Using Directional Antennas for
Medium Access Control
in Ad Hoc Networks
MOBICOM 2002
R. Roy Choudhury et al.
2002.10.16
Prepared by Hyeeun Choi
Contents








Introduction
Related Works
Preliminaries
Basic Directional MAC (DMAC) Protocol
Multi-Hop RTS MAC (MMAC)
Performance Evaluation
Future Work
Conclusion
2/26
Introduction
The Problem of utilizing directional Antennas to
improve the performance of ad hoc networks is
non-trivial
 Pros




Higher gain (Reduced interference)
Spatial Reuse
Cons

Potential possibility to interfere with communications
taking place far away
3/26
Omni-directional Antennas
Silenced
Node
D
B
S
A
C
4/26
Directional Antennas
Not
possible
using Omni
D
B
S
C
A
5/26
Related Works

MAC Proposals differ based on





How RTS/CTS transmitted (omni, directional)
Transmission range of directional antennas
Channel access schemes
Omni or directional NAVs
Gain of directional antennas is equal to the gain of
omni-directional antennas
6/26
Preliminaries (1/2)

Antenna Model

Two Operation modes
: Omni & Directional

Omni Mode:



Omni Gain = Go
Idle node stays in Omni mode.
Directional Mode:


Capable of beamforming in specified direction
Directional Gain = Gd (Gd > Go)
7/26
Preliminaries (2/2)

IEEE 802.11
Physical Carrier
Sensing
Sense
Virtual
Carrier
Sensing
IEEE 802.11 DCF – RTS/CTS access scheme
8/26
Problem Formulation

Using directional antennas

Spatial reuse


Possible to carry out multiple simultaneous
transmissions in the same neighborhood
Higher gain



Greater transmission range than omni-directional
Two distant nodes can communicate with a single hop
Routes with fewer hops
9/26
Basic DMAC Protocol (1/2)

Channel Reservation


A node listens omni-directionally when idle
Sender transmits Directional-RTS (DRTS) using
specified transceiver profile


Physical carrier sense
Virtual carrier sense with Directional NAV

RTS received in Omni mode (only DO links used)
Receiver sends Directional-CTS (DCTS)

DATA,ACK transmitted and received directionally

10/26
Basic DMAC Protocol (2/2)

Directional NAV (DNAV) Table

Tables that keeps track of the directions towards
which node must not initiate a transmission
H
ε = 2ß
RTS
E
2*ß
ε θ
DNAV
C
CTS
B
+Θ
If Θ> 0 ,
New transmission can
be initiated
11/26
Problems with Basic DMAC (1/4)

Hidden Terminal Problems due to asymmetry in
gain

A does not get RTS/CTS from C/B
Data
RTS
A
B
C
12/26
Problems with Basic DMAC (2/4)

Hidden Terminal Problems due to unheard RTS/CTS
D
A
B
C
13/26
Problems with Basic DMAC (3/4)

Shape of Silence Regions
Region of interference for
omnidirectional transmission
Region of interference for
directional transmission
14/26
Problems with Basic DMAC (4/4)

Deafness
Z
RTS
A
B
DATA
RTS
X
X does not know node A is busy.
X keeps transmitting RTSs to
node A
15/26
MMAC Protocol (1/3)

Attempts to exploit the extended transmission
range


Make Use of DD Links
Direction-Direction (DD) Neighbor
A
C
A and C can communication each other directly
16/26
MMAC Protocol (2/3)

Protocol Description : Multi-Hop RTS

Based on Basic DMAC protocol
DO neighbors
RTS
DD neighbors
C
B
G
A
D
DATA
T
F
R
S
17/26
MMAC Protocol (3/3)

Channel Reservation

Send Forwarding RTS with Profile of node F
Fowarding RTS
C
B
A
D
DATA
G
T
F
R
S
18/26
Performance Evaluation (1/6)

Simulation Environment







Qualnet simulator 2.6.1
Beamwidth :45 degrees
Main-lobe Gain : 10 dBi
802.11 transmission range : 250meters
DD transmission range : 900m approx
Two way propagation model
Mobility : none
19/26
Performance Evaluation (2/6)
High Spatial Reuse
D
A
E
F
IEEE 802.11 : 1189.73
Basic DMAC : 2704.18
C
B
Aggregate Throughput (Kbps)
High Directional Interference
Hidden terminal Problem
A
D B
E
C
F
Aggregate Throughput (Kbps)
IEEE 802.11 : 1194.81
Basic DMAC : 1419.51 20/26
Performance Evaluation (3/6)

Aligned Routes
150m
21/26
Performance Evaluation (4/6)

Less aligned Routes
22/26
Performance Evaluation (5/6)

Randomly Chosen Routes
23/26
Performance Evaluation (6/6)

Random Topology
24/26
Future Work
Design of directional MAC protocols that
incorporate transmit power control
 New protocols that rely less on the upper layers for
beamforming information
 Impact of directional antennas on the performance
of routing protocols

25/26
Conclusion

Directional MAC protocols show improvement in
aggregate throughput and delay


Performance dependent on topology


But not always
Random topology aids directional communication
MMAC outperforms DMAC & 802.11

802.11 better in some scenarios
26/26
Related Works

Nasipuri et al


Ko et al


A node may transmit in directions that do not
interfere with ongoing transmission
Bandyopahyay et al


Assume that the gain is equal to the gain of omnidirectional antennas
Present another MAC which uses additional
messages to inform neighborhood nodes about
ongoing communications
Takai et al

Directional virtual Carrier Sensing (DVCS)
27/26
Extra Slides

Gain of Antenna


Used to quantify the directionality of an antenna
Relative power in one direction compared to an
omni-directional antenna
28/26