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Zhi Chen

Publications and source records attributed to Zhi Chen.

At least 325 records · Page 18Linked to original sources
arXiv2015

Mobility-Aware Uplink Interference Model for 5G Heterogeneous Networks

To meet the surging demand for throughput, 5G cellular networks need to be more heterogeneous and much denser, by deploying more and more small cells. In particular, the number of users in each small cell can change dramatically due to users' mobility, resulting in random and time varying uplink interference. This paper considers the uplink interference in a 5G heterogeneous network which is jointly covered by one macro cell and several small cells. Based on the Lévy flight moving model, a mobility-aware interference model is proposed to characterize the uplink interference from macro cell users to small cell users. In this model, the total uplink interference is characterized by its moment generating function, for both closed subscriber group (CSG) and open subscriber group (CSG) femto cells. In addition, the proposed interference model is a function of basic step length, which is a key velocity parameter of Lévy flights. It is shown by both theoretical analysis and simulation results that the proposed interference model provides a flexible way of evaluating the system performance in terms of success probability and average rate.

cs.IT↗
arXiv2015

Fundamental Limits of Caching: Improved Bounds For Small Buffer Users

In this work, the peak rate of the caching problem is investigated, under the scenario that the users are with small buffer sizes and the number of users is no less than the amount of files in the server. A novel coded caching strategy is proposed for such a scenario, leading to a lower peak rate compared to recent results in the literature. Furthermore, it is verified that our peak rates coincides with the cut-set bound analytically in an information-theoretic view.

cs.IT↗
arXiv2015

Weighted Sum-Throughput Maximization for MIMO Broadcast Channel: Energy Harvesting Under System Imperfection

In this work, a MIMO broadcast channel under the energy harvesting (EH) constraint and the peak power constraint is investigated. The transmitter is equipped with a hybrid energy storage system consisting of a perfect super capacitor (SC) and an inefficient battery, where both elements have limited energy storage capacities. In addition, the effect of data processing circuit power consumption is also addressed. To be specific, two extreme cases are studied here, where the first assumes ideal/zero circuit power consumption and the second considers a positive constant circuit power consumption where the circuit is always operating at its highest power level. The performance of these two extreme cases hence serve as the upper bound and the lower bound of the system performance in practice, respectively. In this setting, the offline scheduling with ideal and maximum circuit power consumptions are investigated. The associated optimization problems are formulated and solved in terms of weighted throughput optimization. Further, we extend to a general circuit power consumption model. To complement this work, some intuitive online policies are presented for all cases. Interestingly, for the case with maximum circuit power consumption, a close-to-optimal online policy is presented and its performance is shown to be comparable to its offline counterpart in the numerical results.

cs.IT↗
arXiv2015

On the Secrecy Capacity of a MIMO Gaussian Wiretap Channel with a Cooperative Jammer

We study the secrecy capacity of a helper-assisted Gaussian wiretap channel with a source, a legitimate receiver, an eavesdropper and an external helper, where each terminal is equipped with multiple antennas. Determining the secrecy capacity in this scenario generally requires solving a nonconvex secrecy rate maximization (SRM) problem. To deal with this issue, we first reformulate the original SRM problem into a sequence of convex subproblems. For the special case of single-antenna legitimate receiver, we obtain the secrecy capacity via a combination of convex optimization and one-dimensional search, while for the general case of multi-antenna legitimate receiver, we propose an iterative solution. To gain more insight into how the secrecy capacity of a helper-assisted Gaussian wiretap channel behaves, we examine the achievable secure degrees of freedom (s.d.o.f.) and obtain the maximal achievable s.d.o.f. in closed-form. We also derive a closed-form solution to the original SRM problem which achieves the maximal s.d.o.f.. Numerical results are presented to illustrate the efficacy of the proposed schemes.

