Claims:We Claim:
1. A method for decoding direct sequence spread spectrum samples with frequency offset correction in phase domain, the method comprising:
receiving a modulated signal comprising of preamble and data information spreaded by pseudo random sequence (spread code), wherein the preamble is constructed using zeros or ones;
demodulating the received signal to generate digital I/Q samples using RF demodulator;
computing an arc tan operation on received digital I/Q samples to estimate the instantaneous phase using phase estimator;
estimating the Doppler phase by operating on received I/Q digital samples using Doppler estimator and generates phase component related to Doppler;
compensating the estimated Doppler phase to instantaneous phase by phase rotator;
searching for spread code on the Doppler compensated phase using search engine by phase correlating with reference spread code phases; and
decoding the data by baseband (BPSK) demodulator, if the code search is successful.

2. The method as claimed in claim 1, wherein the Doppler estimator comprises coarse Doppler estimator for rough estimate of Doppler and fine Doppler estimator for fine tuning the coarse Doppler estimate.

3. The method as claimed in claim 2, wherein the coarse Doppler estimator uses FFT operator on decimated received samples to estimate coarse Doppler and fine Doppler estimator uses phase drift estimator for estimating phase drift between partially Doppler compensated phase and reference phases for bit duration.

4. The method as claimed in claim 1, wherein the code search engine is used for searching spread code in Doppler compensated phases, where the search engine is constructed using parallel phase correlators to increase the speed of search and to find the code shifts, if any.

5. The method as claimed in claim 1, wherein the search engine gives out the maximum correlation value along with the code shift.

6. The method as claimed in claim 1, wherein the maximum correlation value which is the output of code search engine given to de-multiplexer, where this de-multiplexer latches input data to baseband demodulator or comparator.

7. The method as claimed in claim 1, wherein the comparator or demodulator compares the maximum correlation value with the preset threshold value, if correlation value is more than preset threshold, it acquires the spread code successfully.

8. The method as claimed in claim 1, wherein the Doppler is tracked by continuously updating the Doppler estimate and spread code search engine output is given to baseband demodulator, where it decides decoded bit is 1 or 0 based on correlation value. , Description:FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10, rule 13)

“A method for Decoding DSSS Signals with Frequency Offset Correction”
By
BHARAT ELECTRONICS LIMITED
Nationality: Indian
M/s. Bharat Electronics Limited, Corporate Office, 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.

