博碩士論文 965402603 詳細資訊




以作者查詢圖書館館藏 以作者查詢臺灣博碩士 以作者查詢全國書目 勘誤回報 、線上人數:51 、訪客IP:3.147.7.185
姓名 蘇柏德(Pratap-Kumar Sahu)  查詢紙本館藏   畢業系所 資訊工程學系
論文名稱
(Destination Discovery based Geographic Routing Protocolin VANET’s Highway and City Scenarios)
相關論文
★ 具多重樹狀結構之可靠性群播傳輸★ 在嵌入式行動裝置上設計與開發跨平台Widget
★ 在 ARM 架構之嵌入式系統上實作輕量化的手持多媒體播放裝置圖形使用者介面函式庫★ 基於網路行動裝置所設計可擴展的服務品質感知GStreamer模組
★ 針對行動網路裝置開發可擴展且跨平台之GSM/HSDPA引擎★ 於單晶片多媒體裝置進行有效率之多格式解碼管理
★ IMS客戶端設計與即時通訊模組研發:個人資訊交換模組與即時訊息模組實作★ 在可攜式多媒體裝置上實作人性化的嵌入式小螢幕網頁瀏覽器
★ 以IMS為基礎之及時語音影像通話引擎的實作:使用開放原始碼程式庫★ 電子書嵌入式開發: 客制化下載服務實作, 資料儲存管理設計
★ 於數位機上盒實現有效率訊框參照處理與多媒體詮釋資料感知的播放器設計★ 具數位安全性的電子書開發:有效率的更新模組與資料庫實作
★ 適用於異質無線寬頻系統的新世代IMS客戶端軟體研發★ 在可攜式數位機上盒上設計並實作重配置的圖形使用者介面
★ Friendly GUI design and possibility support for E-book Reader based Android client★ Effective GUI Design and Memory Usage Management for Android-based Services
檔案 [Endnote RIS 格式]    [Bibtex 格式]    [相關文章]   [文章引用]   [完整記錄]   [館藏目錄]   [檢視]  [下載]
  1. 本電子論文使用權限為同意立即開放。
  2. 已達開放權限電子全文僅授權使用者為學術研究之目的,進行個人非營利性質之檢索、閱讀、列印。
  3. 請遵守中華民國著作權法之相關規定,切勿任意重製、散佈、改作、轉貼、播送,以免觸法。

摘要(中) 近年來眾多將無線網路通訊應用於地面交通運輸系統的相關研究被提出,車輛隨意網路 (VANET, vehicular ad-hoc networks) 便是利用無線區域網路技術所建構而成具備容易實作、富高度價值的應用,例如路面安全檢測、多媒體分享、線上遊戲、車輛網際網路以及各式商業應用。然而在車輛隨意網路上的多點跳躍訊息傳播(Multi-hop information dissemination)將受制於車輛間的移動性以及頻繁的斷線而導致低傳輸成功率。本篇論文提出一個適用於高速公路車輛無線網路環境的路由機制,其中包含以目的節點探索為基礎的單點傳播流程 (unicast destination discovery)、堅固的節點選擇式轉送機制 (forward node selection)以及具有位置性的Hello 機制 (positional hello)。在這篇論文中,為了避免頻繁維護路徑所造成的網路資源浪費,因此沒有設定任何專用的路徑。此外,排除掉洪流法(flooding)以及位置服務將能從本質上減輕控制訊息對網路造成的額外負擔。具有位置性的Hello 機制能夠同時確保節點的連結性與減少控制訊息對網路的額外負擔。模擬結果表現出本論文提出的路由策略(DDOR)相較於過去其他研究所帶來的好處─較高的封包送達比率、減輕路由對網路造成的額外負擔以及縮短延遲時間。現今地理性的路由協定由於並不需要經歷建立與維護路由的階段,因此被廣泛應用在車輛隨意網路上。此外,若加上連接性感知(connectivity awareness)的功能將使得資料傳送更加可靠。但這類協定為了取得目的節點的位置將使用位置服務或是洪流法,然而洪流法將不利於在城市環境中應用,因為其中RTS-CTS 機制無法為探查封包(probe packet)提供安全保護傳送。更進一步,在車輛稀疏甚至淨空的區域,此類協定頻繁的使用復原策略將會不必要的增加傳輸跳躍數(hop-count)。部分地理性的路由協定在距離或連接性上應用最小權重演算法(minimum weighted algorithm)為基礎來選擇中間的交叉點;然而,最短路徑或是擁有較高連接性的路徑將包含許多中間交叉點,這種現象導致這一類協定最終選出較高傳輸跳躍數的路徑。在本論文的第二項研究中,我們提出一個貪婪式跳躍的機制將連接性納入考量來選出擁有最少中間交叉節點的路徑。此外本論文採用了後端骨幹節點的關鍵技術來提供關於交叉點附近的連結狀態,同時追蹤來源端與目的端節點的移動狀況,後端骨幹節點將使得封包能夠往改變後的方向傳送。模擬結果表現出本論文提出的BAHG 路由策略有著較高的封包送達率以及較短的點對點延遲時間。
摘要(英) The emerging adoption of wireless communications on surface transportation systems has generated extensive interest among researchers over the last several years. Using advanced WLAN technologies, vehicular ad-hoc networks have become viable and valuable for their wide variety of novel applications such as road-safety, multimedia content sharing, online gaming, internet on vehicles, and commerce on wheels. Multi-hop information dissemination in vehicular ad hoc networks is constrained by high mobility of vehicles and frequent disconnections. We propose a destination discovery oriented routing (DDOR) scheme for Highway/Freeway VANETs. DDOR consists of a unicast destination discovery process, a robust forward node selection mechanism and a positional hello mechanism. In this work, no dedicated path is framed