Browsing by Author "Wang, LC"

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A cross-layer cost-function based rate adaptation mechanism for the WCDMA system with multi-class services by transport format selection
(2005-06-01)
Wang, LC
;
Chen, MC
;
Chen, YC
;
電信工程研究所
;
Institute of Communications Engineering
The growing demand of high speed multi-class data transmissions poses new challenges on wireless networks. The major objectives of the next generation wireless networks include: (1) achieving high data throughput in the fast varying wireless channel; (2) transmitting multi-class data by service multiplexing; and (3) controlling the delay for delay sensitive service. To achieve these goals, in the context of wideband code division multiple access (WCDMA) system, we propose a cost-function based rate adaptation mechanism by taking account of both physical layer channel impacts and higher layer performance parameters, such as buffer occupancy and service priority. We implement this cross-layer rate mechanism by exploiting the transport format (TF) selection procedure in the medium access control (MAC) layer of the WCDMA system. Through the proposed cross-layer cost function, the TF selection procedure can dynamically adapt suitable spreading factors every transmission time interval (TTI), usually 10-80 ms. Through simulations in a flat Rayleigh fading channel, we show that the proposed cross-layer cost-function based rate adaptation mechanism can effectively improve throughput and reduce the buffer occupancy for multi-class services for the WCDMA system at the cost of slightly higher power efficiency. Copyright (c) 2005 John Wiley & Sons, Ltd.
A joint physical layer multi-state rate adaptation and cost-function based MAC scheduling for the WCDMA system
(2003)
Wang, LC
;
Chen, MC
;
Chen, YC
;
交大名義發表
;
National Chiao Tung University
This paper presents a joint physical layer multi-state rate adaptation algorithm combined with MAC scheduling for the WCDMA system. According to the cumulative distribution function of the received signal to noise ratio (SNR), our proposed multi-state rate adaptation scheme determines a set of state transition thresholds to dynamically adjust spreading factors via the transport format (TF) selection procedures in the MAC layer. Moreover, we proposed a cost function based TF selection procedures to incorporate many important factors in both the MAC layer and physical layer, such as service priority, buffer occupancy, and radio link quality. Through a cross-layer design in both the two layers, the proposed cost function based transport format selection procedures can effectively enhance throughput, save power, and guarantee service fairness for the WCDMA system.
A near real-time signal to interference ratio measurement technique in a frequency-selective multipath fading channel for the WCDMA system
(2001)
Wang, LC
;
Wang, CW
;
電信工程研究所
;
Institute of Communications Engineering
This paper presents a new concept of realtime SIR estimation for CDMA systems. Unlike conventional symbol-based SIR estimator according to the successively received symbols, we propose a new chip-based SIR estimation method. The key contribution of this paper is to propose a mechnism to exploit the spreading codes of CDMA signals to estimate SIR. Under the COST 259 WCDMA channel model, the simulation results demonstrate that the proposed chip-based SIR estimation algorithm can accurately measure SIR in less than 10(-4) second in several chips, which is about over 1,000 times faster than the traditional symbol-based SIR estimation algorithms.
A novel interference-resolving algorithm to support asymmetric services in TDD-CDMA systems with directional antennas
(2002)
Wang, LC
;
Huang, SY
;
Tseng, YC
;
電信工程研究所
;
Institute of Communications Engineering
This paper presents a new cross-slot interference-resolving algorithm for supporting unbalance traffic between downlink and uplink in TDD-CDMA systems. To avoid cross-slot interference between downlink and uplink, traditional algorithms typically require a global control on the transmission direction in each time slot among all cells. Obviously, this requirement significantly limits one of the key advantages in the TDD system -- enabling different rate asymmetry. We observe that the tri-sector cellular architecture possesses additional orthogonality inherently due to the directivity of directional antenna, which can provide an additional degree of freedom in allocating radio resource (i.e., time slots). Thus we propose a new distributed slot allocation algorithm based on the tri-sector cellular architecture. The advantage of the proposed algorithm is to enable a TDD-CDMA system to provide asymmetric services with different rates of asymmetry in all cells, thereby having more flexibility in handling uniform traffic patterns.
