Abstract:
A method and apparatus for controlling the transmit power of a mobile device in a mobile communication network takes the level of self-interference into account to perform inner-loop power control. For normal inner-loop power control, a signal quality estimate is compared to a signal quality target and power control commands are generated based on the comparison. When self-interference is the dominant impairment in the received signal, a “fast break” is introduced to change inner-loop power control command generation. The amount of self-interference is determined based on the delay spread of the channel.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to power control for high speed packet data access in mobile communication networks. 
         [0002]    The current Universal Mobile Telecommunication System (UMTS) standard implements uplink power control to control the transmit power of mobile terminals on uplink channels. The uplink transmit power control procedure simultaneously controls the power of a Dedicated Physical Control Channel (DPCCH) and its corresponding Dedicated Physical Data Channels (DPDCHs), High Speed Dedicated Physical Control Channel (HS-DPCCH), and Enhanced Dedicated Physical Control and Data Channels (E-DPCCH and E-DPDCH). The power control procedure in UMTS includes inner-loop power control and outer-loop power control. Inner-loop power control compares a signal-to-interference ratio (SIR) of a received signal from a mobile terminal with an SIR target to generate transmit power control (TPC) commands to instruct the mobile terminal to either increase or decrease its transmit power. Outer-loop power control adjusts the SIR target to obtain a certain quality of service (QoS). For example, adjustment of the SIR target may be made to maintain a desired block error rate (BLER). 
         [0003]    With higher data rates in the uplink, a higher chip energy-to-noise ratio (E c /N 0 ) is needed in order to support the desired throughput. Multi-path propagation in combination with high transmission power (E c /N 0 &gt;0 dB) may cause severe self-interference that, in some cases, dominates other interference in the received signal and degrades the overall performance of the mobile terminal. When self-interference is dominant, the received SIR may not be able to reach the SIR target, irrespective of the mobile terminal transmit power because increasing the transmit power also increases the self-interference. In this scenario, inner-loop power control continues to ask the mobile terminal to increase its transmit power, which leads to an undesirable power rush, possible system instability, and serious interference that affects other users&#39; performance. 
         [0004]    One possible solution to this problem is to exclude self-interference from the SIR estimation process. For example, an interference suppression receiver, such as a GRAKE receiver, may be used to suppress self-interference when demodulating the received signal. SIR may then be estimated after GRAKE combining. This method has the advantage of being straightforward and the estimated SIR reflects the actual SIR experienced by the modem. However, using an interference suppression receiver to suppress self-interference may not be sufficient to avoid power rushes at high data rates. 
         [0005]    Another possible solution is to compute a modified SIR that excludes self-interference and to use the modified SIR for inner-loop power control. The TPC commands are then generated based on the relation between the modified SIR, with self-interference excluded, and the SIR target. However, there is always some residual self-interference that cannot be removed. Further, it may be difficult to accurately estimate the modified SIR at high data rates. Moreover, signal quality is actually affected by self-interference even if the self-interference is discounted when computing the SIR. Removing the effect of self-interference from the SIR estimate results in worse signal quality for a given SIR target and causes the outer-loop power control to compensate for the self-interference. 
         [0006]    Another possible solution is to take self-interference into account when determining the data transmission rates for the mobile terminal. When self-interference is dominant, the uplink scheduler may avoid scheduling high data rate transmissions. However, there is an inherent delay in the scheduling process. Consequently, the scheduler cannot respond quickly enough to rapidly changing channel conditions. 
         [0007]    U.S. patent application Ser. No. 12/22,346 filed Jan. 30, 2008, titled “Method of Closed Loop Power Control Adjusted by Self-Interference” describes a method of closed loop power control that takes into account the level of self-interference in the received signal when generating power control commands. When the level of self-interference is low, a normal inner-loop power control procedure may be used wherein the signal-to-interference ratio (SIR) of the receive signal is compared to an SIR target to generate power control commands. When the level of self-interference is high, a “fast break” procedure is used for inner-loop power control to constrain further increases in mobile terminal transmit power until the level of self-interference returns to an acceptable level. The “fast break” procedure may reduce the mobile station transmit power, maintain the mobile station transmit power at current levels, or limit further increases in the mobile terminal transmit power. The use of fast break procedures is triggered based on an instantaneous estimate of the orthogonality factor, which can be difficult to compute. 
