrence frequency of the data notification is high (for example, when the occurrence frequency is equal to or greater than the threshold). 
     Alternatively, for example, the base station may configure indication method 1, when the propagation path variation is small (for example, when the propagation path variation is less than a threshold), and may configure indication method 2 when the propagation path variation is large (for example, when the propagation path variation is equal to or greater than the threshold). 
     Note that the switching criterion for the ACK/NACK indication methods is not limited to the occurrence frequency of the data notification and the condition of the propagation path variation, but may be another criterion. In other words, the criterion may be such that indication method 1 is configured in a situation where the number of indication bits for ACK/NACK signals tends to be smaller in indication method 1 than that in indication method 2, and indication method 2 is configured in a situation where the number of indication bits for ACK/NACK signals tends to be smaller in indication method 2 than that in indication method 1. 
     Switching Method 2 
     In switching method 2, the terminal selects an ACK/NACK indication method. 
     For example, the terminal may select one of indication method 1 and indication method 2 in which the number of indication bits is smaller. 
     Here, let the number of processes to be reported be “nP” and let the number of NACKs be “nN.” In this case, the number of indication bits for indication method 1 and the number of indication bits for indication method 2 are as follows: 
     The number of indication bits for indication method 1:nP (bits) 
     The number of indication hits for indication method 2:nN*ceil (log2(nP))+ceil (log2(nP)) (bits) (including indication bits for the number of NACKs) 
     Note that ceil indicates an operation for rounding up the decimal point. 
     The terminal selects one of indication method 1 and indication method 2 in which the number of indication bits is smaller, and indicates the selected indication method to the base station. The base station receives ACK/NACK signals, for example, based on the indication method for ACK/NACK signals indicated from the terminal. 
       FIG. 14  illustrates examples of formats for indicating ACK/NACK signals. In  FIG. 14 , information indicating an indication method is stored in a header, and ACK/NACK signals are stored in the remaining portion (for example, ACK/NACK information field). 
     In the header example 1 of  FIG. 14 , the number of bits of the header is 1, and indication method 1 or indication method 2 is specified by the 1-bit header (one of 0 and 1). Note that in indication method 2, the number of indication bits for ACK/NACK signals differs depending on the number of NACKs. Therefore, the base station may determine the number of indication bits (length) for ACK/NACK signals based on indication bits for NACKs in the ACK/NACK information field illustrated in  FIG. 14 , for example. 
     In the header example 2 of  FIG. 14 , the number of bits for the header is 2, and the absence of NACK (or no data allocation) is specified in addition to indication method 1 and indication method 2 by the 2-bit header (one of 00, 01, 10 and 11). 
     In the header example 2, in a case of the absence of NACK (header: 01), for example, the subsequent ACK/NACK information field does not contain information. Further, in the header example 2, in a case of indication method 2, the header may specify whether the number of NACKs is less than M 1  (header: 10) or equal to or greater than M 1  and less than M 2  (header: 11), When the number of NACKs is less than M 1 , ACK/NACK signals having ceil (log2(M 1 )) bits are stored, and when the number of NACKs is equal to or greater than M 1  and less than M 2 , ACK/NACK signals having ceil (log2(M 2 )) bits are stored. Thus, the base station can determine the number of bits in the ACK/NACK information field by reading the header. 
     Further, in the header example 2, a type of the number of bits of the ACK/NACK information field (for example, four types of the number of bits in  FIG. 14 ) may be defined. Thus, for example, the terminal selects one of radio resource (for example, time and frequency resource) candidates whose number (four in  FIG. 14 ) is determined in accordance with the type of the number of bits, and transmits the ACK/NACK information. The base station blind detects one of radio resources whose number is determined in accordance with the type of the number of bits. In this case, error correction coding can be performed using the header and the ACK/NACK information field as the same block, resulting in a higher coding gain. 
     As described above, in the present embodiment, by collectively indicating ACK/NACK signals for a plurality of received data to the base station, the terminal can indicate the ACK/NACK signals with one of the indication methods in which the number of indication bits is smaller, thereby decreasing the overhead of ACK/NACK indication. 
