Abstract:
Methods and apparatus are described for performing automatic repeat request (ARQ) and hybrid-ARQ (HARQ) assisted ARQ procedures in a relay-based wireless communication system. Triggers for radio link control (RLC)/ARQ retransmissions and RLC/ARQ status reporting are also described.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/188,716 filed Aug. 11, 2008, which is incorporated by reference as if fully set forth. 
     
    
     FIELD OF INVENTION 
       [0002]    This application is related to wireless communications. 
       BACKGROUND 
       [0003]      FIG. 1  shows a third generation partnership project (3GPP) long term evolution (LTE) user-plane protocol stack  100 . A wireless transmit/receive unit (WTRU)  105  and a Node-B  110  transmit and receive data at different protocols/levels, (e.g., physical (PHY) layer  115 , media access control (MAC) layer  120 , radio link control (RLC) layer  125 , and packet data convergence protocol (PDCP) layer  130 ). Each protocol layer performs a variety of functions. 
         [0004]    The MAC layer  120  provides a hybrid automatic repeat request (HARQ) retransmission functionality whereby a transmitting MAC/HARQ entity retransmits failed MAC/HARQ protocol data units (PDUs), depending on the HARQ positive acknowledgement (ACK)/negative acknowledgement (NACK) feedback that is transmitted by a receiving MAC/HARQ entity. 
         [0005]    The RLC layer  125  provides ARQ retransmission functionality whereby the transmitting side of an acknowledged mode (AM) RLC entity retransmits any failed RLC PDUs based an RLC status report transmitted by the receiving side of the AM RLC entity, or based on an indication of a failed MAC/HARQ delivery from a transmitting MAC/HARQ entity. 
         [0006]      FIG. 2  is an example illustrating ARQ and HARQ-assisted ARQ operations in LTE.  FIG. 2  shows a wireless communication system  200  including a transmitter  205  and a receiver  210 . The transmitter  205  includes an RLC/ARQ unit  215  and a MAC/HARQ unit  220 . The receiver  210  includes an RLC/ARQ unit  225  and a MAC/HARQ unit  230 . The term “transmitter” refers to a transmitting node, which resides in a WTRU for uplink data and a Node-B for downlink data. The term “receiver” refers to a receiving node, which resides in a Node-B for uplink data and a WTRU for downlink data. 
         [0007]    An HARQ failure occurs if an HARQ entity does not receive a positive HARQ ACK after a predetermined number of HARQ transmissions. To simplify the example, assume that HARQ delivery failure occurs if the HARQ entity transmits the HARQ PDU twice and does not receive an HARQ ACK. 
         [0008]    The RLC/ARQ entity  215  in the transmitter  205  creates an RLC PDU and submits it to the MAC/HARQ unit  220 , also in the transmitter  205 . The MAC/HARQ unit  220  then transmits a MAC PDU that contains the RLC PDU a predetermined number of times, (e.g. twice), unsuccessfully. Hence, the HARQ process fails to deliver the MAC PDU to the receiver  210 . The HARQ process failure triggers a local NACK, (i.e., HARQ assisted ARQ), indication, whereby the MAC/HARQ unit  220  notifies the RLC/ARQ unit  215  of the failed delivery of the RLC PDU. The RLC/ARQ unit  215  initiates an ARQ retransmission of the failed RLC PDU, and submits the retransmitted RLC PDU to the MAC/HARQ unit  220 . The MAC/HARQ unit  220  then transmits a MAC PDU that contains the RLC PDU once unsuccessfully. An error may occur on the HARQ feedback, whereby the NACK transmitted by the receiver  210  is erroneously received as an ACK at the transmitter  205 . The RLC/ARQ unit  225  in the receiver  210  may transmit an RLC/ARQ status report that positively or negatively acknowledges data, (i.e., ARQ ACK/NACK). The RLC/ARQ status report may be transmitted in several steps, (i.e., via MAC/HARQ, and the like), but for simplifying  FIG. 2 , it is shown via an end-to-end line. The transmitter  205  checks the RLC/ARQ status report it received, and determines that the RLC PDU is not positively acknowledged. Consequently, the RLC/ARQ unit  215  initiates an ARQ retransmission of the failed RLC PDU, and submits the retransmitted RLC PDU to the MAC/HARQ unit  220 . The MAC/HARQ unit  220  transmits the MAC PDU that contains the RLC PDU a predetermined number of attempts, and is successful. The MAC/HARQ unit  230  in the receiver  210  delivers the successfully received packet to the RLC/ARQ unit  225 . The RLC/ARQ unit  225  may transmit an RLC/ARQ status report that positively or negatively acknowledges the data. The transmitter  205  checks the RLC/ARQ status report it received, and determines that the RLC PDU is positively acknowledged. Consequently, successful delivery is confirmed, and no further ARQ retransmission is required. 
