Directional Antennas for Wireless Networks Romit Roy Choudhury

Directional Antennas for Wireless Networks Romit Roy Choudhury 1

Several Challenges, Protocols Applications Internet 2

Omnidirectional Antennas Internet 3

IEEE 802. 11 with Omni Antenna RTS = Request To Send CTS = Clear To Send M S Y RTS D CTS X K 4

IEEE 802. 11 with Omni Antenna silenced M S Data Y D silenced ACK X silenced K 5

IEEE 802. 11 with Omni Antenna E silenced M A silenced F C silenced Y `` Interference management `` silenced S Data D silenced challenge for dense multihop networks A crucial G X B silenced ACK silenced D silenced K silenced 6

Managing Interference n Several approaches § Dividing network into different channels § Power control § Rate Control … New Approach … Exploiting antenna capabilities to improve the performance of wireless multihop networks 7

From Omni Antennas … E silenced M A C silenced F silenced D S silenced G X B silenced Y silenced D silenced K silenced 8

To Beamforming Antennas E M A C F S Y D G X B D K 9

To Beamforming Antennas E M A C F S Y D G X B D K 10

Today n Antenna Systems A quick look n New challenges with beamforming antennas 11

Antenna Systems n Signal Processing and Antenna Design research § Several existing antenna systems • Switched Beam Antennas • Steerable Antennas • Reconfigurable Antennas, etc. § Many becoming commercially available For example … 12

Electronically Steerable Antenna [ATR Japan] n Higher frequency, Smaller size, Lower cost § Capable of Omnidirectional mode and Directional mode 13

Switched and Array Antennas n On poletop or vehicles § Antennas bigger § No power constraint 14

Antenna Abstraction n 3 Possible antenna modes § § § Omnidirectional mode Single Beam mode Multi-Beam mode n Higher Layer protocols select § § Antenna Mode Direction of Beam 15

Antenna Beam n Energy radiated toward desired direction Main Lobe (High gain) A A Sidelobes (low gain) Pictorial Model 16

Directional Reception n Directional reception = Spatial filtering § Interference along straight line joining interferer and receiver C A Signal Interference C B Signal A Interference B D D No Collision at A 17

Will attaching such antennas at the radio layer yield most of the benefits ? Or Is there need for higher layer protocol support ? 18

We design a simple baseline MAC protocol (a directional version of 802. 11) We call this protocol DMAC and investigate its behavior through simulation 19

DMAC Example Y S D X n Remain omni while idle § Nodes cannot predict who will trasmit to it 20

DMAC Example RTS Y S D X n Assume S knows direction of D 21

DMAC Example RTS Y CTS S RTS DATA/ACK D X X silenced … but only toward direction of D 22

Intuitively Performance benefits appear obvious 23

Throughput (Kbps) However … Sending Rate (Kbps) 24

Clearly, attaching sophisticated antenna hardware is not sufficient Simulation traces revealed various new challenges Motivates higher layer protocol design 25

n Antenna Systems A quick look n New challenges with beamforming antennas 26

New Challenges [Mobicom 02] Self Interference with Directional MAC 27

Unutilized Range n Longer range causes interference downstream § Offsets benefits A Data B C D route § Network layer needs to utilize the long range § Or, MAC protocol needs to reduce transmit power 28

New Challenges II … New Hidden Terminal Problems with Directional MAC 29

New Hidden Terminal Problem n Due to gain asymmetry CTS A B RTS Data C n Node A may not receive CTS from C § i. e. , A might be out of DO-range from C 30

New Hidden Terminal Problem n Due to gain asymmetry CTS A Carrier Sense B RTS Data C n Node A later intends to transmit to node B § A cannot carrier-sense B’s transmission to C 31

New Hidden Terminal Problem n Due to gain asymmetry Collision A RTS B Data C n Node A may initiate RTS meant for B § A can interfere at C causing collision 32

New Challenges II … New Hidden Terminal Problems with Directional MAC 33

New Hidden Terminal Problem II Y S Data D X n While node pairs communicate § X misses D’s CTS to S No DNAV toward D 34

New Hidden Terminal Problem II Collision Y S Data D RTS X n While node pairs communicate § X misses D’s CTS to S No DNAV toward D § X may later initiate RTS toward D, causing collision 35

New Challenges III … Deafness with Directional MAC 36

Deafness n Node N initiates communication to S § S does not respond as S is beamformed toward D § N cannot classify cause of failure § Can be collision or deafness M S Data D S T R N 37

Channel Underutilized n Collision: N must attempt less often n Deafness: N should attempt more often § Misclassification incurs penalty (similar to TCP) M S Data D S T R N Deafness not a problem with omnidirectional antennas 38

Deafness and “Deadlock” n Directional sensing and backoff. . . § Causes S to always stay beamformed to D § X keeps retransmitting to S without success § Similarly Z to X a “deadlock” Z S RTS DATA D RTS X 39

New Challenges IV … MAC-Layer Capture The bottleneck to spatial reuse 40

Capture n Typically, idle nodes remain in omni mode § When signal arrives, nodes get engaged in receiving the packet § Received packet passed to MAC § If packet not meant for that node, it is dropped Wastage because the receiver could accomplish useful communication instead of receiving the unproductive packet 41

Capture Example A C C D D B Both B and D are omni when signal arrives from A A B B and D beamform to receive arriving signal 42

Take Away Message n Technological innovations in many areas § Several can help the problem you are trying to solve § Although gains may not come by plug-and-play § New advances need to be embraced with care. n Directional/Beamforming antennas, a case study § Gains seemed obvious from a high level § Much revisions to protocols/algorithms needed § Some of the systems starting to come out today n Above true for projects you will do in class 43