Browsing by Author "Liu, Chun-Hung"

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A Generalized Analytical Framework for Coverage Evaluation in mmWave Heterogeneous Cellular Networks in Urban Areas
(2017-01-01)
Liu, Chun-Hung
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Liang, Di-Chun
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Yang, Jie-Ru
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Chen, Jie
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電機工程學系
In this paper, we introduce a generalized analytical framework to evaluate the coverage probability in an mmWave heterogeneous cellular network (HetNet) consisting of UHF macrocell base stations (BSs) and mmWave small cell BSs deployed in an urban area. A generalized user association scheme that can cover a few user association schemes is formulated and applied in the mmWave HetNet. Two important statistical properties of the associated BS under this scheme are derived for the general line-of-sight and non line-of-sight pathloss channel models and they can be largely simplified if the power-law channel model is applied. Using the two statistical properties, the coverage probability of a downlink multiple-input-single-output channel is found in a compact form, which reveals that the coverage probability in the mmWave HetNet can be significantly improved if compared with the cellular network consisting of a single tier of mmWave small cell BSs. Numerical results not only validate the correctness of the coverage probability derived but also indicate that the mmWave HetNet has the potential to achieve a high coverage probability in an urban area.
Adaptive Downlink CoMP in Heterogeneous Cellular Networks with Imperfect Overhead Messaging
(2014)
Liu, Chun-Hung
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電機學院
;
College of Electrical and Computer Engineering
Coordinated multi-point (CoMP) transmission is an effective means of improving network throughput in heterogeneous cellular networks (HetNets). However, its performance is seriously weakened if imperfect coordination happens between base stations (BSs). Many prior CoMP works do not consider inter-cell overhead message delays such that a seemingly astonishing CoMP throughput gain is attained. In this paper, the quantization error and delay that actually exist in overhead messages was modeled and we developed a much tractable SIR model based on the stochastic geometry framework. We proposed adaptive CoMP that is applied to downlink zeroforcing beamforming (ZFBF) and it can mitigate the interference from the coordinated cells with delayed overhead messages. The bounds on the complementary cumulative distribution function (CCDF) of the SIR of a user are characterized such that the average throughput of a user is able to be analytically evaluated. Numerical results show that the proposed adaptive CoMP scheme can make the throughput gain very robust to the overhead delay and thus significantly increase the throughput even when BSs are not perfectly coordinated.
An Analytical Approach to Coexisting Evaluation in Multi-RAT Heterogeneous Networks with Opportunistic CSMA/CA
(2016)
Tsai, Hong-Cheng
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Liu, Chun-Hung
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Wang, Li-Chun
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電機工程學系
;
Department of Electrical and Computer Engineering
This paper proposes an analytical approach to modeling and analyzing the coexisting transmission performance in a multi-tier heterogeneous network with multiple radio access technologies (Multi-RATs). The coexistence issue is a special phenomenon uniquely existing in a multi-RAT network and it is rarely studied in prior works on heterogeneous wireless network. To simply characterize the coexisting transmission impacts in a multi-RAT network, in this paper we consider a three-tier heterogeneous network using the RATs of L and U, where RATL is adopted by the access points (APs) in the first two tiers, APs in the third tier only use RAT-U and the APs in the second tier can opportunistically use RAT-U. The opportunistic CSMA/CA protocol is used by the APs contending the RAT-U channel and the channel access probabilities for the RAT-U AP in two different tiers are derived under the proposed user association scheme with random weights. The coexisting coverages of RAT-L and RAT-U are defined and found, respectively. Numerical results show that the proposed modeling and analyzing approach can be well applied to evaluate the coexistence transmission performance while LTE and WiFi APs both access the unlicensed band.
