@article{li_townsend_2011, title={Near-Far Resistant Synchronization for UWB Communications}, volume={10}, ISSN={["1558-2248"]}, DOI={10.1109/twc.2010.120610.091659}, abstractNote={This paper proposes a symbol-level synchronization technique for ultra-wideband (UWB) communications in networks without power control. The near-far problem dramatically degrades the performance of synchronization for UWB communications. The technique is based on an easy-to-implement procedure to suppress the interfering waveforms from high-power users without requiring knowledge of their spreading codes. The technique includes two operations, a "suppression" operation, and a subsequent dimension detection operation that determines the desired user's code phase during acquisition in the receiver. Simulation results are provided that show the improved performance of the near-far resistant synchronization procedure proposed in this paper for up to two high-power and four equal-power interfering users when compared to the best alternative from the literature.}, number={2}, journal={IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS}, author={Li, Liping and Townsend, Keith}, year={2011}, month={Feb}, pages={519–529} }
 @article{li_townsend_2010, title={M-ary PPM for Transmitted Reference Ultra-Wideband Communications}, volume={58}, ISSN={["1558-0857"]}, DOI={10.1109/tcomm.2010.07.080675}, abstractNote={In this paper, we investigate the multiple-access (MA) performance of transmitted reference (TR) ultra-wideband (UWB) systems employing M-ary PPM in multipath environments. Because we assume there is no power control in the network, interfering power levels from users can vary over many 10's of dB. We categorize the interfering users into two groups with respect to power levels. A new method for deriving the variance of the Gaussian random variable resulting from multiple-access interference (MAI) is developed using the power delay profile (PDP) of the channel. This makes the theoretical analysis tractable and enables us to predict the system-level performance such as the supported number of users, the achievable data rate, and the required bit-energy-to-noise ratio. Chip discrimination is applied and shown to mitigate the effect of high-power users. Performance is quantified due to the combined effects of equal-power and high-power users.}, number={7}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Li, Liping and Townsend, J. Keith}, year={2010}, month={Jul}, pages={1912–1917} }
 @article{li_townsend_ulman_2008, title={Multiple-Access Performance of Transmitted Reference UWB Communications with M-ary PPM}, ISSN={["1930-529X"]}, DOI={10.1109/glocom.2008.ecp.752}, abstractNote={To increase the data rate for transmitted reference (TR) ultra-wideband (UWB) systems, M-ary pulse position modulation (PPM) is used in this paper. The multiple-access performance of such systems is investigated in a multipath environment. A new method to derive the variance of the Gaussian random variable resulted from multiple-access interference (MAI) is developed using the power delay profile (PDP) of the channel. By applying an upper bound, a general relationship is reached for system parameters such as the number of supported users, the bit error probability, the required Eb/No, and the data rate. Tradeoffs between system parameters are discussed as well in this paper.}, journal={GLOBECOM 2008 - 2008 IEEE GLOBAL TELECOMMUNICATIONS CONFERENCE}, author={Li, Liping and Townsend, J. Keith and Ulman, Robert J.}, year={2008} }
 @article{li_townsend_ulman_2008, title={Transmitted Reference Ultra-Wideband Communications with M-ary PPM}, ISSN={["1930-529X"]}, DOI={10.1109/glocom.2008.ecp.753}, abstractNote={To increase the transmission data rate, we propose to use M-ary pulse position modulation (PPM) for transmitted reference (TR) ultra-wideband (UWB) systems. The performance of TR UWB systems employing M-ary PPM is investigated in multipath, multiuser environments without centralized power control. The interference from high-power users is shown to significantly degrade the performance of TR UWB systems using M-ary PPM. Chip discrimination is used to mitigate the detrimental effect of high-power users. Theoretical analysis on the performance of M-ary PPM applied to TR UWB systems is derived. A tight upper bound for the symbol error probability is obtained. Analysis and numerical results show that TR UWB systems with M-ary PPM using chip discrimination is robust against high-power interfering users in wireless networks where centralized power control is infeasible, such as ad-hoc networks.}, journal={GLOBECOM 2008 - 2008 IEEE GLOBAL TELECOMMUNICATIONS CONFERENCE}, author={Li, Liping and Townsend, J. Keith and Ulman, Robert J.}, year={2008} }
