30,501 research outputs found
Selection biases in empirical p(z) methods for weak lensing
To measure the mass of foreground objects with weak gravitational lensing,
one needs to estimate the redshift distribution of lensed background sources.
This is commonly done in an empirical fashion, i.e. with a reference sample of
galaxies of known spectroscopic redshift, matched to the source population. In
this work, we develop a simple decision tree framework that, under the ideal
conditions of a large, purely magnitude-limited reference sample, allows an
unbiased recovery of the source redshift probability density function p(z), as
a function of magnitude and color. We use this framework to quantify biases in
empirically estimated p(z) caused by selection effects present in realistic
reference and weak lensing source catalogs, namely (1) complex selection of
reference objects by the targeting strategy and success rate of existing
spectroscopic surveys and (2) selection of background sources by the success of
object detection and shape measurement at low signal-to-noise. For
intermediate-to-high redshift clusters, and for depths and filter combinations
appropriate for ongoing lensing surveys, we find that (1) spectroscopic
selection can cause biases above the 10 per cent level, which can be reduced to
5 per cent by optimal lensing weighting, while (2) selection effects in the
shape catalog bias mass estimates at or below the 2 per cent level. This
illustrates the importance of completeness of the reference catalogs for
empirical redshift estimation.Comment: matches published version in MNRA
PPM demodulation: On approaching fundamental limits of optical communications
We consider the problem of demodulating M-ary optical PPM (pulse-position
modulation) waveforms, and propose a structured receiver whose mean probability
of symbol error is smaller than all known receivers, and approaches the quantum
limit. The receiver uses photodetection coupled with optimized phase-coherent
optical feedback control and a phase-sensitive parametric amplifier. We present
a general framework of optical receivers known as the conditional pulse nulling
receiver, and present new results on ultimate limits and achievable regions of
spectral versus photon efficiency tradeoffs for the single-spatial-mode
pure-loss optical communication channel.Comment: 5 pages, 6 figures, IEEE ISIT, Austin, TX (2010
Energy Efficient Transmission over Space Shift Keying Modulated MIMO Channels
Energy-efficient communication using a class of spatial modulation (SM) that
encodes the source information entirely in the antenna indices is considered in
this paper. The energy-efficient modulation design is formulated as a convex
optimization problem, where minimum achievable average symbol power consumption
is derived with rate, performance, and hardware constraints. The theoretical
result bounds any modulation scheme of this class, and encompasses the existing
space shift keying (SSK), generalized SSK (GSSK), and Hamming code-aided SSK
(HSSK) schemes as special cases. The theoretical optimum is achieved by the
proposed practical energy-efficient HSSK (EE-HSSK) scheme that incorporates a
novel use of the Hamming code and Huffman code techniques in the alphabet and
bit-mapping designs. Experimental studies demonstrate that EE-HSSK
significantly outperforms existing schemes in achieving near-optimal energy
efficiency. An analytical exposition of key properties of the existing GSSK
(including SSK) modulation that motivates a fundamental consideration for the
proposed energy-efficient modulation design is also provided
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