Arq system with status and packet acknowledgement

Arch system with status acknowledgment and package.

The present invention relates to a communication system and further relates to primary and secondary stations for use in such a system and to a method for operating such a system. Although the present specification describes a system with particular reference to the universal mobile telecommunications system (UMTS), it should be understood that such techniques can also be applied for use in other communication systems.

There is a growing demand in the area of ​​mobile communications for a system that has the ability to download large blocks of data to a mobile station (MS) on demand at a reasonable rate. Such data could be, for example, Internet web pages, possibly including video segments or the like. Normally, a particular MS will only require such data intermittently, so that dedicated fixed bandwidth links are not appropriate. To meet this requirement in UMTS, a high-speed downlink packet access (HSDPA) scheme is being developed that can facilitate the transfer of packet data to a mobile station at up to 4 Mbps.

A conventional component of a packet data transmission system is an ARQ (automatic replay request) process, for handling data packets received by mistake. For example, consider downlink packet transmission from a base station (BS) to a mobile station (MS) in HSDPA. When the MS receives a data packet it determines whether the packet has been corrupted, for example, using cyclic redundancy check (CRC) information. It then transmits a signal in a field assigned for this purpose to the BS, using a first signal as an acknowledgment (ACK), to indicate that the packet was received successfully, and using a second signal as a negative acknowledgment (NACK), to indicate that the package was received but corrupted. The signals can be, for example, different code words or the same code word transmitted to different powers. The BS requires an appropriate position to establish a decision threshold so that it can correctly decode the ACK / NACK messages.

Since packet transmission is normally intermittent, discontinuous transmission (DTX) can normally be used, so that nothing is transmitted by the MS in the ACK / NACK field unless a data packet has been received. In a typical scenario, the probability that the MS cannot detect a packet of data that has been sent could be 1%. In this case it is desirable that the BS interprets the DTX as if it were a NACK, so that the packet can be retransmitted to the MS. By interpreting the DTX as a NACK it can be achieved that either by diverting the decision threshold in the BS towards the ACK signal, or by means of the MS that transmits a NACK in each ACK / NACK field that does not correspond to a packet with the correct CRC will detect

or not a package.

A problem with the MS that transmits in each ACK / NACK field is that the uplink interference increases significantly, and in addition the battery life of the MS is reduced. This is a particular problem when packet traffic is burst (as is often the case), resulting in the MS being required to transmit in many ACK / NACK fields when no packet had been transmitted to it.

A problem with the deviation of the threshold for deciding between the ACK and NACK orders is that it is necessary to increase the transmission power of the ACK order (as discussed below) in order to achieve an acceptably low probability that interpret an ACK as a NACK. Since the probability of the MS transmitting an ACK must be much greater than that of transmitting a NACK in a well-designed communication system, increasing the ACK transmission power will significantly increase the average transmission power required in the ACK field. / NACK.

Consider a typical communication system that requires that the probability of misinterpreting an ACK as a NACK is less than 1% and the probability of misinterpreting a NACK as an ACK is less than 0.01%. Assuming that the probability that the MS does not detect a packet is 1%, then the probability of misinterpreting the DTX as a NACK should be less than 1% (so that the combined probability of the MS not receiving a packet and of that its DTX is interpreted as an ACK is the same as the probability of misinterpreting a NACK as an ACK, that is, less than 0.01%). Simulations for typical mobile communication channels have shown that the deviation of the decision threshold to an ACK sufficient to ensure that the probability of misinterpreting a DTX as an ACK is less than 1% has the effect of requiring that the ACK power be greater than the power of NACK, as much as 10 or 20 dB in some scenarios.

A partial solution, given in the British patent application being processed together with the present 0207696.6 (reference of the PHGB applicant 020034), is that the MS transmits the NACKs continuously after its initial ACK / NACK whenever a timer is executed. This avoids the need for the BS to divert its decision threshold, thus reducing the required ACK power. However, a problem with this scheme is that the BS still has to divert its ACK / NACK decision threshold for the first of a series of packets, or alternatively tolerate a greater probability of detection error for the DTX after the first packet. .