 @article{lovelace_townsend_2005, title={Threshold discrimination and blanking for large near-far power ratios in UWB networks}, volume={53}, ISSN={["1558-0857"]}, DOI={10.1109/tcomm.2005.855021}, abstractNote={A simple chip-discrimination technique is presented for use with ultra-wideband (UWB) impulse radio (IR) that improves performance for large near-far interference power ratios. A typical spread-spectrum IR that employs a matched-filter sum for bit decisions is susceptible to small numbers of large power pulses that can dominate the bit decision-threshold statistics. This letter describes a technique for chip discrimination prior to the spreading summation, that can greatly reduce the effects of large near-far power ratios among interferers. The technique exploits the very narrow pulsewidth and resulting low-duty-cycle characteristic only achievable with ultra-wide bandwidth. A statistical model is developed that predicts bit-error performance for binary offset pulse position modulation as a function of near-far density and power for varying discrimination thresholds. An analytic solution for perfect chip blanking is developed, and is in good agreement with chip discrimination for large near-far power ratios. We find that even a small number of very near interferers can greatly reduce the performance of a system without blanking or discrimination. Results show substantial improvement using this method for near interferers with near-far power ratios greater than 20 dB.}, number={9}, journal={IEEE TRANSACTIONS ON COMMUNICATIONS}, author={Lovelace, WM and Townsend, JK}, year={2005}, month={Sep}, pages={1447–1450} }
 @article{lovelace_townsend_2002, title={The effects of timing jitter and tracking on the performance of impulse radio}, volume={20}, ISSN={["1558-0008"]}, DOI={10.1109/JSAC.2002.805058}, abstractNote={Impulse radio (IR) is a promising ultra-wideband technique for tactical military communications. A key feature of time-hopping IR are the very narrow pulses used to convey information. Analysis of such time-hopping schemes under a variety of assumptions have been reported in the literature. However, none of these studies to date consider the effects of timing jitter and tracking on time-hopping in a ultra-wideband (UWB) setting. We consider the effects of timing jitter and tracking on the performance of binary and 4-ary UWB communications. We find that the performance of IR is very sensitive to timing jitter and tracking, at least in part due to the very narrow pulses. We also find that in the presence of timing jitter and tracking, orthogonal 4-ary pulse position modulation (PPM) out performs binary offset PPM at all jitter levels in thermal and pulse noise. Simulation results are presented that quantify the sensitivity of binary and 4-ary IR to timing jitter and tracking error.}, number={9}, journal={IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS}, author={Lovelace, WM and Townsend, JK}, year={2002}, month={Dec}, pages={1646–1651} }
 @article{akyamac_freebersyser_townsend_1999, title={Efficient simulation of QoS in ATM switches using connection traffic descriptors}, volume={38}, DOI={10.1016/s0166-5316(99)00040-1}, abstractNote={High speed networks using asynchronous transfer mode (ATM) will be able to carry a broad range of traffic classes and will be required to provide QoS measures, such as the cell loss and cell delay probabilities, to many of these traffic classes. The design and testing of ATM networks and the algorithms that perform connection admission control is difficult due to the rare event nature associated with QoS measures, and the unwieldiness of matching statistical models of the broad range of traffic classes entering the network to the connection traffic descriptors used by the connection admission control algorithms. In this paper, as an alternative to using statistical traffic models, we describe the traffic entering the network by the connection traffic descriptors standardized by the ATM Forum and used by the connection admission control algorithms. We present a Monte Carlo simulation model for estimating the cell loss and cell delay probabilities using a multinomial formulation to remove the correlation associated with estimating bursty events. We develop importance sampling techniques to increase the efficiency of the simulation for ATM networks with heterogeneous input traffic classes, namely constant bit rate and variable bit rate traffic. For the experimental examples considered here, the improvement in simulation efficiency compared to conventional Monte Carlo simulation is inversely proportional to the probability estimate. The efficient simulation methods developed here are suitable for the design and testing of the switches and connection admission control algorithms planned for use in ATM networks.}, number={2}, journal={Performance Evaluation}, author={Akyamac, A. A. and Freebersyser, J. A. and Townsend, J. K.}, year={1999}, pages={105–132} }