in order to prevent frequent path maintenance. In addition, the elimination of flooding and location services substantially reduces the control overhead. Positional hello scheme ensures connectivity and diminishes control overhead concurrently. Simulation results signify the benefits of the proposed routing strategy which has higher packet delivery ratio, reduced routing overhead and shorter delay compared with existing routing protocols. Currently, the geographic routing protocols are widely adopted for city scenarios as they do not require route construction and route maintenance phases. Again, with connectivity awareness they perform well in terms of reliable delivery. To obtain destination position, such protocols employ location service or flooding. Flooding can be detrimental in city environments as probe packets are not safeguarded by RTS-CTS. Further, in case of sparse and void regions, frequent use of recovery strategy in such protocols elevates hop-count. Some of the geographic routing protocols adopt minimum weighted algorithm based on distance or connectivity to select the intermediate intersections. However, the shortest path or the path with higher connectivity may include numerous intermediate intersections. As a result, these protocols yield routing paths with higher hop-count. We propose a back-bone assisted hop-greedy (BAHG) routing scheme to address these problems by selecting a routing path with minimum number of intermediate intersection nodes while taking connectivity into considerations. Besides, we introduce back bone nodes which play a key role in providing connectivity status around an intersection. Apart from this, by tracking the movement of source as well as destination, the back bone nodes enable a packet to be forwarded in the changed direction. Simulation results demonstrate that the proposed routing strategy outperforms state-of-art geographic routing protocols in terms of packet delivery ratio and end-to-end delay.
關鍵字(中) ★ 貪婪路由演算法
★ 高速公路
★ 位置服務
★ 圓周式路由演算法
★ 車輛隨意網路
★ 單點傳送路由演算法
★ 目的節點探索
關鍵字(英) ★ Unicast routing
★ Perimeter Routing
★ Greedy routing
★ Destination Discovery
★ VANET
★ Location service
★ Highway/Freeway
論文目次 Abstract in Chines i
Abstract in English iii
Acknowledgements v
List of Figures xii
List of Tables xv
List of Abbreviations xvi
1. Introduction 1
1.1 Contributions 2
1.1.1 Purpose 2
1.1.2 Motivation 3
1.1.3 Contributions in Brief 4
1.2 Thesis Organization 6
2 Background 7
2.1 VANET and its Characteristics 7
2.1.1 Highly Dynamic Topology 7
2.1.2 Intermittent Connectivity 8
2.1.3 Patterned Mobility 8
2.1.4 Unlimited Battery Power and Storage 8
2.1.5 On-board Sensors, GPS and digital Map 9
2.2 DSRC Standard 9
2.3 Applications of VANET 9
2.4 Challenges of VANET 10
3 Routing Protocols in VANET 11
3.1 Classification 11
3.1.1 Topological Routing 11
3.1.1.1 Proactive Routing 11
3.1.1.2 Reactive Routing 11
3.1.1.3 Hybrid Routing 12
3.1.2 Geographic Routing 13
3.1.3 Delay Torrent Routing 14
