Abstract:
Wasteful use of radio resources is minimized in a packet transmission system which performs retransmission control at a higher layer. When transmission of a segment has failed at a lower layer and, for example, when the number of lower-layer retransmit requests from an error detection unit at the lower layer has exceeded a predetermined threshold, any other segment remaining to be transmitted in a packet of which the failed segment forms a part is discarded and, at the same time, a retransmit request is sent to a retransmission control unit at the higher layer.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a packet transmission apparatus and, more particularly, to a packet transmission apparatus suitable for application to a base transceiver station and a mobile station in a mobile communication system.  
         [0003]     2. Description of the Related Art  
         [0004]     In communications using radio waves and, in particular, communications between base transceiver stations (BTSs) and mobile stations (MSs) in a mobile communication system, it is important that radio resources, such as frequencies and spreading codes, be utilized effectively. The reason is that, in a mobile communication system, limited radio resources linking between each BTS and MSs are shared by many users and, if the radio resources can be utilized effectively, more users can be accommodated in the system.  
         [0005]     On the other hand, in a W-CDMA (Wideband Code Division Multiple Access) system, for example, in addition to the retransmission control at the MAC (Media Access Control) sublayer, a retransmission control mechanism is provided at the RLC (Radio Link Control) sublayer above the MAC sublayer in order to perform error recovery and to guarantee the sequencing of data in a radio transmission. In HSDPA (High Speed Downlink Packet Access) which is a packet transmission scheme for the W-CDMA system, the radio retransmission control at the MAC sublayer is implemented in the BTS, while the retransmission control at the RLC sublayer is implemented in the RNC (Radio Network Controller).  
         [0006]      FIG. 1  schematically shows the retransmission control at the RLC sublayer (the higher layer) implemented in the RNC and the retransmission control at the MAC sublayer (the lower layer) implemented in the BTS. In the RNC, a higher-layer retransmission unit (packet)  10  is segmented into lower-layer retransmission units (segments)  12  and transmitted to the BTS via a wired link. Then, the BTS transmits them as radio signals. The transmitted signals are received by the MS which assembles the received segments  12 ′ into a packet  10 ′, thereby reconstructing the higher-layer signal provided by the RNC. When transmitting the segments from the BTS to the MS, a control scheme that combines, for example, ARQ (Automatic Repeat Request) and error correction coding, known as hybrid ARQ, is used. Using this scheme, if any segment received at the MS is in error and the error is uncorrectable, the MS sends a retransmit request  14  to the BTS to recover from the error. Independently of the above control, retransmission control is also performed on the higher-layer signal from the RNC to the MS. That is, if the packet  10 ′ assembled at the MS is in error, a retransmit request  16  is sent to the RNC via the BTS.  
         [0007]     When the retransmission control is implemented at the higher layer as shown in  FIG. 1 , if a certain segment fails to be received correctly over the radio link between the BTS and the MS, radio resources will be wastefully used. For example, as shown in  FIG. 2 , suppose that, of segments  12 ′- 1  to  12 ′- 4 , the second segment  12 ′- 2  has failed to be received but the subsequent segments  12 ′- 3  and  12 ′- 4  have successfully been received; in this case, the retransmit request  16  is sent to the higher layer, since the packet  10 ′ cannot be assembled. In response to the retransmit request, the packet  10  is retransmitted and divided into segments, and the segments  12 - 1  to  12 - 4  are once again transmitted out from the BTS. As a result, the previously transmitted segments  12 - 3  and  12 - 4  are rendered useless. Here, retransmission of the segment  12 - 1  also renders the previously transmitted segment  12 - 1  useless, but the segment  12 - 1  is always transmitted regardless of whether the segment  12 ′- 2  is received or not received at the MS.  
         [0008]     Even when the retransmission control is also implemented at the lower layer as earlier described, error recovery may not be accomplished; in that case, the result will be the same as described above.  
       SUMMARY OF THE INVENTION  
       [0009]     Accordingly, it is an object of the present invention to minimize wasteful use of the radio resources.  
         [0010]     According to the present invention, there is provided a packet transmission apparatus comprising: a transmitting unit which transmits a packet divided into a plurality of segments; and a transmission control unit which, when it is determined that transmission of a segment has failed, stops transmission of any other segment remaining to be transmitted in the packet of which the failed segment forms a part.  
         [0011]     When the transmission of a segment has failed, as the transmission of any other segment remaining to be transmitted in the packet of which the failed segment forms a part is stopped, wasteful transmissions can be reduced. By reducing wasteful transmissions, the radio resources can be effectively utilized and, as a result, transmission throughput can be increased. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a diagram for explaining retransmission control at a higher layer;  
         [0013]      FIG. 2  is a diagram for explaining a problem associated with the prior art;  
         [0014]      FIG. 3  is a block diagram showing a first embodiment of the present invention;  
         [0015]      FIG. 4  is a block diagram showing a second embodiment of the present invention;  
         [0016]      FIG. 5  is a block diagram showing a third embodiment of the present invention;  
         [0017]      FIG. 6  is a diagram for explaining information to be appended to each segment; and  
         [0018]      FIG. 7  is a block diagram showing a fourth embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]      FIG. 3  shows the configuration of a mobile communication system according to one embodiment of the present invention.  
         [0020]     The retransmission control at the higher layer is performed between a retransmission control unit  20  in the RNC and an error detection processing unit  22  in the MS, while the retransmission control (of the segmented data) at the lower layer is performed between a retransmission control unit  24  in the BTS and an error detection processing unit  26  in the MS.  
