Abstract:
A method for providing power control to a uplink signal, comprising an uplink station transmitting an uplink signal to a satellite. The satellite also transmits a loopback signal representing the received uplink signal back to the uplink station. The satellite also transmits a beacon signal to the uplink station. The uplink station adjusts the power of the uplink signal based on measurements of the beacon signal and the uplink signal.

Description:
[0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/417,874 filed Oct. 11, 2002. Application No. 60/417,874 is hereby incorporated by reference in its entirety. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    Maintaining received power level of carrier signals being transmitted through a wireless channel.  
         BACKGROUND OF THE INVENTION  
         [0003]    Information is sent via a communication channel. A transmitter may communicate the information by modulating a carrier signal (e.g., a wireless carrier). A receiver may demodulate the carrier signal to recover the information. The carrier signal must have sufficient power to ensure that the receiver can demodulate the carrier signal. An example of a carrier signal is an uplink communications carrier signal which may be sent from a transmit terminal at an uplink station to a space based satellite which may then retransmit the signal via downlink communication carrier signal to a receive terminal at a receive station.  
           [0004]    Wireless carriers are subject to fluctuating received power levels due to changes in atmospheric conditions. For example, in satellite communications, transmitted carriers can be affected by changes in the atmospheric conditions on both sides of the communication link (earth to satellite and satellite to earth). This is shown in FIG. 1, which shows the variation in attenuation that a 27 GHz and a 20 GHz carrier signal may experience over time. Atmospheric changes maybe caused by rain, snow, hail, ice, clouds and moisture. In many instances, these changes can cause significant degradation to the received power level of the carrier signals (such as the uplink signal) and result in poor quality of reception.  
           [0005]    The most common approach to correct for the received power level problem is to monitor a non-communications signal and then adjust the transmit carrier power based on the measured degradation in such a non-communications signal. As in the example above the space based satellite may send the non-communication signal (such as a beacon signal, or a pilot tone) to a receive terminal at the uplink station. Although this method can sometimes be effective, degradation that is experienced by the non-communication signal does not always correlate well with degradation experienced by the uplink carrier signals. In addition, fluctuations experienced by carriers in a wireless channel can be highly unpredictable and rapid, causing the power control scheme to lag significantly behind the signal degradation, thus rendering power corrective actions ineffective.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    One aspect of the invention is a method of controlling a first signal that is being transmitted from a first station to a second station. The method utilizes a closed feedback loop and an open control loop for controlling the performance of the first signal. The closed feedback loop and the open control may be implemented separately or in combination. The closed feedback loop and/or the open control loop may take into account historical measurement data.  
           [0007]    The open control loop may utilize a non-communication signal (such as a beacon or a pilot) transmitted from the second station to the first station. The performance of the non-communication signal may indicate the performance of the first signal as received by the second station. The first station may perform a first measurement of the performance of the non-communication signal and use the first measurement in the open control loop in an effort to control the performance of the first signal as received by the second station. This is an open control loop as the first measurement is an indirect measure of the performance of the first signal as received by the second station.  
           [0008]    The closed feedback loop may utilize a loopback communication signal. The second station may transmit the loopback communication signal to the first station. The loopback communication signal comprises a retransmission of the first signal as received by the second station. The first station may perform a second measurement of the performance of the loopback signal. The first station may utilize the second measurement in the closed feedback control loop. This is a closed feedback loop as the second measurement is a direct measure of the first signal as received by the second station.  
           [0009]    Both control loops may be utilized repeatedly in an effort to control the performance of the first signal. Both control loops may control the first signal independently or in sequence. The efforts of both control loops may be combined as explained herein. Each control loop may take into account a historical record of measurements that each control loop has made. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 shows attenuation of a carrier due to atmospheric changes;  
         [0011]    [0011]FIG. 2 is a high-level block diagram of an embodiment the invention;  
         [0012]    [0012]FIG. 3 is an illustration of a system in which an embodiment of the invention may be employed;  
         [0013]    [0013]FIG. 4 is a block diagram of a system in accordance with an embodiment of the invention;  
         [0014]    FIGS.  5 A-G are illustrations of dead zones and how they might be employed in an embodiment of the invention; and  
         [0015]    FIGS.  6 A-C are illustrations of a dead zone and how it might be used in an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    To alleviate the problems with power control as practiced in the prior art, we have developed a method for effective power control of a communication signal. This power control may be of a signal transmitted from a ground station to a satellite or of a signal transmitted from a satellite to a ground station (or to another satellite). Further, while the invention is primarily directed towards satellite communications, it may be applied to transmission of communication signals in other environments. This approach combines the real-time monitoring results of a loopback communications carrier signal with those of the non-communications signals (e.g., satellite beacon and pilot signals in satellite based communications) as indicators for determining the proper amount of transmit power adjustment. Loop-back carriers are defined as those carriers that can be received at their transmit location (e.g., they are sent from a ground station to a satellite and then returned from the satellite to the ground station). In addition, this approach may utilize data from past received power measurements or other performance parameter (such as energy per bit to noise power spectral density ratio (Eb/No)) in a weighted fashion for determining the amount of adjustment.  
