Abstract:
A system and method for adjusting the power control target for a spread-spectrum communication system is disclosed. A preferred embodiment comprises correcting the power control target based upon the estimated slope of the SIR versus quality of service (QoS) curve under current operating conditions. By using the estimated slope of the current SIR versus QoS curve, the power control target converges to the desired value most quickly, and the SIR target overshoot or undershoot is maximally avoided, and the power rise is minimized, 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  
       [0001]    This application is related to the following co-pending and commonly assigned patent applications: Ser. No.______, filed concurrently herewith and entitled “Method and Apparatus for Low Power-Rise Power Control Using Sliding Window Weighted QoS Measurements” (Attorney Docket No. TI-34260) and Ser. No.______, filed concurrently herewith and entitled “Method and Apparatus for Setting the Threshold of a Power Control Target in a Spread Spectrum Communication System” (Attorney Docket No. TI-34262). Both of these applications are hereby incorporated herein by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates generally to a system and method for power control in a communication system, and more particularly to a system and method for updating the power control target to achieve fast convergence and reduce power-rise by using an algorithm to find an estimated slope and correcting the power control target based upon the estimated slope.  
         BACKGROUND  
         [0003]    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.  
           [0004]    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.  
           [0005]    A significant concern in the SIR target  update algorithm is the resulting power-rise. Power rise is a non-negative quantity defined as the difference between the actual average transmitted power for the specified QoS and the minimum transmitted power required to meet the specified minimum QoS. The smaller 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 PCD such as a cellular telephone. The excess transmitted power also produces additional interferences to other PCDs.  
           [0006]    If the transmitted power is lower than that required to warrant the specified minimum QoS, communication will suffer a high error rate or even experience dropouts.  
           [0007]    To reduce power-rise, the power control target is expected to be as constant as possible if the communication channel conditions are steady. On the other hand, when the communication channel conditions are changing, the power control target is expected to follow as fast as possible.  
           [0008]    A prior art SIR target  update algorithm  100  is illustrated in FIG. 1 a . In the prior art, a receiver would receive a series of data blocks, one block at each time. Each block can be determined as good block or bad block based on, for example, the result of a CRC check. Upon decoding the current data block, the block would be checked for errors  102 . If an error occurred, the SIR target  update algorithm would step up SIR target  by an integer multiple K of a fixed increment Δ 104 . If no error occurred, the SIR target  update algorithm would step down SIR target  by the fixed increment Δ 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.  
           [0009]    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 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, responds slowly to the change of channel conditions. The slow convergence of the power control target to the desired target value results in significant overshoot and undershoot, and therefore high power-rise.  
           [0010]    Thus there exists a strong need to reduce the power-rise in a power-controlled communication system by improving the convergence speed in the SIR target  update algorithm.  
         SUMMARY OF THE INVENTION  
         [0011]    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, the present invention reduces power consumption by the PCD and minimizes interference with other PCDs.  
           [0012]    In a first embodiment, the present invention includes a process for updating a current target signal to interference ratio (SIR) in a communication system. An actual error rate is received. The current target SIR is then updated based on the actual error rate and a slope of at least one reference curve (preferably two). The reference curve corresponds to error rates as a function of signal to interference ratios for the communication system.  
           [0013]    In accordance with another embodiment of the present invention, a method for updating the target SIR target  comprises storing at least a first table of SIRs as a function of error rates in a communication system under a first reference channel condition and storing at least a second table of SIRs as a function of error rates under a second reference channel condition. An actual error rate Err act.  is received and an expected error rate Err exp.  is also received. A weighting ratio is computed as a function of a current target signal to interference ratio SIR target , a signal to interference ratio SIR QoS*  from the first table corresponding to Err act.  and a signal to interference ratio SIR ref.,QoS*  from the second table corresponding to Err act. . An estimated slope is then computed as a function of the weighting ratio, a signal to interference ratio SIR QoS  from the first table corresponding to Err exp. , SIR target , a signal to interference ratio SIR ref.,QoS  from the second table corresponding to Err exp.  and SIR ref,QoS* . A correction factor Δ SIR  is computed as a function of the estimated slope and a first predetermined constant k 1  when the target SIR is determined to go up and a second predetermined constant k 2  when the target SIR is determined to go down. The target signal to interference ratio can then be updated based upon Δ SIR .  