Coexisting Success Probability and Throughput of Multi-RAT Wireless Networks With Unlicensed Band Access
(2016-02)
Ding, Xu
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Liu, Chun-Hung
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Wang, Li-Chun
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Zhao, Xiaohui
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電機學院
In this letter, the coexisting success probability and throughput of a wireless network consisting of multiple subnetworks of different radio access technologies (RATs) are investigated. The coexisting success probability that is defined as the average of all success probabilities of all subnetworks is found in closed-form and it will be shown to have the concavity over the number of channels in the unlicensed band. The optimal deployment densities of all different RATs access points (APs) that maximize the coexisting success probability are shown to exist and can be found under the derived constraint on network parameters. The coexisting throughput is defined as the per-channel sum of all spectrum efficiencies of all subnetworks and numerical results show that it is significantly higher than the throughput of the unlicensed band only accessed by WiFi APs.
Coverage Analysis for Dense Heterogeneous Networks with Cooperative NOMA
(2017-01-01)
Liu, Chun-Hung
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Liang, Di-Chun
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Chen, Po-Chia
;
Yang, Jie-Ru
;
電機工程學系
In a heterogeneous cellular network (HetNet) consisting of M tiers of densely-deployed base stations (BSs), consider that each of the BSs in the HetNet that are associated with multiple users is able to simultaneously schedule and serve two users in a downlink time slot by performing the (power-domain) non-orthogonal multiple access (NOMA) scheme. This paper aims at the preliminary study on the downlink coverage performance of the HetNet with the non-cooperative and the proposed cooperative NOMA schemes. First, we study the coverage probability of the NOMA users for the non-cooperative NOMA scheme in which no BSs are coordinated to jointly transmit the NOMA signals for a particular cell and the coverage probabilities of the two NOMA users of the BSs in each tier are derived. We show that the coverage probabilities can be largely reduced if allocated transmit powers for the NOMA users are not satisfied with some constraints. Next, we study and derive the coverage probabilities for the proposed cooperative NOMA scheme in which the void BSs that are not tagged by any users are coordinated to enhance the far NOMA user in a particular cell. Our analyses show that cooperative NOMA can significantly improve the coverage of all NOMA users as long as the transmit powers for the NOMA users are properly allocated.
Coverage Probability of Small Cell Networks with Composite Fading and Shadowing
(2014)
Chen, Jiajia
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Wang, Li-Chun
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Liu, Chun-Hung
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電機工程學系
;
Department of Electrical and Computer Engineering
Coverage analysis of a small cell network is very crucial since small cell deployment will dominates the topology of a cellular in the future. The majority of prior work on the coverage probability is studied based on a simple and consistent Rayleigh fading models in a Poisson-distributed cellular network in order to avoid analytical intractability. In this paper, we study the coverage probability problem in a Poisson small cell network with much more general channel impairments. First a neat expression of the coverage probability with compose Rayleigh fading and log-normal shadowing is derived and it discloses two important facts the coverage performance is not improved by deploying more base stations and it is significantly weakened by shadowing. Then we find the coverage probability with low complexity for the case that a dual-slop path loss is used, the desired signal experiences Nakagami-m fading and interference signals undergo Rayleigh fading. It is able to more practically reflect the coverage performance of a user.
Decentralized Traffic Management for Heterogeneous Networks with Opportunistic Unlicensed Spectrum Sharing
(2017-01-01)
Liu, Chun-Hung
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Tsai, Hung-Chen
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電機工程學系
;
Department of Electrical and Computer Engineering
This paper studies how to maximize the per-user-based throughput in an M-tier heterogeneous wireless network (HetNet) by optimally managing traffic flows among the access points (APs) in the HetNet. The APs in the first M 1 tiers can use the licensed spectrum at the same time whereas they share the unlicensed spectrum with the APs in the M th tier by the proposed opportunistic CSMA/CA protocol. The APs that access the licensed and unlicensed spectra simultaneously are able to integrate their spectrum resources by the carrier aggregation technique. For an AP in each tier, the tight bounds on its mean spectrum efficiencies in the licensed and unlicensed spectra are derived in a low-complexity form for general random channel gain and AP association weight models, and they can provide some insights into how channel gains, AP association weights and void AP probabilities affect the mean spectrum efficiencies of APs. The per-user throughput based on the mean spectrum efficiencies in the licensed and unlicensed spectra is found and maximized by the proposed decentralized traffic management scheme. Numerical results show that the mean spectrum efficiencies and the per-user throughput can be significantly improved by the proposed decentralized traffic management scheme.