In US Patent Application No. 2002/0101835, if a mobile subscriber receiving terminal correctly receives a header, it will send an acknowledgment containing codes assigned to the mobile user, and a transmission terminal will transmit data using this code. assigned.

An object of the present invention is to address the problem identified above.

According to a first aspect of the present invention, a communication system is provided having a downlink indicator channel for the transmission of an indicator signal indicating that a data packet is programmed to be transmitted on a downlink data channel from a primary station to a secondary station, the secondary station having reception means for receiving the signal signal and the data packet, and acknowledgment means for transmitting a signal to the primary station to indicate the status of the received data packet, in which the secondary station comprises means for transmitting a status signal in an uplink channel to indicate the reception of the Indicator signal before the transmission of a positive or negative acknowledgment to indicate the status of the received data packet.

Provided that the secondary station transmits a status signal to indicate reception of the indicating signal, the primary station has at least two opportunities to detect the case in which the secondary station cannot receive the indicating signal (i.e. without receiving anything from the primary station both in the time slot for receiving the initial status signal and without receiving anything in the time slot for an ACK or NACK in response to the reception of the data packet). Therefore, the probability of the primary station misinterpreting the DTX as an ACK or NACK is reduced and the maximum power requirement of the uplink channel can be reduced, thereby reducing interference levels. In a preferred embodiment of the present invention, the status signal is a NACK.

According to a second aspect of the present invention, a primary station is provided for use in a communication system having a downlink indicator channel for the transmission of an indicator signal indicating that a data packet is programmed to be transmitted in a downlink data channel from the primary station to a secondary station, in which means are provided to receive a status signal from the secondary station on an uplink channel to indicate the reception of the indicator signal before the receipt of a positive or negative acknowledgment to indicate the status of the data packet received by the secondary station.

According to a third aspect of the present invention, a secondary station is provided for use in a communication system having a downlink indicator channel for the transmission of an indicator signal indicating that a data packet is programmed to be transmitted in a downlink data channel from a primary station to the secondary station, where reception means are provided to receive the signal signal and the data packet, acknowledgment means are provided to transmit an uplink channel in a signal to the primary station to indicate the status of the received data packet, and means are provided to transmit a status signal to indicate reception of the indicating signal before of the transmission of a positive or negative acknowledgment to indicate the status of the received data packet.

According to a fourth aspect of the present invention, there is provided a method of operation of a communication system having a downlink indicator channel for the transmission of an indicator signal indicating that a data packet is programmed to be transmitted on a communication channel. downlink data from a primary station to a secondary station, the method comprising receiving by the secondary station the signal signal and the data packet, and the transmission in an uplink channel of a status signal to indicate the reception of the indicator signal before the transmission of a positive or negative acknowledgment to indicate the status of the received data packet.

The present invention is based on the recognition, not present in the prior art, that the transmission of a status signal upon receipt of an indication that a data packet is to be transmitted, together with a positive or negative acknowledgment of receipt related to the reception of the data packet itself, the maximum uplink power requirements are reduced, thereby reducing system interference.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic block diagram of a radio communication system;

Figure 2 is a diagram illustrating the operation of a known ARQ stop and wait scheme;

Figure 3 is a diagram illustrating the operation of a basic HSDPA packet transmission scheme; Figure 4 is a diagram illustrating the operation of an improved HSDPA packet transmission scheme that includes repeated NACKs;

Figure 5 is a diagram illustrating the operation of an improved HSDPA packet transmission scheme that includes signaling detection of a packet indication;

Figure 6 is a diagram illustrating the operation of an improved HSDPA packet transmission scheme that includes signaling detection of a repeated packet indication and NACK;

Figure 7 is a graph showing the maximum power requirement for the schemes of Figure 3 (shown as a solid line) and Figure 6 (shown as a dashed line); and

Figure 8 is a flowchart showing a method of operation of a packet data transmission system according to the present invention.

In the drawings, the same reference numbers have been used to indicate corresponding characteristics.

