Abstract:
A system and method for adjusting the power control target for a spread-spectrum communication system  110  is disclosed. A preferred embodiment comprises correcting a power control target  128  based upon the difference between a filtered series of actual error signals  124  and the expected number of errors  126  over a properly determined time window. By using a filtered series of actual error signals, the power control target update step-size is adaptive to the channel conditions, and the power-rise is reduced, thereby reducing power requirements and signal dropouts. The invention finds application, for example, in personal communication devices such as cellular telephones and may be implemented using a digital signal processor (DSP).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is related to the following co-pending and commonly assigned patent applications: Ser. No. 10/303,463, filed concurrently herewith and entitled “Method and Apparatus for Fast Convergent Power Control in a Spread Spectrum Communication System” and Ser. No.10/303,189, filed concurrently herewith and entitled “Method and Apparatus for Setting the Threshold of a Power Control Target in a Spread Spectrum Communication System”. Both of these applications are hereby incorporated herein by reference. 
   TECHNICAL FIELD 
   The present invention relates generally to an apparatus and method for power control in a communication system, and more particularly to an apparatus and method for adjusting the power control target and minimizing power-rise using a sliding-window-filtering algorithm in a spread-spectrum communication system. 
   BACKGROUND 
   Power control is commonly used in communication systems for minimizing transmission power while maintaining the received signal quality at the desired level. In a code division multiple access (CDMA) spread spectrum communication system, since one user&#39;s signal contributes to other users&#39; noise, power control is essential to mitigate the near-far problem and improve the system capacity. Furthermore, in order to minimize power consumption while ensuring a specified minimum quality of service (QoS) under varying channel conditions, the power control target, which is typically a threshold for the received signal to interference ratio (SIR), is updated autonomously to adapt to the change of communication environments. The QoS is typically specified in terms of a block error rate (BLER) or a bit error rate (BER). Examples of such communication systems include those operating under the IS-95, IS-2000, UMTS/WCDMA and TD-SCDMA standards. 
   For example, in a UMTS/WCDMA system (the UMTS/WCDMA standard can be found at http://www.3gpp.org), an open loop power control scheme is used for determining an initial transmission power at the start of a transmission. A closed loop power control scheme is used to adjust the ongoing transmission power to warrant the specified minimum QoS. The closed loop power control scheme includes both an inner loop power control system and an outer loop power control system. The inner loop power control system in a receiver estimates the received SIR and compares it to the power control target SIR target . If the estimated SIR is greater than the target SIR target , the receiver generates a power down command that is sent to the transmitter. Conversely, if the estimated SIR is lower than SIR target , the receiver generates a power up command that is sent to the transmitter. The transmitter then adjusts the transmission power based on the decoded received power control commands. This inner loop power control system operates at a 1,500 Hz update rate. The outer loop power control system uses an algorithm to control SIR target  by adjusting it such that the specified minimum QoS is achieved at minimum power all the time. 
   A significant concern in the SIR target  update algorithm is the resulting power-rise. Power rise is defined as the difference between the actual average transmitted power and the minimum transmitted power required to meet the specified minimum QoS. The smaller (and non-negative) the power-rise, the better the SIR target  update algorithm for several reasons. A larger power-rise results in reduced system capacity due to the nature of a spread spectrum communication system. This excess transmitted power reduces the battery life for a mobile terminal such as a cellular telephone. The excess transmitted power also produces un-necessary interference to other mobile receivers. 
   If the transmitted power is lower than that required to warrant the specified minimum QoS, communication will suffer high error rate or even dropouts may occur. 
   A prior art SIR target  update algorithm  100  is illustrated in  FIG. 1   a . In this prior art, a receiver receives a series of data blocks, one block at each time. Each block can be determined as a good block or a bad block based on, for example, the result of a CRC check. Upon decoding the current data block, the block is checked for errors  102 . If an error occurs, the SIR target  update algorithm steps up SIR target  by an integer multiple K of a fixed increment A as shown by  104 . If no error occurs, the SIR target  update algorithm would step down SIR target  by the fixed increment A as shown by  106 . By using fixed increments, significant overshoot and undershoot occurred. It should also be noted that this prior art SIR target  update algorithm bases its SIR target  update on just the current data block. This memory-less operation will produce large power-rise under steady channel conditions when the SIR target  is expected to be as constant as possible. 