         [0009]    The procedure shown in  FIG. 2  is simplified to illustrate an exemplary procedure, however, more functions may be performed. For example, RLC re-segmentation may be performed, whereby instead of re-transmitting a whole RLC PDU in one transmission, the RLC PDU may be re-segmented into multiple RLC PDU segments. 
         [0010]    In some implementations, a local NACK (HARQ assisted ARQ) may not be implemented, and in this case an ARQ retransmission will only be triggered via RLC/ARQ status reports. 
         [0011]    Recently proposals have been introduced for LTE-advanced, which features additional improvements to LTE. LTE-advanced (LTE-A) will present a significant enhancement over LTE, e.g., peak data rates of 0.5 Gbps in uplink and 1.0 Gbps in downlink. 
         [0012]    The use of “relays” is one of the technologies being considered for LTE advanced.  FIG. 3  shows exemplary uses of relays in a cellular communication system. When referred to herein, the term ‘relay’ may refer to a “relay node”, or an intermediary node, that may provide a link between a Node-B and a WTRU. 
         [0013]    Accordingly, effective, efficient and fast ARQ retransmissions with relays are desired. 
       SUMMARY 
       [0014]    Methods and apparatus are described for performing ARQ and HARQ assisted ARQ procedures in a relay-based wireless communication system. Triggers for RLC/ARQ retransmissions and RLC/ARQ status reporting are also described. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    A more detailed understanding may be had from the following description, given by way of example in conjunction with the accompanying drawings wherein: 
           [0016]      FIG. 1  shows an LTE user-plane protocol stack; 
           [0017]      FIG. 2  shows an example HARQ and ARQ operations in LTE; 
           [0018]      FIG. 3  shows an example of relays in a cellular network; 
           [0019]      FIG. 4  shows an example of a wireless communication system including a plurality of WTRUs, a base station, and a radio network controller (RNC); 
           [0020]      FIG. 5  is a functional block diagram of a WTRU and the base station of  FIG. 4 ; 
           [0021]      FIG. 6  shows a relay that has MAC and PHY functions; 
           [0022]      FIG. 7  shows a relay that has HARQ and PHY functions; 
           [0023]      FIG. 8  shows HARQ and ARQ operations in LTE with a relay; and 
           [0024]      FIGS. 9-14  show examples of enhanced HARQ assisted ARQ operations. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    When referred to hereafter, the terminology “wireless transmit/receive unit (WTRU)” includes but is not limited to a user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, a cellular telephone, a personal digital assistant (PDA), a computer, or any other type of user device capable of operating in a wireless environment. When referred to hereafter, the terminology “base station” includes but is not limited to a Node-B, a site controller, an access point (AP), or any other type of interfacing device capable of operating in a wireless environment. 
         [0026]      FIG. 4  shows a wireless communication system  400  including a plurality of WTRUs  410 , a base station  420 , and an RNC  430 . As shown in  FIG. 4 , the WTRUs  410  are in communication with the base station  420 , which is in communication with the RNC  430 . Although three WTRUs  410 , one base station  420 , and one RNC  430  are shown in  FIG. 4 , it should be noted that any combination of wireless and wired devices may be included in the wireless communication system  400 . For example, although the RNC  430  is shown in the wireless communication system  400 , the RNC  430  may not be included in an LTE system. 
