Vasu Chakravarthy - Academia.edu (original) (raw)

Papers by Vasu Chakravarthy

IEEE Transactions on Communications, 2001

For orthogonal frequency-division multiplexing (OFDM) communication systems, the frequency offset... more For orthogonal frequency-division multiplexing (OFDM) communication systems, the frequency offsets in mobile radio channels distort the orthogonality between subcarriers resulting in intercarrier interference (ICI). This paper studies an efficient ICI cancellation method termed ICI self-cancellation scheme. The scheme works in two very simple steps. At the transmitter side, one data symbol is modulated onto a group of adjacent subcarriers with a group of weighting coefficients. The weighting coefficients are designed so that the ICI caused by the channel frequency errors can be minimized. At the receiver side, by linearly combining the received signals on these subcarriers with proposed coefficients, the residual ICI contained in the received signals can then be further reduced. The carrier-to-interference power ratio (CIR) can be increased by 15 and 30 dB when the group size is two or three, respectively, for a channel with a constant frequency offset. Although the redundant modulation causes a reduction in bandwidth efficiency, it can be compensated, for example, by using larger signal alphabet sizes. Simulations show that OFDM systems using the proposed ICI self-cancellation scheme perform much better than standard systems while having the same bandwidth efficiency in multipath mobile radio channels with large Doppler frequencies.

2007 International Waveform Diversity and Design Conference, 2007

Abstract Several studies have revealed that spectrum congestion is primarily due to the inefficie... more Abstract Several studies have revealed that spectrum congestion is primarily due to the inefficient use of spectrum versus unavailability. Cognitive radio (CR) and ultra wide band (UWB) technologies have been proposed as candidates to address this problem. Currently, a CR determines unused frequency bands and transmits overlay waveforms in these bands, while UWB transmits underlay waveforms that span the entire frequency band while coexisting with primary users. This suggests that most of the spectrum occupied by ...

Open Architecture/Open Business Model Net-Centric Systems and Defense Transformation 2017, 2017

Accurate classification of phase modulated radar waveforms is a well-known problem in spectrum se... more Accurate classification of phase modulated radar waveforms is a well-known problem in spectrum sensing. Identification of such waveforms aids situational awareness enabling radar and communications spectrum sharing. While various feature extraction and engineering approaches have sought to address this problem, the use of a machine learning algorithm that best utilizes these features is becomes foremost. In this effort, a comparison of a standard shallow and a deep learning approach are explored. Experiments provide insights into classifier architecture, training procedure, and performance.

2016 IEEE Radar Conference (RadarConf), 2016

Accurate classification or recognition of phase modulated radar waveforms, typically accomplished... more Accurate classification or recognition of phase modulated radar waveforms, typically accomplished via the combination of pulse parameter estimates and matched filtering, poses a simple problem in ideal conditions. In less than ideal conditions, carrier frequency, time offset, pulse amplitude, initial phase, and bandwidth are unknown to the electronic warfare (EW) receiver rendering the application of a matched filter futile. Recognition of these waveforms is critical in various spectrum management, surveillance, and EW applications. This effort investigates the use of features extracted from the ambiguity function (AF) of an intercepted pulse. Specifically, this effort will expand upon the methodology of previous work done which uses the autocorrelation as a basis for extracting features. To test the efficacy of this work, extensive Monte Carlo testing employed. Simulation results prove that the methodology implemented herein achieves an overall correct classification rate of about 90% at a signal-to-noise ratio (SNR) of -2 dB on data similar to the training data.

Recent studies have suggested that spectrum congestion is mainly due to the inefficient use of sp... more Recent studies have suggested that spectrum congestion is mainly due to the inefficient use of spectrum rather than its unavailability. Dynamic Spectrum Access (DSA) and Cognitive Radio (CR) are two terminologies which are used in the context of improved spectrum efficiency and usage. The DSA concept has been around for quite some time while the advent of CR has created a paradigm shift in wireless communications and instigated a change in FCC policy towards spectrum regulations. DSA can be broadly categorized as using a 1)Dynamic Exclusive Use Model, 2) Spectrum Commons or Open sharing model or 3) Hierarchical Access model. The hierarchical access model envisions primary licensed bands, to be opened up for secondary users, while inducing a minimum acceptable interference to primary users. Spectrum overlay and spectrum underlay technologies fall within the hierarchical model, and allow primary and secondary users to coexist while improving spectrum efficiency. Spectrum overlay in conjunction with the present CR model considers only the unused (white) spectral regions while in spectrum underlay the underused (gray) spectral regions are utilized. The underlay approach is similar to ultra wide band (UWB) and spread spectrum (SS) techniques utilize much wider spectrum and operate below the noise floor of primary users. Software defined radio (SDR) is considered a key CR enabling technology. Spectrally modulated, spectrally encoded (SMSE) multi-carrier signals such as Orthogonal Frequency Domain Multiplexing (OFDM) and Multi-carrier Code Division Multiple Access(MCCDMA) are hailed as candidate CR waveforms. The SMSE structure supports and is well-suited for SDR based CR applications. This work began by developing a general soft decision (SD) CR framework, based on a previously developed SMSE framework that iii combines benefits of both the overlay and underlay techniques to improve spectrum efficiency and maximizing the channel capacity. The resultant SD-SMSE framework provides a user with considerable flexibility to choose overlay, underlay or hybrid overlay/underlay waveform depending on the scenario, situation or need. Overlay/Underlay SD-SMSE framework flexibility is demonstrated by applying it to a family of SMSE modulated signals such as OFDM, MCCDMA, Carrier Interferometry (CI) MCCDMA and Transform Domain Communication System (TDCS). Based on simulation results, a performance analysis of Overlay, Underlay and hybrid Overlay/Underlay waveforms is presented. Finally, the benefits of combining overlay/underlay techniques to improve spectrum efficiency and maximize channel capacity is addressed. xiii Dedication Meaning: Whatever I do through my body, speech, mind, sense organs, reason, will or natural inclination, I offer it all to the Ultimate Being. This oft-recited Sanskrit verse signifies an ardent attitude of a spiritual seeker for whom all life is or must be pervaded by intense desire to reach the acme of spiritual accomplishment. Material achievement is consummated by acquiring something from without; while spiritual accomplishment is marked by selfless giving of one's whole being, together with all of its activities, to the ultimate spiritual being that sustains the universe and stretches far beyond [16].

