Abstract:
A system and method for enhancing the performance of an equalizer in a modem. Multiple techniques are disclosed which improve the modem performance. A first technique uses stored parameters for each burst from each remote site to demodulate a received data stream. A second technique compensates for the gain droop caused by storing parameters across each burst. A third technique minimizes errors caused by adapting the equalizer coefficients for each data burst by analyzing the SN ratio and error rate of the received burst. A fourth technique improves the convergence of the equalizer by using a two-part preamble, whereby both parts are transmitted using different modulation techniques. A fifth technique is provided which performs a soft reset of the modem without performing a complete reset of the modem. A sixth technique determines a modem adaptation factor based on the expected modulation type of an incoming burst transmission. A seventh technique calculates a phase correction value for the stored tap values and applies the value to the incoming signal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a home base modem system in a wireless communication system that provides telephone, data and Internet connectivity to a plurality of users.  
           [0003]    2. Description of Related Art  
           [0004]    Several systems are currently in place for connecting computer users to one another and to the Internet. For example, many companies such as Cisco Systems, provide data routers that route data from personal computers and computer networks to the Internet along conventional twisted pair wires and fiber optic lines. These same systems are also used to connect separate offices together in a wide area data network.  
           [0005]    However, these systems suffer significant disadvantages because of the time and expense required to lay high capacity communications cables between each office. This process is time consuming and expensive. What is needed in the art is a high capacity system that provides data links between offices, but does not require expensive communication cables to be installed.  
           [0006]    Many types of current wireless communication systems facilitate two-way communication between a plurality of subscriber radio stations or subscriber units (either fixed or portable) and a fixed network infrastructure. Exemplary systems include mobile cellular telephone systems, personal communication systems (PCS), and cordless telephones. The objective of these wireless communication systems is to provide communication channels on demand between the subscriber units and the home base in order to connect the subscriber unit with the fixed network infrastructure (usually a wired-line system). Several types of multiple access systems currently exist for wirelessly transferring data between multiple sites. For example, wireless communication systems have typically used a Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA) or Frequency Division Multiple Access (FDMA) type system to facilitate the exchange of information between two users. These access schemes are well known in the art.  
           [0007]    Often in these wireless communication systems a modem is used for modulating and demodulating the information that is exchanged between users. Inside the modem circuit an equalizer is employed to correct distortions either in the wireless channel or fix imperfections in the analog circuitry. A novel equalizer with enhancements to improve performance especially using higher order QAM modulation is described herein.  
         SUMMARY OF THE INVENTION  
         [0008]    In one embodiment, the method comprises demodulating a data stream transmitted between remote sites and a home base using stored parameters for each remote site comprising receiving data from a current remote site at a home base, retrieving a parameter associated with a next remote site from a memory, storing the retrieved parameter for the next remote site in a first buffer, storing a second parameter associated with the current remote site into the memory, determining the initial phase and gain values for the next remote site based on the retrieved parameter, receiving data from the next remote site at the home base, and demodulating the data received from the next remote site using the determined initial phase and gain values.  
           [0009]    In one embodiment, the method comprises compensating for gain droop in a modem which stores equalizer coefficients across data bursts between a remote site and a home base comprising receiving channel data values from an equalizer, calculating the gain for the equalizer based on the received channel data values, and scaling the input to the equalizer to achieve a value of 1 based on the calculated gain.  
           [0010]    In one embodiment, the method comprises minimizing errors caused by adapting equalizer coefficients for each data burst from a remote site to a home base comprising retrieving a first equalizer coefficient for a current remote site, demodulating a data burst from the current remote site, determining the S/N ratio for the received burst, determining the error rate from the demodulated burst, comparing the determined S/N ratio and error rate to defined thresholds, and updating the first equalizer coefficient based on the comparisons.  
           [0011]    In one embodiment, the method comprises improving the convergence of an equalizer in a multi-modulation modem using a two part preamble comprising defining a first part of a preamble with a lower order section, wherein the lower order section is transmitted using QPSK modulation, defining a second part of the preamble with a higher order section, wherein the higher order section is transmitted using QAM  64  modulation, transmitting both the first and second parts of the preamble, receiving the transmitted preamble, and converging the equalizer based on the received preamble.  
           [0012]    In one embodiment, the method comprises providing a soft reset to a modem comprising generating an interrupt to the modem, halting the modem interface, setting a reset bit, flushing a buffer, realigning the buffer, reprogramming the buffer, and restarting the interface.  