     Further, for example, when an ACK/NACK signal of one bit (for example, one process) is transmitted each time, radio resource usage such as spreading or a plurality of repeated transmissions may be increased in order to receive the signal at an error rate (sufficiently low error rate) required in the base station. On the contrary, in the present embodiment, since ACK/NACK signals are collectively fed back, an error correction coding gain or diversity gain is obtained, thereby improving the efficiency of use of radio resources. 
     Furthermore, when radio resources for indication of ACK/NACK signals are used, a more robust transmission method (e.g., transmission at a lower coding rate) can be realized by decreasing the number of indication bits, thereby improving the reliability of ACK/NACK indication. For example, in an NTN, since long-distance transmission is performed, it is to important to improve the reliability. 
     Note that although in the present embodiment, as the example, the case where the indication timing of the ACK/NACK signals is separated for each segment has been described, the present disclosure is not limited thereto. For example, the base station may indicate a feedback periodicity for ACK/NACK signals, and the terminal may indicate ACK/NACK signals based on the feedback periodicity. 
     Embodiment 3 may be combined with, for example, at least one of Embodiment 1 and Embodiment 2. 
     For example, by combining Embodiment 1 and Embodiment 3, terminal  200  (see, for example,  FIG. 5 ) can appropriately perform HARQ of higher-priority data by controlling data save in soft buffer  205  (in other words. HARQ combining) based on a BPF (in other words, priority) indicated from base station  100  (see, for example,  FIG. 4 ). Further, terminal  200  can decrease the number of indication bits by collectively feeding back ACK/NACK signals to base station  100 , as in Embodiment 3. 
     Similarly, for example, by combining Embodiment 2 and Embodiment 3, terminal  200  can appropriately perform HARQ of higher-priority data based on a BPF indicated from base station  100  (see, for example,  FIG. 4 ) without combining data having the same process ID even when a process ID is reused in an RTT Further, terminal  200  can decrease the number of indication bits by collectively feeding back ACK/NACK signals to base station  100 , as in Embodiment 3. 
     Variation 1 of Embodiment 3 
     In Embodiment 3, the case where the terminal collectively indicates the ACK/NACK signals in each segment to the base station has been described. In Variation 1 of Embodiment 3, the terminal may also collectively transmit ACK/NACK signals for each section (called as “subsegment,” for example) obtained by dividing a segment. 
     For example, a segment may be further divided into four subsegments, and the base station and the terminal may configure or select an ACK/NACK indication method for each subsegment. In this case, for example, as illustrated in  FIG. 15 , a header may be added for each subsegment. Thus, for example, a different indication method can be configured for each subsegment. 
     Further, for example, in an environment in which errors occur in a concentrated manner, indication method 1 may be configured in a period of a subsegment in which errors are concentrated, and indication method 2 may be configured in a period of another subsegment. Thus, since one of the indication methods in which the number of indication bits is smaller is selected for each period finer than a segment, the number of ACK/NACK indication bits can be further decreased. 
     The embodiments of the present disclosure have been described above. 
     Note that in Embodiments 1 and 2, terminal  200  may indicate, to base station  100 , information indicating whether or not terminal  200  has saved (or stored) data in soft buffer  205  together with an ACK/NACK signal. Or, terminal  200  may indicate DTX (that is, no ACK or NACK) to base station  100  when terminal  200  does not save data in soft buffer  205 . Base station  100  may determine transmission parameters at the time of retransmission based on the indicated information. For example, for data not saved in soft buffer  205  at terminal  200 , base station  100  may transmit the same RV again so that it can be received in the same way as the previous transmission at terminal  200  or may transmit an RV including Systematic Bits (in other words, the original information bits). Further, for example, for data saved in soft buffer  205  at terminal  200 , base station  100  may transmit a different RV or may transmit an RV that does not include Systematic Bits or has a lower rate of Systematic Bits. When terminal  200  does not save data in soft buffer  205 , terminal  200  may indicate information indicating that the data is not saved to decrease an amount of indication information. 