         [0017]    [0017]FIG. 2 shows a high-level block diagram of a power control method  200  in accordance with one embodiment of the invention. A first step  202  is starting the power control method  200 , which may involve initializing parameters, systems and/or algorithms.  
         [0018]    A step  204  is to measure one or more non-communication signals. Examples of the non-communication signals are: pilot signals, beacon signals, and other signals that may be transmitted along with the communication signal. A step  206  is to determine how much the non-communication signals have degraded. This may be accomplished by comparing a parameter associated with the non-communication signals and a nominal value of said parameter. Examples of said parameter might be: received power level; Eb/No; etc. The use of Eb/No as the parameter associated with the non-communication signal assumes that the non-communication signal is digitally modulated. A step  208  is to determine how the power of the communication signal should be adjusted based on results from step  206  and possibly from a past record of the results from step  206 .  
         [0019]    A step  210  is to measure a loop-back communication signal. A step  212  may be to determine a performance of the loop-back communication signal by characterizing a degradation of the loop-back signal. A step  214  is to determine the power adjustment for the communication signal using the results of the step  212  and possibly from the past results to the step  212 . Finally, steps  204 - 214  are performed repeatedly.  
         [0020]    In one aspect, our approach comprises two distinct elements: (1) utilization of loop-back communications carriers for power control (steps  210 - 214 ) in addition to the utilization of non-communications signals such as beacon and/or pilot signals (steps  204 - 208 ); and (2) a technique in which carrier transmit power adjustments are determined based on prior measured performance and/or prior power adjustments (steps  208  and  214 ). These elements may be implemented separately or in combination.  
         [0021]    Two transmit power adjustment steps ( 208  and  214 ) are shown in FIG. 2. Thus, the method may alternate between: ( 208 ) determining a power adjustment in response to measured performance of a non-communications signal; and ( 214 ) determining a power adjustment in response to a measured performance of a communications signal. The results of these alternating steps may be time-averaged (e.g., by using a weighting function) such that each power adjustment is based on prior power adjustments and/or prior measured performance (of either or both of the communication signal and the non-communication signal).  
         [0022]    It will be apparent that modifications to the method as illustrated in FIG. 2 may be made. For example, multiple power adjustments may be performed in response to measurements of a non-communications signal between adjustments made in response to measurements of the communication signal. Similarly, multiple power adjustments may be made in response to measurements of the communications signal between adjustments made in response to measurements of the non-communications signal. Depending on the circumstances, it may be advantageous to adjust power based only on the communications signal performance, where each adjustment is made based on current and past measurements in a time-averaged fashion. Similarly, power may be adjusted based only on performance of the non-communications signal in a time-averaged fashion.  
         [0023]    Alternatively, measurements of the communications signal and the non-communications signal may be performed between power adjustments. In this case, the power adjustments may be made based on combining both sets of measurements, along with prior measured performance and/or prior power adjustments.  
         [0024]    In one aspect, the invention combines the measurement of the loop-back communications carriers received performance parameters (received power level, Eb/No, etc.) for verifying the power adjustment made based just on the measurement of the non-communication signal. Thus, this method may make an initial adjustment based on the non-communication signal measurement. Then, it will automatically measure the effect on the communications carrier, and make a further adjustment based on the measured received performance parameter on the transmit power level for the carrier.  