           [0013]    In one embodiment, the method comprises determining an adaptation factor for an equalizer based on the expected modulation type of an incoming burst transmission comprising calculating channel characteristics and metrics for a first burst transmission from a remote site, comparing the channel characteristics and metrics, determining an expected modulation type for a second burst transmission from the remote site based on the comparison, selecting an adaptation factor based on the expected modulation type, and applying the selected adaptation factor to the second burst transmission.  
           [0014]    In one embodiment, the method comprises correcting the phase shift caused by the storage of tap values after each incoming burst transmission comprising determining an angle of correction based on correlating the input and output of the equalizer and applying the determined angle of correction to the incoming signal.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a block diagram of one embodiment of a modem.  
         [0016]    [0016]FIG. 2 is a block diagram of one embodiment of a receiver module for the modem.  
         [0017]    [0017]FIG. 3 is a block diagram of one embodiment of a Digital Equalization module from FIG. 2.  
         [0018]    [0018]FIG. 4 is a block diagram of one embodiment of the gain droop compensation module shown in FIG. 3.  
         [0019]    [0019]FIG. 5 illustrates a comparison of 64QAM Error Histograms using a two-type preamble with 2000 FT Cable.  
         [0020]    [0020]FIG. 6 is a zoomed view of FIG. 5.  
         [0021]    [0021]FIG. 7 gives the BER and the probability of the first RS word using the two-type preamble and different cable length.  
         [0022]    [0022]FIG. 8 is a block diagram of one embodiment of a tap update circuit from the digital equalization module shown in FIG. 3.  
         [0023]    [0023]FIG. 9 is a block diagram of one embodiment of a system for estimating the phase shift caused by the digital equalization module shown in FIG. 3.  
         [0024]    [0024]FIG. 10 is an illustration of one state of the digital equalization module shown in FIG. 3.  
         [0025]    [0025]FIG. 11 is an internal block diagram of a parameter storage module.  
         [0026]    [0026]FIG. 12 is a signal flow diagram of the process performed by the parameter storage module in processing uplink bursts.  
         [0027]    [0027]FIG. 13 is a block diagram of one embodiment of a Digital Equalization Parameter Storage and Recovery system.  
         [0028]    [0028]FIG. 14 is a flow chart illustrating the process performed by the error recovery module  968  shown in FIG. 13.  
         [0029]    Like reference numbers and designations in the various drawings indicate like elements.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    Throughout this description, the embodiment and examples shown should be considered as exemplars, rather than as limitations on the present invention.  
         [0031]    As shown in FIG. 1, one embodiment of a modem  100  includes a receiver module  102  and a transmitter module  104 . The transmitter module  104  receives digital data from a communications processor (not shown) and converts the data to an appropriately modulated analog carrier signal, for example, quadrature amplitude modulation (QAM) or quadrature phase shift keying modulation. The analog signal may or may not be up converted to a carrier frequency prior to transmission. The receiver module  102  demodulates an incoming modulated carrier signal and converts it back to its original digital form. A similar or identical modem may be used as the other end of a communication link.  
         [0032]    One embodiment of the receiver module  102  is illustrated in FIG. 2. The receiver module  102  includes a conditioning circuit module  908 , a digital equalization module  930 , a digital phase-locked loop module  950 , a decode/decryption module  959 , a parameter storage memory module  933 , an initial gain adjust module  920 , an adaptive modulation module  980 , a signal to noise ratio (“SNR”) calculator  935 , and an acquisition, tracking, and modulation control module  918 .  
         [0033]    One embodiment of the conditioning circuit module  908  includes an analog to digital converter  910 , a digital quadrature mixer  912 , a matched filter  914 , a correlation timing recovery module  916 , an automatic frequency control module  971 , an automatic gain control module  970 , a dual digital to analog converter  924 , and a voltage controlled oscillator  922 . The down-converted signal from the Intermediate Frequency (IF) is sampled by the digital to analog converter  910  to produce a digital output that is provided to the digital quadrature mixer  912 . In one embodiment, the digital quadrature mixer  912  splits the received signal into it&#39;s quadrature components and translates the signal, for example, from a 20 MHz IF, to baseband.  
         [0034]    The matched filter module  914  maximizes the signal to noise ratio by “matching” the frequency response of the receiver module  102  to the signal generated at the transmitter module  104 . In one embodiment, the matched filter module  914  also rejects adjacent and unwanted signals that could be generated by other transmitters in the system, or interfering spurs generated by imperfections in the analog circuitry.  