cs.IT↗
arXiv2015

Hilbert transform based analyses on ship-rocking signals

The ship-rocking is a crucial factor which affects the accuracy of the ocean-based flight vehicle measurement. Here we have analyzed four groups of ship-rocking time series in horizontal and vertical directions utilizing a Hilbert based method from statistical physics. Our method gives a way to construct an analytic signal on the two-dimensional plane from a one-dimensional time series. The analytic signal share the complete property of the original time series. From the analytic signal of a time series, we have found some information of the original time series which are often hidden from the view of the conventional methods. The analytic signals of interest usually evolve very smoothly on the complex plane. In addition, the phase of the analytic signal is usually moves linearly in time. From the auto-correlation and cross-correlation functions of the original signals as well as the instantaneous amplitudes and phase increments of the analytic signals we have found that the ship-rocking in horizontal direction drives the ship-rocking in vertical direction when the ship navigates freely. And when the ship keeps a fixed navigation direction such relation disappears. Based on these resultswe could predict certain amount of future values of the ship-rocking time series based on the current and the previous values. Our predictions are as accurate as the conventional methods from stochastic processes and provide a much wider prediction time range.

physics.data-an↗
arXiv2015

Computationally Efficient Sparse Bayesian Learning via Generalized Approximate Message Passing

The sparse Beyesian learning (also referred to as Bayesian compressed sensing) algorithm is one of the most popular approaches for sparse signal recovery, and has demonstrated superior performance in a series of experiments. Nevertheless, the sparse Bayesian learning algorithm has computational complexity that grows exponentially with the dimension of the signal, which hinders its application to many practical problems even with moderately large data sets. To address this issue, in this paper, we propose a computationally efficient sparse Bayesian learning method via the generalized approximate message passing (GAMP) technique. Specifically, the algorithm is developed within an expectation-maximization (EM) framework, using GAMP to efficiently compute an approximation of the posterior distribution of hidden variables. The hyperparameters associated with the hierarchical Gaussian prior are learned by iteratively maximizing the Q-function which is calculated based on the posterior approximation obtained from the GAMP. Numerical results are provided to illustrate the computational efficacy and the effectiveness of the proposed algorithm.

cs.IT↗
arXiv2014

Induced radioactivity analysis for the NSRL Linac in China using Monte Carlo simulations and gamma-spectroscopy

The 200-MeV electron linac of the National Synchrotron Radiation Laboratory (NSRL) located in Hefei is one of the earliest high-energy electron linear accelerators in China. The electrons are accelerated to 200 MeV by five acceleration tubes and are collimated by scrapers. The scraper aperture is smaller than the acceleration tube one, so some electrons hit the materials when passing through them. These lost electrons cause induced radioactivity mainly due to bremsstrahlung and photonuclear reaction. This paper describes a study of induced radioactivity for the NSRL Linac using FLUKA simulations and gamma-spectroscopy. The measurements showed that electrons were lost mainly at the scraper. So the induced radioactivity of the NSRL Linac is mainly produced here. The radionuclide types were simulated using the FLUKA Monte Carlo code and the results were compared against measurements made with a High Purity Germanium (HPGe) gamma spectrometer. The NSRL linac had been retired because of upgrading last year. The removed components were used to study induced radioactivity. The radionuclides confirmed by the measurement are: $^{57}$Ni, $^{52}$Mn, $^{51}$Cr, $^{58}$Co, $^{56}$Co, $^{57}$Co, $^{54}$Mn, $^{60}$Co and $^{22}$Na, the first eight nuclides of which are predicted by FLUKA simulation. The research will provide the theoretical basis for the similar accelerator decommissioning plan, and is significant for accelerator structure design, material selection and radiation protection design.

physics.acc-ph↗
arXiv2014

Fading Two-Way Relay Channels: Physical-Layer Versus Digital Network Coding

In this paper, we consider three transmit strategies for the fading three-node, two-way relay network (TWRN) -- physical-layer network coding (PNC), digital network coding (DNC) and codeword superposition (CW-Sup). The aim is to minimize the total average energy needed to deliver a given pair of required average rates. Full channel state information is assumed to be available at all transmitters and receivers. The optimization problems corresponding to the various strategies in fading channels are formulated, solved and compared. For the DNC-based strategies, a simple time sharing of transmission of the network-coded message and the remaining bits of the larger message (DNC-TS) is considered first. We extend this approach to include a superposition strategy (DNC-Sup), in which the network-coded message and the remainder of the longer source message are superimposed before transmission. It is demonstrated theoretically that DNC-Sup outperforms DNC-TS and CW-Sup in terms of total average energy usage. More importantly, it is shown in simulation that DNC-Sup performs better than PNC if the required rate is low and worse otherwise. Finally, an algorithm to select the optimal strategy in terms of energy usage subject to different rate pair requirements is presented.