         [0025]    Thus, an embodiment of the invention is a method of using two independent loops to measure two different signals to maintain the performance of one or more uplink communications traffic carriers. An aspect of the method, is to use one of the loopback communications carriers to verify that any applied power control adjustments are actually maintaining carrier performance. One problem with only using the non-communications carriers, for uplink control as in the prior art, is that there is no verification that the carriers are actually receiving the correct power adjustment. Some of these problems with the method for uplink power control as practiced in the prior art arise due to the satellite beacon being at a very different frequency than the traffic that is being controlled. In some instances, there is very little correlation between the beacon fluctuations and the fluctuation experienced by the traffic carriers when they are at distinctly different frequencies.  
         [0026]    The invention, overcomes the above problem by actually measuring and verifying that the power adjustments made by the beacon are correct. This is accomplished by measuring the performance of the loopback carrier that is under power control. If the performance of the loopback carrier is not being properly adjusted by the beacon power control loop, then the invention adjusts the power level until the loopback carrier&#39;s performance is operating as expected.  
         [0027]    [0027]FIG. 3 is an example of a communication system  300  in which an embodiment of the invention may be implemented. The communication system  300  is a space satellite communication system but the invention is applicable to any system communication system that includes a receiver, a transmitter, and a communication medium in which the attenuation varies with time. An uplink station  302  uses a transmit terminal  304  to transmit an uplink signal  306  to a satellite  308 . Preferably, the uplink signal  306  reaches the satellite  308  with enough power to ensure that the satellite  308  can interpret the uplink signal  306 . Therefore, the uplink signal  306  is transmitted with enough power to ensure that it overcomes any attenuation associate with a transmission medium. For example, the transmission medium may be affected by atmospheric conditions  310  such as rain, snow, hail, ice, clouds, pollution and moisture. The attenuation caused by these conditions  310  varies over time. Thus, one goal of the present invention is to overcome said variable transmission medium.  
         [0028]    An aspect of the invention is the transmission of a non-communication signal  312  from the satellite  308  to a receive terminal  314 . The receive terminal  314  may be connected to, and/or included with the uplink station  302  and may be substantially co-located with the transmit terminal  304 . Examples of the non-communication signal  312  are a beacon signal or a pilot signal that do not have the same carrier frequency as the uplink signal  306 .  
         [0029]    An aspect of the invention is for the uplink station  302  to compare the measured value of the non-communication signal  312  to a nominal value. This assumes that the attenuation experienced by the non-communication signal  312  is related to the attenuation experienced by the uplink signal  306 . Therefore, the uplink station  302  adjusts the power of the uplink signal  306  to compensate for attenuation variations seen in the non-communication signal  312 .  
         [0030]    Unfortunately, attenuation variations experienced by the non-communication signal are not always the same as the attenuation variations experienced by the uplink signal  306 . Thus, a further aspect of the invention is the transmission of a loopback signal  316  which may be sent from the satellite  308  to the receive terminal  314  of the uplink station  302 . The loopback signal  316  is substantially identical to the uplink signal  306  as received by the satellite  308 . The loopback signal  316  has a substantially similar carrier frequency as the uplink signal  306  and experiences substantially similar attenuation as the uplink signal  306 . Thus provides a good indication of the attenuation experienced by the uplink signal  306 .  
         [0031]    The satellite  308  may also retransmit the uplink signal  306  as a downlink signal to a receive terminal at a downlink station  320 .  
         [0032]    [0032]FIG. 4 is a block diagram of a system  400  in accordance with an embodiment of the invention. The system  400  is located at and/or connected to the uplink station  302  with the goal of maintaining the power and/or performance of the uplink signal  306  as received by the satellite  308  at a constant level despite any variation in attenuation due to atmospheric conditions  310  that the uplink signal  306  may experience. The system  400  includes a first power control loop block  402  and a second power control loop block  404 . The first power control loop block  402  makes measurements of the non-communication signal  312  to formulate a first power adjustment  406 . The second power control loop block  404  makes measurements of the loopback signal  316  to formulate a second power adjustment  408 . The first power adjustment  406  and the second power adjustment  408  are combined in a summing block  410  to formulate a combined power adjustment  410 , which is applied to the uplink signal  306 . The first power control loop block  402  and the second power control loop block  404  may operate independently. The power control loop blocks  402  and  404  may sample the signals  312  and  316  at the same rate or at different rates. The power control loop blocks  402  and  404  may also formulate adjustments  406  and  408  at the same rate or different rates.  
