Xueqing Huang

On Green-Energy-Powered Cognitive Radio Networks

A green-energy-powered cognitive radio (CR) network is capable of liberating the wireless access networks from spectral and energy constraints. The limitation of the spectrum is alleviated by exploiting cognitive networking in which wireless nodes sense and utilize the spare spectrum for data communications, whereas dependence on the traditional unsustainable energy is assuaged by adopting energy harvesting through which green energy can be harnessed to power wireless networks. Green-energy-powered CR increases the network availability and thus extends emerging network applications. Designing green CR networks is challenging. It requires not only the optimization of dynamic spectrum access but also the optimal utilization of green energy. This paper surveys the energy-efficient CR techniques and the optimization of green-energy-powered wireless networks. Existing works on energy-aware spectrum sensing, management, and sharing are investigated in detail. The state of the art of the energy-efficient CR-based wireless access network is discussed in various aspects, such as relay and cooperative radio and small cells. Envisioning green energy as an important energy resource in the future, network performance highly depends on the dynamics of the available spectrum and green energy. As compared with the traditional energy source, the arrival rate of green energy, which highly depends on the environment of the energy harvesters, is rather random and intermittent. To optimize and adapt the usage of green energy according to the opportunistic spectrum availability, we discuss research challenges in designing CR networks that are powered by energy harvesters.

Capacity analysis and optimal power allocation for coordinated transmission in MIMO-OFDM systems

Coordinated transmission in multicell coordination networks has attracted much attentions in recent years. In the paper, we investigate one open problem in the above area, which is concerned with how to solve the closed-form optimal power allocation scheme of coordinated transmission in downlink orthogonal frequencydivision multiplexing (OFDM) systems with multiple antenna configurations and individual power constraints. A frequency-selective fading multiple-input multiple-output (MIMO) channel is considered. We derive the closed-form solutions of sum-rate capacity and optimal power allocations for coordinated transmission under the diversity case and the multiplexing case by solving the constrained optimization problems. Numerical simulations are performed to verify the derived theoretical conclusions by means of comparing the proposed optimal power allocation (OPA) scheme with the other two typical power allocation schemes. One is the power allocation scheme combining traditional water-filling with equal power (WS-EF PA). The other is equal power allocation (EPA) scheme.

Optimal Cooperative Power Allocation for Energy-Harvesting-Enabled Relay Networks

In this paper, we present a new power-allocation scheme for a decode-and-forward (DF) relaying-enhanced cooperative wireless system. While both the source node (SN) and relay node (RN) have limited energy storage, the SN can also draw power from the surrounding radio-frequency (RF) signals. In particular, we assume a deterministic RF energy-harvesting (EH) model under which the signals transmitted by the relay serve as the renewable energy source for the SN. The amount of harvested energy is known for a given transmission power value of the forwarding signal and channel condition between the SN and RN. To maximize the overall throughput while meeting the constraints imposed by the initially stored energy and the renewable RF energy source, an optimization problem is formulated and solved. Based on different harvesting efficiency values and channel conditions, closed-form solutions are derived to obtain the optimal joint source and relay power allocation. It is shown that, instead of demanding high on-grid power supply or high green energy availability, the system can achieve compatible or higher throughput by utilizing the harvested energy.

Joint Spectrum and Power Allocation for Multi-Node Cooperative Wireless Systems

Energy efficiency is a growing concern for wireless networks, not only due to the emerging traffic demand from smart devices, but also because of the dependence on the traditional unsustainable energy and the overall environmental concerns. The urgent call for reducing power consumption while meeting system requirements has motivated increasing research efforts on green radio. In this paper, we investigate a new joint spectrum and power allocation scheme for a cooperative downlink multi-user system using the frequency division multiple access scheme, in which arbitrary M base stations (BSs) coordinately allocate their resources to each user equipment (UE). With the assumption that multi-BS UE (user being served by multi-BS) would require the same amount of spectrum from these BSs, we conclude that when the number of multi-BS UEs is limited by M-1, the resource allocation scheme can always guarantee the minimum overall transmit power consumption while meeting the throughput requirement of each UE and also each BSs power constraint. Then, to decide the clusters of multi-BS UEs and the clusters of individual-BS UEs (users being served by individual BSs), we propose a UE-BS association scheme and a complexity reduction scheme. Finally, a novel joint spectrum and power allocation algorithm is proposed to minimize the total power consumption. Simulation results are presented to verify the optimality of the derived schemes.

Energy sharing within EH-enabled wireless communication networks

As energy harvesting (EH) technologies advance, wireless networks will potentially and eminently be powered by harvested energy such that carbon footprints can be reduced. Challenged by the dynamic nature of green energy source availability, various methods have been proposed so that harvested energy can be hoarded for future use or transferred to other devices, such as the storage unit of individual EH devices and energy sharing policy among multiple EH devices within the network. This article provides an overview of the architecture of EH enabled base stations and discusses two energy sharing mechanisms within the wireless communication network: direct energy transfer based schemes (through either the wired power grid or wireless energy transfer), and non-direct energy transfer based schemes (traffic offloading and cooperative transmission). We compare the energy sharing schemes and lay out basic design principles and research challenges on optimizing energy harvesting enabled wireless networks.