EURASIP Journal on Wireless Communications and Networking, 2009

Recommended by Mischa Dohler Cognitive Radio (CR), a hierarchical Dynamic Spectrum Access (DSA) m... more Recommended by Mischa Dohler Cognitive Radio (CR), a hierarchical Dynamic Spectrum Access (DSA) model, has been considered as a strong candidate for future communication systems improving spectrum efficiency utilizing unused spectrum of opportunity. However, to ensure the effectiveness of dynamic spectrum access, accurate signal classification in fading channels at low signal to noise ratio is essential. In this paper, a hierarchical cyclostationary-based classifier is proposed to reliably identify the signal type of a wide range of unknown signals. The proposed system assumes no a priori knowledge of critical signal statistics such as carrier frequency, carrier phase, or symbol rate. The system is designed with a multistage approach to minimize the number of samples required to make a classification decision while simultaneously ensuring the greatest reliability in the current and previous stages. The system performance is demonstrated in a variety of multipath fading channels, where several multiantenna-based combining schemes are implemented to exploit spatial diversity.

EURASIP Journal on Wireless Communications and Networking

2011 IEEE Global Telecommunications Conference - GLOBECOM 2011, 2011

ABSTRACT In this paper, we provide a fundamental channel model to characterize the interference e... more ABSTRACT In this paper, we provide a fundamental channel model to characterize the interference effect inherent in cognitive radio systems. Mutual information rates of our proposed probabilistic block interference channels for both primary and secondary users are derived without assuming that receivers have knowledge on channel interference states. Novel constrained optimization problems are then put forward with a constraint on the performance loss margin tolerated by the primary user. Furthermore, we investigate some special cases where conditions are provided to justify the optimality of adopting Neyman-Pearson rule. Also presented are some scenarios in which randomized decision without using sensing measurement is needed to balance the rateloss for PU and throughput gain for SD.

2008 3rd IEEE Symposium on New Frontiers in Dynamic Spectrum Access Networks, 2008

The under-/un-utilized radio spectrum is an area of emphasis and potential in an effort to increa... more The under-/un-utilized radio spectrum is an area of emphasis and potential in an effort to increase utilization. This paper introduces a new matrix into the frequency evaluation/determination methodology. It will reduce the number of decision and network collisions in a cognitive radio network environment. This is achieved via a new paradigm, data centric prioritization (DCP), the exploitation of the unique

2012 IEEE National Aerospace and Electronics Conference (NAECON), 2012

ABSTRACT Small unmanned aerial vehicles (UAVs) have limits on the weight and size of direction fi... more ABSTRACT Small unmanned aerial vehicles (UAVs) have limits on the weight and size of direction finding (DF) systems that can be used. However, by employing multiple sensors that work cooperatively, simple DF systems need only to provide a rough estimate of the signal's angle of arrival (AOA) and received signal strength (RSS). As the UAVs move, a plurality of data points can be obtained from diverse locations. Combining this information from multiple sensor locations can provide a low-cost and effective solution to electronic emitter location problems, using low-accuracy DF technology. In this paper we present an algorithm, based on the compressed sensing technique, that uses AOA and RSS from mobile sensors (UAVs) to geolocate the source of radio signals on the ground.

