Population adaptation for genetic algorithm-based cognitive radios

Abstract — Genetic algorithms are best suited for optimization problems involving large search spaces. The problem space encountered when optimizing the transmission parameters of an agile or cognitive radio for a given wireless environment and set of performance objectives can become prohibitively large due to the high number of parameters and their many possible values. Recent research has demonstrated that genetic algorithms are a viable implementation technique for cognitive radio engines. However, the time required for the genetic algorithms to come to a solution substantionally increases as the system complexity grows. In this paper, we present a population adaptation technique for genetic algorithms that takes advantage of the information from previous cognition cycles in order to reduce the time required to reach an optimal decision. Our simulation results demonstrate that the amount of information from the previous cognition cycle can be determined from the environmental variation factor (EVF), which represents the amount of change in the environment parameters since the previous cognition cycle. I

Measurements and Analysis of Secondary User Device Effects on Digital Television Receivers

This is the published version. Copyright © 2009 Newman et al.This article presents results from a study of the potential effects of secondary users operating in unoccupied television spectrum. Television spectrum is known within the wireless communications community as being underutilized, making it a prime candidate for dynamic spectrum access. The proposed use of this open spectrum has prompted questions concerning the quantity of available channel space that could be used without negative impact on consumers who view digital television broadcasts and the viability of secondary use of open channels immediately adjacent to a digital television broadcast channel. In this work, we investigate secondary device operation in the channels directly adjacent to a desired television channel, and the effects upon a selection of consumer digital television (DTV) receivers. Our observations strongly suggest that secondary users could operate "White Space Devices" (WSDs) in unoccupied channel bandwidth directly adjacent to a desired digital television (DTV) channel, with no observable adverse impact upon the reception of the desired channel content

Influence of mitral valve repair versus replacement on the development of late functional tricuspid regurgitation

ObjectivesTo study the determinants of functional tricuspid regurgitation (TR) progression after surgical correction of mitral regurgitation, including the influence of mitral valve (MV) repair (MVr) versus replacement (MVR) for degenerative mitral regurgitation.MethodsFrom January 1995 to January 2006, 747 adults with MV prolapse underwent isolated MVr (n = 683) or MVR (n = 64; mechanical in 32). The mean age was 60.8 years, and 491 were men (66.0%). Moderate preoperative functional TR was present in 115 (15.4%). The MVR group had a greater likelihood of New York Heart Association class III or IV (75.0% vs 34.4%, P < .001), atrial fibrillation (20.3% vs 8.3%, P = .002), a lower left ventricular ejection fraction (61.0% vs 65.2%, P < .003), and a higher pulmonary artery pressure (50.1 vs 41.2 mm Hg, P = .001). The patients were monitored for a mean of 6.9 years (MVr) or 7.7 years (MVR; P = .075).ResultsDuring late follow-up, no difference was found between the groups in the development of moderately severe or severe TR: 1 to 5 years (3.0% vs 3.3%, P = .91) and >5 years (6.1% vs 6.5%; P = .93). The univariate predictors of severe TR after 5 years were older age (hazard ratio [HR], 1.1; P = .011), female gender (HR, 6.86; P = .005), higher pulmonary artery pressure (HR, 1.05; P = .022), and larger left atrial size (HR, 2.11; P = .035). Two patients (0.26%) who had undergone initial MVr required reoperation for late functional TR. Another 2 patients had had the tricuspid valve addressed concurrent with reoperation for MVr failure. No tricuspid reoperations were required in the MVR group.ConclusionsThe risk of TR progression was low after MVr or MVR for MV prolapse. Timely MV surgery before the development of left atrial dilatation or pulmonary hypertension could further decrease the risk of TR progression during follow-up