Closed-form solution for minimizing power consumption in coordinated transmissions

The growth in the demand of energy, and its consequent contribution to the greenhouse effect, gives rise to new challenges in the design of future wireless networks. Keeping in mind these requirements, in this article we study the power allocation problem in the downlink of an orthogonal frequency division multiple access (OFDMA) system, where two (or more) coordinated transmission points (CTPs) should find the best way to allocate their transmit power through the multiple orthogonal sub-channels of the system. The ultimate goal of the power allocation scheme is to minimize the joint power consumption of the system, but verifying at the same time the target throughput and the individual power constraint per CTP. The power allocation problem is formulated as a constrained optimization problem, and a group of closed-form power allocation solutions are derived. Based on the derived solutions (that take the form of the traditional water-filling but demanding cooperation among CTPs), a novel power allocation algorithm with joint minimization power consumption (JMPC-PA) is proposed. Numerical results are presented to verify the optimality of the results that were obtained by the JMPC-PA scheme. It is important to note that, due to the flexibility that exist in the definition of CTPs in this article, the derived power allocation scheme is valid for any kind of network that incorporates the coordinated multipoint transmission feature in its design.

Content Caching and Distribution in Smart Grid Enabled Wireless Networks

To facilitate wireless transmission of multimedia content to mobile users, we propose a content caching and distribution framework for smart grid enabled OFDM networks, where each popular multimedia file is coded and distributively stored in multiple energy harvesting enabled serving nodes (SNs), and the green energy distributively harvested by SNs can be shared with each other through the smart grid. The distributive caching, green energy sharing, and the on-grid energy backup have improved the reliability and performance of the wireless multimedia downloading process. To minimize the total on-grid power consumption of the whole network, while guaranteeing that each user can retrieve the whole content, the user association scheme is jointly designed with consideration of resource allocation, including subchannel assignment, power allocation, and power flow among nodes. Simulation results demonstrate that bringing content, green energy, and SN closer to the end user can notably reduce the on-grid energy consumption.

RF energy harvesting enabled power sharing in relay networks

Through simultaneous energy and information transfer, radio frequency (RF) energy harvesting (EH) reduces the energy consumption of the wireless networks. It also provides a new approach for the wireless devices to share each other’s energy storage, without relying on the power grid or traffic offloading. In this paper, we study RF energy harvesting enabled power balancing within the decode-and-forward (DF) relaying-enhanced cooperative wireless system. An optimal power allocation policy is proposed for the scenario where both source and relay nodes can draw power from the radio frequency signals transmitted by each other. To maximize the overall throughput while meeting the energy constraints imposed by the RF sources, an optimization problem is formulated and solved. Based on different harvesting efficiency and channel condition, closed form solutions for optimal joint source and relay power allocation are derived.

Joint power allocation solutions for power consumption minimization in coordinated transmission system

The growth in energy consumption followed with global warming provides new topics and issues in communication systems known as green radio. Hence, how to reduce power consumption while meeting system throughput requirement is an urgent task. In this paper, we investigate a new cooperative power allocation scheme for a multi-user orthogonal frequency division multiplex (OFDM) system, in which two coordinated transmission points (CTP) jointly allocate their power to multiple orthogonal subchannels. In order to minimize the overall transmit power consumption while meeting the system total throughput requirement and also each CTPs power constraint, we formulate the objective task into a constrained optimization problem and derived a group of closed form power allocation solutions. The solutions turn out to take the form of traditional water-filling (WF) and also have a cooperative feature. Based on the derived solution, a novel power allocation algorithm with joint minimization power consumption (JMPC-PA) is proposed. Numerical results are presented to verify the optimality of the derived scheme. Considering the flexibility of CTPs category, e.g., base station or relay station, it is known that the derived scheme can be valid for any coordinated networks such as next-generation cellular networks or ad-hoc networks.

An Effective Uplink Power Control Scheme in CoMP Systems

Coordinated multi-point transmission/reception (CoMP) has been considered in 3GPP LTE-Advanced to improve system performance, especially cell edge throughput. The conventional uplink power control (PC) scheme cannot work well since a UE can be served by multiple cells. This paper analyzes uplink PC issues in detail, and proposes an effective scheme aiming to obtain the reception diversity gain from CoMP. System-level simulation results have shown that CoMP systems with both the conventional scheme and the proposed scheme can achieve a better cell edge performance, compared with non-CoMP systems. Furthermore, the proposed scheme considerably outperforms the conventional scheme, it can bring an additional cell average throughput gain up to 15.78% and an additional cell edge throughput gain up to 69.22% over the conventional scheme, thus is recommended to be adopted in CoMP systems.