Claims:
1. A method of Carrier Frequency Offset (CFO) estimation for OFDM modem, said method comprising:
storing, by a data storing unit (201), a plurality of data samples corresponding to a plurality of OFDM frames in a memory;
fetching, by a data fetching unit (205), the plurality of data samples from the memory of a buffer corresponding to the plurality of OFDM frames;
detecting the preamble location in an OFDM Frame by estimating Start of Frame (SOF) with a peak detection unit (203);
correlating, by a frame-to-frame processing unit (207), the preambles of two OFDM frames of a communication signal wherein the preambles being correlated are separated by a length equal to an integer multiple of the OFDM frame length;
pseudo incrementing, by a pseudo incrementing unit (204), a Start of Frame (SOF) detection point of the first OFDM frame by an integer multiple of frame size, if the Start of Frame (SOF) position estimate of second frame, among the two frames used for estimating the frequency offset, is not an integer multiple of OFDM frame length;
determining, by the device (200), a frequency offset estimate by processing the preambles of said two OFDM frames of the communication signal.
2. The method as claimed in claim 1, wherein the CFO is estimated with any one of the two preambles of each of the two communication frames where preamble length ‘P’ is equal to the length of an OFDM symbol.
3. The method as claimed in claim 2, wherein the first preamble of first communication frame should be used with first preamble of second communication frame or the second preamble of first communication frame should be used with second preamble of second communication frame to estimate the Carrier Frequency Offset (CFO).
4. The method as claimed in claim 2, wherein the first preamble of first communication frame is correlated with second preamble of second communication frame or the second preamble of first communication frame is correlated with first preamble of second communication frame to estimate the Carrier Frequency Offset (CFO) with separation between the preambles being the sum of OFDM symbol length and an integer multiple of OFDM frame length.
5. The method as claimed in claim 1, wherein the preambles of any two communication frames being correlated are of length ‘2P’ and utilizes both the preambles of the independent OFDM frames to improve the accuracy of the CFO estimation.
6. The method as claimed in claim 1, wherein the CFO estimate is refined by averaging the CFO estimates obtained from any n pairs of OFDM frames and n>1.
7. The method as claimed in claim 1, wherein the instantaneous CFO estimate obtained from preambles of the two OFDM communication frames is averaged with the previous average CFO estimate to make the CFO estimate more accurate.
8. The method as claimed in claim 1, wherein the carrier frequency offset estimation method is applicable for any communication frame with preambles.
9. A device for the estimation of Carrier Frequency Offset (CFO) for an OFDM modem comprising:
a data storing unit (201) configured to store the data samples corresponding to the OFDM frame in the memory;
a data fetching unit (205) configured to fetch data samples from the memory locations of the buffer corresponding to the OFDM frame;
a peak detection unit (203) to detect the Start of Frame (SOF) location of an OFDM frame and extract the preambles;
a pseudo incrementing unit (204) configured to pseudo increment the Start of Frame (SOF) location of the first OFDM frame by an integer multiple of the OFDM frame length if the Start of Frame (SOF) position estimate of second frame, among the two frames being correlated, is not an integer multiple of OFDM frame length;
a frame-to-frame processing unit (207) configured to process the preambles of any two OFDM frames consisting of a conjugation unit, a multiplier, an averaging unit;
a phase estimation unit (208) configured to estimate the phase from averaged output of frame-to-frame processing unit (207);
a frequency offset calculation unit (209) configured to calculate the frequency offset from the obtained phase;
a sine-cosine generator unit (210) configured to generate sine and cosine values corresponding to the calculated frequency offset; and
a frequency offset compensation unit (211) configured to compensate the frequency offset by applying the sine and cosine values on the input OFDM frame.
10. The device as claimed in claim 9, wherein frame-to-frame processing unit (207) is further configured to process the preambles corresponding to any two independent OFDM frames comprising the steps of multiplication of preamble of the first OFDM frame with the conjugated preamble of the second OFDM frame using a multiplier (305) where conjugation of preamble of the second OFDM frame is achieved by providing it to the conjugation unit (304) and averaging of the multiplier output with an averaging unit (306).
11. The device as claimed in claim 9, wherein the phase estimation unit (208) is further configured to process the output of the averaging unit (306) and to generate a phase estimate utilizing CORDIC algorithm.
, Description:FORM – 2

THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003

COMPLETE SPECIFICATION
(SEE SECTION 10, RULE 13)

A method and device for Carrier Frequency offset estimation based on preambles of two OFDM frames

BHARAT ELECTRONICS LIMITED

WITH ADDRESS:
OUTER RING ROAD, NAGAVARA, BANGALORE – 560045, KARNATAKA, INDIA

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

TECHNICAL FIELD
The present invention relates to a digital receiver for wireless communication and more particularly to a Carrier Frequency Offset (CFO) estimator of OFDM modem which can estimate the CFO accurately amidst the adverse effects of multipath fading. Furthermore, the present invention can be used for Troposcatter communication where Line-of-Sight doesn’t exist.
BACKGROUND

Wireless communication signals, such as troposcatter communication signals, undergo deep fading which results in signal degradation and loss of information. Fading phenomenon is commonly observed in multipath transmission systems as a consequence of the rapid changes in the signal level of transmitted frequency band. Further, multipath fading leads to phase distortion and Inter Symbol Interference (ISI) of the transmitted data.
Carrier Frequency Offset (CFO) is generated when the transmitter and receiver systems associated with a communication link differ in local oscillator (LO) frequencies. OFDM systems are more sensitive to Carrier Frequency Offset (CFO) which disturbs the orthogonality of subcarriers. Maintaining the orthogonality feature of OFDM is extremely important to guarantee the appropriate reconstruction of original transmitted signal at the receiver. Achieving accurate estimation of Carrier Frequency Offset (CFO) is a challenging task in a heavily faded environment such as troposcatter communication links. If the estimated CFO value gets deviated from actuals and keep on fluctuating, it will degrade the performance of communication links. Precise estimation and compensation of CFO is required for OFDM system to efficiently recover the transmitted data at the receiver. Carrier Frequency Offset (CFO) in an OFDM system increases the bit errors if efficient CFO estimation and correction methods are not employed. CFO can introduce Inter Carrier Interference (ICI) and phase rotation of the symbols which degrades the Bit Error Rate (BER) performance of the OFDM system.
In OFDM modulation scheme, data is transmitted simultaneously over the sub-carriers where orthogonality among the sub-carriers is a necessary and sufficient condition, which prevents Inter Carrier Interference (ICI) between the closely spaced sub-carriers. Efficient CFO estimation and correction methods are required to maintain the orthogonality among the sub-carriers.
There are various conventional techniques in order to estimate the Carrier Frequency Offset (CFO) for OFDM signals.
For example, “A Novel Time and Frequency Synchronization Scheme for OFDM Systems”, IEEE Transactions on Consumer Electronics, Vol. 54, No. 2, MAY 2008, discloses a time and frequency synchronization scheme for orthogonal frequency division multiplexing systems where carrier frequency offset estimate is obtained based on the autocorrelation between adjacently transmitted preambles in a single OFDM frame. Carrier frequency offset is estimated from the phase information at the peak amplitude of autocorrelation output.
US 7039131B2 titled “Carrier Frequency Offset estimation in a wireless communication system” describes a CFO compensation circuit in an OFDM receiver which receives the baseband signal and modifies the phase of the baseband signal in response to a first control signal, a transformation circuit that translates the baseband signal from the CFO compensation circuit into a frequency domain constellation, an equalizer that receives the frequency domain constellation and modifying the frequency domain constellation based at least in part on the reference symbol, and a CFO estimation circuit coupled between an output of the equalizer and the CFO compensation circuit in a feedback configuration. CFO estimation circuit measures difference in phase error between at least two consecutive symbols received from the equalizer and generates the first control signal, the first control signal being representative of the phase error difference.
US6198782 titled “Estimation of Frequency offsets in OFDM communication systems” discloses an apparatus and method for joint estimation of clock and carrier frequency offsets in OFDM receivers. It performs a maximum likelihood estimation on the demodulated signals to jointly estimate the clock and carrier frequency offsets.
Therefore, there is still a need of a technical solution which solves the above defined problems and provides a carrier offset estimator that estimates the Carrier Frequency Offset (CFO) of an OFDM waveform to the expected accuracy in a heavily faded multipath environment like Troposphere and aids in the channel estimation to recover the original information.
SUMMARY
This summary is provided to introduce concepts related to a novel method and a device for Carrier Frequency Offset estimation of a frame based input waveform such as an OFDM waveform in a heavily multipath faded environment. The Carrier Frequency Offset Estimator of the present invention exploits the preambles of any two nearby OFDM communication frames to estimate the CFO accurately in the presence of channel effects like fading, Inter Symbol Interference (ISI), Inter Carrier Interference (ICI) and noise.
In an embodiment of the present invention, the present invention estimates Carrier Frequency Offset (CFO) by correlating the preambles of the above said communication frames and ensures that the preambles being correlated are separated by a length equal to an integer multiple of a single OFDM frame length. Further, if the Start of Frame (SOF) position of second frame, among the two frames being correlated, is not an integer multiple of a single OFDM frame length, then preambles of the second frame for correlation is selected from a location obtained by pseudo incrementing the SOF detection point of the first frame wherein the pseudo increment is an integer multiple of frame size and SOF position indicates the beginning of an independent communication frame.