Decoupled Rate-Optimal User Association for Full-Duplex Heterogeneous Networks
(2018-01-01)
Hu, Heng-Ming
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Liu, Chun-Hung
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電機工程學系
;
Department of Electrical and Computer Engineering
Full-duplex (FD) transmission in a point-to-point (P2P) link, wherein bidirectional traffic flows simultaneously share the same spectrum, has the capability of doubling the link throughput by completely removing self-interferences. However, the throughput gain of FD transmission in an interference-limited network is not as clear as a P2P link due to the interferences induced by complex FD and half-duplex (HD) transmission behaviors in the network. To thoroughly investigate the link throughputs of users and base stations (BSs) in a heterogeneous network (HetNet) with decoupled user association, a general approach is proposed to model a HetNet in which all BSs and users can perform HD or FD transmission depending on their traffic patterns. We first characterize the decoupled rate-optimal user association scheme and use it to define the downlink and uplink throughputs in the HetNet. The tight lower bounds on the link throughputs of the FD users and BSs are found in a neat form that characterizes general channel fading, user and BS intensities and imperfect self-interference cancellation. Numerical results validate that our analytical results are fairly accurate and the proposed decoupled rate-optimal user association indeed significantly improves the link throughputs.
Energy-Efficient Activation and Uplink Transmission for Cellular IoT
(2020-02-01)
Liu, Chun-Hung
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Shen, Yu-Han
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Lee, Chia-Han
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電機工程學系
;
電信工程研究所
Consider a large-scale cellular network in which base stations (BSs) serve massive Internet of Things (IoT) devices. Since IoT devices are powered by a capacity-limited battery, how to prolong their working lifetime is a paramount problem for the success of cellular IoT systems. This article proposes how to use BSs to manage the active and dormant operating modes of the IoT devices via downlink signaling in an energy-efficient fashion and how the IoT devices perform energy-efficient uplink power control to improve their uplink coverage. We first investigate the fundamental statistical properties of an activation signaling process induced by BSs that would like to activate the devices in their cells, which helps to derive the neat expressions of the true, false, and total activation probabilities that reveal joint downlink power control and BS coordination is an effective means to significantly improve the activation performance. We then propose an energy-efficient uplink power control for IoT devices which is shown to save power and ameliorate the uplink coverage probability at the same time. We also propose an energy-efficient downlink power control and BS coordination scheme, which is shown to remarkably improve the activation and uplink coverage performances at the same time.
Energy-Efficient Cell Association and Load Analysis in Heterogeneous Cellular Networks
(2015)
Fong, Kok Leong
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Liu, Chun-Hung
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Tan, Chee Keong
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Lee, Ching Kwang
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電機學院
This paper exploits the fundamental connection between cell association and load in the sense of bits-per-joule energy efficiency. Users adopt a proposed energy-efficiency-based association reward function to associate with their base stations in a heterogeneous cellular network consisting of K independent Poisson point processes (PPPs) of base stations (BSs). We first investigate the cell association probability and the statistics of the kth-tier BSs for a universal association reward function and they can be tailored for any cell association schemes that satisfy the constraints on the association reward function. Under the proposed energy-efficient cell association (EECA) scheme, the statistical properties of an associated BS, coverage probability, average energy efficiency and cell load are all derived and analyzed. We show that EECA can achieve a (much) higher coverage probability, average energy efficiency, and better cell load balancing if compared with maximum received power and nearest BS association schemes.
Enhanced Noisy Sparse Subspace Clustering via Reweighted L1-Minimization
(2018-01-01)
Wu, Jwo-Yuh
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Huang, Liang-Chi
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Yang, Ming-Hsun
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Chang, Ling-Hua
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Liu, Chun-Hung
Sparse subspace clustering (SSC) relies on sparse regression for accurate neighbor identification. Inspired by recent progress in compressive sensing (CS), this paper proposes a new sparse regression scheme for SSC via reweighted l(1)-minimization, which also generalizes a twostep l(1)-minimization algorithm introduced by E. J. Candes al all in [The Annals of Statistics, vol. 42, no. 2, pp. 669-699, 2014] without incurring extra complexity burden. To fully exploit the prior information conveyed by the computed sparse vector in the first step, our approach places a weight on each component of the regression vector, and solves a weighted LASSO in the second step. We discuss the impact of weighting on neighbor identification, argue that a popular weighting rule used in CS literature is not suitable for the SSC purpose, and propose a new weighting scheme for enhancing neighbor identification accuracy. Extensive simulation results are provided to validate our discussions and evidence the effectiveness of the proposed approach. Some key issues for future works are also highlighted.
