Method And Apparatus For Hybrid Automatic Repeat Request In A Wireless Communication System

Abstract

A method and apparatus for handling Hybrid Automatic Repeat reQuest (HARQ) retransmission in a User Equipment (UE) includes being configured with at least one Secondary Cell (SCell), sending an UpLink (UL) transmission corresponding to an uplink grant, and setting a HARQ FEEDBACK variable corresponding to the UL transmission to Acknowledgement (ACK) if a HARQ feedback reception corresponding to the UL transmission is prohibited due to RF retuning.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present Application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/380,331, filed on Sep. 7, 2010, the entire disclosure of which is incorporated herein by reference.

FIELD

[0002] This disclosure generally relates to wireless communication networks, and more particularly, to a method and apparatus for Hybrid Automatic Repeat reQuest (HARQ) in a wireless communication system.

BACKGROUND

[0003] With the rapid rise in demand for communication of large amounts of data to and from mobile communication devices, traditional mobile voice communication networks are evolving into networks that communicate with Internet Protocol (IP) data packets. Such IP data packet communication can provide users of mobile communication devices with voice over IP, multimedia, multicast and on-demand communication services.

[0004] An exemplary network structure for which standardization is currently taking place is an Evolved Universal Terrestrial Radio Access Network (E-UTRAN). The E-UTRAN system can provide high data throughput in order to realize the above-noted voice over IP and multimedia services. The E-UTRAN system's standardization work is currently being performed by the 3GPP standards organization. Accordingly, changes to the current body of 3GPP standard are currently being submitted and considered to evolve and finalize the 3GPP standard.

SUMMARY

[0005] According to an aspect of the disclosure, a method for handling HARQ retransmission in a User Equipment (UE) includes being configured with at least one Secondary Cell (SCell), sending an UpLink (UL) transmission corresponding to an uplink grant, and setting a HARQ FEEDBACK variable corresponding to the UL transmission to Acknowledgement (ACK) if a HARQ feedback reception corresponding to the UL transmission is prohibited due to RF retuning.

[0006] According to another aspect of the disclosure, a UE for use in a wireless communication system includes a control circuit, a processor installed in the control circuit, and a memory installed in the control circuit and coupled to the processor. The processor is configured to execute a program code stored in memory to handle a HARQ retransmission by being configured with at least one SCell, sending an UpLink (UL) transmission corresponding to an uplink grant, and setting a HARQ FEEDBACK variable corresponding to the UL transmission to Acknowledgement (ACK) if a HARQ feedback reception corresponding to the UL transmission is prohibited due to RF retuning.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 shows a diagram of a wireless communication system according to one exemplary embodiment.

[0008] FIG. 2 shows a user plane protocol stack of the wireless communication system of FIG. 1 according to one exemplary embodiment.

[0009] FIG. 3 shows a control plane protocol stack of the wireless communication system of FIG. 1 according to one exemplary embodiment.

[0010] FIG. 4 is a block diagram of a transmitter system (also known as access network) and a receiver system (also known as UE) according to one exemplary embodiment.

[0011] FIG. 5 is a functional block diagram of a UE according to one exemplary embodiment.

[0012] FIG. 6 shows a method for handling HARQ retransmission according to an exemplary embodiment.

DETAILED DESCRIPTION

[0013] The exemplary wireless communication systems and devices described below employ a wireless communication system, supporting a broadcast service. Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), orthogonal frequency division multiple access (OFDMA), 3GPP LTE (Long Term Evolution) wireless access, 3GPP LTE-A (Long Term Evolution Advanced), 3GPP2 UMB (Ultra Mobile Broadband), WiMax, or some other modulation techniques.

[0014] In particular, The exemplary wireless communication systems devices described below may be designed to support one or more standards such as the standard offered by a consortium named "3rd Generation Partnership Project" referred to herein as 3GPP, including Document Nos. R2-104181 and R2-105220. The documents listed above are hereby expressly incorporated herein.

