PIP-OFDM System Design and Application for Cognitive Radio Communications

This thesis defines a new Orthogonal Frequency Division Multiplexing (OFDM) system with Precoded In-band Pilots (PIP) tailored for cognitive radio (CR) communications. The motivation, principle, system design, implementation architecture, and CR application specific considerations of proposed PIP-OFDM system are investigated in this thesis. Principles and limitations of existing spectrum sensing techniques for cognitive radio communications are first analyzed and compared, with a focus on implementation challenges of pilot-based spectrum sensing for OFDM signals due to its robust performance in low signal-to-noise ratio (SNR) conditions. Several technical difficulties which haven’t been well addressed in previous pilot-based OFDM spectrum sensing studies, including impact of cyclic prefix, frequency offset between transmitter and spectrum sensing device, and noise uncertainty in the sensing threshold design, are taken into consideration in the analysis. Considering the poor performance of existing spectrum sensing techniques on user identification in cognitive radio network, where multiple secondary users may coexist, a precoded in-band pilots design is proposed in this thesis to enhance the user identification capabilities at low SNRs. The pilots in proposed PIP-OFDM system consist of uniform pilots and identification pilots. Each secondary user is associated with a unique identification pilot signal for identification purpose. Encoding of identification pilots is investigated, which will be used at the spectrum sensing device to identify the active user on the frequency band of interest. 111 Abstract To demodulate/decode identification pilots for user identification purpose, synchronization between transmitter and spectrum sensing device needs to be established. The synchronization in PIP-OFDM system, which is different from that in traditional OFDM systems, is subsequently investigated. Coarse time and frequency synchronization are achieved by correlation respectively in time and frequency domain. Through phase shift estimation in time domain, fine frequency synchronization is reached using a modified maximum likelihood estimation algorithm exploiting the redundancy in cyclic prefix. Based on this observation, a fine time synchronization algorithm is proposed in this thesis using redundant information on specifically designed uniform pilots. A multiple OFDM symbols processing strategy is used to improve the synchronization performance of PIP-OFDM system considering the poor performance of synchronization at low SNR. With the developed synchronization strategies, channel estimation in PIP-OFDM system is achieved using well developed estimation techniques in frequency domain. User identification is subsequently realized through demodulating the identification pilots. Theoretical performance and simulation results of user identification in PIP-OFDM system are provided to further confirm the effectiveness of the proposed design

Spectrum Sensing für DVB-T2

Lokal von terrestrischen Fernseh-Sendernetzen nicht genutztes Spektrum, auch als TV White Spaces bezeichnet, stellt eine vielversprechende Ressource für zukünftige breitbandige Kommunikationsdienste dar. Wenn sekundäre Spektrumsnutzer auf die TV-Frequenzen zugreifen wollen, muss gewährleistet sein, dass Fernsehempfänger nicht durch unzulässig hohe Interferenzen gestört werden. Eine Möglichkeit, TV White Spaces zu erkennen, bietet Spectrum Sensing. Hierbei tasten die Geräte der Sekundärnutzer das Spektrum ab und treffen autonom die Entscheidung, ob in der Region in einem gegebenen Kanal ein TV-Signal empfangbar ist oder nicht. Im vorliegenden Aufsatz werden erstmalig unterschiedliche Spectrum-Sensing-Verfahren für die neueste Generation des terrestrischen Fernsehens, DVB-T2, untersucht. Da einige der Verfahren die Signalstruktur der digitalen Fernsehsignale auswerten, mussten sie für den Einsatz für DVB-T2 zum Teil neu entwickelt werden. Es wird ein Überblick über die Funktionsweise der einzelnen Algorithmen gegeben. Simulationsergebnisse ermöglichen einen Vergleich der Leistungsfähigkeit der unterschiedlichen Ansätze

