• Journal of Communications and Networks
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• v.18 no.6
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• pp.892-901
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• 2016

In the multi-user multiple-input multiple-output (MIMO) system, orthogonal random beamforming (ORBF) scheme is proposed to serve multiple users simultaneously in order to achieve the multi-user diversity gain. The opportunistic space-division multiple access system (OSDMA-S) scheme performs multiple weighting matrices during the training phase and chooses the best weighting matrix to be used to broadcast data during the transmitting phase. The OSDMA-S scheme works better than the original ORBF by decreasing the inter-user interference during the transmitting phase. To save more time in the training phase, a partly random multiple weighting matrices selection scheme is proposed in this paper. In our proposed scheme, the Base Station does not need to use several unitary matrices to broadcast pilot symbol. Actually, only one broadcasting operation is needed. Each subscriber generates several virtual equivalent channels with a set of pre-saved unitary matrices and the channel status information gained from the broadcasting operation. The signal-to-interference and noise ratio (SINR) of each beam in each virtual equivalent channel is calculated and fed back to the base station for the weighting matrix selection and multi-user scheduling. According to the theoretical analysis, the proposed scheme relatively expands the transmitting phase and reduces the interactive complexity between the Base Station and subscribers. The asymptotic analysis and the simulation results show that the proposed scheme improves the throughput performance of the multi-user MIMO system.

• The Proceeding of the Korean Institute of Electromagnetic Engineering and Science
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• v.5 no.1
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• pp.22-30
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• 1994

The error probability of the DS-CDMA / DPSK cellular mobile communication system with CCI canceller and MRC diversity reception technique has been analyzed in the cellular radio channel which is characterized by AWGN, Multi-User Interference(MUI) and m-distribution fading. System capacity i. e., number of user per cell has been derived and the evaluated results are shown in figures as a function of PN code sequence length, fading index, BER, number of diversity branches and $E_b/N_o$, Here, the voice activity factor is assumed to be 3/8, the number of sectors in a cell 3 and Multi-User Iner- ference is modeled as Gaussian process.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.8 no.1
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• pp.42-51
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• 1997

The satellite channel with a line-of-sight signal component is modeled by a shadowed Rician fading channel. We adopt a direct-sequence / code division multiple access (DS / CDMA), which has the advantage to suppress the multipath effect and increase the user capacity. The performance which is evaluated by bit error probability is subjected to the influence of branch number, multi-user number, and spreading code-length. As the result of the analysis, performance advance is achieved with multi-user number decreasing, number of brnaches increasing, and spreading code-length increasing as chip duration is constant. To use both of diversity combining scheme and channel coding is more efficient for performance improvement than the case using diversity combining scheme only. The use of BCH coding and convolutional coding shows better consequence than Hamming coding. Totally, the performance degradation for heavy shadowing is much larger than that for light and average shadowing as heavy shadowing decreases LOS signal.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.42 no.1
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• pp.39-41
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• 2017

In this letter, we propose a power control mechanism in order to improve secrecy rate defined as the difference between capacity of main link and wiretap link in a wireless multi-cell multiuser network. Through simulations, we verify that the proposed power control mechanism with threshold based user scheduling can significantly increase secrecy rate in a multi-cell environment.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.37A no.9
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• pp.721-727
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• 2012

Since a conventional multi-cell transmit diversity scheme depends on the feedback from the user for the channel gain information, its performance gets to severely degrade when the channel varies fast due to the high mobility of the user. Also, transmit power of the base station cannot be fully used in the conventional scheme because only one transmit antenna is used for data transmission. In this paper, we propose a multi-cell transmit diversity scheme appropriate for fast fading channel. In the proposed scheme, channel-independent precoding vector is applied over all transmit antennas and different precoding vectors are applied for neighboring subcarriers so that the received signal is avoided to experience deep fading over multiple neighboring subcarriers. Simulation results show that the proposed scheme has better detector output signal-to-noise ratio (SNR) and bit error rate (BER) performances than the conventional scheme.