[0015] An exemplary network structure of an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) 100 as a mobile communication system is shown in FIG. 1 according to one exemplary embodiment. The E-UTRAN system can also be referred to as a LTE (Long-Term Evolution) system or LTE-A (Long-Term Evolution Advanced). The E-UTRAN generally includes eNode B or eNB 102, which function similar to a base station in a mobile voice communication network. Each eNB is connected by X2 interfaces. The eNBs are connected to terminals or user equipment (UE) 104 through a radio interface, and are connected to Mobility Management Entities (MME) or Serving Gateway (S-GW) 106 through S1 interfaces.

[0016] Referring to FIGS. 2 and 3, the LTE system is divided into control plane 108 protocol stack (shown in FIG. 3) and user plane 110 protocol stack (shown in FIG. 2) according to one exemplary embodiment. The control plane performs a function of exchanging a control signal between a UE and an eNB and the user plane performs a function of transmitting user data between the UE and the eNB. Referring to FIGS. 2 and 3, both the control plane and the user plane include a Packet Data Convergence Protocol (PDCP) layer, a Radio Link Control (RLC) layer, a Medium Access Control (MAC) layer and a physical (PHY) layer. The control plane additionally includes a Radio Resource Control (RRC) layer. The control plane also includes a Network Access Stratum (NAS) layer, which performs among other things including Evolved Packet System (EPS) bearer management, authentication, and security control.

[0017] The PHY layer provides information transmission service using a radio transmission technology and corresponds to a first layer of an open system interconnection (OSI) layer. The PHY layer is connected to the MAC layer through a transport channel. Data exchange between the MAC layer and the PHY layer is performed through the transport channel. The transport channel is defined by a scheme through which specific data are processed in the PHY layer.

[0018] The MAC layer performs the function of sending data transmitted from a RLC layer through a logical channel to the PHY layer through a proper transport channel and further performs the function of sending data transmitted from the PHY layer through a transport channel to the RLC layer through a proper logical channel. Further, the MAC layer inserts additional information into data received through the logical channel, analyzes the inserted additional information from data received through the transport channel to perform a proper operation and controls a random access operation.

[0019] The MAC layer and the RLC layer are connected to each other through a logical channel. The RLC layer controls the setting and release of a logical channel and may operate in one of an acknowledged mode (AM) operation mode, an unacknowledged mode (UM) operation mode and a transparent mode (TM) operation mode. Generally, the RLC layer divides Service Data Unit (SDU) sent from an upper layer at a proper size and vice versa. Further, the RLC layer takes charge of an error correction function through an automatic retransmission request (ARQ).

[0020] The PDCP layer is disposed above the RLC layer and performs a header compression function of data transmitted in an IP packet form and a function of transmitting data without loss even when an eNB providing a service changes due to the movement of a UE.

[0021] The RRC layer is only defined in the control plane. The RRC layer controls logical channels, transport channels and physical channels in relation to establishment, re-configuration and release of Radio Bearers (RBs). Here, the RB signifies a service provided by the second layer of an OSI layer for data transmissions between the terminal and the E-UTRAN. If an RRC connection is established between the RRC layer of a UE and the RRC layer of the radio network, the UE is in the RRC connected mode. Otherwise, the UE is in an RRC idle mode.

[0022] FIG. 4 is a simplified block diagram of an exemplary embodiment of a transmitter system 210 (also known as the access network) and a receiver system 250 (also known as access terminal or UE in a MIMO system 200. At the transmitter system 210, traffic data for a number of data streams is provided from a data source 212 to a transmit (TX) data processor 214.

[0023] In one embodiment, each data stream is transmitted over a respective transmit antenna. TX data processor 214 formats, codes, and interleaves the traffic data for each data stream based on a particular coding scheme selected for that data stream to provide coded data.

[0024] The coded data for each data stream may be multiplexed with pilot data using OFDM techniques. The pilot data is typically a known data pattern that is processed in a known manner and may be used at the receiver system to estimate the channel response. The multiplexed pilot and coded data for each data stream is then modulated (i.e., symbol mapped) based on a particular modulation scheme (e.g., BPSK, QPSK, M-PSK, or M-QAM) selected for that data stream to provide modulation symbols. The data rate, coding, and modulation for each data stream may be determined by instructions performed by processor 230.