         [0027]      FIG. 5  is a functional block diagram  500  of a WTRU  410  and the base station  420  of the wireless communication system  400  of  FIG. 4 . As shown in  FIG. 5 , the WTRU  410  is in communication with the base station  420  and both are configured to perform a method of ARQ and HARQ assisted ARQ enhancements for relay-based wireless communications. 
         [0028]    In addition to the components that may be found in a typical WTRU, the WTRU  410  includes a processor  415 , a receiver  416 , a transmitter  417 , and an antenna  418 . The processor  415  is configured to perform a method for ARQ and HARQ assisted ARQ enhancements for relay-based wireless communications. The receiver  416  and the transmitter  417  are in communication with the processor  415 . The antenna  418  is in communication with both the receiver  416  and the transmitter  417  to facilitate the transmission and reception of wireless data. 
         [0029]    In addition to the components that may be found in a typical base station, the base station  420  includes a processor  425 , a receiver  426 , a transmitter  427 , and an antenna  428 . The processor  425  is configured to perform a method for ARQ and HARQ assisted ARQ enhancements for relay-based wireless communications. The receiver  426  and the transmitter  427  are in communication with the processor  425 . The antenna  428  is in communication with both the receiver  526  and the transmitter  427  to facilitate the transmission and reception of wireless data. 
         [0030]      FIG. 6  shows a relay incorporating MAC and PHY functions. MAC and PHY transmissions from the Node-B and WTRU are locally terminated at the relay node. RLC and PDCP transmissions can be transparent to the relay node. 
         [0031]      FIG. 7  shows a relay that provides PHY and HARQ functionalities. HARQ and PHY transmissions from the Node-B and WTRU are locally terminated at the relay node. Other MAC, RLC and PDCP transmissions can be transparent to the relay node. The relay may further provide other MAC functionalities (in addition to HARQ) as shown in  FIG. 6 . Other relay architectures may also be used, such as a relay that provides even higher layer functionalities, (i.e., RLC or PDCP functionalities), or a relay that provides only a PHY, (i.e., with no HARQ), functionality. In this case, other higher layer protocols, (i.e., RLC and PDCP), could also be locally terminated at the relay node. The HARQ and MAC protocol termination in the relay node are not necessarily affected by the possible termination of these higher layer protocols in the relay node. 
         [0032]    The WTRU and the Node-B may be configured to provide the PHY, MAC, RLC and PDCP functionalities as shown in  FIG. 6  and  FIG. 7 . 
         [0033]    The HARQ layer is generally modeled as part of the MAC layer.  FIG. 7 , however, shows the HARQ layer in its own box in order to highlight such function. 
         [0034]      FIG. 8  shows an example of HARQ and ARQ operations in LTE using a relay.  FIG. 8  shows a wireless communication system  800  including a transmitter  805 , a receiver  810  and a relay  815 . The transmitter  805  includes an RLC/ARQ unit  825  and a MAC/HARQ unit  830 . The relay  815  includes MAC/HARQ units  835  and  840 . The receiver  810  includes an RLC/ARQ unit  845  and a MAC/HARQ unit  850 . The term MAC PDU Y is used to indicate any MAC PDU that contains the data of RLC PDU X. The multiple ARQ retransmissions of RLC PDU X may be encapsulated in different MAC PDUs each time an RLC/ARQ retransmission is performed, but the term MAC PDU Y may be used to refer to any MAC PDU that contains RLC PDU X or a portion of RLC PDU X. The MAC PDU Y may either concatenate several RLC PDUs including RLC PDU X, or segment RLC PDU X into several MAC PDU Y&#39;s. 