     In Embodiments 1 and 2, a BPF may be explicitly indicated in allocation information included in DCI or a PDCCH, or may be implicitly indicated by another parameter. For the implicit indication, for example, a BPF may be indicated by a DCI format used for terminal  200 , RNTI (Radio Network Temporary ID) used for scrambling a PDCCH, or time and frequency resources called as a search space and CORESET over which a PDCCH was transmitted. Or, terminal  200  may determine BPF=1 when an RV for new transmission including Systematic Bits is indicated for retransmission data and decode the retransmission data without combining the retransmission data, and determine BPF=0 when an RV for retransmission not including Systematic Bits or having a lower rate of Systematic Bits is indicated, combine the retransmission data and perform decoding. Further, terminal  200  may determine BPF=1 when an MCS for new transmission is indicated and decode retransmission data without combining the retransmission data, and determine BPF=0 when an MCS for retransmission is indicated and perform decoding without combining retransmission data. 
     A BPF may also be indicated by a process ID specially defined in a process ID When a particular process ID is indicated, terminal  200  determines BPF=0, and transmits a NACK without saving received data even when there is an error in the received data. When determining BPF=0, for retransmission data, terminal  200  decodes the retransmission data without performing combining. When a process ID other than the particular process ID is indicated, terminal  200  determines BPF=1, saves received data when there is an error in the received data, and transmits a NACK. When determining BPF=1, for retransmission data, terminal  200  combines the retransmission data and perform combining. The particular process ID may be predetermined, or may be indicated from base station  100  to terminal  200  by system information or the like. 
     A BPF may also be indicated by a Logical Channel ID. For example, a higher-priority Logical Channel ID may mean BPF=1 (priority: high) and a lower-priority Logical Channel ID may mean BPF=0 (priority: low). Note that a priority for each Logical Channel ID may be predetermined, or may be indicated from base station  100  to terminal  200 . For example, base station  100  can indicate a Logical Channel ID by DCI each time data is transmitted, so that terminal  200  can identify the Logical Channel ID (in other words, BPF) prior to data decoding. 
     Further, in each embodiment or variation, there is a possibility that there is a method that the terminal can cope with and a method that the terminal cannot cope with, depending on the capability of the terminal (UE). In such a case, each terminal may select a method to be applied according to the terminal capability (UE capability). 
     Although in Embodiments 1 and 2, the case where the BPF is 1 bit has been described, the number of bits for the BPF may be 2 or more. For example, when the number of bits for the BPF is 2, four levels of priority may be indicated as follows. In this example, among BPF=00, 01, 10, and 11, 00 has the highest priority and 11 has the lowest priority. 
     00: Always save data in soft buffer  205  and prohibit overwriting with other data. 
     01: Always save data in soft buffer  205  and allow overwriting with other data 
     10: Save data in soft buffer  205  when there is a free space and allow overwriting with other data 
     11: Not save data in soft buffer  205  even when there is a free space 
     Further, in Embodiments 1 and 2, when BPF=1 is indicated (in other words, higher-priority data is received), and soft buffer  205  of terminal  200  is used for higher-priority data (in other words, there is no free space or there is no saved lower-priority data), terminal  200  may perform the following operations. For example, terminal  200  may discard the oldest data among data saved in soft buffer  205  and save the current received data. Or, terminal  200  may discard the current received data without saving it and may not indicate an ACK/NACK. In other words, terminal  200  may operate as if there was no data transmission this time. In this case, base station  100  may retransmit the same data as new data (NDI toggle indication) because there is no ACK/NACK indication. 
     Or, terminal  200  may operate assuming that higher-priority data (in other words, data with BPF=1) more than a buffer amount implemented in soft buffer  205  is not transmitted. In this case, a terminal operation is not defined and may be configured according to the implementation of terminal  200 . For example, the operation may be different for each terminal. 
