Abstract:
A radio telecommunications network is provided comprising a first station and a second station. The first station is operative to send packets of data to the second station. The packets are of a predetermined length. The second station is operative to receive the data and send to the first station indicators of which packets of data from the first station have been accurately received. The second station is also operative to measure received signal quality and to send information of received signal quality to the first station. The first station is operative to select dependent upon said information a different length for further packets to be sent to the second station.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to a radio telecommunications network comprising a first station and a second station. The present invention also relates to a method of sending packets of data from a first station to a second station in a radio telecommunications network. The present invention also relates to a radio telecommunications station.  
         BACKGROUND OF THE INVENTION  
         [0002]    In a network for mobile telecommunications, the quality of a radio connection depends upon the radio environment. In networks involving code division multiple access (CDMA), the quality of a radio connection is also dependent upon the level of network loading. This is because power is limited and must be shared between radio connections to active users, but, of course, the quality of radio connection tends to decrease as power is reduced or interference from other users increases.  
           [0003]    The performance of a radio interface between a cell and a user varies, of course, dependent upon the quality of the radio connection. For example, in bad radio conditions, a mobile voice user may find received audio is distorted, and a data application user may also find the throughput rate varying and sometimes dropping drastically. Making the radio interface more robust is a challenge.  
           [0004]    One type of CDMA network is a Universal Mobile Telecommunications System (UMTS) network. Background on UMTS networks is provided in, for example, Third Generation Partnership Project Technical Report TR21.905.  
         SUMMARY OF THE INVENTION  
         [0005]    A radio telecommunications network, a method, and a radio telecommunications station according to the present invention are defined in the independent claims to which the reader should now refer. Preferred features are laid out in the dependent claims.  
           [0006]    An example of the present invention is a radio telecommunications network comprising a first station and a second station. The first station is operative to send packets of data of a predetermined length to the second station. The second station is operative to receive the data and send to the first station indicators of which packets of data from the first station have been accurately received. The second station is also operative to measure received signal quality and to send information of received signal quality to the first station. The first station is operative to select dependent upon said information a different length for further packets yet to be sent to the second station and to send said further packets.  
           [0007]    In preferred embodiments, a technique is provided of achieving a higher net data throughput rate by changing data packet length. A complete data packet must be correctly received so as to avoid a repeat attempt at transmitting that data packet. A simple but effective way is thus provided of adapting to changing radio conditions.  
           [0008]    The packets of data are preferably service data units (SDU&#39;s). The SDU length is adjusted in response to the changing radio channel quality. One preferred embodiment involves Acknowledged Mode-Radio Link Control (AM-RLC) transmission in a UMTS network where it is the RLC SDU length that is adjustable.  
           [0009]    The present invention also provides a method of sending packets of data from a first station to a second station in a radio telecommunications network. In an example method, the packets are of a predetermined length. The second station receives the data and sends to the first station indicators of which packets of data from the first station have been accurately received. The second station measures received signal quality and sends information of received signal quality to the first station. The first station selects dependent upon said information a different length for further packets yet to be sent to the second station.  
           [0010]    The present invention also provides a radio telecommunications station operative to send packets of data of a predetermined length. An example is a station also operative to receive indicators of which packets of data have been accurately received and information as to the quality of received signals. The station is operative to select dependent upon said information a different length for further packets yet to be sent and to send said further packets. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    Preferred embodiments of the present invention will now be described by way of example and with reference to the drawings, in which:  
         [0012]    [0012]FIG. 1 is a diagram illustrating a UMTS network and a mobile user terminal,  
         [0013]    [0013]FIG. 2 is a diagram illustrating the protocol stack used in each,  
         [0014]    [0014]FIG. 3 is a diagram illustrating schematically RLC operation in transmission,  
         [0015]    [0015]FIG. 4 is a diagram illustrating schematically RLC operation in reception,  
         [0016]    [0016]FIG. 5 illustrates RLC operation in more detail,  
         [0017]    [0017]FIG. 6 is a diagram illustrating how SDU size is adjusted at various signal quality thresholds, and  
         [0018]    [0018]FIG. 7 is an example illustrating the approach. 
     
    
     DETAILED DESCRIPTION  
       [0019]    The preferred network is a Universal Mobile Telecommunications System (UMTS) terrestrial access network (UTRAN), which is a type of wideband code division multiple access (W-CDMA) network for mobile telecommunications. The UTRAN network is basically as shown in FIG. 1. Only one radio network controller and two base stations of the UTRAN network  2  are shown for simplicity. As shown in this Figure, the UTRAN network  2  includes base stations  4 . Each base station (Node B in UMTS terminology)  4  typically has three cells  6  (i.e. radio coverage areas, also known as sectors) as the base station  4  typically has three directional antennas (not shown) angled at 120 degrees to each other in azimuth. Radio network controllers (RNC)  8  which are themselves connected to the rest of the telecommunications “world” (not shown) each control several base stations  4  and hence a number of cells  6 . A base station  4  is connected to its controlling radio network controller (RNC)  8  via a respective interface  10  known as an luB interface. In use, a mobile user terminal  12  (often referred to as User Equipment (UE) in UMTS terminology) communicates with a serving radio network controller (RNC)  8  via at least one cell  6  of at least one base station  4  (i.e. communicates with the UTRAN network  2 ).  