       SUMMARY 
       [0008]    The present invention provides a reduced complexity method of closed loop power control that takes into account the level of self-interference in the received signal when generating power control commands. A base station varies transmit power control command generation based on the delay spread of the channel. In general, the amount of self-interference increases with increasing delay spread. Therefore, the delay spread can be used as an indication of the self-interference. When the level of self-interference is low, a normal inner-loop power control procedure may be used wherein the signal-to-interference ratio (SIR) of the receive signal is compared to an SIR target to generate power control commands. When the level of self-interference is high, a “fast break” procedure is used for inner-loop power control to constrain further increases in mobile terminal transmit power until the level of self-interference returns to an acceptable level. The “fast break” procedure may reduce the mobile station transmit power, maintain the mobile station transmit power at current levels, or limit further increases in the mobile terminal transmit power. In some embodiments, the SIR target set by outer-loop power control may be modified when the level of self-interference is high to prevent power rushes. Also, the level of self-interference may be provided to an uplink scheduler so that the scheduler may avoid scheduling high data rate transmissions while the level of self-interference is high. 
         [0009]    The use of a delay spread estimate to trigger the fast break procedure significantly reduces the complexity of the closed loop power control mechanism and avoids power rushes and possible system instability when the level of self-interference is high. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is schematic diagram of a mobile communication network. 
           [0011]      FIG. 2  is a flow diagram of an exemplary inner-loop power control procedure. 
           [0012]      FIG. 3  is a block diagram of an exemplary base station in a mobile communication network. 
           [0013]      FIG. 4  is a block diagram of an exemplary power control unit for a base station in a mobile communication network. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Referring now to the drawings, the present invention will be described in the context of a Third Generation (3G) mobile communication network, such as a UMTS network. Those skilled in the art will appreciate, however, that the present invention is applicable to systems implementing other standards. Therefore, the description should not be construed as limiting the present invention to UMTS networks. 
         [0015]      FIG. 1  presents a simplified illustration of a mobile communication network  10  for supporting wireless communications by a plurality of mobile terminals  50 . While  FIG. 1  shows a single base station  20  communicating with the mobile terminals  50 , those skilled in the art will appreciate that a typical communication network  10  comprises many base stations  20 . Base station  20  transmits data to the mobile terminals  50  over one or more downlink channels, and receives data from the mobile terminals  50  over one or more uplink channels. The downlink and uplink channels may comprise dedicated channels, common channels, or a mixture thereof. In the current release of the UMTS standard, the physical channels for the uplink include the dedicated physical control channel (DPCCH), the dedicated physical data channel (DPDCH), the enhanced dedicated physical control channel (E-DPCCH), the enhanced dedicated physical data channel (E-DPDCH), and the high-speed dedicated physical control channel (HS-DPCCH). 
         [0016]    Base station  20  simultaneously controls the transmit power of the mobile terminals  50  on the DPCCH, DPDCH, E-DPCCH, E-DPDCH, and HS-DPCCH. An inner power control loop sets the transmit power of the mobile terminals  50  on the DPCCH by comparing the signal-to-interference ratio (SIR) of the received DPCCH signal to an SIR target. The SIR target is set by outer-loop power control and can be driven by the block error rate (BLER) on the DPDCH, or the number of retransmissions of the E-DPDCH. The mobile terminal transmit power on the DPDCH, E-DPCCH, E-DPDCH, and HS-DPCCH are set relative to the transmit power of the DPCCH. Thus, a single power control mechanism controls the transmit power of the mobile terminal  50  on all of the uplink physical channels. 
         [0017]    The SIR on the DPCCH may be modeled by: 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0000]    where E c  represents the chip energy, I ISI  represents intersymbol interference (e.g., self-interference), I other  represents interference from other users, and N 0  represents thermal noise. A modified SIR, denoted herein as MSIR, is the SIR with self-interference suppressed and is given by: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0018]    According to embodiments of the present invention, the level of self-interference is taken into account in performing uplink power control. When self-interference is the dominant impairment (dominating the total SIR), the power control command generation is modified to avoid power rushes and system instability. Changes in power control command generation may be accomplished by changing the way the inner-loop power control generates power control commands, and/or by adjusting the SIR target used by inner-loop power control. The level of self-interference may be determined based on the delay spread of the channel. For example, whether the self-interference is dominant may be determined by comparing the delay spread of the channel, denoted τ rms , to a threshold ψ rms . A delay spread τ rms  larger than the threshold ψ rms  may be taken as an indication that self-interference is the dominant factor in the SIR estimate. 