End-to-end Precoding Validation over a Live GEO Satellite Forward Link

In this paper we demonstrate end-to-end precoded multi-user multiple-input single-output (MU-MISO) communications over a live GEO satellite link. Precoded communications enable full frequency reuse (FFR) schemes in satellite communications (SATCOM) to achieve broader service availability and higher spectrum efficiency than with the conventional four-color (4CR) and two-color (2CR) reuse approaches. In this scope, we develop an over-the-air test-bed for end-to-end precoding validations. We use an actual multi-beam satellite to transmit and receive precoded signals using the DVB-S2X standard based gateway and user terminals. The developed system is capable of end-to-end real-time communications over the satellite link including channel measurements and precompensation. It is shown, that by successfully canceling inter-user interference in the actual satellite FFR link precoding brings gains in terms of enhanced SINR and increased system goodput.Comment: Submitted to IEEE Access Journa

Novel Approaches in RF/Analog CMOS Spectrum Sensing and Its Applications

Real time spectrum sensing refers to searching for possible signals at a specific time and location, which is applicable to cognitive radio (CR) for primary signal detection and ultra-wideband (UWB) radio for interferer detection. There are several approaches for spectrum sensing. Choosing a proper method for spectrum sensing necessitates evaluating several trade-offs among sensing time, accuracy, power consumption and simplicity of implementation. In this dissertation several approaches for spectrum sensing along with the applications to CR and UWB receivers are presented. A novel simple spectrum sensing technique for detecting weak primary signals with negative signal-to-noise ratio (SNR) is proposed, which is called quasi-cyclostationary feature detection (QCFD) technique. Moreover, a simple, reliable, and fast real-time spectrum sensing technique based on phasers, which are dispersive delay structures (DDSs), is proposed. Lastly, a UWB receiver robust to the narrowband (NB) blockers, in the vicinity of UWB frequency, is presented. To increase the robustness of the UWB receiver towards interferers, a dynamic blocker detector, utilizing a phaser-based real time spectrum sensing technique, is employed. The proposed spectrum sensing methods provide the best solutions for the intended applications, considering the trade-offs, compared to the state-of-the-art CMOS spectrum sensors

Enhanced Spectrum Sensing for Cognitive Cellular Systems

This dissertation aims at improving spectrum sensing algorithms in order to effectively apply them to cellular systems. In wireless communications, cellular systems occupy a significant part of the spectrum. The spectrum usage for cellular systems are rapidly expanding due to the increasing demand for wireless services in our society. This results in radio frequency spectrum scarcity. Cellular systems can effectively handle this issue through cognitive mechanisms for spectrum utilization. Spectrum sensing plays the first stage of cognitive cycles for the adaptation to radio environments. This dissertation focuses on maximizing the reliability of spectrum sensing to satisfy regulation requirements with respect to high spectrum sensing performance and an acceptable error rate. To overcome these challenges, characteristics of noise and manmade signals are exploited for spectrum sensing. Moreover, this dissertation considers system constraints, the compatibility with the current and the trends of future generations. Newly proposed and existing algorithms were evaluated in simulations in the context of cellular systems. Based on a prototype of cognitive cellular systems (CCSs), the proposed algorithms were assessed in realistic scenarios. These algorithms can be applied to CCSs for the awareness of desired signals in licensed and unlicensed bands. For orthogonal frequency-division multiplexing (OFDM) signals, this dissertation exploits the characteristics of pilot patterns and preambles for new algorithms. The new algorithms outperform the existing ones, which also utilize pilot patterns. Additionally, the new algorithms can work with short observation durations, which is not possible with the existing algorithms. The Digital Video Broadcasting - Terrestrial (DVB-T) standard is taken as an example application for the algorithms. The algorithms can also be developed for filter bank multicarrier (FBMC) signals, which are a potential candidate for multiplexing techniques in the next cellular generations. The experimental results give insights for the reliability of the algorithms, taking system constraints v into account. Another new sensing algorithm, based on a preamble, is proposed for the DVBT2 standard, which is the second generation of of DVB system. DVB-T2 systems have been deployed in worldwide regions. This algorithm can detect DVB-T2 signals in a very short observation interval, which is helpful for the in-band sensing mode, to protect primary users (in nearly real-time) from the secondary transmission. An enhanced spectrum sensing algorithm based on cyclostationary signatures is proposed to detect desired signals in very low signal-to-noise ratios (SNRs). This algorithm can be developed to detect the single-carrier frequency division multiple access (SC-FDMA) signal, which is adopted for the uplink of long-term evolution (LTE) systems. This detector substantially outperforms the existing detection algorithms with the marginal complexity of some scalar multiplications. The test statistics are explicitly formulated in mathematical formulas, which were not presented in the previous work. The formulas and simulation results provide a useful strategy for cyclostationarity-based detection with different modulation types. For multiband spectrum sensing, an effective scheme is proposed not only to detect but also to classify LTE signals in multiple channels in a wide frequency range. To the best of our knowledge, no scheme had previously been described to perform the sensing tasks. The scheme is reliable and flexible for implementation, and there is almost no performance degradation caused by the scheme compared to single-channel spectrum sensing. The multiband sensing scheme was experimentally assessed in scenarios where the existing infrastructures are interrupted to provide mobile communications. The proposed algorithms and scheme facilitate cognitive capabilities to be applied to real cellular communications. This enables the significantly improved spectrum utilization of CCSs

