Multiband Spectrum Access: Great Promises for Future Cognitive Radio Networks

Cognitive radio has been widely considered as one of the prominent solutions to tackle the spectrum scarcity. While the majority of existing research has focused on single-band cognitive radio, multiband cognitive radio represents great promises towards implementing efficient cognitive networks compared to single-based networks. Multiband cognitive radio networks (MB-CRNs) are expected to significantly enhance the network's throughput and provide better channel maintenance by reducing handoff frequency. Nevertheless, the wideband front-end and the multiband spectrum access impose a number of challenges yet to overcome. This paper provides an in-depth analysis on the recent advancements in multiband spectrum sensing techniques, their limitations, and possible future directions to improve them. We study cooperative communications for MB-CRNs to tackle a fundamental limit on diversity and sampling. We also investigate several limits and tradeoffs of various design parameters for MB-CRNs. In addition, we explore the key MB-CRNs performance metrics that differ from the conventional metrics used for single-band based networks.Comment: 22 pages, 13 figures; published in the Proceedings of the IEEE Journal, Special Issue on Future Radio Spectrum Access, March 201

Radiated transient interferences in digital communication systems

In the Electromagnetic Compatibility research area, an unsolved interference problem is the measurement and evaluation of the distortion produced by radiated transient disturbances on digital communication systems. This impulsive noise, which is generated by switching devices or by sparks, is a broadband interference that covers the spectrum from DC to several hundreds of megahertz or some gigahertz. Additionally, this man-made noise is characterized by its short and random burst parameters, which make really challenging to measure it correctly. During the thesis, we have explained that impulsive noise is not properly measured and evaluated to prevent interference scenarios, when the EMC standard methodologies are applied. Detectors, such as the quasi-peak, frequency sweep measurements or signal-to-noise limiting evaluation described in the harmonized standards of the electromagnetic compatibility do not enable to determine beforehand the influence of transient interferences. Our strategy to overcome the non-profit measurement has been to perform novel measurement and evaluation techniques beyond EMC standards. The measurement technique developed joins the capabilities of EMI receivers and oscilloscope instrumentations to capture accurately the radiated transient interference. To carry out the measurement, the input stage of the EMI receiver is used for filtering and pre-amplifying purposes, conducting the IF output towards the oscilloscope, which is used for triggering and storage. Furthermore, a final post-processing stage is needed to obtain in time-domain the in-phase and quadrature components of the transient interference. Once the radiated transient interference has been measured properly, an accurate evaluation of the distortion produced to a digital communication system can be estimated. To evaluate the impact of the transient interference, a combination of the time-domain measurement with base-band simulation has been proposed to fulfil the thesis goal. The IQ time-domain measurement enables us to characterize the impulsive-noise present at the communication channel and determine the distortion produced to the digital communication system by means of base-band simulation. The procedure to determine the BER using the base-band simulation has been validated with experimental results, comparing the results reached with the developed methodology with the ones obtained when a communication system device is placed under radiated transient. Excellent results have been obtained employing the developed methodology, considering the interference produced by radiated transient to RFID or GSM communication systems Additionally, a new measurement methodology to obtain the amplitude probability diagram (APD) has been developed, offering the possibility to determine the bit-error-rate including limit lines at the APD diagram. This measurement method, based on captures obtained from a general purpose oscilloscope, makes it possible to obtain the APD measurement at any frequency band with the same accuracy provided by an EMI receiver. Furthermore, the post-processing tools using mathematical software produce the APD results rapidly at any bandwidth, and this makes it more powerful than employing an EMI receiver. The successful APD measurement system created is able to obtain the full-spectrum statistical measurement, employing several time-domain captures which can be acquired in practice immediately. In the final chapter of this thesis, the GSM system is interfered by radiated transients produced by sparks. The results provided by the APD diagram including the limit dots have been especially useful due to its fast capacity to interpret and quantify the degradation produced to the GSM system.En el camp de recerca de la compatibilitat electromagnètica, una de les problemàtiques no resoltes és la mesura i avaluació de les interferències produïdes per transitoris radiats sobre els equips de comunicació digitals. Aquest tipus de soroll impulsiu, que es genera per la commutació d' equips electrònics o guspires, és una interferència de banda ampla que ocupa l' espectre radioelèctric fins a diversos centenars de megahertz o algun gigahertz. A més, aquest soroll es caracteritza per la seva curta durada i l'aleatorietat dels seus paràmetres, i això fa que sigui molt complicat mesurar correctament la interferència. Al llarg de la tesi, hem explicat que el soroll impulsiu no es mesura ni s'avalua adequadament per evitar escenaris d'interferències si s'utilitzen les metodologies definides als estàndards d' EMC. Els detectors, com el de quasi-pic, l'escombratge en freqüència o l'avaluació basada en els límits relacionats amb la relació senyal a soroll no són vàlids per anticipar la influència de les interferències transitòries. La nostra estratègia per solucionar els problemes de les mesures normatives ha estat desenvolupar noves tècniques de mesura i avaluació fora dels estàndards d'EMC. La tècnica de mesura desenvolupada combina les capacitats dels receptors EMI i els oscil·loscopis per capturar la interferència transitòria radiada. Per realitzar les mesures, l'etapa d'entrada del receptor EMI s¿utilitza amb la finalitat de filtrar i preamplificar, enviant la sortida IF cap a l'oscil·loscopi, que es fa servir per detectar i emmagatzemar els transitoris. Per últim, en l'etapa de postprocessament, s'obtenen en el domini del temps els components en fase i en quadratura de la interferència transitòria. Una vegada s'ha mesurat adequadament la interferència radiada, es pot estimar correctament la distorsió produïda sobre els sistemes de comunicació digitals. Per avaluar l'impacte de la interferència transitòria i així complir amb l'objectiu de la tesi, s'ha proposat combinar les mesures en el domini del temps amb la simulació en banda base. La mesura IQ en el domini del temps ens permet caracteritzar el soroll impulsiu present al canal de comunicació i així determinar la distorsió produïda al sistema de comunicació digital mitjançant la simulació en banda base. El procediment per determinar el BER fent servir la simulació en banda base ha estat validat amb resultats experimentals, i s'han comparat els resultats obtinguts utilitzant la metodologia desenvolupada amb els resultats proporcionats directament col·locant un dispositiu sota la influència de transitoris radiats. Els excel¿lents resultats obtinguts considerant interferències produïdes sobre sistemes RFID i GSM han estat publicats. D'altra banda, també s'ha desenvolupat un nou mètode de mesura per adquirir el diagrama de probabilitat d¿amplitud (APD), oferint la possibilitat de determinar la probabilitat d'error en el bit incloent límits en el diagrama APD. Aquest mètode de mesura, basat en captures obtingudes mitjançant un oscil·loscopi de propòsit general, fa possible obtenir la mesura APD en qualsevol banda freqüencial amb la mateixa exactitud que proporciona un receptor EMI. A més, amb les eines de postprocessament desenvolupades és possible produir els resultats APD ràpidament i amb qualsevol amplada de banda, i això fa que sigui un mètode de mesura més potent que no pas utilitzar un receptor EMI. L'exitós sistema de mesura creat per obtenir l'APD és capaç d'obtenir la mesura estadística en tot l'espectre radiolèctric utilitzant poques captures en el domini del temps, les quals a la pràctica soón adquirides de forma inmediata. Al capítol final de la tesi, el sistema GMS és interferit per transistors radiats produïts per guspires. El resultat proporcionat pel diagrama APD, incloent-hi els límits, és especialment útil gràcies a la seva rapidesa per interpretar i quantificar la degradació produïda sobre el sistema GSM

