Enhancing Network Throughput
in Wireless Ad Hoc Networks
using Smart Antennas
Vivek Jain
Outline




Introduction
Antenna System
Smart Antenna System
Enhancing Network Throughput in
Wireless Ad Hoc Networks using:



Directional Antennas
Smart Antennas
Conclusions
Introduction


Throughput is low in Wireless Ad hoc
networks because of using omnidirectional antennas.
Node can forward only a single packet
at a time resulting in poor spatial reuse.
Introduction (Cont.)

Smart Antennas Allow nodes to have
simultaneous reception or transmission
of multiple packets enhancing the
network throughput substantially.
A
D
B
C
Antenna System
Phased Array Antenna

Incident Wave
2
1
3
0 1
2 3 4 5 6 7

d
0
4
5
8 Element Linear Equally Spaced
7
6
Antenna Array
8 Element Equally Spaced
Circular Antenna Array
Greater the number
of elements in the
array, the larger its
directivity
Antenna System (Cont.)

Beam Forming


Antenna Pattern of
7-element uniform
equally spaced
circular array.
Technique in which the gain pattern of an
adaptive array is steered to a desired direction
through either beam steering or null steering
signal processing algorithms.
Adaptive beam forming algorithms can provide
substantial gains (of the order of 10log(M) dB,
where M is number of array elements) as
compared to omni directional antenna system.
Smart Antenna System

Smart Antennas can be classified into
two groups:

Switched Beam

Adaptive Antenna Array
Smart Antenna System (Cont.)

Switched Beam





Consists of a set of
predefined beams.
Allows selection of signal
from desired user.
Beams have narrow main
lobe and small side-lobes.
Signals received from side-lobes can be significantly
attenuated.
Uses a linear RF network, called a Fixed Beam-forming Network
(FBN) that combines M antenna elements to form up to M
directional beams.
Smart Antenna System (Cont.)

Adaptive Beam




Rely on beam-forming
algorithm to steer the main
lobe of the beam.
Can place nulls to the
direction of the interferences.
Linearly equally Space (LES) antenna array
Ability to change antenna pattern dynamically to adjust to
noise, interference, and multipath.
Consists of several antenna elements (array) whose signals are
processed adaptively by a combining network, the signals
received at different antenna elements are multiplied with
complex weights and then summed to create a steerable
radiation pattern.
Smart Antenna System (Cont.)

Switched Beam vs. Adaptive Beam
Switched beam systems may not offer the degree of performance
improvement offered by adaptive systems, but they are often much less
complex and are easier to retro-fit to existing wireless technologies.
Enhancing Network Throughput in Wireless Ad
Hoc Networks using Directional Antennas


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
Enhancing Network Throughput in Wireless Ad
Hoc Networks using Directional Antennas (Cont.)
G
Silenced
Node
E
D
B
C
A
F
With Omni-directional Antennas
H
Enhancing Network Throughput in Wireless Ad
Hoc Networks using Directional Antennas (Cont.)
G
Not
possible
using Omni
E
D
B
C
H
A
F
With Directional Antennas
Enhancing Network Throughput in Wireless Ad
Hoc Networks using Directional Antennas (Cont.)


MAC Proposals differ based on
 How RTS/CTS transmitted (omni, directional)
 Transmission range of directional antennas
 Channel access schemes
 Omni or directional NAVs
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)
Enhancing Network Throughput in Wireless Ad
Hoc Networks using Directional Antennas (Cont.)

IEEE 802.11
Physical Carrier
Sensing
Sense
Virtual
Carrier
Sensing
IEEE 802.11 DCF – RTS/CTS access scheme
Enhancing Network Throughput in Wireless Ad
Hoc Networks using Directional Antennas (Cont.)

Using directional antennas

Spatial reuse


Possible to carry out multiple simultaneous
transmissions in the same neighborhood
Higher gain



Greater transmission range than omnidirectional
Two distant nodes can communicate with a
single hop
Routes with fewer hops
Enhancing Network Throughput in Wireless Ad
Hoc Networks using Directional Antennas (Cont.)
Basic DMAC Protocol

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.
Enhancing Network Throughput in Wireless Ad
Hoc Networks using Directional Antennas (Cont.)
Basic DMAC Protocol

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
Enhancing Network Throughput in Wireless Ad
Hoc Networks using Directional Antennas (Cont.)
Problems with Basic DMAC Protocol

Hidden Terminal Problems due to asymmetry
in gain. A does not get RTS/CTS from C/B
D
B
A
A
Data
RTS

B
C
C
Hidden Terminal Problems
due to unheard RTS/CTS
Enhancing Network Throughput in Wireless Ad
Hoc Networks using Directional Antennas (Cont.)
Problems with Basic DMAC Protocol

