Equalizer performance enhancements for broadband wireless applications

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.

What is claimed is:  
     
       1. A method for operation of a modem system which demodulates data bursts from a plurality of remote sites using stored channel parameters for each remote site, the method comprising:
 receiving a data burst from a current remote site;  
 determining a next remote site which will transmit a next data burst;  
 determining attenuation levels and equalizer tap values for the next remote site based on a previous burst from the next remote site;  
 determining an equalizer phase error from the determined equalizer tap values for the next remote site;  
 replacing attenuation levels and equalizer tap values for the current remote site with the determined attenuation levels and equalizer tap values for the next remote site in an equalizer once the burst from the current remote site is completed;  
 storing the replaced attenuation levels and equalizer tap values for the current remote site for use with a subsequent burst from the current remote site;  
 receiving the next burst from the next remote site;  
 determining an initial phase and an initial gain for the received next burst from the next remote site; and  
 demodulating the received next burst from the next remote site using the determined initial phase, initial gain, and equalizer phase error.  
 
     
     
       2. The method of  claim 1 , further comprising adjusting the replaced attenuation levels and equalizer tap values during receipt of the burst from the current remote site and storing the adjusted values. 
     
     
       3. The method of  claim 1 , wherein demodulating the received next burst from the next remote site comprises:
 applying the determined initial phase and the initial gain for the next remote site to an initial gain adjust module; and  
 applying the determined equalizer phase error for the next remote site to a numerically controlled oscillator.  
 
     
     
       4. The method of  claim 1 , further comprising generating an index of remote sites. 
     
     
       5. The method of  claim 1 , further comprising:
 receiving equalizer tap values associated with the next burst from the next remote site;  
 determining a gain constant for the equalizer based on the received equalizer tap values; and  
 scaling the input signal of a subsequent burst from the next remote site to the equalizer to achieve a gain value of 1 based on the determined gain constant.  
 
     
     
       6. The method of  claim 5 , wherein scaling the input signal of the subsequent burst to the equalizer is performed by assigning a value to at least one of the equalizer tap values at the end of the next burst from the next remote site. 
     
     
       7. The method of  claim 5 , further comprising compensating for gain variation by adjusting a center equalizer coefficient. 
     
     
       8. The method of  claim 5 , further comprising storing at least one scaled equalizer tap value at the end of the next burst from the next remote site. 
     
     
       9. The method of  claim 6 , wherein scaling is performed on all of the equalizer tap values. 
     
     
       10. The method of  claim 6 , wherein scaling is performed on a center equalizer tap value at the end of the next burst from the next remote site. 
     
     
       11. The method of  claim 10 , further comprising determining the gain for the equalizer from an angle of the center equalizer tap value. 
     
     
       12. The method of  claim 11 , wherein the angle includes a sine value. 
     
     
       13. The method of  claim 11 , wherein the angle includes a cosine value. 
     
     
       14. The method of  claim 1 , further comprising:
 determining a noise value for the demodulated next burst from the next remote site;  
 determining an error value for the demodulated next burst from the next remote site;  
 if the determined noise value exceeds a threshold value, invalidating the attenuation levels and equalizer tap values;  
 if the determined error value exceeds a decode error threshold, invalidating the attenuation levels and equalizer tap values; and  
 storing the valid attenuation and equalizer tap values.  
 
     
     
       15. The method of  claim 14 , wherein determining the noise value comprises determining a signal to noise ratio. 
     
     
       16. The method of  claim 1 , further comprising:
 receiving a two part preamble at the start of the next burst from the next remote site, wherein a first part is modulated using a lower order technique and the second part is modulated using a higher order technique than used for the first part; and  
 driving the output of an adaptive filter in the equalizer to minimize errors associated with the demodulation of the next burst which follows the two part preamble.  
 
     
     
       17. The method of  claim 16 , wherein the lower order technique is QPSK. 
     
     
       18. The method of  claim 16 , wherein the higher order technique is QAM 64. 
     
     
       19. The method of  claim 16 , wherein the higher order technique is QAM 16. 
     
     
       20. The method of  claim 16 , wherein the first part and the second part of the preamble includes end points and middle QAM 64 points. 
     