Coded modulation techniques with bit interleaving and iterative processing for impulsive noise channels

Power line communications (PLC) surfers performance degradation due mainly to impulsive noise interference generated by electrical appliances. This thesis studies coded modulation techniques to improve the spectral efficiency and error performance of PLC. Considered in the first part is the application of bit-interleaved coded modulation with iterative decoding (BICM-ID) in class-A impulsive noise environment. In particular, the optimal soft-output demodulator and its suboptimal version are presented for an additive class-A noise (AWAN) channel so that iterative demodulation and decoding can be performed at the receiver. The effect of signal mapping on the error performance of BICM-ID systems in impulsive noise is then investigated, with both computer simulations and a tight error bound on the asymptotic performance. Extrinsic information transfer (EXIT) chart analysis is performed to illustrate the convergence properties of different mappings. The superior performance of BICMID compared to orthogonal frequency-division multiplexing (OFDM) is also clearly demonstrated.Motivated by the successes of both BICM-ID and OFDM in improving the error performance of communications systems in impulsive noise environment, the second part of this thesis introduces a novel scheme of bit-interleaved coded OFDM with iterative decoding (BI-COFDM-ID) over the class-A impulsive noise channel. Here, an iterative receiver composed of outer and inner iteration loops is first described in detail. Error performance improvements of the proposed iterative receiver with different iteration strategies are presented and discussed. Performance comparisons of BI-COFDM-ID, BICM-ID and iteratively decoded OFDM are made to illustrate the superiority of BI-COFDM-ID. The effect of signal mapping on the error performance of BI-COFDM-ID is also studied