Electronic countermeasures applied to passive radar

Passive Radar (PR) is a form of bistatic radar that utilises existing transmitter infrastructure such as FM radio, digital audio and video broadcasts (DAB and DVB-T/T2), cellular base station transmitters, and satellite-borne illuminators like DVB-S instead of a dedicated radar transmitter. Extensive research into PR has been performed over the last two decades across various industries with the technology maturing to a point where it is becoming commercially viable. Nevertheless, despite the abundance of PR literature, there is a scarcity of open literature pertaining to electronic countermeasures (ECM) applied to PR. This research makes the novel contribution of a comprehensive exploration and validation of various ECM techniques and their effectiveness when applied to PR. Extensive research has been conducted to assess the inherent properties of the lluminators of Opportunity to identify their possible weaknesses for the purpose of applying targeted ECM. Similarly, potential jamming signals have also been researched to evaluate their effectiveness as bespoke ECM signals. Whilst different types of PR exist, this thesis focuses specifically on ECM applied to FM radio and DVB-T2 based PR. The results show noise jamming to be effective against FM radio based PR where jamming can be achieved with relatively low jamming power. A waveform study is performed to determine the optimal jamming waveform for an FM radio based PR. The importance of an effective direct signal interference (DSI) canceller is also shown as a means of suppressing the jamming signal. A basic overview of counter-ECM (ECCM) is discussed to counter potential jamming of FM based PR. The two main processing techniques for DVB-T2 based PR, mismatched and inverse filtering, have been investigated and their performance in the presence of jamming evaluated. The deterministic components of the DVB-T2 waveform are shown to be an effective form of attack for both mismatched filtering and inverse filtering techniques. Basic ECCM is also presented to counter potential pilot attacks on DVB-T2 based PR. Using measured data from a PR demonstrator, the application and effectiveness of each jamming technique is clearly demonstrated, evaluated and quantified

OFDM pilot allocation for sparse channel estimation

In communication systems, efficient use of the spectrum is an indispensable concern. Recently the use of compressed sensing for the purpose of estimating Orthogonal Frequency Division Multiplexing (OFDM) sparse multipath channels has been proposed to decrease the transmitted overhead in form of the pilot subcarriers which are essential for channel estimation. In this paper, we investigate the problem of deterministic pilot allocation in OFDM systems. The method is based on minimizing the coherence of the submatrix of the unitary Discrete Fourier Transform (DFT) matrix associated with the pilot subcarriers. Unlike the usual case of equidistant pilot subcarriers, we show that non-uniform patterns based on cyclic difference sets are optimal. In cases where there are no difference sets, we perform a greedy search method for finding a suboptimal solution. We also investigate the performance of the recovery methods such as Orthogonal Matching Pursuit (OMP) and Iterative Method with Adaptive Thresholding (IMAT) for estimation of the channel taps