When the Milky Way turned off the lights: APOGEE provides evidence of star formation quenching in our Galaxy

Haywood, M.,Lehnert, M. D.,Di Matteo, P.,Snaith, O.,Schultheis, M.,Katz, D.,Gó,mez, A. Springer-Verlag 2016 Astronomy and astrophysics Vol.589 No.-

Joint Transmitter and Receiver Optimization for Improper-Complex Second-Order Stationary Data Sequence

여정호,조준호,James S. Lehnert 한국통신학회 2015 Journal of communications and networks Vol.17 No.1

In this paper, the transmission of an improper-complex second-order stationary data sequence is considered over a strictly band-limited frequency-selective channel. It is assumed that the transmitter employs linear modulation and that the channel output is corrupted by additive proper-complex cyclostationary noise. Under the average transmit power constraint, the problem of minimizing the mean-squared error at the output of a widely linear receiver is formulated in the time domain to find the optimal transmit and receive waveforms. The optimization problem is converted into a frequency-domain problem by using the vectorized Fourier transform technique and put into the form of a double minimization. First, the widely linear receiver is optimized that requires, unlike the linear receiver design with only one waveform, the design of two receive waveforms. Then, the optimal transmit waveform for the linear modulator is derived by introducing the notion of the impropriety frequency function of a discrete-time random process and by performing a line search combined with an iterative algorithm. The optimal solution shows that both the periodic spectral correlation due to the cyclostationarity and the symmetric spectral correlation about the origin due to the impropriety are well exploited.

FINANCING GROWTH: COMPARING THE EFFECTS OF FDI, ODA, AND INTERNATIONAL REMITTANCES

MAMOUN BENMAMOUN,KEVIN LEHNERT 중앙대학교 경제연구소 2013 Journal of Economic Development Vol.38 No.2

The world-wide emergence of remittances, in conjunction with challenges surrounding public foreign aid and the theoretical division regarding the development of FDI, prompt important questions as to whether international remittances outperform foreign aid and FDI as a determinant of a country’s economic growth. Using panel data from 1990-2006 and applying System-Generalized Method of Moments (GMM) approach, we show that international remittances, FDI, and ODA are positively and significantly associated with the economic growth rate of low income countries. Specifically, we find that the impact is greater with international remittances. Moreover, international remittances prove to be a greater contributor of economic growth than ODA and FDI even when countries are highly dependent on FDI. We conclude by stressing the need for policy and business responses to stimulate the flow and create an appropriate distribution of international remittances to make full use of international remittances developmental potential.

In Disguise or Out of Reach: First Clues about In Situ and Accreted Stars in the Stellar Halo of the Milky Way from <i>Gaia</i> DR2

Haywood, M.,Di Matteo, P.,Lehnert, M. D.,Snaith, O.,Khoperskov, S.,Gó,mez, A. American Astronomical Society 2018 The Astrophysical journal Vol.863 No.2

Asymptotically Optimal Low-Complexity SC-FDE With Noise Prediction in Data-Like Improper-Complex Interference

Kim, Minuk,Joon Ho Cho,Lehnert, James S. IEEE 2016 IEEE Transactions on Wireless Communications Vol.15 No.3

<P>In this paper, single-carrier (SC) block transmission of improper-complex data symbols is considered over a multiple-input multiple-output (MIMO) frequency-selective channel with data-like cochannel interference (CCI). To exploit the cyclostationarity and impropriety of the desired and the interfering signals, an equalizer is designed that oversamples the received signal and then processes the sampled block by using widely-linear (WL) feedforward (FF) and noise-predicting feedback (NP-FB) filters. For the given equalizer structure, the minimum mean-squared error (MMSE)-optimal WL FF and NP-FB filters have high computational complexity due to the augmented correlation matrix of the data-like CCI in the received signal. Motivated by the asymptotic property of the correlation matrix, we approximate in the frequency domain the matrix by a block matrix with diagonal blocks. This leads to the low-complexity WL design of a frequency-domain FF filter and a causally noise-predicting FB filter. It is shown that this MIMO SC frequency-domain equalizer with noise prediction is asymptotically optimal in the sense that the average mean-squared error converges to that of the MMSE-optimal equalizer as the block length tends to infinity. Numerical results show that the proposed equalizer performs well even with a moderate block length.</P>

Linear suppression of wideband data-like interference in DS/SS communications

Cho, Joon Ho,Jeong, Yeon Kyoon,Lehnert, J.S. IEEE 2005 IEEE journal on selected areas in communications Vol.23 No.5

In this paper, a linear receiver is proposed and its performance is analyzed for direct-sequence spread-spectrum communications with wideband data-like interference. Two groups of communicators employing generalized random spreading are considered, and the signals are assumed to have harmonically related cyclostationarity periods. First, the output signal-to-interference ratio (SIR) of a general correlation receiver is derived in the frequency domain, and the key system parameters that affect the structure of the interference are identified. Then, a linear interference suppression scheme based on the maximum SIR (MSIR) criterion is proposed, and a receiver structure similar to the cyclic Wiener filter is derived. Numerical results show that the proposed linear MSIR receiver significantly outperforms the conventional correlation receiver by effectively suppressing the wideband data-like interference.