2010 International Waveform Diversity and Design Conference, 2010

ABSTRACT This tutorial's main emphasis will be on the physical layer design and implement... more ABSTRACT This tutorial's main emphasis will be on the physical layer design and implementation of cognitive radio and to also provide the audience with an in-depth understanding of cognitive radio and its enabling technologies. Different dynamic spectrum access (DSA) models will be discussed. Next, major components of CR and DSA will be identified. An overview of the candidate Multi-Carrier (MC) waveforms such as Orthogonal Frequency Division Multiplexing (OFDM), MC-Code Division Multiple Access (MC-CDMA), Carrier Interferometry (CI)-MCCDMA, Transform Domain Communication System (TDCS) and their non-contiguous versions will be presented. A general Spectrally Modulated, Spectrally Encoded (SMSE) expression encompassing all the above MC waveforms will be presented. This will be followed by a novel Soft Decision (SD) SMSE framework in the design of two CR enabling waveforms namely, CR-Overlay and CR-Underlay waveforms. Different application of these two CR waveforms will also be examined. Furthermore, hardware implementation of cognitive radio nodes using USRP (Universal Software Radio Peripheral) boards and GNU radio software will be discussed. We will provide hands-on laboratory of a USRP based cognitive radio video transmission in the tutorial as well.

MILCOM 2013 - 2013 IEEE Military Communications Conference, 2013

ABSTRACT Modulation detection is important to many communication and electronic warfare applicati... more ABSTRACT Modulation detection is important to many communication and electronic warfare applications. Recent developments in cognitive radio (CR) and dynamic spectrum access (DSA) network have also brought much attention to modulation detection of unknown radio frequency (RF) signals. It is well known that using second-order cyclostationary features, BPSK modulation can be easily distinguished from higher order modulations such as QPSK and QAM. However, these higher order modulations exhibit similar second-order cyclostationary features, thus theses features cannot be employed to further distinguish higher order modulations. To accurately detect and classify higher order modulations, higher order features such as higher order cumulants are desired. In this paper, we build a automatic blind hierarchical modulation detector to successfully classify the modulations of RF signals. Moreover, we use software defined radio (SDR) to implement and demonstrate a practical blind hierarchical modulation detector which can accurately distinguish among three popular modulations, i.e., BPSK, QPSK and 16-QAM. Specifically, second-order cyclostationary features using detailed spectral coherent function (SOF) are applied first to distinguish between BPSK modulation and non-BPSK modulations (e.g., QPSK and 16-QAM modulations) at first level of the hierarchical modulation detector. Next, the fourth-order cumulant feature is employed to the non-BPSK RF signals to distinguish between QPSK and 16-QAM. The SDR based modulation detector does not require any priori information of the RF transmission, and executes accurate detection in real time. Demonstrations in AWGN channel and realistic multi-path fading channel confirm the effectiveness and efficiency of the proposed modulation detector.

Defense Transformation and Net-Centric Systems 2010, 2010

ABSTRACT In this paper, we demonstrate an adaptive multicarrier multi-function waveform generator... more ABSTRACT In this paper, we demonstrate an adaptive multicarrier multi-function waveform generator for cognitive radio via software defined radio. Using a USRP (universal software radio peripheral) software defined radio boards and GNU radio software, we implement a multi-carrier waveform generator which can generate multi-function waveforms such as OFDM, NC-OFDM, MC-CDMA, NC-MC-CDMA, CI/MC-CDMA, NCCI/ MC-CDMA, TDCS for cognitive radio. Additionally, we demonstrate a portable overlay cognitive radio using this multicarrier multi-function waveform generator. This cognitive radio is capable of detecting primary users in real time and adaptively adjusting its transmission parameters to avoid interference to primary users. More importantly, this cognitive radio can take advantage of multiple spectrum holes by employing non-contiguous multi-carrier transmission technologies. Additionally, we demonstrate that when the primary user transmission changes, the cognitive radio dynamically adjusts its transmission accordingly. We also demonstrate seamless real time video transmission between two cognitive radio nodes, while avoiding interference from primary users and interference to primary users operating in the same spectrum.

2011 IEEE International Symposium on Dynamic Spectrum Access Networks (DySPAN), 2011

ABSTRACT We demonstrate spectrum handoff and spectrum mobility features of cognitive radio using ... more ABSTRACT We demonstrate spectrum handoff and spectrum mobility features of cognitive radio using software defined radio in this demo. Real time video streaming is supported between cognitive radio nodes in dynamic environment. When available spectrum becomes insufficient, the cognitive radio nodes move to a different band and maintain seamless transition when the spectrum handoff occurs.