[0025] The modulation symbols for all data streams are then provided to a TX MIMO processor 220, which may further process the modulation symbols (e.g., for OFDM). TX MIMO processor 220 then provides N.sub.T modulation symbol streams to N.sub.T transmitters (TMTR) 222a through 222t. In certain embodiments, TX MIMO processor 220 applies beam forming weights to the symbols of the data streams and to the antenna from which the symbol is being transmitted.

[0026] Each transmitter 222 receives and processes a respective symbol stream to provide one or more analog signals, and further conditions (e.g., amplifies, filters, and upconverts) the analog signals to provide a modulated signal suitable for transmission over the MIMO channel. N.sub.T modulated signals from transmitters 222a through 222t are then transmitted from N.sub.T antennas 224a through 224t, respectively.

[0027] At receiver system 250, the transmitted modulated signals are received by N.sub.R antennas 252a through 252r and the received signal from each antenna 252 is provided to a respective receiver (RCVR) 254a through 254r. Each receiver 254 conditions (e.g., filters, amplifies, and downconverts) a respective received signal, digitizes the conditioned signal to provide samples, and further processes the samples to provide a corresponding "received" symbol stream.

[0028] An RX data processor 260 then receives and processes the N.sub.R received symbol streams from N.sub.R receivers 254 based on a particular receiver processing technique to provide N.sub.T "detected" symbol streams. The RX data processor 260 then demodulates, deinterleaves, and decodes each detected symbol stream to recover the traffic data for the data stream. The processing by RX data processor 260 is complementary to that performed by TX MIMO processor 220 and TX data processor 214 at transmitter system 210.

[0029] A processor 270 periodically determines which pre-coding matrix to use (discussed below). Processor 270 formulates a reverse link message comprising a matrix index portion and a rank value portion.

[0030] The reverse link message may comprise various types of information regarding the communication link and/or the received data stream. The reverse link message is then processed by a TX data processor 238, which also receives traffic data for a number of data streams from a data source 236, modulated by a modulator 280, conditioned by transmitters 254a through 254r, and transmitted back to transmitter system 210.

[0031] At transmitter system 210, the modulated signals from receiver system 250 are received by antennas 224, conditioned by receivers 222, demodulated by a demodulator 240, and processed by a RX data processor 242 to extract the reserve link message transmitted by the receiver system 250. Processor 230 then determines which pre-coding matrix to use for determining the beamforming weights then processes the extracted message.

[0032] Turning to FIG. 5, this figure shows an alternative simplified functional block diagram of a communication device according to one exemplary embodiment. The communication device 300 in a wireless communication system can be utilized for realizing the UE 104 in FIG. 1, and the wireless communications system is preferably the LTE system, the LTE-A system or the like. The communication device 300 may include an input device 302, an output device 304, a control circuit 306, a central processing unit (CPU) 308, a memory 310, a program code 312, and a transceiver 314. The program code 312 includes the application layers and the layers of the control plane 108 and layers of user plane 110 as discussed above except the PHY layer. The control circuit 306 executes the program code 312 in the memory 310 through the CPU 308, thereby controlling an operation of the communications device 300. The communications device 300 can receive signals input by a user through the input device 302, such as a keyboard or keypad, and can output images and sounds through the output device 304, such as a monitor or speakers. The transceiver 314 is used to receive and transmit wireless signals, delivering received signals to the control circuit 306, and outputting signals generated by the control circuit 306 wirelessly.