           [0014]    An advantage of the preferred embodiment of the present invention is that it improves the power control target convergence speed and 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.  
           [0015]    A further advantage of preferred embodiments of the present invention is that by improving the power control target convergence speed and 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.  
           [0016]    Yet another advantage of the preferred embodiment of the present invention is that by improving the power control convergence speed and reducing power-rise, self-generated interference is reduced. By reducing self-interference, a specified minimum QoS can be maintained at lower transmission power levels.  
           [0017]    An advantage of preferred embodiments of the present invention is that by improving the power control target convergence speed and reducing SIR target  undershoot, signal dropouts are reduced. By reducing the number of signal dropouts, a specified minimum QoS can more readily be maintained.  
           [0018]    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 may 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.  
       
    
    
     DESCRIPTION OF THE DRAWING  
       [0019]    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:  
         [0020]    [0020]FIG. 1 a  is a flowchart of the prior art target SIR control system;  
         [0021]    [0021]FIG. 1 b  is a block diagram of a portion of a prior art communication system;  
         [0022]    [0022]FIG. 2 is an overview of a telecommunications system that can incorporate an embodiment of the present invention;  
         [0023]    [0023]FIG. 3 is a an overview of a personal communication device that can incorporate an embodiment of the present invention;  
         [0024]    [0024]FIGS. 4 a - 4   d  illustrates channel curves based on error rate as a function of SIR for use with an embodiment of the present invention; and  
         [0025]    [0025]FIG. 5 is a flowchart of an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0026]    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.  
         [0027]    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.  
         [0028]    [0028]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  based on the instructions from a transmission power command (TPC) estimator  122 . The TPC is transmitted from the PCD. Performance of the PCD  114  is optimized in part by updating 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 requires estimated slope data  124 , expected error rate data  126 , target SIR update data  128  and a TPC generator  130 . The estimated slope data  124  is used for target SIR update data  128 . The expected error rate data  126  is used in target SIR update data  128 . Lastly, the target SIR update data  128  is used in the TPC generator  130 .  
         [0029]    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 .  
         [0030]    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% or better will typically be required for data communication applications.  
         [0031]    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, actual error rate data is calculated. This actual error rate data preferably includes the number of blocks in error and the total number of blocks in a data frame, thereby allowing calculation of the actual BLER, Err act. . In addition, the PCD  114  must establish the expected BLER data, Err exp .  
         [0032]    During actual operation, the SIR target  for the PCD  114  will vary as operating conditions change in order to guarantee the QoS. These changes may be caused, for example, by changes in the distance between the PCD  114  and the base station  112 , increases or decreases in the number of PCDs in use for a given base station  112 , changes in topology (including intervening hills or buildings) and changes in the speed. The PCD  114  must therefore update the SIR target  as quickly as possible to minimize power-rise.  
         [0033]    [0033]FIG. 4 a  illustrates three BLER curves as a function of SIR. A first reference channel curve  160  indicates the performance of the communication channel under a first set of operating conditions. The first reference channel curve  160  shows that a lower BLER requires a higher SIR as would be expected. A second reference channel curve  164  is also illustrated in FIG. 4 a . The second reference channel curve  164  illustrates the communication channel under conditions that are worse than those of the first reference channel curve  160 . This is clear in that the second reference channel curve  164  shows a higher SIR is required for any given BLER compared to the first reference channel curve  160 . The first and second reference channel curves  160 ,  164  are generated either through modeling of the communication system under certain conditions or are empirically measured.  
         [0034]    Between the first and second reference channel curves  160 ,  164  is a current channel curve  162 . The current channel curve  162  represents the BLER as a function of SIR for the communication system under the current operating conditions. The precise location and shape of the current channel curve will be unknown and will change with changes in operating conditions. Under clear conditions with few obstructions and when few other PCDs are in use, the current channel curve  162  will shift to the left, while adverse current conditions that may include many tall buildings at a time when lots of other PCDs are in use, the current channel curve  162  will shift to the right.  
         [0035]    As the precise location and shape of the current channel curve  162  changes with time, its location and shape can be estimated with respect to the two reference channel curves  160 ,  164 . By estimating the location and shape of the current channel curve, the present invention can more rapidly converge on the SIR target  that is appropriate for the current operating conditions.  