   An alternative SIR target  update algorithm is based upon the proportional-integral-derivative (PID) controller as shown in  FIG. 1   b . This approach filters the difference between the specified minimum QoS (labeled as “Desired QoS”) and the actual QoS and then updates SIR target  based upon this difference. It should be noted that in this prior art the actual QoS is computed from all the previously received data blocks. Under varying channel conditions, the SIR target  is expected to track and compensate the change of channel as quickly as possible. This full-memory operation, however, responded slowly to the change of channel, and results in significant overshoot and undershoot, and therefore high power-rise. 
   Thus there exists a strong need to reduce the power-rise in a power-controlled communication system by using variable step-size based on proper length of history in the SIR target  update algorithm. 
   SUMMARY OF THE INVENTION 
   These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present invention that reduce the target SIR SIR target  overshoot and undershoot. By avoiding SIR target  overshoot or undershoot, embodiments of the present invention reduce power consumption by a PCD and interference with other PCDs. 
   In accordance with a first embodiment of the present invention, a method for controlling SIR target  comprises receiving a series of actual error signals, filtering the series of actual error signals, computing an expected error signal, comparing the filtered series of actual error signals and the expected error signal and generating a correction signal (SIR cor. ) based upon this comparison, and updating SIR target  based upon SIR cor . 
   In accordance with a second embodiment of the present invention, an apparatus for controlling SIR target  comprises a receiver for receiving a series of actual error signals, a filter for filtering the series of actual error signals, a processor for computing an expected error signal, a comparator for comparing the filtered series of actual error signals and the expected error signal, the comparator thus generating SIR cor . and a corrector for adjusting SIR target  based upon SIR cor. . 
   In accordance with a third embodiment of the present invention, a digital signal processor (DSP) for inclusion in a communication device comprises digital signal processing code for receiving a series of actual error signals, filtering the series of actual error signals, computing an expected error signal, comparing the filtered series of actual error signals and the expected error signal thereby generating SIR cor . and adjusting SIR target  based upon SIR cor. . 
   An advantage of the preferred embodiment of the present invention is that it reduces power-rise that consumes transmission power in a PCD. By minimizing transmission power, a battery&#39;s operating time in a PCD can be extended. 
   A further advantage of the preferred embodiment of the present invention is that by minimizing power-rise, more PCDs can operate from a single base station while maintaining a specified minimum QoS, respectively. This increase in the number of PCDs for each base station reduces the number of required base stations, thereby reducing overall communication system costs. 
   Yet another advantage of embodiments of the present invention is that by reducing power-rise, self-generated interference is reduced. By reducing self-interference, a specified minimum QoS can be maintained at lower transmission power levels. 
   Another advantage of embodiments of the present invention is that signal dropouts are reduced by reducing SIR target  undershoot. By reducing the number of signal dropouts, a specified minimum QoS can more readily be maintained. 
   The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description that follows may be better understood. Additional features and advantages of the invention will be described hereinafter, which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed might be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
       FIG. 1   a  is a flowchart of the prior art target SIR control system; 
       FIG. 1   b  is a block diagram of a portion of a prior art communication system; 
       FIG. 2  is an overview of a telecommunications system that can incorporate an embodiment of the present invention; 
       FIG. 3  is a an overview of a personal communication device that can incorporate an embodiment of the present invention 
       FIG. 4   a  illustrates the flow of error signals in an embodiment of the present invention; 
       FIG. 4   b  illustrates the data within each error signal for use with an embodiment of the present invention; and 
       FIG. 5  is a flowchart of an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The process and a system for implementing this process of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. 
   The present invention will be described with respect to preferred embodiments in a specific context, namely a personal communication device (PCD). The invention may also be applied, however, to other communication systems. 
