On detection of OFDM signals for cognitive radio applications

As the requirement for wireless telecommunications services continues to grow, it has become increasingly important to ensure that the Radio Frequency (RF) spectrum is managed efficiently. As a result of the current spectrum allocation policy, it has been found that portions of RF spectrum belonging to licensed users are often severely underutilised, at particular times and geographical locations. Awareness of this problem has led to the development of Dynamic Spectrum Access (DSA) and Cognitive Radio (CR) as possible solutions. In one variation of the shared-use model for DSA, it is proposed that the inefficient use of licensed spectrum could be overcome by enabling unlicensed users to opportunistically access the spectrum when the licensed user is not transmitting. In order for an unlicensed device to make decisions, it must be aware of its own RF environment and, therefore, it has been proposed that DSA could been abled using CR. One approach that has be identified to allow the CR to gain information about its operating environment is spectrum sensing. An interesting solution that has been identified for spectrum sensing is cyclostationary detection. This property refers to the inherent periodic nature of the second order statistics of many communications signals. One of the most common modulation formats in use today is Orthogonal Frequency Division Multiplexing (OFDM), which exhibits cyclostationarity due to the addition of a Cyclic Prefix (CP). This thesis examines several statistical tests for cyclostationarity in OFDM signals that may be used for spectrum sensing in DSA and CR. In particular, focus is placed on statistical tests that rely on estimation of the Cyclic Autocorrelation Function (CAF). Based on splitting the CAF into two complex component functions, several new statistical tests are introduced and are shown to lead to an improvement in detection performance when compared to the existing algorithms. The performance of each new algorithm is assessed in Additive White Gaussian Noise (AWGN), impulsive noise and when subjected to impairments such as multipath fading and Carrier Frequency Offset (CFO). Finally, each algorithm is targeted for Field Programmable Gate Array (FPGA) implementation using a Xilinx 7 series device. In order to keep resource costs to a minimum, it is suggested that the new algorithms are implemented on the FPGA using hardware sharing, and a simple mathematical re-arrangement of certain tests statistics is proposed to circumvent a costly division operation.As the requirement for wireless telecommunications services continues to grow, it has become increasingly important to ensure that the Radio Frequency (RF) spectrum is managed efficiently. As a result of the current spectrum allocation policy, it has been found that portions of RF spectrum belonging to licensed users are often severely underutilised, at particular times and geographical locations. Awareness of this problem has led to the development of Dynamic Spectrum Access (DSA) and Cognitive Radio (CR) as possible solutions. In one variation of the shared-use model for DSA, it is proposed that the inefficient use of licensed spectrum could be overcome by enabling unlicensed users to opportunistically access the spectrum when the licensed user is not transmitting. In order for an unlicensed device to make decisions, it must be aware of its own RF environment and, therefore, it has been proposed that DSA could been abled using CR. One approach that has be identified to allow the CR to gain information about its operating environment is spectrum sensing. An interesting solution that has been identified for spectrum sensing is cyclostationary detection. This property refers to the inherent periodic nature of the second order statistics of many communications signals. One of the most common modulation formats in use today is Orthogonal Frequency Division Multiplexing (OFDM), which exhibits cyclostationarity due to the addition of a Cyclic Prefix (CP). This thesis examines several statistical tests for cyclostationarity in OFDM signals that may be used for spectrum sensing in DSA and CR. In particular, focus is placed on statistical tests that rely on estimation of the Cyclic Autocorrelation Function (CAF). Based on splitting the CAF into two complex component functions, several new statistical tests are introduced and are shown to lead to an improvement in detection performance when compared to the existing algorithms. The performance of each new algorithm is assessed in Additive White Gaussian Noise (AWGN), impulsive noise and when subjected to impairments such as multipath fading and Carrier Frequency Offset (CFO). Finally, each algorithm is targeted for Field Programmable Gate Array (FPGA) implementation using a Xilinx 7 series device. In order to keep resource costs to a minimum, it is suggested that the new algorithms are implemented on the FPGA using hardware sharing, and a simple mathematical re-arrangement of certain tests statistics is proposed to circumvent a costly division operation