         [0036]    In a preferred embodiment, the first reference channel curve  160  corresponds to the communication system operating under ideal conditions. Under ideal conditions, the channel noise will be additive white Gaussian noise (AWGN). Therefore, if the first reference curve is based upon an AWGN channel, the current channel curve  162  will never be further to the left (lower) than the first reference curve  160 .  
         [0037]    In the preferred embodiment, the second reference channel curve  164  shows the BLER as a function of SIR for the communication system under the worst case operating channel conditions. With the two reference channel curves  160 ,  164  thus defined, the current channel curve  162  will of necessity fall between the two reference channel curves  160 ,  164 . In other embodiments, other reference curves may be appropriate based upon alternative channel conditions.  
         [0038]    Because the current channel curve  162  will have the same general shape as either of the two reference channel curves  160 ,  164 , either or both of the these reference channel curves  160 ,  164  can be used to estimate the current channel curve  162 . In a preferred embodiment of the present invention, an estimated slope of the current channel curve  180  is calculated based upon the actual BLER Err act. , the expected BLER Err exp. , the slope of the first reference channel curve  178 , and the slope of the second reference channel curve  182 .  
         [0039]    Continuing with the example PCD  114  of a cellular telephone  140 , the processor/memory unit  146  of the cellular telephone  140  calculates the estimated slope of the current channel curve  180  in a several step process. In a preferred embodiment, the first and second reference channel curves  160 ,  164  are stored in the processor/memory unit  146  as respective first and second tables. The accuracy of the estimated slope of the current channel will depend upon the number of entries in the first and second tables. Table 1, below, is an example table for the first reference channel curve  160  and shows the SIR required to meet a given BLER under a first set of reference channel conditions, and the corresponding BLER.  
                           TABLE 1                                   SIR   BLER                           1.2 dB   1%           1.18 dB    2%           . . .   . . .            0.8 dB   10%                       
 
         [0040]    Since the number of entries stored in the tables is fixed, the processor/memory unit  146  will round the actual BLER to a BLER entry found in the tables. In a preferred embodiment, this rounding may take the form of a floor function, rounding to the next lowest BLER. While the preferred embodiment utilizes tables, the first and second reference channel curves  160 ,  164  may be calculated based on polynomial equations. While calculating the first and second reference channel curves  160 ,  164  avoids rounding the actual BLER when using tables, it will require additional computation time each time the estimated slope of the current channel curve is computed.  
         [0041]    Upon receiving both the actual error rate Err act.  and the expected error rate Err exp. , the processor/memory unit  146  will calculate a weighting ratio r according to Equation 1: 
           r =(SIR target −SIR QoS* )/(SIR ref,QoS* −SIR QoS* ).  Eq. 1 
         [0042]    As shown in FIG. 4 b , SIR target  corresponds to the current target SIR that will be updated when the correction process is completed. SIR QoS*  corresponds to the SIR entry in the first reference channel curve  160  table at the BLER corresponding to Err act. . Lastly, SIR ref.,QoS*  corresponds to the SIR entry in the second reference channel curve  164  table at the BLER corresponding to Err act. . Referring to FIG. 4 a , SIR target  corresponds to the point labeled “Current SIR Target”, SIR QoS*  corresponds to the point labeled A and SIR ref.,QoS*  corresponds to the point labeled C.  
         [0043]    [0043]FIG. 4 c  is provided to more clearly illustrate the two differences used to calculate the weighting factor r in Equation 1. The weighting factor is useful since the shape of the current channel curve  162  will most likely more closely resemble the shape of the reference curve to which it is closest.  
         [0044]    Once the weighting ratio r is computed, the estimated slope of the current channel curve, denoted by s and corresponding to  180  in FIG. 4 a , is calculated according to Equation 2: 
           s =|(1 −r)*(SIR   QoS −SIR QoS* )+ r *(SIR ref.,QoS −SIR ref.,QoS* )|.  Eq. 2 
         [0045]    As shown in FIG. 4 b , SIR QoS  corresponds to the SIR entry in the first reference channel curve  160  at the expected error rate Err exp. . SIR ref.,QoS  corresponds to the SIR entry in the second reference channel curve  164  at Err exp. . Thus, s, the estimated slope of the current channel curve  180  is a weighted average of the first and second reference channel slopes  178 ,  182 . Referring to FIG. 4 a , SIR QoS  corresponds to the point labeled B and SIR ref.,QoS  corresponds to the point labeled D. FIG. 4 d  is provided to more clearly illustrate the two differences used to calculate the estimated slope s in Equation 2.  
