Abstract:
A method and an arrangement for obtaining efficient radio resource utilization in a communication network comprising a first communication network entity ( 10 ), a second communication network entity ( 15 ) connected to said first communication network over a communication interface and one or more user equipments ( 18 ) transmitting data to said second communication network entity ( 15 ) over a radio interface. The user equipments ( 18 ) perform the step of autonomously selecting a hybrid automatic retransmission request (HARQ) operating point in order to efficiently deliver transmitted data.

Description:
TECHNICAL FIELD 
     The present invention relates to methods and arrangements in a Wideband Code Division Multiple Access (WCDMA) communication system, in particular to an enhanced uplink for WCDMA. 
     BACKGROUND OF THE INVENTION 
     Enhanced uplink for WCDMA is currently being standardized within the Third Generation Partnership Project (3GPP). Among the features introduced is fast scheduling and fast hybrid Automatic Retransmission Request (ARQ) with soft combining, both located in the Node B. 
     Hybrid ARQ with soft combining allows the Node B to rapidly request retransmissions of erroneously received data entities, leading to significantly reduced delays compared to earlier releases of the WCDMA specification where the Radio Network Controller (RNC) is responsible for all retransmissions within the radio access network. Soft combining with hybrid ARQ can also be used to enhance the capacity of the system by deliberately target multiple transmission attempts for each data entity and use the soft combining mechanism in the receiver to accumulate the received energy until the data is successfully decoded. This can be viewed as implicit link adaptation and is not possible in earlier releases of the WCDMA specification due to lack of a soft combining mechanism in these releases. Typically, a small number of transmission attempts, i.e. a low Block Error Rate (BLER) for the initial transmission, reduce the transmission delays at the cost of a decreased system capacity. Similarly, by targeting a larger number of transmission attempts, i.e. a high BLER for the initial transmission attempt, the system capacity is increased at the cost of increased delays. The choice of hybrid ARQ operating points (in terms of the targeted number of transmission attempts) thus depends on the system load and the delay requirements for a particular service. The possibility for retransmission by the Radio Link Control (RLC) layer in the RNC remains with the introduction of hybrid ARQ in the Node B. This is useful in situations when the hybrid ARQ mechanism in the Node B cannot deliver error-free data entities to the RNC. 
     Fast scheduling denotes the possibility for the Node B to control when a user equipment is transmitting and at what data rate. Data rate and transmission power is closely related and scheduling can thus also be seen as a mechanism to vary the transmission power used by the user equipment for the enhanced uplink traffic on the E-DPDCH. As the power availability in the user equipment at the time of transmission is not known to the Node B, the final selection of data rate has to be performed by the user equipment itself. The Node B only sets an upper limit on the transmission power the UE may use on the E-DPDCH. 
     Similarly to the uplink in earlier releases of the WCDMA standard, the enhanced uplink uses inner and outer loop power control. The power control mechanism ensures that a user equipment does not transmit with higher power than required for successful delivery of the transmitted data. This ensures stable system operation and efficient radio resource utilization. 
     The power control mechanism consists of two parts: an inner loop, located in the Node B, and an outer loop, located in the RNC. The inner loop is fast and updates the user equipment transmission power 1500 times per second in order to combat fast fading. This is done by measuring the received Signal to Interference Ratio (SIR), comparing it with a SIR target, and sending a power control command to the user equipment. If the received SIR is below the SIR target, the user equipment is instructed to increase the transmission power and vice versa if the received SIR is above the target the user equipment is instructed to decrease. The inner loop power control operates on the DPCCH. The transmission power of the E-DPDCH is set relative to the DPCCH and depends on the instantaneous data rate on the E-DPDCH. 
     The outer loop sets the SIR target in the inner loop and uses statistics available to the RNC, e.g. information whether each data entity for a particular UE delivered to the RNC from the Node B is error-free or not. The outer loop is significantly slower than the inner loop and adapts to slow changes in the radio conditions to match the SIR target to the required quality of service in terms of, e.g., BLER or packet delay. 
     The introduction of a Hybrid ARQ protocol in the Node B requires modifications to the outer loop mechanism compared to previous releases as the hybrid ARQ protocol ideally hides all the error events from the RNC. Solutions to this problem are described in the patent application PCT/SE2004/000541, where different types of statistics on the Hybrid ARQ operation is proposed to be forwarded to the RNC. One possibility is to inform the RNC about the number of transmission attempts required until a packet is successfully received. If the number of attempts indicated to the outer loop mechanism is larger (smaller) than a target value, the SIR target is increased (decreased), resulting in the inner loop requesting a higher (lower) transmission power from the UE. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide an improved method for obtaining efficient radio resource utilization in a communication network comprising a first communication network entity ( 10 ), a second communication network entity ( 15 ) connected to said first communication network over a communication interface and one or more user equipments ( 18 ) transmitting data to said second communication network entity ( 15 ) over a radio interface. 