Full-Duplex Heterogeneous Networks With Decoupled User Association: Rate Analysis and Traffic Scheduling
(2019-03-01)
Liu, Chun-Hung
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Hu, Heng-Ming
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電機工程學系
;
電信工程研究所
;
Department of Electrical and Computer Engineering
Full-duplex (FD) transmission in a point-to-point (P2P) link, wherein bidirectional traffic flows simultaneously share the same spectrum, has the capability of doubling the link rate by completely removing self-interferences. However, the rate performance of an FD heterogeneous network (HetNet) is not as clear as that of an FD P2P link due to the co-channel interferences induced by the complex FD and half-duplex (HD) transmission behaviors in the HetNet. To thoroughly investigate the achievable link rate performances of users and base stations (BSs) in a HetNet with decoupled user association, a hybrid-duplex approach is proposed to model a HetNet in which all BSs and users can perform HD or FD transmission depending on their traffic patterns. We first characterize the decoupled rate-optimal user association scheme and use it to define and evaluate the downlink and uplink rates in the HetNet. The tight lower bounds on the link rates of the FD users and BSs are found in a neat form that characterizes general channel fading, imperfect self-interference cancellation, and the intensities of users and BSs. These bounds outline the rate regions of the FD users that inspire us to propose the opportunistic FD scheduling algorithms that maximize the sum rate of each bidirectional traffic and stabilize each of the queues in the HetNet.
Full-Duplex Heterogeneous Networks: Ergodic Rate Analysis with Realistic Interference Modeling
(2017-01-01)
Hu, Heng-Ming
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Liu, Chun-Hung
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Chang, Kai-Chieh
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電機工程學系
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Department of Electrical and Computer Engineering
In this paper, we adopt a realistic approach to modeling a full-duplex heterogeneous cellular network (HetNet) consisting of a tier of small cell base stations (BSs) that can perform half-duplex (HD) or full-duplex (FD) transmission depending on which duplex transmission is needed by their associated users and a tier of macro BSs that only can perform HD transmission due to their strong self-interference (SI). Two realistic issues, void cell and imperfect SI cancellation, that could exist in this HetNet are also considered in the interference model of this two-tier HetNet. We first study the Laplace transforms of the reciprocals of the signal-to-interference power ratios of users and small cell BSs and derive their explicit expressions. These Laplace transforms lead to the derivations of the ergodic rates of users and small cell BSs found in a very compact form for general power-law channel models. We show that FD does not always outperform HD in terms of ergodic rates and demonstrate how to make FD/HD transmission benefits the ergodic rates by deploying macro and small cell BSs with appropriate intensities.
Fundamentals of Simultaneous Wireless Information and Power Transmission in Heterogeneous Networks: A Cell-Load Perspective
(2019-01-01)
Liu, Chun-Hung
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Hsu, Chi-Sheng
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電機工程學系
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電信工程研究所
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Department of Electrical and Computer Engineering
In a heterogeneous cellular network (HetNet) consisting of multiple different types (tiers) of base stations (BSs), the void cell event in which a BS does not have any users has been shown to exist due to user-centric BS association, and its probability is dominated by the cell load of each tier. Such a void cell phenomenon has not been well characterized in the modeling and analytical framework of simultaneous wireless information and power transmission (SWIPT) in a HetNet. This paper aims to accurately exploit the fundamental performance limits of the SWIPT between the BS and its user by modeling the cell-load impact on the downlink and uplink transmissions of each BS. We first characterize the power-splitting receiver architecture at a user and analyze the statistical properties and limits of its harvested power and energy, which reveals how much of the average energy can be harvested by users and how likely the self-powered sustainability of users can be achieved. We then derive the downlink and uplink rates that characterize the cell-load and user association effects and use them to define the energy efficiency of a user. The optimality of the energy efficiency is investigated, which maximizes the SWIPT performance of the receiver architecture for different user association and network deployment scenarios.