         [0020]    Communications between the mobile user terminal  12  and RNC  8  are undertaken using a series  14  of hierarchically layered protocols (often referred to as a protocol stack), as shown in FIG. 2. The physical layer  16  receives data for transmission from the Medium Access Control (MAC) layer  18 . The physical layer  16  offers various transport channels to the MAC layer  18 . Different types of transport channels are defined by how and with what characteristics data is transferred on the physical layer  16 . The MAC layer  18  offers various logical channels to the Radio Link Control (RLC) layer  20 , where a logical channel is an information stream dedicated to a specific type of information characterising the logical channel. The RLC layer  20  receives data for transmission from a higher layer  22 . The RLC layer  20  consists of processes, known as the User plane, to handle user data, and processes, known as the Control plane, to handle control signalling. The RLC layer  20  can provide an Acknowledged Mode (AM) data transfer service, i.e. a service involving retransmitting data packets until an acknowledgement of receipt is received.  
         [0021]    As shown in FIG. 3, and in more detail in FIG. 5, the transmitting side  24  of the Radio Link Control (RLC) protocol layer  20  operates in Acknowledged Mode (AM). The RLC layer  20  receives (step  26 ) Service Data Units (SDU&#39;s, also known as RLC SDU&#39;s) from upper protocol layers through an Acknowledged Mode-Service Access Point (AM-SAP, not shown). SDU&#39;s are segmented (step  28 ) by the transmitting side  24  of the RLC layer  20  into Acknowledged Mode Data-Protocol Data Units (PDU&#39;s) of a fixed length. An SDU is segmented as its size is larger than the length of the applicable PDU. The PDU size is configured by upper protocol layers, e.g. the higher layer  22 , and is normally fixed. The PDU&#39;s are sent (step  30 ) to the MAC layer  18  for transmission. An SDU discard function (not shown) at the transmitting side  24  is used to remove the SDU&#39;s from a transmit buffer (not shown) when the transmission of one or more of the component PDU&#39;s of an SDU does not succeed despite a predetermined period of time passing or a number of retransmission attempts being made. Such a discard results in a fresh attempt at complete retransmission of the SDU initiated by the higher layer  22 .  
         [0022]    On the receiving side  32 , as shown in FIG. 4 and in more detail in FIG. 5, the Acknowledged Mode (AM) RLC layer  20  receives (step  34 ) incoming PDU&#39;s from the lower protocol layer  18 . The protocol data units (PDU&#39;s) are assembled  36  in a reception buffer (not shown) until a complete service data unit (SDU) has been received, whereupon the successfully received SDU is provided 40 to the higher layer  22 . The receiver, which is either of the radio network controller (RNC)  8  and mobile user terminal  12 , acknowledges successful reception or requests retransmission of the missing PDU&#39;s by sending one or more status reports (known as STATUS PDU&#39;s) to the corresponding RLC layer of the transmitter (i.e. the other of the RNC  8  and mobile user terminal  12 ). Status reports are thus sent to the transmitter acknowledging whether the PDU&#39;s are received correctly or not. Based on the acknowledgements, the transmitter (mobile user terminal  12  or RNC  8 ) removes the PDU from its retransmission buffer if that PDU has been received correctly or retransmits that PDU if not correctly received.  
         [0023]    These acknowledgement and selective retransmission processes occur, of course, at both the mobile user terminal  12  and RNC  8 .  
         [0024]    With the radio link control (RLC) layer  20  operating in Acknowledged Mode (AM) in this way, the net data throughput rate over the air interface depends upon radio conditions. Consider a scenario where the mobile user terminal  12  encounters poor radio conditions. The probability of retransmission is high and the probability of discarding a protocol data unit (PDU) and so a service data unit (SDU) is higher than in normal radio conditions. With the increased probability of SDU discard, the inventors realised that RLC service data unit (SDU) size is a factor critical to the net data throughput rate. The inventors realised that in bad radio conditions, a smaller SDU size would result in less likelihood of an SDU being incorrectly received and so being discarded, and so provide a higher data throughput over the air interface, in particular when using AM RLC transmission.  
         [0025]    Accordingly, radio quality measurements such as of the Bit Error Rate (BER) of the physical channel or transport channel, and/or Block Error Rate (BLER) of a transport channel, are provided (denoted  44  in FIG. 5) as feedback to adjust SDU size (reference numeral  46  in FIG. 5).  