FPGA implementation of a cyclostationary detector for OFDM signals

Due to the ubiquity of Orthogonal Frequency Division Multiplexing (OFDM) based communications standards such as IEEE 802.11 a/g/n and 3GPP Long Term Evolution (LTE), a growing interest has developed in techniques for reliably detecting the presence of these signals in dynamic radio systems. A popular approach for detection is to exploit the cyclostationary nature of OFDM communications signals. In this paper, we focus on a frequency domain cyclostationary detection algorithm first introduced by Giannakis and Dandawate and study its performance in detecting IEEE 802.11a OFDM signals in the presence of practical radio impairments such as Carrier Frequency offset (CFO), Phase Noise, I/Q Imbalance, Multipath Fading and DC offset. We then present a hardware implementation of this algorithm developed using MathWorks HDL Coder and provide implementation results after targeting to a Xilinx 7 Series FPGA device

Algorithmic Framework and Implementation of Spectrum Holes Detection for Cognitive Radios

The ability to dynamically discover portions of unused radio spectrum (spectrum holes) is an important ability of cognitive radio systems. Spectrum holes present a potential opportunity for wireless communication. Detection of holes and signals allows cognitive radios to dynamically access and share the spectrum with minimal interference. This work steps through the design, implementation, and analysis of a spectrum holes detector for cognitive radios. Energy detection and cyclostationary detection algorithms for detecting spectrum holes are compared through computer simulations. Ultimately an energy detection algorithm is proposed which performs better than the cyclostationary detection algorithm and requires no a-priori knowledge of noise power. The energy detection algorithm is implemented on the bladeRF x115 software-defined radio for wideband detection, leveraging on-board FPGA hardware and field-programmable analog hardware to scan a gigahertz-order range of frequencies and discover spectrum holes in real time. Resource utilization and requirements of the implementation are analyzed, and a utilization of 8.8% of the FPGA\u27s logic resources is reported. Experiments are performed on the implementation to measure its detection performance and demonstrate its ability to detect holes over a wide bandwidth with reasonable latency

Spectrum handoff strategy for cognitive radio-based Mac in industrial wirless sensor and actuator networks

