Browsing by Author "Arslan, H."

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Open Access
Adaptation of two types of processing gains for UWB impulse radio wireless sensor networks
(The Institution of Engineering and Technology, 2007) Güvenç, I.; Arslan, H.; Gezici, Sinan; Kobayashi, H.
Ultrawideband impulse radio systems offer two kinds of processing gains that can be adapted based on the interference level in the system so that quality of service requirements are fulfilled. An adaptive assignment scheme for two types of multiple-access parameters in cluster-based wireless sensor networks is investigated. A mathematical framework is developed for asynchronous communications using a Gaussian approximation method to model the multiple-access interference in two cases: one with fixed frame duration, where the goal is to increase the average throughput, and the other with fixed symbol duration, where the goal is to increase the network lifetime. Extension of the analysis to multipath channels is carried out, and the validity of the Gaussian approximation is investigated using the Kullback-Leibler distance.
Open Access
Cognitive-radio systems for spectrum, location, and environmental awareness
(IEEE, 2010) Celebi, H.; Güvenç, I.; Gezici, Sinan; Arslan, H.
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.
Open Access
Evaluation of spontaneous third ventriculostomy by three-dimensional sampling perfection with application-optimized contrasts using different flip-angle evolutions (3D-SPACE) sequence by 3 T MR imaging: preliminary results with variant flip-angle mode
(Elsevier, 2013) Algın, Oktay; Turkbey, B.; Ozmen, E.; Ocakoglu, G.; Karaoglanoglu, M.; Arslan, H.
Purpose This prospective study aimed to evaluate the use of three-dimensional sampling perfection with application-optimized contrasts using different flip-angle evolutions (3D-SPACE) in the diagnosis of spontaneous third ventriculostomy (STV) and to compare it with phase-contrast cine magnetic resonance imaging (PC-MRI). Methods PC-MRI and 3D-SPACE images were obtained in 15 patients with hydrocephalus for evaluation of STV as well as in 10 control cases. The presence of STV was evaluated visually by two experienced radiologists on both PC-MRI and 3D-SPACE images, and the results were statistically compared. Results There was a strong correlation between PC-MRI and SPACE scores for both readers (correlation coefficient [r] = 0.784; P = 0.001). There was also a good correlation between PC-MRI scores and consensus-based results. Interobserver reliabilities were strong for all STV scores. In addition, there was excellent correlation between 3D-SPACE scores and consensus-based results (first reader's kappa value: 0.87; second reader's kappa value: 1). Conclusion 3D-SPACE can provide morphological–physiological information for the evaluation of STV with no need for additional PC-MRI analysis or other sequences. As a non-invasive test, it can also be included among the first line of choices of MRI sequences for patients with obstructive hydrocephalus.
Open Access
Fundamental limits on time delay estimation in dispersed spectrum cognitive radio systems
(IEEE, 2009) Gezici, Sinan; Celebi, H.; Poor, H. V.; Arslan, H.
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.
Open Access
Interference mitigation and awareness for improved reliability
(Cambridge University Press, 2011) Arslan, H.; Yarkan S.; Şahin, M. E.; Gezici, Sinan
Wireless systems are commonly affected by interference from various sources. For example, a number of users that operate in the same wireless network can result in multiple-access interference (MAI). In addition, for ultrawideband (UWB) systems, which operate at very low power spectral densities, strong narrowband interference (NBI) can have significant effects on the communications reliability. Therefore, interference mitigation and awareness are crucial in order to realize reliable communications systems. In this chapter, pulse-based UWB systems are considered, and the mitigation of MAI is investigated first. Then, NBI avoidance and cancelation are studied for UWB systems. Finally, interference awareness is discussed for short-rate communications, next-generation wireless networks, and cognitive radios.Mitigation of multiple-access interference (MAI)In an impulse radio ultrawideband (IR-UWB) communications system, pulses with very short durations, commonly less than one nanosecond, are transmitted with a low-duty cycle, and information is carried by the positions or the polarities of pulses [1-5]. Each pulse resides in an interval called frame, and the positions of pulses within frames are determined according to time-hopping (TH) sequences specific to each user. The low-duty cycle structure together with TH sequences provide a multiple-access capability for IR-UWB systems [6].Although IR-UWB systems can theoretically accommodate a large number of users in a multiple-access environment [2, 4], advanced signal processing techniques are necessary in practice in order to mitigate the effects of interfering users on the detection of information symbols efficiently [6]. © Cambridge University Press 2011.
Open Access
Theoretical limits on time delay estimation for ultra-wideband cognitive radios
(IEEE, 2008-09) Gezici, Sinan; Celebi, H.; Arslan, H.; Poor, H. V.
In this paper, theoretical limits on time delay estimation are studied for ultra-wideband (UWB) cognitive radio systems. For a generic UWB spectrum with dispersed bands, the Cramer-Rao lower bound (CRLB) is derived for unknown channel coefficients and carrier-frequency offsets (CFOs). Then, the effects of unknown channel coefficients and CFOs are investigated for linearly and non-linearly modulated training signals by obtaining specific CRLB expressions. It is shown that for linear modulations with a constant envelope, the effects of the unknown parameters can be mitigated. Finally, numerical results, which support the theoretical analysis, are presented. © 2008 IEEE.
Open Access
Time delay estimation in cognitive radio systems
(IEEE, 2009-12) Koçak, Fatih; Çelebi, H.; Gezici, Sinan; Qaraqe, K. A.; Arslan, H.; Poor, H. V.
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. © 2009 IEEE.
Open Access
Time-delay estimation in dispersed spectrum cognitive radio systems
(SpringerOpen, 2010) Kocak, F.; Celebi, H.; Gezici, Sinan; Qaraqe, K. A.; Arslan, H.; Poor, H. V.
Time-delay estimation is studied for cognitive radio systems, which facilitate opportunistic use of spectral resources. A two-step approach is proposed to obtain accurate time-delay estimates of signals that occupy multiple dispersed bands simultaneously, with significantly lower computational complexity than the optimal maximum likelihood (ML) estimator. In the first step of the proposed approach, an ML estimator is used for each band of the signal in order to estimate the unknown parameters of the signal occupying that band. Then, in the second step, the estimates from the first step are combined in various ways in order to obtain the final time-delay estimate. The combining techniques that are used in the second step are called optimal combining, signal-to-noise ratio (SNR) combining, selection combining, and equal combining. It is shown that the performance of the optimal combining technique gets very close to the Cramer-Rao lower bound at high SNRs. These combining techniques provide various mechanisms for diversity combining for time-delay estimation and extend the concept of diversity in communications systems to the time-delay estimation problem in cognitive radio systems. Simulation results are presented to evaluate the performance of the proposed estimators and to verify the theoretical analysis.