A statistical mobile-to-mobile Rician fading channel model
(2005)
Wang, LC
;
Cheng, YH
;
交大名義發表
;
National Chiao Tung University
The vehicular to vehicular communications is one of important applications for the intelligent transport systems (ITS). In such a communications environment, both the transmitter and receiver are moving on the highway in a multipath fading environment with a certain amount of line-of-sight (LOS) or specular component. In this paper, we present a sum-of-sinusoids method to model a mobile-to-mobile Rician fading channel. The proposed approach incorporates the relative movement between the transmitter and receiver and the LOS effect into a "correlated double-ring" scattering model. Numerical results show that the statistical properties of the proposed simulator match the theoretical values well.
A TCP-physical cross-layer congestion control mechanism for the multirate WCDMA system using explicit rate change notification
(2005)
Wang, LC
;
Lee, CH
;
電信工程研究所
;
Institute of Communications Engineering
In this paper we propose an explicit rate change notification (ERCN) mechanism to improve the transmission control protocol (TCP) performance in the wideband code-division multiple-access (WCDMA) system. The base station in the WCDMA system dynamically adapts transmission data rates according to the wireless channel condition. Therefore, if the TCP sender can not be notified about the rate change in the wireless channel, congestion in the radio link between the base station and mobile terminal occurs quite often. In this work, we first derive the relation between the bandwidth in the physical layer and the maximum queue size in the TCP layer and then obtain a closed-form expression for the average throughput in terms of average queue size, queue delay and channel capacity. We validate results by simulations. The analytical formula can facilitate the design of a cross-layer TCP-physical congestion control mechanism for the rate adaptive wireless system.
A unified capacity analysis for wireless systems with joint antenna and multiuser diversity in Nakagami fading channels
(2004)
Chen, CJ
;
Wang, LC
;
交大名義發表
;
National Chiao Tung University
In this paper, we present a cross-layer analytical framework to jointly investigate antenna diversity and multiuser scheduling under the generalized Nakagami fading channels. We derive a unified capacity formula for the multiuser scheduling system with different multiple-input multiple-output (MIMO) antenna schemes, including the selective combining (SC), maximum ratio combining (MRC) and space-time block codes (STBC). Our analytical results indicate that, in the presence of multiuser diversity, the multiple antennas with the SC method can be viewed as virtual users to amplify multiuser diversity order, while the MRC scheme can even yield greater capacity than the SC scheme due to the increased array gain. However, applying the STBC method on top of the multiuser scheduling system may lead to capacity loss due to the reduced amount of fading.
Adaptive contention window-based cluster head election mechanisms for wireless sensor networks
(2005)
Wang, LC
;
Wang, CW
;
Liu, CM
;
交大名義發表
;
National Chiao Tung University
The clustering architecture is essential in achieving the goal of energy efficiency for a wireless sensor network. In general, a clustering algorithm consists of the cluster head election and the cluster member assignment mechanism. This paper proposes an adaptive contention window (ACW)-based cluster head election mechanism. Unlike other legacy cluster head election mechanisms such as LEACH (Low Energy Adaptive Clustering Hierarchy) protocol, the proposed ACW algorithm can achieve four major goals in cluster head election for wireless sensor networks: 1) high successful probability of cluster head election, 2) appropriate number of cluster heads, 3) uniform distribution of cluster heads, and, 4) equal times to be a cluster head for each sensor, simultaneously.
An analytical framework for capacity and fairness evaluation in high speed wireless data networks
(2003)
Chen, CJ
;
Wang, LC
;
交大名義發表
;
National Chiao Tung University
This paper presents an analytical framework to evaluate the downlink capacity and fairness performance for high speed wireless data networks from both the physical layer and the network layer perspectives. Prom the physical layer standpoint, we take into account of propagation loss, log-normal shadowing and Nakagami fading. From the network layer perspective, we model different scheduling policies and the impact of nonuniform traffic intensity of mobile users into our framework. We analyze the joint effects of radio channel impairments, nonuniform traffic intensity and scheduling algorithms on the downlink capacity and fairness performance of wireless data networks based on the developed framework. Our results indicate that it is crucial for wireless scheduling algorithms to consider not only the radio channel impairments, but also the traffic intensity distribution in the whole network.