In this thesis, a Cognitive Radio(CR)-based MAC for Industrial Wireless Sensor and Actuator Network (IWSAN) applications is proposed. IWSANs are typically used for closed-loop control applications, and they demand strict requirements in terms of time and robustness. Low latency and low error rates are required in order not to endanger persons or machinery. Moreover, these applications operate in industrial environments such as factories or transport scenarios (as aeronautics or railway) where multipath fading and shadowing are present due to metal surfaces. Furthermore, interference from other communication systems or industrial machinery is also common in these environments. The proposed MAC, based on the CR paradigm, is capable of ensuring time and robustness requirements in industrial channels. In the process of designing the CR-based MAC for IWSAN applications, a comparison between several non-CR-based MACs and CR-based MACs has been carried out. This comparison, which allows stating the benefits of CR for these applications, is presented in this thesis. The performance of every MAC is determined theoretically using Network Calculus, and it is validated through OPNET simulations. CR solutions, due to their adaptability characteristics, are capable of avoiding interference and ensuring robustness in industrial environments. However, none of the selected MACs are capable of ensuring robustness without comprising time requirements, since interference is avoided but not in a bounded time. On the other hand, the MAC proposed in this thesis is capable of avoiding interference ensuring time and robustness requirements at the same time. This MAC is therefore suitable for IWSAN applications. To ensure a deterministic behavior against interference, a novel handoff algorithm, which detects interference and hops to another channel, has been proposed. This algorithm has been designed to be used jointly with one of the evaluated MACs. The detection of the interference and the hop to another channel is done in a bounded time, because the proposed algorithm detects interference while the system is transmitting. The performance of this proposal is evaluated using Network Calculus and OPNET simulations, and the results are compared with the system without the proposed handoff algorithm. The comparison of the results shows how the evaluated MAC is only capable of ensuring both time and robustness requirements when the proposed handoff strategy is used. Moreover, the spectrum sensing algorithm used to obtain information about the environment is delved and its performance is measured through MATLAB simulations. An energy detector has been chosen due to its simplicity. Also, a cyclostationary Modulation Classifier is presented and a simplification has been carried out allowing its implementation on real hardware. The Modulation Classifier is capable of distinguishing between OFDM, QPSK and GFSK signals. The performance of the algorithm is presented in this thesis for different signals and for different receiver impairments such as frequency offset, DC offset and I/Q imbalance. Finally, a cognitive platform to validate the spectrum sensing algorithms is presented. This platform has been designed using a Xilinx Virtex 6 FPGA by a working group composed of researchers from IK4-Ikerlan and Mondragon Unibertsitatea. The platform, which uses both spectrum sensing algorithms, is an Ethernet-to-RF bridge. It has been designed to replace an Ethernet wired link by a wireless one for IWSAN applications. The proposed platform ensures a reliable communication link against interference. In the proposed implementation, the energy detector is used by the transmitter in order to find a free channel to transmit data, whereas the modulation classifier is used by the receiver in order to distinguish between the signal transmitted by the RF-Ethernet bridge and other signals. In this way the receiver can find the channel where the transmitter is carrying out the communication.En esta tesis se propone una MAC basada en el paradigma de la Radio Cognitiva (RC) para redes de sensores y actuadores inalámbricos industriales. Estas redes se suelen utilizar para aplicaciones de control en lazo cerrado, que exigen requisitos estrictos de tiempo y robustez. Para no poner en peligro la salud de las personas o la maquinaria es necesario que la red asegure una baja latencia y una tasa baja de errores. Además, al trabajar en ambientes industriales como fábricas o transportes (trenes, aviones, etc.), estas redes tienen que hacer frente a canales con mucho desvanecimiento por multitrayecto y efecto sombra debido a las superficies metálicas. También es común en estos entornos que haya interferencias de otros sistemas de comunicaciones o de la propia maquinaria industrial. La MAC propuesta en esta tesis es capaz de asegurar los requisitos temporales y de robustez demandados trabajando en este tipo de entornos. En el proceso de diseño de la MAC basada en RC para redes de sensores y actuadores inalámbricos industriales, se ha llevado a cabo una comparación de diferentes MACs diseñadas para estas redes. Se han evaluado tanto MACs basadas en RC como no basadas en ella, estableciendo las ventajas de la RC para estas aplicaciones. La evaluación se ha llevado a cabo haciendo un estudio teórico mediante Network Calculus, cuyos resultados se han validado mediante simulaciones en OPNET. Los resultados muestran como la RC es capaz de evitar interferencias y asegurar robustez en ambientes industriales. Sin embargo, ninguna de las MACs seleccionadas ha conseguido asegurar ambos requisitos, temporales y de robustez, al mismo tiempo; se puede evitar las interferencias pero no sin comprometer los requisitos temporales de la aplicación. Sin embargo, la MAC propuesta es capaz de evitar interferencias asegurando al mismo tiempo los requisitos temporales y de robustez. Por lo tanto, la MAC propuesta es apropiada para este tipo de redes. Para asegurar el comportamiento determinista del sistema, se ha propuesto un novedoso algoritmo de handoff que es capaz de detectar una interferencia y saltar a otro canal. Este algoritmo se ha diseñado para ser utilizado conjuntamente con una de las MACs previamente evaluadas. La detección de la interferencia y el salto a otro canal se hace en un tiempo determinado de tiempo, ya que es posible detectar interferencias mientras el sistema está transmitiendo. Su