         [0035]    Still referring to FIG. 2, the filtered output from the matched filter  914  is then driven into the correlation timing recovery module  916 .  
         [0036]    The acquisition, tracking and modulation control module  918  provides control information, such as the modulation type or format of data, for the received burst to the correlation timing recovery module  916 . The acquisition, tracking and modulation control module  918  performs acquisition tracking and modulation control. Multiple data signals are fed to the acquisition, tracking and modulation control module  918  to control the high level operations of the receiver module  102 .  
         [0037]    In one embodiment, the correlation timing recovery module  916  aligns the symbol timing to the samples fed to the digital equalization  930 , and calculates the initial gain and phase offset so that the digital equalization  930  can quickly perform channel inversion and correct any residual distortion due to inaccuracies in the phase and timing estimations. In one embodiment, the correlation timing recovery module  916  calculates the sample timing error by performing correlation between the received preamble with a copy of the expected preamble. The output of the correlation timing recovery module  916  is fed to an interpolation circuit, which further refines the received sample timing offset. After the interpolation filter of the correlated preamble sequence, an accurate calculation, for example, to {fraction (1/16)} symbol time is fed to a polyphase filter which manipulates the received samples to correct for the timing. Even though the samples may be aligned to the correct timing, the constellation may be rotated relative to a phase of zero degrees. The correlation timing recovery module  916  will calculate this angle and feed it to the numerically controlled oscillator  952  so that the timing and phase are corrected enough for the digital equalization  930  to quickly fix any further distortions.  
         [0038]    In one embodiment, the correlation timing recovery module  916  calculates the received gain offset. The gain is adjusted once based on the detected energy in the preamble correlator. Gain is adjusted digitally by feeding a value to the initial gain adjust  920 . The initial gain adjust  920  maps the received quadrature symbols to the proper level before they are equalized by the equalizer. The initial gain is calculated by the correlation timing recovery module  916 . The correlation timing recovery module  916  performs frequency acquisition and automatic frequency correction (AFC) in combination with AFC  971 .  
         [0039]    The modem  135  may be equipped with a process for providing soft resets. A reset signal, while the modem  135  is in transmit mode, may occur in several instances. For example, during uplink transmission, a software module may be too slow in sending data to the modem  135 , as may occur if the data is modulated using quadrature amplitude modulation  64 . In such a case, the limited data received by the modem  135  may cause an interrupt to be generated. The modem  135  then halts the transmit interface and sets a transmit-reset bit. The modem  135  then flushes, realigns, and reprograms the necessary buffers. Once this process is completed, the modem  135  restarts the transmit interface, completing the reset operation.  
         [0040]    Similarly, a reset signal may occur while the modem  135  is in a receive mode. For example, the modem  135  may receive only bad cyclic redundancy check packets. In such a case, the modem  135  halts the receive interface and sets a receive-reset bit. The modem  135  then flushes and realigns the receive buffers and reprograms the necessary buffers. The modem  135  then restarts the receive interface, thus completing the reset operation.  
         [0041]    One embodiment of the digital equalization module  930 , illustrated in FIGS. 2 and 3, includes a gain droop compensation module  941 , an adaptive filter module  932 , a feed forward tap update module  934 , a decision feedback module  936 , a DFE adaptation module  938 , and an EQ Error calculator module  940 .  
         [0042]    The gain droop compensation loop  941  is located between the output of the correlation timing recovery module  916  and the input of the digital equalization module  930 . The gain droop compensation loop  941  accounts for any gain variations due to radio droop in an outdoor unit (not shown) forcing a gain factor which is a ratio to the calculated gain observed in the digital equalization module  930 . In one embodiment, this gain ratio is kept to unity.  
         [0043]    A detailed description of the gain droop compensation loop  941  is shown in FIG. 4. The outputs of the digital equalization module  930 , for example, taps  4  and  5 , are multiplied by 0.625 and 1, respectively. In one embodiment, the values of taps  4  and  5  are received after every symbol. This value is then summed with the resulting value being subtracted from nominal ‘1’. In one embodiment, this final value is 253. An adaptation factor (μ in FIG. 4), is then applied to this final value which is used to increment or decrement a gain constant similar to the adaptive filter  932  described in the digital equalization module  930 . The gain constant multiplies the input signal value to provide a gain to the digital equalization module  930  to be ‘1’. A gain of ‘1’ enhances the stability of the digital equalization module  930 . In one embodiment, the gain constant is reset every burst wherein at the end of each burst, the digital equalization module  930  has a gain of ‘1.’ In one embodiment where the digital equalization module  930  has a gain of ‘1’, the gain estimation module  921  of the Correlation Timing Recovery module  916  assumes that the digital equalization module  930  starts with an initial gain of ‘1.’ 
