• Journal of the Optical Society of Korea
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• v.8 no.1
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• pp.29-33
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• 2004

We propose and demonstrate a novel optical pulse multiplier applicable to OTDM (Optical Time Division Multiplexing) systems using cascaded long-period fiber gratings. We have exploited the fact that each mode in a fiber has a different propagation constant to obtain time delays among optical pulses. The proposed scheme could realize high-frequency optical pulse multiplication for optical short pulse trains. We have successfully implemented two, four, and eight times multiplications with the maximum repetition rate of 416.7 ㎓. The obtained pulse delays are well matched with the simulated ones.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2010.05a
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• pp.956-958
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• 2010

The paper considers MIMO two-way scheme to optimize the end to end capacity in local wireless mesh network. The basic idea is to perform data transmission via intermediate cooperative nodes and cooperative relay therefore higher throughput can be achieved. Each node is equipped with multiple antennas, and has two time slots one for transmission (Tx) and the other is reception (Rx), which are arranged alternatively in the network. In the conventional SISO network, it takes at least four time slots to accomplish the function of two-way relay. Moreover, cooperative technique is used in order to enhance multiplexing of forward and backward streams.

• Convergence Security Journal
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• v.12 no.1
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• pp.35-42
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• 2012

Increase of data traffic and the advent of new real-time services require to change from the traditional TDM-based (Time Division Multiplexing) networks to the optical networks that soft and dynamic configuration. Voice and lease line services are main service area of the traditional TDM-based networks. This optical network became main infrastructure that offer many channel that can convey data, video, and voice. To provide high resilience against failures, Packet-optical networks must have an ability to maintain an acceptable level of service during network failures. Fast and resource optimized lightpath restoration strategies are urgent requirements for the near future Packet-optical networks with a Generalized Multi-Protocol Label Switching(GMPLS) control plane. The goal of this paper is to provide packet-optical network with a hierarchical multi-layer recovery in order to fast and coordinated restoration in packet-optical network/GMPLS, focusing on new implementation information. The proposed schemes do not need an extension of optical network signaling (routing) protocols for support.

• Journal of Internet Computing and Services
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• v.7 no.3
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• pp.41-51
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• 2006

In the case of CDMA-based packet data systems such as 1xEV-DO which are designed to support high rate services, BSs transmit data packets with a maximum power based on time multiplexing mode such that only one user can be serviced at a time. In this paper, we propose a joint power allocation and scheduling algorithm for 1xEV-DO-like systems in which we adopt a code division multiplexing (CDM) transmission method in the downlink common channel in order to utilize channel orthogonality such that we can serve more than one user at a time slot especially when there exist remaining resources after serving the firstly selected user by the scheduler. Simulation results demonstrate that the proposed scheme can improve the performances of conventional schemes such as the maximum rate and the proportional fair algorithms.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.47 no.7
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• pp.85-92
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• 2010

In this paper, we consider the orthogonal frequency division multiplexing (OFDM) based transmission incoorperating with beam-switching and space-time coding. Specifically, we consider three configurations; (1) the beamforming technique, (2) the spatial diversity technique and (3) their combination and evaluate the performance in wireless personal area network (WPAN) environment. For the beam-forming technique, we consider the beam-switching which is performed at RF front-end with a pre-defined set of beams and for the space-time coding, we consider the Alamauti scheme with antenna selection. For the combined scheme, we divide the antennas used into two group to generate two independent beams and apply the two-antenna Alamauti scheme over the two beams. For these three configurations, performance is evaluated in terms of the SNR gain.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.6 no.2
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• pp.236-247
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• 2002

We propose a cell－multiplexing scheme for the real－time communication service in ATM network and a new service discipline guarantee end-to-end delay based on pseudo-isochronous cell switching. The proposed scheme consists of two level frame hierarchy, upper and lower frame, which is used to assign the bandwidth and to guarantee the requested delay bound, respectively. Since the Proposed algorithm employs two level frame hierarchy, it can overcome the coupling problem which is inherent to the framing strategy It can be comparatively reduce the complexity, and still guarantee the diverse delay bounds of end-to-end. Besides, it consists of two components, traffic controller and scheduller, as the imput traffic description model and regulates the input traffic specification. The function of the traffic controller is to shape real -time traffic to have the same input pattern at every switch along the path. The end-to-end delay is bounded by the scheduller which can limit the delay variation without using per-session jitter controllers, and therefore it can decrease the required buffer size. The proposed algorithm can support the QoS's of non-real time traffic as well as those of real time traffic.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.10 no.1
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• pp.44-53
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• 1999

