Computer Networks COE 549 Directional Antennas for Ad-hoc Networks

1. Computer Networks COE 549Directional Antennas for Ad-hoc Networks Tarek Sheltami KFUPM CCSE COE http://faculty.kfupm.edu.sa/coe/tarek/coe549.htm

2. Outline • Introduction • IEEE 802.11 (CSMA/CA) overview • Motivations • Problem statement • Beamforming: Definition, types and advantages. • Basic DMAC • Challenges in Ad-hoc Networks using directional antennas. • Multi-Hop MAC (MMAC) • Beamforming with Power Control • Performance Evaluation

3. Ad Hoc Networks Typically assume Omnidirectional antennas A silenced node C B A D

4. Can Directional Antennas Improve Performance? Not possible using Omni C B A D

5. A Comparison

6. Motivation • Are directional antennas beneficial to medium access control in ad hoc networks ? • To what extent ? • Under what conditions ?

7. IEEE 802.11 • Sender sends Ready-to-Send (RTS) • Receiver responds with Clear-to-Send (CTS) • RTS and CTS announce the duration of the imminent dialogue • Nodes overhearing RTS/CTSdefer transmission for that duration • Network Allocation Vector (NAV) remembers duration

8. IEEE 802.11 RTS = Request-to-Send RTS A B C D E F

9. IEEE 802.11 RTS = Request-to-Send RTS A B C D E F NAV = 10

10. IEEE 802.11 CTS = Clear-to-Send CTS A B C D E F

11. IEEE 802.11 CTS = Clear-to-Send CTS A B C D E F NAV = 8

12. IEEE 802.11 • DATA packet follows CTS. Successful data reception acknowledged using ACK. DATA A B C D E F

13. IEEE 802.11 ACK A B C D E F

14. IEEE 802.11 • Channel contention resolved using backoff • Nodes choose random backoff interval from [0, CW] • Count down for this interval before transmission Random backoff Data Transmit backoff Wait A Random backoff Wait backoff Data Transmit B

15. Antenna Model 2 Operation Modes: OmniandDirectional A node may operate in any one mode at any given time

16. Antenna Model In Omni Mode: • Let us assume that nodes receive signals with Gain Go In Directional Mode: • Directional Gain Gd(Gd > Go)

17. Directional Communication • Received Power  (Tx Gain) * (Rx Gain) • Tx Gain = Transmit gain in the direction of receiver • Rx Gain = Receive gain in the direction of the transmitter A B C Convention: A link shown by overlapping beams along the line joining the transmitter and receiver. Nodes C, A form a link. C, B do not.

18. Directional Neighborhood Receive Beam Transmit Beam B A C • When C transmits directionally • Node A sufficiently close to receive in omni mode • Node C and A are Directional-Omni (DO) neighbors • Nodes C and B are not DO neighbors

19. Directional Neighborhood Transmit Beam Receive Beam A C B • When C transmits directionally • Node B receives packets from C only in directional mode • C and B are Directional-Directional (DD) neighbors

20. Antenna Beamforming • A technique in which the antenna pattern is switched (or steered) to a desired direction. • Two types: switched & steered beam. • Switched beam: • can select one from a set of predefined beams/antennas • Steered beam: • can direct the beam to the desired direction. (cost more but better performance) S S D D

21. Antenna Beamforming 1. Longer range Why? higher antenna gain in the desired direction Benefits: better connectivity and lower end-to-end delay 2. Higher spatial reuse Why? Reduced interference (narrower beamwidth) Benefits: increased capacity and throughput

22. Research Problem Identify the challenges encountered in MAC when beamforming antennas are used in Ad hoc networks and find the possible solutions of those problems in the literature.

23. Challenges in Ad-hoc Networks The two most impacted networking mechanisms as a result of using beamforming antennas are 1. Neighbor discovery identifies the one-hop neighbors 2. MAC provides distributed access to the channel

24. DMAC • DMAC is MAC with directional (beamforming) Antennas. • Two Operation Modes: OmniandDirectional A node may operate in any mode at any given time

25. Basic DMAC • Assumption: Location of neighbors is known. • Sender transmits Directional-RTS (DRTS) • A node listens omni-directionally when idle, • RTS received in Omni mode. • Receiver sends Directional-CTS (DCTS) • DATA, ACK transmitted and received directionally. • Operation is the same as 802.11 but with directional antennas and , and with the use of DNAV (directional NAV)!!

26. Basic DMAC Why DNAV (directional Network allocation Vector)? Asnwer: to combat directional exposed terminal problem.  increased spatial reuse and throughput D B E A C

27. Neighbor discovery New notions of neighbors: C A B Nodes A and B are OO neighbors. Nodes C and A are notOO Nodes C and B are notDO but DO neighbors. but DD neighbors.

28. Neighbor discovery • How to know the direction of the intended node? • CTS, DATA, ACK are much easier than RTS • Two possible ways: • From the AOA (Angle_of_Arrival ) of RTS and CTS. • Or from self location information included in RTS and CTS. • Directing the beam towards the destination for DRTS is challenging. Possible solutions: • Most MAC proposal assumes that this information is available by routing protocol. Each node know its location (by GPS or any location estimation method). • By AoA cashing of overheard packets (ex. Takai et al.[2]) • Circular DRTS • ORTS.