         [0044]    The digital equalization module  930  corrects any signal impairments due to multipath reception or distortion caused by imperfections in the analog receiver circuits. In the modem, the transmitter module  104  sends a known training sequence before the burst so that the digital equalization module  930  can learn what the distortion is and undo it before the data arrives. The length of time the digital equalization module  930  is allowed to observe the impairments must be balanced against the amount of bandwidth that is wasted sending the training sequences over the channel. The digital equalization module  930  decimates the received data from the correlation timing recovery module  916  into a one-sample-per-symbol complex signal.  
         [0045]    Within the equalizer, the acquisition, tracking and modulation control module  918  ascertains the next remote site&#39;s address and its expected modulation type based on comparisons of channel characteristics and metrics. In one embodiment, a reed solomon decoder module  962 , illustrated as a component of the decode/decryption module  959 , and the SNR calculator  935  calculate these channel characteristics and metrics. Once calculated, the adaptive modulation algorithm  980  compares the calculated channel characteristics and metrics and provides the information to the higher layers of the communication system. The higher layers then define what the expected modulation format is for the received burst from the next remote site. The higher layers convey these results to the acquisition, tracking and modulation control module  918 . Depending on what modulation is expected, the digital equalizer uses different adaptation factors (μ&#39;s) for each received burst. Selecting an adaptation factor that is optimized for the received modulation type improves the performance of the modem by increasing the probability that the parameters stored in the parameter memory storage module  933  at the end of the processed burst will be valid.  
         [0046]    Returning to FIG. 3, the digital equalization module  930  is composed of two sections: a feed-forward section and a feedback section. The feed-forward section is composed of an adaptive filter  932  and a feed forward tap update module  934 . In one embodiment, the feed forward tap update  934  adapts the coefficients for the feed-forward section of the digital equalization module  930  based on the Least Mean Squares (LMS) module. This is a gradient descent-based feed forward tap update  934 , which converges to a local minima by forcing the output of the adaptive filter  932  to a known state or a training sequence. The tap update formula for the gradient LMS module is given by:  
           W ( k )= W ( k− 1)−μ∇ J    
         [0047]    W(k) is the value of the new coefficient. W(k−1) is the value of the coefficient at the last instant in time. M is the step-size variable, and ∇J is the complex gradient of the system. The gradient for each dimension (tap) C for time k is given by:  
         ∇ Ck   J=− 2 x ( k−c )ε*( k )  
         [0048]    The equation for the error of the equalizer at time k is given by:  
         ε( k )= d ( k )− W ( k ) H   X ( k )  
         [0049]    Where:  
         [0050]    ε=the error between the desired signal (training or decisions) and the filtered signal complex (I and Q channels) signal X  
         [0051]    d=The desired signal from a training sequence or feedback decisions  
         [0052]    W=The complex vector of the filter taps  
         [0053]    X=The input signal.  
         [0054]    The preambles not only are used for timing recovery but are also used to converge the digital equalization module  930 . The preambles are chosen so that home bases in adjacent sectors do not interfere with one another and that close remote sites will not interfere with one another. There is a different Preamble for uplink, and a different preamble for downlink. There are four types of uplink preambles (1) Unscheduled registration bursts, (2) scheduled 16-QAM bursts, (3) scheduled QPSK bursts, and (4) scheduled 64-QAM bursts. The hardware is designed to detect which preamble is being transmitted and decode the burst using the correct preamble. The preamble for the higher orders are subdivided into a QPSK section, and a higher order section allowing faster convergence in the digital equalization module  930  which is critical in burst modem design and at the same time can be used for symbol timing recovery.  
