Claims:We Claim:

1. An optimal diversity combiner for a troposcatter communication receiver with multiple diversity branches, the optimal diversity combiner comprising:
a down converter 201 which converts RF signals at the diversity receivers to baseband by mixing it with local oscillator signal;
an analog-to-digital converter (ADC) 202 coupled to the down converter which digitizes the baseband signal;
a timing recovery module 203 coupled to the analog-to-digital converter (ADC), wherein the timing recovery module is used for extracting the sample at the precise sampling time instants from the oversampled received signal;
a frame synchronisation module 204 coupled to the timing recovery module, wherein the frame synchronisation is used for estimating the start of data frame (SOF) where signals received by multiple diversity branches of the receiver are organized in frame structure and each frame consists of a header and blocks of data symbols separated by block of pilot symbols;
a frame alignment module 205 coupled to the frame synchronisation, wherein the frame alignment is used for aligning the signals from all the diversity branches based on the SOF;
a carrier frequency offset module 2101,2102,…210N coupled to the frame alignment, wherein the carrier frequency offset module is used for estimating the carrier frequency offset of each of the diversity branches;
an equalizer coupled to the carrier frequency offset module, wherein the equalizer is used for combating the effects of intersymbol interference (ISI), which consist of feed forward 206 and feedback 209 sections; and
a combiner coupled to the equalizer, wherein the combiner is used for effectively combining frequency offset corrected, equalized signal from all the diversity branches so that data can be recovered without any loss.

2. The optimal diversity combiner as claimed in claim 1, wherein the equalizer comprises of feed forward (FFE) 206 and feedback (FBE) 209 sections where the weights are calculated based on the error between the combiner output 211 and header/pilot/decision taken on combiner output depending on whether header/pilot/data is processed.

3. The optimal diversity combiner as claimed in claim 1, wherein the weights of individual equalizers of each of the multiple diversity branches are updated simultaneously based on the combiner output rather than individual equalizer outputs.

4. The optimal diversity combiner as claimed in claim 1, wherein stochastic gradient estimation method is used for weight estimation of equalizer which estimates error based on the difference between the combiner output and header/pilot/decision taken on combiner output depending on whether header/pilot/data is processed.

5. The optimal diversity combiner as claimed in claim 1, further comprises a carrier offset correction applied at input to the equalizer of each diversity branch which is performed jointly with equalizer and combiner.

6. The optimal diversity combiner as claimed in claim 1, wherein the Carrier Frequency Offset module present in each of the multiple diversity branches estimates the carrier offset of the respective diversity branches based on the respective equalizer outputs and header/pilot/decision taken on combiner output depending on whether header/pilot/data is processed.

7. The optimal diversity combiner as claimed in claim 1, wherein the Timing Recovery module uses interpolation to estimate the right sample value from the oversampled received signal.

8. The optimal diversity combiner as claimed in claim 1, wherein the Frame Synchronization module identifies the start of data frame (SOF) by correlating the received signal with the PL (physical layer) header which helps in frame alignment.

9. The optimal diversity combiner as claimed in claim 1, wherein the Frame Alignment module aligns the signals from all the diversity branches based on the SOF without which optimum performance cannot be obtained by the diversity combiner.

10. A method of diversity combining to enhance the signal reception by exploiting multipath faded signals at diversity receivers, the method comprising:
converting RF signals at the diversity receivers to baseband by down converter mixing it with local oscillator signal;
digitizing the baseband signal by an analog-to-digital converter (ADC) coupled to the down converter;
extracting the sample at the precise sampling time instants from the oversampled received signal by a timing recovery module coupled to the analog-to-digital converter (ADC);
estimating the start of data frame (SOF) by frame synchronisation, where signals received by multiple diversity branches of the receiver are organized in frame structure and each frame consists of a header and blocks of data symbols separated by block of pilot symbols;
aligning the signals from all the diversity branches based on the SOF by frame alignment;
estimating the carrier frequency offset of each of the diversity branches by a carrier frequency offset module;
combating the effects of intersymbol interference (ISI), which consist of feed forward and feedback sections by an equalizer; and
combining the frequency offset corrected, equalized signal from all the diversity branches so that data can be recovered without any loss by a combiner.

