1、 ETSI TR 102 862 V1.1.1 (2011-12) Intelligent Transport Systems (ITS); Performance Evaluation of Self-Organizing TDMA as Medium Access Control Method Applied to ITS; Access Layer Part Technical Report ETSI ETSI TR 102 862 V1.1.1 (2011-12) 2Reference DTR/ITS-0040021 Keywords ITS, MAC, TDMA ETSI 650 R
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6、copyright and the foregoing restriction extend to reproduction in all media. European Telecommunications Standards Institute 2011. All rights reserved. DECTTM, PLUGTESTSTM, UMTSTMand the ETSI logo are Trade Marks of ETSI registered for the benefit of its Members. 3GPPTM and LTE are Trade Marks of ET
7、SI registered for the benefit of its Members and of the 3GPP Organizational Partners. GSM and the GSM logo are Trade Marks registered and owned by the GSM Association. ETSI ETSI TR 102 862 V1.1.1 (2011-12) 3Contents Intellectual Property Rights 5g3Foreword . 5g3Introduction 5g31 Scope 7g32 Reference
8、s 7g32.1 Normative references . 7g32.2 Informative references 7g33 Definitions, symbols and abbreviations . 12g33.1 Definitions 12g33.2 Symbols 12g33.3 Abbreviations . 13g34 Introduction 14g34.1 Medium access control in VANETs . 14g34.2 Requirements for road traffic safety applications . 15g34.3 Hid
9、den terminal problem 16g35 CSMA. 19g35.1 Introduction 19g35.2 Channel access procedure and parameters . 19g35.3 Simultaneous transmissions 21g35.4 Summary 21g36 Motivations for time slotted MAC approaches 21g37 Time slotted MAC approaches . 22g37.1 Introduction 22g37.2 STDMA 24g37.2.1 Introduction.
10、24g37.2.1.1 The AIS system . 24g37.2.1.2 Position reports . 25g37.2.1.3 Overhead to run the STDMA algorithm 26g37.2.2 Parameters. 26g37.2.3 Channel access procedure . 28g37.2.3.1 Initialization 28g37.2.3.2 Network entry . 28g37.2.3.3 First frame. 29g37.2.3.4 Continuous operation 29g37.2.3.5 Summary.
11、 30g37.2.4 Simultaneous transmissions 30g37.2.5 Summary . 32g37.3 MS-Aloha . 32g37.3.1 Introduction. 32g37.3.2 Channel access procedure . 33g37.3.2.1 Memory refresh . 34g37.3.2.2 Solutions against protocol overheads 35g37.3.3 Simultaneous transmissions 36g37.3.3.1 Prevention of hidden terminals and
12、unintentional slot re-use . 37g37.3.3.2 Slot reuse at four-hop distance 37g37.3.3.3 Mechanisms for forced slot re-use 38g37.3.3.4 Dynamic mechanisms for the forced slot re-use . 38g37.3.3.5 Pre-emption . 39g37.3.4 Parameters. 40g37.3.5 Summary . 40g37.4 Other time slotted approaches 42g3ETSI ETSI TR
13、 102 862 V1.1.1 (2011-12) 48 Time synchronization . 44g38.1 Introduction 44g38.2 Motivation for GNSS synchronization . 44g38.3 From the accuracy of GNSS synchronization to the required Guard-Times 45g38.4 Fallback solution in absence of GNSS . 47g39 Migration and coexistence in road traffic scenario
14、s . 47g39.1 Introduction 47g39.2 Backward compatibility . 47g39.3 Coexistence with CSMA 48g310 Executive summary 48g3Annex A: Bibliography 50g3History 51g3ETSI ETSI TR 102 862 V1.1.1 (2011-12) 5Intellectual Property Rights IPRs essential or potentially essential to the present document may have been
15、 declared to ETSI. The information pertaining to these essential IPRs, if any, is publicly available for ETSI members and non-members, and can be found in ETSI SR 000 314: “Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in respect of ETSI standards“,
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17、in ETSI SR 000 314 (or the updates on the ETSI Web server) which are, or may be, or may become, essential to the present document. Foreword This Technical Report (TR) has been produced by ETSI Technical Committee Intelligent Transport System (ITS). Introduction By introducing wireless communications
18、 between vehicles and between vehicles and road infrastructure or other fellow road users such as pedestrians and bicyclists, the road environment will become safer and potentially more environmentally friendly. Many different cooperative intelligent transport systems (ITS) applications have been su
19、ggested for the vehicular environment, both for road traffic safety and efficiency. Depending on application area, the resulting communication requirements are quite diverse. Different wireless access technologies have different features and different benefits and all cooperative ITS applications su