Design and Analysis of OFDM System for Powerline Based Communication

Research on digital communication systems has been greatly developed in the past few years and offers a high quality of transmission in both wired and wireless communication environments. Coupled with advances in new modulation techniques, Orthogonal Frequency Division Multiplexing (OFDM) is a well-known digital multicarrier communication technique and one of the best methods of digital data transmission over a limited bandwidth. The main aim of this research is to design an OFDM modem for powerline-based communication in order to propose and examine a novel approach in comparing the different modulation order, different modulation type, application of Forward Error Correction (FEC) scheme and also application of different noise types and applying them to the two modelled channels, Additive White Gaussian Noise (AWGN) and Powerline modelled channel. This is an attempt to understand and recognise the most suitable technique for the transmission of message or image within a communication system. In doing so, MATLAB and embedded Digital Signal Processing (DSP) systems are used to simulate the operation of virtual transmitter and receiver. The simulation results presented in this project suggest that lower order modulation formats (Binary Phase Shift Keying (BPSK) and 4-Quadrature Amplitude Modulation (QAM)), are the most preferred modulation techniques (in both type and order) for their considerable performance. The results also indicated that, Convolutional Channel Encoding (CCE)-Soft and Block Channel Encoding (BCE)-Soft are by far the best encoding techniques (in FEC type) for their best performance in error detection and correction. Indeed, applying these techniques to the two modelled channels has proven very successful and will be accounted as a novel approach for the transmission of message or image within a powerline based communication system

High Speed Turbo Tcm Ofdm For Uwb And Powerline System

Turbo Trellis-Coded Modulation (TTCM) is an attractive scheme for higher data rate transmission, since it combines the impressive near Shannon limit error correcting ability of turbo codes with the high spectral efficiency property of TCM codes. We build a punctured parity-concatenated trellis codes in which a TCM code is used as the inner code and a simple parity-check code is used as the outer code. It can be constructed by simple repetition, interleavers, and TCM and functions as standard TTCM but with much lower complexity regarding real world implementation. An iterative bit MAP decoding algorithm is associated with the coding scheme. Orthogonal Frequency Division Multiplexing (OFDM) modulation has been a promising solution for efficiently capturing multipath energy in highly dispersive channels and delivering high data rate transmission. One of UWB proposals in IEEE P802.15 WPAN project is to use multi-band OFDM system and punctured convolutional codes for UWB channels supporting data rate up to 480Mb/s. The HomePlug Networking system using the medium of power line wiring also selects OFDM as the modulation scheme due to its inherent adaptability in the presence of frequency selective channels, its resilience to jammer signals, and its robustness to impulsive noise in power line channel. The main idea behind OFDM is to split the transmitted data sequence into N parallel sequences of symbols and transmit on different frequencies. This structure has the particularity to enable a simple equalization scheme and to resist to multipath propagation channel. However, some carriers can be strongly attenuated. It is then necessary to incorporate a powerful channel encoder, combined with frequency and time interleaving. We examine the possibility of improving the proposed OFDM system over UWB channel and HomePlug powerline channel by using our Turbo TCM with QAM constellation for higher data rate transmission. The study shows that the system can offer much higher spectral efficiency, for example, 1.2 Gbps for OFDM/UWB which is 2.5 times higher than the current standard, and 39 Mbps for OFDM/HomePlug1.0 which is 3 times higher than current standard. We show several essential requirements to achieve high rate such as frequency and time diversifications, multi-level error protection. Results have been confirmed by density evolution. The effect of impulsive noise on TTCM coded OFDM system is also evaluated. A modified iterative bit MAP decoder is provided for channels with impulsive noise with different impulsivity