Shape of Silence Regions

Z
Deafness
RTS
A
DATA
B
RTS
Region of interference for
omnidirectional transmission
Region of interference for
directional transmission
X
X does not know node
A is busy.
X keeps transmitting
RTSs to node A
Enhancing Network Throughput in Wireless Ad
Hoc Networks using Directional Antennas (Cont.)
MMAC Protocol

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
Enhancing Network Throughput in Wireless Ad
Hoc Networks using Directional Antennas (Cont.)
MMAC Protocol

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
Enhancing Network Throughput in Wireless Ad
Hoc Networks using Directional Antennas (Cont.)
MMAC Protocol

Channel Reservation

Send Forwarding RTS with Profile of node F
Forwarding RTS
C
B
G
A
D
DATA
T
F
R
S
Enhancing Network Throughput in Wireless Ad
Hoc Networks using Directional Antennas (Cont.)
Conclusions

Directional MAC protocols show improvement in
aggregate throughput and delay

But not always

Performance dependent on topology

MMAC outperforms DMAC & 802.11


802.11 better in some scenarios
However, throughput can be further enhanced by
enabling simultaneous transmission/receptions by
using Smart Antennas and exploiting Space Division
Multiple Access.
Enhancing Network Throughput in Wireless Ad
Hoc Networks using Smart Antennas
MAC with Space Division Multiple Access (SDMA)

Space Division Multiple Access (SDMA)

Simultaneous multiple reception (or transmission)
of data at the base station using smart antennas
equipped
with
spatial
multiplexers
and
demultiplexers.
Omni-directional Transmission of RTR packet by R
Directional Reception of RTS packets by R
Enhancing Network Throughput in Wireless Ad
Hoc Networks using Smart Antennas (Cont.)
MAC with Space Division Multiple Access (SDMA)
Directional Transmission of CTS packets by R
Directional Reception of DATA packets by R
Directional Transmission of ACK packets by R
Enhancing Network Throughput in Wireless Ad
Hoc Networks using Smart Antennas (Cont.)
MAC with Space Division Multiple Access (SDMA)



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Nodes that receives the RTR, RTS and CTS adaptively steer
nulls in the appropriate directions.
Spatial Null Angle Vector (SNAV) Table (Analogous to DNAV)
RTR
RTS
CTS
Null Time Duration
yes
no
no
(Avg. Packet Size + ACK) time
yes
no
yes
Duration field in CTS
no
yes
no
(Max. Packet Size + ACK) time
Nodes periodically send the RTR frame
If the receiver node cannot form separate beams in the
direction of the transmitters satisfactorily
 RTS collision : wait for next round contention
Conclusions


Using the smart antenna can significantly increase the
spatial reuse and thus increase the network
throughput.
The protocols are designed to increase network
throughput at the cost of some increased design
complexity.
References
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Jr. J. C. Liberti and T. S. Rappaport, “Smart Antennas for Wireless Communications: IS-95 and
Third Generation CDMA Applications”, Prentice Hall, 1999.
Romit Roy Choudhury, Xue Yang, Nitin H. Vaidya, Ram Ramanathan, “Using directional antennas
for medium access control in ad hoc networks”, Proceedings of the Eighth Annual International
Conference on Mobile Computing and Networking, Atlanta, Georgia, pp 59 – 70, 2002.
Rajesh Radhakrishnan, Dhananjay Lal, James Caffery Jr., and Dharma P. Agrawal, “Performance
Comparison of Smart Antenna Techniques for Spatial Multiplexing in Wireless Ad Hoc Networks,”
in Proceedings of the Fifth International Symposium on Wireless Personal Multimedia
Communications, pp 614-619, Oct. 2002.
Dhananjay Lal, Rishi Toshniwal, Rajesh Radhakrishnan, Dharma P. Agrawal and James Caffery,
Jr., “A Novel MAC Layer Protocol for Space Division Multiple Access in Wireless Ad Hoc
Networks”, Proceedings of IEEE Conference on Computer Communications and Networks (ICCCN
) 2002, pp 421-428, 2002.
Vikram Dham, “Link Establishment in Ad Hoc Networks Using Smart Antennas” MS Thesis,
Alexandria, Virginia, Jan. 15, 2003. [Online] http://scholar.lib.vt.edu/theses/available/etd05072003-180228/unrestricted/etd.pdf
Marwin Sanchez G., “Multiple Access Protocols with Smart Antennas in Multihop Ad Hoc RuralArea Networks” Dissertation, June 2002. [Online]
http://www.s3.kth.se/radio/Publication/Pub2002/Sanchez_Lict2002.pdf
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http://camars.kaist.ac.kr/~hyoon/courses/cs710_2002_fall/2002cas/tp/%5BA22%5D.ppt
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http://thcs-3.cs.nthu.edu.tw/paper/poong/030717.pdf
Thank You!!!