Claims:

1. A method of channel estimation in a wireless Orthogonal Frequency Division Multiplex (OFDM) system, the method comprising:
receiving, by a channel estimation section, a plurality of pilot symbols of a pilot subcarrier channel;
estimating, by the channel estimation section, a channel frequency response of the pilot subcarrier channel including at least one of: Least Square (LS) channel estimate and Minimum Mean Square Error (MMSE) channel estimate;
obtaining, by a spline interpolation section, a channel estimate for a data subcarrier channel based on at least one of: the LS channel estimate and the MMSE channel estimate by spline interpolation;
converting, by an Inverse Discrete Fourier Transform (IDFT) section, the channel estimate into time domain to obtain a channel impulse response;
removing, by a truncation section, one or more noise coefficients in the channel impulse response to obtain a truncated channel impulse response; and
converting, by a Discrete Fourier Transform (DFT) section, the truncated channel impulse response into frequency domain to obtain a smoothed channel estimate.

2. The method as claimed in claim 1, wherein obtaining the LS channel estimate (Ĥ𝐿𝑆) includes minimizing a cost function to obtain the LS channel estimate as:

wherein:
X: transmitted data stream
Y: frequency domain sub-carriers
H: channel vector.

3. The method as claimed in claim 1, wherein obtaining the MMSE channel estimate (Ĥ) includes minimizing a Mean Square Error (MSE) of the channel estimate using:
MSE:

MMSE:

4. The method as claimed in claim 1, wherein the spline interpolation is a cubic spline interpolation.

5. The method as claimed in claim 1, wherein the channel impulse response is smoothed by truncation taps within limits of delay spread to obtain the truncated channel impulse response.

6. A wireless Orthogonal Frequency Division Multiplex (OFDM) system, comprising:
a channel estimation section configured to:
receive a plurality of pilot symbols of a pilot subcarrier channel, and
estimate a channel frequency response of the pilot subcarrier channel including at least one of: Least Square (LS) channel estimate and Minimum Mean Square Error (MMSE) channel estimate,
a spline interpolation section configured to obtain a channel estimate for a data subcarrier channel based on at least one of: the LS channel estimate and the MMSE channel estimate by spline interpolation;
an Inverse Discrete Fourier Transform (IDFT) section configured to convert the channel estimate into time domain to obtain a channel impulse response;
a truncation section configured to remove one or more noise coefficients in the channel impulse response to obtain a truncated channel impulse response;
a Discrete Fourier Transform (DFT) section configured to convert the truncated channel impulse response into frequency domain to obtain a smoothed channel estimate.

7. The OFDM system as claimed in claim 6, wherein obtaining the LS channel estimate (Ĥ𝐿𝑆) includes minimizing a cost function to obtain the LS channel estimate as:

wherein:
X: transmitted data stream
Y: frequency domain sub-carriers
H: channel vector.

8. The OFDM system as claimed in claim 6, wherein obtaining the MMSE channel estimate (Ĥ) includes minimizing a Mean Square Error (MSE) of the channel estimate using:
MSE:

MMSE:

9. The OFDM system as claimed in claim 6, wherein the spline interpolation is a cubic spline interpolation.

10. The OFDM system as claimed in claim 6, wherein the channel impulse response is smoothed by truncation taps within limits of delay spread to obtain the truncated channel impulse response.
, Description:FORM 2
THE PATENTS ACT, 1970
(39 OF 1970)
&
THE PATENTS RULES, 2003

COMPLETE SPECIFICATION
[SEE SECTION 10, RULE 13]

METHOD FOR CHANNEL ESTIMATION IN WIRELESS OFDM SYSTEM AND A WIRELESS OFDM SYSTEM THEREOF

BHARAT ELECTRONICS LIMITED
WITH ADDRESS:
OUTER RING ROAD, NAGAVARA, BANGALORE 560045, INDIA

THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED. 

FIELD OF INVENTION
[0001] The present invention relates generally to wireless communication and particularly to channel estimation techniques in wireless communication networks.

