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IEEE Transactions on Wireless Communications. 01/2011; 10:1992-2003.
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IEEE Transactions on Information Theory. 01/2011; 57:5151-5162.
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IEEE Transactions on Communications. 01/2011; 59:3463-3474.
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ABSTRACT: Hardware constraints, which motivate receive antenna selection, also require that various antenna elements at the receiver be sounded sequentially to obtain estimates required for selecting the `best' antenna and for coherently demodulating data thereafter. Consequently, the channel state information at different antennas is outdated by different amounts and corrupted by noise. We show that, for this reason, simply selecting the antenna with the highest estimated channel gain is not optimum. Rather, a preferable strategy is to linearly weight the channel estimates of different antennas differently, depending on the training scheme. We derive closed-form expressions for the symbol error probability (SEP) of AS for MPSK and MQAM in time-varying Rayleigh fading channels for arbitrary selection weights, and validate them with simulations. We then characterize explicitly the optimal selection weights that minimize the SEP. We also consider packet reception, in which multiple symbols of a packet are received by the same antenna. New suboptimal, but computationally efficient weighted selection schemes are proposed for reducing the packet error rate. The benefits of weighted selection are also demonstrated using a practical channel code used in third generation cellular systems. Our results show that optimal weighted selection yields a significant performance gain over conventional unweighted selection.
IEEE Transactions on Communications 08/2010; · 1.68 Impact Factor
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IEEE Transactions on Wireless Communications. 01/2010; 9:932-938.
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Proceedings of the Global Communications Conference, 2010. GLOBECOM 2010, 6-10 December 2010, Miami, Florida, USA; 01/2010
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Wireless Communications and Mobile Computing. 01/2010; 10:70-86.
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ABSTRACT: Cooperation between the nodes of wireless multihop networks can increase communication reliability, reduce energy consumption, and decrease latency. The possible improvements are even greater when nodes perform mutual information accumulation using rateless codes. In this paper, we investigate routing problems in such networks. Given a network, a source, and a destination, our objective is to minimize end-to-end transmission delay under energy and bandwidth constraints. We provide an algorithm that determines which nodes should participate in forwarding the message and what resources (time, energy, bandwidth) should be allocated to each. Our approach factors into two sub-problems, each of which can be solved efficiently. For any transmission order we show that solving for the optimum resource allocation can be formulated as a linear programming problem. We then show that the transmission order can be improved systematically by swapping nodes based on the solution of the linear program. Solving a sequence of linear programs leads to a locally optimal solution in a very efficient manner. In comparison to the proposed cooperative routing solution, it is observed that conventional shortest path multihop routing typically incurs additional delays and energy expenditures on the order of 70%. Our first algorithm is centralized, assuming that routing computations can be done at a central processor with full access to channel state information for the entire system. We also design two distributed routing algorithms that require only local channel state information. We provide simulations showing that for the same networks the distributed algorithms find routes that are only about two to five percent less efficient than the centralized algorithm.
08/2009;
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Proceedings of IEEE International Conference on Communications, ICC 2009, Dresden, Germany, 14-18 June 2009; 01/2009
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IEEE Transactions on Wireless Communications. 01/2008; 7:155-165.
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IEEE Transactions on Wireless Communications. 01/2008; 7:4142-4154.
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Proceedings of IEEE International Conference on Communications, ICC 2008, Beijing, China, 19-23 May 2008; 01/2008
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WCNC 2008, IEEE Wireless Communications & Networking Conference, March 31 2008 - April 3 2008, Las Vegas, Nevada, USA, Conference Proceedings; 01/2008
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Proceedings of IEEE International Conference on Communications, ICC 2008, Beijing, China, 19-23 May 2008; 01/2008
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ABSTRACT: In cooperative localization systems, wireless nodes need to exchange accurate position-related information such as time-of-arrival (TOA) and angle-of-arrival (AOA), in order to obtain accurate location information. One alternative for providing accurate position-related information is to use ultra-wideband (UWB) signals. The high time resolution of UWB signals presents a potential for very accurate positioning based on TOA estimation. However, it is challenging to realize very accurate positioning systems in practical scenarios, due to both complexity/cost constraints and adverse channel conditions such as multipath propagation. In this paper, a two-step TOA estimation algorithm is proposed for UWB systems in order to provide accurate TOA estimation under practical constraints. In order to speed up the estimation process, the first step estimates a coarse TOA of the received signal based on received signal energy. Then, in the second step, the arrival time of the first signal path is estimated by considering a hypothesis testing approach. The proposed scheme uses low-rate correlation outputs and is able to perform accurate TOA estimation in reasonable time intervals. The simulation results are presented to analyze the performance of the estimator.
EURASIP Journal on Advances in Signal Processing. 01/2008;
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ABSTRACT: In time hopping impulse radio, $N_f$ pulses of duration $T_c$ are transmitted for each information symbol. This gives rise to two types of processing gain: (i) pulse combining gain, which is a factor $N_f$, and (ii) pulse spreading gain, which is $N_c=T_f/T_c$, where $T_f$ is the mean interval between two subsequent pulses. This paper investigates the trade-off between these two types of processing gain in the presence of timing jitter. First, an additive white Gaussian noise (AWGN) channel is considered and approximate closed form expressions for bit error probability are derived for impulse radio systems with and without pulse-based polarity randomization. Both symbol-synchronous and chip-synchronous scenarios are considered. The effects of multiple-access interference and timing jitter on the selection of optimal system parameters are explained through theoretical analysis. Finally, a multipath scenario is considered and the trade-off between processing gains of a synchronous impulse radio system with pulse-based polarity randomization is analyzed. The effects of the timing jitter, multiple-access interference and inter-frame interference are investigated. Simulation studies support the theoretical results.
05/2007;
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IEEE Transactions on Communications. 01/2007; 55:778-789.
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IEEE Transactions on Communications. 01/2007; 55:489-496.
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IEEE Transactions on Wireless Communications. 01/2007; 6:2551-2559.
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Proceedings of IEEE International Conference on Communications, ICC 2007, Glasgow, Scotland, 24-28 June 2007; 01/2007
