• Journal of the Institute of Electronics Engineers of Korea TC
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• v.42 no.10 s.340
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• pp.39-50
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• 2005

In this paper, we propose a performance improving MAC algerian for the IEEE 802.11 DCF. WLAN based IEEE 802.11 uses two control methods called 'Distributed Coordination Function(UF)' and 'Point Coordination Function(PCF)'. The nF controls the Urnsmission based on carrier sense multiple access with collision detection(CSMA/CA), that decides a random backoff time with the range of contention window for each terminal. Normally, each terminal the CW double after collision, and reduces the CW to the minimum after successful transmission. This paper proposes an enhanced DCF algorithm that decreases the CW smoothly after successful transmission in order to reduce the collision Probability by utilizing the current status information of WLAN. We also analyze the throughput and delay performance for the unsaturated case mathematically. Simulation results show that our algorithm enhances the saturation throughput of WLAN. They also coincide well with the analytical results.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.21 no.6
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• pp.40-47
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• 2007

High speed electric trains have nonlinear loads including converters and inverters for the control synchronous motors. Harmonic field measurements have shown that the harmonic contents of a waveform varies with time. Direct application of the harmonic assessment to the snapshot measurements would result in ambiguous conclusions depending on which instant is sampled. A cumulative probablistic approach is the most commonly used method to solve time varying harmonics. Harmonic simulations are performed to evaluate the harmonic voltage and current distortions throughout the system. This paper provides an in depth analysis on harmonics field measurement of the high speed electric train loads, harmonics assessment by the international harmonic standards IEC 61000-3-6 and IEEE Std. 519-l992, and harmonics simulation using EDSA program for the case study.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2015.10a
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• pp.794-797
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• 2015

As the interest in VANET is increased, a study on the beacon message transmission between vehicles is actively being made. IEEE 802.11p/1609.4 standard is based on a multichannel system consisting of multiple service channels (SCH) and a control channel (CCH). Multiple SCHs are defined for nonsafety data transfer, while the CCH is used to broadcast safety messages called beacons and control messages (i.e., service advertisement messages). However, most messages broadcast in the only one CCH belong to safety application that must be contested in dense vehicular network. This paper suggests safety message transfer algorithm in dense vehicular congestion. The proposed algorithm is that the priority of safety messages is decided by decision tree and messages are stored in proper queues according to their priorities. Then, safety messages with higher priorities are sent in turn by CCH in the assigned time. The proposed algorithm decreases the beacon transmission delay and increase on the probability of a successful beacon reception in an IEEE 802.11p/1609.4-based network.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.22 no.1
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• pp.183-189
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• 2018

This paper proposes distributed file placement and coverage expansion techniques for mitigating the traffic bottleneck in backhaul for small-cell networks. In order to minimize the backhaul load with limited memory space, the proposed scheme controls the coverage and file placement of base station according to file popularity distribution and memory space of base stations. In other words, since the cache hit ratio is low when there is small memory capacity or widespread file popularity distribution, the base stations expand its coverage and cache different set of files for the user located in overlapped area to exploit multiple cached file sets of base stations. Our simulation results show that the proposed scheme outperforms the conventional cache strategy in terms of network throughput when there is small memory capacity or widespread file popularity distribution.

• The Transactions of the Korea Information Processing Society
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• v.2 no.5
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• pp.677-690
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• 1995

In this paper, we propose the high efficiency data compression capable of error control using block-sorting, move to front(MTF) and arithmetic code with variable length in to fixed out. First, the substring with is parsed into length N is shifted one by one symbol. The cyclic shifted rows are sorted in lexicographical order. Second, the MTF technique is applied to get the reference of locality in the sorted substring. Then the preprocessed sequence is coded using VF(variable to fixed) arithmetic code which can be limited the error propagation in one codeword. The key point is how to split the fixed length codeword in proportion to symbol probabilities in VF arithmetic code. We develop the new VF arithmetic coding that split completely the codeword set for arbitrary source alphabet. In addition to, an extended representation for symbol probability is designed by using recursive Gray conversion. The performance of proposed method is compared with other well-known source coding methods with respect to entropy, compression ratio and coding times.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.29 no.10A
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• pp.1139-1146
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• 2004

