August 8, 2015 Computer Networks COE 549 Directional Antennas for Ad- hoc Networks Tarek Sheltami KFUPM CCSE COE

2 Outline  Introduction  IEEE (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

Ad Hoc Networks 8/8/20153 A silenced node A B C D Typically assume Omnidirectional antennas

Can Directional Antennas Improve Performance? A B C D Not possible using Omni 8/8/20154

A Comparison IssuesOmniDirectional Spatial ReuseLow High Connectivity LowHigh Interference OmniDirectional Cost & Complexity LowHigh 8/8/20155

Motivation Are directional antennas beneficial to medium access control in ad hoc networks ? –To what extent ? –Under what conditions ? 8/8/20156

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/CTS defer transmission for that duration –Network Allocation Vector (NAV) remembers duration IEEE /8/20157

CFABED RTS RTS = Request-to-Send IEEE /8/20158

CFABED RTS RTS = Request-to-Send IEEE NAV = 10 8/8/20159

CFABED CTS CTS = Clear-to-Send IEEE /8/201510

CFABED CTS CTS = Clear-to-Send IEEE NAV = 8 8/8/201511

CFABED DATA DATA packet follows CTS. Successful data reception acknowledged using ACK. IEEE /8/201512

CFABED ACK IEEE /8/201513

IEEE Channel contention resolved using backoff –Nodes choose random backoff interval from [0, CW] –Count down for this interval before transmission Random backoff Data Transmit Random backoff Wait backoff Data Transmit Wait A B 8/8/201514

Antenna Model 2 Operation Modes: Omni and Directional A node may operate in any one mode at any given time 8/8/201515

Antenna Model In Omni Mode: Let us assume that nodes receive signals with Gain G o In Directional Mode: Directional Gain G d (G d > G o ) 8/8/201516

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. 8/8/201517

B Directional Neighborhood A 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 C Transmit Beam Receive Beam 8/8/201518

Directional Neighborhood A B C When C transmits directionally Node B receives packets from C only in directional mode C and B are Directional-Directional (DD) neighbors Transmit Beam Receive Beam 8/8/201519

A technique in which the antenna pattern is switched (or steered) to a desired direction. Two types: switched & steered beam. 8/8/ Antenna Beamforming - Steered beam: can direct the beam to the desired direction. (cost more but better performance) - Switched beam: can select one from a set of predefined beams/antennas S D S D

21 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 8/8/2015 Antenna Beamforming

22 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. Research Problem 8/8/2015

23 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 Challenges in Ad-hoc Networks 8/8/2015

24 DMAC is MAC with directional (beamforming) Antennas. Omni Two Operation Modes: Omni and Directional A node may operate in any mode at any given time DMAC 8/8/2015

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 but with directional antennas and, and with the use of DNAV (directional NAV)!! 8/8/201525

26 Basic DMAC Why DNAV (directional Network allocation Vector)? Asnwer: to combat directional exposed terminal problem.  increased spatial reuse and throughput A C B E D 8/8/2015

27 Neighbor discovery New notions of neighbors: B A OO Nodes A and B are OO neighbors. OO Nodes C and A are not OO DO Nodes C and B are not DO C DO but DO neighbors. DD but DD neighbors. Transmit antenna Receive antenna OOOmni DODir.Omni DDDir 8/8/2015

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. 8/8/2015

29 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 /8/2015 Neighbor discovery

30 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] 8/8/2015 Neighbor discovery

31 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) D B A C E DO Link DD Link MHR TS DATA MHR TS DRTSDCTS 8/8/2015 Neighbor discovery

Multi Hop RTS – Basic Idea A B C DE F G DO neighbors DD neighbors 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 8/8/201532

MMAC protocol B C DE F G H A transmits RTS towards D A 8/8/201533

MMAC protocol B C DE F G H DNAV A H updates DNAV 8/8/201534

MMAC protocol B C DE F G H A transmits M-RTS to DO neighbor B A 8/8/201535

MMAC protocol C DE F G H A B forwards M-RTS to C (also DO) B 8/8/201536

MMAC protocol B C DE F G H A beamforms toward D – waits for CTS A 8/8/201537

MMAC protocol B DE F G H A C forwards M-RTS to D C 8/8/201538

MMAC protocol B E F G H C A D beamforms towards A – sends CTS D 8/8/201539

MMAC protocol B E F G H C D A A & D communicate over DD link 8/8/201540

MMAC protocol B E F H C A Nodes D and G similarly communicate G D 8/8/201541

42 Problems in DMAC There are two main problems associated with DMAC: 1. New Hidden Terminals 2. Deafness 8/8/2015

43 Case 1. E is out of RTS/CTS range of A/C communication A C E A E D The node is hidden to the ongoing communication of other node when it didn’t hear the RTS/CTS transmission while it can interfere Case 2. Loss in channel state D C Collision The antenna of E is directed twards D RTS/CTS of A/C CANNOT be heard by E Problems in DMAC 1. New Hidden Terminals 8/8/2015

