Standardization of the W-CDMA (UMTS) system, which is one of the third generation mobile communication system, is now under development by the 3GPP (3rd Generation Partnership Project). As a theme of standardization, the HSDPA (High Speed Downlink Packet Access) which can provide a maximum transmission velocity of about 14 Mbps for the downlink is specified.
The HSDPA employs an adaptive modulation and coding (AMC) system which includes, for example, a QPSK modulation method and a 16-level QAM method that are switched adaptively in accordance with the radio communication environment between the base station and mobile station.
Moreover, the HSDPA also adopts the H-ARQ (Hybrid Automatic Repeat request) system. When a mobile station has detected an error in the receiving data received from the base station, the data is re-transmitted from the base station responding to the request from the mobile station, while the mobile station executes the error correction decoding process using both received data and the data received from the re-transmission. In the H-ARQ, as described above, if an error is detected, the gain of the error correction decoding is raised and the number of times of re-transmission is controlled by effectively utilizing the received data.
The major radio channels used for the HSDPA include the HS-SCCH (High Speed-Shared Control Channel), HS-PDSCH (High Speed-Physical Downlink Shared Channel), and HS-DPCCH (High Speed-Dedicated Physical Control Channel).
The HS-SCCH and the HS-PDSCH are shared channels in the downlink direction (i.e., the direction toward a mobile station from a base station), and the HS-SCCH is a control channel for sending various parameters of data transmitted by the HS-PDSCH. The parameters, such as the modulation information which indicates the modulation method used for transmission by the HS-PDSCH, number of spreading codes assigned (number of codes), and information such as the pattern of the rate matching for the transmitting data may all be considered.
Meanwhile, the HS-DPCCH is an dedicated control channel in the uplink direction (i.e., the direction toward a base station from a mobile station) and is used to transmit the ACK signal and NACK signal to the base station from the mobile station in accordance with the acknowledgment or non-acknowledgment of reception of the data received via the HS-PDSCH. If a mobile station has failed to receive the data (the CRC error is detected in the receiving data or the like), the base station executes the re-transmission control because the NACK signal is transmitted from the mobile station or neither the ACK signal nor the NACK signal is received by the base station.
Moreover, the HS-DPCCH may also be used by the mobile station which measured the receiving quality of the signal received from the base station to transmit the result of the measurement to the base station as the CQI (Channel Quality Indicator). The base station determines the environment of the radio communication on the basis of the received CQI. When the communication environment is good, the modulation method is switched to the method for transmitting the data at the higher transmission rate. If the environment is not as good, on the contrary, the modulation method is switched to the method for transmitting the data at a lower transmission rate (namely, adaptive modulation is executed).
“Channel Format”
Next, a channel format in the HSDPA will be described below.
FIG. 1 is a diagram illustrating a channel format in the HSDPA. The W-CDMA introduces the code dividing multiplex system and each channel is therefore separated with the spreading code.
The channels not yet described will be described briefly first.
CPICH (Common Pilot Channel) and P-CCPCH (Primary Common Control Physical Channel) are respectively common channels in the downlink direction.
The CPICH is a channel used by a mobile station for estimation of channel condition, searching of cells, the timing reference of the other downlink physical channels in the same cell, and the channel used for transmitting the pilot signal. The P-CCPCH is the channel for transmitting the broadcasting information.
Next, the timing relationship of channels will be described with reference to FIG. 1.
As illustrated, one frame (10 ms) is formed based on 15 slots in each channel. As described previously, since the CPICH is used as a reference channel, the top of frames of the P-CCPCH and HS-SCCH channels are matched with the top of frame of the CPICH channel. Here, the top of frame of the HS-PDSCH channel is delayed by two slots from the HS-SCCH channel. This may be done to realize demodulation of the HS-PDSCH channel using the demodulating method that corresponds to the received modulation type after the mobile station has received the modulation information via the HS-SCCH channel. Moreover, the HS-SCCH and HS-PDSCH channels may form one sub-frame with three slots.
The HS-DPCCH channel is not synchronized with the CPICH channel but this channel is provided for the uplink direction and is based on the timing generated in the mobile station.
The channel format of the HSDPA channel has been briefly described above. Next, the processes up to transmission of the transmitting data via the HS-PDSCH channel will be described with reference to the block diagram.
“Structure of Base Station”
FIG. 2 illustrates a structure of a base station supporting the HSDPA channel.
In FIG. 2, the reference numeral 1 designates a CRC attachment unit; 2, a code block segmentation unit; 3, a channel coding unit; 4, a bit separating unit; 5, a rate matching unit; 6, a bit collecting unit; 7, a modulating unit.
Next, operations of each block will be described.
The transmitted data transmitted via the HS-PDSCH channel (data accommodated within one sub-frame of the HS-PDSCH channel in FIG. 1) is first subjected to the CRC arithmetic process in the CRC attachment unit 1 and the result of the arithmetic operation is added to the last part of the transmitting data. The transmitting data to which the result of CRC arithmetic operation is added may then be inputted to the code block segmentation unit 2 and is then segmented into a plurality of blocks. This process is required to shorten the data length in unit of the error correction coding, considering the load of decoding process in the receiving side. When the data length exceeds the predetermined length, the code block is equally segmented into a plurality of blocks. An integer 2 or larger may be selected as the number of segmentations but the number of segmentations 2 may be selected to simplify the description. If the data length is rather short, segmentation of the block may be unnecessary.
The segmented and transmitted data are respectively processed as the object data of the individual error correction encoding process in the channel coding unit 3. In other words, the error correction encoding process is executed for the segmented first block and second block. As an example of the channel encoding process, a turbo encoding process may be used.