 @inproceedings{haraszti_townsend_1999, title={Rare event simulation of delay in packet switching networks using DPR-based splitting}, volume={1}, DOI={10.1145/324138.324261}, abstractNote={Article Rare event simulation of delay in packet switching networks using DPR-based splitting Share on Authors: Zsolt Haraszti Ericsson Radio Systems AB, S-12625 Stockholm, Sweden Ericsson Radio Systems AB, S-12625 Stockholm, SwedenView Profile , J. Keith Townsend Center for Advanced Computing and Communication, Department of Electrical & Computer Engineering, North Carolina State University, Raleigh, NC Center for Advanced Computing and Communication, Department of Electrical & Computer Engineering, North Carolina State University, Raleigh, NCView Profile Authors Info & Claims WSC '99: Proceedings of the 31st conference on Winter simulation: Simulation---a bridge to the future - Volume 1December 1999 Pages 402–410https://doi.org/10.1145/324138.324261Online:01 December 1999Publication History 3citation214DownloadsMetricsTotal Citations3Total Downloads214Last 12 Months0Last 6 weeks0 Get Citation AlertsNew Citation Alert added!This alert has been successfully added and will be sent to:You will be notified whenever a record that you have chosen has been cited.To manage your alert preferences, click on the button below.Manage my AlertsNew Citation Alert!Please log in to your account Save to BinderSave to BinderCreate a New BinderNameCancelCreateExport CitationPublisher SiteGet Access}, booktitle={1999 Winter Simulation Conference: proceedings: Pointe Hilton Squaw Peak Resort, Phoenix, AZ, U.S.A., 5-8 December 1999}, publisher={Piscataway, New Jersey: IEEE; New York, NY: Association for Computing Machinery ; San Diego, California: Society for Computer Simulation International}, author={Haraszti, Z. and Townsend, J. K.}, year={1999}, pages={402–410} }
 @article{townsend_haraszti_freebersyser_devetsikiotis_1998, title={Simulation of rare events in communications networks}, volume={36}, ISSN={["0163-6804"]}, DOI={10.1109/35.707815}, abstractNote={Computer simulation is an important tool in the analysis and design of communications networks. In spite of the advances in computational power, using simulation to obtain rare event probabilities such as cell/packet loss or delay in networks still requires prohibitively long execution times. We provide an overview of importance sampling techniques and how they can be used to provide orders of magnitude speedup for many network problems.}, number={8}, journal={IEEE COMMUNICATIONS MAGAZINE}, author={Townsend, JK and Haraszti, Z and Freebersyser, JA and Devetsikiotis, M}, year={1998}, month={Aug}, pages={36–41} }
 @article{alqaq_townsend_1997, title={A stochastic importance sampling methodology for the efficient simulation of adaptive systems in frequency nonselective Rayleigh fading channels}, volume={15}, ISSN={["0733-8716"]}, DOI={10.1109/49.585772}, abstractNote={We present an IS stochastic technique for the efficient simulation of adaptive systems which employ diversity in the presence of frequency nonselective slow Rayleigh fading and additive, white, Gaussian noise. The computational efficiency is achieved using techniques based on importance sampling (IS). We utilize a stochastic gradient descent (SGD) algorithm to determine the near-optimal IS parameters that characterize the dominant fading process. After accounting for the overhead of the optimization algorithm, average speed-up factors of up to six orders of magnitude [over conventional Monte Carlo (MC)] were attained for error probabilities as low as 10/sup -11/ for a fourth-order diversity model.}, number={4}, journal={IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS}, author={AlQaq, WA and Townsend, JK}, year={1997}, month={May}, pages={614–625} }
 @article{akyamac_townsend_1997, title={Efficient simulation of rare jitter probabilities in atm switches}, DOI={10.1109/glocom.1997.644400}, abstractNote={The ATM Forum has defined two different types of cell delay variation (CDV) measures: the 1-Point CDV which is a measure of jitter, is typically specified for constant bit rate (CBR) sources and the 2-Point CDV, which can be specified for both CBR and variable bit rate (VBR) sources. We model the traffic using the ATM Forum standardized traffic descriptors and consider dispersion as a measure of jitter. The events associated with excessive jitter in ATM networks are typically rare (<10/sup -6/), hence Monte Carlo simulation is not feasible. We extend our previous work on remote delay quantiles for heterogeneous systems to generate efficient simulation techniques using importance sampling to estimate the 2-Point CDV for mixed CBR and VBR sources. Subsequently, we present a novel importance sampling simulation methodology to estimate the 1-Point CDV for CBR sources in the presence of background VBR traffic. For both cases, we observe from experimental results that the improvement in simulation efficiency is inversely proportional to the probability being estimated.}, number={4}, journal={TR (Center for Advanced Computing and Communication)}, author={Akyamac, A. A. and Townsend, J. K.}, year={1997} }
 @article{benletaief_biglieri_rappaport_townsend_p._1997, title={Guest editorial: Computer-aided modeling, analysis, and design of communication links}, volume={15}, number={4}, journal={IEEE Journal on Selected Areas in Communications}, author={Benletaief, K. and Biglieri, E. and Rappaport, T. S. and Townsend, J. Keith and P., Taylor D.}, year={1997}, pages={593–596} }
 @article{kolenchery_townsend_bilbro_1997, title={Performance of local power control in peer-to-peer impulse radio networks with bursty traffic}, number={5}, journal={TR (Center for Advanced Computing and Communication)}, author={Kolenchery, S. S. and Townsend, J. K. and Bilbro, G. L.}, year={1997} }