• Journal of the Institute of Electronics Engineers of Korea TC
• /
• v.44 no.4
• /
• pp.80-87
• /
• 2007

In this paper, link-level performance of a OFDMA technique, which has been considered as a standard for 802.16, 802.20, 3G-LTE, WiMax, and WiBro, is evaluated for next-generation wireless communication systems. While many researches have focused on uncoded forward-link performances, this paper provides a coded performance of a reverse-link OFDMA system. Performance degradation due to time offset among reverse-link users and frequency offset during FFT process is investigated. Transmitter and receiver antenna diversity techniques are used to overcome performance degradation. Performance of a OFDMA system is compared with a CDMA system in terms of FER and throughput to emphasize the advantage of OFDMA system for a reverse-link. Finally, under given specification, Eb/No required to achieve the maximum throughput of a reverse-link is proposed considering various coded rates and antenna permutations.

• Journal of Broadcast Engineering
• /
• v.14 no.1
• /
• pp.59-69
• /
• 2009

A bi-directional symmetric prediction technique has been developed to improve coding efficiency of B-slice and to reduce the computational complexity required to estimate two motion vectors. On the contrary to the conventional bi-directional mode which encodes both forward and backward motion vectors, it only encodes a single forward motion vector, and the missing backward motion vector is derived in a symmetric way from the forward motion vector using temporal distance between forward/backward reference frames to and from the current B picture. Since the backward motion vector is derived from the forward motion vector, it can halve the computational complexity for motion estimation, and also reduces motion vector data to encode. This technique always derives the backward motion vector from the forward motion vector, however, there are cases when the forward motion vector is better to be derived from the backward motion vector especially in scene changes. In this paper, we generalize the idea of the symmetric coding with forward motion vector coding, and propose a new symmetric coding with backward motion vector coding and adaptive selection between the conventional symmetric mode and the proposed symmetric mode based on rate-distortion optimization.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.33 no.5A
• /
• pp.542-548
• /
• 2008

This paper proposes a handover scheme using uplink and downlink signals in IEEE 802.16e systems. Exploiting the channel reciprocity in TDD systems, the proposed scheme triggers handover initiation process using uplink signal. In addition, it exploits the uplink and downlink hysteresis to determine the handover direction. Simulation results show that the proposed algorithm with joint hysteresis reduces the outage probability by about 10% compared with the mobile assisted handover in IEEE 802.16e.

• Proceedings of the Korea Information Processing Society Conference
• /
• 2005.11a
• /
• pp.1299-1302
• /
• 2005

TCP 구현의 하나인 Vegas는 패킷의 유실을 망의 혼잡으로 인지하는 Reno와 달리 RTT(Round Trip Time) 측정값을 바탕으로 혼잡을 인지하며 윈도우 크기 등 혼잡 제어를 위한 주요 인자를 결정한다. TCP Vegas는 TCP Reno보다 더 효율적인 네트워크 대역폭과 처리율을 가진다. 그러나 Vegas의 혼잡 회피 방안이 TCP 패킷 경로의 비대칭적 특성을 제대로 반영하지 못하며, 이것은 양방향(순방향,역방향) 패킷 전송 상태를 반영하는 RTT 측정값을 순방향 경로의 상태 해석에 이용하기 때문이다. RTT는 패킷의 왕복 시간만을 측정하기 때문에 패킷의 송수신시 순방향과 역방향에서 어느 정도의 혼잡이 발생하였는지 알 수 없다. 본 논문에서는 리눅스 커널의 TCP 소스에서 RTT 측정값으로 혼잡도를 측정하는 기존의 Vegas 혼잡 제어 알고리즘을 수정하여 순방향 경로의 혼잡과 역방향 경로의 혼잡을 구별할 수 있는 새로운 Vegas 혼잡 제어 알고리즘을 설계하고 구현하여 그 성능을 분석하였다.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.26 no.5B
• /
• pp.615-622
• /
• 2001

본 논문에서는 IMT-2000 3GPP 규격 물리 채널에 따른 역방향 동기식 전송방식(USTS: Uplink Synchronous Transmission Scheme)의 성능을 다양한 채널 환경에 대하여 평가하였으며, 그 결과를 간섭제거 기술에 의한 간섭 제거 효과와 비교 평가하였다. 시뮬레이션 결과 간섭제거 기법에서는 초기 결정값의 정확도에 따라 그 성능이 매우 크게 변화하는데 비하여 역방향 동기식 전송 방식은 안정된 성능을 나타내는 것을 확인할 수 있었다. 또한 매우 많은 사용자 환경에서는 역방향 동기식 전송 방식으로 제거되지 않은 간섭 성분들이 축적되어 그 성능이 열화되는 경향이 있는 반면, 간섭제거 기법에서는 초기 결정값의 정확도만 어느 정도 보장된다고 하면 간섭 신호를 효과적으로 제거할 수 있을 것으로 나타났다.