Field of the invention
The present invention mainly relates to decoding of information and more particularly to a method for decoding the information from the received noisy direct sequence spread spectrum samples with frequency offset correction in phase domain.
Background of the invention
Decoding is well known in the art which is a process of translating received messages into code words of a given code. These are often used to recover messages sent over a noisy channel, such as a binary symmetric channel.
Generally, Spread Spectrum has been defined as a technique whereby an already modulated signal is modulated again to produce a waveform which interferes very less with the any other signal operating in the same band of operation. Specifically in Direct Sequence Spread Spectrum system, base band signal is having a wider bandwidth than digital data stream and is generated by spreading digital data stream with high rate pseudo random code (spread code) which is independent of data stream. This spreaded data is RF modulated then transmitted.
On the reception side, for coherent receiver, the data is acquired by applying de-spread operation on received samples. But, in general, all communication receivers are non-coherent in nature, so received samples are affected by the impairments like channel noise, frequency offset between transmitter and receiver. For a successful reproduction of data at receiver, it needs a processing method for cancelling the effect of above impairments on received samples.
In practice, any communication system (single or multi-carrier) performance greatly depends on the handling capability of Frequency offset between nodes. There are two major reasons for frequency offset between transmitter and receiver: one is the local oscillators drift and second is Doppler shift due to the mobility of the nodes.
In literature, lot of work has been done on estimation of frequency offset for DSSS receivers.
For example, document US 5799034 entitled “Frequency acquisition method for DSSS systems” describes a method to correct frequency offset caused by usage of inaccurate oscillators. An Identification sequence has been used for acquiring frequency offset by computing dot product, cross product and inverted cross product on digital received samples.
Another, document US 6987796 entitled “Method for receiving spreaded spectrum signals with frequency offset correction” estimates the period of the modulation affecting the signal because of the frequency offset. This estimate is computed by calculating the DOT and CROSS signals based on the correlation components. This method is purely based on correlation components but acquiring proper correlation components for Doppler estimation in very noisy (negative SNRs) environment is difficult.
Further, document US 5793794 entitled “Spread spectrum receiving apparatus” describes a spread spectrum receiver comprising clock recovery circuit for reproducing carrier from received signal and it is used in demodulation circuit for analog demodulating the received signal then the output of demodulation circuit is given to judgment circuit to decode the digital information. The developed clock recovery method uses analog techniques which are complex.
Therefore there is a need in the art with method for decoding the information from the received noisy direct sequence spread spectrum samples with frequency offset correction in phase domain and to solve the abovementioned limitations.
Objective of the invention
The main objective of the present invention is to form a method which depends purely on digital processing techniques for decoding information from received spread spectrum samples by correcting the Doppler shift.
Summary of the Invention
An aspect of the present invention is to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below.
Accordingly, in one aspect of the present invention relates to a method for decoding direct sequence spread spectrum samples with frequency offset correction in phase domain. The method including the steps of receiving a modulated signal comprising of preamble and data information spreaded by pseudo random sequence (spread code), wherein the preamble is constructed using zeros or ones, demodulating the received signal to generate digital I/Q samples using RF demodulator, computing an arc tan operation on received digital I/Q samples to estimate the instantaneous phase using phase estimator, estimating the Doppler phase by operating on received I/Q digital samples using Doppler estimator and generates phase component related to Doppler, compensating the estimated Doppler phase to instantaneous phase by phase rotator, searching for spread code on the Doppler compensated phase using search engine by phase correlating with reference spread code phases, and decoding the data by baseband (BPSK) demodulator, if the code search is successful.
Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention.
Brief description of the drawings
The above and other aspects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which:
Figure 1 shows a general transmitter of direct sequence spread spectrum system according to one embodiment of the present invention.
Figure 2 shows a constructional block diagram of direct sequence spread spectrum receiver system according to one embodiment of the present invention.
Figure 3 shows a block diagram of Doppler estimator illustrating Doppler estimation procedure according to one embodiment of the present invention.
Figure 4 illustrates an embodiment of coarse Doppler estimator which generates the rough estimate of Doppler according to one embodiment of the present invention.
Figure 5 illustrates plot of squared samples against time for different SNRs, where Doppler can be visualized as a noisy sine tone according to one embodiment of the present invention.
Figure 6 shows the plot of estimation accuracy against decimation factor for two different Doppler frequencies according to one embodiment of the present invention.
Figure 7 illustrates the functional block diagram of fine Doppler estimator, which further fine tunes the coarse estimate according to one embodiment of the present invention.
Figure 8 shows a constructional block diagram of code search engine according to one embodiment of the present invention.
Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure. Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures.
Detailed description of the invention
The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.
The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.
It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces.
By the term “substantially” it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic is intended to provide.
Figs. 1 through 8, discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way that would limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged communications system. The terms used to describe various embodiments are exemplary. It should be understood that these are provided to merely aid the understanding of the description, and that their use and definitions, in no way limit the scope of the invention. Terms first, second, and the like are used to differentiate between objects having the same terminology and are in no way intended to represent a chronological order, unless where explicitly stated otherwise. A set is defined as a non-empty set including at least one element.
The present invention is a digital technique method for decoding information by nullifying the effect of frequency offset. It is assumed that frequency offset is mostly due to Doppler Effect, however not limiting the scope of this invention to this particular scenario.
The present invention relates to a method which depends purely on digital processing techniques for decoding information from received spread spectrum samples by correcting the Doppler shift. This method assumes that transmitter radiates the zeros, spreaded by spread code when there is no data to transmit. However, it is applicable to preamble followed by data frame structure also, provided preamble should be all zeros (0’s) or all ones (1’s).
The received signal comprises a spreaded all zeros followed by data spread samples. The method operates on spreaded zero samples for estimating Doppler shift and code search. This method operates in two phases: ‘acquisition phase’ and ‘tracking phase’. In acquisition phase, it estimates the Doppler and searches for the code. If it acquires code then it moves to tracking phase. In tracking phase, it works for tracking the Doppler, for any code shifts and de-spreads the samples to acquire information.
The Doppler estimation comprises following steps: Coarse Doppler estimation using FFT on decimated samples of squared received samples and Fine Doppler estimation by computing offset between phase of coarse compensated sample with reference phases (0° and 180°).
The code search process is characterized by the following steps:
The Parallel cross correlators are used for correlating phases of the Doppler compensated samples with spread-code phases (as early late gate method). The output values of correlators decide whether the code search is successful or not based on a threshold value.
Among all parallel correlators the best is chosen based on highest correlation value and shift is incorporated in samples to compensate code shift, if needed.
Figure 1 shows a general transmitter of direct sequence spread spectrum system according to one embodiment of the present invention.
The figure shows a general transmitter of direct sequence spread spectrum which has baseband modulator 11 and RF modulator 12 as major constructs. Baseband Modulator 11 takes the digital data bits (1’s and 0’s) as input and digital modulates it by BPSK (0 is mapped to +1 and 1 is mapped to -1). The digital modulated samples are spreaded using pseudo random sequence (spread code) which is independent of data. Spread code is a high rate NRZ sequence with voltage levels +1 and -1. Spread samples (known as ‘chips’) are constructed by multiplying each digital modulated sample with complete spread code sequence. Let the considered data rate is 9.6kbps, spread code length is 1023 then the chip rate is 9.8208Mcps (9.6kbps*1023). The base band modulator 11 drives the RF modulator 12 with in-phase component (‘I’) and quadrature component (‘Q’) of the chips at a rate of 9.8208 Mcps.
RF modulator 12 is a direct conversion transceiver, which applies some wave shaping filter (i.e. RRC filter) then converts I and Q streams to analog domain and frequency up converts to the desired frequency of operation based on local oscillator 13 clock frequency. The frequency up converted analog signal is given to antenna element 14 to radiate over air.
Figure 2 shows a constructional block diagram of direct sequence spread spectrum receiver system according to one embodiment of the present invention.
The figure shows the constructional block diagram of direct sequence spread spectrum receiver system embodying the invention. The direct sequence spread spectrum receiver system comprises RF Demodulator 21, Local oscillator 22, Doppler estimator 23, Phase estimator 24, Phase rotator 25, Code search engine 26, De-mux 27, Comparator 28, and Baseband Demodulator 29. The RF Demodulator 21 is a direct conversion transceiver, which converts the received analog signal to digital I and Q samples according to the local oscillator 22 clock frequency. It generates the samples at double the chip rate (2*9.8208Mcps). It means there are two samples for chip duration, this interpolation by two helps the method in incorporating half sample shift in tracking phase.
The Doppler estimator 23 estimates the Doppler by operating on received I/Q stream and it generates the phase component related to estimate Doppler. The phase estimator 24 estimates the instantaneous phase of each digital received sample by computing an arc tan operation on I and Q components of the received samples.
The phase rotator 25 modifies the instantaneous phase of each digital sample computed using phase estimator 24 by the estimated Doppler phase component given by Doppler estimator 23. The modified phase component is given to code search engine 26 and back to Doppler estimator 23 for further fine tuning the estimate.
This Doppler compensation can be done in rectangular domain (I/Q domain) where it doesn’t involve with any phase estimators and rotators. But, for compensating Doppler in I/Q domain each received sample should be multiplied with e^(±j?), where ? is angle related to Doppler. It involves complex multiplications and additions which consumes huge hardware resources. The same Doppler compensation in phase domain (angular domain) involves with only addition operations which require fewer resources for realizing in hardware and so the present invention practices phase domain approach.