         [0019]    When self-interference dominates the SIR estimate, a power control unit uses a “fast break” procedure for inner-loop power control. Otherwise, normal SIR-based inner-loop power control is used. In one exemplary embodiment, a “fast break” is introduced into the inner-loop power control process to interrupt normal SIR-based power control command generation. In one exemplary embodiment, the inner-loop power control may be configured to reduce or maintain the current transmit power level of the mobile terminal  50 , irrespective of the relationship between the received SIR and the SIR target. This procedure effectively decouples the power control command generation from the received SIR. 
         [0020]    Various techniques may be used to implement a fast break in the inner-loop power control. In one embodiment, a fast break may be achieved by commanding the mobile terminal  50  to reduce its transmit power even when the SIR is lower than the SIR target. In an alternative embodiment, a “fast break” may be achieved in the inner-loop power control by replacing current SIR estimates with a value that is larger than the SIR target. For example, the current SIR estimate may be replaced with a value that is as high as 10,000 times the current SIR estimate. Replacing the current SIR estimate with a value higher than the SIR target guarantees that the inner-loop power control generates a command to decrease the transmit power. 
         [0021]    In other embodiments of the invention, power control command generation may be changed by adjusting the SIR target. Lowering the SIR target causes the inner-loop power control to issue fewer up commands. SIR target adjustment may be used in place of the fast break procedure for inner-loop power control, or in combination with the fast break procedure. For example, SIR target adjustment may be used when the fast break procedure fails to bring the self-interference level down to an acceptable level. 
         [0022]    Those skilled in the art will appreciate that both the fast break procedure and the SIR target adjustment procedure causes an increase in the error rate. In this case, a retransmission protocol, such as hybrid ARQ, may be used to request retransmission of erroneously received data blocks. However, this effectively reduces the data throughput because more data blocks will be retransmitted when the fast break procedure or the SIR target adjustment procedure is used. 
         [0023]      FIG. 2  illustrates an exemplary inner-loop power control process  100  implemented by the base station  20 . This process is repeated during each power control interval. The process begins by determining the delay spread τ rms  of the channel (block  102 ) and measuring the SIR (block  104 ). After computing the delay spread of the channel, the base station  20  determines whether self-interference dominates other interference in the received SIR by comparing the delay spread to the threshold ψ rms  (block  106 ). If self-interference is not dominant (τ rms ≦ψ rms ), inner-loop power control generates a transmit power control (TPC) command based on the measured SIR on the DPCCH (block  108 ). More particularly, the measured SIR is compared to the SIR target set by outer-loop power control. If the SIR is below the SIR target, the inner-loop power control generates a transmit power control (TPC) command instructing the mobile terminal  50  to increase its transmit power. Conversely, if the received SIR is greater than the SIR target, the inner-loop power control generates a TPC command instructing the mobile terminal  50  to decrease its transmit power. Returning to block  106 , if self-interference is the dominant interference in the received SIR (τ rms &gt;ψ rms ), inner-loop power control uses an alternate “fast break” command generation procedure to generate the TPC commands (block  110 ). For example, the inner-loop power control may be configured to generate a TPC command instructing the mobile terminal  50  to reduce its transmit power regardless of the relationship between the SIR and SIR target. Alternatively, the inner-loop power control may be configured to generate alternating TPC commands to maintain the current transmit power level of the mobile terminal  50 . Other “fast break” procedures may also be used. 
         [0024]    In some embodiments, an uplink scheduler at the base station  20  may be prevented from scheduling high data rate transmissions by the mobile terminal  50  when self-interference dominates the SIR. In this case, the delay spread may be provided to the scheduler so that the scheduler may take the level of self-interference into account when scheduling data rates for the mobile terminal  50  (block  112 ). 
         [0025]    In some embodiments, the MSIR may be used in place of SIR for inner-loop power control. For example, the procedure shown in  FIG. 2  may be implemented using MSIR instead of SIR for normal inner-loop power control. In this case, the MSIR is compared to the SIR target set by the outer-loop power control. The selection of either the SIR or MSIR for inner loop power control can be based on the estimated SIR or the estimated MSIR. 