         [0035]    Referring to  FIG. 8 , the RLC/ARQ unit  825  in the transmitter  805  generates an RLC PDU X and submits it to the MAC/HARQ unit  830  via signal  855 . The MAC/HARQ unit  830  then transmits a MAC PDU Y that contains the RLC PDU X, (or a portion for the RLC PDU), to the MAC/HARQ unit  835  in the relay  815  a predetermined number of times via signals  860 , (e.g., twice in this example), unsuccessfully. However, the HARQ process fails to successfully deliver the MAC PDU Y to the relay  815 , as indicated by HARQ NACK signals  865  sent by the MAC/HARQ  835  in the relay  815  to the transmitter  805 . The HARQ process failure triggers a local NACK  870 , (i.e., HARQ assisted ARQ), indication, whereby the MAC/HARQ unit  830  notifies the RLC/ARQ unit  825  of the failed delivery of the MAC PDU Y. The RLC/ARQ unit  825  initiates an ARQ retransmission of the failed RLC PDU X, and submits the retransmitted RLC PDU X to the MAC/HARQ unit  830  via signal  872 . The MAC/HARQ unit  830  in the transmitter  805  successfully transmits the MAC PDU Y that contains the RLC PDU X to the MAC/HARQ unit  835  in the relay  815  via signal  874 , as indicated by HARQ ARQ signal  876 . In the relay  815 , the MAC/HARQ unit  835  forwards the MAC PDU Y to the MAC/HARQ unit  840  via signal  878 . The MAC/HARQ unit  840  in the relay  815  unsuccessfully transmits the MAC PDU Y that contains the RLC PDU X to the receiver  810  a predetermined number of times  880 , as indicated by HARQ NACK signals  882  sent by the MAC/HARQ unit  850  in the receiver  810  to the MAC/HARQ unit  840  in the relay  815 . Hence, the HARQ process fails to deliver the MAC PDU Y to the receiver  810 . 
         [0036]    The RLC/ARQ unit  845  in the receiver  810  may transmit an RLC/ARQ status report  884  that positively or negatively acknowledges data to the RLC/ARQ unit  825  in the transmitter  805 . Then, the transmitter  805  checks the RLC/ARQ status report  884 , and if the RLC PDU X was not positively acknowledged, the RLC/ARQ unit  825  in the transmitter  805  initiates ARQ retransmission  886  of the RLC PDU X, and submits the retransmitted RLC PDU X to the MAC/HARQ unit  830  in the transmitter  805 . The MAC/HARQ unit  830  transmits the MAC PDU Y containing the RLC PDU X to the to the MAC/HARQ unit  835  in the relay  815  a predetermined number of times  888 , and is successful as indicated by HARQ ACK signals  890  sent by the MAC/HARQ unit  835  in the relay  815  to the MAC/HARQ unit  830  in the transmitter  805 . 
         [0037]    In the relay  815 , the MAC/HARQ unit  835  relays the MAC PDU Y to the MAC/HARQ unit  840  via signal  892 . The MAC/HARQ unit  840  in the relay  815  transmits the MAC PDU Y that contains the RLC PDU X to the MAC/HARQ unit  850  in the receiver  810  a predetermined number of times  894 , (e.g., once in this example), and is successful, as indicated by HARQ ACK signal  896  sent by the MAC/HARQ unit  850  in the receiver  810  to the MAC/HARQ unit  840  in the relay  815 . In the receiver  810 , the MAC/HARQ unit  850  delivers the successfully received packet to the RLC/ARQ unit  845  via signal  898 . The RLC/ARQ unit  845  in the receiver  810  may transmit to the RLC/ARQ unit  825  in the transmitter  805  an RLC/ARQ status report  899  that positively or negatively acknowledges data. The transmitter  805  checks the RLC/ARQ status report and, if the RLC PDU is positively acknowledged, no further ARQ retransmission is required. 