BACKGROUND
[0002] Orthogonal Frequency Division Multiplexing (OFDM) is a widely adopted wireless communication standard known for high data rate transmission capability and its robustness to multipath delay spread. In OFDM, channel estimation is done by estimating time-varying channel frequency response for OFDM symbols. Examples of the channel estimation techniques include least squares (LS) based channel estimation or minimum mean square error (MMSE) based channel estimation.
[0003] In the paper titled “A Comparison of Pilot-Aided Channel Estimation Methods for OFDM Systems”, the estimation of the Channel Impulse Response (CIR) in Orthogonal Frequency Division Multiplex (OFDM) systems is presented. In particular, focus is on two pilot-aided schemes: the Maximum Likelihood Estimator (MLE) and the Bayesian Minimum Mean Square Error Estimator (MMSEE). The advantage of the former is that it is simpler to implement as it needs no information on the channel statistics. On the other hand, the MMSEE is expected to have better performance as it exploits prior information about the channel.
[0004] In the paper titled "Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems", a full review of block-type and comb-type pilot-based channel estimation is presented. The channel estimation based on block-type pilot arrangement with decision feedback equalizer is described. The channel estimation based on comb-type pilot arrangement using Least Square (LS) and Least Mean Square (LMS) algorithms is presented.
[0005] In the paper titled “Channel estimation for OFDM systems based on comb-type pilot arrangement in frequency selective fading channels”, the channel transfer function of pilot tones are estimated by using low-rank MMSE estimator, and the channel transfer function of data tones are interpolated by piecewise linear interpolation method with phase compensation.
[0006] In the patent application titled “Channel Estimation using averaging and interpolation”, interpolation is performed between respective frequency-averaged channel estimates to generate interpolated channel estimates for subcarriers between the respective subcarriers.
[0007] Therefore, there is a need for an efficient channel estimation technique in OFDMA based wireless communication networks.

SUMMARY
[0008] This summary is provided to introduce concepts related to a method of channel estimation in a wireless Orthogonal Frequency Division Multiplex (OFDM) system and a wireless Orthogonal Frequency Division Multiplex (OFDM) system. This summary is neither intended to identify essential features of the present invention nor is it intended for use in determining or limiting the scope of the present invention.
[0009] In an embodiment of the present invention, a method of channel estimation in a wireless Orthogonal Frequency Division Multiplex (OFDM) system is provided. The method includes receiving a plurality of pilot symbols of a pilot subcarrier channel by a channel estimation section. The method includes estimating a channel frequency response of the pilot subcarrier channel including at least one of: Least Square (LS) channel estimate and Minimum Mean Square Error (MMSE) channel estimate, by the channel estimation section. The method includes obtaining a channel estimate for a data subcarrier channel based on at least one of: the LS channel estimate and the MMSE channel estimate by spline interpolation by a spline interpolation section. The method includes converting the channel estimate into time domain to obtain a channel impulse response by an Inverse Discrete Fourier Transform (IDFT) section. The method includes removing one or more noise coefficients in the channel impulse response to obtain a truncated channel impulse response by a truncation section. The method includes converting the truncated channel impulse response into frequency domain to obtain a smoothed channel estimate by a Discrete Fourier Transform (DFT) section.
[0010] In an embodiment of the present invention, a wireless Orthogonal Frequency Division Multiplex (OFDM) system is provided. The wireless OFDM system includes a channel estimation section, a spline interpolation section, an Inverse Discrete Fourier Transform (IDFT) section, a truncation section, and a Discrete Fourier Transform (DFT) section. The channel estimation section is configured to receive a plurality of pilot symbols of a pilot subcarrier channel and estimate a channel frequency response of the pilot subcarrier channel including at least one of: Least Square (LS) channel estimate and Minimum Mean Square Error (MMSE) channel estimate. The spline interpolation section is configured to obtain a channel estimate for a data subcarrier channel based on at least one of: the LS channel estimate and the MMSE channel estimate by spline interpolation. The Inverse Discrete Fourier Transform (IDFT) section is configured to convert the channel estimate into time domain to obtain a channel impulse response. The truncation section is configured to remove one or more noise coefficients in the channel impulse response to obtain a truncated channel impulse response. The Discrete Fourier Transform (DFT) section is configured to convert the truncated channel impulse response into frequency domain to obtain a smoothed channel estimate.
[0011] In an embodiment, obtaining the LS channel estimate (Ĥ𝐿𝑆) includes minimizing a cost function to obtain the LS channel estimate as:

wherein:
X: transmitted data stream
Y: frequency domain sub-carriers
H: channel vector.
[0012] In an embodiment, obtaining the MMSE channel estimate (Ĥ) includes minimizing a Mean Square Error (MSE) of the channel estimate using:
MSE:

MMSE:

[0013] In an embodiment, the spline interpolation is a cubic spline interpolation.
[0014] In an embodiment, the channel impulse response is smoothed by truncation taps within limits of delay spread to obtain the truncated channel impulse response.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
[0015] The detailed description is described with reference to the accompanying figures.
[0016] Figure 1 illustrates a schematic block diagram of a receiver chain in accordance with an embodiment of the present invention.
[0017] Figure 2 illustrates a schematic block diagram of a channel estimation system in accordance with an embodiment of the present invention.
[0018] Figure 3 illustrates a schematic block diagram of a system model in accordance with an embodiment of the present invention.
[0019] Figure 4 illustrates a schematic block diagram of MMSE channel estimation in accordance with an embodiment of the present invention.
[0020] Figure 5 illustrates a flowchart of a method of channel estimation in accordance with an embodiment of the present invention.
[0021] Figure 6 illustrates simulation parameters in accordance with an embodiment of the present invention.
[0022] Figure 7 illustrates experimental values for the mean square error (MSE) for LS estimation in accordance with an embodiment of the present invention.
[0023] Figure 8 illustrates experimental values for the mean square error (MSE) for MMSE estimation in accordance with an embodiment of the present invention.
[0024] Figure 9 illustrates performance improvement of LS-spline with time-domain smoothing in accordance with an embodiment of the present invention.
[0025] Figure 10 illustrates performance improvement of performance improvement of MMSE with time-domain smoothing in accordance with an embodiment of the present invention.
[0026] It should be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative methods embodying the principles of the present invention. Similarly, it will be appreciated that any flow charts, flow diagrams, and the like represent various processes which may be substantially represented in computer readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown.