         [0026]    The delay spread of the channel can be estimated from the power delay profile (PDP) and delays reported by the path searcher. In a conventional RAKE or GRAKE receiver, the path searcher generates a PDP and estimates the delays {circumflex over (τ)} k  of the channel from the PDP. To determine delay spread, the base station  20  may first compute the total power P according to: 
         [0000]    
       
         
           
             
               
                 
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         [0000]    Base station  20  then computes a center of gravity τ COG  for the delays {circumflex over (τ)} k  according to: 
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         [0000]    Finally, the base station  20  computes the delay spread τ rms  based on the computed center of gravity τ COG  according to: 
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         [0027]    The SIR or MSIR can be computed in conventional manner. In a GRAKE receiver, the combining weights may be computed according to: 
         [0000]        w={circumflex over (R)}   −1   ĥ,    (6)
 
         [0000]    where ĥ represents the net channel response vector corresponding to the DPCCH and {circumflex over (R)} represents an impairment covariance matrix. The impairment covariance matrix {circumflex over (R)} in Equation (6) may be computed according to: 
         [0000]        {circumflex over (R)}=f   1   R   ISI ( ĝ )+ f   2   R   n ,   (7)
 
         [0000]    where {circumflex over (R)} ISI  represents a matrix of parametrically-estimated self-interference impairment correlations representing the covariance of the intersymbol interference (ISI), R n  represents a matrix representing the covariance of the thermal noise and other user interference, ĝ represents the medium channel response estimate corresponding to the DPCCH, and the parameters f 1  and f 2  represent fitting parameters related to the base station transmit power and noise power, respectively. In one exemplary embodiment, f 1 =E C /E P  and f 2 =N 0 , ignoring other user interference for simplicity, where E C  represents the transmit power (energy per chip), E P  represents pilot symbol energy, and N 0  represents thermal noise and other user interference power. 
         [0028]    In a GRAKE receiver, the SIR of the received signal on the DPCCH may be computed according to: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    The quantities in Equation (8) may be computed using the parameter estimation portion of a parametric GRAKE as described in G. E. Bottomley, T. Ottosson and Y. E. Wang, A generalized RAKE Receiver for Interference Suppression, IEEE Journal on Selected Areas in Communications, vol. 18, no. 8, August 2000 (Bottomley et al) and in U.S. Pat. No. 7,539,240 titled “Method and apparatus for parameter estimation in a generalized RAKE receiver”, both of which are incorporated herein by reference. 
         [0029]    The SIR estimate given by Equation (8) may be used for inner-loop power control as previously described by comparing the SIR estimate to an SIR target. The outer-loop power control module sets the SIR target in a conventional manner based on the block error rate (BLER) or frame error rate (FER) provided by a decoder  30  ( FIG. 3 ). If MSIR is used instead of SIR for inner-loop power control, the first term in the denominator in Equation (8) is dropped and the MSIR may be computed according to: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0030]      FIG. 3  illustrates an exemplary base station  20  for implementing the power control process as herein described. Base station  20  comprises a Generalized RAKE (GRAKE) receiver  22 , path searcher  24 , decoder  26 , and power control module  30 . The received signal r(t) is input to the GRAKE receiver  22 . GRAKE receiver  22  may, for example, comprises a parametric GRAKE receiver as described in Bottomley et al and in U.S. Published Patent Application No. 2005/0201447. The GRAKE receiver  22  demodulates the received signal and provides a vector of the received symbol estimates z to the decoder  26 . Path searcher  24  generates a PDP of the received signal and determines estimates the signal delays {circumflex over (τ)} k  of the multipath signal, which are used by the GRAKE receiver  22  for demodulation. Path searcher also provides the PDP and signal delays {circumflex over (τ)} k  to the power control module  30 . During the demodulation process, GRAKE receiver  22  computes the SIR, or alternatively the MSIR, of the received signal. The computed SIR or MSIR is provided to the power control module  30 . Decoder  26  decodes the received symbols and generates an estimate of the block error rate (BLER), which is also provided to the power control module  30 . Based on the input from the GRAKE receiver  22 , path searcher  24 , and decoder  26 , the power control module  30  performs inner-loop and outer-loop power control. The base station  20  may also include an uplink scheduler  40  for scheduling uplink transmissions from the mobile terminals  50 . 
         [0031]      FIG. 4  illustrates the main functional elements of the power control module  30 . Power control module  30  comprises delay spread estimator  32 , inner-loop power control unit  34 , and outer-loop power control unit  36 . The functional elements shown in  FIG. 4  may be implemented by one or more processors, microcontrollers, hardware, or a combination thereof. Delay spread estimator  32  generates a delay spread estimate τ max  based on the PDP and signal delays reported by the path searcher  24 , and provides the delay spread estimate to the inner loop power control unit  34 . Inner-loop power control unit  34  performs inner-loop power control as shown in  FIG. 2  based on the SIR and/or MSIR provided by the GRAKE receiver  22  and the delay spread τ rms  provided by the delay spread estimator  32 . Outer-loop power control unit  36  generates the SIR target for the inner-loop power control based on the BLER from the decoder  26 . 
         [0032]    The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.