The code search engine 26 searches for the presence of spread code by operating on Doppler compensated received phases. This search engine is constructed with parallel cross correlators where each correlator correlates the received phases with the reference phases of spread code. Phase correlators are used for its ease of realization in hardware. This search engine gives out the maximum correlation value along with the code shift, if required. Further details about the generation of code shift are described in Fig.8.
The maximum correlation value which is the output of code search engine 26 is given to de-multiplexer 27, this de-multiplexer latches input data to comparator 28 or baseband demodulator 29 based on a selection line A/T. Comparator 28 compares with the preset threshold value, if correlation value is more than preset threshold means it acquired the spread code successfully. Then the complete system will move from ‘acquisition phase’ to ‘tracking phase’ by asserting a signal A/T to 1.
In tracking phase, the Doppler is tracked by continuously updating the Doppler estimate and spread code search engine output is given to baseband demodulator 29 where it decides decoded bit is 1 or 0 based on correlation value.
In one embodiment, the present invention relates to a method for decoding direct sequence spread spectrum samples with frequency offset correction in phase domain, the method comprising: receiving a modulated signal comprising of preamble and data information spreaded by pseudo random sequence (spread code), wherein the preamble is constructed using zeros or ones; demodulating the received signal to generate digital I/Q samples using RF demodulator; computing an arc tan operation on received digital I/Q samples to estimate the instantaneous phase using phase estimator; estimating the Doppler phase by operating on received I/Q digital samples using Doppler estimator and generates phase component related to Doppler; compensating the estimated Doppler phase to instantaneous phase by phase rotator; searching for spread code on the Doppler compensated phase using search engine by phase correlating with reference spread code phases; and decoding the data by baseband (BPSK) demodulator, if the code search is successful.
Figure 3 shows a block diagram of Doppler estimator illustrating Doppler estimation procedure according to one embodiment of the present invention.
The figure shows a detailed block diagram of Doppler Estimator 23 which is a closed loop circuit. It has two major constructional blocks: coarse Doppler estimator 231 and fine Doppler estimator 232. Coarse Doppler estimator 231 takes direct I/Q samples as input and process the samples in I/Q domain to generate near close rough estimate of angle related to Doppler. Fine Doppler estimator 232 further fine tunes the coarse Doppler estimate by processing the output of phase rotator 25. The multiplexer 233 latches the coarse estimate in acquisition phase and in tracking phase it disconnects with coarse estimator. The phase updater 234 uses multiplexer 233 outputs and fine Doppler estimate 232 output to generate updated phase which is related to Doppler.
The coarse Doppler estimator uses FFT operator on decimated received samples to estimate coarse Doppler and fine Doppler estimator uses phase drift estimator for estimating phase drift between partially Doppler compensated phase and reference phases for bit duration. These coarse Doppler estimator 231 and fine Doppler estimator 232 are described in detail in Figure 4 and Figure 7 respectively.
Figure 4 illustrates an embodiment of coarse Doppler estimator which generates the rough estimate of Doppler according to one embodiment of the present invention.
The figure shows a functional diagram of coarse Doppler estimator 231. It is constructed using three major operators: decimator 2311, square operator 2312, FFT operator 2313 and max finder 2314. The decimator 2311 decimates the input I/Q samples based on preset decimation factor (DF), these decimated samples are fed to square operator 2312 which does the squaring operation on decimated I/Q sample.
The purpose of squaring operation is to eliminate the modulation effect on received samples. Actually, the received sample is a noise affected BPSK modulated symbol. So by applying square operation, modulation related information is erased from received digital sample(?±1?^(2 )=1). It means whatever amplitude variations exist on squared samples is due to noise and Doppler. The effect of squaring operation on received samples can be well understood by the following mathematical analysis.
Let the digital observation model for complex received samples at sampling rate f_s(=1/T_s) can be represented as
r_n = s_n * exp (j 2pf_d nT_s ) + w_n
Where, s_n = digital samples of information at rate f_s
f_d = Doppler frequency offset
w_n= Digital complex noise components
if apply squaring operation on each observed sample then
z_n = s_n^2*exp?{ j2p (2*f_d) nT_s } + w_n^'
Where s_n is a binary sequence, so s_n^2 = 1. It means squaring operation is removing the digital modulation effect on samples. Then above Eqn. will be
z_n = exp?{ j2p (2*f_d) nT_s } + w_n^'
It is representing a complex sinusoidal signal which is embedded in white Gaussian noise. And its frequency is representing the frequency offset due to Doppler.
Figure 5 illustrates plot of squared samples against time for different SNRs, where Doppler can be visualized as a noisy sine tone according to one embodiment of the present invention.
The figure illustrates a plot of squared samples against time for different SNRs, where Doppler can be visualized as a noisy sine tone. The problem of finding Doppler frequency in DSSS receivers has become estimating frequency of a noisy complex sinusoidal signal. Estimating frequency of a noisy sinusoidal is a well studied problem in literature, there are so many methods for estimating frequency of noisy sinusoidal based on Maximum likelihood (ML) estimation using FFT and correlation.