         [0038]    Enhanced System Operation 
         [0039]    In some cases, additional time may be required for ARQ retransmission when a HARQ delivery failure occurs on the HARQ process between the relay  815  and the receiver  810 . For example, additional time may be required to generate an RLC/ARQ status report that identifies that the RLC PDU was not successfully received by the RLC/ARQ unit  845  in the receiver  810 , which will delay the eventual ARQ recovery. In order to speed up the ARQ retransmissions, enhancements are proposed as follows. 
         [0040]    Signal from Relay to Transmitter in order to Trigger ARQ by the Transmitter 
         [0041]    As shown in  FIG. 9 , upon a MAC/HARQ failure of the HARQ process that operates between the MAC/HARQ unit  840  in the relay  815  and the MAC/HARQ unit  850  in the receiver  810 , the MAC/HARQ unit  840  may create and transmit an ARQ triggering signal  905  to the RLC/ARQ unit  825  in the transmitter  805 . The ARQ triggering signal  905  may be implemented in any protocol or layer, (e.g. a MAC control element, an RLC control PDU, a radio resource control (RRC) message/information element (IE)), or in any other protocol or layer. The ARQ triggering signal may include one or more of the following information:
       1) the MAC logical channel identity (or logical channel identities) of those packets that were in the failed MAC PDU;   2) the identity (identifier) of the failed MAC PDU;   3) the transmission sequence number (TSN) of the failed MAC PDU;   4) the identity/identities of the RLC PDUs included in the failed MAC PDU, (e.g., RLC PDU SN and/or segment offset/length information);   5) the transmission time interval (TTI) of when failure occurred;   6) time of failure; and   7) HARQ process number of the failed HARQ process.       
 
         [0049]    If the HARQ process in the relay  815  fails, the relay  815  may transmit a signal with minimal information, (i.e., there is no need for the relay  815  to decode (or spoof) the PDU). Alternatively, the relay  815  may decode (or spoof) the MAC PDU, and transmit some or all of the MAC PDU header information to the transmitter  805 , or the relay  815  may decode (or spoof) the RLC PDU, and transmit some or all of the RLC PDU header information to the transmitter  805 . If the relay  815  spoofs the RLC PDU header, then the ARQ triggering signal  905  transmitted from the relay  815  to the transmitter  805  may have the form of an RLC status report, which is transmitted from the relay  815  as opposed to being transmitted from the receiver  810 . 
         [0050]    Alternatively, the relay  815  may not need to transmit the ARQ triggering signal  905  if all of the data in the MAC PDU is not in an acknowledged mode (AM), (i.e., since no ARQ is provided for unacknowledged mode (UM) traffic). 
         [0051]    In another alternative, before transmitting the ARQ triggering signal  905 , the relay  815  may check/verify if any further PDUs have been recently received from transmitter  805 . If the transmitter  805  recently retransmitted the packet that failed, then there is no need to transmit the ARQ triggering signal  905 . 
         [0052]    Upon receiving the ARQ triggering signal  905 , the RLC/ARQ unit  825  in the transmitter  805  may conduct RLC/ARQ retransmissions. If sufficient identifiers are provided in the ARQ triggering signal  905 , then the RLC/ARQ unit  825  may retransmit only the PDUs that are specified or implied from the provided identifiers. Alternatively, the RLC/ARQ unit  825  may estimate the RLC PDUs that need to be retransmitted, or can retransmit those RLC PDUs that have not yet been acknowledged by an RLC/ARQ status report  910 . Alternatively, the RLC/ARQ unit  825  may transmit a polling request to the receiver  810 , in order to receive an updated RLC/ARQ status report  910 , and conduct an RLC/ARQ retransmission(s) based on the information conveyed in the received RLC/ARQ status report  910 . 
         [0053]    The RLC/ARQ retransmission by the transmitter  805  may be conducted/triggered a local NACK  915 , (from the MAC/HARQ unit  830  to the RLC/ARQ unit  825  in the transmitter  805 ). Additionally, it may be triggered by an RLC/ARQ status report  910  transmitted by the receiver  810  to the transmitter  805 , or it may be triggered by an ARQ triggering signal  905  transmitted by the relay  815  to the transmitter  805 . 
