• Journal of the Korea Institute of Information and Communication Engineering
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• v.18 no.7
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• pp.1540-1546
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• 2014

In order to resolve coverage hole problems caused by indoor-BS (IBS) faults, we propose a new frame structure and practical algorithm based on optimization technique. Our main contributions can be described as follows: 1) a frame structure with healing channels for solving abnormal IBS faults; and 2) an efficient heuristic resource allocation algorithm with fixed IBS selection to reduce the complexity for the optimization problem. Through intensive simulations, we evaluate the performance excellency of our proposed algorithm with respect to average cell capacity and user fairness compared with conventional algorithms.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.20 no.12
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• pp.1279-1286
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• 2009

This paper addresses the analysis of the interference produced between the LEO(Low Earth Orbit) satellite constellation and FS(Fixed Service) system operating in the same frequency and area. At first, we calculates the interference of FS system from the LEO satellite constellation depending on the number of LEO satellite antenna beams. Simulation results show that the amount of interference that was calculated from each region. This result can be used to define the carrier level for protecting FS system from total interference by LEO satellite constellation. In the second scenario, we calculates the interference of LEO satellite system earth station by the FS link depending on radius of protection area. The presented results can be used to design FS systems minimizing interference to earth station.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.37 no.7A
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• pp.569-580
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• 2012

In this paper, an enhanced initial ranging algorithm for large-cell coverage fixed wireless communication system is proposed. In typical wireless communication system such as WiBro, because a round-trip delay between a transmitter and a receiver is within one OFDM (Orthogonal Frequency Division Multiplexing) symbol duration, a frequency-domain differential correlation method is generally used. However, the conventional method cannot be applied due to an increase of a maximum time delay in large-cell system. In case of an accumulative differential method, estimation errors can occur because of frequent sign transitions. In this paper, therefore, we propose an algorithm which can estimate a total timing offset in a ranging channel structure for 15 km cell. The proposed method can improve performance by sign comparison based sign error correction rule between the estimated values and using a weighting scheme based on channel correlation, the number of accumulations, and the noise reduction effect in normalization process. Also, it can estimate the integer timing offset of symbol duration by comparing peak-powers after compensating for the fractional timing offset of symbol duration.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2010.05a
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• pp.890-893
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• 2010

In wireless sensor networks, each node generally uses a battery because it is hard to replace or charge. For this reason, study for life time prolongation of each node within the limited energy source has become an important issue. So many ways are suggested to minimize the energy consumption for each node, especially energy efficient MAC protocols have been studied actively. T-MAC of contention based MAC protocol is that added the adaptability on fixed duty cycle of S-MAC. T-MAC allocates the fixed timeout before each node goes to sleep mode from active mode. If no data exchanged in a timeout, each node goes to sleep mode. Because of the timeout is always fixed, the absence of data exchange in a timeout will cause unnecessary energy consumption. In this paper, in order to improve the energy efficiency, we propose a MAC protocol based on adaptive timeout that analyze the probability of the timeout, and provides the modified timeout.

• ETRI Journal
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• v.34 no.5
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• pp.655-665
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• 2012

Based on wideband precoding (WBP) in the multiple-input multiple-output orthogonal frequency division multiplexing system, an adaptive nonuniform codebook is presented in this paper. The relationship between the precoder distribution and spatial correlation is analyzed at first. A closed-form expression based on overlapped isosceles triangles is proposed as an approximation of the precoder distribution. Then, the adaptive codebook design is derived with the approximate distribution to minimize quantization errors. The capacity and bit error rate performance demonstrate that the adaptive codebook with WBP outperforms the conventional fixed uniform codebook.

• Proceedings of the IEEK Conference
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• 2002.07b
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• pp.1346-1349
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• 2002

This paper presents the design and implementation of 16-QAM modem that can be applied to fixed broadband wireless access systenm. It is implemented in the hardware prototype that consist of FPGA(Field Programmable Gate Array) for digital signal processing and analog front end module for analog signal processing. We provide 20.48Mbps data rate using implemented modem and test the modem in KOREA 26㎓ broadband wireless local loop system including IFU(Intermediate Frequency Unit) and RFU(Radio Frequency Unit) via air interface.

• Proceedings of the IEEK Conference
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• 2002.07c
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• pp.1363-1366
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• 2002

The studies of multi-hop wireless networks are active as the new communication media, which does not require any fixed communication infrastructure. One of application of the networks is local information distribution service, which is useful far daily activities in certain geographically restricted region or community within a radius of several kilometers. In this paper, MID-Net, which is the network enabling such distribution service, and effective information distribution algorithm MCMS are proposed. The behavior of the MID-Net is characterized by the waiting time function, three types of the functions are proposed in this paper.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.11 no.3
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• pp.1393-1405
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• 2017

It is well known that duty-cycling control by dynamically adjusting the polling interval according to the traffic loads can effectively achieve power saving in wireless sensor networks. Thus, there has been a significant research effort in developing polling interval adaptation schemes. Especially, Dynamic Low Power Listening (DLPL) scheme is one of the most widely adopted open-looping polling interval adaptation techniques in wireless sensor networks. In DLPL scheme, if consecutive idle (busy) samplings reach a given fixed threshold, the polling interval is increased (decreased). However, due to the trial-and-error based approach, it may significantly deteriorate the system performance depending on given threshold parameters. In this paper, we propose a novel DLPL scheme, called SDL (Sequential hypothesis testing based Dynamic LPL), which employs sequential hypothesis testing to decide whether to change the polling interval conforming to various traffic conditions. Simulation results show that SDL achieves substantial power saving over state-of-the-art DLPL schemes.

• Journal of Communications and Networks
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• v.16 no.3
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• pp.311-321
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• 2014

This paper focuses on contention-based medium access control (MAC) protocols used in wireless local area networks. We propose a novel MAC protocol called adaptive backoff tuning MAC (ABTMAC) based on IEEE 802.11 distributed coordination function (DCF). In our proposed MAC protocol, we utilize a fixed transmission attempt rate and each node dynamically adjusts its backoff window size considering the current network status. We determined the appropriate transmission attempt rate for both cases where the request-to-send/clear-to-send mechanism was and was not employed. Robustness against performance degradation caused by the difference between desired and actual values of the attempt rate parameter is considered when setting it. The performance of the protocol is evaluated analytically and through simulations. These results indicate that a wireless network utilizing ABTMAC performs better than one using IEEE 802.11 DCF.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.21 no.4
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• pp.104-114
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• 2007

In this paper, an Ad-hoc Routing Protocol that works in wireless Ad-hoc communication networks with multiple radios and multiple channels and that controls call admission based on bandwidth measurement is proposed. Unlike the conventional Ad-hoc node with a single radio using a single channel, an Ad-hoc node of the protocol proposed, the MCQosR(Multiple Channel Quality of Service Routing), has multiple radios and uses multiple channels, which allows full duplex transmission between wireless Ad-hoc nodes, and reduces intra interference on the route. Also, a fixed channel only for reception at each node enables the estimation of the available bandwidth, which is used to control the call admission for QoS provision. The performance of the MCQosR was verified by simulation.