rendimiento se ha evaluado mediante Network Calculus y simulaciones en OPNET, y se ha comparado con los resultados obtenidos con la MAC cuando no se utiliza el esquema propuesto. De la comparación se deduce que el esquema de handoff añade a la MAC la capacidad de asegurar a la vez los requisitos temporales y de robustez. Además, en la tesis se explica el algoritmo de spectrum sensing que la MAC utiliza para obtener información del entorno, y su rendimiento se ha estudiado mediante simulaciones en MATLAB. Debido a su simplicidad, se ha optado por un detector de energía para este propósito. También se presenta un clasificador de modulaciones cicloestacionario. Este clasificador ha sido simplificado todo lo posible para posibilitar su implementación en hardware real. El clasificador de modulaciones es capaz de distinguir entre señales OFDM, QPSK y GFSK. Su rendimiento se detalla para diferentes señales y para diferentes deficiencias presentes en el receptor, como son offset de frecuencia, offset de continua o desequilibrios I/Q. Por último, se presenta una plataforma cognitiva que se ha utilizado para validar los algoritmos de spectrum sensing. Un grupo de trabajo compuesto por investigadores de IK4-Ikerlan y Mondragon Unibertsitatea ha diseñado esta plataforma sobre una FPGA Virtex 6 de Xilinx. La plataforma, que utiliza los dos algoritmos de spectrum sensing, es un puente Ethernet-RF. Su objetivo es reemplazar un enlace cableado de Ethernet por uno inalámbrico para aplicaciones de redes de sensores y actuadores industriales. Gracias a los algoritmos de spectrum sensing, la plataforma es capaz de asegurar un enlace robusto ante interferencias. El detector de energía se utiliza en el transmisor para encontrar los posibles canales libres donde transmitir la información. Mientras que el clasificador de modulaciones se utiliza en el receptor para distinguir entre la señal del transmisor y otras posibles señales. Esto permite al receptor saber en qué canal de todos los posibles está el transmisor.Tesi honetan proposatzen da Irrati Kognitiboaren (IK) paradigman oinarritutako MAC bat industriako haririk gabeko sentsore eta eragingailuen sareetarako. Sare horiek begizta itxiko kontrol aplikazioetarako erabili ohi dira, denbora eta sendotasunaren aldetik baldintza ugari eskatzen dute eta. Pertsonen osasuna eta makinak arriskuan ez jartzeko, beharrezkoa da sareak latentzia eta hutsegite tasa txikiak bermatzea. Gainera, industri giroetan lan egiteko direnez, esaterako, lantegietan edo garraioetan (trenak, hegazkinak, etab.), sare horiek gai izan behar dira gainazal metalikoek eragiten dituzten ibilbide aniztunaren eta itzal efektuaren ondorioz asko barreiatzen diren kanalei aurre egiteko. Ingurune horien ohiko ezaugarria da, baita ere, beste komunikazio sistema batzuen edo industriako makinen beraien interferentziak egotea. Tesi honetan proposatzen den MACa gai da honelako inguruetan lan egiteko denborari eta sendotasunari dagokienez eskatzen dituen baldintzak ziurtatzeko. IKan oinarrituta haririk gabeko sentsore eta eragingailu industrialen sareetarako MACa diseinatzeko prozesuan, horrelako sareetarako aurkeztu diren hainbat MAC alderatu dira. IKan oinarritutako MACak zein bestelakoak ebaluatu dira, eta IKak aplikazio hauetarako dituen abantailak ezarri dira. Ebaluaziorako Network Calculus erabili da, zeinaren bidez azterketa teoriko bat egin baita, eta azterketaren emaitzak OPNETen simulazioak eginda baliozkotu dira. Emaitzek erakusten dutenez, IKa gai da industriako inguruneetan interferentziak ekidin eta sendotasuna ziurtatzeko. Halere, aukeratu diren MACetatik batek ere ez du lortu baldintza biak, denborari buruzkoa zein sendotasunari buruzkoa, aldi berean ziurtatzea; interferentziak ekidin daitezke, baina ez aplikazioaren denborari buruzko baldintzak arriskuan jarri gabe. Dena dela, proposatu den MACak portaera determinista bat ziurtatzen du interferentziekiko, eta aldi berean denborari eta sendotasunari buruzko baldintzak ere ziurtatzen ditu. Hortaz, MAC hau egokia da sare mota honetarako. Sistemaren portaera determinista ziurtatzeko, handoff algoritmo berritzailea proposatu da, zeina interferentzia bat antzeman eta beste kanal bat igarotzeko gai den. Algoritmo hori aurretik ebaluatutakoa MACetako batekin batera erabiltzeko diseinatu da. Interferentzia antzeman eta beste kanal batera salto egitea denbora jakin batean egiten da, izan ere, sistema transmititzen ari dela antzeman baitaitezke interferentziak. Network Calculusen bitartez eta OPNETeko simulazioen bitartez ebaluatu da sistemaren errendimendua, eta proposatutako eskema erabiltzen ez denean MACak ematen dituen emaitzekin alderatu da. Alderaketa horretatik ondorioztatzen denez, handoff eskemak denborari eta sendotasunari buruzko baldintzak batera ziurtatzeko ahalmena ematen dio MACari. Gainera, tesiak azaltzen du inguruneari buruzko informazioa eskuratzeko MACak erabiltzen duen spectrum sensing algoritmoa, eta bere errendimendua MATLABen simulazioak eginez aztertu da. Bere sinpletasuna dela eta, energia detektore bat aukeratu da asmo honetarako. Modulazio sailkatzaile zikloegonkor bat ere aurkezten da. Sailkapen hori ahalik eta gehien sinplifikatu da benetako hardwarean inplementatu ahal izateko. Modulazioen sailkatzaileak OFDM, QPSK eta GFSK seinaleak bereizi ditzake. Bere errendimendua hargailuan dauden seinale eta akats desberdinetarako zehazten da, esaterako maiztasunaren offset-a,zuzenaren offset-a edo I/Q desorekak. Bukatzeko, spectrum sensing-eko algoritmoak baliozkotzeko erabili den plataforma kognitibo bat aurkezten da. IK4-Ikerlaneko eta Mondragon Unibertsitateko ikertzailez osatutako lantalde batek diseinatu du plataforma hori Xilinxen Virtex 6 FPGA baten oinarrutz. Plataformak spectrum sensing-eko bi algoritmo erabiltzen ditu eta Ethernet-RF zubi bat da. Bere helburua da Etherneteko kable bidezko lotura bat haririk gabeko batekin ordeztea industriako sentsore eta eragingailuen sareetan aplikatzeko. Spectrum sensing-eko algoritmoei esker, plataformak lotura sendoa bermatu dezake interferentziak gertatzen direnean. Energia detektorea transmisorean erabiltzen da informazioa transmititzeko erabilgarri egon daitezkeen kanalak aurkitzeko. Modulazioen sailkatzailea, berriz, hargailuan erabiltzen da transmisorearen seinalea eta egon daitezkeen beste seinale batzuk bereizteko. Horri esker, hargailuak badaki posible diren kanal guztietatik non dagoen transmisorea