44 A node A is deaf with respect to nodes X, Z, if it cannot receive 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 AB X Z DATA X and Z do not know node A is busy. They keep transmitting RTSs to node A Problems in DMAC 2. Deafness 8/8/2015

45 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] Problems in DMAC 8/8/2015

46 - 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 A B S D X S can send to D but not to X Both schemes send DRTS D S Scheme 2 sends RTS in all directions (ORTS) if no antenna is blocked A B Problems in DMAC DMAC by Ko et al. [5] 8/8/2015

47 Performance Offers about 50% better throughput compared to IEEE , 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 Problems in DMAC DMAC by Ko et al. (Cont.) 8/8/2015

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. A_TONE AB C DATA B_TONE RTS A_TONE 8/8/2015

DMAC Tradeoffs Benefits –Better Network Connectivity –Spatial Reuse Disadvantages –Hidden terminals –Deafness –No DD Links 8/8/201549

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 A B CDA B CD Exposed terminal problem No problem A B A B E No problem Deafness Problem C D E C D 8/8/201550

51 Beamforming with power control Power control by it self can achieve higher performance –Reduce interference –Lower energy consumption Power control + beamforming can substantially improve the performance No power control or beamforming Area = A r/2 r Power control only Area = A/4 r/2 Beamforming only Area = A/6 Power control or beamforming Area = A/144 !!! A rough comparison of relative interference reduction, assuming 10 degrees directional beamwidth, and r 4 propagation. [1] 8/8/2015

Performance Simulation –Qualnet simulator –Constant Bit Rate (CBR) traffic –Packet Size – 512 Bytes – transmission range = 250meters –DD transmission range = 900m approx –Beamwidth = 60 degrees –Channel bandwidth 2 Mbps –Mobility - none 8/8/201552

MMAC Hop Count Max MMAC hop count = 3 –Too many DO hops increases probability of failure of RTS delivery –Too many DO hops typically not necessary to establish DD link A B C DE F G DO neighbors DD neighbors 8/8/201553

MMAC - Concerns Neighbor discovery overheads may offset the advantages of MMAC High traffic – lower probability of RTS delivery Multi-hop RTS may not reach DD neighbor due to deafness or collision No more than 3 DO links is used for each DD link 8/8/201554

Aligned Routes in Grid 8/8/201555

Unaligned Routes in Grid 8/8/201556

“Random” Topology 8/8/201557

“Random” Topology: delay 8/8/201558

Nodes moving out of beam coverage in order of packet-transmission-time –Low probability Antenna handoff required –MAC layer can cache active antenna beam –On disconnection, scan over adjacent beams –Cache updates possible using promiscuous mode –Evaluated in [RoyChoudhury02_TechReport] Mobility 8/8/201559

Broadcast Several definitions of “broadcast” –Broadcast region may be a sector, multiple sectors –Omni broadcast may be performed through sweeping antenna over all directions [RoyChoudhury02_TechReport] A Broadcast Region 8/8/201560

61 References 1.Basagni, M. Conti, S. Giordano, I. Stojmenovic, eds, Mobile Ad Hoc Networking, IEEE Press/Wiley, August M. Takai, et al., “Directional virtual carrier sensing for directional antennas in mobile ad hoc networks”, ACM MobiHoc 2002, pp 39-46, June R.R. Choudhury, X. Yang, N.H. Vaidya, and R. Ramanathan, “Using directional antennas for medium access control in ad hoc networks”, MOBICOM 2002, pp 59-70, September N.S. Fahmy, T.D. Todd and V. Kezys, “Ad hoc networks with smart antennas using IEEE based protocols”, IEEE ICC 2002, pp , May Y-B Ko, V. Shankarkumar and N.H. Vaidya, “Medium access control protocols using directional antennas in ad hoc networks”, IEEE INFOCOM 2000, pp Choudhury, R.R.and Vaidya, N.H., “Deafness: a MAC problem in ad hoc networks when using directional antennas” ICNP 2004, Proceedings of the 12th IEEE International Conference on Network Protocols, pp: , 2004