Fundamentals of the Downlink Green Coverage and Energy Efficiency in Heterogeneous Networks
(2016-12-01)
Liu, Chun-Hung
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Fong, Kok Leong
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資訊工程學系
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電機工程學系
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Department of Computer Science
This paper studies the proposed green (energy-efficient) coverage probability, link and network energy efficiencies in the downlink of a heterogeneous cellular network (HetNet) consisting of K independent Poisson point processes of base stations (BSs). The important statistical properties of the universal (general) cell association functions are first studied, and the cell load statistics for power-law cell association functions, which can characterize the accurate void cell probability of a BS in every tier, are also derived. A simple and feasible green channel-aware cell association (GCA) scheme is proposed and the green coverage probability is also proposed for any particular cell association scheme, such as the maximum received power association (MRPA) and the nearest BS association (NBA) schemes. Then, the link and network energy efficiencies are proposed to characterize the mean spectrum efficiency per unit power consumption for a BS and the mean area spectrum efficiency for the HetNet, respectively. All the tight bounds on the green coverage probability, link, and network energy efficiencies for the GCA, MRPA, and NBA schemes are found. They are theoretically shown to pose the fundamental maximum limits on the link and network energy efficiencies achieved by any other cell association schemes, and such a fact is validated by numerical results as well.
Fundamentals of the Downlink Green Coverage and Energy Efficiency in Heterogeneous Networks
(2016-12)
Liu, Chun-Hung
;
Fong, Kok Leong
;
資訊工程學系
;
電機工程學系
;
Department of Computer Science
This paper studies the proposed green (energy-efficient) coverage probability, link and network energy efficiencies in the downlink of a heterogeneous cellular network (HetNet) consisting of K independent Poisson point processes of base stations (BSs). The important statistical properties of the universal (general) cell association functions are first studied, and the cell load statistics for power-law cell association functions, which can characterize the accurate void cell probability of a BS in every tier, are also derived. A simple and feasible green channel-aware cell association (GCA) scheme is proposed and the green coverage probability is also proposed for any particular cell association scheme, such as the maximum received power association (MRPA) and the nearest BS association (NBA) schemes. Then, the link and network energy efficiencies are proposed to characterize the mean spectrum efficiency per unit power consumption for a BS and the mean area spectrum efficiency for the HetNet, respectively. All the tight bounds on the green coverage probability, link, and network energy efficiencies for the GCA, MRPA, and NBA schemes are found. They are theoretically shown to pose the fundamental maximum limits on the link and network energy efficiencies achieved by any other cell association schemes, and such a fact is validated by numerical results as well.
Heterogeneous Networks With Power-Domain NOMA: Coverage, Throughput, and Power Allocation Analysis
(2018-05-01)
Liu, Chun-Hung
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Liang, Di-Chun
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電信工程研究所
;
Institute of Communications Engineering
In a heterogeneous cellular network (HetNet), consider that a base station in the HetNet is able to simultaneously schedule and serve K users in the downlink by performing the power-domain non-orthogonal multiple access (NOMA) scheme. This paper aims at the preliminary study on the downlink coverage and throughput performances of the HetNet with the non-coordinated NOMA and the proposed coordinated joint transmission NOMA (JT-NOMA) schemes. First, the coverage probability and link throughput of K users in each cell are studied and their accurate expressions are derived for the non-coordinated NOMA scheme in which no BSs are coordinated to jointly transmit the NOMA signals for a particular user. We show that the coverage and link throughput can be largely reduced if transmit power allocations among the K users do not satisfy the constraint derived. Next, we analyze the coverage and link throughput of K users for the coordinated JT-NOMA scheme in which the void BSs without users are coordinated to enhance the farthest NOMA user in a cell. The derived accurate results show that coordinated JT-NOMA can significantly improve the coverage and link throughput of all users. Finally, we show that there exist optimal power allocation schemes that maximize the average cell coverage and throughput under some derived power allocation constraints and numerical results validate our analytical findings.
