• Journal of KIISE:Computing Practices and Letters
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• v.16 no.7
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• pp.794-798
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• 2010

Assuming that a wireless link is mostly used at the network edge and the wireless NIC driver keeps monitoring the error rate of its link, this paper proposes an enhanced TCP congestion control, TCP-L (TCP Link-Aware). TCP-L predicts true congestion losses occurred inside the wired link area by utilizing the wireless link error rate. As a result, it mitigates performance degradation caused from TCP congestion control actions when segments losses occur in a wireless link. Experimental results show that TCP-L provides better performance and fairness in lossy wireless links than existing TCP congestion control schemes. Our approach utilizing the characteristic of the link at TCP could be well adapted to new wireless environments such as Cognitive Radio and ACK-less IEEE 802.11, where a frame may be delivered with a very long delay or lost in the link.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.37A no.9
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• pp.799-808
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• 2012

Bit operation of scrambler in the WAVE System become less efficient because parallel processing is impossible in terms of hardware and software. In this paper, we propose algorism to find the starting position of the matrix table. Also, when bit operation algorithm of scrambler and algorithms for matrix table, algorithm used to find starting position of the matrix table were compared with the performance as 8 bit, 16bit, 32 bit processing units. As a result, the number of processing times per second could be done 2917.8 times more in an 8-bit, 5432.1 times in a 16-bit, 10277.8 times in a 32 bit. Therefore, algorithm to find the starting position of the matrix table improves the speed of the scrambler in the WAVE and the receiving speed of a variety of information gathering and precision over the Vehicle to Infra or Vehicle to Vehicle in the Intelligent Transport Systems.

• The KIPS Transactions:PartC
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• v.15C no.5
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• pp.399-408
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• 2008

Mobile WiMAX provides wireless broadband services for data communication based on IP protocol. The limitation of physical bandwidth in the radio links may cause performance degradation in providing wireless broadband services in WIMAX. To enhance the efficiency of the radio link utilization, Payload Header Suppression (PHS) is defined as an optional header compression mechanism for mobile WiMAX. It has, however, a very limited compression capability since it has very restrictive compression fields. In this paper, hence, we assumed the application of Robust Header Compression (ROHC), a header compression scheme proposed for links characterized by high bit error ratios, long round-trip times (RTT), and scarce resource, to Mobile WiMAX, and studied its performance. Previous studies on ROHC performance merely focused on the impact of high bit error rate. However, bit error is virtually transparent to ROHC in the wireless systems like WiMAX, since the MAC provides the bit error checking function. In order to evaluate the performance of ROHC in the Mobile WiMAX environments, therefore, we evaluated the performance of ROHC with respect to the packet losses instead of bit error. We investigated the impact of the ROHC parameters that are recommended for the implementation in the ROHC and compared the performance of ROHC with PHS.

• International Journal of Contents
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• v.2 no.4
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• pp.19-24
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• 2006

In this paper, the performance of pre-equalization technique which can be applicable for the B-WLL uplink is evaluated and compared to post-equalization technique under three kinds of rain attenuation channels such as rain, intermittent light rain and thundershower. The BER performance comparisons of two algorithms (LMS and RLS) are investigated in the context of channel models and the length of training sequence. From the simulation results, it is shown that the post-equalization outperforms only at quite good channel conditions such as AWGN, while the pre-equalization can guarantee better BER performance at every channel conditions, especially performance gain increases as the severity of channel increases. It is concluded that the pre-equalizer using LMS algorithm is preferable at delay-tolerant situation where the complexity of algorithm is not a strict factor, while one using RLS is suitable for fast burst transmission with a relatively short training sequence.

• ETRI Journal
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• v.38 no.4
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• pp.634-644
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• 2016

Channel-based radio-frequency fingerprinting such as a channel impulse response (CIR), channel transfer function (CTF), and frequency coherence function (FCF) have been recently proposed to improve the accuracy at the physical layer; however, their empirical performance, advantages, and limitations have not been well reported. This paper provides a comprehensive empirical performance evaluation of RF location fingerprinting, focusing on a comparison of received-signal strength, CIR-, CTF-, and FCF-based fingerprinting using the weighted k-nearest neighbor pattern recognition technique. Frequency domain channel measurements in the IEEE 802.11 band taken on a university campus were used to evaluate the accuracy of the fingerprinting types and their robustness to human-induced motion perturbations of the channel. The localization performance was analyzed, and the results are described using the spatial and temporal radio propagation characteristics. In particular, we introduce the coherence region to explain the spatial properties and investigate the impact of the Doppler spread in time-varying channels on the time coherence of RF fingerprint structures.