     
       21. The method of  claim 16 , wherein the received two part preamble is used for symbol timing recovery. 
     
     
       22. The method of  claim 16 , wherein the errors associated with the demodulation of the next burst include a bit error rate and a reed solomon error rate. 
     
     
       23. The method of  claim 1 , further comprising:
 generating an interrupt to the modem in response to a modem system error signal;  
 halting a modem interface once the modem system error occurs;  
 setting a reset bit once the modem is halted;  
 flushing a buffer once the reset bit is set;  
 realigning the buffer once the buffer is flushed;  
 reprogramming the buffer once the buffer is realigned; and  
 restarting the modem interface once the buffer is reprogrammed.  
 
     
     
       24. The method of  claim 23 , wherein the reset bit is a transmit-reset bit. 
     
     
       25. The method of  claim 23 , wherein the reset bit is a receive-reset bit. 
     
     
       26. The method of  claim 23 , wherein the interrupt occurs during modem transmission. 
     
     
       27. The method of  claim 23 , wherein the interrupt occurs during modem reception. 
     
     
       28. The method of  claim 23 , wherein the interrupt is generated in response to the modem transmitting at a rate faster than the data is received by the modem. 
     
     
       29. The method of  claim 28 , wherein the interrupt is generated in response to the modem receiving cyclic redundancy check packets. 
     
     
       30. The method of  claim 29 , wherein the modem is transmitting using QAM 64. 
     
     
       31. The method of  claim 1 , further comprising:
 determining an expected modulation type for a subsequent burst from the next remote site based on the determined initial phase and the initial gain for the next burst from the next remote site;  
 selecting an adaptation factor for the equalizer based on the expected modulation type; and  
 applying the selected adaptation factor to the subsequent burst from the next remote site such that the probability that the attenuation levels and tap values stored after demodulating the subsequent burst are correct is increased.  
 
     
     
       32. The method of  claim 31 , wherein the expected modulation type is QPSK. 
     
     
       33. The method of  claim 31 , wherein the expected modulation type is QAM 16. 
     
     
       34. The method of  claim 31 , wherein the expected modulation type is QAM 64. 
     
     
       35. The method of  claim 1 , further comprising:
 correlating the input and output of the equalizer for the next burst from the next remote site;  
 determining an angle of correction for the subsequent burst from the next remote site based on the correlation; and  
 shifting the subsequent burst by applying the determined angle of correction to the subsequent burst.  
 
     
     
       36. The method of  claim 35 , wherein the angle of correction is determined using the equation ∅ eq =angle (c 0 +0.627c 1 ), wherein C 0  is the center tap value, and wherein c 1  is an adjacent tap value. 
     
     
       37. A method for operation of a modem system which demodulates data bursts from a plurality of remote sites using stored channel parameters for each remote site and compensates for gain droop in the modem transmitter, the method comprising:
 receiving equalizer tap values associated with a burst from a next remote site;  
 determining a gain constant for an equalizer based on the received equalizer tap values; and  
 scaling the input signal of a next burst to the equalizer to achieve a gain value of 1 based on the determined gain constant.  
 
     
     
       38. The method of  claim 37 , wherein scaling the input signal is performed by assigning a value to at least one equalizer coefficient at the end of the burst. 
     
     
       39. The method of  claim 37 , further comprising storing at least one scaled equalizer coefficient value at the end of the burst. 
     
     
       40. The method of  claim 38 , wherein scaling is performed on all of the equalizer coefficients. 
     
     
       41. The method of  claim 38 , wherein scaling is performed on the center equalizer coefficient. 
     
     
       42. The method of  claim 41 , further comprising determining the gain for the equalizer from the angle of the center equalizer coefficient at the end of the burst. 
     
     
       43. The method of  claim 42 , wherein the calculated angle includes a sine value. 
     
     
       44. The method of  claim 42 , wherein the calculated angle includes a cosine value. 
     
     
       45. The method of  claim 37 , further comprising compensating for gain variation is performed by the center equalizer coefficient. 
     