3.2 Comparison of VANET Routing Protocols 14
4 Destination Discovery Oriented Routing in VANET 16
4.1 Motivation 16
4.1.1 Flooding 16
4.1.2 Location Service 17
4.1.3 Periodic Beaconing 18
4.1.4 Gray Zone Problem 19
4.1.5 Scalability 19
4.1.6 Pseudo-Stability 20
4.2 Assumptions 21
4.3 Overview of DDOR 21
4.4 Smart Next Hop Selection Algorithm (SNESA) 22
4.5 Positional Beaconing 24
4.6 Destination Discovery Oriented Routing 24
4.6.1 DDREQ 24
4.6.2 DDREP and Data Dissemination 25
4.6.3 Destination Position Update Mechanism 27
5 Back-bone Assisted Hop Greedy Routing in VANETs City Environments 28
5.1 Motivations 28
5.1.1 Intersection Node Probing 30
5.1.2 Location Service Requirement 31
5.1.3 Distance or Connectivity Based Weighted Graph 32
5.1.4 Packet Swinging in Greedy Forwarding 33
5.2 Assumptions 33
5.3 Zone Formation 34
5.4 Back-bone Creation 35
5.4.1 Back-bone at Intersection 35
5.4.2 Back-bone on Road Segment 37
5.5 Hop Greedy Routing Algorithm 38
5.6 Normalization of delta-count 43
5.7 Analytical Evaluation 45
5.7.1 Proof of Lowest Hop-Count 46
5.7.1 Proof of Higher Connectivity 51
5.8 BAHG Routing 54
5.8.1 Two phase Destination Discovery 54
5.8.2 Reply Mechanism and Data Dissemination 56
5.8.3 Source and Destination movement resilient Update mechanism 57
6 Performance Evaluation 58
6.1 Evaluation Metrics 58
6.2 Simulations in Highway Scenario (Simple) 58
6.2.1 Freeway Mobility Model 58
6.2.2 Protocols Compared 59
6.2.3 Simulation Setup 59
6.2.4 Results and Discussions 60
6.2.4.1 Observation of Packet Delivery Ratio 60
6.2.4.2 Observation of Routing Overhead 60
6.3 Simulations in Highway with intersections 63
6.3.1 Manhattan Mobility Model 63
6.3.2 Protocols Compared 63
6.3.3 Simulation Setup 63
6.3.4 Results and Discussions 64
6.3.4.1 Impact of Beacon Interval and Node Density on GPSR 64
6.3.4.2 Impact of Speed and Node Density on DSR 67
6.3.4.3 Impact of Speed and Node Density on AODV 69
6.3.4.4 Impact of Speed and Node Density on DDOR 71
6.3.4.5 Impact of Speed on Different Protocols 75
6.3.4.6 Impact of Node Density on Different Protocols 79
6.4 Simulations in City Scenario 80
6.4.1 SUMO Mobility 80
6.4.2 Protocols Compared 81
6.4.3 Simulation Setup 81
6.4.4 Results and Discussions 83
6.4.4.1 Observation of End-to-End Delay 83
6.4.4.2 Observation of Packet Delivery Ratio 89
7 Conclusion and Future Works 91
Bibliography 92
參考文獻 [1] M. M. Artimy, W. Robertson, and W. J. Phillips, “Connectivity in inter-vehicle ad hoc
networks,” in Proc. IEEE CCECE, May 2004, pp. 293–298.
[2] U.G. Acer, S. Kalyanaraman, A. A. Abouzeid, “Weak State Routing for Large Scale
Dynamic Networks,” In the Proc. of ACM SIGMOBILE MOBICOM, 2007.
[3] F. Bai, S. Narayanan and A. Helmy, “IMPORTANT: A framework to systematically
analyze the Impact of Mobility on Performance of RouTing protocols for Adhoc
NeTworks”, in Proc. IEEE INFOCOM, 2003, pp. 825-835.
[4] N. Beijar, “Zone Routing Protocol (ZRP),” Online:
http://www.netlab.hut.fi/opetus/s38030/k02/Papers/08-Nicklas.pdf.
