Enhancing Network Throughput in Wireless Ad Hoc Networks using Smart Antennas

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 omni-directional 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

0 1 2 3 4 5 6 7

d

Incident Wave

8 Element Linear Equally Spaced

Antenna Array

0

1

2

3

4

56

7

8 Element Equally Spaced Circular Antenna Array

Greater the number of elements in the array, the larger its directivity

Antenna System (Cont.) Beam Forming

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.

Antenna Pattern of 7-element

uniform equally spaced circular

array.

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.


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.

Linearly equally Space (LES) antenna array

Adaptive Beam Rely on beam-forming

algorithm to steer the main lobe of the beam.

Can place nulls to the direction of the interferences.


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.)

C

D

A

B

Silenced Node

H

With Omni-directional Antennas

E

G

F


C

D

A

B

H

Not possible

using Omni

With Directional Antennas

G

E

F


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)


IEEE 802.11

IEEE 802.11 DCF – RTS/CTS access scheme

Physical Carrier Sense

Physical Carrier Sensing

Virtual Carrier Sensing


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

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

E

H

B

2*ß ε θ

ε = 2ß + Θ

If Θ> 0 ,New transmission can be initiated

DNAV

CCTS

RTS

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

C

A B

DataRTS

CB

D

A 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 Deafness

Region of interference for directional transmissionRegion of interference for

omnidirectional transmission

RTS

RTS

A B

X

Z

DATA

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

C

A

A and C can communication each other directly


Protocol Description : Multi-Hop RTS Based on Basic DMAC protocol

D

R

G

S

T

B

A

C

F

DO neighbors

DD neighborsRTS

DATA


Channel Reservation Send Forwarding RTS with Profile of node F

R

G

S

T

BC

Forwarding RTS

DATA

A FD

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 AntennasMAC 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)

Nodes that receives the RTR, RTS and CTS adaptively steer nulls in the appropriate directions.

Spatial Null Angle Vector (SNAV) Table (Analogous to DNAV)

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

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

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 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/etd-05072003-180228/unrestricted/etd.pdf

Marwin Sanchez G., “Multiple Access Protocols with Smart Antennas in Multihop Ad Hoc Rural-Area Networks” Dissertation, June 2002. [Online] http://www.s3.kth.se/radio/Publication/Pub2002/Sanchez_Lict2002.pdf

http://camars.kaist.ac.kr/~hyoon/courses/cs710_2002_fall/2002cas/tp/%5BA22%5D.ppt

http://thcs-3.cs.nthu.edu.tw/paper/poong/030717.pdf

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Enhancing Network Throughput in Wireless Ad Hoc Networks using Smart Antennas