Abstract:
A method for communication over a communication link, which includes a transmitter and a receiver, includes transmitting and receiving data, which is encoded using a specified forward error correction (FEC) code, modulated using a specified modulation scheme and transmitted at a given power level. 
     One or more parameters of the communication link are monitored. A condition, which is based on values of one or more of the parameters monitored at a plurality of historical time instants, is evaluated. At least one feature of the transmitted data, selected from a group of features consisting of the FEC code, the modulation scheme and the power level, is modified based on the condition.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to communication systems, and particularly to methods and systems for using adaptive coding and modulation (ACM) in communication links. 
       BACKGROUND OF THE INVENTION 
       [0002]    Various methods for predicting the future conditions of a wireless communication channel are known in the art. For example, a channel prediction method is described by Duel-Hallen et al., in “Long Range Prediction of Fading Signals: Enabling Adaptive Transmission for Mobile Radio Channels,” IEEE Signal Processing Magazine, (17:3), May, 2000, pages 62-75, which is incorporated herein by reference. The method computes the linear Minimum Mean Squared Error (MMSE) estimates of future fading coefficients based on past observations. 
         [0003]    Channel prediction is sometimes used for adaptive transmission. For example, an application of the method proposed by Duel-Hallen et al. to adaptive modulation is described by Hu et al., in “Adaptive Modulation Using Long Range Prediction for Flat Rayleigh Fading Channels,” Proceedings of the IEEE International Symposium on Information Theory, Sorrento, Italy, Jun. 25-30, 2000, which is incorporated herein by reference. 
         [0004]    As another example, U.S. Pat. No. 5,541,955, whose disclosure is incorporated herein by reference, describes an adaptive data rate modem, which supports multiple data rates. The modem incorporates an adaptive data rate encoder and an adaptive data rate decoder. Soft decision metrics of the decoder are used to provide an estimate of the signal-to-noise ratio (SNR). An optional predictor receives the SNR estimate from the decoder and predicts the future signal-to-noise ratio, to determine the desired data rate for the modem. 
       SUMMARY OF THE INVENTION 
       [0005]    There is therefore provided, in accordance with an embodiment of the present invention, a method for communication over a communication link that includes a transmitter and a receiver, the method including: 
         [0006]    transmitting and receiving data, which is encoded using a specified forward error correction (FEC) code, modulated using a specified modulation scheme and transmitted at a given power level; 
         [0007]    monitoring one or more parameters of the communication link; 
         [0008]    evaluating a condition based on values of one or more of the parameters monitored at a plurality of historical time instants; and 
         [0009]    based on the condition, modifying at least one feature of the transmitted data, selected from a group of features consisting of the FEC code, the modulation scheme and the power level. 
         [0010]    In some embodiments, the transmitter and the receiver are part of a wireless communication link. Alternatively, the transmitter and the receiver are part of a wireline communication link. 
         [0011]    In another embodiment, transmitting the data includes encoding and modulating the data in accordance with an adaptive coding and modulation (ACM) setting selected from a plurality of predetermined ACM settings, and modifying the at least one feature includes selecting a different ACM setting from among the plurality. 
         [0012]    In yet another embodiment, the modulation scheme defines symbols that are used in modulating the data at the transmitter, and monitoring the parameters includes calculating the parameters over at least one subset of the symbols received by the receiver selected from a group of subsets consisting of a predetermined portion of the symbols and a pseudo-randomly-selected portion of the symbols. 
         [0013]    Modifying the at least one feature may include synchronizing the modified at least one feature between the transmitter and the receiver. 
         [0014]    In some embodiments, evaluating the condition includes predicting a future performance of the communication link assuming the communication link were to continue using the specified FEC code, the specified modulation scheme and the given power level. Predicting the future performance sometimes includes fitting a function to values of the monitored parameters, and calculating a future value of the function. Additionally or alternatively, predicting the future performance may include estimating future values of the monitored parameters using a Kalman filter. 
         [0015]    In another embodiment, the modulation scheme defines symbols that are used in modulating the data at the transmitter, and the monitored parameters include a mean square error (MSE) of at least part of the symbols received by the receiver. In yet another embodiment, the monitored parameters include a radial MSE of at least part of the symbols received by the receiver. 
         [0016]    The monitored parameters may include at least one error rate selected from a group of error rates consisting of a coded bit error rate (BER) after error correction by the FEC code and an uncoded BER before error correction by the FEC code of at least part of the data received by the receiver. Additionally or alternatively, the monitored parameters may include a received signal level (RSL) at the receiver. 
         [0017]    In some embodiments, receiving the data includes equalizing the data using an adaptive equalizer that includes multiple coefficients, and the monitored parameters include a performance metric based on values of the coefficients. The performance metric may include a ratio between a power of a coefficient having a maximum absolute value among the equalizer coefficients and a total power of the equalizer coefficients. Alternatively, the performance metric includes a ratio between a power of a coefficient having a maximum absolute value among the equalizer coefficients and a total power of the equalizer coefficients excluding the coefficient having the maximum absolute value. 