2010 7th IEEE Consumer Communications and Networking Conference, 2010

For orthogonal frequency division multiplexing (OFDM) communication systems, the orthogonality am... more For orthogonal frequency division multiplexing (OFDM) communication systems, the orthogonality among subcarriers is lost in mobile radio channels due to the frequency offsets caused by mobility. As a direct result, intercarrier interference (ICI) is observed on each and every subcarrier, leading to significant performance degradation. Many ICI cancellation methods such as windowing and frequency domain coding have been proposed in the literature to cancel ICI and improve the BER performance of OFDM in mobile channel. However, the performance improvement achieved by all the existing ICI cancellation methods is far from enough: the BER performance after ICI cancellation is still much worse than the BER performance of original OFDM without ICI. Moreover, popular ICI cancellation methods like ICI self-cancellation reduce ICI at the price of lowering the transmission rate and reducing the bandwidth efficiency. Other frequency-domain coding methods do not reduce the data rate, but produce less reduction in ICI as well. In this paper, we propose a novel ICI cancellation scheme which can eliminate the ICI entirely and offer a OFDM mobile system with the same BER performance of a OFDM system without ICI. More importantly, the proposed ICI cancellation scheme, namely Total ICI Cancellation, does not lower the transmission rate or reduce the bandwidth efficiency. Specifically, the Total ICI Cancellation scheme takes advantage of the orthogonality of the ICI matrix and offers perfect ICI cancellation and significant BER improvement at linearly growing cost. Simulation results in AWGN channel and multi-path fading channel confirm the superb performance of the proposed Total ICI Cancellation scheme in the presence of frequency offset or time variations in the channel, outperforming all the existing ICI cancellation methods.

2011 IEEE International Conference on Communications (ICC), 2011

Cognitive radio systems allow secondary users to operate on underutilized licensed spectrum. When... more Cognitive radio systems allow secondary users to operate on underutilized licensed spectrum. When considering highly congested communication channels, however, opportunities for channel access based on time or frequency division can be limited for secondary user networks. In this research, we consider leveraging enhanced spatial diversity through directional steerable antennas to allow secondary user channel access in parallel with licensed spectrum users. Furthermore, we consider effects of mobility on directional secondary user networks and introduce a mechanism for maintaining point-to-point directional communication links in the presence of mobility. We study the trade-offs between spatial diversity and coordination overhead to motivate the use of directional antennas, even in highly mobile cognitive radio networks.

2014 IEEE International Symposium on Dynamic Spectrum Access Networks (DYSPAN), 2014

We present a description of contemporary performance evaluation approaches for spectrum sharing t... more We present a description of contemporary performance evaluation approaches for spectrum sharing technologies (including dynamic spectrum access (DSA)/cognitive radios (CR)), and the perspectives of stakeholders with respect to shortcomings and resultant delayed field transition impacts. The shortcomings are examined to generate a set of desirable new attributes for more capable test and evaluation (T&E) systems and methods. Ongoing research and development (R&D) using the DYnamic Spectrum Environment emulator (DYSE) aimed at addressing these DSA/CR T&E shortcomings is described including a radar-Long Term Evolution (LTE) network sharing example, a TV whitespace sharing example using 802.22, and a position/navigation/timing (PNT) example with shared RF sources for communications and PNT.

Lecture Notes of the Institute for Computer Sciences, Social Informatics and Telecommunications Engineering, 2012

Primary user emulation attack, which targets distabilizing the queuing dynamics of cognitive radi... more Primary user emulation attack, which targets distabilizing the queuing dynamics of cognitive radio networks, is studied using game theoretic argument. The attack and defense are modeled as a stochastic game. The Nash equilibrium of the game is studied. In particular, the Lyapunov drift is considered as the reward in each round. Explicit expressions of the Nash equilibrium strategies are obtained.

2012 IEEE International Symposium on Dynamic Spectrum Access Networks, 2012

In cognitive radio systems, the available frequency bands are usually non-contiguous, which may c... more In cognitive radio systems, the available frequency bands are usually non-contiguous, which may cause impact on the spectrum utilization efficiency (SUE). When the receiver has a limited sampling capability, the aliasing phenomenon may cause interference among the signals at different frequency points, thus causing the loss of channel capacity. Information theoretic analysis is carried out to quantify the impact of spectrum non-contiguity on the channel capacity by analyzing the SUE. Beginning from the single sampler case, the analysis is also extended to the general case of sampler arrays. Numerical results are provided to answer several important questions on the impact of spectrum non-contiguity. 1 Here we consider asymmetric frequency spectrum; i.e., the positive spectrum and negative spectrum are not necessarily identical.

2010 International Waveform Diversity and Design Conference, 2010

Previous communications systems research has demonstrated that the Spectrally Modulated, Spectral... more Previous communications systems research has demonstrated that the Spectrally Modulated, Spectrally Encoded (SMSE) framework is well suited to SDR platforms and operation in both contiguous and non-contiguous spectrum given its ability to generate a wide variety of multicarrier waveforms such as OFDM, NC-OFDM, MC-CDMA, NC-MC-CDMA, CI/MC-CDMA, NCCI/MC-CDMA, and TDCS. In this paper, SMSE waveforms are extended for use in sparse frequency radar systems. Sparse frequency radar can be capable of dynamically sensing available spectrum, and tailoring the transmitted waveforms to suit the instantaneous RF environment conditions, simultaneously transmitting energy in multiple noncontiguous spectrum bands. Conventional radar performance metrics are reviewed, and a new metric is introduced which specifically addresses performance of noncontiguous spectrum waveforms through comparison to contiguous spectrum waveforms. Through simulation, various examples are shown to illustrate and characterize the impairments that are introduced in processing these classes of non-contiguous spectrum waveform returns, specifically the generation of large range sidelobes. I.