In another embodiment of the present invention, the preambles of any two communication frames being correlated are the first preambles of OFDM frames and are of length ‘P’ where ‘P’ is the length of an OFDM symbol.
In another embodiment of the present invention, the preambles of any two communication frames being correlated are of length ‘2P’ and utilizes both the preambles of the independent OFDM frames to improve the accuracy of the CFO estimation.
In another embodiment of the present invention, the CFO estimates of two OFDM communication frames that are separated by a length equal to an integer multiple ‘n’ of a single OFDM frame length are computed where ‘n’=1,2,3…. and then CFO estimates CFON obtained from frames ‘N’ and ‘N+n’, CFON+1 obtained from frames ‘N+1’ and ‘N+n+1’, CFON+2 obtained from frames N+2 and N+n+2 are averaged to make Carrier Frequency Offset estimation more robust.
In another embodiment of the present invention, if the preambles of the two OFDM communication frames being correlated for CFO estimation are labeled ‘Frame ‘N’’ and ‘Frame ‘N+1’’ and are consecutive and separated by a length equal to a single OFDM frame length, the instantaneous CFO estimate obtained is averaged with the previous average CFO estimate to make the CFO estimate more accurate.
In another embodiment of the present invention, the method of Carrier Frequency Offset estimation based on preambles of the independent frame is applicable to any communication frame with preambles.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
The following detailed description of the invention refers to the accompanying figures.
Figure 1 illustrates a block diagram that shows Carrier Frequency Offset (CFO) Estimator and Carrier Frequency Offset (CFO) Correction Module in an OFDM receiver system, in accordance with an exemplary embodiment of the present invention.
Figure 2 illustrates the block diagram of a method and a device for Carrier Frequency Offset estimation based on preambles of two OFDM frames, in accordance with an exemplary embodiment of the present invention.
Figure 3 illustrates a block diagram that demonstrates in detail the operations involved in the method of estimation of Carrier Frequency Offset based on preambles of two OFDM frames, in accordance with an exemplary embodiment of the present invention.
Figure 4 illustrates a structure of an OFDM frame, in accordance with an exemplary embodiment of the present invention.
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
The various embodiments of the present invention describe about a method and a device for Carrier Frequency Offset estimation of a frame based input waveform such as an OFDM waveform in a heavily multipath faded environment.
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. 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.
However, the methods and apparatuses 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 present invention.
Furthermore, connections between components and/or modules within the figures are not intended to be limited to direct connections. Rather, these components and modules may be modified, re-formatted or otherwise changed by intermediary components and modules.
The appearances of the phrase “in an embodiment” in various places in the specification are not necessarily all referring to the same embodiment.
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.
In an embodiment of the present invention, a method and a device method for Carrier Frequency Offset (CFO) estimation of a frame based input waveform such as an OFDM waveform in a heavily multipath faded environment.
In another embodiment, the present invention discloses a method and a device for estimating the Carrier Frequency Offset (CFO) of a frame based input waveform such as an OFDM waveform that requires the detection of frame boundaries. The present invention also describes the method of CFO estimation that detects the location of preambles in the independent OFDM frames and correlates these preambles of the independent frames to compute a Carrier Frequency Offset (CFO) estimate. The present invention also describes the method of extracting the preambles of independent frames for CFO estimation from the locations that are apart from each other by a length equal to an integer multiple of a single OFDM frame length.
In another embodiment, the present invention also discloses the method of finding pseudo Start of Frames (SOF) locations of the present OFDM communication frame by pseudo incrementing the last SOF detection point wherein the pseudo increment is equal to the multiple of a frame size if the Start of Frames (SOF) of the two OFDM frames considered for CFO estimation are not separated by a length equal to integer multiple of single OFDM frame length ‘L’.
Figure 1 illustrates a block diagram that shows Carrier Frequency Offset (CFO) Estimator and Carrier Frequency Offset (CFO) Correction Module in an OFDM receiver system, in accordance with an exemplary embodiment of the present invention. The OFDM receiver system comprises of an antenna 101 which is configured to receive a plurality of OFDM communication frames. Further, the system includes a down converter 103 with an local oscillator 102 which is configured to convert the received pass band signal to a baseband signal. Further, an Analog to Digital Converter (ADC) 104 is configured to digitize the baseband signal and an SOF Detection Module 105 is configured to detect the start of the OFDM frame. The system further includes a Carrier Offset Estimator 106 which is configured to estimate the Carrier Frequency Offset (CFO) and a Carrier Offset Correction module 107 which is configured to compensate the CFO. Further, a Fast Fourier Transform (FFT) module 108 is configured to demodulate the signal in the received OFDM symbol and a decoder 110 is configured to decode the bits of information. The present invention focus on the Carrier Offset Estimator 106 and the Carrier Offset Correction module 107 of Figure 1.
