apparatus and method of compensating for compact digital domain chromatic dispersion

Abstract

A method and apparatus of compensating for compact digital domain chromatic dispersion. The distortion of an optical signal due to chromatic dispersion is compensated by a digital signal processing in the electrical domain, either prior to the optical transmitter or following the receiver. The circular coefficient approximation and sub-band processing reduce the amount of computations to execute a given level of chromatic dispersion compensation compared to a direct finite impulse response filter implementation.

1 . A digital signal processor utilized in a fiber optic communication system, the digital signal processor comprising:
 means for compensating for chromatic dispersion,   means for executing a digital filter function, wherein the digital filter function includes an operation of multiplying a signal value received by the digital signal processor by a coefficient to produce a multiplication result, the multiplication result being used three or more times during the execution of the digital filter function.   
   
   
       2 . The digital signal processor of  claim 1  wherein the digital filter function is a finite impulse response filter function. 
   
   
       3 . The digital signal processor of  claim 2  wherein:
 the multiplication result is stored in a look-up table when the operation of multiplying is executed; and   the multiplication result is drawn from the look-up table to be used for different terms in the finite impulse response filter.   
   
   
       4 . The digital signal processor of  claim 2  wherein the multiplication result is fanned out to one or more locations in the digital signal processor to be used for different terms in the finite impulse response filter. 
   
   
       5 . The digital signal processor of  claim 1  wherein the frequency response associated with the digital filter function is substantially similar to an apodized version of a chromatic dispersion transfer function. 
   
   
       6 . A digital signal processor utilized in a fiber optic communication system compensating for chromatic dispersion, the digital signal processor comprising:
 means for dividing a signal represented by a discrete-time sequence of a certain sample rate into a plurality of sub-bands by an analysis filter bank, each sub-band represented by a sub-band signal discrete-time sequence of a lower sample rate; and   means for separately compensating for the sub-band signals for chromatic dispersion by application of chromatic dispersion compensating digital filter functions.   
   
   
       7 . The digital signal processor of  claim 6  wherein one of the chromatic dispersion compensating digital filter functions is a finite impulse response filter function. 
   
   
       8 . The digital signal processor of  claim 6  wherein one of the chromatic dispersion compensating digital filter functions is an infinite impulse response filter function. 
   
   
       9 . The digital signal processor of  claim 6  wherein the sub-band signals are combined by a synthesis filter bank into a chromatic dispersion compensated output signal, the chromatic dispersion compensated output signal being represented by a discrete-time sequence of a sample rate higher than the sub-band signals. 
   
   
       10 . The digital signal processor of  claim 6  wherein the analysis filter bank comprises a plurality of finite impulse response filters. 
   
   
       11 . The digital signal processor of  claim 6  wherein the analysis filter bank comprises a plurality of infinite impulse response filters. 
   
   
       12 . The digital signal processor of  claim 6  wherein the analysis filter bank comprises a cosine modulated filter bank. 
   
   
       13 . The digital signal processor of  claim 6  wherein the analysis filter bank is a tree structured filter bank. 
   
   
       14 . The digital signal processor of  claim 6  wherein the spectral shapes of the sub-band signals overlap. 
   
   
       15 . A method of compensating for chromatic dispersion experienced by an optical signal propagating through a fiber optic transmission system, said method acting on a discrete-time representation of the optical signal with a digital filter operation, the method including the steps of:
 evaluating a multiplication term to obtain a multiplication result; and   using the multiplication result as the value of three multiplication terms in the digital filter operation.   
   
   
       16 . The method of  claim 15  wherein the step of using the multiplication result as the value of three multiplication terms in the digital filter operation further comprises the steps of:
 writing the multiplication result to a look-up table; and   retrieving the multiplication result from the look-up table to be used as a value of multiplication terms in the digital filter operation.   
   
   
       17 . The method of  claim 15  further including the steps of:
 calculating the inverse Fourier transform of an apodized version of the required chromatic dispersion transfer function; and   setting feedforward digital filter coefficients to be approximately equal to the inverse Fourier transform values.   
   
   
       18 . A method of compensating for chromatic dispersion experienced by an optical signal propagating through a fiber optic transmission system, said method acting on a discrete-time representation of the optical signal, the method including the steps of:
 separating the signal into a plurality of sub-bands, each sub-band having a lower sample rate than the optical signal;   acting on the sub-band signals separately to compensate for chromatic dispersion; and   combining the sub-band signals into a single output signal, the output signal having a sample rate higher than the sub-band signals.   
   
   
       19 . The method of  claim 18  wherein the sub-band signals have overlapping spectra.