• Proceedings of the IEEK Conference
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• 2004.08c
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• pp.667-670
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• 2004

In this paper, we have presented the improved IF transceiver architecture and the implementation and experimental results on re-configurable transceiver based on digital IF for multiple wideband OFDM/TDD base stations for high-speed portable internet-service in which is issued Korea. The implemented IF transceiver has been designed to support multiple frequency allocations and multiple standards by only modifying the programmable software not its hardware like as the software-defined-radio concept. Also, the digital complex quadrature modulation technique has been used for the digital IF transmitter, which is able to combine multiple frequency bands in digital processing block not RF block and to reject the image frequency signals. And the bandpass sampling technique has been used for the digital IF receiver to reduce the sampling rate of ADC. This paper has shown the experiment results on the frequency response and constellation on the base-station implemented using the modified IEEE 802.16a/e physical layer channel structure based on OFDM/TDD.

• Journal of the Korean Operations Research and Management Science Society
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• v.31 no.3
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• pp.145-154
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• 2006

WiBro (Wireless Broadband Internet) system is going to be 9ot down to commercialization. The high portable internet system is able to be connected to the high-speed internet using 2.3 GHz frequency bandwidth, anytime and anywhere. An effective scheduling for UGS, rtPS, nrtPS and BE requiring different QoS, which are service types of the WiBro, is necessary to increase the efficiency of radio channels that are the limiting resource. This paper researches an uplink scheduling that plays a role in assignment for the radio channels from subscribers to base stations. The suggested scheduling provides priorities with each service type considering QoS of them but reserves some bandwidth for lower priory services. After deciding effective amount of bandwidth for reservation, we suggest analytical result on mean delayed time of packet transmission for each terminal, transmission rate for the data, and capacity for uplink. This research can be used as basic data for the standard in the high portable internet system and as data to predict the capacity of base stations.

• Proceedings of the KIEE Conference
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• 2008.10b
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• pp.388-389
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• 2008

이중 이진 (double binary) 터보 디코더는 오류 정정 코드 중 하나로써, 높은 오류 정정 성능으로 인해 IEEE 802.16 표준 (WiMAX)에서 사용되고 있다. Maximum ${\alpha}$ posteriori probability (MAP) 디코딩 블록은 이중 이진 터보 디코더의 가장 핵심적인 블록으로, 본 논문은 이를 구현하기 위한 알고리즘 중 하나인 improved Max-Log-MAP 알고리즘에 대한 효율적인 하드웨어 구조를 제안한다. 제안하는 하드웨어 구조는 기존의 하드웨어 구조와 비교하였을 때, 오류 정정 성능은 동일만 반면, 구떤 복잡도는 감소한다. 0.13um 공정에서 입력 비트폭을 8비트로 가정하고 시뮬레이션 한 결과, 속도와 칩 면적, 그리고 소비전력 측면에서 각각 8.92%, 1845%, 그리고 29.93%의 향상을 보인다. 제안하는 구조를 WIMAX 설계에 적용하여 성능 개선을 이끌어낼 수 있다.

• Proceedings of the KIEE Conference
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• 2009.07a
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• pp.2253_2254
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• 2009

무선 모바일 통신 시장의 성장에 따라 수동 소자로 구성된 모듈에 대한 관심이 높아지고 있다. 본 논문에서는 수동소자를 이용한 Wi-MAX용 트리플렉서를 설계하였다. 트리플렉서는 Wi-MAX의 RF Front-end단 앞부분에 연결되며, IEEE 802.16에 따른 2GHz, 3GHz, 5GHz 대역을 선택, 분리해주는 역할을 한다. 제안된 트리플렉서는 저역통과필터, 대역통과필터, 고역통과필터로 구성하였으며, 수동소자의 최소화로 인해 삽입손실 향상을 중점으로 설계하였다. 회로해석 결과, 각 대역별 (2/3/5GHz) 삽입손실은 각각 -0.5dB, -0.6dB, -0.4dB 였으며, 반사손실은 -20dB, -20dB, -12dB, 격리도는 -20dB, -24dB, -25dB의 특성을 보였다. 수동 소자의 최소화 설계로 인해 모듈 및 시스템의 저가화를 기대할 수 있다.

• The Journal of The Korea Institute of Intelligent Transport Systems
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• v.8 no.4
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• pp.65-72
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• 2009

In this paper, we propose a proxy-based fast handover scheme to improve a performance in the mobile WiMAX network using PMIPv6 for supporting the network-layer mobility. In our proposed scheme, we define three link-layer messages and one network-layer message to reduce the handover latency, and propose a buffering mechanism to minimize packet loss and solve out-of-sequence problem. A performance evaluation indicates that the proposed scheme works more efficiently than PMIPv6 in terms of the handover latency and packet loss.