Figure 2 illustrates the block diagram of a method and a device for Carrier Frequency Offset estimation based on preambles of two OFDM frames, in accordance with an exemplary embodiment of the present invention. A device 200 for Carrier Frequency Offset estimation shown herein consists of a data storing unit 201, a buffer, which is configured to store the digitized OFDM data samples. The device 200 further includes a correlation unit 202 which correlates the data samples and a peak detection unit 203 which detects the peak of the correlated output to detect the Start of Frame position. Further, a pseudo incrementing unit 204 is configured to pseudo increment the Start of Frame (SOF) location of the OFDM frame by an integer multiple of the OFDM frame length. Further a data fetching unit 205 fetches the OFDM data samples from the buffer based on the address location obtained from the pseudo incrementing unit 204 or peak detection unit 203. A Frame boundary detection unit 206 is configured to detect the data samples belonging to an OFDM frame and extract the preambles from the OFDM frame. A Frame to Frame processing unit 207 coupled to the Frame boundary detection unit 206 consisting of a conjugation unit, a multiplier, an averaging unit is configured to correlate the preamble data samples of the OFDM frames. Further, a Phase Estimation Unit 208 is configured to estimate the phase from averaged output of Frame to Frame processing unit 207 and a Frequency Offset Calculation Unit 209 is configured to calculate the frequency offset from the obtained phase. Further, a Sine-Cosine Generator Unit 210 generates sine and cosine values and those values have to be applied on the input OFDM frame with a Frequency Offset Compensation Unit 211.
Figure 3 illustrates a block diagram that demonstrates in detail the operations involved in the method of estimation of Carrier Frequency Offset based on preambles of two OFDM frames, in accordance with an exemplary embodiment of the present invention. The steps followed in the estimation of Carrier Frequency Offset is described in detail herein. The OFDM signals received by the antenna of OFDM communication receiver is down converted to baseband, digitized and further utilized for Carrier Frequency Offset (CFO) estimation. Further, the digitized OFDM samples are stored sequentially in a buffer ‘A’ 301 of size greater than or equal to 2 times of OFDM Symbol length. The incoming digitized OFDM samples are also stored parallel in another buffer ‘B’ 302 of size equal to ‘M’ times of OFDM frame length so that CFO estimation can be performed by correlating the preambles of 1st and (M-1)th OFDM frames stored in buffer ‘B’ if required where M can be 3, 4, 5…. The samples stored in buffer ‘A’ 301 are read and correlated with the samples at locations ‘P’ samples apart where ‘P’ is the separation between the two preambles in a single OFDM frame. ‘P’ is also the size of an OFDM Symbol in a single OFDM frame. Further, the thresholding is performed on the correlated output to find the peak address location that indicates the beginning of the OFDM Frame. The correlation value is high when identical samples are correlated. In the OFDM frame structure, the preambles PR1 and PR2 are transmitted at the beginning of the OFDM frame and they are identical. Further, the preambles PR1 and PR2 are correlated with block 303 that gives a high correlation value which also indicates the Start of the OFDM Frame (SOF) location. Hence, correlating the samples of received OFDM signal from buffer ‘A’ which are ‘P’ samples apart gives a peak value only when the samples getting correlated are preambles PR1, PR2 of the independent OFDM frame. The peak address location ‘X’ obtained refers to the address location in buffer ‘B’ which indicates the beginning of the OFDM Frame. Once Start of Frame (SOF) is identified, the consecutive ‘P’ samples are read from the buffer ‘B’ with ‘X’ being the start address and noted as PR_F1 which indicates the preamble of first OFDM frame. Similarly, Start of Frame (SOF) of the second frame is also identified. The preambles of the second OFDM frame are also read from the buffer ‘B’ from the identified Start of Frame (SOF) location and noted as PR_F2 which indicates the preamble of second OFDM frame. Carrier Frequency Offset is estimated based on the autocorrelation of PR_F1 with PR_F2. Autocorrelation process involves multiplication of the first sample of preamble PR_F1 with the corresponding first sample of conjugated preamble PR_F2* and so on. The Start of Frame (SOF) of the second OFDM frame should be exactly at a distance equal to an integer multiple of the length of the OFDM frame from the Start of Frame (SOF) of the first OFDM frame. Otherwise a pseudo Start of Frame (SOF) is found by pseudo incrementing the SOF detection point of the first frame, ‘first frame’ being the first among the two frames being correlated, wherein the pseudo increment is equal to the multiple of a frame size ‘L’. A phase estimator 307 derives the phase estimate from the preambles PR_F1 and PR_F2 based on CORDIC method. Phase estimation process involves the multiplication of preamble PR_F1 with the conjugated preamble PR_F2* using a multiplier 305 where conjugation of preamble PR_F2 is achieved by providing it to the conjugation unit 304. Multiplier 305 output contains ‘P’ values which are complex in nature. I (in-phase) and Q (quadrature-phase) components of these ‘P’ values from the multiplier is provided to an averaging unit 306 that generates Iavg and Qavg values. Corresponding to these Iavg and Qavg values, a valid flag is also generated. These Iavg, Qavg and valid are given to the CORDIC module to generate a phase estimate θ. θ is stored when enable is high and the frequency offset value ‘f’ is computed according to 308 from theta using the equation ,where f_s is the sampling frequency and ‘L’ is the length of one OFDM frame.

