High luminance and high speed response with the cut-off frequency of more than 50 MHz in multilayer polyfluorene-based light-emitting diodes with an interlayer were achieved. We realized multilayer polyfluorene-based light-emitting diodes for frequency response up to 100 MHz.

In this letter, we study the performance of multi-antenna relay networks with limited feedback beamforming in decode-and-forward (DF) relaying. Closed-form expression for both outage probability and symbol error rate are derived by using the moment generation function (MGF) of the combined signal-to-noise ratio (SNR) at the destination. Subjected to a total power constraint, we also explore adaptive power allocation between source and relay to optimize the performance. Simulations are given to verify the correctness of our theoretical derivations. Results show that the proposed adaptive power allocation solution significantly outperforms the uniform power allocation method.

We demonstrate a single-channel 1.28 Tbit/s-525 km transmission using OTDM of subpicosecond DQPSK signals. In order to cope with transmission impairments due to time-varying higher-order PMD, which is one of the major limiting factors in such a long-haul ultrahigh-speed transmission, we newly developed an ultrafast time-domain optical Fourier transformation technique in a round-trip configuration. By applying this technique to subpicosecond pulses, transmission impairments were greatly reduced, and BER performance below FEC limit was obtained with increased system margin.

Let C be a set of colors, and let ω(c) be an integer cost assigned to a color c in C. An edge-coloring of a graph G is to color all the edges of G so that any two adjacent edges are colored with different colors in C. The cost ω(f) of an edge-coloring f of G is the sum of costs ω(f(e)) of colors f(e) assigned to all edges e in G. An edge-coloring f of G is optimal if ω(f) is minimum among all edge-colorings of G. In this paper, we show that the problem of finding an optimal edge-coloring of a tree T can be simply reduced in polynomial time to the minimum weight perfect matching problem for a new bipartite graph constructed from T. The reduction immediately yields an efficient simple algorithm to find an optimal edge-coloring of T in time O(n1.5Δlog(nNω)), where n is the number of vertices in T, Δ is the maximum degree of T, and Nω is the maximum absolute cost |ω(c)| of colors c in C. We then show that our result can be extended for multitrees.

This research aims to accelerate the computation module in max-plus algebra using CUDA technology on graphics processing units (GPUs) designed for high-performance computing. Our target is the Kleene star of a weighted adjacency matrix for directed acyclic graphs (DAGs). Using a inexpensive GPU card for our experiments, we obtained more than a 16-fold speedup compared with an Athlon 64 X2.

In this paper, we introduce a concise representation, called right-distance sequences (or RD-sequences for short), to describe all t-ary trees with n internal nodes. A result reveals that there exists a close relationship between the representation and the well-formed sequences suggested by Zaks [Lexicographic generation of ordered trees, Theoretical Computer Science 10 (1980) 63-82]. Using a coding tree and its concomitant tables, a systematical way can help us to investigate the structural representation of t-ary trees. Consequently, we develop efficient algorithms for determining the rank of a given t-ary tree in lexicographic order (i.e., a ranking algorithm), and for converting a positive integer to its corresponding RD-sequence (i.e., an unranking algorithm). Both the ranking and unranking algorithms can be run in O(tn) time and without computing all the entries of the coefficient table.

This paper presents a novel time-domain design procedure for fast-settling three-stage nested-Miller compensated (NMC) amplifiers. In the proposed design methodology, the amplifier is designed to settle within a definite time period with a given settling accuracy by optimizing both the power consumption and silicon die area. Detailed design equations are presented and the circuit level simulation results are provided to verify the usefulness of the proposed design procedure with respect to the previously reported design schemes.

This paper describes a novel channel allocation and DBA (Dynamic Bandwidth Allocation) mechanism for ECDM-PON (Electric Code Division Multiplex -- Passive Optical Network) systems. In the current ECDM-PON systems, each ONU (Optical Network Unit) is limited to 2 or 3 CDM channels. This is because (fixed channel) CDM transmitters are expensive, and tunable CDM transmitters even more expensive. With a small number of CDM channels, the bandwidth utilization ratio is restricted by channel blocking. Our proposed mechanisms can reduce the channel blocking ratio without increasing the number of CDM transmitters or using tunable CDM transmitters. To clarify the advantages of the proposed system performance, we have evaluated the channel non-blocking ratio (Rn) and wasted resource ratio (Rw) when some users request bandwidth more than 100%. Evaluation of the non-blocking ratio, Rn shows that the proposed mechanisms approach the performance of a system with tunable CDM transmitters when the number of ONUs with over 100% traffic load is small. We have also simulated throughput for uniform traffic. In addition to these evaluations, we implemented our proposed mechanism on an FPGA (Field Programming Gate Array) and evaluated the calculation speed to allocate timeslots on CDM channels and a timeline.