[0033] Hybrid Automatic Repeat reQuest (HARQ) is a scheme for re-transmitting a traffic data packet to compensate for an incorrectly received traffic packet. A HARQ scheme is used both in UL and DL. In UL transmissions, for example, for each UL packet correctly received by an eNB a positive Acknowledgment (ACK) is transmitted on a Physical Hybrid ARQ Indicator Channel (PHICH) from the eNB to the UE. If a packet is not received correctly, an eNB HARQ entity transmits a Negative Acknowledgement (NACK) on the PHICH in order to request a non-adaptive retransmission of the erroneously received packet. Alternatively, the eNB may transmit a PDCCH carrying an adaptive grant to request an adaptive retransmission from the UE. Thus, the UE performs uplink retransmission according to the HARQ feedback and PDCCH received from the eNB. For implementation, the UE uses a HARQ FEEDBACK variable to store the HARQ feedback received from the eNB and this variable is initialized with NACK.

[0034] The LTE DownLink (DL) transmission scheme is based on Orthogonal Frequency Division Multiple Access (OFDMA), and the LTE UpLink (UL) transmission scheme is based on Single-Carrier (SC) Discrete Fourier Transform (DFT)-spread OFDMA (DFT-S-OFDMA) or equivalently, Single Carrier Frequency Division Multiple Access (SC-FDMA). LTE-Advanced (LTE-A), however, is designed to meet higher bandwidth requirements both in the DL and UL directions. In order to provide the higher bandwidth requirements, LTE-A utilizes component carrier aggregation. A user equipment (UE) with reception and/or transmission capabilities for carrier aggregation (CA) can simultaneously receive and/or transmit on multiple component carriers (CCs). A carrier may be defined by a bandwidth and a center frequency.

[0035] There are several physical control channels used in the physical layer that are relevant to CA operations. A physical downlink control channel (PDCCH) may inform the UE about the resource allocation of paging channel (PCH) and downlink shared channel (DL-SCH), about HARQ information related to DL-SCH. The PDCCH may carry the uplink scheduling grant which informs the UE about resource allocation of uplink transmission. A physical control format indicator channel (PCFICH) informs the UE about the number of OFDM symbols used for the PDCCHs and is transmitted in every subframe. A physical Hybrid ARQ Indicator Channel (PHICH) carries HARQ ACK/NAK signals in response to uplink transmissions. A physical uplink control channel (PUCCH) carries uplink control information such as HARQ ACK/NAK signals in response to downlink transmission, scheduling request and channel quality indicator (CQI). A physical uplink shared channel (PUSCH) carries uplink shared channel (UL-SCH).

[0036] In LTE, a Primary Cell includes the cell operating in the primary frequency in which the UE either performs the initial connection establishment procedure or initiates the connection re-establishment procedure, or the cell indicated as the primary cell in the handover procedure. The UE also uses the PCell to derive the parameters for the security functions and for upper layer system information such as NAS mobility information. A Secondary Cell (SCell) includes the cell operating on a secondary frequency which may be configured once an RRC connection is established and which may be used to provide additional radio resources. System information relevant for operation in the concerned SCell is typically provided using dedicated signaling when the SCell is added to the UE's configuration. Basically a PCell contains an uplink PCC and a downlink PCC, while an SCell configured to a UE may contain a downlink SCC or an uplink SCC along with a downlink SCC.

[0037] At Scell activation/deactivation transitions or measurements on a deactivated Scell, the corresponding Radio Frequency (RF) front end needs to be retuned to adapt the RF bandwidth for the concerned SCell. During RF retuning, all other SCells sharing the same RF front end as the concerned SCell would not be able to receive the HARQ feedback from eNB for an UL transmission because HARQ feedback reception by the UE is prohibited during RF retuning. If the UL transmission has been received successfully, there is no need for the UE to do a retransmission. Otherwise, retransmission is needed. Without reception of the HARQ feedback, it is difficult for the UE to know whether or not the UL transmission has been received successfully to take proper action.

[0038] According to the various embodiments, the UE just sets the HARQ FEEDBACK variable corresponding to an UL transmission to ACK during RF retuning. Accordingly, the eNB has to decide whether retransmission is needed. For example, the eNB can request an adaptive retransmission by a PDCCH signaling if the UL transmission is not successful.