IEEE Transactions on Communications, 2001

For orthogonal frequency-division multiplexing (OFDM) communication systems, the frequency offset... more For orthogonal frequency-division multiplexing (OFDM) communication systems, the frequency offsets in mobile radio channels distort the orthogonality between subcarriers resulting in intercarrier interference (ICI). This paper studies an efficient ICI cancellation method termed ICI self-cancellation scheme. The scheme works in two very simple steps. At the transmitter side, one data symbol is modulated onto a group of adjacent subcarriers with a group of weighting coefficients. The weighting coefficients are designed so that the ICI caused by the channel frequency errors can be minimized. At the receiver side, by linearly combining the received signals on these subcarriers with proposed coefficients, the residual ICI contained in the received signals can then be further reduced. The carrier-to-interference power ratio (CIR) can be increased by 15 and 30 dB when the group size is two or three, respectively, for a channel with a constant frequency offset. Although the redundant modulation causes a reduction in bandwidth efficiency, it can be compensated, for example, by using larger signal alphabet sizes. Simulations show that OFDM systems using the proposed ICI self-cancellation scheme perform much better than standard systems while having the same bandwidth efficiency in multipath mobile radio channels with large Doppler frequencies.

2007 International Waveform Diversity and Design Conference, 2007

Abstract Several studies have revealed that spectrum congestion is primarily due to the inefficie... more Abstract Several studies have revealed that spectrum congestion is primarily due to the inefficient use of spectrum versus unavailability. Cognitive radio (CR) and ultra wide band (UWB) technologies have been proposed as candidates to address this problem. Currently, a CR determines unused frequency bands and transmits overlay waveforms in these bands, while UWB transmits underlay waveforms that span the entire frequency band while coexisting with primary users. This suggests that most of the spectrum occupied by ...

Open Architecture/Open Business Model Net-Centric Systems and Defense Transformation 2017, 2017

Accurate classification of phase modulated radar waveforms is a well-known problem in spectrum se... more Accurate classification of phase modulated radar waveforms is a well-known problem in spectrum sensing. Identification of such waveforms aids situational awareness enabling radar and communications spectrum sharing. While various feature extraction and engineering approaches have sought to address this problem, the use of a machine learning algorithm that best utilizes these features is becomes foremost. In this effort, a comparison of a standard shallow and a deep learning approach are explored. Experiments provide insights into classifier architecture, training procedure, and performance.

2016 IEEE Radar Conference (RadarConf), 2016

Accurate classification or recognition of phase modulated radar waveforms, typically accomplished... more Accurate classification or recognition of phase modulated radar waveforms, typically accomplished via the combination of pulse parameter estimates and matched filtering, poses a simple problem in ideal conditions. In less than ideal conditions, carrier frequency, time offset, pulse amplitude, initial phase, and bandwidth are unknown to the electronic warfare (EW) receiver rendering the application of a matched filter futile. Recognition of these waveforms is critical in various spectrum management, surveillance, and EW applications. This effort investigates the use of features extracted from the ambiguity function (AF) of an intercepted pulse. Specifically, this effort will expand upon the methodology of previous work done which uses the autocorrelation as a basis for extracting features. To test the efficacy of this work, extensive Monte Carlo testing employed. Simulation results prove that the methodology implemented herein achieves an overall correct classification rate of about 90% at a signal-to-noise ratio (SNR) of -2 dB on data similar to the training data.

Recent studies have suggested that spectrum congestion is mainly due to the inefficient use of sp... more Recent studies have suggested that spectrum congestion is mainly due to the inefficient use of spectrum rather than its unavailability. Dynamic Spectrum Access (DSA) and Cognitive Radio (CR) are two terminologies which are used in the context of improved spectrum efficiency and usage. The DSA concept has been around for quite some time while the advent of CR has created a paradigm shift in wireless communications and instigated a change in FCC policy towards spectrum regulations. DSA can be broadly categorized as using a 1)Dynamic Exclusive Use Model, 2) Spectrum Commons or Open sharing model or 3) Hierarchical Access model. The hierarchical access model envisions primary licensed bands, to be opened up for secondary users, while inducing a minimum acceptable interference to primary users. Spectrum overlay and spectrum underlay technologies fall within the hierarchical model, and allow primary and secondary users to coexist while improving spectrum efficiency. Spectrum overlay in conjunction with the present CR model considers only the unused (white) spectral regions while in spectrum underlay the underused (gray) spectral regions are utilized. The underlay approach is similar to ultra wide band (UWB) and spread spectrum (SS) techniques utilize much wider spectrum and operate below the noise floor of primary users. Software defined radio (SDR) is considered a key CR enabling technology. Spectrally modulated, spectrally encoded (SMSE) multi-carrier signals such as Orthogonal Frequency Domain Multiplexing (OFDM) and Multi-carrier Code Division Multiple Access(MCCDMA) are hailed as candidate CR waveforms. The SMSE structure supports and is well-suited for SDR based CR applications. This work began by developing a general soft decision (SD) CR framework, based on a previously developed SMSE framework that iii combines benefits of both the overlay and underlay techniques to improve spectrum efficiency and maximizing the channel capacity. The resultant SD-SMSE framework provides a user with considerable flexibility to choose overlay, underlay or hybrid overlay/underlay waveform depending on the scenario, situation or need. Overlay/Underlay SD-SMSE framework flexibility is demonstrated by applying it to a family of SMSE modulated signals such as OFDM, MCCDMA, Carrier Interferometry (CI) MCCDMA and Transform Domain Communication System (TDCS). Based on simulation results, a performance analysis of Overlay, Underlay and hybrid Overlay/Underlay waveforms is presented. Finally, the benefits of combining overlay/underlay techniques to improve spectrum efficiency and maximize channel capacity is addressed. xiii Dedication Meaning: Whatever I do through my body, speech, mind, sense organs, reason, will or natural inclination, I offer it all to the Ultimate Being. This oft-recited Sanskrit verse signifies an ardent attitude of a spiritual seeker for whom all life is or must be pervaded by intense desire to reach the acme of spiritual accomplishment. Material achievement is consummated by acquiring something from without; while spiritual accomplishment is marked by selfless giving of one's whole being, together with all of its activities, to the ultimate spiritual being that sustains the universe and stretches far beyond [16].