Various kinds of content replication strategies in pure P2P networks have recently been examined. However, such strategies have not been thoroughly considered in hybrid P2P networks. In a hybrid P2P network, the target content can easily be found because there is a server that controls each peer and its content. Therefore, it is important to decrease futile storage resource consumption, since the data search cost through the network is relatively small. This paper proposes an effective content replication strategy that takes into account storage capacity restrictions. In brief, this method restricts the number of contents replicas possessed by a peer using threshold-based control by relocating or deleting excess replicas. Furthermore, the effectiveness of the proposal is evaluated using computer simulations.

This paper evaluates the preimage resistance of the Tiger hash function. To our best knowledge, the maximum number of the attacked steps is 17 among previous preimage attacks on Tiger, where the full version has 24 steps. Our attack will extend the number of the attacked steps to 23. The main contribution is a pseudo-preimage attack on the compression function up to 23 steps with a complexity of 2181 following the meet-in-the-middle approach. This attack can be converted to a preimage attack on 23-step Tiger hash function with a complexity of 2187.5. The memory requirement of our attack is 222 words. A Tiger digest has 192 bits. Therefore, our attacks are faster than the exhaustive search.

In terrain visualization, the quadtree is the most frequently used data structure for progressive mesh generation. The quadtree provides an efficient level of detail selection and view frustum culling. However, most applications using quadtrees are performed on the CPU, because the pointer and recursive operation in hierarchical data structure cannot be manipulated in a programmable rendering pipeline. We present a quadtree-based terrain rendering method for GPU (Graphics Processing Unit) execution that uses vertex splitting and triangle splitting. Vertex splitting supports a level of detail selection, and triangle splitting is used for crack removal. This method offers higher performance than previous CPU-based quadtree methods, without loss of image quality. We can then use the CPU for other computations while rendering the terrain using only the GPU.

In this paper, we design a practical time-reversal quasi-orthogonal space-time block code (TR-QO-STBC) system for broadband multi-input multi-output (MIMO) communications. We first modify the TR-QO-STBC encoding structure so that the interference between the transmitted blocks can be completely removed by linear processing. Two low complex decision-feedback equalization (DFE) schemes are then proposed. One is built from the frequency-domain decision-feedback equalization (FD-DFE). The derived bi-directive FD-DFE (BiD-FD-DFE) cancels the interference among the successive symbols along the time axis. The other one is the enhanced V-BLAST, which cancels the interference between the real and imaginary parts of the spectral components. They have distinct performance characteristics due to the different interference-cancellation strategies. The underlying orthogonal and symmetric characters of TR-QO-STBC are exploited to reduce the computational complexity. Computer simulations confirm that the proposed equalizers can achieve better performance than the existing schemes.

Broadcasting information to users about new resource assignments generates a substantial mapping overhead. The mapping overhead influences the system throughput and, in particular, seriously affects the performance of voice-over-Internet protocol (VoIP) services. Persistent scheduling was introduced to reduce the mapping overhead. However, up to now no studies have mathematically analyzed the performance of the persistent scheduling. This paper develops analytical and simulation models and evaluates the performance of the persistent scheduling for VoIP services in mobile WiMAX systems.

In the IEEE 802.11p WAVE system, applications can directly control the transmission power of the messages sent in WAVE Short Message Protocol (WSMP). This feature enables the vehicles to control the transmission range based on the application requirements and/or the vehicle density. Seemingly straightforward, however, the distributed power control between vehicles can easily go awry. Unless carefully coordinated, the power assignments can irrevocably deviate from the vehicle density pattern. In this letter, we first show that such anomaly happens for a straightforward power control where the power level reacts to the number of messages heard from ambient vehicles. Then in order to resolve the anomaly, we propose an application layer scheme that adapts the WSMP transmission power so that the power assignments precisely reflect the vehicle density pattern.

