JP3924574B2 - Hybrid ARQ method and hybrid ARQ transmitter - Google Patents

Description

  The present invention relates to a hybrid ARQ technique for data transmission, and more particularly to a hybrid ARQ type 2/3 scheme for operating a transmitter that transmits data units to a receiver over a radio channel of a communication system. The invention further relates to a corresponding transmitter and communication system. The present invention is applicable to mobile communication systems, particularly cellular systems. In particular, the present invention is applicable to universal mobile telephone systems (UMTS).

  UMTS is one of the technologies that are candidates for the next generation mobile communication system, and its structure is as shown in FIG. UMTS has a configuration in which a core network CN100 is connected to a radio access network UTRAN110 (UMTS terrestrial radio access network) via an interface lu. On the other hand, the UTRAN 110 is connected to the user apparatus UE120 via the interface Uu.

  As shown in FIG. 2, the UTRAN 110 is composed of a set of radio network subsystems RNS200 connected to the core network CN100 via an interface lu. Each RNS 200 is composed of a radio network controller RNC 210 that serves as a handover decision that requires signal transmission to the user equipment UE 120. Furthermore, the radio network subsystem RNS 200 includes a base station (Node B) 220 connected to the radio network controller RNC 210 via an interface lub. Within the UTRAN 110, the radio network controllers RNCs 210 of the radio network subsystem RNS200 can be connected to each other via an interface lur. Interfaces lu and lur are logical interfaces. The lur can be realized via a direct physical connection between the radio network controllers RNCs 210 or via a virtual network using any suitable transport network.

  Signal transmission in the layer 3 control plane between the user equipment UE 120 and the UTRAN 110 is processed in the radio resource control RRC layer. In addition to transmission of broadcast information, establishment of radio carriers, and control of radio resources, the RRC also plays a role of measurement notification and notification control of the user equipment UE. The measurement performed by the user apparatus UE120 is controlled by the RRC including the UMTS radio interface and other systems regarding the measurement object, measurement timing, and notification method. The RRC layer also performs measurement notification from the user apparatus UE120 to the network.

  In a data communication system, error detection included in an automatic repeat request (ARQ) is widely used for error control. The most common technique for non-real-time service error detection is based on a hybrid ARQ scheme that combines ARQ and error correction decoding (FEC).

  FEC introduces redundancy into an information bit block of length k and forms a coded block of length n before transmission. Redundancy contributes to error suppression at the receiver.

  Details of a transmitter that performs error correction decoding are shown in FIG. Input data to be transmitted is first put in the buffer 300. When data is present in the buffer 300 and a physical channel for transmission is assigned to the transmitter, the data is encoded by the FEC encoder 310, thereby generating a mother code. The mother code or all codewords (code segments) of the mother code are then sent to modulator 330 and spreader 340 (in the case of a code division multiple access CDMA system) and converted to radio frequency RF by RF circuit 350 And transmitted from the antenna 360. When using Type 2/3 ARQ (shown below), the transmitter further includes a codeword buffer 320 because different codewords are transmitted in the retransmission.

  Turning now to ARQ technology, the most frequently used schemes in mobile communications are stop-and-wait (SAW) and selective repetition (SR) continuous ARQ schemes. If an error is detected in the cyclic redundancy check (CRC), the receiver requests the transmitter to send a new bit. The retransmission unit of the radio link control RLC layer is called “protocol data unit PDU”.

  A transmitter configured to operate according to the ARQ scheme is shown in FIG. Since the transmitter must be able to receive requests from the receiver, the transmitter includes a duplexer 400 that enables the antenna 360 to be used for both transmission and reception. When the transmitter receives the signal, the RF circuit 410 converts the signal into a baseband signal, despreads the signal by the despreader 420, transmits the despread signal to the demodulator 430, and generates an ACK / NAK signal from the demodulated data. Take out. The ACK message notifies the transmitter that the receiver has successfully decoded the transmitted PDU. The NAK message notifies the transmitter of a decoding error. Depending on whether the transmitter receives an ACK or a NAK, the ACK / NAK extractor 440 accesses the codeword buffer 320 for the purpose of retransmission, or releases the memory if an ACK is received.