FPGA IMPLEMENTATION OF A REALTIME CYCLOSTATIONARY FEATURE DETECTOR FOR OFDM SIGNALS

The demand for wireless connectivity has prompted regulatory authorities in the United States to investigate spectrum sharing of the DSRC band with U-NII operators. However, DSRC operation has public safety implications, and moreover, time-critical requirements due to the vehicular nature of its application. The field of cognitive radio has identified several sensing techniques for the identification of licensed operators in a given band. This thesis explores cyclostationary detection techniques for primary users. A method will be identified for the detection of the 802.11p OFDM modulation used for DSRC communications. A test statistic will be given that is invariant to the signal noise covariance to allow simple and robust operation. Finally, the detection algorithm will be implemented in FPGA digital logic in order to demonstrate the methods ability to be employed in a commercial radio chipset with minimum resource requirements, yet still provide real-time detection

Energy-detection based spectrum sensing for cognitive radio on a real-time SDR platform

There has been an increase in wireless applications due to the technology boom; consequently raising the level of radio spectrum demand. However, spectrum is a limited resource and cannot be infinitely subdivided to accommodate every application. At the same time, emerging wireless applications require a lot of bandwidth for operation, and have seen exponential growth in their bandwidth usage in recent years. The current spectrum allocation technique, proposed by the Federal Communications Commission (FCC) is a fixed allocation technique. This is inefficient as the spectrum is vacant during times when the primary user is not using the spectrum. This strain on the current available bandwidth has revealed signs of an upcoming spectrum crunch; hence the need to find a solution that satisfies the increasing spectrum demand, without compromising the performance of the applications. This work leverages on cognitive radio technology as a potential solution to the spectrum usage challenge. Cognitive radios have the ability to sense the spectrum and determine the presence or absence of the primary user in a particular subcarrier band. When the spectrum is vacant, a cognitive radio (secondary user) can opportunistically occupy the radio spectrum, optimizing the radio frequency band. The effectiveness of the cognitive radio is determined by the performance of the sensing techniques. Known spectrum-sensing techniques are reviewed, which include energy detection, entropy detection, matched-filter detection, and cyclostationary detection. In this dissertation, the energy sensing technique is examined. A real-time energy detector is developed on the Software-Defined Radio (SDR) testbed that is built with Universal Software Radio Peripheral (USRP) devices, and on the GNU Radio software platform. The noise floor of the system is first analysed to determine the detection threshold, which is obtained using the empirical cumulative distribution method. Simulations are carried out using MATrix LABoratory (MATLAB) to set a benchmark. In both simulations and the SDR development platform, an Orthogonal Frequency Division Multiplexing (OFDM) signal with Quadrature Phase Shift Keying (QPSK) modulation is generated and used as the test signal

