OFDM channel estimation system and method components

Abstract

Channel estimation for high mobility OFDM channels is achieved by identifying a set of channel path delays from an OFDM symbol stream including carrier data, inter-channel interference noise and channel noise; determining the average channel impulse response for the identified set of channel path delays in each symbol; generating a path delay curvature for each channel path delay in each symbol based on stored average channel impulse responses for the identified channel path delays; estimating the carrier data in the symbols in the OFDM symbol stream in the presence of inter-channel interference noise and channel noise from the OFDM symbol steam and the average impulse responses for the identified channel path delays; reconstructing the inter-channel interference noise in response to the path delay curvature, the identified set of channel path delays and estimated carrier data to produce a symbol stream of carrier data and channel noise with suppressed inter-channel interference noise.

1 . A path delay estimator circuit responsive to an OFDM symbol stream including carrier data, inter-channel interference noise and channel noise for identifying a set of channel path delays in a group of non-overlapping windows which are above a predetermined energy threshold comprising:
 a threshold setting circuit for setting a local predetermined threshold for sets of channel path delays in each said window in accordance with their energy levels; and   a threshold circuit for selecting channel path delays in each said window meeting their local predetermined threshold and combining the selected channel path delays, from all said windows, to determine the total channel path delays.   
   
   
       2 . The path delay estimator circuit of  claim 1  further including a Fourier transform circuit for performing Fourier transform on an OFDM symbol. 
   
   
       3 . The path delay estimator circuit of  claim 1  further including a normalizing circuit for extracting the channel frequency response for known carriers and inserting zeros for unknown carriers. 
   
   
       4 . The path delay estimator circuit of  claim 1  further including an inverse Fourier transform for performing inverse Fourier transform on the channel frequency response. 
   
   
       5 . The path delay estimator circuit of  claim 1  further including a noise estimator circuit for determining the channel noise level. 
   
   
       6 . An average channel estimator circuit, responsive to the OFDM symbol stream and an identified set of channel path delays, for determining the average channel impulse response for the identified set of channel path delays in each symbol comprising:
 an estimator circuit for determining average path gains based on least squares and known noise.   
   
   
       7 . The channel estimator circuit of  claim 6  further including a normalizing circuit for extracting the channel frequency response for known carriers. 
   
   
       8 . The channel estimator circuit of  claim 6  further including an Fourier transform circuit for performing an Fourier transform on an OFDM symbol. 
   
   
       9 . A curve generator circuit, responsive to stored average impulse responses, for generating a path delay curvature for required channel path delay in each symbol comprising:
 a selection circuit for selecting from storage the average channel gains of neighboring OFDM symbols;   a rate determining circuit for determining the rate of change of the neighboring average channel gains;   a model selection circuit for identifying a best fit average free curve for the stored channel impulse responses.   
   
   
       10 . A carrier data estimator circuit, responsive to an OFDM symbol stream and average impulse responses from an average channel estimator circuit, for estimating the carrier data in the symbols in the OFDM symbol stream in the presence of inter-channel interference and channel noise comprising:
 a vector generating circuit for creating a vector with zeros and inserting average path gains in associated delay locations; and   an equalization circuit for calculating equalization coefficients.   
   
   
       11 . The carrier data estimator circuit of  claim 10  further including an Fourier transform circuit for performing a Fourier transform on said vector. 
   
   
       12 . The carrier data estimator circuit of  claim 10  further including an averaging circuit for calculating noise level. 
   
   
       13 . The carrier data estimator circuit of  claim 10  further including a slicer circuit for matching the equalized symbols to a predefined grid of levels. 
   
   
       14 . A regenerator circuit, responsive to a curve generator, path delay estimator circuit and carrier data estimation circuit, for reconstructing inter-channel interference noise comprising;
 local OFDM symbol generator, responsive to estimated carrier data to generate locally OFDM symbol replicas; and   an ICI distortion generator for shifting an OFDM symbol replica by each associated channel path delay, multiplying it by the associated path delay curvature and summing the shifted, multiplied symbol replicas to produce local inter-channel interference noise.   
   
   
       15 . A method for identifying a set of channel path delays from an OFDM symbol stream including carrier data, inter-channel interference noise and channel noise comprising:
 setting a threshold for groups of channel path delays in accordance with their energy levels; and   selecting channel path delays meeting a predetermined threshold.   
   
   
       16 . The method of  claim 15  further including performing Fourier transform on an OFDM symbol. 
   
   
       17 . The method of  claim 15  further including extracting the channel frequency response for known carriers and inserting zeros for unknown carriers. 
   
   
       18 . The method of  claim 15  further including performing IFT on the channel frequency response. 
   
   
       19 . The method of  claim 15  further including determining the channel noise level. 
   
   
       20 . A method for determining the average channel impulse response for an identified set of channel path delays in each symbol including determining average path gains based on least squares and known noise. 
   
   
       21 . The method of  claim 20  further including extracting the channel frequency response for known carriers. 
   
   
       22 . The method of  claim 20  further including performing a FT on an OFDM symbol. 
   
   
       23 . A method for generating a path delay curvature for each channel path delay in each symbol based on stored average channel impulse responses for the identified channel path delays comprising:
 averaging the channel gains of neighboring OFDM symbols;   determining the rate of change of the neighboring average channel gains; and   identifying a best fit average free curve for the stored channel impulse responses.   
   
   
       24 . A method for estimating the carrier data in the symbols in the OFDM symbol stream in the presence of inter-channel interference noise and channel noise from the OFDM symbol stream and average impulse responses for the identified channel path delays comprising:
 creating a vector with zeros and inserting average path gains in associated delay locations; and   calculating equalization coefficients in response to an FT and applying them to the associated symbol.   
   
   
       25 . A method of  claim 24  further including performing FT on the vector. 
   
   
       26 . A method of  claim 24  further including calculating noise level. 
   
   
       27 . A method of  claim 24  further including matching the equalized symbols to a predefined grid of levels. 
   
   
       28 . A method of reconstructing the inter-channel interference noise in response to the path delay curvature, the identified set of channel path delays and estimated carrier data comprising;
 generating locally OFDM symbol replicas from said estimated carrier data; and   shifting an OFDM symbol replica by each associated channel path delay, multiplying it by the associated path delay curvature and summing the shifted, multiplied symbol replicas to produce local inter-channel interference noise.   
   
   
       29 . A system for identifying a set of channel path delays from an OFDM symbol stream including carrier data, inter-channel interference noise and channel noise for identifying a set of channel path delays in a group of non-overlapping windows which are above a predetermined energy threshold comprising:
 a threshold setting circuit for setting a local predetermined threshold for sets of channel path delays in each said window in accordance with their energy levels; and   a threshold circuit for selecting channel path delays in each said window meeting their local predetermined threshold and combining the selected channel path delays, from all said windows, to determine the total channel path delays.   
   
   
       30 . A method for identifying a set of channel path delays from an OFDM symbol stream including carrier data, inter-channel interference noise and channel noise for identifying a set of channel path delays in a group of non-overlapping windows which are above a predetermined energy threshold comprising:
 setting a local predetermined threshold for sets of channel path delays in each said window in accordance with their energy levels; and   selecting channel path delays in each said window meeting their local predetermined threshold and combining the selected channel path delays, from all said windows, to determine the total channel path delays.