20、ggested for the vehicular environment cannot be solved with one single technology due to resource constraints and diverse requirements. Vehicular ad hoc networks (VANETs) based on, e.g. IEEE 802.11p i.2, will be used for road traffic safety applications, i.1, i.2. However, other wireless carriers su
21、ch as cellular technology (e.g. 3G, LTE) will also be used to support different cooperative ITS applications in general. The major difference between VANETs and cellular technology is that there is no central controller in the former. The central controller usually has perfect knowledge about the no
22、des within range and it can distribute and optimize the available resources. However, in cellular technology there is a central controller in the form of a base station present, otherwise communication is not possible. VANETs do not need coverage by base stations - instead if there is someone to com
23、municate with, communication will take place directly in between any two nodes within range of each other. The ad hoc structure is advantageous, since it does not require coverage by base stations, but without a central control mechanism, problems with scalability may arise. Due to the lack of a cen
24、tral coordinator, all nodes typically transmit on a common frequency channel. This frequency channel, called the control channel, is known a priori to all nodes. For road traffic safety applications, this channel is where the most important data will be transmitted. To facilitate additional cooperat
25、ive ITS applications with higher bandwidth requirements, two or more service channels are also available. However, the control channel is the core of a VANET. Many emerging road traffic safety applications will be based purely on broadcast communication, i.3, i.e. one-to-many. Due to the broadcast c
26、ommunication, the assurance of sufficient reliability is limited. A sender does not know if the transmitted data has arrived at the intended receiver because no acknowledgments of successful reception are possible in broadcast mode (receivers cannot send an acknowledgment to the sender since the num
27、ber of intended receivers is not known and this may flood the network). One way to increase the reliability in broadcast mode is instead to repeat the same message several times. Ultimately, cooperative ITS applications for enhancing road traffic safety should be designed taking the characteristics
28、of a VANET into account. These characteristics can be summarized by: a decentralized network topology, a common control channel and broadcast as the preferable communication mode. The utilization of the control channel should be carefully designed so it can be used to its maximum. The medium access
29、control (MAC) protocol schedules access to the shared control channel. A MAC protocol suitable for road traffic safety applications in VANETs should be decentralized such that it functions without a central controller, it should support broadcast such that channel access is fair and predictable for
30、all participating nodes and it should aim to minimize interference between transmitters to maximize scalability. Further, as road traffic safety typically involves interaction with vehicles located in the vicinity of each other, the MAC method should maximize the packet reception probability for the
31、 closest neighbouring nodes. ETSI ETSI TR 102 862 V1.1.1 (2011-12) 6ETSI has standardized a VANET protocol based on a profile of IEEE 802.11p i.2, called ITS-G5 i.1, which uses the MAC method carrier sense multiple access (CSMA). CSMA has some of the desired properties, i.e. it is decentralized and
32、aims at minimizing interference between any transmitters. However, it does not necessarily maximize the packet reception probability for the closest neighbouring nodes or provide fair and predictable channel access for broadcast. The present document therefore scrutinize time slotted MAC protocols,
33、to determine if these can utilize the common control channel more efficiently than the current proposed MAC from IEEE 802.11p i.2. ETSI ETSI TR 102 862 V1.1.1 (2011-12) 71 Scope The present document describes the use of time slotted MAC algorithms in VANETs. Two specific MAC methods, self-organizing
34、 time division multiple access (STDMA) and mobile slotted Aloha (MS-Aloha), are described in detail, not excluding other time slotted approaches. Time slotted approaches are suitable for road traffic safety applications as the maximum delay is predictable and channel access can be made fair among al