         [0018]    In a disclosed embodiment, the FEC code includes an iteratively-decoded code, receiving the data includes decoding the FEC code at the receiver using an iterative decoding process that produces a code metric, and monitoring the parameters includes monitoring the code metric. The code metric may include at least one metric selected from a group of metrics consisting of a likelihood ratio (LR) and a log likelihood ratio (LLR). 
         [0019]    There is also provided, in accordance with an embodiment of the present invention, a communication link, including: 
         [0020]    a transmitter, which is arranged to transmit data, which is encoded using a specified forward error correction (FEC) code, modulated using a specified modulation scheme and transmitted at a given power level; 
         [0021]    a receiver, which is arranged to receive the data transmitted by the transmitter and to monitor one or more parameters of the communication link responsively to the received data; and 
         [0022]    a controller, which is associated with at least one of the transmitter and the receiver and is arranged to evaluate a condition based on values of the parameters monitored at a plurality of historical time instants and, based on the condition, to modify at least one feature selected from a group of features consisting of the FEC code, the modulation scheme and the power level. 
         [0023]    There is additionally provided, in accordance with an embodiment of the present invention, a method for communication over a communication link that includes a transmitter and a receiver, the method including: 
         [0024]    transmitting and receiving data, which is encoded using a forward error correction (FEC) code, modulated using a modulation scheme and transmitted at a power level; 
         [0025]    monitoring coefficient values of an adaptive equalizer used for equalizing the modulated data received by the receiver; and 
         [0026]    responsively to the monitored coefficient values, modifying at least one feature of the transmitted data, selected from a group of features consisting of the FEC code, the modulation scheme and the power level. 
         [0027]    There is further provided, in accordance with an embodiment of the present invention, a method for communication over a communication link that includes a transmitter and a receiver, the method including: 
         [0028]    transmitting and receiving data, which is encoded using a forward error correction (FEC) code, modulated using a modulation scheme and transmitted at a power level; 
         [0029]    monitoring a radial mean square error (MSE) of at least part of the modulated data received by the receiver; and 
         [0030]    responsively to the monitored radial MSE, modifying at least one feature of the transmitted data, selected from a group of features consisting of the FEC code, the modulation scheme and the power level. 
         [0031]    The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  is a block diagram that schematically illustrates a communication link, in accordance with an embodiment of the present invention; and 
           [0033]      FIG. 2  is a flow chart that schematically illustrates a method for adapting the coding and/or modulation of a communication link based on predicted performance, in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
       [0034]    Some communication links adapt their modulation scheme, error correction code and/or transmit power level in order to provide reliable performance under varying channel conditions. In some cases, the link predicts the future channel conditions in order to reduce the response time of the adaptation, i.e., the time from the occurrence of a change in channel conditions until the link begins to operate at the updated setting. 
         [0035]    Embodiments of the present invention provide improved methods and systems for adapting the modulation and coding scheme, as well as the power level used by a communication link. While the communication link operates in a certain modulation/coding/power configuration, the receiver monitors one or more parameters, such as performance metrics of the link. The monitored parameters may comprise performance metrics such as bit error rate (BER) and mean square error (MSE). Based on the monitored performance metrics, a system controller predicts the expected future performance of the link, assuming the link were to continue using the current configuration. Typically, the system controller bases the decision on a number of performance metric values, monitored at multiple time instants in a time interval that precedes the decision instant. These metric values are also referred to as historical metric values. 
         [0036]    If the controller determines that the predicted link performance using the current configuration violates a certain predetermined condition, the controller selects a different modulation/coding/power configuration and the link switches to the new configuration. 
         [0037]    Unlike some known adaptation methods which first predict the future conditions of the communication channel and then adapt the coding, modulation or power to match the predicted channel conditions, the methods and systems described herein explicitly and directly predict the anticipated future performance of the link, if it were to continue using the currently-used coding and modulation. Therefore, the methods and systems described herein are expected to provide a more reliable and accurate adaptation in comparison with known methods. 
       System Description 
       [0038]      FIG. 1  is a block diagram that schematically illustrates a communication link  20 , in accordance with an embodiment of the present invention. Link  20  comprises a transmitter  24 , which accepts input data and transfers it to a receiver  28 . The link may comprise a microwave link, a millimeter-wave link or any other suitable wireless or wired link. 
