Abstract:
Systems and methods for packet re-transmission in multi-hop wireless networks are provided. In some embodiments, RLP packet re-transmission only starts from the hop where L 1  ARQ fails. This can result in an increased efficiency of radio resource utilization, such as in implementations where the last hop is more unstable than the remaining hops. In some embodiments, a short RLP recovery delay enables a higher number of re-transmissions of lost RLP packets which, in turn, translates into a higher target physical layer FER (frame error rate) being allowed and/or an increased system capacity. Alternately or additionally, a short RLP recovery delay may reduce the possibility of TCP re-transmission and slow start.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 13/470,646, filed May 14, 2012, which is a continuation of U.S. patent application Ser. No. 10/894,035, filed Jul. 20, 2004, the disclosures of which are incorporated by reference herein in their entirety. 
     
    
     BACKGROUND 
       [0002]    In conventional cellular wireless access networks, a cell is covered by a BTS (base station transceiver) and all mobile terminals communicate with the BTS directly. With the addition of relays, a multi-hop network, including the BTS, relay nodes and mobile terminals, is set up. With relay nodes involved, the coverage of a wireless access network is improved. In such a wireless access network, there may be multiple routes for communicating between a terminal and the network. For example, a terminal can communicate directly with the BTS or indirectly via one or more relay nodes. 
         [0003]    An example of a two-hop scenario is shown in  FIG. 1A . Shown is a base station  10  having a coverage area  12 , and a relay  16  having a coverage area  18 . There is a link  22  between the base station  10  and the relay  16  which is typically a high capacity and very reliable wireless link. Shown are a number of mobile terminals  14  communicating directly with the BTS  10 , and a number of mobile terminals  20  communicating with the relay  16  which relays communications for these mobile terminals to and from the BTS  10 . 
         [0004]    In such a network, the fixed relay node  16  is added to improve the coverage in the edge of a cell. Since the relay  16  is a fixed node, the channel between the base station  10  and the relay  16  can be a high quality channel implemented using any one of many advanced channel technologies, such as MIMO, which can provide improved capacity. However, the quality of the channel between the relay node  16  and a mobile terminal  20  is typically lower and less stable due to the mobility of the mobile terminals  20  and due to height differences in the location of the relays  16  and the mobile terminals  20 . Because of this, it is possible that data will accumulate in the relay  16  after having been transmitted over the reliable channel between the BTS  10  and the relay  16 . This requires the relay node  16  to have a significant buffer capacity, particularly in the cases where a long delay bound is allowed and where there are a lot of mobile terminals  20  that are served by the relay  16 . While the illustration shows the relay  16  extending the physical coverage of the cell, this may not always be the case. The relay  16  may provide service to an area within coverage area  12  to enhance service in that area for example to improve rate coverage within coverage area  12 . In that case, a decision to communicate directly or indirectly via the relay can be made dynamically. 
         [0005]      FIG. 1B  shows an example of a three-hop scenario. In this example, there is an additional relay  30  shown communicating with another wireless terminal  32 . There are three wireless hops to get from the BTS  10  through to the wireless terminal  32 . 
         [0006]    As indicated above, in such networks, the communication between the BTS  10  and the mobile terminal  20 , 32  may be possible via more than one route. The route can be selected dynamically by a L 2  (layer  2 ) function. 
         [0007]    Compared to wire-line networks, a concern on a wireless access network is the unreliability of wireless links. In order to provide comparable quality of wire-line networks, two re-transmission protocols are implemented to improve the reliability of a wireless access network. L 1  HARQ (layer  1  hybrid automatic repeat request) is implemented in layer  1  and the Radio Link Protocol (RLP, layer  2  ARQ) is implemented in layer  2 . The L 1  HARQ is used to improve the quality of each hop individually. Each relay implements the L 1  HARQ function. The L 2  ARQ is used to improve the quality of a wireless end-to-end connection. Thus the L 2  ARQ is implemented in the BTS and terminals but not in the relay nodes. This results in a simple relay node function, which can be beneficial for a low cost wireless access network. 
