A Reliable MAC Protocol for Broadcast VANETs

Transcription

1 A Reliable MAC Protocol for Broadcast VANETs Cristina Rico García, Andreas Lehner, Thomas Strang Institute of Communications and Navigation Folie Cristina Rico García. Institute for Communication and Navigation

2 Index MAC layers in Broadcast Vehicular Ad-hoc Networks (VANETs) Special constraints in highly dynamic VANETs Cell-based Orientation-aware MANET Broadcast MAC layer: COMB Conclusions and Outlook Folie

3 MAC layer in highly dynamic Broadcast VANETs MAC layer: Medium Access Control Layer. No infrastructure available (ad-hoc) Distributed protocol. No information about the receivers Broadcast. Highly dynamic network Unknown network configuration. Hi, I'm Aircraft C and my position is X, Y, Z Hi, I m Aircraft AXX, and my position is X,Y, Z Hi, I'm Aircraft AE and my position is X, Y, Z Automatic Dependence Surveillance- Broadcast CarCar Automatic Identification System Folie

4 MAC layers in Broadcast Vehicular Ad-hoc Networks (VANETs) Special constraints in highly dynamic VANETs Cell-based Orientation-aware MANET Broadcast MAC layer: COMB Conclusions and Outlook Folie

5 MAC layers for Broadcast VANETs Protocol group Based on CSMA/CA Based on TDMA or CDMA Disadvantages Assume static networks and/or a priori knowledge of the network Hidden and exposed terminal problem No optimal MAC layer protocol for broadcast VANETs Folie

6 SOTDMA. Self Organized Time Division Multiple Access Folie

7 MAC layers in Broadcast Vehicular Ad-hoc Networks (VANETs) Special constraints in highly dynamic VANETs Cell-based Orientation-aware MANET Broadcast MAC layer: COMB Conclusions and Outlook Folie

8 Problems in highly dynamic VANETs. B B Nodes in different geographical zones produce the Hidden terminal problem. A B C Moving nodes may produce massive collisions due to contention. Folie

9 MAC layers in Broadcast Vehicular Ad-hoc Networks (VANETs) Special constraints in highly dynamic VANETs Cell-based Orientation-aware MANET Broadcast MAC layer: COMB Conclusions and Outlook Folie

10 Some extra information that we could use Periodic beacons Time (GNSS) Location (GNSS+Map) Direction (GNSS) ORIENTATION Speed (GNSS) Folie 0

11 A Solution for the hidden terminal problem A B D C Time C Divide the world map in hexagonal cells. A channel is assigned to each cell. A B D Intracell: SOTDMA Intercell: CDMA location awareness Folie

12 Number of Necessary Codes R Minimum number of codes that avoid the hidden terminal problem Relation cell size minimum tx range. In a range of at least R there must not be any repeated code. All the nodes inside a cell must hear each other. The range should be at least the maximum diameter of a cell = R Folie 0

13 Conclusions and Outlook Conclusions No optimal MAC protocol for high dynamic broadcast VANETs. COMB uses additional information. It avoids all collisions in ideal conditions. Outlook Analyze the effect of the near-far problem in the protocol. Optimization of the cells dimension. Folie

14 Thank you for your attention Questions? Folie

15 BACKUP Folie

16 Nodes crossing to a new cell C B C A Time When two nodes cross to a same target cell, they send with the same code, and they might try to access the SOTDMA structure in the same time slot. COLLISION. Folie

17 Nodes crossing to a new cell D C D C Code F F Code A B A B Code E E E F Code Past Slot Frame Present Slot Frame Future Slot Frame Time The nodes infer to which cell they are going to cross from their speed, direction and position information. They observe at least two target SOTDMA frames, and one frame of the neighbors. A F E B D C They reserve the first free slot in the target cell according to their cell priority. Folie

18 Number of Necessary Codes a =sqrt()r R b = R/ All the nodes inside a cell can see each other, they are in range : The range should be at least the maximum diameter of a cell = R. A map can be painted with four colors. How many colors (codes) do we need? In a range of at least R there shouldn t be any repeated code Folie 0

19 Consideration about the range: Minium-maximum range Maximum range for codes: R(+sqrt()) Minimum range: R+frames(maximum speed) Margin: 0,R-frames = few Kilometers (%) Folie