[5] J. Bernsern, D. Manivannan, “Unicast routing protocols for vehicular ad hoc networks: A
critical comparison and classification,” Pervasive and Mobile Computing, Sep. 2008.
[6] Car2Car Communication Consortium. [Online]. Available: www.car-tocar.org
[7] C. H. Chou, K.F. Ssu, H.C. Jiau, “Geographic Forwarding with Dead-End Reduction in
Mobile Ad Hoc Networks,” IEEE Trans. on Vehicular Technology, Vol. 57, No. 4, Jul.
2008.
[8] T.H. Clausen, G. Hansen, L. Christensen and G. Behrmann, “The Optimized Link State
Routing Protocol, Evaluation through Experiments and Simulation,” IEEE Symposium
on Wireless Personal Mobile Communications, September 2001.
[9] Communications for eSafety. [Online]. Available: www.comesafety.org
[10] Cooperative Vehicle-Infrastructure Systems. [Online]. Available: www.cvisproject.org
[11] Coopers. [Online]. Available: http://www.coopers-ip.eu
[12] T. H. Cormen, C. E. Leiserson, R. L. Rivest, C. Stein, "Section 24.3: Dijkstra’’s
algorithm".Introduction to Algorithms(Second ed.). MIT Press and McGraw-Hill. 2001,
pp.595–601
[13] Dedicated Short Range Communications. [Online]. Available:
http://www.leearmstrong.com/DSRC/DSRCHomeset.htm
[14] Y. Ding, C. Wang, L. Xiao, “A Static-Node Assisted Adaptive Routing Protocol in
Vehicular Networks,” in Proc. ACM VANET ‘07, 2007, pp. 59-68.
[15] eSafety. [Online]. Available: http://www.esafetysupport.org
[16] H. Fuβler, Martin Mauve, Hannes Hartenstein, Dieter Vollmer, “A Comparison of
Routing Strategies in Vehicular Ad-Hoc Networks,” Technical Report, TR-02-003, Dept.
of Computer Science, Univ. of Mannheim, Jul. 2002.
[17] J. Gong, C.Z. Xu, J. Holle, “Predictive Directional Greedy Routing in Vehicular Ad hoc
Networks,” IEEE ICDCSW, 2007
[18] C.C. Hung, H. Chan. E. H. K. Wu, “Mobility Pattern Aware Routing for Heterogeneous
Vehicular Networks,” IEEE WCNC, 2008.
[19] IEEE Draft Std. P802.11p /D9.0, Sep. 2009. [Online]. Available:
http://ieeexplore.ieee.org/servlet/opac?punumber=5325056
[20] M. Jerbi, S.M. Senouci, R. Meraihi, Y.G. Doudane, “Towards Efficient Geographic
Routing in Urban Vehicular Networks,” IEEE Trans. Veh. Technol., vol. 58, pp. 5048 –
5059, Nov. 2009.
[21] M. Jerbi, S.M. Senouci, R. Meraihi, Y.G. Doudane, “An improved vehicular ad hoc
routing protocol for city environments,” In IEEE ICC, Jun. 2007.
[22] X. Jiang, T. Camp, “An Efficient Location Server for an Ad Hoc Networks,” Technical
Report, MCS-03-06, The Colorado School of Mines, May 2003.
[23] D.B. Johnson, D.A. Maltz, J. Broch, “DSR: The Dynamic Source Routing Protocol for
Multi-Hop Wireless Ad Hoc Networks,” in Ad Hoc Networking, edited by Charles E.
Perkins, Chapter-5, Adison-Wesley 2001.
[24] B. Karp, H.T. Kung, “GPSR: Greedy Perimeter Stateless Routing for Wireless Networks,”
ACM MOBICOM, 2000.
[25] M. Kasemann, H. Fuβler, H. Hartenstein, M. Mauve, “A Reactive Location Service for
Mobile Ad Hoc Networks,” Technical Report, TR-14-2002, Dept. of Computer Science,
Univ. of Mannheim, Nov. 2002.
[26] W. Kies, H. Fusler, J. Widmer, “Hierarchical Location Service for Mobile Ad-Hoc
Networks,” ACM SIGMOBILE Mobile Computing and Communications Review, Oct.
2004.