     The disclosed embodiments are applicable regardless of the type of satellite, such as GEO (Geo-stationary Earth Orbit), MEO (Medium Earth Orbit), LEO (Low Earth Orbit), HAPS, or the like. Further, the embodiments of the present disclosure are not limited to an NTN, but can be applied to, for example, a terrestrial network in an environment where a cell size is large and a propagation delay between a base station and a terminal is long g an environment where the propagation delay is equal to or greater than a threshold). 
     In the above embodiments, the ACK/NACK signal may be called as HARQ-ACK or HARQ-Feedback information. 
     Further, even when the terminal for which HARQ-Feedback is configured to be disabled (inactivated) does not transmit an ACK/NACK signal, an error rate can be improved by transmitting data from the base station a plurality of times and HARQ combining the data at the terminal. Even in this case, by indicating a BPF indicating whether or not to combine the data at the terminal, a limited HARQ buffer can be effectively utilized. For example, when the base station transmits data a plurality of times, BPF=1 may be indicated, and the terminal may store the data in the HARQ buffer. On the other hand, when the base station does not transmit any more data, BPF=0 may be indicated, and the terminal may not store the data in the HARQ buffer. Also, the indication of the BPF may be applied only to an HARQ process for which HARQ-Feedback is configured to be disabled. Since indication of HARQ related information such as RV information is not necessary in the HARQ process for which HARQ-Feedback is configured to be disabled, the indication is possible without increasing the data size of control information by indicating the BPF instead of the HARQ related information. 
     Furthermore, although in each of the above-described embodiments, the downlink data transmission from the base station to the terminal has been described, the present disclosure is applicable not only to this but also to uplink data transmission from the terminal to the base station. In the uplink data, transmission, whether or not data hold in the terminal transmission buffer is needed may be instructed by indicating a BPF′ together with uplink data allocation indication from the base station to the terminal. In this case, the BPF is indicated by DCI. Alternatively, the terminal may indicate, to the base station, a BPF together with uplink transmission data, thereby instructing whether or not the data needs to be held in a reception buffer of the base station. In this case, the BPF is indicated by UCI (Uplink Control Information). 
     In addition, although the above-described embodiments assume Dynamic scheduling in which data is allocated using DCI or a PDCCH for each data transmission as data allocation in uplink and downlink to the terminal, the present disclosure is also applicable to a case where downlink semi-persistent scheduling (SPS) or uplink Configured grant (CG) that periodically allocates radio resources to the terminal in advance is used. In this case, as in Rel. 15 NR, the initial transmission data is transmitted in radio resources allocated periodically in advance, but at the time of HARQ retransmission, Dynamic scheduling of allocating data using DCI or a PDCCH may be used. A BPF may be indicated when allocation of periodic radio resources (periodicity) is indicated from the base station to the terminal, or may be indicated by DCI or a PDCCH at the time of HARQ retransmission. RRC reconfiguration, MAC CE or DCI can be used for the indication of the allocation of the periodic radio resources (periodicity). Further, in Embodiment 2, in a case where a HAW) process is determined based on a transmission timing, with respect to SPS or CG for which the same HARQ process ID is configured, the HARQ process may be determined based on a transmission timing (for example, slot number or segment) of the initial transmission data transmitted using SPS or CG resources. 
     Also, in Rel. 15 NR, when uplink CG is used, configuredGrantTimer operates after the initial transmission data is transmitted, and during the time period until the timer expires (for example, duration of the timer), the new initial transmission data transmission from the terminal using CG resources is prohibited, and only allocation of retransmission data by dynamic scheduling is performed. On the other hand, since an RTT is long in an NTN, a delay is increased while waiting for a retransmission instruction from the base station. For this reason, configuredGrantTimer may be set to a value equal to or less than the RTT data transmitted from the terminal using CO resources within the duration of configuredGrantTimer may be treated as retransmission data, and received data may be combined. Thus, the terminal can retransmit data transmission that needs to be improved in reliability without waiting for an instruction to allocate the retransmission data by dynamic scheduling from the base station. On the other hand, configuredGrantTimer may be disabled in a case where high reliability is not required. In this case, disablement of HARQ feedback or disablement of configuredGrantTimer may be indicated by a ConfiguredGrantConfig message for configuring CG. 