         [0054]    In case more than one trigger is generated, conflicts may be handled using an RLC/ARQ status report trigger. The RLC/ARQ status report trigger may be configured to over-ride/supersede any other triggers. 
         [0055]    Alternatively, any trigger that contains a NACK may lead to PDU retransmission, or a timer may be used to prevent multiple retransmissions of the same PDU, if multiple triggers are received within a short time period. 
         [0056]    Signal from Relay to Receiver in order to Trigger Status Reporting by the Receiver 
         [0057]    RLC/ARQ status reporting by the receiver  810  may be conducted/triggered by a status report triggering signal transmitted by the relay to the receiver. 
         [0058]      FIG. 10  shows a proposed enhanced HARQ assisted ARQ operation. As shown in  FIG. 10 , upon MAC/HARQ failure of the HARQ process that runs between the relay  815  and the receiver  810 , the MAC/HARQ unit  840  in the relay  815  will generate and transmit a status report triggering signal  1005  to the receiver  810  in order to trigger transmission of an RLC status report. 
         [0059]    The status report triggering signal  1005  may include the MAC logical channel identity (or logical channel identities) of those packets that were in the failed MAC PDU. Alternatively, the status report triggering signal  1005  may include one or a combination of the following: 
         [0060]    1) identity (identifier) of the failed MAC PDU; 
         [0061]    2) TSN of the failed MAC PDU; 
         [0062]    3) identity/identities of the RLC PDUs included in the failed MAC PDU, (e.g., RLC PDU SN and/or segment offset/length information); 
         [0063]    4) TTI when failure occurred; 
         [0064]    5) time of failure; and 
         [0065]    6) HARQ process number of the failed HARQ process. 
         [0066]    When an HARQ process fails in the relay  815 , the relay  815  may be configured to transmit a signal with minimal information, (i.e., no need for relay to decode (or spoof) the PDU). Alternatively, the relay  815  may be configured to decode (or spoof) the MAC PDU, and transmit some or all of the MAC PDU header information to the receiver  810 , or the relay  815  may decode (or spoof) the RLC PDU, and transmit some or all of the RLC PDU header information to the receiver  810 . 
         [0067]    Alternatively, the relay  815  optionally does not need to transmit the status report triggering signal  1005  if all of the data in the MAC PDU Y is not associated with an RLC PDU X operating in AM, (i.e., not UM or transparent mode (TM)). 
         [0068]    Alternatively, before transmitting the status report triggering signal  1005 , the relay  815  may check/verify if any further PDUs have been recently received from the transmitter  805 . If the transmitter  805  recently retransmitted the packet that failed, then there is no need to transmit the status report triggering signal  1005 . 
         [0069]    Upon receiving the status report triggering signal  1005 , the RLC/ARQ unit  845  of the receiver  810  may generate an RLC/ARQ status report  1010 , (for the identified logical channels, or for all AM logical channels if detailed information identifying the specific RLC AM instances are not included in the status report triggering signal  1005 ), and transmit the RLC/ARQ status report  1010  to the transmitter  805 . The transmitter  805  may conduct an RLC/ARQ retransmission based on the information conveyed in the received RLC/ARQ status report  1010 . 
         [0070]    Additional Enhanced Schemes 
         [0071]      FIGS. 11-13  show alternative proposed enhanced schemes for the relay to trigger retransmissions when an HARQ process transmission failure occurs between the relay and the receiver. As shown in the Figures, an HARQ-NACK2 signal  1105  includes a new message that is transmitted from the MAC/HARQ unit  835  in the relay  815  to the MAC/HARQ unit  830  in the transmitter  805 . The NACK2 signal  1105  is used to indicate that the MAC PDU Y was not received correctly. This signal updates the status HARQ process transmission status of MAC PDU Y that may have already received successful HARQ feedback, (i.e., the HARQ status changes from ACK to NACK). 