11. The optimal diversity combiner as claimed in claim 10, wherein the method of diversity combining is applicable for space diversity or frequency diversity or combination of these diversity input channels.
, Description:FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10, rule 13)
“An optimal diversity combiner for troposcatter communication receiver“
By
Bharat Electronics Ltd
“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 digital receiver for Troposcatter communication and more particularly to an optimal diversity combiner for a troposcatter communication receiver which enhances the signal to noise ratio (SNR) of the received signal and nullifies the effects of channel fading by exploiting multipath faded signals at multiple diversity receivers.
Background of the invention
Tropospheric scatter (also known as troposcatter) is well known method in the art which is used for long distance wireless communications often up to 300 km with microwave radio signals. Microwaves are radio waves with frequencies in the range 300MHz to 300GHz. Troposcatter radio systems communicate through troposphere by using a radio path beyond the horizon. The troposphere is the lowest layer of earth's atmosphere, about 8 to 15 km above the earth’s surface.
Signals propagating through troposphere undergo phenomenon like scattering due to atmospheric irregularities in this region. Some portion of this scattered energy will be directed towards earth which can be picked up by receiver stations. Scattering phenomena introduces multipath fading to the signals propagated through troposphere. Usually, most radio communications links are affected in one form or another by multipath fading. The multipath fading may create deep notches in the transmission frequency band resulting in signal degradation, i.e. the signals fade completely away. Further, multipath fading may also introduce phase distortion and intersymbol interference to the data transmitted.
Diversity techniques like spatial diversity, frequency diversity, etc. can be applied at the receivers to combat the effects of multipath fading. In a typical diversity receiver, the signals received at the diversity branches from a common source experience uncorrelated fading most of the time. Therefore, it is unlikely that the signals received at all the diversity branches undergo deep fade at the same time. The uncorrelated signals received at these diversity branches can be combined using an appropriate algorithm to recover the original transmitted signal nullifying the effects of channel fading and eventually improving SNR (Signal to Noise Ratio). In diversity approach, signals received at the diversity branches are appropriately weighted and combined together using suitable signal processing technique to recover the original transmitted signal. However, achieving flat transmission frequency band in the presence of frequency selective fading is a difficult task even with diversity receiver. In other words, data loss may occur which will affect the receiver performance.
A practical solution to the problem is improving the signal processing methods for appropriate weighting of signals received from diversity branches which in turn will mitigate the effects of channel fading. Diversity receiver consists of modules like timing recovery module, carrier recovery module, equalizer module and diversity combiner modules. Appropriate positioning of these modules which determine the architecture of digital receiver also can improve the performance of diversity receiver. Design and implementation of an optimum diversity combiner is required to recover the original transmitted data at the receiver without any data loss.
Various techniques for diversity combining have been suggested in the prior art in order to extract the original transmitted signal in the presence of fading by improving the SNR of the received signals.
For example, in US3879664 titled “High speed digital communication receiver” discloses a digital receiver system with pre-detection diversity combiner. The signals received from the diversity branches are weighted using forward adaptive filter equalizer and then combined. The diversity combiner output is then demodulated and passed through a detector to recover the original data. The generated error signal is modulated and correlated with each tap outputs of forward transversal equalizer which in turn adjusts the attenuator weights. The detector output and unmodulated error signal are provided to backward adaptive filter equalizer which utilizes it to generate a cancellation signal that removes the intersymbol interference and source correlation effects from the demodulated diversity combiner output signal. An automatic gain control system is also described that determines the gain to be applied at the IF amplifiers based on the strongest received signal.
Further, in US4890298 titled “Troposcatter modem receiver” describes a troposcatter modem receiver with pre-detection maximal ratio diversity combiner. Here incoherent combining of receiver signals occurs at RF level. The combined output is further demodulated and filtered to obtain baseband signal. This signal is then equalized using a decision feedback equalizer (DFE) and provided to a detector circuit, the output of which is fed back as secondary input to the equalizer. The error difference between the detector input and output is provided to a calculator circuit to determine the optimum weights for the adaptive equalization of the received signals so as to combat the effects of intersymbol interference.
Further, in US4334316 titled “Pre-detection maximal ratio combining system for diversity reception of radio frequency signals” discloses a pre-detection combining system where the phase and amplitude of the received diversity signals are controlled based on the correlation between mutually orthogonal components of each of the pre-detection signals at the diversity receivers with a reference comparison signal derived from the combined output. Amplitude of maximal ratio combiner output is normalized and provided as the reference comparison signal.
Furthermore, in US4271525 titled “Adaptive diversity receiver for digital communications” discloses an adaptive digital diversity receiver with a pre-detection combiner. Each diversity branch includes a transversal filter whose tap gains are updated to determine the channel impulse response of each diverse channel based on detected data output. Output signals from the transversal filters are linearly combined and the combined output is then equalized using decision feedback equalizer (DFE) which consists of adaptive feed-forward and feedback transversal filters whose tap gains are updated based on the error signal. Signal from feed-forward transversal filter is further demodulated and provided to a subtractor which takes output of backward transversal filter as the second input. Detector circuit takes this equalized signal as input and generates an error signal which updates the tap gains of DFE.
Also, in US20100309970 titled “DVB-S2 Demodulator” discloses a method for demodulating n-PSK or n-APSK modulated signals which follow a frame structure consisting of a header followed by blocks of data symbols separated by blocks of pilot symbols. Pre-equalization of header, pilot and data symbols is performed prior to determining the phase of header and pilot blocks. Phase determination of header and pilot blocks helps in predicting the evolution of signal phase wherein the phase of data symbols can be corrected based on this signal phase evolution. Phase corrected data symbols are then equalized using equalization coefficients generated based on known or estimated symbols of the signal.
Moreover, in US3633107 titled “Adaptive signal processor for diversity radio receivers” discloses a diversity radio receiver system with adaptive signal processors that performs demodulation, equalization, diversity combining of the signals received at the diversity receivers. Here weighting is performed on the analogy demodulated received signal where the attenuators are adjusted to minimize the error.
Therefore there is a need in the art for an optimal diversity combiner which enhances the signal to noise ratio (SNR) of the received signal and nullifies the effects of channel fading.