  Referring now to FIG. 5, this flowchart details the process performed by the receiver. In step 500, the receiver receives the codeword and stores it in step 510. If the codeword has been previously transmitted, the received and stored codeword is combined with the previously transmitted codeword of the same data unit at step 520. Next, in step 530, it is determined whether the PDU can be decoded. If possible, an acknowledgment message ACK is returned to the transmitter, and all stored codewords for that PDU are released (step 540). If decryption is impossible, a negative acknowledgment message NAK is transmitted (step 550) to request retransmission.

ARQ is classified into three types according to the retransmitted bits.
Type 1: Abandon a PDU containing an error, retransmit a new copy of the PDU, and decode alone. The received new and old PDUs are not combined.
Type 2: A PDU that contains an error and needs to be retransmitted is not abandoned but is combined with an incremental redundant bit prepared on the transmitter side, and subsequently decoded. The retransmitted PDU may have a higher coding rate and is combined with the coding rate stored on the receiver side. That is, only a little redundancy is added for each retransmission.
Type 3: This ARQ type differs from Type 2 ARQ only in that each retransmission PDU can be automatically decoded. This means that the PDU can be decoded without combining with the previous PDU. This is useful when a part of the PDU is damaged and the information is almost unusable.

  Type 2 and Type 3 systems have higher processing power compared to Type 1 in that the coding rate is adjusted according to the changing radio environment and the redundancy of the previously transmitted PDU can be reused. Excellent performance. Such type 2/3 ARQ scheme is hereinafter referred to as “incremental redundancy”. Different PDUs are encoded separately at the physical layer, increasing the coding gain for the synthesis process. These different parts of the overall code are called “code blocks” or “codewords”.

  Since the ARQ type 2 and type 3 systems impose strict requirements on the memory size for storing soft decision values for later synthesis, the introduction of very fast feedback channels has been proposed. The feedback channel is used to send ACK and NAK information from the receiver to the transmitter. Usually, this information is collected in a status report, so some round trip delay is included until an ACK or NAK can be sent. Therefore, it has been found useful to perform feedback very fast at the physical layer without going through higher layers such as radio link control RLC. If a NAK is received, the transmitter can transmit the next code block with minimal delay. Thus, the number of code blocks that need to be stored is kept at a very small value, reducing the overall delay.

  Due to the limited frequency resources, future communication systems will be adapted to the wireless environment. Transmission parameters such as modulation, data transfer rate, spreading factor, and number of spreading codes are based on the current channel state.

  However, in the frequency division bidirectional FDD scheme, the transmitter usually has little knowledge of the channel state of the receiver. If there is a communication data flow from the receiver to the transmitter on the uplink, this data flow measurement is performed at a different frequency and is not reliable. There are also measurements that can be performed at the transmitter. As an example of such a measurement, for the Node B, there is a measurement of transmission code power corresponding to the transmission power for a certain user apparatus UE120. Since the transmission power is controlled by the UE power control command, correction corresponding to channel attenuation such as path loss and fading is performed according to the channel state. However, such transmitter measurements may not make sense under certain conditions.

  Conventional adaptation techniques are often based on measurement reports to be transmitted from the receiver to the transmitter. The radio resource control RRC constitutes such a measurement, which is communicated from the UE 120 to the radio network controller RNC 210. These measurement reports add new signal overhead that must be transmitted wirelessly. Therefore, continuous measurement notifications are disadvantageous for adaptation purposes because they cause excessive collisions on the uplink. On the other hand, if notification is not performed continuously, a delay occurs in the transmitter, and adaptation cannot be accurately performed according to the current channel state.

  Conventional adaptation techniques in simple ARQ systems are based on ACK / NAK transmissions already available at the transmitter. If a large number of NAK messages are received, the transmitter can reduce the coding rate, for example. However, in systems that use incremental redundancy, this is disadvantageous because hybrid ARQ type 2/3 inherently involves a large number of retransmissions, such as a large number of NAK messages.

  Incremental redundancy can already be considered as an adaptive coding scheme, but requires further adaptation. The following are examples of why further adaptation is required.

