7,247 research outputs found
Multiband Spectrum Access: Great Promises for Future Cognitive Radio Networks
Cognitive radio has been widely considered as one of the prominent solutions
to tackle the spectrum scarcity. While the majority of existing research has
focused on single-band cognitive radio, multiband cognitive radio represents
great promises towards implementing efficient cognitive networks compared to
single-based networks. Multiband cognitive radio networks (MB-CRNs) are
expected to significantly enhance the network's throughput and provide better
channel maintenance by reducing handoff frequency. Nevertheless, the wideband
front-end and the multiband spectrum access impose a number of challenges yet
to overcome. This paper provides an in-depth analysis on the recent
advancements in multiband spectrum sensing techniques, their limitations, and
possible future directions to improve them. We study cooperative communications
for MB-CRNs to tackle a fundamental limit on diversity and sampling. We also
investigate several limits and tradeoffs of various design parameters for
MB-CRNs. In addition, we explore the key MB-CRNs performance metrics that
differ from the conventional metrics used for single-band based networks.Comment: 22 pages, 13 figures; published in the Proceedings of the IEEE
Journal, Special Issue on Future Radio Spectrum Access, March 201
Swift BAT Survey of AGN
We present the results of the analysis of the first 9 months of data of the
Swift BAT survey of AGN in the 14-195 keV band. Using archival X-ray data or
follow-up Swift XRT observations, we have identified 129 (103 AGN) of 130
objects detected at |b|> 15 deg and with significance >4.8 sigma. One source
remains unidentified. These same X-ray data have allowed measurement of the
X-ray properties of the objects. We fit a power law to the log N - log S
distribution, and find the slope to be 1.42+/-0.14. Characterizing the
differential luminosity function data as a broken power law, we find a break
luminosity log L_*(erg/s) = 43.85+/-0.26, a low luminosity power law slope
a=0.84^{+0.16}_{-0.22}, and a high luminosity power law slope
b=2.55^{+0.43}_{-0.30}, similar to the values that have been reported based on
INTEGRAL data. We obtain a mean photon index 1.98 in the 14-195 keV band, with
an rms spread of 0.27. Integration of our luminosity function gives a local
volume density of AGN above 10^{41} erg/s of 2.4x10^{-3}/Mpc^3, which is about
10% of the total luminous local galaxy density above M_*=-19.75. We have
obtained X-ray spectra from the literature and from Swift XRT follow-up
observations. These show that the distribution of log n_H is essentially flat
from n_H=10^{20}/cm^{2} to 10^{24}/cm^2, with 50% of the objects having column
densities of less than 10^{22}/cm^{2}. BAT Seyfert galaxies have a median
redshift of 0.03, a maximum log luminosity of 45.1, and approximately half have
log n_H > 22.Comment: 15 pages, 15 figures, 2 tables; to appear in the Astrophysical
Journal, July 10, 2008, v. 68
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