Quantity-based event reliability protocols have been proposed for reliable event detection in wireless sensor networks. They support the event reliability by achieving the desired number of data packets successfully transmitted from sensor nodes sensing an event to a sink by controlling the transport process. However, since many data collisions and buffer overflows frequently happen due to data congestions on limited data delivery paths from an event to a sink, the quantity-based event reliability protocols are hard to achieve the desired number due to lost data packets. Thus, this letter proposes a Quality-based Event Reliability Protocol (QERP) utilizing a property that the data packets from sensor nodes have different Contribution Degree (CD) values for event detection according to their environmental conditions. QERP selects sensor nodes to forward their data packets according to CD, and differentially transports the data packets by CD-based buffer management and load balancing.

Post-wall waveguide with a linear array of reflection-canceling slot pairs and center-feed is designed to cancel the frequency dependent tilting of the main beam and enhance the bandwidth of the antenna boresight gain. The array is fed at the center of the waveguide from the backside; the length of the radiating waveguide is halved and the long line effect in traveling wave operation is suppressed. Authors establish the array design procedure in separate steps to reduce the computational load in the iterative optimization by using Ansoft HFSS simulator. A center-feed linear array as well as an end-feed equivalent with uniform excitation is designed for 25.6 GHz operation and measured. The measured performances confirm the design and the advantage of the centre-feed; a frequency independent boresight beam is observed and the frequency bandwidth for 3 dB gain reduction is enhanced by 1.5 times compared to the end-feed array.

Achievable rates of two different transmission schemes of the two-cell cooperative single user (CSU) multi-input multi-output (MIMO) system with hybrid feedback is studied, in which one cell has the channel state information (CSI) from a mobile station (MS) and the other has channel covariance information (CCI) from it. Disjoint encoding over two base stations (BSs) is shown to achieve the capacity of the CSU-MIMO with hybrid feedback. Rather than finding an optimal transmission scheme, a suboptimal one is proposed such that the transmit directions of the BSs with CSI and CCI are eigen directions of the instantaneous channel correlation matrix and transmit covariance matrix respectively. The optimum power allocation for these transmit directions is derived as an iterative power allocation (IPA) similar to that of the MIMO multiple access channel (MAC) with CCI only. We also propose a simple disjoint power allocation (DPA). The simulation results show that the proposed transmit directions and IPA for the CSU-MIMO with hybrid feedback outperforms the MIMO-MAC with CCI only, while the DPA achieves almost the same performance as the IPA, only when the SNR is low.

An improvement of estimating sound power spectra located in a particular 2-dimensional area is proposed. We previously proposed a conventional method that estimates sound power spectra using multiple fixed beamformings in order to emphasize speech located in a particular 2-dimensional area. However, the method has one drawback that the number of areas where the active sound sources are located must be restricted. This restriction makes the method less effective when many noise source located in different areas are simultaneously active. In this paper, we reveal the cause of this restriction and determine the maximum number of areas for which the method is able to simultaneously estimate sound power spectra. Then we also introduce a procedure for investigating areas that include active sound sources to reduce the number of unknown power spectra to be estimated. The effectiveness of the proposed method is examined by experimental evaluation applied to sounds recorded in a practical environment.

Recently, network coding (NC) has been popularly applied to wireless networks in order to improve scarce wireless capacity. In wireless LANs, NC can be applied to packet retransmission, and a base station can simultaneously retransmit multiple packets destined to different wireless stations by a single retransmission trial. On the other hand, NC creates additional packet delay at both base station and wireless stations, and hence, packet transfer delay may increase seriously. However, existing NC-based retransmission methods do not consider this additional delay explicitly. In addition, when the number of flows is small, NC exhibits less benefit because the chances of NC-based retransmission are highly reduced. Therefore, in this paper, we propose a novel NC-based retransmission method in order to improve packet transfer delay and jitter of received packets. Moreover, to achieve further improvement of delay, jitter and retransmission efficiency even when there exist a small number of traffic flows, we propose a retransmission method in which NC-based retransmission cooperates with the typical ARQ method. We overcome the disadvantage of NC-based retransmission by combining with ARQ cooperatively. Finally, we show the effectiveness of the proposed methods by extensive computer simulation.

A mixed-mode high-speed binary phase-shift keying (BPSK) demodulator for IEEE802.15.3c mm-wave wireless personal area network (WPAN) application is realized with 0.18-µm CMOS process. The proposed demodulator scheme does not require any analog-to-digital converters (ADC) and, consequently, can have advantages over the conventional schemes for high-data-rate demodulation. The demodulator core consumes 53.8 mW from 2.5-V power supply while the chip area is 380500 µm2. The fabricated chip is verified by 60-GHz wireless link tests with 1.6-Gb/s data.