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A Framework for Distributed Spatio Temporal Communications in Mobile Ad hoc Networks Gentian 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 prashant tele pitt edu oercetin sabanciuniv edu Abstract Space time communications can help combat fading and hence can significantly increase the capacity of ad hoc networks Cooperative diversity or virtual antenna arrays facilitate spatio temporal communications without actually requiring the deployment of physical antenna arrays Virtual MISO entails the simultaneous transmission of appropriately encoded information by multiple nodes to effectively emulate a transmission on an antenna array We present a novel multi layer approach for exploiting virtual MISO links in ad hoc networks The approach spans the physical medium access control and routing layers and provides a a significant improvement in the end to end performance in terms of throughput and delay and b robustness to mobility and interference induced link failures The key physical layer property that we exploit is an increased transmission range due to achieved the diversity gain Except for space time signal processing capabilities our design does not require any additional hardware We perform extensive simulations to quantify the benefits of our approach using virtual MISO links As compared to using only SISO links we achieve an increase of up to 150 in terms of the end to end throughput and a decrease of up to 75 in the incurred end to end delay Our results also demonstrate a reduction in the route discovery attempts due to link failures by up to 60 a direct consequence of the robustness that our approach provides to link failures I I NTRODUCTION The use of antenna arrays in conjunction with space time codes can significantly improve signal quality and thereby enhance the capacity of ad hoc networks Depending on whether multiple transmitting antennas inputs and or multiple receiving antennas outputs are used one could have a Multi Input Single 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 however is infeasible due to the required size of these antennas More specifically the space between two elements of a multiple element antenna array must be at least of the order of 2 being the wavelength used for transmissions For the commonly used 2 4 GHz frequency band the required inter element distance is 6 125 cm Therefore even an antenna with four elements can be too big to be mounted on a laptop and even more so on a PDA or a low cost sensor node A new paradigm that has emerged is the use of virtual antenna arrays also called cooperative diversity With cooperative diversity nodes that are in the same vicinity simultaneously transmit and or jointly receive appropriately encoded signals i e the individual antennas on the multiple nodes are used together to This work is supported in part by the U S Army Research Office under the Multi University Research Initiative MURI grant W911NF 04 1 0224 and the NSF CAREER Grant 0237920 1 Traditional systems wherein nodes have a simple single antenna element are referred to as Single Input Single Output SISO systems form an antenna array With this method one could create virtual MISO SIMO or MIMO links The use of virtual antenna arrays can yield the spatial diversity benefits possible with a traditional antenna array housed on a single node While physical layer research on the use of virtual antenna arrays has been fairly extensive 36 34 35 17 30 there are no mature higher layer protocols which can translate the advantages of using virtual antenna arrays to enhance network and application performance Our overarching objective in this paper is to translate the advantages of using virtual MISO at the physical layer into higher layer performance benefits In this work we define a virtual MISO link to be established when a group of nodes transmitters jointly enable space time communications with a single receiver For the virtual MISO link to be formed the receiver needs to have an estimate of the channel state We do not assume feedback i e the transmitters do not have any knowledge of channel state the diversity benefits are achieved due to the use of space time codes The key advantage provided by a virtual MISO transmission is an increase in the transmission range with only a small increase in the channel interference This is possible due to the improvement in signal quality on the virtual MISO link However exploiting this key benefit requires the establishment and use of such links in a networked setting This is not trivial and requires significant changes at both the routing and the underlying MAC layers In this paper we propose a multi layer approach to exploit virtual MISO links in mobile ad hoc networks Our approach is based on the development of a synergy between the layers of the protocol stack lower layers export appropriate information and optimization handles to higher layers while higher layers allow for the refinement of the performance parameters of lower layers In particular we take advantage of the extended range virtual MISO links to establish shorter paths which in turn leads to an increase in throughput and a reduction in latency First we develop a new MAC protocol that closely ties in with the underlying physical layer to enable virtual MISO links In particular the MAC layer facilitates coordination between the collaborating nodes that transmit jointly on a virtual MISO link Second we design a routing protocol that can construct a path with virtual MISO links Our approach has two attractive properties a it is completely decentralized and nodes do not need more than local one hop information and b it provides robustness to link failures due to both mobility and interference effects The latter property is facilitated via a dynamic anycast mechanism to be discussed later for establishing virtual MISO links We perform extensive simulations with physical layer models that include fading effects to evaluate our approach We observe that our schemes can successfully help form and exploit virtual MISO links They provide a significant improvement in higherlayer performance in terms of the observed end to end throughput and delay In particular in mobile scenarios the


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