arXiv subjects

Limin Xiao

Publications and source records attributed to Limin Xiao.

33 records · Page 2Linked to original sources

arXiv2020

Transmission Scheduling for Multi-loop Wireless Networked Control Based on LQ Cost Offset

In this paper, transmission scheduling for multiloop wireless networked control systems sharing the wireless channel is considered. A linear quadratic cost offset has been proposed to evaluate the performance gap induced by the nonideal communication. A functional relationship between linear quadratic offset metric and Age of Information has been built up. Based on the offset metric, we come up with an age-based scheduling policy and numerical simulations show that there is a significant improvement compared to the former work.

cs.IT↗

arXiv2019

Precise Measures of Orbital Period, Before and After Nova Eruption for QZ Aurigae

For the ordinary classical nova QZ Aurigae (which erupted in 1964), we report 1317 magnitudes from 1912--2016, including four eclipses detected on archival photographic plates from long before the eruption. We have accurate and robust measures of the orbital period both pre-eruption and post-eruption, and we find that the orbital period decreased, with a fractional change of -290.71+-0.28 parts-per-million across the eruption, with the orbit necessarily getting smaller. Further, we find that the light curve outside of eclipses and eruption is flat at near B=17.14 from 1912--1981, whereupon the average light curve starts fading down to B=17.49 with large variability. QZ Aur is a robust counter-example against the Hibernation model for the evolution of cataclysmic variables, where the model requires that all novae have their period increase across eruptions. Large period decreases across eruptions can easily arise from mass imbalances in the ejecta, as are commonly seen in asymmetric nova shells.

astro-ph.SR↗

arXiv2019

Fast Analog Beam Tracking in Phased Antenna Arrays: Theory and Performance

The directionality of millimeter-wave (mmWave) communications introduces a significant challenge in serving fast-rotating/moving terminals, e.g., mobile AR/VR, high-speed vehicles, trains, UAVs.This challenge is exacerbated in mmWave systems using analog beamforming, because of the inherent non-convexity in the analog beam tracking problem. In this paper, we obtain the Cramér-Rao lower bound (CRLB) of beam tracking and optimize the analog beamforming vectors to get the minimum CRLB. Then, we develop a low complexity analog beam tracking algorithm that simultaneously optimizes the analog beamforming vector and the estimate of beam direction. Finally, by establishing a new basic theory, we provide the theoretical convergence analysis of the proposed analog beam tracking algorithm, which proves that the minimum CRLB of the MSE is achievable with high probability. Our simulations show that this algorithm can achieve faster tracking speed, higher tracking accuracy and higher data rate than several state-of-the-art algorithms. The key analytical tools used in our algorithm design are stochastic approximation and recursive estimation with a control parameter.

cs.IT↗

arXiv2018

How to Mobilize mmWave: A Joint Beam and Channel Tracking Approach

Maintaining reliable millimeter wave (mmWave) connections to many fast-moving mobiles is a key challenge in the theory and practice of 5G systems. In this paper, we develop a new algorithm that can jointly track the beam direction and channel coefficient of mmWave propagation paths using phased antenna arrays. Despite the significant difficulty in this problem, our algorithm can simultaneously achieve fast tracking speed, high tracking accuracy, and low pilot overhead. In static scenarios, this algorithm can converge to the minimum Cramér-Rao lower bound of beam direction with high probability. Simulations reveal that this algorithm greatly outperforms several existing algorithms. Even at SNRs as low as 5dB, our algorithm is capable of tracking a mobile moving at an angular velocity of 5.45 degrees per second and achieving over 95\% of channel capacity with a 32-antenna phased array, by inserting only 10 pilots per second.

cs.IT↗

arXiv2017

Analog Beam Tracking in Linear Antenna Arrays: Convergence, Optimality, and Performance

The directionality of millimeter-wave (mmWave) communications creates a significant challenge in serving fast-moving mobile terminals on, e.g., high-speed vehicles, trains, and UAVs. This challenge is exacerbated in mmWave systems using analog antenna arrays, because of the inherent non-convexity in the control of the phase shifters. In this paper, we develop a recursive beam tracking algorithm which can simultaneously achieve fast tracking speed, high tracking accuracy, low complexity, and low pilot overhead. In static scenarios, this algorithm converges to the minimum Cramér-Rao lower bound (CRLB) of beam tracking with high probability. In dynamic scenarios, even at SNRs as low as 0dB, our algorithm is capable of tracking a mobile moving randomly at an absolute angular velocity of 10-20 degrees per second, using only 5 pilot symbols per second. If combining with a simple TDMA pilot pattern, this algorithm can track hundreds of high-speed mobiles in 5G configurations. Our simulations show that the tracking performance of this algorithm is much better than several state-of-the-art algorithms.