Load-Aware Coordinated Multipoint Joint Transmission in Dense Heterogeneous Networks: Downlink Coverage and Throughput Limits
(2017-01-01)
Liu, Chun-Hung
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Chen, Po-Chia
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電機工程學系
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Department of Electrical and Computer Engineering
In this paper, we propose a (cell) load-aware approach to performing the downlink coordinated multi-point joint transmission (JT-CoMP) scheme in a dense heterogeneous cellular network (HetNet) consisting M tiers of different kinds of base stations (BSs). Inspiring from the fact that there could exist a considerable number of the void BSs in a dense HetNet that are not associated by any users, this load-aware JT-CoMP scheme is able to coordinate the void BSs so as to enhance the downlink transmission of the non-void BSs. Using such a CoMP scheme, the fundamental upper and lower limits on the coverage and ergodic link throughput of a user with downlink MISO channel are accurately found in a low-complexity approximated form and they straightforwardly show that the load-aware JT-CoMP scheme has the capability of significantly increasing the coverage and ergodic link throughput of a user as long as the user intensity is not very much higher than the total intensity of the BSs. Numerical results validate the accuracy and correctness of the analytical results and demonstrate the huge gains of the coverage and the ergodic link throughput exploited by the proposed load-aware JT-CoMP scheme.
MmWave UAV Networks With Multi-Cell Association: Performance Limit and Optimization
(2019-12-01)
Liu, Chun-Hung
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Ho, Kai-Hsiang
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Wu, Jwo-Yuh
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電信工程研究所
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Institute of Communications Engineering
This paper aims to exploit the fundamental limits on the downlink coverage and spatial throughput performances of a cellular network comprised of a tier of unmanned aerial vehicle (UAV) base stations (BSs) using the millimeter wave (mmWave) band and a tier of ground BSs using the ultra high frequency (UHF) band. To reduce handover signaling overhead, the ground BSs take charge of control signaling delivery whereas the UAVs are in charge of payload data transmission so that users need to be simultaneously associated with a ground BS and a UAV in this network with a control-data plane-split architecture. We first propose a three-dimensional (3D) location distribution model of the UAVs using stochastic geometry which is able to generally characterize the positions of the UAVs in the sky. Using this 3D distribution model of UAVs, two performance metrics, i.e., multi-cell coverage probability and volume spectral efficiency, are proposed. Their explicit low-complexity expressions are derived and their upper limits are found when each of the UAVs and ground BSs is equipped with a massive antenna array. We further show that the multi-cell coverage probability and the volume spectral efficiency can be maximized by optimally deploying and positioning the UAVs in the sky and thereby their fundamental maximal limits are found. These important analytical findings are validated by numerical simulations.
Modeling and Analysis of Coexisting Multiple Radio Access Technologies in Heterogeneous Wireless Networks
(2016)
Liu, Chun-Hung
;
Wang, Li-Chun
;
電機學院
;
College of Electrical and Computer Engineering
This paper introduces a stochastic geometry framework for modeling and analysis of a heterogeneous wireless network with cellular and WiFi radio access technologies (RATs). Our focus here is to study the coexistence performance of the cellular and WiFi subnetworks in the WiFi frequency spectrum (band). The cellular subnetwork consists of macro and small cell access points (APs) with WiFi spectrum access. We first define the coexisting success probability in the WiFi frequency band as the sum of the density-weighted success probabilities of two subnetworks. The closed-form coexisting success probability is found and it is shown to have the concavity over the number of channels in the WiFi frequency band. The optimal deployment densities of small cell and WiFi access points (APs) that maximize the coexisting success probability are shown to exist and found under the derived constraint on network parameters. We then define the coexisting throughput as the sum of the throughputs of the two subnetworks and claim its unique maximum can be attained by the appropriate setting of AP densities. Numerical results indicate that the coexisting throughput is significantly higher than the throughput when only WiFi APs can access the WiFi frequency band.