Abstract:
The present invention provides a solution for maintaining a reliable power control in a telecommunications network during soft handover between a serving and non serving base station when a dedicated physical control channel power is boosted due to an increased data rate.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a solution for uplink power control in a cellular telecommunication network. In particular, the present invention relates to a method and an arrangement for uplink power control during soft handover conditions. 
       BACKGROUND 
       [0002]    High-Speed Packet Access (HSPA) is a collection of mobile telephony protocols that extend and improve the performance of existing Universal Mobile Telephony System (UMTS) protocols. Two standards, High-Speed Downlink Packet Access and High-Speed Uplink Packet Access also referred to as Enhanced Uplink (EUL) have been established. The enhanced uplink introduces a new transport channel, the Enhanced Dedicated Channel (E-DCH). A dedicated channel (DCH) is assigned to only one UE at a time. The DCHs are power controlled which implies that the transmitter power is increased if the channel is too poor and the power is reduced if an unnecessary high power level is used. 
         [0003]    At the physical layer, the Enhanced Uplink introduces e.g. the E-DCH Dedicated Physical Control Channel (E-DPCCH) and the E-DCH Dedicated Physical Data Channel (E-DPDCH). The E-DPDCH is used to carry the E-DCH transport channel and the E-DPCCH is used to carry the control information associated with the E-DCH such as the E-DCH Transport Format Combination Indicator (E-TFCI). The Dedicated Physical Control Channel DPCCH is used to carry pilot symbols used for channel estimation. 
         [0004]    To increase the data rate in the uplink, higher order modulation (HOM) based on 16 QAM (Quadrature Amplitude Modulation) is introduced to the uplink E-DCH. The introduction of 16 QAM doubles the data rate with respect to Release 6 of the 3GPP specifications concerning Enhanced Uplink and allows peak data rates up to 11.5 Mbps (with coding rate equal 1). The transmission power of the data channel, E-DPDCH, depends on the transport format used and is adjusted relative to the DPCCH power. The DPCCH power is set by the inner power control loop to reach the SIR target set by the outer loop power control. 
         [0005]    The Open loop power control is the ability of the User Equipment (UE) transmitter to set its output power to a specific value. It is used for setting initial uplink and downlink transmission powers when a UE is accessing the network. The Inner loop power control (also called fast closed loop power control) in the uplink is the ability of the UE transmitter to adjust its output power in accordance with one or more Transmit Power Control (TPC) commands received in the downlink, in order to keep the received uplink Signal-to-Interference Ratio (SIR) at a given SIR target. 
         [0006]    Reliable demodulation of high rate signals requires a good phase reference (by using pilot symbols for channel estimation. It has been shown that the current power settings in Release 6 of the 3GPP specifications are not sufficient to guarantee good performance. A better phase reference can be obtained by scaling the control channel (DPCCH) power according to the transport block size which indicates the current bit rate, wherein the transport blocks are transmitted by the E-DPDCH. The DPCCH is then transmitted at higher power for high data rate transmission. The DPCCH carries pilot symbols that are used as a phase reference for channel estimation as illustrated in  FIG. 1 . 
         [0007]      FIG. 1  illustrates a network comprising a plurality of radio base stations  110   a,b,c  connected to a radio network controller (RNC)  100 . The radio base stations  110   a,b,c  are adapted to communicate wirelessly with the UEs  120  (only one UE is illustrated). One UE  120  may be connected to more than one radio base station simultaneously referred to as soft handover (SHO) as illustrated in  FIG. 1 . 
         [0008]    Assume that the boosting of the DPCCH transmission power when high data rates are transmitted is applied. A problem with the power control loop arises when the UE is in SHO. Consider the case when the UE  120  is in SHO with a first base station  110   a  and a second base station  110   b . The first base station  110   a  is the serving base station, i.e. the first base station  110   a  is responsible for the scheduling of the user. The UE  120  increases the power of DPCCH according to the transmission data rate negotiated with the first base station  110   a . The second base station  110   b  has no knowledge that the UE  120  has boosted its power and the SIR target at the second base station  110   b  will be set at a value lower than the correct value. The power control loop with the second base station  110   b  then will react to the increased received DPCCH power by sending “down” power commands to the UE  120 . Since the UE  120  listens to the power control commands of both base stations  110   a,b  and acts according to the “OR of the down commands”, the power is lowered as soon as at least one TPC indicates a lower power. Thus, the UE  120  will lower the transmitted power even when the serving base station commands otherwise. 
         [0009]    Hence, the problem is the generation of incorrect power control commands sent by the non-serving base station which is not aware of that the UE has boosted the DPCCH power. This leads to a too low receive power, and to an increased probability that transport blocks cannot be correctly decoded. Hence the system capacity is degraded. 