cs.IT↗

arXiv2016

Deployment of 5G Networking Infrastructure with Machine Type Communication Considerations

Designing optimal strategies to deploy small cell stations is crucial to meet the quality-of-service requirements in next-generation cellular networks with constrained deployment costs. In this paper, a general deployment framework is proposed to jointly optimize the locations of backhaul aggregate nodes, small base stations, machine aggregators, and multi-hop wireless backhaul links to accommodate both human-type and machine-type communications. The goal is to provide deployment solutions with best coverage performance under cost constraints. The formulated problem is shown to be a multi-objective integer programming for which it is challenging to obtain the optimal solutions. To solve the problem, a heuristic algorithm is proposed by combining Lagrangian relaxation, the weighted sum method, the $ε$-constraint method and tabu search to obtain both the solutions and bounds, for the objective function. Simulation results show that the proposed framework can provide solutions with better performance compared with conventional deployment models in scenarios where available fiber connections are scarce. Furthermore, the gap between obtained solutions and the lower bounds is quite tight.

cs.IT↗

arXiv2013

Capacity Region Bounds and Resource Allocation for Two-Way OFDM Relay Channels

In this paper, we consider two-way orthogonal frequency division multiplexing (OFDM) relay channels, where the direct link between the two terminal nodes is too weak to be used for data transmission. The widely known per-subcarrier decode-and-forward (DF) relay strategy, treats each subcarrier as a separate channel, and performs independent channel coding over each subcarrier. We show that this per-subcarrier DF relay strategy is only a suboptimal DF relay strategy, and present a multi-subcarrier DF relay strategy which utilizes cross-subcarrier channel coding to achieve a larger rate region. We then propose an optimal resource allocation algorithm to characterize the achievable rate region of the multi-subcarrier DF relay strategy. The computational complexity of this algorithm is much smaller than that of standard Lagrangian duality optimization algorithms. We further analyze the asymptotic performance of two-way relay strategies including the above two DF relay strategies and an amplify-and-forward (AF) relay strategy. The analysis shows that the multi-subcarrier DF relay strategy tends to achieve the capacity region of the two-way OFDM relay channels in the low signal-to-noise ratio (SNR) regime, while the AF relay strategy tends to achieve the multiplexing gain region of the two-way OFDM relay channels in the high SNR regime. Numerical results are provided to justify all the analytical results and the efficacy of the proposed optimal resource allocation algorithm.

cs.IT↗

arXiv2012

Optimal Power Allocation for Two-Way Decode-and-Forward OFDM Relay Networks

This paper presents a novel two-way decode-and-forward (DF) relay strategy for Orthogonal Frequency Division Multiplexing (OFDM) relay networks. This DF relay strategy employs multi-subcarrier joint channel coding to leverage frequency selective fading, and thus can achieve a higher data rate than the conventional per-subcarrier DF relay strategies. We further propose a low-complexity, optimal power allocation strategy to maximize the data rate of the proposed relay strategy. Simulation results suggest that our strategy obtains a substantial gain over the per-subcarrier DF relay strategies, and also outperforms the amplify-and-forward (AF) relay strategy in a wide signal-to-noise-ratio (SNR) region.

cs.IT↗

arXiv2010

Discovery of the 2010 Eruption and the Pre-Eruption Light Curve for Recurrent Nova U Scorpii

We report the discovery by B. G. Harris and S. Dvorak on JD 2455224.9385 (2010 Jan 28.4385 UT) of the predicted eruption of the recurrent nova U Scorpii (U Sco). We also report on 815 magnitudes (and 16 useful limits) on the pre-eruption light curve in the UBVRI and Sloan r' and i' bands from 2000.4 up to 9 hours before the peak of the January 2010 eruption. We found no significant long-term variations, though we did find frequent fast variations (flickering) with amplitudes up to 0.4 mag. We show that U Sco did not have any rises or dips with amplitude greater than 0.2 mag on timescales from one day to one year before the eruption. We find that the peak of this eruption occurred at JD 2455224.69+-0.07 and the start of the rise was at JD 2455224.32+-0.12. From our analysis of the average B-band flux between eruptions, we find that the total mass accreted between eruptions is consistent with being a constant, in agreement with a strong prediction of nova trigger theory. The date of the next eruption can be anticipated with an accuracy of +-5 months by following the average B-band magnitudes for the next ~10 years, although at this time we can only predict that the next eruption will be in the year 2020+-2.