     
       46. A method for operation of a modem system which demodulates data bursts from a plurality of remote sites using stored channel parameters for each remote site and improves the convergence of the modem, the method comprising:
 transmitting a first part of a preamble that is modulated using a lower order technique;  
 transmitting a second part of the preamble after transmitting the first part of the preamble, wherein the second part of the preamble is modulated using a higher order technique than the modulation technique used for the first part;  
 receiving the transmitted preamble at the modem; and  
 driving the output of an adaptive filter in communication with the modem to a known state based on the received preamble.  
 
     
     
       47. The method of  claim 46 , wherein the lower order technique is QPSK. 
     
     
       48. The method of  claim 46 , wherein the higher order technique is QAM 64. 
     
     
       49. The method of  claim 46 , wherein the higher order technique is QAM 16. 
     
     
       50. The method of  claim 46 , wherein the first part and the second part of the preamble includes end points and middle QAM 64 points. 
     
     
       51. The method of  claim 46 , wherein the received preamble is used for symbol timing recovery. 
     
     
       52. The method of  claim 46 , wherein the method is implemented in a Time Division Duplex system. 
     
     
       53. A method for operation of a modem system which demodulates data bursts from a plurality of remote sites using stored channel parameters for each remote site which corrects the phase shift caused by the storage of equalizer tap values, the method comprising:
 correlating the input and output of an equalizer for a received data burst from a next remote site based on equalizer tap values;  
 determining an angle of correction for an incoming data burst from the next remote site based on the correlation using the equation ∅ eq =angle (c 0 0.627c 1 ), wherein c 0  is the center tap value, and wherein c 1  is an adjacent tap value; and  
 shifting the incoming data burst by applying the determined angle of correction to the incoming data burst.  
 
     
     
       54. The method of  claim 53 , wherein the method is implemented in a Time Division Duplex system. 
     
     
       55. A modem system which demodulates data bursts from a plurality of remote sites using stored channel parameters for each remote site, the modem comprising:
 means for receiving a data burst from a current remote site;  
 means for determining a next remote site which will transmit a next data burst;  
 means for determining attenuation levels and equalizer tap values for the next remote site based on a previous burst from the next remote site;  
 means for determining an equalizer phase error from the determined equalizer tap values for the next remote site;  
 means for replacing attenuation levels and equalizer tap values for the current remote site with the determined attenuation levels and equalizer tap values for the next remote site in an equalizer once the burst from the current remote site is completed;  
 means for storing the replaced attenuation levels and equalizer tap values for the current remote site for use with a subsequent burst from the current remote site;  
 means for receiving the next burst from the next remote site;  
 means for determining an initial phase and an initial gain for the received next burst from the next remote site; and  
 means for demodulating the received next burst from the next remote site using the determined initial phase, initial gain, and equalizer phase error.  
 
     
     
       56. A modem system which demodulates data bursts from a plurality of remote sites using stored channel parameters for each remote site and compensates for gain droop in the modem transmitter, the modem comprising:
 means for receiving equalizer tap values associated with a burst from a next remote site;  
 means for determining a gain constant for an equalizer based on the received equalizer tap values; and  
 means for scaling the input signal of a next burst to the equalizer to achieve a gain value of 1 based on the determined gain constant.  
 
     
     
       57. A method to demodulate data bursts from a plurality of transmission sites, the method comprising:
   determining a second transmission site from which to receive a next data burst, said next data burst to be received following receipt of a data burst from a first transmission site;        obtaining attenuation levels and equalizer tap values associated with the second transmission site based, at least in part, on a previous data burst received from the second transmission site;        determining an equalizer phase error from said obtained equalizer tap values;        replacing, in an equalizer, attenuation levels and/or equalizer tap values associated with said first transmission site with the obtained attenuation levels and/or equalizer tap values associated with the second transmission site;        storing the replaced attenuation levels and/or equalizer tap values associated with the first transmission site;        determining an initial phase and/or an initial gain for the next burst received from the second transmission site; and        demodulating the received next burst received from the second transmission site using the determined initial phase, initial gain, and/or equalizer phase error.     
     