[0039] Referring to FIG. 6, a method 400 for handling HARQ retransmission according to an exemplary embodiment is shown. The method 400 includes a UE being configured with at least one SCell via a RRC procedure by an eNB at 402. The UE receives a PDCCH with an uplink grant at 404 and accordingly sends an uplink transmission at 406. The HARQ feedback corresponding to the uplink transmission occurs at 408 during a period of RF retuning in the UE. If the HARQ feedback reception corresponding to an UL transmission is prohibited due to RF retuning, the UE sets the HARQ FEEDBACK variable corresponding to the UL transmission to ACK at 410. Then, at 412 the eNB may transmit a PDCCH with an adaptive uplink grant to the UE if the uplink transmission has not been received correctly by the eNB. Accordingly, the UE sends an uplink retransmission at 414. No adaptive uplink grant will be received by the UE if the uplink transmission has been received correctly by the eNB.

[0040] In one embodiment, the HARQ FEEDBACK variable is initialized with NACK. A non-adaptive retransmission corresponding to an UL transmission will be generated in the UE if the HARQ FEEDBACK variable corresponding to the UL transmission is set to NACK and no PDCCH with adaptive UL grant for the UL transmission is received. As a result, if the eNB has received the uplink transmission correctly and the corresponding HARQ feedback of ACK is not received by the UE due to RF retuning, the UE may generate a non-adaptive retransmission corresponding to the UL transmission unnecessarily and it may cause interference to other UE's transmission. In contrast, a non-adaptive retransmission corresponding to the UL transmission will not be generated in the UE if the HARQ FEEDBACK variable corresponding to an UL transmission is set to ACK. Therefore, by the UE setting the HARQ FEEDBACK variable to ACK during RF retuning, the UE is no longer concerned with not receiving HARQ feedback and unnecessary UL retransmission as well as potential interference to other UE's transmission can be avoided.

[0041] In another embodiment, the corresponding RF front end is retuned to adapt the RF bandwidth at Scell activation/deactivation transitions or measurements on a deactivated Scell.

[0042] Referring to FIG. 5, which is a functional block diagram of a UE according to one exemplary embodiment, the UE 300 includes a program code 312 stored in memory 310. The CPU 308 executes the program code 312 to handle HARQ retransmission by being configured with at least one SCell, sending an UpLink (UL) transmission corresponding to an uplink grant, and setting the HARQ FEEDBACK variable corresponding to the UL transmission to ACK if the HARQ feedback reception corresponding to the UL transmission is prohibited due to RF retuning. The CPU 308 can also execute the program code 312 to perform the additional steps of method 400 as described herein.

[0043] Various aspects of the disclosure have been described above. It should be apparent that the teachings herein may be embodied in a wide variety of forms and that any specific structure, function, or both being disclosed herein is merely representative. Based on the teachings herein one skilled in the art should appreciate that an aspect disclosed herein may be implemented independently of any other aspects and that two or more of these aspects may be combined in various ways. For example, an apparatus may be implemented or a method may be practiced using any number of the aspects set forth herein. In addition, such an apparatus may be implemented or such a method may be practiced using other structure, functionality, or structure and functionality in addition to or other than one or more of the aspects set forth herein. As an example of some of the above concepts, in some aspects concurrent channels may be established based on pulse repetition frequencies. In some aspects concurrent channels may be established based on pulse position or offsets. In some aspects concurrent channels may be established based on time hopping sequences. In some aspects concurrent channels may be established based on pulse repetition frequencies, pulse positions or offsets, and time hopping sequences.

[0044] Those of skill in the art would understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof.

[0045] Those of skill would further appreciate that the various illustrative logical blocks, modules, processors, means, circuits, and algorithm steps described in connection with the aspects disclosed herein may be implemented as electronic hardware (e.g., a digital implementation, an analog implementation, or a combination of the two, which may be designed using source coding or some other technique), various forms of program or design code incorporating instructions (which may be referred to herein, for convenience, as "software" or a "software module"), or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present disclosure.