     FIG. 2  shows an overview of a communication system  110 . The system includes both a base station  112  and a PCD  114 . The base station  112  and the PCD  114  transmit and receive data via a down link channel  116  and an up link channel  118 . Performance of the base station  112  is optimized in part by a power adjustment  120  received from a transmission power command (TPC) estimator  122 . Performance of the PCD  114  is optimized in part by adjusting the target signal to interference ratio (SIR target ) in an outer loop power control and generating the TPC in an inner loop power control. This optimization uses filtered error signal data  124 , expected error calculation data  126 , target SIR adjustment data  128  and a TPC generator  130 . The filtered error signal data  124  is used for target SIR adjustment  128 . The expected error calculation data  126  is used in target SIR adjustment  128 . Lastly, the output signal of the target SIR adjustment  128  is used in the TPC generator  130 . 
   An example PCD  114  in the form of a cellular telephone  140  is illustrated in  FIG. 3 . The cellular telephone  140  includes an antenna  142 , an input/output section  144 , a processor/memory unit  146 , a speaker  148 , a display panel  150 , a keypad  152 , and a microphone  154 . Data frames are received by the antenna  142 , modified by the input/output section  144 , and provided to the processor/memory unit  146 . The processor/memory unit  146  may also receive data from the keypad  152  or the microphone  154 . The processor/memory unit  146  may display data on the display panel  148  or output sounds to the speaker  148 . While the processor/memory unit  146  is illustrated as a single element, a separate processor and a separate memory may also be used. A digital signal processor (DSP) may also be used as the processor/memory unit  146 . 
   As the specified minimum quality of service (QoS) is frequently a function of, or equal to, the Block Error Rate (BLER) or the Bit Error Rate (BER), the BLER will be used to represent the QoS without loss of generality throughout the remainder of this description. A BLER of 1% may be adequate for voice-only communication applications while a BLER of 10% will typically be required for data communication applications. 
   Referring back to  FIG. 2 , the PCD  114  receives a series of data frames from the base station  112  via the down link channel  116 . After processing the series of data frames, a series of actual error signals  160  is generated as shown in  FIG. 4   a . The series of actual error signals  160  includes individual actual error signals  162 - 172 . Actual error signal ES 0    162  is the error signal for the current data frame. Actual error signal ES 1    164  is the error signal for the previous data frame. The present invention adjusts SIR target  using a limited number of actual error signals. The sliding window  174  of  FIG. 4   a  illustrates the use of L+1 actual error signals. The actual error signal ES 0    162  comprises the quantity of data blocks N 0    180  in the current data frame and the quantity of data blocks in error N error,0    182  in the current data frame as illustrated in  FIG. 4   b . The length of the sliding window L is an integer greater than 0 and is only limited by the amount of memory within the PCD  124 . In a preferred embodiment of the present invention L is selected such that the expected number of data blocks in error within the window is between 3 and 12. 
   A loop of the process steps  200  of the present invention is shown in  FIG. 5 . The loop of process steps  200  comprises five primary steps that are repeated continuously during the course of data transmission and receipt. In a preferred embodiment, the loop of process steps  200  would be executed at a proper rate (e.g., 100 Hz or 50 Hz for WCDMA). First, the next actual error signal is received in step  202 . The received actual error signals are then filtered in step  204  to create N error,filter . An expected error signal (N error,exp ) is computed based upon the received error signals in step  206 . The filtered actual error signals N error,filter  and the expected error signal N error,exp. are then compared, thereby generating the correction signal SIR cor. in step  208 . Lastly, in step  210  SIR cor. is used to adjust SIR target . The process is then iterated as needed. Referring to the cellular telephone example illustrated in  FIG. 3 , the loop of process steps  200  will generally be conducted within the input/output section  144  and the processor/memory unit  146 . 
   The process steps will now be described in greater detail. The received actual error signals of step  202  create the series of actual error signals  160  shown in  FIG. 4   a . Step  204  computes N error,filter  according to Equation 1: 
                     N     error   ,   filter       =         ∑     i   =     1   ⁢           ⁢   …   ⁢           ⁢   L         ⁢       w   i     *     N     error   ,   i           +     N     error   ,   0           ,           Eq   .           ⁢   1               
where w t  is a weighting factor for the ith actual error signal and N error,i  is the quantity of data blocks in error in the ith data frame of the sliding window  174 . The weighting factors w i  will typically range from 0.0 to 1.0. As an example, the most recent data frames may have weighting factors w i  between 0.8 and 1.0 and the oldest data frames may have weighting factors w i  between 0.0 and 0.2.
 
