Abstract:
Automatic control of transmitted power of a transmitted signal at a local transmitter in response to intermittent signal quality reports from a remote receiver about a received communication signal originating as the transmitted communication signal is provided by distinguishing between improving link conditions and degrading link conditions and at least derating a scheduled signal power change if there are changes in the signal quality reports, such as if the power change is scheduled to occur too soon after a previous signal power change. A fast attack power increase and slow decay power decrease may also be employed. The method is designed to maintain signal quality (e.g. distant Es/No) in the presence of these variations and uncertainties, for time-varying link conditions that have a fade rate range from less than 0.01 dB/s to more than 1 dB/s.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present application claims benefit under 35 USC 119(e) of U.S. provisional Application No. 60/984,989, filed on Nov. 2, 2008, entitled “Method for Power Control in Wireless Communications,” the content of which is incorporated herein by reference in its entirety. 
     
    
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK 
       [0003]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0004]    This invention relates to control of link performance through a communications link, typically a satellite, to maximize efficiency of use of valuable channel resources while assuring adequate signal reception quality. In particular, the invention relates to implementation of link performance control in an enhanced bandwidth efficient modem for transponded satellite communication. 
         [0005]    In order to assist in understanding of the invention, the following definitions are offered. 
         [0006]    AUPC—Automatic Uplink Power Control—a method for maintaining distant-end signal quality by varying local transmit power on an uplink channel, typically in a satellite communication system. 
         [0007]    Es/No—Energy per symbol relative to noise density, in dB—A common signal quality measure for wireless communications. 
         [0008]    Link margin—Es/No in excess of the minimum required to maintain link performance. By way of example, a system producing a measured bit error rate (BER) of 1E-8 at 9 dB Es/No while operating at 10 dB Es/No has 1 dB link margin. 
         [0009]    Static link margin—The link margin that is provisioned when a link is configured. For example, a link may be configured with 2 dB static link margin, and then operate with a operating link margin between 0.5 dB and 3 dB, variable as a function of time, as the overall system responds to weather and other propagation impairments cause changes in the link. 
         [0010]    A power amplifier can transmit information modulated onto a signal carrier over a distance via communication channel subject to uncontrolled propagation to a remote receiver. For many systems, especially transponded satellite communications, it is desirable to maintain link performance with the minimum transmit power to conserve system resources and to minimize interference among users sharing system resources. Static link margin, namely the set fixed power level at a local transmitter that is above a minimum required power level to maintain a certain quality of communication at a remote receiver, is the metric typically used to maintain link performance in the presence of variations in received signal quality due to equipment power, signal gain, self-induced noise variations and uncertainties in the propagation characteristics of wireless communication channel, including channel impairments such signal attenuation (fading) due to weather and thus interference due to signal absorption by water vapor at the frequencies of interest. 
         [0011]    Automatic uplink power control (AUPC) methods are commonly used in satellite modems to adapt the transmitted signal power to relatively slow variations in signal quality due to equipment and weather in commonly-used satellite frequency bands (C-, and Ku-band, 5 to 14 GHz). The known methods typically support fade rates of up to 0.1 dB/s with fade depths of 2 to 8 dB. The prior art methods typically rely on signal quality reports received at the local transmitter received every few seconds (e.g., 4 to 60 seconds) from the remote receiver. In the past, the interval between reports has been such that power adjustments could be made so that each report is independent of any previous power change. A variety of relatively simple methods have been used to meet these operating requirements, including proportional change back to the desired Es/No (signal-to-noise ratio) at the receiver on every report, changes of a fixed amount (e.g. 0.5 dB) when the reported Es/No differs from the desired Es/No by more than a threshold, etc. The known methods are found in commercially available devices. 
         [0012]    The emergence of Ka-band (30 GHz) band for communication using geo-synchronous satellites poses a fundamentally more difficult problem. The typical fade in this band exhibit rates that are an order of magnitude greater (1 dB/s) than in other bands and the fade depths can exceed 20 dB. Therefore, to cope with these dynamics, signal quality reports must be received at a much faster rate (e.g. once per second) in order to maintain link performance while minimizing static link margins. Given a typical 260 ms end-to-end RF propagation delay, plus processing delays on the order of half a second, the signal quality reports are found to be correlated with previous power changes, increasing the problem of maintaining system stability in a highly dynamic link quality environment. In other words, the control system controlling signal power level can be caused to oscillate uncontrollably. 