[0046] In addition, the various illustrative logical blocks, modules, and circuits described in connection with the aspects disclosed herein may be implemented within or performed by an integrated circuit ("IC"), an access terminal, or an access point. The IC may comprise a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, electrical components, optical components, mechanical components, or any combination thereof designed to perform the functions described herein, and may execute codes or instructions that reside within the IC, outside of the IC, or both. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.

[0047] It is understood that any specific order or hierarchy of steps in any disclosed process is an example of a sample approach. Based upon design preferences, it is understood that the specific order or hierarchy of steps in the processes may be rearranged while remaining within the scope of the present disclosure. The accompanying method claims present elements of the various steps in a sample order, and are not meant to be limited to the specific order or hierarchy presented.

[0048] The steps of a method or algorithm described in connection with the aspects disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module (e.g., including executable instructions and related data) and other data may reside in a data memory such as RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, a hard disk, a removable disk, a CD-ROM, or any other form of computer-readable storage medium known in the art. A sample storage medium may be coupled to a machine such as, for example, a computer/processor (which may be referred to herein, for convenience, as a "processor") such the processor can read information (e.g., code) from and write information to the storage medium. A sample storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in user equipment. In the alternative, the processor and the storage medium may reside as discrete components in user equipment. Moreover, in some aspects any suitable computer-program product may comprise a computer-readable medium comprising codes relating to one or more of the aspects of the disclosure. In some aspects a computer program product may comprise packaging materials.

[0049] While the invention has been described in connection with various aspects, it will be understood that the invention is capable of further modifications. This application is intended to cover any variations, uses or adaptation of the invention following, in general, the principles of the invention, and including such departures from the present disclosure as come within the known and customary practice within the art to which the invention pertains.

Claims

1. A method for handling Hybrid Automatic Repeat reQuest (HARQ) retransmission in a User Equipment (UE) comprising: being configured with at least one Secondary Cell (SCell); sending an UpLink (UL) transmission corresponding to an uplink grant; and setting a HARQ FEEDBACK variable corresponding to the UL transmission to Acknowledgement (ACK) if a HARQ feedback reception corresponding to the UL transmission is prohibited due to RF retuning.

2. The method of claim 1, wherein the uplink grant is carried by a physical downlink control channel (PDCCH) received from an eNB.

3. The method of claim 1, wherein the HARQ FEEDBACK variable is initialized with Negative Acknowledgement (NACK).

4. The method of claim 1, wherein the RF retuning occurs at SCell activation/deactivation transitions.

5. The method of claim 1, wherein the RF retuning occurs at measurements on a deactivated SCell.

6. The method of claim 1, wherein a non-adaptive retransmission corresponding to the UL transmission is not generated in the UE if the HARQ FEEDBACK variable corresponding to the UL transmission is set to ACK.

7. A User Equipment (UE) for use in a wireless communication system, the UE comprising: a control circuit; a processor installed in the control circuit; and a memory installed in the control circuit and coupled to the processor; wherein the processor is configured to execute a program code stored in memory to handle a Hybrid Automatic Repeat reQuest (HARQ) retransmission by: being configured with at least one Secondary Cell (SCell); sending an UpLink (UL) transmission corresponding to an uplink grant; and setting a HARQ FEEDBACK variable corresponding to the UL transmission to Acknowledgement (ACK) if a HARQ feedback reception corresponding to the UL transmission is prohibited due to RF retuning.

8. The device of claim 7, wherein the uplink grant is carried by a physical downlink control channel (PDCCH) received from an eNB.

9. The device of claim 7, wherein the HARQ FEEDBACK variable is initialized with Negative Acknowledgement (NACK).

10. The device of claim 7, wherein the RF retuning occurs at SCell activation/deactivation transitions.

11. The device of claim 7, wherein the RF retuning occurs at measurements on a deactivated SCell.

12. The device of claim 7, wherein a non-adaptive retransmission corresponding to the UL transmission is not generated in the UE if the HARQ FEEDBACK variable corresponding to the UL transmission is set to ACK.