   Using a voice communication system as a more specific example, the following parameters are typical. Assume that the communication system requires a BLER of 1% with 1 data block per data frame. In one example, the sliding window would have a length L+1 of 500 to provide an expected number of data blocks in error within the window of 5. The weighting factors for a simple sliding window filter could be all 1. 
   While a sliding window filter has been described in detail, other more general filter routines are possible. As an example, a single pole infinite impulse response (IIR) filter may be used. This IIR filter has the advantage of reducing memory costs and computation time, but is less flexible than the sliding window filter. The sliding window filter itself is but one type of finite impulse response (FIR) filter, and other FIR filters may be more suitable depending upon the application and time or memory constraints. 
   Step  206  first computes the total number of data blocks in the last L+1 data frames (N total ) according to Equation 2: 
                     N   total     =       ∑     i   =     0   ⁢           ⁢   L         ⁢     N   i         ,           Eq   .           ⁢   2               
where N t  is the quantity of data blocks in the ith data frame.
 
   Next, step  206  computes the expected number of blocks in error Nerror,exp. according to Equation 3:
 
 N   error,exp   =BLER*N   total .  Eq. 3
 
   At this point, the system will determine whether the SIR target is to increase or decrease. This step is labeled with reference numeral  208  in  FIG. 5 . If, in the current frame, the number of blocks in error is greater than the product of the total number of blocks and the desired block error rate (i.e., N error,0 &gt;=BLER*N 0 ), then SIR target will increase. Otherwise, the SIR target will decrease. 
   Step  210  generates the correction signal SIR cor  in one of several ways, depending upon the application. In general, SIR cor . is proportional to the difference between the filtered series of actual error signals N error,filter  and the expected error signal N error,exp . according to Equations 4A and 4B:
 
 SIR   cor.   =k   1 *(N error,filter   −N   error,exp. )*Δ SIR  and  Eq. 4A
 
 SIR   cor.   =k   2 *(N error,filter   −N   error,exp. )*Δ SIR,   Eq. 4B
 
where Equation 4A applies when the SIR target needs increasing and Equation 4B applies when the SIR target needs decreasing. In these equations, k 1  and k 2  are predetermined constants and Δ SIR  is the minimum SIR target increment, which may be constant or variable. Typically, k 1  and k 2  are positive. In a preferred embodiment, 1&lt;=k 1 &lt;10 and 0&lt;k 2 &lt;=1. While not generally case, it is possible that k 1 =k 2 .
 
   While Equations 4A and 4B appear to base the correction signal SIR cor  on just the difference between the actual error signals and the expected error signal, this is not the case. In this particular embodiment, the correction signal is based upon the difference between the filtered actual error signals over a properly chosen sliding window and the expected error signal over the same sliding window. This filtering of the actual error signals and determining the sliding window size provides greater flexibility and allows the present invention to reduce power-rise relative to a target SIR control algorithm based on the PD method. 
   In the preferred embodiment, the correction signal SIR cor  is found according to Equations 5A and 5B:
 
 SIR   cor.   =k   1 *max [0, ( N   error,filter   −N   error,exp. )]*Δ SIR   Eq. 5A
 
SIR cor   =k   2 *min [0, ( N   error,filter   −N   error,exp. )]*Δ SIR   Eq. 5B
 
where Equation 5A applies when the SIR target will increase, and equation 5B applies when the SIR target will decrease as determined in  208 .
 
   Lastly, in step  212  the correction signal SIR cor  is used to adjust the target SIR target . The process is then iterated as needed. The new target SIR SIR target,new  is the current SIR target  updated according to Equation 6:
 
 SIR   target,new   =SIR   target   +SIR   cor. .  Eq. 6
 
   Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, means, methods, or steps.