         [0013]    Correlated automatic uplink power control (AUPC) techniques could prove useful in other communication environments, such as in Medium Earth Orbit (MEO) satellites and line-of-sight (LOS) communication systems wherein high fade rates and depths are manifest, although there are significantly different RF propagation delays. It is desirable that the same AUPC method be used for these applications without imposing a system engineering burden on the equipment operator. 
         [0014]    In particular, users desire a simple, easy-to-use system for power control management that minimizes the number of corrections over a given time and that simplifies human or remote monitoring of the system. A key user constraint is therefore that power corrections are made only when the reported signal quality (Es/No) differs from the desired value by more than a threshold. 
         [0015]    There is a need to provide a simple, robust, stable, and high-performance power control method for operating environments with high fade rates and fade depths. 
       SUMMARY OF THE INVENTION 
       [0016]    According to the invention, in a communication system involving an uncontrolled propagation link, such as a satellite communication channel, a method and control system for automatically controlling transmitted power of a transmitted communication signal at a local transmitter in response to intermittent signal quality reports from a remote receiver about a received communication signal originating as the transmitted communication signal at the local transmitter, where the intermittent reported signal quality is subject to systemic delays and may be impacted by power variations responsive to prior reported signal quality, by distinguishing between improving link conditions and degrading link conditions and at least derating a scheduled signal power change if there are changes in the signal quality reports, such as if the power change is scheduled to occur too soon after a previous signal power change. In a specific embodiment, the method adapts the transmit power at a local transmitter transmitting via a link in response to a sequence of intermittent signal quality reports from a distant receiver (typically Es/No) on the link. The method takes into account variations in accuracy and timeliness of signal quality reports, which can vary due to modem configuration (mode, modulation, coding, data rate) and to variations in time latencies inherent in the distributed processing and changes in the propagation path, as well as variations in the nature of the messaging elements of the reports in a real system. 
         [0017]    The method is designed to maintain signal quality (e.g. distant Es/No) in the presence of these variations and uncertainties, for time-varying link conditions that have a fade rate range from less than 0.01 dB/s to more than 1 dB/s. 
         [0018]    In a specific embodiment of a system according to the invention, the system implements an embedded control method comprising three elements: code for a “fast attack” algorithm that is responsive to a sequence of non-continuous signal reports indicative of degrading link quality; code for a “slow decay” algorithm that is operative to maintain a current link margin for an interval during which non-continuous signal reports indicative of improved signal quality are confirmed; and code for a “time-derating” algorithm that reduces or eliminates planned power level changes that occur too soon after a previous change was implemented. 
         [0019]    The combination of these aspects of the invention work together to provide a method that is simple, stable, and robust over a wide range of operating conditions. 
         [0020]    The invention will be better understood by reference to the following detailed description in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a diagram of a communication system with an uplink and a return link in which the present invention is implemented. 
           [0022]      FIG. 2  is a logical block diagram of a system according to the invention. 
           [0023]      FIG. 3  is a histogram of message inter-arrival distribution. 
           [0024]      FIG. 4  is a flow chart of an algorithmic implementation of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]      FIG. 1  illustrates a communication system  10  having a communication uplink  12 ,  14  between a local transmitter  16  and control system  18  and a remote receiver  20  and user terminal  22  via a satellite  24  providing a relay with a return channel  26  or  28 ,  30  for carrying signal quality reports. The control system  18  according to the invention is provided for adjusting transmit power. The control system  18  includes an input element  32  for receiving and interpreting signal quality reports obtained at the remote receiver(s)  20  about signals via the link  12 ,  14  originating from the local transmitter  16 . The control system  18  includes decision analysis tools  34  according to the invention and a power adjustment message output  36  to inform the transmitter  16  of the need to adjust transmit power of the transmitter  16 . This is a feedback control system impacted by uncontrolled delays in processing and propagation, as for example when the length of the propagation path on the link  12 ,  14  is changing, and by fluctuations in link quality that are more rapid that the response time of the control system. 