In this invention ML estimation using FFT is incorporated. The maximum bin index of FFT computed on squared samples gives the Doppler estimate. But, there is a trade-off between FFT size and accuracy of estimate in presence of noise where accuracy of estimate is proportional to FFT length but using more length FFT is computationally complex. For reducing computational complexity, this invention applies FFT on decimated squared samples which enable less FFT size for good estimation accuracy in noisy scenarios.
Figure 6 shows the plot of estimation accuracy against decimation factor for two different Doppler frequencies according to one embodiment of the present invention.
The figure shows the plot of estimation accuracy against decimation factor for two different Doppler frequencies. For a successful Doppler estimation there is an inequality between decimation factor (DF), Doppler frequency (f_d) and received sampling rate (f_s) which is as below:
f_s/DF=2*f_d
So decimation factor can be chosen such that above inequality should be satisfied.
The decimator 2311 decimates the received samples based on the chosen decimation factor, the square operator 2312 applies square operation on decimated samples, then FFT 2313 operates on squared samples to generate spectral components, max finder 2314 finds the maximum bin index by finding peak spectral component and generates phase corresponding to Doppler.
Figure 7 illustrates the functional block diagram of fine Doppler estimator, which further fine tunes the coarse estimate according to one embodiment of the present invention.
The figure illustrates the functional block diagram of fine Doppler estimator 232, which further fine tunes the coarse estimate. It has two functional blocks phase drift estimator 2321 and bit integrator 2322. The phase drift estimator 2321 operates on each previous Doppler compensated phase to find drift with reference phase 0° or 180° corresponding to voltage levels ±1. If the previous Doppler compensated phase is in quadrant I or IV, then the drift is computed by comparing with 0° and if it is in quadrant II or III then drift is computed by comparing with 180°.
The each estimated phase drift is fed to bit integrator 2322 such that it integrates and generates the average drift for bit duration (over 1023 chips, assumed code length is 1023) as a fine Doppler estimate. This has been updated to coarse estimate for further fine tuning Doppler estimate.
Figure 8 shows a constructional block diagram of code search engine according to one embodiment of the present invention.
The figure shows the constructional block diagram of the code search engine 26 where it is constructed using parallel cross correlators 261,262,263 to increase the speed of search and to find the code shifts, if any. Each cross correlator correlates the phases of reference spread code with Doppler compensated phases and integrate it for bit duration. The code search engine is used for searching spread code in Doppler compensated phases. The search engine gives out the maximum correlation value along with the code shift which is given to de-multiplexer, where this de-multiplexer latches input data to baseband demodulator or comparator.
Each cross correlators is build using a phase comparator and a bit integrator. Comparator compares the Doppler compensated phase with spread code phase, bit integrator integrates the comparator output for bit duration and gives out the phase correlation value. Each correlator works on bit duration data which are shifted in time. The comparator or demodulator compares the maximum correlation value with the preset threshold value, if correlation value is more than preset threshold, it acquires the spread code successfully.
The Doppler is tracked by continuously updating the Doppler estimate and spread code search engine output is given to baseband demodulator, where it decides decoded bit is 1 or 0 based on correlation value.
Out of all parallel correlators whichever gives the maximum value indicates that correlator phase is matched with reference code phase. So the max finder 264 estimates the maximum among all correlator outputs and generates the code shift based on the maximum correlator. If centre correlator is producing maximum then code shift is zero otherwise code shift will be ±1 . According to generated code shift value, the shift is incorporated in samples such that it aligns with phase of reference spread code.
The maximum value generated by max finder 264 will be compared with preset threshold value by comparator 28. If it crosses the threshold value then the method will move from acquisition phase to track phase (A/T = 1).
In track phase, computed maximum correlation value is given to baseband demodulator 29 which generates the demodulated bit information. And generated code shift is incorporated to correct the code shifts, if any, by half sample shift such that it tracks the changes in Doppler frequency.
The code search engine is implemented using three parallel correlators, it may be implemented using more correlators also. The more correlators’ implementation reduces the code search time (known as ‘acquisition time’).
This method has applicability in continuous and burst mode of communication systems which are affected by very noisy channels along with more frequency offsets.
Those skilled in this technology can make various alterations and modifications without departing from the scope and spirit of the invention. Therefore, the scope of the invention shall be defined and protected by the following claims and their equivalents.
FIGS. 1-8 are merely representational and are not drawn to scale. Certain portions thereof may be exaggerated, while others may be minimized. FIGS. 1-8 illustrate various embodiments of the invention that can be understood and appropriately carried out by those of ordinary skill in the art.
In the foregoing detailed description of embodiments of the invention, various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments of the invention require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus, the following claims are hereby incorporated into the detailed description of embodiments of the invention, with each claim standing on its own as a separate embodiment.
It is understood that the above description is intended to be illustrative, and not restrictive. It is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined in the appended claims. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein,” respectively.

