1A Framework for Distributed Spatio-Temporal Communications in Mobile Ad hocNetworksGentian Jakllari+, Srikanth V. Krishnamurthy+, Michalis Faloutsos+Prashant V. Krishnamurthy†and Ozgur Ercetin∗+University of California, Riverside,†University of Pittsburgh,∗Sabanci University, Turkey{jakllari, krish, michalis}@cs.ucr.edu, [email protected], [email protected]— Space-time communications can help combat fadingand hence can significantly increase the capacity of ad hoc net-works. Cooperative diversity or virtual antenna arrays facilitatespatio-temporal communications without actually requiring the de-ployment of physical antenna arrays. Virtual MISO entails the si-multaneous transmission of appropriately encoded information bymultiple nodes to effectively emulate a transmission on an antennaarray. We present a novel multi-layer approach for exploiting vir-tual MISO links in ad hoc networks. The approach spans the phys-ical, medium access control and routing layers and provides: (a)a significant improvement in the end-to-end performance in termsof throughput and delay and, (b) robustness to mobility and inter-ference induced link failures. The key physical layer property thatwe exploit is an increased transmission range due to achieved thediversity gain. Except for space-time signal processing capabilities,our design does not require any additional hardware. We performextensive simulations to quantify the benefits of our approach us-ing virtual MISO links. As compared to using only SISO links,we achieve an increase of up to 150% in terms of the end-to-endthroughput and a decrease of up to 75% in the incurred end-to-enddelay. Our results also demonstrate a reduction in the route discov-ery attempts due to link failures by up to 60%, a direct consequenceof the robustness that our approach provides to link failures.I. INTRODUCTIONThe use of antenna arrays in conjunction with space-timecodes can significantly improve signal quality and thereby en-hance the capacity of ad hoc networks. Depending on whethermultiple transmitting antennas (inputs) and/or multiple receiv-ing antennas (outputs) are used, one could have a Multi-InputSingle-Output (MISO) system, a Single-Input Multi-Output(SIMO) system or a Multi-Input Multi-Output (MIMO) system1.The deployment of antenna arrays on small mobile nodes, how-ever, is infeasible due to the required size of these antennas.More specifically, the space between two elements of a multipleelement antenna array must be at least of the order ofλ2, λ beingthe wavelength used for transmissions. For the commonly used2.4 GHz frequency band, the required inter-element distance is6.125 cm. Therefore, even an antenna with four elements can betoo big to be mounted on a laptop and even more so on a PDAor a low cost sensor node.A new paradigm that has emerged is the use of virtual antennaarrays (also called cooperative diversity) With cooperative di-versity, nodes that are in the same vicinity simultaneously trans-mit and/or jointly receive appropriately encoded signals, i.e., theindividual antennas on the multiple nodes are used together toThis work is supported in part by, the U. S. Army Research Office under theMulti-University Research Initiative (MURI) grant W911NF-04-1-0224 and theNSF CAREER Grant 0237920.1Traditional systems wherein nodes have a simple single antenna element arereferred to as Single-Input Single-Output (SISO) systems.form an antenna array. With this method, one could create vir-tual MISO, SIMO or MIMO links. The use of virtual antennaarrays can yield the spatial diversity benefits possible with a tra-ditional antenna array housed on a single node. While physicallayer research on the use of virtual antenna arrays has been fairlyextensive [36], [34], [35], [17], [30], there are no mature higherlayer protocols, which can translate the advantages of using vir-tual antenna arrays to enhance network and application perfor-mance.Our overarching objective in this paper is to translate the ad-vantages of using virtual MISO at the physical layer into higherlayer performance benefits. In this work, we define a virtualMISO link to be established when a group of nodes (transmit-ters) jointly enable space-time communications with a single re-ceiver. For the virtual MISO link to be formed, the receiverneeds to have an estimate of the channel state. We do not as-sume feedback i.e., the transmitters do not have any knowledgeof channel state; the diversity benefits are achieved due to theuse of space-time codes. The key advantage provided by a vir-tual MISO transmission is an increase in the transmission rangewith only a small increase in the channel interference. This ispossible due to the improvement in signal quality on the virtualMISO link. However, exploiting this key benefit requires the es-tablishment and use of such links in a networked setting. Thisis not trivial and requires significant changes at both the routingand the underlying MAC layers.In this paper, we propose a multi-layer approach to exploitvirtual MISO links in mobile ad hoc networks. Our approachis based on the development of a synergy between the layers ofthe protocol stack; lower layers export appropriate informationand optimization “handles” to higher layers, while higher layersallow for the refinement of the performance parameters of lowerlayers. In particular, we take advantage of the extended rangevirtual MISO links to establish shorter paths, which in turn, leadsto an increase in throughput and a reduction in latency. First, wedevelop a new MAC protocol that closely ties in with the under-lying physical layer to enable virtual MISO links. In particular,the MAC layer facilitates coordination between the collaboratingnodes that transmit jointly on a virtual MISO link. Second, wedesign a routing protocol that can construct a path with virtualMISO links. Our approach has two attractive properties: (a) itis completely decentralized and nodes do not need more than lo-cal (one-hop) information, and (b) it provides robustness to linkfailures due to both mobility and interference effects. The latterproperty is facilitated via a dynamic anycast mechanism (to bediscussed later) for establishing virtual MISO links.We perform extensive simulations with physical layer modelsthat include fading effects to evaluate our approach. We observethat our schemes can successfully help form and exploit virtualMISO links. They provide a significant improvement in