Comparison of downlink power allocation mechanisms in soft handoff for the WCDMA system with heterogeneous cell structures
(2005-09-01)
Liao, CY
;
Wang, LC
;
Chang, CJ
;
電信工程研究所
;
Institute of Communications Engineering
Handoff in heterogeneous cellular networks is one of the hot topics for wireless networks beyond the third generation. We observe that a power exhausting issue may occur in a code division multiple access (CDMA) system with mixed-sized cells. During soft handoff in the downlink transmission, a number of base stations transmit signals to a user simultaneously. Usually, a microcell has a more stringent limitation on the total available power than a macrocell. Thus, ignoring the impact of various cell sizes, the traditional downlink power allocation techniques for soft handoff may easily consume excessive power to serve soft handoff users, while leaving insufficient power for serving other regular users. To resolve such an power exhausting issue in CDMA systems, we investigate different downlink power allocation techniques used in soft handoff subject to the impact of mixed-sized cells. For the single-site power allocation technique we consider the site selection diversity transmission (SSDT) technique, while for the multi-site power allocation we study the link proportional power allocation (LPPA), the quality balancing power allocation (QBPA), and the equal power allocation (EPA) techniques. We find that the multi-site LPPA technique can more efficiently allocate power to both handoff and non-handoff users than others. In an example with the ratio of the micrcocell radius/macrocell radius equal to 1/3, it is demonstrated that LPPA can improve the capacity over EPA, QBPA, and SSDT by 125, 30, and 5%, respectively. By taking account of measurement errors in the same case, the capacity improvements of LPPA over EPA, QBPA, and SSDT become 180, 41, and 23%, respectively.
Comparisons of link adaptation based scheduling algorithms for the WCDMA system with high speed downlink packet access
(2004-01-01)
Wang, LC
;
Chen, MC
;
電信工程研究所
;
Institute of Communications Engineering
The wideband code division multiple access (WCDMA) system with high speed downlink packet access (HSDPA) is an important next generation wireless system. By adopting adaptive modulation, efficient scheduling, and hybrid automatic repeat request technologies, it can support data rates up to 10 Mbps in the mobile cellular environment. Among these techniques, the scheduling algorithm plays a key role in realizing the HSDPA concept A good scheduling algorithm should consider all the important factors including channel impact, delay issue, and fairness. In this paper, we suggest a fairness index to examine the fairness performance of current link adaptation based scheduling algorithms, including the maximum CA, round robin, proportional fair, and exponential rule schedulers. When supporting multi-type services, we find that the fairness performance of current scheduling algorithms, including the round robin scheduler, can be improved further even though the round robin scheduler is viewed as the scheduler of the greatest fairness. Thus, we are motivated to develop a new scheduling algorithm, namely the queue-based exponential rule scheduler. Through simulations, we show that in the context of multi-type services the fairness performance of the queue-based exponential rule scheduler can outperform all the schedulers in the time-multiplexing fashion, while maintaining good throughput and delay performance.
Comparisons of link-adaptation-based scheduling algorithms for the WCDMA system with high-speed downlink packet access
(2004-01-01)
Wang, LC
;
Chen, MC
;
電信工程研究所
;
Institute of Communications Engineering
The wideband code division multiple access (WCDMA) system with high-speed downlink packet access (HSDPA) is an important next-generation wireless system. By adopting adaptive modulation, efficient scheduling, and hybrid automatic repeat request technologies, it can support data rates of up to 10 Mb/s in the mobile cellular environment. Among these techniques, the scheduling algorithm plays a key role in realizing the HSDPA concept. A good scheduling algorithm should consider all the important factors, including channel impact, delay issues, and fairness. In this paper, a fairness index is adopted to examine the fairness performance of current link-adaptation-based scheduling algorithms, including the maximum carrier-to-interference (C/I), round-robin, proportional fair, and exponential rule schedulers. It is found that when multi-type services are supported, the fairness performance of current scheduling algorithms, including the round-robin scheduler, can be further improved even though the round-robin scheduler is viewed as the scheduler of the greatest fairness. Thus, a new scheduling algorithm, namely the queue-based exponential rule scheduler, is developed. Through simulations, it is shown that in the context of multi-type services the fairness performance of the queue-based exponential rule scheduler can surpass that of all the other schedulers in the time-multiplexing fashion, while maintaining good throughput and delay performance.
