MobiSteer : Using Steerable Beam Directional Antenna for Vehicular Network Access

1. MobiSteer: Using Steerable Beam Directional Antenna for Vehicular Network Access Vishnu Navda, AnandPrabhu Subramanian, KannonDhanasekaran, Andreas Timm-Giel, & Samir R. Das Originally Presented at MobiSys ‘07 Reviewed by Lauren Cohen on 2/12/08

2. Motivation • Wireless communication between moving vehicles and roadside access points • Three application types • Traffic safety and information • Mobile sensors • Internet access for vehicle occupants • Current connectivity is poor • Access point inter-arrival time >> median connection time • Link layer delivery rate ~80%

3. Design Goals • Directional antennas to increase gain and reduce interference • Steerable to maintain best link quality over longest duration • Optimize handoff between access points

4. Omnidirectional: • Directional:

5. Assumptions • APs use omnidirectional antennas • Vehicles access stationary network using one-hop links • …in other words, we’ll be considering applications #2 (mobile sensors) and #3 (Internet access)

6. MobiSteer Architecture Overview

7. Hardware/Software Setup • Multi-beam 2.4 GHz antenna • One omnidirectional beam • 16 45⁰ directional beams • Computer-controlled steering commands via serial • Data on captured packets logged to RF signature database during idle times

8. Operational Modes • Cached • Familiar territory • RF signature database used to determine optimal steering and AP selection • Databases can be downloaded from a server • Online • Previously untravelled routes • Scans all beams/channels and determines best based in SNR value

9. Data Collection • Link quality of received frames stored in RF signature database • Passive scanning • Monitors each beam/channel for any frame • Active probing • Periodic probe requests sent • Responses from APs recorded • Allows quicker sampling

10. Cached Mode Operation

11. Optimal AP and Beam Selection • Computes best AP and beam for every point in trajectory • Segments of length ∆ • RF signature database queried for SNR • Still need to deal with handoff latency

12. Optimal Handoff Algorithm • Estimate speed of vehicle from RF signature database to calculate handoff latency between possible APs • Use dynamic programming to select best APs to minimize latency between segments

13. Experimental Scenarios • Controlled scenarios • Single AP in empty parking lot • Multiple APs in apartment complex • Typically two within hearing range • All on same channel • In situ scenario • Existing APs along campus roadways • Again all on same channel • No actual data transferred

14. Experimental Results • Controlled scenario • Data collected on # of packets received and rate of transmission • MobiSteer greatly increased both • In situ scenario • MobiSteer improved average SNR and distance from which each beam could be heard • Alas, no actual data rate results

15. Online Mode Operation

16. Intricacies • Constantly uses active probing • Simple heuristic used to choose best AP/beam combination from probed data • Only steering considered, as handoff and AP selection covered in other literature

17. Experimental Results • Used controlled scenario setup • Again measured # of packets received and data rate • MobiSteer again improved both measurements over using an omnidirectional beam in online mode

18. Conclusions • MobiSteer provides good alternative to omnidirectional beams for vehicular networking by improving connectivity duration and data rate • Cached mode is superior to online mode • Future ideas • Use for localization of roadside APs • Interface with cellular modem networks for additional connectivity

19. Questions • What about communications between moving vehicles? • Can this be used to improve cellular networks as well? • How do we alleviate the overhead of active probing (since it’s necessary to build the RF signature database)?