         [0026]    Referring to  FIG. 2 , there is a logical depiction of the feedback system according to the invention. A complicating factor in the design of a high-performance power control method is time jitter in the signal quality report delivery process. Modern communication systems are complex, with many internal modules that communicate information. For example, there will be a module  38  for a signal quality measurement function that produces metrics (Es/No estimates) as often as the data are available. Another module  40  will generate a report for transmission using an asynchronous process, and the message for this report may be queued in a transmission queue  44  behind other messages that have already started transmission to the distant modem. Hence there is a communication channel  26  or  28  or through an additional link  42  with both processing delay and radio frequency propagation delay, particularly if the channel  28  is through a long path such as via a satellite relay. Upon receipt at the distant modem  48 , the message will received at an element  50  and extracted from the channel and passed between other software modules using internal message processing functions until it is acted upon, as for example in a power change module  52 . The resulting control message is then processed by a transmit power change manager  54  such that the transmit power at the transmitter  16  is changed, resulting is change in the power on the uplink  12 ,  14 , which is subject to propagation delay. 
         [0027]    A histogram of message delivery times in a typical configuration is shown in  FIG. 3 . For message generation every 980 milliseconds, queuing delay on the channel causes 99.5% of the messages to be received at intervals ranging from 0 to 2 seconds, with an average of one second. The distribution is mostly symmetric, because queuing delay that makes report “N” late will tend to mean that report “N+1” will be early. Time jitter is a significant problem and exacerbates the already challenging problem of maintaining system stability in a highly dynamic link quality environment. 
         [0028]    The functional elements of  FIG. 2  are described below. The use of terms “Distant” and “Local” are meant to provide clarity for a hypothetical configuration when Modulator A (Local) is running in accordance with this invention to maintain Es/No at a at modem B (Distant)  56 . The embedded channel  26  from B to A is used to return Modem B&#39;s (Distant) AUPC Es/No Estimates. Since present invention is useful in one or both directions of a bi-directional circuit through the satellite  24 , the algorithm should be considered as two independent processes. The following provides a summary of each functional block. 
         [0029]    AUPC Es/No Estimator  38 : the distant or remote demodulator  56  generates real-time estimates of its received Es/No. The estimates are generated as a data-driven process as often as possible given accuracy and modem configuration constraints. The time between estimates will be a function of modem configuration and symbol rate. 
         [0030]    Report generator  40 : At regular intervals (e.g. 1 second), the most current estimate of the distant receive Es/No is encapsulated and transmitted over an embedded channel. Message transmission is subject to queuing delay behind other embedded channel messages that have already started transmission as represented by a transmit queue  44 . 
         [0031]    RF propagation and processing delay: Delay in embedded channel message receipt is dependent both on the RF propagation to the local modem via the channel  26 ,  28  or  42 , in addition to processing delay. The processing delay varies based on the modem configuration. 
         [0032]    Local demodulator embedded channel receiver  50 : Distant receive Es/No estimate is extracted from the received embedded channel message. 
         [0033]    AUPC Power Change Module  52 : A power change is calculated to maintain distant Es/No within a desired range as herein described. 
         [0034]    Transmitter Power Change Manager  54 : The calculated power change is implemented by adjusting the output power at the transmitter in accordance with commands from the power change module  52 . 
         [0035]    According to the invention automatic uplink power is controlled by steering the distant signal quality (Es/No) to a desired level called the target Es/No. The target Es/No is the minimum Es/No required to maintain link performance plus a static link margin. The distant Es/No is allowed to vary about the target Es/No (between a Minimum and Maximum Es/No) without changing local power. The Minimum and Maximum Es/No approach provides hysteresis to prevent the system from making small corrections after every distant Es/No report. System performance is based on its ability to maintain link performance while minimizing Target Es/No (i.e. static link margin) for a given link configuration and fading channel scenario. 
         [0036]    The AUPC Power Change Module  52  is a control loop that responds quickly to a fading channel in order to preserve the link while mitigating overcorrection that could result in excessive link margin or oscillating power changes. The module decouples time uncertainties by taking advantage of the user&#39;s desire to make power corrections only when the reported Es/No differs from the desired Es/No by a threshold. The module accommodates time uncertainties in normal operation by means of static link margin. The module explicitly manages time uncertainty in the immediate aftermath of a power correction event. The module reduces static link margin requirements by ensuring that power changes and channel fades have ample time to propagate through the control loop prior to reducing power. 
