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Ofdm channel estimation system and method componentsOfdm channel estimation system and method components description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20080095256, Ofdm channel estimation system and method components. Brief Patent Description - Full Patent Description - Patent Application Claims
RELATED APPLICATIONS [0001]This application is a continuation-in-part of U.S. patent application Ser. No. 11/789,180 filed Apr. 24, 2007 which claims benefit of and priority to U.S. Provisional Application Ser. No. 60/852,607 filed Oct. 18, 2006 each of which are incorporated herein by this reference. FIELD OF THE INVENTION [0002]This invention relates to channel estimator system and method components for high mobility OFDM channels. BACKGROUND OF THE INVENTION [0003]Binary phase shift keying (BPSK) is a conventional data modulation scheme that conveys data by changing, the phase of a reference carrier signal, for example, during each BPSK symbol period carrier data in the form of either a positive or negative sine wave is transmitted. A positive sine wave represents a data "1", a negative sine wave a data "0". When the symbol stream arrives at the receiver it is decoded by multiplying with a positive sine wave. The multiplying of it by another positive sine wave produces a average positive level; if the symbol period contains a negative sine wave the multiplexing by a positive sine wave produces an average negative level. Orthogonal Frequency Division Multiplexing (OFDM) employs the same idea but instead of one carrier wave per bit, the bit stream to be transmitted is split into several parallel low-rate bit streams, two, ten or any number; presently over 8k (8192). Each low-rate bit stream is transmitted over one sub-channel by modulating a sub-carrier using a standard modulation scheme, for example BPSK. The sub-carrier frequencies are chosen so that the modulated data streams are orthogonal to each other. The demodulation at the receiver is done in the same way with the symbol period sine waves being multiplied selectively by a positive sine wave of each of the frequencies transmitted. By virtue of orthogonality it is possible to distinguish between the various carrier sine waves. OFDM is thus a much higher density data encoding technique. OFDM has shortcomings but works well especially where the transmitter and received are fixed or not moving fast with respect to each other and so the transmitter channel between them remains constant or fairly constant. That is, the amplitude and phase of the various sine waves transmitted over that channel within a symbol period do not vary significantly over the symbol period. However in high mobility situations where the channel does change over the time of a symbol period, e.g. video streaming to a receiver on a moving vehicle or train, different sine waves can experience different channel paths resulting in variations in their phase and/or amplitude. Such variations referred to as inter-carrier or inter-channel interference (ICI) noise interferes with the orthogonality of the sine waves and can cause errors in the data decoding causing "1"s to appear to be "0"s and "0"s to appear as "1"s. This ICI noise accompanies but is different then the conventional channel noise that accompanies the carrier data. BRIEF SUMMARY OF THE INVENTION [0004]It is therefore an object of this invention to provide improved OFDM estimator system and method components for high mobility OFDM channels. [0005]It is a further object of this invention to provide such improved OFDM estimator system and method components which make efficient use of memory and power. [0006]It is a further object of this invention to provide such improved OFDM estimator system and method components which are power adaptive to channel conditions. [0007]The invention results from the realization that a channel estimation for high mobility OFDM channels can be achieved with improved system and method components for identifying a set of channel path delays from an OFDM symbol stream including carrier data, inter-channel interference noise and channel noise; for determining the average channel impulse response for the identified set of channel path delays in each symbol; for generating a path delay curvature for each channel path delay in each symbol from the stored average channel impulse responses for the identified channel path delays; for estimating the carrier data in the symbols in the OFDM symbol stream in the presence of inter-channel interference noise from the OFDM symbol stream and said average impulse responses for the identified channel path delays; for reconstructing the inter-channel interference noise in response to 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. [0008]The subject invention, however, in other embodiments, need not achieve all these objectives and the claims hereof should not be limited to structures or methods capable of achieving these objectives. [0009]This invention features 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 including a threshold setting circuit for setting a local predetermined threshold for sets of channel path delays in each window in accordance with their energy levels and a threshold circuit for selecting channel path delays in each window meeting their local predetermined threshold and combining the selected channel path delays, from all the windows, to determine the total channel path delays. [0010]In a preferred embodiment the path delay estimator circuit may include a Fourier transform circuit for performing Fourier transform on an OFDM symbol. The path delay estimator circuit may include a normalizing circuit for extracting the channel frequency response for known carriers and inserting zeros for unknown carriers. The path delay estimator circuit may include an inverse Fourier transform for performing inverse Fourier transform on the channel frequency response. The path delay estimator circuit may include a noise estimator circuit for determining the channel noise level. [0011]This invention also features 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 including an estimator circuit for determining average path gains based on least squares and known noise. [0012]In a preferred embodiment the channel estimator circuit may include a normalizing circuit for extracting the channel frequency response for known carriers. The channel estimator circuit may include a Fourier transform circuit for performing a Fourier transform on an OFDM symbol [0013]This invention also features a curve generator circuit, responsive to stored average impulse responses, for generating a path delay curvature for required channel path delay in each symbol. There is 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 and a model selection circuit for identifying a best fit average free curve for the stored channel impulse responses. [0014]This invention also features 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 including 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. [0015]In a preferred embodiment the carrier data estimator circuit may include a Fourier transform circuit for performing a Fourier transform on the vector. The carrier data estimator circuit may include an averaging circuit for calculating noise level. The carrier data estimator circuit may include a slicer circuit for matching the equalized symbols to a predefined grid of levels. [0016]This invention also features a regenerator circuit, responsive to a curve generator, path delay estimator circuit and carrier data estimation circuit, for reconstructing inter-channel interference noise including a 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. [0017]This invention also features 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 including setting a threshold for groups of channel path delays in accordance with their energy levels and selecting channel path delays meeting a predetermined threshold. [0018]In a preferred embodiment the method may include performing a Fourier transform on an OFDM symbol. The method may include extracting the channel frequency response for known carriers and inserting zeros for unknown carriers. The method may include performing IFT on the channel frequency response. The method may include determining the channel noise level. [0019]This invention also features 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. [0020]In a preferred embodiment the method may include extracting the channel frequency response for known carriers. The method may include performing a FT on an OFDM symbol. Continue reading about Ofdm channel estimation system and method components... 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