cs.IT↗
arXiv2013

Utility-maximization Resource Allocation for Device-to-Device Communication Underlaying Cellular Networks

Device-to-device(D2D) underlaying communication brings great benefits to the cellular networks from the improvement of coverage and spectral efficiency at the expense of complicated transceiver design. With frequency spectrum sharing mode, the D2D user generates interference to the existing cellular networks either in downlink or uplink. Thus the resource allocation for D2D pairs should be designed properly in order to reduce possible interference, in particular for uplink. In this paper, we introduce a novel bandwidth allocation scheme to maximize the utilities of both D2D users and cellular users. Since the allocation problem is strongly NP-hard, we apply a relaxation to the association indicators. We propose a low-complexity distributed algorithm and prove the convergence in a static environment. The numerical result shows that the proposed scheme can significant improve the performance in terms of utilities.The performance of D2D communications depends on D2D user locations, the number of D2D users and QoS(Quality of Service) parameters.

cs.IT↗
arXiv2013

Multifractality can be a universal signature of phase transitions

Macroscopic systems often display phase transitions where certain physical quantities are singular or self-similar at different (spatial) scales. Such properties of systems are currently characterized by some order parameters and a few critical exponents. Nevertheless, recent studies show that the multifractality, where a large number of exponents are needed to quantify systems, appears in many complex systems displaying self-similarity. Here we propose a general approach and show that the appearance of the multifractality of an order parameter related quantity is the signature of a physical system transiting from one phase to another. The distribution of this quantity obtained within suitable (time) scales satisfies a $q$-Gaussian distribution plus a possible Cauchy distributed background. At the critical point the $q$-Gaussian shifts between Gaussian type with narrow tails and L$\acute{\text{e}}$vy type with fat tails. Our results suggest that the Tsallis $q$-statistics, besides the conventional Boltzmann-Gibbs statistics, may play an important role during phase transitions.

cond-mat.stat-mech↗
arXiv2013

On Feasibility of Generalized Interference Alignment with Partial Interference Cancelation

We study a new IA strategy which is referred to as "Partial Interference Cancelation-based Interference Alignment" (PIC-IA). Unlike the conventional IA strategy, PIC-IA does not strive to eliminate interference from all users. Instead, it aims to remove the most significant interference signals. This PIC-IA strategy generalizes the conventional IA concept by addressing partial, instead of complete, interference cancelation. The feasibility of this new strategy is studied in this paper. Our results show that for a symmetric, single-stream system with $N_t$ transmit antennas and $N_r$ receive antennas, the PIC-IA is feasible when the number of significant interference signals to be removed at each receiver is no more than $N_t+N_r-2$, no matter how many users are in the network. This is in sharp contrast to the conventional IA whose feasibility is severely limited by the number of users $K$.

cs.IT↗
arXiv2012

Digital Network Coding Aided Two-way Relaying: Energy Minimization and Queue Analysis

In this paper, we consider a three node, two-way relay system with digital network coding over static channels where all link gains are assumed to be constant during transmission. The aim is to minimize total energy consumption while ensuring queue stability at all nodes, for a given pair of random packet arrival rates. Specifically, we allow for a set of transmission modes and solve for the optimal fraction of resources allocated to each mode, including multiaccess uplink transmission mode and network coding broadcasting mode. In addition, for the downlink, we find the condition to determine whether superposition coding with excess data over the better link and network coded data for both users is energy efficient and the corresponding optimization is formulated and solved. To tackle the queue evolution in this network, we present a detailed analysis of the queues at each node using a random scheduling method that closely approximates the theoretical design, through a two-dimensional Markov chain model.

cs.IT↗
arXiv2012

Optimal Precoding Design and Power Allocation for Decentralized Detection of Deterministic Signals

We consider a decentralized detection problem in a power-constrained wireless sensor networks (WSNs), in which a number of sensor nodes collaborate to detect the presence of a deterministic vector signal. The signal to be detected is assumed known \emph{a priori}. Given a constraint on the total amount of transmit power, we investigate the optimal linear precoding design for each sensor node. More specifically, in order to achieve the best detection performance, shall sensor nodes transmit their raw data to the fusion center (FC), or transmit compressed versions of their original data? The optimal power allocation among sensors is studied as well. Also, assuming a fixed total transmit power, we examine how the detection performance behaves with the number of sensors in the network. A new concept "detection outage" is proposed to quantify the reliability of the overall detection system. Finally, decentralized detection with unknown signals is studied. Numerical results are conducted to corroborate our theoretical analysis and to illustrate the performance of the proposed algorithm.

