36,236 research outputs found
A DHT-Based Multicarrier Modulation System with Pairwise ML Detection
This paper presents a complex-valued discrete multicarrier modulation (MCM) system based on the real-valued discrete Hartley transform (DHT) and its inverse (IDHT). Unlike the conventional discrete Fourier transform (DFT), the DHT cannot diagonalize multipath fading channels due to its inherent properties, and this results in mutual interference between subcarriers of the same mirror-symmetrical pair. We explore this interference pattern in order to seek an optimal solution to utilize channel diversity for enhancing the bit error rate (BER) performance of the system. It is shown that the optimal channel diversity gain can be achieved via pairwise maximum likelihood (ML) detection, taking into account not only the subcarrier's own channel quality but also the channel state information of its mirror-symmetrical peer. Performance analysis indicates that DHT-based MCM can mitigate fast fading effects by averaging channel power gains of each mirror-symmetrical pair of subcarriers. Simulation results show that the proposed scheme has a substantial improvement in BER over the conventional DFT-based MCM system
Multi-wavelength variability study of the classical BL Lac object PKS 0735+178 on timescales ranging from decades to minutes
We present the results of our power spectral analysis for the BL Lac object
PKS 0735+178 utilizing the Fermi-LAT survey at high-energy -rays,
several ground-based optical telescopes, and single-dish radio telescopes
operating at GHz frequencies. The novelty of our approach is that, by combining
long-term and densely sampled intra-night light curves in the optical regime,
we were able to construct for the first time the optical power spectrum of the
blazar for a time domain extending from 23 years down to minutes. Our analysis
reveals that: (i) the optical variability is consistent with a pure red noise,
for which the power spectral density can well be approximated by a single
power-law throughout the entire time domain probed; (ii) the slope of power
spectral density at high-energy -rays (), is significantly
flatter than that found at radio and optical frequencies () within the
corresponding time variability range; (iii) for the derived power spectra we
did not detect any low-frequency flattening, nor do we see any evidence for
cut-offs at the highest frequencies down to the noise floor levels due to
measurement uncertainties. We interpret our findings in terms of a model where
the blazar variability is generated by the underlying single stochastic process
(at radio and optical frequencies), or a linear superposition of such processes
(in the -ray regime). Along with the detailed PSD analysis, we also
present the results of our extended (1998-2015) intra-night optical monitoring
program and newly acquired optical photo-polarimetric data for the source.Comment: The Astrophysical Journal, in pres
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