On Combined Coding and Modulation

In the treatment of channel coding as a separate operation independent of the modulation, the coded set of sequences generally has a smaller channel symbol duration than the uncoded set of sequences for the same information rate. Accordingly, the power spectrum density (PSD) of the channel signals changes essentially. On the other hand, if the modulation is designed in conjunction with the channel coding, error correction can be achieved without leading to any essential changes in the PSD. In this thesis, two combined coding and modulation schemes are studied. Narrowband powerline communication (PLC) is considered as a practical application. The thesis can be divided into two parts. In the first part, combined coding and modulation scheme based on the single carrier modulation is proposed. An run-length limited (RLL) encoder is introduced between the channel encoder and the constant envelope modulator to control the minimum channel symbol duration (the minimum duration in which the channel symbol stays constant) of a set of block waveforms defined in a constant time. As a single carrier modulation, noncoherent FSK and PSK are considered. Accordingly, it is shown that high coding gains can be achieved at the same information rate without leading to an essential change in the PSD. The maximum-likelihood (ML) receiver structures are derived and investigated for the additive white Gaussian noise (AWGN) and the impulsive noise channel models. In the second part of the thesis, OFDM modulation is considered. If the discrete Fourier transform (DFT) of the transmitted OFDM symbol contains a small number of zeros or known data, there is a similarity between the inverse DFT (IDFT) and RS encoder. In practice, not all subcarriers are used to carry information. Some subcarriers are set to zero or known data (pilot symbols) for different purposes, that include channel estimation, synchronization or cancelation of the DC value. An iterative impulsive noise suppression algorithm is proposed, which exploits the impulsive noise structure in the time and frequency domain and uses the existing redundancy to decode the errors. The simulation results show that the influence of impulsive noise can be essentially reduced.Bei der klassischen Kanalcodierung wird die Datenrate durch Einfügen von Redundanzen bewusst erhöht, um dadurch eine Absicherung gegen auftretende Fehler zu erreichen. Die Erhöhung der Datenrate erfordert eine wesentliche Änderung im Power-Spektrum. Aber bei vielen Anwendungen ist eine möglichst effiziente Nutzung des Power-Spektrums gefordert. In dieser Arbeit werden Codierung und Modulation gemeinsam betrachtet, so dass die Verbesserung der Übertragungsqualität keine wesentliche Änderung in dem Power-Spektrum erfordert. Das Power-Spektrum wird durch Power-Spektrum-Dichte analysiert. Als praktische Anwendung der Arbeit wird die Schmalband-Powerline-Kommunikation (Narrowband Power Line Communication) betrachtet. Powerline ist der Begriff für die Übertragung von Daten über Stromkabel. Im Gegensatz zu konventionellen Kommunikationskanälen kann die Störung auf Stromnetze nicht als additives weißes Gaußsches Rauschen (AWGN) modelliert werden. Das ist darauf zurückzuführen, dass neben Hintergrundrauschen auch Schmalbandstörungen und insbesondere Impulsstörungen vorkommen. Beim Auftreten eines Impulses sind Bit- oder Burstfehler bei einer Datenübertragung sehr wahrscheinlich. Im ersten Teil werden Einzelträgerverfahren betrachtet. Ein sogenannter RLL- (Run-length Limited) Code wird verwendet, um die Anzahl aufeinanderfolgender Symbole mit gleichem Wert nach unten zu begrenzen. Dementsprechend wird gezeigt, dass im gemeinsamen blockweisen RLL Encoder/Modulator der minimale euklidische Abstand erhöht werden kann, ohne wesentliche Änderung in der Power-Spektrum-Dichte. Im Empfänger erfolgen Demodulation und Decodierung nicht getrennt, sondern in einem Schritt, wobei alle Vorteile der Maximum-Likelihood-Decodierung mit Verwendung von Soft-Decision erhalten bleiben. Ein wesentlich größerer Codierungsgewinn ergibt sich bei der Verkettung mit einem RS- (Reed-Solomon) Code. Zunächst werden die Auswirkungen von Impulsstörungen auf RLL-codierte Einzelträgerverfahren erläutert. Die Modellierung von Impulsstörungen als nicht-Gauß'sche Verteilungen wurde in der Literatur durch verschiedene Ansätze vorgestellt. In der Arbeit wird das Klasse-A Modell von Middleton angewendet. Im zweiten Teil der Arbeit wird OFDM- (Orthogonal Frequency Division Multiplexing) Verfahren betrachtet. Insbesondere durch die Impulsstörungen werden in der Datenübertragung erhebliche Störeffekte hervorgerufen. In OFDM werden die Modulation bzw. Demodulation mit Hilfe einer IDFT bzw. DFT (Inverse Discrete Fourier Transform, Discrete Fourier Transform) ausgeführt. Die bisherigen Überlegungen zur Kompensation von Impulsstörungen behandeln nur das OFDM-Verfahren mit einer großen Anzahl von Unterträgern (>256). In diesem Fall wird die Energie des Störimpulses durch die DFT auf viele Unterträger verteilt. Wenn die Anzahl der Unterträger kleiner als 256 ist, ist die Verteilung der Störung nicht uniform. Wenn die DFT der gesendeten OFDM-Symbol eine kleine Anzahl von Nullen oder bekannten Daten enthält, gibt es eine Ähnlichkeit zwischen der IDFT und RS-Encoder. Die OFDM-Signale beinhalten häufig Pilotinformationen und zu Null gesetzte Träger in der Signalstruktur. Es werden Verfahren zur Kompensation von Impulsstörungen durch Pilotinformationen und zu Null gesetzte Träger untersucht bzw. entwickelt, die eine Steigerung der Robustheit der Datenübertragung ermöglichen. Als Zielkriterium wird dabei die Senkung der Bitfehlerrate bei einer impulsgestörten Übertragung herangezogen. Für die Modellierung von Impulsstörungen wird das vereinfachte "Klasse-A"\, Modell von Middleton verwendet