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UCLA COMSCI 218 - On the Capacity Improvement of Ad Hoc Wireless Networks Using Directional Antennas

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On the Capacity Improvement of Ad Hoc WirelessNetworks Using Directional AntennasSu YiECSE DepartmentRensselaer PolytechnicInstituteTROY, NY [email protected] PeiECE DepartmentUniversity of MiamiCoral Gables, FL [email protected] DepartmentRensselaer PolytechnicInstituteTROY, NY [email protected] capacity of ad hoc wireless networks is constrainedby the interference between concurrent transmissions fromneighboring nodes. Gupta and Kumar have shown that thecapacity of an ad hoc network does not scale well with theincreasing number of nodes in the system when using om-nidirectional antennas [6]. We investigate the capacity ofad hoc wireless networks using directional antennas. In thiswork, we consider arbitrary networks and random networkswhere nodes are assumed to be static.In arbitrary networks, due to the reduction of the interfer-ence area, the capacity gain is proven to be2παwhen usingdirectional transmission and omni reception. Because of thereduced probability of two neighbors pointing to each other,the capacity gain is2πβwhen omni transmission and direc-tional reception are used. Although these two expressionslook similar, the proof technique is different. By taking ad-vantage of the above two approaches, the capacity gain is2π√αβwhen both transmission and reception are directional.For random networks, interfering neighbors are reduceddue to the decrease of interference area when directionalantennas are used for transmission and/or reception. Thethroughput improvement factor is2πα,2πβand4π2αβfor direc-tional transmission/omni reception, omni transmission/direc-tional reception, and directional transmission/directional re-ception, respectively.We have also analyzed hybrid beamform patterns that area mix of omnidirectional/directional and a better model ofreal directional antennas.Categories and Subject DescriptorsC.2 [Computer-Communication Networks]: Miscella-neousPermission to make digital or hard copies of all or part of this work forpersonal or classroom use is granted without fee provided that copies arenot made or distributed for profit or commercial advantage and that copiesbear this notice and the full citation on the first page. To copy otherwise, torepublish, to post on servers or to redistribute to lists, requires prior specificpermission and/or a fee.MobiHoc’03, June 1–3, 2003, Annapolis, Maryland, USA.Copyright 2003 ACM 1-58113-684-6/03/0006 ...$5.00.General TermsTheoryKeywordsCapacity, Ad hoc networks, Directional antenna1. INTRODUCTIONAd hoc wireless networks are wireless networks withoutfixed base stations or any wireline backbone infrastructure.The nodes use peer-to-peer packet transmissions and mul-tihop routes to communicate with each other. Throughputcapacity is a key characteristic of ad hoc networks. Con-sider an ad hoc network with n nodes randomly located ina domain of area one square meter. It was shown by Guptaand Kumar in [6] that under a Protocol Model of interfer-ence, such a network could provide a per node throughputof Θ(1√nlogn) bits/sec. It was also shown there that evenunder the best possible placement of nodes, such a net-work could not provide a per-node throughput of more thanO(1√n) bits/sec. In this case, the total end-to-end capacityis roughly O(n√n), which is O(√n).The key reasons why the overall capacity is reduced are:a) interference in a zone around the receiver prevents anyother node in the zone from receiving data from any trans-mitter.b) as the number of hops increases, the “forwarding bur-den” of nodes increases; i.e., they spend a fraction of theircapacity relaying other nodes’ traffic rather than their own.Even if the interference zone around receivers is of area 0,due to range limits of any one-hop, the multi-hops necessaryfor a large network may in general grow as√n.Several works study how these reasons affect the capacityof the network and try to find ways to complement theseeffects. Li et al.[9] examine interactions of the 802.11 MACand ad hoc forwarding and the effect on capacity for sev-eral simple configurations and traffic patterns. It is shownthat for total capacity to scale up with network size the av-erage distance between source and destination nodes mustremain small as the network grows. In [5], Grossglauser andTse propose a scheme that takes advantage of the mobil-ity of the nodes. By exploiting node mobility as a type ofmultiuser diversity, they show that the throughput can in-crease dramatically when nodes are mobile rather than fixed.Gastpar and Vetterli [4] study the capacity under a different108traffic pattern. There is only one active source and desti-nation pair, while all other nodes serve as relay, assistingthe transmission between this source-destination pair. Thecapacity is shown to scale as O(log n). Liu et al. [10] studythe throughput capacity of hybrid wireless networks formedby placing base stations in a ad hoc network. This is not apure wireless ad hoc network since these base stations areconnected by a high-bandwidth wired network. They showif the number of base stations m grows faster than√n, thethroughput capacity increases linearly with the number ofbase stations.Such research on the capacity of wireless ad hoc networksand the popular IEEE 802.11 protocol typically assume theuse of omnidirectional antennas at all nodes. An outcome ofthis assumption is that all nodes lying in the vicinity of a pairof communicating nodes are required to stay silent. How-ever, with directional antennas, more than one pair of nodeslocated in each other’s vicinity may potentially communicatesimultaneously, depending on the directions of transmission.This can increase spatial reuse of the wireless channel.Nasipuri et al. [11] propose a MAC protocol for an adhoc network of mobile wireless nodes that are equipped withmultiple directional antennas. Their protocol uses a varia-tion of the RTS/CTS exchange to let both source and desti-nation nodes determine each other’s directions. Simulationexperiments indicate that by using four directional antennasin each node, the average throughput in the network can beimproved 2∼3 times over that obtained by using CSMA/CAwith omnidirectional antennas.Ko et al. [7] present a DMAC protocol that exploits thecharacteristics of both directional and omnidirectional an-tennas to allow simultaneous transmissions that are not al-lowed in the 802.11 protocol. Choudhury et


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UCLA COMSCI 218 - On the Capacity Improvement of Ad Hoc Wireless Networks Using Directional Antennas

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