DETAILED DESCRIPTION
[0027] The various embodiments of the present invention provide a method of channel estimation in a wireless Orthogonal Frequency Division Multiplex (OFDM) system and a wireless OFDM system.
[0028] In the following description, for purpose of explanation, specific details are set forth in order to provide an understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these details.
[0029] One skilled in the art will recognize that embodiments of the present invention, some of which are described below, may be incorporated into a number of systems.
[0030] However, the systems and methods are not limited to the specific embodiments described herein. Further, structures and devices shown in the figures are illustrative of exemplary embodiments of the presently invention and are meant to avoid obscuring of the presently invention.
[0031] It should be noted that the description merely illustrates the principles of the present invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described herein, embody the principles of the present invention. Furthermore, all examples recited herein are principally intended expressly to be only for explanatory purposes to help the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass equivalents thereof.
[0032] The present invention relates to a method of channel estimation in a wireless OFDM system. The method includes Fast Fourier Transform (FFT), Inverse FFT (IFFT), and cubic spline interpolation to improve the channel estimation performance. In the present invention, the LS/MMSE channel estimates for the pilot sub-carriers are used in the spline interpolation process to obtain the channel estimates for the data sub-carriers. The interpolated channel estimate is then converted into the time domain by the Inverse Discrete Fourier Transform (IDFT). A truncation operation is then applied in the time domain assuming that the maximum multi-path delay is kept within the Cyclic Prefix (CP) of the OFDM symbol. As a consequence, the noise power is reduced in the time domain. The DFT is finally applied to return to the frequency domain.
[0033] Referring now to Figure 1, a schematic block diagram of a receiver chain (100) is shown in accordance with an embodiment of the present invention. The receiver chain (100) includes an antenna (102), a Radio Frequency (RF) receiver (104), an Analog to Digital Converter (ADC) (106), a synchronization and Cyclic Prefix (CP) removal section (108), a serial to parallel converter (110), a Fast Fourier Transform (FFT) section (112), a pilot subcarrier extraction section (114), a channel estimation section (116), a spline interpolation section (118), an Inverse Discrete Fourier Transform (IDFT) section (120), a truncation section (122), a DFT section (124), a demodulator (126), and a decoder (128).
[0034] The RF receiver (104) receives a data stream x through the antenna (102), i.e., y with added noise z and attenuated by channel h. The ADC (106) converts the analog data stream into digital data stream. The synchronization and CP removal section (108) synchronizes the digital data in time and frequency domains. The serial to parallel converter (110) splits the serial stream of the digital data into parallel sets. The FFT section (112) converts the time domain samples into frequency domain values and generates frequency-domain sub-carriers Y. The pilot subcarrier extraction section (114) parses the channel reference subcarriers (pilot symbols) and the data sub-carriers. The channel estimation section (116) receives the extracted channel reference subcarriers (pilot symbols) and estimates the channel frequency response Ĥ𝐿𝑆. The spline interpolation section (118) uses the LS/MMSE channel estimates for the pilot sub-carriers Ĥ𝐿𝑆[k] to obtain the channel estimates for the data sub-carriers, i.e., 𝐻̃. The IDFT section (120) converts the interpolated channel estimate 𝐻̃ into the time domain by the IDFT, i.e., ĥ. The truncation section (122) truncates the ĥ in time domain to form ĥtrunc assuming that the maximum multi-path delay is kept within the CP of the OFDM symbol. This reduces the power of the noise component in the time domain. The DFT section (124) converts the ĥtrunc into frequency domain to form Ĥtrunc. The demodulator demodulates the frequency domain signal Ĥtrunc to recover the original data signal X.
[0035] Referring now to Figure 2, a schematic block diagram of a channel estimation system (200) is shown in accordance with an embodiment of the present invention. The channel estimation system (200) includes a channel estimation section (202), a spline interpolation section (204), an N-Point IDFT section (206), a time domain smoothing section (208), and an N-Point DFT section (210).
[0036] The training symbols used for the channel estimation provides good performance. The present method uses the LS and MMSE techniques.
[0037] It is assumed that all subcarriers are orthogonal (i.e., Inter Channel Interference (ICI) free). Then, the training symbols for N subcarriers can be represented by the following diagonal matrix:
………………………………. equation (1)
[0038] Where X[k] denotes a pilot tone at the kth subcarrier, with E{X[k]}=0 and Variance Var{X[k]} = 𝜎𝑥2, k=0,1,2…N−1.
[0039] As shown in the Figure 3, X is given by a diagonal matrix since it is assumed that all subcarriers are orthogonal. Given that the channel gain is H[k] for each subcarrier k, the received training signal Y[k] can be expressed as:
………… equations (2,3)
[0040] Where H is a channel vector given as 𝐻=[ 𝐻[0],𝐻[1],…,𝐻[𝑁−1]]𝑇 and Z is a noise vector given as Z=[ 𝑍[0],𝑍[1],…,𝑍[𝑁−1]]𝑇 with E{Z[k]} = 0 and Var{Z[k]} = 𝜎𝑧2, k=0,1,2…N−1. In the following discussion, let Ĥ denote the estimate of channel H.
[0041] The LS channel estimation method finds the channel estimate in such a way that the following cost function is minimized:
………………….……. equation (4)
which gives the solution to the LS channel estimation as
…….………… equation (5)