We Claim:
1. A method for decoding direct sequence spread spectrum samples with frequency offset correction in phase domain, the method comprising:
receiving a modulated signal comprising of preamble and data information spreaded by pseudo random sequence (spread code), wherein the preamble is constructed using zeros or ones;
demodulating the received signal to generate digital I/Q samples using RF demodulator;
computing an arc tan operation on received digital I/Q samples to estimate the instantaneous phase using phase estimator;
estimating the Doppler phase by operating on received I/Q digital samples using Doppler estimator and generates phase component related to Doppler;
compensating the estimated Doppler phase to instantaneous phase by phase rotator;
searching for spread code on the Doppler compensated phase using search engine by phase correlating with reference spread code phases; and
decoding the data by baseband (BPSK) demodulator, if the code search is successful.

2. The method as claimed in claim 1, wherein the Doppler estimator comprises coarse Doppler estimator for rough estimate of Doppler and fine Doppler estimator for fine tuning the coarse Doppler estimate.

3. The method as claimed in claim 2, wherein the coarse Doppler estimator uses FFT operator on decimated received samples to estimate coarse Doppler and fine Doppler estimator uses phase drift estimator for estimating phase drift between partially Doppler compensated phase and reference phases for bit duration.

4. The method as claimed in claim 1, wherein the code search engine is used for searching spread code in Doppler compensated phases, where the search engine is constructed using parallel phase correlators to increase the speed of search and to find the code shifts, if any.

5. The method as claimed in claim 1, wherein the search engine gives out the maximum correlation value along with the code shift.

6. The method as claimed in claim 1, wherein the maximum correlation value which is the output of code search engine given to de-multiplexer, where this de-multiplexer latches input data to baseband demodulator or comparator.

7. The method as claimed in claim 1, wherein the comparator or demodulator compares the maximum correlation value with the preset threshold value, if correlation value is more than preset threshold, it acquires the spread code successfully.

8. The method as claimed in claim 1, wherein the Doppler is tracked by continuously updating the Doppler estimate and spread code search engine output is given to baseband demodulator, where it decides decoded bit is 1 or 0 based on correlation value.

Abstract
A method for Decoding DSSS Signals with Frequency Offset Correction
A method for decoding Direct-Sequence Spread Spectrum (DSSS) signals, in noisy Doppler scenario, estimates the initial frequency offset by applying FFT on decimated digital received samples and is fine-tuned by computing phase drift with reference phases. It compensates the same by modifying the phase of receiving samples then de-spreads the samples by computing phase correlation between reference spread-code phases with phases of Doppler compensated samples to acquire the information.

Figure 2 (for publication)