arXiv subjects

Xin Zhang

Publications and source records attributed to Xin Zhang.

1,292 records · Page 72Linked to original sources

arXiv2007

Cooperative Transmission Protocols with High Spectral Efficiency and High Diversity Order Using Multiuser Detection and Network Coding

Cooperative transmission is an emerging communication technique that takes advantages of the broadcast nature of wireless channels. However, due to low spectral efficiency and the requirement of orthogonal channels, its potential for use in future wireless networks is limited. In this paper, by making use of multiuser detection (MUD) and network coding, cooperative transmission protocols with high spectral efficiency, diversity order, and coding gain are developed. Compared with the traditional cooperative transmission protocols with single-user detection, in which the diversity gain is only for one source user, the proposed MUD cooperative transmission protocols have the merits that the improvement of one user's link can also benefit the other users. In addition, using MUD at the relay provides an environment in which network coding can be employed. The coding gain and high diversity order can be obtained by fully utilizing the link between the relay and the destination. From the analysis and simulation results, it is seen that the proposed protocols achieve higher diversity gain, better asymptotic efficiency, and lower bit error rate, compared to traditional MUD and to existing cooperative transmission protocols.

cs.IT↗

arXiv2007

Reconstructing holographic quintessence

The holographic dark energy model is an attempt for probing the nature of dark energy within the framework of quantum gravity. The dimensionless parameter $c$ determines the main property of the holographic dark energy. With the choice of $c\geq 1$, the holographic dark energy can be described completely by a quintessence scalar field. In this paper, we show this quintessential description of the holographic dark energy with $c\geq 1$ and reconstruct the potential of the quintessence as well as the dynamics of the scalar field.

astro-ph↗

arXiv2007

Reconstructing generalized ghost condensate model with dynamical dark energy parametrizations and observational datasets

Observations of high-redshift supernovae indicate that the universe is accelerating at the present stage, and we refer to the cause for this cosmic acceleration as ``dark energy''. In particular, the analysis of current data of type Ia supernovae (SNIa), cosmic large-scale structure (LSS), and the cosmic microwave background (CMB) anisotropy implies that, with some possibility, the equation-of-state parameter of dark energy may cross the cosmological-constant boundary ($w=-1$) during the recent evolution stage. The model of ``quintom'' has been proposed to describe this $w=-1$ crossing behavior for dark energy. As a single-real-scalar-field model of dark energy, the generalized ghost condensate model provides us with a successful mechanism for realizing the quintom-like behavior. In this paper, we reconstruct the generalized ghost condensate model in the light of three forms of parametrization for dynamical dark energy, with the best-fit results of up-to-date observational data.

astro-ph↗

arXiv2007

Statefinder diagnosis in a non-flat universe and the holographic model of dark energy

In this paper, we study the holographic dark energy model in non-flat universe from the statefinder viewpoint. We plot the evolutionary trajectories of the holographic dark energy model for different values of the parameter $c$ as well as for different contributions of spatial curvature, in the statefinder parameter-planes. The statefinder diagrams characterize the properties of the holographic dark energy and show the discrimination between this scenario and other dark energy models. As we show, the contributions of the spatial curvature in the model can be diagnosed out explicitly by the statefinder diagrams. Furthermore, we also investigate the holographic dark energy model in the $w-w'$ plane, which can provide us with a useful dynamical diagnosis complement to the statefinder geometrical diagnosis.

gr-qc↗

arXiv2006

A Note on Noncommutative Brane Inflation

In this paper, we investigate the noncommutative KKLMMT D3/anti-D3 brane inflation scenario in detail. Incorporation of the brane inflation scenario and the noncommutative inflation scenario can nicely explain the large negative running of the spectral index as indicated by WMAP three-year data and can significantly release the fine-tuning for the parameter $β$. Using the WMAP three year results (blue-tilted spectral index with large negative running), we explore the parameter space and give the constraints and predictions for the inflationary parameters and cosmological observables in this scenario. We show that this scenario predicts a quite large tensor/scalar ratio and what is more, a too large cosmic string tension (assuming that the string coupling $g_s$ is in its likely range from 0.1 to 1) to be compatible with the present observational bound. A more detailed analysis reveals that this model has some inconsistencies according to the fit to WMAP three year results.

hep-th↗

arXiv2006

Dynamical vacuum energy, holographic quintom, and the reconstruction of scalar-field dark energy

When taking the holographic principle into account, the vacuum energy will acquire dynamical property that its equation of state is evolving. The current available observational data imply that the holographic vacuum energy behaves as quintom-type dark energy. We adopt the viewpoint of that the scalar field models of dark energy are effective theories of an underlying theory of dark energy. If we regard the scalar field model as an effective description of such a holographic vacuum theory, we should be capable of using the scalar field model to mimic the evolving behavior of the dynamical vacuum energy and reconstructing this scalar field model according to the fits of the observational dataset. We find the generalized ghost condensate model is a good choice for depicting the holographic vacuum energy since it can easily realize the quintom behavior. We thus reconstruct the function $h(ϕ)$ of the generalized ghost condensate model using the best-fit results of the observational data.

astro-ph↗

arXiv2006

Noncommutative chaotic inflation and WMAP three year results

Noncommutative inflation is based upon the consideration of some effects of the space-time uncertainty principle motivated by ideas from string/M theory. The CMB anisotropies may carry a signature of this very early Universe correction from the space-time uncertainty principle and can be used to place constraints on the parameters of the noncommutative inflation model. In this paper we analyze the noncommutative chaotic inflation model by means of the WMAP three year results. We show that the noncommutative chaotic inflation model can produce a large negative running of spectral index within a reasonable range of $e$-folding number $N$, provided that the value of $p$ in the potential $V(ϕ)\propto ϕ^p$ is enhanced roughly to $p\sim 12-18$. Applying a likelihood analysis to the WMAP results, we find the best-fit values of parameters for the noncommutative chaotic inflation model. In addition, this model predicts a rather big gravitational wave which may be tested by the upcoming experiments.

