1 MAC for Directional Antenna Redes Ad Hoc Sem Fio Prof. Marco Aurélio Spohn DSC UFCG

2 MAC for Directional Antenna Benefits of Directional Antenna More spatial reuse With omni-directional antenna, packets intended to one neighbor reaches all neighbors as well Node A Node B Node C Node A Node B Node C  Omni-directional transmissionDirectional transmission

3 MAC for Directional Antenna Benefits of Directional Antenna Increase “range”, keeping transmit power constant Reduce transmit power, keeping range comparable with omni mode Reduces interference, potentially increasing spatial reuse

4 More Spatial Reuse AB CD Omni-directional antenna While A is transmitting to B, C cannot transmit to D AB CD Directional antenna Both A and C can transmit simultaneously

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

6 Antenna Model In Omni Mode: Nodes receive signals with gain G o While idle a node stays in omni mode In Directional Mode: Capable of beamforming in specified direction Directional Gain G d (G d > G o )‏ Symmetry: Transmit gain = Receive gain

7 Multi-Beam Adaptive Array (MBAA) directional Antennas An MBAA antenna can successfully receive and transmit one or more overlapping packets at the same time by pointing its beams toward individual packet directions, while annulling all other unwanted directions.

8 Multi-Beam Adaptive Array (MBAA) directional Antennas Node d transmitting to nodes b and c at the same time.

9 Two “directional” models Steerable: can point to any direction (there is a minimum beam-width)‏ Switched-beam: many antenna components cover all directions (i.e., 4 antenna elements covering each 90 degrees)‏

10 Directional Packet Transmission A B B’s omni receive range D-O transmission AB B’s directional receive beam D-D transmission

11 MAC Designs for Directional Antenna Most proposals use RTS/CTS dialog They differ in how RTS/CTS are transmitted Omni-directional transmit: ORTS, OCTS Directional transmit: DRTS, DCTS Current proposals: ORTS/OCTS [Nasipuri’00] DRTS/OCTS [Ko’00] DRTS/DCTS [Choudhury’02]

12 ORTS/OCTS Sender sends omni-directional RTS Receiver sends omni-directional CTS Receiver also records direction of sender by determining the antenna on which the RTS signal was received with highest power level Similarly, the sender, on receiving CTS, records the direction of the receiver All nodes overhearing RTS/CTS defer transmissions Sender then sends DATA directionally to the receiver Receiver sends directional ACK

13 ORTS/OCTS – cont. Protocol takes advantage of reduction in interference due to directional transmission/reception of DATA All neighbours of sender/receiver defer transmission on receiving omni-directional RTS/CTS  spatial reuse benefit not realized

14 D-MAC Uses directional antenna for sending RTS, DATA and ACK in a particular direction, whereas CTS sent omni-directionally Directional RTS (DRTS) and Omni-directional CTS (OCTS)‏ RTS and CTS block antenna elements, making it possible to transmit/receive even though a node hear a RTS/CTS

15 DATA DRTS(B)‏ OCTS(B,C)‏ ACK A BCE D DRTS(D)‏ DATA ACK OCTS(D,E)‏ DMAC: DRTS/OCTS

16 Directional NAV Physical carrier sensing still omni- directional Virtual carrier sensing directional – directional NAV When RTS/CTS received from a particular direction, record the direction of arrival and duration of proposed transfer Channel assumed to be busy in the direction from which RTS/CTS received

17 Nodes overhearing RTS or CTS set up directional NAV (DNAV) for that Direction of Arrival (DoA)‏ X D Y C CTS Directional NAV (DNAV)‏

18 Nodes overhearing RTS or CTS set up directional NAV (DNAV) for that Direction of Arrival (DoA)‏ X Y Directional NAV (DNAV)‏ D C DNAV

19 Directional NAV (DNAV)‏ A B C θ DNAV D New transmission initiated only if direction of transmission does not overlap with DNAV, i.e., if (θ > 0)‏ RTS

20 Deafness: A MAC problem with directional antennas

21 ToneDMAC Use out-of-band tone acting as a feedback mechanism to all neighbors After the completion of every dialog, a communicating node transmits an out-of-band tone, OMNIDIRECTIONALLY! This tone helps other nodes to distinguish deafness from congestion If the tone is received from the intended receiver, a node deduces deafness and sets backoff timer appropriately

22 Acknowledgments Presentation adapted from the following sources:  Prasant Mohapatra, UC Davis,  :// ://  Katia Obraczka and J.J. Garcia-Luna-Aceves, UCSC 