• Journal of Communications and Networks
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• 제18권4호
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• pp.639-648
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• 2016

Next-generation (4G) systems are designed to support universal frequency reuse (UFR) to achieve best use of valuable spectra. However, it leads to undesirable interference level near cell borders. To control this, 4G systems adopt techniques, such as network multiple-input multiple-output (MIMO) and inter-cell interference coordination (ICIC), to improve cell-edge throughput. Network MIMO aims at mitigating inter-cell interference towards cell-edge users (CEUs) through multi-cell cooperation, where each collaborative base station serves both cell-center users (CCUs) and CEUs, including other cells' CEUs, under a power constraint. The present ICIC strategies cannot be directly applied to network MIMO because they were designed in absence of multi-cell coordination. In the presence of network MIMO, this paper investigates antenna orientations in ICIC and the method of power management. Results show that a proper antenna orientation can improve the cell-edge capacity and meantime lower the interference to CCUs. Capacity inconsistency between CCUs and CEUs is detrimental to mobile communications. Simulation results show that the proposed power management for ICIC in network MIMO systems can achieve a uniform data rate regardless users' position.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제11권2호
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• pp.650-669
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• 2017

Coordinated multiple points transmission and reception (CoMP) technology is used to mitigate the inter-cell interference, and increase cell average user normalized throughput and cell edge user normalized throughput. There are two kinds of radio resource schedule strategies in LTE-A/5G CoMP system, and they are called centralized scheduling strategy and distributed scheduling strategy. The regional centralized scheduling cannot solve interference of inter-region, and the distributed scheduling leads to worse efficiency in the utilize of resources. In this paper, a novel distributed scheduling scheme named 9-Cell alternate authorization (9-CAA) is proposed. In our scheme, time-domain resources are divided orthogonally by coloring theory for inter-region cooperation in 9-Cell scenario [6]. Then, we provide a formula based on 0-1 integer programming to get chromatic number in 9-CAA. Moreover, a feasible optimal chromatic number search algorithm named CNS-9CAA is proposed. In addition, this scheme is expanded to 3-Cell scenario, and name it 3-Cell alternate authorization (3-CAA). At last, simulation results indicate that 9/3-CAA scheme exceed All CU CoMP, 9/3C CU CoMP and DLC resource scheduling scheme in cell average user normalized throughput. Especially, compared with the non-CoMP scheme as a benchmark, the 9-CAA and 3-CAA have improved the edge user normalized throughput by 17.2% and 13.0% respectively.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제9권2호
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• pp.528-544
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• 2015

Base Station Cooperation (BSC) has been a promising technique for combating the Inter-Cell Interference (ICI) by exchanging information through a high-speed optical fiber back-haul to increase the diversity gain. In this paper, we propose a novel pilot symbol assisted data fusion scheme for distributed Uplink BSC (UBSC) based on Differential Evolution (DE) algorithm. Furthermore, the proposed scheme exploits the pre-defined pilot symbols as the sample of transmitted symbols to constitute a sub-optimal Weight Calculation (WC) model. To circumvent the non-linear programming problem of the proposed sub-optimal model, DE algorithm is employed for searching the proper fusion weights. Compared with the existing equal weights based soft combining scheme, the proposed scheme can adaptively adjust the fusion weights according to the accuracy of cooperative information, which remains the relatively low computational complexity and back-haul traffic. Performance analysis and simulation results show that, the proposed scheme can significantly improve the system performance with the pilot settings of the existing standards.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제9권12호
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• pp.4799-4818
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• 2015

In this paper, we investigate the problem of achieving global optimization for distributed carrier aggregation (CA) in small cell networks, using a game theoretic solution. To cope with the local interference and the distinct cost of intra-band and inter-band CA, we propose a non-cooperation game which is proved as an exact potential game. Furthermore, we propose a spatial adaptive play learning algorithm with heterogeneous learning parameters to converge towards NE of the game. In this algorithm, heterogeneous learning parameters are introduced to accelerate the convergence speed. It is shown that with the proposed game-theoretic approach, global optimization is achieved with local information exchange. Simulation results validate the effectivity of the proposed game-theoretic CA approach.

• 한국통신학회논문지
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• 제39B권3호
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• pp.151-161
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• 2014

3GPP LTE-Advanced 시스템(Release 10)의 Carrier Aggregation (CA) 규격은 고속의 전송률을 지원하기 위해 다수의 주파수 밴드를 통합하면서, 이를 고려한 다양한 배치 시나리오를 제시하고 있다. 본 논문에서는 각 주파수 밴드 별로 상이한 방향성 섹터 안테나를 사용하는 배치 시나리오를 고려하며, 이때 단말이 인접 셀의 주파수 밴드를 선택 가능하게 함으로써 셀 경계 사용자의 수율 성능을 향상할 수 있는 셀 선택 방식을 제안한다. 그리고 이러한 방식 하에 셀 간에 비례공정성(proportional fairness)이 확보될 수 있도록 하는 분산 방식의 셀간 협력 스케쥴링 알고리즘을 제시하며, 이를 통해 기존 방식보다 셀 경계 사용자의 수율이 약 50% 향상되는 것을 확인한다.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제13권2호
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• pp.635-656
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• 2019

