Femtocell networks, consisting of a conventional macro cellular deployment and overlaying femtocells, forming a hierarchical cell structure, constitute an attractive solution to improving the macrocell capacity and coverage. However, the inter-and intra-tier interferences in such systems can significantly reduce the capacity and cause an unacceptably high level of outage. This paper treats the uplink interference problem in orthogonal frequency-division multiple-access (OFDMA)-based femtocell networks with partial cochannel deployment. We first propose an inter-tier interference mitigation strategy without the femtocell users power control by forcing the femto-interfering macrocell users to use only some dedicated subcarriers. The noninterfering macrocell users, on the other hand, can use either the dedicated subcarriers, or the shared subcarriers which are also used by the femtocell users. We then propose subcarrier allocation schemes based on the auction algorithm for macrocell users and femtocell users, respectively, to independently mitigate the intra-tier interference. The proposed interference mitigation scheme for femtocell networks offers significant performance improvement over the existing methods by substantially reducing the inter-and intra-tier inferences in the system.

The availabilities of debt for Chinese firms have been increased since 2008 due to the expansionary monetary policies. These policies triggered concerns over the impact of debt financing on firms’ technological innovation activities. Based on a sample of 225 listed computer and telecommunications equipment firms in China within 2008-2015, this study explored the effect of debt financing on two types of technological innovation, namely radical and incremental innovation. Specifically, both the direct effect of debt financing on technological innovation and its moderating effect on the relationship between R&D intensity and technological innovation were investigated. Results of this study reveal that radical innovation decreases with debt financing at decreasing rates, while incremental innovation is not affected by debt financing itself. In addition, debt financing interacted with R&D intensity exerts positive effect on both radical and incremental innovation. This study adds meaningful insights to the literature on financing technological innovation and builds a bridge connecting macroeconomic policies to firm activities.

It is well known that chaos phenomenon is harmful for a nonlinear mechanical vibration system. For a running vehicle, chaotic vibrations may lead to early damage to road and the component of vehicle or even bring physical injury to crew member. In this paper, we first studied the possibility of chaotic vibration of the four degree-of-freedom half vehicle model under consecutive speed control humps on the highway. Then, a direct variable feedback control is proposed to eliminate the influence of chaos on vehicle nonlinear vibrations and how to select proper control parameters is discussed.

Since the characteristics of wireless channel are open and broadcasting, wireless networks are very vulnerable to be attacked via eavesdropping, jamming, and interference. As traditional secure technologies are not suitable for PHY layer of wireless networks, physical-layer security issues become a focus of attention. In this paper, we firstly identify and summarize the threats and vulnerabilities in PHY layer of wireless networks. Then, we give a holistic overview of PHY layer secure schemes, which are divided into three categories: spatial domain-based, time domain-based, and frequency domain-based. Along the way, we analyze the pros and cons of current secure technologies in each category. In addition, we also conclude the techniques and methods used in these categories and point out the open research issues and directions in this area.