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

8 Element Equally Spaced Circular Antenna Array Antenna System Phased Array Antenna Incident Wave 2 1 3 Greater the number of elements in the array, the larger its directivity 1 2 3 4 5 6 7 4 d 7 5 6 8 Element Linear Equally Spaced Antenna Array 8 Element Equally Spaced Circular Antenna Array

Antenna Pattern of 7-element uniform equally spaced circular array. 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.

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.) B Silenced Node H With Omni-directional Antennas E G F

Not possible using Omni Enhancing Network Throughput in Wireless Ad Hoc Networks using Directional Antennas (Cont.) C D A B H Not possible using Omni With Directional Antennas G E F

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 IEEE 802.11 DCF – RTS/CTS access scheme Physical Carrier Sense Physical Carrier Sensing Virtual Carrier Sensing

Using directional antennas 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 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.

Directional NAV (DNAV) Table 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 C CTS 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 Data RTS C B 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 transmission omnidirectional transmission RTS A B X Z DATA X does not know node A is busy. X keeps transmitting RTSs to node A

A and C can communication each other directly 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 Enhancing Network Throughput in Wireless Ad Hoc Networks using Directional Antennas (Cont.) MMAC Protocol Protocol Description : Multi-Hop RTS Based on Basic DMAC protocol D R G S T B A C F DO neighbors DD neighbors RTS DATA

Enhancing Network Throughput in Wireless Ad Hoc Networks using Directional Antennas (Cont.) MMAC Protocol Channel Reservation Send Forwarding RTS with Profile of node F R G S T B C Forwarding RTS DATA A F D

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.

Space Division Multiple Access (SDMA) 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) 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 (Avg. Packet Size + ACK) time Duration field in CTS (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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