JP2004282792A - Hybrid arq method and hybrid arq transmitter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform novel measurement using a transmitter and to reduce overhead without requiring a signal measuring report on an uplink. <P>SOLUTION: An encoding parameter is used to encode a data unit into at least two first and second codewords, the first codeword is then transmitted, and ACK or NAK is received from a receiver. When NAK is received, the second codeword is transmitted. Thus, a measured value indicating a present channel status is obtained by evaluating the encoding parameter. It is preferable that the measured values are an entire encoding rate, an average resending number for each data unit and an average resending number for each codeword. In the embodiment, the transmitter is equipped with a NAK counter 600. <P>COPYRIGHT: (C)2005,JPO&NCIPI

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 for transmitting data units to a receiver over a wireless 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, especially 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. The 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 UE 120 via the interface Uu.

  As shown in FIG. 2, the UTRAN 110 includes a set of radio network subsystems RNS200 connected to the core network CN100 via an interface lu. Each RNS 200 comprises a radio network controller RNC 210 which plays a role in handover decisions requiring signal transmission to the user equipment UE120. Further, the radio network subsystem RNS 200 includes a base station (Node B) 220 connected to the radio network controller RNC 210 via the interface lub. Within the UTRAN 110, the radio network controllers RNC 210 of the radio network subsystem RNS 200 are further connectable to each other via an interface lur. Interfaces lu and lur are logical interfaces. The lur can be realized by a direct physical connection between the radio network controllers RNC 210 or via a virtual network using any suitable transport network.

  Signal transmission of 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 of the user apparatus UE and control of the notification. The measurement performed by the user apparatus UE120 is controlled by the RRC including the UMTS radio interface and other systems with respect to the measurement target, the measurement timing, and the 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 blocks of information bits of length k and forms encoded blocks of length n before transmission. Redundancy contributes to suppressing errors at the receiver.

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

  Turning now to the ARQ technology, the most frequently used schemes in mobile communications are stop-and-wait (SAW) and selective repetition (SR) continuous ARQ schemes. If a cyclic redundancy check (CRC) detects an error, the receiver requests the transmitter to transmit 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 has 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 with a despreader 420, transmits the despread signal to a demodulator 430, and converts the ACK / NAK signal from the demodulated data into demodulated data. Take out. The ACK message informs the transmitter that the receiver has successfully decoded the transmitted PDU. The NAK message notifies the transmitter of the decoding error. Depending on whether the transmitter receives an ACK or a NAK, the ACK / NAK extractor 440 accesses the codeword buffer 320 for retransmission, or frees memory if an ACK is received.

  Referring now to FIG. 5, this flowchart details the process performed by the receiver. At step 500, the receiver receives the codeword and stores it at step 510. If the codeword has been previously transmitted, the received and stored codeword is combined in step 520 with a previously transmitted codeword of the same data unit. 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 the PDU are released (step 540). If decoding is not possible, a negative acknowledgment message NAK is transmitted (step 550), requesting retransmission.

ARQ is classified into three types according to bits to be retransmitted.
Type 1: The PDU containing the error is discarded, and a new copy of the PDU is retransmitted and decoded alone. The new and old PDUs received are not combined.
Type 2: PDUs containing errors and requiring retransmission are not discarded, but are combined with incremental redundant bits prepared by the transmitter, and decoding is continued. The retransmitted PDU may have a higher coding rate and is combined with the stored coding rate at the receiver. That is, each retransmission adds only a small amount of redundancy.
Type 3: This ARQ type differs from type 2 ARQ only in that each retransmitted PDU is auto-decodable. This means that the PDU can be decoded without combining with the previous PDU. This is useful when part of the PDU is damaged and its information is almost unusable.

  Type 2 and Type 3 schemes have higher processing power than Type 1 in that the coding rate can be adjusted according to the changing wireless environment and the redundancy of previously transmitted PDUs can be reused, Excellent in performance. Such a 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 combining process. These different parts of the overall code are called "code blocks" or "code words".

  Since ARQ Type 2 and Type 3 schemes impose strict requirements on the memory size for storing soft decision values for later combining, 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. Typically, this information is collected in a status report, which includes some round-trip delay before an ACK or NAK can be sent. Thus, it has been found beneficial to provide very fast direct feedback 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. In this way, the number of code blocks that need to be stored is kept at a very small value, and the overall delay is reduced.