         [0037]      FIG. 4  is a flow chart of the operation of the Power Change Module  52 . Es/No reports are received from the receiver  50  (Step A), which causes the module  52  to wake up (Step B) and compute an improving channel derating metric (Step C) (if not already established) and to test to see if the current Es/No report is less than a known or accepted minimum indicative of impaired link quality (Step D). If yes, the “fast attack” process is invoked wherein a sub-module computes a power correction value designed to restore the distant Es/No to the Target Es/No (Step E). This can be done in multiple ways. First, the correction can be simply the difference between distant Es/No report and Target Es/No. Second, the correction can account for time delays in the report. If the report is late, the distant Es/No information is “stale” and the actual distant Es/No may be worse than the reported value. In this case, the sub-module could correct by an additional amount to account for the lateness of the report. 
         [0038]    If the Es/No report shows no deficiency (Step D), then the Es/No report is tested to see if it exceeds a predetermined maximum indicating improved link quality (Step F). One of the innovations is based on the realization that it is not time-critical to reduce power in the face of an improving link condition. It is possible and desirable to be conservative when reducing power to maintain link performance. This is important because reaction to a spurious Es/No report could reduce power below the level needed to maintain the link. The slow-decay sub-module can be designed in multiple ways. First, it can reduce power when a number of consecutive Es/No reports (e.g. 2, 3, 4, 5 reports) all show distant Es/No is now above the Maximum Es/No. Second, it can reduce power when the latest Es/No report and the average Es/No (averaged over the past several reports) are both above the Maximum Es/No. Another approach would be to use messaging on the embedded channel to request confirmation from the distant end that Es/No has improved. As herein contemplated, the improving channel derating metric is tested to see if it exceeds a maximum (Step G) and if so, a decision is made whether that result should be overridden (Step H). If not, then the time delay aspects are invoked, first by computing the recommend power change of a constant B times the difference between the target Ex/No and the reported Ex/No (Step J). 
         [0039]    Time derating permits the maintaining of system stability in highly dynamic link conditions. The time derating element is activated when a change is planned soon after a previous change. From this point on the fast attack and slow decay processing are merged. A test is thus made to determine if there has been a recent previous change by comparing the current time to the time when the previous power change was initiated (Step K). A loop response time is also computed, which is based on the size of the previous change, RF propagation delay, end-to-end throughput delay, data rate, and other factors including a time margin. If the time since the last power change exceeds the loop response time, then the full planned correction is initiated (Step L). If the last change occurred within the loop response time, then the planned correction is reduced (Step M). It may be eliminated altogether if the computed amount of reduce power change is zero. The amount of reduction can be proportional to the time remaining in the loop response time, or can be a fixed percentage of the planned correction. The planned correction is eliminated (set to 0 dB) for specific ranges of modem configurations when the distant Es/No is above Maximum Es/No (an improving channel). This is represented by the defer power change step (Step N). The transmit power change manager then adjusts transmitter uplink power in accordance with the instructed power change (Step P and the power change module goes to sleep (Step Q) until the next Es/No reports are received (Step A). 
         [0040]    As described above, the time derating sub-module modifies planned power corrections in order to eliminate power oscillations and over-corrections. The combination of the fast attack sub-module and the time derating sub-module allows the AUPC system to minimize the number of corrections while also eliminating over-corrections in the presence of a sudden decrease in channel quality. Without the time derating sub-module, the AUPC system would have to be more conservative when increasing transmit power in order to prevent over-correction. 
         [0041]    The following pseudo-code listing is provided to illustrate a specific implementation of the invention. 
         [0042]    Receive an Es/No message over the embedded channel. 
         [0043]    Determine if the message contains a valid Es/No (the Es/No message field contains either numeric Es/No (valid) or a carrier out of lock indicator (invalid). 
         [0044]    If the message is valid, compute the Improving Channel Derating Metric using the following equation: 
         [0000]      movAvg=10*log 10 [1/5*Σ10̂( X   n /10)] 
         [0000]    where X 0 , X 1 , X 2 , X 3 , and X 4  are the values (in dB) of the last five received Es/No messages. 
