ABSTRACT
With the deployment of fifth-generation (5G) wireless networks worldwide, research on sixth-generation (6G) wireless communications has commenced. It is expected that 6G networks can accommodate numerous heterogeneous devices and infrastructures with enhanced efficiency and security over diverse, e.g. spectrum, computing and storage, resources. However, this goal is impeded by a number of trust-related issues that are often neglected in network designs. Blockchain, as an innovative and revolutionary technology that has arisen in the recent decade, provides a promising solution. Building on its nature of decentralization, transparency, anonymity, immutability, traceability and resiliency, blockchain can establish cooperative trust among separate network entities and facilitate, e.g. efficient resource sharing, trusted data interaction, secure access control, privacy protection, and tracing, certification and supervision functionalities for wireless networks, thus presenting a new paradigm towards 6G. This paper is dedicated to blockchain-enabled wireless communication technologies. We first provide a brief introduction to the fundamentals of blockchain, and then we conduct a comprehensive investigation of the most recent efforts in incorporating blockchain into wireless communications from several aspects. Importantly, we further propose a unified framework of the blockchain radio access network (B-RAN) as a trustworthy and secure paradigm for 6G networking by utilizing blockchain technologies with enhanced efficiency and security. The critical elements of B-RAN, such as consensus mechanisms, smart contract, trustworthy access, mathematical modeling, cross-network sharing, data tracking and auditing and intelligent networking, are elaborated. We also provide the prototype design of B-RAN along with the latest experimental results.
ABSTRACT
In this work, we propose a satellite-aided permissionless consensus protocol for scalable space-terrestrial blockchains. We design its working principle and workflow by taking full advantage of satellites for extensive coverage and ubiquitous connectivity. Based on the proposed protocol, we demonstrate how such a space-terrestrial blockchain grows and evolves through several typical cases in the presence of adversarial nodes, user misbehavior, and transmission outage. Taking proof of work (PoW) as a benchmark, we assess the system security by considering both adversarial miners and possible colluding satellites. Then, we analyze the maximum blockchain throughput under network capacity limits and evaluate the impact of information propagation delay via a Markov model. Simulation results support that the proposed satellite-aided consensus protocol achieves higher throughput and exhibits greater scalability than PoW.
ABSTRACT
Direct-current-biased optical orthogonal frequency-division multiplexing (DCO-OFDM) is widely used in high-speed visible light communication (VLC). Due to the limited dynamic range of light-emitting diode (LED) and the unipolarity for the intensity modulation (IM), double-sided clipping is inevitably imposed on the time-domain signal in VLC OFDM systems. Consequently, it calls for proper DCO-OFDM signal shaping by selecting an appropriate bias and time-domain signal power to reduce the clipping distortion and achieve a higher transmission rate. In this paper, we deep dive into the signal shaping design problem for double-sided clipping DCO-OFDM over both flat and dispersive channels. We derive the optimal bias for flat and dispersive channels, and explain its optimality from the perspectives of effective signal-to-noise ratio (SNR) and information theory. We then analytically characterize the optimal power for flat channels and propose a useful algorithm for dispersive channels enlightened by the optimal solution to the flat case. Furthermore, we uncover an inherent relationship between the considered double-sided clipping and the downside-clipping only DCO-OFDM regarding signal shaping optimization, and develop an in-depth understanding of the impact of top clipping based on the established connection. Practical simulations are provided to validate the superiority of our proposed signal shaping over the existing shaping schemes.
ABSTRACT
Metal-organic frameworks (MOFs) as versatile templates for preparing transition metal compounds has received wide attention. Benefiting from their diversified spatial structure and controllable chemical constituents, they have become a research hotspot in the field of electrocatalytic water splitting. Herein, Fe2Ni-MIL-88B MOF on nickel foam (Fe2Ni MOF/NF) has been prepared through a one-pot method growth process. Compared with Fe MOF/NF and Ni MOF/NF, the interaction between Fe3+ and Ni2+ in Fe2Ni MOF/NF accelerates the electron transfer through the oxygen of the ligand, leading to increased 3d orbital electron density of Ni, which enhances the activity of the oxygen evolution reaction (OER) in alkaline solution. Fe2Ni MOF/NF provides a current density of 10 mA cm-2 at a low overpotential of 222 mV, and its Tafel slope is also very small, reaching 42.39 mV dec-1. The success of the present Fe2Ni MOF/NF catalyst is attributed to the abundant active centers, the bimetallic clusters Fe2Ni-MIL-88B, the positive coupling effect between Ni and Fe metal ions in the MOF, and synergistic effect between the MOF and NF. Besides, Fe2Ni MOF/NF possesses excellent stability over 50 h of continuous operation, providing feasibility for commercial use.
ABSTRACT
Orthogonal frequency-division multiplexing (OFDM) is a practical technology in visible light communication (VLC) for high-speed transmissions. However, one of its operational limitations is the peak-to-average power ratio (PAPR) of the transmitted signal. In this paper, we analyze the PAPR distributions of four VLC OFDM schemes, namely DC-biased optical OFDM (DCO-OFDM), asymmetrically clipped optical OFDM (ACO-OFDM), pulse amplitude modulated discrete multitone (PAM-DMT), and Flip-OFDM. Both lower and upper clippings are considered. We analytically derive the complementary cumulative distribution functions (CCDFs) of the PAPRs of the clipped VLC OFDM signals, and investigate the impact of lower and upper clippings on PAPR distributions. Our analytical results, as verified by numerical simulations, provide useful insights and guidelines for VLC OFDM system designs.
