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ABSTRACT: Non-coherent ultra-wideband (UWB) receivers offer an attractive low-complexity solution to ranging with UWB radios. They use
a simple receiver architecture that can operate at very low sampling rates compared to the Nyquist rate. In this paper, time-of-arrival
(TOA) estimation performances of two different searchback algorithms for non-coherent impulse radio UWB systems are analyzed:
jump back and search forward (JBSF) and serial backward search (SBS). Average ranging errors for both algorithms are formulated
using noise-based ranging thresholds and at sub-Nyquist sampling rates. For JBSF, a lower bound on the average ranging error
is obtained using an error-minimizing threshold. Also, a semi-analytic technique for the selection of the searchback window
length is presented. For SBS, noise-based threshold selection for both single-cluster and multi-cluster channels are introduced.
Computer simulations in IEEE 802.15.4a channels provide insights on the selection of some of the critical parameters for accurate
ranging. The simulation results show that JBSF typically performs better than SBS if the inter-cluster delay parameter (K) is not selected appropriately for SBS. In both cases, it is shown that even at sampling rates much smaller than the Nyquist
rate, ranging accuracies on the order of 33cm can be obtained using a non-coherent UWB receiver.
Wireless Personal Communications 04/2012; 48(4):585-603. · 0.46 Impact Factor
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ABSTRACT: Beside providing solutions to the interoperability problem of the first responders, location awareness feature of the cognitive radios can be beneficial for victim and first responder location estimation under extreme conditions. Location and environment based services can be employed to detect and locate victims even when the core wireless communications networks are down. In this paper, a location awareness framework is proposed for victim signal detection and direction estimation. A wideband, multi-channel sensing method is proposed along with a two-antenna multiple signal classification (MUSIC) direction finding algorithm. The performance evaluation of proposed methods are discussed with both simulation and measurement results.
Wireless and Microwave Technology Conference (WAMICON), 2011 IEEE 12th Annual; 05/2011
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ABSTRACT: In order to perform reliable communications, a system needs to have sufficient information about its operational environment, such as spectral resources and propagation characteristics. Cognitive-radio technology has capabilities for acquiring accurate spectrum, location, and environmental information, due to its unique features such as spectrum, location, and environmental awareness. The goal of this paper is to give a comprehensive review of the implementation of these concepts. In addition, the dynamic nature of cognitive-radio systems - including dynamic spectrum utilization, transmission, the propagation channel, and reception - is discussed, along with performance limits, challenges, mitigation techniques, and open issues. The capabilities of cognitive-radio systems for accurate characterization of operational environments are emphasized. These are crucial for efficient communications, localization, and radar systems.
IEEE Antennas and Propagation Magazine 09/2010; · 0.97 Impact Factor
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ABSTRACT: Coexistence of narrowband (NB) and multicarrier technologies will be a major concern in next generation wireless communication systems due to the co-channel interference (CCI) problem. In this paper, an efficient CCI cancellation method is proposed that may be utilized for an improved coexistence. The method treats both co-channel signals as desired signals and enhances them in an iterative manner. Through computer simulations, it is demonstrated that it yields significant gains in the symbol error rate (SER) performance of both the NB and multicarrier systems. Its computational complexity is compared with the complexity of the joint demodulation technique and it is shown to be much lower.
Wireless Communications and Networking Conference (WCNC), 2010 IEEE; 05/2010
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ABSTRACT: Opportunistic spectrum access feature of cognitive radio systems is a method to improve frequency underutilization of wireless spectrum. One of the techniques for detecting the unused bands is the energy detection for which selection of the threshold defines detection performance. In this paper, the adaptive threshold method is proposed as an alternative approach to estimate the threshold as a function of first and second order statistics of recorded signals. The proposed method does not require estimation of noise variance or signal to noise ratio and aims to minimize the effects of impairments introduced by wireless channel and non-stationary noise. The simulation results indicate that adaptive threshold has low false alarm and missed detection rates that can satisfy the detection requirements of multi-channel cognitive radios for either narrow or wideband spectrum sensing, when the standard deviation coefficient is selected properly.
