Abstract:
A radio apparatus is disclosed for improving the throughput of data transfer using a HARQ-based automatic retransmission. The radio apparatus performs a HARQ-based automatic retransmission. In this event, a redundancy version control unit identifies a redundancy version in the retransmission based on a redundancy version in the preceding transmission in which the need for retransmission arose. A HARQ processing unit performs rate matching on each of the systematic bits and parity bits of data to be retransmitted in accordance with the redundancy version identified by the redundancy version control unit.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a radio apparatus which performs an HARQ-based automatic retransmission. 
   2. Description of the Related Art 
   The Third-Generation Partnership Project (3GPP) defines specifications for high speed downlink packet access (HSDPA) in order to enable high speed data transfers. 
   One feature of HSDPA is automatic retransmission using hybrid ARQ (HARQ: Hybrid Automatic Repeat Request). According to HARQ, a terminal performs error detection on data from a radio base station, and requests a retransmission from the radio base station if an error is detected. Based on the nature of the response from the terminal, the radio base station determines whether a retransmission is necessary and, if necessary, retransmits the data. 
     FIG. 1  is a diagram illustrating the flow of signal processing on the transmission side of a physical layer by a conventional general radio base station. 
   Referring to  FIG. 1 , the radio base station performs, on data to be transmitted, CRC attachment processing  81 , bit scrambling processing  82 , code block segmentation processing  83 , and channel coding processing  84 , and then performs HARQ (Physical Layer Hybrid-ARQ functionality) processing  85 . HARQ processing  85  is the processing for automatic retransmission using HARQ. 
   After this processing, the radio base station performs physical channel segmentation processing  86 , HS-DSCH interleaving processing  87 , constellation re-arrangement processing  88 , and physical channel mapping processing  89 , and transmits data to a terminal on a physical channel (phCH). 
     FIG. 2  is a block diagram illustrating the configuration of an HARQ processing unit which performs HARQ processing in the signal processing of the physical layer illustrated in  FIG. 1 . Referring to  FIG. 2 , HARQ processing unit  90  has bit separation unit  91 , first rate matching unit  92 , virtual IR buffer  93 , second rate matching unit  94 , and bit collection unit  95 . Then, HARQ processing unit  90  performs rate matching on input data c twice, before and after the virtual buffer, using parameters s, r of redundancy version (hereinafter called the “RV parameters”) that are applied, for example, from a scheduler (not shown) in the MAC layer. 
   Virtual IR buffer  93  is a virtual buffer which performs buffering with a predefined buffer size. 
   Bit separation unit  91  separates input data c from channel coding processing  83  into systematic bits, parity  1  bits, and parity  2  bits. 
   First rate matching unit  92  performs rate matching processing on the systematic bits, parity  1  bits, and parity  2  bits that are from bit separation unit  91 , and sends the resulting respective bits to virtual IR buffer  93 . In this event, first rate matching unit  92  actually performs the processing only on the parity  1  bits and parity  2  bits (RM_P 1 _ 1 , RM_P 2 _ 1 ), but passes the systematic bits, as they are, therethrough. 
   Second rate matching unit  94  performs the rate matching processing on data from virtual IR buffer  93 , and sends the resulting data to bit collection unit  95 . In this event, second rate matching unit  94  performs the processing on the systematic bits, parity  1  bits, and parity  2  bits (RM_S, RM_P 1 _ 2 , RM_P 2 _ 2 ). In the rate matching in second rate matching unit  94 , the number of bits Ndata of output data w is used as a parameter in addition to the RV parameters. 
   Bit collection unit  95  interleaves the systematic bits, parity  1  bits, and parity  2  bits from second rate matching unit  94 , and delivers them as output data w. 
   Ndata used in second rate matching unit  94  can be derived from the number of codes and a modulation scheme after the scheduler decides to sent the data to a user who has been selected to receive it. Specifically, Ndata can be calculated by Equation (1):
 
Ndata=3×(Number of Bits of Modulation Scheme)×(Number of Codes of HS-PDSCH)  (1)
 
   In Equation (1), the number of bits of the modulation scheme is 320 bits for QPSK, and 640 bits for 16QAM. 