         [0010]    To address this problem, it has been proposed that the UE should not act on the power control commands from the non-serving base stations for 2 or 3 time slots when boosting or lowering the power of DPCCH according to the granted rate. Or, according to an alternative solution, the UE should not act on any of the received power control commands for a few slots if the boosting of DPCCH is set according to the actual transmitted rate. The drawbacks of these proposals is that the convergence time for the SIR target value may be longer than the required time for the UE to ignore its power commands according to the prior art solution above. Furthermore, the power control procedure of these proposals is user dependent and can create instability in the system. 
       SUMMARY 
       [0011]    Thus, the object of the present invention is to achieve an improved solution for handling power control during soft handover when the UE power on the DPCCH is boosted. 
         [0012]    The present invention relates to the determination of the amount of DPCCH boosting from an estimate of the UE transmission data rate in order to be able to adjust the SIR target at the non-serving base station. The power control commands sent to the UE are then processed according to the procedures of prior art, independently if the UE is in SHO or not. 
         [0013]    In accordance with a first aspect of the present invention, a method for a radio base station of a mobile telecommunication network for controlling power of a DPCCH, used by a UE, connected to said radio base station is provided. The UE is configured to transmit data on one or several Dedicated Physical Data Channels, (E-DPDCH) and to transmit reference information for channel estimation on a DPCCH. In the method a first SIR target (SIR target A) for the DPCCH power used by said UE is used. A change of the UE data transmission rate on the E-DPDCH is detected, and the SIR target for the DPCCH power received from the UE is adjusted from a first SIR target (SIR target A) to a second SIR target (SIR target B). The adjustment is based on a pre-determined mapping between a new UE data transmission rate and the SIR target. 
         [0014]    In accordance with a second aspect of the present invention, a radio base station of a mobile telecommunication network for controlling power of a DPCCH used by a UE connected to said radio base station is provided. The UE is configured to transmit data on one or several Dedicated Physical Data Channels, E-DPDCH and to transmit reference information for channel estimation on a DPCCH. The radio base station comprises means for using a first SIR target (SIR target A) for the DPCCH power used by said UE and means for detecting a change of the UE data transmission rate on the E-DPDCH. In addition, means for adjusting the SIR target for the DPCCH power received from the UE is adjusted from a first SIR target (SIR target A) to a second SIR target (SIR target B) are provided. The adjustment is based on a pre-determined mapping between a new UE data transmission rate and the SIR target. 
         [0015]    Hence the advantage with the embodiments of the present invention is that the problem of maintaining a reliable power control when the UE is in SHO and that the DPCCH power is boosted according to the data rate. If the present invention would not be used, the non-serving radio base station would not be aware of that the DPCCH power was boosted and may therefore generate incorrect power control commands. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    In the following the invention will be described in a non-limiting way and in more detail with reference to exemplary embodiments illustrated in the enclosed drawings, in which: 
           [0017]      FIG. 1  illustrates schematically a network wherein the embodiments of the present invention may be implemented; 
           [0018]      FIGS. 2   a  and  2   b  illustrates schematically a flowchart of the method according to embodiments of the present invention; 
           [0019]      FIG. 3  illustrates schematically in a block diagram of a radio base station according to embodiments of the present invention; 
           [0020]      FIG. 4  illustrates schematically in a block diagram a method according to an embodiment of the present invention; 
           [0021]      FIG. 5  illustrates schematically in a block diagram a method according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    In  FIG. 1 , reference numeral  130  generally refers to a cellular telecommunication network wherein the present invention may be implemented. At least one mobile unit (referred to as a user equipment, UE)  120  may be connected wirelessly to the radio network controller (RNC)  100  via one or more base stations  110   a,b,c . If the UE  120  is connected to the RNC via more than one base station  110   a,b,c , one  110   a  of the base stations acts as a serving base station and the other  110   b  acts as a non-serving base station to the UE  120 . The serving base station  110   a  is responsible for scheduling, rate control etc. However, both the serving  110   a  and the non-serving base station  110   b  send transmit power control (TPC) commands to the UE  120 . 
         [0023]    The present invention will now be discussed by the exemplary embodiments described below. 
         [0024]    Consider a UE  120  which is in SHO with a first base station  110   a  and a second base station  110   b . The first base station  110   a  is the serving base station and the second base station  110   b  is the non-serving base station and the UE  120  is transmitting at a certain rate r A . A change in UE transmission rate occurs and a new negotiated higher data rate r B  wherein r B &gt;r A  with the first base station  110   a  is established. (In this specification a data rate increase is assumed, but the same principles can be applied in case of a data rate decrease.). The UE  120  transmits then at rate r B . The DPCCH transmission power is increased accordingly when transmitting at the higher rate r B . The non-serving base station  110   b  has the SIR target set to SIR targetA . When transmission at rate r B  starts, the measured SIR meas  at the second base station  110   b  is compared to the SIR targetA  which was computed for the lower rate transmission (r B ) and can be lower than SIR meas  independently of the quality of the link. 