35、l participating nodes even during broadcast. However, time slotted approaches do require synchronization between nodes to build a common framing structure for transmissions, something that is not needed for non-time slotted approaches, e.g. CSMA as used by ITS G5 i.1. In the literature of time slott
36、ed MAC protocols for VANETs, synchronization is provided by a global navigation satellite system (GNSS) such as the global positioning system (GPS) or Galileo. The present document also describes the GNSS synchronization issue as well as proposals for dealing with synchronization when the GNSS signa
37、l is absent or weak, which can occur in urban environments and tunnels. Further, time slotted approaches use fixed-length time slots for transmissions, implying that packet lengths are fixed. However, as the physical (PHY) layer suggested for VANETs offers several transfer rates, this means that dif
38、ferent packet sizes can be obtained in the fixed time slots. The analysis of the most preferable configuration in this context constitutes the second technical topic covered by the present document. Finally the present document also deals with the coexistence between CSMA and time slotted MAC approa
39、ches nodes. The backward compatibility and coexistence are of crucial importance since the first generation of VANETs will use CSMA technology. This represents the third and final topic of the present document. 2 References References are either specific (identified by date of publication and/or edi
40、tion number or version number) or non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the reference document (including any amendments) applies. Referenced documents which are not found to be publicly available in the expected loc
41、ation might be found at http:/docbox.etsi.org/Reference. NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee their long term validity. 2.1 Normative references The following referenced documents are necessary for the application of the pres
42、ent document. Not applicable. 2.2 Informative references The following referenced documents are not necessary for the application of the present document but they assist the user with regard to a particular subject area. i.1 ETSI ES 202 663: “Intelligent Transport Systems (ITS); European profile sta
43、ndard for the physical and medium access control layer of Intelligent Transport Systems operating in the 5 GHz frequency band“. i.2 IEEE 802.11p: 2010: “IEEE Standard of Information Technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Spec
44、ific requirements; Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications; Amendment 6: Wireless Access in Vehicular Environments“. i.3 ETSI TR 102 638: “Intelligent Transport Systems (ITS); Vehicular Communications; Basic Set of Applications; Definitions“. ETSI ET
45、SI TR 102 862 V1.1.1 (2011-12) 8i.4 IEEE 802.11: 2007: “IEEE Standard of Information Technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements; Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Spe
46、cifications“. i.5 ETSI TS 102 637-3: “Intelligent Transport Systems (ITS); Vehicular Communications; Basic Set of Applications; Part 3: Specifications of Decentralized Environmental Notification Basic Service“. i.6 ETSI TS 102 637-2: “Intelligent Transport Systems (ITS); Vehicular Communications; Ba
47、sic Set of Applications; Part 2: Specification of Cooperative Awareness Basic Service“. i.7 ETSI TR 101 683: “Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; System Overview“. i.8 ETSI TS 102 687: “Intelligent Transport Systems (ITS); Decentralized Congestion Control Mechanisms for Intellig
48、ent Transport Systems operating in the 5 GHz range; Access layer part“. i.9 ITU-R Recommendation M.1371-1:2001: “Technical characteristics for universal shipborne automatic identification system using time division multiple access in the VHF maritime mobile band“. i.10 F. Tobagi, and L. Kleinrock, “
49、Packet switching in radio channels: Part II - the hidden terminal problem in carrier sense multiple access and the busy tone solution“, IEEE Transactions on Communications, vol. 23, no. 12, pp. 1417-1433, December, 1975. i.11 H. Lans, “Position Indicating System“, US patent 5,506,587, issued 1996. i.12 ITU-T std G.811, G812, G.813; series G: “Transmission systems and media digital transmission systems - digital networks - design objectives for digital networks“. i.13 S. Ganeriwal, R. Kumar, and M.B. Srivastava, “Timing-sync protocol f