Referring to FIG. 1, a radio communication system comprises a primary station 100 (BS) and a plurality of secondary stations 110 (MS). The BS 100 comprises a microcontroller 102 (! C), transceiver means 104 (Tx / Rx) connected to antenna means 106, power control means (PC) 107 to alter the transmitted power level, and connection means 108 for connection to the PSTN or other suitable network. Each MS 110 comprises a microcontroller 112 (! C), transceiver means 114 (Tx / Rx) connected to antenna means 116, and power control means (PC) 118 to change the transmitted power level. Communication from BS 100 to MS 110 takes place on a downlink channel 122, while communication from MS 110 to BS 100 takes place on an uplink channel 124.

An example of the operation of a known stop and wait ARQ scheme is illustrated in Figure 2. The data packets 202, identified as Pn where n is a one-bit sequence number, are transmitted in time slots assigned in a channel. 122 downlink (DL) from a BS 100 to an MS 110. The first data packet P0, with sequence number 0, is received in a state corrupted by the MS 110, which therefore transmits a negative acknowledgment 204 (N) in a field reserved for the transmission of positive acknowledgments and negative In response to this, the BS 100 retransmits the first data packet 202, which this time is correctly received by the MS 100 which transmits an acknowledgment 206 (A). Then, the BS 100 transmits the following packet, with sequence number 1. The BS 100 also retransmits a data packet 202 if it receives no acknowledgment within a predetermined period of time (in case the MS 110 does not receive the entire package or the acknowledgment is lost). If the MS 110 did indeed receive the previously transmitted packet 202, it may determine that the received packet 202 is a retransmission since it has the same sequence number as the previous packet.

The operation of HSDPA as currently specified is shown in Figure 3, which shows in a simplified manner the approximate synchronization relationships between the various channels used to provide HSDPA. The presence of a data packet 202 programmed for transmission to MS 110 is signaled by the transmission of an indicator signal 302 I in subframe N of a downlink indicator channel DL1 (the high-speed shared control channel, HS -SCCH). This is followed by the transmission of the data packet 202 in a downlink data channel DL2 (the high-speed downlink shared channel, HS-DSCH). If MS 110 correctly decodes packet 202, it sends an ACK 206 in subframe N of an uplink channel, a dedicated high-speed physical control channel (HS-DPCCH), as shown. If the package is not decoded correctly, a NACK 204 is sent instead.

If the MS 110 cannot detect the indicating signal 302, it will not transmit anything (ie use discontinuous transmission, DTX) in the subframe N of the uplink channel 124. If the BS 100 then incorrectly detects the DTX as an ACK 206, the BS will not perform a physical layer retransmission of the packet. This means that higher layer protocols are required if the MS 110 can correctly receive the missing packet; however, such protocols generate significant additional signaling traffic (and therefore more interference) and may be too slow for real-time applications.

In order for the BS to limit the likelihood of incorrectly detecting the DTX as an ACK 206 (a probability of 0.01 seems acceptable), it must divert its ACK / NACK decision threshold to favor the detection of NACK. However, this means that the power required for ACK messages is increased, and can be as high as 10-20 dB above that required for a normal DPCCH uplink.

A partial solution to this problem, disclosed in the British patent application being processed together with the present 0207696.6 (applicant reference PHGB 020034), is described with reference to Figure 4. In this scheme, the MS 110 transmits an ACK 206 (as shown) or NACK 204 in the subframe N of the uplink channel 124, and then continues to transmit the NACK 204 in each uplink subframe corresponding to a subframe HS -DSCH in which a packet was not detected through the HS-SCCH, while a timer is running. In other words, while the timer is running, a NACK 204 is transmitted in subframe N of uplink channel 124 unless a packet of data 202 is properly decoded in subframe N of downlink HS-DSCH. Thus, during contiguous (or almost contiguous) packet bursts, the BS 100 does not have to deflect its detection threshold (except perhaps for the first packet in each burst), thus reducing the required ACK power.

However, a drawback of this scheme is that the BS 100 has yet to divert its ACK / NACK decision threshold for the first packet in each burst, or else to tolerate a greater probability of detection error for DTX after the first packet. . Therefore, the maximum transmission power required for uplink channel 124 is not improved (unless the BS 100 also does not divert its detection threshold for the first packet, in which case the first packet in each burst will be subjected to a higher probability of misinterpretation of DTX). In addition, if packets are produced only individually (that is, separated by a period longer than the duration of the timer), then the NACK 204 transmission scheme while the timer is running offers no benefit.