   As described above, the automatic retransmission is performed using the HARQ processing in HSDPA, but it is important to appropriately select the redundancy version in the retransmission in accordance with a change in the environment of a propagation path in order to improve the throughput of the system. A variety of proposals have been made so far for methods to select the redundancy version (for example, see JP-A-2004-112597). 
   However, no method has been established at present for selecting an appropriate redundancy version, and standardization has not been achieved. Unless an appropriate redundancy version is selected whenever retransmission occurs, an appropriate resource will not be allocated to appropriate bits, resulting in an increase in the number of times of retransmission. 
   For example, in the retransmission, the number of bits (Nsys) of the systematic bits applied to HARQ unit  90  is the same as that in the first transmission. If there is a significant deterioration in the propagation path environment (reduction in CQI (Channel Quality Indicator) value) in the retransmission, as compared with the first transmission, and if limits are imposed on the number of available codes, only part of the systematic bits, rather than all of them, can be retransmitted depending on the selected redundancy version. 
   Also, even if retransmissions are repeated from the radio base station with a redundancy version which handles the systematic bits as nonpriority in a state where the systematic bits have not yet been correctly received by a terminal, correct decoding is not accomplished in the terminal, resulting in repeated retransmissions. 
   Since appropriate resources are not allocated in the HARQ-based automatic retransmission due to the failure to establish a method of selecting a redundancy version in transmission, it is difficult to improve the data throughput. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a radio apparatus which improves the throughput of data transfer using an HARQ-based automatic retransmission. 
   To achieve the above object, the radio apparatus of the present invention is a radio apparatus that performs a HARQ-based automatic retransmission, and has a redundancy version control unit and an HARQ processing unit. 
   The redundancy version control unit identifies a redundancy version in a retransmission based upon determination criteria which differ depending on whether the redundancy version in the preceding transmission, in which the need for retransmission arose, handled systematic bits as priority or as nonpriority. The HARQ processing unit performs rate matching on each of the systematic bits and parity bits of data to be retransmitted in accordance with the redundancy version identified by the redundancy version control unit. 
   According to the present invention, the redundancy version control unit determines a new redundancy version when a retransmission occurs, based on the determination criteria which differ depending on whether the systematic bits were handled as priority or nonpriority, so that an appropriate redundancy version can be determined in accordance with a situation in which the retransmission is needed. 
   The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram illustrating the flow of signal processing on the transmission side of the physical layer by a conventional general radio base station; 
       FIG. 2  is a block diagram illustrating a HARQ processing unit which performs HARQ processing in the signal processing of the physical layer illustrated in  FIG. 1 ; 
       FIG. 3  is a block diagram illustrating a radio base station of one embodiment; 
       FIG. 4  is a flow chart illustrating the flow of a redundancy version selection process in an automatic retransmission by a redundancy version control unit shown in  FIG. 3 ; and 
       FIG. 5  is a state transition diagram of the redundancy version carried out by the operation shown in  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3  is a block diagram illustrating the configuration of a radio base station in one embodiment. This radio base station is used in a W-CDMA mobile communications system, and performs a HARQ-based automatic retransmission in HSDPA. 
   Referring to  FIG. 3 , radio base station  11  has HARQ processing unit  90 , ACK/NACK receiver unit  12 , CQI receiver unit  13 , and scheduler  14 . Scheduler  14  has redundancy version control unit  15 . 
   HARQ processing unit  90 , which is the same as the conventional HARQ processing unit illustrated in  FIG. 2 , performs rate matching on input data c twice, before and after the virtual buffer, using parameters s, r of a redundancy version (hereinafter called the “RV parameters”) given by scheduler  14 . 
   Referring to  FIG. 2 , virtual IR buffer  93  is a virtual buffer which performs buffering with a predefined buffer size. Bit separation unit  91  separates input data c from channel coding processing  83  into systematic bits, parity  1  bits, and parity  2  bits. 