29. Neighbor discovery DMAC by Takai et al. [2] • Goals: send RTS directionally without location knowledge. • Employs DNAV • It is set according to AoA of the RTS/CTS dialog • Employs AoA cashing • The direction of neighbors is cashed based on the estimation of AoA of the overheard packets. • RTS is send directionally if the direction of the intended destination is available in the cash • RTS is sent omnidirectionally if the direction of the destination is not available in the AoA cash or CTS is not received after directional RTS transmission. • 3 to 4 times improvement in throughput compared to 802.11

30. Neighbor discovery • Extended transmission range • Beamforming enables longer range • Advantages: reduced # of hops, e2e delays and better connectivity (sparse networks) • Most of MAC proposals are not able to achieve the maximum possible range • OO, OD link only, • For Maximum range: • DD link • MMAC by Choudhury et al. [3]

31. Neighbor discovery MMAC by Choudhury et al.[3] - Knowledge of neighbors location is assumed - Goal: improve system performance (e2e delay and throughput) by extending the range of transmission (DD link). • Similar to basic DMAC + DD link • DD link can be established by multi-hop RTS (MHRTS) MHRTS C B DO Link MHRTS MHRTS DD Link D E A DRTS DCTS DATA

32. DO neighbors D E DD neighbors F C A B G Multi Hop RTS – Basic Idea A source-routes RTS to D through adjacent DO neighbors (i.e., A-B-C-D) When D receives RTS, it beamforms towards A, forming a DD link

33. A transmits RTS towards D MMAC protocol D E H F C A B G

34. MMAC protocol H updates DNAV D E DNAV H F C A B G

35. A transmits M-RTS to DO neighbor B MMAC protocol D E H F C A B G

36. B forwards M-RTS to C (also DO) MMAC protocol D E H F C A B G

37. A beamforms toward D – waits for CTS MMAC protocol D E H F C A B G

38. C forwards M-RTS to D MMAC protocol D E H F C A B G

39. D beamforms towards A – sends CTS MMAC protocol D E H F C A B G

40. MMAC protocol A & D communicate over DD link D E H F C A B G

41. MMAC protocol Nodes D and G similarly communicate D E H F C A B G

42. Problems in DMAC There are two main problems associated with DMAC: 1. New Hidden Terminals 2. Deafness

43. Problems in DMAC 1. New Hidden Terminals The node is hidden to the ongoing communication of other node when it didn’t hear the RTS/CTS transmission while it can interfere Collision Case 1.E is out of RTS/CTS range of A/C communication Case 2. Loss in channel state D E A D C The antenna of E is directed twards D RTS/CTS of A/C CANNOT be heard by E Collision A C E

44. Problems in DMAC2. Deafness • A node A is deaf with respect to nodes X,Z, if it cannotreceive from nodes X, Z due to beam direction while it can receive if it was in omni mode. • Effects: • Waste the capacity and energy (due unproductive control packets). • Introduce unfairness (increased backoff interval). RTS A B X DATA RTS X and Z do not know node A is busy. They keep transmitting RTSs to node A Z

45. Problems in DMAC • Hidden terminals and deafness are the two critical problems in DMAC. • Possible Solution: • Send RTS and/or CTS omnidirectionally while DATA/ACK are sent directionally. Example: DMAC by Ko et al. [5]

46. Problems in DMACDMAC by Ko et al. [5] - Knowledge of neighbors location is assumed - Multiple directional antennas for each nodes (switched beam) - Goal: increase spatial reuse while reducing control packet collisions. - DATA/ACK is directional - CTS is omnidirectional = OCTS - Two schemes for RTS: • Scheme 1 : DRTS (Directional RTS) only • Scheme 2 : ORTS/DRTS X D D A A S S B B Scheme 2 sends RTS in all directions (ORTS) if no antenna is blocked S can send to D but not to X Both schemes sendDRTS

47. Problems in DMACDMAC by Ko et al. (Cont.) Performance • Offers about 50% better throughput compared to IEEE 802.11, depends on Topology • Scheme 1 vs. Scheme 2: • Scheme 2 tries to reduce collision of control packets at the source while scheme 1 tries maximize spatial reuse in the vicinity of the source. • No significant performance difference

48. Problems with DMAC Possible Solution to unfairness caused by Deafness: ToneDMAC by Choudury et al. [6] • Goal: to reduce the effect of unfairness caused by Deafness by identify Deafness from congestion • RTS/CTS/DATA/ACK are sent directionally • After RTS/CTS/DATA/ACK exchange, A and B send their tones omnidirectinally. • neighboring nodes that overhear the tones will know that node A or B was engaged in communication. • Throughput is 2 times better than DMAC. • Fairness is improved. C will know that B was deaf. It will reset the backoff window to the minimum value. B_TONE A_TONE A B RTS DATA C B_TONE A_TONE

49. DMAC Tradeoffs • Disadvantages • Hidden terminals • Deafness • No DD Links • Benefits • Better Network Connectivity • Spatial Reuse

50. A A A A B B B B C C C C D D Exposed terminal problem No problem D D E E No problem Deafness Problem Impact of Beamforming on Ad-hoc Networking: MAC , Neighbor discovery, Route discovery Our Goal is to study the impact of Antenna beamforming on MAC. Examples: (Assume CSMA/CA ) Without beamforming With beamforming