         [0039]    Link  20  may comprise a standalone point-to-point link or may be part of a point-to-multipoint communication system. For the sake of conceptual clarity, the description that follows refers to a unidirectional link. Typically, however, link  20  is part of a bidirectional link between two communication systems, wherein each system comprises a transmitter similar to transmitter  24  and a receiver similar to receiver  28 . 
         [0040]    The data input to transmitter  24  is formatted and encapsulated in data frames by a framer  30 . The data frames are encoded and modulated by a transmit (TX) modem  32 . In some embodiments, the TX modem encodes the input data with a forward error correction (FEC) code. Typically, the FEC has a code rate between  0 . 5  and approaching unity, although any suitable FEC code and code rate can be used. 
         [0041]    The TX modem modulates the encoded data in accordance with a particular modulation scheme, typically by mapping bits or groups of bits to symbols selected from a particular signal constellation. For example, TX modem  32  may use quaternary phase shift keying (QPSK), 16-symbol quadrature-amplitude modulation (16-QAM), 256-QAM, or any other suitable modulation scheme. 
         [0042]    The modulated symbols produced by TX modem  32  are converted to an analog signal by a digital-to-analog (D/A) converter  36 . The analog signal is filtered, amplified and up-converted to a suitable radio frequency by a transmitter front-end (TX FE)  40 . The radio signal is amplified by a power amplifier (PA)  44  and transmitted to receiver  28  via a transmit (TX) antenna  48 . 
         [0043]    The signal transmitted by transmitter  24  is received by a receive (RX) antenna  52 . A receiver front end (RX FE)  56  down-converts the signal to a suitable intermediate frequency (IF) or to baseband. The RX FE may also perform functions such as low-noise amplification, filtering, gain control, equalization, synchronization and carrier recovery. The signal produced by the RX FE is digitized by an analog-to-digital (A/D) converter  60 . The digitized signal is provided to a receive (RX) modem  64 . The RX modem demodulates the received symbols and decodes the FEC, so as to reconstruct the data frames. The RX modem may also perform functions such as filtering, gain control, equalization, synchronization and carrier recovery. A de-framer  66  extracts the data from the data frames and provides the extracted data as output. 
         [0044]    TX modem  32  and RX modem  64  comprise variable-rate modems. In some embodiments, modems  32  and  64  use adaptive coding and modulation (ACM). In ACM, the FEC code rate and the signal constellation used for modulation are jointly selected to produce the desired data rate and/or quality of service. Typically, two or more combinations of code rate and signal constellation are predefined. Each combination of code rate and signal constellation is referred to herein as an ACM setting. A suitable ACM setting is selected and coordinated between the TX and RX modems. 
         [0045]    As will be explained in detail below, the desired ACM setting is selected by monitoring the link performance, predicting the future performance of the link based on the monitored performance, and selecting an ACM setting based on the predicted performance. 
         [0046]    Transmitter  24  comprises a TX controller  70 , and receiver  28  comprises an RX controller  74 . The TX and RX controllers respectively manage the operation of the transmitter and receiver, and in particular monitor the performance of the link and coordinate ACM setting changes. Controllers  70  and  74  can be jointly viewed as a system controller, which carries out the methods described herein. The different performance monitoring, prediction and ACM setting adaptation functions can be partitioned between controllers  70  and  74  as desired. 
         [0047]    The TX and RX controllers coordinate the ACM setting changes, and otherwise communicate with one another, by exchanging management information over a management channel  78 . For example, the TX controller may send information to the RX controller by embedding management information in the data frames produced by framer  30 . When link  20  is part of a bidirectional communication link, the RX channel may send information to the TX controller by embedding management information in data frames of the opposite link direction. 
       ACM Setting Adaptation Based on Predicted Link Performance 
       [0048]    Link  20  adapts its ACM settings in order to provide reliable performance under varying channel conditions. When channel conditions are good, the link may select ACM settings having a high number of constellation symbols and/or a high code rate, thus increasing the transmitted data rate. When channel conditions deteriorate, ACM settings having fewer constellation symbols and/or lower code rate are selected, thus improving the robustness of the link at the expense of reduced data rate. Typically, ACM settings are selected so that the link complies with certain quality of service (QoS) requirements. Alternatively, the ACM setting can be adapted to comply with any other suitable condition. 
         [0049]    Adapting the ACM setting in response to changing channel conditions has a certain response time. In the present context, the response time interval is defined as the time period from the occurrence of a change in channel conditions, until the link begins to operate at an updated ACM setting that matches this change. In many practical cases, channel conditions change rapidly. For example, when the channel experiences fading, its attenuation sometimes changes at a rate on the order of 100 dB per second or more. 
         [0050]    It is generally desirable that link  20  be able to switch to a new ACM setting before the currently-used ACM setting stops matching the channel conditions and causes degraded performance (e.g., BER degradation). Therefore, the length of the response time interval may have a significant effect on the link performance. 