An abrupt increase of traffic-load in cellular CDMA systems can cause overload, as a result, degrade the quality of service (QoS) and the spectrum utilization due to lacking radio resources in base stations. In this paper, we propose a joint coverage and resource management (JCRM) scheme which can improve the QoS degradation and the spectrum utilization. The JCRM scheme hands over the overloaded traffic to neighboring cells by virtually reducing overloaded cell coverage and extending neighboring cell coverage, as well the scheme allocates radio resources based on the necessary handover probability. The proposed scheme can be applied to the existing cellular CDMA systems as well as adaptive coverage management schemes for next generation mobile communication systems.

• Aerospace Engineering and Technology
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• v.9 no.1
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• pp.67-71
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• 2010

In this study, an algorithm for blind modulation detection in EDGE systems is presented. EDGE introduces an 8PSK modulation to provide high-speed data rates in addition to the existing GSM system. A transmitter may switch dynamically the modulation and coding schemes for transmission of data according to the channel quality. To decode the data correctly, the receiver has to detect using only training sequence which modulation is being used. Basically the property of one radio block composed of four bursts to detect effectively the modulation scheme even under severe conditions is used. More specifically, the reference value calculated for received burst is accumulated with previous reference value to minimize statistically the false detection probability in one radio block. Also each burst data having different modulation from the modulation of the fourth burst is set to zero to improve the decoding performance because the reference of the fourth burst has the highest reliability.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.31 no.2B
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• pp.146-153
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• 2006

It is required to have researches on efficient resource allocation schemes in an integrated voice and data CDMA system with the spreading of high-speed wireless internets. In this paper, we proposed a efficient resouce allocation scheme for providing a high speed data service in an integrated CDMA system. In an integrated voice/data CDMA system, resources for voice users are allocated with high priority and residual resources are allocated to the data service. In this case, it is necessary to use a resource allocation scheme for minimizing interference. In this paper, we first explain about a interference minimizing method and define QoS requirements. Based on the method, we proposed a efficient resource allocation scheme which satisfy the QoS requirements. The proposed scheme controls the transmission rate and delay of data users with a priority information such as the number of packets in a queue. From the simulation results, we show that the proposed scheme reduce the blocking probability and delay and improve the performance.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.13 no.1
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• pp.94-101
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• 1999

In this paper the problems associated with the switching surge of PWM inverter devices were analysed. As the application of ASD for efficient speed control of AC rmtors is increased, so is the rrntor and ASD-related failure increased. In the industrial rmtor awlications, the introduction of PWM drives can sometmes cause insulation breakdown between the phase windings due to high transient voltage peak. The motor cable length combined with the high switching frequency becomes more sensitive issue for proper drive operation. Effects of the cable length and high switching frequency influence both on rmtor and inverter. When the insulation level of rmtor winding is low, the failure probability caused by high peak voltage and fast voltage rise times(dv/dt) is high. Voltage refloctions are simulated far rmtor capacities, ASD and rise times and are presented graphically. The filtering techniques are discussed to reduce the rmtor terminal overvoltage and dv/dt in inverter fed AC rrntor drive systems. We confirmed that the lower motar capacity and rmtar insulation level, the shorter switching time and cable length and the higrer the probability of insulation breakdown .kdown .

• The Journal of Korean Institute of Communications and Information Sciences
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• v.32 no.11B
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• pp.709-717
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• 2007

Pricing a network service aims for congestion control of the network as well as economic efficiency. A monopolistic supplier providing users with a network service on a shared link needs a pricing schedule that maximizes revenue under the link's bandwidth constraint and guarantees the bandwidth purchased by the users. In that case, nonlinear pricing is an efficient scheme which meets both requirements. This study reviews how nonlinear pricing can be applied to the network service under the constraint and shows that the nonlinear pricing may result in a fixed unit price of bandwidth as linear pricing when demand characteristics of the users follow a power law. Also, the way how the provider with incomplete information on the demand distribution seeks for the optimal pricing from the degree of the network congestion is introduced and the relationship between the development direction of the Internet and internet pricing is considered based on the results of the study.