Using Directional Antennas for Medium Access Control in Ad Hoc Networks

1. Using Directional Antennas for Medium Access Control in Ad Hoc Networks MOBICOM 2002 R. Roy Choudhury et al. 2002.10.16 Presented by Hyeeun Choi

2. Contents • Introduction • Related Works • Preliminaries • Basic Directional MAC (DMAC) Protocol • Multi-Hop RTS MAC (MMAC) • Performance Evaluation • Future Work • Conclusion

3. Introduction • The Problem of utilizing directional Antennas to improve the performance of ad hoc networks is non-trivial • Pros • Higher gain (Reduced interference) • Spatial Reuse • Cons • Potential possibility to interfere with communications taking place far away

4. Omni-directional Antennas Silenced Node B D S C A

5. Directional Antennas Not possible using Omni B D S C A

6. Related Works • MAC Proposals differ based on • How RTS/CTS transmitted (omni, directional) • Transmission range of directional antennas • Channel access schemes • Omni or directional NAVs • Gain of directional antennas is equal to the gain of omni-directional antennas

7. Preliminaries (1/2) • Antenna Model • Two Operation modes : Omni & Directional • Omni Mode: • Omni Gain = Go • Idle node stays in Omni mode. • Directional Mode: • Capable of beamforming in specified direction • Directional Gain = Gd (Gd > Go)

8. Physical Carrier Sense Physical Carrier Sensing IEEE 802.11 DCF – RTS/CTS access scheme Preliminaries (2/2) • IEEE 802.11 Virtual Carrier Sensing

9. Problem Formulation • Using directional antennas • Spatial reuse • Possible to carry out multiple simultaneous transmissions in the same neighborhood • Higher gain • Greater transmission range than omni-directional • Two distant nodes can communicate with a single hop • Routes with fewer hops

10. Basic DMAC Protocol (1/2) • Channel Reservation • A node listens omni-directionally when idle • Sender transmits Directional-RTS (DRTS) using specified transceiver profile • Physical carrier sense • Virtual carrier sense with Directional NAV • RTS received in Omni mode (only DO links used) • Receiver sends Directional-CTS (DCTS) • DATA,ACK transmitted and received directionally

11. Basic DMAC Protocol (2/2) • Directional NAV (DNAV) Table • Tables that keeps track of the directions towards which node must not initiate a transmission H ε = 2ß + Θ If Θ> 0 , New transmission can be initiated RTS 2*ß θ ε E B DNAV CTS C

12. Data RTS Problems with Basic DMAC (1/4) • Hidden Terminal Problems due to asymmetry in gain • A does not get RTS/CTS from C/B C B A

13. Problems with Basic DMAC (2/4) • Hidden Terminal Problems due to unheard RTS/CTS D B C A

14. Problems with Basic DMAC (3/4) • Shape of Silence Regions Region of interference for directional transmission Region of interference for omnidirectional transmission

15. Problems with Basic DMAC (4/4) • Deafness Z RTS A B DATA RTS X X does not know node A is busy. X keeps transmitting RTSs to node A

16. MMAC Protocol (1/3) • Attempts to exploit the extended transmission range • Make Use of DD Links • Direction-Direction (DD) Neighbor C A A and C can communication each other directly

17. DO neighbors DD neighbors MMAC Protocol (2/3) • Protocol Description : Multi-Hop RTS • Based on Basic DMAC protocol RTS C B G T D F A DATA S R

18. MMAC Protocol (3/3) • Channel Reservation • Send Forwarding RTS with Profile of node F Fowarding RTS C B G T D F A DATA S R

19. Performance Evaluation (1/6) • Simulation Environment • Qualnet simulator 2.6.1 • Beamwidth :45 degrees • Main-lobe Gain : 10 dBi • 802.11 transmission range : 250meters • DD transmission range : 900m approx • Two way propagation model • Mobility : none

20. Performance Evaluation (2/6) High Spatial Reuse Aggregate Throughput (Kbps) IEEE 802.11 : 1189.73Basic DMAC : 2704.18 D E F C A B High Directional Interference Hidden terminal Problem Aggregate Throughput (Kbps) IEEE 802.11 : 1194.81Basic DMAC : 1419.51 D C A B F E

21. Performance Evaluation (3/6) • Aligned Routes 150m

22. Performance Evaluation (4/6) • Less aligned Routes

23. Performance Evaluation (5/6) • Randomly Chosen Routes

24. Performance Evaluation (6/6) • Random Topology

25. Future Work • Design of directional MAC protocols that incorporate transmit power control • New protocols that rely less on the upper layers for beamforming information • Impact of directional antennas on the performance of routing protocols

26. Conclusion • Directional MAC protocols show improvement in aggregate throughput and delay • But not always • Performance dependent on topology • Random topology aids directional communication • MMAC outperforms DMAC & 802.11 • 802.11 better in some scenarios

27. Related Works • Nasipuri et al • Assume that the gain is equal to the gain of omni-directional antennas • Ko et al • A node may transmit in directions that do not interfere with ongoing transmission • Bandyopahyay et al • Present another MAC which uses additional messages to inform neighborhood nodes about ongoing communications • Takai et al • Directional virtual Carrier Sensing (DVCS)

28. Extra Slides • Gain of Antenna • Used to quantify the directionality of an antenna • Relative power in one direction compared to an omni-directional antenna