         [0008]    L 2  functionality typically includes multiplexing/scheduling functions (implemented by the so-called multiplex sub-layer) and flow control functions. If RLP flow control were running on the relay and the base station, two instances of RLP would be required on the wireless terminals to accommodate dynamic route selection. Conventional wireless terminals have only one such instance. 
         [0009]    RLP is NACK based and run end-to-end, meaning that the receiver generates a NACK when it receives an out of sequence packet, but otherwise does nothing. L 1  HARQ is run on a per-hop basis and is ACK based meaning that for each L 1  attempt, the receiver responds to indicate success or failure of the attempt. Typically after some number of failed L 1  attempts, the transmitter for that hop moves on to the next packet resulting in an out of sequence packet at the receiver which in turn causes a NACK to be generated. 
       SUMMARY 
       [0010]    Systems and methods for packet re-transmission in multi-hop wireless networks are provided. 
         [0011]    Advantageously, in some embodiments RLP packet re-transmission only starts from the hop where L 1  ARQ fails. This results in an increased efficiency of radio resource utilization. The benefit increases with a larger number of hops. In particular, the benefit is greatest for implementations where the last hop is more unstable than the remaining hops. This is the case for the cellular downlink where the last hop is between a relay and a mobile terminal, and this hop is typically the most unstable. However, applications are not limited to this particular case. 
         [0012]    The improvement in RLP recovery delay can be translated into an improvement in system capacity and per-terminal throughput. A short RLP recovery delay enables the possibility of a higher number of re-transmissions of lost RLP packets. This in turn translates into a higher target physical layer FER (frame error rate) being allowed which in turn translates into an increased system capacity. Furthermore, a short RLP recovery delay may reduce the possibility of TCP re-transmission and slow start. 
         [0013]    According to one aspect, some embodiments provide a method of performing re-transmission for a multi-hop communications path, the method comprising: performing a first re-transmission protocol on a per hop basis; performing a second re-transmission protocol on a wireless end-to-end basis; for a particular hop of the multi-hop communications path: upon failure of the first re-transmission protocol for a particular packet for the particular hop, performing at least one additional first re-transmission protocol re-transmission of the particular packet on that hop. 
         [0014]    In some embodiments, the at least one additional first re-transmission protocol re-transmission is performed before transmitting a next packet out of sequence on the particular hop. 
         [0015]    In some embodiments, a method further comprises giving the at least one additional first re-transmission protocol re-transmission priority. 
         [0016]    In some embodiments, the first re-transmission protocol comprises L 1  HARQ (layer  1  hybrid automatic repeat request). 
         [0017]    In some embodiments, the second re-transmission protocol comprises RLP (radio link protocol). 
         [0018]    In some embodiments, performing at least one additional first re-transmission protocol re-transmission of the packet on that hop comprises: re-initiating the first re-transmission protocol for the particular hop after failure of the first re-transmission protocol until success of the first re-transmission protocol, and until a predetermined number of re-initiations of the first re-transmission protocol have been performed in the event of no success of the first re-transmission protocol. 
         [0019]    In some embodiments, a method comprises: maintaining at least one parameter allowing a determination of a maximum additional number of first re-transmission protocol re-transmissions on a particular hop to be performed. 
         [0020]    In some embodiments, the at least one parameter comprises: a count of a number of second re-transmission protocol re-transmissions to be allowed for each packet which is then used to determine if additional first re-transmission protocol re-transmissions are allowed. 
         [0021]    In some embodiments, the at least one parameter comprises: a count of a number of additional first re-transmission protocol re-transmissions to be allowed; further comprising adjusting the count for a given packet for each additional first re-transmission protocol re-transmission of the packet until either the packet is successfully delivered or the count indicates no further first re-transmission protocol re-transmissions are to be performed. 
         [0022]    In some embodiments, a method is applied in a multi-hop wireless network, wherein the particular hop comprises a last hop of a communications path through the multi-hop wireless network. 