cs.IR↗
arXiv2012

A Plethysm formula on the characteristic map of induced linear characters from $U_n(\mathbb F_q)$ to $GL_n(\mathbb F_q)$

This paper gives a plethysm formula on the characteristic map of the induced linear characters from the unipotent upper-triangular matrices $U_n(\mathbb F_q)$ to $GL_n(\mathbb F_q)$, the general linear group over finite field $\mathbb F_q$. The result turns out to be a multiple of a twisted version of the Hall-Littlewood symmetric functions $\tilde{P}_n(Y,q)$. A recurrence relation is also given which makes it easy to carry out the computation.

math.CO↗
arXiv2011

Supercharacters, symmetric functions in noncommuting variables, and related Hopf algebras

We identify two seemingly disparate structures: supercharacters, a useful way of doing Fourier analysis on the group of unipotent uppertriangular matrices with coefficients in a finite field, and the ring of symmetric functions in noncommuting variables. Each is a Hopf algebra and the two are isomorphic as such. This allows developments in each to be transferred. The identification suggests a rich class of examples for the emerging field of combinatorial Hopf algebras.

math.CO↗
arXiv2011

Flat connections and Brauer Type Algebras

We introduce a Brauer type algebra $B_G (Υ) $ associated with every pseudo reflection group and every Coxeter group $G$. When $G$ is a Coxeter group of simply-laced type we show $B_G (Υ)$ is isomorphic to the generalized Brauer algebra of simply-laced type introduced by Cohen, Gijsbers and Wales ({\it J. Algebra}, {\bf 280} (2005), 107-153). We also prove $B_G (Υ)$ has a cellular structure and be semisimple for generic parameters when $G$ is a dihedral group or the type $H_3$ Coxeter group. Moreover, in the process of construction, we introduce a further generalization of Lawrence-Krammer representation to complex braid groups associated with all pseudo reflection groups.

math.RT↗
arXiv2010

Basis of Diagonally Alternating Harmonic Polynomials for low degree

Given a list of $n$ cells $L=[(p_1,q_1),...,(p_n, q_n)]$ where $p_i, q_i\in \textbf{Z}_{\ge 0}$, we let $Δ_L=\det |{(p_j!)^{-1}(q_j!)^{-1} x^{p_j}_iy^{q_j}_i} |$. The space of diagonally alternating polynomials is spanned by $\{Δ_L\}$ where $L$ varies among all lists with $n$ cells. For $a>0$, the operators $E_a=\sum_{i=1}^{n} y_i\partial_{x_i}^a$ act on diagonally alternating polynomials and Haiman has shown that the space $A_n$ of diagonally alternating harmonic polynomials is spanned by $\{E_λΔ_n\}$. For $t=(t_m,...,t_1)\in \textbf{Z}_{> 0}^m$ with $t_m>...>t_1>0$, we consider here the operator $F_t=\det\big\|E_{t_{m-j+1}+(j-i)}\big\|$. Our first result is to show that $F_tΔ_L$ is a linear combination of $Δ_{L'}$ where $L'$ is obtained by {\sl moving} $\ell(t)=m$ distinct cells from $L$ in some determined fashion. This allows us to control the leading term of some elements of the form $F_{t_{(1)}}... F_{t_{(r)}}Δ_n$. We use this to describe explicit bases of some of the bihomogeneous components of $A_n=\bigoplus A_n^{k,l}$ where $A_n^{k,l}=\hbox{Span}\{E_λΔ_n :\ell(λ)=l, |λ|=k\}$. More precisely we give an explicit basis of $A_n^{k,l}$ whenever $k<n$. To this end, we introduce a new variation of Schensted insertion on a special class of tableaux. This produces a bijection between partitions and this new class of tableaux. The combinatorics of those tableaux $T$ allows us to know exactly the leading term of $F_TΔ_n$ where $F_T$ is the operator corresponding to the columns of $T$ and whenever $n$ is bigger than the weight of $T$.

math.CO↗