     This object is achieved by the the independent claims. 
     Accordingly it is an object of the present invention to provide an improved arrangement for obtaining efficient radio resource utilization in a communication network comprising a first communication network entity ( 10 ), a second communication network entity ( 15 ) connected to said first communication network over a communication interface and one or more user equipments ( 18 ) transmitting data to said second communication network entity ( 15 ) over a radio interface. 
     This other object is achieved by the independent claims. 
     Due to the provision of a system where the user equipments autonomously select a hybrid automatic retransmission request operation point, an improved link efficiency is obtained where the user equipment selects operating point depending of the existing circumstances, such as power availability, granted upper limit of power offset and/or on which logical channel the user equipment has received data. 
     Still other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  shows the communication network architecture according: to the present invention. 
         FIG. 2  illustrates the uplink channels after the introduction of the enhanced uplink according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A network according to a standard like 3GPP comprises a Core Network (CN), Radio Access Networks (RAN) and User Equipments (UE) attached to a RAN, such as the UMTS Terrestrial Radio Access Network (UTRAN) architecture.  FIG. 1  shows an exemplary network like this, wherein the UTRAN comprises one or more Radio Network Controllers (RNCs)  10  and one or more radio base stations  15 , in the following denoted as Node B, which are connected to the RNC  10  through the lub-interface. The UTRAN connects to the core network  12  through the lu-interface. The UTRAN and the CN  12  provide communication and control for a plurality of user equipments  18 . 
     Node B  15  is the function within the UTRAN that provides the physical radio link between the user equipments  18  and the network. Along with the transmission and reception of data across the radio interface the Node B  15  also applies the codes that are necessary to describe channels in a CDMA system. In Node B  15 , there is provided a scheduler which controls when a user equipment is transmitting and at what data rate. There is also provided the Hybrid Automatic Retransmission Request (HARQ), which allows Node B  15  to rapidly request retransmissions of erroneously received data entities. 
     The RNC  10  comprises an Outer Loop Power Controller (OLPC) which sends a SIR target level to an inner loop power controller provided in Node B  15  as described above. 
     In the uplink direction, several channels from each UE  18  will be transmitted with the introduction of the enhanced uplink as illustrated in  FIG. 2 . The Dedicated Physical Control Channel (DPCCH) carries pilot symbols and parts of the outband control signalling. Remaining outband control signalling for the enhanced uplink is carried on the Enhanced Dedicated Physical Control Channel (E-DPCCH) which is a new control channel, while the Enhanced Dedicated Physical Data Channel (E-DPDCH) carries the data transmitted using the enhanced uplink features. As the High Speed Dedicated Physical Control Channel (HS-DPCCH) is not related to the enhanced uplink it is not discussed further. 
     As stated above, the scheduler in the Node B cannot set exactly which power the user equipment may use on the E-DPDCH, but can only set an upper limit on the E-DPDCH/DPCCH power ratio. Consequently, the probability of successful decoding of the transmitted data will vary, depending on the power ratio used by the user equipment. The power ratio used is not known to the RNC, which thus cannot differentiate between variations in the number of transmission attempts due to channel variations and due to the user equipment varying the E-DPDCH/DPCCH power ratio. 
     There are several reasons why the user equipment may vary the E-DPDCH/DPCCH transmission power for a given size of a packet:
         High priority delay sensitive data. In this case, the user equipment may want to use a higher E-DPDCH/DPCCH power ratio to target a smaller number of transmission attempts than typically used in the outer loop power control.   The scheduler may have granted the user equipment to use a higher upper limit on the power ratio (data rate) than can be used with respect to the amount of data in the user equipment buffer. One reason could be that the network is lightly loaded and the excess capacity can be used to lower the data transmission delays by trying to obtain a successful transmission with a smaller number of transmission attempts than typically used. Therefore, it is typically desirable if the user equipment uses as much of the granted power as possible.   At the time of retransmission, the amount of power available may be larger than for the initial transmission. Provided that the user equipment is allowed to exploit this additional power for the retransmission, the data transmission delays can be reduced.       

