Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
  • H03H17/00—Networks using digital techniques
  • H03H17/02—Frequency selective networks
  • H03H17/06—Non-recursive filters

Definitions

• the number of vectors of the transform matrix should preferably be commensurate with the number of data elements in the input data block. Accordingly, if there 256 data elements in the input data block, the transform matrix should preferably have 256 vectors each having 256 elements.
• the output of the inverse vector domain transform 22 is an output data block having a number of data elements commensurate with the number of data elements of the input data block. Thus, if there are 256 data elements in the input data block 32, the output data block has 256 data elements.
• Figure 1 shows an interference pattern which could result when two signals in the same frequency band are received by a receiver at substantially the same time
• Figure 2 illustrates the response of an FIR filter which is commonly used as a time domain equalizer in a receiver in order to eliminate ghosts
• Figure 3 illustrates a frequency domain equalizer which is used in a receiver in order to eliminate ghosts
• Figure 4 illustrates an exemplary set of coefficients A ⁇ that are used by the frequency domain equalize of Figure 3 in order to cancel ghosts;
• Figure 5 illustrates guard intervals which may be used between transmitted vectors in systems employing equalizers ;
• Figure 7 illustrates a first embodiment of an equalizer in accordance with the present invention
• Figure 8 illustrates a second embodiment of an equalizer in accordance with the present invention
• Figure 9 illustrates a first embodiment of the response of the pre-processor of the equalizers shown in Figures 7 and 8 ;
• Figure 12 illustrates the imaginary part of the response of a multiplier of the equalizer shown in Figure 8
• Figure 13 illustrates a first embodiment of the response of a post-processor of the equalizers shown in Figures 7 and 8 ;
• Figure 14 illustrates a second embodiment of the response of the pre-processor of the equalizers shown in Figures 7 and 8 ;
• Figure 15 illustrates a second embodiment of the response of a post-processor of the equalizers shown in Figures 7 and 8 ;
• Figure 17 illustrates the output of the preprocessor response in the time domain
• Figure 18 illustrates a third embodiment of an equalizer in accordance with the present invention.
• Figure 20 illustrates the real part of the response of a multiplier of the equalizer shown in Figure 19;
• Figure 21 illustrates the imaginary part of the response of a multiplier of the equalizer shown in Figure 19 ; and, Figure 22 illustrates a response of a postprocessor of the equalizer shown in Figure 19.
• the finite filter 104 is a convolver 108. Accordingly, the multiplication results of the pre-processor 102 are convolved in the convolver 108 with coefficients a. The convolution performed by the convolver 108 eliminates the ghost from the multiplication results of the pre-processor 102.
• the post-processor 106 multiplies the convolution results from the convolver 108 by coefficients c so that the output of the post-processor 106 is the data transmitted into the channel .
• the data at the output of the post-processor 106 is designated in Figure 7 as Data Out.
• the post-processor 106 reverses the effects of the modulation imposed by the pre-processor 102 and applies a window function to the output of the convolver 108. This window function has a duration which is substantially equal to the duration of a Data In block.
• the convolver 108 may be implemented, for example, as an FIR filter, such as that described above in connection with Figure 2. That is, because of the window function applied by the post-processor 106, the number of taps of an FIR filter need not be infinite but may be limited to a reasonable number. For example, these taps may have a duration that is twice the duration of a Data In block.
• a controller 109 is provided to measure the time interval, d, separating the received main signal and its ghost .
• the interval d may be used in shaping the coefficients b, a, and c.
• the controller 109 supplies the coefficients b to the pre-pro- cessor 102, supplies the coefficients a to the convolver 108, and supplies the coefficients c to the post-processor 106.
• the controller 109 also synchronizes the preprocessor 102, the convolver 108, and the post-processor 106 to each block of data moving through the equalizer 100. Each two blocks of data may be separated by a guard interval .
• Figure 8 illustrates an equalizer 110 which is equivalent to the equalizer 100 shown in Figure 7 and which includes a pre-processor 112, a finite filter 114, and a post-processor 116.