An improved scheme, according to the present invention, is described with reference to Figure 5. In this scheme, when the MS 110 detects a packet indication 302 on the downlink indicator channel, it transmits a NACK 204 as a status signal in the subframe prior to the subframe in which the ACK 206 or NACK 204 would normally be transmitted concerning the data packet 202. In the illustrated scenario, an indicator signal 302 is transmitted in subframe N of the indicator channel and, in response, MS 110 transmits a NACK 204 in subframe N-1 of uplink channel 124 (unless it has been successfully decoded a data packet of subframe N-1 of the downlink data channel). In other embodiments of the present invention, it is not necessary for the status signal to be a NACK 204. For example, it may be an ACK 206

or some other suitable signal.

By using this scheme, a communication system can be designed so that the probability that the BS 100 cannot detect that the MS 110 has not been able to detect a packet indication 302 transmitted on the downlink indicator channel DL1 is shared between at least two transmissions on uplink channel 124. Therefore, if the total probability of the DTX being detected as an ACK 206 is required to be less than 0.01, the power of the NACK transmission in the N-1 subframe can be set so that the probability that the BS 100 incorrectly detects the DTX in subframe N-1 as a NACK 204 is 0.1, and the power of the ACK transmission in subframe N can be set so that the probability that the BS 100 incorrectly detects the DTX in subframe N as an ACK 206 is also 0.1. In this way, the maximum power requirement for uplink channel 124 is minimized. It follows that the transmission power used for NACK transmission in the N-1 subframe may be different from the transmission power used for other NACKs. In fact, it is preferable that the transmission power used for NACK transmission in the N-1 subframe is the same as the transmission power normally used for ACK transmissions.

In a preferred embodiment, the scheme of Figure 5 can be combined with that of Figure 4. In particular, a special case of the behavior in Figure 4 can be used, whereby the timer is executed exactly for a subframe after the subframe. N. Therefore, when an ACK or NACK has been transmitted in subframe N, the MS 110 will always additionally transmit a NACK 204 in subframe N + 1, unless another packet immediately follows in subframe N + 1 in the data channel downlink and decode correctly, in which case an ACK is transmitted in subframe N + 1 on uplink channel 124. Therefore, the BS 100 never has to distinguish the DTX from the ACK 206 in a single subframe. A particular advantage of this embodiment of the present invention is that it is not necessary for the timer to run longer than a subframe to obtain the benefit of the reduced ACK power requirement.

According to current HSDPA specifications, it is also possible for a system to require ACK or NACK to be repeated several times (up to three repetitions) in subsequent subframes in order to increase its reliability without increasing its transmission power. No packet may be transmitted in the downlink data channel in any subframe corresponding to a subframe in the uplink channel 124 that contains a repeat of an ACK / NACK of a previous packet.

In this case, the present invention could be applied so that the MS 110 transmits NACK 204 in both the N-1 subframe and the N-2 subframe with the detection of a packet indication in the indicator channel, in order to maintain the power required for this preliminary NACK 204 similar to the power required for normal ACK / NACK transmissions. However, there is not enough time between an indicator signal 302 in subframe N of the indicator channel and subframe N-3 in uplink channel 124 for a NACK 204 to be transmitted in subframe N-3 of the uplink channel in a system in which the number of ACK / NACK repetitions is set to a value greater than 1.

In such a system, the timer could still be used to cause an additional NACK 204 to be transmitted after the normal ACK / NACK (although it would be necessary to run the timer for longer than a subframe). In this case, this additional NACK would be repeated in the same number of subframes as normal ACK / NACKs, and would follow the last repeat of the normal ACK / NACK. This is illustrated in Figure 6 for the case in which the number of repetitions of each AC K / NACK is set to 1.