         [0051]    For example, when channel conditions deteriorate and the response time is long, data transmitted during the response interval may be fully or partially lost. Moreover, the effective data rate of the link may be reduced, the bit error rate (BER) may be degraded and the data latency may increase. In some cases, the link may lose synchronization and some of the receiver loops (e.g., carrier recovery and automatic gain control) may unlock. As another example, when channel conditions improve, such as when recovering from a fade, the link may operate at an unnecessarily-low data rate during the response time. 
         [0052]    Moreover, when the response time is long, the user may revert to using excessive link budget margins before making the switch, in order to compensate for the lengthy adaptation. As a result, the link may operate at an unnecessarily low ACM setting (i.e., slower data rate). Such a selection of ACM setting causes significant reduction in link capacity. 
         [0053]    In order to shorten the response time of adapting ACM settings, link  20  adapts its ACM setting by predicting the anticipated future performance of the link. In some embodiments, the receiver monitors one or more performance metrics of the link over time. Using the monitored metrics, the system controller (i.e., TX controller  70 , RX controller  74  or both) predicts the future performance of the link, assuming the link were to continue operating at the currently-used ACM setting. If the predicted link performance is not acceptable, or otherwise violates a predetermined condition, the system controller changes the ACM setting of the link accordingly. 
         [0054]    The system controller may use any suitable performance metric for predicting the future performance of the link. For example, the controller may calculate or estimate the mean square error (MSE) of the received signal. In some cases, the MSE can be estimated over only part of the received symbols. The selection of symbols, which participate in the MSE estimation, may be fixed (e.g., the symbols located at a certain region in each data frame) or variable (e.g., pseudo-randomly changing locations in the data frame). 
         [0055]    When symbols are received by the RX modem, each received symbol is represented as a two-dimensional signal point in the In-phase/Quadrature (I/Q) plane. The RX modem demodulates the received symbols by mapping each signal point to one of the nominal constellation symbols. The difference vector between the received symbol signal point and the corresponding constellation symbol is referred to as the error vector. 
         [0056]    The MSE is defined as the average error vector magnitude, squared and averaged over multiple symbols. A large MSE value typically corresponds to a high error rate, i.e., degraded link performance, and vice versa. Thus, a prediction of the future MSE of the link can serve as a good indication of future link performance. 
         [0057]    The MSE metric can be calculated over all received symbols or over only part of the symbols. The RX modem can also make ACM setting switching decisions based on current MSE values, calculated over any desired group of symbols, instead of predicting future MSE values. 
         [0058]    In some embodiments, the RX modem estimates the radial MSE of the received symbols, and the radial MSE is used as a performance metric and as the basis for prediction. The radial MSE is calculated by considering only the radial components of the error vectors. In other words, for each received symbol, only the component of the error vector along a ray, which projects from the origin of the I/Q plane and crosses the received symbol&#39;s signal point, is measured. Radial MSE is particularly suitable for differentiating between thermal noise and phase noise (which is usually channel-independent). Thus, radial MSE is sometimes more accurate in predicting the link performance under high phase noise scenarios. Radial MSE can be calculated over some or all of the received symbols. The RX modem can also make ACM setting switching decisions based on the current radial MSE value, instead of predicting future radial MSE values. 
         [0059]    In some embodiments, an estimate of the bit error rate (BER) is used as a basis for prediction. For example, the RX modem can calculate the number of bit errors corrected by the FEC in each data frame. The RX modem may compare the uncorrected data bits at the input of the FEC decoder to the corresponding bits at the decoder output, and count the number of corrected errors per frame. 
         [0060]    The received signal level (RSL), as measured by the RX modem, can also serve as a performance metric and as a basis for prediction of the future link performance. 
         [0061]    In some embodiments, the RX model comprises an adaptive equalizer. The equalizer typically comprises a digital filter with programmable coefficients, whose values are adapted in order to equalize the response of the channel. In some cases, the values of the equalizer coefficients can provide a good indication of an approaching performance degradation, and can be used as performance metrics. 
         [0062]    For example, the metric may comprise a ratio between the energy of the coefficient having the largest absolute value and between the total energy of all equalizer coefficients. Alternatively, the metric may comprise a ratio between the energy of the largest absolute value coefficient and between the total energy of all other equalizer coefficients, excluding the largest coefficient. The metric may be based either on current values or on predicted (e.g., extrapolated) values of the equalizer coefficients. 
         [0063]    In many cases, exceedingly low values of these ratios indicate that the channel response is non-uniform and may be used as an early indication of performance degradation. Thus, the metric value can be compared to a predetermined threshold in order to determine whether or not performance degradation is approaching. Let C i , i=1 . . . N, denote the equalizer coefficients and C max  denote the coefficient having the maximum absolute value, the two alternative metrics can be written as 
         [0000]    
       