Coverage and capacity enhancement in multiuser MIMO systems with scheduling
(2004)
Chen, CJ
;
Wang, LC
;
交大名義發表
;
National Chiao Tung University
Recent studies have revealed that the capacity improvement resulting from an open-loop multiple-input-multiple-output (MIMO) spatial multiplexing system may come at the sacrifice of reducing link reliability. The tradeoff of antenna multiplexing gain against antenna diversity gain may lead to smaller cell coverage sizes. In this paper, we propose a scheduling scheme, called the strongest-weakest-normalized-subchannel-first (SWNSF) scheduling, to replenish the diversity-deficient MIMO system with multiuser diversity. Our analysis and results indicate that the SWNSF scheduling can significantly increase both cell coverage and system capacity requiring only limited amount of feedback.
Coverage and capacity of a wireless mesh network
(2005)
Huang, JH
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Wang, LC
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Chang, CJ
;
電信工程研究所
;
Institute of Communications Engineering
Wireless mesh networks (WMN) will play an important role in the next-generation Wireless communication systems because it can support broadband services with ubiquitous coverage by low transmission power. In this paper, we propose a multi-channel ring-based wireless mesh network and develop an analytical framework to evaluate the capacity and coverage of such a network. In the analytical framework, we first establish a physical (PHY)/medium access control (MAC) cross-layer throughput performance model by including the carrier sense multiple access (CSMA) MAC protocol and distance-based rate adaptation with multi-hop connections. Based on the derived throughput model, we apply the mixed-integer nonlinear programming (MINLP) optimization approach to maximize the capacity (throughput) and service coverage of a mesh cell, in which the number of rings in a mesh cell and the radius for each ring are determined.
Cross-layer design for determining the optimal number of clusters in a wireless sensor network
(2004)
Wang, LC
;
Wang, CW
;
Liu, CM
;
交大名義發表
;
National Chiao Tung University
The clustering concept can help a wireless sensor network establish hierarchy among numerous sensor nodes. Clustering architecture is essential in achieving the goal of saving power for wireless sensor networks because a cluster representative node can forward data to sink for the covered cluster area. One of key parameters in the clustering design is the number of clusters in a entire wireless sensor network. To this end, we suggest a cross-layer approach to determine an optimal number of clusters in an observation area from the perspectives of physical (PHY) layer, medium access control (MAC) sub-layer, scheduling (SCH) sublayer and network (NET) layer.
Cross-layer goodput analysis for rate adaptive IEEE 802.11 a WLAN in the generalized Nakagami fading channel
(2004)
Wang, LC
;
Lin, YW
;
Liu, WC
;
交大名義發表
;
National Chiao Tung University
This paper aims to evaluate the goodput performance of the IEEE 802.11a wireless local area network (WLAN) from both the media access control (MAC) layer and physical (PHY) layer perspectives. From the physical layer perspective, we analyze the packet error rate performance of the orthogonal frequency division multiplexing (OFDM) based WLANs under the generalized Nakagami fading channel. According to the Chernoff bound analysis in the Nakagand fading channel, we derive the approximate packet error rate for the IEEE 802.11a WLAN with different data rates and fading parameters. Furthermore, we propose an efficient channel driven rate adaptation (CDRA) scheme. Through a PHY/MAC cross-layer analysis, we demonstrate that the goodput of the CDRA scheme indeed approaches to that of the optimal dynamic programming based rate adaptation method, while avoiding the complex calculations in selecting transmission parameters as the optimal method.