         [0033]    These rates may be independent of the rates at which the signals  312  and  316  are measured. For example, the first block  402  might formulate power adjustments at a rate of once per second. The second block  404  might formulate power adjustments at a rate of once every 5 seconds. The numbers stated here serve only as examples, and are likely to be adjusted based on parameters such as transmit frequency and look angle of the satellite antenna.  
         [0034]    Since the loops  402  and  404  may run at different rates it is possible for either of the adjustments  406  or  408  to be zero at the instance that the combined power adjustment  412  is calculated during the summing block  410 .  
         [0035]    In an alternate embodiment of the invention, block  410  is skipped and the combined power adjustment  412  is not calculated. Instead both power adjustments are applied to the uplink signal in series or in parallel. The second power control loop  404  may be used to verify performance and to make additional power adjustments if necessary.  
         [0036]    The first power control loop block  402  includes two functions: one that estimates a non-communication signal level; and another that formulates a first power adjustment  406 . The signal level of the non-communication signal is representative of the performance of the non-communication signal  312 .  
         [0037]    The following equation may be used to estimate the non-communication signal level at a time n, as in step  204  of FIG. 2 and block  402  of FIG. 4.  
               B        (   n   )       =       ∑     k   =   0       M   -   1                         W        (   k   )            b        (     n   -   k     )                   (     Eqn   .              1     )                               
 
         [0038]    Where,  
         [0039]    B(n) is an estimated non-communication signal level at time n;  
         [0040]    W(k) are a set of M weighting factors of a trend filter W;  
         [0041]    b(n−k) are M discrete measurement samples of the non-communication signal  312  at time (n−k); and  
         [0042]    M is a number of points in the trend filter W.  
         [0043]    The value b(x)=is a measurement of the non-communication signal at a time x. When x is greater then or equal to zero then b(x) is a sample of the non-communication signal  312  at time x. When x is less then zero then b(x) is typically set to a reference level or to the expected level of the non-communication signal. This reference level is often referred to as the “blue-sky” signal level.  
               b        (   x   )       ≡     {           x   ≥   0           the                 sample                 of                 the                 non        -        communication                 signal                 at                 time                 x               x   &lt;   0           the                 reference                 level                     (     Eqn   .              2     )                               
 
         [0044]    The trend filter W is normalized such that  
                 ∑     k   =   0       M   -   1                       W        (   k   )         =   1           (     Eqn   .              3     )                               
 
         [0045]    An estimate of the non-communication signal  312  may be calculated by calculating a weighted average of a plurality of measurements of the non-communication signal. The plurality of measurements may be power, noise or some other performance metric of the non-communication signal  312 .  
         [0046]    In one form, the weights can be set to all the same value, which would represent a simple average of the M samples. In a preferred embodiment of the invention, the weights will actually be unequal and will be disproportionately weighted toward the newest measured samples. This form of filter is referred to as a “forgetting filter”, because it weights the oldest samples lightly and the newest samples more heavily. The formulation of this filter has been designed so that the power control loop is responsive to fade changes, but at the same time, provides some averaging of the potentially varying input samples. Examples of possible disproportionately weighting schemes are linear, non-linear or exponential.  
         [0047]    The following equation may be used for formulating the power adjustment:  
         Δ P   1   =B ( n− 1)− B ( n )  (Eqn. 4)  
         [0048]    Where,  
         [0049]    ΔP 1  is the calculated first power adjustment  406 .  
         [0050]    B(n−1) is a previous non-communication signal level estimate.  
         [0051]    B(n) is the current non-communication signal level estimate.  
         [0052]    At time n=0, the value of B(n−1) is undefined and is typically set to a reference level or to the expected level of the beacon. This reference level is often referred to as the “blue-sky” signal level.  
         [0053]    The first power adjustment  406  may be equal to a previous estimate of the non-communication signal  312  minus a current estimate of the non-communication signal  312 . The first power adjustment may be calculated in step  208  of FIG. 2 or by the block  402  of FIG. 4  
         [0054]    The estimating step and the formulating step may be combined into one step. In which case the following equation may be used for formulating the power adjustment.  