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

  However, in the frequency division bidirectional FDD scheme, the transmitter usually does not know much about the channel state of the receiver. If there is some communication data flow on the uplink from the receiver to the transmitter, the measurement of this data flow is performed at different frequencies and is therefore unreliable. 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 a transmission code power corresponding to a 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 or fading is to be performed according to the channel state. However, measurements by such a transmitter may not be meaningful 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 signaled from the UE 120 to the radio network controller RNC 210. These measurement reports add the overhead of a new signal that must be transmitted wirelessly. Thus, continuous measurement notification is disadvantageous for adaptation purposes as it causes excessive collisions on the uplink. On the other hand, if the notification is not performed continuously, a delay occurs in the transmitter, and the adaptation cannot be performed accurately 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 may for example reduce the code rate. However, in systems that use incremental redundancy, this is disadvantageous because hybrid ARQ type 2/3 inherently involves many retransmissions, such as many NAK messages.

  While incremental redundancy can already be considered an adaptive coding scheme, it requires more adaptation. The following is an example of why further adaptation is needed.

  In mobile communication systems that do not use incremental redundancy, the coding rates are typically about 1/2 and 1/3. Type 2/3 uses a lower code 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 code rate (after subsequent combining) decreases with each retransmission, such as r = 1, 1/2, 1/3, 1/4.

  Therefore, as compared with the type 1 ARQ, the number of retransmissions in the incremental redundancy scheme is larger in the type 2/3 scheme because the redundancy added to each code block is smaller (every retransmission). To improve the granularity of adaptation (the coding rate for the channel state), the coding rate of a single code block must be high. The number of retransmissions increases, but at this particular moment the overall code rate approaches the optimal code value.

  There are several problems with such design criteria.

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

  In addition to incremental redundancy, other adaptive coding techniques are known. However, such a conventional adaptive coding system does not consider the operation of the ARQ type 2/3 system capable of dividing a code into a number of code blocks. The required coding rate for the hybrid ARQ type 1 scheme is different from that for the hybrid ARQ type 2/3 scheme because the gain is caused by the temporal diversity of many code blocks.

  As described above, in the conventional apparatus, when hybrid ARQ is used, a signal measurement report is required on the uplink, and there is a problem that communication causes overhead.

  The present invention has been made in view of such a point, and an object of the present invention is to provide a new measurement by a transmitter without needing a signal measurement report on the uplink to overcome the above-mentioned drawbacks of the conventional method. An object of the present invention is to provide a hybrid ARQ method and a hybrid ARQ transmitter capable of providing a possible hybrid ARQ technique and thereby reducing overhead.

  This object is achieved by the present invention as set forth in the independent claims. That is, the hybrid ARQ method of the present invention is a hybrid ARQ method for transmitting a data unit, wherein the method converts the data unit into at least two first and second codewords using at least one coding parameter. Encoding; transmitting the first codeword; receiving a positive or negative acknowledgment message indicating whether a receiver has successfully decoded the data unit; and receiving the negative acknowledgment message. Transmitting the second codeword, and evaluating the at least one coding parameter to obtain a measurement indicating the channel condition. The hybrid ARQ transmitter of the present invention also includes an encoder for encoding a data unit into at least two first and second codewords using at least one encoding parameter, and transmitting the first codeword. A transmitting device, a receiving device for receiving an affirmative or negative acknowledgment message indicating whether the receiver has successfully decoded the data unit, and transmitting the second codeword when receiving the negative acknowledgment message. The configuration includes means for operating the transmitting device, and a measuring device for evaluating the at least one coding parameter to obtain a measured value indicating the channel state.

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

  Embodiments are defined in the dependent claims.

  As described above, the incremental redundancy scheme originally receives ACK / NAK information on the feedback channel. Therefore, the expression 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 a previous PDU response. This information can be derived by summing the number of retransmissions with the corresponding coding rate of the corresponding PDU. This makes it possible, for example, to obtain the required FEC parameter estimates without the need for a new measurement report from the receiver to the transmitter.

  In addition to new transmitter measurements, the present invention utilizes this measurement in one embodiment to perform adaptation. The number of retransmissions required for decoding the PDU changes depending on the channel state. In the case of incremental redundancy, the performance improvement over ARQ type 1 is only significant if there is a certain percentage of NAK messages in the first retransmission (eg, greater than 50%). For good channel conditions, a code rate close to 1 (i.e. no redundancy) is appropriate, while in very bad situations (e.g. fading, shadowing) even a code rate of 1/3 is small. Can only be correctly decoded.

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

  According to the present invention, it is possible to provide a hybrid ARQ technique capable of performing a new measurement by a transmitter without requiring a signal measurement report in an uplink, thereby reducing overhead.