EURASIP Journal on Wireless Communications and Networking, 2009

Recommended by Mischa Dohler Cognitive Radio (CR), a hierarchical Dynamic Spectrum Access (DSA) m... more Recommended by Mischa Dohler Cognitive Radio (CR), a hierarchical Dynamic Spectrum Access (DSA) model, has been considered as a strong candidate for future communication systems improving spectrum efficiency utilizing unused spectrum of opportunity. However, to ensure the effectiveness of dynamic spectrum access, accurate signal classification in fading channels at low signal to noise ratio is essential. In this paper, a hierarchical cyclostationary-based classifier is proposed to reliably identify the signal type of a wide range of unknown signals. The proposed system assumes no a priori knowledge of critical signal statistics such as carrier frequency, carrier phase, or symbol rate. The system is designed with a multistage approach to minimize the number of samples required to make a classification decision while simultaneously ensuring the greatest reliability in the current and previous stages. The system performance is demonstrated in a variety of multipath fading channels, where several multiantenna-based combining schemes are implemented to exploit spatial diversity.

EURASIP Journal on Wireless Communications and Networking

2011 IEEE Global Telecommunications Conference - GLOBECOM 2011, 2011

ABSTRACT In this paper, we provide a fundamental channel model to characterize the interference e... more ABSTRACT In this paper, we provide a fundamental channel model to characterize the interference effect inherent in cognitive radio systems. Mutual information rates of our proposed probabilistic block interference channels for both primary and secondary users are derived without assuming that receivers have knowledge on channel interference states. Novel constrained optimization problems are then put forward with a constraint on the performance loss margin tolerated by the primary user. Furthermore, we investigate some special cases where conditions are provided to justify the optimality of adopting Neyman-Pearson rule. Also presented are some scenarios in which randomized decision without using sensing measurement is needed to balance the rateloss for PU and throughput gain for SD.

2008 3rd IEEE Symposium on New Frontiers in Dynamic Spectrum Access Networks, 2008

The under-/un-utilized radio spectrum is an area of emphasis and potential in an effort to increa... more The under-/un-utilized radio spectrum is an area of emphasis and potential in an effort to increase utilization. This paper introduces a new matrix into the frequency evaluation/determination methodology. It will reduce the number of decision and network collisions in a cognitive radio network environment. This is achieved via a new paradigm, data centric prioritization (DCP), the exploitation of the unique

2012 IEEE National Aerospace and Electronics Conference (NAECON), 2012

ABSTRACT Small unmanned aerial vehicles (UAVs) have limits on the weight and size of direction fi... more ABSTRACT Small unmanned aerial vehicles (UAVs) have limits on the weight and size of direction finding (DF) systems that can be used. However, by employing multiple sensors that work cooperatively, simple DF systems need only to provide a rough estimate of the signal's angle of arrival (AOA) and received signal strength (RSS). As the UAVs move, a plurality of data points can be obtained from diverse locations. Combining this information from multiple sensor locations can provide a low-cost and effective solution to electronic emitter location problems, using low-accuracy DF technology. In this paper we present an algorithm, based on the compressed sensing technique, that uses AOA and RSS from mobile sensors (UAVs) to geolocate the source of radio signals on the ground.