         [0072]    In  FIG. 11 , the HARQ NACK 2 generates a Local NACK in the transmitter, which results in an RLC PDU X and associated MAC PDU Y retransmission to the relay. 
         [0073]    As shown in  FIGS. 12 and 13 , upon HARQ NACK 2 reception and optionally local NACK processing for RLC PDU X retransmission, instead of retransmitting the associated MAC PDU Y, a C-MAC PDU Y control signal  1205  is transmitted from the MAC/HARQ unit  830  in the transmitter  805  to the MAC/HARQ unit  835  in the relay  815 . The C-MAC PDU Y control signal  1205  is used to request a re-transmission of a MAC PDU Y, or some of the contents of MAC PDU Y. Since the relay has previously successfully received a MAC PDU Y that previously was not successfully sent to the receiver, it is not necessary to retransmit this data to the relay. The C-MAC PDU Y control signal  1205  requests the relay to retransmit the previously unsuccessfully transmitted MAC PDU Y to the receiver  810 . 
         [0074]    Referring to  FIGS. 12 and 13 , the relay  815  stores the MAC PDU Y, even after a HARQ retransmission failure in the relay  815 , since it may have to retransmit the MAC PDU Y upon receiving the C-MAC PDU Y control signal  1205  from the transmitter  805 . The exemplary procedures shown in  FIGS. 12 and 13  may provide improved efficiency since the transmitter  805  does need not to retransmit the MAC PDU Y or some of its contents to the relay  815 . Instead, the transmitter  805  transmits a control signal that requests/orders the relay to retransmit the stored PDU or some of its contents. 
         [0075]      FIG. 14  shows another proposed enhanced scheme whereby an ARQ triggering signal may be supported by existing HARQ feedback signaling. In this scheme, an HARQ status relay indicator  1405  is transmitted by the MAC/HARQ unit  840  to the MAC/HARQ unit  835  in the relay  815 . The HARQ operation in the transmitter  805  may be common with non-relay operation, (i.e., there is no need to apply new signaling for an ARQ triggering signal). This scheme may utilize asynchronous HARQ feedback since it may not be possible to guarantee the time of the relayed feedback from the receiver. The period from the HARQ transmission to transmission feedback may be variable. To support this, the HARQ process ID may be included in the feedback status. It may also be advantageous to apply feedback aggregation where feedback from multiple HARQ processes is identified in a single message. 
         [0076]    A method of using a relay  815  to provide PHY and HARQ functionalities to a transmitter  805  and a receiver  810  is described below. 
         [0077]    As shown in  FIG. 8 , a first RLC/ARQ unit  825  in the transmitter  805  generates an RLC PDU and forwards the RLC PDU to a first MAC/HARQ unit  830  in the transmitter  805  via signal  855 . The first MAC/HARQ unit  830  in the transmitter  805  sends a MAC PDU that contains at least a portion of the RLC PDU to a second MAC/HARQ unit  835  in the relay  815  via signal  860 . The second MAC/HARQ unit  835  in the relay  815  forwards the MAC PDU to a third MAC/HARQ unit  840  in the relay  815  via signal  878  and provides HARQ feedback to the first MAC/HARQ unit  830  in the transmitter  850  via signal  865  in response to receiving the MAC PDU. The third MAC/HARQ unit  840  in the relay  815  transmits the MAC PDU to a fourth MAC/HARQ unit  850  in the receiver  810  via signal  880 . The third MAC/HARQ unit  840  in the relay  815  receives HARQ feedback sent by the fourth MAC/HARQ unit  850  in response to receiving the MAC PDU via signal  882 . 
         [0078]    A second RLC/ARQ unit  845  in the receiver  810  may transmit an RLC/ARQ status report  884  to the first RLC/ARQ unit  825  in the transmitter  805  to initiate a retransmission of the RLC PDU on a condition that an HARQ transmission failure occurs at the receiver  810 . 