  In mobile communication systems that do not use incremental redundancy, coding rates are typically about 1/2 and 1/3. Type 2/3 uses a lower coding rate for the first code block. Conventional systems use a fixed code rate for each code block for incremental redundancy. For example, the coding rate of each code block can be fixed to 1. Assuming no response, the overall coding rate (after subsequent synthesis) decreases as r = 1, 1/2, 1/3, 1/4 for each retransmission.

  Therefore, compared with type 1 ARQ, the redundancy added to each code block is smaller in the type 2/3 system (every retransmission), and therefore the number of retransmissions in the incremental redundancy system is larger. In order to improve the granularity of adaptation (coding rate for channel conditions), the coding rate of a single code block must be high. Although the number of retransmissions increases, the overall coding rate approaches the optimum coding value at this particular moment.

  There are several problems associated with such design criteria.

  First, the number of retransmissions that need to be requested in the feedback channel is large, resulting in increased signal overhead in the feedback channel. Further, the delay until the decoding of the entire PDU is increased by the round trip delay RTD time for each retransmission. Also, the memory required increases as the number of retransmissions increases in proportion to the time required to store a single code block. Furthermore, assuming a high coding rate for the first code block, for example r = 1, this data rate is designed for good channel conditions, so retransmission in bad channel conditions is close to 100%. Become. On the other hand, when a low coding rate is assumed for the first code block, for example, when r = 1/2, the number of retransmissions is reduced, but the gain is relatively smaller than the hybrid type 1 ARQ.

  In addition to incremental redundancy, other adaptive coding techniques are known. However, such a conventional adaptive coding method does not consider the operation of the ARQ type 2/3 method that can divide the code into a large number of code blocks. Since a gain is generated due to temporal diversity of a large number of code blocks, a necessary coding rate for the hybrid ARQ type 1 scheme is different from that of the hybrid ARQ type 2/3 scheme.

  As described above, in the case of using the hybrid ARQ, the conventional apparatus requires a signal measurement report on the uplink, and there is a problem that overhead is generated in communication.

  The present invention has been made in view of such points, and an object of the present invention is to perform a new measurement by a transmitter without the need for a signal measurement report on the uplink in order to overcome the above-described drawbacks of the conventional method. It is to provide a hybrid ARQ method and a hybrid ARQ transmitter that can provide a possible hybrid ARQ technique and thereby reduce overhead.

This object is achieved by the present invention as described in the independent claims. That is, the hybrid ARQ method of the present invention is a hybrid ARQ method for transmitting data units in a radio channel, said method, at least the data unit using at least one encoding parameter first codeword and the An acknowledgment message or a negative acknowledgment message indicating whether the data unit was successfully transmitted, a step of encoding the first codeword using transmission parameters, and a step of transmitting the first codeword using transmission parameters. receiving from the receiver, when receiving the negative acknowledgment message comprises the steps of: transmitting the second code words, at least one of the coding parameters and the transmission parameters, the first Based on the measured values of the codewords evaluated . The hybrid ARQ transmitter of the present invention is a hybrid ARQ transmitter for transmitting data units in a radio channel, at least one encoding parameter of the data unit at least a first codeword and a second using An encoder that encodes into a codeword, a transmission unit that transmits the first codeword using transmission parameters, and an acknowledgment message or a negative response message that indicates whether the data unit was successfully transmitted. A reception unit that receives from the receiver; and an operation unit that operates the transmission unit to transmit the second codeword when the negative acknowledgment message is received , the encoding parameter and the transmission parameter At least one of, based on the measured values obtained by evaluating the first codeword, a configuration

  The present invention is advantageous in that it provides an adaptive coding scheme with incremental redundancy by utilizing information already available at the transmitter. Thus, the present invention provides transmitter measurements that overcome many of the disadvantages of conventional schemes.

  Embodiments are defined in the dependent claims.

  As described above, the incremental redundancy scheme inherently receives ACK / NAK information through a feedback channel. Therefore, the expressiveness is not so high because many NAK messages need to be received for good adaptation. In an embodiment, the present invention provides a technique for deriving the overall code rate required for previous PDU responses. This information can be derived by summing the number of retransmissions having the corresponding coding rate of the corresponding PDU. As a result, for example, a necessary FEC parameter estimate can be obtained without requiring a new measurement report from the receiver to the transmitter.