2010 International Waveform Diversity and Design Conference, 2010

ABSTRACT This tutorial's main emphasis will be on the physical layer design and implement... more ABSTRACT This tutorial's main emphasis will be on the physical layer design and implementation of cognitive radio and to also provide the audience with an in-depth understanding of cognitive radio and its enabling technologies. Different dynamic spectrum access (DSA) models will be discussed. Next, major components of CR and DSA will be identified. An overview of the candidate Multi-Carrier (MC) waveforms such as Orthogonal Frequency Division Multiplexing (OFDM), MC-Code Division Multiple Access (MC-CDMA), Carrier Interferometry (CI)-MCCDMA, Transform Domain Communication System (TDCS) and their non-contiguous versions will be presented. A general Spectrally Modulated, Spectrally Encoded (SMSE) expression encompassing all the above MC waveforms will be presented. This will be followed by a novel Soft Decision (SD) SMSE framework in the design of two CR enabling waveforms namely, CR-Overlay and CR-Underlay waveforms. Different application of these two CR waveforms will also be examined. Furthermore, hardware implementation of cognitive radio nodes using USRP (Universal Software Radio Peripheral) boards and GNU radio software will be discussed. We will provide hands-on laboratory of a USRP based cognitive radio video transmission in the tutorial as well.

MILCOM 2013 - 2013 IEEE Military Communications Conference, 2013

ABSTRACT Modulation detection is important to many communication and electronic warfare applicati... more ABSTRACT Modulation detection is important to many communication and electronic warfare applications. Recent developments in cognitive radio (CR) and dynamic spectrum access (DSA) network have also brought much attention to modulation detection of unknown radio frequency (RF) signals. It is well known that using second-order cyclostationary features, BPSK modulation can be easily distinguished from higher order modulations such as QPSK and QAM. However, these higher order modulations exhibit similar second-order cyclostationary features, thus theses features cannot be employed to further distinguish higher order modulations. To accurately detect and classify higher order modulations, higher order features such as higher order cumulants are desired. In this paper, we build a automatic blind hierarchical modulation detector to successfully classify the modulations of RF signals. Moreover, we use software defined radio (SDR) to implement and demonstrate a practical blind hierarchical modulation detector which can accurately distinguish among three popular modulations, i.e., BPSK, QPSK and 16-QAM. Specifically, second-order cyclostationary features using detailed spectral coherent function (SOF) are applied first to distinguish between BPSK modulation and non-BPSK modulations (e.g., QPSK and 16-QAM modulations) at first level of the hierarchical modulation detector. Next, the fourth-order cumulant feature is employed to the non-BPSK RF signals to distinguish between QPSK and 16-QAM. The SDR based modulation detector does not require any priori information of the RF transmission, and executes accurate detection in real time. Demonstrations in AWGN channel and realistic multi-path fading channel confirm the effectiveness and efficiency of the proposed modulation detector.

Defense Transformation and Net-Centric Systems 2010, 2010

ABSTRACT In this paper, we demonstrate an adaptive multicarrier multi-function waveform generator... more ABSTRACT In this paper, we demonstrate an adaptive multicarrier multi-function waveform generator for cognitive radio via software defined radio. Using a USRP (universal software radio peripheral) software defined radio boards and GNU radio software, we implement a multi-carrier waveform generator which can generate multi-function waveforms such as OFDM, NC-OFDM, MC-CDMA, NC-MC-CDMA, CI/MC-CDMA, NCCI/ MC-CDMA, TDCS for cognitive radio. Additionally, we demonstrate a portable overlay cognitive radio using this multicarrier multi-function waveform generator. This cognitive radio is capable of detecting primary users in real time and adaptively adjusting its transmission parameters to avoid interference to primary users. More importantly, this cognitive radio can take advantage of multiple spectrum holes by employing non-contiguous multi-carrier transmission technologies. Additionally, we demonstrate that when the primary user transmission changes, the cognitive radio dynamically adjusts its transmission accordingly. We also demonstrate seamless real time video transmission between two cognitive radio nodes, while avoiding interference from primary users and interference to primary users operating in the same spectrum.

2011 IEEE International Symposium on Dynamic Spectrum Access Networks (DySPAN), 2011

ABSTRACT We demonstrate spectrum handoff and spectrum mobility features of cognitive radio using ... more ABSTRACT We demonstrate spectrum handoff and spectrum mobility features of cognitive radio using software defined radio in this demo. Real time video streaming is supported between cognitive radio nodes in dynamic environment. When available spectrum becomes insufficient, the cognitive radio nodes move to a different band and maintain seamless transition when the spectrum handoff occurs.