   First rate matching unit  92  performs rate matching processing on the systematic bits, parity  1  bits, and parity  2  bits from bit separation unit  91 , and sends the resulting respective bits to virtual IR buffer  93 . In this event, first rate matching unit  92  actually performs the processing only on the parity  1  bits and parity  2  bits (RM_P 1 _ 1 , RM_P 2 _ 1 ), but passes the systematic bits, as they are, therethrough. 
   Second rate matching unit  94  performs the rate matching processing on data from virtual IR buffer  93 , and sends the resulting data to bit collection unit  95 . In this event, second rate matching unit  94  performs the processing on the systematic bits, parity  1  bits, and parity  2  bits (RM_S, RM_P 12 , RM_P 2 _ 2 ). In the rate matching in second rate matching unit  94 , the number of bits of output data w (Ndata) is used as a parameter in addition to the RV parameters. 
   Bit collection unit  95  interleaves the systematic bits, parity  1  bits, and parity  2  bits from second rate matching unit  94 , and delivers them as output data w. 
   In the HARQ-based automatic retransmission of HARQ processing unit  90 , a selection can be made as to whether the systematic bits are handled as priority or as nonpriority by the RV parameters. By appropriately selecting systematic bits as priority/nonpriority in accordance with a change in the environment of a propagation path, the data throughput can be improved. When RV parameter s is set to “1,” the systematic bits are prioritized. When RV parameter s is set to “0,” the systematic bits are handled as nonpriority, and parity bits (parity 1 bits and parity 2 bits) alone are transmitted. 
   ACK/NACK receiver unit  12  receives a response signal from a terminal (not shown), and sends the response signal to redundancy version control unit  15  of scheduler  14 . The response signal may be an ACK signal or a NACK signal. The ACK signal is a signal which indicates that data from radio base station  11  can be normally decoded. The NACK signal is a signal which requests a retransmission due to a failure in normal decoding of data from radio base station  11 . 
   CQI receiver unit  13  receives a CQI signal from a terminal, and sends the CQI signal to redundancy version control unit  15 . The CQI signal includes a CQI value. The CQI value is a value indicative of the state of a downlink propagation path. The CQI value becomes smaller as the environment deteriorates on the propagation path. 
   Scheduler  14  determines a user to whom data will be sent at the next transmission timing using the response signal from ACK/NACK receiver unit  12 , and the CQI signal from CQI receiver unit  13 . 
   Also, redundancy version control unit  15  of scheduler  14  adaptively selects a redundancy version using the state of the redundancy version in the preceding transmission, and sends RV parameters s, r of the redundancy version to HARQ processing unit  90 . 
     FIG. 4  is a flow chart illustrating the flow of a redundancy version selection process in the automatic retransmission by the redundancy version control unit shown in  FIG. 3 . Referring to  FIG. 4 , redundancy version control unit  15  first determines during the retransmission process whether or not the systematic bits were prioritized in the preceding transmission (step  101 ). 
   If the systematic bits were prioritized in the preceding transmission, redundancy version control unit  15  next determines whether or not Nsys-Ndata is positive and whether or not there are limits to the number of available codes (step  102 ). 
   The number of bits in input data c (Nsys) to HARQ processing unit  90  is the number of input bits to the physical layer, i.e., the number of output bits from the MAC layer. Nsys in a retransmission has the same value as Nsys in the first transmission. The number of bits in output data w (Ndata) from HARQ processing unit  90  is determined from a modulation scheme and the number of codes. Specifically, Ndata is calculated by the aforementioned Equation (1). 
   If Nsys is positive and there are limits to the number of available codes, redundancy version control unit  15  sets the redundancy version such that the systematic bits are handled as nonpriority, and the parity bits alone are transmitted (step  103 ). In this event, redundancy version control unit  15  may set RV parameter s to “0.” As retransmission is performed with this setting, redundancy version control unit  15  determines whether or not retransmission is needed (step  104 ), followed by a return to processing at step  101  if needed, or followed by termination of the process if not needed. The retransmission is determined to be needed unless the ACK signal is received at ACK/NACK receiver unit  12 . 
