Xianren Wu

Optimal waveform design for UWB radios

With transmit power spectra strictly limited by regulatory spectral masks, the emerging ultra-wideband (UWB) communication systems call for judicious pulse shape design in order to achieve optimal spectrum utilization, spectral mask compatibility, and coexistence with other wireless services. Meanwhile, orthogonal pulse sets are often desired in order to apply high-rate multidimensional modulation and (carrier-free) orthogonal frequency-division multiple access. Motivated by these considerations, we suggest a digital finite impulse response (FIR) filter approach to synthesizing UWB pulses and propose filter design techniques by which optimal waveforms that satisfy the spectral mask can be efficiently obtained. For single pulse design, we develop a convex formulation for the design of the FIR filter coefficients that maximize the spectrum utilization efficiency in terms of both the bandwidth and power allowed by the spectral mask. For orthogonal pulse design, a sequential strategy is derived to formulate the overall pulse design problem as a set of convex subproblems, which are then solved in a sequential manner to yield a set of mutually orthogonal pulses. Our design techniques not only provide waveforms with high spectrum utilization and guaranteed spectral mask compliance but also permit simple modifications that can accommodate several other system objectives.

A unified analysis of routing protocols in MANETs

This paper presents a mathematical framework for the evaluation of the performance of proactive and reactive routing protocols in mobile ad hoc networks (MANETs). This unified framework provides a parametric view of protocol performance, which in turn provides a deeper insight into protocol operations and reveals the compounding and interacting effects of protocol logic and network parameters. The parametric model comes from a combinatorial model, where the routing logic is synthesized along with the characterization of MAC performance. Each wireless node is seen independently as a two-customer queue without priority, where the two types of customers are unicast and broadcast packets. The model captures the essential behavior and scalability limits in network size of both classes of routing protocols, and provides valuable guidance on the performance of reactive or proactive routing protocols under various network configurations and mobility conditions. The analytical results obtained with the proposed model are in close agreement with simulation results obtained from discrete-event Qualnet simulations.

Routing Overhead as A Function of Node Mobility: Modeling Framework and Implications on Proactive Routing

rdquoThe paper presents a mathematical framework for quantifying the overhead of proactive routing protocols in mobile ad hoc networks (MANETs). We focus on situations where the nodes are randomly moving around but the wireless transmissions can be decoded reliablely, when nodes are within communication range of each other. We explicitly present a framework to model the overhead as a function of stability of topology and analytically characterize the statistical distribution of topology evolutions. The OLSR protocol is further singled out for a detailed analysis, incorporating the proposed analytical model. Results are compared against Qualnet simulations for random movements, which corroborate the essential characteristics of the analytical results. The key insight that can be drawn from the analytical results of this paper is that nodal movements will drive up the overhead by a penalty factor, which is a function of the overall stability of the network.

Asymptotically optimal UWB receivers with noisy templates: design and comparison with RAKE

For pulsed ultra-wideband (UWB) radios, a major challenge in receiver design is to collect sufficient energy from ultrashort pulses exposing to strong multipath scattering. We develop a UWB receiver structure along with low-complexity timing synchronization and data demodulation schemes based on noisy templates (NTs). The NT receiver enjoys full multipath diversity, and achieves asymptotically optimal detection performance with robustness to mistiming. The detection error performance of the NT receiver is analyzed and compared with that of RAKE receivers, under realistic channel and timing estimation errors. Insights on the design tradeoffs of NT versus RAKE reception are provided, using unifying metrics that capture the relative importance of various performance-critical factors of individual receivers. The NT receiver outperforms the RAKE with a limited number of fingers under practical operating conditions.

Modeling of topology evolutions and implication on proactive routing overhead in MANETs

We present a mathematical framework for quantifying the impact of node mobility on the overhead of proactive routing protocols in mobile ad hoc networks (MANETs). We focus on MANETs in which nodes move randomly. The analytical model we introduce models signaling overhead as a function of stability of topology, and characterizes the statistical distribution of topology evolutions. Although we could apply our analytical framework to any proactive routing scheme, we use the OLSR protocol as an example of our model, because it is a leading example of proactive routing for ad hoc networking. We corroborate the accuracy of the results obtained analytically by means of results obtained with discrete-event simulations using the same parameters adopted in the analytical model.

Link Lifetime as a Function of Node Mobility in MANETs with Restricted Mobility: Modeling and Applications

We present statistical models to accurately evaluate the distribution of the lifetime of a wireless link in a mobile ad hoc network (MANET) in which nodes move randomly within constrained areas. We show that link lifetime can be computed through a two-state Markov model and further apply the computed statistics to the optimization of segmentation schemes of information stream. Summarizing all these results, we further provide comprehensive analysis on throughput, delay, and storage requirements for MANETs with restricted node mobility.

Orthogonal waveform design for UWB radios

Pulse waveforms with overlapping yet orthogonal spectra are desired in high-rate multidimensional modulation and (carrier-free) orthogonal frequency division multiple access. To this end, we propose a digital filter solution to orthogonal pulse design for UWB impulse radios. A sequential design strategy is used in which the filter tap coefficients generating a new pulse are imposed with linear constraints to guarantee (spectral) orthogonality to all previously designed pulses. By formulating the overall pulse design problem as a set of convex subproblems, a sequence of orthogonal waveforms can be efficiently obtained using convex optimization methods. Complying with a prescribed (FCC) spectral mask, our orthogonal waveforms enjoy high power utilization efficiency compared with other orthogonal waveform alternatives.

Optimized Data Fusion in Bandwidth and Energy Constrained Sensor Networks

This paper considers the problem of decentralized data fusion (DDF) for large wireless sensor networks with stringent bandwidth requirements. To reduce the power and bandwidth costs of wireless transmissions, each sensor node is confined to quantize its sensing data and send 1-bit information only. Under this setting, we derive the maximum likelihood (ML) data fusion rule for decentralized parameter estimation, and analyze its Cramer-Rao lower bound (CRLB) of the fusion performance in the sense of mean square distortion. Depending on the underlying noise characteristics, our 1-bit DDF scheme can achieve estimation performance competitive to or even surprisingly better than that of centralized fusion over unquantized data. There is considerable saving in communication costs, which in turn reduces network energy consumption. Furthermore, we investigate network optimization, for which a worst-case robust design methodology is adopted to formulate a well-behaved min/max optimization problem. From the information processing viewpoint, the resulting optimized network offers robust fusion performance at minimal costs of communication resources

A hybrid view of mobility in MANETs: Analytical models and simulation study

We study the effects of node mobility on the wireless links and protocol performance in mobile ad hoc networks (MANETs). First we examine the behavior of links through an analytical framework and develop statistical models to accurately characterize the distribution of lifetime of such wireless links in MANETs. We compute the lifetimes of links through a two-state Markov model, and use these results to model multi-hop paths and topology changes. We show that the analytical solution follows closely the results obtained through discrete-event simulations for two mobility models, namely, random direction and random waypoint mobility models. Finally, we present a comprehensive simulation study that combines the results from the findings in simulations with the analytical results to bring further insight on how different types of mobility translate into protocol performance.

Link dynamics in MANETS restricted node mobility: modeling and applications

We present statistical models to accurately evaluate the distribution of the lifetime of wireless links in a mobile ad hoc network (MANET) in which nodes move randomly within constrained areas. We show that link lifetime can be computed through a two-state Markov model and further apply the computed statistics to the optimization of segmentation schemes of an information stream. Summarizing all these results, we further provide a comprehensive analysis on throughput, delay, and storage requirements for MANETs with restricted node mobility.