• Current Optics and Photonics
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• 제3권4호
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• pp.311-319
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• 2019

A hybrid free-space-optical/radio-frequency (FSO/RF) communication system is considered, with the help of amplify-and-forward (AF) relaying. We consider various weather conditions to investigate their effects on the system's performance. We begin by obtaining the cumulative distribution function and probability density function of the end-to-end signal-to-noise ratio for the AF dual-hop FSO communication system with RF backup link. Then, these results are used to derive closed-form expressions for the outage probability, average bit-error rate, and average ergodic capacity. The results show that the considered system efficiently employs the complementary nature of FSO and RF links, resulting in impressive performance improvements compared to non-hybrid systems.

• International Journal of Advanced Culture Technology
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• 제10권3호
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• pp.307-312
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• 2022

The commercialization of the fifth generation (5G) mobile systems has quested enabling technologies, such as intelligent reflecting surface (IRS) transmissions, towards the sixth generation (6G) networks. In this paper, we present a bit-error rate (BER) performance analysis on IRS transmissions in 5G non-orthogonal multiple access (NOMA) networks. First, we derive a closed-form expression for the BER of IRS-NOMA transmissions under Rician fading channels. Then, by Monte Carlo simulations, we validate the proposed approximate BER expression, and show numerically that the derived BER expression is in good agreement with Monte Carlo simulations. Furthermore, we also analyze the BER performance of IRS-NOMA networks under Rician fading channels with different numbers of reflecting elements, and demonstrate that the performances improve monotonically as the number of reflecting devices increases.

• Journal of Astronomy and Space Sciences
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• 제37권4호
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• pp.219-228
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• 2020

A CubeSat platform has become a popular choice due to inexpensive commercial off-the-shelf (COTS) components and low launch cost. However, it requires more power-efficient and higher-data rate downlink capability for space applications related to remote sensing. In addition, the platform is limited by the size, weight and power (SWaP) constraints as well as the regulatory issue of licensing the radio frequency (RF) spectrum. The requirements and limitations have put optical communications on promising alternatives to RF communications for a CubeSat platform, owing to the power efficiency and high data rate as well as the license free spectrum. In this study, we analyzed the performance of optical downlink communications compatible with CubeSat platforms in terms of data rate, bit error rate (BER) and outage probability. Mathematical models of BER and outage probability were derived based on not only the log-normal model of atmospheric turbulence but also a transmitter with a finite extinction ratio. Given the fixed slot width, the optimal guard time and modulation orders were chosen to achieve the target data rate. And the two performance metrics, BER and outage data rate, were analyzed and discussed with respect to beam divergence angle, scintillation index and zenith angle.

• 한국통신학회논문지
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• 제29권6B호
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• pp.548-553
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• 2004

무선 이동 통신 기술의 발전으로 인하여 기존 인터넷에서 제공받던 고품질의 서비스를 무선 인터넷을 통하여 이동 환경에서도 제공 받고자하는 욕구가 늘고 있다. 하지만 무선 이동 환경에서는 유선 구간과 달리 무선 구간의 특성상 전파에 대한 간섭과 노이즈 현상이 심하여 성능이 떨어지게 된다. 현재 인터넷에서 별리 사용되고 있는 TCP는 유선 환경에 최적화되어 만들어진 프로토콜이기 때문에 전송 오류나 전파 간섭 등의 영향으로 비트 에러가 상대적으로 많은 무선 환경에서 성능 저하를 가져온다. 특히 전파 환경이 열악한 지하나 지하철 이동시 그 성능은 더 떨어진다. 본 논문에서는 무선 링크에서 빈번히 발생되는 전송 오류로 인한 TCP 성능 저하를 최소화하기 위하여 Robust TCP 알고리즘을 제안한다. Robust TCP는 무선 환경에서 TCP성능 저하의 주요 원인인 버스트 에러 상황에서 우수한 성능을 보임을 성능정가를 통하여 검증하였다.