         
           
             
               
                 
                    
                   
                     C 
                     max 
                   
                    
                 
                 2 
               
               
                 
                   ∑ 
                   
                     i 
                     = 
                     
                       1 
                        
                       
                           
                       
                        
                       … 
                        
                       
                           
                       
                        
                       N 
                     
                   
                 
                  
                 
                   
                      
                     
                       C 
                       i 
                     
                      
                   
                   2 
                 
               
             
              
             
                 
             
              
             and 
              
             
                 
             
              
             
               
                 
                   
                      
                     
                       C 
                       max 
                     
                      
                   
                   2 
                 
                 
                   
                     ∑ 
                     
                       
                         i 
                         = 
                         
                           1 
                            
                           
                               
                           
                            
                           … 
                            
                           
                               
                           
                            
                           N 
                         
                       
                       , 
                       
                         i 
                         ≠ 
                         max 
                       
                     
                   
                    
                   
                     
                        
                       
                         C 
                         i 
                       
                        
                     
                     2 
                   
                 
               
               . 
             
           
         
       
     
         [0064]    In some embodiments, the FEC code used by the TX and RX modems comprises an iteratively-decoded code, such as a low density parity check (LDPC) code or a turbo-code. Iteratively-decoded FEC codes are decoded by iterative decoding processes, which compute and use statistical metrics of the received symbols, such as likelihood ratios (LR) and log-likelihood ratios (LLR). LDPC codes are described, for example, by Gallager in “Low-Density Parity-Check Codes,” IRE Transactions on Information Theory, Volume 7, January, 1962, pages 21-28, and by Ryan and Vasic in “An Introduction to LDPC Codes,” GlobeCom 2003, San Francisco, Calif., Dec. 5, 2003, which are incorporated herein by reference. 
         [0065]    Iterative decoding processes are described, for example, by Worthen and Stark in “Unified Design of Iterative Receivers using Factor Graphs,” IEEE Transactions on Information Theory, (47:2), February, 2001, pages 843-849, and by Richardson and Urbanke in “An Introduction to the Analysis of Iterative Coding Systems,” Proceedings of the 1999 Institute for Mathematics and its Applications (IMA) Summer program: Codes, Systems and Graphical Models, Minneapolis, Minn., Aug. 2-6, 1999, which are also incorporated herein by reference. 
         [0066]    When the data transmitted over link  20  is encoded with an iteratively-decoded FEC code, the statistical metrics produced by the iterative decoding process can be used as performance metrics, and predicting their future values can be used as good indication of the future link performance. The statistical metrics (e.g., LR and LLR) are usually adapted during the iterative decoding process. Both the initial values and the final values of these metrics, i.e., before and after the decoding process is carried out, can be used as performance metrics. The number of uncorrected errors and/or the percentage of code words which contain uncorrected errors can also be used as prediction metrics. These parameters, which are often provided by the iterative decoding process, are indicative of the residual error rate of the link after error correction. 
         [0067]    The performance metrics used for prediction are typically dependent on the ACM setting being used. For example, MSE-based metrics and BER estimation are modulation-dependent. Other metrics, such as metrics derived from an iterative FEC decoding process, depend on both the specific modulation and the specific code rate being used. As such, basing the prediction on these metrics often provides a more explicit and accurate estimate as to how the currently-used ACM setting will perform at a future point in time. 
         [0068]    The system controller may use various methods for predicting the future link performance based on the monitored performance metrics. For example, the controller may fit the time-dependence of the monitored performance metric values with a suitable function, such as a linear, polynomial or rational function. The controller then extrapolates the function to produce expected values of the metric. Alternatively, the system controller may predict the future metric value using a Kalman filter, as is known in the art, or using any other suitable filtering or prediction process. 