Objective of the invention
The main objective of the present invention is to provide an optimal diversity combiner for combating the effects of fading, ISI and noise.
Another objective of the present invention is to enhance the data recovering capability at the receiver by providing improved signal processing techniques of diversity combining with frequency correction and equalization thereby improving the link availability.
Further objective of the present invention is to enhance the signal reception by exploiting multipath faded signals at multiple diversity receivers. The present invention deals with the techniques of equalization, frequency correction and combination of diversity signals to combat the channel fading effects in tropospheric channel
Furthermore the present invention mitigates the effects of fading like frequency selective fading, where equalizer weights are computed based on the error difference between combiner output and header/pilot/decision taken on Combiner output depending on whether header/pilot/data is processed. Frequency offset error is determined by comparing the respective equalized output of each diversity branch with the header/pilot/decision and frequency correction is applied at the input side prior to equalization.
Summary of the Invention
An aspect of the present invention relates to an optimal diversity combiner for a troposcatter communication receiver with multiple diversity branches, the optimal diversity combiner comprising: a down converter 201 which converts RF signals at the diversity receivers to baseband by mixing it with local oscillator signal, an analog-to-digital converter (ADC) 202 coupled to the down converter which digitizes the baseband signal, a timing recovery (TR) module 203 coupled to the analog-to-digital converter (ADC), wherein the timing recovery module is used for extracting the sample at the precise sampling time instants from the oversampled received signal, a frame synchronisation module 204 coupled to the timing recovery module, wherein the frame synchronisation is used for estimating the start of data frame (SOF) where signals received by multiple diversity branches of the receiver are organized in frame structure and each frame consists of a header and blocks of data symbols separated by block of pilot symbols, a frame alignment module 205 coupled to the frame synchronisation, wherein the frame alignment is used for aligning the signals from all the diversity branches based on the SOF, a carrier frequency offset module 2101,2102,…210N couple to the frame alignment, wherein the carrier frequency offset module is used for estimating the carrier frequency offset of each of the diversity branches, an equalizer coupled to the carrier frequency offset module, wherein the equalizer is used for combating the effects of intersymbol interference (ISI), which consist of feed forward 206 and feedback 209 sections and a combiner coupled to the equalizer, wherein the combiner is used for effectively combining frequency offset corrected, equalized signal from all the diversity branches so that data can be recovered without any loss.
Another aspect of the present invention relates to a method of diversity combining to enhance the signal reception by exploiting multipath faded signals at diversity receivers, the method comprising: converting RF signals at the diversity receivers to baseband using down converter by mixing it with local oscillator signal, digitizing the baseband signal by an analog-to-digital converter (ADC) coupled to the down converter, extracting the sample at the precise sampling time instants from the oversampled received signal by a timing recovery module coupled to the analog-to-digital converter (ADC), estimating the start of data frame (SOF) by frame synchronisation, where signals received by multiple diversity branches of the receiver are organized in frame structure and each frame consists of a header and blocks of data symbols separated by block of pilot symbols, aligning the signals from all the diversity branches based on the SOF by frame alignment, estimating the carrier frequency offset of each of the diversity branches by a carrier frequency offset module, combating the effects of intersymbol interference (ISI), which consist of feed forward and feedback sections by an equalizer and combining the frequency offset corrected, equalized signal from all the diversity branches so that data can be recovered without any loss by a combiner.
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 illustrates the block diagram of Troposcatter modem receiver according to the prior art.
Figure 2 illustrates the block diagram of diversity receiver with optimum combiner for Troposcatter communication receiver according to one embodiment of the present invention.
Figure 3 illustrates the novel method of diversity combining 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 the 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 was intended to provide.