θ_1,θ_2,θ_3,…………..θ_L values are generated according to equation (1) and provided to the sine-cosine generator 309 with corresponding enables that generates sin⁡〖θ_1 〗,cos⁡〖θ_1,sin⁡〖θ_2 〗,〗 cos⁡〖θ_2 〗,………. according to the CORDIC algorithm. With the first enable corresponding to θ_1, the first data sample of the OFDM frame is read from buffer B and multiplied with cos⁡〖θ_1 〗+1i*sin⁡〖θ_1 〗 to compensate the frequency offset which is performed by the module 310. Similarly, the second data sample of the OFDM frame is read from buffer B with the enable of θ_2 and multiplied with cos⁡〖θ_2 〗+1i*sin⁡〖θ_2 〗.

Figure 4 illustrates a structure of an OFDM frame, in accordance with an exemplary embodiment of the present invention. The OFDM frame shown in the Figure 4 is composed of OFDM symbols 405 of length ‘P’ and consists of L/P OFDM symbols where ‘L’ is the length of a single OFDM frame. Each independent OFDM frame consists of two OFDM preamble symbols PR1 and PR2. The figure shows two consecutive OFDM frames; OFDM Frame1 401 and OFDM Frame2 402. The preambles PR_F1 403 of the OFDM Frame1 is correlated with preambles PR_F2 404 of the OFDM Frame2 to estimate the Carrier Frequency Offset (CFO) where preambles PR_F1 and PR_F2 can be PR1 or {PR1 PR2}. Hence the length of PR_F1 and PR_F2 can be ‘P’ or ‘2P’ depending on whether PR1 or {PR1 PR2} is used.
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.