2010 7th IEEE Consumer Communications and Networking Conference, 2010

For orthogonal frequency division multiplexing (OFDM) communication systems, the orthogonality am... more For orthogonal frequency division multiplexing (OFDM) communication systems, the orthogonality among subcarriers is lost in mobile radio channels due to the frequency offsets caused by mobility. As a direct result, intercarrier interference (ICI) is observed on each and every subcarrier, leading to significant performance degradation. Many ICI cancellation methods such as windowing and frequency domain coding have been proposed in the literature to cancel ICI and improve the BER performance of OFDM in mobile channel. However, the performance improvement achieved by all the existing ICI cancellation methods is far from enough: the BER performance after ICI cancellation is still much worse than the BER performance of original OFDM without ICI. Moreover, popular ICI cancellation methods like ICI self-cancellation reduce ICI at the price of lowering the transmission rate and reducing the bandwidth efficiency. Other frequency-domain coding methods do not reduce the data rate, but produce less reduction in ICI as well. In this paper, we propose a novel ICI cancellation scheme which can eliminate the ICI entirely and offer a OFDM mobile system with the same BER performance of a OFDM system without ICI. More importantly, the proposed ICI cancellation scheme, namely Total ICI Cancellation, does not lower the transmission rate or reduce the bandwidth efficiency. Specifically, the Total ICI Cancellation scheme takes advantage of the orthogonality of the ICI matrix and offers perfect ICI cancellation and significant BER improvement at linearly growing cost. Simulation results in AWGN channel and multi-path fading channel confirm the superb performance of the proposed Total ICI Cancellation scheme in the presence of frequency offset or time variations in the channel, outperforming all the existing ICI cancellation methods.

2011 IEEE International Conference on Communications (ICC), 2011

Cognitive radio systems allow secondary users to operate on underutilized licensed spectrum. When... more Cognitive radio systems allow secondary users to operate on underutilized licensed spectrum. When considering highly congested communication channels, however, opportunities for channel access based on time or frequency division can be limited for secondary user networks. In this research, we consider leveraging enhanced spatial diversity through directional steerable antennas to allow secondary user channel access in parallel with licensed spectrum users. Furthermore, we consider effects of mobility on directional secondary user networks and introduce a mechanism for maintaining point-to-point directional communication links in the presence of mobility. We study the trade-offs between spatial diversity and coordination overhead to motivate the use of directional antennas, even in highly mobile cognitive radio networks.

2014 IEEE International Symposium on Dynamic Spectrum Access Networks (DYSPAN), 2014

We present a description of contemporary performance evaluation approaches for spectrum sharing t... more We present a description of contemporary performance evaluation approaches for spectrum sharing technologies (including dynamic spectrum access (DSA)/cognitive radios (CR)), and the perspectives of stakeholders with respect to shortcomings and resultant delayed field transition impacts. The shortcomings are examined to generate a set of desirable new attributes for more capable test and evaluation (T&E) systems and methods. Ongoing research and development (R&D) using the DYnamic Spectrum Environment emulator (DYSE) aimed at addressing these DSA/CR T&E shortcomings is described including a radar-Long Term Evolution (LTE) network sharing example, a TV whitespace sharing example using 802.22, and a position/navigation/timing (PNT) example with shared RF sources for communications and PNT.

Lecture Notes of the Institute for Computer Sciences, Social Informatics and Telecommunications Engineering, 2012

Primary user emulation attack, which targets distabilizing the queuing dynamics of cognitive radi... more Primary user emulation attack, which targets distabilizing the queuing dynamics of cognitive radio networks, is studied using game theoretic argument. The attack and defense are modeled as a stochastic game. The Nash equilibrium of the game is studied. In particular, the Lyapunov drift is considered as the reward in each round. Explicit expressions of the Nash equilibrium strategies are obtained.

2012 IEEE International Symposium on Dynamic Spectrum Access Networks, 2012

In cognitive radio systems, the available frequency bands are usually non-contiguous, which may c... more In cognitive radio systems, the available frequency bands are usually non-contiguous, which may cause impact on the spectrum utilization efficiency (SUE). When the receiver has a limited sampling capability, the aliasing phenomenon may cause interference among the signals at different frequency points, thus causing the loss of channel capacity. Information theoretic analysis is carried out to quantify the impact of spectrum non-contiguity on the channel capacity by analyzing the SUE. Beginning from the single sampler case, the analysis is also extended to the general case of sampler arrays. Numerical results are provided to answer several important questions on the impact of spectrum non-contiguity. 1 Here we consider asymmetric frequency spectrum; i.e., the positive spectrum and negative spectrum are not necessarily identical.

2010 International Waveform Diversity and Design Conference, 2010

Previous communications systems research has demonstrated that the Spectrally Modulated, Spectral... more Previous communications systems research has demonstrated that the Spectrally Modulated, Spectrally Encoded (SMSE) framework is well suited to SDR platforms and operation in both contiguous and non-contiguous spectrum given its ability to generate a wide variety of multicarrier waveforms such as OFDM, NC-OFDM, MC-CDMA, NC-MC-CDMA, CI/MC-CDMA, NCCI/MC-CDMA, and TDCS. In this paper, SMSE waveforms are extended for use in sparse frequency radar systems. Sparse frequency radar can be capable of dynamically sensing available spectrum, and tailoring the transmitted waveforms to suit the instantaneous RF environment conditions, simultaneously transmitting energy in multiple noncontiguous spectrum bands. Conventional radar performance metrics are reviewed, and a new metric is introduced which specifically addresses performance of noncontiguous spectrum waveforms through comparison to contiguous spectrum waveforms. Through simulation, various examples are shown to illustrate and characterize the impairments that are introduced in processing these classes of non-contiguous spectrum waveform returns, specifically the generation of large range sidelobes. I.