         [0055]    1) The preamble is a mixture of both End Points and Middle QAM- 64  Points  
         [0056]    2) The Preamble is chosen for Uplink and Downlink Differently so that Adjacent home bases will not detect each other as transmitting remote sites  
         [0057]    [0057]FIG. 5 illustrates a comparison of QAM- 64  Error Histograms using a two-type preamble with 2000 FT Cable. FIG. 6 is a zoomed view of FIG. 5. FIG. 7 gives the BER and the probability of the first reed-solomon (“RS”) word using the two-type preamble and different cable length. For example, if the digital equalization module  930  is pre compensate over 1000 FT cable using Preamble type  2  we get the following results: BER of 5.81e-4 and an RS Error Rate of 1.5e-5.  
         [0058]    [0058]FIG. 8 illustrates an embodiment of a feed forward tap update module  934  (illustrated in FIG. 3). Upon initial acquisition, remote sites Reset, or remote sites register, the taps are reset such that the center tap is set to ‘1’ and the other taps are reset to ‘0’. If the taps have been initialized and adapted in past history, the tap update circuit receives a stored coefficient from the medium access control processor. The taps are updated to their new value by converging with either a training sequence (preamble) or the decision feedback  936  data from the decision device  958 . After remote site processing, the medium access control processor is responsible for storing the updated coefficients to a remote site database.  
         [0059]    Preferably, the digital equalization module  930  coefficients are initialized to take advantage of the knowledge gained during the previous bursts. Initialization relieves some of the concerns related to the digital equalization module  930  convergence time. One approach is to initialize the taps with the values from the end of the previous burst. However, this results in the digital equalization module  930  possibly having adapted to correct a phase shift (e.g., one introduced by phase noise or frequency offset) during the previous burst. At the beginning of the new burst, the correlation timing recovery module  916  re-calculates the initial phase and sets the symbol rotator to properly rotate the signal. This calculation assumes that the digital equalization module  930  does not introduce any overall phase rotation.  
         [0060]    A method for estimating the phase shift caused by the digital equalization module  930  and the mathematical basis will now be described.  
         [0061]    [0061]FIG. 9 illustrates a simplified block diagram of the system. For purposes of simplicity, the transmit and matched filters are assumed to have 2 samples per symbol. The odd samples of x are assumed to be zero (i.e., the pulse shape filter does the interpolation from 1 sample/symbol to 2 samples/symbol). The digital equalization module  930  is assumed to be T/ 2  spaced.  
         [0062]    If the channel is assumed to have no intersymbol interference, and the cascade of the transmit and matched filters is assumed to create an ideal Nyquist filter, then the even values of y are simply phase-shifted versions of the transmitted symbols, x. This is true because the even values of h are zero (except for h 0 ). If the digital equalization module  930  is also assumed to do a decimation (as our implementation does), then the digital equalization module  930  outputs a value when the even values of y line up with the main tap. To determine the phase shift induced by the digital equalization module  930 , the correlation between the input and output of the digital equalization module  930  at zero lag is desired. This is illustrated by the calculations described below.  
         [0063]    The output of the expectation operator for each tap is calculated individually (denoted it as r m ):  
               r   2     =                E        {       c   2          y     k   +   2              y   _     k       }                   =                  c   2        E        {       x     n   +   2                 j                 θ              x   _     n                 -   j                   θ         }                   =                  c   2        E        {       x     n   +   2              x   _     n       }                                  but                 transmit                 symbols                   (   x   )                   are                 uncorrelated     ,   so                 =                    c   2        0     =   0                   r   1     =                E        {       c   1          y     k   +   1              y   _     k       }                   =                  c   1        E        {            j                 θ              x   _     k                 -   j                   θ              ∑     m   =       -   2        N         2      N              x     k   +   1   -   m            h   m           }                   =                  c   1        E        {     …   +       h     -   2              x   _     k          x     k   +   3         +       h     -   1              x   _     k          x     k   +   2         +       h   0            x   _     k          x     k   +   1         +       h   1            x   _     k          x   k       +   …     }                   =                  c   1          h   1        E        {            x   k          2     }                     r   0     =                E        {       c   0          y   k            y   _     k       }                   =                  c   0        E        {       x   k             jθ            x   _     k                 -   j                   θ                         =                  c   0          h   0        E        {            x   k          2     }                     r     -   1       =                E        {       c     -   1            y     k   -   1              y   _     k       }                   =                  c     -   