     
       58. The method of  claim 57 , further comprising using said stored and replaced attenuation levels and/or equalizer tap values to process a next data burst received from said first transmission site. 
     
     
       59. A method, comprising:
   receiving a preamble from one of a plurality of transmission sites, the preamble including a first part modulated according to a lower order modulation technique and a second part modulated according to a modulation technique of an order higher than said lower order modulation technique; and        driving an output signal of an adaptive filter to a state based, at least in part, on said received preamble.     
     
     
       60. The method of  claim 59 , further comprising storing channel parameters associated with two or more of said transmission sites. 
     
     
       61. The method of  claim 59 , wherein the lower order modulation technique comprises QPSK. 
     
     
       62. The method of  claim 59 , wherein the higher order modulation technique comprises QAM  64 . 
     
     
       63. The method of  claim 59 , wherein the higher order modulation technique comprises QAM  16 . 
     
     
       64. The method of  claim 59 , wherein the first part and the second part of the preamble include end points and middle QAM  64  points. 
     
     
       65. The method of  claim 59 , further comprising recovering symbol timing based, at least in part, on said received preamble. 
     
     
       66. The method of  claim 59 , wherein the method is implemented in a Time Division Duplex system. 
     
     
       67. The method of  claim 59 , wherein said output signal comprises a training sequence. 
     
     
       68. An apparatus, comprising:
   a demodulator to demodulate a preamble received from one of a plurality of transmission sites, said demodulator being adapted to:      demodulate a first portion of said preamble according to a first modulation technique; and        demodulate a second portion of said preamble according to a second modulation technique, said second modulation technique having an order higher than an order of said first modulation technique; and          a circuit to drive an output signal of an adaptive filter in communication with the demodulator to a state based, at least in part, on said received preamble.     
     
     
       69. The apparatus of  claim 68 , wherein said output signal comprises a training sequence. 
     
     
       70. The apparatus of  claim 68 , further comprising storing channel parameters for two or more of said transmission sites. 
     
     
       71. The apparatus of  claim 68 , wherein the lower order modulation technique comprises QPSK. 
     
     
       72. The apparatus of  claim 68 , wherein the higher order modulation technique comprises QAM  64 . 
     
     
       73. The apparatus of  claim 68 , wherein the higher order modulation technique comprises QAM  16 . 
     
     
       74. The apparatus of  claim 68 , wherein the first portion and the second portion of the preamble include end points and middle QAM  64  points. 
     
     
       75. The apparatus of  claim 68 , further comprising recovering symbol timing based, at least in part, on said received preamble. 
     
     
       76. The apparatus of  claim 68 , wherein the apparatus is implemented in a Time Division Duplex system. 
     
     
       77. A method, comprising:
   storing equalizer tap values associated with a plurality of transmission sites;        determining a correlation between an input signal and an output signal based, at least in part, on equalizer tap values associated with a transmission site of said plurality of transmission sites, said input signal being based, at least in part, on an incoming data burst received from said transmission site;        determining an angle of correction associated with said correlation based, at least in part, on a weighted sum of a center tap value and one or more adjacent tap values of said equalizer tap values; and        shifting said incoming data burst by an amount based, at least in part, on said determined angle of correction.     
     
     
       78. The method of  claim 77 , wherein said weighted sum substantially comprises c 0   + 0 . 627 c   1    wherein c   0    comprises said center tap value and c   1    comprises an adjacent tap value.   
     
     
       79. An apparatus, comprising:
   a memory to store equalizer tap values associated with a plurality of transmission sites; and        an equalizer adapted to:      determine a correlation between an input signal and an output signal based, at least in part, on equalizer tap values associated with a transmission site of said plurality of transmission sites, said input signal being based, at least in part, on an incoming data burst received from said transmission site;        determine an angle of correction associated with said correlation based, at least in part, on a weighted sum of a center tap value and one or more adjacent tap values of said equalizer tap values; and        shift said incoming data burst by an amount based, at least in part, on said determined angle of correction.       
     