astro-ph.SR↗

arXiv2010

Long-Term Light Curve of Highly-Variable Protostellar Star GM Cep

We present data from the archival plates at Harvard College Observatory and Sonneberg Observatory showing the field of the solar type pre-main sequence star GM Cep. A total of 186 magnitudes of GM Cep have been measured on these archival plates, with 176 in blue sensitivity, 6 in visible, and 4 in red. We combine our data with data from the literature and from the American Association of Variable Star Observers to depict the long-term light curves of GM Cep in both B and V wavelengths. The light curves span from 1895 until now, with two densely sampled regions (1935 to 1945 in B band, and 2006 until now in V band). The long-term light curves do not show any fast rise behavior as predicted by an accretion mechanism. Both the light curves and the magnitude histograms confirm the conclusion that the light curves are dominated by dips (possibly from extinction) superposed on some quiescence state, instead of outbursts caused by accretion flares.Our result excludes the possibility of GM Cep being a FUor, EXor, or McNeil's Nebula type star. Several special cases of T Tauri stars were checked, but none of these light curves are compatible with that of GM Cep. The lack of periodicity in the light curve excludes the possibility of GM Cep being a KH 15D system.

astro-ph.CO↗

arXiv2009

Estimating Redshifts for Long Gamma-Ray Bursts

We are constructing a program to estimate the redshifts for GRBs from the original Swift light curves and spectra, aiming to get redshifts for the Swift bursts \textit{without} spectroscopic or photometric redshifts. We derive the luminosity indicators from the light curves and spectra of each burst, including the lag time between low and high photon energy light curves, the variability of the light curve, the peak energy of the spectrum, the number of peaks in the light curve, and the minimum rise time of the peaks. These luminosity indicators can each be related directly to the luminosity, and we combine their independent luminosities into one weighted average. Then with our combined luminosity value, the observed burst peak brightness, and the concordance redshift-distance relation, we can derive the redshift for each burst. In this paper, we test the accuracy of our method on 107 bursts with known spectroscopic redshift. The reduced $χ^2$ of our best redshifts ($z_{best}$) compared with known spectroscopic redshifts ($z_{spec}$) is 0.86, and the average value of $log_{10}(z_{best}/z_{spec})$ is 0.01, with this indicating that our error bars are good and our estimates are not biased. The RMS scatter of $log_{10}(z_{best}/z_{spec})$ is 0.26. For Swift bursts measured over a relatively narrow energy band, the uncertainty in determining the peak energy is one of the main restrictions on our accuracy. Although the accuracy of our $z_{best}$ values are not as good as that of spectroscopic redshifts, it is very useful for demographic studies, as our sample is nearly complete and the redshifts do not have the severe selection effects associated with optical spectroscopy.

astro-ph.CO↗

arXiv2009

The Behavior of Novae Light Curves Before Eruption

In 1975, E. R. Robinson conducted the hallmark study of the behavior of classical nova light curves before eruption, and this work has now become part of the standard knowledge of novae. He made three points; that 5 out of 11 novae showed pre-eruption rises in the years before eruption, that one nova (V446 Her) showed drastic changes in the variability across eruptions, and that all but one of the novae (excepting BT Mon) have the same quiescent magnitudes before and after the outburst. This work has not been tested since it came out. We have now tested these results by going back to the original archival photographic plates and measuring large numbers of pre-eruption magnitudes for many novae using comparison stars on a modern magnitude scale. We find in particular that four out of five claimed pre-eruption rises are due to simple mistakes in the old literature, that V446 Her has the same amplitude of variations across its 1960 eruption, and that BT Mon has essentially unchanged brightness across its 1939 eruption. Out of 22 nova eruptions, we find two confirmed cases of significant pre-eruption rises (for V533 Her and V1500 Cyg), while T CrB has a deep pre-eruption dip. These events are a challenge to theorists. We find no significant cases of changes in variability across 27 nova eruptions beyond what is expected due to the usual fluctuations seen in novae away from eruptions. For 30 classical novae plus 19 eruptions from 6 recurrent novae, we find that the average change in magnitude from before the eruption to long after the eruption is 0.0 mag. However, we do find five novae (V723 Cas, V1500 Cyg, V1974 Cyg, V4633 Sgr, and RW UMi) that have significantly large changes, in that the post-eruption quiescent brightness level is over ten times brighter than the pre-eruption level.