         [0045]    If the received message indicates that the estimate of the distant Es/No is below the Minimum Distant Es/No, calculate a New Tx Power as follows: 
         [0000]      If (a power change completed within (790 ms+modem processing delay)), Power Change=0.5* (Target  Es/No −Received  Es/No  Message) 
         [0000]      Else, Power Change=Target  Es/No −Received  Es/No  Message 
         [0000]      New  Tx  Power=min[(Current  Tx  Power+Power Change), Maximum  Tx  Power] 
         [0046]    If a message is received that indicates the estimate of the distant Es/No is above the Maximum Distant Es/No, a New Tx Power is calculated as follows: 
         [0000]      If ((a power change completed within (2175 ms+modem processing delay)) AND (pre-distortion is NOT closed-loop)), Power Change=0 
         [0000]      Else If ((a power change completed within (3175 ms+modem processing delay)) AND (pre-distortion is closed-loop)), Power Change=0 
         [0000]      Else If (Improving Channel Derating Metric&gt;Maximum Distant  Es/No ) AND (a power change completed within (790 ms+modem processing delay)), Power Change=0.5*(Target  Es/No −Received  Es/No  Message) 
         [0000]      Else If (Improving Channel Derating Metric&gt;Maximum Distant  Es/No ), Power Change=Target  Es/No −Received  Es/No  Message 
         [0000]      Else, Power Change=0 
         [0000]      If (pre-distortion is NOT closed-loop), New  Tx  Power=max[(Current  Tx  Power+Power Change), (Current  Tx  Power−4.25 dB), Minimum  Tx  Power] 
         [0000]      * Else, New  Tx  Power=max[(Current  Tx  Power+Power Change), (Current  Tx  Power−5.75 dB), Minimum  Tx  Power] 
         [0047]    If a message is received that indicates the estimate of the distant Es/No is between the Minimum Es/No and the Maximum Es/No, the transmit power is not changed. 
         [0048]    The following parameters are defined to help the reader understand the algorithm above:
       “power change completed” is the time it takes the local modulator to slew power from the old to the new setting.   “modem processing delay” is the one-way throughput delay for the embedded channel messaging excluding embedded channel buffering and queuing.       
 
         [0051]    One specific embodiment of the subject invention is described below for a Ka-band geo-synchronous satellite modem system operating at fade rates of up to 1 dB/s, where power changes are implemented at 3 dB/s, where X 0 , X 1 , X 2 , X 3 , and X 4  are the values (in dB) of the last five received Es/No messages: 
         [0052]    At a frequency of 1 Hz, the distant modem transmits an estimate of the distant receive Es/No to the local modem. Upon receipt, the local modem updates the Improving Channel Derating Metric, defined by the expression: 10*log 10 [1/5*Σ10̂(Xn/10)]. If the received message is below the Minimum Es/No, the message is passed to the “fast attack” sub-module. The “fast attack” sub-module will recommend a power change equal to (Target Distant Es/No−Es/No Message) before passing it to the “time derating” sub-module. The time derating sub-module compares the time since last power change to the loop response time of (625 ms+modem processing delay+(previous power change)/3 dB/s). If the time difference is greater than the loop response time, then the full correction is made, otherwise the correction is reduced by 50% before implementation. At the end of this process, the time derating sub-module will implement either 50% of the planned change (recent power change), or 100% of the planned change (previous change is not a factor in this update). 
         [0053]    If the received message is above the Maximum Es/No, the message is passed to the “slow decay” sub-module. The “slow decay” sub-module will only change power if both the received Es/No Message and the Improving Channel Derating Metric indicate that the Es/No is above the Maximum Es/No. If both are true, then the slow decay sub-module will recommend a power change equal to (Target Distant Es/No−Es/No Message) before passing it to the “time derating” sub-module. The “time derating” sub-module will implement the same conditional 50% reduction logic as described above, and then check to see if the change should be reduced to 0. The sub-module will not change power if the data rate is less than or equal to 128 kbps and a previous power change completed within (1000 ms+modem processing delay+(previous power change)/3 dB/s). At the end of this process, the time derating sub-module will implement either a 0 dB change (low data rates and recent change), 50% of the planned change (high data rates and recent change), or 100% of the planned change (previous change is not a factor in this update). 
         [0054]    If the received message is between the Minimum and Maximum Es/No, the AUPC module will not implement a power correction. 
         [0055]    The invention has now been explained with reference to specific embodiments. Other embodiments will be evident to those of skill in the art. It is therefore not intended that this invention be limited, except as indicated by the appended claims.