  In addition to new transmitter measurements, the present invention uses this measurement in one embodiment to perform adaptation. Depending on the channel state, the number of retransmissions required for PDU decoding varies. In the case of incremental redundancy, the performance improvement compared to ARQ type 1 is only significant if there is a proportion of NAK messages in the first retransmission (eg greater than 50%). For good channel conditions, a code rate close to 1 (ie no redundancy) is appropriate, whereas in very bad situations (eg fading, shadowing) even a code rate of 1/3 is small. Only correct packets can be decoded.

  The present invention further provides adaptive coding corresponding to the coding rate of each code block. In particular, the coding rate of the first code block can be adjusted to provide a certain target retransmission rate in several channel conditions.

  According to the present invention, it is possible to provide a hybrid ARQ technique that enables a transmitter to perform a new measurement without requiring a signal measurement report on the uplink, thereby reducing overhead.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings, but the same components are denoted by the same reference numerals.

  The coding rate r is the ratio of the number of information bits k to the number of transmitted code bits n. Different coding rates and codewords can be easily generated using rate compatible punctured codes such as RCPT and RCPC codes. The code word is punctured from a common mother code. The mother code needs to be low code rate so that several high code rate codewords can be generated therefrom. Since retransmission is required, the PDU or mother code must be stored in the transmitter and a new codeword must be transmitted. It will be apparent to those skilled in the art that the present invention is not particularly limited to the embodiments described below, but here the entire mother code (ie, all codewords) is stored in the transmitter and retransmitted. This reduces the coding complexity but requires more transmitter memory.

  Referring to FIG. 6 showing the first embodiment of the transmitter according to the present invention, the configuration of FIG. 4 is different in that it includes a NAK counter 600 and a measuring device 610. The operation of the transmitter will be described in detail with reference to FIG.

After encoding the PDU with the FEC encoder 310 in step 700, the codeword is placed in the codeword buffer 320 (step 710). Next, at step 720, the first codeword is transmitted. If the receiver cannot decode the data, it sends a NAK message to the transmitter to send the next new code, requesting that additional redundancy be added to the received data. In this case, the transmitter receives the NAK message at step 730 and selects the next codeword from the codeword buffer 320 at step 740. This next codeword is queued waiting for transmission. After reception, the receiver _{combines the} new codeword with the already received codeword so that the overall coding rate of the receiver is r _tot = k / (n _cw1 + n _cw2 ). In this way, redundancy is added and the possibility of decoding a packet increases.

  When the transmitter receives an ACK message at step 730, all codewords of the respective PDUs are cleared at step 760.

As shown in the flowchart, at step 750, the NAK counter 600 is incremented each time a NAK message is received. Thus, the NAK counter 600 counts all NAK messages in the PDU until the PDU is accepted. By knowing the coding rate of each codeword, the total number of PDU coding, the average number of retransmissions per PDU, or the average number of retransmissions per codeword is calculated using the number of retransmissions N _NAK per PDU in the measuring device 610 can do. When the code rate of the code word is fixed, the measuring apparatus 610 preferably includes a memory for storing the code rate of each code word. As will be described in detail below, measurements can be averaged over a number of PDUs or over a period of time. For this reason, it is preferable that the measuring apparatus 610 has a filter function. It is preferable to perform averaging according to the round trip delay until retransmission becomes possible and according to the rate of change of the channel state. In the filter function, weighting can be applied to the measuring device 610 to add weight to more recent measurements than previous measurements.

  In the present embodiment, the measurement apparatus 610 notifies the measurement value to the upper layer unit or other elements. The measured value is preferably notified to the base station (Node B) 220 or the radio network controller RNC 210. Notification can be performed periodically upon request or based on a predetermined threshold.

  An example of measurement values that the transmitter can calculate based on the number of retransmissions without making a new measurement in step 770 is shown in detail. It can be seen that due to the original operation of the hybrid ARQ type 2/3 system, the total coding rate required for decoding the PDU can be derived. Any adaptation to the wireless environment can be made based on such measurements.