Space-time-frequency methods for interference-limited communication systems

textTraditionally, noise in communication systems has been modeled as an additive, white Gaussian noise process with independent, identically distributed samples. Although this model accurately reflects thermal noise present in communication system electronics, it fails to capture the statistics of interference and other sources of noise, e.g. in unlicensed communication bands. Modern communication system designers must take into account interference and non-Gaussian noise to maximize efficiencies and capacities of current and future communication networks. In this work, I develop new multi-dimensional signal processing methods to improve performance of communication systems in three applications areas: (i) underwater acoustic, (ii) powerline, and (iii) multi-antenna cellular. In underwater acoustic communications, I address impairments caused by strong, time-varying and Doppler-spread reverberations (self-interference) using adaptive space-time signal processing methods. I apply these methods to array receivers with a large number of elements. In powerline communications, I address impairments caused by non-Gaussian noise arising from devices sharing the powerline. I develop and apply a cyclic adaptive modulation and coding scheme and a factor-graph-based impulsive noise mitigation method to improve signal quality and boost link throughput and robustness. In cellular communications, I develop a low-latency, high-throughput space-time-frequency processing framework used for large scale (up to 128 antenna) MIMO. This framework is used in the world's first 100-antenna MIMO system and processes up to 492 Gbps raw baseband samples in the uplink and downlink directions. My methods prove that multi-dimensional processing methods can be applied to increase communication system performance without sacrificing real-time requirements.Electrical and Computer Engineerin

Cognitive Radio for Emergency Networks

In the scope of the Adaptive Ad-hoc Freeband (AAF) project, an emergency network built on top of Cognitive Radio is proposed to alleviate the spectrum shortage problem which is the major limitation for emergency networks. Cognitive Radio has been proposed as a promising technology to solve todayâ?~B??~D?s spectrum scarcity problem by allowing a secondary user in the non-used parts of the spectrum that aactully are assigned to primary services. Cognitive Radio has to work in different frequency bands and various wireless channels and supports multimedia services. A heterogenous reconfigurable System-on-Chip (SoC) architecture is proposed to enable the evolution from the traditional software defined radio to Cognitive Radio

CYCLOSTATIONARY DETECTION FOR OFDM IN COGNITIVE RADIO SYSTEMS

Research on cognitive radio systems has attracted much interest in the last 10 years. Cognitive radio is born as a paradigm and since then the idea has seen contribution from technical disciplines under different conceptual layers. Since then improvements on processing capabilities have supported the current achievements and even made possible to move some of them from the research arena to markets. Cognitive radio implies a revolution that is even asking for changes in current business models, changes at the infrastructure levels, changes in legislation and requiring state of the art technology. Spectrum sensing is maybe the most important part of the cognitive radio system since it is the block designed to detect signal presence on the air. This thesis investigates what cognitive radio systems require, focusing on the spectrum sensing device. Two voice applications running under different Orthogonal Frequency Division Multiplexing (OFDM) schemes are chosen. These are WiFi and Wireless Microphone. Then, a Cyclostationary Spectrum Sensing technique is studied and applied to define a device capable of detecting OFDM signals in a noisy environment. One of the most interesting methodologies, in terms of complexity and computational requirements, known as FAM is developed. Study of the performance and frequency synchronization results are shown, including the development of a blind synchronization technique for offset estimation.