[27] F. Kuhn, R. Wattenhofer, Y. Zhang, and A. Zollinger, “Geometric ad-hoc routing: Of
theory and practice,” in Proc. ACM Symp. PODC, Jul. 2003, pp. 63–72.
[28] K.C. Lee, J. Haerri, U. Lee, M. Gerla, “Enhanced Perimeter Routing for Geographic
Forwarding Protocols in Urban Vehicular Scenarios,” IEEE GlobeCom Workshops,
2007.
[29] U. Lee, J. Lee, J. S. Park, M. Gerla, “FleaNet: A Virtual Market Place on Vehicular
Networks”, IEEE Trans. on Veh. Technol., vol. 59, pp. 344 – 355, Jan. 2010.
[30] J. Li, J. Jannotti, D.S.J. De Couto, D.R. Karger, R. Morris, “A Scalable Location Service
for Geographic Ad Hoc Routing,” in Proc. ACM MOBICOM, 2000, pp. 120-130.
[31] G. Liu, B.S. Lee, B.C. Seet, C.H. Foh, K.J. Wong, K.K. Lee, “A Routing Strategy for
Metropolis Vehicular Communications,” Lecture notes in computer science ISSN 0302-
9743, Aug. 2004.
[32] C. Lochert, M. Mauve, H. Fuβler, Hannes Hartenstein, “Geographic Routing in City
Scenarios,” ACM SIGMOBILE Mobile Computing and Communications Review, Jan.
2005.
[33] C. Lochert, H. Hartenstein, J. Tian, H. Fusler, D. Hermann, M. Mauve, “A Routing
Strategy for Vehicular Ad Hoc Networks in City Environments,” In proc. of the IEEE
Intelligent Vehicles Symposium, 2003.
[34] H. Lundgren, E. Nordstrom, C. Tschudin, “The Gray Zone Problem in IEEE 802.11b
based Ad hoc Networks,” ACM SIGMOBILE Mobile Computing and Communications
Review, 2002.
[35] M. Mauve, J. Widner, H. Hartenstein, “A Survey on Position-Based Routing in Mobile
Ad-Hoc Networks,” IEEE Network, Nov/Dec. 2001.
[36] X. Ma, M.T. Sun, G. Zhao, Z. Liu, “An Efficient Path Pruning Algorithm for
Geographical Routing in Wireless Networks,” IEEE Trans. on Vehicular Technology,
Vol. 57, No. 4, Jul. 2008.
[37] H. Menouar, M. Lenardi, F. Filali, “Improving Proactive Routing in VANETs with the
MOPR Movement Prediction Framework,” in Proc. ITST’07, 2007, pp. 1-6.
[38] H. Menouar, M. Lenardi, F. Filali, “A Movement Prediction-based Routing Protocol for
Vehicle-to-Vehicle Communications,” V2VCOM, 1st International Vehicle-to
Vehicle Communications Workshop, San Diego, California, USA, July 2005.
[39] H. Menouar, M. Lenardi, F. Filali, “Movement Prediction-based Routing (MOPR)
Concept for Position-based Routing in Vehicular Networks,” IEEE VTC, 2007.
[40] Z. Mo, H. Zhu, K. Makki, N. Pissinou, “MURU: A Multi-Hop Routing Protocol for
Urban Vehicular Ad Hoc Networks,” IEEE MOBIQUITOUS, 2006.
[41] V. Namboodiri, L. Gao, “Prediction-Based Routing for Vehicular Ad Hoc Networks,”
IEEE Trans. on Vehicular Technology, Vol. 56, No. 4, Jul. 2007.
[42] V. Naumov, T.R. Gross, “Connectivity-Aware Routing (CAR) in Vehicular Ad Hoc
Networks,” IEEE INFOCOMM, 2007.
[43] V. Naumov, R. Baumann, and T. Gross, “An evaluation of inter-vehicle ad hoc networks
based on realistic vehicular traces,” in Proc. ACM MOBIHOC’06, 2006, pp. 108–119.
[44] S.Y. Ni, Y.C. Tseng, Y.S. Chen, J.P. Sheu, “The Broadcast Storm Problem in a Mobile
Ad Hoc Network,” in Proc. ACM/IEEE MOBICOM, 1999, pp. 151–162.
[45] J. Nzouonta, N. Rajgure, G. Wang, C. Borcea, “VANET Routing on City Roads using
Real-Time Vehicular Traffic Information”, IEEE Trans. Veh. Technol., vol. 58, pp.