The first example of calculation of the measured value is to calculate the overall coding rate. Therefore, when the overall coding rate cannot be clearly derived, the transmitter counts the number of retransmissions N _NAK per PDU and the coding rate r _cwi of each code block. The total PDU coding rate r _tot can be calculated from the following equation.

Here, _ncwi is the number of encoded bits of codeword i. The coding rate can be averaged according to the accuracy and time variation of the mobile channel.

Here, N _PDU is the number of packets used for averaging. In the embodiment, the coding rate of the first code block is adjusted according to the average coding rate used in the previously transmitted PDU.

Another example of calculating the measurement is to calculate the average number of retransmissions per PDU. The number of retransmissions required for decoding for each PDU is N _NAKi . The average value can be calculated by the following formula.

This value is zero if no retransmission has occurred. In the embodiment, the coding rate of the first code block is adjusted according to the number of retransmissions required for each PDU. Compared to the ARQ type 1 adaptive coding scheme, the coding is adjusted not to retransmit (ie N _NAK = 0) but to ensure reception of the specified average number of retransmissions. The average number of retransmissions is preferably about 1 or more.

Yet another example of calculating the measured value is to calculate the average number of transmissions per codeword. The average number of retransmissions per codeword x averaged over N _PDUs (common to all NAK messages) can be calculated from:

  While the transmitter measurement of the present invention has been described in detail above, it will be noted that the following description uses this measurement for adaptation. FIG. 8 will be referred to in this regard, but FIG. 8 is different from the configuration of FIG. 6 in that an FEC control device 800 is provided. The transmitter of FIG. 8 performs adaptation of FEC parameters used to encode the data of the FEC encoder 310.

  In FIG. 8, the adaptation is performed by the FEC controller 800. Based on the measurement values provided by the measurement device 610, the FEC controller 800 adapts the FEC parameters. When the adaptation is performed, the measurement apparatus 610 is notified of a new coding rate for the codeword. The process for performing the adaptation is shown in FIG.

  The initial value of FEC obtained in step 900 is preferably based on measurements performed before transmission. In another embodiment, the initial parameter is set to a high coding rate. Upon transmission, the FEC parameters are adjusted to apply to the environment. For this reason, it is necessary to monitor a predetermined parameter used as an adaptation standard. Examples of this parameter include the total number of retransmissions for each PDU, the number of retransmissions for each codeword, or the overall coding rate (see above). However, the adaptation may be based on any other parameter measured at the transmitter or received from the receiver.

  In the adaptation process shown in FIG. Some degree of freedom can be provided to avoid excessive switching between different FEC parameter combinations. The parameters monitored in step 910 are then compared to threshold values in steps 930 and 950. When the monitored parameter such as the average number of retransmissions for each codeword is larger than the threshold, the coding rate is decreased. On the other hand, if the monitored parameter is less than the threshold, the coding rate is increased.

  In an embodiment, the adaptation process is limited to several codewords or one of them. When the adaptation is performed only on the first codeword, the coding rate (or FEC parameter) of the next codeword can be fixed to a higher coding rate. This guarantees a small granularity of the overall coding rate and a near-optimal overall coding rate. Also, the coding rate of the next codeword can be clearly obtained from the coding rate of the first codeword. As a result, the receiver can obtain the coding rate of the subsequent code block from the coding rate of the first code block, so that the signal overhead can be reduced.

  In another embodiment, the code block coding rate is adjusted so that only a certain percentage of PDUs per retransmission can be decoded. This proportion is preferably 25%.

  Referring again to FIG. 8, in yet another embodiment of the present invention, there is an adapted transmission parameter instead of or in addition to the adaptation of the coding parameter. In some embodiments, the modulation format is adapted. For this reason, the FEC controller 800 makes a new access (not shown in FIG. 8) to the modulator 330. In another embodiment, the adapted transmission parameter is a spreading factor or number of spreading codes. Therefore, the FEC controller 800 makes a new access (not shown in FIG. 8) to the spreader 340.