         [0023]    In some embodiments, a method is applied in a multi-hop wireless network, wherein the particular hop comprises a relatively unreliable hop of the multiple hops. 
         [0024]    In some embodiments, a method further comprises: a first end of the second re-transmission protocol ignoring messages from a second end of the second re-transmission protocol in respect of packets for which the additional first re-transmission protocol re-transmissions are to take place. 
         [0025]    In some embodiments, a method further comprises: performing dynamic route selection to transmit some packets on a single hop directly between a first end of the second re-transmission protocol and a second end of the second re-transmission protocol, and to transmit some packets on multiple hops indirectly between the first end of the second re-transmission protocol and the second end of the second re-transmission protocol; a first end of the second re-transmission protocol ignoring messages from the second end of the second re-transmission protocol in respect of packets that were transmitted indirectly. 
         [0026]    In some embodiments, the communications path is between a base station and a mobile device via a relay, the particular hop comprising a wireless link between the relay and the mobile device. 
         [0027]    In some embodiments, the second re-transmission protocol is established between the base station and the mobile device, the first re-transmission protocol is established between the relay and the mobile device, the method further comprising: for each packet transmitted from the base station to the relay, including at least one parameter indicating a number of second re-transmission protocol re-transmissions to be allowed for the packet; the relay using the at least one parameter to indicate how many additional first re-transmission protocol re-transmissions are allowed. 
         [0028]    According to another broad aspect, some embodiments provide a base station adapted to transmit packets to wireless devices via a relay, the base station comprising: a packet processor adapted to add at least one parameter to each packet sent to the relay, the at least one parameter allowing a determination of how many additional first re-transmission protocol re-transmissions are allowed at the relay after failure of the first re-transmission protocol between the relay and a wireless device for the packet. 
         [0029]    In some embodiments, a base station is further adapted to transmit packets directly from the base station to wireless devices, the base station further comprising: a second re-transmission protocol processor adapted to perform an end-to-end re-transmission protocol for packets sent from the base station to wireless devices, the second re-transmission protocol processor being adapted to ignore particular second re-transmission protocol messages received from wireless devices in respect of packets sent via the relay. 
         [0030]    According to another broad aspect, some embodiments provide a relay comprising: a receiver for receiving packets; a transmitter for transmitting the packets on a wireless hop of a multi-hop communications path; the relay being adapted to implement a first re-transmission protocol over the wireless hop; the relay being further adapted to, upon failure of the first re-transmission protocol for a particular packet, perform at least one additional first re-transmission protocol re-transmission of the particular packet on the wireless hop. 
         [0031]    In some embodiments, a relay is adapted to perform the at least one additional first re-transmission protocol re-transmission of the particular packet on the wireless hop before transmitting an out of sequence packet on the wireless hop. 
         [0032]    In some embodiments, the transmitter is adapted to transmit packets to a plurality of receivers on a respective wireless hop of a respective multi-hop communications path for each receiver; the relay being adapted to give priority to re-transmissions above normal traffic. 
         [0033]    In some embodiments, a relay is adapted to extract at least one parameter from each packet and to perform the additional first re-transmission protocol re-transmissions in accordance with the at least one parameter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    Various embodiments will now be described with reference to the attached drawings in which: 
           [0035]      FIG. 1A  is a schematic of an example of a two hop network; 
           [0036]      FIG. 1B  is a schematic of an example of a three hop network; 
           [0037]      FIG. 2  is a schematic diagram of an L 1  and L 2  scheme within a network featuring a relay; 
           [0038]      FIGS. 3A and 3B  are packet flow diagrams illustrating how unnecessary L 2  re-transmissions may occur as a result of transmission failures at the relay; 
