A More Reliable and Efficient Usage of the 5,9 GHz Band for Cooperative ITS

Transcription

1 A More Reliable and Efficient Usage of the 5,9 GHz Band for Cooperative ITS Katrin Sjöberg, Halmstad University 4 th ETSI TC IT Workshop, February 7-9, 2012

2 Cooperative ITS Wireless communication is one of the key components to achieve cooperative ITS No communication no cooperation Focus herein is on ad hoctechnologies, i.e., ITS-G5/IEEE p Road traffic safety applications Broadcasted position messages and hazard warnings 2 Cooperative = acting together willingly for a common goal

3 ETSI ITS-G5 3 Profile standard of IEEE p Physical and medium access control layers ES Adapting p to the European spectrum Requirement on Decentralized Congestion Control (DCC) Management Applications Facilities Network&Transport Access Technologies Security

4 ETSI ITS-G5 MAC Carrier sense multiple access with collision avoidance(csma/ca) Senses the channel before transmitting Predetermined listening period Backoff procedure invoked if channel sensed busy Only invoked once due to broadcast communicatoin Problems: Cannot handle overloaded situations! Scalability problem leads to unbounded channel access delay and simultaneous transmissions 4 Medium access control (MAC) is responsible for scheduling transmissions and minimize interference.

5 Challenges for Cooperative ITS supporting road traffic safety [ad hoc, ad hoc, ad hoc, ad hoc, ad hoc, ad hoc, ad hoc, ad hoc, ad hoc] SCALABILITY The performance of the vehicular ad hoc network (VANET) is very much dependent on the number of ITS equipped vehicles within radio range! 5

6 STF395 The scalabilityissueleadtoinvestigationswithinwg4 in ETSI TC ITS Phase 1 (finished) Develop decentralized congestion control(dcc) for ITS-G5 Main goal: tokeepthe networkstable(controlthe networkload) Tools: restrictdata trafficinjected, usedifferent transfer rates and power control Co-existence between ITS-G5 and CEN DSRC Phase 2 (finished, described herein) InvestigateiftimeslottedMAC approachesfor VANET is able to cope with the scalability issue 6 STF = Specialist Task Force

7 Time-slotted MAC approaches Time Division Multiple Access Carrier Sense Multiple Access 7

8 STF395 phase 2 Two time slotted MAC approaches were investigated in the VANET scenario with broadcast data traffic Self-organizing time division multiple access (STDMA) Mobile slotted Aloha(MS-Aloha) Both STDMA and MS-Aloha have been benchmarked against CSMA/CA of ITS-G5 Two technical reports(tr) have been produced No DCC mechanisms have been investigated 8

9 STDMA Alreadyin commercialusein a collisionavoidancesystems for the shipping industry Automatic Identification System (AIS) NodesareALWAYS grantedchannelaccess regardlessofthe numberofnodesin the system More nodes than slots: The nodesselecta slotfor transmission at the same timeas the node situated furthest away from itself Channel access is always GUARANTEED! Can handle overloaded situations! Needssynchronization, position information and supports fixed packet sizes determined by the transfer rate 9

10 MS-Aloha Specifically designed for VANET Each node appends to its frames a frame information(fi) field containing info about the perceived slot allocation Prevention of URBAN HIDDEN TERMINALS 3-hop coordination to MINIMIZE INTERFERENCE Implicit ACKNOWLEDGEMENTS also in broadcast More nodes than slots: Slot reuse based on received power levels Channel access is always GUARANTEED! Pre-emption: leveraged for COEXISTENCE with CSMA/CA Needs synchronization and support fixed packet sizes determined by the transfer rate 10

11 STDMA simulations 2 Hz/800B, 25 dbm Highway scenario 12 lanes In the blue curve of CSMA, it is not restricted to 25% channel load, as usually foreseen in ITS-G5. STDMA can reach more than 4 times better performance than CSMA! The red curve shows CSMA result for 25% channel load. 44 vehicles/lane/km ~ 260 vehicles within 500 m from every TX 11

12 MS-Aloha simulations Urban scenario 4 Hz/800B, 20 dbm Lightly loaded network: No benefits expected from DCC If PDR is restricted to the reception by nodes in the centre of crossroads, the difference gets higher: ~30% CSMA/CA and ~90% MS-Aloha at 0-20metres Here interference by HT is heavier and reception is safety critical HT prevention is coupled with scalability under heavy congestion (additional simulations) 12

13 Conclusions STDMA and MS-Aloha utilize the frequency band more efficiently due to scheduling STDMA and MS-Alohacanloadthe channelto100% and aboveif necessary STDMA and MS-Aloha provide a higher reliability Transmissions are synchronized and scheduled in space STDMA and MS-Aloha are scalable Allows coordinated simultaneous transmissions CSMA needs congestion control methods Candegradeperformanceofroad trafficsafetyapplicationsin the long run A time-slottedmac approach is a viablesolution for a second generation of ITS equipped vehicles A migration pathfrom CSMA toa time-slottedmac protocolis not yet specified 13

14 STF395 phase 2 information Twotechnicalreports(TR) havebeenproducedand they are publicly available at TR STDMA recommendedparameters and settings for cooperative ITS TR Performance evaluation of self-organizing TDMA as medium access control method applied to ITS STF395 phase 2 members Katrin Sjöberg, Halmstad University, Sweden (STF leader) Riccardo Scopigno, ISMB, Italy Dieter Smely, Kapsch, Austria Elisabeth Uhlemann, Halmstad University, Sweden 14

15 Thankyou for your attention Questions? 15