astro-ph↗

arXiv2005

New generalized Chaplygin gas as a scheme for unification of dark energy and dark matter

We propose in this paper a new model for describing the unification of dark energy and dark matter. This new model is a further generalization of the generalized Chaplygin gas (GCG) model, thus dubbed new generalized Chaplygin gas (NGCG) model. The equation of state of the NGCG is given by $p = - \tilde{A}(a)/ρ^α$, where $a$ is the scale factor and $\tilde{A}(a)=-w_XAa^{-3(1+w_X)(1+α)}$. We show that the NGCG model is totally dual to an interacting XCDM parametrization scenario, in which the interaction between dark energy and dark matter is characterized by the constant $α$. We discuss the cosmological consequences led by such an unified dark sectors model. Furthermore, we perform a statefinder analysis on this scenario and show the discrimination between this scenario and other dark energy models. Finally, a combined analysis of the data of Type Ia supernovae, cosmic microwave background, and large scale structure provides a fairly tight constraint on the parameters of the NGCG model.

astro-ph↗

arXiv2005

Constraints on holographic dark energy from X-ray gas mass fraction of galaxy clusters

We use the Chandra measurements of the X-ray gas mass fraction of 26 rich clusters released by Allen et al. to perform constraints on the holographic dark energy model. The constraints are consistent with those from other cosmological tests, especially with the results of a joint analysis of supernovae, cosmic microwave background, and large scale structure data. From this test, the holographic dark energy also tends to behave as a quintom-type dark energy.

astro-ph↗

arXiv2005

Black hole evaporation based upon a q-deformation description

A toy model based upon the $q$-deformation description for studying the radiation spectrum of black hole is proposed. The starting point is to make an attempt to consider the spacetime noncommutativity in the vicinity of black hole horizon. We use a trick that all the spacetime noncommutative effects are ascribed to the modification of the behavior of the radiation field of black hole and a kind of q-deformed degrees of freedom are postulated to mimic the radiation particles that live on the noncommutative spacetime, meanwhile the background metric is preserved as usual. We calculate the radiation spectrum of Schwarzschild black hole in this framework. The new distribution deviates from the standard thermal spectrum evidently. The result indicates that some correlation effect will be introduced to the system if the noncommutativity is taken into account. In addition, an infrared cut-off of the spectrum is the prediction of the model.

hep-th↗

arXiv2005

Statefinder diagnostic for holographic dark energy model

In this paper we study the holographic dark energy model proposed by Li from the statefinder viewpoint. We plot the evolutionary trajectories of the model with $c=1$ in the statefinder parameter-planes. The statefinder diagrams characterize the properties of the holographic dark energy and show the discrimination between this scenario and other dark energy models. We also perform a statefinder diagnostic to the holographic dark energy model in cases of different $c$ which given by three fits to observational data. The result indicates that from the statefinder viewpoint $c$ plays a significant role in this model and should thus be determined seriously by future high precision experiments.

astro-ph↗

arXiv2005

Coupled Quintessence in a Power-Law Case and the Cosmic Coincidence Problem

The problem of the cosmic coincidence is a longstanding puzzle. This conundrum may be solved by introducing a coupling between the two dark sectors. In this Letter, we study a coupled quintessence scenario in which the scalar field evolves in a power law potential and the mass of dark matter particles depends on a power law function of $ϕ$. It is shown that this scenario has a stable attractor solution and can thus provide a natural solution to the cosmic coincidence problem.

astro-ph↗

arXiv2005

Constraints on holographic dark energy from type Ia supernova observations

In this paper, we use the type Ia supernovae data to constrain the holographic dark energy model proposed by Li. We also apply a cosmic age test to this analysis. We consider in this paper a spatially flat Friedmann-Robertson-Walker Universe with matter component and holographic dark energy component. The fit result shows that the case $c<1$ ($c=0.21$) is favored, which implies that the holographic dark energy behaves as a quintom-type dark energy. Furthermore, we also perform a joint analysis of SNe+CMB+LSS to this model; the result is well improved, and still upholds the quintom dark energy conclusion. The best fit results in our analysis are $c=0.81$, $Ω_m^0=0.28$, and $h=0.65$, which lead to the present equation of state of dark energy $w_0=-1.03$ and the deceleration/acceleration transition redshift $z_T=0.63$. Finally, an expected SNAP simulation using $Λ$CDM as a fiducial model is performed on this model, and the result shows that the holographic dark energy model takes on $c<1$ ($c=0.92$) even though the dark energy is indeed a cosmological constant.

astro-ph↗

arXiv2005

Statefinder diagnostic for coupled quintessence

The problem of the cosmic coincidence is a longstanding puzzle. This conundrum may be solved by introducing a coupling between the two dark sectors. In this Letter, we study two cases of the coupled quintessence scenario. $(a)$ Assume that the mass of dark matter particles depends exponentially on the scalar field associated to dark energy and meanwhile the scalar field evolves in an exponential potential; $(b)$ Assume that the mass of dark matter particles depends on a power law function of the scalar field and meanwhile the scalar field evolves in a power law potential. Since the dynamics of this system is dominated by an attractor solution, the mass of dark matter particles is forced to change with time as to ensure that the ratio between the energy densities of dark matter and dark energy becomes a constant at late times, and one thus solve the cosmic coincidence problem naturally. We perform a statefinder diagnostic to both cases of this coupled quintessence scenario. It is shown that the evolving trajectory of this scenario in the $s-r$ diagram is quite different from those of other dark energy models.

astro-ph↗