Browsing by Author "Liao, Wanjiun"

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A Cost-Effective Strategy for Road-Side Unit Placement in Vehicular Networks
(2012-08-01)
Wu, Tsung-Jung
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Liao, Wanjiun
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Chang, Chung-Ju
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電機工程學系
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Department of Electrical and Computer Engineering
In this paper, we study the Roadside Unit (RSU) placement problem in vehicular networks. We focus on the highway-like scenario in which there may be multiple lanes with exits or intersections along the road. In our model, each vehicle can access RSUs in two ways: 1) direct delivery, which occurs when the vehicle is in the transmission range of the RSUs, and 2) multi-hop relaying, which takes place when the vehicle is out of RSU transmission range. We account for both access patterns in our placement strategy and formulate this placement problem via an integer linear programming model such that the aggregate throughput in the network can be maximized. We also take into account the impact of wireless interference, vehicle population distribution, and vehicle speeds in the formulation. The performance of the proposed placement strategy is evaluated via ns-2 simulations together with VanetMobisim to generate vehicle mobility patterns. The results show that our strategy leads to the best performance as compared with the uniformly distributed placement and the hot spot placement. More importantly, our solution needs the least number of RSUs to achieve the maximal aggregate throughput in the network, indicating that our scheme is indeed a cost effective yet highly efficient placement strategy for vehicular networks.
Adaptive Downlink and Uplink Channel Split Ratio Determination for TCP-Based Best Effort Traffic in TDD-Based WiMAX Networks
(2009-02-01)
Chiang, Chih-He
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Liao, Wanjiun
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Liu, Tehuang
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Chan, Iam Kin
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Chao, Hsi-Lu
In this paper, we study the determination of downlink (DL) and uplink (UL) channel split ratio for Time Division Duplex (TDD)-based IEEE 802.16 (WiMAX) wireless networks. In a TDD system, uplink and downlink transmissions share the same frequency at different time intervals. The TDD framing in WiMAX is adaptive in the sense that the downlink to uplink bandwidth ratio may vary with time. In this work, we focus on TCP based traffic and explore the impact of improper bandwidth allocation to DL and UL channels on the performance of TCP. We then propose an Adaptive Split Ratio (ASR) scheme which adjusts the bandwidth ratio of DL to UL adaptively according to the current traffic profile, wireless interference, and transport layer parameters, so as to maximize the aggregate throughput of TCP based traffic. Our scheme can also cooperate with the Base Station (BS) scheduler to throttle the TCP source when acknowledgements (ACKs) are transmitted infrequently. The performance of the proposed ASR scheme is validated via ns-2 simulations. The results show that our scheme outperforms static allocation (such as the default value specified in the WiMAX standard and other possible settings in existing access networks) in terms of higher aggregate throughput and better adaptivity to network dynamics.
Joint Routing and Spectrum Allocation for Multi-Hop Cognitive Radio Networks with Route Robustness Consideration
(2011-09-01)
Shih, Chao-Fang
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Liao, Wanjiun
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Chao, Hsi-Lu
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資訊工程學系
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Department of Computer Science
In this paper, we introduce the concept of "route robustness" for path selection in multi-hop cognitive radio networks. We demonstrate that the aggregate throughput and the robustness of routes determined by the proposed route selection strategy are superior to existing rate-based selection strategies. The rationale behind our approach is to guarantee a basic level of robustness for a set of routes (referred to as skeletons in this paper). Then, we select some routes from this robust route set and determine the spectrum to be allocated on each link along these routes such that the system throughput is maximized. We also design a polynomial time algorithm for this problem, and evaluate our proposed mechanism via simulations. The results show that our proposed algorithm indeed achieves a near optimal solution of this problem for multi-hop overlay CR networks.
Tight Lower Bounds for Channel Hopping Schemes in Cognitive Radio Networks
(2016-08)
Chang, Cheng-Shang
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Liao, Wanjiun
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Wu, Tsung-Ying
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傳播研究所
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Institute of Communication Studies
In this paper, we consider the two-user multichannel rendezvous problem in a cognitive radio network (CRN) and derive tight lower bounds for maximum time-to-rendezvous (MTTR) and maximum conditional time-to-rendezvous (MCTTR) of various channel hopping (CH) schemes under a channel loading constraint. In the symmetric and synchronous setting, we propose a novel Cycle-Adjustable Channel Hopping (CACH) scheme to achieve the MTTR lower bound (when the channel loading is bounded above by with being a prime power). Thus, the MTTR lower bound is tight and the CACH scheme is optimal in minimizing MTTR among all the symmetric and synchronous CH schemes under the same channel loading constraint. In the asymmetric setting, we show that the classical wait-for-mommy strategy can be used to achieve the MCTTR lower bound, and thus it is optimal. In the symmetric and asynchronous setting, we also show a hierarchical construction of an asynchronous CH sequence by using two smaller asynchronous CH sequences. To further understand the effect of channel loading to the other performance metrics in a CRN, we perform various computer simulations for various CH schemes. Our simulation results show that the average time-to-rendezvous of CACH is independent of the total number of channels, and it is also robust to the disturbance of primary users.