Figures 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 depicts an optimal diversity combiner for optimal combining of received signals in the presence of fading for a troposcatter communication receiver. Signals received by multiple antennas are organized in frame structure where each frame consists of a header and blocks of data symbols separated by block of pilot symbols. Received signals are amplified and their gains are adjusted with the help of an AGC which is digitized further and provided to a timing recovery circuit. Timing recovery module outputs the proper symbol sampled at the correct timing instant with intersymbol interference eliminated to the possible extent which is then correlated with known header/pilot symbols to find the Start Of Frame (SOF) referred as Frame Synchronization. The Frame Synchronization module identifies the start of data frame (SOF) by correlating the received signal with the PL (physical layer) header which helps in frame alignment. Frame alignment is then performed by aligning the SOF of the signals received by diversity branches of the receiver. The present invention describes a novel method of diversity combining performed in conjunction with carrier recovery and equalization which is capable of removing intersymbol interference and mitigate the effects of fading to a greater extent thereby improving the data recovering performance at the receiver.
The present invention depicts an optimal diversity combiner to effectively recover the data at the receiver by optimally combining signals received by the diversity branches at the receiver. Transmitted signals, on propagation through the troposphere, undergo multipath fading, especially Rayleigh fading and these signals when received by the diversity receiver will be degraded in signal quality due to the effects of intersymbol interference (ISI). In accordance with the invention, the optimal diversity combiner described consists of timing recovery, carrier offset, equalizer and combiner modules. These modules have to be placed and utilized appropriately to effectively combine the signals received by the diversity branches and recover the transmitted data mitigating the effects of fading and noise. The present invention provides methods for achieving better link availability for a communication link. Further, the invention provides better results in a fading environment for signals with lower signal-to-noise ratio (SNR) by developing a novel method of diversity combining which is performed together with carrier offset correction and equalization.
Radio frequency (RF) signals received by diversity branches of the receiver, which are uncorrelated most of the time due to the uncorrelated fading experienced at the different diversity channels, are amplified, down converted to baseband, digitized and provided to the timing recovery module. Timing recovery module estimates the sample at the precise sampling time instants from the oversampled input and provides it as output. The symbol synchronized signals from all the diversity branches are then frame synchronized and frame aligned. Frequency correction, equalization and diversity combining are then performed together according to the novel method depicted in the present invention.
The present invention provides improved methods for enhancing the data recovering capability at the receiver, mitigating the effects of multipath fading, ISI and noise by providing improved signal processing techniques of diversity combining with frequency correction and equalization thereby improving the link availability.
The invention, herein described, utilizes diversity combining technique for mitigating the effects of fading like frequency selective fading, where equalizer weights are computed based on the error difference between Combiner output and header/pilot/decision taken on Combiner output depending on whether header/pilot/data is processed. Frequency offset error is determined by comparing the respective equalized output of each diversity branch with the header/pilot/decision and frequency correction is applied at the input side prior to equalization.
The invention discloses a novel method to enhance the signal reception at the receiver by exploiting multipath faded signals at diversity receivers. Novelty in the invention dwells on how frequency offset correction, equalization and diversity combining is performed jointly to achieve enhanced signal quality at receiver. The invention uses the technique of diversity combining together with frequency offset correction and equalization and improve the link availability. Enhancement in data recovery at the receiver is achieved by the diversity combining of frequency offset corrected equalized signals whose weights are updated according to an adaptive algorithm that makes use of known sequence of PL header/pilot symbols.
Figure 1 illustrates the block diagram of Troposcatter modem receiver according to the prior art.