         [0025]    The non-serving base station  110   b  retrieves information of the transport block size by decoding the E-TFCI as illustrated in step  401  of  FIG. 4 . The transport block size indicates the data rate and the DPCCH target SIR may then be scaled according to the amount of DPCCH power boosting required for that data rate as shown in step  402 . For each E-TFCI, or alternatively for each transport block size, the base station has knowledge of the power offset between E-DPDCH and DPCCH, and the DPCCH SIR target. The non-serving base station then computes the difference in DPCCH SIR between the two rates, Δ DPCCH     —     SIR  (step  403 ), and corrects the SIR target (step  404 ), i.e. 
         [0000]      SIR targetB =SIR targetA +Δ DPCCH     —     SIR    
         [0026]    A power control command is then generated as follows: 
         [0000]    If SIR meas &gt;Δ DPCCH     —     SIR +SIR targetA  then a down transmit power command (TPC) is transmitted.
 
If SIR meas &lt;Δ DPCCH     —     SIR +SIR targetA  then an up transmit power command (TPC) is transmitted.
 
         [0027]    The above described embodiment is described assuming that the UE  120  boosts the power of the DPCCH according to the granted rate. In the scenario below, the serving base station  110   a  has knowledge of the transmitted transport block size and the granted rate, and adjusts the DPCCH SIR target accordingly. 
         [0028]    If the UE  120  boosts the DPCCH power according to the actual transmission rate rather than according to the granted rate, both the serving  110   a  and the non-serving base station  110   b  may decode the E-TFCI and can then adjust the SIR target in accordance with the corresponding transport block (TB) size (i.e. in accordance with the data rate). This could be performed when all slots in a Transmission Time Interval (WI) have been received, which implies a delay of three slots. In accordance with a further alternative, both the serving  110   a  and the non-serving base station  110   b  may estimate the transmitted TB size from the physical channel power levels and then adjust the SIR target in accordance with this TB size. This can already be performed before the entire TTI has been received. Hence a delay shorter than three slots is possible with this alternative. In accordance with a yet further alternative, it may be assumed that the received TB size, or the E-TFCI) is the same as the one sent in the previous TTI. This alternative results in no delay. Furthermore, the serving base station may assume that the received TB size is the same as the TB size that the base station has scheduled the UE to transmit with. This implies no delay. A combination of these above described approaches may also be applied. 
         [0029]    Hence, an embodiment of the present invention relates to a method for a radio base station of a mobile telecommunication network for controlling power of a DPCCH, used by a UE connected to the radio base station. The radio base station acts as a non-serving radio base station which implies that the UE is also connected to a further radio base station acting as a serving radio base station. The UE is configured to transmit data on one or several E-DPDCH and to transmit reference information for channel estimation on a DPCCH. The method is illustrated by the flowchart of  FIGS. 2   a  and  2   b  and comprises the steps of: 
         [0000]      201 . Use a first SIR target (SIR target A) for the DPCCH power used by said UE.
   202 . Detect that the UE data transmission rate on the E-DPDCH is changed from a first data transmission rate to a second data transmission rate.
   203 . Adjust the first SIR target (SIR target A) for the DPCCH power to a second SIR target (SIR target B) for the DPCCH power used by the UE. The SIR target is adjusted based on a pre-determined mapping between the new UE data transmission rate, i.e. second data transmission rate, and the SIR target.
 
         [0030]    If the E-TFCI can be decoded correctly, the step  203  may comprise the further steps of: 
         [0000]      204 . Determine a Transport Block size indicative of the data transmission rate for the transmitted data.
   205 . Map the new transmission rate to the second SIR target.
 
         [0031]    It should however be noted that the steps above also are applicable for a radio base station acting as a serving radio base station, which is further described below. 
         [0032]    In some cases the E-TFCI may not be decoded correctly. When the E-TFCI is not decoded correctly at the non-serving base station, the step  203  comprises the further steps of: 
         [0000]      206 . Estimate a Transport Block size indicative of the data transmission rate for the transmitted data as the Transport Block size of a previously sent Transport Block.
   207 . Map the new transmission rate to the second SIR target.
 
         [0033]    Two possible alternatives are described below for this scenario. 
         [0034]    In the first alternative, the non-serving base station receives the signal at very low power and is unable to decode E-TFCI. In this alternative the measured DPCCH SIR is lower than the target SIR and the non-serving base station will send a TPC command “up”. This is not a problem since the serving base station would control the power control loop. 