Telecommunications (ICT), 2010 IEEE 17th International Conference on; 05/2010
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ABSTRACT: In cognitive radio systems, secondary users can utilize multiple dispersed bands that are not used by primary users. In this paper, time delay estimation of signals that occupy multiple dispersed bands is studied. First, theoretical limits on time delay estimation are reviewed. Then, two-step time delay estimators that provide trade-offs between computational complexity and performance are investigated. In addition, asymptotic optimality properties of the two-step time delay estimators are discussed. Finally, simulation results are presented to explain the theoretical results.
Computational Advances in Multi-Sensor Adaptive Processing (CAMSAP), 2009 3rd IEEE International Workshop on; 01/2010
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ABSTRACT: Cognitive radio is an enabling technology for efficient utilization of radio spectrum. In this paper, an orthogonal frequency division multiple access (OFDMA) based cognitive (opportunistic) network is considered, which co-exists with a macrocell network through utilizing its unused subcarriers. We particularly consider the uplink opportunity detection problem by the opportunistic network, where accurate synchronization to the macrocell network is crucial for minimizing the interference received from the macrocell users. After demonstrating the impact of synchronization on the inter-carrier-interference (ICI) observed at the opportunistic network, an improved blind first-user synchronization technique is proposed, and its statistics are analyzed. Through computer simulations it is shown that the proposed technique yields less interference and better opportunities for the opportunistic network.
Global Telecommunications Conference, 2009. GLOBECOM 2009. IEEE; 01/2010
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ABSTRACT: Femtocells have a strong potential for increasing the efficiency and coverage of next-generation broadband wireless networks. In this paper, a co-channel framework for the coexistence of orthogonal frequency division multiple access (OFDMA) based macrocell and femtocell wireless networks is proposed. It is based on utilizing the resource blocks of macrocell-associated mobile stations (mMSs) that are far away to a femtocell base station (fBS), therefore avoiding strong interference that may occur between a femtocell and close-by mMSs. An avoidance method that jointly utilizes the spectrum sensing results as well as scheduling information obtained from the macrocell base station (mBS) is introduced. Moreover, the impact of inter-carrier interference (ICI) from the mMSs in the uplink is discussed and evaluated through simulations.
IEEE Transactions on Consumer Electronics 12/2009; · 0.94 Impact Factor
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ABSTRACT: Accurate detection of spectrum opportunities within the frequency band of an orthogonal frequency division multiple access (OFDMA) system carries critical importance for OFDMA-based cognitive radios. In this paper, we analyze the opportunity detection performances of energy detection and ESPRIT (estimation of signal parameters by rotational invariance techniques) algorithms in the presence of timing misalignments in uplink (UL) OFDMA. For the energy detector, the statistics of subcarrier power are derived considering timing misalignments, and they are verified through computer simulations. Using these statistics, which take inter-carrier-interference (ICI) effects into account, receiver operating characteristics (ROCs) of the energy detector receiver are obtained. It is shown that energy detection has a considerably better performance than ESPRIT, especially when the subcarrier assignment changes frequently. Moreover, a closed form expression is derived for the UL-OFDMA synchronization point that minimizes the ICI. Finally, it is shown that employing resource allocation blocks with larger sizes in the primary network yields better opportunities for the cognitive radio.
IEEE Transactions on Wireless Communications 11/2009; · 2.59 Impact Factor
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ABSTRACT: Cognitive Radio (CR) systems with spectrum awareness feature is a promising approach to use spectrum effectively. However, accurate modeling of spectrum utilization is crucial for the development and performance evaluation of such systems. Hence, in this paper, a wideband multidimensional spectrum occupancy measurement campaign is conducted to study the evolution of RF spectrum over time, frequency, and space dimensions simultaneously. The measurements are performed over three consecutive days considering 700-3000 MHz frequency band at four different locations concurrently. The measurement results show low utilization of the 700-3000 MHz frequency band with different utilization percentage at each location. Furthermore, the measurements confirm that the spectrum occupancy highly depends on the time, frequency, and location. As a result, multidimensional spectrum measurement and analysis are vital for accurate spectrum utilization modeling and performance evaluation of CR systems.