         [0069]    In some embodiments, the receiver monitors two or more performance metrics, and the system controller predicts the future link performance and/or bases its decision to change the ACM setting on the multiple metrics. In some cases, using two or more metrics based on uncorrelated mechanisms, such as combining MSE prediction and equalizer coefficient prediction, may improve the prediction accuracy. 
         [0070]    In many practical cases, predicting the performance of the link approximately  10  ms in advance enables the link to operate with sufficiently small margins and perform reliable ACM setting switches. Alternatively, any other suitable prediction interval can be used. 
         [0071]      FIG. 2  is a flow chart that schematically illustrates a method for adapting the coding and/or modulation of a link  20  based on predicted future performance, in accordance with an embodiment of the present invention. The method begins with link  20  communicating using a certain ACM setting, at a communication step  80 . During communication, RX modem  64  measures one or more performance metrics, at a monitoring step  82 . 
         [0072]    Based on the monitored metrics, the system controller predicts the future performance of link  20 , at a prediction step  84 . The controller estimates the anticipated performance of the link at a future point in time, assuming the link were to continue using the currently-used ACM setting. For example, the controller may predict that the BER performance of the link is anticipated to drop to an unacceptable level. As another example, the controller may predict that the MSE at the receiver is expected to deteriorate beyond a predetermined threshold. 
         [0073]    The system controller checks whether, based on the performance prediction, an ACM change is required, at an adaptation checking step  86 . If the prediction results indicate that the currently-used ACM setting can be maintained, the method loops back to communication step  80  above. 
         [0074]    If, on the other hand, the performance prediction indicates that the ACM setting should be changed, the controller initiates an ACM setting change, at an adaptation step  88 . The system controller may initiate a change, for example, when the performance of the current ACM setting is predicted to deteriorate to an unacceptable level, or when the performance of the current ACM setting is predicted to be sub-optimal in terms of the achievable data rate. Alternatively, the system controller may evaluate any other condition in order to determine whether the performance prediction indicates a need to change the current ACM setting. 
         [0075]    At step  88 , the controller selects a different ACM setting to replace the current setting. The set of possible ACM settings used in link  20  is usually sorted in ascending order of robustness, which usually corresponds to a descending order of data rate. When the predicted performance indicates a need to improve the link robustness at the expense of data rate, the controller may select the ACM setting that immediately follows the current setting in the order of robustness, or any other suitable ACM setting. When the predicted performance indicates a possibility to increase the link data rate, the controller may select the ACM setting that immediately follows the current setting in terms of data rate, or any other suitable setting. The system controller may predict the future performance of any of the ACM settings, in order to determine to which ACM setting to switch. 
         [0076]    The system controller causes the indication of the new ACM setting to be propagated to framer  30 , TX modem  32 , RX modem  64  and de-framer  66 . The system controller may use any suitable method for coordinating and synchronizing the change in ACM setting between the elements of transmitter  24  and receiver  28 . For example, in some embodiments each data frame comprises an ACM field, which indicates the ACM setting used in the next frame. Alternatively, the ACM field may indicate the ACM setting of the current frame or of a frame having any other offset with respect to the current frame. The system controller, using framer  30 , inserts the desired ACM setting indication into the ACM setting fields of the data frames. When the data frames traverse the link, the modems and the de-framer extract the contents of the ACM setting fields and configure their ACM settings accordingly. 
         [0077]    After changing to the new ACM setting, the method loops back to communication step  80  above. The link then begins to communicate using the newly-updated setting. Because the change in ACM setting is based on a prediction of the link performance, the response time is reduced, enabling smooth transition between ACM settings. 
         [0078]    In some embodiments, other properties of link  20  can be changed based on the prediction of future link performance, either instead of or in addition to changing the ACM setting of the link. For example, the output power level of the transmitter can be increased or decreased based on the predicted future link performance. 
         [0079]    Although the embodiments described herein mainly address wireless point-to-point links, the principles of the present invention can also be used in other applications, such as satellite links and Digital Subscriber Line (DSL) links. 
         [0080]    It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.