         [0026]    The SDU size used at the radio network controller (RNC)  8  is changed on the basis of BER and/or BLER measurements of the radio connection quality in the downlink direction (i.e. to the mobile user terminal) made by the mobile user terminal  12  and sent to the RNC  8 . The SDU size used at mobile user terminal (UE)  12  is changed on the basis of BER and/or BLER measurements of the radio connection quality in the uplink direction (i.e. from the mobile user terminal) made by the base station (NodeB)  4  on behalf of the RNC  8  and sent to the UE 12 through control channels. The physical layer  16  carries out BER measurements on physical or transport channels. BLER measurement is carried out on transport channels by the Medium Access Control (MAC) protocol layer  18 .  
         [0027]    The measurements are taken frequently at the receiving side  32  (in the mobile user terminal  12  or RNC  8 ) and the information passed back to the corresponding transmitting side (in the other of the mobile user terminal  12  and RNC  8 ) frequently so that, where appropriate, rapid i.e. dynamic adjustment of SDU size is possible. The setting (step  46 ) of SDU size is performed either by an intermediate protocol layer (not shown) such as Packet Data Convergence Protocol (PDCP), or by the higher layer  22 .  
         [0028]    The proposed thresholds for measured signal quality at which SDU size is altered are illustrated schematically in FIG. 8. SDU size is reduced from its largest Size 0 to Size1 when bit error rate (BER) is greater than a first threshold  50 . Similarly, SDU size is reduced from Size1 to Size 2 when BER is greater than a second threshold  52 , and so on. For example in the case of Internet Protocol as the higher layer  22 , the SDU sizes are selected dependent on measured BER as shown in Table 1 below.  
                                         TABLE 1                                   Bit Error Rate (BER)   Service Data Unit (SDU) size in bytes                                        0-5%   1500           5-7%   800           7% or higher   500                      
 
         [0029]    In some embodiments, the thresholds are set taking into account such factors as the delays involved so as to maximise the net throughput rate gain.  
       EXAMPLE SCENARIO  
       [0030]    [0030]FIG. 7 further illustrates the proposed approach. Say a data packet (i.e. RLC SDU)  56  received from a higher layer is of size 500 bytes and each data packet is segmented into a series of PDU&#39;s small enough to be readily transmitted over the air-interface. For example, an SDU of 500 bytes is typically broken into 13 PDU&#39;s, each of 320 bits (there being 8 bits per byte and the last of the 13 PDU&#39;s including 160 bits of padding). T is the time duration over which the transmission of the PDU&#39;s corresponding to one SDU takes place through the channel as shown in FIG. 7.  
         [0031]    When the channel condition is sufficiently good, (all of its component PDU&#39;s and hence) the first SDU  56  of 500 bytes is transmitted successfully over the air interface. However, if it is known through measurement of received signal quality at the receiving end that the channel condition is becoming bad, then action is taken by the higher layer  22  controlling the RLC layer  20  to reduce the service data unit (SDU) size. In the example shown in FIG. 7, a packet  56 ′ of 500 bytes is segmented into five SDU&#39;s  58  each of 100 bytes. As shown in FIG. 7, the second 100 byte SDU  60  and the fifth 100 byte SDU  62  are not received correctly first time, as one or more of their respective component PDU&#39;s are still received in error after either a predetermined time-out period of PDU retransmission attempts (known as timer SDU discard) or a maximum number of attempts at successfully transmitting a PDU has been made (known as max-retransmission discard). Accordingly, the second 100 byte SDU  60  and fifth 100 byte SDU  62  are discarded. Assuming that these two 100 byte SDU&#39;s  60 , 62  are received correctly in the next attempt at their transmission, a total of at least 1200 bytes of data is required to be sent over the air interface to correctly deliver 1000 bytes of data to higher layers at the receiving end. Of course, in practice, there will also be some additional retransmission of PDU&#39;s.  
         [0032]    In contrast, if the prior art approach were taken whereby SDU size is unchanged, during the transmission of the second SDU  56 ′of 500 bits, if the channel condition became bad then SDU  56 ′ would be considered not to have been received correctly and so be discarded at the receiving side. In consequence the whole second SDU  56 ′of 500 bytes would need to be retransmitted. Assuming that the SDU  56 ′ is received correctly upon first retransmission, a total of at least 1500 bytes of data are then required to be sent over the air interface to correctly deliver 1000 bytes of data to higher layers to the receiving end. Of course, in practice, there will also be some additional retransmission of PDU&#39;s.  
         [0033]    Accordingly, we can see from this comparison that by using smaller data units in bad channel conditions, it is possible to send less retransmitted data so as to get the information across, resulting in a better net data throughput rate. Of course, net data throughput rate is one of the main parameters for gauging Quality of Service (QoS), i.e. overall performance of a network.  
         [0034]    The preferred embodiment described above relates to a Universal Mobile Telecommunications System (UMTS) network. Other embodiments relate to other types of CDMA or W-CDMA networks.