• the finite filter 114 includes a Fast Fourier Transform 118, a multiplier 120, and an inverse Fast Fourier Transform 122.
• the finite filter 104 operates in the time domain
• the finite filter 114 operates substantially in the frequency domain where the multiplier 120 applies complex coefficients A (described below) to the frequency domain output of the Fast Fourier Transform 122.
• the post-processor 116 multiplies the output from the finite filter 114 by the coefficients c in order to reverse the effects of the modulation imposed by the pre-processor 102 and to apply a window function to the output of the inverse Fast Fourier Transform 122, as described above.
• the coefficients b applied by the pre-proces- sors 102 and 112 may be discrete steps as shown by way of example in Figure 9. Each of these steps has a width along the time axis equal to the interval d, which is the time interval separating the received main signal and its ghost. Also, the ratio of the amplitude of any one step to the amplitude of the next previous step is ⁇ , .where ⁇ is a constant and is preferably less than one. In the example shown in Figure 9, is 0.8.
• the coefficients b are applied as a block to each Data In block and, therefore, the difference between t 0 at the beginning of the block of coefficients b and t b+d at the end of the block of coefficients b is commensurate with the length in time of a Data In block plus d, where d, as discussed above, is the time interval separating the received main signal and its ghost. For example, if each Data In block has a duration of 256 samples times and d has a duration of 32 sample times, then the difference between t 0 and t b is 288 sample times, as shown in Figure 9. In addition, there should be an appropriate guard interval on each side of the block of coefficients b.
• the coefficients c are applied as a block to the output of the finite filter 104 and the output of the inverse Fast Fourier Transform 120 and, therefore, the difference between t 0 at the begin- ning of the block of coefficients c and t c at the end of the block of coefficients c is commensurate with the length in time of a Data In block.
• the difference between t 0 and t c is not required to include d which, as discussed above, is the length of time separating the received main signal and its ghost, because the ghost has already been eliminated. For example, if a Data In block has a duration of 256 samples times, then the difference between t 0 and t c is also 256 sample times.
• the coefficients b and c as described above in relation to Figures 9 and 13 generally require a priori knowledge of d.
• the coefficients b and c described below in relation to Figures 14 and 15 require no a priori knowledge of d.
• the curve for the coefficients b as shown by way of example in Figure 14 is such that the ratio of the amplitude of the curve at any point xl along the time axis to the amplitude of the curve at a point x2 is the constant ⁇ , where xl and x2 are separated by d, where d may have any value, and where x2 occurs earlier along the time axis than xl .
• the constant is preferably less than one. In the example shown in Figure 14, ⁇ is 0.8.
• the coefficients b are applied as a block to a Data In block and, therefore, the difference between t 0 at the beginning of the curve and t b+d at the end of the curve is commensurate with the length in time of a Data In block plus d where d, as discussed above, is the length of time separating the received main signal and its ghost.
• d is the length of time separating the received main signal and its ghost.
• x is a point along the time axis between t 0 and t b+d , is as described above, k 0 is a constant such that b has a desired value at the point t 0 , and k-,_ is related to d.
• the curve for the coefficients c as shown by way of example in Figure 15 is such that the ratio of the amplitude of the curve at any point xl along the time axis to the amplitude of the curve at a point x2 is , where xl and x2 are separated by d, where d may be any value, and where x2 occurs later along the time axis than xl . As shown in Figure 15, is 0.8.
• the coefficients c are applied as a block to the output of the finite filter 104 and the inverse Fast Fourier Transform 120 and, therefore, the difference between t 0 at the beginning of the block of coefficients c and t c at the end of the block of coefficients c is commensurate with the duration of a Data In block.
• the difference between t 0 and t c is not required to include d because the ghost has already been eliminated.
• the coefficients c reverse the modulation imposed on the signal by application of the coefficients b.