Diagram showing UMTS configuration Diagram showing UTRAN configuration Block diagram showing a transmitter capable of FEC encoding of data to be transmitted Block diagram showing a transmitter adapted to the FEC / ARQ scheme Flow diagram showing the operation of a receiver operating with ARQ technology The block diagram which shows the transmitter which concerns on the 1st Embodiment of this invention Flow chart for explaining the operation process of the transmitter of the above embodiment The block diagram which shows the transmitter which concerns on the 2nd Embodiment of this invention. Flow chart for explaining the operation process of the transmitter of the above embodiment

Explanation of symbols

300 PDU buffer 310 FEC encoder 320 codeword buffer 330 modulator 340 spreader 350 transmission RF circuit 360 antenna 400 duplexer 410 reception RF circuit 420 despreader 430 demodulator 440 ACK / NAK extractor 600 NAK counter 610 measuring device 800 FEC controller

Claims (23)

A hybrid ARQ method for transmitting data units over a radio channel, the method comprising:
Encoding the data unit into at least a first codeword and a second codeword using at least one encoding parameter;
Transmitting the first codeword using transmission parameters;
Receiving an acknowledgment message or a negative acknowledgment message from the receiver indicating whether the data unit was successfully transmitted;
Receiving the negative response message, transmitting the second codeword;
At least one of the encoding parameter and the transmission parameter is based on a measured value of evaluating the first codeword;
Hybrid ARQ method.   The hybrid ARQ method according to claim 1, wherein the measured value is evaluated for each codeword.   The hybrid ARQ method according to claim 1, wherein the measurement value indicates a state of the radio channel.   The hybrid ARQ method according to any one of claims 1 to 3, wherein the measurement value is the number of the negative acknowledgment messages received until the acknowledgment message is received.   The hybrid ARQ method according to claim 1, wherein the measurement value is the number of the negative acknowledgment messages for each codeword. Step averaging the measured values over the data units of a certain number, or further comprising applying a weighting filter function to attach a small weight to previous data unit, as claimed in any one of claims 5 Hybrid ARQ method. The hybrid ARQ method according to claim 1, wherein the at least one coding parameter is an FEC coding rate. The hybrid ARQ method according to claim 1, further comprising adapting the at least one coding parameter to the measurement value. The hybrid ARQ method according to claim 1, further comprising adapting the transmission parameter to the measurement value. The hybrid ARQ method according to claim 1, wherein the transmission parameter is a modulation format. The hybrid ARQ method according to claim 1, wherein the transmission parameter is a spreading factor. The hybrid ARQ method according to claim 1, wherein the transmission parameter is a number of spreading codes. The at least one coding parameter is a codeword coding rate;
The method further includes initializing the coding rate to a high value,
The hybrid ARQ method according to any one of claims 1 to 12 . The hybrid ARQ method according to any one of claims 1 to 13 , wherein an encoding rate of a codeword following the first codeword is derived from an encoding rate of the first codeword. The hybrid ARQ method according to claim 1, further comprising a step of transmitting only a coding rate of the first codeword to the receiver. The hybrid ARQ method according to any one of claims 1 to 15 , wherein the first codeword is different from the second codeword in terms of redundancy. A hybrid ARQ transmitter for transmitting data units over a radio channel,
An encoder that encodes the data unit into at least a first codeword and a second codeword using at least one encoding parameter;
A transmission unit for transmitting the first codeword using transmission parameters;
A receiving unit for receiving from the receiver an acknowledgment message or a negative acknowledgment message indicating whether the data unit was successfully transmitted;
An operation unit that operates the transmission unit to transmit the second codeword when the negative acknowledgment message is received, and
At least one of the encoding parameter and the transmission parameter is based on a measured value of evaluating the first codeword;
Hybrid ARQ transmitter. A hybrid ARQ transmitter configured to perform the hybrid ARQ method according to any of claims 1-16. The hybrid ARQ transmitter according to claim 17 or 18 , which is a base station. 20. The hybrid ARQ transmitter according to any one of claims 17 to 19 , further comprising means for transmitting the measured value to a radio network controller in response to a request. 20. A hybrid ARQ transmitter as claimed in any of claims 17 to 19 , further comprising means for periodically transmitting the measurement values to a radio network controller. The hybrid ARQ transmitter according to any one of claims 17 to 19 , further comprising means for transmitting the measurement value to a radio network controller in response to an event. A system comprising the hybrid ARQ transmitter and receiver according to any one of claims 17 to 22 .

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