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The next 6th generation (6G) mobile communication technologies should solve the out-of-band emission problem of cyclic prefix (CP) orthogonal frequency division multiplexing (OFDM) that the 4th generation technology did not solve in order to use frequency resources efficiently. In this study, we propose the spectral encapsulation (SE) that shapes the spectrum of OFDM signals. We also derive the orthogonalizing matrix and thoroughly analyze its characteristic. Since vector (v) SE-OFDM is a spectral shaping technique based on zero insertion without using any long filter, it can be applied to even short OFDM symbols. We build the precoder with the accurate orthogonal matrix and show that it has perfect blocking at the frequency bandwidth boundaries. Since the information transmission part of the proposed vSE-OFDM is the same as the CP-OFDM, it maintains the bit error rate (BER) performance of CP-OFDM as it is, and the complexity can be significantly improved because it can be implemented using IFFT.

In underwater acoustic communications, it is difficult to find the starting time of received signals because of noise, multipath, transmission time delay, and Doppler frequency. In this paper, we solve the problem by using 2-STEP frame synchronization in the receiver. The transmitted signal is modulated by using burst PSK. The frame is composed of preamble of PN code and random data. The 2-STEP frame synchronization consist of non-coherent correlation, sliding FFT, and coherent correlation. First, in the STEP 1, the non-coherent correlation and sliding FFT are simultaneously performed to obtain an approximate start time of a frame in time domain and a Doppler frequency in frequency domain. However, through the results of the marine experiments, we can see that it is difficult to guarantee the starting time point in STEP 1 when the received signal power is weak and there is high Doppler frequency. Therefore, in the STEP 2, we use the coherent correlation to get more accurate start time stably after compensating the Doppler frequency of FFT in STEP 1. Finally, we can assure that the performance of the proposed 2-STEP frame synchronization is robust to low power of received signal and high Doppler frequency through marine experiments.

A binder-free electrode for flexible supercapacitor applications was fabricated using carbon fiber papers (CFPs) via a facile paper making method. CV curves and galvanostatic charge/discharge profiles exhibited ideal capacitive behavior and linear voltage–time function with small IR drop. O2 activation process greatly affected the porosity and surface structures. The CFP electrodes activated at 325 °C for 45 min by O2 oxidizing gas showed highest specific capacitance of 156.9 F g−1 and excellent cycle stability (∼90.2%) after 5000 cycles at 1.0 A g−1 in 1.0 M H2SO4, which presented the possibilities as flexible electrodes with excellent electrochemical performances for EDLCs.

It is sometimes required that acoustic communication be covert while at the same time maintaining reliability. We proposed a covert underwater acoustic communication method which was robust against channel fading and Doppler shift in our previous paper. In this paper we verify the previously proposed method through the sea experimental results at range of 60 km.

In UWAC (Underwater Acoustic Communications), underwater acoustic channels change rapidly due to various environment condition. AMC (Adaptive Modulation and Coding) technique is efficient method for improving the system efficiency by changing transmission parameters according to channel conditions in underwater acoustic channels. In this paper, we propose four modes and three threshold detection algorithms to determine the transmission mode in AMC technique. In transceiver model, convolutional coding is used to free the size of the information bits and turbo equalization is applied to improve the performance. In addition, the relationship between the modes and the threshold is analyzed through underwater experiments.

In this paper, we propose a covert underwater acoustic communication method that is robust to fading using a chirp signal combined with a frequency-hopping spread spectrum scheme. A fractional Fourier transform, which estimates the slope of the signal frequency variation, is applied to the receiver to enable a robust and reliable symbol estimation with respect to the frequency and irregular phase variations. In addition, since the recursive symbol synchronization can be implemented using a chirp signal, compression and expansion effects due to the Doppler shift can be mitigated. Simulation and lake trials were performed to verify the performance of the proposed method. The simulation was performed by two different methods.

In this paper, we propose a precoding scheme for 6th generation (6G) mobile communications, which can enhance the spectrum efficiency by overcoming the shortcoming of OFDM of power flowing out to adjacent channels and estimate the channel state with pilots. The proposed scheme is the first trial to have spectral efficiency by having orthogonality between subcarriers in the in-band as well as out-of-band (OOB) spectral efficiency. The precoding is realized with a matrix that consists of a cyclic prefix matrix, a central matrix, and a cyclic postfix matrix. The cyclic prefix and postfix matrices, and the central matrix play roles to provide the packet with the guard interval and to carry an information-bearing signal, respectively. The fade in and fade out properties of cyclic prefix and postfix can confine the spectrum of OFDM signals and prohibit the power leakage to adjacent channels. A good autocorrelation characteristic of the precoding scheme can guarantee the frequency offset estimation with high quality. Since the columns of the central matrix in the time domain are orthogonal to each other, the pilot symbols can be inserted in the same manner as in the conventional OFDM scheme.We will show that the proposed precoding scheme is useful in terms of out-of-band power leakage blocking, autocorrelation, and BER performance compared with OFDM.