1          E        {            j                 θ              x   _     k                 -   j                   θ              ∑     m   =       -   2        N         2      N              x     k   -   1   -   m            h   m           }                   =                  c     -   1          E        {                …   +       h     -   2              x   _     k          x     k   +   1         +       h     -   1              x   _     k          x   k       +       h   0            x   _     k          x     k   -   1         +       h   1            x   _     k          x     k   -   2         +   …                }                   =                  c     -   1            h     -   1          E        {            x   k          2     }                     r     -   2       =                E        {       c     -   2            y     k   -   2              y   _     k       }                   =                  c     -   2          E        {       x     n   -   2                 j                 θ              x   _     n                 -   j                   θ                         =                  c     -   2          E        {       x     n   -   2              x   _     n       }                   =                    c     -   2          0     =   0                                 
 
         [0064]    Summing all the contributors to the correlation, only those from the main tap and the two adjacent to the main tap are non-zero:  
             r   =                  r     -   1       +     r   0     +     r     +   1                     =                E          {            x   k          2     }     ·     (         c     -   1            h     -   1         +       c   0          h   0       +       c   1          h   1         )                                     
 
         [0065]    Calculating for the angle of the correlation value:  
          eq =angle( c   −1   h   −1   +c   0   h   0   +c   1   h   1 )  
         [0066]    for raised cosine filter,  
         [0067]    with α of 0.25  
          eq =angle(0.627 c   −1   +c   0 +0.627 c   1 )  
         [0068]    A Mathcad simulation has been used to check the results shown above. With no timing error, the module shown above has been verified to be accurate. Although timing error was not included in the analysis above, the Mathcad simulation has shown that with reasonable timing error (e.g., T/ 8 ), the errors in the method shown above are very small (&lt;0.3°).  
         [0069]    One embodiment of the adaptation module  938  in the digital equalization module  930  (illustrated in FIGS. 2 and 3) uses a T/ 2  feed-forward filter, followed by a T-spaced decision feedback section. If the correct decisions are assumed to have been made, the adaptation module  938  can be illustrated as shown in FIG. 10.  
         [0070]    By calculating the correlation as described above with reference to FIG. 9, the angle of the correlation is arrived at as follows:  
          eq =angle( c   0   h   0   +c   1   h   1 )  
         [0071]    for raised cosine filter,  
         [0072]    with α of 0.25  
          eq =angle( c   0 +0.627 c   1 )  
         [0073]    If the channel has non-zero intersymbol interference, the above descriptions can be characterized as approximately true. Intersymbol interference will change the weighting factor for the two taps adjacent to the main tap, and the contribution of the other taps to the digital equalization module  930  phase shift becomes non-zero. Simulation results have shown that the adaptive module  938  described above is effective with all intermediate frequency filter models. Thus, since the magnitude of the intersymbol interference is typically small, it can be neglected.  
         [0074]    As illustrated in FIG. 2, the output of the digital equalization module  930  is fed to the digital phase-locked loop module  950 . One embodiment of the digital phase-locked loop module  950  includes a numerically controlled oscillator  952 , a loop filter module  954 , a phase error module  956 , and a decision device  958 . The decision device  958  receives the signal from the digital equalization module  930  and maps each complex signal pair into a sliced constellation symbol.  
         [0075]    The digital phase-locked loop  950  uses the input and output of the decision device  958  to continuously correct any phase error using hardware complex multipliers (or rotators)  953 . The output of the decision device  958  is also fed back into the decision feedback module  936  of the digital equalization module  930 , where it is used to adapt the equalizer coefficients.  
         [0076]    [0076]FIG. 2 illustrates a parameter storage module  933 . The parameter storage module  933  contains channel characteristics, in the form of taps, corresponding to each remote site. In one embodiment, each remote site has different tap values, each of which corresponds to a recent communication parameter by the remote site. Since each remote site has different channel characteristic, each remote site&#39;s tap values must be stored and retrieved independently. The digital equalization module  930  receives, from the parameter storage module  933 , the tap values corresponding to the remote site that is presently sending an uplink burst to the modem  135 . Once received, the digital equalization module  930  sets the correct channel characteristics for the incoming burst from the remote site which helps the modem  135  perform by keeping estimates of the channel characteristics for each transmission from each remote site. In one embodiment, the acquisition, tracking and modulation control module  918  generates an index of remote sites and their associated tap values to expedite their retrieval from the parameter storage module  933 . After receiving a burst from one of the remote sites, the parameter storage module  933  updates the stored tap values for that remote site.  