         [0035]    In the second alternative, the non-serving base station is unable to decode the E-TFCI correctly but the received power is high enough such that the DPCCH SIR is higher than the SIR target. The non-serving base station will send a TPC command “down” and destroy the power control loop of the serving base station. 
         [0036]    Furthermore, the following method as illustrated in  FIG. 5  may be used to estimate the TB size when the E-TFCI is not correctly decoded at the non-serving base station: 
         [0000]      501 . Select the lowest TB size is selected.
   502 . Set a SIR target according to the selected TB size.
   503 . Compute the DPCCH SIR and check if the DPCCH SIR is greater or lower than the SIR target.
   504 . If the DPCCH SIR is lower, then a TPC “up” command is sent and if DPCCH SIR is higher,
         505 . Estimate DPCCH power. The estimation may be done based on pilot bits.     506 . Estimate the received power for (all) the E-DPDCH(s).     507 . Estimate the used TB size or E-TFCI.
           The quotient between the received power for (all) the E-DPDCH(s) and the received power for the DPCCH could be used to estimate the TB size or the E-TFCI, since each TB size value corresponds to a quotient between the transmitted power for (all) the E-DPDCHs and the transmitted power for the DPCCH, as specified in the “Setting of the uplink E-DPCCH and E-DPDCH powers relative to DPCCH power” procedure in 3GPP TS 25.214, “Physical layer procedures (FDD) (Release 6)” and in 3GPP TS 25.321, “Medium Access Control (MAC) protocol specification” and in the “E-TFC selection” procedure in 3GPP TS 25.321, “Medium Access Control (MAC) protocol specification”         508 . Set SIR target according to the estimated TB size or the E-TFCI.     509 . Compute the DPCCH SIR and check if the DPCCH is greater or lower than SIR target.     510 . Send the TPC command.         
         [0044]    In an alternative embodiment of the invention, when the E-TFCI cannot be decoded in the non-serving base station, the received TB size may be assumed to be the same as the one corresponding to the last correctly decoded E-DPCCH instead of basing the TB size estimate on power estimates as described in step  507  above. 
         [0045]    If the E-TFCI is not decoded correctly at the serving base station, several alternative actions may be taken: 
         [0046]    The DPCCH SIR target may be adjusted according to the granted rate. If the UE is instead transmitting at a different rate, always a lower rate, and the DPCCH SIR target is not set correctly, the DPCCH SIR and the E-DPDCH will increase to a larger value than intended. It may also be assumed that the received TB size is the same as the one sent in the previous TTI. Further, it is also possible to do as when the E-TFCI was not correctly decoded at the non-serving base station as described earlier. 
         [0047]    It should be noted that this may cause a renegotiation of the granted rate. 
         [0048]    Accordingly, the embodiments of the present invention solve the problem of maintaining a reliable power control in SHO when the DPCCH power is boosted according to the data rate. 
         [0049]      FIG. 3  illustrates a radio base station  110   a ; 110   b  according to embodiments of the present invention. The radio base station  110   a ; 110   b  comprises at least one computational unit  301  and at least one memory  302  (volatile and/or non-volatile). It comprises further a communication interface  303  towards the UE and a communication interface  305  towards the RNC  100 . It further comprises functions/components  304  required by the present invention, wherein the functions/components  304  can interact with the memory  302 , computational unit  301  and interfaces  303 ,  305 . The base station is arranged to adjust a signal to interference ratio (SIR) target depending on the amount of change of power applied. A power control command is sent to the UE  120  and the system processes the power control commands according to standard procedures. There are different ways of taking a decision to adjust the SIR target depending on situation. The function/components  304  required by the present invention comprises means for using a first SIR target (SIR target A) for the DPCCH power  304   a  used by said UE, means for detecting  304   b  a change of the UE data transmission rate on the E-DPDCH, and means for adjusting  304   c  the first SIR target (SIR target A) for the DPCCH power to a second SIR target (SIR target B) for the DPCCH power used by the UE based on a pre-determined mapping between a new UE data transmission rate and the SIR target. 
         [0050]    The present invention may be implemented as software in a computational unit in the base station or as part of an ASIC (application specific integrated circuit) in the base station. 
         [0051]    It should be noted that the word “comprising” does not exclude the presence of other elements or steps than those listed and the words “a” or “an” preceding an element do not exclude the presence of a plurality of such elements. It should further be noted that any reference signs do not limit the scope of the claims, that the invention may be implemented at least in part by means of both hardware and software, and that several “means”, “units” or “devices” may be represented by the same item of hardware. 
         [0052]    The above mentioned and described embodiments are only given as examples and should not be limiting to the present invention. Other solutions, uses, objectives, and functions within the scope of the invention as claimed in the below described patent claims should be apparent for the person skilled in the art.