         [0021]     In the RNC, a packet receiving unit  28  receives a packet from the higher node, and stores it in a buffer  30 . Then, the retransmission control unit  20  retrieves the data stored in the buffer  30  and supplies it to an error detection code appending unit  32  where an error detection code is appended; the data is then segmented by a segmenting unit  34  and a transmission format is generated by a transmitting unit  36  for transmission to the BTS.  
         [0022]     In the BTS, a receiving unit  38  receives the data from the RNC, and stores it in a buffer  40  in the form of segments as generated by the RNC. Then, the retransmission control unit  24  retrieves the data stored in the buffer  40  on a segment-by-segment basis, appends an error detection code ( 42 ), and transmits the data to the MS via a transmitting unit  44 .  
         [0023]     In the MS, a receiving unit  46  receives the data from the BTS, and the error detection processing unit  26  performs error detection for the lower layer by using the error detection code appended by the BTS. If there is no error, a packet is reconstructed ( 48 ) from the received segments, and the error detection processing unit  22  performs error detection for the higher layer by using the error detection code appended by the RNC. If there is no error, the packet is taken to be the received data.  
         [0024]     If an uncorrectable error from the higher layer is detected by the error detection processing unit  22 , the error detection processing unit  22  generates a higher-layer retransmit request signal, which is transmitted from a transmitting unit  50  to the BTS. The request signal is received by a receiving unit  52  in the BTS and transmitted from a transmitting unit  54  on to a receiving unit  56  in the RNC. In response, the retransmission control unit  20  retransmits the higher-layer data.  
         [0025]     If an uncorrectable error from the lower layer is detected by the error detection processing unit  26 , the error detection processing unit  26  generates a lower-layer retransmit request signal, which is transmitted from the transmitting unit  50  to the BTS. In response, the retransmission control unit  24  retransmits the lower-layer data. The procedure up to this point is the same as that known in the art.  
         [0026]     Here, if a preset number of lower-layer retransmissions (or a preset timer count) is reached, the transmission of the data that follows that segmented data is stopped, and the data stored in the buffer  40  connected to the retransmission control unit  24  is discarded.  
         [0027]     In this case, the higher-layer data (higher-layer retransmission unit) is retransmitted when the preset timer count in the retransmission control unit  20  is reached.  
         [0028]      FIG. 4  shows the configuration of a mobile communication system according to a second embodiment of the present invention. The difference from the embodiment of  FIG. 3  is that, when the preset number of retransmissions or the preset timer count is reached in the retransmission control unit  24  at the lower layer, not only is the data stored in the buffer  40  discarded but, at the same time, a higher-layer retransmit request is issued from the retransmission control unit  24  and transmitted to the higher layer via the transmitting unit  54  and the receiving unit  56 . Here, as described above, in response to the data delivery failure and the data discarding at the lower layer, the retransmission from the higher layer is initiated after a prescribed time has elapsed but, in this embodiment, the retransmission can be initiated without delay by issuing the retransmit request from the retransmission control unit  24  at the lower layer to the retransmission control unit  20  at the higher layer upon discarding the data.  
         [0029]      FIG. 5  shows the configuration of a mobile communication system according to a third embodiment of the present invention. The difference from the embodiment of  FIG. 3  is that when a notification of the occurrence of an uncorrectable error is received from the error detection processing unit  26  at the lower layer, the unit  24  does not retransmit the segment, but immediately discards the contents of the buffer  40 . In this way, in the present invention, the retransmission control at the lower layer is not an essential requirement.  
         [0030]     In the embodiment of  FIG. 5 , the retransmit request may be issued to the higher layer upon discarding the contents of the buffer  40 , as in the embodiment of  FIG. 4 .  
         [0031]     When applying the packet transmission method of the present invention, necessary information associated with segmentation must be appended to each segment. One example is shown in  FIG. 6 . As shown in  FIG. 6 , when dividing the higher-layer packet into segments, information identifying the segment position, i.e., the starting segment, an intermediate segment, or the ending segment, is appended to each segment. Based on this information, the packet is reconstructed from the segments starting from the segment labeled START and ending with the segment labeled END. When the retransmission control unit  20  in the RNC initiates the higher-layer retransmission, the MS at the receiving end receives the segment labeled START without receiving the segment labeled END, and the MS thus knows that the retransmission has been initiated in the middle of the transmission of the higher-layer data. Accordingly, at the MS, the higher-layer data assembled up to that point is discarded, and the data is reassembled starting from the segment labeled START, thereby reconstructing the correct higher-layer data. Further, at the BTS, by discharging the buffer contents up to the segment labeled END, it becomes possible to locate the segment for starting the next higher-layer data.  
         [0032]      FIG. 7  shows an example in which the packet transmission method shown in  FIG. 4  is applied to the uplink data transmission from the MS to the RNC, not to the downlink data transmission from the RNC to the MS. In this example, the component elements provided in the RNC and BTS in  FIG. 4  are provided in the MS. Accordingly, the transmitting units  36  and  54  and the receiving units  38  and  56  are eliminated. In  FIG. 7 , the component elements corresponding to those in  FIG. 4  are designated by the same reference numerals as those used in  FIG. 4 . On the other hand, the component elements provided in the MS in  FIG. 4  are divided between the BTS and the RNC, the boundary being between the packet reconstruction unit  48  and the error detection processing unit  26 , and transmitting units  60  and  64  and receiving units  62  and  66  are added.  
         [0033]     Similarly, the packet transmission methods shown in  FIGS. 3 and 5  can also be applied to the uplink data transmission.  
         [0034]     Further, in the examples of FIGS.  3  to  5 , as in the example of  FIG. 7 , the component elements contained in the RNC and BTS can be housed in a single cabinet, for example, in the BTS. In this case also, the transmitting units  36  and  54  and the receiving units  38  and  56  can be eliminated.