The figure illustrates a troposcatter modem receiver with pre-detection maximal ratio diversity combiner as described in the prior art, where the incoherent signals received at each of the ‘L’ antenna array elements are combined at RF level. Received signals are amplified by Low Noise Amplifiers (LNA) 101 and their gains are adjusted with the help of an Automatic Gain Controller (AGC) 102. According to prior art, the signals combined by the diversity combiner 103 are of analog nature. Diversity combined signal is then demodulated by the demodulator 104 circuit by mixing it with intermediate frequency signal and filtered by a baseband filter 105. The filtered baseband signal is equalized by using Decision Feedback Equalizer (DFE) which consists of feed forward (FF equalizer) section 106 and feedback (FB equalizer) section 107 and decision is taken by providing the equalized signal to a detector circuit 108. A calculator circuit 109 is used to determine the optimum weights for the adaptive equalization of received signals. Timing recovery is also achieved which allows receiver clock to be aligned with the transmitter clock variations.
Figure 2 illustrates the block diagram of diversity receiver with optimum combiner for Troposcatter communication according to one embodiment of the present invention.
The figure illustrates a simplified block diagram of an optimal diversity combiner for troposcatter communication which is capable of recovering the data efficiently at the receiver in the presence of fading and noise by eliminating the effects of intersymbol interference (ISI) in accordance with the present invention. Optimal diversity combiner of present invention consists of diversity receiver and the signals received by the diversity branches of the receiver are utilized for diversity combining thereby improving the signal reception at the receiver. Received signals consist of symbols organized into frames. Each frame constitute a PL header (Physical Layer header) followed by blocks of data symbols separated by blocks of pilot symbols.
Each diversity branch consists of down converter 201 which converts RF signals at the diversity receivers to baseband by mixing it with local oscillator signal, analog-to-digital converter (ADC) 202 which digitizes the baseband signal, a timing recovery module 203 coupled to the analog-to-digital converter (ADC), wherein the timing recovery module is used for extracting the sample at the precise sampling time instants from the oversampled received signal, a frame synchronisation module 204 coupled to the timing recovery module, wherein the frame synchronisation is used for estimating the start of data frame (SOF) where signals received by multiple diversity branches of the receiver are organized in frame structure and each frame consists of a header and blocks of data symbols separated by block of pilot symbols, a frame alignment module 205 coupled to the frame synchronisation, wherein the frame alignment is used for aligning the signals from all the diversity branches based on the SOF, carrier frequency offset modules 2101,2102,…210N couple to the frame alignment, wherein the carrier frequency offset module is used for estimating the carrier frequency offset of each of the diversity branches, an equalizer coupled to the carrier frequency offset module, wherein the equalizer is used for combating the effects of intersymbol interference (ISI), which consist of feed forward 206 and feedback 209 sections and a combiner coupled to the equalizer, wherein the combiner is used for effectively combining frequency offset corrected, equalized signal from all the diversity branches so that data can be recovered without any loss.
The Timing Recovery module uses interpolation to estimate the right sample value from the oversampled received signal. The timing recovery (TR) module 203 which estimates the appropriate sample at the precise sampling time instants from the oversampled input that closely represents the transmitted symbol, frame synchronization (FS) module 204 which detect the start of data frames (SOF) with the help of known PL header/pilot symbols. Frame synchronized signals are then provided to the frame alignment module 205, where signals from all the diversity branches are aligned based on SOF. The Frame Alignment module aligns the signals from all the diversity branches based on the SOF without which optimum performance cannot be obtained by the diversity combiner.
Frame aligned signals from the diversity branches are further processed according to the novel method of diversity combining. Frequency offset correction is performed on each diversity branch signal by multiplying with respective frequency offset estimated since each diversity branch will be having different frequency offset. Block diagram includes Carrier Frequency Offset (CFO) Estimator modules, CFO Estimator1 2101, CFO Estimator2 2102,……., CFO Estimator N 210N which estimates frequency offset errors of each diversity channel, N is the number of available diversity channels. The carrier offset correction is performed jointly with equalizer and combiner and applied at the input to the equalizer. The Carrier Frequency Offset module present in each of the multiple diversity branches estimates the carrier offset of the respective diversity branches based on the respective equalizer outputs and header/pilot/decision taken on combiner output depending on whether header/pilot/data is processed.
