Impact of Directional Antennas on Ad Hoc Routing

Romit Roy Choudhury

Nitin H. Vaidya

Ad Hoc Networks Typically assume Omnidirectional antennas

A silenced node

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D

Using Directional Antennas …

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D

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C

D

Spatial reuse increases

Wireless interference reduces

Range extension possible

MAC layer performance shown to improve.[Zander, Ramanathan, Takai,RoyChoudhury, Kalyanaraman]

Are directional antennas also beneficial to ad hoc routing ?

Do routing protocols need to be adapted to suit directional antenna systems ?

This Paper Proposes a simple DiMAC protocol

Evaluates impact of DSR over DiMAC Identifies key tradeoffs

Proposes optimizations to suit directional antennas – Directional DSR (DDSR)

Discusses issues where directional antennas may or may not be suitable

Antenna Model

2 Operation Modes: Omni and Directional

A node may operate in any one mode at any given time

Antenna Model

In Omni Mode:• Nodes receive signals with Gain Go

• While idle a node stays in Omni mode

In Directional Mode:• Beamforms in any one of N static beams (switched)• Directional Gain Gd (Gd > Go)

CB

RTSCTS

Directional MAC – DiMAC A node listens omni-directionally when idle Sender transmits Directional-RTS (DRTS)

– Receiver receives RTS in the omni mode (DO links)

Receiver sends Directional-CTS (DCTS) DATA,ACK transmitted and received directionally

CB

DataACK

Directional MAC – DiMAC A node listens omni-directionally when idle Sender transmits Directional-RTS (DRTS)

– Receiver receives RTS in the omni mode (DO links)

Receiver sends Directional-CTS (DCTS) DATA,ACK transmitted and received directionally

Directional MAC – DiMAC Directional Network Allocation Vector (DNAV)

Defer only in the direction of ongoing communication

Broadcast implemented through sweeping

Beam Handoffs (due to node mobility) handled through scanning Send probe packets on recently used beams Update neighbor cache based on replies to probes

Routing Protocols

• Many routing protocols for ad hoc networks rely on broadcast messages– For instance, flood of route requests (RREQ)

• Using omni broadcast will not discover far-away neighbors

• Need to implement broadcast using directional transmissions– A directional transmission, Omni reception = DO link

Dynamic Source Routing [Johnson]

• Sender floods RREQ through the network

• Nodes forward RREQs after appending their names

• Destination node receives RREQ and unicasts a RREP back to sender node, using the route in which RREQ traveled

Route Discovery in DSR

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S E

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H

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Represents a node that has received RREQ for D from S

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Route Discovery in DSR

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S E

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D

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IK

Represents transmission of RREQ

Z

YBroadcast transmission

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[S]

[X,Y] Represents list of identifiers appended to RREQ

Route Discovery in DSR

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S E

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H

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[S,E]

[S,C]

Route Discovery in DSR

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S E

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H

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D

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G

IK

• Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once

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[S,C,G]

[S,E,F]

Route Discovery in DSR

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• Nodes J and K both broadcast RREQ to node D

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[S,C,G,K]

[S,E,F,J]

Route Reply in DSR

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RREP [S,E,F,J,D]

Represents RREP control message

DSR over DiMAC DiMAC broadcast – RREQ transmitted

sequentially on all N beams – sweeping Sweeping allows DO links

Higher delay Higher Overhead

Tradeoffs

Higher tx range Fewer hop routes Lower end to end delay Fewer link failures Narrow beamwidth

Narrow beamwidth High sweeping delay High sweeping overhead Frequent handoffs

Motivation to evaluate impact of directional antennas on routing

Evaluation Simulation

– Qualnet simulator 3.1– Constant Bit Rate (CBR) traffic– Packet Size – 512 Bytes– 802.11 transmission range = 250meters– Channel bandwidth 2 Mbps

– DSR DSR + 802.11 + Omni Antenna – DDSRx DSR + DiMAC + x-Beam Antenna

• E.g., DDSR6 DSR over DiMAC, with beamwidth = 60 degrees

Route discovery latency … Single flow, grid topology (200 m distance)

