Browsing by Author "Lee, Chang-Shen"

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A geometry based efficient decoder for underdetermined MIMO systems
(2015-06-01)
Huang, Chung-Jung
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Lee, Chang-Shen
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Chung, Wei-Ho
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Lee, Ta-Sung
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電機資訊學士班
In this paper, a geometry based decoder with low decoding complexity and exact maximum-likelihood (ML) performance is proposed for underdetermined multiple-input multiple-output (MIMO) systems. In the proposed decoder, an underdetermined MIMO system can be divided into a multiple-input single-output (MISO) sub-system and a regular MIMO sub-system in which numbers of transmit and receive antennas are equal. An efficient slab search algorithm (ESSA) is adopted to efficiently obtain valid candidate points in the MISO sub-system. By adopting ESSA in the MISO sub-system and sphere decoding algorithm (SDA) in the MIMO sub-system, ML solution of underdetermined MIMO system can be obtained with low computational complexity. To further reduce the computational complexity, a near-ML SDA is proposed to more efficiently find the candidate points in the MIMO sub-system. In addition, an optimal preprocessing technique is proposed from the geometrical perspective and the comprehensive analysis on the complexity reduction is also provided. Simulation results indicate that the proposed approach significantly reduces the complexity as compared to existing ML decoders, particularly for systems with large number of antennas and/or high-order constellations. (C) 2015 Elsevier Inc. All rights reserved.
Distributed Channel Access Schemes for Multi-Channel ALOHA Cognitive Radio Networks
(2015)
Lee, Chang-Shen
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Chung, Wei-Ho
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Lee, Ta-Sung
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
In this paper, the distributed channel access schemes for ALOHA-based cognitive radio networks are considered. In the considered system, time is divided into frames, which are further divided into sensing phase and transmission phase. We derive channel sensing policy in the sensing phase, and channel access policy in the transmission phase for a secondary user (SU) to maximize its throughput. To mitigate high complexities of the above scheme, we propose a threshold-based channel access scheme. In this scheme, an appropriate threshold is set based on channel occupancy information and channel state information, and only channels providing potentially high throughput will be sensed and accessed. The proposed schemes are fully distributed, i.e., no extra information exchange is needed among SUs, which is a highly desired and beneficial property. Simulation results confirm that the proposed schemes outperform prior random access schemes.
Dynamic, Data-Driven Spectrum Management in Cognitive Small Cell Networks
(2014)
Lee, Chang-Shen
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Chen, Wei-Chong
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Bhattacharyya, Shuvra S.
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Lee, Ta-Sung
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電子工程學系及電子研究所
Recently, the deployment of small cells is considered as an effective solution to enhance the capacity in existing cellular networks. However, massive deployment of small cells also incurs severe interference and increased energy consumption, which degrades the energy efficiency of the system. In this paper, we analyze the energy efficiency in cognitive small cell network, and propose a traffic-aware distributed sensing and access scheme for cognitive small cells base stations (SBSs). The proposed scheme adopts the concept of dynamic data driven applications systems (DDDAS). In the DDDAS paradigm, a model of the underlying design space is managed dynamically, updated periodically based on measurements of data, and used to drive measurement functions and adaptation of system configurations. Through careful integration of DDDAS-based design principles, SBSs have the ability to configure their sensing and access parameters according to the traffic patterns that are actually encountered. Simulation results show that our proposed DDDAS-based scheme can achieve significantly higher energy efficiency compared to conventional spectrum sharing schemes in cognitive small cell networks.
Sectorization with Beam Pattern Design Using 3D Beamforming Techniques
(2013)
Lee, Chang-Shen
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Lee, Ming-Chun
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Huang, Chung-Jung
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Lee, Ta-Sung
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電機工程學系
This paper presents a framework for a three-dimensional (3D) beam pattern design with a load-balanced cell sectorization strategy. First, characteristics of the 3D beam pattern were observed, and convex optimization was used to provide a solution based on the criteria that describe the observations. In addition, a user equipment (UE) determination problem that arose because of cell sectorization was also addressed. By incorporating a beam pattern design with a proposed UE determination scheme, a framework of a backward compatible system was completed. The network performance regarding throughput for cell sectorization with a dedicated beam pattern design was evaluated and the simulation results show that the proposed system is superior compared to the conventional system and other proposed sectorization schemes.