     Although in the above embodiments, the communication between the single base station and the single terminal has been described, the present disclosure is not limited thereto. For example, one base station may perform the operations of each embodiment with respect to each of a plurality of terminals. One terminal may perform the operations of each embodiment with respect to a plurality of base stations. 
     Further, among Embodiments 1, 2 and 3 described above and the variations of the respective embodiments, at least two or more methods may be combined and operated. 
     In addition, the values of the RTT length, the buffer size, the number of processes, the number of process Ms, the number of segments, the BPF and the like described in the above embodiments are examples, and are not limited to thereto. 
     The notation “-or” or “-er” in the above-described embodiments may be replaced to with another notation such as “circuitry,” “device,” “unit,” or “module.” 
     The present disclosure can be realized by software, hardware, or software in cooperation with hardware. Each functional block used in the description of each embodiment described above can be partly or entirely realized by an LSI such as an integrated circuit, and each process described in each embodiment may be controlled partly or entirely by the same LSI or a combination of LSIs. The LSI may be individually formed as chips, or one chip may be formed so as to include a part or all of the functional blocks. The LSI may include a data input and output coupled thereto. The LSI here may be referred to as an IC, a system LSI, a super LSI, or an ultra LSI depending on a difference in the degree of integration. Technique of implementing an integrated circuit is not limited to the LSI and may be realized by using a dedicated circuit, a general-purpose processor, or a special-purpose processor. In addition, an FPGA (Field Programmable Gate Array) that can be programmed after the manufacture of the LSI or a reconfigurable processor in which the connections and the settings of circuit cells disposed inside the LSI can be reconfigured may be used. The present disclosure can be realized as digital processing or analogue processing. If future integrated circuit technology replaces LSIs as a result of the advancement of semiconductor technology or other derivative technology, the functional blocks could be integrated using the future integrated circuit technology. Biotechnology can also be applied. 
     The present disclosure can be realized by any kind of apparatus, device or system having a function of communication, which is referred to as a communication apparatus. The communication apparatus may comprise a transceiver and processing/control circuitry. The transceiver may comprise and/or function as a receiver and a transmitter. The transceiver, as the transmitter and receiver, may include an RF (radio frequency) module including amplifiers, RF modulators/demodulators and the like, and one or more antennas. Some non-limiting examples of such a communication apparatus include a phone (e.g, cellular (cell) phone, smart phone), a tablet, a personal computer (PC) (e.g, laptop, desktop, netbook), a camera (e.g, digital still/video camera), a digital player (digital audio/video player), a wearable device (e.g, wearable camera, smart watch, tracking device), a game console, a digital book reader, a telehealth/telemedicine (remote health and medicine) device, and a vehicle providing communication functionality (e.g., automotive, airplane, ship), and various combinations thereof. 
     The communication apparatus is not limited to be portable or movable, and may also include any kind of apparatus, device or system being non-portable or stationary, such as a smart home device (e.g, an appliance, lighting, smart meter, control panel), a vending machine, and any other “things” in a network of an “Internet of Things (IoT).” 
     The communication may include exchanging data through, for example, a cellular system, a wireless LAN system, a satellite system, etc., and various combinations thereof. 
     The communication apparatus may comprise a device such as a controller or a sensor which is coupled to a communication device performing a function of communication described in the present disclosure. For example, the communication apparatus may comprise a controller or a sensor that generates control signals or data signals which are used by a communication device performing a communication function of the communication apparatus. 
     The communication apparatus also may include an infrastructure facility, such as a base station, an access point, and any other apparatus, device or system that communicates with or controls apparatuses such as those in the above non-limiting examples. 
     A terminal according to an embodiment of the present disclosure, includes: reception circuitry, which, in operation, receives control information on buffering for retransmission control; and control circuitry, which, in operation, controls the buffering based on the control information. 
