Jaeyoung Lee

Facet-controlled hollow Rh2S3 hexagonal nanoprisms as highly active and structurally robust catalysts toward hydrogen evolution reaction

Developing highly active and structurally robust electrocatalysts for hydrogen evolution reaction (HER) is of paramount importance for sustainable and clean production of hydrogen. Metal sulphides exposing catalytically active sites, in particular, have been actively pursued as advanced HER catalysts. Herein we report high-performance Rh2S3-based HER catalysts with excellent activity and durability. Hollow Rh2S3 hexagonal nanoprisms with controlled size and thickness could be conveniently prepared by one-step formation of core–shell nanoprisms followed by the etching of the core, and they show high surface areas and highly exposed edge sites. The hollow Rh2S3 nanoprisms exhibit very high HER activity and excellent stability under harsh acidic conditions.

Cactus-Like Hollow Cu2-xS@Ru Nanoplates as Excellent and Robust Electrocatalysts for the Alkaline Hydrogen Evolution Reaction

The development of Pt-free electrocatalysts for the hydrogen evolution reaction (HER) recently is a focus of great interest. While several strategies are developed to control the structural properties of non-Pt catalysts and boost their electrocatalytic activities for the HER, the generation of highly reactive defects or interfaces by combining a metal with other metals, or with metal oxides/sulfides, can lead to notably enhanced catalytic performance. Herein, the preparation of cactus-like hollow Cu2-x S@Ru nanoplates (NPs) that contain metal/metal sulfide heterojunctions and show excellent catalytic activity and durability for the HER in alkaline media is reported. The initial formation of Ru islands on presynthesized Cu1.94 S NPs, via cation exchange between three Cu+ ions and one Ru3+ , induces the growth of the Ru phase, which is concomitant with the dissolution of the Cu1.94 S nanotemplate, culminating in the formation of a hollow nanostructure with numerous thin Ru pillars. Hollow Cu2-x S@Ru NPs exhibit a small overpotential of 82 mV at a current density of -10 mA cm-2 and a low Tafel slope of 48 mV dec-1 under alkaline conditions; this catalyst is among state-of-the-art HER electrocatalysts in alkaline media. The excellent performance of hollow Cu2-x S@Ru NPs originates from the facile dissociation of water in the Volmer step.

Random access transport capacity of multihop AF relaying: A throughput-reliability tradeoff

To determine the capacity of distributed wireless networks (i.e., ad hoc networks), the random access transport capacity was proposed as the average maximum rate of successful end-to-end transmission in the distance. In this article, we consider the random access transport capacity for multihop relaying to find the end-to-end throughput of a wireless ad hoc network, where each node relays the signal using an amplify-and-forward (AF) strategy. In particular, we analyze the exact outage probability for multihop AF relaying in the presence of both co-channel interference and thermal noise, where interferers are spatially distributed following a Poisson distribution. In our numerical results, it is observed that the maximum random access transport capacity is achieved at a specific spatial density of transmitting nodes due to the throughput-reliability tradeoff as the number of transmitting nodes (=interferers) increases. We compute the optimal spatial density of transmitting nodes that maximize their random access transport capacity. As a result, we can obtain the actual random access transport capacity of multihop AF relaying and predict the maximum number of transmitting nodes per unit area to maximize their performance.

Transmission capacity for dual-hop relaying in wireless ad hoc networks

To account for randomly distributed nodes in a wireless ad hoc network, the transmission capacity is defined as the number of successful transmissions taking place in the network per unit area under an outage constraint. In this paper, we analyze the transmission capacity for dual-hop relaying in a wireless ad hoc network in the presence of both cochannel interference and thermal noise, where interferers are spatially distributed following a Poisson distribution. Specifically, we first present the exact outage probability for amplify-and-forward and decode-and-forward protocols in a Poisson field of interferers without neglecting noise at all nodes. We then derive the transmission capacity of such networks, which determines the maximum allowable density of transmitting nodes for each relay strategy at a specified outage probability and data rate. Numerical results demonstrate that the dual-hop relaying is still beneficial in terms of the transmission capacity in wireless ad hoc Poisson networks.