           [0039]      FIG. 4  is a schematic diagram of a relay provided by at least one embodiment associated with a multi-hop network; 
           [0040]      FIGS. 5A and 5B  illustrate packet flows that may result using the relay of  FIG. 4  so as to avoid unnecessary L 2  re-transmissions; 
           [0041]      FIG. 6  is a flowchart of an example method implemented within the relay of  FIG. 4 ; and 
           [0042]      FIG. 7  is a flowchart of NACK processing in the base station in accordance with another embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0043]      FIG. 2  shows an example of a two-hop wireless connection.  FIG. 2  will be used to illustrate how L 1  ARQ typically HARQ, and L 2  ARQ, typically RLP may be implemented in a multi-hop wireless network. Shown is a BTS transmitter  50 , a relay  51  having a relay receiver  52  and a relay transmitter  54 , and a terminal receiver  56 . Each device  50 , 52 , 54 , 56  has a respective L 1  function  72 , 80 , 88 , 92  and respective L 1  ARQ function  70 , 82 , 86 , 94 . These functional entities are responsible for performing ARQ on a per-hop basis, sometimes referred to as L 1  ARQ. Thus there is a first L 1  ARQ implemented between the BTS transmitter  50  and the relay receiver  52  as indicated at  73 . There is a second L 1  ARQ implemented between the relay transmitter  54  and the terminal receiver  56  as indicated by  87 . The BTS transmitter  50  and the terminal receiver  56  each have respective full L 2  implementations  76 , 96  with L 2  ARQ functions  74 , 98 . These functional entities are responsible for end-to-end ARQ, sometimes referred to as layer  2  ARQ, typically RLP between the BTS transmitter  50  and the terminal receiver  56  as indicated at  75 . The relay receiver  52  and the relay transmitter  54  are not involved in the L 2  ARQ. Rather, the L 2  ARQ is performed end-to-end. 
         [0044]    Typically, when the relay  51  fails to transmit a packet successfully to terminal receiver  56  after L 1  HARQ, the relay will simply discard the packet. Sometime later, the terminal receiver  56  will realize that there is a packet loss, for example by detecting a sequence number gap, and will issue an L 2  NACK (negative acknowledgement). The L 2  NACK is transmitted from the terminal receiver  56  all the way back to the BTS transmitter  50  and the impacted packet is transmitted again from the BTS transmitter  50  to the relay  51 . It can be seen that although the packet is only lost on the last hop, the packet is re-transmitted again starting from the first hop. 
         [0045]    The problem becomes more severe with increasing numbers of hops where again typically most of the packet loss will occur on the last hop. If an L 1  packet failure occurs in the first hop, this is not a significant problem since the packet needs to be re-transmitted from the start in any case. 
         [0046]      FIGS. 3A and 3B  show an illustration of this problem. Shown is a series of transmissions between the BTS  50 , the relay  51  and the terminal  56  of  FIG. 2 . The scenario begins at  106  with the transmission of three packets P 1 ,P 2 ,P 3  from the BTS  50  to the relay  51 . These are shown being buffered at  107  in the relay  51 . At  108 , the relay  51  transmits packet P 1  to the terminal  56  and the new contents of the buffer are indicated at  109 . At  110 , the relay  51  attempts to transmit packet P 2 , but this fails after L 1  ARQ. Packet P 3  is transmitted at  112 , and shortly thereafter, the terminal  56  will detect a gap in the sequence numbers of the packets received and will generate an L 2  NACK  114  which is sent right back to the BTS  50 . In response to this, the BTS  50  will re-send the packet P 2  as indicated at  116  to the relay  51 . The relay  51  then sends the packet P 2  at  118  to the terminal  56 . 
         [0047]    Consider  FIG. 4 , which shows an example of a two hop wireless connection in accordance with one or more embodiments. Shown is a BTS transmitter  450 , a relay  451  having a relay receiver  452  and a relay transmitter  454 , and a terminal receiver  456 . Each device  450 , 452 , 454 , 456  has a respective L 1  function  472 , 480 , 488 , 492  and respective L 1  ARQ functions  470 , 482 , 486 , 494 . These functional entities are responsible for performing L 1  ARQ on each wireless hop. Thus there is a first L 1  ARQ implemented between the BTS transmitter  450  and the relay receiver  452  as indicated at  473 . There is a second L 1  ARQ implemented between the relay transmitter  454  and the terminal receiver  456  as indicated by  487 . The BTS transmitter  450  and the terminal receiver  456  each have respective full L 2  implementations  476 , 496  with L 2 -ARQ functions  474 , 498 . These functional entities implement layer  2  ARQ, typically RLP between the BTS transmitter  450  and the terminal receiver  456  as indicated at  475 . Some relay implementations may include minimized L 2  functions as indicated at  484  for the relay receiver  452  and  490  for the relay transmitter  454 . This might include a multiplex sub-layer as in the illustrated example. 
