237 research outputs found
Asymmetric electron energy sharing in strong-field double ionization of helium
With the classical three-dimensional ensemble model, we have investigated the
microscopic recollision dynamics in nonsequential double ionization of helium
by 800 nm laser pulses at 2.0 PW/cm^2. We demonstrate that the asymmetric
energy sharing between the two electrons at recollision plays a decisive role
in forming the experimentally observed V-shaped structure in the correlated
longitudinal electron momentum spectrum at the high laser intensity [Phys. Rev.
Lett. 99, 263003 (2007)]. This asymmetric energy sharing recollision leaves
footprints on the transverse electron momentum spectra, which provide a new
insight into the attosecond three-body interactions
Multiphoton Rabi Oscillations of Correlated Electrons in Strong Field Nonsequential Double Ionization
With quantum calculations, we have investigated the multiphoton nonsequential
double ionization of helium atoms in intense laser fields at ultraviolet
wavelengths. Very surprisingly, we find a so-far unobserved double-circle
structure in the correlated electron momentum spectra. The double-circle
structure essentially reveals multiphoton Rabi oscillations of two electrons,
which are strongly supported by the oscillating population of a certain doubly
excited state and by the oscillating double ionization signals. This
two-electron multiphoton Rabi effect provides profound understandings of
electronic correlations and complicated multiphoton phenomena and is expected
to be a new tool for broad applications, such as quantum coherent control.Comment: 5 pages, 5 figure
BER Analysis of Decision-Feedback Multiple Symbol Detection in Noncoherent MIMO Ultra-Wideband Systems
In this paper, we investigate noncoherent multiple-input multiple-output
(MIMO) ultra-wideband (UWB) systems where the signal is encoded by differential
space-time block code (DSTBC). DSTBC enables noncoherent MIMO UWB systems to
achieve diversity gain. However, the traditional noncoherent symbol-by-symbol
differential detection (DD) for DSTBC-UWB suffers from performance degradation
compared with the coherent detection. We introduce a noncoherent multiple
symbol detection (MSD) scheme to enhance the performance of DSTBC-UWB system.
Although the MSD scheme can boost the performance more as the observation
window size gets to larger, the complexity of the exhaustive search for MSD
also exponentially increases in terms of the window size. To decrease the
computational complexity, the concept of decision-feedback (DF) is introduced
to MSD for DSTBC-UWB in this paper. The resultant DF-MSD yields reasonable
complexity and also solid performance improvement. We provide the bit error
rate (BER) analysis for the proposed DF-MSD. Both theoretical analysis and
simulation results validate the proposed scheme.Comment: 6 pages, 3 figures, accepted by IEEE TV
Revisiting the tunnelling site of electrons in strong field enhanced ionization of molecules
We investigated electron emissions in strong field enhanced ionization of
asymmetric diatomic molecules by quantum calculations. It is demonstrated that
the widely-used intuitive physical pic- ture, i.e., electron wave packet direct
ionization from the up-field site (DIU), is incomplete. Besides DIU, we find
another two new ionization channels, the field-induced excitation with
subsequent ionization from the down-field site (ESID), and the up-field site
(ESIU). The contributions from these channels depend on the molecular asymmetry
and internuclear distance. Our work provides a more comprehensive physical
picture for the long-standing issue about enhanced ionization of diatomic
molecules
Blind Channel Separation in Massive MIMO System under Pilot Spoofing and Jamming Attack
We consider a channel separation approach to counter the pilot attack in a
massive MIMO system, where malicious users (MUs) perform pilot spoofing and
jamming attack (PSJA) in uplink by sending symbols to the basestation (BS)
during the channel estimation (CE) phase of the legitimate users (LUs). More
specifically, the PSJA strategies employed by the MUs may include (i) sending
the random symbols according to arbitrary stationary or non-stationary
distributions that are unknown to the BS; (ii) sending the jamming symbols that
are correlative to those of the LUs. We analyze the empirical distribution of
the received pilot signals (ED-RPS) at the BS, and prove that its
characteristic function (CF) asymptotically approaches to the product of the
CFs of the desired signal (DS) and the noise, where the DS is the product of
the channel matrix and the signal sequences sent by the LUs/MUs. These
observations motivate a novel two-step blind channel separation method, wherein
we first estimate the CF of DS from the ED-RPS and then extract the alphabet of
the DS to separate the channels. Both analysis and simulation results show that
the proposed method achieves good channel separation performance in massive
MIMO systems
Quenching effect in below-threshold high harmonic generation
We theoretically demonstrate the quenching effect in below-threshold high
harmonic generation (HHG) by using the time-dependent density-functional theory
(TDDFT) and solving the time-dependent Schr\"{o}dinger equation (TDSE). It is
shown that the HHG is substantially suppressed in particular harmonic orders in
the below-threshold region when multi-electron interaction comes into play. The
position of the suppression is determined by the energy gap between the highest
occupied orbital and the higher-lying orbital of the target. We show that the
quenching effect is due to a new class of multi-electron dynamics involving
electron-electron energy transfer, which is analog to the fluorescence
quenching owing to the energy transfer between molecules in fluorescent
material. This work reveals the important role of the multi-electron
interaction on HHG especially in the below-threshold region.Comment: 5 figure
Exit momentum and instantaneous ionization rate of nonadiabatic tunneling ionization in elliptically polarized laser fields
Based on the strong-field approximation, we obtain analytical expressions for
the initial momentum at the tunnel exit and instantaneous ionization rate of
tunneling ionization in elliptically polarized laser fields with arbitrary
ellipticity. The tunneling electron reveals a nonzero offset of the initial
momentum at the tunnel exit in the elliptically polarized laser field. We find
that the transverse and longitudinal components of this momentum offset with
respect to the instantaneous field direction are directly related to the time
derivatives of the instantaneous laser electric field along the angular and
radial directions, respectively. We further show that the nonzero initial
momentum at the tunnel exit has a significant influence on the laser phase
dependence of the instantaneous ionization rate in the nonadiabatic tunneling
regime.Comment: 7 page, 4 figures, accepted by Phys. Rev.