• Journal of Internet Computing and Services
• /
• v.9 no.4
• /
• pp.21-27
• /
• 2008

TCP receivers deliver ACK packets to senders for reliable end-to-end transfer. When ACK packets are not transferred properly because of network congestion, the performance of TCP degrades. This paper proposes a reverse congestion warning mechanism and a congestion handling mechanism in heterogeneous networks with heavy background traffic in the backward direction. Help from TCP receivers or hardware such as routers and gateways other than the ACK packets themselves is not necessary. TCP senders compare the arrival intervals of ACK data passed from receivers and the difference in t imestamp values echoed by receivers. According to the simulation results using the NS-2 network simulator, the proposed scheme shows a performance elevation of 20% greater than Reno, 150% greater than New Reno, and 450% greater than Westwood, respectively, under heterogeneous networks and that the error rate of the radio link is 1% when the backward network is congested.

• The Transactions of the Korea Information Processing Society
• /
• v.6 no.2
• /
• pp.408-414
• /
• 1999

In this paper, we propose the method using coherent detection with time division multiple pilot signal for each users I reverse CDMA channel as the structure of reverse CDMA channel to apply CDMA to upstream channel of bidirectional LMDS. By using coherent detection with pilot signal in reverse CDMA channel, capacity can be improved when compared with noncoherent detection, but it is required to allocate pilot signal for all user. And then interference is increased due to allocate continuously the pilot signals. In this case channel capacity is decreased. The method f pilot allocation is needed that reduce this interference and keep synchronization effect using coherent detection, In the reverse link, the pilot allocated to each users at different time and not overlapped is possible solution for it. this pilot is proposed and analyzed its performance and capacity. The result is that proposed method has more increased capacity compared wit others.

• Journal of the Institute of Electronics Engineers of Korea TC
• /
• v.42 no.2 s.332
• /
• pp.17-22
• /
• 2005

This paper presents our investigation for the effect of symbol timing errors in orthogonal frequency division multiple access (OFDMA) uplink systems. We express the symbol timing errors between users as the symbol timing misalignments with respect to the desired user. Then, we derive an explicit expression of the average effective signal-to-interference-plus-noise ratio. (SINR) as a function of the maximum value of the symbol timing misalignments. Based on the resulting SINR degradation, we evaluate the SINR gain with guard subcarriers in order to mitigate the effect of the symbol timing misalignments.

• Journal of the Institute of Electronics Engineers of Korea SD
• /
• v.40 no.6
• /
• pp.384-392
• /
• 2003

The algorithm for calculating the base-collector breakdown voltage of NPN BJT for integrated circuits is proposed. The method of three-dimensional mesh generation to minimize the time required for device simulation is presented and the method for calculating the breakdown voltage using solutions of the Poisson´s equation is presented. To verify the proposed method, the breakdown voltage between base and collector of NPN BJT using 20V process and 30V process is compared with the measured data. The breakdown voltage from the proposed method of NPN BJT using 20V process shows an averaged relative error of 8.0% compared with the measured data and the breakdown voltage of NPN BJT using 30V process shows an averaged relative error of 4.3% compared with the measured data.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.23 no.4
• /
• pp.901-913
• /
• 1998

This paper presents a new analytical approach for the evaluation of the reverse link interference in a CDMA cellular system with unequal cell loading. In particular, the dynamic nature of the reverse link power control has been taken into account in the proposed approach so that a realistic effect of unequal cell loading, including the shadowing and soft handoff, on system capacity may be precisely captured. This is aspect that has not beensuccessfully formmulated and rarely attempted tosolve in any previous analytical approach. Our computational result shows that as the loading of every tier of surrounding cells is doubled, the subscriber capacity of the reference cell can be reduced as much as 27% of that obtained in the case that the effect of power control is not appropriately modeled.