• Journal of the Korea Institute of Information and Communication Engineering
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• v.18 no.5
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• pp.1061-1066
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• 2014

Transmission characteristics of the sound propagation is very complicate and sparse in shallow water. To increase the performance of underwater acoustic communication system, lots of channel estimation technique has been proposed. In this paper, we proposed the channel estimation based on LMS(Least Mean Square) algorithm which has faster convergence speed than conventional sparse-aware LMS algorithms. The proposed method combines $L_p$-norm LMS with soft decision process. Simulation was performed by using the sound velocity profile which acquired in real sea trial. As a result, we confirmed that the proposed method shows the improved performance and faster convergence speed than conventional methods.

• The Journal of the Acoustical Society of Korea
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• v.23 no.1
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• pp.24-29
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• 2004

In ocean acoustic tomography (OAT), the pulse compression techniques using M-sequences are employed in the many studies for investigating the ocean structures. M-sequences can provide the good time and Doppler resolution in the process of demodulation using matched-filter. The signal-to-noise (SNR) performance at the output of receiver may be improved by manipulating received signal, i. e. coherently averaging. The processing time can be significantly reduced by using fast hadarmard transform (FHT) or fast Fourier transform (FFT). In this paper, we estimate the multipath arrival structures and delay times using the East Korean Sea experiment data and explore the compensation method for the detrimental effects on performance due to sampling rate error. We also analyze the characteristics of the ocean acoustic channels through scattering function, delay power profile, and time dispersions.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.10 no.8
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• pp.3551-3567
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• 2016

The Low Earth Orbit satellite network has the unique characteristics of the non-uniform and time-variant traffic load distribution, which often causes severe link congestion and thus results in poor performance for delay-sensitive flows, especially when the network is heavily loaded. To solve this problem, a novel adaptive routing algorithm, referred to as the delay-oriented adaptive routing algorithm (DOAR), is proposed. Different from current reactive schemes, DOAR employs Destination-Sequenced Distance-Vector (DSDV) routing algorithm, which is a proactive scheme. DSDV is extended to a multipath QoS version to generate alternative routes in active with real-time delay metric, which leads to two significant advantages. First, the flows can be timely and accurately detected for route adjustment. Second, it enables fast, flexible, and optimized QoS matching between the alternative routes and adjustment requiring flows and meanwhile avoids delay growth caused by increased hop number and diffused congestion range. In addition, a retrospective route adjustment requesting scheme is designed in DOAR to enlarge the alternative routes set in the severe congestion state in a large area. Simulation result suggests that DOAR performs better than typical adaptive routing algorithms in terms of the throughput and the delay in a variety of traffic intensity.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.22 no.2
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• pp.253-260
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• 2018

The acoustic communication channel in shallow underwater is typically shown as time-varying multipath fading channel characteristics. The received signal through channel transmission cause inter-symbol interference (ISI) owing to multiple components of different time delay and amplitude. To compensate for this, several techniques have been used, and one of them is acoustic equalizer. In this study, we used four equalizers - feed forward equalizer (FFE), decision directed equalizer (DDE), decision feedback equalizer (DFE) and combination DDE with DFE to compensate ISI. And we applied two adaptive algorithms to adjust coefficient of equalizers - normalized least mean square algorithm and recursive least square algorithm. As result, we found that it has a significant performance improvement over 6 dB on SNR in nonlinear equalizer. By combination of DFE and DDE has almost best performance in any case.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.20 no.7
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• pp.1243-1248
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• 2016

This paper investigated the channel link budget for Wireless Avionics Intra-Communication (WAIC) in 4.4GHz. The band 4.2~4.2GHz was allocated for the communication service among aeronautical applications in world radiocommunication conference (WRC). Wireless channels in aircraft was modelled by the combination of path loss, shadowing effect caused by obstacles, and fading caused by multipath signals. In addition, wireless channels in aircraft are categorized into 6 groups according to various locations of transmitter and receiver. We analyzed the channel link budget for the 6 channel groups in terms of maximum transmission distance and outage probability. Our analysis and intensive computer simulation results show that the propagation characteristics of group A, B, and F is superior to group C, D, E, and the propagation of group E is the most vulnerable. Also, these results can be utilized as basic reference for the channel analysis of intra-aircraft or similar environment.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.11 no.6
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• pp.2196-2201
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• 2010