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

  The coding rate r is a ratio of the number of information bits k to the number of transmitted code bits n. Different code rates and codewords can be easily generated using rate compatible punctured codes such as RCPT and RCPC codes. The codeword performs puncturing from a common mother code. The mother code must have a low code rate so that some high code rate codewords can be generated therefrom. For cases where retransmission is required, the PDU or mother code must be stored in the transmitter and a new codeword needs to be transmitted. It will be clear 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 differs from the configuration in FIG. 4 in that a NAK counter 600 and a measuring device 610 are provided. The operation of the transmitter will be described in detail with reference to FIG.

After encoding the PDU with FEC encoder 310 in step 700, the codeword is placed in codeword buffer 320 (step 710). Next, at step 720, the first codeword is transmitted. If the receiver is unable to decode the data, it sends a NAK message to the transmitter to send the next new code requesting to add more redundancy to the received data. In this case, the transmitter receives the NAK message at step 730 and selects the next codeword from codeword buffer 320 at step 740. This next codeword is queued for transmission. After receiving, 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 ). Thus, the redundancy is added, and the possibility of decoding the packet is increased.

  If the transmitter receives the ACK message in step 730, all codewords of each PDU are cleared in step 760.

As shown in the flowchart, at step 750, the count of NAK counter 600 is incremented each time a NAK message is received. Thus, NAK counter 600 counts all NAK messages in a PDU until the PDU is accepted. Knowing the coding rate of each codeword, the number of retransmissions per PDU, N _NAK , is used by the measurement device 610 to calculate the overall PDU coding rate, the average number of retransmissions per PDU, or the average number of retransmissions per codeword. can do. When the coding rate of the codeword is fixed, it is preferable that measuring device 610 includes a memory for storing the coding rate of each codeword. As detailed below, the measurement can be averaged over a certain number of PDUs or over time. Therefore, the measuring device 610 preferably has a filter function. It is preferable to perform averaging according to the round-trip delay until the retransmission becomes possible, and according to the rate of change of the channel state. In the filter function, weights can be applied to the measurement device 610 to add weight to more recent measurements than previous measurements.

  In the present embodiment, the measurement device 610 notifies a measurement value to an upper layer unit or another element. The measurements are preferably communicated to the base station (Node B) 220 or the radio network controller RNC 210. Notification can be performed periodically on demand or based on a predetermined threshold.

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

A first example of calculating the measurement value is to calculate the overall code rate. Therefore, if it is not possible to direct the entire coding rate clearly, the transmitter counts the coding rate r _CWI retransmission number N _NAK and each code block per PDU. The total PDU coding rate r _tot can be calculated from the following equation.

Here, _ncwi is the number of coding bits of codeword i. The coding rate can be averaged by the accuracy and time change of the mobile channel.

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

Another example of calculating measurements is to calculate the average number of retransmissions per PDU. The number of retransmissions required for decoding for each PDU is _NNAKi . The average value can be calculated by the following equation.

This value is zero if no retransmission was performed. In the embodiment, the coding rate of the first code block is adjusted according to the required number of retransmissions for each PDU. Compared to the ARQ type 1 adaptive coding scheme, the coding is adjusted to ensure reception of the specified average number of retransmissions rather than to avoid retransmissions (ie, N _NAK = 0). The average number of retransmissions is preferably approximately one or more.

Yet another example of calculating a measurement 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 the following equation:

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

  In FIG. 8, the adaptation is performed by the FEC controller 800. The FEC controller 800 adapts the FEC parameters based on the measurements provided by the measurement device 610. Upon performing the adaptation, the new coding rate for the codeword is notified to the measuring device 610. The process of performing the adaptation is shown in FIG.

  Preferably, the initial value of the FEC determined in step 900 is based on measurements taken before transmission. In another embodiment, the initial parameters are 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 predetermined parameters used as adaptation criteria. These parameters include, for example, the total number of retransmissions per PDU, the number of retransmissions per codeword, or the overall coding rate (see above). However, the adaptation may be measured at the transmitter or based on any other parameters received from the receiver.

  In the adaptation process shown in FIG. 9, a threshold value is obtained in step 920. Some degree of freedom can be provided to avoid excessive switching between different combinations of FEC parameters. The parameters monitored in step 910 are then compared to thresholds in steps 930 and 950. If the monitored parameter such as the average number of retransmissions for each codeword is larger than the threshold, the coding rate is reduced. 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 ensures 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. This allows the receiver to determine the coding rate of the subsequent code blocks from the coding rate of the first code block, thereby reducing the signal overhead.