   If Nsys-Ndata is not positive or there are no limits to the number of available codes, as determined at step  102 , redundancy version control unit  15  maintains the setting which prioritizes the systematic bits (step  105 ). In this event, redundancy version control unit  15  may maintain RV parameter s equal to “1.” As a retransmission is performed with this setting, redundancy version control unit  15  proceeds to processing at step  104 . 
   If the systematic bits were nonpriority in the preceding transmission, as determined at step  101 , redundancy version control unit  15  next determines whether or not the number of times of retransmission is equal to or larger than a predetermined threshold (step  106 ). This threshold is intended to determine, based on the number of times of retransmission, whether the systematic bits have not been normally received by the terminal, and the threshold is set to adapt to the system. 
   If the number of times of retransmission is equal to or larger than the threshold, redundancy version control unit  15  sets the redundancy version such that the systematic bits are handled as priority (step  107 ). In this event, redundancy version control unit  15  may set RV parameter s to “1.” As a retransmission is performed with this setting, redundancy version control unit  15  proceeds to processing at step  104 . 
   If the number of times of retransmission is smaller than the threshold, as determined at step  106 , redundancy version control unit  15  maintains the setting of the redundancy version which handles the systematic bits as nonpriority (step  108 ). In this event, redundancy version control unit  15  may maintain RV parameter s equal to “0.” As a retransmission is performed with this setting, redundancy version control unit  15  proceeds to processing at step  104 . 
     FIG. 4  illustrates only the control process for RV parameter s, wherein there are no particular limits to the control process for RV parameter r. 
     FIG. 5  is a state transition diagram of the redundancy version carried out by the operation shown in  FIG. 4 . Referring to  FIG. 5 , the systematic bits are handled as priority in the first retransmission. 
   In this state, if Nsys-Ndata is positive, and if there are limits to the number of available codes (condition A 1 ), the redundancy version transitions to a state in which the systematic bits are handled as nonpriority. On the other hand, if Nsys-Ndata is not positive or if there are no limits to the number of available codes (condition A 2 ), the redundancy version is maintained in the state in which the systematic bits are handled as priority. 
   On the other hand, in a state in which the systematic bits are handled as nonpriority, if the number of times of retransmission is smaller than the threshold (condition A 4 ), the redundancy version is maintained in the state in which the systematic bits are handled as nonpriority. Conversely, if the number of times of retransmission is equal to or larger than the threshold (condition A 3 ), the redundancy version transitions to the state in which the systematic bits are handled as priority. 
   As described above, in this embodiment, since redundancy version control unit  15  determines a new redundancy version based on the determination criteria which differs depending on whether the redundancy version in the preceding transmission handled the systematic bits as priority or nonpriority, an appropriate redundancy version can be determined in accordance with a situation in which a retransmission is needed, thus improving the data throughput. 
   Also, according to this embodiment, when the systematic bits were handled as priority in the preceding transmission, redundancy version control unit  15  sets the redundancy version which handles the systematic bits as nonpriority and transmits the parity bits alone if the value of Nsys-Ndata is positive, and if there are limits to the number of available codes. Thus, even if the number of output bits from the physical layer is smaller than the number of bits input to the physical layer, its influence can be reduced to improve the coding rate. Then, as a result, data is correctly decoded at a terminal with a higher possibility to improve the throughput. 
   Also, according to this embodiment, when the systematic bits were handled as nonpriority in the preceding transmission, redundancy version control unit  15  sets the redundancy version such that systematic bits are prioritized if the number of times of retransmission is equal to or larger than the threshold. When the retransmission has been repeated a number of times equal to or larger than the threshold, it can be thought that the systematic bits have not been correctly received by the terminal, so that the redundancy version is set to prioritize the systematic bits. As a result, data is correctly decoded at the terminal with higher possibility to improve the throughput. 
   As appreciated, the embodiment is only a preferred example of the present invention, and the present invention is not limited to this embodiment, but can be widely applied when HARQ is utilized. As another example to which the present invention can be applied, there is a radio apparatus which employs HARQ for EUDCH (Enhanced Uplink DCH) that is under investigation in 3GPP, like HSDPA. 
   While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.