The presence of a data packet is signaled by an indicator signal 302 in the normal manner in subframe N. Then, a NACK 204 is transmitted in subframes N-2 and N-1 (unless a decoder has already been correctly decoded packet in the downlink data channel in subframe N-2, in which case an ACK 206 is sent in subframes N-2 and N-1). If the packet is decoded correctly, then an ACK 206 is sent on the uplink channel 124 in subframe N, and repeated in subframe N + 1. No packet can be transmitted in the downlink data channel in subframe N + 1. If the packet is not decoded correctly, a NACK 206 is sent on uplink channel 124 in subframe N and repeated in subframe N + 1. Additionally, depending on the use of the timer, a NACK 204 is always sent in the N + 2 and N + 3 subframes of the uplink channel 124, unless a packet of the N + 2 subframe is correctly decoded in the data channel downlink, in which case an ACK 206 is sent in subframes N + 2 and N + 3.

It may be desirable to be able to switch the transmission of a NACK 206 to indicate the reception of a signal 302 indicating activation and deactivation by signaling from BS 100. This signaling can be combined with the activation and deactivation of the timer for the transmission of NACK 204 after acknowledgment of normal packet receipt (ie both aspects of activation and deactivation together), alternatively the two aspects can be activated and deactivated independently of each other . The switching can be determined by the state of the MS 110, for example, whether or not it is in continuous transfer, or in the selected number of ACK and NACK repetitions.

An example in which it may be desirable to deactivate the use of the present invention is whether BS 100 is specifically trying to detect DTX as a separate case from NACK. This may be the case if, for example, different versions of redundancy are used for retransmissions, in which case they cannot be combined directly into the same flexible buffer in MS 110. This, however, would not present any problem if it were used. a combination of Chase.

The simulation results showing the benefit (in terms of the maximum uplink channel 124 power requirement of the present invention, in combination with a 1-frame timer), are shown in Figure 7. This is a graph of P, the maximum uplink power requirement in relation to the normal uplink transmission power (DPCCH), in dB versus V, the speed of the MS 110 in km / h. In this simulation, the total probability of incorrectly detecting the DTX as an ACK 206 is required to be 0.01. The power requirements without an initial NACK or subsequent timer are shown as a continuous line, and the requirements with an initial NACK and a subsequent timer of duration of a subframe are shown as a dashed line. It can be seen that the use of the two techniques together provides a benefit of between 3 and 6 dB.

The operation of such a combined scheme is summarized by the flow chart shown in Figure 8. The method is started, in step 802, when the MS 110 is ready to receive the data packets 202. Test 804 refers to the determination of MS 110 of whether an indicator signal 302 has been received for a data packet. If so, the MS 110 transmits, in step 806, a negative acknowledgment and proceeds to receive the data packet. Test 808 refers to the determination of MS 110 of whether the data packet has been successfully received. If a data packet 202 is received, test 808 is passed, the timer is reset, in step 810, an acknowledgment 206 is transmitted, in step 812, and MS 110 returns to test 804 to check a signal 302 indicator. Resetting the timer may involve starting a timer, if it is not already running, or restarting a timer that is already running.

If the data packet is not received successfully, test 808 is not passed and an additional 814 test is performed to determine if the timer is running. If the timer is running, test 814 is passed and MS 110 transmits, in step 816, a negative acknowledgment 204 in the corresponding ACK / NACK field, then returns to test 804. If the timer is not running , test 814 is not passed and MS 110 returns directly to test 804.

The above description aims at the FDD (frequency division duplex) mode of UMTS. The invention could also be applied in TDD mode (time division duplex). In this case, the fact that the uplink and downlink channels use different time slots on the same frequency (ie, reciprocal channel) may reduce the need for signaling of the channel information.

This above description referred to BS 100 which performed a variety of functions relating to the present invention. In practice, these tasks may be the responsibility of a variety of parts of the fixed infrastructure, for example, in a "node B", which is the part of the fixed infrastructure that is directly interconnected with an MS 110, or in a top level in the radio network controller (RNC). In this specification, the use of the term "base station" or "primary station", therefore, should be understood to include parts of the fixed network infrastructure involved in an embodiment of the present invention.

5 Upon reading this description, other modifications will be apparent to those skilled in the art. Such modifications may involve other features that are already known in the design, manufacture and use of communication systems and component parts thereof, and that may be used in place of or in addition to the features already described herein.

10 In the present specification and the claims the word "a / or" or "a" preceding an element does not exclude the presence of a plurality of such elements. In addition, the expression "comprising / understanding" does not exclude the presence of elements or stages other than those listed.