[0042] Each component of the LS channel estimate Ĥ𝐿𝑆 is denoted by Ĥ𝐿𝑆[k],l = 0,1,3,…,N−1. Since X is assumed to be diagonal due to the ICI-free condition, the LS channel estimate Ĥ𝐿𝑆 can be written for each subcarrier as
………..…….…… equation (6)
The Mean Square Error (MSE) of this LS channel estimate is given as
……………… equation (7)
[0043] Using the weight matrix W, Ĥ ≜ W 𝐻̃ which corresponds to the MMSE estimate. The MSE of the channel estimate Ĥ is given as
…………….……… equation (8)
[0044] As shown in the Figure 4, the MMSE channel estimation method finds a better (linear) estimate in terms of W in such a way that the MSE in Equation is minimized. The orthogonality principle states that the estimation error vector 𝒆=𝑯−𝑯̂ is orthogonal to 𝐻̃, such that
……………… equation (9)
[0045] 𝐻̃ is the LS channel estimate given as
………………… equation (10)
[0046] Solving Equation for W yields
…………………..…… equation (11)
[0047] Where 𝑅𝐻̃𝐻̃ is the autocorrelation matrix of 𝐻̃ given as
…………………… equation (12)
[0048] And RH~H is the cross-correlation matrix between the true channel vector and temporary channel estimate vector in the frequency domain. Using Equation, the MMSE channel estimate follows as
………………… equation (13)
[0049] The channel estimates obtained by using either the LS or MMSE channel estimation at the pilot subcarriers are used to obtain the channel estimates at the data subcarriers by using the method of Cubic spline interpolation. The cubic spline interpolation is a piecewise continuous curve, passing through each of the channel estimates at the pilot subcarriers . There is a separate cubic polynomial for each interval between adjacent pilot subcarriers, each with its own coefficients:
……… equation (14)
where 𝑥𝑖 denotes adjacent pilot subcarrier locations.
[0050] The above discussed methods for channel estimation operate entirely in the frequency domain and do not use the time domain parameters of the channel. In the time domain, the channel impulse response has a few non-zero taps. The present method improves the accuracy of the channel estimate by using the delay spread of the channel.
[0051] The present method performs the time domain smoothing to improve the performance of channel estimation by eliminating the effect of noise outside the maximum channel delay. Let 𝐻̂(𝑘) denote the estimate of channel gain at the kth subcarrier, obtained by either MMSE or LS channel estimation method. Taking the IDFT of the channel estimate {𝐻̂(𝑘)}𝑘=0𝑁−1
…… equation (15)
[0052] where 𝑧[𝑛] denotes the noise component in the time domain. Ignoring the coefficients {ℎ̂[𝑛]} that contain the noise only, the truncated channel impulse response is
…….… equation (16)
[0053] The truncated channel impulse response vector is then transformed back to the frequency domain by using the DFT operation as follows
………………………… equation (17)
[0054] The smoothed channel estimate in the frequency domain is used by the Zero-forcing or MMSE equalizer to remove the impact of channel in the frequency domain.
[0055] Referring now to Figure 5, a flowchart of a method of channel estimation is shown in accordance with an embodiment of the present invention.
[0056] At step 502, the channel estimation section (116) receives the pilot symbols of a pilot subcarrier.
[0057] At step 504, the channel estimation section (116) estimates the channel frequency response of the pilot subcarrier channel. The channel estimation is performed based on the Least Square (LS) channel estimate and/or the Minimum Mean Square Error (MMSE) channel estimate.
[0058] At step 506, the spline interpolation section (118) obtains the channel estimate for the data sub-carrier channel based on the LS channel estimate and the MMSE channel estimate by the spline interpolation.
[0059] At step 508, the IDFT section (120) converts the channel estimate into time domain to obtain a channel impulse response.
[0060] At step 510, the truncation section (122) removes the noise coefficients in the channel impulse response to obtain a truncated channel impulse response.
[0061] At step 512, the DFT section (124) converts the truncated channel impulse response into frequency domain to obtain a smoothed channel estimate.
[0062] Referring now to Figure 6, the simulation parameters are shown in accordance with an embodiment of the present invention.
[0063] Referring now to Figure 7, the experimental values for the mean square error (MSE) for LS estimation are shown in accordance with an embodiment of the present invention.
[0064] Referring now to Figure 8, the experimental values for the mean square error (MSE) for MMSE estimation are shown in accordance with an embodiment of the present invention.
[0065] Referring now to Figure 9, the performance improvement of LS-spline with time-domain smoothing is shown in accordance with an embodiment of the present invention.
[0066] Referring now to Figure 10, the performance improvement of performance improvement of MMSE with time-domain smoothing is shown in accordance with an embodiment of the present invention.
[0067] In operation, the present invention provides a method of improving the channel estimation in wireless OFDM systems. The method includes interpolating the LS/MMSE channel estimates obtained from the pilot subcarriers. The channel estimates are interpolated using the cubic spline interpolation. The interpolated channel estimates are transformed into the time domain using IDFT. The time domain channel estimates are smoothed by truncation taps within the delay spread. The smoothed channel estimates in the time domain may be transformed back into the frequency domain using DFT. The channel estimates are truncated in the time domain to remove the noise coefficients. The channel estimate is truncated to within the maximum delay spread. The truncated channel estimate is transformed into the frequency domain using FFT.
[0068] The foregoing description of the invention has been set merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to person skilled in the art, the invention should be construed to include everything within the scope of the invention.