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

  Referring again to FIG. 8, in yet another embodiment of the present invention, instead of or in addition to the adaptation of the coding parameters, there are adapted transmission parameters. In one embodiment, the modulation type is adapted. Therefore, the FEC controller 800 makes a new access to the modulator 330 (not shown in FIG. 8). 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 to the spreader 340 (not shown in FIG. 8).

Diagram showing UMTS configuration Diagram showing UTRAN configuration FIG. 2 is a block diagram showing a transmitter capable of performing FEC encoding of data to be transmitted. FIG. 2 is a block diagram showing a transmitter adapted to the FEC / ARQ scheme. Flow chart showing the operation of a receiver operating by ARQ technology FIG. 1 is a block diagram illustrating a transmitter according to a first embodiment of the present invention. Flow chart for explaining the operation process of the transmitter according to the above embodiment FIG. 4 is a block diagram showing a transmitter according to a second embodiment of the present invention. Flow chart for explaining the operation process of the transmitter according to the above embodiment

Explanation of reference numerals

300 PDU buffer 310 FEC encoder 320 Codeword buffer 330 Modulator 340 Spreader 350 Transmit RF circuit 360 Antenna 400 Duplexer 410 Receive RF circuit 420 Despreader 430 Demodulator 440 ACK / NAK extractor 600 NAK counter 610 Measuring device 800 FEC controller

Claims (21)

A hybrid ARQ method for transmitting data units, the method comprising:
Encoding the data unit into at least two first and second codewords using at least one encoding parameter;
Transmitting the first codeword;
Receiving a positive or negative acknowledgment message indicating whether the receiver has successfully decoded the data unit;
Transmitting the second codeword when receiving the negative acknowledgment message,
Determining a measurement indicative of a channel condition by evaluating the at least one coding parameter;
Hybrid ARQ method.   The hybrid ARQ method according to claim 1, wherein the measurement value is an overall coding rate obtained from a coding rate of each codeword transmitted until the acknowledgment message is received.   The hybrid ARQ method according to claim 1 or 2, further comprising averaging the measurements over a number of data units, or applying a weighting filter function to weight the previous data units less.   4. The hybrid ARQ method according to claim 1, wherein the at least one coding parameter is an FEC coding rate. 5.   5. The hybrid ARQ method according to claim 1, further comprising the step of adapting at least one coding parameter to the measured value.   The hybrid ARQ method according to one of claims 1 to 5, further comprising the step of adapting the transmission parameters to the measured values.   The hybrid ARQ method according to claim 6, wherein the transmission parameter is a modulation format.   The hybrid ARQ method according to claim 6, wherein the transmission parameter is a spreading factor.   The hybrid ARQ method according to claim 6, wherein the transmission parameter is a spreading code number.   The method according to claim 1, wherein the at least one coding parameter is a codeword coding rate, and wherein the method further comprises initializing the coding rate to a high value. Hybrid ARQ method.   The hybrid ARQ method according to claim 10, wherein the code rate of the second codeword is derived from the code rate of the first codeword.   The hybrid ARQ method according to claim 10 or 11, further comprising transmitting only a coding rate of the first codeword to a receiver. An encoder for encoding the data unit into at least two first and second codewords using at least one encoding parameter;
A transmitting device for transmitting the first codeword;
A receiver for receiving a positive or negative acknowledgment message indicating whether the receiver has successfully decoded the data unit,
Means for operating a transmitting device for transmitting the second codeword when receiving the negative acknowledgment message;
A measuring device for determining a measurement indicative of a channel condition by evaluating said at least one coding parameter;
A hybrid ARQ transmitter comprising:   A hybrid ARQ transmitter configured to perform the hybrid ARQ method of claim 1.   The hybrid ARQ transmitter according to claim 13 or 14, which is a base station.   The hybrid ARQ transmitter according to claim 15, further comprising means for transmitting a measurement to a radio network controller on demand.   17. The hybrid ARQ transmitter according to claim 15 or claim 16, further comprising means for transmitting measurements to the radio network controller periodically.   The hybrid ARQ transmitter according to any one of claims 15 to 17, further comprising means for transmitting a measurement value to the radio network controller in response to an event.   A system comprising the hybrid ARQ transmitter according to claim 13 and a receiver.   19. A hybrid ARQ transmitter according to any of claims 13 to 18, comprising means for adapting at least one coding parameter to a measurement.   19. The hybrid ARQ transmitter according to any of claims 13 to 18, comprising means for adapting transmission parameters to measured values.

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