Optimal Linear Multihop System for DF Relaying in a Poisson Field of Interferers

This paper provides a strategy for multihop transmission with decode-and-forward (DF) in a Poisson field of interferers. We analyze the optimal linear multihop system such as optimal resource allocation, optimal placement of the relay nodes, and optimal number of hops for multihop DF relaying which minimize their outage probability.

Power Allocation and Transmission Period Selection for Device-to-Device Communication as an Underlay to Cellular Networks

Efficient design of device-to-device (D2D) communication calls for D2D users to propose adaptive power allocation strategy and to establish reliable communication links while protecting the QoS of cellular communications. In this paper, we consider the D2D communication as an underlay to relay-assisted cellular networks. To maximize the ergodic capacity, we derive an optimal transmission power under an average power constraint. With the derived optimal transmission power, a transmission period selection strategy for D2D communication is firstly introduced to improve reliability. We derive the outage probability in closed forms and evaluate the ergodic capacity to show performances of the proposed system. Numerical results show that the D2D system can achieve high capacity gains by flexibly allocating transmission power based on channel state information and significantly enhance reliability by selecting a transmission period, while satisfying various QoS conditions for cellular communication.

Efficient cooperative spectrum sensing for Wi-Fi on TV spectrum

Cognitive Radio has become an effective theory to solve inefficient spectrum usage. The traffic of Wi-Fi data increases rapidly because of smart phones and netbooks. IEEE 802.22 standard supposes a cognitive radio system which consists of one licensed primary user (PU) and multiple unlicensed secondary user(SU)s. The SUs utilize the licensed spectrum while the PU is absence. In this paper, we consider to reuse the fallow TV (PU) spectrum for Wi-Fi (SU) data transmission without causing any harmful interference to incumbents. The optimal threshold number of SUs to detect the presence of PU in the constraint of minimizing detection error probability is derived. As a result it increases the capacity of traffic in the Wi-Fi zone.

Performance analysis and optimal power allocation for hybrid incremental relaying

Relaying technique has been developed considerable attention in response to improve reliability and to extend wireless network coverage. One of conventional relaying technique, incremental relaying which uses limited feedback from the destination has attracted research attention because of performance and minimization of relay usage. Moreover, optimal power allocation for various relaying schemes have been recently studied due to limited battery lifetimes in emerging wireless applications, e.g. sensor networks. In this paper, for proposing suitable relay scheme in the practical communication and improving performance of conventional relaying, we apply hybrid relaying to incremental relaying, called hybrid incremental relaying. This scheme adaptively uses the amplify-and-forward and decode-and-forward whether a relay decodes successfully or not. This paper derives outage probability and bit error probability of hybrid incremental relaying. We also compute optimal power allocation for proposed scheme and simulation results show a noticeable performance improvement compared to the equal power allocation scheme.

Random access transport capacity of dual-hop AF relaying in a wireless ad hoc networks

To account for randomly distributed nodes in a wireless ad hoc network, the random access transport capacity is defined as the average maximum rate of successful end-to-end transmission over some distance. In this paper, we consider a random access transport capacity for a dual-hop relaying to find the end-to-end throughput of wireless ad hoc network, where each node relays using Amplify-and-Forward (AF) strategy. In particular, we also present the exact outage probability for dualhop AF relaying in the presence of both co-channel interference and thermal noise, where interferers are spatially distributed following a Poisson distribution. Intriguingly, even though transmitting nodes increase, numerical results demonstrate that the overall throughput of dual-hop AF relaying decreases due to interference. Moreover, it is noted that the dual-hop AF relaying is still beneficial in terms of the random access transport capacity in wireless ad hoc networks.

Multi-hop Decode-and-Forward relaying in a wireless ad hoc networks

Multi-hop relaying over long distance is efficient to solve transmitter-power problem and mitigate wireless channel impairment. This paper provides a multi-hop relaying with Decode-and-Forward(DF) strategy in a wireless ad hoc network considering both noise and interference. Furthermore, we consider a realistic communication model where interferers are randomly scattered and uncoordinated with Poisson distribution. We analyze exact outage probability and spectral efficiency of a multi-hop DF relaying. From numerical results, multi-hop DF relaying has better performance than dual-hop relaying in terms of both outage probability and spectral efficiency.