               Δ                   P   1       =       ∑     k   =   0       M   -   1                         W        (   k   )            [       b        (     n   -   k   -   1     )       -     b        (     n   -   k     )         ]                 (     Eqn   .              5     )                               
 
         [0055]    In a preferred embodiment of the invention, a “no adjust” region  502  or “dead zone” is incorporated in to the adjustment criteria of step  208  of FIG. 2 and the block  402  of FIG. 4. FIG. 5A is an illustration of the no adjust region  502 . The no adjust region  502  is a region above and below a blue sky reference level  504  wherein no power adjustments are made. For example, if the estimated signal level falls within the no adjust region  502  then no power adjustment is applied to the uplink signal  306 .  
         [0056]    In the formulas below a letter “R” is defined as the blue sky reference level  504 . A letter “U” is defined as the upper bound of the no adjust region  502 . A letter “L” is defined as the lower bound of the no adjust region  502 .  
         [0057]    An embodiment of the invention may employ the following rules 1-4, in place of equations 4 or 5. B(n) and B(n−1) may be calculated using Equation 1. These rules may be practiced by block  402  of FIG. 4 or during step  208 .  
         (( L&lt;B ( n )≦ U )         ( L≦B ( n− 1)≦ U ))         Δ P   1 =0 (Rule 1)  
         [0058]    Rule 1 states that if B(n) and B(n−1) are both within the no adjust region  502 , then ΔP 1 =0 and no adjustment is made.  
         (( B ( n )&lt; L )         ( B ( n )&gt; U ))         ( L≦B ( n− 1)≦ U ))&gt;Δ P   1   
         [0059]    Rule 2 states that if B(n) is outside the no adjust region  502  and B(n−1) is inside the no adjust region  502  then ΔP 1 =reference level−B(n). This situation represents the non-communication signal  312  moving outside the no adjust region  502 .  
         [0060]    ti (( L≦B ( n )≦ U )         (( B ( n− 1)&gt; U )         ( B ( n− 1)&lt; L )))=&gt;Δ(Rule 3)  
         [0061]    Rule 3 states that if B(n) is inside the no adjust region  502  and B(n−1) is outside the no adjust region  502  then ΔP 1 =B(n−1)—reference level. This situation represents the non-communication signal  312  moving inside the no adjust region  502 .  
         ((( B ( n )&lt; L )         ( B ( n )&gt; U ))         (( B ( n− 1)&lt; L )         ( B ( n−(Rule  4)  
         [0062]    Rule 4 states that if B(n) and B(n−1) are outside the no adjust region  502  then use equation 4.  
         [0063]    In summary:  
         [0064]    if B(n) and B(n−1) are both within the no adjust region  502 , then ΔP 1 =0;  
         [0065]    else if B(n) is outside the no adjust region  502  and B(n−1) is inside the no adjust region  502  then ΔP 1 =reference level−B(n);  
         [0066]    else if B(n) is inside the no adjust region  502  and B(n−1) is outside the no adjust region  502  then ΔP 1 =B(n−1)−reference level.  
         [0067]    else if B(n) and B(n−1) are outside the no adjust region  502  then ΔP 1 =B(n−1)−B(n).  
         [0068]    An advantage of the method outlined above is that it produces unbiased power control results when performing uplink power control when using a non-communication signal and a dead zone.  
         [0069]    In an embodiment of the current invention is the second power control loop  404 , which determines a second power adjustment  408  based on measurements of the loopback signal  316 .  
         [0070]    Equation 6 is a representation of how the second power adjustment  408  may be calculated during the step  214  of FIG. 2 and/or by the block  404  of FIG. 4:  
         ΔP 2   =CP   p   −CP   m   (Eqn. 6)  
         [0071]    Where,  
         [0072]    ΔP 2  is the second power adjustment  408 ;  
         [0073]    CP p  is a planned or a desired performance of the loopback signal  316 ; and  
         [0074]    CP m  is a currently measured performance of the loopback signal  316 .  
         [0075]    This may be used to maintain the mean level of the uplink signal  306  at a desired level. A drawback of this is the potential for introducing noise into the uplink signal  306 . A source of such noise might be rapid variations in CP m , such as from atmospheric scintillation, or due to a delay between the time that CP m  is measured and the time at which the uplink signal is adjusted. To compensate for these possible errors, the second power adjustment  408  is preferably based not only on the magnitude of the error (difference from the planned or desired performance level), but also on the direction of the error. FIG. 6A is an illustration of a “no-adjust” region  602  (i.e. “dead zone”) that may be employed as part of the loopback adjustment criteria of a preferred embodiment of the invention. The no adjust region  602  is a region above and below a desired value  604  wherein no adjustment are made. The value  604  is CP p . The letter “A” represents an upper bound of the no adjust region  602 . The letter “B” represents a lower bound of the no adjust region  602 . The region above A is an “ABOVE” region. The region below B is a “BELOW” region.  
