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Recent Research Highlights

 

 

This page contains brief summaries of recent research conducted by CSI researchers. This content will change each time the page is loaded or you may view all summaries.

Dynamic Control of Satellite and Wireless Networks
Prof. Michael J. Neely

Satellite and wireless networks operate over time-varying channels that depend on attenuation conditions, power allocation decisions, and inter-channel interference. In order to reliably integrate these systems into a high speed data network and meet the increasing demand for high throughput and low delay, it is necessary to develop efficient network layer strategies that fully utilize the physical capabilities of each network element.

We develop the notion of network layer capacity and describe capacity achieving power allocation and routing algorithms for general networks with wireless links and adaptive transmission rates. The algorithms do not require knowledge of arrival rates or channel statistics. Fundamental issues of delay, throughput optimality, fairness, implementation complexity, and robustness to time varying channel conditions and changing user demands are discussed. Analysis is performed at the packet level and fully considers the queueing dynamics in systems with arbitrary, potentially bursty, arrival processes.

We further consider ad-hoc mobile networks with a special cell-partitioned structure and a simplified mobility model. Exact expressions for capacity and end-to-end delay are derived. To reduce delay, a transmission protocol which sends redundant packets over multiple paths is developed. For large networks, the protocol reduces delay by orders of magnitude at the cost of decreasing throughput. A fundamental delay/rate tradeoff curve is established, and our protocols are shown to operate on distinct boundary points of this curve.

Learn More:
  • M.J. Neely, E. Modiano, and C.E. Rohrs, "Dynamic Power Allocation and Routing for Time Varying Wireless Networks," in IEEE INFOCOM Proceedings, April 2003.

  • M.J. Neely, E. Modiano, and C. E. Rohrs, "Power Allocation and Routing in Multi-Beam Satellites with Time Varying Channels," IEEE Transactions on Networking, Feb. 2003.

  • M.J. Neely, Jun Sun, Eytan Modiano, "Delay and Complexity Tradeoffs for Dynamic Routing and Power Allocation in a Wireless Network," Proceedings of the 40th Annual Allerton Conference on Communication, Control, and Computing, Oct. 2002.

Ultra-Wideband Ranging

Ranging in a Dense Multipath Environment Using an UWB Radio Link
Dr. Joon-Yong Lee and Prof. Robert. A. Scholtz

GMLE algorithm performance for UWB ranging

The very short pulses used in ultra-wideband (UWB) radio, often less than a nanosecond in duration, result in the receiver being able to resolve the UWB signal's time-of-arrival (ToA) with very fine resolution.  This enables potential applications in high-resolution ranging, applications as varied as search and rescue and warehouse inventory control.

Unfortunately, in many such applications, the shortest, direct signal path between the UWB transmitter and receiver is not the strongest path received due to blockage. Hence the challenge for accurate ranging is to identify the time-of-arrival of the direct path, when that path is typically not the strongest path and in fact could be attenuated to a level close to the noise floor.

To achieve that goal this paper introduces a ToA measurement algorithm using generalized maximum-likelihood estimation, employing an iterative nonlinear programming technique to reduce complexity.  In addition, hundreds of blocked line-of-sight observations were analyzed to determine the probability distributions of the amplitude and time-of-arrival of an UWB signal over the direct path. These are used to provide a set of equations that determine the probability of over- and under-estimating range.

In verification experiments using the algorithm, it was found that the previously neglected effect of slowing of the speed of propagation that is caused by line-of-sight blockages resulted in significant excess delay, particularly at long distances, and hence in consistent overestimation of range by up to 5%.

Learn More:
  • J.-Y. Lee and R.A. Scholtz, "Ranging in a Dense Multipath Environment Using an UWB Radio Link," IEEE Journal on Selected Areas in Communications, Vol. 20, Dec 2002, pp. 1677-1683.

  • M.Z. Win and R.A. Scholtz, "Energy Capture versus Correlator Resources in Ultra-wide Bandwidth Indoor Wireless Communications Channels", Proc. MILCOM, Vol. 3, Nov. 1997, pp. 1277-1281.

  • J.M. Cramer, R.A. Scholtz and M.Z. Win, "Evaluation of an Ultra-Wideband Propagation Channel," IEEE Transactions on Antennas and Propagation, Vol. 50, May 2002, pp. 561-570.  (This paper received the 2003 A. Shelkunoff Transactions Prize Paper Award of the IEEE Antennas and Propagation Society.)

Space-Time Coding for Wireless Communications
Prof. Urbashi Mitra, Ph.D. Candidates Mr. Jifeng Geng and Mr. Madhavan Vajapeyam

Signals in a wireless channel typically suffer from fading--amplitude fluctuations due to multiple copies of the received signal being added constructively and destructively. If proper compensation is not considered, fading can severely degrade the performance of a wireless system. One method for dealing with fading, is to introduce diversity, or multiple independent paths across which the signal is transmitted. Space-time systems exploit specifically designed signals for transmission over multiple transmit and receive antennae to generate and capture different copies of signal and improve performance.

Our work in space-time systems has considered: channel equalization for space-time systems, performance analysis, and space-time modulation and code construction. Our equalization work specifically exploits the structure of a popular space-time modulation scheme. The unitary space-time modulation was designed for simple flat fading channels; however with our proposed low complexity equalizer, such modulations can now be employed in multipath fading channels. We have developed a host of tools for analyzing space-time systems both at low and high signal-to-noise ratios, thus removing the need for simulation to understand system performance and also providing methods by which systems can be designed for low signal-to-noise ratio environments. We have also designed optimized space-time modulations for multiuser, spread spectrum systems as well as narrowband systems. Our code constructions are systematic and readily lend themselves for use in trellis-coded modulation. Thus, we have been able to devise coded modulation for an arbitrary number of transmit antennae, block sizes, rates and complexity.

Learn More:

  • J. Geng, M. Vajapeyam and U. Mitra, "Union Bound of Space-Time Block Codes and Decomposable Error Pattern", ISIT 2003.

  • E. Aktas and U. Mitra, "Blind equalization for an application of unitary space-time modulation in ISI channels", in IEEE Transactions on Signal Processing, Volume: 51 Issue: 11, Nov. 2003, Pages: 2931-2942.

  • J. C. Guey, M. P. Fitz, M. R. Bell and W-Y. Kuo, "Signal Design for Transmitter Diversity Wireless Communication Systems over Rayleigh Fading Channels", in IEEE Transactions on Communications, Volume: 46, pages: 527-537, April 1999.

  • V. Tarokh, N. Seshadri and A. R. Calderbank, "Space-Time Codes for High Data Rate Wireless Communication: Performance Criterion and Code Construction", in IEEE Transactions on Information Theory, March 1998.