Recently, the applications of carbon fiber reinforced plastics (CFRPs) have become broader than everwhen it comes to such industries as automotive, ships, aerospace and military because of their lightweight-ness andhigh mechanical properties. Thermosetting plastics like epoxy are frequently used as the binding matrix in CFRPs dueto their high hardness, wetting characteristics and low viscosity. However, they cannot melted and remolded. For thisreason, thermosetting plastic wastes have caused serious environmental problems with the production of fiberreinforced plastics. Thus, many studies have focused on the carbon fiber reinforced thermoplastics (CFRTPs) andrecycling carbon fiber. In this study, recycled carbon fiber (RCF) was prepared from CFRPs using a pyrolysis method,which was employed to separate resin and carbon fiber. The degree of decomposition for epoxy resin was confirmedfrom thermal gravimetric analysis (TGA) and scanning electron microscope (SEM). The RCF was cut and ground toprepare a carbon fiber composite sheet (CFCS). CFCS was manufactured by applying recycled carbon fibers andvarious thermoplastic fibers. Various characterizations were performed, including morphological analyses of surfaceand cross-section, mechanical properties, and crystallization enthalpy of CFCS at different cooling conditions.초록

Recently, unmanned systems are largely utilized in various fields due to the persistency and the least operational risk and an unmanned surface vehicle(USV) is the one of the representative application in the naval field. To assign multiple roles to an USV, we developed a sonar system which consists of a forward detecting sonar for the long-range detection, a downward detecting sonar for the small target scan and identification, and a strut type body for mounting sonar systems. In this paper, we described the developed sonar system for USV and the sea test results for verifying system performance. The test results showed that the developed sonar system was able to detect the underwater target about several kilometers away and could recognize a small object at the downside of the sonar system. We expect that the developed sonar system will be easily applied to other unmanned platforms without serious consideration.Key Words : Unmanned Surface Vehicle(무인수상정), Sonar System Development(소나시스템 개발), Forward Detecting Sonar(전방탐지소나), Downward Detecting Sonar(하방탐지소나)

In this paper, we have described the characteristics of the Underwater Distributed Sensor Network (UDSN) and proposed the conceptual design guideline by an effectiveness analysis. To perform the effectiveness analysis, we defined an battlefield environment, and then analyzed principal components which compose the UDSN to find out simulation parameters and system constraints. We have chosen a measure of effectiveness based on a target trajectory, which could enhance intuitive understanding about current status, and performed various simulations to reveal critical design parameters in terms of sensor node types, arrangement, cost and combination of detection information.Key Words : Underwater Distributed Sensor Network(수중분산 센서망), Effectiveness Analysis(효과도 분석), Network Centric Warfare(네트워크 중심전), Surveillance System(감시 체계) 1. 서 론 천해지역에서 잠수함과 같은 위협 세력에 효율적으로 대응하기 위해서는 상시 감시가 가능한 경계 체계 구축이 요구되나, 초계기, 초계함 및 잠수함과 같은 전통적인 유인 플랫폼만으로는 소기의 목적을 달성하기가 쉽지 않다. 이에 미국에서는 수중에 설치된 음향/비 * Corresponding author, E-mail: belfre@add.re.krCopyright

Fibrids are materials of a complex fibrous, film-like, or intermediate form. The diameter of the particles var-ies between 20–50 µm, and 100–10,000 µm. The optimal form and size of the particles depends in each case on thearea of application. The fibrid form is the result of two processes, viz. a wave process and diffusion. The life span andbreak-up pattern of the polymer depend on the interphase surface tension, the viscosity of the polymer, and the diffu-sion speed ratio of the polymer and medium. In this study, m-aramid was polymerized at low temperature and in solu-tion with an equal molar ratio of MPD and IPC in DMAc. The meta-aramid solution was prepared by dissolution inDMAc, and ethanol was used as a coagulation bath. The rotation speed was the main factor affecting the formation andmorphology of the fibrid. Meta-aramid paper was produced from m-aramid fiber with meta-aramid fibrid by wet lay-ing. The results showed that fibrid particle sizes decreased with increasing rotation speeds and decreasing concentra-tions of m-aramid in the solution. The properties of the m-aramid fibrid as a function of the processing conditions wereinvestigated using an image analyzer, optical microscopy, and SEM. The resulting meta-aramid paper was analyzed interms of its morphology, thickness, air permeability, and mechanical properties.Keywords: fibrid, m-aramid, ethanol, meta-aramid paper

In the advanced T-DMB (AT-DMB) system, the performance of the base layer is affected by both modulation noise and a demodulation method. The amount of modulation noise is determined by the hierarchy parameter of the transmitter, and thus it is not controllable at the receiver. On the other hand, a demodulation method for the base layer can be freely selected by the designer and it dominates the performance of the base layer. This paper focuses on the performance analysis of the base layer in the AT-DMB system according to the choice of a demodulation method and provides useful information for designing receivers for the AT-DMB system.

A novel pre-equalization method for terrestrial DTV transmitters is presented. A pre-equalizer is used in transmitters to correct group delay and amplitude distortions caused by a channel filter. In the proposed pre-equalizer, an equation-error adaptive IIR filtering scheme is adopted unlike the conventional pre-equalization using FIR filtering. The pole-zero modeling property of IIR filters improves the signal-to-noise ratio by compensating for various linear distortions existing in DTV transmitters as well as the channel filter distortion. Simulation results show that the proposed IIR pre-equalizer performs better than the FIR pre-equalizer in terms of the residual mean-square error