         [0076]    The following rules 5-9 may be used to implement a no adjust region to determine a power adjustment based on the performance of the loopback signal  316  as in step  214  of FIG. 2 or by block  404  of FIG. 4.  
           B≦CP   m ( n )≦ A=&gt;ΔP   2 =0  (Rule 5)  
         [0077]    Rule 5 states if CP m (n) is within the no adjust region  602  as shown in FIG. 6B, then ΔP 2 =0  
         ( CP   m ( n )&lt; B )         ( B≦CP   m ( n− 1))         Δ P   2 =( CP   p   −CP   m ( n (Rule  6)  
         [0078]    As stated in rule 6 if CP m (n) is in the BELOW region and CP m (n−1) is within the no adjust region  602  or the ABOVE region as shown in FIG. 6C, then the second power adjustment would be equal to a difference between the desired value and the measured value times a first weighting factor.  
         ( CP   m ( n )&lt; B )         ( CP   m ( n− 1)&lt; B )         ( B≦CP   m ( n− 2))         Δ(Rule 7)  
         [0079]    As stated in rule 7 if CP m (n) and CP m (n−1) are in the BELOW region and CP m (n−2) is within no adjust region  602  or the ABOVE region as shown in FIG. 6D, then the second power adjustment is equal to a difference between the desired value and the measured value times a second weighting factor.  
                   (            k   =   0       N   -   1                       (         CP   m          (     n   -   k     )       &lt;   B     )       )     ⋀     (     B   ≤       CP   m          (     n   -   N     )         )       ⇒     Δ                   P   2         =       (       CP   p     -       CP   m          (   n   )         )     *     W   N               (     Rule                 8     )                               
 
         [0080]    Rule 8 is an extension of the rules 6 and 7 to N steps. Rule 6 is rule 8 if N=1. Rule 7 is rule 8 if N=2. If CP m (n), CP m (n−1), . . . , CP m (n−N−1) are in the BELOW region and CP m (n−N) is within no adjust region  602  or the ABOVE region, then the second power adjustment is equal to a difference between the desired value and the measured value times an Nth weighting factor.  
         [0081]    In addition, a rule may be formulated to apply to the case of CP m (n) being in the ABOVE region.  
                   (            k   =   0       N   -   1                       (         CP   m          (     n   -   k     )       &gt;   A     )       )     ⋀     (     A   ≥       CP   m          (     n   -   N     )         )       ⇒     Δ                   P   2         =       (       CP   p     -       CP   m          (   n   )         )     *     W   N               (     Rule                 9     )                               
 
         [0082]    Rule 9 essentially states that if CP m (n), CP m (n−1), . . . , CP m (n−N+1) are in the ABOVE Region and CP m (n−N) is within no adjust region  602  or the BELOW region, then the second power adjustment is equal to a difference between the desired value and the measured value times an Nth weighting factor.  
         [0083]    In an embodiment of the invention, N is equal to 3, and weights, W, may be assigned the following values, W 1 =0.25, W 2 =0.5, W 3 =1. This embodiment would employ the above rules for N=1, 2 and 3.  
         [0084]    An embodiment of the invention that incorporates features of rules 8 and 9 would be less susceptible to noise then a method following equation 6. The reason is that only correlated directional changes will be significantly adjusted for. In other words, if the measured loopback signal is persistently in a particular region, the power control loop will quickly compensate for the bias. When the measured performance of the loopback carrier is constantly changing regions, the magnitude of the power changes will be very small (if not zero).  
         [0085]    It is important to note, that although we are applying the loopback control technique for uplink power control, it is actually a formulation of closed loop link power control. The formulation described above can also be applied to any situation where closed loop link power control can be achieved.  
         [0086]    The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of the principles of construction and operation of the invention. Such reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended hereto. It will be apparent to those skilled in the art that modifications may be made in the embodiment chosen for illustration without departing form the spirit and scope of the invention.