• The Journal of Korean Institute of Communications and Information Sciences
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• v.30 no.9A
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• pp.749-754
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• 2005

It is well-known that mobile communication systems are usually limited in their performance and capacity by three major impairments, which are multipath fading, delay spread and co-channel interference (CCI). OFDMA (OFDM-FDMA) system can cope with the multipath fading and delay spread easily due to the beneficial properties of OmM technology. Though OFDMA scheme avoids intra-cell interference using the orthogonality among subcarriers, the scheme contains difficulty of reducing co-channel interference. Therefore, in this paper, adaptive antenna techniques are deployed into OFDMA/TDD system to minimize the co-channel interference induced by adjacent cells and to enhance the uplink performance. For the improvement of downlink performance, we apply TxAA (Transmit Adaptive Array), a kind of transmit diversity technique, utilizing preamble symbols for training antenna may into OFDMA/TDD transmitter side. Simulation results show that the uplink and downlink performance under multipath Rayleigh fading channel improved 9dB and 7dB each compared with the case of single antenna system at target BER $10^{-3}$ .

• The Journal of Korean Institute of Communications and Information Sciences
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• v.30 no.2A
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• pp.34-42
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• 2005

In orthogonal frequency-division multiple access (OFDMA) uplink environments, asynchronously received signals can cause multiple access interference (MAI). This paper focuses on the performance degradation due to the MAI over frequency-selective fading channels. We first introduce the timing misalignment, which is defined as the relative timing difference between asynchronous timing error of uplink user and reference time of the base station, and analytically derive the MAI using the power delay profile of wide-sense stationary uncorrelated scattering (WSSUS) channel model. Then, the effective signal-to-noise ratio (SNR) and the average symbol error probability (SEP) are derived. The proposed analytical results are verified through simulations with respect to the region of the timing misalignment and the number of asynchronous users.

• Proceedings of the IEEK Conference
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• 2008.06a
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• pp.239-240
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• 2008

A new frequency offset estimator using low cross-property of PN sequences is proposed in the orthogonal frequency division multiple access (OFDMA) uplink systems. Simulation results show that the proposed estimator improves the mean square error (MSE) performance compared to conventional estimator [2] where each user have both different timing offset and different frequency offset. In addition, the MSE performance is improved as the length of PN sequence increases.

• Proceedings of the IEEK Conference
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• 2006.06a
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• pp.159-160
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• 2006

We present a multiuser synchronization scheme for OFDMA uplink systems. First a new preamble structure is proposed by which timing and frequency offsets estimation is developed. Based on proposed structure a new joint synchronization scheme is presented and the simulation results show that the performance of this scheme don't suffer from the number of users.

• Proceedings of the Korean Institute of Communication Sciences Conference
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• 2004.11a
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• pp.340-340
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• 2004

• The Journal of Korean Institute of Communications and Information Sciences
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• v.30 no.7A
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• pp.557-567
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• 2005

We evaluate the BER performance of OFDMA system for Wibro services in frequency selective Rayleigh fading channel, considering the carrier frequency offset and the nonlinear high power amplifier. In the uplink OFDMA, nonlinear amplifier and frequency offset introduce larger BER degradation than in the downlink. We explain the reason and obtain the required output back-off of the nonlinear amplifier and value of frequency offset for good BER performance.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.28 no.12C
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• pp.1217-1224
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• 2003

In this paper, we propose an edge side-lobe suppressor (ESS) for the uplink of OFDMA systems, which mitigates the interference introduced by different frequency offsets between a desired user and the other users. We evaluate the proposed ESS scheme by measuring average signal to interference ratio (SIR) and average bit error rate (BER). The simulation results confirm that the proposed ESS scheme improves system performance by approximately 5∼10 dB SIR as compared to conventional OFDMA systems. Because the additional operations for the proposed scheme can be performed by a real-valued look-up table, the implementation of the ESS hardly requires increasing transmitter and receiver complexity.

• Journal of Internet Computing and Services
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• v.9 no.2
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• pp.25-33
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• 2008

In this paper we propose uplink packet transmission scheme based on Synchronized RA. The scheme can afford to support diverse QoS required from diverse multimeda traffics. In order to minimze the collision probability occurred in contention based channel access, we assign a reservation access channel additionally to the traffic channels. Access probability function for controlling collisions occurred in the reservation access channel is applied. Through assigning different access probability to each traffic user with respect to priority issued by his required QoS, the proposed scheme guarantees minimum throughput required by higher priority users in the whole range of system load.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.10 no.1
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• pp.15-22
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• 2006

In OFDMA system, even if the number of allocated sub-channel in mobile station varies from one to the whole sub-channel as in base station, while because of mobile station's transmit power is lower than that of base station, therefore full loading range(FLR) constraint occurs where whole sub-channel can be used and the conventional open-loop power control scheme can not be used beyond FLR. We propose a new scheme that limits the maximum sub-channel allocation number and uses power concentration gain(PCG) depending on location of mobile station, which is based on ranging in OFDMA system. Simulation results show that the proposed scheme provides solutions for optimum utilization of radio resource depending on the location of mobile station and enables open-loop power control beyond FLR without extra hardware complexity.

• Proceedings of the Korean Society of Broadcast Engineers Conference
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• 2010.07a
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• pp.174-176
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• 2010

이 논문에서는 직교 주파수 분할 다중 접속방식(orthogonal frequency division multiple access : OFDMA)을 이용한 상향 링크 시스템에서 수신된 신호에 주파수 옵셋이 존재할 때 나타나는 영향을 보상하는 방법을 제안한다. OFDMA에서 각 사용자가 전송한 신호에서 발생하는 주파수 옵셋은 다중 접속 간섭(multi-user interference : MUI)을 발생시킨다. 이러한 다중접속 간섭은 OFDMA 시스템의 성능을 저하시키는 주요한 원인 중에 하나이다. 이 논문에서는 시간영역에서 특정 사용자의 신호에 대한 주파수 옵셋의 보상을 한 뒤, 주파수 영역에서 일반적인 채널간간섭 제거 방법을 적용하여 다중 접속 간섭에 의한 영향을 효과적으로 제거하는 방법을 제안한다. 또한 부반송파에 위치에 따라 달리 나타나는 다중 접속 간섭의 크기에 따라 간섭 제거 방법을 달리하여 보다 적은 계산량으로 간섭을 제거하는 방법을 제안한다.