Deployment strategies of access points for outdoor wireless local area networks
(2005)
Huang, JH
;
Wang, LC
;
Chang, CJ
;
電信工程研究所
;
Institute of Communications Engineering
In this paper, we investigate the issues of deploying access points for wireless local area networks (WLANs) in an outdoor environment, where several adjacent access points (APs) will form a cluster and in each cluster access points are connected through wireless repeaters to ease deployment. Access points are usually placed aiming to maximize its cell coverage. Nevertheless, a larger coverage distance will lead to a lower throughput for the wireless repeater. We apply the mixed-integer nonlinear programming (MINLP) optimization approach to determine the number of access points in a cluster and the best separation distance between access points. The objective is to maximize the ratio of the total carried traffic load to the total cost for a cluster of APs connected by wireless repeaters. In this paper, the uniform-spacing and the increasing-spacing placement strategies for access points are proposed and compared. Our results show that the increasing-spacing placement strategy outperforms the uniform-spacing placement strategy in terms of an objective function considering total carried traffic load, the number of access points, and the cost connecting an AP to the backbone network.
Downlink soft handover and power allocation for CDMA heterogeneous cellular networks
(2002)
Wang, LC
;
Liao, CY
;
Chang, CJ
;
電信工程研究所
;
Institute of Communications Engineering
Handover in heterogeneous cellular networks is one of the hottest topics for the wireless networks beyond the third generation. This paper presents a new downlink soft handover algorithm to solve the serious "power exhausting" problem during the traditional handover process in heterogeneous cellular systems. The power exhausting problem occurs when in the traditional handover process, the serving base stations usually adopt the equal power allocation (EPA) strategy for different sizes of cells. Thus we propose a constrained unequal power allocation (CUPA) technique to serve handover users by taking account of the effects of different cell sizes and shadowing. Compared to the EPA method, our simulation results show that the proposed soft handover algorithm combining with the CUPA technique can improve total system capacity by 5% in a homogeneous cellular system, and 73% in a heterogeneous cellular system.
Dynamic bandwidth allocation for multimedia traffic with rate guarantee and fair access in WCDMA systems
(2005-09-01)
Chao, CM
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Tseng, YC
;
Wang, LC
;
資訊工程學系
;
電信工程研究所
Packet scheduling in a WCDMA system poses a new challenge due to its nature of variable bit rates and location-dependent, time-varying channel conditions. In this work, three new downlink scheduling algorithms for a WCDMA base station are proposed to support multimedia transmissions. Using a credit management and a compensation mechanism, our algorithms provide rate guarantee and fair access to mobile terminals. In particular, we propose to allow a user to simultaneously use multiple OVSF codes in a time-sharing manner, which we call a multicode, shared model. Using multiple codes allows us to compensate those users suffering from bad communication quality or even errors. The proposed schemes can tolerate a multistate link condition (compared to the typically assumed two-state, or good-or-bad, link condition) by adjusting the number of OVSF codes and the spreading factor of each code. Simulation results show that the proposed schemes do achieve higher bandwidth utilization while keeping transmission delay low.
Error statistics of closed-loop power control in multirate DS-CDMA cellular systems
(2002)
Wang, LC
;
Chang, CW
;
電信工程研究所
;
Institute of Communications Engineering
In this paper, we investigate the error statistics of the close-loop power control scheme with variable spreading factors for the multi-rate services in the third generation wideband CDMA (WCDMA) system. We demonstrate that the long scrambling pseudo-noise code, besides its well known feature in differentiating users and base stations, can improve power control false command over a frequency-selective fading channel as well. Our computer simulation results indicate that the SIR measurement error can be reduced by 3 dB in a typical case, and the power control false command can be reduced from 27 % to 10 % in comparison to using pure short Walsh codes. It will be shown that the close-loop power control error is a composite function of spreading factor, target E-b / N-o and Doppler frequency. The statistics of closed loop power control false command can be easily characterized by a Bernoulli distribution with a parameter provided by our results.