Directional Antennas for Ad-Hoc Networks

1. Directional Antennas for Ad-Hoc Networks Andy Collins UW CSE 802.11 Seminar 23 July 2003

2. Problem and opportunity • Ad-hoc networks are limited in capacity • More power = fewer hops but more interference • Technology will (soon) allow steerable antennas • Using phased arrays • High gain (10X to 1000X) • Electronically steerable (no physical rotation) • Less interference to nodes in other directions • Can we break the capacity bounds? • More gain ?= fewer hops and less interference

3. Major challenges • Neighbor discovery • Goal is to use hops that require directional antennas at one or both ends • How does sender know where to aim • How does receiver know to aim (and where)? • Hidden transmitters • Cannot hear nearby nodes pointing away • More interesting than geography because it changes fast • Must rethink MAC protocol

4. Antenna basics • Antennas are governed by two properties: • Efficiency: fraction of input power not lost as heat • Gain: concentration of energy in one direction • We focus only on gain • Can only reshape energy • The sum of power over the sphere must stay constant • Implies a direct tradeoff between gain and beamwidth • Some energy always ends up in “sidelobes” off the main beam

5. Antenna terminology • Gain is typically measured in decibels (dB) • True dB is a unitless ratio describing gain or loss through a component • Also used for amplifiers and attenuators, and path loss • Logarithmic: dB = 10 log10 (power out/power in) • 3dB is approx. twice as much power, 10dB exactly 10X • Antenna gain is figured relative to isotropic (dBi) • An ideal omnidirectional radiator • This is the gain in the center of the beam

6. A gain antenna

7. Phased arrays • A phased array is a group of individual antennas grouped and fed together • Usually each element is omnidirectional • Pattern is determined by geometry and phase delay • Magic is in varying phase delay to each element to steer the beam without moving the elements • Long history of military use for radar • Can also use as omni receiver, and sense arrival direction • Very different from other sorts of directional antennas you’ve probably seen • Yagis, Log-periodic arrays, parabolic dishes

8. Modeling gain antennas • Both papers use a simplified model • Beam is uniform gain within its beamwidth • Single, uniform sidelobe • Can calculate gains and beamwidth • Max width a function of gain • Sidelobe gain a function of gain and selected beamwidth

9. Meanwhile, back in computer science… • Step 1: directional antennas for interference only • Use omni antennas for routing and MAC • Step 2: directional transmit / omni receive • Use geometry information to know where to send • Will use the term “DO” for “directional omni” • Step 3: double directional • Create a protocol to arrange for the receiver to aim back at the sender • Termed “DD” for “directional directional”

10. Interference reduction • What happens if we use normal omni MAC and directional data transmission? • Implies no improvement in hop count • Except for some ideas on power or processing tradeoffs • Can dramatically reduce the likelihood of collision • Especially if we also reduce power • Data transmission is most of the time spent • Must relax MAC rules to get this benefit • Ramanathan shows useful benefit

11. Ramanathan results Setup: 40 nodes, steered beams, “aggressive” CA, power control

12. Basic DMAC algorithm • A MAC algorithm for “DO” operation • Must know where receiver is to point beam • Can only get “half” the gain benefit • Although they also propose using receiver gain for data • Basically like 802.11, but per-direction • Sender aims at receiver and sends RTS • As always, after doing aimed carrier sense • Non-receivers update DNAV for arrival direction • Receiver sends aimed CTS

13. DNAV table operation

14. Basic DMAC problems • Hidden terminals • “Asymmetry in gain”: can only hear RTS/CTS within “DO” range, but can interfere out to “DD” • Can’t hear nearby RTS/CTS when focused away • Can’t hear nearby transmissions in same direction • Deafness • Can’t tell when receiver is focused away and doesn’t hear RTS

15. Multi-hop RTS MAC algorithm • Can we use “DO” routing to set up “DD” hops? • Assume network is dense enough, and we have map

16. MMAC algorithm continued • Basically the same steps, but each repeated for “DO” and “DD” • Sender first aims a “DD” RTS towards receiver • Probably won’t be heard at receiver, unless it happens to be aimed properly for some reason • Point is to alert others along the path (who update DNAV for both arrival direction and opposite (where the receiver is) • Sender sends a “DO” RTS along the “DO” route to receiver • This is forwarded, but must be dropped rather than delayed in any queues • Receiver aims a “DD” CTS towards sender • This will be heard, because sender is expecting it • Also alerts intermediates near the receiver

17. MMAC problems • Hidden terminals • MMAC inherits all the DMAC hidden terminals • Directly, since it also does “DO” communication • Plus some “DD” analogs • “DD” signal may not reach all in-between nodes • It is not the case that all nodes between a pair of “DD” nodes can hear even one endpoint using “DO” reception • If gain > 6 dBi, then adding receiver directivity increases range by more than a factor of two • Nodes directly between “DD” pairs may not hear either end • But any node between a pair can interfere if it beamforms • And carrier sense will only work if it points towards the sender

18. Applicability of beamforming • What kinds of networks are a good match? • Nodes must know location and orientation • Although some algorithms may work purely by sensing incoming direction, and building only a logical map • Antenna arrays are larger than simple antennas • Typical setup is a circle of elements and 1/2l spacing • Or about 16cm diameter for 2.4 GHz • But much smaller for X band and above • Higher gain does require larger arrays • As well as more stable platforms • Fixed systems make a lot of sense • Vehicular too, if we can fold in mobility