Classical simulations including electron correlations for sequential double ionization
With a classical ensemble model that including electron correlations during
the whole ionization process, we investigated strong-field sequential double
ionization of Ar by elliptically polarized pulses at the quantitative level.
The experimentally observed intensity-dependent three-band or four-band
structures in the ion momentum distributions are well reproduced with this
classical model. More importantly, the experimentally measured ionization time
of the second electrons [A. N. Pfeiffer et al., Nature Phys. 7, 428 (2011)],
which can not be predicted by the standard independent-electron model, is
quantitatively reproduced by this fully classical correlated model. The success
of our work encourages classical description and interpretation of the complex
multi-electron effects in strong field ionization where nonperturbative quantum
approaches are currently not feasible
An Improved Diversity Combining Receiver for Layered ACO-FOFDM in IM/DD Systems
In this paper, an improved receiver based on diversity combining is proposed
to improve the bit error rate (BER) performance of layered asymmetrically
clipped optical fast orthogonal frequency division multiplexing (ACO-FOFDM) for
intensity-modulated and direct-detected (IM/DD) optical transmission systems.
Layered ACO-FOFDM can compensate the weakness of traditional ACO-FOFDM in low
spectral efficiency, the utilization of discrete cosine transform in FOFDM
system instead of fast Fourier transform in OFDM system can reduce the
computational complexity without any influence on BER performance. The BER
performances of layered ACO-FOFDM system with improved receiver based on
diversity combining and DC-offset FOFDM (DCO-FOFDM) system with optimal DC-bias
are compared at the same spectral efficiency. Simulation results show that
under different optical bit energy to noise power ratios, layered ACO-FOFDM
system with improved receiver has 2.86dB, 5.26dB and 5.72dB BER performance
advantages at forward error correction limit over DCO-FOFDM system when the
spectral efficiencies are 1 bit/s/Hz, 2 bits/s/Hz and 3 bits/s/Hz,
respectively. Layered ACO-FOFDM system with improved receiver based on
diversity combining is suitable for application in the adaptive IM/DD systems
with zero DC-bias.Comment: This manuscript has been published in Photonic Network
Communications, we replace the previous version submitted on March 12th due
to the modification of manuscript titl
Faster-than-Nyquist Non-Orthogonal Frequency-Division Multiplexing for Visible Light Communications
In this paper, we propose a faster-than-Nyquist (FTN) non-orthogonal
frequency-division multiplexing (NOFDM) scheme for visible light communications
(VLC) where the multiplexing/demultiplexing employs the inverse fractional
cosine transform (IFrCT)/FrCT. Different to the common fractional Fourier
transform-based NOFDM (FrFT-NOFDM) signal, FrCT-based NOFDM (FrCT-NOFDM) signal
is real-valued which can be directly applied to the VLC systems without the
expensive upconversion. Thus, FrCT-NOFDM is more suitable for the
cost-sensitive VLC systems. Meanwhile, under the same transmission rate,
FrCT-NOFDM signal occupies smaller bandwidth compared to OFDM signal. When the
bandwidth compression factor is set to , bandwidth saving
can be obtained. Therefore, FrCT-NOFDM has higher spectral efficiency and
suffers less high-frequency distortion compared to OFDM, which benefits the
bandwidth-limited VLC systems. As the simulation results show, bit error rate
(BER) performance of FrCT-NOFDM with of or is better than
that of OFDM. Moreover, FrCT-NOFDM has a superior security performance. In
conclusion, FrCT-NOFDM shows great potential for application in the future VLC
systems.Comment: Under review of Journal of Lightwave Technolog
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