Personal, Indoor and Mobile Radio Communications, 2009 IEEE 20th International Symposium on; 10/2009
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ABSTRACT: In cognitive radio systems, information is transmitted and received over multiple dispersed bands using dispersed spectrum utilization approach whereas information is transmitted and received over a single band using whole spectrum utilization approach. In this paper, performance of both approaches are compared theoretically considering CRLB and maximum likelihood (ML) method for time delay estimation. The results show that dispersed spectrum utilization methods have a great potential to exploit the efficiency of spectrum utilization and support goal driven and autonomous cognitive radio systems.
Cognitive Radio Oriented Wireless Networks and Communications, 2009. CROWNCOM '09. 4th International Conference on; 07/2009
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ABSTRACT: Direct conversion receivers are attractive for low cost systems as they avoid intermediate frequency (IF) filters. However, the direct conversion from radio frequencies (RF) to complex inphase (I) and quadrature (Q) baseband signals in one mixing step introduces additional front-end distortions. These IQ distortions lead to a degradation in the system performance. The problem becomes more significant in orthogonal frequency division multiple access (OFDMA) systems where multiple user signals with different IQ impairments are combined in the uplink (UL) signal. In this paper, detection methods for OFDMA-UL signals corrupted by IQ distortions are investigated. The received signal as a function of transmitted signals, IQ parameters, and communication channels is mathematically formulated. We designed a novel pilot pattern that is used by two proposed estimation and compensation methods of IQ impairments to the signal. Proposed methods were shown to significantly improve the system performance.
Communications, 2009. ICC '09. IEEE International Conference on; 07/2009
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ABSTRACT: The recent growing interest in underwater acoustic communication (UAC) requires an in depth understanding of a vast and diverse water medium. The UAC channel presents many difficulties such as high frequency, time, and space selectivity, frequency dependent noise, and significant range and band limitation on transmission. The traditional channel parameter measurement and estimation methods may not be sufficient and sometimes inapplicable to UAC, as many of the underwater situations are beyond human reach. This paper will first identify and classify the UAC channel environment, its parameters, parameter measurement and estimation methods. Then two novel cognitive intelligent algorithms will be presented to measure and estimate the parameters and map them to appropriate UAC channel environment models. To the best knowledge of the authors, the concept of cognitive intelligence (CI) and the proposed CI algorithms in UAC channel parameter measurement, estimation, and mapping ha ve not been addressed in literature to current date. Thus this research presents a pioneering work in this domain.
OCEANS 2009 - EUROPE; 06/2009
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ABSTRACT: Error vector magnitude (EVM) is one of the widely accepted figure of merits used to evaluate the quality of communication systems. In the literature, EVM has been related to signal-to-noise ratio (SNR) for data-aided receivers, where preamble sequences or pilots are used to measure the EVM, or under the assumption of high SNR values. In this paper, this relation is examined for nondata-aided receivers and is shown to perform poorly, especially for low SNR values or high modulation orders. The EVM for nondata-aided receivers is then evaluated and its value is related to the SNR for quadrature amplitude modulation (QAM) and pulse amplitude modulation (PAM) signals over additive white Gaussian noise (AWGN) channels and Rayleigh fading channels, and for systems with IQ imbalances. The results show that derived equations can be used to reliably estimate SNR values using EVM measurements that are made based on detected data symbols. Thus, presented work can be quite useful for measurement devices such as vector signal analyzers (VSA), where EVM measurements are readily available.
IEEE Transactions on Wireless Communications 06/2009; · 2.59 Impact Factor
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ABSTRACT: In this paper, we explore a method for interference suppression in EGPRS (also commonly known as EDGE) systems, wherein 8PSK is used as the modulation for the desired signal and the interferer is modulated using GMSK. Previous methods for single antenna interference cancellation (SAIC) have concentrated on the case when both the desired and interfering signals are GMSK modulated, or have relied on highly complex methods such as joint demodulation. We derive a simple method that exploits the redundancy inherent in the interferer signal to suppress it. The interference suppression method causes a well- modeled nonlinear distortion to the 8PSK signal. This non-linear distortion is handled using a modified equalizer that deals with a time-varying input symbol constellation applied to a matrix channel. Simulation results using the method show great promise.