     
       80. The apparatus of  claim 79 , wherein said weighted sum substantially comprises c 0   + 0 . 627 c   1    wherein c   0    comprises said center tap value and c   1    comprises an adjacent tap value.   
     
     
       81. A system to demodulate data bursts from a plurality of transmission sites including at least a first transmission site and a second transmission site, the system comprising:
   a memory to store channel parameters associated with said plurality of transmission sites;        an equalizer adapted to:      obtain attenuation levels and equalizer tap values associated with said second transmission site based, at least in part, on a previous data burst received from said second transmission site;        determine an equalizer phase error based, at least in part, on said obtained equalizer tap values;        replace attenuation levels and/or equalizer tap values associated with a data burst received from said first transmission site with said obtained attenuation levels and/or equalizer tap values;        store said replaced attenuation levels and/or equalizer tap values in said memory; and        determine an initial phase and/or an initial gain for a next data burst received from the second transmission site; and          a demodulator to demodulate the next data burst received from the second transmission site based, at least in part, on said determined initial phase, initial gain, and/or equalizer phase error.     
     
     
       82. The system of  claim 81 , further comprising circuitry to adjust the replaced attenuation levels and/or equalizer tap values during receipt of the burst from the first transmission site. 
     
     
       83. The system of  claim 81 , wherein said demodulator is further adapted to:
   apply the determined initial phase and the initial gain to an initial gain adjust module; and        apply the determined equalizer phase error to a numerically controlled oscillator.     
     
     
       84. The system of  claim 81 , further comprising logic to generate an index of transmission sites. 
     
     
       85. The system of  claim 81 , wherein said equalizer is further adapted to obtain equalizer tap values associated with the next data burst from the second transmission site, and wherein said system further comprises gain control circuitry adapted to:
   determine a gain based, at least in part, on the obtained equalizer tap values; and        scale an input signal of a next data burst from the second transmission site to the equalizer to achieve a gain value based, at least in part, on the determined gain.     
     
     
       86. The system of  claim 85 , wherein said gain control circuitry is further adapted to scale said input signal of the next data burst from said second transmission site by assigning a value to at least one of the equalizer tap values at an end of said next data burst from said second transmission site. 
     
     
       87. The system of  claim 85 , wherein said gain control circuitry is further adapted to compensate for gain variation by adjustment of a center equalizer coefficient. 
     
     
       88. The system of  claim 85 , further comprising a memory to store at least one scaled equalizer tap value at an end of the next data burst from the second transmission site. 
     
     
       89. The system of  claim 86 , wherein said gain control circuitry is further adapted to scale all of said equalizer tap values. 
     
     
       90. The system of  claim 86 , wherein said gain control circuitry is further adapted to scale said input signal at a center equalizer tap value at the end of the next data burst from the second transmission site. 
     
     
       91. The system of  claim 90 , wherein said gain control circuitry is further adapted to determine the gain for said equalizer based, at least in part, on an angle of the center equalizer tap value. 
     
     
       92. The system of  claim 91 , wherein said angle includes a sine value. 
     
     
       93. The system of  claim 91 , wherein the angle includes a cosine value. 
     
     
       94. The system of  claim 81 , wherein said system is further adapted to:
   determine a noise value for the demodulated next data burst from the second transmission site;        determine an error value for the demodulated next data burst from the second transmission site;        invalidate the attenuation levels and/or equalizer tap values in response to said determined noise value exceeding a threshold value;        invalidate the attenuation levels and/or equalizer tap values in response to said determined error value exceeding a decode error threshold; and        storing valid said attenuation and equalizer tap values.     
     
     
       95. The system of  claim 94 , wherein said system is further adapted to determine the noise value based, at least in part, on a signal to noise ratio. 
     
     
       96. The system of  claim 81 , wherein said demodulator is further adapted to:
   demodulate a first part of a two part preamble received at a start of the next data burst from the second transmission site according to a first demodulation technique and demodulate a second part of said received preamble according to a second demodulation technique, said second demodulation technique having an order higher than an order of said first demodulation technique; and        drive an output signal of an adaptive filter in the equalizer to reduce errors associated with demodulation of said next data burst received from said second transmission site.     
     