     It is proposed to solve the problem outlined above by informing the outer loop power control of a momentarily change in the power offset, or targeted number of transmissions. With this information, the outer loop can determine whether a change in the number of transmission attempts compared to the configured value for a particular packet depends on variations in the channel quality or on temporary changes in the E-DPDCH power decided upon by the user equipment. For example, if the power offset is larger than configured, the outer loop power control can treat the packet as if the targeted number is achieved, although a smaller number of transmissions are needed. 
     The user equipment is allowed to select autonomously between a number of power offset values for each transport block size (TB size) according to a transport format (TF) table. The table below shows an example of a transport format table containing two possible offsets per transport block size. In said table two levels are available, a “normal” mode and a “boost” mode. The offset Δ corresponds to a block error rate, BLER, after N transmission attempts while the offset Δ′ corresponds to a block error rate BLER′ after N′ transmission attempts. 
     
       
         
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                   
                 Power 
                   
                 Power 
                   
               
               
                   
                   
                   
                 offset 
                 N tgt   
                 offset 
                 N tgt   
               
               
                   
                 TF 
                 TB size 
                 normal 
                 normal 
                 boost 
                 boost 
               
               
                   
                   
               
             
             
               
                   
                 1 
                 320 
                 Δ 
                 N 
                 Δ′ 
                 N′ 
               
               
                   
                 2 
                 2 × 320 
                 2 × Δ 
                 N 
                 2 × Δ′ 
                 N′ 
               
               
                   
                 3 
                 3 × 320 
                 3 × Δ 
                 N 
                 3 × Δ′ 
                 N′ 
               
               
                   
                 4 
                 4 × 320 
                 4 × Δ 
                 N 
                 4 × Δ′ 
                 N′ 
               
               
                   
                 5 
                 5 × 320 
                 5 × Δ 
                 N 
                 5 × Δ′ 
                 N′ 
               
               
                   
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                   
                   
               
             
          
         
       
     
     The Node B can either estimate the power ratio for each transmission attempt or be informed by the UE about the ratio used through control signalling on the E-DPCCH. The number of offsets allowed for the UE can be limited in order to limit to signalling requirements or, in case the offset is estimated, simplify the estimation. The power ratio and/or the TF (or a similar quantity) that has been used by the UE is sent to the outer loop. The outer loop uses this information to adjust the SIR target according to the block error rate and number of transmission attempts corresponding to the offset. 
     Alternatively, if the same BLER is assumed for the different number of target attempts, the Node B can estimate the number of transmission attempts that the UE targeted, T tgt , based on the power ratio used by the UE. The outer loop can use some relation between the actual number of transmission attempts needed, N tx  with the number of transmission attempts targeted by the UE, N tgt , to decide on the SIR target setting. 
     If the outer loop power control is situated in the RNC some information is signalled from the HARQ entity to the outer loop power control to inform the RNC about the power offset (or a similar quantity) the UE is using for a particular packet. This signalling occurs between the Node B and the RNC; so this information should be transmitted using the lub frame protocol. The needed information should preferably be included in an existing user plane frame protocol frames as a new information field. Alternatively a new frame protocol control signalling between Node B and RNC could be used. 
     The information signalled in the frame protocol could, for example, contain an indication on what power offset is used or weather or not the UE has used a higher offset than the minimum or some other reference level. In another solution the signalling contains a relation between the actual number of transmission attempts needed, N tx , and the number targeted by the UE, N tgt . For example the ratio, N tx /N tgt , or the difference, N tx −N tgt  can be signalled. 
     The UE may select HARQ operating point based on which logical channel said data is received. Data with different priorities is often mapped on different logical channels, i.e. each logical channel represents different degree of priority. The skilled person realizes that the HARQ operating point may be chosen directly depending on the priority of the data was received.