Phylogeny of the Milky Way’s inner disk and bulge populations: Implications for gas accretion, (the lack of) inside-out thick disk formation, and quenching

Haywood, Misha,Di Matteo, Paola,Lehnert, Matthew,Snaith, Owain,Fragkoudi, Francesca,Khoperskov, Sergey Springer-Verlag 2018 Astronomy and astrophysics Vol.618 No.-

<P>We show that the bulge and the disk of the Milky Way (MW) at <I>R</I> ≲ 7 kpc are well described by a unique chemical evolution and a two-phase star formation history (SFH). We argue that the populations within this inner disk, not the entire disk, are the same, and that the outer Lindblad resonance (OLR) of the bar plays a key role in explaining this uniformity. In our model of a two-phase SFH, the metallicity, [<I>α</I>/Fe] and [<I>α</I>/H] distributions, and age-metallicity relation are all compatible with the observations of both the inner disk and bulge. The dip at [Fe/H] ∼ 0 dex seen in the metallicity distributions of the bulge and inner disk reflects the quenching episode in the SFH of the inner MW at age ∼8 Gyr, and the common evolution of the bulge and inner disk stars. Our results for the inner region of the MW, <I>R</I> ≲ 7 kpc, are consistent with a rapid build-up of a large fraction of its total baryonic mass within a few billion years. We show that at <I>z</I> ≤ 1.5, when the MW was starting to quench, transitioning between the end of the <I>α</I>-enhanced thick disk formation to the start of the thin disk, and yet was still gas rich, the gas accretion rate could not have been significant. The [<I>α</I>/Fe] abundance ratio before and after this quenching phase would be different, which is not observed. The decrease in the accretion rate and gas fraction at <I>z</I> ≤ 2 was necessary to stabilize the disk allowing the transition from thick to thin disks, and for beginning the secular phase of the MW’s evolution. This possibly permitted a stellar bar to develop which we hypothesize is responsible for quenching the star formation. The present analysis suggests that MW history, and in particular at the transition from the thick to the thin disk - the epoch of the quenching - must have been driven by a decrease of the star formation efficiency. We argue that the decline in the intensity of gas accretion, the formation of the bar, and the quenching of the star formation rate (SFR) at the same epoch may be causally connected thus explaining their temporal coincidence. Assuming that about 20% of the gas reservoir in which metals are diluted is molecular, we show that our model is well positioned on the Schmidt-Kennicutt relation at all times.</P>

Capacity of an Orthogonal Overlay Channel

Jeongho Yeo,Byung Wook Han,Joon Ho Cho,Lehnert, James S. IEEE 2015 IEEE Transactions on Wireless Communications Vol.14 No.11

<P>In this paper, the design of a selfless overlay system is considered for a frequency-selective bandlimited linear time-invariant channel already occupied by a legacy system. It is assumed that the transmitted signal by the legacy system is well modeled by a zero-mean second-order cyclostationary (SOCS) Gaussian random process. It is also assumed that the legacy receiver employs a linear-conjugate linear (LCL) filter to capture possible impropriety of the signal. Our objective is to maximize the throughput of the overlay system under the constraint that no interfering signal component is observed at the output of the legacy LCL filter sampled at a uniform rate. A relevant problem of deriving the constrained capacity of a discrete-time vector Gaussian noise channel is first examined. Then, it is shown that the optimal input to the overlay channel must be a zero-mean SOCS Gaussian random process. Using these results, the optimization problem is formulated in the frequency domain, and the procedure named orthogonal cyclic water filling (O-CWF) is derived to find the optimal channel input distribution. Numerical results are provided to show that the proposed selfless overlay scheme enables reliable communication even over a channel fully occupied by a legacy system.</P>

Using neutron methods SANS and PGAA to study evolution of structure and composition of alkali-doped polybenzimidazole membranes

Babcock, E.,Szekely, N.,Konovalova, A.,Lin, Y.,Appavou, M.-S.,Mangiapia, G.,Revay, Z.,Stieghorst, C.,Holderer, O.,Henkensmeier, D.,Lehnert, W.,Carmo, M. Elsevier 2019 Journal of membrane science Vol.577 No.-

<P><B>Abstract</B></P> <P>Potassium hydroxide (KOH) doped polybenzimidazole (PBI) membranes are investigated as compelling candidates for water electrolysis applications, drastically reducing the ohmic losses in contrast to thick ZrO<SUB>2</SUB> based diaphragms. Using small angle neutron scattering (SANS) we have found that the structure of the (KOH doped) PBI changes with doping time on a minute time scale, and that the development of the structure is highly dependent on the KOH concentration. This data is correlated with macroscopic measurements of membrane swelling resulting from the doping process which also occurs on a minute time scale. Then, using prompt gamma activation analysis (PGAA) to follow the changes in time of the chemical composition, we have found that the K concentration of these samples only increases slightly with doping times after a very rapid initial uptake, reaching a saturation value that is relatively independent of KOH concentration for long doping times of up to 24 h. However measurements of similarly doped samples show increases in ion-conductivity of nearly 3 fold, and resistivity reductions of over 2 fold on the same time scales. These measurements prove that PGAA is a sensitive method to follow changes in the chemical compositions during doping, while SANS can give information on the sub-micro structural changes of polymer electrolyte membranes. Since these methods can be correlated with <I>ex-situ</I> measurements of composition, resistance, ion-conductivity and macro-structure, the combined use of PGAA and SANS provides a promising means for <I>in-operando</I> study in order to elucidate changes in membrane performance due to electrochemical cycling, as well as to help characterize and optimize doping parameters though in-situ doping measurements, by enabling real-time study of such membrane systems.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Elucidating the doping of Alkali-doped Polybenzimidazole membranes. </LI> <LI> Complementary structure and composition information from SANS and PGAA to study functional membranes. </LI> <LI> Structure varies over doping time for KOH doped Polybenzimidazole membranes. </LI> <LI> New possibilities for in-situ and in-operando characterization. </LI> </UL> </P>