Frequency offset errors are estimated separately for each diversity channel based on the equalized signal of each diversity branch and the symbol detected (DS) on providing the diversity combiner 211 output to the detector 208. Frequency offset estimated for each diversity branch is represented as CC1, CC2…CCN, N is the number of diversity channels. Equalized signal of each diversity branch is denoted as E1 2071, E2 2072,….EN 207N, N being the number of diversity channels. Frequency corrected signal is equalized with adaptive Decision Feedback Equalizer (DFE) where the weighting coefficients for the equalizer are updated using stochastic gradient descent method. The stochastic gradient estimation method is used for weight estimation of equalizer which estimates error based on the difference between the combiner output and header/pilot/decision taken on combiner output depending on whether header/pilot/data is processed.
DFE consists of feed forward (FFE) 206 and feedback (FBE) 209 sections where the weights are calculated based on the error between the combiner output and header/pilot/decision taken on combiner output depending on whether header/pilot/data is processed. The weights of individual equalizers of each of the multiple diversity branches are updated based on the combiner output rather than individual equalizer outputs. The individual weights of all the equalizers of the multiple diversity branches are updated simultaneously. FFE section consists of a tapped-delay-line filter which acts on the frequency corrected received baseband signal and FBE section consists of a tapped-delay-line filter which acts on the previous detected symbols (DS). FFE section is weighted by coefficients computed by utilizing the error and frequency corrected received baseband signal. FBE section is weighted by coefficients computed by utilizing the error and previous detected symbols. Combined frequency offset correction, equalization and diversity combining as explained above will give good data recovery performance at the receiver.
Figure 3 illustrates the novel method of diversity combining according to one embodiment of the present invention.
The figure illustrates the novel method of optimal diversity combining where frequency offset correction, equalization and diversity combining is performed combinedly to achieve better link availability. The method to enhance the signal reception at the receiver by exploiting multipath faded signals at diversity receivers, the method comprising: converting RF signals at the diversity receivers to baseband using down converter by mixing it with local oscillator signal, digitizing the baseband signal by an analog-to-digital converter (ADC) coupled to the down converter, extracting the sample at the precise sampling time instants from the oversampled received signal by a timing recovery module coupled to the analog-to-digital converter (ADC), estimating the start of data frame (SOF) by frame synchronisation module, where signals received by multiple diversity branches of the receiver are organized in frame structure and each frame consists of a header and blocks of data symbols separated by block of pilot symbols, aligning the signals from all the diversity branches based on the SOF by frame alignment module, estimating the carrier frequency offset of each of the diversity branches by a carrier frequency offset module, combating the effects of intersymbol interference (ISI), which consist of feed forward and feedback sections by an equalizer and combining the frequency offset corrected, equalized signal from all the diversity branches so that data can be recovered without any loss by a combiner.
The algorithm for optimum diversity combining involves frequency offset correction and equalization performed jointly with the combiner. This method makes successive corrections to the weight vector of the equalizer of all the diversity branches simultaneously based on the combiner output rather than individual equalizer outputs.
Novelty of the present invention lies in the manner in which frequency offset correction, equalization and diversity combiner modules are combined. Channel 1, Channel 2……., Channel N represents frequency/spatial diversity channels where the signals received by these channels can be utilized properly by performing diversity combining to retrieve the original transmitted data at the receiver without data loss in a Rayleigh faded environment at low SNR. Signals received at each of the N channels are multiplied 301 respectively with the frequency offsets (CC1, CC2,…..CCN) estimated for each individual channel to compensate the frequency offset errors which vary for different channels. Frequency offset corrected signals are then passed through Decision Feedback Equalizers (DFE) to remove intersymbol interference (ISI). Equalization for each channel is not performed independently since tap weights of feedback (FBE) 306 section of equalizer (DFE) are updated based on the decision taken on the MRC output which is obtained by summing signals from all the diversity channels. Tap weights of feed-forward (FFE) 303 section of each channel are updated based on the received frequency corrected baseband signal. FFE section of equalizer consists of