In this paper, we investigate the degrees of freedom (DoF) of a multi-cell multi-user multiple-input multiple-output (MIMO) interference broadcast channel (IBC) with non-cooperation distributed base stations (BS), where each BS serves users of its corresponding cell. When all BSs simultaneously transmit their own signals over the same frequency band in the MIMO IBC, the edge users in each cell will suffer the inter-cell interference (ICI) and inter-user interference (IUI) signals. In order to eliminate the ICI and IUI signals, a distributed space time interference alignment (DSTIA) approach is proposed where each BS has only limited access to distributed moderately-delay channel state information at the transmitter (CSIT). It is shown that the DSTIA scheme can obtain the appreciate DoF gains. In addition, the DoF upper bound is asymptotically achievable as the number of antenna at each BS increases. It is shown that the DSTIA method can get DoF gains over other interference alignment schemes with delayed CSIT in literature. Moreover, the DSTIA method can attain higher DoFs than the IA schemes with global CSIT for certain antenna configurations.

• Journal of Communications and Networks
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• 제10권2호
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• pp.118-126
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• 2008

The use of beamforming is effective for users in limited power environments. However, when it is applied to the downlink of a cellular system with universal frequency reuse, users near the sector boundary may experience significant interference from more than one sector. The use of a minimum mean square error (MMSE)-type receiver may not sufficiently cancel out the interference unless a sufficient number of receive antennas are used. In this paper, we consider the use of inter-sector beamforming that cooperates with a neighboring sector in the same cell to mitigate this interference problem in time-division duplex (TDD) environments. The proposed scheme can avoid interference from an adjacent sector in the same cell, while enhancing the transmit array gain by using the TDD reciprocity. The performance of the proposed scheme is analyzed in terms of the output signal-to-interference-plus-noise power ratio (SINR) and the output capacity when applied to an MMSE-type receiver. The beamforming mode can be analytically switched between the inter-sector and the single-sector mode based on the long-term channel information. Finally, the effectiveness of the proposed scheme is verified by computer simulation.

• 한국산학기술학회논문지
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• 제11권7호
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• pp.2565-2569
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• 2010

실리콘 태양전지 셀을 제작하고 효율을 측정하는데 있어 texturing 효과에 대하여 상관성을 조사하였다. Texturing은 $HNO_3$, KOH, NaOH를 각각 사용한 IPA혼합 에칭용액을 만들어서 샘플을 제작하였다. 셀의 분석은 FTIR 과 XRD를 이용하였으며, 실리콘 태양전지 셀의 효율과 화학적인 특성 사이의 상관성을 조사하였다. FTIR 데이터의 950~1350 $cm^{-1}$ 영역에서 픽의 생김새와 셀 표면의 반사도 사이에는 상관성이 있으며, 픽의 쪼개짐이 많으면 표면에서 결합의 상호작용이 활발하지 않다는 것을 의미하며 이러한 표면에서 반사가 잘 일어나지 않으면서 효율이 증가하는 것을 확인하였다.

• Journal of Communications and Networks
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• 제18권5호
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• pp.773-783
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• 2016

Both coordinated multi-point transmission and frequency reuse are effective approaches to mitigate inter-cell interference and improve network coverage. The motivation of this work is to explore the manner to effectively utilize the spectrum resource by reasonably combining cooperation and frequency reuse. The $Mat{\acute{e}}rn$ cluster process, which is appropriate to model networks with hot spots, is used to model the spatial distribution of base stations. Two cooperative mechanisms, coherent and non-coherent joint transmission (JT), are analyzed and compared. We also evaluate the effect of multiple antennas and imperfect channel state information. The simulation reveals that the proposed approach to combine cooperation and frequency reuse is effective to improve the network coverage for users located at both the center and the boundary of the cooperative region.

• 한국통신학회논문지
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• 제32권6A호
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• pp.624-631
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• 2007

본 논문에서는 OFDM 기반 셀룰러 시스템에서 기지국 송신 안테나가 2일 때 브로드캐스트 패킷 서비스 성능을 향상시키기 위한 셀간 협력 전송 방식을 제안한다. 제안한 기법은 동일한 정보를 갖는 부호화된 브로드캐스트 패킷 데이터를 여러 부분으로 나누고 각 부분마다 송신 다양성 부호의 두 직교 성분을 전송하는 셀 그룹 및 송신안테나를 다르게 할당하는 것이다. 제안한 방법은 부호화 다양성 이득을 증가시킴으로써 셀 경계 성능을 왜 좋게 한다는 것을 이론적으로 얻은 쌍 오류율의 상한과 모의실험으로 얻은 터보 부호 적용 시의 성능으로 보인다.