         [0077]    [0077]FIG. 12 is a signal flow diagram of the process performed by the parameter storage module  933 , illustrated in FIG. 2, in processing uplink bursts after the correlation timing recovery module  916  but before the decision device module  958 . The process stores and retrieves parameters to/from memory when processing bursts from different remote sites at the home base receiver module  102 .  
         [0078]    The process begins at a start state  2540 . Next at a state  2542  the home base receiver retrieves a remote site number from the acquisition, tracking and modulation control  918 . Next, at a state  2544 , the parameter storage module  933  retrieves parameters for this (next) remote site, for example, the equalizer taps from the previous burst. Next, at a state  2546 , the parameter storage module  933  retrieves parameters for this (next) remote site, for example, the attenuation levels. In one embodiment, these parameters are stored in a first temporary buffer. Flow proceeds to a state  2550  where the parameter storage module  933  waits until the current burst is completed. Flow continues to a decision state  2552  where the process determines whether the current burst is completed. If the home base receiver is currently processing an uplink burst, the parameter storage module  933  waits until the current burst is finished. In one embodiment, the acquisition, tracking and modulation control module  918  informs the parameter storage module  933  that the current burst is finished. If the current burst is not completed, the process loops back to state  2550 . Otherwise, if the current burst is completed, flow proceeds to a state  2554  where the retrieved parameters for the current remote site are stored into a second temporary buffer.  
         [0079]    In one embodiment, the decision whether to store the parameters depends on the state of the processed error metrics calculated by the error recovery module  968 . For example, if the error thresholds are not exceeded, the parameter storage module  933  stores the retrieved parameters. Next at a state  2556 , the attenuation and EQ taps for the current remote site retrieved from the feed forward tap update  934  and DFE adaptive algorithm  938  are stored into a second temporary buffer. Flow proceeds to a state  2558  where the parameters from the first temporary buffer for the next remote site burst are transferred to the feed forward tap update  934  and DFE adaptive algorithm  938  modules.  
         [0080]    Continuing to a state  2560 , the modem  100  waits for the next remote site burst to arrive. Also at state  2560 , the parameter storage module  933  writes the parameters from the second temporary buffer to the parameter storage memory  933 . Next at a decision state  2562 , the correlation timing recovery module  916  sends a message to the modem processing blocks when the next remote site burst is present. If a burst is not present, flow loops back to state  2560  as discussed above. Otherwise, if the next remote site burst is present, the process continues to a state  2564  where the correlation timing recovery module  916  calculates the initial phase and gain of the burst. Next at a state  2566 , the correlation timing recovery module  916  applies the gain and phase to the initial gain adjust module  920  and  952  respectfully. Next at a state  2568  the parameter storage memory  933  calculates the Equalizer phase offset from the retrieved EQ taps and then applies this offset to the numerically controlled oscillator  952 . Next at a state  2570 , the burst is ready to be processed by the receive chain of the home base receiver module  102 . Flow then proceeds to an end state  2572 . In one embodiment, this process is performed for each received uplink burst.  
         [0081]    To increase the likelihood that the stored channel parameters, or taps, will be valid for an incoming burst, factors, such as, symbol timing, interpolator timing, end of a burst gain value, and any phase offset generated by the digital equalization module  930 , may be optimized. In one embodiment, locking the symbol timing within ⅛ symbol, locking the timing interpolator, providing a nominal gain of ‘1’ for the taps at the end of a burst, and compensating for any phase offset generated by the digital equalization module  930  at the beginning of the following burst are done to increase the likelihood that an incoming burst will be recognized and found valid.  
         [0082]    [0082]FIG. 13 illustrates a Digital Equalization Parameter Storage and Recovery system. The Digital Equalization Parameter Storage and Recovery system includes the acquisition, tracking, and modulation control module  918 , the digital equalization module  930 , the parameter storage memory module  933 , an error recovery module  968 , the reed solomon decoder module  962 , and the SNR calculator  935 . In one embodiment, the SNR calculator  935  calculates the mean square error between the received symbol and the expected optimal symbol. By accumulating and averaging these calculations, an accurate measurement of the noise or interference of the system is measured. For example, when the SNR is high, the system can operate at a high order of modulation, say QAM- 64 . If the SNR is Low, the system must operate at a more robust modulation scheme such as QPSK. The output of the SNR calculator  935  is fed to the DFE adaptive modulation algorithm  938 , which based on particular SNRs will feed the acquisition, tracking and modulation control module  918  to specify which modulation scheme is optimal for the current channel characteristics.  
