Hasari Celebi

Utilization of Location Information in Cognitive Wireless Networks

Location awareness is an essential characteristic of cognitive radios as well as networks. In this article a location awareness engine architecture is proposed for the realization of location awareness in cognitive radios and networks. The proposed architecture consists of location estimation and/or sensing, seamless positioning and interoperability, statistical learning and tracking, security and privacy, mobility management, and location-based applications. However, the focus of this article is on location-based applications where we demonstrate the utilization of location information in cognitive wireless networks by presenting some representative location-assisted network optimization applications such as location-assisted spectrum management, network planning and expansion, and handover. Our study unveils that location information can be used in cognitive wireless networks to optimize network performance. Possible solutions to the implementation issues are proposed, and the remaining open issues are also addressed.

Cognitive Positioning Systems

Location awareness is one of the essential characteristics of cognitive radios. In this paper, an innovative cognitive positioning system (CPS) that achieves accuracy adaptation in both indoor and outdoor environments is proposed as a step towards realization of location awareness in cognitive radios. The proposed CPS is composed of two modes; bandwidth determination and enhanced dynamic spectrum management (EDSM). Bandwidth determination equations are derived through Cramer-Rao Lower Bound (CRLB) for both additive white Gaussian noise (AWGN) and multipath channels. An EDSM system providing the optimum available bandwidth to the CPS is proposed. Overlay spectrum access based EDSM (O-EDSM) and hybrid overlay and underlay spectrum access based EDSM (H-EDSM) are two schemes that are introduced. A switching mechanism that manages the transition between underlay and overlay spectrum usage modes for the H-EDSM algorithm is presented. The theoretical analysis of the mechanism is carried out using Two-slope model. Simulation results, challenges and complexity options for the implementation of the CPS are outlined. Our study reveals the existence of a trade-off between the accuracy and complexity in the cognitive positioning systems.

Wireless IPTV over WiMAX: Challenges and Applications

Internet protocol television (IPTV) is gaining recognition as a viable alternative for the delivery of video by telecommunications and cable companies. It features bandwidth efficiencies, and management; therefore, it is ideally suited for broadcast, multicast, unicast interactive and multimedia services (IMS - IP multimedia). Core networks are packet-based, however, access networks are not always packet-based. Furthermore, they are wired as either fiber-coax or fiber-twisted pair topologies. Design, construction, and maintenance costs of such topologies are exorbitant, making the IPTV deployment economically unviable, especially in underdeveloped countries and rural areas. Worldwide interoperability for microwave access (WiMAX) offers a wireless solution in the access networks that can support IPTV services. WiMAX is capable of very high data rates, longer reach, and operates in non-line of sight (NLOS) modes in licensed and license exempt frequencies. Bandwidth is scalable and managed. Contents are encrypted for secured transmission and mobility is supported at vehicular speeds. Installation and maintenance costs of WiMAX systems are at a fraction of the costs of wired access networks. Taking advantage of these features, IPTV services can be designed, delivered, and managed cost effectively without compromising the video and audio quality. In this paper, the authors present IPTV over WiMAX implementation in such scenarios. The authors identify challenges and discuss ways to overcome them, especially those in medium access control (MAC), physical layer (PHY) and radio sub-systems. Despite of these challenges, it is observed that IPTV over WiMAX is very feasible.

Fundamental limits on time delay estimation in dispersed spectrum cognitive radio systems

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.

Enabling location and environment awareness in cognitive radios

Location and environment awareness are two prominent features of cognitive radios and networks enabling them to interact with and learn the operating environment. A cognitive radio architecture with location and environment awareness engines is introduced in this paper. Architectural framework of both engines along with their components are presented. The proposed architecture is a promising model to support advanced and autonomous location and environment-aware applications (e.g. advanced location-based services (LBS)). Implementation options, design challenges, issues, and potential solutions for the realization of both engines are discussed. An underlying method for both engines, which is range accuracy adaptation, is presented. Finally, concluding remarks with future research directions are provided.

An adaptive threshold method for spectrum sensing in multi-channel cognitive radio networks

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.

Empirical results for wideband multidimensional spectrum usage

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 700–3000MHz frequency band at four different locations concurrently. The measurement results show low utilization of the 700–3000MHz 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.

Adaptive Positioning Systems for Cognitive Radios

Location awareness is a prominent characteristic of cognitive radio technologies. Realizing such awareness in cognitive radios require an underlying advanced positioning system that have cognition capabilities. In this paper, an innovative cognitive positioning system (CPS) that have cognition capabilities by means of adjusting positioning accuracy adaptively in both indoor and outdoor environments is proposed. Bandwidth determination and hybrid overlay and underlay enhanced dynamic spectrum management (H-EDSM) are two main components that form the proposed CPS technique. Bandwidth determination equations are derived through Cramer-Rao lower bound (CRLB) for additive white Gaussian noise (AWGN). H-EDSM that provides optimum available bandwidth to the CPS is proposed. A switching mechanism that manages the transition between overlay and underlay spectrum usage modes for the H-EDSM algorithm is presented. Theoretical analysis of the mechanism is carried out using two-slope model. Simulation results, challenges and complexity options for the implementation of the CPS are outlined. Our study reveals the existence of a trade-off between accuracy and complexity in the adaptive positioning systems for cognitive radios.

Time-Delay estimation in dispersed spectrum cognitive radio systems

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.

An autoregressive approach for spectrum occupancy modeling and prediction based on synchronous measurements

Inefficient spectrum usage is a crucial issue in wireless communications and methods for dynamic spectrum access are proposed based on spectrum sensing methodology of the cognitive radio systems. Beside the detection and estimation methods, spectrum sensing procedures can also benefit from the modeling and prediction of the wireless spectrum usage. Markovian, regressive and other approaches are introduced for time or frequency domain channel modeling however, the research on the spectrum allocation methods indicates that location information has also an important influence on the spectrum occupancy characterization. In this paper, linear autoregressive prediction approach for binary time series is employed to investigate channel occupancy prediction performance based on spectrum measurements conducted in four different locations synchronously. Through the modeling procedure, dependency in frequency domain is also taken into consideration by modeling the adjacent frequency bands together. The model order is selected based on mean residual magnitudes and Akaike information criterion, mode order parameters are tabulated, and comparative prediction analysis considering the observation time is given for each location. The performance of the proposed linear modeling method is also compared with continuous-time Markov chain modeling in one of the locations.