         [0048]    Also shown in  FIG. 4  is a tag processor  491 . This is illustrated to be part of the layer  2  of the protocol stack. However, this is an arbitrary distinction. It is noted for the example implementation of  FIG. 4 , the method is shown being implemented between the relay  451  and the terminal receiver  456 . More generally, it can be implemented on any hop of a multi-hop wireless network, even including the first hop. However, in most implementations, the method does not need to be implemented on the first hop between the transmitter  450  and relay receiver  452  because if an L 1  ARQ failure occurs in the first hop, the sender L 1  ARQ can directly communicate with the sender L 2  ARQ to immediately determine whether the lost packet needs to be re-transmitted. In a scenario with more than two hops, some embodiments implement the new methods on the last hop. This is because typically it is the last hop that has a decreased reliability. However, in implementations in which others of the links making up a multi-hop link were also not reliable, and may also be advantageous to implement the method on those links. 
         [0049]    More generally, some embodiments are applicable to perform re-transmission for a multi-hop communications path in which there is a first re-transmission protocol on a per hop basis, and there is a second re-transmission protocol on an end-to-end basis. In the above example, the first re-transmission protocol is L 1  HARQ, and the second re-transmission protocol is RLP. However, other re-transmission protocols can alternatively be employed. The second re-transmission protocol has a first end of the protocol and a second end of the protocol. Since this is run end-to-end, any multi-hop wireless network the first end would be a base station, and a second end a mobile terminal. 
         [0050]      FIG. 4  shows a detailed block diagram for the BTS transmitter  450 , the relay  451  and the terminal receivers  456 . This is an example of a functional layout for the purpose of illustration only. It is to be understood that the interconnection between various components may be different, and that there may be fewer or additional components than those shown specifically in  FIG. 4 . It is also noted that the functionality may be included in a fewer or larger number of functional blocks. Hardware, software and/or firmware can be employed. By “BTS transmitter”, it is meant a transmitter and antenna. A single base station may have multiple sectors and as such multiple transmitters and antennas. A base station that is not sectorized will have only a single transmitter. In some cases, the BTS  450  adds a tag to each packet indicating a number of L 2  re-transmissions allowed. More generally, in some embodiments the BTS  450  may be considered to have a second re-transmission protocol processor responsible for implementing the end-to-end protocol, and a packet processor responsible for adding at least one parameter to each packet to allow a downstream relay to determine if additional first protocol re-transmissions are allowed. 
         [0051]    In some embodiments, the tag processor functional entity maintains a tag for each RLP packet that indicates a delay bound and/or a maximum number of re-trials for the RLP packet. In some embodiments, each re-trial is effectively a new L 2  attempt, but initiated by the relay instead of the base station. Each such re-trial may allow for multiple L 1  attempts in accordance with the L 1  ARQ scheme being used. In the examples that are detailed below, it is assumed that the re-trials are equivalent to L 2  re-transmissions. An example of this is shown in Table 1 below. 
         [0052]    More generally, in some embodiments, the relay has a packet processor which extracts at least one parameter for each packet received from the BTS, the at least one parameter allowing a determination of additional first re-transmission protocol re-transmissions to be performed. The packet processor can be implemented in any suitable manner, such as hardware, software, firmware, etc. In the illustrated example, the second re-transmission protocol processor is realized with the L 2  ARQ  474 , and L 2   476  and may include part of L 1  ARQ  470  and L 1   472 , depending on where the at least one parameter is added. 
         [0053]    Alternatively, since as detailed below in some embodiments L 2  NACKs are ignored, each re-trial can be some number of additional L 1  attempts not tied to the L 2  protocol, or the number of re-trials can be an arbitrary number of L 1  re-trials. 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 RLP Packet Sequence 
                 Maximum Number of 
                   