• Journal of Satellite, Information and Communications
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• v.6 no.2
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• pp.20-25
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• 2011

The authors have proposed novel TDD-CDMA systems with Pre-Rake transmit diversity schemes (system 1 and system 2) using multiple transmit antennas in [2] and have also evaluated the system performance through the theoretical analysis and computer simulation. However, the performance of system 2 which transmit a signal using all antennas has not been evaluated for multi-user environment. Therefore in this paper, we analyze the performance of system 2 for multi-user environment and compare the performance with that of the already proposed system 1 which chooses only one antenna. From the numerical results, it is found that system 2 outperforms system 1 as the number of users increases while system 1 outperforms system 2 at a small number of users. Therefore in order to achieve the best system performance, the Pre-Rake transmit diversity type should be selected at the base station according to the number of users.

• Journal of Communications and Networks
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• v.6 no.4
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• pp.317-330
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• 2004

In this paper, we explore the utility of recently discovered multiple-antenna techniques (namely MIMO techniques) for medium access control (MAC) design and routing in mobile ad hoc networks. Specifically, we focus on ad hoc networks where the spatial diversity technique is used to combat fading and achieve robustness in the presence of user mobility. We first examine the impact of spatial diversity on the MAC design, and devise a MIMO MAC protocol accordingly. We then develop analytical methods to characterize the corresponding saturation throughput for MIMO multi-hop networks. Building on the throughout analysis, we study the impact of MIMO MAC on routing. We characterize the optimal hop distance that minimizes the end-to-end delay in a large network. For completeness, we also study MAC design using directional antennas for the case where the channel has a strong line of sight (LOS) component. Our results show that the spatial diversity technique and the directional antenna technique can enhance the performance of mobile ad hoc networks significantly.

• Proceedings of the Korean Society of Broadcast Engineers Conference
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• 2008.11a
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• pp.103-105
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• 2008

차세대 양방향데이터방송 (BIFS: BInary Format for Scenes) 서비스에서는 하향링크(downlink) 데이터 전송률에 상응하는 높은 상향링크(uplink) 데이터 전송률이 요구되고 있다. 이에 무선 전송 채널의 한정된 자원 환경 속에서 상향링크 전송용량 증대를 위한 주파수 효율성 제고 기술의 필요성이 대두되고 있다. 본 논문에서는 차세대 양방향데이터방송을 위한 다중 모바일(mobile) 사용자의 클러스터(cluster) 기반 상향링크 협동 다이버시티(cooperative diversity) 기법을 제안한다. 제안된 기법은 하나의 단일 안테나(single antenna) 방송 기지국과 다수의 단일 안테나 모바일 사용자가 있는 상향링크 시분할다중접속 (TDMA: Time Division Multiple Access) 시스템에서 데이터 지연허용(delay-tolerant)에 따라 클러스터를 구성하고 제곱 다이버시티 차수(square diversity order)의 협동 프로토콜(cooperative protocol)을 통해 높은 공간다이버시티 이득(spatial diversity gain)을 얻는다. 또한, 비대칭(asymmetric) 무선 채널 환경에서 공정성(fairness)을 보장한다.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.35 no.7A
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• pp.632-638
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• 2010

In this paper, we propose a fixed-complexity sphere encoder (FSE) for multi-user MIMO (MU-MIMO) systems. The Proposed FSE accomplishes a scalable tradeoff between performance and complexity. Also, because it has a parallel tree-search structure, the proposed encoder can be easily pipelined, leading to a tremendous reduction in the precoding latency. The complexity of the proposed encoder is also analyzed, and we propose two techniques that reduce it. Simulation and analytical results demonstrate that in a $4\times4$ MU-MIMO system, the complexity of the proposed FSE is 16% that of the conventional QRD-M encoder (QRDM-E). Also, the encoding throughput of the proposed endoder is 7.5 times that of the QRDM-E with tolerable degradation in the BER performance, while achieving the optimum diversity order.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2000.05a
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• pp.183-187
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• 2000

In this paper, the performance of DS-CDMA system and FH-CDMA system has been comparision analyzed in a channel environment which is characterized by Multi-User environment and Rayleigh fading environment. The techniques of improvement of the performance has been comperision analysis evaluated when adopting MRC diversity techniques. In same condition, We are analyzed the BER(Bit Error Rate) as increase Use's number due to comparing performance of two systems by Same communication band(300KHz), jamming signal(JSR) 10∼20dB, user data-rate 300bps. In the result, the performance of DS and FH systems in multiuser and Rayleigh fading environment is improved performance when adopting MRC diversity techniques. Especially DS system has been improved performance about 9.5 times than FH system when adopting MRC diversity techniques.