         [0079]    As shown in  FIG. 9 , the relay  815  may transmit an ARQ triggering signal  905  to the first RLC/ARQ unit  825  in the transmitter  805  to initiate a retransmission of the RLC PDU on a condition that an HARQ transmission failure occurs at the receiver  810 . 
         [0080]    The ARQ triggering signal  905  may be signaled by a MAC control element (CE), an RLC control PDU or an RRC message. The ARQ triggering signal  905  may include an RLC logical channel, an RLC sequence number (SN), RLC segment information or HARQ process information and time of failure. The ARQ triggering signal  905  may cause RLC status report polling by the transmitter  805 . 
         [0081]    The transmitter  805  may reside in a WTRU and the receiver  810  may reside in a Node-B. Alternatively, the transmitter  805  may reside in a Node-B and the receiver  810  may reside in a WTRU. 
         [0082]    As shown in  FIG. 10 , the relay  815  may transmit a status report triggering signal  1005  to the second RLC/ARQ unit  845  in the receiver  810  to initiate a transmission of an RLC/ARQ status report from the second RLC/ARC unit  845  in the receiver  810  to the first RLC/ARQ unit  825  in the transmitter  805 . 
         [0083]    The status report triggering signal  1005  may be signaled by a MAC CE, an RLC control PDU or an RRC message. The status report triggering signal  1005  may include an RLC logical channel, an RLC SN, RLC segment information or HARQ process information and time of failure. The status report triggering signal  1005  may cause RLC status report polling by the transmitter  805 . 
         [0084]    As shown in  FIG. 11 , the second MAC/HARQ unit  835  in the relay  815  may send an HARQ NACK2 indication  1105  to the first MAC/HARQ unit  830  in the transmitter  805  on a condition that an HARQ transmission failure occurs at the receiver  810 . The first MAC/HARQ unit  830  in the transmitter  805  then sends a local NACK2 indication to the first RLC/ARQ unit  825  in the transmitter  805  to initiate a retransmission of the RLC PDU. As shown in  FIG. 12 , the first MAC/HARQ unit in the transmitter may then transmit a C-MAC PDU control signal  1205  to the second MAC/HARQ unit  835  in the relay  815  to initiate retransmission of the MAC PDU from the relay  815  to the receiver  810 . 
         [0085]    Although features and elements are described above in particular combinations, each feature or element can be used alone without the other features and elements or in various combinations with or without other features and elements. The methods or flow charts provided herein may be implemented in a computer program, software, or firmware incorporated in a computer-readable storage medium for execution by a general purpose computer or a processor. Examples of computer-readable storage mediums include a read only memory (ROM), a random access memory (RAM), a register, cache memory, semiconductor memory devices, magnetic media such as internal hard disks and removable disks, magneto-optical media, and optical media such as CD-ROM disks, and digital versatile disks (DVDs). 
         [0086]    Suitable processors include, by way of example, a general purpose processor, a special purpose processor, a conventional processor, a digital signal processor (DSP), a plurality of microprocessors, one or more microprocessors in association with a DSP core, a controller, a microcontroller, Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs) circuits, any other type of integrated circuit (IC), and/or a state machine. 
         [0087]    A processor in association with software may be used to implement a radio frequency transceiver for use in a wireless transmit receive unit (WTRU), user equipment (UE), terminal, base station, radio network controller (RNC), or any host computer. The WTRU may be used in conjunction with modules, implemented in hardware and/or software, such as a camera, a video camera module, a videophone, a speakerphone, a vibration device, a speaker, a microphone, a television transceiver, a hands free headset, a keyboard, a Bluetooth® module, a frequency modulated (FM) radio unit, a liquid crystal display (LCD) display unit, an organic light-emitting diode (OLED) display unit, a digital music player, a media player, a video game player module, an Internet browser, and/or any wireless local area network (WLAN) or Ultra Wide Band (UWB) module.