               
               
                 Numbers 
                 Re-Trials 
                 Packet Payload 
               
               
                   
               
             
             
               
                 1 
                 2 
                 P1 
               
               
                 2 
                 2 
                 P2 
               
               
                 3 
                 2 
                 P3 
               
               
                 4 
                 2 
                 P4 
               
               
                 5 
                 2 
                 P5 
               
               
                   
               
             
          
         
       
     
         [0054]    In the example above, there are five packets buffered, namely packets having sequence numbers  1 ,  2 ,  3 ,  4  and  5 , and the maximum number of re-trials for each RLP packet is indicated at  2 . 
         [0055]    After detecting a failed transmission of an RLP packet (i.e. L 1  ARQ has failed), the relay determines whether or not to perform an additional RLP packet re-transmission based on the information in the tag of the failed packet. A failed re-transmission of an RLP packet is typically detected by receiving an HARQ NACK for the last HARQ re-transmission. More generally, for whatever L 1  protocol is being employed, once the L 1  protocol fails to deliver the packet over the wireless hop, that transmission has failed. For the failed packet, the tag of the lost packet is consulted, and if the maximum number of re-trials for the packet is still non-zero, then the packet is re-transmitted. At this point, the maximum number of re-trials is adjusted. 
         [0056]    In the illustrated example, L 1  ARQ  486  in the relay informs the tag processor  491  as indicated at  495  that the failure has occurred. The tag processor  491  decides if an additional L 2  re-trial is allowed and if so informs the multiplex sub-layer to re-transmit the packet. In the examples given, it is assumed that L 2  packets map one to one to L 1  packets. However, it is to be understood the solutions provided herein extend to cases where the relationship is not one to one. 
         [0057]    The remaining functional entities of  FIG. 4  perform in a conventional manner with one exception. When L 1  ARQ fails, the receiving terminal  456  will still generate an L 2  NACK that will be transmitted all the way back to the BTS transmitter  450 . In at least one embodiment, the BTS transmitter  450  is configured to simply ignore this L 2  NACK. A flowchart of example functionality associated with BTS transmitter  450  is given below in  FIG. 7 . This is only one example implementation of the base station. Furthermore, a flowchart of an example method implemented in the relay  451  is given below in  FIG. 6 . This is only one example of how the method can be implemented. 
         [0058]    Referring now to  FIGS. 5A and 5B , shown is an example packet flow illustrating how the tag processor works. This is again a two-hop example with a terminal  300 , relay  302  and BTS  304 . At  306 , the BTS  304  transmits three packets P 1 , P 2  and P 3  and these are shown being buffered at  307  with their respective tags  305 . It is noted that the wide arrows in  FIG. 5A  represent L 2  transmission, and each may involve multiple L 1  re-transmissions. At  308 , the relay  302  transmits P 1  to the terminal. The updated contents of the buffer are shown at  309 . Here, the fact that P 1  is no longer shown in the buffer indicates that successful layer  1  transmission has been achieved. Sometime later, a first attempt to transmit to P 2  is made at  310 . However, the buffer contents  311  still show P 2  present. In this case, the attempt to transmit P 2  has failed after L 1  ARQ. The tag processor determines if additional L 2  re-trials are allowed by consulting the packet&#39;s tag. Assuming additional re-trials are allowed, the tag for P 2  is then decremented, and a further L 2  re-transmission is made for packet P 2  at  312 . This involves re-initiating L 1  ARQ for the packet. This time, the re-transmission is successful, and the buffer contents are shown at  313  to only include P 3 . The L 2  transmission of P 3  then occurs at  314 . In the illustrated example, the receiver receives all packets in the current sequence so no L 2  NACK is generated. If after all of the L 2  re-trials fail, an out of sequence packet is sent on the last hop, the receiver will then generate a NACK. 
         [0059]    Referring now to  FIG. 6 , shown is an example method implemented by a relay, in accordance with one or more embodiments. At step  6 - 1 , the relay receives packets and buffers them. This is done on an ongoing basis. At step  6 - 2 , L 1  ARQ is performed for the next packet. If no L 1  ARQ failure occurs, (no path step  6 - 3 ), then the method continues from  6 - 1 . If an L 1  ARQ failure occurs, (yes path step  6 - 3 ), then a check is made at  6 - 4  to see if additional L 2  re-transmissions are allowed. In the above described example embodiment, this involves checking a tag maintained for each packet to ascertain whether or not additional re-transmissions were allowed. More generally, any appropriate method of making this decision can be employed. If no additional L 2  re-transmissions are allowed, (no path  6 - 4 ), then the packet is discarded at step  6 - 6 . On the other hand, if additional L 2  re-transmissions are allowed, (yes path step  6 - 4 ), then at step  6 - 5  the number of additional L 2  re-transmissions allowed is adjusted, and the method then continues at step  6 - 2  with L 1  ARQ being performed for the same packet again. In some instances, for a packet that is being re-transmitted in an additional L 2  re-transmission, that packet is moved to the head of the buffer. Alternately or additionally, re-transmission packets are sent before out of sequence packets. In some embodiments, the relay is relaying packets to multiple terminals on a respective wireless hop of a respective multi-hop communications path. At times, a higher priority is given to re-transmission packets than to other packets to other terminals. 