Here, the turbo encoding process will be described briefly. In an exemplary turbo encoding process, when the data is defined as U, the data U′ obtained by the convolutional encoding of the data U, and the data U″ obtained by the convolutional encoding of the data U after the interleave (re-arrangement) process of the data U may be outputted. Here, the data may be referred to as the systematic bits and may be understood, in the turbo decoding process, as the data used in two element decoders and the data having a higher degree of importance because the application frequency is high. On the other hand, the data U′ and U″ are parity bits. These bits are data used in one of the two element decoders and have a degree of importance that is lower than that of the data U because the application frequency is low.
Namely, it can be said that since the systematic bits have a higher degree of importance than that of the parity bits, and the systematic bits are received with greater accuracy, the more accurate decoding result can be obtained with the turbo decoder.
The systematic bits and parity bits that are generated as described above may be inputted as the serial data to the bit separating unit 4. The bit separating unit 4 separates the input serial data into the data U, U′, and U″ of three systems, and then outputs this data as parallel data.
The rate matching unit 5 preferably performs the puncture process for deleting the bits with the predetermined algorithm and also executes the repetition process to repeat the bits in order to store the data within the sub-frame formed of three slots of the HS-PDSCH channel.
As described above, the bits having completed the bit adaptation process to the sub-frame are then inputted in parallel to the bit collecting unit 6.
The bit collecting unit 6 preferably generates bit sequences of four bits indicating each signal point, for example, of 16-level QAM modulation based on the input data and then outputs these bit sequences. At the time of generation of bit sequences, the systematic bits are preferably arranged, for the first transmission, in the side of upper bits in which an error is not easily generated.
The modulating unit 7 outputs the signal of the 16-level QAM modulation to provide the amplitude and phase corresponding to the signal points indicated with the input bit sequences and then provides the signal to the side of the antenna (not illustrated) after conversion to the radio frequency through the frequency conversion.
“Detailed Description of the Rate Matching Unit 5”
FIG. 3 illustrates one embodiment of the rate matching unit 5. The rate matching unit 5 comprises, as illustrated, a first rate matching unit 51, a virtual buffer 52, and a second rate matching unit 53.
The first rate matching unit 51 executes the rate matching process (puncture process) to the first parity bits (U′) and second parity bits (U″) separated in the bit separating unit 4. This process may be executed, considering the capability of the mobile station in the receiving side described below (capacity of memory or the like to store the data obtained by combining the received data and re-transmitted data), in order to keep the maximum amount of data which can be transmitted including the re-transmission under the capability of the mobile station. Accordingly, bits are deleted on the basis of a predetermined rule.
The virtual buffer unit 52 may be provided to store the systematic bits. The first and second parity bits may have completed the puncture process (deletion of bits) in accordance with the capability of the mobile station.
The second rate matching unit 53 performs the puncture process and the repetition process. The puncture process deletes the bits based on the predetermined rule, and the repetition process repeats the bits for the data read from the virtual buffer in order to store the data within the sub-frame formed from the three slots of the HS-PDSCH channel.
The structure and operations of the rate matching unit 5 are described above and one embodiment of data due to the process performed by the rate matching unit 5 will be described with reference to FIG. 4.
The block A in FIG. 4 illustrates the systematic bits (U), first parity bits (U′), and second parity bits (U″) inputted to the first rate matching unit 51.
The first rate matching process unit 51 executes the puncture process to the block A to attain the predetermined amount of data determined in accordance with the capability of the mobile station and then provides an output. Namely, the bits are deleted to result in the amount of data indicated in the block B considering the capability of the mobile station.
Various methods may be assumed for deletion of bits. For example, the block B illustrated in FIG. 4 may be formed by not applying the puncture process to the systematic bits. These systematic bits are important, and the puncture process is applied to the first and second parity bits (indicated as U′ (r) and U″ (r) in order to show execution of the puncture process as the rate matching process). In FIG. 4, the left half bits are deleted but it is preferable to delete the bits at the positions which a dispersed to a certain degree. For example, the even number bits or odd number bits are deleted.
The second rate matching unit 53 executes the rate matching process to store the data within the sub-frame on the basis of the data block B puncture-processed in the first rate matching unit 51 and then outputs the transmitting data.
For example, with the first transmission, the systematic bits U are outputted as the block C after execution of the puncture process. With the second transmission (first re-transmission), the first parity bits U′ (r) and the second parity bit U″ (r) are outputted as block D after execution of the puncture process. The number of times that the re-transmission may occur may be a predetermined number equal to 1 or larger. However, when the third transmission is assumed to be the last re-transmission which includes the first transmission, the block C obtained by the puncture process is transmitted again, for example, to the block B in the third transmission (second re-transmission).
The items regarding the HSDPA channel are disclosed, for example, in the non-patent document “3G TS 25. 212 (3rd Generation Partnership Project: Technical Specification Group Radio Access Network; Multiplexing and channel coding (FDD))”.
According to the background technology described above, the amount of data transmitted to a mobile station is previously limited (limited to the block B), with the (first) rate matching, to the value below the capability of the receiving apparatus (capacity of memory or the like to store the data obtained by combining the received data and re-transmitted data). Thus, the receiving apparatus does not receive data exceeding the capability (capacity of memory) and is capable of executing the error correction decoding process by completely utilizing the first receiving data and the re-transmitted receiving data.
However, such limitation will close the way of utilization of the data part exceeding the capacity of memory of the receiving apparatus. This will result in the disadvantage that the sufficient capability of the error correction decoding process cannot be ensured.
Elimination of such limitation may be achieved by increasing the capacity of the memory of the receiving apparatus. However, when reduction in the size of the mobile station is alienated due to the increase in capacity of memory and particularly when soft-determination data is used for the error correction decoding process, the amount of data increases drastically. Therefore, elimination of the limitation cannot be employed directly.