tap weights akj, a(k-1)j,….., a0j multiplied with delayed frequency corrected baseband input Xkj, X(k-1)j,….., X0j where j=1,2,………..N, and N represents the number of diversity channels, k represents the number of delay elements. FBE section of equalizer consists of tap weights bi,……., b1 multiplied with latest ‘i’ decisions/detected symbols made on combiner output 307 represented as DSi,……., DS1 where ‘i’ denotes the number of previous decisions. FBE section output is added with the FFE section of each channel to equalize each channel. These equalized outputs are represented as E1, E2,………., EN for N diversity channels. A Carrier Frequency Offset (CFO) Estimator is associated with each diversity channel to compensate the respective frequency offset errors of each channel. CFO Estimator1 3021 module takes the first channel equalized output E1 and detected symbol DS or known symbols PL header/pilot as inputs and calculates the frequency offset. This estimated frequency offset error is applied to the received baseband signal at the diversity branch which corrects the frequency offset associated with the signal. This frequency offset corrected signal is then provided to the equalizer module. Diversity Combining 304 is then performed on equalized outputs E1,E2, …..,EN by combining them together. Decision is taken on the diversity combiner output by providing it to a detector module 308 and is denoted as DS. Error calculated by taking the difference between diversity combiner output and DS is utilized to estimate the weights for the equalizer. Weight estimation block 305 shown in the fig. takes the instantaneous error as one of the inputs and tap weights akj, a(k-1)j,….., a0j of FFE section of equalizer are updated with the delayed frequency corrected baseband signal as the other input and tap weights bi,……., b1 of FBE section of equalizer are updated with latest ‘i’ detected symbols made on diversity combiner output as the second input. Weights are updated using stochastic gradient descent method. From the above explanation, it is obvious that frequency correction, equalization and diversity combining modules have to operate jointly to achieve the expected target of data retrieval without loss at the receiver.
Advantages of the present invention
The optimal diversity combiner with an appropriate algorithm to recover the original transmitted signal nullifying the effects of channel fading and eventually improving SNR
The combiner performs well for heavily Rayleigh faded multipath channels with low SNR. Further, the combiner provides better SNR, BER (Bit Error Rate) and ISI mitigation performance for the digital troposcatter communication receiver.
The present invention enhances the signal reception at the receiver with an optimal diversity combiner by exploiting multipath faded signals at diversity receivers.
The present invention diversity combining is applicable for space diversity or frequency diversity or combination of these diversity input channels.
The present invention achieves an enhanced data recovery performance at the digital troposcatter communication receiver by mitigating the effects of multipath fading, ISI and noise.
Figures are merely representational and are not drawn to scale. Certain portions thereof may be exaggerated, while others may be minimized. Figures 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. An optimal diversity combiner for a troposcatter communication receiver with multiple diversity branches, the optimal diversity combiner comprising:
a down converter 201 which converts RF signals at the diversity receivers to baseband by mixing it with local oscillator signal;
an analog-to-digital converter (ADC) 202 coupled to the down converter which digitizes the baseband signal;
a timing recovery module 203 coupled to the analog-to-digital converter (ADC), wherein the timing recovery module is used for extracting the sample at the precise sampling time instants from the oversampled received signal;
a frame synchronisation module 204 coupled to the timing recovery module, wherein the frame synchronisation is used for estimating the start of data frame (SOF) where signals received by multiple diversity branches of the receiver are organized in frame structure and each frame consists of a header and blocks of data symbols separated by block of pilot symbols;
a frame alignment module 205 coupled to the frame synchronisation, wherein the frame alignment is used for aligning the signals from all the diversity branches based on the SOF;
a carrier frequency offset module 2101,2102,…210N coupled to the frame alignment, wherein the carrier frequency offset module is used for estimating the carrier frequency offset of each of the diversity branches;
an equalizer coupled to the carrier frequency offset module, wherein the equalizer is used for combating the effects of intersymbol interference (ISI), which consist of feed forward 206 and feedback 209 sections; and
a combiner coupled to the equalizer, wherein the combiner is used for effectively combining frequency offset corrected, equalized signal from all the diversity branches so that data can be recovered without any loss.