• 한국정보통신학회논문지
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• 제16권2호
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• pp.246-252
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• 2012

STBC 방식에서는 여러 개의 송신 안테나를 사용함으로서 우수한 성능의 전송이 가능하다. 본 논문에서는 파일럿 심볼을 이용하여 채널을 추정하는 OFDM 시스템에 STBC 안테나 다이버시티 기법을 적용할 때의 시스템 성능을 분석한다. OFDM 전송표준 IEEE 802.16e에서 전송채널의 정보를 송신 측에 다시 보내어 STBC 시스템 성능을 더욱 향상 시키는 방식이 제시되고 있다. 한편, 파일럿 심볼을 이용하여 전송채널을 추정하는 현 시스템에서는 전송채널 추정에러로 인하여 시스템 성능이 열화 된다. 그러므로 채널추정 에러의 영향을 고려하여, 송신 신호가 다중경로 페이딩 채널을 통하여 전송될 때, 채널정보를 이용하는 STBC OFDM 시스템의 데이터 비트 오류율 성능을 분석한다.

• ETRI Journal
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• 제31권5호
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• pp.619-621
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• 2009

In this letter, we study the influence of receiver imperfections on bit error rate (BER) degradations in detecting low-density parity-check coded multilevel phase-shift keying signals transmitted over a Rician fading channel. Based on the analytical system model which we previously developed using Monte Carlo simulations, we determine the BER degradations caused by the simultaneous influences of stochastic phase error, quadrature error, in-phase-quadrature mismatch, and the fading severity.

• Journal of Electrical Engineering and Technology
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• 제10권6호
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• pp.2406-2412
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• 2015

Multi-user multi-input multi-output (MU-MIMO) system has attracted the 4^th generation wireless network as one of core technique for performance enrichment. In this system rate control is a challenging problem and another problem is optimization. Proper scheduling can resolve these problems by deciding which set of user and at which rate the users send their data. This paper proposes a new multi-parameter based scheduling (MPS) for downlink multi-user multiple-input multiple-output (MU-MIMO) system under space-time block coding (STBC) transmissions. Goal of this MPS scheme is to offer improved link level performance in terms of a low average bit error rate (BER), high packet delivery ratio (PDR) with improved resource utilization and service fairness among the user. This scheme allows the set of users to send data based on their channel quality and their demand rates. Simulation compares the MPS performance with other scheduling scheme such as fair scheduling (FS), normalized priority scheduling (NPS) and threshold based fair scheduling (TFS). The results obtained prove that MPS has significant improvement in average BER performance with improved resource utilization and fairness as compared to the other scheduling scheme.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제2권6호
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• pp.333-351
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• 2008

Transmission control protocols have to overcome common problems in wireless networks. TCP employing both packet loss discrimination mechanism and available bandwidth estimation algorithm, known as the good existing solution, shows significant performance enhancement in wireless networks. For instance, TCP New Jersey which exhibits high throughput in wireless networks intends to improve TCP performance by using available bandwidth estimation and congestion warning. Even though it achieves 17% and 85% improvements in terms of goodput over TCP Westwood and TCP Reno, respectively, we further improve it by exploring maximized available bandwidth estimation, handling bit-error-rate error recovery, and effective adjustment of sending rate for retransmission timeout. Hence, we propose TCP NJ+, showing that for up to 5% packet loss rate, it outperforms other TCP variants by 19% to 104% in terms of goodput when the network is in bi-directional background traffic.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2003년도 하계학술대회 논문집 Vol.4 No.2
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• pp.1212-1216
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• 2003

In this study, the ${\pi}/4$ shift QPSK(quadrature phase shift keying) with NEC(nonredundant error correction) on the multipath channel can detect the burst error as well as random error one by using the second and L-th order phase difference. Therefore, the BER(bit error rate) performance in ${\pi}/4$ shift QPSK is more improved than that of the ${\pi}/4$ shift QPSK without NEC structure. Also, this performance become a bit better in Rayleigh fading channel.

• 한국항행학회논문지
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• 제9권2호
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• pp.156-163
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• 2005

본 논문에서는 ICAO (International Civil Aviation Organization)에서 규정한 VDL 모드 3 시스템 계통도와 제원을 구성하여 BER(Bit Error Rate), 전송지연시간, 버스트 재전송율, 그리고 throughput에 대한 성능을 분석하였다. 성능 분석 결과, BER과 전송지연시간, throughput, 버스트 재전송율의 관계를 명확히 정의할 수 있었고, 또한 down link에서 V/D 재전송율과 throughput에 강한 상관이 있음이 판명되었다.