The video transmission system using OFDM method at 18GHz frequency band was developed for wireless video transmission from subway to monitoring center when subway running. It is composed of digital modulation board and RF board. And it has a robust characteristics of multipath fading in subway tunnels at 18GHz. To verify the system, we used spectrum analyzer for analysing the output of transmission system and get the results that it has 350MHz frequency, 8MHz bandwidth and -19dBm channel power. Also we tested video receiving using MPEG-2 encoder and got good quality images.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.28 no.5A
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• pp.305-315
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• 2003

MC-CDMA(Multicarrier code division multiple access), which is based on a combination of OFDM(orthogonal frequency division multiplexing) and CDMA(code division multiple access), has gained a lot of interests in wireless multimedia communications, as high speed data transmission is required for mobile services. MC-CDMA has many advantages for broadband high speed data transmission in multipath environment because it can offer both advantages of the CDMA and the OFDM. However, A high PAPR(peak to average power ratio) problem, which is a major drawback of OFDM, is also shown in the MC-CDMA. In this paper, we propose a new phase factor optimization scheme to reduce complexity in PTS(partial transmit sequence) to reduce PAPR. We also analyze the performance of the MC-CDMA with various PTS schemes to investigate the relations between PAPR characteristics and effect of nonlinear distortion of a high power amplifier. Our simulation results reveal that the proposed PTS scheme reduces PAPR about 0.2∼0.5 dB even with 25% reduced- complexity compared to the conventional scheme.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.37B no.11
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• pp.975-983
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• 2012

Multi network interface has become common phenomenon for mobile devices such as smart phone which has 3G, LTE-advanced, WiFi. Consequently, there are researches for a transmission strategies using multiple paths below on end-to-end connection. MPTCP which is proposed and being standardized by the IETF as a new transport protocol can perform concurrent multipath transfer using multiple network interfaces. However, current MPTCP has performance degradation when it use heterogeneous networks which have quite different network characteristics. Therefore, this paper proposes the packet scheduling scheme and receiver-based recovery scheme to reduce the performance degradation due to reordering problem. Also, simulation results show that the proposed scheme can improve throughput and retransmission performance.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.16 no.10
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• pp.2095-2100
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• 2012

In this paper, we propose the channel sounding scheme which is made for ideal communication between some application as well as the short distance of high speed data transmission in MIMO-OFDM system for Wireless PAN. This method is able to perceive the duration of the impulse response through the delaying of power delay profile, modeled a power delay profile which has an attenuate characteristic, and obtained the coefficient of channel response by ML (maximum likelihood). Through the amplitudes, phases and delays associated with each multipath component which were acquired from this channel sounding scheme, we can describe the wave propagation characteristics of channels between the transmitter and receiver so that the receiver could enhance not only the reliability but also the ability of communication link. Multi agent system models can be used to analyze the path of the system within any time frame. Further, parameter values can be perturbed to examine how the path of the system changes in response to exogenous shocks.

• The Journal of the Institute of Internet, Broadcasting and Communication
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• v.9 no.6
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• pp.23-28
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• 2009

In this paper, we proposed and analyzed the performance of the adaptive switching equalization for SC-FDMA system. It is well known that SC-FDMA system have a fairly similar structure to OFDMA system. Furthermore, SC-FDMA system has great advantage of low PAPR compare to OFDM system. However, this system often suffers from wireless channel characteristics such as multipath fading and increased channel impulse response and so on. To reduce this channel influence, it strongly requires efficient adaptive equalization. Therefore, the proposed system operated upon two modes namely, ZF mode for slow speed and MMSE mode for high speed. From the simulation results, we can confirm that the proposed scheme has more efficient performance from the system complexity point of view. So we can expect that the proposed system will be applied design of 3GPP LTE uplink.