DSR

DDSR4DDSR6

Throughput

DDSR18

DDSR9

DSR

Observations• Advantage of higher transmit range significant only

at higher separation between source-destination

• Grid distance = 200 m -- thus no gain with higher tx range of DDSR4 (350 m) over 802.11 (250 m).– However, DDSR4 has sweeping delay. Thus route

discovery delay higher

• Sub-optimal routes chosen by DDSR because destination misses shortest RREQ, when beam-formed

Sub-optimal Routes in DDSRF

J

D receives RREQ from J, and replies with RREPMeanwhile, D misses RREQ from K – called Deafness

N

J

RREP

RREQ

D

K

Delayed RREP Optimization

• Due to sweeping – earliest RREQ need not have traversed shortest hop path.– RREQ packets “sweep-ed” to different neighbors at different

points of time

• If destination replies to first arriving RREP, it can miss shorter-path RREQ

• Optimize by having DSR destination wait before replying with RREP– Waiting allows destination to gather all early RREQs

Bridging “Voids” using DDSR For randomly located nodes

Using DDSR can be beneficial in sparse networks. Higher transmission range of directional antennas can communicate across “voids” in the topology.

Throughput and BeamwidthFor randomly located nodes

Routing Overhead

Using omni broadcast, nodes receive multiple copies of same packet – Redundant Broadcast Storm Problem

Using directional Antennas – can do better ?– Forward packets radially outward

Use K antenna elements to forward broadcast packet. K = N/2 in simulations

(No. Ctrl Tx) (Footprint of Tx) No. Data Packets

Ctrl Overhead =

Selective-Forward Optimization

Footprint of Tx

Selective-Forward OptimizationControl overhead reduces

Beamwidth of antenna element (degrees)

Mobility

• Link lifetime increases using directional antennas.– Higher transmission range - link failures are less

frequent

• Handoff: Nodes moving out of beam coverage in order of packet-transmission-time– Low probability

• Antenna handoff– If no response to RTS, MAC layer uses n adjacent

antenna elements to transmit same packet

– Route error avoided if communication re-established

[RoyChoudhury02UIUC Techrep]

Mobility

Aggregate throughput Over random mobile scenarios

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Agg

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DSRDDSR4DDSR6DDSR9DDSR18

Observations

• Randomness in topology aids DDSR.

• Voids in network topology bridged by higher transmission range (prevents partition)

• Higher transmission range increases link lifetime – reduces frequency of link failure under mobility

• Antenna handoff due to nodes crossing antenna elements – not too serious

Future work

• Directional route repair possible in DDSR

• Incorporate Anycasting in DDSR

• Reducing route alignment– Choosing zig-zag routes increase spatial reuse

• Power control based on the knowledge of neighborhood

Conclusion

• Directional antennas can be beneficial to routing– Fewer hop-count

– Bridges network “voids” in sparse scenarios

– Higher link lifetime

• However tradeoffs exist– Broadcast overhead higher

– Handoffs possible when node moves beam beams

– Deafness can cause sub-optimality

Conclusion

• Evaluation shows DDSR better than DSR when– Sparse networks

– Large src-dest separation

– Moderately narrow beamwidth

Thank you

www.crhc.uiuc.edu/~nhv

Issues Broadcast storm: Using broadcasts, nodes receive

multiple copies of same packet

Optimize by using K out of N beams to forward broadcast packets

Performance

• Results indicate that routing performance can be improved using directional antennas

Issues: Sub-optimal Routes Due to sweeping, shortest path RREQ may reach

destination late Sub-optimal routes may be chosen if destination node

misses shortest request, while beamformed

D receives RREQ from JD beamforms to send RREP

D misses RREQ from K

Using Omni, D gets all RREQs

F

J

N

JD

K

RREP

RREQ

PerformanceControl overhead

0

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0 5 10 15 20 25 30

mobility (m/s)

Agg

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hrou

ghpu

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DSRDDSR4DDSR6DDSR9DDSR18

Throughput Vs Mobility

• Control overhead higher using DDSR• Throughput of DDSR higher, even under mobility• Latency in packet delivery lower using DDSR