2. The optimal diversity combiner as claimed in claim 1, wherein the equalizer comprises of feed forward (FFE) 206 and feedback (FBE) 209 sections where the weights are calculated based on the error between the combiner output 211 and header/pilot/decision taken on combiner output depending on whether header/pilot/data is processed.

3. The optimal diversity combiner as claimed in claim 1, wherein the weights of individual equalizers of each of the multiple diversity branches are updated simultaneously based on the combiner output rather than individual equalizer outputs.

4. The optimal diversity combiner as claimed in claim 1, wherein stochastic gradient estimation method is used for weight estimation of equalizer which estimates error based on the difference between the combiner output and header/pilot/decision taken on combiner output depending on whether header/pilot/data is processed.

5. The optimal diversity combiner as claimed in claim 1, further comprises a carrier offset correction applied at input to the equalizer of each diversity branch which is performed jointly with equalizer and combiner.

6. The optimal diversity combiner as claimed in claim 1, wherein the Carrier Frequency Offset module present in each of the multiple diversity branches estimates the carrier offset of the respective diversity branches based on the respective equalizer outputs and header/pilot/decision taken on combiner output depending on whether header/pilot/data is processed.

7. The optimal diversity combiner as claimed in claim 1, wherein the Timing Recovery module uses interpolation to estimate the right sample value from the oversampled received signal.

8. The optimal diversity combiner as claimed in claim 1, wherein the Frame Synchronization module identifies the start of data frame (SOF) by correlating the received signal with the PL (physical layer) header which helps in frame alignment.

9. The optimal diversity combiner as claimed in claim 1, wherein the Frame Alignment module aligns the signals from all the diversity branches based on the SOF without which optimum performance cannot be obtained by the diversity combiner.

10. A method of diversity combining to enhance the signal reception by exploiting multipath faded signals at diversity receivers, the method comprising:
converting RF signals at the diversity receivers to baseband by down converter mixing it with local oscillator signal;
digitizing the baseband signal by an analog-to-digital converter (ADC) coupled to the down converter;
extracting the sample at the precise sampling time instants from the oversampled received signal by a timing recovery module coupled to the analog-to-digital converter (ADC);
estimating the start of data frame (SOF) by frame synchronisation, where signals received by multiple diversity branches of the receiver are organized in frame structure and each frame consists of a header and blocks of data symbols separated by block of pilot symbols;
aligning the signals from all the diversity branches based on the SOF by frame alignment;
estimating the carrier frequency offset of each of the diversity branches by a carrier frequency offset module;
combating the effects of intersymbol interference (ISI), which consist of feed forward and feedback sections by an equalizer; and
combining the frequency offset corrected, equalized signal from all the diversity branches so that data can be recovered without any loss by a combiner.

11. The optimal diversity combiner as claimed in claim 10, wherein the method of diversity combining is applicable for space diversity or frequency diversity or combination of these diversity input channels.

AN OPTIMAL DIVERSITY COMBINER FOR TROPOSCATTER COMMUNICATION RECEIVER

Abstract

The present invention mainly relates to an optimal diversity combiner for a troposcatter communication receiver which enhances the signal reception at the receiver by exploiting multipath faded signals. The novel method of diversity combining is applicable for space diversity or frequency diversity or combination of these diversity input channels. Signals received by diversity branches of the receiver are uncorrelated most of the time due to the uncorrelated fading experienced at the different diversity channels and this property is used by the optimal diversity combiner with an appropriate algorithm to recover the original transmitted signal nullifying the effects of channel fading and eventually improving SNR. The algorithm for optimal diversity combining involves frequency offset correction and equalization performed jointly with the combiner. This method makes successive corrections to the weight vector of the equalizer of all the diversity branches simultaneously based on the combiner output rather than individual equalizer outputs. Optimal diversity combiner gives better performance at low SNR for heavily Rayleigh faded multipath channels. It achieves an enhanced data recovery performance at the digital troposcatter modem receiver by mitigating the effects of multipath fading, ISI and noise.

Figure 2 (for publication)