In this paper, we propose a pilot based time-averaging channel estimation method and analyze error performances for efficient transmission of OFDM(Orthogonal Frequency Division Multiplexing) in multipath fading environment. Frequency domain channel estimations have been used in OFDM systems to compensate signal distortions due to fading on each subcarrier. The frequency domain estimation scheme uses scattered pilot to estimate channel response by simple interpolation. This implies that the estimated channel response includes signal distortions due to the noise. In this paper, we propose time-averaged channel estimation method to remove the distortion by noise. The proposed scheme can effectively remove noise components by taking time-average of the estimated channel response after estimating frequency domain channel. The computer simulations were performed to evaluate the performance of the proposed channel estimator. For the Rician channel, we compared the performance of the proposed method to that of a conventional one using channel estimation by gaussian interpolation when SER(Symbol Error Rate) = $10^{-4}$, and compared to perfect channel estimation case. The proposed method showed differences of 0.07 dB, 0.6 dB compared to perfect channel estimation and improvements of 1.7 dB, 1.9 dB for 16 QAM, 64 QAM respectively compared to conventional method.

• Journal of information and communication convergence engineering
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• v.4 no.2
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• pp.71-74
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• 2006

A basic access method about IEEE 802.11 MAC layer protocol using IEEE 802.11 wireless LANs is the DCF thatis based on the CSMA/CA. But, cause of IEEE 802.11 MAC layer uses original backoff algorithm (exponential backoff method), when collision occurred, the size of contention windows increases the double size Also, a time of packet transmission delay increases and efficienty is decreased by original backoff scheme. In this paper, we have analyzed TCP packet transmission time of IEEE 802.11 MAC DCF protocol for wireless LANs a proposed enhanced backoff algorithm. It is considered the transmission time of transmission control protocol (TCP) packet on the orthogonal frequency division multiplexing (OFDM) in additive white gaussian noise (A WGN) and Rician fading channel. From the results, a proposed enhanced backoff algorithm produces a better performance improvement than an original backoff in wireless LAN environment. Also, in OFDM/quadrature phase shift keying channel (QPSK), we can achieve that the transmission time in wireless channel decreases as the TCP packet size increases and based on the data collected, we can infer the correlation between packet size and the transmission time, allowing for an inference of the optimal packet size in the TCP layer.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2006.05a
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• pp.530-533
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• 2006

Recently, a basic access method about IEEE 802.11 MAC layer protocol using IEEE 802.11 wireless LANs is the DCF thatis based on the CSMA/CA. But, cause of IEEE 802.11 MAC layer uses original backoff algorithm (exponential backoff method), when collision occurred, the size of contention windows increases the double size. Also, a time of packet transmission delay increases and efficiency is decreased by original backoff scheme. In this paper, we have analyzed TCP packet transmission time of IEEE 802.11 MAC DCF protocol for wireless LANs a proposed enhanced backoff algorithm. It is considered the transmission time of transmission control protocol (TCP) packet on the orthogonal frequency division multiplexing (OFDM) in additive white gaussian noise (AWGN) and Rician fading channel. From the results, a proposed enhanced backoff algorithm produces a better performance improvement than an original backoff in wireless LAN environment. Also, in OFDM/quadrature phase shift keying channel (QPSK), we can achieve that the transmission time in wireless channel decreases as the TCP packet size increases and based on the data collected, we can infer the correlation between packet size and the transmission time, allowing for an inference of the optimal packet size in the TCP layer.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.16 no.10 s.101
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• pp.995-1002
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• 2005

An orthogonal frequency division multiplexing(OFDM) system has the problem of the peak-to-average power ratio(PAR). In general, in order to obtain optimal PAR reduction using the partial transmitted sequence(PTS), the total search for the number of sub-blocks and the rotation factors must be accomplished. As the number of sub-blocks and rotation factors increases, PAR reduction improves, such that complexity increases exponentially and the process delay occurs simultaneously. Therefore a technique that reduces PAR, which is almost close to optimal, and the amount of calculation is desired. In this paper a new method using genetic algorithm(GA), which is widely used to search for a point that is globally optimal in many problems, is proposed to search for a rotation factor that reduces simultaneously both the PAR and the amount of calculation, such that the complexity of calculation and the process time are reduced at the same time, Comparison is performed between the proposed method and the various techniques developed previously. The superiority of proposed method is presented by demonstrating the reduction of complexity while a similar PAR reduction is obtained.