         [0060]    At step  6 - 5 , decrementing of the number of additional L 2  re-transmissions allowed is shown as an example method of updating this statistic. Other methods may alternatively be employed. For example, a single flag may be employed to indicate that one additional re-transmission is allowed. Once the re-transmission takes place, the flag is cleared. The re-transmission information can be stored in the buffer as a prefix or suffix to the particular packet. Alternatively, the information can be stored separately. 
         [0061]    In some embodiments, all packets start with the same number of allowed re-transmissions in which it may not be necessary to include any additional information in packets sent from the BTS. In other embodiments, the number of allowed re-transmissions is a per packet parameter which is sent to the relay by the transmitter as in the above examples. An example of this is shown in  FIG. 5B  where a tag  320  is shown being transmitted from the BTS  304  to the relay  302  in association with packet P 2 . 
         [0062]    Referring now to  FIG. 7 , shown is a flowchart of functionality implemented in the base station transceiver in accordance with one or more embodiments. At step  7 - 1 , the base station transceiver is shown transmitting packets to the relay or directly to the mobile terminal, bypassing the relay. This is done on an ongoing basis. The base station would in fact be transmitting packets to multiple users, and potentially to multiple relays. At step  7 - 2 , if an L 2  NACK is not received, “no” path, the process continues from the step  7 - 1 . At step  7 - 2 , if an L 2  NACK is received, “yes” path, then the L 2  NACK is ignored at step  7 - 4  if the NACK is for a packet sent to the relay (yes path, step  7 - 3 ). The transmitter should not ignore the L 2  NACK unless it can confirm that the packet has been transmitted to the relay. It can do this by verifying that L 1  transmission to the relay was successful. By “ignore”, it is meant that the decision is not immediately made to initiate L 2  packet re-transmission. In some embodiments, all L 2  NACKs are ignored in respect of traffic sent via the relay once it is successfully sent to the relay. This is because the L 2  re-transmissions are being taken care of by the relay. Once the relay has exhausted its re-trials, this is equivalent to the end-to-end L 2  failing. At this point, it is up to the next layer to deal with the problem. For example, TCP (transmission control protocol) might be responsible after failure of L 2 . The BTS should still pay attention to NACKs in respect of direct, non-relayed traffic, as the BTS is responsible for executing L 2  re-trials. If the NACK was not for a packet sent to a relay (no path, step  7 - 3 ), then normal NACK processing is conducted at step  7 - 5 . In another embodiment, the transmitter only ignores L 2  NACKs satisfying one or more ignore parameters. This might be a number of L 2  NACKs to ignore, or a time window for ignoring NACKs of a given packet to name two specific examples. 
         [0063]    In the above described embodiments, it is assumed that a number of re-transmissions is maintained in a tag, or a flag is used to indicate a single re-transmission is possible. In another embodiment, a maximum re-transmission time is maintained. In a similar manner to the above described embodiment, this maximum re-transmission time is checked each time L 2  re-transmission is to take place, and once the maximum re-transmission time has expired, the relay gives up and discards the packet. More generally, any appropriate parameter(s) may be employed to decide whether or not to make another attempt. 
         [0064]    In some embodiments, L 1  ARQ is performed using “HARQ channels”. In such embodiments, the tag processor may maintain a mapping between the RLP packet and the HARQ channel used for re-transmitting this RLP packet. At the time the relay L 1  ARQ receives the last HARQ NACK of an HARQ channel, which indicates a layer  1  HARQ failure in transmitting an RLP packet, the relay layer  1  ARQ indicates the HARQ channel to the tag processor. The tag processor will identify the corresponding RLP packet and further check the parameter in the tag of the RLP packet. If the parameter indicates that one or more re-transmission is still possible, the tag processor will instruct the multiplex sub-layer in the relay to put the packet in the head of the transmission buffer; otherwise the tag processor will instruct the multiplex sub-layer to discard this RLP packet. 
         [0065]    Numerous modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, various embodiments may be practiced otherwise than as specifically described herein.