astro-ph.SR↗

arXiv2009

Discovery of a Second Nova Eruption of V2487 Ophiuchi

A directed search for previously-undiscovered nova eruptions was conducted in the astronomical plate archives at Harvard College Observatory and Sonneberg Observatory. We found that an eruption of V2487 Oph (Nova Oph 1998) occurred on 1900 June 20. V2487 Oph was previously classified as a classical nova, which we identified as a probable recurrent nova based on its large expansion velocities and the presence of high excitation lines in the outburst spectrum. The event was recorded on Harvard plate AM 505, at a B magnitude of 10.27 +/- 0.11, which is near peak. The outburst can only be seen on one plate, but the image has a characteristic dumbbell shape (caused by a double exposure) that is identical to the other star images on the plate, and thus is not a plate defect. We conclude that this is in fact a previously-undiscovered nova outburst of V2487 Oph, confirming our prediction that it is a recurrent nova. We also examine the discovery efficiency for eruptions of the system and conclude that a randomly-timed outburst has, on average, a 30% chance of being discovered in the past century. Using this, we deduce a recurrence time for V2487 Oph of approximately 18 years, which implies that the next eruption is expected around 2016.

astro-ph.HE↗

arXiv2009

Are Swift Long-Lag Gamma-Ray Bursts in the Local Supercluster?

A sample of 18 long-lag (tau_{lag} > 1 s) Gamma-Ray Bursts (GRBs) has been drawn from our catalog of all Swift long GRBs. Four different tests are done on this sample to test the prediction that a large fraction of long-lag GRBs are from our Local Supercluster. The results of these four tests come out that: (1) the distribution of these GRBs shows no tendency towards the Supergalactic plane; (2) the distribution shows no tendency towards the Virgo or Coma Cluster; (3) no associated bright host galaxies (m <=15) in the Local Supercluster are found for any of the 18 GRBs; (4) 17 of these 18 GRBs have redshifts of z>0.5, which are too far to be in the Local Supercluster. All these results disproved the hypothesis that any significant fraction of long-lag GRBs are from Local Supercluster. Hence these long-lag GRBs can not be counted in the calculation of LIGO detection rates. An explanation of why we can detect long-lag GRBs at high redshift is presented.

astro-ph.CO↗

arXiv2006

GRB060614 is at High Redshift, So No New Class of Gamma-Ray Bursts is Required

Long duration Gamma-Ray Bursts (GRBs) have been strongly connected with core collapse supernovae, so it was surprising when the recent GRB060614 (with a reported redshift of 0.125) was found to have no visible supernova to deep limits. Three separate groups have reached the same conclusion that this event forces the existence of a new previously-unsuspected class of GRBs. The problem with this conclusion is that the redshift is not secure, as the measured galaxy emission lines simply give the redshift of the brightest galaxy near the line-of-sight to the GRB afterglow. Fortunately, eight different luminosity indicators are known which can give the luminosity (and hence redshift) of the GRB itself. In combination, these luminosity indicators have already proven to give redshifts with average one-sigma errors of 26% as based on 69 GRBs with spectroscopic redshifts. For GRB060614, the luminosity indicators all uniformly give a high luminosity for the burst which implies a high redshift. This can be seen by the spiky many-peaked light curve, the high E_{peak} value, the near-zero spectral lag, and the timing of the jet break. Quantitatively, we derive a redshift of 1.97 (1.44<z<2.81 one-sigma) from the eight luminosity indicators. Additional information from the detection of the afterglow in all Swift UVOT bands further constrains the redshift to be <1.71, so the final range of allowed redshift is 1.44<z<1.71, which readily explains why no supernova was visible. Thus, GRB060614 is at high redshift and is certainly not evidence for a new class of GRBs.

astro-ph↗