     In the terminal according to an embodiment of the present disclosure, the control information includes information indicating a priority of the buffering between a plurality of data. 
     In the terminal according to an embodiment of the present disclosure, the control circuitry, in operation, controls save of the data in a buffer and discard of the data based on the information indicating the priority. 
     In the terminal according to an embodiment of the present disclosure, when there is no free space in the buffer, the control circuitry, in operation, overwrites data having a lower priority than received data with the received data. 
     In the terminal according to an embodiment of the present disclosure, when there is no data having the lower priority in the buffer, the control circuitry, in operation, discards the received data. 
     In the terminal according to an embodiment of the present disclosure, the control information includes information indicating the priority of data relating to new transmission and information indicating whether or not to perform combining processing on data relating to a retransmission request. 
     In the terminal according to an embodiment of the present disclosure, the control information includes information indicating a priority of the buffering between different data associated with same identification information related to a retransmission control process. 
     In the terminal according to an embodiment of the present disclosure, the different data associated with the same identification information is transmitted in different periods of a plurality of periods obtained by dividing a time interval corresponding to a propagation delay time of the data. 
     In the terminal according to an embodiment of the present disclosure, when the data transmitted in a first period is retransmitted in a second period different from the first period of the plurality of periods, the control information for the data retransmitted in the second period includes information indicating that combining processing is not performed on the data retransmitted. 
     In the terminal according to an embodiment of the present disclosure, the control circuitry, in operation, determines the identification information based on a time resource and an offset value to the time resource. 
     In the terminal according to an embodiment of the present disclosure, the offset value is determined based on a difference between a number of the time resource corresponding to data relating to new transmission and a number of the time resource corresponding to data relating to retransmission, and the control information for the data relating to the retransmission includes information indicating that combining processing is performed. 
     In the terminal according to an embodiment of the present disclosure, the offset value is determined based on a difference between a number of the time resource of a first data saved in a buffer of the terminal and a number of the time resource of a second data associated with identification information same as the identification information associated with the first data, and the control information for the second data includes information indicating that buffering of the second data is performed. 
     In the terminal according to an embodiment of the present disclosure, the terminal further includes: transmission circuitry, which, in operation, collectively transmits response signals for a plurality of data, and the control circuitry, in operation, switches between a first method of transmitting the response signals each including information indicating an error detection result for a corresponding one of the plurality of data and a second method of transmitting the response signals including information on erroneous data of the plurality of data. 
     In the terminal according to an embodiment of the present disclosure, the control circuitry, in operation, switches between the first method and the second method based on indication from a base station. 
     In the terminal according to an embodiment of the present disclosure, the control circuitry, in operation, selects one of the first method and the second method in which a number of transmission bits for the response signals is smaller. 
     A base station according to an embodiment of the present disclosure, includes: control circuitry, which; in operation, generates control information on buffering for retransmission control; and transmission circuitry, which, in operation, transmits the control information. 
     A receiving method according to an embodiment of the present disclosure, includes the following performed by a terminal: receiving control information on buffering for retransmission control; and controlling the buffering based on the control information. 
     A transmitting method according to an embodiment of the present disclosure, includes the following performed by a base station: generating control information on buffering for retransmission control; and transmitting the control information. 
     The disclosure of Japanese Patent Application No. 2019-084513, filed on Apr. 25. 2019, including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     INDUSTRIAL APPLICABILITY 
     One embodiment of the present disclosure is useful for a radio communication system. 
     REFERENCE SIGNS LIST 
     
         
           100  Base station 
           101  Contoller 
           102 .  207  Coder and modulator 
           103 ,  208  Radio transmitter 
           104 ,  201  Antenna 
           105 ,  202  Radio receiver 
           106 ,  203  Demodulator and decoder 
           107  ACK/NACK determiner 
           200  Terminal 
           704  Soft buffer controller 
           205  Soft buffer 
           206  ACK/NACK generator