3609 – 3626, Sept. 2008.
[46] OpenStreetMap. [Online].Available: http://www.openstreetmap.org/
[47] M.R. Pearlman, Z.J. Haas, “Determining the Optimal Configuration for the Zone Routing
Protocol,” IEEE JSAC, Vol. 17, No. 8, Aug. 1999.
[48] C.E. Perkins, E.M. Royer, “Ad-Hoc On-Demand Distance Vector Routing,” Proceedings
of the 2nd IEEE Workshop on Mobile Computing Systems and Applications, New
Orleans, LA, February 1999.
[49] C.E. Perkins, P. Bhagwat, “Highly Dynamic Destination-Sequenced Distance-Vector
Routing (DSDV) for Mobile Computers”, Sigcomm’’94, 1994.
[50] PTV simulation VISSIM. http://www.english.ptv.de/
[51] Safespot. [Online]. Available: http://www.safespot-eu.org
[52] R.A. Santos, O. Alvarez, A. Edwards, “Performance Evaluation of two Location-Based
Routing Protocols in Vehicular Ad-Hoc Networks,” IEEE VTC, 2005.
[53] P.K. Sahu, E.H. Wu, J. Sahoo, M. Gerla, “DDOR: Destination Discovery Oriented
Routing in Highway/Freeway VANETs", Springer Telecommunication Systems, Special
Issue on Vehicular Communications, Networks, and Applications, 2010.
[54] C. Sommer, F. Dressler, “The DYMO Routing Protocol in VANET Scenarios,” IEEE
VTC, 2007.
[55] SUMO Simulation of Urban Mobility. [Online].Available: http://sumo.sourceforge.net/
[56] A. Takahashi and N. Asanuma, “Introduction of Honda ASV-2 (Advanced Safety
Vehicle phase 2),” in Proc. IEEE Intell. Vehicles Symp., 2000, pp. 694–701.
[57] T. Taleb, E. Sakhaee, A. Jamalipour, K. Hashimoto, N. Kato, Y. Nemoto, “A Stable
Routing Protocol to Support ITS Services in VANET Networks,” IEEE Trans. on
Vehicular Technology, Nov. 2007.
[58] The CitySense Sensor Network Project. [Online]. Available:http://www.citysense.net.
[59] The Network Simulator-ns-2. [Online]. Available: http://www.isi.edu/nsnam/ns/
[60] TIGER (Topologically Integrated GEographic Encoding and Referencing). [Online].
Available: http://www.census.gov/geo/www/tiger/.
[61] C.K. Toh, “Associativity-Based Routing For Ad-Hoc Mobile Networks,” Wireless
Personal Communications, 1997.
[62] Vehicle Infrastructure Initiative. [Online]. Available: www.vehicleinfrastructure.org
[63] N. Wisitpongphan, F. Bai, P. Mudalige, V. Sadekar, and O. Tonguz, “Routing in sparse
vehicular ad hoc wireless networks,” IEEE J. Sel. Areas Commun., vol. 25, no. 8, pp.
1538–1556, Oct. 2007.
[64] E.H. Wu, P.K. Sahu, J. Sahoo, “Destination Discovery Oriented Position Based Routing
in VANET” in Proc. IEEE APSCC’08,2008,pp.1606-1610.
[65] Q Yang, A Lim, S Li, J Fang, P Agrawal, “ACAR: Adaptive Connectivity Aware
Routing for Vehicular Ad Hoc networks in City Scenarios,” Mob. Netw. Appl., vol. 15,
pp. 36–60, Feb. 2010.
[66] J. Zhao, G. Cao, “VADD: Vehicle-Assisted Data Delivery in Vehicular Ad Hoc
Networks,” IEEE Trans. Veh. Technol. vol. 57, May 2008.
指導教授 吳曉光(Hsiao-kuang Wu) 審核日期 2012-7-27
推文 facebook   plurk   twitter   funp   google   live   udn   HD   myshare   reddit   netvibes   friend   youpush   delicious   baidu   
網路書籤 Google bookmarks   del.icio.us   hemidemi   myshare   

若有論文相關問題,請聯絡國立中央大學圖書館推廣服務組 TEL:(03)422-7151轉57407,或E-mail聯絡  - 隱私權政策聲明