         [0046]    If only a single reference curve is used, the calculation of the slope s would be simplified. In this case, the slope would be computed as the difference between SIR at the desired error rate and the SIR at the measured error rate. With only one curve, no weighting factor r would be needed. Similarly, if more than two reference curves were to be used, then a corresponding number of weighting factors would be used.  
         [0047]    A SIR correction factor Δ SIR  is computed based upon the estimated slope s according to Equation 3: 
         Δ SIR   =k   2   *s,   Eq. 3 
         [0048]    when the target SIR is to step up and according to Equation 4: 
         Δ SIR   =k   2   *s,   Eq. 4 
         [0049]    when the target SIR is to step down. The factors k 1  and k 2  correspond to predetermined constants with k 1 &gt;0 and k 2 &lt;0. While the magnitude of k 1  and k 2  may typically range from 0 to 5, in a preferred embodiment the values of k 1  and k 2  will be in the approximate range of 0.5&lt;k 1 &lt;5 and −2&lt;k 2 &lt;0. Both k 1  and k 2  may have the same magnitude and the most typical magnitude is 1 for both.  
         [0050]    Lastly, the target SIR SIR target  is updated by computing a new target SIR SIR target,new  according to Equation 5: 
         SIR target,new =SIR target +Δ SIR .  Eq.5 
         [0051]    Referring to FIG. 4 a , SIR target,new  will rapidly converge on the point labeled with the words “Desired SIR Target” after several updating iterations.  
         [0052]    [0052]FIG. 5 illustrates the overall process flow  200  for finding the estimated slope s and updating the current target SIR SIR target  to the new target SIR SIR target,new . Step  202  corresponds to storing the first table of SIR and BLER values for the first reference channel curve  160 . Step  204  corresponds to storing the second table of SIR and BLER values for the second reference channel curve  164 .  
         [0053]    In step  206 , the actual error rate Err act.  and the expected error rate Err exp  are received. In step  208 , the weighting ratio r is computed according to equation 1. Step  210  corresponds to computing the estimated slope according to equation 2. In step  212 , the correction Δ SIR  is computed according to Equations 3 or 4 depending upon the relationship between Err act.  and Err exp. . In step  214  the current target SIR SIR target  is updated to the new target SIR SIR target,new  according to equation 5. As the current operating conditions are dynamic, step  216  causes the process steps  206 - 214  to be repeated, thereby ensuring minimal power-rise.  
         [0054]    While FIG. 5 shows only process steps  206 - 214  being repeated, the first and second reference channel curve  160 ,  164  tables could be updated as needed. In this case, step  216  would cause process steps  202 - 214  to be repeated. While steps  202 - 214  could be repeated each time, it is unlikely that the reference channel curve  160 ,  164  tables would need updating this frequently. In a preferred embodiment, the first and second reference channel curve  160 ,  164  tables would be updated as part of the initialization process upon powering up the PCD  114 . Furthermore, while a single table for each of the first and second reference channel curves  160 ,  164  is preferred, a set of tables corresponding to each of the first and second reference channel curves is possible. For example, a first table for the first reference channel curve  160  may span the BLER range of 1-10%, while a second table for the first reference channel curve  160  may span the BLER range of 0.01-1.0%.  
         [0055]    In another embodiment of the present invention, the current channel curve  162  is estimated using only one of the reference channel curves  160 ,  164 . For example, the current channel curve is estimated to have the same shape as the first reference channel curve  160 , but be shifted to higher SIR values to the right. The advantage of this embodiment is that only a single table need be stored in memory and that no weighting ratio r need be calculated and the estimated slope of the current channel curve  180  will equal the slope of the reference channel curve  178 . However, this simplified approach will not converge as rapidly as the weighted, two reference channel curve approach described above.  
         [0056]    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. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, means, methods and steps described in the specification. 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.