OFDM-based cognitive radio for DSA networks

Dissertation (Ph.D.)--University of Kansas, Electrical Engineering & Computer Science, 2007.With the advent of new high data rate wireless applications, as well as growth of existing wireless services, demand for additional bandwidth is rapidly increasing. Existing "command-and-control" spectrum allocations defined by government regulatory agencies, such as federal communications commission (FCC), prohibit unlicensed access to licensed spectrum, constraining them instead to several heavily populated, interference-prone frequency bands. However, it has been shown that the spectrum is not utilized efficiently across time and frequency. Therefore, FCC is currently working on the concept of unlicensed users "borrowing" spectrum from incumbent license holders temporarily to improve the spectrum utilization. This concept is called dynamic spectrum access (DSA). Cognitive radios offer versatile, powerful, and portable wireless transceivers enabling DSA. This dissertation investigates physical layer techniques to enhance a cognitive radio performance. Multi-carrier techniques, such as orthogonal frequency division multiplexing (OFDM), support huge data rates that are robust to channel impairments. However, with a growing demand for spectrum access, it may be difficult for any single transmission to obtain a large contiguous frequency spectrum block in DSA environment. This dissertation proposes a novel non-contiguous OFDM (NC-OFDM) technique, where the implementation achieves high data rates via collective usage of a large number of non-contiguous subcarriers while simultaneously avoiding any interference to the existing transmissions by turning off the subcarriers corresponding to these spectrum bands. Moreover, the performance of the proposed NC-OFDM technique is compared with other multi-carrier technique, such as multi-carrier code division multiple access (MC-CDMA). One of the major drawbacks of an OFDM signal is that it may exhibit large peak-to-average power ratio (PAPR) values resulting in expensive transceivers. This dissertation presents an extensive overview and taxonomy of the PAPR reduction algorithms proposed in the literature. This dissertation presents five novel techniques for reducing PAPR of an OFDM signal viz., subcarrier phase adjustment algorithm, adaptive-mode PAPR reduction algorithm, subcarrier power adjustment algorithm, variable throughput algorithm, and low correlation phase sequences-based algorithm. Due to the noncontiguous nature of the spectrum in DSA environment, the statistical properties of the PAPR of an NC-OFDM signal are different from that of an OFDM signal. Additionally, the conventional PAPR reduction algorithms need to be carefully chosen and modified to be used for reducing the PAPR of an NC-OFDM signal. This dissertation investigates the statistical properties of the PAPR of an NC-OFDM signal and lay out design requirements for the PAPR reduction algorithms for an NC-OFDM signal. OFDM technique implements fast Fourier transform (FFT) algorithms for modulating and demodulating the OFDM signal. This dissertation proposes an efficient general FFT pruning algorithm for improving the efficiency of the FFT evaluation by reducing the arithmetic operations, when a large number of subcarriers are deactivated. Given that the cognitive radios employing NC-OFDM need to quickly adapt to the changing operating environment, and that the hardware resources of small form factor cognitive radios are limited, such an algorithm would be beneficial

DSA Networks

With the advent of new high data rate wireless applications, as well as growth of existing wireless services, demand for additional bandwidth is rapidly increasing. Existing “command-and-control ” spectrum allocations defined by government regulatory agencies, such as federal communications commission (FCC), prohibit unlicensed access to licensed spectrum, constraining them instead to several heavily populated, interference-prone frequency bands. However, it has been shown that the spectrum is not utilized efficiently across time and frequency. Therefore, FCC is currently working on the concept of unlicensed users “borrowing ” spectrum from incumbent license holders temporarily to improve the spectrum utilization. This concept is called dynamic spectrum access (DSA). Cognitive radios offer versatile, powerful, and portable wireless transceivers enabling DSA. This dissertation investigates physical layer techniques to enhance a cognitive radio performance. Multi-carrier techniques, such as orthogonal frequency division multiplexing (OFDM), support huge data rates that are robust to channel impairments. However, with a growin

An efficient implementation of NC-OFDM transceivers for cognitive radios

In this paper, we present an efficient implementation of a non-contiguous orthogonal frequency division multiplexing (NC-OFDM) transceiver for cognitive radio systems. NC-OFDM is designed to transmit information in the presence of incumbent users, deactivating subcarriers located in the vicinity of these users to avoid interference. Given that the core component of an NC-OFDM transceiver is the fast Fourier transform (FFT), and that several of the subcarriers are deactivated, it is possible to reduce the execution time by “pruning ” the FFT. We propose an algorithm that efficiently and quickly prunes the FFT for NC-OFDM transceivers. Results show that the proposed algorithm substantially outperforms other FFT pruning algorithms when a medium to large number of subcarriers have been deactivated.

SUBCARRIER POWER ADJUSTMENT TECHNIQUE FOR PEAK-TO-AVERAGE POWER RATIO REDUCTION OF OFDM SYSTEMS

Abstract — In this paper, we propose a novel peakto-average power ratio (PAPR) reduction algorithm for OFDM systems that employs a two-step approach for adjusting the subcarrier power levels. The first step redistributes the power level across all subcarriers, which slightly reduces the error robustness of the system, while the second step ensures the subcarrier power distribution obeys a subband power constraint. The resulting power levels satisfy spectrum regulatory requirements across the transmission spectrum while simultaneously reducing the PAPR and maintaining a degree of error robustness. Since no overhead information is required, the proposed algorithm does not incur a throughput penalty. Simulation results for a 256-subcarrier OFDM system employing QPSK symbols and the proposed algorithm show a PAPR reduction of 1.5 dB for a complementary cumulative distribution of 0.1%. I

Adaptive-Mode Peak-to-Average Power Ratio Reduction Algorithm for OFDM-based Cognitive Radio

Abstract — In this paper, we present a novel low complexity algorithm for reducing the peak-to-average power ratio (PAPR) occurring in OFDM-based cognitive radios. Although several PAPR reduction algorithms exist in the literature, they are often only effective for specific scenarios. Our proposed algorithm exploits the agility of cognitive radio technology to rapidly choose and employ the appropriate PAPR reduction approach from a set of approaches to achieve a large decrease in PAPR, given the current operating conditions. The results show that for a wide range of operating conditions, the proposed algorithm achieves a large decrease in PAPR, unlikely the PAPR results when only a single reduction approach is employed across the same wide range. I