         [0083]    The reed Solomon decoder module  962  calculates the number of errors in a stream of 300-bits to measure the decode error rate. At the end of a burst from a remote site, both of these calculated error values are then compared to threshold values by the error recovery module  968 . If either of the calculated values exceed the threshold values, the error recovery module  968  does not update the stored channel parameters, or taps, in the parameter storage memory  933  for that remote site. In one embodiment, the stored channel parameters in the parameter storage memory  933  are reset when the threshold is exceeded. In another embodiment, the stored channel parameters are replaced with zeros if the calculated values exceed the threshold values. If the threshold values are not exceeded, the new calculated channel parameters are stored in the parameter storage memory  933 . These new channel parameters are available to the digital equalization module  930  to set the correct channel characteristics for the next incoming burst from the remote site. Thus, the Digital Equalization Parameter Storage and Recovery system provides a recovery system should a catastrophic channel disruption occur.  
         [0084]    [0084]FIG. 14 illustrates the Digital Equalization Parameter Storage and Recovery process. The process begins at a start state  2900  and continues to a state  2902  where the set of stored channel parameters associated with the transmitting remote site is retrieved. Next, at a decision state  2904 , a determination is made as to whether a burst the remote site is being received. If a burst is not received, the process loops back to decision state  2904  until a burst is received. If a burst is received, the process moves to a state  2906  where the incoming burst is demodulated. At a next state  2908 , the SNR calculator  935  calculates the noise of the demodulated signal. Continuing to a state  2910 , the error in the decoded signal is calculated by the reed solomon decoder module  962 . Next at a decision state  2912 , if the received burst is not at an end, flow continues to state  2906  as discussed above. Returning to decision state  2912 , if the received burst is at an end, flow continues to a decision state  2914  to determine if the calculated noise from state  2908  exceeds a threshold value. If the calculated noise exceeds the threshold value, the process proceeds to a state  2916  where the current channel characteristics are invalidated. Flow then ends at an end state  2918 .  
         [0085]    Returning to decision state  2914 , if the calculated noise does not exceed the noise threshold, flow moves to a decision state  2920  to determine if the calculated error in the decode signal exceeds the decode error threshold. If the decode error exceeds the threshold, the process continues to state  2916  as described above. Otherwise, if the calculated error in the decode signal does not exceed the decode threshold, the process moves to a state  2922  where the channel characteristics for the remote site burst are stored in the parameter storage memory  933 . Flow then proceeds to the end state  2918 .  
         [0086]    In downlink communication, the equalization and channel estimation is simpler than in uplink communication. In the downlink direction, immediately after the training sequence is sent, data follows in a QPSK-modulation format. In the uplink direction, on the other hand, a higher-order modulation immediately follows the training sequence, worst case being QAM- 64  modulation. In the downlink direction, the probability of making a bit error due to the equalizer not fully undoing the signal impairments is low since the Euclidean distance of received symbols for a QPSK constellation is much greater than the Euclidean distance of symbols for a QAM- 64  constellation.  
         [0087]    Preambles are not only chosen to converge the digital equalization module  930 , but also used for timing recovery. Preambles are chosen such that uplink communication and downlink communication do not interfere with each other. The preamble structure is split into two sections. For the downlink preamble and the uplink-unscheduled preambles, a QPSK portion is repeated twice so that when the received signal is correlated with the local copy, a low probability of preamble false detection will occur when trying to acquire the downlink or when demodulating an unscheduled uplink burst. The receiver module  102  correlator followed by an interpolator filter bank is used to recover the timing by comparing the energy.  
         [0088]    In one embodiment, the above noted techniques are used in combination by the modem  100  to perform higher order modulation techniques, for example, QAM  64 . These techniques include performing the initial phase calculation on the incoming burst, performing droop compensation, performing a phase calculation on the stored taps, and storing channel characteristics, or taps, after each burst. These techniques are further combined with a method of performing symbol clock synchronization as described in U.S. patent application ______, filed Mar. 20, 2001 and entitled “SYNCHRONIZING CLOCKS ACROSS A COMMUNICATION LINK” which is hereby incorporated by reference. The combination of these techniques enhances the operation of the modem  100  when performing QAM  64  modulation.  
         [0089]    Aspects of the present invention have been disclosed in one or more exemplary embodiments. These embodiments are not to be construed as limiting, but rather as showing a way to practice the invention. The scope of the invention is defined by the claims which follow.