Vehicular Technology Conference, 2009. VTC Spring 2009. IEEE 69th; 05/2009
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ABSTRACT: Separating user signals in the uplink (UL) of an orthogonal frequency division multiple access (OFDMA) system without access to the subcarrier assignment scheme (SAS) is a challenging task, but it can have a number of useful applications. In this paper, a semi-blind user separation algorithm is proposed, which assumes time synchronization and availability of information on the type of SAS used. The proposed algorithm estimates the carrier frequency offsets and time delays of each frequency allocation block by exploiting the cross-correlations over pilot subcarriers. A clustering method to group the blocks is employed, where each group belongs to a different user. Mathematical model of the proposed algorithm is presented, and possible user separation applications in OFDMA-based femtocells are discussed. Feasibility of the algorithm is proved through practical simulations.
Vehicular Technology Conference, 2009. VTC Spring 2009. IEEE 69th; 05/2009
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ABSTRACT: Cognitive radio is a novel concept that enables wireless systems to sense the environment, adapt, and learn from previous experience to improve the quality of the communication. However, CR requires a flexible and adaptive physical layer in order to perform the required tasks efficiently. In this article, CR systems and their requirement of a physical layer are discussed, and the orthogonal frequency division multiplexing technique is investigated as a candidate transmission technology for CR. The challenges that arise from employing OFDM in CR systems are identified. The cognitive properties of some OFDM-based wireless standards also are discussed to indicate the trend toward a more cognitive radio.
IEEE Wireless Communications 05/2009; · 2.58 Impact Factor
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ABSTRACT: The number of multiple-input-multiple-output (MIMO) orthogonal frequency-division multiple-access (OFDMA) systems is expected to sharply increase in the near future. Engineers who need to test these systems face two difficulties: 1) the lack of descriptive instructions to conduct reliable measurements and 2) the increased hardware cost due to the need for multiple transmitters and receivers. This paper first introduces all the measurable parameters of MIMO-OFDMA systems and provides a clear guide in performing the measurements specific to various parts of the system. Then, it proposes the implementation of the reception of MIMO-OFDMA signals using a single receiver rather than multiple receivers. For this purpose, impairments related to each of the radio frequency front-end components are investigated. The challenges of MIMO-OFDMA measurements are addressed in comparison with single-input single-output measurements. A complete procedure is provided to receive and perform impairment estimation for Worldwide Interoperability for Microwave Access (WiMAX) MIMO signals using a single receiver according to the IEEE 802.16 Standards.
IEEE Transactions on Instrumentation and Measurement 04/2009; · 1.21 Impact Factor
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ABSTRACT: In this paper, fundamental limits on time delay estimation are studied for cognitive radio systems, which facilitate opportunistic use of spectral resources. First, a generic Cramer-Rao lower bound (CRLB) expression is obtained in the case of unknown channel coefficients and carrier-frequency offsets (CFOs) for cognitive radio systems with dispersed spectrum utilization. Then, various modulation schemes are considered, and the effects of unknown channel coefficients and CFOs on the accuracy of time delay estimation are quantified. Finally, numerical studies are performed in order to verify the theoretical analysis.
IEEE Transactions on Wireless Communications 02/2009; · 2.59 Impact Factor
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ABSTRACT: In this study, the impact of mobility is investigated in low-speed environments such as femtocells and picocells for wireless networks. Given that there is interference on the uplink of a FDD system, this study solely focuses on how interference evolves with respect to mobility of terminals which move in a random fashion. Wiener-Levy process is used as a stochastic tool for characterizing the impact of mobility on the future behavior of interference. The results show that there is a trade-off between short and long interference observation (measurement) interval. On the one hand, choosing a short interval leads to a waste of processing power, since the interference level to be observed is not expected to deviate drastically from the previous observations. Choosing a long interval, on the other hand, increases the variance of the density of future interference level. In addition, results show that if there is more than one interference source in motion, the interference level observed has a tendency to increase in the future in low mobility environments. It is also shown that mean value of the interference level density to be observed in the future increases, whereas its standard deviation decreases with respect to the number of interference sources in motion.
Global Telecommunications Conference, 2008. IEEE GLOBECOM 2008. IEEE; 01/2009