     
       97. The system of  claim 96 , wherein said first demodulation technique comprises QPSK. 
     
     
       98. The system of  claim 96 , wherein said second demodulation technique comprises QAM  64 . 
     
     
       99. The system of  claim 96 , wherein said second demodulation technique comprises QAM  16 . 
     
     
       100. The system of  claim 96 , wherein the first part and the second part of the preamble comprise end and middle QAM  64  points. 
     
     
       101. The system of  claim 96 , further comprising timing recovery circuitry to recover symbol timing based, at least in part, on said received preamble. 
     
     
       102. The system of  claim 96 , wherein said errors associated with the demodulation of said next data burst from said second transmission site include a bit error rate and/or a reed solomon error rate. 
     
     
       103. The system of  claim 81 , wherein said system is further adapted to:
   halt a modem interface in response to detection of an error;        set a reset bit in response to said halt;        flush a buffer in response to said set;        realign said buffer in response to said flush;        reprogram said buffer in response to said realign; and        restart said modem interface in response to said reprogram.     
     
     
       104. The system of  claim 103 , wherein the reset bit comprises a transmit- reset bit.   
     
     
       105. The system of  claim 103 , wherein the reset bit comprises a receive- reset bit.   
     
     
       106. The system of  claim 81 , wherein said system is further adapted to:
   determine an expected modulation type for said next data burst from the second transmission site based, at least in part, on the determined initial phase and/or the initial gain for the next data burst from the second transmission site;        select an adaptation factor for the equalizer based, at least in part, on the determined expected modulation type; and        apply the selected adaptation factor to the next data burst from the second transmission site.     
     
     
       107. The system of  claim 106 , wherein the expected modulation type comprises QPSK. 
     
     
       108. The system of  claim 106 , wherein the expected modulation type comprises QAM  16 . 
     
     
       109. The system of  claim 106 , wherein the expected modulation type comprises QAM  64 . 
     
     
       110. The system of  claim 81 , wherein said system is further adapted to:
   correlate an input signal of said equalizer with an output signal of said equalizer for the next data burst from the second transmission site;        determine an angle of correction for the next data burst from the second transmission site based, at least in part, on said correlation; and        shift the next data burst received from said second transmission site by applying the determined angle of correction to the next data burst received from said second transmission site.     
     
     
       111. The system of  claim 110 , wherein the angle of correction is based, at least in part, on a weighted sum of a center equalizer tap value and one or more adjacent equalizer tap values. 
     
     
       112. The system of  claim 111 , wherein said weighted sum substantially comprises c 0   + 0 . 627 c   1    wherein c   0    comprises said center tap value and c   1    comprises an adjacent tap value.   
     
     
       113. The system of  claim 81 , wherein said equalizer is further adapted to use said stored and replaced attenuation levels and/or equalizer tap values to process a next data burst received from said first transmission site. 
     
     
       114. A system to demodulate data bursts from a plurality of transmission sites, the system comprising:
   a memory to store equalizer tap values associated with a data burst from a transmission site of said plurality of transmission sites; and        a gain control circuit adapted to:      determine a gain for an equalizer based, at least in part, on said stored equalizer tap values; and        scale an input signal of a next data burst to the equalizer to achieve a gain value of substantially  1  based, at least in part, on said determined gain.       
     
     
       115. The system of  claim 114 , wherein said equalizer includes a digital equalizer. 
     
     
       116. The system of  claim 114 , wherein said gain control circuit is included in a modem. 
     
     
       117. An apparatus, comprising:
   means for receiving a preamble, the preamble including a first part modulated according to a lower order modulation technique and a second part modulated according to a modulation technique of an order higher than said lower order modulation technique; and        means for driving an output signal of an adaptive filter to a state based, at least in part, on said received preamble.     
     
     
       118. The apparatus of  claim 117 , wherein said output signal includes a training sequence. 
     
     
       119. The apparatus of  claim 117 , wherein said lower order modulation technique includes QPSK. 
     
     
       120. The apparatus of  claim 117 , wherein said higher order modulation technique includes QAM  64  or QAM  16 .