• the coefficients c provide a window function so that a Data Out block at the output of the finite filters 104 and 114 has a duration that substantially matches the duration of a corresponding Data In block.
• x is a point along the time axis between t 0 and t c
• ⁇ is as described above
• k 0 is a constant such that c has a desired value at the point t 0
• k-,_ is related to d.
• the number of calculations performed by the transforms shown in Figure 6 increases in accordance with n 2 as n increases, where n is the number of data elements in a data block. It is further noted that the number of calculations performed by a convolver, such as the convolver 108 of Figure 7, also increases in accordance with n 2 as n increases. However, the number of calculations performed by the finite filter 114 of Figure 8 increases in accordance with nlogn as n increases. Thus, the calculations performed by the equalizer 110 are considerably fewer than the calculations performed by the transforms of Figure 6.
• Figure 19 illustrates an equalizer 160 which includes a finite filter 162 and a post-processor 164.
• the finite filter 162 includes a Fast Fourier Transform 166, a multiplier 168, and an inverse Fast Fourier Transform 170.
• the signal received from the channel is transformed to the frequency domain by the Fast Fourier Trans- form 166, the multiplier 168 multiplies the frequency domain signal from the Fast Fourier Transform 166 by complex coefficients A in order to eliminate the ghost from the received signal, and the inverse Fast Fourier Transform 170 transform ' s the ghost-free, frequency domain signal to the time domain.
• the coefficients A applied by the multiplier 168 are shown in Figures 20 and 21 by way of example. Because the output of the Fast Fourier Transform 166 is complex, the coefficients A must also be complex. Accordingly, the coefficients A have a real part shown in Figure 20 and an imaginary part shown in Figure 21. As can be seen from Figures 20 and 21, the coefficients A are based upon the interval d and the ratio . Again, each of the real and imaginary parts of the coefficients A preferably have a duration that is twice as long as the duration of a Data In block. As a result of the application of the coefficients A by the multiplier 168, the ghost in the output from the pre-processor 102 is eliminated.
• the coefficients c applied by the post-processor 164 are shown by way of example in Figure 22. Because there is no pre-processor in the equalizer 160 that modulates both the received main signal and the ghost, the coefficients c are not required to undo the effects of any modulation. Therefore, the coefficients may have a constant non-zero value within the window from t 0 and t c .
• the coefficients c shown in Figure 22 are applied as a block to the output of the finite filter 162 and, therefore, the difference between t 0 at the beginning of the coefficients c and t c at the end of the coefficients c is commensurate with the duration of each Data In block.
• a Data In block has a duration of 256 samples times
• the difference between t 0 and t c is also 256 sample times.
• the coefficients c provide a window function that limits each Data Out block at the output of the finite filter 162 to a duration that substantially matches the duration of its corresponding Data In block. Accordingly, the number of impulses in the response of the finite filter 162 need not be infinite in order to eliminate a 100% ghost, but may instead be a practicable number .
• coefficients b have been shown above as non-complex coefficients. However, the coefficients b may be complex, such as where the received main signal is a QAM signal.

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• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Computer Hardware Design (AREA)
• Mathematical Physics (AREA)
• Picture Signal Circuits (AREA)
• Noise Elimination (AREA)
• Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)

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Citations (3)

• 2000
  • 2000-05-17 WO PCT/US2000/013620 patent/WO2001089087A1/en not_active Application Discontinuation
  • 2000-05-17 CA CA002409518A patent/CA2409518A1/en not_active Abandoned
  • 2000-05-17 BR BR0015868-2A patent/BR0015868A/pt not_active Withdrawn
  • 2000-05-17 MX MXPA02011140A patent/MXPA02011140A/es active IP Right Grant
  • 2000-05-17 AU AU2000251406A patent/AU2000251406A1/en not_active Abandoned
  • 2000-05-17 CN CNB008195420A patent/CN1193501C/zh not_active Expired - Fee Related
• 2004
  • 2004-03-18 HK HK04102008A patent/HK1059154A1/xx not_active IP Right Cessation

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